A POST-MAZAMA SEDIMENTATION RECORD FOR THE NORTH CASCADES FROM GLACIAL LYMAN LAKE, WASHINGTON, USA

Sediment cores from Lyman Lake (North Cascades, WA) preserve a detailed post-Mazama sedimentation record related to Lyman Glacier, a small (0.20 km²) alpine glacier. Outwash from the glacier drains through the forefield before dropping into the bedrock-dammed lake. The lake is bisected by a shallowly submerged late-glacial moraine that traps glacial bedload in the lake’s upper basin. The only other fluvial sediment source is a small stream that enters the lake at the submerged moraine. Thus, the lower basin of the lake is well situated for preserving the glacier's rock flour record.

Last summer, we collected three sediment core sequences (~4m deep) from the lake's lower basin. Analyses (visual stratigraphy, magnetic susceptibility - MS, loss on ignition - LOI) of the sediments record three episodes of increased clastic sedimentation in the lake since deposition of the Mazama ash (~7630 cal yr B.P.) The oldest interval involves multiple high-MS, low-organic beds of coarse tephra that span 60 - 70 cm of the core. The intermediate interval, located directly above the Mount St. Helens Py ash (~2600 cal yr B.P.), is ~35 cm thick and comprises 5-15 mm thick beds of high-MS clastic sand interspersed with lower-MS organic fine silts. The youngest interval spans the top 30 cm of the core sequence and is characterized by finely bedded, medium-MS, low-organic silts.

Whereas the oldest pulse of clastic sedimentation is related to the Dusty Creek eruptive interval (~5710-6100 cal yr B.P.) from nearby Glacier Peak, the younger two events must be linked to changes in basin sediment production or mobility. Preliminary ages indicate that the youngest clastic interval occurred during the Little Ice Age, when the Lyman Glacier was at or near its Holocene maximum. The origin of the intermediate clastic interval, however, remains unclear. The coarse grain size suggests a local hillslope (i.e., non-glacial) origin, possibly related to a change in climate, fire regime, or sediment storage. Ongoing analyses of particle size, ¹⁴C, and ²¹⁰Pb in the cores will improve our understanding of these events and their relationship to fluctuations of the Lyman Glacier and climatic events in the North Cascades.