Radium Inputs Into the Arctic Ocean From Rivers: A Basin‐Wide Estimate

Radium isotopes have been used to trace nutrient, carbon, and trace metal fluxes inputs from ocean margins. However, these approaches require a full accounting of radium sources to the coastal ocean including rivers. Here, we aim to quantify river radium inputs into the Arctic Ocean for the first time for 226Ra and to refine the estimates for 228Ra. Using new and existing data, we find that the estimated combined (dissolved plus desorbed) annual 226Ra and 228Ra fluxes to the Arctic Ocean are [7.0–9.4] × 1014 dpm y−1 and [15–18] × 1014 dpm y−1, respectively. Of these totals, 44% and 60% of the river 226Ra and 228Ra, respectively are from suspended sediment desorption, which were estimated from laboratory incubation experiments. Using Ra isotope data from 20 major rivers around the world, we derived global annual 226Ra and 228Ra fluxes of [7.4–17] × 1015 and [15–27] × 1015 dpm y−1, respectively. As climate change spurs rapid Arctic warming, hydrological cycles are intensifying and coastal ice cover and permafrost are diminishing. These river radium inputs to the Arctic Ocean will serve as a valuable baseline as we attempt to understand the changes that warming temperatures are having on fluxes of biogeochemically important elements to the Arctic coastal zone. Plain Language Summary Radium (Ra) isotopes can be dissolved in water that comes into contact with minerals. Since they have varying half‐lives, the ratio of Ra isotopes in water masses can help us understand where and how long ago Ra became dissolved in the water. As a result, these isotopes are powerful tracers of freshwater and shelf inputs into the ocean, which are important for nutrient, carbon, and trace metal fluxes. However, quantifying these inputs requires an understanding of all other radium sources to the coastal ocean, including rivers. The net ocean input of Ra isotopes from rivers must consider the freshwater dissolved load, as well as Ra desorbed from suspended particles in river estuaries. This study quantifies river radium inputs into the Arctic Ocean using dissolved concentrations and desorption experiments on riverine sediment. We find that Arctic rivers may have removal processes impacting Ra transport through estuaries that have not been observed in other regions. We also estimate the total riverine Ra inputs to the global ocean by adding data from 20 major rivers. These findings show that ∼11% of all riverine Ra inputs to the global ocean come from Arctic rivers. Key Points This s