Late Quaternary environmental change in the Lake O'Hara region, Yoho National Park, British Columbia

Abstract

Palaeoenvironmental reconstructions for the upper Cataract Brook Valley are derived from radiocarbon and tephra dated cores recovered from subalpine Lake O'Hara and alpine Opabin Lake located near the Continental Divide in the central Canadian Rocky Mountains. Late Quaternary vegetational and climatic changes are based on pollen and macrofossil data and are supplemented by preliminary diatom data. The postglacial colonizing vegetation prior to ca. 10,100 yr B.P. at Lake O'Hara and before ca. 8530 yr B.P. at Opabin Lake was a shrub herb community dominated by Artemisia, Gramineae and Alnus. Pioneering forests at both sites were composed of Pinus cf. albicaulis/flexilis and Abies with lesser abundances of Picea and Pinus cf. contorta. Timberline remained above the elevation of Opabin Lake, at least 90 m above modern timberline elevation, during the period ca. 8500 yr B.P. to ca. 3000 yr B.P. in response to warmer climatic conditions associated with restricted glacial activity. Forest compositions resembling the modern subalpine Picea-Abies forest had developed by the end of this period. The period ca. 3000 yr B.P. to present was marked by deteriorating climatic conditions associated with renewed glacial activity in the Opabin Cirque and declining timberlines to below the elevation of Opabin Lake. Glacigenic sediments in the Opabin record from the post-Bridge River (ca. 2350 yr B.P.) interval are associated with reduced diatom production and a change in dominant diatom taxa to those characteristic of turbid water conditions and unstable catchment areas.