Award: OCE-1737224

Insoluble substances are removed from the ocean by adsorption to sedimenting particles, a process that oceanographers refer to as scavenging. The intensity, or rate, of scavenging can be monitored by measuring the concentrations of dissolved thorium and protactinium isotopes, 230Th and 231Pa, which are produced at a known rate throughout the ocean by radioactive decay of dissolved uranium. Therefore, concentrations of these substances vary inversely with the intensity of scavenging. This award provided support for one of three collaborating labs (University of Minnesota, University of Southern Mississippi and Lamont-Doherty) to undertake measurements on the US GEOTRACES Pacific Meridional Transect (PMT) of the dissolved and particulate concentrations of230Th,232Th and231Pa.Full water column depth profiles of each of these isotopes have been measured at each full-depth station along the ship track at 152 west from Alaska to Tahiti.In addition, we have analyzed surface sediments collected along the transect for these radionuclides. One of the principal hypothesis motivating our work was to test the idea that scavenging would be more intense throughout the Subarctic gyre, from the Alaska coastal current south to about 40 north, than within the Subtropical gyre, between about 10 north and 30 north. Our results supported this hypothesis, which relates the greater scavenging intensity within the Subarctic gyre to the community of phytoplankton that live there, but we also found that throughout the entire transact, the scavenging intensity in the upper 2000 m of the water column was inversely proportional to the abundance of particles, regardless of their source. Higher biological productivity within the Subarctic gyre produces a greater abundance of particles, and this is a major factor influencing the difference in scavenging intensity between these two major biogeographic resumes.In addition, we found a greater abundance of particles, mirrored by more intense scavenging, around the equator and at the confluence between the Subarctic and Subtropical gyres.This tight relationship between scavenging intensity and particle abundance was not surprising, but to our knowledge it has never been demonstrated as clearly as with this data set. Another objective of this work was to determine whether or not there was any evidence at 152 west of far field removal, or scavenging, of heavy metals by hydrothermal plumes. We did see evidence of enhanced removal within the plumes coming from the East Pacific rEse, thousands of kilometers to the east, at a depth centered at about 2500 m, but we saw no evidence for enhanced scavenging associated with hydrothermal activity at the Juan de Fuca Ridge or even very close to the Loihi Seamount. Nowhere along the section at 152 west, from Alaska to Tahiti, did we see the signs of intense near bottom removal, or scavenging, like we saw in the South Pacific Ocean, or in the north west Atlantic Ocean.Some stations showed evidence for mild near bottom removal and some stations showed evidence for a near bottom source. We are still investigating the processes responsible for these features that are characteristic of the deepest 1000 m of the water column in the north and equatorial Pacific Ocean. Last Modified: 01/28/2024 Submitted by: RobertFAnderson