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

We dated a continuous, similar to 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 C-14 dates on bulk-organic matter, combined with a Pb-210 chronology for recent sediments and corrected for a variable old-carbon age offset. This offset was estimated by i) pairing bulk-organic C-14 dates with either Pb-210-derived time markers or C-14 dates on grass charcoal, and ii) wiggle-matching high-density series of bulk-organic C-14 dates. Variation in the old-carbon age offset through time is relatively modest, ranging from similar to 450 yr during glacial and late glacial time to similar to 200 yr during the early and mid-Holocene, and increasing again to similar to 250 yr today. The screened and corrected C-14 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 C-14 date had been calibrated on its own. The smooth-spline age-depth model has 95% age uncertainty ranges of similar to 50-230 yr during the Holocene and similar to 250-550 yr in the glacial section of the record. The delta C-13 values of paired bulk-organic and grass-charcoal samples, and additional C-14 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. (C) 2011 Elsevier Ltd. All rights reserved.