reproduction dinoflagellate produce gametes, pairs of which fuse to produce a hypnozygote (Fig. 1.4). The hypnozygote is often protected by a so-called resting cyst (Dale, 1976, 1983), most of which are of an organic composition, although there are also small numbers of calcareous or siliceous cysts known (Kokinos et al., 1998).
Figure 1.4. Simplified life cycle of cyst-producing dinoflagellates (from Bockelmann, 2007; after Dale, 1983)
Cysts formation mostly occurs during or shortly after periods with very h igh motile cell concentrations and is accompanied by a high nutrient concentration in the water (Ishikawa and Taniguchi, 1996; Montresor et al., 1998). However, encystment could be influenced also by temperature, day length, irradiance and an endogenous rhythm (Anderson and Keafer, 1987). Cysts may survive various adverse environmental conditions such as anoxia, low temperatures and light/nutrient limitation and remain viable for many years under conditions unfavourable for excystment (Dale, 1983). After the dormancy period
excystment occurs, during which the protoplast hatches through an opening in the cyst wall (so called archeopyle). Following the excystment, the hatchling joins the motile community and the empty cyst may undergo burial and become fossilised within marine deposits (Dale, 1983). There are around 2000 living dinoflagellate species, but only about 16% of them are known to form fossilisable dinocysts (Head, 1996). The dinocyst fossil record begins in the Mesozoic, with a major species radiation in the Triassic and
Jurassic (Fig. 1.5; MacRae et al., 1996).
Their long geological history has
established dinocysts as useful stratigraphic markers used commonly in the petroleum exploration industry. Dinocysts biostratigraphy (e.g. Woollam and Riding, 1983; Dimter and Smelror, 1990; Louwye et al., 2004) is widely used to determine the age of sediments on the basis of marker dinocyst species. They are also very important for palaeoenvironmental reconstructions, since
dinocysts distribution and assemblages are related to sea-surface temperature, sea-surface salinity, sea-ice cover and nutrient concentration.
Figure 1.5. Dinocyst diversity throughout the time (after MacRae et al., 1996). 1.3.2. Dinocyst application As a constituent of the phytoplankton, dinoflagellates together with diatoms and coccolithophorids, are responsible for the major part of the marine primary production (Parsons et al., 1984). Their cysts often dominate fossil
assemblages of primary producer origin within marine sediments (Devillers and de Vernal, 2000). Dinocysts are found in deep and shallow marine environments of all climatic zones (Marret and Zonneveld, 2003), and are recognized as useful indicators of palaeoenvironmental conditions. Dinocystbased palaeoenvironmental reconstructions include, for instance, estimation of temperatures, salinity and sea-ice cover and past bottom-water O2

Rochon, A., de Vernal, A., Sejrup, H-P., Haflidason, H., 1998. Palynological Evidence of Climatic and Oceanographic Changes in the North Sea during the Last Deglaciation. Quaternary Research 49, 197207. Rullktter, J., 2006. Organic Matter: The driving Force for Early Diagenesis. In H. D. Schulz, M. Zabel (Eds) Marine Geochemistry, 2nd Edition. Springer-Verlag BerlinHeidelberg, pp 125-168. Schmitz, B., 1987. Barium, equatorial productivity, and the northwardwandering of the Indian continent. Paleoceanpgraphy 2, 63-77. Schnepf, E., Elbrchter, M., 1992. Nutritional strategies in dinoflagellates. European Journal of Protistology 28, 3-24. Schnetger, B., Brumsack, H-J., Schale, H., Hinrichs, J., Ditter, L., 2000. Geochemical characteristics of deep-sea sediments from the Arabian Sea: a highresolution study. Deep-Sea Research II 47, 2735-2768. Schrank, E., 1988. Effects of chemical processing on the preservation of peridinioid dinoflagellates: a case from the late Cretaceous of NE Africa. Review of Palaeobotany and Palynology 56, 123-140. Shemesh, A., Burckle, L. H., Froelich, P. N., 1989. Dissolution and preservation of Antarctic diatoms and the effect on sediment thanatocoenoses. Quaternary Research 31, 288-308. Siegenhalter, U., Sarmiento, J. L., 1993. Atmospheric carbon dioxide and the ocean. Nature 365, 119-125. Suess, E., 1980. Particulate organic carbon flux in the ocean-surface productivity and oxygen utilization. Nature 288, 260-263. Sundquist, E. T., 1985. Geological perspectives on carbon dioxide and the carbon cycle. In E. T. Sundquist, W. S. Broecker (Ed) The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. Geophysical Monographies 32, Washington, D. C., American Geophysical Union. Taylor, F. J., R., Pollingher, U., 1987. Ecology of dinoflagellates. In F. J. R. Taylor (Ed) The biology of dinoflagellates. Botanical Monographs 21, Blackwell, London, pp 398-529. Tribovillard, N., Algeo, T.J., Lyons, T., Riboulleau, A., 2006. Trace metals as paleoredox and paleoproductivity proxies: An update. Chem. Geol. 232, 12-32. Tromp, T. K., van Cappellen, P., Key, R. M., 1995. A global model for the early diagenesis of organic carbon and organic phosphorus in marine sediments. Geochimica et Cosmochimica Acta 59 (7), 1259-1284. Turekian, K. K., Wedepohl, L. H., 1961. Distribution of elements in some major units of the earths crust. Bulletin of the Geological Association of America 72, 175-192. Turgeon, S., Brumsack, H-J., 2006. Anoxic vs dysoxic events reflected in sediment geochemistry during the Cenomanian-Turonian Boundary Event (Cretaceous) in the Umbria-Marche Basin of central Italy. Chemical Geology 234, 321-339. Versteegh, G. J. M., Zonneveld, K. A. F., 2002. Use of selective degradation to separate preservation from productivity. Geology 30 (7), 615-618. Wakeham, S. G., Lee, C., Farrington, J. W., Gagosian, R. B., 1984. Biogeochemistry of particulate organic matter in the oceans: results from sediment trap experiments. Deep-Sea Research 31 (5), 509-528. Walker, J. C. J., 1993. Biogeochemical cycles of carbon on hierarchy of time scales. In R. S., Oremland (Ed) Biogeochemistry of Global Change: Radiatively Active Trace Gases. Chapman and Hall, New York, pp 3-28. Wall, D., Dale, B., Lohmann, G. P., Smith, W. K., 1977. The environmental and climatic distribution of dinoflagellate cysts in modern marine sediments ffrom regions in

heterotrophic peridinioids. Again, these vulnerable species form only a part of the heterotrophic species and species with a peridinioid plate configuration. To get insight in the intrinsic properties of the cysts bringing about the selective preservation, we continue with reviewing the understanding of algal cell walls and dinoflagellate cyst walls at the molecular level. The review documents that cysts of Mesozoic age have different preservation characteristics than Late Cainozoic to Modern species. We propose that over a long periods, taphonomic processes on a molecular level substantially change the cyst wall macromolecular structure and herewith cyst degradability. Having described the impact of selective preservation on the dinoflagellate cyst assemblages, we continue summarising the methods presently available for the recognition of and correction for this diagenetic overprint. Subsequently, we take advantage of the selective preservation by using it for reconstructing past export-production. Since the rates of dinoflagellate cyst degradation are strongly related to the bottom water oxygen concentration, this opens the way for a new proxy to reconstruct deep ocean oxygen concentrations. The importance of the rate of deep ocean ventilation within the marine global carbon cycle and its relationship with climate change, make this use of selective dinoflagellate cyst preservation an important though unexpected application.
Keywords: dinoflagellate cyst, selective preservation, proxy, macromolecular chemistry, diagenesis, ocean ventilation.
2.1. Introduction Organic-walled dinoflagellate cysts are widespread in modern marine sediments and are known to have a long geological history (MacRea et al., 1996). Their high diversity and fast evolution make them very useful for stratigraphic purposes, especially in sediments where calcareous and/or siliceous microfossils are scarce or absent (Huber et al., 2004). The cyst morphology of virtually all dinoflagellates is species-specific. Upon
sedimentation the cysts reflect the distribution of their respective motile stages
in the upper water column at low taxonomic level. Therefore, cyst associations in sediments enable detailed reconstructions of the upper water column. During the last decades methods to establish such reconstructions improved considerably, allowing quantitative estimates of environmental parameters (e.g. Mudie, 1992; Peyron and de Vernal, 2001; de Vernal et al., 2005). Because of their widespread distribution in modern sediments and their prominent abundance in fossil sediments from the Jurassic onward dinoflagellate cysts have been considered to be extremely resistant against degradation (e.g. Dale, 1996). However, recent studies have shown that this does not hold for all modern dinoflagellate cyst species (e.g. Marret, 1993; Hopkins and McCarthy, 2002; Mudie and McCarthy, 2006). It appears that species of some genera are indeed extremely resistant against aerobic degradation whereas others are very vulnerable (Zonneveld et al., 1997; Versteegh and Zonneveld, 2002). In this paper we give a review and update on the present knowledge about aerobic degradation of organic walled dinoflagellate cysts and its chemical background on the basis of laboratory and field experiments, as well as observations from surface sediments and sediment cores. Furthermore, we discuss how the above-mentioned information can be applied in

based on the assumption of a continuum of k-values of the different components (Middelburg, 1989; Middelburg et al., 1997; Moodley et al., 2005). Other factors might also influence the degradation of OM such as bioturbation rates, sedimentation rate and water depth (Middelburg, 1989; Hartnett et al., 1998; Hedges et al., 1999). These factors are considered to influence the oxygen exposure time (OET). Laboratory experiments on the effect of bioturbation on the degradation of freshly produced OM in an aerobic water column with and without bioturbating organisms show that increased bioturbation in a oxygenated water column might result in increased preservation of OM (Sun et al., 2002). The reason is that freshly produced OM, that still contains many components that are easily degradable, is quickly transported to deeper sediment layers that are anoxic. Although simultaneously older organic matter from the deeper anoxic zone becomes exposed to oxygen rich waters again, this does not compensate for the effect of the burying of the labile component. However, so far this could not clearly be shown in natural settings. Furthermore, natural environments with sites characterised by oxygenated bottom/pore water where benthic life is absent are extremely rare and non-bioturbated sediments are generally only found in anoxic
environments. Consequently, total OM degradation in natural environments is higher at bioturbated sites compared to non-bioturbated sites.
grams 20 cyears 4 cyears 4 total mass grams 100 total mass 80 c40 cc3
Figure 2.1. Changes in initial mass versus time of organic matter material composed of three components (C1, C2, C3) that have first order decay constants k of 0.2, 1.0 and 5.0 respectively. A. weight percent, B. relative changes (%). Redrawn after Hedges and Prahl (1993).
The effect of oxygen concentration on OM concentrations might be more complex. Oxygen may affect the growth or maintenance efficiency of organisms that mineralise OM. Recent studies suggest that oxygen concentration does not have a linear relationship to bacterial biomass-production (Middelburg, 1989; Dauwe et al., 1999; Dauwe et al., 2001; Lee et al., 2004; Moodley et al., 2005). Considering a mixture of components, such as TOC or a dinoflagellate cyst association, and the logarithmic nature of the degradation process, the most labile components or species will disappear first so that the more refractory components or species will dominate the composition/association with time (Fig. 2.1b). In rich OM samples, the disappearance of the most reactive components/species, results in a noticeable increase in the weight percentage of the most refractory components/species. When other organic components as dinoflagellate cysts are more reactive to aerobic degradation, exposure to oxygen-rich conditions of a sediment sample with a dinoflagellate cyst association that contains species with different k-values, would result in an increase of weight percentage (cyst per gram sediment) of species with low k whereas the calculated accumulation rates remain constant. T because the his other components would disappear from the sediment increasing the amount of weight percent of the refractive dinoflagellate cysts. Accumulation rates are calculated independent from sediment weight and would therefore not change. For species with high k-values, the relative abundances, amounts of cyst per gram dry sediment and the calculated accumulation rates would decrease over time.

