HR: 1340h
AN: PP43B-1251    [Abstracts]
TI: Mid-Holocene Climate Variability and Coastal Upwelling: Geochemical Evidence From Mytilus californianus
AU: * Dyck, K
EM: kdyck@ucsc.edu
AF: University of California, Santa Cruz, Earth and Planetary Sciences 1156 High St, Santa Cruz, CA 95064, United States
AU: Koch, P
EM: pkoch@es.ucsc.edu
AF: University of California, Santa Cruz, Earth and Planetary Sciences 1156 High St, Santa Cruz, CA 95064, United States
AU: Schellenberg, S
EM: schellenberg@geology.sdsu.edu
AF: San Diego State University, Geological Sciences 5500 Campanile Dr, San Diego, CA 92182, United States
AU: Ford, H
EM: hford@rohan.sdsu.edu
AF: San Diego State University, Geological Sciences 5500 Campanile Dr, San Diego, CA 92182, United States
AB: 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 proxy for coastal precipitation and runoff. We conducted geochemical studies (δ¹⁸O, Mg/Ca, and ¹⁴C) on shells of Mytilus californianus from archaeological middens near Pt. Año Nuevo, on the central California coast, to study shifts in upwelling and moisture regimes from ~1,000 to ~5,500 BP. The dated specimens were sectioned along the growth axis and the prismatic calcite layer sampled down the length of visible growth bands with samples split for Mg/Ca and stable isotope analysis. Seven shells have been analyzed for δ¹⁸O so 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 sampling approach. Under normal Walker circulation in central California, conditions are characterized by strong summer upwelling, relatively cool summer SSTs, 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 SSTs, 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 will calculate the δ¹⁸O value of coastal water as a proxy for precipitation and runoff.
DE: 1055 Organic and biogenic geochemistry
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 4870 Stable isotopes (0454, 1041)
DE: 4964 Upwelling (4279)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting