Geochemical Fingerprints of Paleoceanographic Variability in the Subarctic Pacific Over the Last 500,000 Years

摘要

Marine sediments are a storehouse of the geochemical, biological, and physical changes in the ocean over thousands to millions of years. Intensive study of the Atlantic Ocean has well constrained the role of this basin in global climate change, but the vast Pacific Ocean, deeper and more corrosive to carbonate, has remained more elusive. This thesis leverages a new suite of sediment cores collected on the Juan de Fuca Ridge in the East Subarctic Pacific Ocean (~45°N, 135°W) to better understand how the paleoceanographic history of this region has evolved over the past 500kyr. In Chapter 1, I developed age models for multiple cores using benthic delta18O and physical properties of sediment as stratigraphic markers. Despite the proximity of the cores (within 50km2), the sedimentation rates varied by an order of magnitude, likely reflecting remobilization of sediment caused by the high relief of the mid-ocean ridge bathymetry. In Chapter 2, I analyzed uranium series disequilibria in the sediment in order to investigate the processes generating the highly variable sedimentation rates. This chapter presents evidence that the particle flux settling through the water column (based on excess 230Th) is relatively constant at six different sites, and the variability in sedimentation rates is largely driven by lateral sediment remobilization along the rough bathymetry of the ridge. Chapter 3, entitled "Trace element (Mn, Zn, Ni, V) and authigenic uranium (aU) geochemistry reveal sedimentary redox history on the Juan de Fuca Ridge, North Pacific Ocean", presented high-resolution x-ray fluorescence records of metal diagenesis in response to changing oxygen conditions in the sediment. This study is the first to show strong evidence for low sedimentary oxygen conditions during interglacial periods in the North Pacific, which we suggest may be linked to hydrothermal sulfide deposition. In Chapter 4, I returned to uranium series disequilibria by utilizing 231Pa/ 230Th records from the Juan de Fuca Ridge to reconstruct productivity in the East Subarctic Pacific Ocean over the last 200kyr. Productivity across much of the Subarctic Pacific is low during glacial periods and high during interglacial periods, which is usually associated with changes in stratification. I investigated several different mechanisms for increasing stratification during glacial periods, and conclude that a combination of surface freshening, weak winds, and reduced subsurface nutrient concentrations likely created the stratification that led to low glacial productivity.;Finally, in Chapter 5, "Dust deposition in the East Subarctic Pacific on glacial-interglacial timescales", I reconstructed the patterns of dust fluxes in the East Subarctic Pacific Ocean over the last 500kyr to assess the climatic effects on the spatial distribution of dust in the North Pacific Ocean. I predict that migration of the westerlies would have caused a shift in dust provenance away from Asian dust and towards higher North American contributions during glacial periods. Although lithogenic endmembers are currently poorly constrained in this region, I present some evidence for variable provenance over time that may be consistent with the influence of the westerlies on dust fluxes in the East Subarctic.