Estimating Air-Sea Fluxes of CO2 in the River Influenced Northern Gulf of Mexico Shelf Utilizing 226Ra-222Rn Disequilibria

Air-water CO₂ fluxes were estimated from the shallow shelf region of the Mississippi River-influenced northern Gulf of Mexico using a traditional wind-based approach and a ²²⁶Ra-²²²Rn disequilibria approach. Flux measurements were carried out from two contrasting regions of the shelf: the Wax Lake Delta (WLD, a propagating delta) and Barataria Bay (BB, a degrading delta) during early summer (high river flow) and fall (low river flow) in 2019. On average, surface water pCO₂ for WLD varied between 275–1,487 μatm in early summer and 381–1,267 μatm in fall, while BB was mostly CO₂ undersaturated, ranging from 212 to 289 μatm and 336 to 414 μatm, respectively. The freshwater discharge largely controlled pCO₂ in the near-shore region, whereas community metabolism was the dominant process farther offshore. Wind-based estimates of CO₂ fluxes varied between −1–28 and −9–0.2 mmol m⁻² d⁻¹ whereas radiometric-based estimates varied between −6–6 and −48–5 mmol m⁻² d⁻¹ for WLD and BB, respectively. The near-shore region was found to be a source of CO₂ for the atmosphere. Our results show that gas transfer velocities estimated by the radiometric approach were consistently higher than the wind-based approach at water depths of 20–40 m. This suggests that processes other than wind velocity, such as turbulence, fetch, and chemical enhancement, can play an important role in air-sea exchange of CO₂ in the shallow shelf region.