HOLOCENE SEDIMENT RECORDS FROM SAN FRANCISCO ESTUARY TIDAL MARSHES AND THEIR RELEVANCE TO CALIFORNIA PALEO-CLIMATE VARIABILITY

As depositional environments, tidal marshes provide excellent opportunities for long-term records of changes in physical conditions and biological responses. Late Holocene sediment records have been analyzed from several of the tidal marshes surrounding the San Francisco Estuary, which drains most of northern California. Changes in physical conditions include local events, such as changes in the relative elevation of the marsh surface, changes in sediment supply, and regional changes in climate affecting the amount of fresh water inflow (from 40% of California) and thus estuarine salinity.

Results from marsh cores collected from Benicia State Park, a small marsh in the Carquinez strait between Suisun bay and San Pablo bay, are highlighted here. The sediment stratigraphy and chronology of the cores indicate dramatic changes in physical conditions experienced on this marsh, with periods of both very high and very low sediment accumulation that appear to be reflecting local changes, e.g., from 1250-1050 cal yr BP sedimentation was over 4 mm/yr, and from 1050-300 cal yr BP, sedimentation was less than 0.5 mm/yr. These periods of anomalous sedimentation are seen at other sites in Suisun bay (see also Starratt, S. Abstract, INQUA XVI 2003, and Chin, J.L., et al. Abstract AGU 2003 meeting).

The biological effects of changes in fresh water inflow to the San Francisco Estuary and its adjacent marshlands were assessed using total organic carbon ?13C, and pollen analysis. Drought events recorded in the watershed are recorded in marshes as periods of increased salt-tolerant plants, e.g., several paleo-records across California indicate severe drought centered ca. A.D. 1300 and in the Estuary marshes this period was marked by increased salt tolerant plants.

More recent changes in the Estuary marsh vegetation may not be related to climate variability, but to human modifications of the watershed hydrology. Increases in the ?13C values of organic carbon and the in pollen from salt tolerant plants seen in the tidal marshes suggest an increase in estuarine salinity occurred within the last 60 years, most likely resulting from dam construction along the major tributaries to the Sacramento and San Joaquin rivers.