A Late Holocene History of Vegetation Change in San Francisco Estuary Marshes Using Stable Carbon Isotopes and Pollen Analysis
Stable carbon isotopes and pollen analysis provide a multiproxy record of vegetation change in four tidal marshes in Northern San Francisco Bay Estuary, including two island marshes in Suisun Bay, a pocket marsh at Benicia State Park in the Carquinez Strait and a salt marsh at China Camp State Park on the western edge of San Pablo bay. Marsh vegetation responds to changes in physical conditions in the Estuary, particularly salinity. The 013C value of plants occupying the marsh is preserved in the organic sediments, providing a permanent record of vegetation change, with some residual differences which are discussed. Marsh pollen types are easily identified in the sediments and provide another, complementary, record of vegetation change in the marshes around the Estuary. Combining these two methodologies, a record of late Holocene vegetation change can be reconstructed, and interpreted in terms of changes in the salinity of the San Francisco Estuary.
Near surface sediments at all sites show consistent evidence of human modifications of the Estuary. Increases in the isotopic compositions at each site and increases in pollen from salt-tolerant plant taxa suggest that dam construction along the major tributaries to the Sacramento and San Joaquin rivers, the major source of fresh-water flow through the Estuary, have had significant impacts on the wetland ecosystems around the Estuary. Earlier periods of relatively high salinity have also been detected at each site, though with some differences in the timing of events. The first of these earlier periods was experienced ca. 465 cal yr B.P. to 340 cal yr B.P. at China Camp; ca. 725 cal yr B.P. to 120 cal yr B.P. at Benicia State Park; ca. 625 cal yr B.P. to 480 cal yr B.P. and ca. 350 cal yr B.P. to 200 cal yr B.P. at Roe Island, and ca. 870 cal yr B.P. to 565 cal yr B.P. at Browns Island. The record at Benicia State Park detected an earlier high salinity event from ca. 1350 cal yr B.P. to 1140 cal yr B.P. and the record at Roe Island detected a similar event ca. 1650 cal yr B.P. to 1300. This earlier event was not detected at Browns Island, and the China Camp site was subtidal during this period. The timing of these periods of increased estuarine salinity is in agreement with similar periods detected in other estuary marsh sites.
Rates of venical accumulation at the two sites, when compared to rates at other estuary sites, reveal intriguing estuary-wide trends, possibly related to changes in rates of relative sea level rise during the late Holocene. In panicular, estuary-wide rates of sea level rise were depressed to less than I mrnlyear for a prolonged period centered around 1100 cal. yr. B.P. to 750 cal. yr. B.P. Local tectonic events may also be reflected in abrupt changes in sedimentation rates detected at 4 marsh sites in the northern reaches of the San Francisco Bay Estuary.