Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile

Magnetic and geochemical core data spanning the last 17,000 years are correlated with new seismic stratigraphy from Lake Tana, Ethiopia, to infer past lake-level change and hence effective precipitation. The data confirm that low lake-level coincides with Heinrich Event 1 (H1) in the North Atlantic, as previously shown from diatom and pollen evidence (Lamb et al., 2007). The lake deepened at 15.3 cal kyr BP and abruptly returned to freshwater conditions, when the lake overflowed into the Blue Nile. Low runoff and lake levels and therefore rainfall are inferred between 13.0 and 12.5 cal kyr BP and may represent southerly suppression of the ITCZ and the associated monsoon front at the time of the Younger Dryas. Two drought episodes occurred at 8.4 and 7.5 cal kyr BP, and are also interpreted as a southward shift in the monsoon front. The first of these events appears to have preceded and been more significant than the 8.2 cal kyr BP. Precipitation declined after 6.8 cal kyr BP, although we do not see an abrupt end to the African Humid Period. This period culminated in a dry episode at ~ 4.2 cal kyr BP, supporting the view that reduced Nile flow was a contributing factor to the demise of the Egyptian Old Kingdom.