Holocene environmental history and evolution of a tidal salt marsh in San Francisco Bay, California

Analyses of three sediment cores collected from a tidal salt marsh located on the western edge of San Pablo Bay in the San Francisco Bay estuary have produced long-term records of late Holocene marsh development. The records from these cores include a suite of elements, organic carbon content, fossil seeds, pollen, and stable carbon isotopes. The stratigraphy indicates fresher water conditions than present between 3400 and 2000 cal YBP. A tidal marsh became established at China Camp after about 2000 cal YBP; between 2000 cal YBP and approximately 700 cal YBP, conditions in the estuary were apparently more saline and variable. This interval was terminated by at least three or possibly five flood events, as evidenced by coarse clastic materials most likely washed down from the surrounding uplands. These floods represent high rainfall events, possibly El Niño years, and occurred during the late Medieval Climatic Anomaly. Greater plant diversity and pollen from some species with low salt tolerance are found in a core collected near the upland edge of the marsh and date to about 200 years ago, suggesting fresher conditions than today. Over the past 50 years, the diversity of marsh vegetation has decreased, and salt tolerant plants (especially Salicornia virginica) have become the dominant species. These changes are likely a result of the impacts of water diversions and upstream dams in the San Francisco Bay watershed.