2.4. Selective preservation of cysts in natural environments The first firm indications that modern cyst assemblages might be altered by aerobic degradation in natural environments were published a decade ago as a pilot study on sediments of the Madeira Abyssal Plain (Zonneveld et al., 1997) in a reaction on a paper by Keil et al. (1994b). Sediments of this Plain are characterized by the occurrence of turbidites that consist of a organic-rich ungraded mud that, at times of deposition, had a homogenous chemical and organic composition (Colley et al., 1984; Buckley and Cranston, 1988; de Lange
et al., 1994; Keil et al., 1994a; Cowie et al., 1995). After deposition, oxygen started to penetrate the turbidite from above resulting in partial decomposition of organic matter in the top 50 cm of the studied f-turbidite. This process stopped at the moment a next turbidite covered the site, resulting in a so called fossil oxidation front. By comparing the cyst concentrations in sediments of the unoxidised part of the turbidite with those in the oxidized part, Zonneveld et al. (1997) observed that cysts of Protoperidinium species were selectively degraded whereas cyst concentrations of several gonyaulacoids did not show any change over the front (Fig. 2.3). These results were confirmed by a study of diffusion-limited aerobic decay of organic matter, often referred to as burndown in sediments of the last Eastern Mediterranean Sapropel S1, where similar association and concentration changes were observed when comparing sediments above and below the still active oxidation Front (Fig. 2.4; Zonneveld et al., 2001). Based on these studies Zonneveld et al. (2001) established a table where cyst species were grouped according to their vulnerability (Appendix 2.1). In comparing the different degradation rates of cyst groups with other organic components in the sediments, Versteegh and Zonneveld (2002) showed that the species classified as extremely resistant were more refractory than all other organic components measured. The sensitive species however were among the most reactive components (Appendix 2.2).

vis. S1

Dept h (m m)

p ost.

2.4.1 Sediment traps When the cyst content of sediment trap material is studied from sites where bottom waters are oxygenated, generally a discrepancy can be found between the associations found in the traps compared to those in the underlying sediments. In comparison to the surface sediment samples, trap material is generally enriched in (round brown) Protoperidinium cysts (Fig. 2.5). The first to notice this paradox was Barrie Dale in his pioneering study of trap material that had been collected during one year in the Pacific, equatorial Atlantic and North Atlantic regions (Dale, 1986; Dale, 1992). In the trap sediments of mooring sites in the western equatorial Atlantic Ocean and in the Panama Bight, the organic walled dinoflagellate cyst association was completely formed by round brown cysts of Protoperidinium species (Brigantedinium spp.), with the exception of a single finding of I. sphaericum. Also in the trap material collected in the northern North Atlantic Ocean, the cyst association is dominated by brown Protoperidinium cysts and cysts of the genus Islandinium (probably also formed by dinoflagellates with a peridinioid plate configuration). Although Dale discusses the possibility of the differences in accumulation time between trap samples and surface sediments (one year and up to several 100 to 1000 years), and the possibility of lateral transport altering the cyst association, he did not consider selective preservation as a possible explanation for his observations.

Weddel Sea Mooring VIII

Scotia Sea Mooring IX

Scotia Sea Mooring XI

Bellinghausen Sea Mooring ST-02
surface sediment at mooring site (oxic bottom water)
Northern North Atlantic Mooring NA
Northern North Atlantic Mooring LB
Northern North Atlantic Mooring BI
Northern North Atlantic Mooring FS
Northern North Atlantic Mooring GB
West equatorial Atlatic Mooring E
Arabian Sea Mooring MST-9
off NW Africa Mooring Cap Blanc 9
Arabian Sea Mooring MST-8
surface sediment at mooring site (anoxic bottom water)
= resistant cysts = sensitive cysts = other cysts
Figure 2.5. Relative abundances of three groups of cyst species of sediment trap sediments and underlying surface sediments. Sensitive group includes all cysts of the genera Brigantedinium, Echinidinium, Islandinium, Lejeunecysta, Protoperidinium, Selenopemphix, Trinovantedinium. Resistant groups include all cysts of the genera Impagidinium and Nematosphaeropsis dalei, and

Xf =final cyst concentration (cysts/cm2/ky) and Xi=initial cyst concentration (cysts/cm2/ky).
Figure 2.9. Relationship between accumulation rates of R-cysts and mean annual upper water chlorophyll-a concentrations. Estimated linear relationship with 99.9% confidence limits of mean. Redrawn from Zonneveld et al. (2007). 2.6.2. The use of species selective degradation to recognise differential preservation-states in the past There are only few examples where selective preservation of organic-walled dinoflagellate cysts has been used in pre-Quaternary palaeoceanographic studies. One study focuses on the Late Pliocene to Early Pleistocene sediments of northwest North Pacific Ocean ODP Site 1179. Here, intervals occur that are characterised by high calcium carbonate contents although the site is more than 1km below the modern calcium compensation depth (McCarthy et al., 2004). Within these intervals high concentrations of Brigantedinium cysts are present whereas the intervals without calcium carbonate contain mainly Gonyaulacoid cysts. Considering the different preservation potentials of the dinoflagellate cyst groups and by comparing the foraminifera, pollen, spore, dinoflagellate cyst and dust contents, McCarthy at al. (2004) concluded that this interesting phenomenon results from rapid burial of the calcareous and organic microfossils probably caused by an increased flux of land-derived nutrients, enhancing the sea-surface productivity. The authors suggest that the
associated increase in trace metals triggered algal blooms of e.g. gonyaulacoid dinoflagellates and grazers like the planktic foraminifera and peridinioid dinoflagellates. The enhanced bioproduction in upper waters combined with an increased eolian dust input might have accelerated the sinking of particles resulting in extremely good preservation of microfossils. 2.6.3. Dinoflagellate cysts as bottom water oxygen concentration indicators By grouping cysts with different ecologies, those environmental factors that influence the cyst production, transport and preservation of all species in the group in a similar way, have a strong relationship with the total cyst accumulation of that group. On the contrary, a damped effect occurs for factors that influence only part of the species within the group, or influence individual species of the group in different ways. Recently it has been shown that the degradation rate, kt of the group of dinoflagellate cysts classified as extremely sensitive shows an S-shape curve in relationship to bottom-water oxygen concentration according to the equation O2=5.17/1+e-1.23(kt-2.058); r2=0.85 (Fig. 2.10; Zonneveld et al., 2007). This suggests degradation of dinoflagellate cysts by aerobic bacteria with the bottom water oxygen concentration being a limiting factor for bacterial growth (Jorge and Livingston, 1999; Guerra-Garcia and Garca-Gmez, 2005). Upon anoxia, degradation is absent. When oxygen concentrations increase, the amount of degrading bacteria can increase resulting in higher degradation rates. At a certain point, all S-cysts are being consumed and the kt value will increase to 8. Therefore, the above mentioned relationship can form the basis for establishing quantitative estimates of past deep-ocean oxygen concentrations. Such estimates have been quite

problematic when calculated from other proxies such as those based on sediment structure, the (bio-) chemical content of sediments and the chemical and isotopic composition of microfossils or on numerical models (e.g. Francois et al., 1997; Toggweiler, 1999; Ninnemann and Charles, 2002; Matear and Hirst, 2003; McManus et al., 2004; Ivanochko and Pedersen, 2004).

8 kt (c/cm 2/ky)

