Contrasting Pathways of Organic Matter Dynamics in a Pair of Mesoscale Eddies in the Northwestern Pacific Ocean and Their Implications for Carbon Sequestration

Mesoscale eddies are active physical features of the ocean that exert a strong influence on the biogeochemical cycling of organic matter. However, their integrated roles in structuring coupled particulate and dissolved organic matter (POM and DOM) dynamics remain poorly understood, limiting the mechanistic understanding of their net effect on carbon export and sequestration. To address this knowledge gap, we conducted a multi‐parameter investigation across a cold‐warm pair of mesoscale eddies in the northwestern Pacific Ocean, combining hydrographic observations with stable isotope analyses of POM (δ 13 C‐POC and δ 15 N‐PN) and optical characterization of chromophoric and fluorescent dissolved organic matter (CDOM and FDOM). Our results show that the cold and warm eddies represent two contrasting regimes of organic matter production and transformation. The cold eddy, despite enhanced nutrient supply through upwelling that stimulated the production of fresh phytoplankton‐derived POM, exhibited relatively low inventories due to rapid biological degradation, indicative of a high‐turnover process. In contrast, the warm eddy functioned as a degradation reactor, where downwelling and subduction of surface organic matter and rich‐oxygen water fueled elevated microbial reprocessing, as evidenced by 13 C‐enriched particulate organic carbon and enhanced production of recalcitrant humic‐like DOM. These findings indicate that in the specific pair of eddies investigated, cold and warm eddies can support complementary carbon pathways, with cold eddies promoting a rapid biological pump and warm eddies fostering microbial reprocessing that shifts organic matter toward the dissolved pool. The relative importance of these pathways is therefore likely to influence the carbon sequestration potential of eddy‐rich regions in the open ocean.