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, 112 (19), 5915-20

Reappraisal of Hydrocarbon Biomarkers in Archean Rocks

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Reappraisal of Hydrocarbon Biomarkers in Archean Rocks

Katherine L French et al. Proc Natl Acad Sci U S A.

Abstract

Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼ 2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories.

Keywords: Great Oxidation Event; Pilbara; cyanobacteria; eukaryotes; oxygenic photosynthesis.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Hopanes and steranes in rock extracts from AIDP-2 compared with RHDH2A. Comparison and lithological correlation of the intersected stratigraphy of AIDP-2 (Center Left) and RHDH2A (Center Right) based on previous descriptions of RHDH2A (3, 19). The age references are listed in SI Appendix, and red markers denote the stratigraphic depths of analyzed samples (SI Appendix, Table S1). The MRM analyses of hopanes and steranes monitored the transitions from the molecular ion to the m/z 191 and m/z 217 fragments, respectively. In an RHDH2A sample from the Carawine Dolomite, the C30–34 hopane and C26–29 sterane concentrations are higher in the core surfaces (surf.) compared with the interior (Right). In contrast, hopanes and steranes were not detected in MRM analyses of either interiors or exteriors of a correlative sample from the Carawine Dolomite recovered at 170.4 m in AIDP-2 (Top Left) or from the underlying Roy Hill Member of the Jeerinah Formation at 325.5 m (Bottom Left).
Fig. 2.
Fig. 2.
Compound specific and bulk organic δ13C data. Stable carbon isotopic measurements of bulk organic matter from AIDP-2 and RHDH2A (3, 39) are compared (Center). The RHDH2A depth was converted to the AIDP-2 depth scale by a correction factor of 45 m due to the stratigraphic offset (see Fig. 1). Rock extract and HyPy full scan chromatograms are plotted with compound-specific isotopic analysis (CSIA) of PAH δ13C. AIDP-2 133.55 m (SI Appendix, Fig. S3) is the only sample that displayed an isotopic match between the extractable PAHs and the bulk organic matter (Top Right). The extractable PAH δ13C from the other Carawine Dolomite samples diverged from the corresponding bulk organic matter δ13C (Top Left). The hydropyrolysate PAHs from the Carawine Dolomite and the Jeerinah Formation match the bulk organic carbon isotopic composition (Bottom Left and Bottom Right). The range of previously published (Pub.) Archean PAH δ13C is plotted for reference (21, 24, 25). The analytical uncertainty is plotted for the PAH δ13C measurements, but it is smaller than the symbol in most cases. The different GC methods yield different retention times.

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