Tracing twilight zone organic carbon remineralization and paleoproductivity with particulate barium proxies: insights and limitations

The biological pump is a fundamental component of the oceanic carbon cycle, in which export production from the euphotic zone and subsequent organic carbon remineralization in the twilight zone jointly regulate carbon sequestration in the ocean interior. However, the magnitude, spatial variability, and tracers of these coupled processes remain incompletely understood. Here, we investigate the linkage between export production, twilight zone remineralization, and particulate barium in the western North Pacific (wNP) and the South China Sea (SCS). Organic carbon remineralization fluxes in the twilight zone (150-600 m) are quantified using a newly developed transfer function relating particulate excess barium (PBa_xs) to oxygen utilization rates, revealing pronounced spatial heterogeneity, with PBa_xs concentrations and remineralization fluxes increasing from the subtropical gyre to the North Pacific transition zone. Satellite-derived net primary production (NPP) and export production (EP) exhibit spatial patterns broadly consistent with the inferred remineralization fluxes, indicating a strong association between upper-ocean productivity and mesopelagic carbon degradation. Estimates of the e-ratio and r-ratio based on NPP, EP, and remineralization fluxes demonstrate contrasting biological pump efficiencies, with low e-ratios and high r-ratios in the subtropical gyre reflecting weak carbon sequestration, and high e-ratios and low r-ratios in the transition zone indicating a more efficient biological pump. We further evaluate the potential of particulate barium isotopes as tracers of EP by establishing a calibration between twilight zone particulate barium isotopic composition and euphotic-zone EP in the modern ocean, which reveals a significant negative relationship. However, this relationship does not persist in sedimentary archives: barium isotopic compositions show no systematic response to glacial-interglacial variations in paleoproductivity, and EP reconstructed using the modern calibration exhibits no correlation with sedimentary total organic carbon fluxes. Overall, this study provides an integrated assessment of the applicability and limitations of barium-based proxies from the water column to sediments, highlighting the tight association between Ba, export production, and twilight zone remineralization while emphasizing the challenges and limitations in extending modern barium-based proxies to reconstruct past biological pump dynamics.