Quantifying Organic Carbon Remineralization in the Twilight Zone of the Western North Pacific Using Particulate Excess Barium

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Abstract The twilight zone plays a pivotal role in the oceanic carbon cycle because large amounts of organic carbon (OC) exported from the euphotic zone are degraded within this layer. However, the magnitude and variability of this remineralization flux remain incompletely understood, particularly across varying regional scales. Using a new particulate excess barium (PBa xs )‐oxygen utilization rate transfer function, this study examines spatial patterns of twilight zone OC remineralization fluxes (F OC_remineral ) in the western North Pacific (wNP) and the South China Sea (SCS). Results reveal a pronounced latitudinal gradient in wNP open waters, with PBa xs concentrations and F OC_remineral in the upper 150–600 m increasing from the oligotrophic western North Pacific Subtropical Gyre (NPSG) to the nutrient‐rich North Pacific Transition Zone (NPTZ). Despite its subtropical location, the SCS basin shows values comparable to those of the NPTZ. Satellite‐derived net primary production (NPP) and export production (EP) distributions generally align with F OC_remineral patterns, indicating a strong association between upper and mesopelagic ocean carbon cycling. Using NPP, EP, and F OC_remineral data, we estimated the twilight zone OC remineralization ratio (r‐ratio) relative to the euphotic zone biological pump carbon export efficiency (e‐ratio). The NPTZ shows enhanced carbon storage potential, with higher e‐ratio and lower r‐ratio values, while the western NPSG exhibits an opposite trend, indicating reduced carbon sequestration capacity. This spatial variability underscores the need for a comprehensive understanding of twilight zone OC remineralization and its connection to upper‐ocean carbon cycling, in order to accurately assess the biological pump's role in the long‐term pelagic carbon sink.

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