2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 14
Presentation Time: 1:30 PM-5:30 PM


GREEN, Ryan R.1, PECK, John1, KING, John2, WHEELER, C. Winston3 and OVERPECK, Johnathan4, (1)Geology, Univ of Akron, University of Akron, Department of Geology, Akron, OH 44325-4101, (2)Marine Geology & Geophysics, Univ of Rhode Island Graduate School of Oceanography, Narragansett Bay Campus, Narragansett, RI 02882, (3)Univ of Arizona, 1040 E. Fourth Street, room 208, Tucson, AZ 85721-0077, (4)Univ of Arizona, Tucson, AZ, rrg3@uakron.edu

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.