EVIDENCE OF LATE HOLOCENE GLACIER ACTIVITY FROM PROGLACIAL SUNDAY LAKE, AHKLUN MOUNTAINS, SOUTHWEST ALASKA

Sediment cores recovered from Sunday Lake are used to reconstruct upvalley Holocene glacier activity. The valley is presently ice-free, however, two sharp crested Holocene moraines fronting the cirque headwall testify to late Holocene glacial activity. Several tephra units within the cores, originating from the Aleutian volcanic chain, provide important chronological control.

A 7 m core was recovered from Sunday Lake. The basal 292 cm (425-717 cm) of the core consists of well-layered, blue-gray mud exhibiting low organic matter content and elevated magnetic susceptibility values. This unit is interpreted as late-Wisconsinan deglacial sediment and is constrained by a radiocarbon date (peaty organic matter at 414 cm) of 9510 +/- 80 14C yr BP.

Organic-rich Holocene sediment overlies the "glacial" mud and contains several prominent stratigraphic units and tephra layers. The principle tephra in the core (225-159 cm, a likely candidate for the Aniakchak Tephra dated to ~3.5 14C kyr BP) is an important marker-bed for the late Holocene. This tephra immediately overlies two distinct "blue layers" (226-238 cm, and 245-251 cm) exhibiting low organic matter content and elevated magnetic susceptibility.

These units are similar to the glacial sediment in appearance, organic matter content and geochemical signature, and are interpreted as periods of renewed glacial growth. This suggests that Neoglaciation in the Ahklun Mountains commenced just prior to 3.5 kyr. Curiously, there is no prominent stratigraphy above the Aniakchak tephra, which can be attributed to glacier activity during the Little Ice Age. Either reworking of the Aniakchak Tephra has completely masked this sediment record, or, if the Little Ice Age did occur in the Ahklun Mountains,it was less significant than earlier Neoglacial advances.

Lichen (Rhizocarpon geographicum s.l.) diameters measured on the outer moraine ridge measured upto 64 mm suggest an age (using the growth curve for the Brooks Range) of 1.8 kyr B.P. - significantly younger than the core ages. This discrepancy has yet to be resolved!