Holocene records of climate and pollution from the Northern Rocky Mountains: Integrating perspectives from glacial ice, lake sediments, and tree rings
AuthorChellman, Nathan J.
AdvisorMcConnell, Joseph R.
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Reconstructions of past climate and pollution often rely on chemical records preserved in glacial ice, lake sediments, and tree rings. The goal of this dissertation is to combine perspectives on paleoclimate and pollution from paired records developed from these archives to obtain a more complete understanding of past environmental change, as well as to leverage the strengths of each archive to address shortcomings of another. Paired ice-core and tree-ring records were developed from the Upper Fremont Glacier (UFG) and groves of nearby whitebark pines in the Wind River Range, Wyoming. The depth-age scale for the difficult-to-date ice cores was constrained by synchronizing the ice-core water isotopes to the absolutely-dated tree-ring chronology, both proxies for temperature. On the revised depth-age scale, trends in black carbon, thallium, bismuth, and mercury (Hg) in the ice paralleled those observed in Greenland documenting widespread industrial pollution. The record of ice-core Hg subsequently was used to evaluate the viability of tree rings for reconstructing past atmospheric Hg concentrations. Hg trends observed in the trees were temporally offset from the ice core, suggesting radial movement of Hg across ring boundaries. A well-dated ice core record of water isotopes and ice accumulation was developed from a shallow ice patch on the Beartooth Plateau, Wyoming. Radiocarbon dating indicated the ice patch has persisted for over 10,000 years, and these records represent the first climate records developed from such an archive. In conjunction with a nearby lake sediment record, the ice patch records were shown to document Holocene climate variability, including peak warmth around 4,200 years ago followed by a 1,500 year-long era of colder and wetter winters, generally paralleling human activity in the nearby Bighorn Mountains. Black carbon (BC) profiles from two Wyoming lakes were used to assess how lake characteristics affect interpretations of lake sediment BC records. Comparisons of BC to charcoal data from Island Lake indicate that BC can be subject to similar depositional mechanisms as charcoal, leading to locally-biased records. At North Lake, which has a comparatively smaller and less vegetated watershed of the two lakes, comparisons to the UFG BC record and modelling results indicate that the North Lake BC record is primarily atmospherically deposited and therefore represents regional burning. The climate and pollution records presented in this dissertation demonstrate how improved analytical techniques and new paleo archives can lead to a better understanding of environmental changes in the northern Rocky Mountains over past centuries and millennia.