Transient impacts of Little Ice Age glacier expansion on sedimentation processes at glacier-dammed Iceberg Lake, southcentral Alaska

Abstract

Iceberg Lake, a glacier-dammed proglacial lake in southern Alaska, contains a 1,500+ year varve record complicated by a history of episodic lake-level changes associated with fluctuations in ice-dam thickness and position. To better understand the basinwide glaciolacustrine response to late Holocene climate variability, we collected five cores from two areas in the lake, including a previously unexamined deepwater area distal from inlet streams. Based on eight AMS 14C dates, and correlations among our cores and previously documented outcrops, we describe ~1,000 years of stratigraphy from each area. Deposition at both areas was dominated by fine-grained varves, but cores from the distal area uniquely contain coarser deposits, including rhythmites and graded sand beds, that we attribute to deposition of a subaqueous outwash fan-delta between ~1250 and 1650 AD. We attribute this event to thickening of the impounding glacier and consequent incursion of the glacier margin, and an associated lateral moraine, into the lake. This result suggests an early onset of the Little Ice Age (LIA) glacial advance in this region. Changes in basinwide circulation and sedimentation associated with this event probably caused minor thickening of varves used previously to reconstruct summer temperatures, reducing sensitivity of that record to early LIA cooling. The basinwide impact of this event illustrates the potentially significant spatial and temporal variability of lacustrine sedimentary processes in dynamic glacial landscapes.