Loveley, Matthew Robert (2017-08). Constraints and Applications of U-Th Systematics to Climate Reconstructions. Doctoral Dissertation. Thesis uri icon

abstract

  • The objective of this dissertation is to understand the dynamics of U-Th isotope behavior in deep sea marine sediment cores extracted from the Panama Basin located in the eastern Equatorial Pacific. Specifically, I attempt to further our understanding of the potential biases inherent in these proxies which are used to reconstruct past global climate change (export production, continental aridity, atmospheric circulation, deep-ocean circulation) over the past several hundred thousand years. First, I have investigated the effects that biasing of specific grain size fractions during deep-sea sediment redistribution processes have on the ???Th-normalization technique. This method is used frequently in the paleoceanographic community to reconstruct past changes in a number of climate-related processes. Recent studies have shown that sediment fluxes calculated by the ???Th method may be biased because of the fractionation of ???Th-enriched fine-grained particles (i.e. clay-sized) during sediment focusing processes. The results of my study show that there is biasing of ???Th-normalized sediment fluxes at sites that have experienced extremes in sediment redistribution both spatially and temporally. However, in general terms, the biasing remains insufficient to alter interpretations estimated by this method, especially when corrections of biasing can be made. Secondly, I have investigated the use of redox-sensitive authigenic uranium in marine sediments over the past 25 kyr in the Panama Basin. Authigenic U has been used previously as both a proxy for biologic productivity and bottom water oxygen content. However, the ability to directly separate out which process controls the distribution of authigenic U at our Panama Basin sites is essential to reconstructing past climate conditions. It appears at our Panama Basin sites, only 1 of the 4 cores analyzed remain unaffected by early sediment diagenesis (i.e. burn down of relict organic carbon and migration of the uranium signal to shallower or deeper depths). Thirdly, I have applied a multiproxy approach to investigate the relationship shared between global climate and the iron fertilization hypothesis on millennial time scales. The Panama Basin is a region where biologic productivity is limited by essential micronutrients (i.e. iron). I suggest that through increased deposition and dissolution of continentally-derived material (i.e. dust that is rich in iron), iron limitation within the Panama Basin is relaxed and biological productivity is increased. For the first time, I present multiple proxy records that suggest increases in productivity are correlated with increases in both upwelling and increased deposition of iron through the dissolution of dust. The timing of each of these increases is tightly linked to Northern Hemisphere Heinrich stadial events.
  • The objective of this dissertation is to understand the dynamics of U-Th isotope behavior in deep sea marine sediment cores extracted from the Panama Basin located in the eastern Equatorial Pacific. Specifically, I attempt to further our understanding of the potential biases inherent in these proxies which are used to reconstruct past global climate change (export production, continental aridity, atmospheric circulation, deep-ocean circulation) over the past several hundred thousand years.

    First, I have investigated the effects that biasing of specific grain size fractions during deep-sea sediment redistribution processes have on the ???Th-normalization technique. This method is used frequently in the paleoceanographic community to reconstruct past changes in a number of climate-related processes. Recent studies have shown that sediment fluxes calculated by the ???Th method may be biased because of the fractionation of ???Th-enriched fine-grained particles (i.e. clay-sized) during sediment focusing processes. The results of my study show that there is biasing of ???Th-normalized sediment fluxes at sites that have experienced extremes in sediment redistribution both spatially and temporally. However, in general terms, the biasing remains insufficient to alter interpretations estimated by this method, especially when corrections of biasing can be made.

    Secondly, I have investigated the use of redox-sensitive authigenic uranium in marine sediments over the past 25 kyr in the Panama Basin. Authigenic U has been used previously as both a proxy for biologic productivity and bottom water oxygen content. However, the ability to directly separate out which process controls the distribution of authigenic U at our Panama Basin sites is essential to reconstructing past climate conditions. It appears at our Panama Basin sites, only 1 of the 4 cores analyzed remain unaffected by early sediment diagenesis (i.e. burn down of relict organic carbon and migration of the uranium signal to shallower or deeper depths).

    Thirdly, I have applied a multiproxy approach to investigate the relationship shared between global climate and the iron fertilization hypothesis on millennial time scales. The Panama Basin is a region where biologic productivity is limited by essential micronutrients (i.e. iron). I suggest that through increased deposition and dissolution of continentally-derived material (i.e. dust that is rich in iron), iron limitation within the Panama Basin is relaxed and biological productivity is increased. For the first time, I present multiple proxy records that suggest increases in productivity are correlated with increases in both upwelling and increased deposition of iron through the dissolution of dust. The timing of each of these increases is tightly linked to Northern Hemisphere Heinrich stadial events.

publication date

  • August 2017