Sr isotope-salinity modelling constraints on Quaternary Black Sea connectivity

Abstract

Reconstruction of the Black Sea's connectivity history during the Quaternary is based on high-resolution isotope data and longer-term biotic records. These demonstrate that the Black Sea experienced rapidly changing hydrologic configurations involving, at different times, isolation from and/or connection with the Mediterranean and Caspian seas. Here, we simulate these connectivity scenarios using a numerical box model of Black Sea hydrography. This allows us to explore the magnitude of the different water fluxes and their consequences for Black Sea salinity and Sr isotope ratio; parameters that we can measure today and reconstruct in the past. We show that the model is able to replicate the present-day hydrography using new Sr isotopic data measured on modern Black Sea water (0.709143 ± 0.000008) and that the Black Sea has a homogenous Sr isotope ratio despite its strongly stratified water column. We then create a 1.2 Myr record of paired Sr isotope (ostracods) and palaeosalinity (dinoflagellate cysts) data, using samples from DSDP Site 379. We also provide the first constraints on the Sr isotope ratio of the Amu Darya River, a substantial fluvial system that previously fed the Caspian Sea and therefore influenced the Black Sea when the two basins were connected. Modelling the different connectivity scenarios and incorporating both new and previously published geochemical and biotic data indicates that influx from the Caspian Sea dominated Black Sea hydrography between 1.2 to around 0.5 Ma. In contrast, Mediterranean input has exerted increasing influence since 0.6 Ma, particularly during interglacial periods. The hypothesis that an interval of anomalously high Sr isotope ratios in the Black Sea during the late Saalian deglaciation (MIS 6) was caused by input from the Amu Darya is demonstrably unrealistic. Instead the high Sr ratios may have been driven by variations in glacial extent within the Dnieper catchment and the resultant erosion of Sr from granitic basement rocks. Our new combined model-data approach tests and refines interpretations of the glacio-hydrographic development of the Black Sea during the second half of the Quaternary.