OSTRACODE GEOCHEMICAL RECORD OF HOLOCENE CLIMATIC-CHANGE AND IMPLICATIONS FOR VEGETATIONAL RESPONSE IN THE NORTHWESTERN ALASKA RANGE

Trace-element analysis of the calcareous shells of ostracodes in a sed iment core from Farewell Lake provides the first limnogeochemical reco rd for climatic reconstructions in Alaska. When compared with pollen d ata from the same site, this record offers new insights into climatic controls over vegetation dynamics during the Holocene. The low Mg/Ca r atios and high Sr/Ca ratios suggest that a relatively cold dry climate prevailed in this region between 11,000 and 9000 pr B.P. (uncalibrate d C-14 ages are used throughout the paper). This result contrasts with previous interpretations of a thermal maximum at this time, correspon ding to the widespread establishment of Populus woodland/forest. The t race-element record suggests, instead, that the warmest period of the early Holocene at Farewell Lake was between 8500 and 8000 yr B.P. duri ng the decline of Populus. Marked decreases in Sr/Ca and Mg/Ca suggest a major increase in effective moisture around 6500 yr B.P., which coi ncided with the establishment of Picea boreal forests in the Farewell Lake region, This climatic change was probably widespread throughout m uch of Alaska and adjacent Canada and might have induced the rapid spr ead of Alnus and the shift from Picea glauca to F. mariana dominance a cross that region. Our geochemical record also suggests that the late- Holocene climate history was more complex than previously thought on t he basis of palynological studies, According to this record, growing-s eason temperatures increased 6000-4500 pr B.P., decreased 4500-1500 yr B.P., and increased with fluctuations afterward. After 6000 pr B.P. s tratigraphic changes in pollen percentages of Picea appear to be posit ively related with those of Mg/Ca. This relationship implies that once the threshold of effective moisture was crossed for the establishment of Picea forests temperature was the primary control of Picea populat ion density. (C) 1998 University of Washington.