The Change in Black Sea Water Composition and Hydrology during Deglaciation from Multiproxy Reconstructions

Abstract:

This study presents a reconstruction of changes in the water column from the last glacial into the early Holocene using stable isotope, ⁸⁷Sr/⁸⁶Sr, ¹⁴C, and trace element ratios from mollusks from the shelf area and ostracods from the basin of the Black Sea. The stable isotope record is compared to a thoroughly U/Th dated terrestrial stable isotope record of a nearby cave, Sofular cave in northwestern Turkey. The combination of deep, surface, and terrestrial signals gives valuable insight towards the behavior of the lake water during the deglaciation in multiple dimensions, specifically the water column stratification and hydrological dynamics. The comparison of the stable isotope records of two independent proxies allows to make inferences on the changes in the ¹⁴C reservoir of the Black Sea-Lake. Results show that during the glacial period, water from the Black Sea-Lake was outflowing to the Sea of Marmara but the ventilation of the water column was weak as old ¹⁴C was not removed and allowed to accumulate giving the lake a large ¹⁴C reservoir age. A deglacial pulse of meltwater released from the Eurasian Fennoscandian pro-glacial lakes increased ventilation of the water column. This is seen in lighter δ¹⁸O and a spike in radiogenic ⁸⁷Sr/⁸⁶Sr in both deep and shallow parts of the water column. The dynamic ventilation and outflow of water into the Sea of Marmara continued until the onset of the Bolling/Allerod as the radiogenic ⁸⁷Sr/⁸⁶Sr was almost completely flushed out in a couple hundred years time. During the Bolling/Allerod and Preboreal warming, δ¹⁸O got heavier whereas the ⁸⁷Sr/⁸⁶Sr stayed constant and the ¹⁴C again accumulated and contributed to an older reservoir age. The Younger Dryas period, sandwiched in between the two warming periods, shows a return to glacial conditions in the δ¹³C and that the water outflowed to the Sea of Marmara as the δ¹⁸O only showed a slight change towards a more heavy value.