Reappraisal of hydrocarbon biomarkers in Archean rocks
Katherine L. Frencha,1,2, Christian Hallmannb,c, Janet M. Hoped, Petra L. Schoone, J. Alex Zumbergee, Yosuke Hoshinof, Carl A. Petersf, Simon C. Georgef, Gordon D. Lovee, Jochen J. Brocksd, Roger Buickg, and Roger E. Summonsh
aJoint Program in Chemical Oceanography, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, Cambridge, MA 02139;
bMax Planck Institute for Biogeochemistry, 07745 Jena, Germany;
cCenter for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany;
dResearch School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia;
eDepartment of Earth Sciences, University of California, Riverside, CA 92521;
fDepartment of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia;
gDepartment of Earth & Space Sciences and Astrobiology Program, University of Washington, Seattle, WA 98195-1310; and
hDepartment of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
Edited by Andrew H. Knoll, Harvard University, Cambridge, MA, and approved March 16, 2015 (received for review October 21, 2014)
Significance
The advent of oxygenic photosynthesis set the stage for the evolution of complex life on an oxygenated planet, but it is unknown when this transformative biochemistry emerged. The existing hydrocarbon biomarker record requires that oxygenic photosynthesis and eukaryotes emerged more than 300 million years before the Great Oxidation Event [∼2.4 billion years ago (Ga)]. We report that hopane and sterane concentrations measured in new ultraclean Archean drill cores from Australia are comparable to blank concentrations, yet their concentrations in the exteriors of conventionally collected cores of stratigraphic equivalence exceed blank concentrations by more than an order of magnitude due to surficial contamination. Consequently, previous hydrocarbon biomarker reports no longer provide valid evidence for the advent of oxygenic photosynthesis and eukaryotes by ∼2.7 Ga.
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.
oxygenic photosynthesis eukaryotes cyanobacteria Great Oxidation Event Pilbara
Footnotes
1Present address: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
2To whom correspondence should be addressed. Email: kfrench{at}whoi.edu.
Author contributions: K.L.F., C.H., S.C.G., G.D.L., J.J.B., R.B., and R.E.S. designed research; K.L.F., C.H., J.M.H., P.L.S., J.A.Z., Y.H., C.A.P., and R.B. performed research; K.L.F., C.H., Y.H., S.C.G., G.D.L., J.J.B., and R.E.S. analyzed data; and K.L.F., C.H., Y.H., S.C.G., G.D.L., J.J.B., R.B., and R.E.S. wrote the paper.
The authors declare no conflict of interest.
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