Fossilised Bacteria In Barberton Area

J. Seckbach and M. Walsh (eds.), From Fossils to Astrobiology, 25-37. © Springer Science + Business Media B. V. 2009

DISENTANGLING THE MICROBIAL FOSSIL RECORD IN THE BARBERTON GREENSTONE BELT: A CAUTIONARY TALE

MAUD M. WALSH1 AND FRANCES WESTALL2

1School of Plant, Environmental, and Soil Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, USA 2Centre de Biophysique Moléculaire, Centre National de la Recherche Scientifique, 45071 Orléans cedex France

Abstract Morphological remains of microbes are one of several lines of evidence for the presence and nature of life on Earth and elsewhere. It is therefore critical to establish the timing of microbial influences on the rock record. This paper describes two examples of post-lithification colonization of rock surfaces by microbes that might confuse an authentic Archean biogenic signal in samples from the Barberton Greenstone Belt.

1. Introduction

The timing and environmental setting of early life on Earth is important for understanding past and present ecosystems on our planet as well as for exploring the possibility of life on other planets. The record of this early life, therefore, must hold up to the most rigorous scrutiny. One line of evidence for biological activity on the early Earth comes from microfossils preserved in sedimentary rocks from the eastern Pilbara block of Western Australia and the Barberton Greenstone Belt of South Africa that are up to 3,500 million years old (Altermann and Kazmierczak, 2003; Awramik et al., 1983; Rasmussen, 2000; Schopf, 1993; Schopf et al., 2002, 2007a, b; Walsh, 1992; Walsh and Lowe, 1985; Westall et al., 2001, 2006a, b). Recent studies have cast doubt on the genesis of some of the Australian fossils, however, with the contention that the structures formed abio-logically in hydrothermal veins (Brasier et al., 2002) and other studies suggest that a variety of post-Archean microbial processes have affected the fossiliferous rocks. As new chemical techniques for detecting biomarkers emerge (Summons et al., 1999) it is essential that care be taken to discriminate between chemical and physical signals produced by syndepositional microbial activity and those resulting from later biological influences.

This paper describes two cases of post-Archean microbial growth on and within rocks from the Barberton Greenstone Belt. The first case reveals evidence of fungal colonization of internal rock surfaces and the second indications of bacterial precipitation of post-greenstone belt (probably) modern-redeposition of Archean iron and manganese oxides.

2. Geologic Setting

The Barberton Greenstone Belt, located in the eastern part of the Kaapvaal Craton, South Africa contains rocks that are remarkably well preserved, often displaying only low greenschist facies metamorphism and little or no strain or recrystallization (Xie et al., 1997). It has been the focus of many studies of the early Earth because many primary fabrics and textures are preserved down to submicron scales. The Onverwacht Group is a predominantly volcanic succession, but contains important sedimentary deposits of both clastics and precipitates (Lowe and Byerly, 1999, 2007b). The two formations in the Onverwacht Group that have been the focus of this study are the Hooggenoeg and Kromberg Formations. The Hooggenoeg Formation is about 3,000 m thick and includes lavas that have been dated at 3,470 Ma (Byerly et al., 2002). The Kromberg Formation is a 1,700 m thick sequence of komatiites and basalts with minor inter-bedded sediments. The base of the unit is dated at 3,416 Ma and the top at 3,310 Ma (Byerly et al., 1996). The sediments of the Hooggenoeg and Kromberg Formations were deposited either in shallow-water and subaerial environments, where they tend to be associated with komatiitic or dacitic volcanic units, or under quiet, slightly deeper-water conditions associated with basaltic volcanism (Lowe and Byerly, 1999, 2007b). In their long history, the rocks of the Barberton Greenstone Belt may have been subject to subaerial exposure, weathering and microbial colonization in a variety of climates several times since their formation over 3 billion years ago (Beukes, 1999; Van Niekerk et al., 1999). The rocks are presently exposed in the temperate climate of southern Africa.

