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

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 ¹²CH⁻/¹²C₂⁻ 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.