121-4 Raman spectroscopic evidence for abiogenic origins of spheroidal biomorphs in Mesoproterozoic chert: a call for critical evaluation of microfossil-like structures

Session: Caves and Karst Through Space and Time: Biogeochemistry, Climate, and Astrobiology

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Authors:

Abstract:

Early diagenetic chert provides exceptional preservation of microbial communities, offering windows into Earth's earliest microbial ecosystems. Clear identification of coccoidal communities, however, can be obscured by early-to-late diagenetic processes that produce convincing abiogenic mimics. The presence of spherulitic structures in silicified materials does not necessarily indicate biological origins, as such features frequently develop through natural mineralization processes. Using conventional light microscopy and confocal Raman imaging spectroscopy, we characterize and compare the morphological and compositional signatures of both unambiguous coccoidal microfossils and ambiguous features herein termed biomorphs: Type 1 (radial-fibrous structures with or without centrally concentrated organic staining), Type 2 (clear spherules, defined by concentrations of organic matter at their surfaces), and Type 3 (complex spherules consisting of interior granules surrounded by a discrete isopachous rim).

            Critically, hyperspectral Raman mapping reveals significant heterogeneity in organic matter thermal maturity within individual samples and between neighboring structures of identical taxonomy and taphonomic state. Analysis of bulk intensity ratios and distribution mapping of fitted peak centers and FWHM quantifies heterogeneity between individual coccoids within mats and among biomorph types. Raman geothermometry demonstrates that organic matter associated with biomorphs experienced higher thermal alteration compared to genuine microfossils from similar depositional environments. This thermal signature, combined with their association with cross-cutting fractures and absence from preserved microbial mat fabrics, strongly supports abiogenic formation during later diagenetic events rather than primary biological origin. Our findings show that superficially biological morphologies can result from secondary incorporation and mobilization of organic matter during diagenesis. The spatial heterogeneity revealed by hyperspectral Raman mapping of coccoidal mats highlights the complexity of organic preservation and diagenetic overprinting in ancient chert.

            These results highlight need for researchers to approach spheroidal organic-bearing structures with increased skepticism, particularly when occurring in contexts suggesting later diagenetic formation. Hyperspectral Raman analysis of organic matter offers a valuable diagnostic approach for distinguishing primary microbial preservation from secondary organic incorporation. This can improve confidence in microfossil identifications and reduce misinterpretation of abiogenic structures as evidence for early life, while providing insights into organic matter preservation mechanisms in Mars analog environments. These findings contribute to developing criteria for distinguishing biogenic from abiogenic mimics in ancient rocks, with potential applications for future life detection missions on Mars and beyond.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11359

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