Climate-driven connectivity changes of the Black Sea and their applications in stratigraphy

Biostratigraphy is often a vital tool for establishing reliable stratigraphic schemes. However, in basins such as the Black Sea, which have been partially or completely isolated from the global ocean system for long periods of geological time, global biostratigraphic schemes often cannot be applied directly without consideration of ecological filtering effects. This is due to the existence of distinct environmental conditions in the water of landlocked basins when compared to the contemporaneous open ocean, resulting in different biotic compositions on either side of the gateway. Differences in the compositions and timings of assemblages can potentially lead to the attribution of incorrect ages to strata in marginal or isolated basins such as the Black Sea.

Due to the potential for introducing significant error to age estimates in Black Sea sediments as a result of ecological filtering effects, it is crucial that both taxonomy and palaeoenvironments are well constrained before erecting biostratigraphic frameworks. Regional biostratigraphic schemes may be useful to minimise the error related to long distance or trans-basin correlation. However, fossils from landlocked basins also tend to display a large degree of morphological variability related to irregular and fluctuating environmental conditions (e.g. salinity, temperature and nutrient availability). Such ecophenotypic plasticity is observed in many biotic groups and may lead to difficulties in establishing robust taxonomy, with knock-on effects on biostratigraphy.

Here, we present an example using dinoflagellate cysts (dinocysts) from the Deep Sea Drilling Project (DSDP) Site 379 record, which was drilled in the central part of the Black Sea. Dinocysts are ideal marker fossils in Black Sea sediments because they are often highly abundant and their dinosporin casings lead to excellent preservation in anoxic and/or acidic settings (such as those often observed in the Black Sea geological record), where calcareous fossils can be altered (or even absent) due to dissolution. We combine this record with strontium isotopes (87Sr/86Sr) on ostracod valves where carbonate was preserved.

In common with many biotic groups dinocyst morphology is related not only to genetics but is also dependent on a range of environmental factors. In basins such as the Black Sea, where environmental change can be extreme and occurs on relatively short (millennial) timescales, the taxonomy governing their description is particularly challenging. Morphological continua can be observed between described forms, displaying a large range of intermediate phenotypes. We review some frequently occurring morphotypes and propose that using matrices to show the gradual variation of phenotypes between endmember forms is the most pragmatic approach until cyst–theca studies and genetic sequencing can be used to demonstrate relationships between genotypes and morphotypes.

Using this tool, in conjunction with previously published dinocyst taxonomic work, we then present an example record of climate-driven connectivity changes in the Black Sea since 430 ka. We use Black Sea sediments as a natural laboratory to test a dual palynological and geochemical approach using dinocysts and 87Sr/86Sr on ostracod valves. Dinocysts provide direct evidence for properties of surface waters and 87Sr/86Sr constrains the source(s) of water entering the basin. The new dinocyst and 87Sr/86Sr records suggest that the Black Sea was isolated from the Mediterranean during global sea-level low stands that were associated with glacial periods during MIS 4, 8 and 10. Both proxies also strongly suggest that marine (Mediterranean) water flowed into the Black Sea during the eustatic high stands that were associated with peak interglacial periods during MIS 5 and 9. All four interglacial periods studied contain different dinocyst assemblages, suggesting that different conditions may have prevailed during each warm period. These differences may be useful for stratigraphic fingerprinting of different climatic events. Assemblages dominated by species of Paratethyan lineage across parts of the succession are compatible with input from the Caspian Sea, which may also be of stratigraphic significance.

The principles demonstrated in this Quaternary example are equally important for understanding of earlier periods of Black Sea sedimentary history, such as those of the Pontian, Maeotian and Maikop. The Black Sea has experienced sporadic connections and disconnections to/from the open ocean throughout much of the Cenozoic, driven by a combination of global sea level variation and tectonic alteration of sill-points and basin bathymetry. As such, environmental control on biota has been significant and a strong understanding of both palaeoenvironments and connectivity histories is therefore necessary for the creation of robust biostratigraphic frameworks.