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Forty-eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern Vancouver Island, British Columbia. Sea level fell rapidly from its high stand of about +75 m elevation just before 14 000 cal BP (12 000 radiocarbon yrs BP) to below the present shoreline by 13 200 cal BP (11 400 radiocarbon years BP). The sea fell below its present level 1000 years later in the central Strait of Georgia and 2000 years later in the northern Strait of Georgia, reflecting regional differences in ice sheet retreat and downwasting. Direct observations only constrain the low stand to be below −11 m and above −40 m. Analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the Cascadia subduction zone, however, places the low stand at −30 ± 5 m at about 11 200 cal BP (9800 BP). The inferred low stand for southern Vancouver Island, when compared to the sea-level curve previously derived for the central Strait of Georgia to the northwest, generates differential isostatic depression that is consistent with the expected crustal response between the two regions. Morphologic and sub-bottom features previously interpreted to indicate a low stand of −50 to −65 m are re-evaluated and found to be consistent with a low stand of −30 ± 5 m. Submarine banks in eastern Juan de Fuca Strait were emergent at the time of the low stand, but marine passages persisted between southern Vancouver Island and the mainland. The crustal uplift presently occurring in response to the Late Pleistocene collapse of the southwestern sector of the Cordilleran Ice Sheet amounts to about 0.1 mm/yr. The small glacial isostatic adjustment rate is a consequence of low-viscosity mantle in this tectonically active region.

The Strait of Georgia is situated between Vancouver Island and the western Canadian mainland. It was deglaciated quickly, around 12,500 14C yr BP, by in situ downwasting. The subsequent marine invasion has left glacimarine sediments with foraminifers, diatoms and dinocysts characterizing successive paleoenvironments recognizable basinwide. The first paleoenvironment represents near-ice conditions under the influence of the turbid meltwater plume marked by low algal productivity and few foraminifers, mostly Elphidium excavatum. Just off the plume, the second environment shows a peak of Nonionellina labradorica possibly resulting from an observed sudden increase in diatom numbers, mostly ice-dwelling forms implying a long annual sea-ice cover. Further away, Cassidulina reniforme increases, N. labradorica decreases while other calcareous foraminifer species colonize the area; there are fewer ice diatoms, more marine planktonic diatoms and dinocysts become more abundant. Finally, after 12,000 14C yr BP, calcareous foraminifers are gradually replaced because of dissolution of calcium carbonate by a low-diversity arenaceous assemblage. At the same time, abundant planktonic diatoms and dinocysts indicate warmer surface temperature. The succession of fossils and environments compares well with modern, horizontal, off-glacier successions observed in fjords of the European Arctic.