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C chronology 14 C age offsets Bayesian ageedepth modelling a b s t r a c tWe dated a continuous, w22-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 bulkorganic 14 C dates with either 210 Pb-derived time markers or 14 C dates on grass charcoal, and ii) wigglematching high-density series of bulk-organic 14 C dates. Variation in the old-carbon age offset through time is relatively modest, ranging from w450 yr during glacial and late glacial time to w200 yr during the early and mid-Holocene, and increasing again to w250 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 w50e230 yr during the Holocene and w250e550 yr in the glacial section of the record.The d 13C 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.

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Sediments from Lake Tswaing (25°24'30'' S, 28°04'59'' E) document hydrological changes in southern Africa over the last 200 kyr. Using high-resolution XRFscanning, basic geochemistry (TIC, TOC, TN), organic petrology and Rock-Eval pyrolysis, we identify intervals of decreased carbonate precipitation, increased detrital input, decreased salinity and decreased autochthonous (algal and bacterial) organic matter content that represent periods of less stable water column stratification and