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

Abstract

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 δ 18O 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‰. Ostracode δ 18O values show a gradual increase by 5‰ between 500 yr BP and the present, probably reflecting rising sea level and increased evaporation in the marsh. Superimposed on this trend are higher frequency Mg/Ca and δ 18O variations (3–4‰), probably reflecting changes in freshwater inflow and evaporation. A period of low Mg/Ca occurred between about 100–300 cal yr BP, suggesting wetter and cooler conditions during the Little Ice Age. Higher Mg/Ca ratios occurred 600–700 cal yr BP, indicating drier and warmer conditions during the end of the Medieval Warm Period. Both ostracode and gastropod δ 13C values decrease up-core, reflecting decomposition of marsh vegetation, which changes from C 4 (δ 13C ∼ −12‰) to CAM (δ 13C = −26‰)-type vegetation over time.