Tropical mangrove forests as a source of dissolved rare earth elements and yttrium to the ocean

Rare earth elements (REEs) and Y, or REYs, are commonly used as geochemical proxies for water chemistry, history of the continental crust and provenance studies. At the continent-ocean interface, the estuarine geochemistry of REYs is commonly thought to be driven by large-scale removal of the dissolved fraction. Consequently, contributions of river-borne dissolved REYs to the marine budget are assumed to represent a minor fraction of the total flux. Here, we report a significant release of dissolved REYs, together with a fractionation between light REEs, heavy REEs and Y, in the tropical mangrove estuaries of New Caledonia. These observations were associated with biogeochemical processes of the redox-dynamic mangrove system, rather than a possible contribution of REYs from a fresh groundwater source, based on stable water isotope compositions. These findings imply that tropical mangrove estuaries may act as a sizeable source of REYs to the ocean rather than buffer zones, at least for the dissolved fraction. We also extrapolated our data to compute the contributions of dissolved REY fluxes from the mangroves to global oceans. This preliminary calculation suggests that the mangrove system supplies 2.6-5% of global river-borne dissolved Nd, an REE with the most comprehensive mass balance. Therefore, given that the ocean mass budget of REYs remains unbalanced with fluxes missing on a global scale, considering the mangrove system as an input of oceanic mass balance models may help improve our understanding of the global distribution of REYs.