East African climate change and orbital forcing during the last 175 kyr BP

Variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long-term fluctuations in African lake levels. The understanding of such relationships is essential for designing climate-prediction models for the tropics. Our assessment of the nature and timing of East African climate change is based on lake-level fluctuations of Lake Naivasha in the Central Kenya Rift (0°55′S 36°20′E), inferred from sediment characteristics, diatoms, authigenic mineral assemblages and 17 single-crystal 40Ar/39Ar age determinations. Assuming that these fluctuations reflect climate changes, the Lake Naivasha record demonstrates that periods of increased humidity in East Africa mainly followed maximum equatorial solar radiation in March or September. Interestingly, the most dramatic change in the Naivasha Basin occurred as early as 146 kyr BP and the highest lake level was recorded at about 139–133 kyr BP. This is consistent with other well-dated low-latitude climate records, but does not correspond to peaks in Northern Hemisphere summer insolation as the trigger for the ice-age cycles. The Naivasha record therefore provides evidence for low-latitude forcing of the ice-age climate cycles.