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    Biogeosciences

    FTIR microspectroscopy of carbonaceous matter in ~ 3.5 Ga seafloor hydrothermal deposits in the North Pole area, Western Australia

    Igisu M, Ueno Y, Takai K

    FTIR microspectroscopy, Carbonaceous matter, Dresser formation, Archean

    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.