2002 ARCSS All-Hands Workshop

Pronounced Climatic and Ecological Changes in Alaska during past 2000 years

Feng Sheng Hu¹, Willy Tinner^2
1Departments of Plant Biology and Geology, University of Illinois, 505 South Goodwin Avenue, Urbana, IL, 61801, USA, Phone 217-244-2983, Fax 217-244-7246, fshu@life.uiuc.edu
²University of Illinois (current: U. Bern, Switzerland), USA

High-resolution geochemical, pollen, and charcoal analyses of lake-sediment cores from two Alaskan lakes provide new evidence for marked environmental variations during the past two millennia. Paired oxygen-isotopic analyses of abiotic carbonate and benthic-ostracode shells from the sediments of Farewell Lake (62° 33'N, 153° 38'W, 320 m a.s.l.) reveal three time intervals of comparable warmth: AD 0-300, 850-1200, and post-1800, the latter two of which correspond to the Medieval Climatic Anomaly and climatic amelioration following the end of the Little Ice Age (LIA). A marked climatic cooling occurred around AD 600, coinciding with extensive glacial advances in Alaska. Comparisons of this temperature record with ostracode trace-element ratios (Mg/Ca, Sr/Ca) suggest that colder periods were wetter and warmer periods wetter. Pollen data from this site, which is well below the altitudinal limits of any tree species in that region, do not show clear signals of vegetational change related to the LIA or the 20th-century warmth. In contrast, a high-resolution pollen record from Grizzly Lake (62° 43' N, 144° 12' W, 720 m a.s.l.), located near the altitudinal limits of Picea mariana (black spruce) and Betula papyrifera (paper birch), suggests abrupt vegetation shifts in the past 1000 years. At the onset of the LIA, these tree species declined markedly in favor of species characteristic of alpine tundra and disturbed sites. Vegetation recovered abruptly in response to climatic warming at the end of the LIA. Charcoal analysis of the same sediment core suggests that the LIA climatic cooling caused vegetation dieback, leading to increases in fuel availability and fire occurrence. Overall variations in this paleoecological record are similar to those in the average annual temperature of the Northern Hemisphere.

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