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

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

Oct 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998gecoa..62.3229i&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 62, Issue 19-20, pp.3229-3237

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Stable isotope ( 18 O/ 16 O and 13 C/ 12 C) 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 18 O 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 18 O values show a gradual increase by 5 between 500 yr and the present, probably reflecting rising sea level and increased evaporation in the marsh. Superimposed on this trend are higher frequency Mg/Ca and 18 O variations (3-4 ), probably reflecting changes in freshwater inflow and evaporation. A period of low Mg/Ca occurred between about 100-300 cal yr , suggesting wetter and cooler conditions during the Little Ice Age. Higher Mg/Ca ratios occurred 600-700 cal yr , indicating drier and warmer conditions during the end of the Medieval Warm Period. Both ostracode and gastropod 13 C values decrease up-core, reflecting decomposition of marsh vegetation, which changes from C 4 ( 13 C ~ -12 ) to CAM ( 13 C = -26 )-type vegetation over time.

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