Quantifying contrasting nutrient and dissolved organic carbon transport in mesoscale eddies using dual water isotopes

Abstract Mesoscale eddies are ubiquitous swirling oceanic structures. These dynamic processes play a critical role in supplying biogenic elements to the upper ocean to stimulate new productivity, thereby dominating regional marine productivity and biogeochemical processes. Despite their importance in shaping marine productivity and carbon export, the lack of quantitative understanding of the vertical transport of the biogenic elements in mesoscale eddies hinders advances in our understanding of the eco-environmental effects and efficient carbon export mechanisms. This study addresses this gap by employing dual water isotopes (δD and δ18O) to trace water mass mixing and quantify nutrient and dissolved organic carbon (DOC) transport within a cold and a warm eddy pair in the western North Pacific Ocean. Our results reveal that upwelling driven by cold eddies can provide over 70% of dissolved inorganic nutrients to the upper ocean (<200 m), which is much higher than the contribution of in situ subsurface water (∼20%), resulting in nutrient concentrations nearly double those in the warm eddy. By contrast, the warm eddy contributed little to nutrient enrichment but transported a substantial amount of surface-derived DOC (>40%) to deeper layers (∼200 m). These quantitative estimates provide novel insights into the complementary roles of mesoscale eddies, where cold eddies stimulate new production and warm eddies enhance the vertical export of carbon (e.g. through DOC subduction), thereby deepening our understanding of their collective impacts on marine carbon cycling.