SEDIMENTARY EVIDENCE FOR THE LATE ONSET OF NEOGLACIATION IN NORTHWEST BRITISH COLUMBIA COAST MOUNTAINS

Sediment cores from White Pass on the British Columbia-Alaska border were used to reconstruct Holocene glacier cover and meltwater production. Two cores obtained from separate basins (fed by different glacial streams) in Summit Lake indicate a similar Holocene stratigraphy of basal diamicton overlain by gyttja that grades upwards into clay-rich sediment. The clay-rich sediment becomes consistently laminated near the White River tephra (1200 yr BP) and continues to the present. The absence of substantial clay in the lake prior to c. 1800 yr BP (estimated from sedimentation rates) suggests that glacial meltwater was absent from the watersheds during most of the Holocene. The cirque glaciers currently found in the basin represent the inland limit of ice cover in the Coast Mountains and are thus, likely to be highly sensitive to the changing frequency of Pacific weather systems. The evidence from Summit Lake suggests that glaciation of the border region began at c. 1800 yr BP and intensified after 1200 yr BP, when sedimentation rates increased sufficiently to generate clear varved sediments. Previously published pollen records from an adjacent lake show increased exotic western hemlock (Tsuga heterophylla) pollen at this time and other records in the region also suggest an increased influence by Pacific air masses in the late Holocene. Compared to other areas in northwestern North America, the onset of neoglaciation was comparatively late and driven by increased coastal weather systems and precipitation in this region.