3. Preservation of Archean Microbial Signatures in the Barberton Greenstone Belt

Layers of cohesive fine laminations of carbonaceous matter, mainly found in black cherts and in banded black and white cherts represent the remains of layers of microbial mats interspersed with a chemical precipitate that is now very finegrained silica (Walsh and Lowe, 1999) or silicified volcaniclastic sediments (Westall et al., 2008). Composite grains of clotted organic matter cemented with silica are commonly associated with the laminations. These pellets appear to have acted as loosely bound particles probably reflecting the influence of microbially-produced exopolymers. Within the laminated layers, including ones associated with evaporate deposits, are preserved rare fossil bacteria (Walsh, 1992; Walsh and Lowe, 1985; Westall et al., 2001, 2006a, b). Filamentous microfossils have been reported by Walsh and Lowe (1985) and (Westall et al., 2006a, b). One occurrence is in an approximately 60-cm-thick black and white banded chert overlying silici-fied volcaniclastic sands in the upper Hooggenoeg Formation (Walsh and Lowe, 1985). The chert containing filamentous fossils is made up mainly of fine kerog-enous laminations interlayered with thin accumulations of detrital kerogenous grains (Fig. 1). Pyrite grains 5-25 |im in size are scattered throughout the thin section. Solid threadlike filaments composed of kerogen and fine pyrite grains

Hooggenoeg Formation
Figure 1. Microbial laminations preserved in carbonaceous chert. Top: Laminations in carbonaceous chert sample, Hooggenoeg Formation. Scale bar equals 200 |im. Bottom: Filamentous microfossils in carbonaceous chert, Kromberg Formation. Scale bar equals 20 |im.

occur mainly in clusters within both the fine laminae and the detrital layers. The filaments have cross-sectional diameters ranging from less than 0.2-2.5 |m and lengths up to 200 |im. A more complex community of filamentous microbes is preserved in a sample from the Kromberg Formation. The microfossils are preserved within layers of fine carbonaceous laminae that contain carbonaceous and lithic intraclasts. Hollow cylindrical filaments range from 1.4 to 1.2 |im in diameter and 10-150 |im in length and solid threadlike filaments range from less than 0.2 to 2.5 |im and lengths up to 200 |im. Most are non-septate, with a few exhibiting slight constrictions at intervals of approximately 1 ||m, or breakage at intervals of several micrometers. The walls of the filaments are composed of carbonaceous matter and fine pyrite grains.

The filaments are commonly oriented subparallel to bedding, but in some cases extend downward between layers, or radiate from a tangle of filaments (Fig. 1). The filamentous microbes may represent only a portion of the original community, but seem to have had a role in the construction of the mats. Morphologically the preserved microbes are similar to both filamentous cyanobacteria and to filamentous sulfur bacteria, but their size range is closer to that of filamentous bacteria.

Yet another example of filamentous microorganisms occurs in a superbly preserved microbial mat from a biolaminite in the uppermost part of the Kromberg Formation (Westall et al., 2006a, b). In this case the microbial mat was formed in a littoral, evaporitic environment by microorganisms having diameters in the range of 0.25-0.3 ccm and lengths up to tens of micrometers. The streamlined microorganisms are embedded in copious quantities of exopolymeric substances (EPS). Interlayered within the filamentous strata are very thin horizons of a suite of evaporite minerals that include aragonite, gypsum, Mg calcite and a halide (probably carrobite). Together with desiccation textures, the evaporite layers indicate periodic exposure of the mat. FIB-cut sections through the microbial mat show that it was coated by a thin layer (10-20 nm) of silica that perfectly preserved the filament and evaporite mineral morphologies on top of the mat. Beneath the surface, however, the mat is characterized by partly mineralized kero-gen presenting an alveolar texture, similar to that found in modern photosyn-thetic microbial mats (Defarge et al., 1996) and representing the degraded remains of the microbial biofilm.

