Research ArticleAPPLIED SCIENCES AND ENGINEERING

Carbon speciation in organic fossils using 2D to 3D x-ray Raman multispectral imaging

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Science Advances  30 Aug 2019:
Vol. 5, no. 8, eaaw5019
DOI: 10.1126/sciadv.aaw5019

Figures

  • Fig. 1 Carbon XRS mapping and spectroscopy of a fragment of Lepidodendron trunk from the Upper Carboniferous (ca. 305 Ma ago) of Noyelles-lez-Lens, France.

    (A) Optical photograph of the studied object. (B) Schematic view of the experimental XRS setup. SDD, silicon drift detector. (C) Close-up on the studied area. The dashed line represents the boundaries identified in (D). (D) Carbon map from the dotted box area in (A) (scan area, 40 mm by 20 mm; 20,000 pixels; scan step, 200 μm by 200 μm; beam size, 15 μm by 15 μm). The box corresponds to the area analyzed in Fig. 2. a.u., arbitrary units. (E) Normalized background-corrected carbon K-edge XRS spectra from the locations indicated by asterisks in (D) (sum of four spectra; 500 ms per energy step; beam size, 15 μm by 15 μm), and pure graphite (denoted as “G”) for energy calibration and reference; spectra were vertically shifted for an increased readability. Scale bars, 1 cm. (Photo credit: Rafaella Georgiou, CNRS IPANEMA)

  • Fig. 2 2D XRS carbon K-edge speciation mapping of a fragment of Lepidodendron trunk from the Upper Carboniferous (ca. 305 Ma ago) of Noyelles-lez-Lens, France.

    (A) Spectral decomposition in two Gaussians and an arctangent of edge features of the normalized background-corrected XRS carbon K-edge XANES spectrum from the location indicated as point “2” in Fig. 1D. (B) Carbon intensity maps collected at 270, 280, 285, 288, 293, and 350 eV from the solid box area in Fig. 1D (scan step, 300 μm by 300 μm; 6000 pixels; beam size, 15 μm by 15 μm). Note how accurately the intensities in the fossils match the full spectra collected and how the intensities decrease following the Compton scattering background in the shale. (C) Spectral decomposition of the reduced spectrum collected at the exact same location as spectrum point “2” in (A) and Fig. 1D. (D) Distribution of the (1s-π*)/(1s-σ* + arctangent) ratio within the Lepidodendron trunk (calculated from the spectral decomposition of the reduced spectrum at each pixel). The white crosses indicate the location of the full spectra shown in Fig. 1E. (E) Histogram and kernel density of the ratio, allowing to pinpoint a few pixels with a speciation different from the full spectra collected. (F) Mean (reduced) spectra from the different classes of ratio identified by their respective colored boxes in (E).

  • Fig. 3 XRS 3D carbon K-edge speciation mapping of an Eocene (ca. 53 Ma ago) ant entrapped in amber from Oise, France.

    (A) Schematic view of the experimental XRS setup. (B) Optical photograph of the specimen. (C) Isosurface of the raw, energy-integrated, intensity data. (D) 3D rendering of the ant cuticle (brown) and internal void (transparent gray) classes of voxels based on the total signal intensity; image with interpolation [voxel size, 50 μm3; 23.409 voxels (amber voxels not shown); beam size, 10 μm by 20 μm). Oblique, dorsal, lateral right, and ventral views of the 3D rendering after smoothing (averaged voxel-distance interpolation). (E) Clustering of the ant cuticle voxels [brown voxels shown in (D)] based on the Achit parameter allows chemically distinguishing two classes of voxels: one in dorsal right position (negative Achit values in blue) and the other in ventral left position (positive Achit values in red), here shown in oblique, dorsal, lateral right, and ventral 3D views (after smoothing). (Photo credit: Rafaella Georgiou, CNRS IPANEMA)

  • Fig. 4 XRS virtual cross section and spectra of an Eocene ant entrapped in amber from Oise (France, ca. 53 Ma old).

    (A) Total intensity virtual cross-sectional image (pixel size, 50 μm; beam size, 10 μm by 20 μm) of the ant entrapped in the Eocene amber (image with interpolation). (B) Spatial distribution of the quadratic error showing the diverse chemical regions of the sample when performing a fit based on the amber reference (image with interpolation). (C) Spatial distribution of Achit when performing a fit based on the reference compounds amber and chitin (image with interpolation). (D) From top to bottom, distinctive normalized mean XRS spectral profiles corresponding to the ventral (Achit > 0.007, ExV) and the dorsal (Achit < −0.007, ExD) areas of the exoskeleton, as indicated with the arrows in (C), Oise amber collected from the bulk specimen (amber), and chitin standard used as a reference material (chitin standard). Scale bars, 500 μm.

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