STRONTIUM ISOTOPIC COMPOSITION OF ESTUARINE SEDIMENTS AS PALEOSALINITY-PALEOCLIMATE INDICATOR

The strontium isotopic composition of biogenic precipitates that occur in estuarine sediments can be used as proxy indicator of paleosalinity and for assessing precipitation and river discharge rates over thousands of years. In the San Francisco Bay estuary, river water with low Sr-87/Sr-86 ratio (average, 0.7065) and low Sr concentration (0.13 parts per million) mixes with seawater with a higher Sr-87/Sr-86 ratio (0.7092) and Sr concentration (7.9 parts per million). The predicted mixing relation between salinity and Sr isotopic composition is confirmed by measurements of modern estuarine surface waters. A paleosalinity record obtained from foraminifera for the ancestral San Francisco Bay during oxygen isotope substage 5e of the last interglacial reflects a global rise and fall of sea level, and short time-scale variations related to fluctuations in discharge rates of the Sacramento and San Joaquin rivers.