| Summary: | The purpose of the dataset is to report the results from NSF grant support 1503846 (https://www.nsf.gov/awardsearch/showAward?AWD_ID=1503846&HistoricalAwards=false). Before our study, temperature reconstructions for the region north of the Brooks Range in Alaska were based on pollen transfer functions (which could be complicated by non-analog taxa), which suggest a warmer-than-present Last Glacial Maximum (LGM, 26,500 to 19,000 years ago). Global climate models differ greatly in their simulations of this region, with some suggesting a warmer and others a colder climate. Robust temperature reconstructions spanning the LGM to present are therefore needed to test the outputs from climate models and to understand regional sensitivity to climate forcing. Associated with the ambiguous temperature history is uncertainty in the regional fire history, as revealed by the unexpected discovery of three major Alaskan tundra fires in last 150 years. These fires contradict the conventional notion that tundra ecosystems rarely, if ever, burn and stimulate a key question: What are the relationships between climate change, fire, and vegetation since the LGM? This project intends develop careful reconstructions of temperatures in the region using records obtained from lake cores. The resulting data will then be compared with a variety of climate model outputs. The data sets are measurements of sediment cores from four lakes, Toolik Lake, Lake 5, Upper Capsule, and Fog 2 on the North Slope of Alaska. These include age models based on radiocarbon, Pb-210, Cs-137, sediment property scans (magnetic susceptibility, color spectra, XRF measurements of elemental abundances), organic geochemical proxies (alkenone concentrations and paleoclimate proxy measurements, branched tetraether lipids and index calculations, polycyclic aromatic hydrocarbons, hydrogen and carbon isotopic values of plant leaf waxes and short chain fatty acids, sterol concentrations), diatom assemblage counts, charcoal counts. Among these data sets, Lake E5 records spans the past 30,000 years, whereas other lake records cover only Holocene. Major findings include a general rising spring temperature, but declining summer temperature during the Holocene. LGM summer temperature is only moderately colder (4 to 5 degrees centigrade) than the early Holocene - the amplitude is much smaller than other Arctic regions such as Siberia and Greenland. This observation can be probably explained by influence of Laurentide Ice Sheet (LIS) on the atmospheric circulation. Both charcoal and PAHs from Lake E5 indicate extensive fires during the glacial period, contradicting with prevalent ecological modeling results suggesting more fires in warmer conditions (e.g., warmer conditions as a result of anthropogenic global warming) due to increased lightning ignition. Along with the increased coprostanol concentrations (human fecal sterols) in the glacial sediments, it is possible the pervasive fires during the last glacial maximum may originate from human ignitions. The conclusion would support the "Standstill Hypothesis", indicating "the First Americans" once inhabited behind the LIS in the unglaciated landscape of the northern Alaska. However, it is also possible the glacial fires resulted from dry conditions (as indicated by hydrogen isotopic values of long and short chain fatty acids), as well as the productive marmmoth steppe vegetation around the last glacial maximum.
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