Eastern Mediterranean surface water temperatures and δ18O composition during deposition of sapropels in the late Quaternary

Abstract

Water column stratification increased at climatic transitions from cold to warm periods during the late Quaternary and led to anoxic conditions and sapropel formation in the deep eastern Mediterranean basins. High‐resolution data sets on sea‐surface temperatures (SST) (estimated from U37k′ indices) and δ18O of planktonic foraminifer calcite (δ18Ofc) across late Pleistocene sapropel intervals show that δ18Ofc decreased (between 1 and 4.6‰) and SST increased (between 0.7° and 6.7°C). Maximal δ18Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0‰, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in δ18Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon‐derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is “glacial” sapropel S6; its largest δ18Oseawater depletion (3‰) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment.