Pumice from the ∼3460 Ma Apex Basalt, Western Australia: A natural laboratory for the early biosphere
Highlights
► Abundant pumice clasts are found in the ∼3460 Ma Apex Basalt. ► This discovery enables the testing of the hypothesis that pumice provided a cradle for early life. ► Pumice clasts contain potential biominerals including sulfides and phosphates, plus intimate associations of C, N, P and S. ► Pumice clasts contain catalytic minerals such as titanium oxide, plus altered clays and zeolites. ► Catalysts and molecules needed for the earliest stages of life were found in pumice rafts in the earliest oceans on Earth.
Introduction
Was there an unusual kind of geological substrate, still preserved in the early Archaean geological record, that could have concentrated, selected and catalysed the diversity of chemical reactants needed for life? Was this environment not only widespread and abundant but also capable of providing reaction chambers with maximum surface area over sufficiently long periods of time? In a recent article, Brasier et al. (2011b) have suggested that the answer to these questions could be ‘yes’, within the vastly abundant mineral vesicles of pumice and related rocks (Fig. 1). The special properties of pumice have been discussed in detail elsewhere (Brasier et al., 2011b) but a summary is given here to enable the reader to understand the context for the ‘pumice hypothesis’ with regard to the origin of life.
Pumice has the highest surface area to volume ratio of any rock type. Today this allows for high levels of biological colonisation (van Houten et al., 1995, Di Lorenzo et al., 2005) while, in a prebiotic world, pumice would have a great capacity for adsorbing biologically relevant elements and compounds, and maximising the surface area available for chemical reactions. Pumice is the only rock type known to float on the surface of the ocean for sustained periods, owing to its high pore-volume and gas-filled vesicles. It can then come to sit within the intertidal zone of beaches for many thousands of years. This unique position at the air water interface is potentially ideal for the mixing of atmospheric and trapped volcanic gases with water. Pumice would also be able to adsorb organic complexes of the kind thought to have existed locally as oily slicks at the surface of the Archaean ocean (Nilson, 2002, Deamer et al., 2002, Bada, 2004), creating lipid lined vesicles, precursors to the synthesis of peptides, proteins, ATP, and nucleic acids. Thermal conditions would not have been severe on the surface of the ocean, and the vesicular nature of pumice would have provided protection from harsh UV radiation.
Pumice also experiences an unusually wide variety of conditions from eruption to burial via floating (rafting) and benthic stages (Fig. 2; see also Brasier et al., 2011b). These include: exposure to triboelectric charges, Leidenfrost boiling (see James et al., 2008) and UV light, raising the possibility of Miller–Urey syntheses (e.g. Miller, 1992); hydrocarbon production via electric discharges through mixtures of ‘primitive’ gases (Navarro-González and Basiuk, 1996, Segura and Navarro-Gonzalez, 2001); and production of bio-available phosphorus and fixed nitrogen by lightning (Yamagata et al., 1991, Schwartz, 2006, Mather and Harrison, 2006). During its life cycle pumice would also likely experience energy-releasing cycles of heat, light and tides at diurnal to seasonal scales, alkaline waters, freeze–thaw conditions of the kind favoured by some for nucleic acid syntheses (cf. Miller, 1992, Menor-Salvan et al., 2007), episodic changes in salinity (hence osmotic gradients), plus hydration and dehydration of organic compounds held within its vesicles. These types of cycle would be particularly prevalent in pumice beached along marine or lake shorelines.
