Chapter I: Epistemic Challenges
I. Epistemic Challenges to Archaean Geobiology
“It may be [...] that all enquiry on our part is set so as to exempt certain propositions from doubt, if they are ever formulated. They lie apart from the route travelled by enquiry.”
- Ludwig Wittgenstein, Über Gewißheit(1969)
1.Introduction
Archaean geobiologists employ a host of different techniques to scrutinize the Archaean record for clues on early surface conditions and signs of early life. Severe limitations are placed upon any study of ancient rocks, however, careful consideration of which infuses derived conclusions with important caveats. It is the brief purpose of the present chapter to emphasize these caveats.
“[…] I continue to think of the conceptual scheme of science as a tool, ultimately”, writes an eminent 20th Century philosopher of science, “for predicting future experience in the light of past experience” (Quine, 1951). This highly general conceptualization of science is highlighted here in order to illustrate the unique epistemic status held by Archaean geology, which is tasked with predicting (as it were) the deep past on the basis of present experience.A method towards this end was proposed by Charles Lyell, who subtitled his Principles of Geology (1830-1833) with the words: “An attempt to explain the former changes of the Earth's surface by reference to causes now in operation” (Figure 1). This method, known today as the ‘uniformitarian doctrine’, was likely inspired by James Hutton, who earlier (1785) stated that, in geology, “[…] no powers are to be employed that are not natural to the globe, no action to be admitted except those of which we know the principle” (cited in Holmes, 1965).
The applicability of uniformitarianism to Archaean geology is vehemently defended by adherents today (e.g. Eriksson et al., 1994). But what are we to do when presented, as in the case of banded-iron formation, with geological phenomena the ‘action’ or ‘causes’ of which are no longer in operation? Would strictly applied uniformitarianism, to give one further example, not bias our interpretation of early continental differentiation processes towards alignment with the modern, that is, Phanerozoic plate-tectonic paradigm? In this manner, overemphasis of the present may be the key to misunderstanding the past.
2.Over-Representationthrough Preservation
Dominant amongst these potential epistemic pitfalls is therepresentativeness of the available data, which is of limited spatial and temporal extent:very old rocks are scarce (Figure 2, 3). As rocks mature, their potential exposure to agents of weathering and erosion increases, as does the probability of structural deformation, with or without accompanying metamorphic and metasomatic overprinting (Figure 4). Consider that, while the age of the Earth is fairly well constrained at 4.56 Ga(Patterson, 1956; Zhang, 2002), the oldest crustal rocks found to-date have an age of ‘merely’ 3.85 Ga(Allaart, 1976; Cates and Mojzsis, 2007; O'Neill et al., 2007). This leaves a gap of well over half a billion years of Earth history for which the Earth’s rock record cannot provide insight.
The Archaean geobiologist, then, has a limited number of outcrops to work with, all of which may represent different depositional environments, ages and locations. Just how representative of early crustal conditions are ancient rocks?Doubtless, the surviving rock record is heavily skewed towards tectonically quiescent environments amenable to preservatory processes. For illustrative purposes, some pertinent examples of preservation bias follow.
Tectonic preservation
As evidenced by the affiliation of ancient rocks with durable cratons, supra-continental emplacement allows prolonged protection from destructive convergent tectonism and mantle recycling. In consequence, epi- and peri-continental settings enjoy preferential preservation, heavily skewing geological interpretations towards allied diastrophic processes.
Structural preservation
In the face of the leveling effects of erosion, sedimentary bedding and other planar features approaching vertical orientation will enjoy preferential persistence. The sub-vertical bedding and foliation typifying greenstone belt outcrop (Figure 5) may partly be a preservational artifact. Seismic cross-sections of greenstone belts reveal sheet-like geometries (Figure 6), possibly supporting the interpretation that the diagnostic vertical fabric in fact represents an anomalous and local feature of Archaean granitoid margins.
Chemical preservation
Mineral assemblages that equilibrated under conditions that represent a significant departure from those encountered on or in the modern crust will have faced preferential removal from the geological record: exactly those assemblages that are the most telling of exotic Archaean conditions are most vulnerable. Chemical sediments precipitated on the floor of a more acidic and less oxidizing Archaean ocean, for instance, will be disproportionately affected by exposure to higher pH and fO2 conditions prevalent today.
Of particular relevance here is the protective role that certain metasomatic and diagenetic phenomena can play. Silicification, in particular, obliterates chemical information but preserves otherwise vulnerable textures to microscopic scale and smaller (Figure 7, consider furthersilicified oncoids in Chapter 7). Such secondary cryptochrystalline silica is largely impregnable to migrating fluids, invulnerable to redox reactions, insoluble within a wide pH range, and resistant to all but the highest grades of metamorphism. Consequently, Archaean environments conducive to silicification will be strongly over-represented in the rock record.Such a depositional preservation bias could be expected in marginal regions experiencing high silica influx, such as near-shore environments, and in environments where abrupt decreases in silica solubility cause pervasive silicification, such as at sediment/water interfaces where underlying hot acidic silica-saturated hydrothermal fluids meet overlying cold alkaline marine bottom-waters.
Under some circumstances, the dehydration reactions that accompany progressive low-grade metamorphism also have the paradoxical ability to enhance the preservation potential of rocks relative to less weathering-resistant hydrous protolith. An example of this effect is seen through contact metamorphism of the Pilbara’s lowermost Coonterunah Group, where hitherto undiscovered deep-water metapelites (Chapter 5) have survived only at upper-greenschist facies within the contact aureole of the intrusive Carlindi batholith.
Environments conducive to rapid burial, meanwhile, protect rocks from aggressive surface weathering reactions and enhance lithification rates. The eruption of a ~4 km pile of basaltic and lesser gabbroic flows shortly following deposition likely allowed for preservation of the oldest sedimentary carbonate unit yet discovered (Chapter 5).
3.Variability and Secularism
As contested by a slew of ongoing controversies, some over a century old, the interpretation of a fragmentary – and, as argued above, preservationally-biased - rock record faces considerable challenges. One further issue pertinent to geological evidence for ancient environments merits consideration: how variable were Early Archaean conditions? To what extent is it meaningful, for instance, to speak of ‘the Archaean surface ocean temperature’ or ‘the Archaean climate’ or ‘the Archaean atmospheric composition’?
Indeed, the case could be made that compared to the Earth System today, a less evolved Archaean prokaryotic ecto-sphere, subject to the vigorous and stochastic behaviour (Nelson, 2004) of a nascent and intermittently bombarded (Anbar et al., 2001; Simonson et al., 2004) planetary crust, would be a less efficient dampener of perturbations and consequently experience more frequent and higher amplitude oscillations (Lenton, 2004).
Furthermore, a majority of biogeochemically important ions have residence times < 107 years in modern oceans (Holland, 1978; Holland, 1984). The Archaean Era spanned 1.3109 years, and its ocean(s) likely experienced enhanced turnover rates in many hydrothermally fluxed ions.
Faced with a scarcity of data, the human mind reaches all too eagerly for secularism.Perhaps some consolation can be found in the observation that“[f]alse facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do little harm, for everyone takes a salutary pleasure in proving their falseness; and when this is done, one path towards error is closed and the road to truth is often at the same time opened.” (Darwin, 1871)
4.Candidate Evidence for Life
An obvious approach to early Life would seem to be to examine the rock for microfossils. However, early biological structures – if preserved - may well have been far simpler and perhaps structurally and functionally different from those encountered today.A second, more subtle approach is to search for chemical fossils, or biosignatures.
Despite a diverse plethora of putative evidence, no unambiguous proof exists for Early Archaean life (Figure 8). Purported evidence for life includes: (i) microfossils; (ii) stromatolitic trace fossils; (iii) microstructures housed in volcanic glass; and (iv) isotopically fractionated carbon, sulphur and/or nitrogen.
