222Rn as a tracer of air-sea gas exchange across the river influenced northern Gulf of Mexico shelf

Large oceanic- river convergent zones are considered some of the major players in the global carbon cycling and fluxes. However the carbon fluxes in these coastal margins are poorly constrained in global budgets due to lack of estimates that can fully resolve the seasonality or spatial heterogeneity in CO₂ fluxes. Coastal ocean margins such as Louisiana shelf in the northern Gulf of Mexico that receive input from large rivers represent extremes of continental shelf systems in carbon cycling and fluxes. These coastal zones see higher productivity and carbon transport due to nutrient rich river water mixing with the ocean causing a draw down of surface pCO₂ and acting as an overall net sink of atmospheric CO₂. In order to understand the spatial and temporal variability in pCO₂ fluxes across the Louisiana shelf, the pCO₂ of surface water and air were measured along two transects across the Louisiana shelf during 2019 summer, in conjunction with ²²²Rn-²²⁶Ra disequilibria in the water column. The average gas transfer velocity of ²²²Rn, integrated over three-week timescale can be determined using the naturally-occurring radioactive gas, ²²²Rn, which is constantly produced by radioactive decay of ²²⁶Ra found in seawater and can be utilized to understand the air-sea transfer of CO₂. Preliminary data indicate high surface water pCO₂ of 1800 ppm and 600 ppm respectively near the Wax Lake outlet and Barataria Pass, two important regions of freshwater discharge in the region other than the Mississippi river, which progressively decreases with distant from the coast. Similarly, ²²²Rn activities were also found to decreases significantly offshore from 350 Bg/m³ to near zero about 50 miles offshore. Preliminary calculation indicate an overall negative pCO₂ fluxes in the region with a gradual decrease in magnitude progressively offshore.