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 Fr ancisco Bay are used to derive a record of paleosalinity and paleostre amflow for the past 5,900 years. The delta(18)O and delta(13)C values of river water (-12 parts per thousand and -9 parts per thousand) are markedly different than seawater (0 parts per thousand and 1 parts per thousand), and vary systematically as a function of salinity in the e stuary. The data show that annu ally averaged salinity in the south-ce ntral part of the Bay was very close to the modern 'diversion-correcte d' value of 26.8 parts per thousand over the past 2,700 years, and 4 p arts per thousand lower than modern between 3,800 and 5,100 yr B.P. Ba sed on those salinities, the average annual river inflow to San Franci sco Bay is calculated to have been 1290 m(3)/s over the past 2,400 yea rs, and 1990 m(3)/s between 3,800 and 5,100 yr B.P., 1.8 times greater than the modern 'diversion-corrected' value of 1100 m(3)/s, assuming a constant bay volume. The inferred river discharge record generally c orroborates independent paleohydrologic records in California, includi ng tree-ring, treeline and lake level records.