Dissolved Inorganic Carbon Export, Biological Pump Efficiency, and Mesopelagic Carbon Sequestration in the South Atlantic Subtropical Gyre: Results From a 15-Month Time-Series Mooring

This study investigates dissolved inorganic carbon export flux, biological pump efficiency, and seasonal carbon sequestration patterns in the South Atlantic Subtropical Gyre mesopelagic zone from a 15-month sediment trap and sensor mooring within the context of marine biogeochemistry and carbon cycle research, an area of growing scientific importance given its implications for Southern Ocean carbon budget, IPCC biological pump parameterization update, and carbon sequestration potential assessment for South Atlantic marine protected area design. Using sediment trap particulate organic carbon (POC) flux quantification, BGC-Argo float pCO2 and O2 measurements for net community production, DIC flux from surface to 1000 m computed by mass balance, and Martin curve b-value biological pump efficiency parameterization, we examine seasonal deepening of mixed layer in austral winter entraining nutrients stimulating phytoplankton blooms that drive sinking POC export below the permanent thermocline, with efficient biological pump transferring carbon to mesopelagic sequestration depths >500 m in 15-month time series (July 2022 - September 2023) with monthly sediment trap recovery at 3 depths, 6 BGC-Argo floats profiling at 10-day intervals (n=284 profiles), and meteorological buoy forcing data drawn from South Atlantic Subtropical Gyre center (24 S, 22 W) — a subtropical oligotrophic region with seasonal mixed layer variability driven by winter deep mixing (Oct-Mar southern summer). Results indicate that annual mean POC flux at 150 m = 2.84 mmol C/m2/day with austral winter peak 4.84 mmol/m2/day; Martin b-value 0.84 (efficient pump, vs. 1.04 subtropical North Atlantic); annual carbon sequestration = 18.4 g C/m2/yr with 38.4% buried below 1000 m (p < 0.001), with POC flux 2.84 mmol/m2/day mean; b=0.84 vs. 1.04 N. Atlantic; 38.4% below 1000 m as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to marine biogeochemistry and carbon cycle research and carry actionable implications for the design of programs and policies targeting Southern Ocean carbon budget, IPCC biological pump parameterization update, and carbon sequestration potential assessment for South Atlantic marine protected area design.