Evolution of winter precipitation in the Nile-River watershed since the last glacial
Abstract. Between 11.5 and 5 ka BP, the Sahara was vegetated owing to a wet climate during the African Humid Period (AHP). However, the climatic factors sustaining the “Green Sahara” are still a matter of debate. Particularly the role of winter precipitation is poorly understood. Using the stable hydrogen isotopic composition (δD) of high molecular weight (HMW) n-alkanoic acids in a marine sediment core from the Eastern Mediterranean (EM), we provide a continuous record for winter precipitation in the Nile-River delta spanning the past 18 ka. Pairing the data with regional δD records from HMW n-alkanes, we show that HMW n-alkanoic acids constantly derive from the delta while the HMW n-alkanes also receive significant contributions from the headwaters between ~15–1 ka BP due to enhanced fluvial runoff. This enables us to reconstruct the evolution of Mediterranean (winter) and monsoonal (summer) rainfall in the Nile River watershed in parallel. Heinrich Stadial 1 (HS1) evolved in two phases with a dry spell between ~17.5–16.0 ka BP followed by wet conditions between ~16–14.5 ka BP owing to movements of the Atlantic storm track. Winter rainfall enhanced substantially between 11–6 ka BP lagging behind the intensification of the summer monsoon by ca. 3 ka. Heavy winter rainfall resulted from a southern position of the Atlantic storm track combined with elevated sea-surface temperatures in the EM reinforcing local cyclogenesis. We show that during the “Green Sahara” monsoon precipitation and Mediterranean winter rainfall were simultaneously enhanced and infer that the winter-rainfall zone extended southwards delivering moisture to the Sahara. Our findings corroborate recent hypotheses according to which southward extended winter rains were a crucial addition to the northward displacement of the summer monsoon helping to sustain a “Green Sahara”.
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