7 bottom water oxygen (ml/l)
Figure 2.10. Relationship between the degradation expressed of S-cysts by kt and bottom water O2. Redrawn from Zonneveld et al. (2007). 2.7. References
Aken, M E. and Pienaar, R. N., 1985. Preliminary investigations on the chemical. composition of the scale-boundary and cyst wall of Pyramimonas pseudoparkeae (Prasinophyceae). South African Journal of Botany 51, 408-416. Allard, B. and Templier, J., 2000. Comparison of neutral lipid profile of various trilaminar outer cell wall (TLS)-containing microalgae with emphasis on algaenan occurrence. Phytochemistry 54, 369-380. Allard, B., Templier, J. and Largeau, C., 1998. An improved method for the isolation of artifact-free algaenans from microalgae. Organic Geochemistry 28, 543-548. Baas, M., Briggs, D. E. G., van Heemst, J. D. H., Kear, A. J. and de Leeuw, J. W., 1995. Selective preservation of chitin during the decay of shrimp. Geochimica et Cosmochimica Acta 59, 945-951. Bertheas, O., Metzger, P. and Largeau, C., 1999. A high molecular weight complex lipid, aliphatic polyaldehyde tetraterpenediol polyacetal from Botryococcus braunii (L race). Phytochemistry 50, 85-96. Blokker, P., Schouten, S., de Leeuw, J. W., Sinninghe Damst, J. S. and van den Ende, H., 1999. Molecular structure of the resistant biopolymer in the zygospore cell walls of Chlamydomonas monoica. Planta 207, 539-543. Blokker, P., Schouten, S., van den Ende, H., de Leeuw, J. W., Hatcher, P. G. and Sinninghe Damst, J. S., 1998. Chemical structure of algaenans from the fresh water algae Tetraedron minimum, Scenedesmus communis and Pediastrum boryanum. Organic Geochemistry 29, 1453-1468. Blokker, P., van den Ende, H., de Leeuw, J. W., Versteegh, G. J. M. and Sinninghe Damst, J. S., 2006. Chemical fingerprinting of algaenans using RuO4 degradation. Organic Geochemistry 37, 871-881.
Bockelmann, F.-D., Zonneveld, K. A. F. and Schmidt, M., 2007. Assessing environmental control on dinoflagellate cyst distribution in surface sediments of the Benguela upwelling region (eastern South Atlantic). Limnology and Oceanography. Boom A. 2004. A geochemical study of lacustrine sediments: towards palaeoclimatic reconstructions of high Andean biomes in Colombia. Ph.D. Dissertation, University of Amsterdam, Amsterdam. Briggs, D. E. G., Kear, A. J., Baas, M., de Leeuw, J. W. and Rigby, S., 1995. Decay and composition of the hemichordate Rhabdopleura: implications for the taphonomy of graptolites. Lethaia 28, 15-23. Brunner, U. and Honegger, R., 1985. Chemical and ultrastructural studies on the distribution of sporopollenin like biopolymers in six genera of lichen phycobionts. Canadian Journal of Botany 63, 2221-2230. Buckley, D. E. and Cranston, R. E., 1988. Early diagenesis in deep sea turbidites: the imprint of paleo-oxidation zones. Geochimica et Cosmochimica Acta 52, 29252939. Burdige, D. J., 2007. Preservation of organic matter in marine sediments: Controls, mechanisms, and an imbalance in sediment organic carbon budgets? Chemical Reviews 107, 467-485. Colley, S., Thomson, J., Wilson, T. R. S. and Higgs, N. C., 1984. Post-depositional migration of elements during diagenesis in brown clay and turbidite sequences in the North East Atlantic. Geochimica et Cosmochimica Acta 48, 1223-1235. Combaz, A., 1971. Thermal degradation of sporopollenin and genesis of hydrocarbons. In: Brooks, J., Grant, P., Muir, M. D., Shaw, G., and Van Gijzel, P. (Eds.). Sporopollenin. Academic Press, London, pp. 621-653. Cowie, G. L., Hedges, J. I., Prahl, F. G. and de Lange, G. J., 1995. Elemental and major biochemical changes across an oxidation front in a relict turbidite: an oxygen effect. Geochimica et Cosmochimica Acta 59, 33-46. Dale, A. L. and Dale, B., 1992. Dinoflagellate contributions to the open ocean sediment flux. In: Dale, B. and Dale, A. L. (Eds.). Dinoflagellate Contributions to the Deep Sea. Woods Hole Oceanographic Institution, Woods Hole, pp. 45-73. Dale, B., 1976. Cyst formation, sedimentation, and preservation: factors affecting dinoflagellate assemblages in recent sediments from Trondheimsfjord. Norway. Review of Palaeobotany and Palynology 22, 39-60. Dale, B., 1986. Life cycle strategies of oceanic dinoflagellates. UNESCO Technical Papers in Marine Science 49, 65-72. Dale, B., 1992. Dinoflagellate contributions to the open ocean sediment flux. In: Dale, B. and Dale, A. L. (Eds.). Dinoflagellate Contributions to the Deep Sea. Woods Hole Oceanographic Institution, Woods Hole, pp. 1-23. Dale, B., 1996. Dinoflagellate cyst ecology: modeling and geological applications. In: Jansonius, J. and McGregor, D. C. (Eds.). Palynology: Principles and Applications. AASP Foundation, Salt Lake City, pp. 1249-1275. Dauwe, B., Middelburg, J. J. and Herman, P. M. J., 2001. Effect of oxygen on the degradability of organic matter in subtidal and intertidal sediments of the North Sea area. Marine Ecology Progress Series 215, 13-22. Dauwe, B., Middelburg, J. J., Herman, P. M. J. and Heip, C. H. R., 1999. Linking diagenetic alteration of amini acids and bulk organic matter reactivity. Limnology and Oceanography 44, 1809-1814. de Graaf, W., Sinninghe Damst, J. S. and de Leeuw, J. W., 1995. Lowtemperature addition of hydrogen polysulfides to olefins: formation of 2,2'-dialkyl polysulfides from alk-1-enes and cyclic (poly)sulfides and polymeric organic sulfur

decomposition of OC is dependent on the bottom and pore water O2 concentrations.

Keywords: degradation

organic-walled

dinofllagelate

cysts,

oxygen,

organic

carbon,

4.1. Introduction Southern Ocean is often believed to play an important role in modulating atmospheric CO2 levels and hence in respect to global climate changes. Different reconstructions show that atmospheric CO2 during last glacial period was reduced by ~80 ppm in comparison to preindustrial modern times (e.g. Siegenthaler and Wenk; Moore et al., 2000). However, the mechanism responsible for lowering CO2 level is not yet well understood. For example one hypothesis assumes that sea-ice expansion caused permanent surface water stratification south of the Antarctic Polar Front and resulted in reduced vertical mixing and thus preventing the ventilation of CO2-rich deep water and CO2 release to the atmosphere (e.g. Francois et al., 1997; Sigman and Boyle, 2000). Another hypothesis links lower CO2 level with carbon sequestration in marine sediments as a result of higher primary productivity caused by enhanced dust delivery to the Southern Ocean during the last glacial stage (e.g. Martin et al., 1990). However, high primary productivity alone does not influence carbon sequestration. It is the export production and more importantly the organic carbon (OC) burial in marine sediments that can remove carbon from the global carbon cycle at a longer time scale. Unfortunately only 0.1% of the produced OC is ultimately preserved whereas the major part is oxidised back to CO2, H2O and nutrients (Hedges et al., 1997) and, thus introducing carbon back to the sea-water and the atmosphere therefore it is important to estimate both primary productivity and the degradation rates of OC as accurately as possible. Recently a method to separate productivity from preservation and thus a way to quantify OC degradation was proposed based on the selective degradation of organic-walled dinoflagellate cysts (dinocysts) under oxic conditions (Versteegh and Zonneveld, 2002). According to the authors the
production of dinocyst sensitive to aerobic decay (S-cysts) is related to production of dinocyst resistant to oxic decomposition (R-cysts) and depends on the degradation constant k and the oxygen exposure time (OET). Proposed method was successfully used to decouple preservation from productivity in southeastern Atlantic sediments over the past 145 ky, however, assumption of k being constant makes the given method a rather qualitative approach (Versteegh and Zonneveld, 2002). Further work revealed that dinocyst degradation in the sediments is strongly related to O2 concentrations in the bottom waters (Zonneveld et al., 2007). Dinocysts are significant contributors of OC in marine sediments hence their degradation is based on the same premises as entire OC pool decay. Rabouille and Gaillard (1991) and Arthur et al. (1998) hypothesised that O concentrations may influence OC degradation. 2 OC degradation in marine sediments is commonly considered a first order process dependent only on OC concentration and OET (Middelburg, 1989; Hedges and Prahl, 1993; Canfield, 1994; Hartnett et al., 1998; Sun et al., 2002; Keil et al., 2004). Here we investigate two short cores from the Atlantic sector of the Southern Ocean to further explore if O2 concentration is one of the factors influencing degradation of dinocysts and OC in general. To obtain this information we compare the calculated degradation constant k with the pore water O2 concentrations. Additionally we assess the applicability of the dinocysts as a proxy in areas characterised by variable primary production and O2 profiles. 4.2. Regional setting Atlantic sector of the Southern Ocean is characterised by the eastward flowing Antarctic Circumpolar Current (ACC) that is driven by strong westerly winds (Orsi et al., 1995). The ACC is bound to the north by the Subtropical Front (STF) that is positioned on average at 4140S (Lutjeharms and Valentine, 1984). At the STF, the northward flowing Subantarctic Surface Water (SASW) sinks beneath the much warmer and saltier Subtropical Surface Water (STSW) producing large temperature and salinity gradients (Orsi et al., 1995). Within the ACC three frontal systems can be distinguished: the Subantarctic Front (SAF),

6.7. References

Arthur, M. A., Dean, W. E., Laarkamp, K., 1998. Organic carbon accumulation and preservation in surface sediments on the Peru margin. Chemical Geology 152, 273286. Assmy, P., Henjes, J., Klaas, C., Smetacek, V., 2007. Mechanisms determining species dominance in a phytoplankton bloom induced by the iron fertilisation experiment EisenEx in the Southern Ocean. Deep-Sea Research I 54, 340-362. Bakker, D. C. E., De Baar, H. J. W., Bathmann, U. V., 1997. Changes of carbon dioxide in surface waters during spring in the Southern Ocean. Deep-Sea Research 44 (1/2), 91-127. Bockelmann, F-D., Zonneveld, K. A. F., Schmidt, M., Holzwarth, U., subm. Relating organic-walled dinoflagellate cysts to bottom water oxygenation: A case study and paleoceanographic application from the southeast Atlantic. Submitted to Paleoceanography. Brinkhuis, H., Bujak, J. P., Smit, J., Versteegh, G. J. M., Visscher, H., 1998. Dinoflagellate-based sea surface temperature reconstructions across the CretaceousTertiary boundary. Palaeogeography, Palaeoclimatology, Palaeoecology 141, 67-83. Canfield, D. E., 1994. Factors influencing organic carbon preservation in marine sediments. Chemical Geology 114, 315-329. de Vernal, A., Larouche, A., Richard, P. J. H., 1987. Evaluation of palynomorph concentrations : do the aliquot and the marker-grain methods yield comparable results ? Pollen et Spores 29 (2/3), 291-304. de Vernal, A., Rochon, A., Turon, J-L., Matthiessen, J., 1998. Organic-walled dinoflagellate cysts : palynological tracers of sea-surface conditions in middle to high latitude marine environments. GEOBIOS 30 (7), 905-920. de Vernal, A., Eynaud, F., Henry, M., Hillaire-Marcel, C., Londeix, L., Mangin, S., Matthiessen J., Marret, F., Radi, T., Rochon, A., Solignac, S., Turon, J-L., 2005.Reconstruction of sea-surface conditions at middle to high latitudes of the Northern Hemisphere during Last Glacial Maximum (LGM) based on dinoflagellate cyst assemblages. Quaternary Science Review 24, 897-924. Esper, O., Zonneveld, K. A. F., 2002. Distribution of organic-walled dinofalgellate cysts in surface sediments of the Southern Ocean (eastern Atlantic sector) between the Suptropical Front and the Weddell Gyre. Marine Micropaleontology 46, 177-208. Esper, O., Zonneveld, K. A. F., 2007. The potential of organic-walled dinoflagellate cysts to reconstruct past sea-surface conditions in the Southern Ocean. Marine Micropaleontology 65 (3/4), pp 185-212. Ferdelman, T. G., Thamdrup, B., Canfield, D. E., Nhr Glud, R., Kuever, J., Lillebk, R., Birger Ramsing, N., Wawer, C., 2006. Biogeochemical controls on the oxygen, nitogen and sulfur distributions in the water column of Golfo Dulce: an anoxic

