TOO CLEVER TO BE UNDERSTOOD: LATE HOLOCENE CENTRAL CALIFORNIA CLIMATE RECORDS FROM SAN FRANCISCO BAY MARSH SEDIMENTS

Sediment cores collected from three marshes on the northern margin of San Francisco Bay each provide a somewhat different record of late Holocene climate variation in central California. The timing of changes in the diatom floras illustrates the complex interaction between local and regional climatic processes.

Results from Rush Ranch and Petaluma marsh suggest that conditions along the central coast began to dry out prior to the Medieval Climatic Anomaly, and that fresh water became increasingly abundant during the transition from the Medieval Climate Anomaly to the Little Ice Age. In contrast, the Benicia State Park site is dominated by a freshwater flora during the Medieval Climate Anomaly, and conditions become brackish at the beginning of the Little Ice Age.

The Rush Ranch site shows periods of increased salinity between 3,000 to 2,700 cal yr B.P., 1,750 to 750 cal yr B.P., and from about A.D. 1930 to the present. The 3,700 year Benicia State Park record shows a rapid decrease in salinity around 3,200 cal yr B.P. Fresher conditions continue until 500 cal yr B.P., after which salinity increases. During this later period, sediment deposition occurred in the summer and fall when river flow rates were lower, resulting in a more brackish diatom flora.

Differences in the timing and duration of the fresher and more saline intervals at Rush Ranch and Benicia State Park are largely controlled by proximity to the main channel of the Sacramento-San Joaquin River system. The short (about 15 km) distance from the mouth of the tidal channel to Rush Ranch appears to have a dampening effect on the expression of climate variation. For example, the transition from brackish to fresher conditions takes place between 3,200 and 3,100 cal yr B.P. at Benicia State Park, but does not occur until several hundred years later at Rush Ranch. The subsequent transition from fresher to more brackish conditions occurs 150 to 200 years earlier at Rush Ranch.

The record at Petaluma marsh is strongly controlled by precipitation in the Coast Ranges. Beginning about 1550 cal yr B.P., conditions become more saline, and, with the exception of a 200-year period from ~700 to 500 cal yr B.P., continued to increase in salinity to the present. This shift in the diatom flora may be influenced by marsh accretion, resulting in longer periods of exposure during the summer and fall.