Geochemical formation of organosulphur compounds (thiols) by addition of H2S to sedimentary organic matter

Abstract

Organosulphur compounds constitute a significant fraction of organic matter in both recent and ancient iron-poor marine sediments as well as in petroleum generated in such sediments1,2. Most studies on organosulphur compounds in marine sediments have focused on hydrophobic compounds, for example, thiophenes3–6, sulphur-containing hopanoids7 and volatile compounds. Recently, hydrophilic, organic thiols were found in significant concentrations in anoxic marine sediments8,9. Several studies10–13 have shown that H2S formed from bacterial sulphate reduction can become incorporated into organic matter during early diagenesis, however, the exact chemical mechanisms by which this sulphur reacts with sedimentary organic matter are not well understood. Here we describe the widespread occurrence of 3-mercaptopropionic acid in coastal marine sediments (Biscayne Bay, Florida) and present evidence for its formation by an abiotic reaction between hydrogen sulphide and acrylic acid, which is a cleavage product of the common algal osmolyte β-dimethylsulphoniopropionate14,15. The reaction probably occurs by nucleophilic addition of bisulphide (HS−) ion to the activated double bond in the α,β-unsaturated car bony 1 system of acrylic acid. Reactions between hydrogen sulphide and activated unsaturated bonds in organic molecules are probably responsible for the formation of other thiols in sediments and could represent a major pathway for the incorporation of sulphur into sedimentary organic matter during early diagenesis.