Mid-Holocene Climate Variability and Coastal Upwelling: Geochemical Evidence from Mytilus Californianus

Oxygen isotope values in carbonate shells from the coastal zone are influenced by temperature and by the δ¹⁸O value of water, which could be ¹⁸O-depleted relative to average ocean water due to input of freshwater runoff. If Mg/Ca ratios are a reliable independent proxy for past sea-surface temperature (SST), we can reconstruct the δ¹⁸O value of coastal waters from the δ¹⁸O value of biogenic calcite as a signal of coastal precipitation and runoff. We conducted geochemical studies (δ¹⁸O, δ¹³C, Mg/Ca, and ¹⁴C) on shells of Mytilus californianus from archaeological middens on the central California coast to study shifts in upwelling and moisture regimes from ~1000 to ~5,500 BP. The dated specimens were sectioned along the growth axis and the prismatic calcite layer sampled along visible growth bands with samples split for Mg/Ca and stable isotope analysis. Twelve shells have been analyzed for δ¹⁸O thus far and six of these have also been analyzed for Mg/Ca ratios. Shells typically contain 3-7 years of accretionary growth that can be resolved at the monthly scale with our approach.

Under normal Walker circulation in central California, conditions are characterized by strong summer upwelling, relatively cool summer SST's, and rainy winters. Interruption of normal Walker circulation, due to a weak North Pacific high pressure system and therefore reduced offshore Ekman transport, is reflected by warmer summer and winter SST's, suppressed summer upwelling, and even higher winter precipitation. Overall biogenic calcite Mg/Ca ratios show a negative seasonal covariation with δ¹⁸O values, as expected due to shifts in ocean temperature. Several specimens show geochemical values consistent with a breakdown in Walker circulation during several winters. With additional coupled Mg/Ca and δ¹⁸O analyses we will calculate the frequency of these events through time and estimate the δ¹⁸O value of coastal water as a proxy for precipitation and runoff.