The 1 Ma Lake Bosumtwi (West Africa) Paleoclimate Record: Comparisons to Marine and Polar Records

Lake Bosumtwi is a hydrologically closed lake occupying a 1.07 Ma impact crater in Ghana, West Africa. The lake lies beneath the path of the seasonal migration of the ITCZ and therefore can provide a sedimentary record of monsoon variability in West Africa. Scientific drilling recovered a 291-m long sediment section that spans the full 1 Ma history of the lake. This long continental record is ideal for comparison to long marine and ice-core records at both glacial-interglacial and abrupt-change timescales. Oxygen-isotope stratigraphy, derived from calcareous fossils, often provides age control and a way to place individual marine sediment cores into a global stratigraphic framework. Lacking a direct tie-in to the marine oxygen-isotope stratigraphy, individual lacustrine basins can present challenges for global correlation. Through radiocarbon, optically stimulated luminescence and paleomagnetic dating, limited age control has been established for the 1 Ma Lake Bosumtwi sediment sequence. Within a Bosumtwi sediment sequence that is mostly laminated occur intervals of non-laminated sediment having increased density, decreased organic content and a high-coercivity magnetic mineral assemblage. Some of these massive layers contain slump-folding and intraformational clasts. These lithologies are interpreted to represent lake-level lowstands when a diminished West African summer monsoon resulted in decreased moisture balance and lake-level regression. Some Bosumtwi lake-level lowstands match intervals of increased sea surface salinity in the Gulf of Guinea resulting from reduced river discharge (Weldeab et al. 2007, Science, 316, 1303-1307). However, during other intervals (MIS2) there are differences between the two records. Corresponding to glacial stages and stadials, increased amounts of high-coercivity magnetic minerals are present in the Lake Bosumtwi sediment. Elevated aerosol dust export from arid Sahel sources, possibly accompanied by enhanced magnetic-mineral diagenesis during lake- level lowstands, is interpreted to have produced this magnetic signature. The Bosumtwi dust proxy record displays variability similar to that of published dust records from marine and polar settings. Comparing this continental dust record to a marine dust record off the coast of West Africa provides additional constraints on identifying glacial-interglacial variability in the 1 Ma long lacustrine sediment sequence.