TESTING MODERN CONTROLS ON SEASONAL 14C VARIATIONS IN SEAWATER DIC AND MYTILUS CALIFORNIANUS SHELLS: THE POTENTIAL FOR A NEW UPWELLING PROXY

Marine radiocarbon (¹⁴C) is a widely used tracer of past ocean circulation, but very few high-resolution records have been obtained. Seasonally resolved ¹⁴C variations have been utilized to reconstruct ENSO related upwelling variability from tropical Pacific corals (Druffel et al., 2007), yet no similar records exist in higher latitudes. Seasonal records from the NE pacific could significantly improve our understanding of the complex interactions between the California Current strength, coastal upwelling, and coupled climate modes, such as ENSO, which is currently limited by the short duration of instrumental records and a lack of seasonal-resolution archives. Seasonal ¹⁴C variations in marine mollusk shells collected from Holocene shell middens in coastal North American sites hold great potential to fill this gap. We have conducted a detailed modern calibration study to investigate the link between upwelling intensity and the Δ¹⁴C of dissolved inorganic carbon in surface seawater, and how faithfully this seawater signal is recorded in the calcite growth increments of living California mussel (Mytilus californianus) shells. We present new data from surface seawater samples collected bi-weekly from Newport Beach Pier for ¹⁴C, salinity, ΣCO₂, and stable isotope (δ¹³C ,δ¹⁸O, and δD) analysis from March 2008 to March 2009. Comparison of this new data, and published data on samples collected since Nov. 2004 (Hinger et al., in press), reveals a link between upwelling intensity and the Δ¹⁴C of marine DIC. Living mussels were collected from Newport Beach, California. Sequential samples were micromilled from the outer calcite layer of shell cross-sections and analyzed for stable isotopes, trace elements and ¹⁴C. By comparing the geochemical profiles produced from the shells with instrumental records and the ocean geochemical measurements, it is possible to identify empirical relationships between shell geochemistry, upwelling and other oceanic variables. We also present results from Mytilus californianus shells collected from Mexico to Oregon following the strong El Niño event of 1997-1998. This event caused a collapse of upwelling and provides an ideal test of the fidelity of these empirical relationships along the west coast of North America at a time of dramatic change within the California Current system.