NATURAL CLIMATE VARIABILITY AND HUMAN IMPACT ON A BRACKISH TIDAL MARSH IN NORTHERN SAN FRANCISCO BAY, CA

A qualitative record of late Holocene fresh water flux into northern San Francisco Bay has been reconstructed using pollen, total carbon, ¹³C, diatoms, and chrysophyte stomatocysts. Because the rivers that flow through the Sacramento delta and into San Francisco Bay drain more than 40 percent of California, variation in their discharge is an indication of changes in the characteristics of regional precipitation. Results from two coring sites at Benicia State Park, a small marsh in the Carquinez Strait located between Suisun and San Pablo Bays, reflect temporal changes in fresh water flow. Sedimentation rates vary from 0.5 mm/yr to more than 4 mm/yr. Similar variations in sedimentation rate have been documented in other marshes in San Francisco Bay, although the magnitude of the variation is not as large. The more than eight-fold fluctuation may be due to the relatively close proximity of the coring sites to the river. Variations in density, magnetic susceptibility, and total carbon are indicative of both natural and human-induced changes in sediment composition. The lowest parts of the cores are dominated by organic-poor, pre-marsh sediments. Marsh accretion began about 3,300 years ago and appears to have continued with some variation in magnitude, until European contact caused a rapid increase in inorganic sediment deposition, primarily from hydraulic mining, dredging, and farming. Climate variability is evident from the total carbon, ¹³C, pollen, and siliceous microfossil records. Periods of decreased precipitation and/or increased temperatures lasting several years to several centuries have been detected. These variations in climate appear in records from marshes in northern San Francisco Bay as well as other Sierra Nevada watersheds such as Mono Lake and the Lake Tahoe-Truckee River-Pyramid Lake system suggesting regional-scale changes in climate patterns.