Tracking hidden organic carbon in rocks using chemometrics and hyperspectral imaging

Ludovic Duponchel,Céline Pisapia,Bénédicte Ménez,Frédéric Jamme

doi:10.1038/s41598-018-20890-4

Abstract

Finding traces of life or organic components of prebiotic interest in the rock record is an appealing goal for numerous fields in Earth and space sciences. However, this is often hampered by the scarceness and highly heterogeneous distribution of organic compounds within rocks. We assess here an innovative analytical strategy combining Synchrotron radiation-based Fourier-Transform Infrared microspectroscopy (S-FTIR) and multivariate analysis techniques to track and characterize organic compounds at the pore level in complex oceanic rocks. S-FTIR hyperspectral images are analysed individually or as multiple image combinations (multiset analysis) using Principal Component Analyses (PCA) and Multivariate Curve Resolution – Alternating Least Squares (MCR-ALS). This approach allows extracting simultaneously pure organic and mineral spectral signatures and determining their spatial distributions and relationships. MCR-ALS analysis provides resolved S-FTIR signatures of 8 pure mineral and organic components showing the close association at a micrometric scale of organic compounds and secondary clays formed during rock alteration and known to catalyse organic synthesis. These results highlights the potential of the serpentinizing oceanic lithosphere to generate and preserve organic compounds of abiotic origin, in favour of the hydrothermal theory for the origin of life.

Highlights

Spectroscopic techniques and especially Synchrotron-based Fourier-Transform Infrared microspectroscopy (S-FTIR) have been used in geosciences for decades[1,2,3]
SEM observations, SEM-EDX and Synchrotron radiation-based Fourier-Transform Infrared microspectroscopy (S-FTIR) analyses indicated the presence of a partly serpentinized olivine rimed with saponite and Fe-rich yellow serpentine, all embedded in a white serpentine matrix (Fig. 1b)
Four typical S-FTIR spectra were identified (Fig. 2b). They presented, respectively: (i) a dominant band at 1774 cm−1, which was found to be systematically associated with the presence of olivine, (ii) aliphatics’ vibration bands at 2850, 2920 and 2956 cm−1 associated with numerous vibration bands in the 1400–1800 cm−1 range that might be attributed either to organic compounds or minerals, (iii) a broad vibration band at 1629 cm−1 in the Fe-rich serpentine mineral, and (iv) 3 main vibration bands (1472, 1543, 1596 cm−1) associated with two weaker ones (1984 and 2077 cm−1) in the white serpentine mineral

Summary

Introduction

Spectroscopic techniques and especially Synchrotron-based Fourier-Transform Infrared microspectroscopy (S-FTIR) have been used in geosciences for decades[1,2,3]. The combined use of S-FTIR microspectroscopy and multivariate image analysis is a very powerful tool that should lead to the accurate characterization of the global and local composition of a complex sample along with the identification of its components[20,21,22] Among these chemometric techniques, Principal Component Analysis (PCA) should allow describing the chemical structure and spatial distribution of components of a natural sample[20]. In order to properly detect and describe impurities by chemometric analysis of hyperspectral images, it is mandatory to increase the variance of these low concentration compounds This can be achieved through multiset analysis by simultaneously combining and analysing several images representative of the global variability of one sample. It provides a new analytical strategy for the search and characterization of organic compounds within a mineralized environment in which close spatial relationships between organic and mineral phases may suggest potential genetic links

Methods

Results

Conclusion