The role of suspended particulate matter in 234Th scavenging and 234Th-derived export fluxes of POC in the Canada Basin of the Arctic Ocean

Abstract

The concentration and composition of suspended particulate matter (SPM) plays a major role in the removal of particle-reactive nuclides in the ocean. In the Arctic Ocean, SPM concentrations in the deep basins vary widely, over an order of magnitude. Unique features, such as the presence and release of algae from sea ice to the upper waters may result in differing scavenging behavior of suspended particulate matter. In this study, we collected a suite of water samples across a horizontal transect in the western Arctic Ocean, from the coast to the deep Canada Basin, and measured concentrations of SPM, particulate organic carbon (POC) and nitrogen, and activities of dissolved and particulate 234Th. Our data indicate that the concentrations and inventories of SPM is about an order of magnitude higher than those reported for the permanently ice-covered Arctic region, but comparable to the value reported recently for another deep station in the Canada Basin. There is disequilibrium between 238U and total 234Th in the upper waters (∼100 m) indicating active scavenging in the deep basins. Variations in the specific activity of 234Th in the upper ∼100 m indicate differences in the remineralization rates in the deep Canada Basin. Plots between particulate and dissolved removal rate constants and SPM indicate that the biochemical composition of particulate matter could play a significant role on the removal rates of Th. The POC export rate calculated for the upper 50 m of the deep Canada Basin was found to be 1.4 mmol m −2 day −1. The residence times and removal fluxes of particulate and dissolved 234Th calculated for the upper 50 m in our study is compared to the calculated values from published data for other regions in the Arctic Ocean.