Early claims of microfossils at Isua (Pflug, 1978; Pflug, 1979; Pflug and Jaeschke-Boyer, 1979) have been convincingly refuted (Roedder, 1981). More recent purported microfossils elsewhere (Awramik et al., 1983; Schopf, 1993; Schopf et al., 2002; Schopf and Packer, 1987; Walsh, 1992; Walsh and Lowe, 1983; Walsh and Lowe, 1985; Westall et al., 2001) are steeped in controversy (Brasier et al., 2002; Brasier et al., 2005).Although compelling, stromatoloids of purported stromatolitic origin (Allwood et al., 2006; Hofmann et al., 1999; Lowe, 1980; Lowe, 1983; Van Kranendonk et al., 2003; Walter, 1980) remain susceptible to non-biological interpretations (Buick et al., 1981; Grotzinger and Knoll, 1999; Grotzinger and Rothman, 1996; Lowe, 1994). Endolithic microborings (Banerjee et al., 2006; Furnes et al., 2004) cannot definitively be distinguished from ambient inclusion trails (Knoll and Barghoorn, 1974).
Chemical evidence is, in some ways, more susceptible to ambiguity (e.g. contamination, abiotic non-equilibrium processes – see Chapter 3). Although the conditions facing Early Archaean microbes were very different, the mechanisms of preservation and geological evolution of their decay products was likely similar (Figure 9). Geochemical analysis of some ancient kerogen has yielded surprisingly good biomarker results (Brocks et al., 1999; but see Rasmussen et al., 2008), although it may prove optimistic to expect biomarkers from the older and higher-grade rocks in South Africa and Australia. Kerogen undergoes thermodynamic re-equilibration to less-informative graphite following diagenesis and metamorphism.
Isotopically light carbon, in the form of graphite, has been reported in Akilia and Isua (Mojzsis et al., 1996; Rosing, 1999; Schidlowski, 2001) but may have arisen through the non-biological decarbonation of siderite (van Zuilen et al., 2002; van Zuilen et al., 2003) or through non-equilibrium thermodynamics (see also Chapter 3). Isotopically depleted kerogen at Barberton and in the Pilbara could be the product ancient Fischer-Tropsch-type (‘FTT’) synthesis (Brasier et al., 2002; Brasier et al., 2005; Fischer, 1935; Ueno et al., 2001; Ueno et al., 2003; Ueno et al., 2004). Although sulphur isotopic evidence is promising (Shen et al., 2001), little certainty exists regarding ancient sulphur cycling and isotopic fractionation incurred (Philippot et al., 2007; Shen et al., 2009; Ueno et al., 2008). High mobility, the lack of mineralogical sequestration and susceptibility to devolatilization hampers the use of nitrogenisotopes (both as 15Norg and 15NH4+) as a biosignature, although this has not deterred all workers(Beaumont and Robert, 1998; Beaumont and Robert, 1999; Garvin et al., 2009; Pinti and Hashizume, 2001; Pinti et al., 2007; van Zuilen et al., 2005).
The Precautionary Null Hypothesis
In the context of Figure 3, it should be clear that the timing of the transition (slide? step? skip?) from prebiotic to biotic synthesis is of more than cursory interest. The Hadaean was ostensibly characterized by more-or-less continuous surface bombardment by comets and meteorites (Kring and Cohen, 2002; Schoenberg et al., 2002), hindering the nascence of Life everywhere – excepting, speculatively, the deep subsurface (Krumholz et al., 1997; Teske, 2005). How long did it take for a biosphere to develop once conditions were suitable? And what are the implications for the existence of life elsewhere?
Because of the great weight which these questions carry, it is proposed that a null-hypothesis be adopted as the point of departure: any potential evidence of Life that could have formed by non-living processes must be considered as non-evidence.
5.Structure and Objectives
With the foregoing caveats firmly in mind, the following chapters seek to report on several novel discoveries in old rocks, together with their inferred bearing on early life and environments.
