SILICEOUS MICROFOSSILS AS PROXIES FOR LATE HOLOCENE CLIMATE CHANGE: THE RECORD FROM NORTHERN SAN FRANCISCO BAY AND THE SACRAMENTO DELTA, CALIFORNIA, USA

A qualitative record of late Holocene fresh water flux through the Sacramento delta and northern San Francisco Bay has been reconstructed using diatoms and chrysophyte stomatocysts. Coring sites are located in fluvial, subtidal, and marsh environments located between the normal marine environment of San Francisco Bay and the freshwater environment of the Sacramento and San Joaquin Rivers. Because the rivers that flow through the Sacramento delta and into northern San Francisco Bay drain more than 40 percent of California, variation in their discharge from the watershed is an indication of both the volume and timing of precipitation over a significant part of the state. The modern diatom assemblages in subtidal, freshwater, brackish, and saltwater marshes were used to calibrate the late Holocene salinity record. More than 600 diatom taxa have been identified from the modern sediments of San Francisco Bay, with additional taxa identified in core material. Approximately 150 of these taxa comprise more than two percent of any assemblage. Species diversity is lowest in normal marine environments (>30 psu) and highest in brackish marsh environments (2-30 psu). At the Rush Ranch coring site, located near the middle of the salinity gradient, there is evidence of centennial- to millennial-scale salinity cycles over the past 3,000 years. Human impact on the Sacramento and San Joaquin Rivers over the past 150 years, is indicated by a rapid increase in brackish and marine taxa. Prior to European contact about 150 years ago, there are two intervals dominated by freshwater taxa and two intervals dominated by more marine taxa. The intervals that indicate more saline conditions (3,000 cal yr B.P. to 2,500 cal yr B.P. and 1,750 cal yr B.P. to 750 cal yr B.P.) may correspond to periods of lower precipitation suggested by other California climate records. Smaller-scale variations within the salinity cycles correlate with variations in the lake level record at Mono Lake, as well as shorter precipitation cycles in the Lake Tahoe-Truckee River-Pyramid Lake system. Dendroclimatological records from California generally support the record of the past 400 years, although there are discrepancies between records from the Sacramento and San Joaquin Valleys, which may indicate variations in the source of the runoff.