TIME SCALES OF VARIATION IN SAN FRANCISCO BAY, NORTHERN CALIFORNIA, AND MEASUREMENT PROXIES

San Francisco Bay's physical properties vary on a range of time scales from sub-daily to glacial/interglacial. It is well known that the Bay exists as an estuary only during sea level high stands, but details concerning the extent to which the Holocene Bay resembles that which existed during the Last Interglacial are of continuing interest. On short time scales, salinity and temperature at a given Bay site vary on a semi-diurnal basis due to tidal influence, and strong diurnal temperature variation is also typical, particularly for near-surface water. Pronounced seasonal fluctuation in salinity and temperature are driven by systematic variation in runoff and insolation. In addition, previous studies have documented dramatic century-scale variation in bay salinity, suggesting corresponding shifts in regional precipitation regimes.

In order to investigate the estuary's variation on these time scales, we use several methods and proxies. To track century-scale secular change and to compare the Holocene Bay with its predecessor, we have developed oxygen isotope records using fossil foraminiferal tests. Our Holocene data extend the record of century-scale trends and confirm the pattern of significant variation over at least the past 4 ka. Foraminiferal species shifts in two sediment cores also identify changes at this time scale, disclosing dramatic site variability in water depth and salinity due to post-glacial sea level rise or seismic activity. Since the isotopic and elemental compositions of biogenic carbonate formed in the Bay are functions of the salinity and temperature of the waters from which they were precipitated, and since mollusk shells are deposited sequentially during the lifetime of the organism, large shell fragments provide records of water temperature and salinity over a period of about 1-2 years. We are using incremental sampling of these shells, analyzed by isotope ratio mass spectrometry and ICP-AES to investigate the history of the annual salinity cycle of the Bay as an indication of variation in regional hydrology. Finally, elemental analysis of the same shells, sampled at a very fine spatial scale by ion microprobe, will allow us to infer daily and tidal-scale temperature changes.