The REPEAT Project:

Rates and Evolution of Peat Accretion Through Time in the Sacramento-San Joaquin Delta of California

During the Holocene, a 1,400 km² tidal marsh region formed in the Sacramento-San Joaquin Delta, which is situated at the landward end of the San Francisco Bay Estuary.

Over 90 percent of this region was drained for agriculture during the past 100 to 150 years. Nevertheless, small relict marshes remain, which constitute an historical archive of information on peat formation processes, geomorphological change, carbon storage, and anthropogenic disturbance.

The purpose of the REPEAT project is to use this archive to determine past rates and processes of peat accretion in order to benefit future Delta wetland restoration efforts.

During the summer of 2005, we collected peat cores from four pairs of islands (drained farmed islands and relatively undisturbed, marsh islands) in the Delta in order to discern the differences between them.  Peat cores were sectioned and analyzed for bulk density, percentage of organic matter, percentage of organic carbon, and a suite of major and trace elements.  Achenes (fruiting bodies of bulrushes), charcoal, and plant fragments other than roots were analyzed for radiocarbon content. 

Project Chief: Dr. Judith Drexler, U.S. Geological Survey, California Water Science Center (CWSC), Sacramento, CA

Co-Principal Investigators: Dr. Charles Alpers, CWSC; Dr. Steven Deverel, Hydrofocus, Inc., Davis, CA; Dr. Kenneth Verosub, University of California, Davis, Department of Geology (UCD); and Dr. Irina Delusina (UCD)

Reports and Publications

Deverel, S.J., Drexler, J.Z., Ingrum, T., and Hart, C. 2008. Simulated Holocene, recent, and future accretion in channel marsh islands and impounded marshes for subsidence mitigation, Sacramento-San Joaquin Delta, California, USA. REPEAT Project Final Report to the CALFED Science Program of the Resources Agency of California, 60 pp.

Drexler, J.Z., de Fontaine, C.S. and Knifong, D. 2007. Age determination of the remaining peat in the Sacramento-San Joaquin Delta, CA. U.S. Geological Survey Open File Report, 2007-1303.

Drexler, J.Z., de Fontaine, C.S. and Deverel, S.J. 2009. The legacy of wetland drainage on the remaining peat in the Sacramento-San Joaquin Delta, CA, USA. Wetlands in press.

What's next

The second phase of the REPEAT project will be beginning in the summer of 2009. This phase of the project is concerned with characterizing the paleosalinity of the Delta. Concentrations of Na and Sr will be used to obtain a qualitative measure of the variability of salinity through time. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) in peat, vegetation, ocean water, and river water will be used to form a mixing model of salinity in the San Francisco Bay Estuary. This model will be used to quantify salinity in the Delta during the past 6000 years.

Map of Delta and coring sites