Precambrian Research 158 (2007) 156–176 Comparing petrographic signatures of bioalteration in recent to Mesoarchean pillow lavas: Tracing subsurface life in oceanic igneous rocks Harald Furnes a,∗, Neil R. Banerjee b, Hubert Staudigel c, Karlis Muehlenbachs d, Nicola McLoughlin a, Maarten de Wit e, Martin Van Kranendonk f a Centre for Geobiology and Department of Earth Science, University of Bergen, Allegt. 41, 5007 Bergen, Norway b Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7 c Scripps Institution of Oceanography, University of California, La Jolla, CA 92093-0225, USA d Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 e AEON and Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa f Geological Survey of Western Australia, 100 Plain St. East Perth, Western Australia 6004, Australia Received 31 October 2006; received in revised form 12 March 2007; accepted 28 April 2007 Abstract Bioalteration of basaltic glass in pillow lava rims and glassy volcanic breccias (hyaloclastites) produces several distinctive traces including conspicuous petrographic textures. These biologically generated textures include granular and tubular morphologies that form during glass dissolution by microbes and subsequent precipitation of amorphous material. Such bioalteration textures have been described from upper, in situ oceanic crust spanning the youngest to the oldest oceanic basins (0–170 Ma). The granular type consists of individual and/or coalescing spherical bodies with diameters typically around 0.4 ␮m. These are by far the most abundant, having been traced up to ∼550 m depths in the oceanic crust. The tubular type is defined by distinct, straight to irregular tubes with diameters most commonly around 1–2 ␮m and lengths exceeding 100 ␮m. The tubes are most abundant between ∼50 m and 250 m into the volcanic basement. We advance a model for the production of these bioalteration textures and propose criteria for testing the biogenicity and antiquity of ancient examples. Similar bioalteration textures have also been found in hyaloclastites and well-preserved pillow lava margins of Phanerozoic to Proterozoic ophiolites and Archean greenstone belts. The latter include pillow lavas and hyaloclastites from the Mesoarchean Barberton Greenstone Belt of South Africa and the East Pilbara Terrane of the Pilbara Craton, Western Australia, where conspicuous titanite-mineralized tubes, have been found. Petrographic relationships and age data confirm that these structures developed in the Archean. Thus, these biologically generated textures may provide an important tool for mapping the deep oceanic biosphere and for tracing some of the earliest biological processes on Earth and perhaps other planetary surfaces. © 2007 Elsevier B.V. All rights reserved. Keywords: Bioalteration textures; Volcanic glass; Oceanic crust; Ophiolites; Greenstone belts; Evidence for early life 1. Introduction Life on Earth may have evolved well before the old- est preserved rocks, prior to 3.8 Ga, and most likely in ∗ Corresponding author. Tel.: +47 5558 3530; fax: +47 5558 3660. the vicinity of hydrothermal vents in the oceanic crust E-mail address: [email protected] (H. Furnes). (Nisbet and Sleep, 2001; Canfield et al., 2006). Evidence 0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.04.012 H. Furnes et al. / Precambrian Research 158 (2007) 156–176 157 for the earliest (∼3.8 Ga) purported life on Earth is solely et al., 1998; Furnes and Staudigel, 1999; Banerjee geochemical and consists of isotopically light carbon and Muehlenbachs, 2003). Furthermore, Furnes and in graphite contained within amphibolite- to granulite- Staudigel (1999) have demonstrated that the bioalter- grade supracrustal rocks in the Itsaq Gneiss Complex ation process can be traced as deep as ∼550 m into the of the North Atlantic Craton in southwest Greenland oceanic crust and that these biotic alteration processes (Schidlowski, 1988, 2001; Mojzsis et al., 1996; Rosing, dominate in the upper ∼350 m of the volcanic crust. 