PHYTOLITH AND CARBON-ISOTOPE EVIDENCE FOR LATE QUATERNARY VEGETATION AND CLIMATE-CHANGE IN THE SOUTHERN BLACK-HILLS, SOUTH-DAKOTA

Analyses of phytoliths and carbon isotopes document change in late Qua ternary grasslands in the Red Valley of the southern Black Hills. Late Pleistocene grassland composition was equivalent to the C-3 grass par klands of modern central Alberta. The rise of mixed grassland occurred rapidly between 11,000 and 9000 yr B.P. Early Holocene mixed grasslan ds included both short and tall C-4 grasses. A mid-Holocene erosional unconformity (ca. 8000 to 4500 yr B.P.) precludes phytolith or isotope analysis, but suggests lack of vegetation and landscape denudation ca used by a drier climatic. Basin-wide stability and soil development fo llowed the erosional episode (ca. 4500 to 3600 yr B.P.). Mesic-adapted C-4 panicoid grasses increased during this period of soil development . Low-magnitude fluctuation in the C-4-dominated mixed grassland occur red throughout the late Holocene (3600 yr B.P. to present). Rise in de lta(13)C values during the last 1000 yr without corresponding change i n phytoliths may indicate a decrease in woodlands caused by increased fire frequency. (C) 1997 University of Washington.