Lipid taphonomy in the Proterozoic and the effect of microbial mats on biomarker preservation

The pre-Ediacaran biomarker record has several characteristic features that distinguish it from its Phanerozoic counterpart, including high concentrations of unresolved complex mixture (UCM), low concentrations or absence of eukaryotic steranes, and a conspicuous carbon isotopic enrichment of n-alkyl lipids relative to bulk sedimentary carbon. We propose that these derive from a common, non-actualistic taphonomy based on the pervasive presence of microbial mats prior to the "Cambrian substrate revolution." Such mats would have formed a significant mechanical and biochemical barrier to in-falling plankton, leaving a biomarker record dominated by benthic microbial lipids, most of which would have originated from the lower, heterotrophically reworked layers of the mat. Most Proterozoic biomarker assemblages studied in this work derive from shallow-water sediments, where cyanobacteria-dominated upper mats represent a focused source of corrosive molecular oxygen. This "mat-seal effect" was broken with the onset of bioturbation in the Ediacaran and early Cambrian, at which point the primary source of fossil biomarkers switched from the benthos to the plankton.