Chapters 2 and 3 are introductory in nature. Chapter 2 introduces key aspects of Early Archaean geobiochemical cycling, while Chapter 3 tackles generalized and specific aspects of Early Archaean geology. The latter includes consideration of geological, structural and metamorphic aspects of field areas in Australia, Greenland and South Africa, in decreasing order of geological detail.
Through application of the precautionary null hypothesis, outlined above, thermodynamic modeling detailed in Chapter 4 seeks to place quantitative bounds on carbon isotope biosignature studies.
Chapters 5 and 6 reportnew metasediment discoveries in Greenland and Australia, together with accompanying inferences into ancient depositional environments.
Chapters 7 and 8 respectively report new isotopic- and trace-fossil- evidence for Early Archaean life from the Pilbara Craton.
A brief concluding synthesis is presented in Chapter 9.
Chapter 10, an unscientific postscript, argues that the notion of life as presently conceived is philosophically fallacious.
FIGURE CAPTIONS
Figure 1: Cover of Charles Lyell’sPrinciples of Geology (1830-1833), subtitled: “An attempt to explain the former changes of the Earth's surface by reference to causes now in operation”.
Figure 2:The distribution of Archaean outcrop today.
Figure 3:The beginning of geobiological history. Numbered references: 1: Brocks et al.(1999); 2: Shen et al. (2001); 3: Wilde et al. (2001); 4: Bowring et al. (1999); 5: Zhang(2002); 6: Patterson (1956); 7: Kroner et al. (1996), Kruner et al. (1991); 8: Buick et al. (1995); 9: Whitehouse et al. (1999); 10: Nutman et al. (2000), Mojzsis et al. (2002).
Figure 4: (a) Silicified magnesian pillow basalt immediately overlying the Strelley Pool Chert, eastern Pilgangoora syncline, Pilbara; (b) Severely flattened, metamorphosed and metasomatized basalt pillows in the central eastern arc of the Isua Supracrustal Belt, scaled to hammer in photograph (a).
Figure 5: In typical Early Archaean greenstone belts, such as the Pilbara’s Pilgangoora belt (above, note vehicle for scale) and Barberton’s Hoogenoeg Formation (below), sub-vertically dipping silicified sedimentary units protrude as prominent ridges above surrounding mafic extrusive and lesser felsic and hyperbyssal volcanics.
Figure 6:Cross-sections of greenstone belts, illustrating sheet-like geometry. Figure after M.C. Dentith (In: de Wit and Ashwal, 1997).
Figure 7: Textural preservation through silicification. (a)Silicified basalt pillow, complete with quartz-filled vesicles, preferentially preserved within a metamafic mylonitic fabric at upper amphibolite facies conditions, northwestern Pilgangoora syncline, Pilbara Craton; (b) Silicified diagenetic gypsum rosettes, North Pole dome, Pilbara Craton; (c) Silicified digitate structures after sulphate, Strelley Pool chert, Pilbara Craton. Although chemical information was pervasively destroyed, textural information was shielded from ~3.5 billion years of alteration, deformation, erosion, metamorphism and weathering through silicification.
Figure 8: (a) Silicified dolomitic isoclinal stromatoloids, Strelley Pool chert, central Pilgangoora Syncline, Pilbara; (b) Kerogenous cherty silicified dolomitic stromatolite, North Pole dome, Pilbara; (c) Late Archaean stromatolite, 2.73 Ga (Blake et al., 2004)Tumbiana Formation, Fortescue Group, western Australia; (d) Silicified haematite-stained domal stromatolite, North Pole dome, Pilbara; (e) Mildly silicified dolomitic stromatolite/loid, North Pole dome, Pilbara; (f) Cross-section through Late Archaean domal stromatolite, Tumbiana Formation, Fortescue Group, Pilbara; (g) Abiogenic ferruginous ooids in Fortescue Formation, Pilbara; (h) Living stromatolites at Shark Bay, Western Australia; (i) Controversial kerogenous chert bearing spheroidal microfossil-like structures (Schopf and Packer, 1987), Chert VIII, Pilgangoora Syncline, Pilbara; (j) Living lacustrine stromatolites in Thesis Lake, Western Australia; (k, l) Early graphite viewed in (k) plane-polarized light and (l) reflected light in newly discovered metasediments from the Isua Supracrustal Belt. This graphite carries a possibly biogenic carbon isotope signature of 13CPDB ≈ -23 ‰, compatible with photoautotrophic metabolism; (m, n) granular kerogen remobilized during early growth of chalcedonic quartz in a Neptunian fissure of Strelley Pool chert age, Coonterunah Group, Pilbara. This kerogen carries a carbon isotope signature of 13CPDB ≈ -35 ‰, suggestive of chemotrophy and out of the range of abiotic synthesis reactions.
