Work

A plethora of hopanoid transformation products co-exist in a thermally immature Jurassic black shale I: hopanoid hydrocarbons

Year: 2025

Type: article

Abstract: Hopanoids are among the most abundant natural products on Earth. Despite their widespread occurrence in soils, coals, sediments, sedimentary rocks and crude oils, their diagenetic pathways have not been fully resolved. Here, we present a diverse suite of aliphatic and aromatic hopanoids from a single sample of a thermally immature Lower Jurassic black shale. All extended saturated and Δ 17,21 -unsaturated hopanoid species occur as 2α-methyl analogues, which exhibit a carbon number distribution matching that of the desmethyl hopanes. This suggests similar precursor organisms for 2-methyl and desmethyl hopanoids favouring the genesis of C 30+ hopanoids. Potential precursor organisms are assumed to include marine diazotrophic cyanobacteria or α-proteobacteria both affiliated with the nitrogen-cycle. Polyaromatic hopanoids occur in high concentrations, especially the C 24 tetraaromatic hopane, and deviate from the extended hopanoid species in terms of carbon number distribution and A-ring methylation, indicating different origins for these hopanoids. The high aromaticity of these hopanoids suggests an origin from soil bacteria, which is consistent with an accelerated hydrological cycle and evidence for enhanced soil erosion during the early Toarcian. The co-occurrence of a multitude of hopanoid species within one sample exemplifies the complexity of hopanoid transformation products during stages of very early diagenesis. Preservation of this hopanoid complexity during diagenesis in the geosphere over a period of 183 Ma is exceptional because labile hopanoids commonly are destroyed by thermal maturation. It must be assumed that other sediments contained comparably complex hopanoid distributions, which upon later diagenesis suffered reduced variability seen in most sedimentary rocks and oils.

Source: Organic Geochemistry

DOI: 10.1016/j.orggeochem.2025.105120

Authors: Tim Marten, Lorenz Schwark

Institutions: Christian-Albrechts-Universität zu Kiel, Curtin University

Language: English

Topic: Paleontology and Stratigraphy of Fossils

Subfield: Paleontology

Field: Earth and Planetary Sciences

Domain: Physical Sciences

Open Access status: hybrid

Estimated APC paid: $3,270

Funder: Deutsche Forschungsgemeinschaft