Constraints and Applications of U-Th Systematics to Climate Reconstructions

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 ²³⁰Th-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 ²³⁰Th method may be biased because of the fractionation of ²³⁰Th-enriched fine-grained particles (i.e. clay-sized) during sediment focusing processes. The results of my study show that there is biasing of ²³⁰Th-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

Constraints and Applications of U-Th Systematics to Climate Reconstructions

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 ²³⁰Th-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 ²³⁰Th method may be biased because of the fractionation of ²³⁰Th-enriched fine-grained particles (i.e. clay-sized) during sediment focusing processes. The results of my study show that there is biasing of ²³⁰Th-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

Mechanisms of doping graphene

We distinguish three mechanisms of doping graphene. Density functional theory is used to show that electronegative molecule like F4-TCNQ and electropositive metals like K dope graphene p- and n-type respectively. These dopants are expected to lead to a decrease in carrier mobility arising from Coulomb scattering but without any hysteresis effects. Secondly, a novel doping mechanism is exhibited by Au which dopes bilayer graphene but not single layer. Thirdly, electrochemical doping is effected by redox reactions and can result in p-doping by humid atmospheres and n-doping by NH3 and toluene.Comment: submitted to Physica Status Solid

Escherichia coli as a model active colloid:A practical introduction

The flagellated bacterium Escherichia coli is increasingly used experimentally as a self-propelled swimmer. To obtain meaningful, quantitative results that are comparable between different laboratories, reproducible protocols are needed to control, `tune' and monitor the swimming behaviour of these motile cells. We critically review the knowledge needed to do so, explain methods for characterising the colloidal and motile properties of E.coli, cells, and propose a protocol for keeping them swimming at constant speed at finite bulk concentrations. In the process of establishing this protocol, we use motility as a high-throughput probe of aspects of cellular physiology via the coupling between swimming speed and the proton motive force.Comment: 18 pages, 16 figures, 4 table