Evidence for surface organic matter modulation of air-sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; gas exchange

M Ll Calleja,S Agustí,G J Herndl,Y T Prairie,C M Duarte

doi:10.5194/bg-6-1105-2009

Evidence for surface organic matter modulation of air-sea CO&lt;sub&gt;2&lt;/sub&gt; gas exchange

M Ll Calleja, S Agustí + Show 3 more

Open Access

https://doi.org/10.5194/bg-6-1105-2009

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Abstract

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.

Highlights

The role of air-sea exchange as a sink for atmospheric CO2 is assessed through a major concerted international effort to examine CO2 fluxes across the air-sea boundary layer
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
The dependency of the residuals of k with total surface organic matter concentration (TOC) content in the top cm were analyzed at different wind regimes, and we found the residuals to be strongly and significantly negatively correlated with TOC in the top cm of the ocean surface at wind speeds below 5 m s−1 (R2=0.86, p

Summary

Introduction

The role of air-sea exchange as a sink for atmospheric CO2 is assessed through a major concerted international effort to examine CO2 fluxes across the air-sea boundary layer. The calculation of air-sea CO2 flux (F CO2) is typically based on an indirect bulk method where data sets of global distributions of surface water CO2 partial pressure, increasingly available (Takahashi et al 1997, 2002), and wind speed (U ) are used as model input variables. Where pCO2 is the difference between CO2 partial pressure in the surface ocean and that in the lower atmosphere ( pCO2 = pCO2w − pCO2a) and represents the thermodynamic driving force that determines both the potential extent and direction of the flux. K is the gas transfer velocity, itself driven by near-surface turbulence in the water boundary layer (Donelan 1990), which represents the kinetic term and affects the magnitude of CO2 exchange. While pCO2 can be accurately obtained by in situ measurements, k is difficult to quantify and is, in practice, predicted from one of the multiple parameterizations deriving k from wind (U )

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