234U/238U and delta 87Sr in peat as tracers of paleosalinity in the Sacramento-San Joaquin Delta of California, USA

• Published in: Applied geochemistry 2014-01, Vol.40, p.164-179
• Main Authors: Drexler, J Z, Paces, J B, Alpers, C N, Windham-Myers, L, Neymark, LA, Bullen, T D, Taylor, HE
• Format: Article
• Language: English
• Subjects: Brackish; Freshwater
• ISSN: 0883-2927
• DOI: 10.1016/j.apgeochem.2013.10.011

The purpose of this study was to determine the history of paleosalinity over the past 6000+ years in the Sacramento-San Joaquin Delta (the Delta), which is the innermost part of the San Francisco Estuary. We used a combination of Sr and U concentrations, delta 87Sr values, and 234U/238U activity ratios (AR) in peat as proxies for tracking paleosalinity. Peat cores were collected in marshes on Browns Island, Franks Wetland, and Bacon Channel Island in the Delta. Cores were dated using 137Cs, the onset of Pb and Hg contamination from hydraulic gold mining, and 14C. A proof of concept study showed that the dominant emergent macrophyte and major component of peat in the Delta, Schoenoplectus spp., incorporates Sr and U and that the isotopic composition of these elements tracks the ambient water salinity across the Estuary. Concentrations and isotopic compositions of Sr and U in the three main water sources contributing to the Delta (seawater, Sacramento River water, and San Joaquin River water) were used to construct a three-end-member mixing model. Delta paleosalinity was determined by examining variations in the distribution of peat samples through time within the area delineated by the mixing model.