Molecular preservation of 1.88 Ga Gunflint organic microfossils as a function of temperature and mineralogy

Abstract

The significant degradation that fossilized biomolecules may experience during burial makes it challenging to assess the biogenicity of organic microstructures in ancient rocks. Here we investigate the molecular signatures of 1.88 Ga Gunflint organic microfossils as a function of their diagenetic history. Synchrotron-based XANES data collected in situ on individual microfossils, at the submicrometre scale, are compared with data collected on modern microorganisms. Despite diagenetic temperatures of ∼150-170 °C deduced from Raman data, the molecular signatures of some Gunflint organic microfossils have been exceptionally well preserved. Remarkably, amide groups derived from protein compounds can still be detected. We also demonstrate that an additional increase of diagenetic temperature of only 50 °C and the nanoscale association with carbonate minerals have significantly altered the molecular signatures of Gunflint organic microfossils from other localities. Altogether, the present study provides key insights for eventually decoding the earliest fossil record.

Figure 1. X-ray diffraction patterns of the five Gunflint cherts investigated.

The five samples investigated display a similar mineralogical composition at the bulk scale (they are mainly composed of α quartz).

Figure 2. Photomicrographs of thin sections of the five Gunflint cherts investigated.

These five samples contain spheroidal microfossils that appear more or less permineralized by silica and exhibit a more or less thick wall: (a,b) Discovery Point, (c,d) Mink Mountain, (e,f) Triple Junction, (g,h) Schreiber Beach, (i,j) Kakabeka Falls. Scale bars, 50 μm.

Figure 3. Raman mapping and FIB–SEM imaging of Schreiber Beach organic microfossils.

(a) Photomicrograph of spheroidal organic microfossils from the Schreiber Beach chert and (b) corresponding Raman map showing the distribution of organic carbon. Red lines indicate the location of the cross-sections shown in c,d. Scale bars, 5 μm. (c,d) FIB–SEM images of cross-sections of the microfossils shown in a, illustrating that Gunflint spheroidal microfossils can be more or less permineralized by silica or filled by organics (which appear dark) and micrometric mineral phases (which appear bright). Scale bars, 5 μm.

Figure 4. SEM and TEM analyses of Schreiber Beach and Mink Mountain organic microfossils.

(a,b) SEM images of organic microfossils (dark). Red lines indicate where the FIB sections shown in c and d were extracted. Scale bars, 15 μm. (c,d) Scanning TEM (STEM) images of FIB sections extracted from the carbonaceous microfossils showing the textural relationships between the carbonaceous matter and the associated mineralogy. Organic carbon appears dark. Spots indicate where the EDXS data shown in e and f were collected. Scale bars, 5 μm. (e,f) TEM–EDX spectra acquired on the organic matter composing the microfossils. Schreiber Beach organic microfossils exhibit high nitrogen (N) and sulfur (S) contents compared to Mink Mountain organic microfossils.

Figure 5. TEM analyses of Discovery Point organic microfossils.

(a) Scanning TEM (STEM) image showing the nanoscale mixing of organic carbon and carbonates. Organic carbon appears dark. Scale bar, 250 nm. (b–d) High-resolution TEM images showing details of this nanoscale association. Spots indicate where the EDXS data were collected. Scale bars, 200 nm (b), 100 nm (c) and 20 nm (d). (e,f) TEM–EDX spectra acquired on the organic matter constituting the microfossils and on the associated calcium carbonates.

Figure 6. Representative Raman spectra of Gunflint organic microfossils.

These spectra are typical of disordered carbons, with a composite G band (G+D2), a composite D band (D1+D4) and a D3 band in between. Grey spectra are data from the calibration series published by Lahfid et al.. The peak at ∼1,090 cm⁻¹ in the Raman spectrum of Discovery Point organic microfossils is attributed to iron-rich calcium carbonates such as ankerites.

Figure 7. X-ray absorption spectra of Gunflint organic microfossils.

The atomic N/C ratios (±0.010) shown here have been estimated following the methodology developed by Alleon et al.. The absorption spectra of modern cyanobacteria (G. violaceus) and of modern micro-algae (E. gracilis) are shown for comparison. Absorption features observed in the 340–360 eV range indicate the presence of calcium.

Figure 8. C-XANES spectra of Gunflint organic microfossils.

The C-XANES spectra of modern cyanobacteria (G. violaceus) and of modern micro-algae (E. gracilis) are shown for comparison. All spectra are normalized to the total carbon quantity. Absorption features at 285.1, 286.4, 286.7, 288.2, 288.6, 289.4 and 290.3 eV are attributed to electronic transitions of aromatic and/or olefinic carbons, carbons bonded with heteroatoms, carbonyl or phenolic groups, amide groups, carboxylic groups, hydroxylated carbons and carbonate groups, respectively.

Figure 9. N-XANES spectra of Gunflint organic microfossils.

The N-XANES spectra of modern cyanobacteria (G. violaceus) and of modern micro-algae (E. gracilis) are shown for comparison. All spectra are normalized to the total nitrogen quantity. Absorption features at 398.8, 399.8, 401.4 and 402.2 eV are attributed to the presence of aromatic nitrogen, amide, imine and nitrile groups. Absorption peaks at 401.7 and 405.4 eV are attributed to inorganic nitrogen contained in nitrates.