STABLE-ISOTOPE AND SALINITY SYSTEMATICS IN ESTUARINE WATERS AND CARBONATES - SAN-FRANCISCO BAY

Salinities, delta D and delta(18)O values of water samples collected b imonthly from two stations in San Francisco Bay estuary during 1991-19 93, and along a salinity transect in March of 1992, indicate a linear mixing relation between the isotopic compositions of the waters and th eir salinities. The salinities and stable isotope compositions of samp les from two locations in San Francisco Bay vary in response to change s in freshwater inflow. The data from these locations indicate simple mixtures of Pacific Ocean water (salinity approximate to 33, delta(18) O approximate to 0 to -1 parts per thousand, delta D M 0 to -10 parts per thousand) and Sacramento-San Joaquin River water (salinity approxi mate to: 0, delta(18)O = -10 to - 12 parts per thousand, SD = -75 to - 85 parts per thousand). Preliminary water balance estimates, using iso topic differences between local and upland runoff, suggest that local runoff (including waste water) comprises less than 20% of total freshw ater entering the bay. The average delta(18)O values of mussel shells (Mytilus edulis) collected live from eight locations in San Francisco Bay primarily reflect the delta(18)O of the water in which they grew. Shells subsampled along growth bands show that seasonal shifts in sali nity and delta(18)O are recorded in the shells. Therefore, the use of stable isotope measurements should be useful in reconstructing pre-ins trumental bay salinity and associated freshwater inflow (both annual a verage values and seasonal variations) to the San Francisco Bay, as we ll as potentially other estuarine systems.