Multiple sulfur isotope analysis of volatile organic sulfur compounds and their sulfonium precursors in coastal marine environments

Abstract

Volatile methylated sulfur compounds emitted from terrestrial and aquatic ecosystems play a significant role in the global sulfur cycle, yet no satisfactory methods are available to trace their source and transformation in natural systems. Here we present a method for quantification and multiple sulfur isotopic analysis of a variety of volatile sulfur species as well as their natural precursors via hydrodesulfurization with a Raney nickel catalyst. The detection limit of this method for methanethiol (MT), dimethylsulfide (DMS), dimethyldisulfide (DMDS), and carbon disulfide (CS 2) is 0.2 mg of sulfur per sample. Average recovery of ~ 95% was attained for samples containing more than 1.3 mg of these sulfur compounds. Triplicate to quadruplicate sulfur isotopic analyses of reduced standard materials yield average standard deviations of 0.3‰, 0.02‰, and 0.1‰, respectively, for δ 34S, ∆ 33S, and ∆ 36S. The method developed here was used for determination of sulfur isotopic compositions of volatile organic sulfur compounds (VOSCs) and their precursor dimethylsulfoniopropionoate (DMSP) in sediment cores and a C 4 plant Spartina alterniflora collected from the Delaware Great Marsh. Application of the method to these natural samples indicates that the S-isotope compositions of VOSCs and DMSP-S are similar to, but slightly 34S-depleted (~0.6–0.9), relative to porewater sulfide. These compounds are 34S-enriched (~ 1.7–2.0‰) relative to the compositions of the coexisting sulfide. Both suggest a relationship between source sulfide and these organic sulfur compounds.