Biosignatures and abiotic constraints on early life

Abstract

Arising from: Y. Ueno, K. Yamada, N. Yoshida, S. Maruyama & Y. Isozaki Nature 440, 516–519 (2006); Ueno et al. reply

Ueno et al.¹ contend that methane found in fluid inclusions within hydrothermally precipitated quartz in the Dresser Formation of western Australia (which is roughly 3.5 Gyr old) provides evidence for microbial methanogenesis in the early Archaean era. The authors discount alternative origins for this methane, suggesting that the range of ${\text{δ}}^{\text{34}}{\text{C}}_{{\text{CH}}_{\text{4}}}$ values that they record (−56 to −36‰) is attributable to mixing between a primary microbial end-member with a ${\text{δ}}^{\text{34}}{\text{C}}_{{\text{CH}}_{\text{4}}}$ value of less than −56‰ and a mature thermogenic gas enriched in ¹³C (about −36‰). However, abiotic methane produced experimentally^2,3 and in other Precambrian greenstone settings^4,5 has ¹³C-depleted ${\text{δ}}^{\text{13}}{\text{C}}_{{\text{CH}}_{\text{4}}}$ values, as well as ${\text{δ}}^{\text{13}}{\text{C}}_{{\text{CO}}_{\text{2}}{\text{-CH}}_{\text{4}}}$ relationships that encompass the range measured for the inclusions by Ueno et al. — which suggests that an alternative, abiotic origin for the methane is equally plausible. The conclusions of Ueno et al. about the timing of the onset of microbial methanogenesis might not therefore be justified.

Main

The range of isotopic values for abiotic methane shown in Fig. 2c of Ueno et al.¹ differs distinctly from those from thermogenic and microbial sources, and does not encompass the methane in their fluid inclusions. However, the range of ${\text{δ}}^{\text{13}}{\text{C}}_{{\text{CH}}_{\text{4}}}$ for abiotic hydrocarbons is larger than that used in their comparison. Abiotic methane end-members in Precambrian Shield rocks extend to much lighter ${\text{δ}}^{\text{13}}{\text{C}}_{{\text{CH}}_{\text{4}}}$ values (−47 to −28‰)^4,5. Also, as noted by Ueno et al., experimental studies² indicate that abiotic synthesis using a Ni–Fe alloy catalyst may produce methane with a range of isotopic compositions (${\text{δ}}^{\text{13}}{\text{C}}_{{\text{CH}}_{\text{4}}}$ values of −54 to −19‰ at 200–300 °C) that is similar to the range in the inclusions¹. Although those results have not been duplicated², they have not been disputed either. In fact, hydrothermal abiotic methane synthesis has been confirmed with Ni–Fe alloy⁶ and other potential mineral catalysts^7,8.

The values of ${\text{Δ}}^{\text{13}}{\text{C}}_{{\text{CO}}_{\text{2}}{\text{-CH}}_{\text{4}}}$ measured by Ueno et al. in their inclusions (34–52‰; see their Supplementary Table 2) span a range that is identical to, or even smaller than, those produced in abiotic synthesis experiments (33–66‰)^2,3. Furthermore, the experimental range would encompass the larger value the authors infer for primary inclusions (${\text{Δ}}^{\text{13}}{\text{C}}_{{\text{CO}}_{\text{2}}{\text{-CH}}_{\text{4}}}$ >52‰). The authors also contend that the lack of C₂⁺ compounds in those inclusions is in agreement with a microbial origin; however, C₂⁺ hydrocarbons were not detected during methane synthesis in abiotic experiments² either, so such reactions can also produce methane with a low C₂⁺/(C₁+C₂⁺) ratio (<<0.01). Therefore, neither an absence of C₂⁺ nor isotopic compositions can distinguish between microbial and abiotic origins for this data set, even when both factors are considered together.

Ueno et al. also discount an abiotic source on the grounds that Fe–Ni alloys and other catalysts for Fischer–Tropsch-type synthesis reactions would not have existed in the surrounding rocks at conditions prevailing during silica precipitation. But abiotic methane could have been synthesized during interaction of migrating hydrothermal fluids with ultramafic rocks elsewhere in the system in the presence of either Ni–Fe alloy or other mineral catalysts, and transported to the site of quartz deposition^3,9,10, so the absence of suitable catalysts immediately adjacent to the fluid inclusion-bearing quartz would not preclude an abiotic origin. Indeed, Ueno et al. explain the inferred thermogenic origin of methane in the secondary inclusions using migration from surrounding rocks.

The key criteria used by Ueno et al. to establish a microbial origin for methane in the inclusions from the Dresser Formation are therefore equally consistent with an abiotic origin, and their conclusion about the antiquity of the onset of microbial methanogenesis should be viewed with caution. Biosignatures that are mimicked by abiotic processes need to be analysed closely to evaluate not only signs of life in rocks from the early Earth but also the search for life on other planets.