HR: 1340h
AN: PP33B-0952    [Abstracts]
TI: Diatom, Total Organic Carbon, And Sediment Geochemistry Record From Southampton Bay, A Brackish Tidal Marsh In Northern San Francisco Bay, California
AU: * Starratt, S W
EM: sstarrat@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Sediments from a 3.8-m-long core collected from the marsh in Southampton Bay in northern San Francisco Bay, California, record both locally and regionally controlled variations in the flow of the Sacramento-San Joaquin river system through the Carquinez Strait. Because this river system drains more than 40 percent of California, variation in its discharge is an indication of changes in the magnitude and timing of precipitation in the region. Sedimentation rates at the site range from a high of 2.5 mm/yr from about 3,800 to 3,300 cal yr B.P. to about 0.8 mm/yr from about 1,800 cal yr B.P. to the present. Relatively high levels (20-30 percent) of total organic carbon (TOC) indicate that the primary interval of marsh accretion was between 3,300 and 300 cal yr B.P. A rapid decrease in TOC about 300 years ago suggests a decrease in marsh accretion, non-accretion, or erosion. Prior to the establishment of the marsh, between 3,800 and 3,300 cal yr B.P., and after about 300 cal yr B.P., overall accretion rates were lower, TOC values fell below 10 percent, and freshwater diatoms comprised less than 20 percent of the assemblage. These lower accretion rates occurred during periods of increased river flow which resulted in seasonal erosion and/or limited deposition of sediment. During the interval of marsh accretion, the abundance of freshwater diatoms parallels the TOC data, with freshwater taxa accounting for between 60 and 70 percent of the assemblage. The diatom and TOC data were supplemented with analysis of the silt and clay fraction using inductively coupled plasma-mass spectrometry and cold-vapor atomic absorption spectrometry. During the interval of marsh accretion high TOC, Na, Ca, and Mg values are higher, whereas during the intervals of lower TOC, the amount of Al, K, Fe, and Ti are higher. It is suggested that marsh accretion results in a greater contribution of sediments from local sources ({\it i.e.} Great Valley sequence and mafic units). During periods of lower rates of marsh accretion a greater part of the sediment budget is derived from weathered granitic sources in the Sacramento and San Joaquin Valleys. Values for Hg and Ag were more than an order of magnitude above background levels between 100 and 150 years ago, indicative of sediments deposited during the period of hydraulic mining in the central Sierra Nevada.
DE: 4875 Trace elements
DE: 4235 Estuarine processes
DE: 3022 Marine sediments--processes and transport
DE: 3030 Micropaleontology
DE: 1890 Wetlands
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting