High-resolution 14C dating of a 25,000-year lake-sediment record from equatorial East Africa

Abstract We dated a continuous, ∼22-m long sediment sequence from Lake Challa (Mt. Kilimanjaro area, Kenya/Tanzania) to produce a solid chronological framework for multi-proxy reconstructions of climate and environmental change in equatorial East Africa over the past 25,000 years. The age model is based on a total of 168 AMS 14 C dates on bulk-organic matter, combined with a 210 Pb chronology for recent sediments and corrected for a variable old-carbon age offset. This offset was estimated by i ) pairing bulk-organic 14 C dates with either 210 Pb-derived time markers or 14 C dates on grass charcoal, and ii ) wiggle-matching high-density series of bulk-organic 14 C dates. Variation in the old-carbon age offset through time is relatively modest, ranging from ∼450 yr during glacial and late glacial time to ∼200 yr during the early and mid-Holocene, and increasing again to ∼250 yr today. The screened and corrected 14 C dates were calibrated sequentially, statistically constrained by their stratigraphical order. As a result their constrained calendar-age distributions are much narrower, and the calibrated dates more precise, than if each 14 C date had been calibrated on its own. The smooth-spline age-depth model has 95% age uncertainty ranges of ∼50–230 yr during the Holocene and ∼250–550 yr in the glacial section of the record. The δ 13 C values of paired bulk-organic and grass-charcoal samples, and additional 14 C dating on selected turbidite horizons, indicates that the old-carbon age offset in Lake Challa is caused by a variable contribution of old terrestrial organic matter eroded from soils, and controlled mainly by changes in vegetation cover within the crater basin.