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

• Published in: Geochimica et cosmochimica acta Vol. 62; no. 19; pp. 3229 - 3237
• Main Authors: Ingram, B.L., De Deckker, P., Chivas, A.R., Conrad, M.E., Byrne, A.R.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-10-1998
• Subjects: Freshwater; Gastropoda; INE, USA, California, San Francisco Bay
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(98)00240-3

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.