Paleo-Environmental Change in the San Francisco Estuary Using Marsh Vegetation Records

Stable carbon isotopes, fossil pollen and stratigraphic analyses are used to reconstruct the paleo-environmental history of the San Francisco Bay Estuary. Research sites were selected along a salinity gradient spanning the northern reaches of the Estuary, from the west end of San Pablo Bay to the mouth of the Sacramento-San Joaquin Delta. We collected sediment cores from tidal marshes which contain the history of marsh development in the accumulated sediments. The results presented here demonstrate a strong and consistent correspondence between periods of increased salinity in the Estuary and changes in the local marsh vegetation. In particular, the vegetation has responded rapidly to the changing environmental conditions of the last century. These recent changes can be attributed to extreme El Nino events in the 1980s, and to human modifications of regional hydrology. Earlier periods of high salinity in the Estuary have been detected in the combined records from brackish marsh sites, occurring at ca. 875 cal. yr. B.P. and 1225 cal. yr. B.P. Conditions at our sites were considerably fresher preceding European contact approximately 150 years ago. Changes in the sediment cores from some sites also indicate possible tectonic activity occurring circa 1700 cal. yr. B.P. and 1300 cal. yr. B.P. Sedimentation rates from dated cores suggest sea level rose at an average rate of 1.5 mm/year, significantly less than predicted rates under global warming. Rising sea level poses a serious threat to local wetlands both through loss of habitat and high salinity. Land use and water use management around the San Francisco Bay must consider options for protecting local marshes and preserving species diversity. Research such as is presented here can help assess the sedimentation rates of local marshes as well as their relative robustness.