Ultrastructural Heterogeneity of Carbonaceous Material in Ancient Cherts: Investigating Biosignature Origin and Preservation

Abstract

Opaline silica deposits on Mars may be good target sites where organic biosignatures could be preserved. Potential analogues on Earth are provided by ancient cherts containing carbonaceous material (CM) permineralized by silica. In this study, we investigated the ultrastructure and chemical characteristics of CM in the Rhynie chert (c. 410 Ma, UK), Bitter Springs Formation (c. 820 Ma, Australia), and Wumishan Formation (c. 1485 Ma, China). Raman spectroscopy indicates that the CM has experienced advanced diagenesis or low-grade metamorphism at peak metamorphic temperatures of 150-350°C. Raman mapping and micro-Fourier transform infrared (micro-FTIR) spectroscopy were used to document subcellular-scale variation in the CM of fossilized plants, fungi, prokaryotes, and carbonaceous stromatolites. In the Rhynie chert, ultrastructural variation in the CM was found within individual fossils, while in coccoidal and filamentous microfossils of the Bitter Springs and formless CM of the Wumishan stromatolites ultrastructural variation was found between, not within, different microfossils. This heterogeneity cannot be explained by secondary geological processes but supports diverse carbonaceous precursors that experienced differential graphitization. Micro-FTIR analysis found that CM with lower structural order contains more straight carbon chains (has a lower R3/2 branching index) and that the structural order of eukaryotic CM is more heterogeneous than prokaryotic CM. This study demonstrates how Raman spectroscopy combined with micro-FTIR can be used to investigate the origin and preservation of silica-permineralized organics. This approach has good capability for furthering our understanding of CM preserved in Precambrian cherts, and potential biosignatures in siliceous deposits on Mars.

Carbonaceous biosignatures of diverse chemotrophic microbial communities from chert nodules of the Ediacaran Doushantuo Formation

“…3), which is commonly used to characterize the structural order of organic matter (e.g. Tuinstra and Koenig, 1970;Bonal et al, 2006;Foucher et al, 2015;Qu et al, 2015). In summary, the I-1350/1600 values of organic matter in nodule center, rim, and host rock vary across large ranges of 0.49-0.62, 0.51-0.71, and 0.49-0.88, respectively (Fig.…”

“…hexagonal ring, heteroatoms, dangling bonds, tetrahedral coordinated carbons, and aromatic substances) give rise to specific bands in the Raman spectra (Tuinstra and Koenig, 1970;Beyssac et al, 2002;Sadezky et al, 2005;Lahfid et al, 2010;Kouketsu et al, 2014;RomeroSarmiento et al, 2014). In particular, two-dimensional Raman mapping is an efficient technique to quantify the lm-scale ultrastructural heterogeneities of organic matter within individual microfossils (Foucher et al, 2015;Qu et al, 2015), recording different compounds from co-existing organisms (e.g. plants, fungi, cyanobacteria).…”

“…-band, and it has been used to describe the branching index of carbon chains in fossilized carbonaceous material (Igisu et al, 2006(Igisu et al, , 2009Qu et al, 2015). It has been proposed that the R 3/2 ratio shows systematic variations between biomass of archaeans, bacteria, and eukaryotes due to their different chemical compositions within cellular constituents, especially the membrane lipids (Igisu et al, 2009).…”

“…It has also been demonstrated that different subcellular components (e.g. cell wall and protoplasm) in a single cell may have different R 3/2 ratios (Qu et al, 2015).…”

Contributions of Professor Martin Brasier to the study of early life, stratigraphy and biogeochemistry

“…Martin and colleagues were inspired to re-examine other benchmark microfossils from the Precambrian, such as the Gunflint Chert, the Bitter Springs Formation and selected Phanerozoic horizons, including the Rhynie Chert, producing new insights into microfossil origins and preservation (e.g. Wacey et al 2011;Qu et al 2015). In short, one of Martin's most significant contributions was to emphasize a more rigorous and sceptical approach to testing and documenting the earliest evidence for life on Earth and to explore new windows onto early life beyond cherts, which had traditionally been the favoured exploration target.…”