A Global, Seasonally Resolved BGC-Argo Estimate of Deep Particulate Organic Carbon Flux

Quantifying the global magnitude and variability of the biological carbon pump's deep flux remains challenging due to sparse, historically limited sediment-trap observations. Biogeochemical Argo (BGC-Argo) floats drifting at 1000 m depth offer a scalable alternative, combining particulate backscattering (bbp) sensors, transmissometers used as optical sediment traps (OST), and, on a subset of floats, Underwater Vision Profiler 6 (UVP6) cameras resolving particle size distributions. Using observations from 500 BGC-Argo floats (2012-2026), we derive regional and seasonal climatologies of particulate organic carbon (POC) flux at 1000 m and develop log10-linear models relating daily-averaged bbp spike height to OST-derived flux. A general model, fit across regions, explains substantial variance (R2=0.54), but a region-specific model is required south of the Polar Front in the Southern Ocean (SPF_SO; R2=0.89), consistent with distinct, diatom-dominated export dynamics; without this isolation, SPF_SO flux would be underestimated by about a factor of two. Applying these models globally, we obtain regional and seasonal POC flux climatologies broadly consistent with independent sediment-trap observations. The resulting global annual POC flux at 1000 m is 0.95-1.34 PgC yr⁻1, of which nearly 65% is estimated to be sequestered for a century or more. These results demonstrate the potential of the growing BGC-Argo fleet to deliver a sustained, high-frequency, global view of deep-ocean carbon flux, complementing sparse trap coverage and supporting the monitoring of the current and future deep ocean biological carbon pump.