Lopez, Grecia Ivonne (2015-08). Dissolved and Particulate 230Th - 232Th Systematics in the Central Equatorial Pacific Ocean : Evidence for Far-Field Transport of the East Pacific Rise Hydrothermal Plume. Master's Thesis.
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.