-2 (~270 g C m a-1), but was punctuated by some periods of strongly enhanced
productivity. Their calculations for the interval investigated here yield a maximum export production of ~300 g C m-2a-1, which has to be regarded as intermediate to low compared to modern values. Tyson (2004), in his work on TOC variation in the KCF, provides even lower palaeoproductivity estimates in the uppermost part of the KCF, below 50 g C m-2a-1. In the light of elevated OM values inferred from our EFs and palynofacies data, we suggest an intermediate marine productivity, supported by nutrients derived from proximal sediment source areas. From the observed good to very good linear correlation of detrital elements (Appendix 5.2), we infer that the source of terrigenous material did not change through the period of deposition (e.g. Tribovillard et al., 1994). This is confirmed by rather stable proportions of the phytoclasts. As expected, the detrital fraction
shows a clear negative correlation towards carbonate, indicating that m arine biogenic and terrigenous components were diluting each other. Such a dilution effect can be seen in the middle of the studied interval, where we observe a significant shift from terrestrial-dominated to marine-dominated palynofacies assemblage. This change results mainly from much higher recovery of marine palynomorphs, in particular dinocysts, while the abundance of terrestrial particles remains relatively stable. The TOC pattern parallels the distribution of marine palynomorphs, and groups together with marine palynomorphs on the same side of PCA axis. In contrast it correlates negatively with the detrital elements and plots on the other end of the PCA axis from terrestrial particles, indicating that the TOC record is primarily bound to marine primary productivity rather than to continental OM input. Higher TOC content and marine palynomorph abundances in the central part of the studied interval in comparison to the upper part, imply higher primary palaeoproductivity in the surface waters of the Wessex Basin, enhanced preservation of OM under oxygen-depleted bottom water and sediment conditions, or a combination of both. Changes in Cu values that parallel the TOC profile indicate that Cu is primarily bound to the OM export flux, and is not significantly remobilised diagenetically due to hydrogen sulphide production. This implies that
differences in palaeoproductivity may have been triggered by variations in the availability of micronutrients like Cu, and was thus the reason for the shift in marine palynomorphs and TOC content of the sediments. However, if
enhanced productivity is indeed responsible, elevated values of other nutrientrelated elements such as P would be expected, which is not the case. In addition, abundances of terrestrial particles, such as brown wood, do not vary significantly throughout the interval, suggesting that run-off and hence nutrient delivery from the hinterland was stable. The pattern of the C/P ratio indicates a preferential removal of P from OM in the centre of the interval. Selective P recycling under anoxic conditions has been reported from various modern and ancient anoxic environments (e.g.

grossi, Senoniasphaera jurassica and Systematophora spp. experienced a stronger decrease than Glossodinium dimorphum and Cribroperidinium sp. 1. The abundances of Circulodinium spp., Cyclonephelim spp., S. grossi, S. jurassica and Systematophora spp. closely parallel the TOC pattern and group together with TOC during PCA analysis, which suggests they are possibly affected by changes in redox conditions. Both a general and a differential degradation of dinocysts suggest that dinocyst assemblages are affected by species-selective preservation/degradation. The process of species-selective dinocyst preservation is known from modern and quaternary sediments and documented by natural and laboratory experiments (Zonneveld et al., 1997, 2001; Hopkins and McCarthy, 2002; Chapter 3). For Cretaceous dinocysts, laboratory experiments have also confirmed species-specific degradation potential (Schrank, 1988). However, no similar studies have yet been carried out on Jurassic species. In the data collected for this study there is a major shift in relative dinocyst abundances which occurs at ~122.37 m depth. Changes in absolute abundances of dinocyst species are visible at the same depth, but are much less pronounced. Mediterranean sapropels and Atlantic turbidites have also been the subject of palynological studies that reported a rapid decrease in concentrations of dinocysts and pollen at the OBFs (Zonneveld et al., 1997, 2001; Versteegh and Zonneveld, 2002). For example, the total dinocyst concentrations of the oxidised part of an Atlantic turbidite yielded only 10% of the dinocyst abundances from the unoxidised turbidite section (Zonneveld, 1997). Although we observe a drop in total concentrations of dinocysts and pollen in the oxidised part of our KCF interval, this decrease is, however, rather gradual, unlike the rapid decrease seen in the case of the turbidites and sapropels. The fact that the TOC and marine palynomorph records do not show a similar, very rapid change as observed at oxidation fronts in Mediterranean sapropels or turbidites from the Madeira Abyssal Plain - might be due to the relatively large contribution of more refractory terrestrial organic matter to the total organic carbon pool in the KCF. In addition, very high S values argue strongly for production of hydrogen sulphide, and thus euxinic conditions, in the

Appendix 2.1. Dinoflagellate cyst species grouped with respect to their sensitivity to oxygen availability in pore waters according to Zonneveld et al. (2001).

Extremely sensitive

Moderately sensitive

Resistant

Cysts of Protoperidinium species (a.o. Brigantedinium spp.) Echinidinium species Lingulodinium machaerophorum Protoceratium reticulatum Pyxidinopsis reticulatum Spiniferites species (including Spiniferites bentorii, Spiniferites mirabilis, Spiniferites pachydermus and Spiniferites ramosus) Nematosphaeropsis labyrinthus Impagidinium aculeatum Impagidinium paradoxum Impagidinium patulum Impagidinium plicatum Impagidinium sphaericum Operculodinium israelianum Pentapharsodinium dalei Polysphaeridium zoharyi
Appendix 2.2. Degradation factors of organic matter types. Cyst and Pollen in numbers per dry gram sediment. ox., oxidised; unox., unoxidized; Df, degradation factor; k, degradation constant; LCK, long chain ketones; LL, loliolide and isololiolide; n.d., not detectable. Data after Prahl et al. (1997) and Zonneveld et al (1997, 2001). *C 29 and C31 alkanes; C26 and C28 alkanes; C20 to C 30 alkanes.
Sample Mediterranean S1 Sapropel ox. unox. Df k ox. unox. Df
TOC% 0.44 2.65 6.02 4.2 0.185 0.973 5.26
Resistant cysts 1.33 0.1032 1.37
Sensitive cysts 5.885.5 15.9 15.558

Pollen 16.7 6.2070 25.2

n-alkanes (g/g) 1.17* 3.92* 3.3 2.8 61.05
n-acids (g/g) # 0.83 5.87 7.1 4.6 64.75

LCK n.d. abund

LL n.d. abund
Madeira Abyssal Plain f-turbidite

96.52^ 1.58

389^ 6.01
Appendix 3.1. Count data of organic-walled dinoflagellate cyst
Sub-sample Namibia original Namibia ox UB Namibia anox UB Namibia ox BB Namibia anox BB Sapropel original Sapropel ox UB Sapropel anox UB Sapropel ox BB Sapropel anox BB 1 331.5 200.5 160.0 217.5 75.0 186.5 133.5 139.5 168.0 177.6.0 1.0 0.0 6.0 0.0 0.0 0.0 0.0 0.0 0.12.0 7.0 6.0 3.0 2.5 0.0 0.0 0.0 1.0 1.1.5 0.5 1.0 1.0 0.0 1.0 0.0 0.0 4.0 1.3.5 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1.0 0.0 1.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0.0 0.0 0.0 0.0 0.0 4.5 2.0 6.0 4.0 4.50.5 47.5 25.0 50.5 11.0 0.0 0.0 0.0 1.0 1.43.5 21.0 21.5 20.0 9.5 0.0 0.0 0.0 1.0 0.61.5 53.0 28.5 45.0 13.0 8.0 2.0 4.5 4.0 5.4.0 5.0 1.5 5.0 1.0 6.0 17.0 7.0 15.5 9.5.5 0.5 3.5 2.0 2.0 0.0 0.0 0.0 0.0 0.0

MONTHLY WEATHER REVIEW.

JUNE, 1914
TBE ~ E B S T O R M AND ITS PHENOmNA. By W. J. HUIPEREYR, Professor of Meteorological Physics. [DW, Weather Burem, Waahiugton, D C, July 17,1814.1.
Introduction.-A thunderst.orm, as its name implies, is a storm characterized by t.hunder and lighhing, just as a dust storm is one charsct?rizcd by.& great quantit,y of flyin dust. But the dust is never in any sense tho cause o the storm that carries-it along, nor, so far as known, does either thunder or lzhtning have any influence on the course-genesis, deqelopnient, or termination-of even those storms of which. they forni, in some respects, the most impo.rt,?nt feat-ures. No mat.t.er how impressive nor how tprrifymg these phenomena may be, thoy never are anybhmg more t.han mere incidents t.o or pr0duct.s o the pecuhar storms the accompany, as f w i l l be made clear by what follows. f n short, t.hey are never any sense either storm-originating or stormcontrokng factors. Origin o thunderstorm e?ectri&y.-A knowledge, or at f least a good working hypothesis, of how t.he great amount of electricity incident to t;hunderstorms is generated, is absolutely essential to their lo cal explanation; t,hat is, to a clear understanding of t e probable int,errelations between their many phenomena. Fortunately such an hypothesis, or theory, rat.her, since it is abundantly supported bv observat,ions and b laboratory experimentis, 18 available as a result of done on t.his subject m India bv Dr. G. C. Simpson (1) of the Indinn Moteorolo ' c d Department. %r. Simpson's observations ust referred to, were obtained at Simla, India, at an e evat.ion of about 7,000 feet above sea level, and covered all of the monsoon seasons, B a t is, roughly, April 15 to Sept.ember 15, of 1908 and 1909. -He also obtained observations of the electrical conditions of the snow at Simla during the winter of 1908-9. A tipping-bucket rain gage gave an automatic continuous record of the rate and time of rainfall, while a Benndorf (2) self-registering electrometer marked the sign and potential of the char e acquired during each twominute interval. A seconc Benndorf electrometer registered the potential gradient near the earth, and a coherer o the t e used in radiotelegraphy registered the occurf rence o each lightning discharge. All obvious sources of error were esamined and carefully guarded against. Hence it would seem that the conclusions drawn. from the thousands of observations e;iven in the memoir are fully justified; and especially so since several mdependent series of similar observations made a t different times, by different people, and at places widely se arated, have given confirmatory results m every case. 8impson's records show that-
(7) As stated in paragraph (3) a b m , positive electrki wae recorded more frequentl than negative, but the exceee wao leae marked t,he higher the czarge on the rain. (8) With all rates of rainfall positively ch rain occurred more frequently than negatively charged rain, and e relative frequencyof rain increased rapidly with increased rate of mhpmtive1.y ch-ed all. With ramfall of less than about 1 millimeter in two minutes; poeitively charged rain occurred twice aa often a~ negatively charged rain, while with greater intensities it occurred 11 times as often. (9) When the rain was falling at a less rate than about 0.6 millimeter i two minutes, the charge per cubic centimeter of water decressed 88 n the intensity of the rain incread. (10) With rainfall of greater intensity than about 0.6 millimeter in two minutes the positive charge carried per cubic centimeter of water waa independent of the rate of rainfall, while the negabve charge carried decreased aa the rate of rainfall incretwed. (11) During periods of rainfall the potentaal gradient was more often negatwe than positive, but there were no clear indications of a relation@hip between the sign of the charge on the rain and the e n of the potential gradient. (12) The data do not s g x a that the negative electricity occurs more u gef frequently during any particular period of a storm than during any other.

