Carbon-isotope, diatom, and pollen evidence for Late Holocene salinity change in a brackish marsh in the San Francisco Estuary

Analysis of diatoms, pollen, and the carbon-isotopic composition of a sedim ent core from a brackish marsh in the northern part of the San Francisco Es tuary has provided a paleosalinity record that covers the past 3000 yr. Cha nges in marsh composition and diatom frequencies are assumed to represent v ariations in freshwater inflow to the estuary. Three periods of relatively high salinity (low freshwater inflow) are indicated, 3000 to 2500 cal yr B. P., 1700 to 730 cal yr B.P., and ca. A.D. 1930 to the present. The most rec ent period of high salinity is primarily due to upstream storage and water diversion within the Sacramento-San Joaquin watershed, although drought may also have been a factor. The two earlier high-salinity periods are likely the result of reduced precipitation. Low salinity (high freshwater flow) is indicated for the period 750 cal yr B.P. to A.D. 1930. (C) 2001 University of Washington.