Can NanoSIMS probe quantitatively the geochemical composition of ancient organic-walled microfossils? A case study from the early Neoproterozoic Liulaobei Formation

Abstract

Assessing the biogenicity of Precambrian putative remnants of life requires solid criteria. Among possible criteria, searching for evidence of pristine biological signatures and identifying various biological organic matter (OM) precursors in close association with microfossil morphology are of interest. Nano-scale Secondary Ion Mass Spectrometry (NanoSIMS) can provide a quantitative geochemical proxy at the scale of the individual microfossil but its use has remained limited because of potential analytical biases related to matrix effects and microtopography that may result in inaccurate NanoSIMS-derived measurement. No study so far has assessed whether these potential analytical biases were strong enough to preclude any identification of pristine OM degradation products and of organic precursors in ancient sediments. In this study, we characterized the geochemical composition of organic-walled microfossils from the early Neoproterozoic Liulaobei Formation in North China using NanoSIMS. The 12 CH − / 12 C 2 − ionic ratio allows us to distinguish filament from spheroid acritarchs, revealing the co-occurrence of two distinct pristine OM signatures that differ by their H and/or aliphatic contents. In addition, NanoSIMS data show that morphological degradation was tightly linked to a loss of H and/or hydrogenated organic compounds in spheroid acritarchs. In contrast, in situ N/C atomic ratios are homogeneous across all organic-walled microfossils studied. Although highly coherent with Proterozoic N/C atomic ratios from the literature, such homogeneity may alternatively reflect (i) a similar N content for different organic precursors or (ii) an extensive homogenization related to early degradation. Overall, these data obtained on microfossils from the Proterozoic Liulaobei Formation are the first to demonstrate that the quantitative capability of NanoSIMS can be used to track ancient OM precursors and to probe the effects of degradation on pristine OM. These findings open up tremendous perspectives and put forward new criteria for assessing the biogenicity of the putative early traces of life found in Archean metasediments.

New preparation techniques for molecular and in‐situ analysis of ancient organic micro‐ and nanostructures

“…Spatially resolved studies have investigated the composition of organic matter through characterization of heteroatom content (i.e., N/C, H/C ratio; see Delarue et al, , ; Duda et al, ; Oehler et al, , ) and functional group abundances (i.e., aromatic, aliphatic compounds; N‐, O‐, and S‐bearing functional groups Alleon et al, , ; Arouri et al, ; Igisu et al, ; Lemelle et al, ; Lepot et al, , ; Marshall et al, ; Steemans et al, ). Here, we detected a wealth of molecular fragments from Orbagnoux stromatolites including polycyclic aromatic hydrocarbons (PAH) (i.e., benzene C 6 H 5 + , tropylium C 7 H 7 + , naphthalene C 10 H 8 + , phenanthrene/anthracene C 14 H 10 + , pyrene C 16 H 10 + ), fragments of aliphatic molecules (C 4 H 7 + , C 5 H 9 + ions and other fragments with a ratio H/C > 1), and O‐bearing organic fragments (C 2 H 3 O + and C 3 H 5 O + ) (Figure ).…”

“…Spatially resolved studies have investigated the composition of organic matter through characterization of heteroatom content (i.e., Delarue et al, 2017Delarue et al, , 2018Duda et al, 2016;Oehler et al, 2006Oehler et al, , 2009) and functional group abundances (i.e., aromatic, aliphatic compounds; N-, O-, and S-bearing functional groups Alleon et al, 2016Alleon et al, , 2018Arouri et al, 1999;Igisu et al, 2009;Lemelle et al, 2008;Lepot et al, 2009Lepot et al, , 2019Marshall et al, 2005;Steemans et al, 2010 9). Associated with organic fragments, we also detected sulfur ions (S + , HS + , and H 3 S + ) in positive mode (Figure 9), and organic sulfur compounds such as C 4 S − ion (Figure 7c).…”

Mid-latitudinal habitable environment for marine eukaryotes during the waning stage of the Marinoan snowball glaciation

“…In post-Ordovician sedimentary rocks, C/N ratios can be elevated by terrestrial organic matter fluxes 36 , including organic matter derived from terrestrial plants, which became ecologically important since the Ordovician 37 . Precambrian sedimentary rocks typically have higher C/N ratios 38 , but they are unlikely to have been influenced by terrestrial organic matter fluxes; instead, differential preservation of organic carbon vs organic nitrogen, as well as a biological response to high productivity and hence N limitation 39 , may have contributed to the high C/N ratios in Precambrian sedimentary rocks. Despite the possibility of differential preservation of organic carbon vs organic nitrogen, the consistent carbon and nitrogen isotope values in the SBS, as well as the lack of correlation between C/N ratios and δ 13 C or δ 15 N values (Supplementary Fig.…”

Identifying microbial life in rocks: Insights from population morphometry

“…These biogenicity criteria require that (a) microstructures occur in a sedimentary rock of constrained age, (b) are hollow and contain a mineral filling, (c) include traces of altered organics, (d) display a biologic‐like shape, and (e) occur spatially associated with similar specimens reflecting a biologic population. In recent years, the evaluation of biogenicity criteria has been greatly improved with the high‐resolution, in situ chemical and structural characterization of potential microfossils and of their mineral matrix (Baghekema et al, ; Brasier, Antcliffe, Saunders, & Wacey, ; Delarue et al, , ; Fadel, Lepot, Busigny, Addad, & Troadec, ; Guo, Peng, Czaja, Chen, & Ta, ; House, Oehler, Sugitani, & Mimura, ; Javaux, Knoll, & Walter, ; Kempe et al, ; Lepot et al, ; Pang et al, ; Schopf et al, , ; Schopf & Kudryavtsev, ; Wacey, Kilburn, Saunders, Cliff, & Brasier, ; Wacey et al, , ; Williford et al, ). This characterization also facilitated the determination of the indigenous character of a carbonaceous microfossil, enabling the exclusion of modern organic contaminants and modern endoliths.…”