Coastal marsh sediments from Bodega Harbor: Historical environmental changes at the tidal land

Coastal marsh sediments provide an important archive of environmental changes at the environment where they deposited and in this case, the studied area is the tidal land in Bodega Harbor. Over the last century, humans have significantly altered the coastal environment near Bodega Bay, California, through alteration of natural hydrologic patterns (dredging and filling), sediment sources (stabilization of sand dunes), and the dominant ecosystem (land use and management). Previous investigations of recent coastal marsh sediments (\u3c 50 years) suggest that physical barriers, such as roads, which limit the connection between Bodega Bay and the marshes, alters biogeochemical cycling (including carbon storage) in the coastal environment. The present study extends the record of changes in biogeochemical cycling in the coastal marshes back more than 100 years (approximately 90 cm) through the use of grain size analysis, C and N isotopes, and age dating. Sediments were analyzed for grain size distribution, the amount of carbon and nitrogen, and the stable isotopes of carbon and nitrogen in 1 cm intervals throughout the core. In addition, a subset of eight samples was analyzed for sediment age using a combination of Pb-210 and Cs-137 techniques. Sediments from the top third and bottom third down the core have a lower percentage of finer grained sediment (\u3c2%). In addition, these sediments also contain lower levels of total organic carbon and nitrogen, lower C:N ratios, as well as lighter carbon and nitrogen isotopic signatures. The sediments likely correspond to a pre-1900 depositional environment based on Pb-210 dates, when development induced human activities in the region was increasing. The observed results also suggest that there are several significant transitions occurring at the depth of 18-19cm, 41-42cm, 47-48cm and 71-72cm. These results suggest a stronger influence of the marine environment during that time and the significant transitions are associated with massive human activities such as dredging, filling and stabilization of sand dunes historically. The significant shift in sediment properties suggests a less stable environment with greater communication between the terrestrial and marine environments. Results from this investigation suggest that the Bodega Bay coastal marshes are continually evolving in response to environmental changes, and insights from this research will lead to greater understanding of the impacts of increasing population, construction, and changing vegetation and hydrology on the coastal environment