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. The resultant increase in monsoon precipitation over the Sahara caused an increase in low-δ¹⁸O freshwater runoff into eastern Mediterranean surface waters, which is reflected by negative δ¹⁸O anomalies in the records of planktic foraminiferal calcite. However, despite extensive research on sapropels, the magnitude of monsoon intensification and freshwater runoff, along with its influence on δ¹⁸O, remains elusive. Here, we present a quantification of African monsoon freshwater runoff into the eastern Mediterranean for the period of deposition of last interglacial sapropel S5 (~128.3–121.5 ka). Our method uses a box model of the Mediterranean Sea, which represents different water masses, and has been calibrated using δ¹⁸O from planktic foraminiferal species of different depth and seasonal habitats. The model was constrained with existing records of sea level and sea surface temperature then inverted to deconvolve the δ¹⁸O signal of the surface-dwelling foraminiferal species Globigerinoides ruber (w) and calculate the freshwater runoff volume. Our calculated African monsoon runoff suggests large increases in freshwater discharge to the eastern Mediterranean (up to ~8.8 times the modern pre-Aswan Nile discharge). Rapid onset of S5 deposition following the estimated increase in runoff strongly suggests a preconditioning of the eastern Mediterranean for sapropel deposition. Our study also provides insight into the stratification and warming of eastern Mediterranean surface waters during the S5 interval.

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