Summary
Surface sediments (0-25 cm) from Ace Lake (eastern Antarctica), a saline euxinic lake, were analyzed to study the early incorporation of reduced inorganic sulfur species into organic matter. The apolar fractions were shown to consist predominantly of dimeric (poly)sulfide linked C-27-C-29 steroids. These steroid moieties were identified by GC-MS analysis of the apolar fractions after cleavage of polysulfide linkages using MeLi and Mel and after desulfurisation. The polar fractions contained the oligomeric analogues. The S-bound steroids are most likely formed by sulfur incorporation into steroidal ketones formed from Delta(5) sterols by biohydrogenation by anaerobic bacteria. The concentrations of these sulfurised steroids increased with depth in the sediment. The sulfurisation reaction is completed in 1000-3000 years. Despite a wide range of functionalised lipids present in these sediments that are potentially available for sulfurisation, there is a very strong preference for the incorporation of sulfur into steroidal compounds. A predominance of sulfurised C-27 steroids contrasted with the distribution of free sterols, which showed a strong predominance of C-29 sterols. This indicates that the incorporation of sulfur is biased towards C- 27 sterols. The results demonstrate that intermolecular sulfurisation of organic matter can occur in surface sediments at low temperatures and in the absence of light.