Quantification of African Monsoon Runoff During Last Interglacial Sapropel S5

Abstract

Organic‐rich sapropel layers punctuate the eastern Mediterranean sedimentary sequence,recording deep‐sea anoxic events. The timing of sapropel deposition coincides with precession minima,which are associated with the northward migration of the monsoon rain belt over North Africa. Theresultant increase in monsoon precipitation over the Sahara caused an increase in low‐δ18O freshwaterrunoff into eastern Mediterranean surface waters, which is reflected by negativeδ18O anomalies in therecords of planktic foraminiferal calcite. However, despite extensive research on sapropels, the magnitudeof monsoon intensification and freshwater runoff, along with its influence onδ18O, remains elusive. Here,we present a quantification of African monsoon freshwater runoff into the eastern Mediterranean for theperiod of deposition of last interglacial sapropel S5 (~128.3–121.5 ka). Our method uses a box model of theMediterranean Sea, which represents different water masses, and has been calibrated usingδ18O fromplanktic foraminiferal species of different depth and seasonal habitats. The model was constrained withexisting records of sea level and sea surface temperature then inverted to deconvolve theδ18O signal of thesurface‐dwelling foraminiferal speciesGlobigerinoides ruber(w) and calculate the freshwater runoff volume.Our calculated African monsoon runoff suggests large increases in freshwater discharge to the easternMediterranean (up to ~8.8 times the modern pre‐Aswan Nile discharge). Rapid onset of S5 depositionfollowing the estimated increase in runoff strongly suggests a preconditioning of the eastern Mediterraneanfor sapropel deposition. Our study also provides insight into the stratification and warming of easternMediterranean surface waters during the S5 interval.