A LATE QUATERNARY MAGNETIC MINERAL RECORD FROM THE SEDIMENTS OF LAKE BOSUMTWI, GHANA

Lake Bosumtwi is a hydrologically-closed crater lake located in the tropical forest lowlands of Ghana, Africa. The hydrologic closure and relatively small drainage basin make for a strong linkage between lake level and regional moisture balance. This study characterizes the rock-magnetic properties of recently collected Bosumtwi sediment cores. The goal of this study is to determine if rock-magnetic parameters can provide additional insight on West African climate variability since the last glacial maximum. Rock-magnetic measurements, made on an alternating gradient magnetometer, include hysteresis loops, isothermal remanent magnetization and coercivity of remanence. These rock-magnetic parameters were used to characterize cores spanning the past 22 ¹⁴C ka. Several radiocarbon dates provide a preliminary age model for the long cores, whereas varve counting and ²¹⁰Pb dates have provided an age model for the most recent 160 years of sediment.

Distinct zones of differing magnetic mineralogies, grain-sizes and concentrations are readily identifiable down core and may relate to climatically driven changes in lithology. For example, between 94 – 255 cm an organic-rich sapropel lithology dates to approximately 3.1 – 8 ¹⁴C ka. This lithologic zone is characterized by very high concentrations of moderate coercivity, pseudo single domain grains. This organic-rich sapropel represents a very humid period and lake level high stand (Talbot and Johannsen, EPSL 110, p. 23, 1992). Beneath 255 cm the sediment has extremely low magnetic concentrations of very coarse-grained (multidomain) magnetic minerals. However, there are three distinct zones at 255 – 280, 450 – 570 and 820 – 850 cm that have high concentrations of very fine-grained (single domain), extremely high coercivity minerals. Ongoing work is focused on determining if these pronounced magnetic mineral zones relate to the existing Bosumtwi lake-level curve and regional climate records.