Preservation Potential And Some Ancient Analogues

As most of the organic matter produced by the cyanobacteria is soon decomposed and mineralized so that organic components or even carbon are rarely preserved in the ancient siliciclastic record, preservation of mat-related structures is largely dependent on the volume of inorganic material present in the mat or trapped by the structure.

A major process to introduce silt- to sand-sized sediment grains into the mat is 'trapping and binding' as described before. In the same way, clay minerals may be trapped, but formation of authigenic clay minerals by biogeochemical processes (Krumbein and Werner, 1983), e.g., related to bacterial lysis, and possibly microbial trapping of clay minerals from the groundwater (Draganits and Noffke, 2004) may also contribute to the accumulation of clay. Furthermore, formation of iron sulfides and pyrite is not uncommon in peritidal microbial mat systems and results from the degradation of organic material by heterotrophic, sulfate-reducing or methanogenic bacteria (e.g. Berner, 1984; Gerdes et al., 2000). Lastly, microbially mediated precipitation of carbonates is a well-known process (e.g., Gerdes and Krumbein, 1987 and references therein).

The mat-related structures themselves, being either positive or negative features on the mat surface, may act as sediment traps. Obviously, shrinkage cracks may become filled with sediment from currents or eolian action, whereas involute (curled) crack margins may trap grains in their windings. Curled margins have a fairly high preservation potential due to their involute, firm structure and are occasionally found as detached 'roll-up structures' in the ancient record (see Section 3.3.4).

Another mechanism by which some mat-related structures, e.g., domes and bulges on the mat surface, may become preserved is 'filling from below'. This process requires comparatively high hydraulic upward pressure enabling liquefaction of the sediment below the sealing mat so that grains are lifted and moved upwards. As a result, a previously flat surface of a sedimentary layer may be transformed into an irregular surface mimicking the morphological features of the mat.

Of course, preservation of mat-related structures does not depend on the processes listed above only, but also on burial processes and related hydrodynamic conditions, compaction and dewatering etc. Therefore not many of the numerous structures observed with modern mats are clearly identified in the ancient record

Figure 9. Ancient analogues of structures related to mat desiccation and erosion. All photographs are from the Neoproterozoic (Ediacaran) Vingerbreek Member, Schwarzrand Subgroup of the Nama Group, Farm Haruchas, Namibia. (A) Small sigmoidal, sand-filled shrinkage cracks with tapering ends, preserved on upper surface of sandstone layer. (B) Upper surface of finegrained sandstone layer exhibiting linear, triradiate and subcircular shrinkage cracks with compacted, involute margins. (C) Upper rippled surface of sandstone bed with 'sand clasts' in ripple troughs. (D) Upper surface of sandstone bed showing features related to a previously existing thin mat: [1] a patch of mat subsurface that was exposed after local removal (erosion) of the mat; [2] domal protrusions of the sandstone bed resulting from 'upward filling' of respective domes developed in the overlying mat; [3] subcircular crack with compacted involute margin.

Figure 9. Ancient analogues of structures related to mat desiccation and erosion. All photographs are from the Neoproterozoic (Ediacaran) Vingerbreek Member, Schwarzrand Subgroup of the Nama Group, Farm Haruchas, Namibia. (A) Small sigmoidal, sand-filled shrinkage cracks with tapering ends, preserved on upper surface of sandstone layer. (B) Upper surface of finegrained sandstone layer exhibiting linear, triradiate and subcircular shrinkage cracks with compacted, involute margins. (C) Upper rippled surface of sandstone bed with 'sand clasts' in ripple troughs. (D) Upper surface of sandstone bed showing features related to a previously existing thin mat: [1] a patch of mat subsurface that was exposed after local removal (erosion) of the mat; [2] domal protrusions of the sandstone bed resulting from 'upward filling' of respective domes developed in the overlying mat; [3] subcircular crack with compacted involute margin.

(see Schieber et al., 2007 for several discussions of this). Among the few that may be considered as reasonable proxies of the former presence of supratidal, thin microbial mats, are: (1) various types of small 'sand cracks' (Fig. 9A); (2) impressions of circular cracks with curled margins (Fig. 9B); (3) 'roll-up structures' representing fragments of curled margins (see Fig. 6); (4) 'sand clasts' and other mat fragments (Fig. 9C); and (5) irregular subsurface structures (Fig. 9D).

