The co-evolution of Black Sea level and composition through the last deglaciation and its paleoclimatic significance

Abstract

The strontium and oxygen isotopic compositions of carbonate shells are a measure of the water delivered to the Black Sea lake since the last glacial maximum. Commencing at ~18 ka BP cal with the arrival of substantial meltwater from the Alpine and northern European ice sheets and overflow via the Caspian Sea from the disintegrating Siberian ice cover, the 87Sr/86Sr ratio rose rapidly from a glacial minima around 0.7087 to reach a set of peaks near 0.7091 in layers of conspicuous reddish-brown clay with a mineralogy of Eurasian provenance. The 87Sr/86Sr ratio oscillates between high in the red-brown layers to low in interbedded gray clays with glacial era mineralogy, indicative that the meltwater came in pulses. On the other hand, the rise of the δ18O ratio from glacial low values of -7 per mil was delayed until15.2 ka BP cal, after the last meltwater pulse. The rising δ18O of the Black Sea lake corresponds with two episodes of calcite precipitation whose interruption corresponds to the Younger Dryas cold period. During each interval of calcite precipitation the δ18O increased a further 2 per mil, without variation in the 87Sr/86Sr composition. During cooling the 87Sr/86Sr ratio trended back toward its glacial value with little change in the δ18O. The disparity between the Sr and O isotope behavior demonstrates that δ18O is not simply a signal of end-member mixing, but instead the δ18O record reflects changes in atmospheric moisture delivered to the Black Sea watershed. At 9.4 ka BP cal the 87Sr/86Sr composition shifted to that of the global ocean and remained there to the present. Since lake water is significantly depleted in strontium relative to seawater, any earlier leakage from the Mediterranean should have left a corresponding signal.