Surfactants and chromophoric dissolved organic matter (CDOM) in the Atlantic Ocean surface microlayer and the corresponding underlying waters

Abstract

The sea surface microlayer (SML; depth < 400 μm) is a physically and biogeochemically distinct interface covering the entire ocean surface. Biologically-derived surfactants are ubiquitous in the SML, where they limit air-sea gas exchange and the formation of marine boundary layer aerosols that impact atmospheric chemistry and climate. Total surfactant activity (SA) and chromophoric dissolved organic matter (CDOM) were measured in the SML, in depth profiles (≤ 100 m) and semi-continuously in sub-surface water (SSW: 7 m non-toxic seawater supply) on Atlantic Meridional Transect (AMT) cruises 24 (2014) and 25 (2015), from 50°N to 50°S. On-board estimates of the gas transfer velocity (kw) of CH4 (custom gas exchange tank) were related to SA distributions in the SML to evaluate surfactant control of air-sea gas exchange. SML and SSW SA (mg L-1 eq. T-X-100) was always higher in the Northern Hemisphere than in the Southern Hemisphere (0.10 - 1.76 in the Northern Hemisphere; 0.08 - 0.63 in the Southern Hemisphere). A constant enrichment of SA in the SML was observed at all wind speeds encountered. SA enrichment factors (EF = SASML/SASSW) ranged between 0.95 – 4.25 in the Atlantic Ocean, higher in the Northern Hemisphere than in the Southern Hemisphere. EF >1 up to the maximum mean wind speed recorded (~13 m s-1) challenges the idea that high latitude wind speeds > 12 m s-1 preclude high EFs and implies that the SML is self-sustaining concerning SA. CDOM absorption coefficient (a300) in general was higher in the Northern Hemisphere (range 0.10 - 1.52 m-1) than in the Southern Hemisphere (range 0.17 - 0.82 m-1). CDOM spectral slope (S275-295) showed an inverse correlation with CDOM (a300) and was significantly lower (t-test, p < 0.001) in the SML than in the SSW (SML; 0.033 ± 0.005 nm-1, SSW; 0.038 ± 0.007 nm-1) suggesting in-situ CDOM production in the SML and more refractory CDOM in the SSW. CH4 k660 (kw for CO2 in seawater at 20°C) derived from the gas exchange tank (6.9 - 9.8 cm h-1) gave film factors (R660´; sample kw / surfactant-free MilliQ kw) that strongly correlated with SML SA (r2 = 0.63, p = 0.001, n = 13). Corresponding R660´ suppressions ~ 25% imply a strong control of Atlantic Ocean gas exchange by surfactant.