Late Holocene diatom and geochemical evidence of freshwater flow variation in northern San Francisco Bay, California

A qualitative record of late Holocene fresh water flux through the Sacramento delta and northern San Francisco Bay was constructed using diatoms and sediment geochemistry. Because the Sacramento and San Joaquin Rivers drain more than 40% of California through the Sacramento delta and into northern San Francisco Bay, variation in their discharge is an indication of both the volume and timing of precipitation over a significant part of the state.; More than 600 species and varieties of diatoms were identified in this study. Species diversity was lowest in the normal marine (>30‰) waters of San Francisco Bay and reached the highest diversity in the brackish tidal marshes west of the confluence of the two rivers. Diatom taxa were used to construct a paleosalinity record in cores collected along a salinity gradient from the west (higher) to the east (lower). The western site, Petaluma marsh, displayed a mixed record of both regional and local salinity control. At the central site, located in a marsh near Southampton Bay (Benicia State Park), salinity was primarily controlled by the flow of the combined Sacramento and San Joaquin rivers. This site was the least sensitive to the small-scale changes in river flow brought about by short-term regional precipitation changes, and the record may have been affected by fluvial erosional processes. The third site, Rush Ranch, is the most sensitive to changes in precipitation due to the location near the end of a tidal channel. The timing and magnitude of changes in salinity at this site are correlative to a number of other proxies for precipitation change in northern and central California.; The sediment from the Benicia State Park core was analyzed using inductively-coupled mass spectrometry (ICP-MS) and cold-vapor atomic absorption (CVAA) in order to identify variations in provenance. Sediment derived from the Sierra Nevada (high in Al, K, Ti, and associated trace elements) was generally correlated with periods of higher flow and lower rates of marsh accretion. During periods of increased marsh accretion the sediment (high Na, Ca, Mg, Co, Ni) represented several sources, including the Great Valley sequence and local mafic units in the Coast Range. Sediment deposited during hydraulic mining in the second half of the 19th century was also recorded.