Computer Science
Scientific paper
Aug 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.119..103l&link_type=abstract
Earth and Planetary Science Letters, Volume 119, Issue 1-2, p. 103-119.
Computer Science
9
Scientific paper
Strontium isotopic compositions of carbonate mollusk shells from estuarine sediments cored beneath San Francisco Bay are used to derive a record of mean annual salinity and average freshwater inflow to the estuary for intervals during the past 4300 yrs. The large difference in the 87Sr/86Sr ratio between seawater (0.7092) and the average freshwater entering the estuary (0.7065) produces a correlation between 87Sr/86Sr and salinity in bay waters that can be detected with high-precision measurements. Paleosalinity is inferred from the 87Sr/86Sr ratio measured in fossil carbonate shells of bivalves preserved in the sediment. 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 using 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‰), and from San Pablo Bay, located about 30 km upstream (modern MAS of ca. 18-20‰). 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 represented.
The Richardson Bay data indicate extended periods of low salinity relative to modern, indicating higher freshwater inflow, at ca. 80, 220, 310, 440-500, 3100-3400 and 4300 yrs ago. The data indicate high salinity relative to modern, hence lower freshwater inflow, at about 40, 140-210, 270, 2100 and 3450-3700 yrs B.P. For San Pablo Bay, where salinity resolution is better, salinity was lower than modern during 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 annual discharge to San Francisco Bay typically varies between values that are below the modern value (600 m3/s) and values 2 to 3 times as high; the average paleosalinity is typically substantially lower than the modern values. The 4300 yr average freshwater inflow is estimated to be >= 1200 +/- 200 m3/s. The record indicates that quasi-cyclic variations in mean annual discharge occur naturally with a period of about 200 yrs or less.
Depaolo Donald J.
Lynn Ingram B.
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