FTIR microspectroscopy of carbonaceous matter in ~\u20093.5 Ga seafloor hydrothermal deposits in the North Pole area, Western Australia

Motoko Igisu,Ken Takai,Yuichiro Ueno

doi:10.1186/s40645-018-0242-1

Abstract

Carbonaceous matter in ~ 3.5 Ga hydrothermal vein deposits from the Dresser Formation, Western Australia, was analyzed using Fourier transform infrared (FTIR) microspectroscopy. Based on the spectroscopy, the carbonaceous matter was mainly composed of disordered aromatic structures, with minor aliphatic C–H functional groups. Spatially resolved analysis supports that the aliphatic C–H signatures are derived from kerogenous macro-molecule and not from free bitumen or other artificial sources. The intensity ratios of the asymmetric aliphatic CH3 to the asymmetric aliphatic CH2 (R3/2 value) in the carbonaceous clots range from 0.22 to 0.51. Thermal alteration may increase or not change the R3/2 value of organic matter, as suggested by previous experiments, but it is unlikely to be the cause of the substantially lower R3/2 values when compared with those of primary organic matter. In particular, the low R3/2 values (< ~ 0.4) suggest that the carbonaceous matter mainly contains aliphatic C–H bonds derived from bacterial cells. The carbonaceous clots may have been possibly produced by abiotic reaction such as Fischer-Tropsch-type (FTT) synthesis. However, the organic matter source only produced by the FTT synthesis is inconsistent with the R3/2 values for the analyzed carbonaceous clots. The results obtained by combining these spectroscopic features of the carbonaceous clots together with the previously reported isotopic features may possibly suggest that both bacteria and archaea were colonized in the ~ 3.5 Ga Dresser hydrothermal system.

Highlights

Hydrothermal systems have been considered as the origin of life (Holm and Andersson 1998) and the locations of the earliest evolution of life and metabolism (Nisbet 1995)
The organic matter has been characterized by Raman microspectroscopy, Fourier transform infrared (FTIR) spectroscopy, solid-state 13C nuclear magnetic resonance, and pyrolysis analysis (e.g., Ueno et al 2001; Derenne et al 2008; Duda et al 2018)
The pyrolysis fragments of the organic matter comprise long-chain aliphatic hydrocarbons and thiophenes, which are likely related to the existence of microbes including sulfate-reducing bacteria (Derenne et al 2008; Duda et al 2018)

Summary

Introduction

Hydrothermal systems have been considered as the origin of life (Holm and Andersson 1998) and the locations of the earliest evolution of life and metabolism (Nisbet 1995). The organic matter has been characterized by Raman microspectroscopy, FTIR spectroscopy, solid-state 13C nuclear magnetic resonance, and pyrolysis analysis (e.g., Ueno et al 2001; Derenne et al 2008; Duda et al 2018). We first report the FTIR microspectroscopic features of early Archean carbonaceous matter obtained from two ~ 3.5 Ga silica veins in the Dresser Formation in the North Pole area of Western Australia. Petrographic investigation of the thin section indicated that the silica vein sample was mainly composed of microcrystalline quartz (< ~ 10 μm), carbonaceous matter, sulfides, and carbonates (see for example Fig. 2g, h). We chose the D/G band intensity ratio (I1350/I1600) for estimation of the degree of thermal alteration of the carbonaceous matter in order to compare with the previous spectral data of the Proterozoic samples (Qu et al 2015). The analytical errors for the Raman spectral features were determined by taking duplicate measurements within one area analyzed by FTIR microspectroscopy

Results and discussion

C Carbonaceous matter

Conclusions