DIATOM AND SEDIMENT GEOCHEMISTRY RECORD FROM SOUTHAMPTON BAY, A BRACKISH TIDAL MARSH IN NORTHERN SAN FRANCISCO BAY, CALIFORNIA

Sediments from a 3.8-m-long core collected from the marsh in Southampton Bay record both regionally and locally 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 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.

During this 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 during intervals when the TOC values average between 20 and 30 percent. Between 3,800 and 3,300 cal yr B.P. and from about 300 cal yr B.P. to the present, 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 prior to initial marsh formation (3,800-3,300 cal yr B.P.) and during a period of increased river flow which resulted in seasonal erosion and(or) limited deposition of sediment.

Sediment analysis using ICP-MS and ICP-AES identified several trends in sediment composition. The abundance of Na, Ca, and Mg is higher in intervals when TOC values are higher; Fe, Al, K, and Ti are higher during periods when TOC is lower. These variations may be due to changes in sediment source between the local rocks of the Great Valley sequence and a more regional signal from sediment derived from the Sierra Nevada, Cascades, and east side of the Coast Range. Values for Hg and Ag were relatively high (more than an order of magnitude for Hg) between 100 and 150 years ago, indicating sediments deposited during hydraulic mining.

The relatively low variability of fresh water flux at this site between 3,300 and 300 cal yr B.P. may be due to its proximity to the river channel. Other sites in northern San Francisco Bay may be more useful for identifying climatically controlled variations in fresh water flow on decadal and centennial timescales.