Deciphering hydroclimate fluctuations of Nile watershed from Holocene sediment of Manzala lagoon: Bio-geochemical implications

A 19.5-m long Holocene sediment core (MZ-1) was retrieved from Manzala lagoon coast, the depocenter of the Nile Delta. AMS-dating shows the complete Holocene strata of MZ-1, mostly consisting of lagoon muds. The high-resolution dated geochemical elements (major and trace) and bio-markers (n-alkanes) of MZ-1 serve as effective hydroclimate proxies that oscillated during the Africa Humid Period (AHP). Our measurements show the highest value of diagnostic geochemical elements of provenance implications (e.g. Ti/Al, Zr/Al, Cr/Al) prior to 7.5 ka - Period I, indicating the peak period of the AHP, when the African monsoon was intense over the Nile watershed, mainly the East African Plateau (EAP). This is immediately followed by Period II ca. 7.5–4.5 ka, characterized by the highly variable hydroclimatic conditions when the Intertropical Convergence Zone (ITCZ) withdrew southwards. It is denoted by a sharp drop of many diagnostic elements, then succeeded by a gentle, but highly-fluctuated rise, especially the metallic elements Fe, Ti and Cr of basaltic origins of the Ethiopian Highlands. It ascribes to an instability of the hydroclimate in the Blue Nile watershed due to eastward shift of the Congo Air Boundary (CAB) during the AHP receding. In response, the biomarker (n-alkanes) showed remarkably high terrestrial derived primary productivity in Manzala lagoon during 7.5–4.5 ka, evidently reversal to the Early-Holocene marine-enhanced ecological setting. The high Nile floods triggered by unstable hydroclimate of the Nile watershed, particularly in the Ethiopian Highlands of Period II delivered much terrestrial organic matter to coast, making lagoon water less saline. Minor geo-elemental and biomarker fluctuations occurred after ca. 4.5 ka in response to the post-AHP termination in the upper Nile Basin.