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3.4–3.5 Ga Microbial Biomarkers in Pillow Lavas and Hyaloclastites from the Barberton Greenstone Belt, South Africa

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3.4–3.5 Ga Microbial Biomarkers in Pillow Lavas and Hyaloclastites from the Barberton Greenstone Belt, South Africa Earth and Planetary Science Letters 241 (2006) 707–722 www.elsevier.com/locate/epsl Preservation of ~3.4–3.5 Ga microbial biomarkers in pillow lavas and hyaloclastites from the Barberton Greenstone Belt, South Africa Neil R. Banerjee a,b,*, Harald Furnes a, Karlis Muehlenbachs b, Hubert Staudigel c, Maarten de Wit d a Department of Earth Science, University of Bergen, Allegt. 41, 5007 Bergen, Norway b Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 c Scripps Institution of Oceanography, University of California, La Jolla, CA 92093-0225, USA d AEON and Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa Received 11 April 2005; received in revised form 3 November 2005; accepted 4 November 2005 Available online 19 December 2005 Editor: H. Elderfield Abstract Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Greenstone Belt in South Africa contain textural, geochemical, and isotopic biomarkers indicative of microbially mediated alteration of basaltic glass in the Archean. The textures are micrometer-scale tubular structures interpreted to have originally formed during microbial etching of glass along fractures. Textures of similar size, morphology, and distribution have been attributed to microbial activity and are commonly observed in the glassy margins of pillow lavas from in situ oceanic crust and young ophiolites. The tubes from the Barberton Greenstone Belt were preserved by precipitation of fine-grained titanite during greenschist facies metamorphism associated with seafloor hydrothermal alteration. The presence of organic carbon along the margins of the tubes and low d13C values of bulk-rock carbonate in formerly glassy samples support a biogenic origin for the tubes. Overprinting relationships of secondary minerals observed in thin section indicate the tubular structures are pre-metamorphic. Overlapping metamorphic and igneous crystallization ages thus imply the microbes colonized these rocks 3.4–3.5 Ga. Although, the search for traces of early life on Earth has recently intensified, research has largely been confined to sedimentary rocks. Subaqueous volcanic rocks represent a new geological setting in the search for early life that may preserve a largely unexplored Archean biomass. D 2005 Elsevier B.V. All rights reserved. Keywords: early life; biomarker; volcanic glass; pillow lava; greenstone belt; Archean 1. Introduction is a habitat for microorganisms. In this environment microbes colonize fractures in the glassy selvages of During the last decade several studies have shown pillow lavas, extracting energy and/or nutrients from that the upper volcanic part of the modern oceanic crust the glass by dissolving it, leaving behind biomarkers that reveal their former presence [1–12]. The biomar- kers consist of (1) corrosion structures (commonly * Corresponding author. Present address: Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada filled by secondary minerals) that have textural criteria N6A 5B7. indicative of a biogenic origin (size, morphology, dis- E-mail address: [email protected] (N.R. Banerjee). tribution as populations), (2) enrichment of C, N, P, and 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.11.011 708 N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722 S associated with the corrosion structures, (3) charac- (several tens of micrometers) tubular structures, the teristically low d13C values of disseminated carbonate experiment by Thorseth et al. [18] demonstrates the within the altered glass rims of pillows compared to onset of a dissolution process. We suggest that given their crystalline interiors, and (4) presence of DNA enough time the etching process, ultimately might associated with corrosion structures. produce the long tubular structures. Over the past The methods developed for tracing biomarkers in decade numerous studies have shown that microbe- modern oceanic crust have been successfully applied to sized corrosion structures are commonly produced by pillow lava sections of ophiolites. The ophiolites inves- biological activity in natural basaltic glasses through- tigated so far range in age from Cretaceous to Middle out the upper few hundreds of meters of the oceanic Proterozoic, range in metamorphic grade from near crust of any age, including the oldest oceanic crust in unmetamorphosed to lower amphibolite facies, and con- the western Pacific Ocean [18,2,21,3,4,22,5–7,9,11, tain all the principal components of a Penrose-type 10,23]. These structures are very distinct and cannot ophiolite (summarized in [13]). Further, in a recent be explained by abiotic processes, as supported by study of pillow lavas of the ~3.2–3.5 Ga Barberton evidence from petrography, geochemistry and molec- Greenstone Belt (BGB) in South Africa, Furnes et al. ular biology. [14] reported biomarkers related to the initial alteration Key petrographic arguments for a biogenic origin for of glassy pillow lava rims. Most products of biological the corrosion structures include their size similarity to activity are too delicate to survive geological processes microbes, their biotic morphology, and distribution as like weathering, erosion, and dynamothermal-metamor- populations. In particular, these structures commonly phism. As such destructive processes compound through occur as irregular tubes that consistently originate from geological time, it has proven to be increasingly difficult fractures. The structures are also observed to bifurcate to find preserved evidence for life as the age of a rock and never occur with a symmetric counterpart on the approaches the age of the oldest rocks on Earth. Studies other side of the fracture. Geochemical evidence of the earliest history of life are plagued also by pro- includes the common enrichment of biologically im- blems of fossil preservation and poor and ambiguous portant elements such as C, N, P, K, and S associated evidence for fossil material. In this paper we build upon with the microbial alteration structures (e.g. [4,6,7,11]) our previous work, present a new dataset of the biomar- and characteristically low d13C values of disseminated kers found in the volcanic rocks of the BGB, and stress carbonate within microbially altered basaltic glass the importance of how the study of basaltic volcanic [4,8,11]. Molecular arguments include the presence of rocks in Archean greenstone belts may contribute to the DNA associated with biological corrosion textures (e.g. discussion of the early life on Earth. [21,4,11]). As to the timing of formation of microbial alteration structures and subsequent filling of the struc- 2. Basaltic glass as a geological setting for microbial tures, it is important to mention that we have found life filled tubules in the glassy rinds of Quaternary pillow lavas (e.g. Fig. 3A of [8]). This shows that microbial Biologically mediated corrosion of synthetic glass etching and subsequent filling of the empty structures is a well-known phenomenon [15] that has also been can be a penecontemporaneous process. In the absence proposed for the pitting of natural volcanic glass [16]. of abiotic explanations for these phenomena, microbial Thorseth et al. [17] first suggested that bio-corrosion etching is the most likely explanation for these petro- was produced by colonizing microbes that cause local graphic, geochemical, and biomolecular biomarkers in variations in pH which allows them to actively dis- the glassy margins of submarine lavas. The breadth of solve the natural basaltic glass substrates thereby pro- these arguments and the abundance of these features ducing tubular structures. This process was later make it unlikely that microbial processes do not play an verified in experiments [18–20]. The microbial disso- important role during alteration of basaltic glass on the lution experiments by Thorseth et al. [18] showed that present-day seafloor. Recent work by Lysnes et al. [24] etch marks on the basaltic glass surface were produced on the microbial community diversity in young (V1 after a relatively short time (46 days). The etch marks Ma) seafloor basalts has revealed eight main phyloge- produced were of uniform size (0.3–0.5 Am in diam- netic groups of Bacteria and one group of Archaea that eter) and they had a chain or bcolonyQ shape, similar differ from those of the surrounding seawater including to the size and arrangement of the live bacteria that autolitotrophic methanogens and iron reducing bacteria. were removed from the glass surface. Although we are It should be stressed, however, that it has not yet been unaware of any experiment that has produced long possible to identify specific microbes or specific meta- N.R. Banerjee et al. / Earth and Planetary Science Letters 241 (2006) 707–722 709 bolic processes that cause the tubular corrosion struc- micromorphs (F. Westall, personal communication tures described here. 2004). These searches for early life in Greenland, Australia, 3. Evidence for early life and South Africa show very clearly that geochemical or morphological evidence for life is controversial and The evidence for earliest life on Earth fall in three underscores the need for more and better evidence for main categories: chemical evidence (e.g., carbon isoto- Archean life in the oldest rock sequences. In this paper, pic evidence), micro-morphological evidence (e.g., mi- we describe textures and
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