Differences in the radiocarbon ages of coeval marine shells and wood from emergent glaciomarine sediments in the Fraser Lowland of southwestern British Columbia and northwest Washington indicate that a total marine radiocarbon reservoir value of ∼−1.1 k.y. is applicable in the region. This is 40% greater (300 yr) than the −800 yr value currently used to correct the ages of both modern and late Pleistocene marine shells from this area. The glaciomarine sediments were deposited rapidly between ca. 12.5 and 11.5 14C k.y. B.P. Paleodeviations of the marine reservoir value may illustrate that rapid environmental changes occurred during the last glacial to interglacial transition. These variations are thought to have been caused by changes in both deep and surface ocean circulation and by variations in the production of atmospheric radiocarbon (Δ14C). These findings may indicate a rapid reordering of oceanic-atmospheric circulation in the region.