A 1 MA ENVIRONMENTAL MAGNETIC RECORD FROM LAKE BOSUMTWI, WEST AFRICA

Lake Bosumtwi occupies a 1.07 Ma impact crater in Ghana, West Africa. This 78 m deep, hydrologically-closed lake has a water budget extremely sensitive to the moisture balance and is located in the path of the seasonal migration of the ITCZ, thus it is well situated to provide a long record of monsoon variability. Using the GLAD800 lake drilling rig, the 1 Ma history of the lake was recovered by drilling in the crater's annular moat to recover 291 m of lacustrine sediment.

Magnetic hysteresis measurements reveal two contrasting lithologies that were deposited over glacial-interglacial cycles. Organic-rich sediment with low concentrations of low-coercivity magnetic minerals accumulated during wetter interglacials when increased boreal summer insolation intensified the rainy summer monsoon. However, during periods of decreased summer insolation and the accompanying weaker summer monsoon, lake levels fell and a mineral rich sediment accumulated. The magnetic mineral assemblage is characterized by high concentrations of remanence bearing minerals and a high-coercivity magnetic mineral assemblage dominated by hematite, goethite, and/or greigite. During these drier glacial periods, an increase in aerosol dust export from arid Sahel sources as well as lower lake levels promoting sulfate reduction both contributed to the distinct magnetic mineralogy. The Bosumtwi sedimentary magnetic record is readily correlated to the marine oxygen isotope record of global ice volume thus allowing for improved age control for the Bosumtwi drill cores. The magnetic mineral proxy of dust in the Bosumtwi drill cores readily correlates to dust records from the marine and ice sheet environments thereby allowing the Bosumtwi record to be placed in a global stratigraphic framework. Strong eccentricity and obliquity periodicities in the Bosumtwi climate proxies reveal the response of tropical climate to high latitude ice sheets and a connection between high and low-latitude climate change.