Ostracode Geochemical Record of Holocene Climatic Change and Implications for Vegetational Response in the Northwestern Alaska Range

INTRODUCTION Trace-element analysis of the calcareous shells of ostracodes The effects of climatic change on the boreal forest have in a sediment core from Farewell Lake provides the first limnobeen the subject of many recent investigations because of geochemical record for climatic reconstructions in Alaska. When the concern about the sensitivity of the northern high-latitude compared with pollen data from the same site, this record offers new insights into climatic controls over vegetation dynamics durclimate to increased atmospheric CO2 and the potential coming the Holocene. The low Mg/Ca ratios and high Sr/Ca ratios plex feedbacks of ecosystem changes in these regions to the suggest that a relatively cold dry climate prevailed in this region global climate (e.g., Bonan et al., 1990, 1992; Foley et al., between 11,000 and 9000 yr B.P. (uncalibrated C ages are used 1994). One important means for improving our understandthroughout the paper). This result contrasts with previous intering of the possible responses of boreal forests to climatic pretations of a thermal maximum at this time, corresponding to forcing is to decipher climatic and ecological records arthe widespread establishment of Populus woodland/forest. The chived in lake sediments (e.g., MacDonald et al., 1993). In trace-element record suggests, instead, that the warmest period of Alaska, pollen data from lake sediments have greatly enthe early Holocene at Farewell Lake was between 8500 and 8000 hanced our knowledge of the regional vegetation history yr B.P. during the decline of Populus. Marked decreases in Sr/Ca during the late Quaternary (Anderson and Brubaker, 1994; and Mg/Ca suggest a major increase in effective moisture around Edwards and Barker, 1994). Comparisons of these pollen 6500 yr B.P., which coincided with the establishment of Picea boreal forests in the Farewell Lake region. This climatic change data and AGCM (atmospheric general circulation model) was probably widespread throughout much of Alaska and adjacent simulations have served as a major approach to interpreting Canada and might have induced the rapid spread of Alnus and the climatic cause of these vegetation changes (e.g., Barthe shift from Picea glauca to P. mariana dominance across that nosky et al., 1987). Although these comparisons have subregion. Our geochemical record also suggests that the late-Holostantially improved our understanding of the complex relacene climate history was more complex than previously thought tionships between climate and ecosystems (COHMAP, on the basis of palynological studies. According to this record, 1988; Anderson and Brubaker, 1994), this approach suffers growing-season temperatures increased 6000–4500 yr B.P., defrom major differences in the spatial and temporal scales of creased 4500–1500 yr B.P., and increased with fluctuations afterexisting simulations and pollen data (Anderson and Bruward. After 6000 yr B.P. stratigraphic changes in pollen percentbaker, 1994). ages of Picea appear to be positively related with those of Mg/ In order to assess the effects of climatic forcing on past Ca. This relationship implies that once the threshold of effective moisture was crossed for the establishment of Picea forests tempervegetation changes, it is necessary to acquire proxy climatic ature was the primary control of Picea population density. q 1998 records independent of pollen data but with comparable spa-