Sulfur isotope homogeneity of oceanic DMSP and DMS
2013
Amrani, Alon | Said-Ahmad, Ward | Shaked, Yeala | Kiene, Ronald P.
Oceanic emissions of volatile dimethyl sulfide (DMS) represent the largest natural source of biogenic sulfur to the global atmosphere, where it mediates aerosol dynamics. To constrain the contribution of oceanic DMS to aerosols we established the sulfur isotope ratios (³⁴S/ ³²S ratio, δ ³⁴S) of DMS and its precursor, dimethylsulfoniopropionate (DMSP), in a range of marine environments. In view of the low oceanic concentrations of DMS/P, we applied a unique method for the analysis of δ ³⁴S at the picomole level in individual compounds. Surface water DMSP collected from six different ocean provinces revealed a remarkable consistency in δ ³⁴S values ranging between +18.9 and +20.3‰. Sulfur isotope composition of DMS analyzed in freshly collected seawater was similar to δ ³⁴S of DMSP, showing that the in situ fractionation between these species is small (<+1‰). Based on volatilization experiments, emission of DMS to the atmosphere results in a relatively small fractionation (−0.5 ± 0.2‰) compared with the seawater DMS pool. Because δ ³⁴S values of oceanic DMS closely reflect that of DMSP, we conclude that the homogenous δ ³⁴S of DMSP at the ocean surface represents the δ ³⁴S of DMS emitted to the atmosphere, within +1‰. The δ ³⁴S of oceanic DMS flux to the atmosphere is thus relatively constant and distinct from anthropogenic sources of atmospheric sulfate, thereby enabling estimation of the DMS contribution to aerosols.
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