MON'I'HLY

WEATHER REVIEW.
thunderstom, and, therefore, the !toms themselves, occur most.frequently dunng the T n t e r and least frequently dunng the summer. This IS because the temerature of the air- a t some distance above the surface., geing largely what it was when i t left the windward continent, greatly changes from season to season while that of the watcr, and, of course, the air in contact with it, cha es but little through the year. 'ihat is, over the oceans t e average decrease of temperature with increbe of eleva-

"%

the same relation to the annual average windward tempcrature that the total annual precipitation over the entire world does to the annual average world temperature. I n each case the amount of evaporation or amount of water vapor taken into the atmosphere, and, therefore, the amount of subsequent recipitation, clearly niust increase dnd decrease with t e tcmpernture. Au excellent test and complete support of this deduction is furnished by figure 3,in which the full line represents the anioothed
Fro. %-Relatbn of European rainfall to eastern U. 8. temperature.
tion obviously is least and, therefore, thundemtorms fewest in summer! and greatest, with such storm. most numerous, in winter. flyclic land period.-Since thunderstormq are accompanicd by rain and since over land thev are most iiuiiierous during summer, it would appear that they niust occur most frequently either in warm or in wet yenrv and lewt frequently in cold or in dry yoarv. Further, if it should happen, as it actually does, that, for the earth as
annual European precipitation (5), and the dotted line smoothed annual average eastern American temperatures. Beyond a reasonable doubt, therefore, for the world as a whole, warm years are wet and cold ones are d. Hence, as above stated, it is practically certain that x e maxima of thunderstorms occur during years that are wet, or warm, if we prefer, for the two are synchronous, and the minima during years that are dry, or cold. A partial and, so far as it goes, a confirmatory statistical

-Holland. The uppermast, m v y curveshows the vsrfstlon in the smoothed numbm Fro. S.-Relation of snnusl number of thunderstom days to total annual pmipitatbnOf destructive thuride~tormnn Germany. i
a whole, warm years are also wet years and dry yenrs cold years, it would appear logical1 certain that, for the entire world, the maxlmum num er of thundemtornlr must belong to the ears that are wet and warm and the minimum to thoise t a t are cold and dr. A complcte statistical examination o these statements is not possible, owing to the fact that nieteorological data are available for onl portions of the earth's surface and not for the whole o it. Nevertheless, well-nigh conclusive data do exist. The annual rainfall, for instance, to the leeward of a large body of water obviously must bear
test of this conclusion is given by figure 3. T h e lower group of curves is based on m exhaustive study by Dr. von Gulik (6) of thunderstorms and lightning injuries in Holland. The continuous zigz line 'ves the actual annual number of thunderstorm ays an the continuous curved line the lame numbers smoothed. The broken lines give,respectively, the actual and the smoothed values of the annual average preci itation. The up er curve represents the variations in t e smoothed num er of destructive thundorstorms (7) (number of thunderstorm days not readily available) i Germany. n
MONTFIIJY WEATHER REVIEW.

JUNE;1914

m Theoriginal dataonwhichthislastcurveishasedindicate storms did so increase, which seems i robable, or else there was, on the average, an increase o attention given tiveness. Presuma ly, however, this feature is real only to this particular phenomenon. At any rate, so continto the extent that the country has become more densely uous and so great an increase in the average numbor of populated and more thick1 studded with destructible thundeistorms can h a d l be accepted without abundant li property. At any rate, this e ement has been omitted from confirmation, and for t! is reason the earlier thunderstorm records provisionally have been rejected. the curve and on1 the variation factor retained. Obviously a niuch closer relation between the number It will be note that the curve of thunderstorm freuencp for all Holland closely parallelv the curve of of thunderstorms and total reci itation would hold for %understorm injury in all German. Hence, it seenis some months and seasons t an or others, but no such safe to infer that the frequency of understornLq vnries subgrouping of the data has been made, though, sumably, it would give interesting results. The w ole pretty much the same way over both-countries, and, presumably, also over many other portions Of Europe; purpose of this portion of the study was to arrive at that IS, roughly as the rainiall varies, or, conadenng the some definite idea in regard to the cyclic change of thunderstorni frequency, to see with what other meteworld as R whole, roughly as the tem erature varies. Additional statistical evidence of t e relation between orolo ical phenomena this change is associated, and, if the annual number ot thunderstorms and the total annual possi le, to determine its cause. Diviprecipitation, kindly assembled by the Climatologicn.1 sion of the United States Weather Bureau, P. C. Day in chargo,.is shown by figure 4, in which the upper line gives, in rmlbmeters, the smoothed aveiage annual preci itations of 127 stations widely scattered over the who e of

a continuous and ra id 'increase of thunderstormn:destruc-
Fro. 4.-E&tlon of snnual number of thunderstom,

P, United States.

T ,to total snnual preclpltation,
age annual number of thunderstorms a t these same statlona. It was thou' h t a t first that this relation might differ greatly for g o s o gortions of the United States
the United States, and the lower line the smoothed aver-
whose climates are radica y dissimilar, and tor this reason the stations east of the one hundredth mendmi provisionally were classed separate!y from those west of it; but the resulk for the two sechons, being substantially alike, show that for this purpose their division is entirely unnecessary. As will be seen from the figure the statistics of only the past 10 years have been used. This is because the annual number of such storms reported ra idly decreases from 1904 back to about 1S90. Indeed, t ie annual number of thunderstorms re orted er station during the past 10 years is almost ouble t e annual number per station (practically the same stations) from 1880 to 1890. The transition from tho smaller to the lar er number was due in great measure, doubtless, to an a teration in station regulations that chan ed the official deaition of a thunderstorm rrom " thun er with rain to "thunder with or without rain." This, however, does not account for the fact that from 1890 to 1904 the average annual number of thunderstorms reported per station increased, a t a nearly constant rate, almost 100 per cent. Either the
frequency of thunderstorms, and many other phenomena must perforce undergo exactly the same irregular cyclic variation. As already stated the statistical evidence bearing on these conclusions neither is nor can be com lete, but the deductions are so obvious and the sta,tistictt data already exanlined so confirniatory that but little doubt can exist of their general accuracy. Cyclic oct-an period.-The record of thunderstorms over the oceans is not sufficiently full to justify any conclusions in regard to their cyclic changes. Possibly, as in the yeiirly and the daily periods, the ocean cyclic period may be just the reverse of that of the land, but this is not certain. Grographic distribu tion.-The geographic distribution of the thunderstorni may safely be inforred from the fact tshat it is caused b the strong vertical convection of huinid air. From t e nature of its formation one would assume, and the assumption is supported by observstion, that the thunderstorm niust be rare beyond either polar circle, especially over Greenland and over the Antarctic continent, rare over great desert regions wherever situated, and, on the other hand, increasmgly abundant with increase of tenipecature and humidity, and, therefore, in general, most. abundant in the more rainy portions of the equatorial regions. The east coast of South America from Pernambuco to Bahia is said to be an exception. Pressure and tcm erature dktribu.tion.-In illustrating the occurrence of t understorms with reference to the disposition of isobars and isotherms, or the distribution of atmospheric pressure and temperature, typical weather maps OF the United States,' figures 5-1 9, have been used, not because the thunderstorms of this country are different in any essential particular From those of other countries, but chiefly as a matter of convenience in making the clrttwings. To facilitate their stud each of the several types discussed is illustrated with t Fee consecutive maps. The first shows the 12-hour antecedent

1 The author wishes lo acknowledge the courteous cooperatbn of the Forecsst D i d slon U 8. \\eather Bureau, I selecting mnps typical 01 thunderstorm condltiozu In n thr trnited statas.

JUNE,191.1.

MONTHLY WEATKER REVIEW.
MONTRLY W E A m R REVIEW.

JUNB,1914

J m , 1014.
conditions, the second the particular pressuretern erature distribution in question, and the third the E l o u r subsequent conditions. In these figures the isobars, in corrected inches of , and the isotherms, in Fahrenheit degrees, are marke by full and by dotted lines, res ect,ively. The legend LOWis written over a region rom which, for some distance in every horizontal direction, the pressure increases. Similarly, the legend HIGH apphes to a region from which, in every direction, the pressure decreases. The arrows, as is custoniaxy on such maps, f y with the wind, while the state of weather is indicated l by the usual U. S. Weather Bureau synibols. Obviously, the key to the geographic distribution of thunderstorms, vertical convection of humid air, is also the key to their location with reference to the existing distribution of baronietric pressure. From this standpoint the places of their most frequent ocmrrence are: a. Regions of high temperature and widely estended nearly uniform pressure. (See f i ~ 5., 6, and 7.) The conditions are still more avor&ble when the air is humid and the ressure, perhaps because of blie humidity, sli htly be ow normal or, at most, but litt,le above normaf When the ressure is approsimately uniform the winds are light anc every opportunity is given for the surface air to become strongly heated ancl thereby finally to establish thunderstorni convections. Such storms, always favored by mountmain regions, and part.icularly by steep mountain peaks and stroiigly heated valleys, are, of course, most frequent of summer afternoons and are especially liable to occur a t the end of two or three days of unusually warm weather. They develop here and there sporadically, hence the name LZocaZ thunderstorm; last, as tt rule, only an hour or two, and travel neither rapidly nor far. Those that form over mountain peaks oft,en do not travel at all. The necessary initial convection is essentially, if not wholly, due to surface heating ancl therefore they frequently are referred to as heat thunderstorms. They are well-ni h the only type of thunderstorm in the tropics, an$ perha s, the most common type in the warmer portions of t e temperate zones. b. The southeast quadrant (Southern Hemisphere, northeast), or, less fre uently, the southwest (Southern Hemis here, northwes8, of a regularly formed low, or typica cyclonic storni. (See figs. 8, 8, a.nd lo.) I n this case the temperature gradient essential to a rapid vertical convection is not produced chiefly by local surface heating, as it is during the genesis of lieat thunderstorms, but, in great measure, result,s from the more or less crossed directions of the under and over currents of air. The surface air of the quadrant in question normally flows from lower and warmer latitudes, while with increasin altitude the winds conie more ancl more nearly from t e west, or even northwest. This crossing of the air currents, then, the lower from w a r i e r sectionsand the u perfroin regions not so much warme? possibly even colc er-progressively increases the vertica.1 temperature gradient, or rate of temperature decrease with increase of altitude, and therefore may frequent.ly be, and doubtless often finally is, the determining cause of a rapid vertical convection and the formation of a thunderstorm. This particular type of thunderstorm, commonly known as the yclonic thunderstorm, is almost wholly c.onfined to the temperate and higher zones, for the simple reason that the well-dehed cyclone, essential to its creation, seldom occurs in tropical or equatorial regions.