Microbial mats themselves are occasionally preserved as thin layers of seric-itic argillite with isolated silt-sized grains ('floating grains'), associated with argillaceous siltstone in which individual grains are surrounded by sericite and thus not in direct contact ('coated grains'). In this doublet, the sericitic argillite likely represents the 'felty' upper mat layers including trapped and bound grains, whereas the argillaceous siltstone likely stands for the underlying EPS-rich horizons dominated by coccoid cyanobacteria and/or sulfur bacteria.

5. Discussion

Using the example of the c.2-1.9 Ga Waterberg Group in South Africa, we have briefly investigated typical desert interdune-playa type microbial mat features and accounted for their formation and nature through the conditions implicit in such a setting. For these desert settings, interdune ephemeral lakes or playas are critical for mats to survive, and they may grow episodically after flooding and may remain living as long as groundwater is available. Since they will soon become subaerially exposed, however, shrinkage and cracking will definitely occur. And as the mats likely will not be very thick, curling of crack margins also will be a common feature. Finally, the mat will completely desiccate and shrink into a hard and brittle, tightly folded and crumpled tissue, detached from the substratum. Fragments of this will easily be transported by wind and will break down into smaller pieces. Alternatively, the next desert rainstorm, leading to the beginning of the next mat cycle, will rework these desiccated and rolled up mat fragments into the wadi-fluvial deposits which precede playa formation and the next generation of mat features. As we have identified four such playa-interdune flood deposit complexes, separated by dune deposits, the cycles of mat development were severely interrupted. We cannot determine which species of bacteria may have been responsible for the mats in these ancient (c.1.8 Ga) environments, as only proxy mat structures have been preserved within these Waterberg outcrops.

We feel, for desert settings, that the evolution of the mats was relatively simple, because we are dealing with a unidirectional development, starting with an event (the flooding), followed by evaporation, total desiccation of the surface and finally cessation of groundwater flow. But if flooding events and the following desiccation and mat destruction happened repeatedly, a specific chemistry of the brines (including groundwater) may have developed and respective evaporite minerals (like thenardite, gaylussite, shortite etc.) may have been precipitated during progressive evaporation. Unfortunately, except for trace amounts of evaporate minerals identified by XRD, no such minerals have been preserved in these ancient desert sediments.

In the supratidal zone, the story is much more complex due to much more common periodic (plus episodic) inundations and more or less continuous groundwater flow. Mats can survive for quite long times in such environments and exhibit on their surfaces a kind of "memory" of the various events of drying and wetting. Structures may be manifold, varying from simply flat to blistered, crinkled and highly deformed mat surfaces. There will further be an interplay of microbial growth and evaporitic mineral precipitation, which in combination sometimes may lead to peculiar domal structures. Strangely enough, conspicuous desiccation cracks and networks of such cracks are not really typical of supratidal mats. Rather the cracks are small and soon overgrown. If larger cracks occur, the mat is almost destroyed and may easily be eroded (leading to mat fragments like in the desert setting). Depending on the overall setting, the chemistry may involve carbonate, anhydrite/gypsum, halite, or mainly Fe-sulfides. What is important for both settings, of course, is the grain size of the mat substratum, which should be in the silt to fine-grained sand range, because of the combined favorable porosity and capillarity.

Comparing the two environments, (ancient) paleodesert interdune-pan settings and (modern) supratidal flats, differences in the set of preserved mat features are noted: in the Waterberg desert, mat proxy features reflect either desiccation-related structures, or those due to rapid physical destruction resulting from flash floods, and these two sets of features are physically set apart in preserved outcrops. Within the modern supratidal flats, incipient destruction of mats due to desiccation is commonly "healed" due to crack infilling and overgrowth features. In these settings a more complex set of mat proxy features is preserved, including dome- and ridge-like positive features, and reflecting also a complex history of repeated alternations of wet and desiccating environmental conditions, thus enabling the "healing" to become a common feature. Once mats become fatally desiccated in suptratidal flats, however, their physical destruction by high and spring tides is ensured and the proxy features thus formed closely resemble their paleodesert counterparts. The desiccation-related cracks formed in mats in both settings have much in common, with the combination of spindle-shaped, triradiate and complex-sinuous cracks being seen in both environments; this reflects a similar effect of desiccation on the (probably analogous) microbial organisms in both cases.

