J. Seckbach andM. Walsh (eds.), From Fossils to Astrobiology, 181-210. © Springer Science + Business Media B. V. 2009



'Department of Applied Geology, Geowissenschaftliches Zentrum Göttingen, Universität Göttingen, Goldschmidtstrasse 3, D-37077, Göttingen, Germany

2Department of Geology, University of Pretoria, Pretoria 0002, South Africa

1. Introduction

Up till about 3,850 Ma, planet-sterilising impact events would have made Earth effectively inhospitable to life (Maher and Stevenson, 1988; Sleep et al., 1989). There is no record of the origin of life, but it can be assumed that it began on Earth under extreme conditions: very hot, only trace amounts to no oxygen, possibly saltier oceans than now, higher UV flux, but with a wide range of potential habitats for life, varying from subaerial to deep water conditions (Nisbet, 1995; Nisbet and Sleep, 2001; Westall, 2004). Cyanobacteria, inferred in the rock record from at least 3.5 Ga (Schopf, 2004), were the pioneering oxygenic phototrophs within this framework of early Earth's evolution (Paerl et al., 2000 and references therein) and their further evolution was punctuated by critical biochemical transitions such as the oxygenic atmosphere, which many put at c. 2.3 Ga, others much earlier (cf., Ohmoto, 2004).

By c.2.0-1.8 Ga, all known sedimentary environments were active on Earth, including large hot desert settings (Eriksson and Simpson, 1998; Eriksson et al., 1998) and a large number of researchers agree on an at least partially oxygenic paleo-atmosphere (Ohmoto, 2004). Despite this transformation to a less extreme Earth, extreme environments for cyanobacteria and the mats they form, persisted and occur up till today. In this paper, we will examine the record of microbial mat features preserved within two such siliciclastic (fine sandstones to siltstones) settings, from the c.2.0-1.8 Ga Waterberg Group paleodesert (Kaapvaal craton, South Africa) and modern supratidal flats. In the former, physically separated mat proxy features related to either desiccation or rapid flash-flood events predominate. In the latter, a more complex association of mat proxy features is preserved, concomitant with a more complex cyclical wetting and drying history; crack healing rather than propagation is common, yet fatal desiccation results in analogous features to those from the paleodesert.

2. Microbial Mat Features in Clastic Sedimentary Environments

Microbially-formed structures (cf. stromatolites) are well known from carbonate rocks, particularly of Precambrian age, but features formed from microbial mats in clastic rocks are less well known (e.g., Schieber et al., 2007). Initial biofilms of microorganism clusters and extracellular polymeric substances (EPS) over time become transformed into microbial mats at the clastic sediment-water interface (Schieber et al., 2007). The cyanobacteria are the most successful group in enlarging such biofilms into mats and have a high capacity for biostabilisation on clastic sedimentary surfaces exposed to sunlight (Gerdes, 2007). With time, tough and even leathery mats may form, which provide cohesion to silt and sand grains, thereby helping to bind them and provide resistance to erosion (Schieber et al., 2007).

Such mats and their effects on the clastic sediment within and upon which they grow, leave subtle traces and less evident proxies of their presence within the clastic sedimentary record (Gerdes, 2007). Their wide paleoenvironmental adaptation in modern clastic environments is largely mirrored in the preserved rock record, particularly in that of Precambrian age when predators of mats were absent (Schieber et al., 2007, and references therein). However, in terms of preservation, features directly attributable to microbial mats in terrigenous clastics are either rare or very localized; in contrast, proxy structures resulting from processes such as mat-induced sediment binding, grain agglutination, and chemical compartmentalization of the sediment are common in shallow marine sandstones and offshore shales, especially those of Precambrian affinity (Schieber et al., 2007). These proxy structures largely owe their formation to EPS (Decho, 1990, 2000) secreted by the cyanobacteria and other microorganisms. EPS and microbial filaments make sand and silt cohesive, enabling trapping and binding of clastic particles, which will then respond differently to stress, often behaving more like mud, and forming a host of features (more than 50 are known) generally not expected in sands/sandstones: e.g., desiccation cracks, sand-curls, and flat, pebble-sized rolled-up fragments upon erosion (Fig. 1) (Schieber et al., 2007 and references therein).

The known record of microbial mats in siliciclastic settings extends to c.3.2 Ga (Noffke et al., 2006) and a wide occurrence in Proterozoic siliciclastic sedimentary lithologies probably also reflects the vast epeiric seas of that era (e.g., Eriksson et al., 2005) where low sedimentation rates enhanced the growth of microbial mats. The importance of low sedimentation rates for the transition from an initial and fragile biofilm to a much denser and more robust mat, able to withstand erosive processes, is emphasized by Gerdes (2007); based on experimental work on modern mats, several weeks of non-burial are generally required. However, motile bacteria in well-established mats can easily move upwards through a thin sediment cover (a few millimetres of silt or sand) to re-establish the mat within only a few days; once again, low sedimentation rates are implicit. Terrestrial mat systems have been documented as far back as 1.8 Ga in the fully continental rock record (Eriksson et al., 2000). In this paper, we will briefly describe microbial mat-related features from the oldest known terrestrial example, the

Figure 1. Summary figure of main types of known proxy structures resulting from microbial mat-related processes within sandy sediments, arranged according to genesis of the features. (Reprinted from Schieber, 2004, Fig. 7-9.1. With permission from Elsevier.)

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