Water column dynamics during the last interglacial anoxic event in the Mediterranean (sapropel S5)

[ 1] Marine organic-rich sediments are important for long-term carbon sequestration, and as a source of fossil hydrocarbons, but the processes underlying their formation remain elusive. We present a multiproxy analysis of a relatively recent, well-preserved Mediterranean organic-rich deposit known as sapropel S5, which formed 124 - 119 thousand years ago ( ka B. P., where B. P. references year 1950). Combining planktonic foraminiferal abundance records with stable isotope and organic biomarker data, we reconcile the apparently contradicting coexistence of deep-dwelling foraminiferal faunas with green sulphur bacteria that indicate photic zone euxinia. S5 started with a rapid freshwater-induced increase of density stratification that inhibited vertical mixing and deepwater ventilation. Through the first 900 years, anoxic to euxinic deepwater conditions developed and extended up to roughly 200 m water depth. Then followed a 4100-year period of variability in the depth/ intensity of winter mixing with a roughly 1400-year periodicity, coincident with fluctuations in water column stability. S5 terminated with a marked decrease in density stratification and consequently renewed convective overturn.