Stable isotopes, Sr/Ca, and Mg/Ca in biogenic carbonates from Petaluma Marsh, northern California, USA

Stable isotope (O-18/O-16 and C-13/C-12) and minor-element compositions (Sr /Ca and Mg/Ca ratios) of ostracodes and gastropods separated from marsh sed iments from San Francisco Bay, Northern California, were used to reconstruc t paleoenvironmental changes in Petaluma Marsh over the past 700 yr. The va lue of delta(18)O in the marsh carbonates reflects changes in freshwater in flow, evaporation, and temperature. Mg/Ca and Sr/Ca in ostracode calcite re flect changes in both freshwater inflow and temperature, although primarily reflect temperature changes in the salinity range of about 10-35 parts per thousand. Ostracode delta(18)O values show a gradual increase by 5 parts p er thousand between 500 yr BP and the present, probably reflecting rising s ea level and increased evaporation in the marsh. Superimposed on this trend are higher frequency Mg/Ca and delta(18)O variations (3-4 parts per thousa nd), probably reflecting changes in freshwater inflow and evaporation. A pe riod of low Mg/Ca occurred between about 100-300 cal yr BP, suggesting wett er and cooler conditions during the Little Ice Age. Higher Mg/Ca ratios occ urred 600-700 cal yr BP, indicating drier and warmer conditions during the end of the Medieval Warm Period. Both ostracode and gastropod delta(13)C va lues decrease up-core, reflecting decomposition of marsh vegetation, which changes from C-4 (delta(13)C similar to -12 parts per thousand) to CAM (del ta(13)C = -26 parts per thousand)-type vegetation over time. Copyright (C) 1998 Elsevier Science Ltd.