Pushing Raman spectroscopy over the edge: purported signatures of organic molecules in fossils are instrumental artefacts

Abstract

Claims for the widespread preservation of fossilized biomolecules in many fossil animals have recently been reported in six studies, based on Raman microspectroscopy. Here, we show that the putative Raman signatures of organic compounds in these fossils are actually instrumental artefacts resulting from intense background luminescence. Raman spectroscopy relies upon the detection of photons scattered inelastically by matter as a result of its interaction with a laser beam. For many natural materials, this interaction also generates a luminescence signal that is often orders of magnitude more intense than the light produced by Raman scattering. Such luminescence, coupled with the transmission properties of the spectrometer, induced quasi-periodic ripples in the measured spectra that have been incorrectly interpreted as Raman signatures of organic molecules. Although several analytical strategies have been developed to overcome this common issue, Raman microspectroscopy as used in the studies questioned here cannot be used to identify fossil biomolecules.

Taphonomic patterns mimic biologic structures: diagenetic Liesegang rings in Mesozoic coleoids and coprolites

“…7 ), while Ca is a bit more abundant in the light gray parts ( Table 1 ). Raman spectroscopy confirms, despite a strong fluorescence associated to a quasi-periodic ripple-artefact ( Alleon et al (2020) and references therein), the presence of C in the circles evidenced by the so-called graphite (G) and defect (D) bands typical of fossil organic materials ( Fig. 9C ).…”

“…(C) Raman spectroscopy (using a 532-nm laser) of the circles in the Doratheutis sample, showing the so-called graphite (G) and defect (D) bands typical of organic materials preserved in sedimentary rocks. The bands identified by an asterisk are instrumental quasi-periodic ripples resulting from intense background luminescence (see Alleon et al (2020) and references therein). (D) Diffractograms obtained for both samples, compared to references for minerals commonly associated with fossils.…”

Validation of biosignatures confirms the informative nature of fossil organic Raman spectra

“…Alleon et al ( 21 ) presented different observations and arguments in support of their assertion that the disputed spectra are artefactual in nature. Their arguments relied primarily on a frequency contribution analysis which, as far as we can determine, has not been applied by others to baselined Raman spectra.…”

“…Alleon et al ( 21 ) downloaded a freely available RazorEdge filter transmission spectrum ( 21 , fig. 2a), which resembles a periodic, sinusoidal wave function, from the commercial website ‘Semrock’ (www.semrock.com/FilterDetails.aspx?id=LP03-532RE-25).…”

“…A diversity of studies conducted by different laboratories ( 3-15 ) have recovered very similar patterns in the molecular makeup of fossil organic matter in independently acquired in situ Raman spectra. However, a recent Perspective by Alleon et al ( 21 ) claimed that the results of a selected subset of these studies ( 3, 4, 6, 11, 12, 15 ) are invalid, based on the assertion that published spectra, despite resembling previous independent Raman analyses ( 7-10, 13, 14 ), are the result of instrumental artefacts reflecting sinusoidal edge filter ripples. The disputed studies are based on in situ Raman spectra for a diversity of modern, experimentally matured, and fossil tissues of different types, as well as sedimentary host rocks.…”