Not least among the virtues of pumice and scoria for early life research is their capacity to host secondary minerals with known catalytic potential. Interaction with hydrothermal fluids readily produces microporous aluminosilicates called zeolites. These can grow alongside smectite clays when siliceous pumice reacts with alkaline waters (e.g. Tomita et al., 1993), forming amygdale infillings. Such zeolite minerals can greatly boost the catalytic activity of pumice during industrial processes today (Brito et al., 2004). Hence zeolites (or ‘silicalite zeolites’ with periodic Ti atoms in place of Si; e.g. Yamashita et al., 2007) are used for the synthesis of organic polymers (e.g. de Vos and Jacobs, 2001) as well as for the cracking of hydrocarbons and the commercial liberation of hydrogen gas (van Bekkam et al., 2001). Of equal interest is the potential of pumice to generate clay minerals that could have acted as potential templates for replication within early protocells (e.g. Cairns-Smith, 2005). Of these, smectite clays formed by hydrothermal alteration of volcanic glass, provide important catalysts, bringing about abiotic synthesis of polycyclic aromatic hydrocarbons and long-chain esters (Wiliams et al., 2010), while montmorillonite clays can catalyse the condensation of activated mononucleotides towards much longer RNA oligomers (Ferris, 2002). Of considerable interest, too, is the potential of pumice to convert into halloysite clay nanotubes. These ultra-tiny hollow tubes occur naturally and abundantly during the weathering of pumice towards kaolinite clays (Aomine and Wada, 1962), producing nanotubes typically 1–30 nm in diameter, with lengths from c. 500 nm to over 1.2 μm. Such nanotubes have noted capacities to act as molecular sieves for the uptake of aqueous ‘pollutants’ (Du et al., 2010).
The pumice hypothesis is now available for testing against the geological record, and that is the aim of this contribution. Volcaniclastic rocks are abundant in Archaean cratons, and some of these are now being investigated and evaluated as possible sites for early life (Walsh, 2004, Brasier et al., 2010, Westall et al., 2011). Indeed, pumice has been reported from a putative tidal beach setting in rocks as old as 3490 Ma (Buick and Dunlop, 1990). Even so, no biogeochemical studies of Archaean pumice have yet been performed. Below, we focus upon pumice from the 3460 Ma Apex Basalt to test the following postulates of the pumice hypothesis: 1. That highly porous pumice was common on the early Earth; 2. That Archaean pumice attracted concentrations of vital elements essential for life, such as C, H, O, N, P and S; 3. That Archaean pumice played host to secondary minerals with the potential to catalyse the formation of organic polymers.
Section snippets
Materials and methods
Our pumice samples come from the ∼3460 Ma Apex Basalt of the Warrawoona Group in Western Australia (Fig. 3a). This unit is of great value for studies of the origins of life because it combines evidence not only for komatiitic lavas, tuffs and porous volcanic rocks but it also contains some of the best preserved silica-rich hydrothermal and fissure eruption systems known from the Archaean (Brasier et al., 2002, Brasier et al., 2005, Brasier et al., 2011a, Van Kranendonk et al., 2002, Van
Context and petrography
Felsic rock fragments, tuff, pumice and other vesicular pyroclastic rock debris have been widely reported from the Archaean of the eastern Pilbara (e.g. DiMarco and Lowe, 1989a, DiMarco and Lowe, 1989b, DiMarco and Lowe, 1989c). The presence of features consistent with stranded pumice rafts in the Warrawoona Group at North Pole, Western Australia has even been reported by Buick and Dunlop (1990), based on coarse, pure, matrix-free pumice sand without grading of the kind associated with
Conclusion
This study of ∼3460 Ma pumice clasts allows us now to address the three geologically testable aspects of the pumice hypothesis outlined at the start of this paper.
- 1.
We confirm that clasts with the texture and mineralogy of pumice can be widespread in Archaean rock successions. Although volumetrically small when compared with the vast thicknesses of pillow lava and komatiite (see Fig. 3a and b), pumice-dominated pyroclastic deposits formed beds several metres thick and many kilometres wide. These
Acknowledgements
We thank Dave Waters, Graham Cairns-Smith, Harold Morowitz, Norman Sleep, Ian Parsons and Lynn Margulis for much appreciated encouragement and advice; Nicola McLoughlin, Martin van Kranendonk, Cris Stoakes, Arthur Hickman, Leila Battison, Kate Hendry, Owen Green and John Lindsay for invaluable assistance with field work in Australia and Canada; David Pyle and Tamsin Mather for essential guidance on pumice and volcaniclastic rocks; Scott Bryan for providing examples of modern pumice; Owen Green,
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