4. Fungal Colonization of Rock Surfaces

Scanning electron microscopy (SEM) was used to examine carbonaceous chert samples with filamentous fossils that had been detected by light microscopy. Rock sample chips were washed in distilled water and placed in an ultrasonic bath to eliminate superficial dust and rock particles. Some were etched in the fumes of HF acid for 1-3 h (for possible in situ microfossil investigation), then thoroughly rinsed with distilled water. When dry the etched and non-etched samples were coated with either Au or Pt. The chips were studied using a JEOL JSM-840A. One sample examined is the fossiliferous Hooggeneog sample described in the preceding section. Some of the same filaments observed with light microscopy were detected by SEM. They appear to be kerogen-lined tubes completely encased by silica. Also observed on two contiguous fracture surfaces of this sample were abundant wide, septate filaments. Several protuberances at regularly spaced intervals along the filaments are similar to clamp connections in fungi of the Phylum Basidiomycota (Fig. 2). In addition, possible fungal spores with spiny exterior walls are present on the fracture surfaces. The fungal structures appear in some

Fossilised Bacteria Barberton Area
Figure 2. Fungus colonizing sample from the Barberton Greenstone Belt. Top: Collapsed segmented filament. Bottom: Clamp connections typical of the Phylum Basidiomycota. Scale bars equal 1 |m.

cases to be encased within the silica matrix, suggesting that they have been fossilized. It appears that two generations of filamentous structures are present in one sample: (1) Archean fossil bacteria and (2) younger fungi. Fungi are not present in the fossil record until the late Proterozoic, and clamp connections are not known before the Permian, approximately 290 Ma (Alexopoulos et al., 1996; Blackwell, 2000). The fungi clearly represent a younger episode of secondary contamination associated with the rocks containing primary Early Archean fossils. Since the Barberton Greenstone Belt has most likely experienced several episodes of sub-aerial exposure, including during the late Carboniferous to early Permian, the timing of the colonization of the fracture surfaces cannot be determined with any certainty. A similar situation in which younger silicified endolithic cyanobacteria and fungal hyphae and spores occur in fractures and crevasses around grain boundaries in very ancient sediments from Greenland (Isua, 3.8 Ga) was described by Westall and Folk (2003).

The presence of fungal contamination on interior surfaces of apparently unfractured samples reinforces the need for a combined use of light microscopy, valuable in determining the spatial relationships of fossil-like structures to sedimentary and petrologic features, and SEM, which allows examination of much finer structural features as well as chemical analysis, in the study of fossil bacteria. The SEM provides topographic information, but only surface features are visible. Transmitted light microscopy, which provides a view into the 40-60 |im depth of a thin section and gives a three-dimensional picture of the relationship of fossils or possible fossils with the surrounding rock matrix, yields valuable information about the context of the fossils and their mode of preservation. Veins and cracks that may be the venue of post-lithification colonization are usually readily apparent in transmitted light, particularly because of the difference in crystal size between matrix and veins.

5. Pleistocence Microbially-Mediated Mineral Precipitation on Barberton Greenstone Belt Rocks

Ironstone deposits that had been interpreted as remains of the world's oldest hydrothermal vents (de Ronde et al., 1994) have recently been reassessed and reinterpreted as much younger deposits that formed as subaerial spring deposits (Lowe and Byerly, 2003, 2007a; Roy et al., 2005). Evidence for Pleistocene ages for the deposits includes slope-parallel bedding and vertical dripstone (Fig. 3).

A microbial influence on the precipitation of the minerals is suggested by the presence in the goethite layers of a variety of structures that resemble filamentous bacteria and/or fungi. The manganese oxide layers do not appear to contain microbe-like structures, but the opacity of these minerals prevents ruling out a microbial presence. Delicate branches of goethite 10-20 |im wide and several hundred microns in length contain at their centers dark filamentous structures or linear arrangements of spherical structures that are 1-3 |im wide. The symmetrical

Figure 3. Pleistocene ironstone deposits associated with rocks of the Barberton Greenstone Belt. Top: outcrop showing horizontal terracing. Bottom: Close-up of vertical dripstone in ironstone deposit.

