Multiple generations of carbonaceous material deposited in Apex chert by basin-scale pervasive hydrothermal fluid flow Alison Olcott Marshall a, ⁎, Jan Jehlička b , Jean-Noel Rouzaud c , Craig P. Marshall a a Department of Geology, University of Kansas, Lawrence, KS 66045, United States b Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic c Laboratoire de Géologie, UMR CNRS 8538, Ecole Normale Supérieure, 24 Rue Lhomond, 75231 Paris Cedex 5, France abstract article info Article history: Received 13 January 2013 Received in revised form 21 April 2013 Accepted 26 April 2013 Available online 4 May 2013 Handling Editor: M. Santosh Keywords: High-resolution transmission electron microscopy Carbonaceous material Apex chert Pilbara Craton Archean The Pilbara Craton in Western Australia contains the best-preserved and most complete record of Archean rocks in the world. As such, they are some of the most studied rocks in the world; paleontologists, isotopic geochemists, geologists and geobiologists have all investigated these rocks for clues about the early biosphere and atmosphere. Here we show using high-resolution transmission electron microscopy that the carbona- ceous material found in the Apex chert, and potentially in other associated units, was formed by multiple pro- cesses such as abiotic catalytic synthesis and/or biological synthesis. We use these data as well as the geological history of the craton to demonstrate that when the rocks of the Pilbara Craton experienced a high degree of post-depositional hydrothermal alteration, carbonaceous material could have been remobilized and redeposited. As the carbonaceous material within the Apex chert samples was formed over nearly a billion years, bulk chemistry, even at the micron level, will be unable to unambiguously delin- eate the presence of life in these ancient rocks, although nanoscale observations may provide a way forward in the search for ancient life. © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction The Pilbara Craton of Western Australia contains supracrustal green- stone successions dated from 3660 Ma to 2830 Ma (Hickman, 2010; Brasier et al., 2011) unconformably overlain by 2800–2400 Ma volcano- sedimentary successions (Rasmussen et al., 2005a)(Fig. 1). The Marble Bar greenstone belt, located in the northeast of the craton, is part of the 3530–3170 Ma East Pilbara Terrane and is primarily composed of the 3525 Ma–3427 Ma Warrawoona Group (Van Kranendonk et al., 2007). Found within the upper part of the Warrawoona Group is the Apex Basalt which contains the stratiform chert layer informally referred to as the Apex chert (Brasier et al., 2011)(Fig. 1). The Apex chert has been the cen- ter of many debates about whether these rocks, and by extension, other Early Archean rocks, contain unambiguous evidence of an early biosphere (Schopf and Packer, 1987; Schopf, 1993; Brasier et al., 2002; De Gregorio et al., 2009; Pinti et al., 2009; Brasier et al., 2011; Marshall et al., 2011; Olcott Marshall et al., 2012; Schopf and Kudryavtsev, 2012; Olcott Marshall and Marshall, 2013; Pinti et al., 2013; Schopf and Kudryavtsev, 2013). Originally, these rocks were described to contain cyanobacterial microfossils (Schopf and Packer, 1987; Schopf, 1993), although later stud- ies have described these features as having a morphology (Brasier et al., 2002, 2011) and mineralogy (Marshall et al., 2011) inconsistent with life. Furthermore, researchers have since discovered that these micro- structures were not collected from the stratiform sedimentary chert, but rather from a synsedimentary hydrothermal fracture-fill vein (Van Kranendonk et al., 2007; Brasier et al., 2011)(Fig. 2). In addition to the controversial microstructures described from this vein (hereafter referred to as the Apex chert vein, or ACV), these rocks contain carbonaceous ma- terial (CM) of unknown origin. Some have described this CM as being of abiotic catalytic origin, similar to Fischer–Tropsch-type synthesis (Brasier et al., 2002, 2011), and other researchers have used Raman spec- troscopy (Schopf et al., 2002) and synchrotron radiation-based spectros- copy (De Gregorio et al., 2009) to characterize this CM as biogenic in origin. Although Raman spectroscopy is definitively not sufficient in and of itself to determine the source of CM, including whether it is biogenic (Pasteris and Wopenka, 2002, 2003; Marshall et al., 2010), we recently used Raman spectroscopy in a paragenetic framework to demonstrate that the CM from the ACV is from at least two separate populations (Olcott Marshall et al., 2012). The uncertainty about the source of the CM is not unique to this locality; Early Archean CM has been analyzed with many techniques, including stable isotopic measurements, Raman spectroscopy, hydrogen-pyrolysis gas chromatography/mass spectrome- try, nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, electron energy loss spectroscopy, and X-ray absorption near-edge spectroscopy, but none of the data have unambiguously delin- eated any Archean CM as of biological origin (see Marshall et al., 2007 for further discussion). Gondwana Research 25 (2014) 284–289 ⁎ Corresponding author. Tel.: +1 7858641917. E-mail address: olcott@ku.edu (A. Olcott Marshall). 1342-937X/$ – see front matter © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gr.2013.04.006 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr