Multi-proxy indicators in a Pontocaspian system: a depth transect of surface sediment in the SE Caspian Sea

Abstract

The response of large water-bodies to global change in terms of ecosystem services and economical value is a major concern. The Caspian Sea, the world's largest enclosed water-body, has a poorly-known water-level history, but observed changes are a hundred times faster than recent global sea-level rise. This ancient lake, characterised by brackish waters, is rich in endemic species; some of them have spread to similar environments worldwide. However, the ecology of Pontocaspian species remains poorly understood and must be studied in their original habitat. This work aims at improving the capacity to reconstruct Quaternary environments of the Pontocaspian region and to provide a benchmark for biodiversity turnover studies. A transect of surface sediment across a wide shelf was subjected to multidisciplinary analyses: stable isotopes, pollen, dinocysts, diatoms, foraminifers, ostracods and molluscs and vertical oceanographic profiles. Three depositional environments with characteristic communities were found: shore face, shelf and slope. Invasion impact was strongly felt by the molluscs. All biota groups, except diatoms, reflected high endemism. The radiocarbon reservoir effect is highlighted in differential 14 C ages for different groups. Understanding such discrepancies require detailed insight into reworking processes. Tephra presence in the sediment shows a potential for tephrochronology. Stable isotope ratios in ostracods appear to reflect temperature depth gradients. Our results provide a baseline for calibrating proxy data to the present Pontocaspian environment.

Mollusc species from the Pontocaspian region – an expert opinion list

“…A substantial number of Pontocaspian species has been described from empty shells from beach material or derive from grab samples. Such samples typically are dominated by time-averaged Holocene shell assemblages, which may or may not yield living specimens and in very rare occasions also contain older (Pleistocene) material (see, e.g., Leroy et al 2018). For the Black Sea Basin, the Holocene time interval largely coincides with the date of the marine flooding through the Bosphorus and subsequent marginalisation of Pontocaspian species to the NW coastal zone (Danube Delta to Dnieper Estuary) and the Sea of Azov (Mordukhay-Boltovskoy 1960).…”

Human responses to environmental change on the southern coastal plain of the Caspian Sea during the Mesolithic and Neolithic periods

“…Hence it is difficult to decide 964 what calibration curve to choose (INTCAL13 or MAR13). Moreover, it is already known by investigations north of the Miankaleh Spit along a surface transect that the reservoir effect is higher than the 400 yr used for the MAR13 calibration curve and may vary between 670 and 750 yr (Leroy et al, 2018). When we examined the chronological sequence of the two pollen 969 diagrams in the context of other Caspian sequences, we came to doubt the 970 radiocarbon ages obtained from the Shahkileh and Gharasoo cores.…”

“…1A). In the east, the Gorgan Bay is connected to the CS by a narrow inlet (Leroy et al, 2018;Kouhanestani et al, 2019). The freshening of the bay is due to water inflow from rivers originating in the Alborz Mountains.…”

“…The large coastal plain in front of the caves extended hundreds km northward (Fig. 1A), as the bathymetry north of the Miankaleh Spit is very shallow (mostly less than 20 m) (Leroy et al, 2018), Gorgan Bay itself would have been much reduced or non-existent. The oldest group of dates with seals and highstand indicators in the Late Pleistocene corresponds to the Khvalynian highstand, whereas the youngest group of dates suggest the post-Mangyshlak highstand (Leroy et al, 2019;in press).…”

Caspian Sea levels over the last 2200 years, with new data from the S-E corner

“…The taxonomy and the ecological preferences of the Caspian dinocysts have been detailed in Mudie et al (2017) and in Leroy et al (2018). An additional form with a morphology between Galeacysta etrusca and Spiniferites cruciformis A was found.…”