Tracking hidden organic carbon in rocks using chemometrics and hyperspectral imaging

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.

Habitability of the marine serpentinite subsurface: a case study of the Lost City hydrothermal field

“…In a drill core sample from the Atlantis Massif, the amino acid tryptophan was detected and proposed to be abiotically derived [82]. It, along with additional unidentified organic compounds [82,83], was closely associated with saponites, which form at temperatures below 150°C during aqueous alteration of serpentinized ultramafic rocks. Biologically derived compounds have also been detected in Atlantis Massif rocks and proposed to have been deposited from seawater organic matter during circulation [79].…”

“…Therefore, late-stage fluid circulation may have an important role in altering the organic content of the rocky subsurface. Given the heterogeneous distribution of these admittedly tiny amounts of organic compounds [83], it is as yet unclear how important they would be for sustaining subseafloor ecosystems.…”

A Robust Stochastic Approach to Mineral Hyperspectral Analysis for Geometallurgy

“…The main idea here is to infer contributions from all pure components present in a complex spectrum to build a matrix of individual pure spectra and corresponding concentration profiles which best fit experimental data. An interesting example of this technique is shown in Pisapia et al [108], where tracking of organic carbon hidden in rocks has been explored by integrating chemometrics and a specific type of spectral data (synchrotron radiation-based Fourier-transform infrared microspectroscopy (S-FTIR)). The analysis has been interestingly performed using a combination of principal component analyses (PCA) and the mixed approach called multivariate curve resolution-alternating least squares (MCR-ALS).…”

New Perspectives on Abiotic Organic Synthesis and Processing during Hydrothermal Alteration of the Oceanic Lithosphere

“…[70][71][72] A similar association was recently highlighted at a micrometric scale using FTIR spectroscopy in oceanic serpentinites collected at ~170 m depth below seafloor during the IODP Expeditions 304/305 targeting the Atlantic Massif. 73 Overall, all of these studies highlight that hydrothermally derived organic carbon trapped within the upper lithosphere as heavy and aromatic compounds can be chemically and structurally diverse, although formation mechanisms are not yet well-known. Whether aromatization occurs in situ from CO 2 /CO and H 2 during serpentinization or derives from progressive polymerization and polycondensation reactions of light aliphatic AE aromatic hydrocarbons inherited from higher-T FTT reactions still needs to be addressed.…”