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Structural and chemical heterogeneity of Proterozoic organic microfossils of the ca. 1 Ga old Angmaat Formation, Baffin Island, Canada

DOI: 10.1111/gbi.12463 DOI Help

Authors: Sami Nabhan (Université de Paris, Institut de Physique du Globe de Paris, CNRS) , Linda C. Kah (University of Tennessee) , Bhoopesh Mishra (University of Leeds) , Kilian Pollok (Friedrich Schiller University Jena) , Ashley R. Manning‐berg (University of Tennessee at Chattanooga) , Mark A. Zuilen (Université de Paris, Institut de Physique du Globe de Paris, CNRS)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Geobiology , VOL 212

State: Published (Approved)
Published: July 2021
Diamond Proposal Number(s): 23583

Abstract: Organic microfossils in Meso- and Neoproterozoic rocks are of key importance to track the emergence and evolution of eukaryotic life. An increasing number of studies combine Raman spectroscopy with synchrotron-based methods to characterize these microfossils. A recurring observation is that Raman spectra of organic microfossils show negligible variation on a sample scale and that variation between different samples can be explained by differences in thermal maturation or in the biologic origin of organic precursor material. There is a paucity of work, however, that explores the extent to which the petrographic framework and diagenetic processes might influence the chemical structure of organic materials. We present a detailed Raman spectroscopy-based study of a complex organic microfossil assemblage in the ca. 1 Ga old Angmaat Formation, Baffin Island, Canada. This formation contains abundant early diagenetic chert that preserves silicified microbial mats with numerous, readily identifiable organic microfossils. Individual chert beds show petrographic differences with discrete episodes of cementation and recrystallization. Raman spectroscopy reveals measurable variation of organic maturity between samples and between neighboring organic microfossils of the same taxonomy and taphonomic state. Scanning transmission X-ray microscopy performed on taphonomically similar coccoidal microfossils from the same thin section shows distinct chemical compositions, with varying ratios of aromatic compounds to ketones and phenols. Such observations imply that geochemical variation of organic matter is not necessarily coupled to thermal alteration or organic precursor material. Variation of the Raman signal across single samples is most likely linked to the diagenetic state of analyzed materials and implies an association between organic preservation and access to diagenetic fluids. Variation in the maturity of individual microfossils may be a natural outcome of local diagenetic processes and potentially exceeds differences derived from precursor organic material. These observations stress the importance of detailed in situ characterization.

Journal Keywords: Angmaat Formation; Mesoproterozoic; organic microfossils; Raman spectroscopy

Subject Areas: Earth Science

Instruments: I08-Scanning X-ray Microscopy beamline (SXM)

Added On: 26/07/2021 09:21

Discipline Tags:

Earth Sciences & Environment Palaeontology Geology Geochemistry

Technical Tags:

Microscopy X-ray Microscopy Scanning X-ray Microscopy