A 4300-YEAR STRONTIUM ISOTOPE RECORD OF ESTUARINE PALEOSALINITY IN SAN-FRANCISCO BAY, CALIFORNIA

Strontium isotopic compositions of carbonate mollusk shells from estua rine sediments cored beneath San Francisco Bay are used to derive a re cord of mean annual salinity and average freshwater inflow to the estu ary for intervals during the past 4300 yrs. The large difference in th e Sr-87/Sr-86 ratio between seawater (0.7092) and the average freshwat er entering the estuary (0.7065) produces a correlation between Sr-87/ Sr-86 and salinity in bay waters that can be detected with high-precis ion measurements. Paleosalinity is inferred from the Sr-87/Sr-86 ratio measured in fossil carbonate shells of bivalves preserved in the sedi ment. Because salinity in San Francisco Bay is primarily controlled by the freshwater inflow from the Sacramento and San Joaquin rivers, the paleosalinity record can be converted to a paleodischarge record usin g a transfer function derived from historical data. Salinity data are referenced to modern values estimated for the estuary after correcting for the lower discharge caused by diversion since 1850 A.D. Data are presented from Richardson Bay, located near the estuary mouth (modern mear arrival salinity (MAS) of ca. 24 parts-per-thousand), and from Sa n Pablo Bay, located about 30 km upstream (modern MAS of ca. 18-20 par ts-per-thousand). For Richardson Bay, the data cover the periods 0-600 yrs B.P. and 2200-4300 yrs B.P., based on radiocarbon dating. For San Pablo Bay, the periods 0-130 yrs B.P. and 2500-2800 yrs B.P. are repr esented. The Richardson Bay data indicate extended periods of low sali nity relative to modern, indicating higher freshwater inflow, at ca. 8 0, 220, 310, 440-500, 3100-3400 and 4300 yrs ago. The data indicate hi gh salinity relative to modern, hence lower freshwater inflow, at abou t 40, 140-210, 270, 2100 and 3450-3700 yrs B.P. For San Pablo Bay, whe re salinity resolution is better, salinity was lower than modern durin g the period at ca. 50 and 2550-2650 yrs B.P., and higher than modern at ca 90-110, 155 and 2510-2530 yrs B.P. The data suggest that mean an nual discharge to San Francisco Bay typically varies between values th at are below the modern value (600 m3/s) and values 2 to 3 times as hi gh; the average paleosalinity is typically substantially lower than th e modern values. The 4300 yr average freshwater inflow is estimated to be greater-than-or-equal-to 1200 +/- 200 m3/s. The record indicates t hat quasi-cyclic variations in mean annual discharge occur naturally w ith a period of about 200 yrs or less.