arrangement of goethite around these central structures suggests that iron oxide was precipitated on the cell walls of the microbes. Distinctly tubular structures with well-defined dark (carbonaceous?) boundaries are also found in some botry-oidal goethite layers. These structures either form central cores to symmetrical precipitate layers or cross-cut the laminations (Fig. 4). In some cases they are branching or appear to have originated from other structures (Fig. 4). The branching of these structures indicates that they are most likely the remains of filamentous fungi, and possibly associated bacteria. The tubes range in diameter from 2-10 |im, with lengths ranging from approximately 60-400 |im. Larger tubular structures are commonly associated with these filaments. The larger tubes, 10-25 |im in diameter and up to 300 |im in length, have indistinct, bumpy outlines. The size of the larger structures is greater than is commonly found amongst filamentous bacteria or fungi. The tubular structure and the wavy boundaries suggest that these represent filamentous microbial remains coated with mineral precipitates (primarily goethite, an iron oxyhydroxide, with minor gibbsite, an alumina oxyhydroxide, and todorokite, a manganese (+4 valence) oxyhydroxide). Similar goethite-encrusted filaments have been reported from stalactites in Lechuguilla Cave, New Mexico (Provencio and Polyak, 2001).

Modern weathering processes commonly produce coatings or varnishes on rock surfaces that contain iron and/or manganese (Adams et al., 1993; Perry et al., 2006). Because most geological samples are, of necessity, collected in the modern weathering zone, the possibility of modern mineral deposits always exists. The presence of iron and manganese, in particular, should be evaluated to eliminate the possibility of post-lithification mineral coatings being interpreted as syngenetic with the rocks being studied. It is essential to consider the outcrop context of a sample, including relationship to modern weathering surface, in establishing the timing of mineral formation.

6. Conclusions and Implications for Astrobiology

As new chemical techniques for detecting biomarkers emerge, extreme care must be taken to discriminate between chemical and physical signals produced by syndepositional microbial activity and those resulting from later biological influences. Both large-scale observations of outcrops and light microscopy of petrographic thin sections should be used in conjunction with electron microscope and chemical studies of terrestrial and, when possible, extraterrestrial samples to ensure that microbial remains are contemporaneous with parent rock formation.

7. Acknowledgements

MMW acknowledges support from the Louisiana Board of Regents/LaSPACE under the NASA Space Training Grant award NNG05GH22H. M. Blackwell, Louisiana State University, identified fungal structures. D. R. Lowe and G. R. Byerly provided access to ironstone samples.

Barberton Bacteria

Figure 4. Branching filaments replaced and/or coated by goethite from ironstone deposit associated with rocks in the Barberton Greenstone Belt. Top: Petrographic light photomicrograph showing filaments cross-cutting botryoidal layers of goethite. Scale bar equals 50 |m. Bottom: Scanning electron microscope image of filaments in pore spaces. Scale bar equals 20 |im.

Figure 4. Branching filaments replaced and/or coated by goethite from ironstone deposit associated with rocks in the Barberton Greenstone Belt. Top: Petrographic light photomicrograph showing filaments cross-cutting botryoidal layers of goethite. Scale bar equals 50 |m. Bottom: Scanning electron microscope image of filaments in pore spaces. Scale bar equals 20 |im.

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Biodata of David Wacey, Nicola McLoughlin and Martin Brasier, authors of "Looking Through Windows onto the Earliest History of Life on Earth and Mars"

Dr. David Wacey is a biogeochemist at Oxford University. He obtained his D.Phil. from Oxford University in 2003 studying the effects of sulfate-reducing bacteria on biomin-eralization. His current research focuses on morphological, chemical and isotopic tracers of primitive Archean life, using cutting edge techniques such as NanoSIMS and Laser Raman Spectroscopy. He is the author of a new introductory textbook on Archean life entitled 'Early Life on Earth: A Practical Guide', released in 2008.

E-mail: [email protected]

Dr. Nicola McLoughlin is a geobiologist at the University of Bergen in Norway. Her current research focuses on the nature of Archean Earth environments and the emergence of life on Earth. She is currently involved in field mapping, sampling and drilling projects in the Pilbara Craton of W Australia, the Barberton Mountain land of South Africa and the Pechenga Greenstone Belt of N Russia. 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. She is the author of a paper 'On Biogenicity Criteria for Endolithic Microborings on Early Earth and Beyond' which explains the application of this work to the field of

Prof. Martin Brasier

Astrobiology.

E-mail: [email protected]

Prof. Martin Brasier

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