MON'I'HLY WEATHER REVIEW.

J m , 1914

MONTHLY WEAlTlXR REVIEW.

JUNE,1914

There does not appear to be any independent. or dis- any of the familiar gush and other thunderstorm phetiiictive name for the thundeistorm enerated mcler this nomena. Hence we must infer that somehow or other t,ype of pressure distribution. Per aps it midit, with the rain is an important factor both in starting and in some justification, be called the anticyclonic *thunder- maintaining the winds in question, for they do not exist storm, or even t,lie trough storin. before tlie rain begins nor continue after it has ceased. e. The boundary between warin and cold waves. (See On the other hand, i t cannot be assumed that the rain is figs. 17, 18, and 19.) the whole cause of these winds, for they do not accomAlong such a boundn.ry the direction of flow of thc pany other and ordinary showers, however heavy the warm humid layers of air is more or less opposite, iis 21low11 downpour may be. on the maps, t.0 that of the colder ones. Therefore i t Jlie w t u d course of evcnts, illustrated by figure 20, must frequently happen that at irregular int.ervals along ti9cn froiii the rccords obtained at Washington, D. C., dursuch a boundary t.he upper air, coming froin the cold arc:\., overruns a section of surface air beloiiging t o t.lie \va.rili region; but, of course, only where tlic upper ai;. is potentially warnier than the lowcr-if potciitially colder it 90 would underrim. Xow, whercv er this overriiiining on t.he ptirt of t.he cold air does ocoiir t.he vertical t.e:~:]~~rnture 80 gradient obviously is abrupt,ly and greatly incrrr.iscd. niid tlic wherever, in the course of its fi.irt.litlri?;o~eii:e~;t., n c w 0 gradient esceeds the ticlinbat.ic rate of t.miq)crtitiire 7 b, i chnge, as analogous t.0 c.~~.sc?t 0ft.r.n IiitiSt., ulitlr:. tdic 9.9 given conditions, verticd coiivcction with r:.i.iiiq tIiuiicl(:r, s M and iightning i apt to occur. Ecncc, : s t ; ? t d , t.iic 9.8 boundary betm-oen warni and cold WBXW i.5 wot.lier p1ac.e favorable to the thunde-wtorni, wliich, uiidcr tiic;;? con60 ditions, >ossiblyniiglit bc c d e d thc ibori7~1! storill. he a ore five distinct t:,-pe:s of. wratlii~rcunclitiona, I so together with their innuinrri~bl mriztions and C O J ~ &ii:c :~ tions, protinbly iiicludc d l t.1tc t. nrc distinctly fiivornhlr 43 to tho production of tl:uiiclcist,or!rIs. E:!.ch t.iiic1.G t,o establish a.11ndiabiitic or (?veil:iuy,eriidiabtitic. tecqwrtit,ii;:> 3.5 gradimt up to the cloud !evc-l--tl:e O I ~ Pthins ~ : ~ R C Y I t,o tial the production of 6 stroiig ycrticn! coiivcction, tlir lira20 genitor, as we 1in.c-e :wen, o thc t!liiiid:rat,i)rl;~. f Thmderstonn uvXs.--Sliortly, sag 20 niiiiu t,cs or IO so, before the rain of R thuntlei.storm reaches 8. e v e n LIcality the wind a t that place, which generdy is light itiitl 0 from the south or southwest across the p i t h of the storm, begins to die down to an approsimate c:ilni nnd 2J to change its direction. Alien this cliaiigc is complete, it blows for a few minutes, rather gently, dircctlv t.:m-ard 1.5 the nearest portion of the storm front, nitti ihaIlj-, as t,hc. rain is almost at. hand, again, but this time ahrupt,lT ant1 1.0 in rather violent, ust.s, away from trlie storm aitc! 111 t.ho same direction t a t i t i3 traveling, n directii.ni tliut 0.5 usually differs appreciably from that of t.lie original sur0 face wiiicl. Generally this violent gusty miid l:ist,s through only the earlier portion of the strmi, :111il t,licn is graclually but rather quickly succeeded by a contparntively gentle wiiid that, though following the storm a t ?cl.--rJurse of meteorological elements on a thunderstorm day, at IVashhgton, first, frequently, after an hour or so, blows in tlie saiiit: FIG. D.C. (July 30,1913). general hrection RS the original surfiice wind. The cause of the thunderstorm winds needs to be care- in:: tl:c p t s s g e of the notable thunder squall of July 30, f idly considered if one would iiiiderstnnd nt all clearly the In!:.;, s(v?tisto be about as follows: mechanism of tlie storm itself. I;ii:.t. approximately adiabatic temperature gradiAs already explained, this typo of storin o m s it.s origin vnt i.: ~.stiibli+:lird a wide area, rough1 up to the base over to that vertical con-rection w1iic.h rcsults from a inore or Iewl of tlw cui-!iulus clouds. But whi e the uprising less superndiabatsic temperature graclien t. I t is t.liis grn- II~:?J:c]w~ of tdic Psisting convection currents, due to su erdient, no matter how established, uc-liet,lierby siniplc sur- adi.Lbntio gr:;clicnti, iiuiy be localized aiid here and t ere face heating or by the oveI aiid uncler running of uii- mthcr r:q~id,tile return or downflow, though real1 the equally heated layers of air, that permits, or rather forccs: c n u w of t h uy1r;l.ft, is widcsp-ead mid correspon ingly the pIoduction of the cuniulcs cloud in which m d by the gcntlc. Tlic condition cssantial to a local and ra id motions of which the electricity that chm-actcrixes the flo~~.iiflo~~~-t,lint local dccidcd cooling at a high a tii4, a storm in question is geneiated. tticlc-does not exist, and therefore the counterpart to Nevertheless, as everyone knows: the passap of a tlic 1ipwa.rc1currents is nowhere conspicuous. cumulus cloud overhead, however large, so long as no Second. Aftw n time! RS a result of strong convection rain is falling from it, does not greatly affect tlie direction in a cunmlus cloud, rain is formed a t a considerable altiand mamitude of the surface wind-does not bring. on tude where, of course, the air is quite cold, in fact so cold --

shower, even thundershower, have the base of a heaT surface ofaut,terly, becacse of it appears quite clou and yet, fail evaporation, t o reach the tlie earth. Hence
It is a common thing in semiarid regions to see a
certain that. in t.he aver e tliiinderstorm a considerable ortion of t,lie rain t.liat eaves tlie cloud is eva orated efore it, reaches tlie ground, and thwefore that t e temperat.ure decrease of the at.mosp1iere is largely owing to t.liis fact. But if so, why? t,lien, one might properly ask, does not. an eqi:ally great temperat.ure drop accompany all heavy rains ? The answer is obvious, becaim, as a rule, the temperatiire is Iiiglier and the relative humidity lower during a thunderst.orm than at, tlie time of any other ordinary rain. The chief, perhaps t.he soh, reaSon for this difference in relative humidity is the difference in the two cases, bet,ween the movements of the air. I n the thunderstorm t,he descending air, wliich can be no more than saturated at, top, dynamically warms EO rapidly and is so continuously renewed that. evaporation into it can not keep pace with its vapor capacity. During other rains, however, where t,liere is no atmospheric descent., and t,liereforeno dynamical heat,ing, ap roximate saturat.ion mcst soon obt.ion; hence but 1itt.le urt.her evaporation and, of course, but. little cooling. We will now return to the numerical values and compute a probable magnitude of cooling due to eva oration. As before, let, a hentimeter rain leave tlie c oud, but let one-fourth of the rain that,started, or half a cent.het.er, be evapciratod. This would consiime 303 heat, units from an aircolumn 3,000meters highwhose heat capacit.yis that of only 50 cubic centimeters of water. Hence, as a result of eva oration alone, the temperat.ure of the air cclumn \\vo1ild e lowered on die average by about Go C. Evapcrat,ion, therefore, appears to be both necessar and sufficient to produce all or nearly all the cooling o a tliunderstorm. Since t.he molecular weight of water is 1s while t,be average mcolecditr weight of air is approsimately 39, it follows that, tlie amount, of evaporation above assumed woiild decrease the density of t.he atniospliere by, roughly, one part in a thousand. On the other hand, a decrease in temperature of Go C., that would be proclnc.ecl by the evaprat ion assumed, wolild increase t.liedensity by about one part. in flfty. Hence the resdtant of t.1iek.etwo cpposing effects is substantially that of the sec.oncl alone; that is, a distinct. increase in the density. Doubtless, as already stated, the evaporation of thunderstorni rain, and t,herefore the drop in t.emperature and the consequent, gtiin in density, all increase wit,h decrease of elevation. I n some nieusure, however, this effect is counteracted hy the increasing rate of dynamicltl heating in the lower layers result.ing froni the correspondingly increased rate of ressure gain to change in elevation. But no matter ow nor to wh3t estent the details may vary, it seeiiis quite certain that t.he cold rain of a thunderstorm and its evaporation together must establish a local downrush of cold air-an observed important and characteristic phenomenon, redly the immediate cause of the vigorous circulation, whose rational esplanation has been atteinptcd in the ast few paragraphs. As the colunin or shcet o cold air flows down it maintains in great nieasure its origind velocity and, therefore, on reaching the earth rushes forward in the direction of the storni movement, underrunning and buoying up the adjacent warm air. And this condition, largely due, as ex lained, to condensation and evaporation, once establishec! necessarily is self-perpetuating, so long t u the general temperature gradient, humidity, and wind

MONTHLY W E A m R REVIEW.

.JUNE, 1814

hmr, 1014.

MONTHLY WEA!IXBR REVIEW.

