A HIGH-RESOLUTION PALEOCLIMATIC RECORD FOR THE LAST 27,500 YEARS IN TROPICAL WEST AFRICA FROM THE CARBON AND NITROGEN ISOTOPIC COMPOSITION OF LACUSTRINE ORGANIC-MATTER

Variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of Lake Bosumtwi, Ghana reflect climatically induced changes to the lake and the catchment flora. Cores spanning the last 27.5 kyr of sedimentation in the lake show large oscillations in delta-C-13(org) PDB and delta-N-15(org) Air. The late Pleistocene record is particularly detailed, showing changes of ca. 20 parts per thousand in carbon and ca. 10 parts per thousand in nitrogen isotopic compositions. These variations are of complex origin. Although different in magnitude, major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the Last Glacial Maximum. The Allerod-Younger Dryas period also seems to have been marked by generally dry conditions in this part of tropical West Africa. Nitrogen isotopic evidence suggests that during the period 9.2-3.2 kyr the lake had an extremely stable water column, probably due to the absence of a windy or cool, dry season. or both. Regular circulation of the water column recommenced during the late Holocene and has persisted until the present day.