Seasonality in equatorial climate over the past 25 k.y. revealed by oxygen isotope records from Mount Kilimanjaro

Multi-proxy analysis of a well-dated 25,000-year (25 ka) lake-sediment sequence from Lake Challa, on the eastern flank of Mt Kilimanjaro, reveal the climatic controls which govern both the lake’s palaeohydrology and the climate-proxy record contained in the mountain’s receding ice cap. The oxygen-isotope record extracted from diatom silica (d18Odiatom) in Lake Challa sediments captured dry conditions during the last glacial period and a wet late-glacial transition to the Holocene interrupted by Younger Dryas drought. Further, it faithfully traced gradual weakening of the southeastern monsoon during the Holocene. Overall, d18Odiatom matches the branched isoprenoid tetraether (BIT) index of rainfall-induced soil run-off, except during 25–22 ka BP and the last 5 ka when insolation forcing due to orbital precession enhanced the northeastern monsoon. This pattern arises because during these two periods, a weakened southeastern monsoon reduced the amount of rainfall during the long rain season and enhanced the opposing effect of evaporation intensity and/or length of the austral winter dry season. Importantly, our lake-based reconstruction of moisture-balance seasonality in equatorial East Africa also helps understand the oxygen-isotope record contained in Mt. Kilimanjaro ice. Negative correlation between ice-core d18O and Lake Challa d18Odiatom implies that temperature, not moisture balance, is the primary climate control on the long-term trend in ice-core d18O