Figure 9: Generalized scheme of the evolution of organic matter(modified after Schopf, 1983).
References
Allaart, J. H., 1976. The pre-3760 m.y. old supracrustal rocks of the Isua area, central West Greenland, and the associated occurrence fo quartz-banded ironstone. In: Windley, B. F. (Ed.), The Early History of the Earth. Wiley, London.
Allwood, A. C., Walter, M. R., Kamber, B. S., Marshall, C. P., and Burch, I. W., 2006. Stromatolite reef from the Early Archaean era of Australia. Nature441, 714-718.
Anbar, A. D., Zahnle, K., Arnold, G. L., and Mojzsis, S. J., 2001. Extraterrestrial iridium, sediment accumulation and the habitability of the early Earth's surface. J. Geophys. Res.102 [E2], 3219-3237.
Awramik, S. M., Schopf, J. W., and Walter, M. R., 1983. Filamentous Fossil Bacteria from the Archean of Western Australia. Precamb. Res.20, 357-374.
Banerjee, N. R., Furnes, H., Muehlenbachs, K., Staudigel, H., and de Wit, M., 2006. Preservation of similar to 3.4-3.5 Ga microbial biomarkers in pillow lavas and hyaloclastites from the Barberton Greenstone Belt, South Africa. Earth Planet. Sci. Lett.241, 707-722.
Beaumont, V. and Robert, F., 1998. Nitrogen isotopic composition of organic matter from Precambrian cherts: new keys for nitrogen cycle evolution? Bulletin Societe geologique France169, 211-220.
Beaumont, V. and Robert, F., 1999. Nitrogen isotope ratios of kerogens in Precambrian cherts: a record of the evolution of atmospheric chemistry? Precamb. Res.96, 63-82.
Blake, T. S., Buick, R., Brown, S. J. A., and Barley, M. E., 2004. Stratigraphic geochronology of a late Archaean
flood basalt province in the Pilbara Craton, Australia: constraints on basin evolution, mafic and felsic
volcanism and continental drift rates. Precamb. Res.133.
Bowring, S. A. and Williams, I. S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada. Contrib. Mineral. Petrol.134, 3-16.
Brasier, M. D., Green, O. R., Jephcoat, A. P., Kleppe, A. K., Van Kranendonk, M., Lindsay, J. F., Steele, A., and Grassineau, N. V., 2002. Questioning the evidence for Earth's oldest fossils. Nature416, 76-81.
Brasier, M. D., Green, O. R., Lindsay, J. F., McLoughlin, N., Steele, A., and Stoakes, C., 2005. Critical testing of earth's oldest putative fossil assemblage from the similar to 3.5 Ga Apex Chert, Chinaman Creek, western Australia. Precamb. Res.140, 55-102.
Brocks, J. J., Logan, G. A., Buick, R., and Summons, R. E., 1999. Archean molecular fossils and the early rise of eukaryotes. Science285, 1033-1036.
Buick, R., Dunlop, J. S. R., and Groves, D. I., 1981. Stromatolite Recognition in Ancient Rocks - an Appraisal of Irregularly Laminated Structures in an Early Archean Chert-Barite Unit from North-Pole, Western-Australia. Alcheringa5, 161-181.