The development of a sediment-based methodology for the identification of high resolution Late Holocene environmental change in a barrier estuarine system : Pescardero Marsh, San Mateo County, California

Pescadero Marsh is a back-barrier wetland environment located on the central coast of California. In an attempt to reconstruct sub-annual trends in back-barrier environmental change over the late Holocene, driven by the presence and integrity of a coastal barrier system, carefully targeted sections of the sediment record from this site were analysed at high resolution (2mm) for their grain size and geochemical characteristics. Where possible, diatom analysis supplemented these data. The sediment record, and the methodology employed, proved to be sufficiently sensitive for this purpose by distinguishing between a series of barrier estuarine and lagoonal sedimentary environments. Grain size data were the primary basis for environmental interpretation. At this resolution, the grain size distribution curves of individual samples revealed sensitive environmental information regarding deposition through different transport processes, i.e. saltation vs suspension loads. Distinctions between samples were driven by the interaction of the lagoonal pool, tidal ingress, river flow and the integrity of the barrier system. Geochemical data indicated sediment provenance, distinguishing between samples primarily sourced by marine and terrestrial waters, and grain size related controls on many geochemical characteristics. Diatom assemblages reflected high energy inputs from the ocean and the feeding creeks whilst recording limited change in the salinity of the back-barrier environment. A confident chronology was constructed for Euro-American era sediments, with a sedimentation rate of 144.2 days per 2mm sample established above a 1963 'weapons testing' peak in 137Cs. A similar rate was demonstrated to continue to at least 1900 through the presence of geochemical markers of anthropogenic activity (pollution, catchment destabilisation and development of infrastructure). This represented a marked increase above the prehistoric rate (~0.8mm per year). During the late-Holocene period of relative sea-level stability, the environmental trends established are characteristic of a morphologically resilient coastal system. A barrier estuarine environment persisted, despite changing boundary conditions and a perturbation of very high magnitude/Iow frequency, and was observed to shift though a series of 'static equilibrium' operational states. From wavelet analysis, the ENSO phenomenon emerged as the likely principal driver of variability in the most recent of these static equilibra. The research presented greatly improves understanding of the sites prehistoric functioning and suggests significant anthropogenic impacts on the system during the Euro-American era. Such information can inform management of this and other analogous sites.EThOS - Electronic Theses Online ServiceGBUnited Kingdo