Changes in Upwelling Along the Northern California Coast From C-14 Reservoir Ages

Coastal upwelling along the California coast occurs in response to alongshore winds driven by contrasts in heat capacity between land and sea. As a source of nutrients and cold water to the surface ocean, upwelling stimulates marine ecosystems and promotes formation of fog, affecting terrestrial ecosystems as well. Variability in coastal upwelling could thus provide information related to changes in marine productivity and the nature of near-shore terrestrial ecosystems, as well as insights into how these resources may have been used by early Californians. We evaluate changes in coastal upwelling over the last ~8500 years near Bodega Bay by determining the local offset from the global average radiocarbon reservoir age (ΔR) based on C-14 ages of contemporaneous Mytilus californianus shell and charcoal samples from Duncan's Landing, an archaeological site near Bodega Bay. Since ΔR is greater in deep ocean waters long isolated from the atmosphere, higher ΔR values are associated with upwelled waters. Our results indicate that the C-14 ΔR at 8540 YBP (mean calibrated age; ΔR = 425±165 years) was not significantly different from modern (267±19 years). Subsequently, ΔR varied from a maximum of 550±220 years at 4,940 YBP to 4±108 years at 2280 YBP. Between 2280 and 940 YBP, ΔR was significantly lower (20±22 years) than the average over the period from 4420 to 3130 years (305±66 years). By 550 YBP, ΔR had returned to modern values. These results suggest that from 4420 to 3130 years, upwelling rates were similar to modern, but were significantly reduced between 2280 to 940 YBP before returning to modern rates by 550 YBP.