Doctoral Dissertation Research: Paleo-Environmental History of San Francisco Bay Estuary Using Stable Carbon Isotopes and Pollen

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Growing interest in understanding the processes of environmental change have highlighted the need for deeper understanding of complex interactions among different natural and human systems in critical regions and localities. This doctoral dissertation research project aims to produce a history of environmental change in the San Francisco Bay Estuary using stable carbon isotopes, fossil pollen, sediment analysis and carbon-14 dating spanning the last 2,000 to 3,000 years. This research has two distinct but interrelated primary objectives: (1) to elucidate the history of environmental change in the Bay Estuary; and (2) to test and elaborate models of vegetation responses to environmental change which are based on modern distributions. The spatial distribution of marsh plant species in the San Francisco Estuary is primarily determined by salinity, with other physical and biological factors of lesser importance. A series of sediment cores have been recovered from four marsh sites selected along a salinity gradient extending from the western edge of San Pablo Bay (highest salinity) to the eastern edge of Suisun Bay (near fresh water conditions). The carbon isotopic composition (13C/12C) of organic carbon in marsh sediments is a direct reflection of the source plants, with only slight changes over time due to decomposition. The two salt marsh grasses, Distichlis spicata and Spartina foliosa, operate with the C4-type photosynthetic pathway, with distinct carbon isotope signatures. The ability to distinguish these C4 grass species in the sediment record is an important complement to pollen analysis, as grass pollen is indistinguishable below the family level. Records produced in this research will be compared with other published records of paleosalinity in the estuary, which have revealed alternating periods of increased and decreased water salinity (relative to the present) during the late Holocene. This comparison will ascertain the degree to which temporal distributions of plant species at specific sites are analogous to modern spatial distributions. The record of vegetation change from marshes can then be used to expand the record of environmental change. Preliminary results from two marsh sites in this research indicate that the plant assemblages have responded consistently to past changes in estuarine salinity. An important part of this research will be to determine what natural process (including tectonic activity, sea level rise or climate variation) was dominant in causing changes detected in the sediment records. A strategy that establishes high-resolution chronologies from several sites will be used. The sites must be carefully selected relative to tectonic faults and compared not only for timing of vegetation changes, but also for sedimentation rates. Integral to this approach will be comprehensive dating of the sediment cores, employing both radiocarbon dating and lead-210 dating.This project will result in records that are important and relevant because present knowledge about rates of long-term fresh water inflow, salinity variability, and vegetation responses in the San Francisco Bay Estuary is limited. Earlier published studies of Estuarine paleosalinity are chronologically incomplete and do not include vegetation responses. Successful application of combining these methods in the San Francisco Estuary can be transferred to other estuarine settings. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.