Stable isotope record of late Holocene salinity and river discharge in San Francisco Bay, California

Oxygen and carbon isotopic measurements of fossil mollusks from San Francisco Bay are used to derive a record of paleosalinity and paleostreamflow for the past 5,900 years. The δ18O and δ13C values of river water (−12‰ and −9‰) are markedly different than seawater (0‰ and 1‰), and vary systematically as a function of salinity in the estuary. The data show that annually averaged salinity in the south-central part of the Bay was very close to the modern ‘diversion-corrected’ value of 26.8‰ over the past 2,700 years, and 4‰ lower than modern between 3,800 and 5,100 yr B.P. Based on those salinities, the average annual river inflow to San Francisco Bay is calculated to have been 1290 m3/s over the past 2,400 years, and 1990 m3/s between 3,800 and 5,100 yr B.P., 1.8 times greater than the modern ‘diversion-corrected’ value of 1100 m3/s, assuming a constant bay volume. The inferred river discharge record generally corroborates independent paleohydrologic records in California, including tree-ring, treeline and lake level records.