SEDIMENT MAGNETIC HYSTERESIS MEASUREMENTS AS A PALEOCLIMATE PROXY FROM LAKE BOSUMTWI, GHANA

The West African country of Ghana contains Lake Bosumtwi which occupies a 1.07 Ma impact crater. This hydrologically closed lake has a small drainage basin creating a strong coupling between regional moisture balance and lake level. The one million year history of the lake was recovered by drilling in the crater's annular moat to recover 294 m of lacustrine sediment. The cores were measured for magnetic hysteresis properties at ~1 m (about 3 to 4 ka) intervals. X-ray diffraction (XRD) analysis every ~3 m was also performed to determine mineralogical variations. Magnetic hysteresis properties provide a measure of the magnetic concentration, magnetic grain size, and magnetic mineralogy in the sediment and have been successfully used as paleoclimate proxies in a wide range of depositional settings. Fluctuating environmental conditions change components of the depositional system such as sediment source and transport pathways. These components, in turn, influence the type of sediments accumulating therefore altering the magnetic hysteresis and XRD properties of the sediment.

The most recent interglacial and glacial periods have been well dated in Lake Bosumtwi sediment. S ratios greater than 0.8 and Hc values less than 20 mT indicate a low coercivity magnetic mineral assemblage accumulated during the wet interglacial period. This time period also includes the moist Holocene North African Humid Period when Lake Bosumtwi overflowed its surrounding crater wall. The glacial-age sediment contains high coercivity minerals indicated by S ratios less then 0.7 and Hc values above 30 mT. The increase in high coercivity minerals likely results from increased aeolian dust flux from arid Sahel sources at a time of greatly weakened summer monsoons. Identifying these two magnetic mineral assemblages deeper down, beyond the well-dated portion of the core, provides a means of identifying interglacial/glacial cycles for the lake's entire 1 Ma history. Correlating the Bosumtwi magnetic climate proxy to other climate records provides a way of dating the entire drill core record. The Bosumtwi record shows a change in the North African Monsoon from periods dominated by obliquity prior to 600 ka to periods dominated by eccentricity afterwards. A similar switch in orbital periodicity is also observed in published oceanic records of ice volume.