HOLOCENE PALEOCLIMATE RECORDS FROM A LARGE CALIFORNIA ESTUARY AND ITS WATERSHED REGION: LINKING WATERSHED CLIMATE AND ESTUARY CONDITIONS

The San Francisco Bay estuary includes a vast watershed that covers 40 percent of the area of California and paleoclimate records from the Bay and its watershed are reviewed here to give a more complete understanding of natural climate variations and the linkages between those variations and the salinity and ecosystems of the estuary. These records put into context instrumental records documenting climate variability and change. Initial comparison of paleoclimate records from the estuary and its watershed illustrate effects of climate changes in the upstream watershed on estuarine salinity and ecosystems. Paleoclimate records from the Bay estuary and its watershed include Central Valley lowland floodplain rivers, Sierra Nevada mountain lakes and tree ring chronologies from the entire watershed region.

Paleoclimate records from the S.F. Bay spanning the last 3,000 to 6,000 years include cores collected from the open bay, and from seven marshes spanning the salinity gradient of the northern reaches of the Estuary, and archeological reports from prehistoric shellmound middens. A long-term trend towards higher salinity in the Bay estuary (due to steadily rising sea level) has been punctuated by periodic shorter term variations in estuarine salinity (largely due to climate variability) detected in all the Bay archives.

The archives from the San Francisco Bay estuary and its watershed (primarily sediment cores, tree-rings, geomorphic evidences, and lake sediments), all contain evidence of large fluctuations in climate over the past 5,000 years, with unusually wet and dry periods lasting decades (or centuries). In general, conditions were wetter and cooler in the period of 4000 – 2000 cal yr B.P., and drier and warmer over much of the past 2000 years, punctuated by several abnormally wet and dry periods. Climate during the Medieval Climate Anomaly (ca. A.D. 900 - 1200) appeared to be unusually dry, and coastal surface waters were warmer, while the Little Ice Age (ca. A.D. 1400 – 1700) was a time of unusual wetness, with cooler than average coastal waters. A notably benign period, with relatively little variability has been seen in many records from about A.D. 1850 – 1950. Many of the records showed interdecadal variability with the most common periodicities of ca. 55, 70, 90, 100, 150, and 200 years.