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

Tuesday, 16 December 2014
Grecia Ivonne Lopez, Texas A & M University, College Station, TX, United States and Franco Marcantonio, Texas A&M University, College Station, TX, United States
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.