Nanoscale 3D quantitative imaging of 1.88 Ga Gunflint microfossils reveals novel insights into taphonomic and biogenic characters

Abstract

Precambrian cellular remains frequently have simple morphologies, micrometric dimensions and are poorly preserved, imposing severe analytical and interpretational challenges, especially for irrefutable attestations of biogenicity. The 1.88 Ga Gunflint biota is a Precambrian microfossil assemblage with different types and qualities of preservation across its numerous geological localities and provides important insights into the Proterozoic biosphere and taphonomic processes. Here we use synchrotronbased ptychographic X-ray computed tomography to investigate well-preserved carbonaceous microfossils from the Schreiber Beach locality as well as poorly-preserved, iron-replaced fossil filaments from the Mink Mountain locality, Gunflint Formation. 3D nanoscale imaging with contrast based on electron density allowed us to assess the morphology and carbonaceous composition of different specimens and identify the minerals associated with their preservation based on retrieved mass densities. In the Mink Mountain filaments, the identification of mature kerogen and maghemite rather than the ubiquitously described hematite indicates an influence from biogenic organics on the local maturation of iron oxides through diagenesis. This non-destructive 3D approach to microfossil composition at the nanoscale within their geological context represents a powerful approach to assess the taphonomy and biogenicity of challenging or poorly preserved traces of early microbial life, and may be applied effectively to extraterrestrial samples returned from upcoming space missions. Understanding Precambrian fossilized microorganisms, where preserved, can provide critical insights into the earliest records of life on Earth and its paleoenvironment 1-6 , especially in light of controversies surrounding the origin of chemical biosignatures such as isotopic fractionation 7,8 and biomolecules 9. Nonetheless, imaging the morphologies of these micrometric structures demands high spatial resolution, while the composition of

Deciphering the Chemistry of Cultural Heritage: Targeting Material Properties by Coupling Spectral Imaging with Image Analysis

“…Additional examples are the discovery of an unexpected lowdensity phase in 3D images of 1.88-Ga-old gunflint microfossils, later attributed to kerogens resulting from diagenesis of the original organic matter, 4 or the 3D distinction of two levels of preservation of the exoskeleton in a fossil ant preserved in amber. 2 In the latter, Georgiou et al 2 reported the unprecedented use of a hard X-ray probe based on X-ray Raman scattering for imaging in three dimensions the carbon chemistry of an ∼53 Ma ant from France (Figure 8).…”

“…L.BerenguerF.WestallF.BertrandL.GalanteD. Maldanis, L. Hickman-Lewis, K. Verezhak, M. Gueriau, P. Guizar-Sicairos, M. Jaqueto, P. Trindade, R. Rossi, A. L. Berenguer, F. Westall, F. Bertrand, L. Galante, D. 8163202010 …”

“…32−36 The advent of coherence-based tomography made it possible to couple imaging and phase identification in three dimensions with nanometric resolution. 4 (d) Infrared spectroscopy has enabled high signal-to-noise microscale characterization of organic and in some cases inorganic systems and has been used extensively for the study of archeological and historical paint samples. 37,38 (e) By giving access in particular to the deep UV, UV/vis photoluminescence (PL) spectroscopy of ancient materials on synchrotron has opened new avenues for the study of semiconductor compounds and paint alteration phases at the micrometer and submicrometer scales.…”

“…Soft XAS has provided chemical speciation of light elements such as carbon, nitrogen, and oxygen and of heavier elements through the study of their L absorption edges, opening up the possibility of nanoresolved speciation and identification of organic molecules in heritage systems. − Recent developments in X-ray Raman spectroscopy have extended the scope of X-ray absorption near-edge spectroscopy (XANES) for light elements to bulk spectroscopy and three-dimensional (3D) imaging. , (c) X-ray microtomography (μCT) enabled 3D internal morphological rendering of many heritage objects based on contrasts in their internal density and, more generally, their complex optical index. Edge enhancement by beam propagation after traversing the analyzed object or phase reconstruction, both of which result from the very small photon source of third-generation synchrotrons, have greatly facilitated segmentation and morphological recognition, particularly for paleontological specimens. − The advent of coherence-based tomography made it possible to couple imaging and phase identification in three dimensions with nanometric resolution . (d) Infrared spectroscopy has enabled high signal-to-noise microscale characterization of organic and in some cases inorganic systems and has been used extensively for the study of archeological and historical paint samples. , (e) By giving access in particular to the deep UV, UV/vis photoluminescence (PL) spectroscopy of ancient materials on synchrotron has opened new avenues for the study of semiconductor compounds and paint alteration phases at the micrometer and submicrometer scales. ,, …”

From petrography to nanoscale: new approach for 3D investigations of fossilized microorganisms from Mars analogue deposits

“…The investigation of putative preserved microbes in rocks returned from Mars will require multiscale methods. Due to the difficulty of undisputedly identifying fossils at the cellular level, the access to nanoscale and 3D information are often critical [2][3][4]. Here we present a novel approach that allows micron-to-nanoscale 3D investigation of microfossils within 3x3 cm petrographic thin sections, in a non-destructive and non-invasive way.…”