Lawrence Berkeley National Laboratory

Stable isotope (18O/16O and 13C/12C) and minor-element compositions (Sr/Ca and Mg/Ca ratios) of ostracodes and gastropods separated from marsh sediments from San Francisco Bay, Northern California, were used to reconstruct paleoenvironmental changes in Petaluma Marsh over the past 700 yr. The value of d18O in the marsh carbonates reflects changes in freshwater inflow, evaporation, and temperature. Mg/Ca and Sr/Ca in ostracode calcite reflect changes in both freshwater inflow and temperature, although primarily reflect temperature changes in the salinity range of about 10-35 o/oo. Ostracode d18O values show a gradual increase by 5 o/oo between 500 yr BR and the present, probably reflecting rising sea level and increased evaporation in the marsh. Superimposed on this trend are higher frequency Mg/Ca and d18O variations (3-4 o/oo), probably reflecting changes in freshwater inflow and evaporation. A period of low Mg/Ca occurred between about 100-300 cal yr BR, suggesting wetter and cooler conditions during the Little Ice Age. Higher Mg/Ca ratios occurred 600-700 cal yr BR, indicating drier and warmer conditions during the end of the Medieval Warm Period. Both ostracode and gastropod d13C values decrease up-core, reflecting decomposition of marsh vegetation, which changes from C4 (d13C ~; -12 o/oo) to CAM (d13C = -26 o/oo)-type vegetation over time.