Evidence for surface organic matter modulation of air-sea CO₂ gas exchange

Abstract. Air-sea CO₂ exchange depends on the air-sea CO₂ gradient and the gas transfer velocity (k), computed as a function of wind speed. Large discrepancies among relationships predicting k from wind suggest that other processes also contribute significantly to modulate CO₂ exchange. Here we report, on the basis of the relationship between the measured gas transfer velocity and the organic carbon concentration at the ocean surface, a significant role of surface organic matter in suppressing air-sea gas exchange, at low and intermediate winds, in the open ocean, confirming previous observations. The potential role of total surface organic matter concentration (TOC) on gas transfer velocity (k) was evaluated by direct measurements of air-sea CO₂ fluxes at different wind speeds and locations in the open ocean. According to the results obtained, high surface organic matter contents may lead to lower air-sea CO₂ fluxes, for a given air-sea CO₂ partial pressure gradient and wind speed below 5 m s⁻¹, compared to that observed at low organic matter contents. We found the bias in calculated gas fluxes resulting from neglecting TOC to co-vary geographically and seasonally with marine productivity. These results support previous evidences that consideration of the role of organic matter in modulating air-sea CO₂ exchange may improve flux estimates and help avoid possible bias associated to variability in surface organic concentration across the ocean.