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Late Pleistocene chronology and paleoclimate of Vancouver Island determined from cave deposits

1981; NRC Research Press; Volume: 18; Issue: 11 Linguagem: Inglês

10.1139/e81-152

1480-3313

M. Gascoyne, Derek Ford, Henry P. Schwarcz,

Groundwater and Isotope Geochemistry

Speleothems from four caves in south-central Vancouver Island have been analysed by the 230 Th/ 234 U method. Only speleothems from one cave, Cascade Cave, near Port Alberni, contained sufficient uranium for them to be dated. Twenty-seven ages determined for seven speleothems were found to cluster in two periods: 67–28 ka, corresponding to the Olympia interstadial, and <23–10 ka (or <15 ka if corrected for detrital thorium contamination), corresponding to the Fraser (late Wisconsin) deglaciation and Holocene periods. Two speleothems were found to be deposited in isotopic equilibrium with their seepage waters. Profiles of variations in δ 18 O of the calcite (δ 18 O c ) of each of the two deposits show a decrease of 1.3‰ over the growth period, 64–28 ka. At all times, δ 18 O c was less than δ 18 O of modern calcite in the cave. Using modern cave temperature and the variation of δ 18 O of seawater over the dated period, the profiles of δ 18 O c are interpreted in terms of a paleotemperature record for the Olympia interstadial in Vancouver Island. The results show a gradual cooling from 4 °C at 64 ka, to 0 °C between 35 and 28 ka. These results are consistent with conditions necessary for speleothem growth and with published work on surficial Wisconsin deposits in the area. No distinct, short-period warming or cooling events are seen in the record, probably due to thermal buffering by the adjacent ocean.