International Geology Review, Vol. 48, 2006, p. 78–88. Copyright © 2006 by V. H. Winston & Son, Inc. All rights reserved. 0020-6814/06/854/78-11 $25.00 78 Coccoid-Like Microstructures in a 3.0 Ga Chert from Western Australia Y UICHIRO UENO, 1 Research Center for the Evolving Earth and Planets, Department of Environmental Science and Technology, Tokyo Institute of Technology, Post No. S2-17, Midori-ku, Yokohama 226-8503, Japan Y UKIO ISOZAKI, Department of Earth Science and Astronomy, University of Tokyo, Meguro, Tokyo 153-8902, Japan AND KENNETH J. MCNAMARA Department of Earth and Planetary Sciences, Western Australia Museum, Perth, WA 6000, Australia Abstract Organic-walled spheroidal microstructures were discovered in a 3.0 Ga chert of the Cleaverville Formation, Western Australia. The spheroids are composed of solitary or paired cell-like units enclosed by an outer envelope, which are apparently similar to cyanobacterial microfossils. How- ever, some of the spheroidal structures appear to be related to the arrangement of the surrounding minerals, and some to overprint diagenetic fabrics. Despite several cell-like characteristics, such as organic composition, paired nature, and multi-layered envelopes, at least some spheroids were formed during diagenesis, and thus are of diagenetic origin rather than being fossilized cells. It has been generally difficult to distinguish Archean microfossils from abiotic carbonaceous structures, however, and such microscopic observations could demonstrate the abiotic origin of some superfi- cially fossil-like structures produced during diagenesis. Introduction MORPHOLOGICALLY PRESERVED ARCHEAN micro- fossils are direct evidence for the existence of life on the early Earth (e.g., Altermann and Kazmierczak, 2003). The putative microfossils so far have been reported from Archean strata, such as the 3.5–3.0 Ga Pilbara Supergroup, Australia (Awramik et al., 1983; Schopf, 1993; Rasmussen, 2000; Ueno et al., 2001) and the penecontemporaneous Swaziland Supergroup, South Africa (Knoll and Barghoorn, 1977; Walsh and Lowe, 1985; Westall et al., 2001). They show filamentous and coccoidal morphologies, some of which resemble cyanobacteria (Awramik et al., 1983; Schopf, 1993). In contrast to well-pre- served microfossils in younger Proterozoic strata, the biological origins of some Archean microfossils are still a matter of debate (e.g., Brasier et al., 2002; Schopf et al., 2002). Inorganic minerals coated by carbonaceous materials can possibly show prokary- ote-like morphologies (Schopf and Oehler, 1976; Buick, 1990; Brasier et al., 2002; Garcia-Ruiz et al., 2003). It is not yet well established how to distin- guish real microfossils from these abiologically produced fossil-like structures (Cady et al., 2003; Hofmann, 2004). Here, we report new types of spheroidal carbon- aceous structures in the 3.0 Ga chert from the Cleaverville Formation in the Pilbara craton, West- ern Australia (Fig. 1), which resemble coccoidal prokaryote. The purpose of this study is to document their mode of occurrence and morphological varia- tions. Both biological and abiological interpreta- tions are plausible for the origin of the structures. We further discuss the two possibilities in order to search for the criteria distinguishing authentic microfossils. Cleaverville Formation The Cleaverville Formation is composed of bed- ded chert, banded iron formation, siltstone, sand- stone, and minor volcaniclastic rocks (Hickman et al., 2001). The unit overlies pillowed basaltic green- stone of the Regal Formation, and is unconformably overlain by the 2775 ± 10 Ma Mt. Roe Basalt (Hick- man et al., 2001). The minimum depositional age of the Cleaverville Formation is constrained by a 3014 1 Corresponding author: email: ichiro@depe.titech.ac.jp