The “glacial” sapropel S6 (172 ka; MIS 6): A multiproxy approach to solve a Mediterranean “cold case”

Abstract

Sequences of dark, organic-rich sediment layers (sapropels) exist throughout the Neogene of the Mediterranean Sea sedimentary record. While the mechanisms behind their cyclical deposition are not entirely understood, they have been found to correspond with precession minima (Northern Hemisphere [NH] insolation maxima). This causes NH summer monsoon to shift northward and intensify, which increases precipitation over North Africa and alters Mediterranean freshwater budget, leading to restricted bottom-water ventilation and anoxia. Most Mediterranean sapropels were deposited during interglacial periods, but sapropel S6 formed during the penultimate glaciation of Marine Isotopic Stage 6 (MIS 6; 190–130 ka), during which the Eurasian ice sheet extended to its maximum size of the Quaternary. Eurasian ice-sheet melting may have provided an additional input of freshwater to the Mediterranean during S6 deposition. To test this hypothesis, we present a multiproxy paleoecological (planktic foraminifera, calcareous nannofossils, pollen, dinocysts) and geochemical (foraminiferal δ18O) study of S6 from the Ionian Sea. We confirm that S6 deposition resulted from an interaction of two different mechanisms of freshwater input to the Mediterranean, in which: (1) local ice-sheet meltwater discharge preconditioned the basin for stratification; and (2) increased monsoon activity over North Africa caused intense precipitation and river runoff that exacerbated water-column stratification. Our results provide new evidence for the prevalence of mild/temperate and humid conditions during S6 deposition, dispelling the notion that this “glacial” sapropel formed under cold and dry conditions and we document signals of warm (interstadial) and cold (stadial) conditions within S6 in the eastern Mediterranean basin.