1999), but this evidence has been controversial (van Thus, a large body of evidence collected over the last Zuilen et al., 2002; Lepland et al., 2005). The earliest decade has convincingly demonstrated that the upper candidate fossilized microorganisms, on the other hand, volcanic part of the in situ oceanic crust is a habitat for are found in rocks ∼3.5 Ga from the Pilbara Craton in microbial life. Moreover, the bioalteration of pillow lava Australia (Walter et al., 1980; Schopf, 1993; Hoffman et glass is a widespread and common process that may have al., 1999; Ueno et al., 2001; Van Kranendonk et al., 2003; a profound effect on the chemical reactions, fluxes and Allwood et al., 2006) and the Barberton Greenstone products of seawater–rock interactions (e.g. Staudigel Belt in South Africa (Muir and Grant, 1976; Knoll and and Furnes, 2004; Staudigel et al., 2004). Barghoorn, 1977; Walsh and Lowe, 1985; Walsh, 1992; Textural studies of pillow lavas from in situ oceanic Westall et al., 2001, 2006), but many of these claims have crust can only record bio-interaction with pillow lavas likewise proved controversial (e.g. Lowe, 1994; Brasier dating back to the oldest intact example of ∼170 Ma et al., 2002, 2005; Garcia-Ruiz et al., 2003). (Fisk et al., 1999). This represents only a small fraction of Until recently, only sediments were considered to Earth’s history and to extend the record of bioalteration provide habitats for microbial activity, leaving volcanic further back in Earth’s history, it is therefore necessary rocks largely unexplored by biogeoscience research. to investigate pillow lavas of former oceanic crust repre- Recent studies have shown that submarine glassy basaltic sented by ophiolites and greenstone belts. These studies rocks also provide habitats for microbial life, first con- have so far confirmed that similar microbe-rock interac- vincingly shown by Thorseth et al. (1992). Moreover, tions have taken place within formerly glassy volcanic it has been suggested that soon after eruption, when rocks since the Mesoarchean (Furnes and Muehlenbachs, the ambient temperature (<113 ◦C) is tolerable for life 2003; Furnes et al., 2004, 2005; Banerjee et al., 2006a). to exist (Stetter et al., 1990; Stetter, 2006), coloniza- Thus bioalteration textures provide a new search tool for tion of the glassy rim of pillow lavas by microorganisms the earliest signs of life on Earth and other planetary sur- occurs contemporaneously wherever seawater has access faces (e.g. Banerjee et al., 2004a,b, 2006b; McLoughlin (Thorseth et al., 2001). et al., 2007). Microbial colonization of the glassy selvages of pil- In this paper, we first describe the range of alteration low lavas is most commonly observed along fractures, textures that are found within the glassy rims of pillow leaving behind several traces of their former presence. lavas from in situ oceanic crust. We then present a model The most ubiquitous are microscopic alteration tex- for the biotic alteration of oceanic pillow lavas and pro- tures found within the fresh glass at the interface with posed criteria for testing the antiquity and biogenicity of altered glass. These are empty or mineral-filled pits and these bioalteration textures. We then proceed to demon- channels with sizes and shapes that are comparable to strate that similar bioalteration textures are preserved in modern microbes. Furthermore, samples with these pet- ancient pillow lavas from ophiolites and greenstone belts rographic alteration textures commonly show very low back to 3.5 billion years ago. We outline the petrographic δ13C values (e.g. Furnes et al., 1999, 2001a; Banerjee and characteristics of mineralised bioalteration structures in Muehlenbachs, 2003), elevated concentrations of ele- ancient pillow lava rims and hyaloclastites and review ments such as C, N, P, K and S (e.g. Furnes et al., 2001b; what is currently known about how these biostructures Banerjee and Muehlenbachs, 2003), and in younger sam- are preserved. Lastly, we explore how bioalteration tex- ples the presence of DNA (e.g. Torsvik et al., 1998; tures found in terrestrial pillow lavas may be sought in Furnes et al., 2001a; Banerjee and Muehlenbachs, 2003), extraterrestrial rocks. all of which are strongly suggestive of a biogenic origin. Several studies have documented alteration textures, 2. Alteration textures in pillow lava of the element distributions and carbon isotope compositions modern oceanic crust of pillow lavas from in situ oceanic crust world-wide that are indicative of microbial alteration processes There are two fundamentally different modes of alter- (Thorseth et al., 1995a, 2001, 2003; Furnes et al., ation of basaltic glass in modern seafloor setting, i.e. 1996, 1999, 2001a,b; Fisk et al., 1998, 2003; Torsvik abiotic and biotic alteration. The abiotic alteration results 158 H. Furnes et al. / Precambrian Research 158 (2007) 156–176 in the formation of the long-recognized, but enigmatic as palagonite. It appears as banded material of approx- material termed palagonite. The other alteration mode imately equal thickness on both sides of fractures, with is the more recently-recognized biotic etching gener- smooth alteration fronts between the fresh and altered ated by the microbial colonization of rock surfaces. glass that are symmetric with respect to the fracture. The two alteration processes may be