ANTHROPOCENE OVERPRINTS IN TRILOBITE BIOMARKERS FROM THE CAMBRIAN

Molecular biomarkers are a useful tool for biogeochemistry in both modern and fossil systems. While some studies have found molecular biomarkers in the exoskeleton of calcareous fossil groups such as crinoids, trilobites remain an unexplored potential biomarker source. Trilobite biomarkers could yield information about paleoenvironments throughout the Paleozoic, including climatically dynamic periods such as the Cambrian. The Wheeler Shale of Utah contains well-preserved Cambrian trilobites that can provide a test of biomarker preservation. We analyzed biomarkers from two abundant and ubiquitous trilobites from the Wheeler Shale: four benthic Elrathia kingii and 24 putatively pelagic Peronopsis interstrictus quarried from deposits buried under five meters of overburden. Because anthropogenic polymers such as phthalates are a major potential biomarker contaminant, two Elrathia and eight Peronopsis were deliberately protected from plastics during collection and storage and treated to remove plasticizers before analysis. Specimens were powdered, solvent-extracted, and derivatized to isolate biomarkers for chromatography, and extract aliquots were analyzed using gas chromatography mass spectrometry.

We found evidence of 55 unique biomarkers. Recent biomarkers were the most abundant, indicative of terrestrial vegetation like gymnosperms, biomass burning, terrestrial animal and microbial biomass. Some biomarkers may indicate a marine microbial origin, possibly though not demonstrably Cambrian. Anthropogenic biomarkers were ubiquitous, even in specimens seemingly shielded by overburden, protected from plastic contaminants and rigorously cleaned. Anthropogenic molecules may still have been introduced during handling, or may have been environmental. Although the Wheeler Shale trilobites are exceedingly well preserved, they reveal more about ecological disturbance and regional environment in the Anthropocene than the Cambrian. Thus modern biomarkers in fossils, rather than being thought of as contaminants, may serve as a potential record for modern natural and anthropogenic changes in the regions where fossils occur.