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

• Published in: Biogeosciences
• Main Authors: Le Gland, Guillaume, Memery, Laurent, Aumont, Olivier, Resplandy, Laure
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Gesellschaft Mbh 2017
• Subjects: Arctic; Arctic Ocean; Indian; Kerguelen; Kerguelen Islands; Pacific; North Atlantic
• Online Access: https://archimer.ifremer.fr/doc/00616/72811/72571.pdf; https://archimer.ifremer.fr/doc/00616/72811/72572.pdf; https://doi.org/10.5194/bg-14-3171-2017; https://archimer.ifremer.fr/doc/00616/72811/

Radium-228 (Ra-228), 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 Ra-228 dataset (including GEOSECS, TTO and GEOTRACES programs, and, for the first time, data from the Arctic and around the Kerguelen Islands) to compute the total Ra-228 fluxes toward the ocean, using the ocean circulation obtained from the NEMO 3.6 model with a 2 degrees resolution. We optimized the inverse calculation (source regions, cost function) and find a global estimate of the Ra-228 fluxes of 8.01-8.49 x 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 x 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 Ra-228 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 Ra-228 budget, the global estimate of SGD is rather unconstrained, between 1.3 and 14.7 x 10(13) m(3) yr(-1), due to high uncertainties on the other sources of Ra-228, 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.