Award: OCE-1434479

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, concentration of these substances varies inversely with the intensity of scavenging. We have measured concentrations of 230Th and 231Pa throughout the western Arctic Ocean to determine the rate of scavenging and compare the results from the Arctic with our previous data from the Atlantic and Pacific Oceans. We found that the intensity of scavenging throughout the western Arctic Ocean is greater than observed previously in the Atlantic or Pacific Oceans. We attribute this greater intensity in the Arctic Ocean to the widespread presence of manganese dioxide coatings on particles in the Arctic. This reflects the mobilization of manganese in Arctic margin sediments by reductive dissolution of manganese dioxide caused by the high flux of organic matter to Arctic margin sediments. On average, we find that the intensity of scavenging in the Arctic Ocean is five times greater than that of the North Atlantic Ocean. Therefore, concentrations of 230Th are lower throughout the Arctic Ocean than in the Atlantic even though the concentrations of particles that are responsible for scavenging are lower in the Arctic Ocean due to the extensive ice cover that prevents production of biogenic particles and input of lithogenic particles from dust or from rivers. This shows the importance of manganese mobilization from sediments and the resulting manganese dioxide coatings of particles throughout the basin. A special case that illustrates the importance of the manganese oxide coating on particles can be found in the halocline, the depth interval between about 100 and 200 m in the Arctic Ocean, filled with water that has accumulated high concentrations of manganese as it passed over the continental shelf north of Alaska and Siberia. Within the halocline we find the intensity of scavenging of 230Th and 231Pa to be as much as a factor of five greater than in waters above or below this depth interval. This illustrates the importance of manganese oxide as an agent that removes dissolved insoluble substances from the ocean. In a separate part of our study we looked at the dissolved 232Th concentration in waters of the Transpolar Drift (TPD), which occupies the upper 50 m of the water column along a broad swath that exits the Siberian shelf, crosses the North Pole, and then flows out of the Arctic Ocean past Greenland. Within the TPD we find that the concentrations of dissolved 232Th are much greater than in surrounding regions and furthermore the concentrations are positively correlated with the fraction of meteoric water (i.e., from precipitation). We attribute this correlation to the supply of dissolved 232Th by the large Siberian rivers that feed meteoric water into the TPD. Furthermore, the concentration of dissolved 232Th is positively correlated with the concentration of dissolved organic carbon. Future studies will test whether this correlation is caused by stabilization of dissolved 232Th by organic matter against the scavenging processes that remove it from the water column. Last Modified: 05/17/2020 Submitted by: Robert Anderson