Holocene Carbon Sequestration and Ecosystems of Wetlands in the San Joaquin Watershed, California

Sediment cores were collected from wetlands along a headwaters-to-estuary gradient within the San Joaquin watershed, California, to examine environmental changes, and carbon (C) storage and sequestration rates over the Holocene. Local hydrogeomorphic conditions were the primary drivers of C sequestration at seven wetland systems analyzed Paleoecological assays of C and multiple paleo-indices were conducted on two cores from the Sacramento-San Joaquin Delta, three cores from Yosemite National Park, and on two cores from Sequoia-Kings Canyon National Park to reconstruct C storage and ecosystem functioning of wetland systems in the watershed over the Holocene. Bayesian age-depth models were developed using cesium 137/lead 210 and radiocarbon dating. Paleo proxies such as loss-on-ignition, pollen, x-ray fluorescence, and charcoal showed that local hydrogeomorphic conditions were the primary drivers of variability in C sequestration at 7 wetland systems analyzed. There is some evidence of C sequestration responding to past warming by changing vegetation types, altering geochemical concentrations of crustal elements, and varying the frequency of wildfires. A 300 cm sediment core retrieved from Dana Meadow, Yosemite National Park, spans the past 9000 years and indicates carbon storage was ≥ 24 g m2 yr-1 during the Holocene Thermal Maximum (~8000 – 3000 years ago). By the mid-Holocene, terrestrial plants like Quercus and Tsuga mertensiana expanded upslope amid prolonged warming. Average rates of C sequestration and vertical accretion from the five Yosemite and Sequoia-Kings Canyon wet meadows indicate regional similarities to rates of C storage from subalpine fens in the mountains of western North America as far north as Alberta, Canada. The two cores from the Sacramento-San Joaquin Delta present no clear relationships between C content and climate change. In the Delta, decreases in C content and sedimentation rates occurred after Euro-American settlement in the 19th century, trends that continue during the 21st century. The findings of this dissertation on the whole suggest: 1) no regionally coherent climatic response to past climatic changes was shown in the paleo indices measured for the seven sediment cores from the Delta and Sierra; 2) there is much local variability in the histories of these wetlands and their C sequestration records; 3) local variations in hydrogeomorphology appears to be a primary driver of natural C sequestration and storage for each site; 4) such variability likely reflects differing responses and sensitivities to the impacts past climatic change as well as local hydrogeomorphology; and 5) although this study detected no widespread coherent response of the San Joaquin wetlands C dynamics to past Holocene climatic changes, documenting and helping to understand the potential variability of such environmental systems isarguably just as important in anticipating California’s climate change future and its potential variability of responses.

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