Dissolved and particulate Th-230-Th-232 in the Central Equatorial Pacific Ocean: Evidence for far-field transport of the East Pacific Rise hydrothermal plume
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2015 Elsevier B.V. We assess the distribution of 230Th and 232Th along a latitudinal gradient in the Central Equatorial Pacific Ocean (~155W-159W) at two sites: 8N and the equator. The dissolved 230Th concentration profile at 8N 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 increasing slightly 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 8N. The mid-depth-water deviation from equilibrium reversible scavenging between 2 and 3 km in the 230Th profiles (lower concentrations than expected) at both sites occurs in the interval of the water column that is consistent with an interval that has high concentrations of 3He and dissolved Fe at other nearby sites. This 3He- and Fe-rich signal has been traced to hydrothermal plumes from the East Pacific Rise, thousands of kilometers away. We hypothesize that the lower concentrations of 230Th in mid-depth waters of the Central Equatorial Pacific are a result of a 5000-km transit of waters that have had their 230Th scavenged by Fe-Mn particulates close to the EPR.Oceanic residence times of thorium combined with dissolved 232Th concentrations suggest dust fluxes of about ~0.5-0.6 gm-2yr-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.
