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  California Water Science Center

The REPEAT Project:

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

Repeat project coring crew: Dr. Judith Drexler, Jacob Fleck, and Christian de Fontaine

Dr. Judith Drexler, right, with scientists Jacob Fleck, center, and Christian de Fontaine, collecting a peat core on the Tip of Mandeville Tip, a marsh island in the Delta.

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 archive of information on peat formation processes, geomorphological change, carbon storage, and anthropogenic disturbance.

The purpose of the REPEAT project was to use this peat archive to study marsh formation processes in order to inform current wetland restoration efforts in the Delta. In addition, our research shed light on environmental contamination and changes in salinity through the millennia, which will also prove useful to managers tasked with restoring Delta habitats.

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. For the next three years, we analyzed the cores 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.

Additional funding was obtained for a study on paleosalinity using the same cores collected for the first part of the project. Concentrations and isotopic values of Sr and U in peat and source waters from the Delta were analyzed. These data were used to construct a mixing model for salinity. Delta paleosalinity through the millennia was determined by examining variations in the distribution of peat samples through time within the area delineated by the mixing model.

During the entire REPEAT project, which ran from 2005-2011, we determined:

  • Peat accretion histories for the entire lifetimes of 3 marshes
  • How inorganic vs. organic constituents of peat vary over the millennia
  • How peat on drained farmed islands differs from undisturbed peat
  • The contaminant history of lead and mercury in the Delta
  • Predicted time frames for bringing elevations of subsided islands back to sea level by using wetland restoration to re-build peat soils
  • Paleosalinity in the Delta over the past 6000+ years

Project Chief: Dr. Judith Drexler, U.S. Geological Survey, California Water Science Center (CWSC), Sacramento, CA
Phone: 916-278-3057
Email: jdrexler@usgs.gov

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)

A Primer on the Livingstone Corer

View a series of videos on the Livingstone corer.

Reports and Publications

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

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 Deverel, S.J. 2009. The legacy of wetland drainage on the peat resource in the Sacramento San Joaquin Delta, CA, USA. Wetlands, 29(1): 372-386.

Drexler, J.Z., de Fontaine, C.S., and Thomas A. Brown. 2009. Peat accretion histories during the past 6,000 years in marshes of the Sacramento-San Joaquin Delta of California, USA. Estuaries and Coasts 32: 871-892.

Drexler, J.Z. 2011. Peat formation processes through the millennia in tidal marshes of the Sacramento-San Joaquin Delta, California, USA. Estuaries and Coasts DOI 10.1007/s12237-011-9393-7.

Drexler, J.Z., Paces, J., Alpers, C.N., Windham-Myers, L., Neymark, L., and Taylor, H.E. 2013. 234U/238U and δ87Sr in peat as tracers of paleosalinity in the Sacramento-San Joaquin Delta of California. Applied Geochemistry, published online at http://dx.doi.org/10.1016/j.apgeochem.2013.10.011.

Map of Delta and coring sites

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