| Abstract: | The goals of this thesis are to assess the distribution of ²³⁰Th and ²³²Th along a latitudinal gradient in the Central Equatorial Pacific Ocean (~155° W - 159°W) at two sites: 8°N and the equator. In so doing, I test for (1) the extent of advection or diffusion of dissolved ²³⁰Th from the oligotrophic North Pacific gyre (low particle flux) to the more productive equatorial region (high particle flux), and (2) the efficacy of using ²³²Th - ²³⁰Th concentration systematics as a proxy for estimating dust fluxes to the sea surface in Central Equatorial Pacific Ocean. The dissolved ²³⁰Th concentration profile at 8°N increases nearly linearly from the surface to 2000 m, exhibiting behavior consistent with thermodynamic reversible scavenging. However, from 2000 m to 3000 m, the dissolved ²³⁰Th concentrations exhibit little change, before linearly increasing once more from 3000 m to the bottom. At this site dissolved ²³⁰Th concentrations range from 1.1 fg/kg at 100 m to 55.2 fg/kg at 4600 m. At the equator, dissolved ²³⁰Th concentrations are slightly lower, and range from undetectable at 25 m to 19.1 fg/kg at 3038 m. The pattern in the dissolved ²³⁰Th concentration profile at the equator is indistinguishable from that at 8°N. The deep-water deviation from linearity between 2 and 3 km in the ²³⁰Th profiles (lower concentrations than expected) at both sites occur in the interval of the water column that has the highest concentrations of ³He and dissolved Fe. This ³He- and Fe-rich signal has been traced to hydrothermal plumes from the EPR, thousands of km away. We hypothesize that the lower concentrations of ²³⁰Th in mid-depth waters of the Central Equatorial Pacific are a result of scavenging of water-column ²³⁰Th by Fe-Mn particulates transported within the EPR hydrothermal plume during its 5000 km transit from the EPR to 160° W. Oceanic residence times of thorium combined with dissolved ²³²Th concentrations suggest dust fluxes of about ~ 0.5 - 0.6 g m^-2 yr^-1 to the sea surface. These fluxes are in agreement with other empirical studies in the Pacific, but are higher than those suggested by global atmospheric circulation models. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/155633 |
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