Reassessing the biogenicity of Earth’s oldest trace fossil with implications for biosignatures in the search for early life
Eugene G. Grosch1 and Nicola McLoughlin
Author Affiliations
Department of Earth Science and Centre for Geobiology, University of Bergen, N-5007 Bergen, Norway
Edited* by Norman H. Sleep, Stanford University, Stanford, CA, and approved April 28, 2014 (received for review February 10, 2014)
Significance
It has been argued that Archean subseafloor pillow lava sequences provide an environment in which to seek evidence for the earliest traces of life. Candidate titanite biosignatures of microbial activity have been reported in ∼3.45-Ga metavolcanic glass from the Barberton greenstone belt of South Africa. In this paper we present new in situ U–Pb age data, metamorphic constraints, and morphological observations on these titanite microtextures. Our data challenges a biological origin for these oldest purported trace fossils, with implications for the ecological niches where life may have first emerged. We therefore suggest alternative biosignatures and approaches should be considered in the search for subsurface life on early Earth and in extraterrestrial mafic–ultramafic rocks, for example, in martian basalts.
Abstract
Microtextures in metavolcanic pillow lavas from the Barberton greenstone belt of South Africa have been argued to represent Earth’s oldest trace fossil, preserving evidence for microbial life in the Paleoarchean subseafloor. In this study we present new in situ U–Pb age, metamorphic, and morphological data on these titanite microtextures from fresh drill cores intercepting the type locality. A filamentous microtexture representing a candidate biosignature yields a U–Pb titanite age of 2.819 ± 0.2 Ga. In the same drill core hornfelsic-textured titanite discovered adjacent to a local mafic sill records an indistinguishable U–Pb age of 2.913 ± 0.31 Ga, overlapping with the estimated age of intrusion. Quantitative microscale compositional mapping, combined with chlorite thermodynamic modeling, reveals that the titanite filaments are best developed in relatively low-temperature microdomains of the chlorite matrix. We find that the microtextures exhibit a morphological continuum that bears no similarity to candidate biotextures found in the modern oceanic crust. These new findings indicate that the titanite formed during late Archean ca. 2.9 Ga thermal contact metamorphism and not in an early ca. 3.45 Ga subseafloor environment. We therefore question the syngenicity and biogenicity of these purported trace fossils. It is argued herein that the titanite microtextures are more likely abiotic porphyroblasts of thermal contact metamorphic origin that record late-stage retrograde cooling in the pillow lava country rock. A full characterization of low-temperature metamorphic events and alternative biosignatures in greenstone belt pillow lavas is thus required before candidate traces of life can be confirmed in Archean subseafloor environments.
Archean habitats astrobiology Archean Earth ichnofossil bioalteration
Footnotes
1To whom correspondence should be addressed. E-mail: geogene@gmail.com.
Author contributions: E.G.G. designed research; E.G.G. and N.M. performed research; E.G.G. and N.M. contributed new reagents/analytic tools; E.G.G. and N.M. analyzed data; and E.G.G. wrote the paper.
The authors declare no conflict of interest.
*This Direct Submission article had a prearranged editor.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1402565111/-/DCSupplemental.
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