Preservation of biotic and palaeoenvironmental signatures in organosulfur compounds of immature fine-grained sedimentary rocks

Abstract

Organosulfur compounds (OSCs) in sedimentary rocks are considered to have formed primarily by abiotic incorporation of inorganic sulfur species into biogenetic functionalized molecules during early diagenesis, thus preserving carbon skeletons of appropriate biomolecules. OSCs could be characterized at a molecular level by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) even when they are small in amounts and large in molecular weights. Herein, to reveal the palaeoecological and palaeoenvironmental information that OSCs have recorded but might not be contained in other biomarkers, OSCs’ composition in solvent extracts of rocks with different geological histories were determined utilizing FT-ICR-MS. The analyzed natural laboratories are the marine Schöneck, Dynow and Posidonia formations and the lacustrine Wealden Formation which are in the late diagenetic or early catagenetic stages.The prevailing iron-deficient sulfidic depositional settings of the marine Posidonia and Schöneck formations are reflected by abundant OSCs bearing up to three sulfur atoms. The high ratios of reduced relative to oxidized forms (Sz versus SzOx) further illustrate the restricted presence of oxidants at the oxic-anoxic interfaces. The high average oxygen numbers of S1Ox compounds and the exclusive presence of nitrogenous OSCs (SzNy and SzNyOx) found in the marine rock extracts document the high abundances of polyoxygenated compounds and proteinaceous moieties in marine organisms that can be sulfurized. In contrast, the lacustrine organisms contain abundant sulfurizable aliphatic moieties.The observed prominent enrichment of Sz and SzOx compounds containing 40, 35, 30, 25 carbon atoms are associated with the selective and efficient preservation of polyfunctionalized biomolecules via sulfurization, such as C40 carotenoids, C35 bacteriohopanepolyols, C30 unsaturated tetracyclic polyprenoid alcohols, or C25 or C30 highly branched isoprenoid (HBI) polyenes. The strong enrichment of sulfurized C35 bacteriohopanepolyols can be developed as an indicator of the low levels of oxygen exposure prior to sulfurization, which occur only in the Dynow and Schöneck formations. The prominent enrichment of sulfurized carotenoids is generally associated with high primary productivity. While the strongly enriched sulfurized HBI polyenes in the Upper Schöneck Formation is indicative for diatom blooms, the precursors of C30 pentacyclic polyprenoid organosulfur compounds are more abundant in fresh/brackish water algae.