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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. INSTAAR, University of Colorado andThe Regents of the University of Colorado, a body corporate, contracting on behalf of the University of Colorado at Boulder for the benefit of INSTAAR are collaborating with JSTOR to digitize, preserve and extend access to Arctic and Alpine Research.ABSTRACT A comparison of modern climates at late Pleistocene glacier equilibrium lines in the Colorado Rocky Mountains with the range of climates which occur at the equilibrium lines of modern glaciers worldwide allows an evaluation of the combinations of temperature and precipitation change which would have been necessary to sustain the late Pleistocene glaciers. Modern climatic conditions at late Pleistocene equilibrium lines at 12 sites in 7 ranges are approximated using instrumental and snow survey data.The comparison provides paired values of temperature and precipitation changes which would maintain the glaciers at their late Pleistocene maximum positions. If no change occurred in total precipitation or in seasonal distribution of precipitation between the late Pleistocene and the present, an approximately 8.5?C summer temperature depression would have been necessary to sustain the late Pleistocene glaciers. A 10 to 13?C late Pleistocene temperature depression, which has been suggested on the basis of other lines of evidence, would have been accompanied by a reduction of at least 44%o in fall-through-spring precipitation compared to present-day values.

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Cosmogenic 10 Be surface-exposure dating and numerical glacier modeling are used to reconstruct glacial chronology and climate in the Colorado Sangre de Cristo Mountains during the local last glacial maximum (LLGM) and the subsequent deglaciation. Twenty-two surface-exposure ages on moraine boulders and polishedbedrock outcrops in the Willow Creek valley and ten in two adjacent valleys indicate that glaciers were at or near their maxima from ~21ka until 17-16 ka, and then retreated rapidly, nearly deglaciating the Willow Creek valley entirely by ~14 ka. Coupled energy/mass-balance and flow modeling of two of the glaciers indicates that, if changing ice extent was driven only by temperature and insolation changes, temperature depressions of 5.0 and 5.1°C from modern conditions, with an uncertainty ofapproximately +1.5 −1.0°C , would have sustained the glaciers in massbalance equilibrium at their LLGM extents. Doubling or halving of modern precipitation during the LLGM would have been associated with 2.7-3.0°C and 6.9-7.0°C temperature depression respectively. Approximately half of the subsequent LLGM-to-modern climate change was accomplished by ~14 ka. If the rapid main phase of deglaciation betweenabout 16 kaand 14 ka was driven solely by temperature and insolation changes, it would have been associated with a temperature rise of about 2.5°C, at a mean rate of approximately 1.1°C/ky. This new chronology of the last glaciation is generally consistent with others developed recently in the Colorado Rocky Mountains. The numerical modeling, 2 however,suggests a lesserLLGM temperature depression from modern conditions than have most previous studies in Colorado.

Bottom sediments from three lakes in the Canadian Rocky Mountains were examined with the aim of evaluating the usefulness of downvalley sediment studies in reconstruction of Holocene glacial histories. Analyses of organic carbon and carbonate contents of core sediments provide information on changing sedimentation rate and changing relative importance of glacial and non-glacial sediment sources. Sedimentary histories of the three lakes are similar, suggesting that they record regional glacial/climatic forcing, rather than localized events, and thus that they may be useful in reconstructing Holocene glacial history. Lacustrine sediments indicate a period of high sedimentation rates and relatively large glacial sediment contribution prior to 7500-7000 yr B.P., with much reduced rates and decreased glacial sediment contribution between about 6000 and 4000 yr B.P., possibly interrupted by a brief period of increased glacial sediment output shortly after 5000 yr B.P. Sometime after 4000 yr B.P., sedimentation rates and glacial sediment output began to rise again, reaching approximately present levels by 2750-2650 yr B.P., and have not since returned to low mid-Holocene levels. In detail over the last 3000 yr there is some indication of a slight decrease in sedimentation rate for more than 1000 yr after about 2200 yr B.P. Sedimentation rates and glacial sediment input into all three lakes rose between about 900 and 750 yr B.P. and have remained very high since. If the lake sediments are interpreted as a proxy record of upvalley glacial activity, they allow the development of a glacial chronology which is at once generally consistent with, and more complete and easily datable than, the surficial glacial record.