MONTHLY WEA!I'HER REVIEW.
direction am favorable. It must be remembered, however, that thunderstorm convection, rising -air just in front and descending air with the rain, does not occur in a closed circuit, for the air that goes up does not return nor does the air that comes down immediately go up again, there simply is an interchange between the surface air in front of the storm and the upper air initsrear. The travel of the storm, by kee ing u with the underrunning cold current, just as e ectua y maintains the temperature contrast essential to.this open-circuit convechon as does continuous heatmg on one side and cooling on the other maintain the temperature contrast essentd to a closed circuit convection. The movements of the warm air in front of the rain, the lull, the i d o w , and the updraft resemble somewhat those of a horizontal cylinder restin on the earth where the air is quiet and rolling forwar% with the speed of the storm. Similarly, the cold air in its descent and forward rush, together with the updraft of warm air, also
foglike condensation which, of course,. renders my d s tached vortex at this position uite visible. This squall cloud, in whic.h t e direction of motion on top is against the storm, ma be regarded as a thiid horizontal thunderstorm cylin er much smaller but more complete than eithcr of the others. Schematic illustrutions.-The above conceptions of the mechanism of a thunderstorm can, perhaps, be made a little clearer with thc aid of illustrations. Figure 21, a in the makin cloud from whic to fall, and the stream lines of

Fro.!2l.--prlncipal

a t movements in the development ora cumulus cloud.
resembles a horizontal cylinder, but one sliding on the earth and turning in the o posite direction from that of the forward r o b or alrwarm cylinder. I n neither case, however, is t e analogy complete, for, as above ex lained, the air that goes up remains aloft while the GO d air that comes down is kept by its greater density to the lower levels. The concbtion of flow persists, as do cataracts and crest clouds, but here, too, as in their case, the material involved is ever renewed. The squall cloud.-Between the u rising sheet of warm a r and the adjacent descending s eet of cold air horii zontal vortices are sure to be formed in which the two currents are more 01: less mixed. The lower of these vortices can only be znferred as a necessary consequence of the opposite directions of flow of the adjacent sheets of warm and cold air, for there is nothing to render them visible. Neither can any vortices that may exist within the cloud be seen. Near the front lower ed e of the cumulo-nimbus system, however, and immezately in front of the sheet of rain, or rain and hail, the rising air has so nearly reached its dew point that the somewhat lower temperature, produced by the admixture of the dewending cold air, ui~ouffieient to produce i it e iight n

concentrated or local down current, only an impcrceptible counter settling of the air round about, because as viously explained, the air cataract requires locai cooEg to subpotential temperatures, and this in turn requires local rain. Figure 33 scheniatically represents a well-developed thunderstorm in rogress. T h e falling rain, often mixed with hail, cools t le air throu h which i t falls, and as the temperature oTaclient was a ready closely adiabatic it follows that &e actual temperatures will be subpotential from the sui.fac.e of the earth to within the cloud, or throughout and a little beyond the nonsaturated or evaporating levels. As soon, then, as this column or sheet of air is suflicientl cooled i t flows down and forward and a l the atmosp eric movements peculiar to the l thundeistorm are established substantially as shown. Referrin to the ure: The warm ascendin air is in the region the co3descending air a t D; the ust cloud (in dry weather) at D', the s uall cloud at S-the storm collar a t C; the thunder hea s at T; the haif a t H; the primary rain, due to initial convection at R ; and the secondary rain at R'. This latter phenomenon, the recondary rain, b 8 thbg of frequent occurrence and

J m , 1914.

often is due, M indicated in the figure, to the coalescence and quiet settling of drops from an abandoned portion of the cumulus in which and below which winds-and conmction are no lon er active. Mmmato-cumu% rarely, false cirri frequently, and cap-cbuds occasionally, accom any thunderstorms,. but as tbp are not essential to i t g e y therefore are omitted from e above schematic illustration. Thundmstomn pressurF.-Before the onset of a thunderstorm there usually if not always is a distinct fall in the barometer. At times this fall is extended over several hours, but whether the period be loiig or short the rate of fall usually is greatest at the near approach of the storm. Just as the storm breaks, however, the pressure rises very rapidly, almost abruptly, usual1 from 1 to 2 ally a , the , millimeters, fluctuates irregularly, and storm asses again becomes rather steady but at a somewhat figher pressure than prevailed before the storm began. The cause of these pressure changes is, doubtless, rather complex. The decrease in the absolute humidity

one&fth its former value. This would reduce the total flow by about 1 part in 400, and thereby increase the barometric reading by nearly 2 millimeters. It would seem, then, that the friction of the thunderstorm gust on the surface of the earth, through the consequent decrease in the total linear momentum of the atmosphere and, therefore, its total flow, must be an important contnbuting cause of the rapid and marked increase of the barometric pressure that accompanies the onset of a hcavy thunderstorm. To sum u : The chief factors increase of t e barometric storm appear to be, possibly tude: a. Decrease of horizontal friction. b. Vertical wind pressure, due to descending air. c. Lower temperature. d. Decrease in absolute
the bulk of the cumulus cloud happens to be located. Hence as the wind at this level is faster by ni ht than by day and faster over the ocean than over lan%, it follows that exactly the same relations hold for the thunderstorm, that it travels faster over water than over land and faster by night than by day. The nctual velocity of the thunderstorm, of course, varies greatly, but its aver e velocity in Europe is 30 to 50 kilometers per hour; in #e United States, 50 to 65 kilometers per hour. Hail.-Hail, consisting of lumps of roughly concentric layers of compact snow and solid ice, is a conspicuous and well-known phenomenon that occurs during the early portion of most severe thunderstorms. But in what ortion of tlie cloud it is formed and b what process the fayers of ice and snow are built up are acts that, far from being obvious, become clear only when the mechanism of the storm itself is understood. As before, let the surface temperature be 30 C. and the Thun erstomn temperutures.-Before the onset of the storm the temperature commonly is high, but it begins absolute huniidit 40 per cent, or tlie dew point 15 C. rapidly to fall with the first outward gust and soon drops Under these conc itions saturation will obtain, and, thereoften as much as 5C. to 10 C. because, as already es- fore, cloud formation will begin wlien the surface air has lained, this gust is a portion of the descending air cooled risen to an elevation of 1.5 kilometers. Immediately gy the cold rain and by its eva oration. As the storm above this level the latent heat of condensation reduces passes the temperature genera ly recovers somewhat, the rate of temperature decrease with elevation to about half its former value, nor does this rate rapidly increase though it seldom regains its original value. Thunderstm. hu.m.idihj.-As previously explained,heavy with further gain of height. Hence, usually, for the rain, at least up in the clouds, and therefore much hu- abwre assuniptions correspond in general to avera e midity, and a temperature contrast sufficient to pro- thunderstorm conditions, it is only beyond the 4-ki 0duce rapid vertical convection, are essential to the genesis meter level that freezing ternpcratures are reached. It of a thunderstorm. Hence during the early forenoon of a is therefore only in the upper portions of cumulus clouds, thunderstorm day both the absolute and the relative the portions that clearly must consist of snow ail can humidity are likely to be high. Just before the storm, and undercooled fog or cloud droplets, that Barticla however, when the temperature has greatly increased, either originate or greatly grow. But whnt, then, is the process by which the nucleus of though the absolute humidity still is high, the relative humidity is likely to be rather low. Onthe other hand, the hailstone is formed and its layer upon layer of snow during and immediately after the storm, because chiefly and ice built u p ? Obrioudy such drops of rain as the of the decrease in temperature, the absolute humidity IS strong upclraft within the cloud may blow into the region of freezing teniperatures will quickly congeal and also corn arativel low and tlie relative humidity hi h. fZainqu8~--It has frequently been n o t e 8 that the gather coatings of snow and frost. After a time each rainfall is greatest after heavy claps of thunder, a fact inci ient hailstone mill get into a weaker updraft, for this that appears to have given much comfort and great en- is a ways irregular and ufly, or else will tumble to the couragement to those who maintain the efficacy of mere edoe of the ascending co umn. In either case it will then : noise to induce precipitation-to jostle cloud particles fa17 back into the region of liquid dro., where it will ather a coatina of water, a portion of w ich will at once togother into raindrops. The correct explanation, however, of this phenomenon seems obvious: The violent %e frozen by tge low temperature of the kernel. But turmoil and spasmodic movements within a large cumulus again it meets an upward gust, or falls back where the or thunderstorm cloud cause similar irregularities in t-lie ascendin drnft is stronger, and again the cyclic journe condensation and resulting number of raindrops a t any from rea m of ra.in to region of snow is begun; and eac given level. These in turn, as broken by the air currents, time-there may be several-the journey 1: completed a give local excess of electrification and of electric dischar e new layer of ice a.11~1 fresh Inyer of snow are added. In z or li htning flash. We have, then, startin toward t8e general the size of the hailstone3 will be roughly proporeart at the same time and from racticaly the same tional to the strength of the convection current, but smce (they are not homolevel, mass, sound, and light. The i g h t travels with the their weights vary approxinintmely greatest velocity, about 300,000 kilometers per second, geneous) as the cube of their diameters while the supand therefore the lightning flash is seen before the thunder porting force of the upward air current varies, also is heard; its velocity bein , roughly, only 330 meters per a11 rosimat8ely,as only the square of their diameters it second. But the rain fa& much slower still and there- fol ows that n limiting size i;j quickly reached. It is also fore reaches the ground after the thunder is heard. I n evident, from the fact that a strong convection current reality it is the excessive condensation or rain formation is essential to the forination of hail, that it can occur only up in the cumulus cloud that causes the vivid lightning where this convection exists; that is, in the front portion and the heavy thunder. According only to the order in of a heavy to violent thunderstorm. Some meteorologists hold that the roll scud between which their several velocities cause them to reach tshe surface of the earth it mi h t appear, and has often been the awending warm and descending cold air is the seat so int.er reted, that the lig tning, first perceived, was the of hail formation, but this is a mistaken assumption. cause o the thunder, which, indeed, i t is, and that the Centrifugal force would throw a solid object, like a hailheavy thunder, next in order, was the cause of the esces- stone, out of this roll probably before a single turn had sive rain, which most certainly it is not. been completed. Besides, and this objection is, Jerhaps, Thundptorm vdoeit.-The velocity of the thunder- more obviouvl fatal than the one just given, t e temstorm is simply the ve ocity of the atmosphere in which perature of d e roll scud, because of its position, the

humidiY.

