Hydroclimate Variability at Lake Tana, Ethiopia, Over the Past 6,000 Years: A Critical Synthesis of Geochemical and Isotopic Records

Lake Tana, the source of the Blue Nile in the northwestern Ethiopian highlands, is a climatically sensitive archive at the northern margin of the Intertropical Convergence Zone (ITCZ). Its sediments record changes in the balance between Atlantic/Congo Basin moisture and Indian Ocean/Red Sea moisture, making it a key site for understanding Afro-Asian monsoon dynamics. This review critically synthesizes geochemical (XRF element ratios: Zr/Rb, Ca/Ti, Rb/K) and isotopic (leaf-wax δD) records from Lake Tana to reconstruct hydroclimate variability over the past 6,000 years, evaluate proxy reliability, and identify research gaps. Available evidence indicates that the African Humid Period (AHP) terminated at Lake Tana by approximately 8,000 cal BP, significantly earlier than in the Sahara and North Africa (~5,000 cal BP), δDwax records indicate that this transition was driven by eastward migration of the Congo Air Boundary (CAB), not solely by ITCZ southward retreat. Late Holocene aridification culminated in a pronounced low-stand at ~4,200 cal BP, coeval with the Egyptian Old Kingdom collapse. Existing XRF data (366–150,190 yr) provide a foundation but lack the sampling density needed to resolve centennial-scale variability in the past 6,000 years, and no dedicated high-resolution study exists for the past 2,000 years, a period encompassing the Aksumite civilization and the Little Ice Age. A dedicated high-resolution geochemical and isotopic study is urgently needed to address outstanding questions about natural variability versus anthropogenic impact, and to provide a longer historical context for water resource management in the Blue Nile basin under future global warming.