非表示:
キーワード:
palynology; palaeolimnology; diatoms; sedimentary;
geochemistry; stable isotopes; Holocene; Pleistocene; savanna;
Nigeria; West Africa
West-african sahel; pollen; record; history; forests
要旨:
The Lake Tilla crater lake in northeastern Nigeria (10degrees23'N, 12degrees08'E) provides a ca. 17,000 C-14 yr multiproxy record of the environmental history of a Sudanian savanna in West Africa. Evaluation of pollen, diatoms, and sedimentary geochemistry from cores suggests that dry climatic conditions prevailed throughout the late Pleistocene. Before the onset of the Holocene, the slow rise in lake levels was interrupted by a distinct dry event between ca. 10,900 and 10,500 C-14 yr B.P., which may coincide with the Younger Dryas episode. The onset of the Holocene is marked by an abrupt increase in lake levels and a subsequent spread of Guinean and Sudanian tree taxa into the open grass savanna that predominated throughout the Late Pleistocene. The dominance of the mountain olive Olea hochstetteri suggests cool climatic conditions prior to ca. 8600 C-14 yr B.P. The early to mid- Holocene humid period culminated between ca. 8500 and 7000 C-14 yr B.P. with the establishment of a dense Guinean savanna during high lake levels. Frequent fires were important in promoting the open character of the vegetation. The palynological and palaeolimnological data demonstrate that the humid period terminated after ca. 7000 C-14 yr B.P. in a gradual decline of the precipitation/evaporation ratio and was not interrupted by abrupt climatic events. The aridification trend intensified after ca. 3800 C-14 yr B.P. and continued until the present. (C) 2002 University of Washington.
