Contrasting organic carbon remineralisation rates revealed by particulate excess barium in the North Pacific and South China Sea

17 December 2025

Yuan and co-workers (2025, see reference below) quantify organic carbon (OC) remineralisation in the twilight zone (150-600 m) of the western North Pacific and the South China Sea (SCS) using particulate excess barium (PBa_xs) as a proxy. By developing a new PBa_xs-oxygen utilisation rate transfer function, the study constrains twilight zone OC remineralisation fluxes (F_{OC_remineral}) and reveals pronounced spatial heterogeneity across the study area. The dataset, acquired from multiple cruises including three GEOTRACES cruises — GP09, GPpr15 summer, and GPpr15 winter — shows substantially higher F_{OC_remineral} in the nutrient-rich North Pacific Transition Zone (NPTZ) than in the oligotrophic western North Pacific Subtropical Gyre (NPSG) and SCS. F_{OC_remineral} patterns also closely follow satellite-derived net primary production (NPP) and export production (EP), and higher euphotic zone biological pump carbon export efficiency (e‐ratio) coupled with lower twilight zone OC remineralisation ratio (r‐ratio) in the NPTZ indicates enhanced carbon sequestration potential relative to the NPSG and SCS. Overall, these results demonstrate the utility of PBa_xs as a proxy for twilight zone OC remineralisation and provide a framework for assessing cross-regional variability in the biological pump.

**Figure 1.** Relationship of depth‐weighted average particulate excess barium (PBaxs‐DWA) to depth‐weighted average oxygen utilisation rate (OURDWA) between 150 and 600 m water depth along the GEOTRACES‐GP09 section.

**Figure 2.** Spatial distribution of twilight zone organic carbon remineralization flux (F_{OC_remineral}; 150-600 m), euphotic zone export production (EP), and euphotic zone net primary production (NPP) across different regions of the North Pacific, including the western North Pacific Subtropical Gyre (NPSG), North Pacific Transition Zone (NPTZ), South China Sea (SCS), station ALOHA, and Site K2. Error bars indicate one standard deviation of the mean, mainly reflecting spatial variability among sampling stations within a given region.

Reference:

Yuan, Y., Zhao, S., Lin, W., Li, Y., Yu, J., Huang, Y., et al. (2025). Quantifying organic carbon remineralization in the twilight zone of the western North Pacific using particulate excess barium. Global Biogeochemical Cycles, 39, e2025GB008755. Access the paper: 10.1029/2025GB008755

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