The Tangled Web – Records of the Medieval Climate Anomaly (A.D. 900-1350) from Northern San Francisco Bay Marshes

The record of late Holocene climate variability in San Francisco Bay marshes reflects both regional and local processes. Runoff from more than 40 percent of California passes through northern San Francisco Bay, resulting in the deposition of a diatom record that reflects variations in salinity that are controlled by the timing and volume of runoff . Changes in runoff from local watersheds, which is more strongly connected to coastal marine evaporative moisture source, complicates the salinity record.

Due to the proximity of its watershed to the coast, Petaluma Marsh is most strongly affected by local variations in precipitation and evaporation. The record at Benicia State Park, adjacent to the main channel of the Sacramento River, reflects only major shifts in regional fresh water flow. Rush Ranch is located on a on a slough, several miles from the main channel of the Sacramento River, in an area with limited local runoff.

The diatom assemblage at Rush Ranch is dominated by brackish and marine species throughout the Medieval Climate Anomaly (MCA), with marine taxa more abundant during the later part of the interval. This dominance by brackish and marine species reflect the continuation of a gradual increase in salinity that began at about 1,750 cal yr B.P. The abundance of freshwater species decreased to less than five percent of the assemblage by about 800 cal yr B.P.

Brackish species are the largest component of the assemblage at Petaluma Marsh, with an increase in freshwater taxa in the later half of the MCA. Freshwater species comprise as much as eighty percent of the assemblage at Benicia State Park throughout the MCA.

Comparison of the salinity records from these localities with other regional climate proxies suggests the record from Rush Ranch most strongly reflects climate variations along the west coast of the U.S.