Paleoenvironmental Change During the Holocene and Last Interglacial in San Pablo Bay, Northern California

The San Francisco Bay exists as an estuary during sea level highstands and during this time its salinity field reflects the runoff regime of its watershed. Since the latter comprises about 40% of the state of California, study of the changing state of the estuary can provide insight into the climate history of the region. Previous studies have demonstrated significant secular variation in runoff during the late Holocene. The goals of the current work were to extend the record of the estuary's Holocene history and to investigate corresponding variation during the previous interglacial period (ca 130 - 115 ka). Changes at our study site indicative of variation in sediment budgets or tectonic activity were also of interest. Two boreholes were drilled along the southernmost margin of San Pablo Bay, a northern reach of San Francisco Bay. Estuarine sediments were examined for organic constituents; foraminifers were counted and shifts in species assemblages noted; their tests were subjected to stable isotope analysis so that variations in their isotopic signature could be tracked. Beneath recent fill and remnants of marsh deposits, each borehole showed a sequence of three stratigraphic units: Holocene bay mud; alluvial deposits; and estuarine mud. This stratigraphy resembles the typical south Bay sequence: Holocene mud; alluvium deposited during the last glacial period; estuarine mud deposited during the 5e highstand. Foraminifers recovered from the Holocene and older estuarine deposits support the correlation with south Bay deposits. On the basis of stratigraphic position and this correlation, we consider the older estuarine muds to date from the 5e highstand. Foraminiferal assemblages in both the Holocene and Last Interglacial sediments showed systematic variation indicative of environmental change at the site. The 5e samples document a shift in dominant species from Trochammina inflata in the lowermost sample to Elphidium gunteri in the middle of the section and to E. excavatum in the upper samples. This transition indicates a gradual change from shallow intertidal mudflat conditions to a subtidal environment, a shift similar to that indicated by the south Bay sediments. The Holocene interval shows two changes in dominant taxa. T. inflata predominates in the lowermost sample beneath a short non-estuarine interval. E. excavatum is dominant in the middle of the section but gives way abruptly to T. inflata in the upper samples. These shifts indicate sudden changes in water depth that produced rapid transitions from intertidal to subtidal and back to intertidal conditions at the site, likely a result of tectonic movements or changes in sediment input. Stable isotopic measurements of both foraminifers and mollusk shells are in progress. These results are expected to provide information about past salinity at this location, which reflects the volume of freshwater inflow to the estuary. Two ¹⁴C dates have been obtained on Holocene shell fragments. These samples, spaced 420 cm apart vertically, show (uncorrected) ages of 3545 and 1775 radiocarbon years, providing an estimated average accumulation rate at the site of about 2.4 mm/yr. If this rate applied throughout the Holocene at the site, the more recent shift noted above occurred about 2760 radiocarbon years (uncorrected for reservoir effect) ago, and the entire 12.8 m thick Holocene unit represents about a 5300 year record.