Effects of Hydrothermal Scavenging of 230Th in the Eastern Equatorial Pacific Translated to the Deep Waters of the Central Equatorial Pacific

Abstract:

Although we were not involved in the 2013 US GEOTRACES zonal transect in the eastern tropical south Pacific, our results in the central tropical Pacific are complementary in that they shed light on the extensive hydrothermal plume emanating from the East Pacific Rise (EPR). Specifically, we have analyzed dissolved ²³⁰Th concentrations at high-resolution within the water column at two locations (ML1208-12CTD; 8° 19.989’ N, 159° 18.000’ W, and ML1208-03CTD; 00° 13.166’ S, 155° 57.668’ W) sampled as part of a cruise to the Line Islands. The pattern of the dissolved ²³⁰Th concentration profile at 8°N is essentially linear from the surface to 2000 m and generally follows a reversible scavenging model. However, from 2000 m to 3000 m, the dissolved ²³⁰Th concentrations are constant, before linearly increasing again from 3000 m to the bottom. At this site dissolved ²³⁰Th concentrations range from 1.06 fg/kg at 100 m to 55.15 fg/kg at 4600 m. At the equator, dissolved ²³⁰Th concentrations are slightly lower, and range from undetectable at 25 m to 19.07 fg/kg at 3038 m. A nearly indistinguishable pattern in dissolved ²³⁰Th concentrations occurs in the profile at the equator compared to that from 8°N. The deep-water deviation from linearity between 2 and 3 km in the ²³⁰Th profiles (lower concentrations than expected) at both sites coincides well with the interval of the water column which has the highest concentrations of ³He. This ³He-rich signal has been traced to hydrothermal plumes from the EPR, thousands of km away (Lupton et al., 1998). We hypothesize that the lower concentrations of ²³⁰Th in deep waters of the central equatorial Pacific are a result of: 1) scavenging of water-column ²³⁰Th by Fe-Mn particulates contained within the EPR hydrothermal plume, and 2) lateral export of these ²³⁰Th-deficient deep waters approximately 7000 km westward. We will discuss the implications that the transport of this signature across vast distances has on water residence and transport times.

Reference: Lupton, J. (1998) J. Geophys. Res. 103, 15,853-15,868.