The microbial mat-related features preserved within modern and ancient sedimentary environments are thus both varied and subtle, analogous to the better known physically-formed sedimentary structures, the use of which to build paleoenvironmental models is equally subject to the niceties of interpretation and overlap of features from one setting to another. The facies-specificity of mat-related structures and their physically formed counterparts is thus also open to the subtleties of scientific interpretation.

Within the Waterberg paleodesert, preservation is essentially through incorporation of larger desiccated mat fragments into rapidly formed flash-flood deposits. Without these latter, larger desiccated mat fragments would break up further, and along with earlier smaller fragments would be picked up by the predominant winds, dispersed and incorporated as small, discrete fragments within wind-deposited sediment. Their chances of identification as mat proxies would be very limited. The major control on their potential preservation would thus appear to be climatic, dependent specifically on a succeeding wet desert flash-flood event occurring before desiccated mat fragments had been left to the vagaries of the predominant wind regime for too long. A celestial body such as the planet Mars, where strong wind regimes may have been important in forming surficial sedimentary deposits, may thus make the search for evidence of pre-existing life difficult; preservation of any mat proxies would be largely dependent on wet climatic events occurring when suitably desiccated mat material happened to be present.

In contrast, epeiric marine shorelines where mats can be expected to flourish under relatively low wave energies and enhanced tidal effects, will produce a much larger range of potential mat proxy features to be preserved. Desiccation effects will be reduced by the common "healing" of cracks. However, once fatal desiccation of mats occurs, the resultant mat fragments will be rapidly dispersed and incorporated into relatively localised littoral clastic deposits, and the different hydrodynamic properties of desiccated mat fragments as opposed to sediment grains, may result in preferential concentration of the former, making identification and preservation more likely. On a planet such as Mars, shallow aqueous deposits reflecting tidal and wave action should be readily identifiable through resultant physically formed sedimentary structures, without recourse to mat proxy features. The problem, as in the rock record on Earth, would be to distinguish lacustrine from littoral marine deposits (e.g., Eriksson et al., 1998). Lacustrine water bodies would tend to become much more desiccated through paleoclimatic variation, with similarities to the Waterberg scenario discussed above. In contrast, shallow marine water bodies should then preserve a much greater range and quantity of mat proxy features. Clastic sediment mat proxies could thus be expected, potentially, to play an important role in indicating evidence for life on a planet such as Mars, as direct preservation of organic matter might have been problematic.

6. Acknowledgments

The authors are grateful to G. Gerdes for critical reading and for helpful suggestions to improving the text. P.G.E. acknowledges research funding from the National Research Foundation and the University of Pretoria, South Africa, and H.P. is grateful for research funding from the Volkswagen Foundation. Igor Tonzetic and Adam Bumby are acknowledged for the photographs in Figs. 3-6. Two anonymous referees are thanked for their constructive comments, and Professor Joseph Seckbach for his editorial skills and encouragement.

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Biodata of Jonathan Antcliffe and Nicola McLoughlin authors of "Deciphering Fossil Evidence for the Origin of Life and the Origin of Animals: Common Challenges in Different Worlds"

Dr. Jonathan Antcliffe is a palaeobiologist at the University of Oxford in the UK. He obtained his Ph.D. in 2008 at Oxford University. His primary area of research is the Ediacaran to Cambrian transition and the emergence of the animal phyla. In particular he is interested in the development of methodologies to understand enigmatic fossil phylogenies with applicability to a range of palaeobiological problems. He is currently also involved in conservation projects for fossil sites across the UK and Canada. He has also applied a variety of new techniques to the analysis of Ediacaran fossils including high resolution three dimensional mapping of their morphology with lasers.

E-mail: [email protected]

Dr. Nicola McLoughlin is a geobiologist at the University of Bergen in Norway. She obtained her Ph.D. in 2006 at Oxford University. Her current research focuses on the nature of Archean earth environments and the emergence of life on earth. She is currently involved in field based projects in the Pilbara Craton of W Australia, the Barberton Mountain land of S Africa and the Pechenga Greenstone Belt of N Russia. She employs field mapping, microscopy and an array of geochemical techniques to investigate putative microfossils and stromatolites remains in Archean cherts. She also studies the microbial alteration of recent volcanic glass and the fossil record of these euendolithic organisms in pillow lavas from Phanerozoic ophiolites and Precambrian greenstone belts.

E-mail: [email protected]

Jonathan Antcliffe Nicola McLoughlin
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