Early diagenetic sequestration of microbial mat lipid biomarkers through covalent binding into insoluble macromolecular organic matter (IMOM) as revealed by sequential chemolysis and catalytic hydropyrolysis

Abstract

Analyses of lipids in modern microbial mat ecosystems provide valuable insights into the taxonomic affinities of the dominant biological producers and consumers as well as carbon flow within the mat community. Such microbial mat biomarker assemblages also provide a window into the ancient biomarker record, facilitating paleobiological and paleoenvironmental reconstructions of analogous ecosystems on early Earth. Our focus was to investigate the covalent binding of lipid biomarkers into insoluble macromolecular organic matter (IMOM) during diagenesis in the well-studied hypersaline microbial mat ecosystems and underlying sediments at Guerrero Negro, Baja California Sur, Mexico. Following exhaustive solvent extraction, three different chemical degradation reagents of varying reactivity (acid methanolysis, trichloroacetic acid extraction, and periodate oxidation) were applied to two shallow sedimentary layers prior to fragmentation of the recalcitrant insoluble residues using catalytic hydropyrolysis (HyPy). The hydrocarbons released by HyPy included a complex variety of linear, branched and polycyclic alkane structures, including hopanes, methylhopanes and steranes. Our findings indicate that a significant fraction of the bound biomarker pool was strongly linked by covalent binding through functional groups into IMOM but that a portion of this bound pool was not recoverable by chemolysis treatment. These results provide new insights about the mode and timing of chemical binding of steroids and hopanoids, and their early diagenetic transformation products, into IMOM. The formation of sedimentary IMOM from the earliest stages of organic matter diagenesis, commencing on a timescale of only years, aids the long-term preservation of biomarker lipids in the geological record.