Reconstruction of Holocene Climate Change Using Multiproxy Analysis of Sediments from Pyramid Lake, British Columbia, Canada

Abstract

Sediment cores from Pyramid Lake, an alpine tarn in the Cassiar Mountains of northwestern British Columbia, were investigated for changes in pollen, plant macrofossils, charcoal, and clastic sediment, which are used to infer changes in climate throughout the Holocene. Radiometric dating has yielded a chronology of high-magnitude rainstorm events and timberline migration for the Pyramid Lake basin since deglaciation at about 10600 B.P. Fifteen distinct minerogenic layers represent material delivered to the lake by runoff events. The frequency of minerogenic layer deposition, and by analogy of storms, has changed throughout the Holocene. Four large-magnitude rainstorm events occurred between 4400 and 5100 B.P. During this period white spruce (Picea cf. glauca) was likely present near the lake, although a closed forest stand did not develop around the lake at any point during the Holocene. The macrofossil record indicates that subalpine fir (Abies lasiocarpa) has been present, likely as krummholz, above the elevation of the lake since at least 9400 B.P. Pollen of western hemlock (Tsuga heterophylla) is represented from ca. 1500 B.P. to the present and may be a consequence of changes in regional air-mass circulation patterns.