STABILITY(?) OF THE MATANUSKA GLACIER OVER THE LAST 14.5 CAL KA AND YOUNGER DRYAS COOLING IN SOUTH-CENTRAL ALASKA

Multi-proxy data from two cores at Hundred Mile Lake (“HML”, 61.808°N, 147.842°W, elevation = 506.3 m) in the Matanuska Valley of south-central Alaska were used to investigate climate and vegetation change over the last 13,500 cal years (Walker et al., 2005, GSA-NE Abstract, vol. 35, no 1, p. 24.; Yu et al., in prep.). Here we discuss the implications of the HML cores with respect to the deglaciation history of the Matanuska Valley. HML is located in a series of well-developed, nested end moraines, 5 km from the terminus of the modern Matanuska Glacier. The timing of deglaciation, and the Younger Dryas (YD) cooling, is controlled by six AMS dates – one on organic-rich sediment, one on a terrestrial macrofossil and four on Pisidium mollusk shells. The 14C dates on shells were corrected for the “old carbon effect”, and all dates were calibrated using INTCAL 98 dataset. An age model was developed based on linear interpolation of five accepted dates. Extrapolation of the age-depth model, 0.5m, to the base of the lacustrine sediment provides a minimum deglaciation age of 14.5 ka (1 ka = 1000 cal yr BP). Both carbonate content and oxygen isotopes (on Pisidium shells) show clear excursions between 12.4 and 11.3 ka, probably representing YD cooling. Perhaps the most surprising and important conclusion to be derived from this study is that the terminus of Matanuska Glacier was located only 5 km down valley of its present position 14.5 ka. This would also require that the YD ice margin and end moraine, if there is one, is located between HML and the modern Matanuska Glacier margin, or up-glacier of the modern margin, and that the Matanuska did not advance far enough down valley during the YD to overrun the HML site. The “up-glacier” scenario is consistent with the emerging picture of early post-glacial warmth in high northern latitudes far from the melting ice sheets, including peak temperatures before YD in parts of Alaska (Kaufman et al., 2004, Quat. Sci. Rev.), in response to peak summer insolation near YD time. Any YD readvance of Matanuska Glacier may have been muted both because the YD signal was small so far from the north Atlantic center of action, and because YD cooling in widespread regions was concentrated in wintertime and glaciers respond primarily to summer temperatures.