GLACIAL STRATIGRAPHY AND LITTLE ICE AGE HISTORIES OF SHERIDAN AND SHERMAN GLACIERS, SOUTHERN ALASKA

Glacial stratigraphy and geomorphology constrained by tree-ring dates shows three distinct expansions of maritime glaciers in southern Alaska during the past 1000 years. These three Little Ice Age (LIA) advances spanned, respectively, from ~AD 1190-1300, 1610-1715, and 1820-1900. New data from Sheridan and Sherman glaciers show how adjacent glaciers have differed in their specific response to this climatic forcing.

Sheridan Glacier is a 24 km-long valley glacier that flows southwestwards to end as a 6 km-wide terminal lobe. Tree stumps in the western forefield record outwash aggradation from 1180s to 1250s, and logs crushed into a bedrock outcrop in 1284 indicate that ice reached an early LIA maximum at least 1.3 km beyond the present terminus position. Ice retreated ~1 km before readvancing to a middle LIA maximum 1.8 km beyond the present terminus. This middle LIA stand was the largest expansion of Sheridan Glacier during the late Holocene. Retreat preceded formation of a large recessional moraine during the late LIA. Twentieth century retreat has been accelerated by formation of large proglacial lakes that have allowed rapid disintegration of areas of the terminal lobe.

Sherman Glacier is a 13 km-long valley glacier that flows west-southwest towards the eastern edge of the Sheridan terminal lobe. In contrast to Sheridan, tree-ring dates show evidence for only a late LIA advance of Sherman Glacier. Tree stumps 0.2 km from the present terminus were alive from 1512 to 1820, and were killed by the ice margin expanding farther in the 19th century than during the middle LIA. The different LIA history of Sherman Glacier may be explained in part by the absence of proglacial lakes that would allow rapid terminus retreat following maxima.