Barberton Microbial Mats And Microfossils

Climbing the stratigraphy slightly, there are reports from the Buck Reef Chert (K1 in Fig. 3b) of putative microbial mat remains in a sedimentary setting (Tice and Lowe, 2004, 2006). This contrasts with many recent studies of Archean cherts that have been interpreted as formed from either hydrothermal exhalites or injections, or as hydrothermally modified sediments (e.g. Hofmann and Bolhar, 2007; Van Kranendonk, 2006; Brasier et al., 2005). In the case of the Buck Reef Chert, however, Tice and Lowe argue that the thickness, lateral extent, trace metal profiles and development across a range of depositional environments represents deposition under "normal" marine conditions followed by silicification by supersaturated marine waters. If this is indeed the case, then the associated microbial matlike laminations, carbonaceous grains and 'rip up' clasts (Fig. 3e) of this 3,416 Ma unit may represent one of the first environments truly analogous to a modern day shallow marine setting where it may have been possible for life to gain a foothold (cf. Westall, 2005).

Combined with measured carbon isotope fractionations of 513C = -20%o to -35%o and apparent ecological control on the distribution of the laminated matlike structures and carbonaceous grains, this is at least consistent with a biogenic component to these structures (Tice and Lowe, 2006).

Similar reports of fossilized microbial mats with more numerous filamentous and coccoid microfossils come from carbonaceous cherts lower in the Barberton succession from within the upper Hooggenoeg and lower Kromberg Formations (Walsh and Lowe, 1985, 1999; Walsh, 1992). These include examples of plastically deformed carbonaceous fragments interpreted as microbial mat rip-up clasts and two examples of purported filamentous microfossils. Both threadlike (0.1-0.6 |m in diameter) and tubular (1.4-2.2 |im in diameter) filaments are present (Fig. 3f), and are composed of carbonaceous matter with some exhibiting slight constrictions at 1 |im intervals (Walsh and Lowe, 1985). These putative wrinkle-mat textures and microfossil morphologies are suggestive of microbial processing, but their morphological similarity to laminar and wispy textures in the hydrothermal cherts of the 3,460 Ma 'Apex chert' (cf. Brasier et al., 2005) raises the possibility of an abiotic origin for this carbonaceous material which needs to be more thoroughly tested. The carbon isotopic analyses are from 'bulk rock' analyses, so younger contamination cannot be completely eliminated. In addition, only weak evidence (the colour of the carbonaceous matter) is presented for syngenicity of the 'microfossils' with their host rock. A further structure of note within these cherts is the discovery of ~40|im diameter 'spindles' (Fig. 3d). These intriguing morphologies have been variously interpreted as being biogenic and the outer sheaths of colonies of bacterial cells or, alternatively, as abiogenic and the carbonaceous coatings of ghosted gypsum crystals. These scenarios may merit further investigation, especially in light of the recent discovery of similar structures in the 3,400 Ma Strelley Pool Chert in Western Australia (Sugitani et al., 2007).

Westall et al. (2001) also describe spherules around 1 |im in size (Fig. 3c) commonly occurring as pairs, 'zig-zags' or 'strings' which exhibit a uniform size range and are associated with very fine, dark, discontinuous, wavy laminae and lenticular clots. Sausage shaped, rod shaped, oval mould structures and films are also described and are interpreted to represent fossil bacteria or phenomena associated with bacteria by direct analogy to modern equivalents. Here, detailed geological and petrographic context like that presented for the Buck Reef Chert is lacking. (It is not clear, for instance, from which chert units the samples precisely originate, nor whether they are from primary fabrics. Neither is detailed geological mapping presented to support the postulated depositional environment of a shallow water, periodically sub-aerial depo-sitional setting.) Carbon isotope data are provided to show 513C values as low as -27%o, but caution must be used with this interpretation because such values can be derived abiogenically (cf. Horita and Berndt, 1999; McCollum and Seewald, 2006). Indeed, Westall et al. (2006a) acknowledge this problem in a more recent discussion of these structures. On the other hand, a recent study of the isotopic, chemical and structural characteristics of selected carbonaceous material from the Hooggenoeg and Kromberg cherts concluded that this is indeed indigenous to the rock and has all the characteristics of metamorphosed biological material (Van Zuilen et al., 2007). However, this study also showed that small scale migration and re-deposition of organics had occurred and so morphological interpretations of putative microfossils and micro-laminae should be made with caution.

There are several reports of wrinkle structures in Archean clastic sediments. Amongst the best-preserved are those from the 2,900 Ma Mozaan Group (Noffke et al., 2003) and the 3,200 Ma Moodies Group (Noffke et al., 2006) of South Africa. These horizons contain organic carbon remains with 513C values consistent with biological processing and sufficiently good micro-textural preservation to suggest the presence of filamentous microbial mats that trapped and bound sediment grains in shallow marine to tidal flat environments (Noffke et al., 2003, 2006). Older cherts from South Africa and Australia also contain purported wrinkle mat-remains (e.g. Westall et al., 2001, 2006a, b), but the degree of preservation is typically less good in these early Archean cherts. The paucity of detrital sedimentary fabrics and widespread graphitization of carbonaceous remains in these early Archean cherts makes testing the biogenicity of wrinkle structures more challenging. Furthermore, the experimental work of McLoughlin et al. (2008a) has synthesized abiotic wrinkle structures from colloidal media that may be comparable to the silica gel precursors of some Archean cherts. This urges caution when inferring the biogenicity of wrinkle structures.

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