Organic molecular heterogeneities can withstand diagenesis

Abstract

Reconstructing the original biogeochemistry of organic fossils requires quantifying the extent of the chemical transformations that they underwent during burial-induced maturation processes. Here, we performed laboratory experiments on chemically different organic materials in order to simulate the thermal maturation processes that occur during diagenesis. Starting organic materials were microorganisms and organic aerosols. Scanning transmission X-ray microscopy (STXM) was used to collect X-ray absorption near edge spectroscopy (XANES) data of the organic residues. Results indicate that even after having been submitted to 250 °C and 250 bars for 100 days, the molecular signatures of microorganisms and aerosols remain different in terms of nitrogen-to-carbon atomic ratio and carbon and nitrogen speciation. These observations suggest that burial-induced thermal degradation processes may not completely obliterate the chemical and molecular signatures of organic molecules. In other words, the present study suggests that organic molecular heterogeneities can withstand diagenesis and be recognized in the fossil record.The fossil record contains crucial information about the evolution of Life on Earth 1, 2 . Molecular investigations regarding ancient Life are yet limited by the poor quality of the 'biogeochemical' signals preserved in the fossil record: in addition to biodegradation, burial-induced alteration processes (i.e. thermal maturation) inevitably modify the original biochemical signatures of fossilized organic molecules 1, 3 . With increasing temperature, weaker organic bonds are thermally broken and significant deoxygenation and dehydrogenation reactions occur 1, 4 . Thermal degradation eventually promotes increasing structural reorganization and may ultimately lead to the formation of pure graphite 5,6 .It has long been recognized that biomacromolecules exhibit conspicuous differences in decay in natural environments 7,8 . For instance, cell-wall biopolymers that protect some algae, cysts, spores and pollen grains are intrinsically more resistant than polysaccharides, proteins and nucleic acids 9-11 . In some contexts, taxon-specific chemosystematic data can even be preserved in the fossil record 12,13 . Still, the general perception in paleobiology remains that thermal maturation processes lead to a converging composition of organic materials from different origins, thereby limiting the use of chemical composition for discriminating between possibly different fossilized taxa 14 .Taking advantage of advanced spectroscopic tools (including synchrotron-based techniques), a number of studies have demonstrated that organic molecules may undergo only partial degradation during diagenesis in natural settings [15][16][17][18][19][20][21][22][23][24] . In parallel, laboratory experiments have helped evaluating the influence of key factors such as the pressure-temperature conditions, the redox conditions, the presence or absence of a fluid or of certain mineral phases on the degradation of organics 22,[25][26][27][28...

Chemical nature of the 3.4 Ga Strelley Pool microfossils

“…Similarly, the N-XANES spectra display a broad absorption feature at 401.4 eV, which is related to the presence of amide, imine and/or nitrile groups. The peak at 399.9 eV and shoulder around 398.8 eV indicate the presence of nitrogen within aromatic moieties (Leinweber et al, 2007;De Gregorio et al, 2011;Alleon et al, 2017). The two absorption features at 401.7 and 405.4 eV can be attributed to the presence of trace quantities of potassium and/or calcium nitrates (Leinweber et al, 2007;Alleon et al, 2016a).…”

“…The peak at 286.7 eV reveals the presence of imine (C=N), nitrile (CN), carbonyl (C=O) and/or phenolic (Ar-OH) groups. Whereas, the much smaller peak centred at 285.1 eV signals the presence of aromatic and/or olefinic groups (De Gregorio et al, 2011;Alleon et al, 2017). Similarly, the N-XANES spectra display a broad absorption feature at 401.4 eV, which is related to the presence of amide, imine and/or nitrile groups.…”

“…3). Recent laboratory experiments have demonstrated that thermal degradation of chemically distinct organic precursors can lead to chemically distinct residues (Alleon et al, 2017). The spectral differences between the lenticular and film-like microfossils may thus signal differences in the initial molecular composition of organic material now composing the Strelley Pool microfossils.…”

“…molecular investigations of the Strelley Pool microfossils at the submicrometre scale. N/C atomic ratio values were estimated following the methodology and calibration outlined in Alleon et al (2015) and successfully used in Alleon et al (2016aAlleon et al ( , 2017.…”

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

“…Even if this geochemical pattern has been addressed at the scale of bulk OM (see Vandenbroucke and Largeau, 2007 and references therein), it remains poorly documented at the scale of individual microfossils. Indeed, only a few studies have dealt with the characterization of the elemental and isotopic compositions of individual Precambrian microfossils (House et al, 2000;Kaufman and Xiao, 2003;Oehler et al, 2009Oehler et al, , 2010Alleon et al, 2016;Delarue et al, 2017), despite the potential crucial linking of geochemical with morphological preservation states to assess the biogenicity of microfossils through the distinction of various biological OM precursors, and of degradation patterns of pristine biological signature(s) (Alleon et al, 2017).…”