Improving the inverse modeling of a trace isotope: how precisely can radium-228 fluxes toward the ocean and submarine groundwater discharge be estimated?

International audienceRadium-228 (228 Ra), an almost conservative trace isotope in the ocean, supplied from the continental shelves and removed by a known radioactive decay (T 1/2 = 5.75 years), can be used as a proxy to constrain shelf fluxes of other trace elements, such as nutrients, iron, or rare earth elements. In this study, we perform inverse modeling of a global 228 Ra dataset (including GEOSECS, TTO and GEO-TRACES programs, and, for the first time, data from the Arc-tic and around the Kerguelen Islands) to compute the total 228 Ra fluxes toward the ocean, using the ocean circulation obtained from the NEMO 3.6 model with a 2 • resolution. We optimized the inverse calculation (source regions, cost function) and find a global estimate of the 228 Ra fluxes of 8.01-8.49 × 10 23 atoms yr −1 , more precise and around 20 % lower than previous estimates. The largest fluxes are in the western North Atlantic, the western Pacific and the Indian Ocean, with roughly two-thirds in the Indo-Pacific Basin. An estimate in the Arctic Ocean is provided for the first time (0.43-0.50 × 10 23 atoms yr −1). Local misfits between model and data in the Arctic, the Gulf Stream and the Kuroshio regions could result from flaws of the ocean circulation in these regions (resolution, atmospheric forcing). As radium is enriched in groundwater, a large part of the 228 Ra shelf sources comes from submarine groundwater discharge (SGD), a major but poorly known pathway for terrestrial mineral elements , including nutrients, to the ocean. In contrast to the 228 Ra budget, the global estimate of SGD is rather uncon-strained, between 1.3 and 14.7 × 10 13 m 3 yr −1 , due to high uncertainties on the other sources of 228 Ra, especially diffusion from continental shelf sediments. Better precision on SGD cannot be reached by inverse modeling until a proper way to separate the contributions of SGD and diffusive release from sediments at a global scale is found