Microbial communities constrain the organic δ13C variations in the Lower Cambrian mudstones

Abstract

The Lower Cambrian organic-rich mudstones preserve extensive records of microbial life, potentially contributing to the ‘Cambrian explosion’ (∼540 to 520 Ma) and the formation of ancient petroleum resources. However, the link of organic δ¹³C variations to microbial communities in the Lower Cambrian Formation within the Tarim Craton, part of the Central Asian Orogenic Belt, remains incompletely understood. Here, we conducted a comprehensive biomarker and δ¹³C analysis of mudstone extracts. The prevalence of eukaryotic organisms relative to prokaryotes, together with increased primary productivity, results in substantial ¹²C enrichment in organic matter, with the converse relationship being equally valid. The Lower Cambrian mudstones formed in more reducing yet confined stratified environments. The presence of cholestane, ergostane, stigmasterane, and 4-methylsterane (e.g., dinosterane) indicates the occurrence of red algae, prasinophytes, green algae, and dinoflagellates, respectively. Notably, the predominance of ergosterane and stigmasterane over cholestane suggests a greater contribution from green algae over red algae. The even-carbon preference across the C₁₄-C₂₀ range is indicative of limited contributions from Gloeocapsomorpha prisca. The presence of 3β-methylhopane, 7-+8-monomethylheptadecane and 2α-methylhopane points to contributions from methanotrophs and cyanobacteria. Since methanotrophs are active in an oxygen- and sulfate-depleted environment, we speculate that ¹³C-rich organic matter with a high value of 3-methylhopane index (3-MHI) may be deposited in waters with expanded anoxic zones and a scarcity of photoautotrophic sulfur bacteria (e.g., Chlorobiaceae and Chromatiaceae). This study highlights the constraints of microbial communities on organic δ¹³C variations and enhances our understanding of the evolution of ecology and primary productivity during the Precambrian-Cambrian transition.