Radium as a tracer in the Arctic Ocean

Abstract

The Arctic Ocean is heavily influenced by continental margin sources, in part due to the wide continental shelves that make up over 50% of its area. Rising temperatures are causing shifts in the Arctic system that could increase communication between the shelves and the central basin, such as permafrost thawing, increased river discharge, and reduced ice cover. Radium isotopes are soluble in seawater and continuously produced through the decay of thorium in sediments, making them particularly useful tracers of shelf-derived material inputs to ocean basins. Here we show that²²⁸Ra(t_1/2= 5.75 y) activities in surface waters of the central Arctic Ocean have approximately doubled over the last decade, indicative of a substantial increase in shelf-ocean exchange processes. Activities of²²⁶Ra (t_1/2= 1600 y) have also increased by a smaller amount. Using a mass balance approach, we conclude that this change can only be explained by an increase in the shelf²²⁸Raflux, most likely due to increased wind-driven vertical mixing and the thawing of permafrost. Our findings imply that the fluxes of other shelf-derived materials, including nutrients and trace metals, have also increased, and could lead to increases in productivity of the Arctic Ocean.