Measurement of Be from Lake Malawi (Africa) drill core sediments and implications for geochronology

a r t i c l e i n f o The cosmogenic radionuclide 10 Be was measured from drill core sediments from Lake Malawi in order to help construct a chronology for the study of the tropical paleoclimate in East Africa. Sediment samples were taken every 10 m from the core MAL05-1C to 80 m in depth and then from that depth in core MAL05-1B to 382 m. Sediment samples were then later taken at a higher resolution of every 2 m from MAL05-1C. They were then leached to remove the authigenic fraction, the leachate was processed to separate out the beryllium isotopes, and 10 Be was measured at the TAMS Facility at the University of Arizona. The 10 Be/ 9 Be profile from Lake Malawi sediments is similar to those derived from marine sediment cores for the late Pleistocene, and is consistent with the few radiocarbon and OSL IR measurements made from the same core. Nevertheless, a strong correlation between the stable isotope 9 Be and the cosmogenic isotope 10 Be suggests that both isotopes have been well mixed before deposition unlike in some marine sediment cores. In addition, the correlation of beryllium isotopes to a proxy of lake level TOC (Total Organic Matter) from Lake Malawi indicates that the concentrations of 10 Be in the lake sediments result from the combined effects of global and local climates on lake level, local hydrology, and sediment transport in the Lake Malawi basin rather than as a direct response to its production in the atmosphere modulated by the intensity of the Earth's dipole. Therefore, a direct correlation of the 10 Be/ 9 Be to a chronology derived from the paleomagnetic variations measured from marine sediments was not possible. Nevertheless, a comparison of the 10 Be/ 9 Be chronology, allowing for decay, at Lake Malawi to that of the global marine paleomagnetic record suggests that the bottom of core MAL05-1B is no more than 750 ka in age. We investigated using the cosmogenic radionuclide 10 Be to date sediments from Lake Malawi because its half life duration is appropriate for the time frame in which the lake sediments were deposited, as well as the possibility of tying this record to that of marine sediments. More than 600 m of the core have been obtained from Lake Malawi (Scholz et al., 2006, and Scholz et al., 2011-this issue), located in the southern …