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.

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

“…The observed relationship -α N = 9.8051 × N/ C + 0.0029 -is equivalent to those previously determined by Nano-SIMS on flattened OM (Thomen et al, 2014;Alleon et al, 2016). This is consistent with a wealth of literature data showing that the determination of the N/C atomic ratio in situ is not biased by matrix effects (Van Zuilen et al, 2007;Thomen et al, 2014;Alleon et al, 2016;Delarue et al, 2017).…”

“…6). On the other hand, most organicwalled microfossils from the 1.9 Gyr-old Gunflint Formation (Alleon et al, 2016) present anomalously high in situ N/C atomic ratios compared to literature data (Fig. 6).…”

“…Consequently, it has also been used as a proxy for the geochemical preservation at the scale of individual Precambrian microfossils (Oehler et al, 2009(Oehler et al, , 2010Alleon et al, 2016). As stressed before, α N values did not allow any distinction of precursors and degradation effects among the organic-walled microfossil populations investigated.…”

“…Apart from stable isotope analysis, NanoSIMS has so far been mainly used to investigate the distribution of "organic elements" such as C or N across Precambrian organic-walled microfossils (Oehler et al, 2006;Wacey et al, 2010;McLoughlin et al, 2011;Peng et al, 2016). In parallel, it has been demonstrated that the emission of secondary ionic species used to monitor elements such as C or N can also be quantitatively associated with the geochemical elemental composition of both bulk OM (Thomen et al, 2014;Alleon et al, 2016) and individual organicwalled microfossils (Oehler et al, 2009(Oehler et al, , 2010Delarue et al, 2017). For instance, the 12 C 14 N − / 12 C 2 − secondary molecular ion ratio allows the determination of the N/C atomic ratio (Thomen et al, 2014), which is often used as a proxy for OM degradation during early diagenesis (Watanabe et al, 1997;Beaumont and Robert, 1999).…”

“…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).…”