The 5th International Conference of Geobiology

Microbialites, as some of the oldest records of the early biosphere on Earth¹, provide a comprehensive archive of geobiological evolution across geological times. Oncolites, a common constituent of microbialites, are typically formed through a combination of rolling and biogenic encrusting processes, resulting in spheroid shapes with intricate microbialite architectures². These structures have garnered significant attention due to their intriguing formation mechanisms and environmental implications. However, interpreting microbial metabolism in fossil oncolites has largely relied on morphological analysis due to the scarcity of preserved microbial remains³. While modern oncoids often exhibit similar macro-morphologies to their geological counterparts, their microstructures differ markedly, complicating our understanding of organomineralization and paleoenvironments through oncolites⁴. In this study, we report on Oligocene oncolites from the Junggar Basin, which have undergone moderate diagenesis and thus retain well-preserved lipid archives that offer insights into their formation. Thin-section observations reveal that the spheroid oncolites are composed of alternating dark and light layers with distinct microbialite architectures. Mineral analysis indicates that the oncolites are primarily composed of calcite, with minor constituents of quartz and Fe-Mn hydroxides. Elemental mapping of fresh oncolite surfaces highlights the presence of Fe-Mn-rich cortex and the surfaces of individual coating layers. Conventional analysis of free lipids demonstrates a predominance of prokaryotic over eukaryotic biomarkers, with cyanobacteria and heterotrophic bacteria being particularly diagnostic. Notably, a significant portion of these lipids was bound within the carbonate matrix and only released upon acid dissolution, suggesting continuous incorporation during oncoid growth. Intriguingly, saturated derivatives of fatty acids were also liberated through acid treatment. Collectively, these biomarker data lead us to hypothesize that oncoids are colonized by a complex microbial community that mediates calcification during the formation of oncolites.

References

1. Allwood, A.C., Rosing, M.T., Flannery, D.T., Hurowitz, J.A. and Heirwegh, C.M., 2018. Reassessing evidence of life in 3,700-million-year-old rocks of Greenland. Nature, 563(7730), 241-244.
2. Flügel, E. and Munnecke, A., 2010. Microfacies of carbonate rocks: analysis, interpretation and application (Vol. 976, p. 2004). Berlin: springer.
3. Vodrážková, S., Kumpan, T., Vodrážka, R., Frýda, J., Čopjaková, R., Koubová, M., Munnecke, A., Kalvoda, J. and Holá, M., 2022. Ferruginous coated grains of microbial origin from the Lower Devonian (Pragian) of the Prague Basin (Czech Republic)–Petrological and geochemical perspective. Sedimentary Geology, 438, 106194.
4. Kazmierczak, J. and Kempe, S., 2006. Genuine modern analogues of Precambrian stromatolites from caldera lakes of Niuafoʻou Island, Tonga. Naturwissenschaften, 93, 119-126.

carbonate associate lipids,microbialites,biomarker,Junggar Basin,Oligocene