Changing elevation, accretion, and tidal marsh plant assemblages in a South San Francisco Bay tidal marsh

Analyses of organic content, pollen, and the carbon-isotopic composition of a 3.5-m sediment core collected from a subsided tidal marsh located in South San Francisco Bay, California, have provided a 500-yr record of sediment accretion and vegetation change before, during, and after a rapid I m increase in sea level. Core chronology was established using C-14 dating of fossil plant material, the first appearance of pollen types produced by plants not native to California, and changes in lead concentrations coincident with anthropogenic contamination. Prior to the mid 19th century, rates of sediment accretion were between I and 4 nun yr(-1); sediment accretion accelerated to an average of 22 mm yr(-1) following the initiation of subsidence. Changes in tidal marsh vegetation also accompanied this depositional change. Vegetation shifted from a high to low marsh assemblage, as indicated by a larger percentage of grass pollen, rhizomes of Spartina foliosa, and a strong C-4 signal. Between 1980 and 200 1, Triangle Marsh again developed high marsh vegetation, as indicated by higher percentages of the Amaranthaceae pollen type, seed deposition, including Salicarnia spp., and more negative carbon isotopic ratios.