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Climate change in the Pacific North America region over the past millennium : development and application of novel geochemical tracers


Decadal climate variability in the Pacific North America (PNA) region largely determines the fresh-water supply of the western United States and fisheries production of the northeast Pacific Ocean through tightly coupled ocean- atmosphere interactions. Documenting the historical manifestations of these interactions, over at least the past millennium, is an essential step towards anticipating the ramifications of future climate change on these resources. This dissertation aims to extend efforts at PNA paleoclimate reconstruction through the development of interannually resolved geochemical proxy records from annually laminated marine and lacustrine sediments. Comparison with instrumental climate measurements provides constraint on the driving mechanisms for observed geochemical variability in each record. The radiocarbon content ([Delta]¹⁴C) of benthic foraminiferal tests in the Santa Barbara Basin (SBB) proves a sensitive tracer for decadal fluctuations in vertical density structure along the southern California margin that are forced by north Pacific ocean-atmosphere interactions and tropical- midlatitude teleconnections. Enumeration of cosmopolitan benthic foraminifera in the SBB over the past 2̃50 years suggests that, on decadal time scales, the overall size and distribution of the SBB benthic foraminiferal community is largely a function of benthic carbon oxidation rate --- a property previously linked to regional surface productivity. In the terrestrial realm, stable hydrogen isotope ratios ([delta]D) of plant lipid biomarkers preserved in the sediments of Swamp Lake, Yosemite National Park, were measured over the 20th century and late medieval period (1160-1432 A.D). Plant lipid [delta]D exhibits a complex relationship with environmental parameters but nevertheless reflects interannual to multidecadal changes in amount of wintertime precipitation in the Sierra Nevada Mountains over the 20th century. Consistent with this observation, multidecadal [delta]D variability during the late medieval generally follows that of concomitant tree ring-derived hydrologic reconstructions and captures droughts of greater severity than any witnessed in the Sierra Nevada over the past 100 years. Comparison of the two isotopic records in terms of medieval vs. modern drought severity, however, requires further constraint on the driving mechanisms for long-term shifts in plant lipid [delta]D. Overall, this dissertation documents the novel application of benthic foraminiferal [Delta]¹⁴C and plant lipid [delta]D as a means for reconstructing decadal scale PNA climate from high-resolution sedimentary archives

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