A POSSIBLE RECORD OF BOLLING-ALEROD/YOUNGER DRYAS CLIMATE FLUCTUATIONS FROM THE BLACK SEA

The Black Sea, the largest anoxic basin in the world, was transformed from a lacustrine to a marine depositional system between ~7.5 and 8.5 kyr B.P. by post-glacial eustatic rise. The 332.5-m-thick lacustrine succession of Unit III consists of relatively homogenous clayey silts prior to ~7.5 kyr B.P. To date, the warm Bolling-Alerod interstadial (14.7-12.9 kyr B.P.) and the ensuing cold Younger Dryas stadial (12.9-11.7 kyr B.P.) have only been recognized in sediment cores of the Black Sea’s shelf. In this study, we sampled at 2-cm intervals the uppermost 103 to 148 cm of two drillcores recovered during DSDP Leg 42, representing the central basin floor (Site 379A) and southwestern slope of the basin, across from the Bosporus (Site 380). Units I and II were lost during the rotary drilling process, and the tops of the cores are known to contain upper Unit III sediments, although the exact ages of the core tops were not determined by the DSDP.

Most samples of Core 379A and all samples of Core 380 exhibit typical lithogenic elemental concentration patterns and C_org:P ratios (15-30) consistent with aerobic bottomwaters. However, core 379A shows two unusual intervals: (1) between 65 and 43 cm, C_org:P ratios repeatedly rise to 50-110 and Cr, Cu, and Ni exhibit modest enrichments (2-3× background) in sub-sapropelic sediment layers, and (2) between 43 and 30 cm, Na and Cl concentrations each rise to ~20-35% (versus <2% Na and ~0% Cl in the rest of the core). We infer that suboxic bottomwater conditions developed in the 65-43-cm interval in an otherwise well-oxygenated lake basin, linked to increased humidity of the Bolling-Allerod warming. The 43-30-cm interval is inferred to represent an evaporative halite layer formed during the Younger Dryas event. The latter is particularly significant given the generally humid conditions that prevailed within the Black Sea watershed. Given an average sedimentation rate for Unit III of ~20±10 cm kyr^-1, these two core intervals represent ~0.7-2.2 kyr and 0.4-1.3 kyr, respectively.

These unusual sedimentary signals suggest control by extreme climate fluctuations. Although exact ages are lacking, the existing age constraints suggest the Bolling-Allerod and Younger Dryas events are present in the study cores.