lowest of the whole storm cloud, clearly must be many de ees,above the' freezing point. Indeed, as-the above c culation shorn, temperatures low enough for the formation of hail cmi not often obtain at levels much less than three times that of the scud, and therefore it clearly is in the higher levels of the cumulus and not in the low scud that hail must have its genesis and make its growth. &Mning.-About the middle of the eighteenth century F a k i and others clearly demonstrated that the rnln lightning.of a thunderstorm and the discharge of an ordinary electric machine are identical in nature, and
simultaneous but locally disconnected streaks. Frequently the dischaye continues flickeringly (on rare occasions evm steady, like a white-hot wire) during a perceptible time-occasionally a full second. But all these phenomena are best studied by means of the camera, and have been so studied by several ersons, among whom Walter, of Hamburg, and Larsen, of &cago,
are two of the most persistent and successful. Stationary cameras, revolving cameras. stereoscopic cameras, cameras with revolving lates, and cameras with s ectrographic attachments l a v e all been used, separate y and jointly, and the results have abundantly justified the time and the labor devoted to the work. Figure 34, eopied by permission from one of Walter's un ublished negatives, shows the ordinary tracery of a lig tning dischar e when photographed with a stationary camera. It is on y a perinanent record of t.he ap marance of the lighning to the unaided eye. Figure 35, owever, also copied by Walter's kind perinission from one of his unpublished photo raphs, is a record of the same discharge obtained wit a rotating caiiiera. It will he noted that the more nearly vertical discharge occurred but once or was single; that this discharge was quickly followed by a second along the same path to about onefourth of the way to the sarth where it branched off on a new course; that the second discharge was followed in turn at short but irregiilar intervals by R whole series of sequent discharges; that most of the dischar es appeared as narrow intensely luminous streaks, and t at one of the sequent discharges appeared, not to t.he eye, but on the plate of the rotating camera, as a broad band or ribbon. On close inspection it will be obvious that the plaidlike ribbon effect is due, the warp to irre ularities in the more or less continuous discharge, and t e woof t.0 rou lily end-on and therefore brighter ortions of the strea -. Another point particular1 wort y of attention is the fact that while the first dy ischarge has several side branches the following ones remain entire from end to end and am nowhere subdivided. Figure 36, taken. froni a hotogra h obtained by Mr. Larsen, of Chicago, and kin ly loane for use here by the Smithsonian Institution, shows another series of sequent discharges siniilar to those of figure 25, exce t that in t,his case there was no ribbon discharge. T l e time of the whole discharge, as calculated by Mi. Larsen, was 0.315 second. Here, too, side branches occur with tlie fint but. only tlie first discharge. This, however, is not an invariable rule for occasionall , as illustrated by figure 27, copied from apublished p otograph by Walter, the side branches ersiilt through two or three o the f first successive disc aroes, but not through all. I n such case each tributary &en re eated follows, as does the nisin stream, its own oiigin channel. Tile phenonienon of sequent discharges, all along the same path, and the clisn psarance of the side branches with or quick1 after the 1st discharge both seem reason:hly clear. $he first discharge, however produced, obviously takes place against very great resistance, and therefore under conditions the most favorable for the occurrence of side branches or ramifications. But the clischarge itself leaves the air along its path temporaril highly ionized-puts a temporary line conductor wit here and there a poorer conductin branch, in the atniospliere. This conductor is not on y temporary (half the icms are reunitedin about 0.15 second, the air being dust ) but also so estreniely fragile as to be liable to rupture y tlie atmospheric violence it itself creates. Because partly, perhaps, of just such interruptions, and because also of the volunie distribution of the electricity which prevents n sudden and coiiiplete discharge, the actual discharge is tlivitbd into a nuiiiber of artials that occur sequent1. Ol>viously,the breaks in t le conducting (ionized) pat , if they esist, are only here and there and but little more than sufficient to interru t the flow. Hence the next discharge, if it occurs uicE1 must follow the wnducting and, therefore, origin3 discgarge path. Besides, in the

?Pit a

MONTHLY WEATHER RETrlEW.
indeed is just what they are ac.cording t.o t.he ahove apeculat.ioii, a specula.t.ion tmhat. reco izes no difference in kind between streak, rocket, and?22ll Ijghining,. only differences in the amounts of ionization, quant.it.ies of availa.ble electricity and steepness of pot.entia1 grdients. SA.eet Zigh.tning.-When a dist.ant. t~liuadercloud is observed at-night one is uite certnin to see in it. bcautiful illuminations, looking ike great eheet,s of flnme, that. often flicker and glow 111 exactly the snnie inmner as does streak lightning for well-nigh a whole second. In the daytime and in full sunlight the phenomenon when seen at all ap ears like a sudden sheen t,liat tmivels :u:cl spreails here an there over the surface of the cloucl. Certninly in most cases, so far 8 s definitely known in all cases this is only reflect.ionfrom t.lie body of t.lic cloud of streak lightning in ot.her and invisible portions. C'onceivnbly a brush or coronal discha.rge may take >lnce from the upper surface of a tohunderat.ornicloud, u t one w-ould expect this to be either a faint cont.inuuus glow c)r else a momentary flash coincident wit.11 a discharge from the lower port-ion of the cloud to earth or to eunie other cloud. But, as already stated, only reflection is delinitely known to be the cause of sheet lightning. C'oroiiid effectss seem occasionally possiblet but that they are ever the cause of the henumenon in q u e s h n has ncver clearly been establis led and appeals verS doubtful. It has often been asserted, too, that tlierc? is ti railicnl difference between the spectra of storl.eali n : sheet 1d lightning, but even this does not tippear ever t.cr h a ~ e been phot.ogra.phically >roved. Beaded ~~g~.t~~g.-$iscontiiiuous 'or beaded streaks of lightnin have been reported from tinie to tinie. Indeed the au or liniaelf has several tinics seen, or hud t.he impression of seeing, this phenomenon, but with one or two doubtful esceptions lie felt prwticnliy ccrtaiii that it was only an o tical illusion. I n nddition to visual observations of t ie kind just described iiiiuiy Iiliotographs showing streaks of light broken i1it.o inore or less evenly spaced dashes have been obtained and rcporbetl aa photographs of beaded lightning. Without exception, however, these seein certainly to be nothing other than the photo ra lis of alternating current elect,ric lights, taken wi t if tfie ctiineru in motion. The objectire reality, therefore, of beaded ligiitning does not, seein at :ill well established, at least, not sufficiently well to justify an serious effort to esplain it. Ziqhtniii.-This is comnionly referred to as the return shock, a (9is only those relatively sinal! electrical n discharges that take plrtce here and i.lisre from objects on the surface of the earth coinciclentl-~ wit,li liglit4iiing flashes, and as a result of the suddenly chitnged elect.ricn1 strain. These discharges are always siiinll in coinparison with the main lightning flash, but st, tinies they are suiiicient to induce explosions, to start fires, and e v ~ n t,o take life. Dark Zightwin.g.-When t~ hotogpplic. plate is esposecl to a succession of lightning ashe!, it occctsionnlly lispjmis that one or more of the streak images, on development, exhibits the "Clayden effectJJ-tliats is, a ~ p e t m coiiipletrly reversed-while the others show no suc i tendency. 1viously, then, on prints froni such a negative the reversed streaks must a pear as dark lines, and for that reiisoii the lightning ashes that rodiiced thein have been called "dark liglitning." '&ere is, of course, no such thing as dark lightning, but tlie pliotogttpliic. phenomenon that ave rise. to the nanie IS real, interesting, and 1 n reproducib e at m 1 i the laboratory(l5).

MONTHLY WEATmR REVIEW.

fact, the thunderstorm is especiallylikely itself to establish the second set of the above conditions, or those least favorable to the f a r carrying of sound. Then, too, when a cannon, say, is f i e d the noise a l l starts from the same place, the energy is concentrated, while in the case of thunder it IS stretched out over the entire length of the lightning path. I n the h t case the energy is confined to a single shell; in the second it is diffused throii h an estensive volume. It is these differences in t e concentration and the conservation of tlie energy that cause the cannon to be heard much farther than tlie heaviest thtinder, even though the latter almost certainly produces much the greater total atmospheric disturbance. N o m 1 atmospheric electrici.ty.-The only reason for mentioning norninl atmospheric elmtricity rn connection with thunderstorms is to emphaslze the fact that, contrary to what man suppose to be tlie case, there is but little relation, in t ie sense of cause and effect, between these two phenomena. Thus while the difference in electrical potential between the surface of the earth and a point at constant elevation is roughly the same at all parts of the world, tlie nniiiber and intensity of thunderstorms vary greatly from place to place. Further, while the potmtinl gradient at any given place is in winter the number of thunderstorms is most requent in siiiiinier, and while the gradient, in the lower layer of the atmosphere, at ninny places, usually is.greatest from S to 10 oclock, both morning and evening, and least at 3 to 3 oclock p. ni. and 3 to 4 oclock a. m., no closely analooous relations hold for tlie thunderstorm. cles of which he is t.he center, while ot.her portions are Probably tKe most interesting conclusion in regard to directed more or less rndinlly from him. This would normal atmospheric electricity so far drawn from obseraccount for, and doubt.lessin a iiiensureis t.hocoi.rec,t.8s )In- vation and esperiment is this: That the earth everynat.ion of, soinn of t,heloud booming effects or crashes t mt where, land nncl water and from pole to pole, is a negaaccompany t.hunder. tively charged s >here of practical1 constant surface Surcession of discharges. When, as often happens, sev- density, surrom ed by nn ntmosp iere so conducting erd discharges follow each ot.herin rapid succession t,liere that it is constantly carrying away a current that is every op ortunity for all sorts of irregular mut.ualinter- amounts on the whole to about 1,000 amperes. ference an reinforcement of t.he compression waves or Where the sup ly of negative electricity comes from that keeps the su ace of the earth on the whole negatively sound impulses they send out. Reflect.ion. Under favorable conditions t.he echo of charged in spite of this steady great loss, or how the outthunder from clouds, hills, and ot,lier reflecting objects going current is compensated, no one knows. Rain does cert.ainly is effect,ive in accentuating and prolonging the not help matters for, as we have seen, that is prevailingly ositive, whereas we need, to com ensate the loss, to noise and rumble. But. t,he import.ance of this factor generally is g r e d y overestimated, for ordinarily the rumble grin back negative electricity a n l a great deal of it. is sub$t.ant.iaUyt.he same whet,her over t,he ocean, on t.he Neit er, so f a r as known, is compensation supplied by means of the lightning, for, in the great majority of cases, prairies, or among the mountains. Distance heud-The distance to which thunder can this, too, is positive from cloud to earth. And so the be heard seldom esceeds 35 kilometers, while ordinarily, puzzle remains. As Simpson (22) puts it: perhaps, it is not heard more than half so far. To most A flow of negative electricity takes place from the mfme of the ersons, familiar with tlie great distances to which the whole globe into the atmosphere above it, and this neceasitatesa return h n of large caniion is still perceptible, the relatively current of more than 1,000 amperes; et not the Eli htest indication of smafi distances to which thunder is audible is quite a any mch current has 80 far been founi, and no eatisf&9.ory explanation surprise. It shoiild be remembered, however, that both for ita absence has been given. the origin of the sound and often the air itself as a sound Much more, of course, might be said on this subject, conductor are radically different in the two cases. The for i t is a big one on which many have labored, but erfiring of cannon or any other surface disturbance is heard haps the above is sufficient for the purpose of this farthest when the air is still and when, through temperature section, namely, to show that, contrar to opinions often inversion or otherwise, it is so stratxed as in a measure held, there is no obvious and close re ation between the to conserve the sound eiier y between horizontal planes. thundemtorm and normal atmospheric electricity; that, Conversely, sound is hear to the least distance when according to our best evidence, they are distinct and indethe atmosphere irregular in respect to either its tem- pendent phenomena. perature or moisture distribution, or both, for these conditions favor the production of internal sound REFERENCES. reflections and the dissipation of energy. Now the former Memoirs, Indian metl. dept., Simk, 1910, 20, pt. 8. or -favorable conditions occasional1 obtain during the Physikal. Ztachr., Lei zi , 1906, 7 98. : production of ordinary.noises, including the firing of Sitzber., R. preuse. d. Wiss., Berlin, 1892, 8: 279-309. cannon, but never obtam during a thunderstorm. In Braak, Beitr. z. Phyaik d. f. Atmoaph., Le~pzig, r 1914,8: 141,