Carbon and sulfur isotopic composition of alkyl- and benzo-thiophenes provides insights into their origins and formation pathways

Abstract

Thiophenic compounds can provide significant geochemical information and may be used as paleoenvironmental indicators as long as their biological origins and formation pathways are understood. To this end, we investigated the structures, distributions, δ13C and δ34S values, of specific thiophenic compounds extracted from immature sedimentary rocks from the Upper Cretaceous (Ghareb Formation, Shefela Basin, Israel). Isoprenoidal alkylthiophenes (ATs) showed a general trend of depletion in their 13C and 34S values relative to the normal (linear) ATs extracted from the studied samples. In addition, a consistent enrichment of up to 2‰ in 13C and 8‰ in 34S of methylated ATs (m/z = 111), relative to non-methylated ones (m/z = 97), was recorded. This suggests that AT precursors derived from different organisms and diagenetic pathways, which later affected their sulfurization mechanisms. The large variation, of ∼15‰, in the δ34S values of individual ATs, along with the general trend of 13C depletion with increasing Tmax, suggests that ATs formed by a set of diagenetic processes of mild thermochemical alteration. The δ13C values of benzothiophenes (BTs) were enriched in 13C relative to the rest of the organic sulfur compounds; by ∼3‰ on average. This in turn suggests that BTs were generated mainly by sulfurization and subsequent ring closure, or aromatization of 13C-enriched aromatic/alkyl cyclohexane compounds with unsaturation and/or functional groups in their alkyl chain. Such 13C-enriched precursors can be generated from carotenoids and terrestrial compounds (e.g., by lignin degradation). The large variation in δ34S values of individual BTs, of ∼15‰, suggests that the BTs in our samples were formed during diagenesis and did not experience advanced thermal alteration processes. Therefore, their occurrence in immature sedimentary rocks might be used as a proxy for early thermochemical alteration that cannot be detected using conventional geochemical indicators, such as Tmax. The combination of structural, 13C and 34S investigations of individual organic sulfur compounds enhanced an understanding of their sulfurization pathways and origins, which in turn may expand their utilization as paleoenvironmental indicators.