Maud M. Walsh

Prolonged magmatism and time constraints for sediment deposition in the early Archean Barberton greenstone belt: evidence from the Upper Onverwacht and Fig Tree groups

The single zircon evaporation, SHRIMP ion-microprobe and conventional dissolution techniques were used to determine 207Pb/206Pb and UPb ages on samples from the Upper Onverwacht and Fig Tree groups of the early Archean Barberton greenstone belt, South Africa. Zircons from dacitic rocks of the upper Hooggenoeg Formation yield ages of ∼ 3445–3452 Ma. A tuff in the basal Kromberg Formation has a mean age of 3416 ± 5 Ma. A tuffaceous band, 5 cm thick, in the uppermost Kromberg Formation contains igneous zircons with a mean age of 3334 ± 3 Ma. The 1700 m section of Kromberg Formation between these two samples is composed of basaltic lavas, minor komatiites and cherty metasediments. The overlying Mendon Formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m. A cherty, stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 ± 3 Ma. The basal Fig Tree Group has units as old as 3259 ± 3 Ma, and upper units in the Fig Tree are as young as 3225 ± 3 Ma. Xenocrystic zircons in the Upper Onverwacht and overlying Fig Tree Group samples suggest that successive igneous units inherited zircons from underlying units and that, over several hundred million years, episodes of intermediate to felsic igneous activity took place at 20–40 Ma intervals. Structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution, but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the Barberton greenstone belt.

Polymeric substances and biofilms as biomarkers in terrestrial materials: Implications for extraterrestrial samples

Organic polymeric substances are a fundamental component of microbial biofilms. Microorganisms, especially bacteria, secrete extracellular polymeric substances (EPS) to form slime layers in which they reproduce. In the sedimentary environment, biofilms commonly contain the products of degraded bacteria as well as allochthonous and autochthonous mineral components. They are complex structures which serve as protection for the colonies of microorganisms living in them and also act as nutrient traps. Biofilms are almost ubiquitous wherever there is an interface and moisture (liquid/liquid, liquid/solid, liquid/gas, solid/gas). In sedimentary rocks they are commonly recognized as stromatolites. We also discuss the distinction between bacterial biofilms and prebiotic films. The EPS and cell components of the microbial biofilms contain many cation chelation sites which are implicated in the mineralization of the films. EPS, biofilms, and their related components thus have strong preservation potential in the rock record. Fossilized microbial polymeric substances (FPS) and biofilms appear to retain the same morphological characteristics as the unfossilized material and have been recognized in rock formations dating back to the Early Archaean (3.5 b.y.). We describe FPS and biofilms from hot spring, deep-sea, volcanic lake, and shallow marine/littoral environments ranging up to 3.5 b.y. in age. FPS and biofilms are more commonly observed than fossil bacteria themselves, especially in the older part of the terrestrial record. The widespread distribution of microbial biofilms and their great survival potential makes their fossilized remains a useful biomarker as a proxy for life with obvious application to the search for life in extraterrestrial materials.

Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration

The magnitude of isotopic variations between organic and inorganic nitrogen was examined in samples from three stacked hydrocarbon reservoirs in the Fordoche Field (Louisiana Gulf Coast Basin, USA). Measurements were made of δ 15N in kerogen, bitumen, oil, formation water, and fixed-NH4 extracted from mudstones, nonproductive sandstones, and productive sandstones. Nitrogen isotope fractionation occurs because 14N is released preferentially to 15N from organic molecules during thermal maturation. Released 14N goes into solution, or may be adsorbed by minerals, leaving crude oil enriched in 15N. Diagenetic clay minerals (e.g., illite) commonly form in the temperature range of hydrocarbon generation, and NH4+ may be fixed in clay interlayers with an isotopic ratio similar to that of the migrating fluids. Results indicate that the influence of organic matter on mineral δ 15N depends on the timing of authigenic mineral formation relative to fluid migration. The average δ 15N of kerogen (3.2 ± 0.3‰) and fixed-NH4 from mudstones (3.0 ± 1.4) is similar, while bitumen increases from +3.5 to +5.1‰ with depth. In deep reservoir sandstones (>100°C), the δ 15N of crude oil averages +5.2 ± 0.4‰, similar to the δ 15N of bitumen in the proposed source rocks. Formation waters are 14N-enriched with an average δ 15N of −2.2 ± 2.6‰. Fixed-NH4 δ 15N values lie between that of the oil and water. The average δ 15N of fixed-NH4 is 3.0 ± 1.2‰ in productive sandstones, and 0.2 ± 2.4‰ innonproductive sandstones. In the shallower reservoir sandstones (<90°C) fixed-NH4 is apparently not influenced by the presently associated fluids. Productive and nonproductive sandstones have distinctly low average δ 15N values (−1.2 ± 0.8‰), yet crude oil (+11.1 ± 0.3‰) and water (+3.8 ± 0.1‰) have been 15N-enriched by ∼6‰ relative to the deeper reservoirs. This suggests that the present fluids migrated into the reservoir after authigenic illite had formed. Fluids become enriched in 15N during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments.

Species-specific detection of hydrocarbon-utilizing bacteria.

Rapid detection and quantitative assessment of specific microbial species in environmental samples is desirable for monitoring changes in ecosystems and for tracking natural or introduced microbial species during bioremediation of contaminated sites. In the interests of developing rapid tests for hydrocarbon-degrading bacteria, species-specific PCR primer sets have been developed for Pseudomonas aeruginosa, Stentrophomonas (Xanthomonas) maltophilia, and Serratia marsescens. Highly variable regions of the 16S rRNA gene were used to design these primer sets. The amplification products of these primer sets have been verified and validated with hemi-nested PCR and with ligase chain reaction (LCR) techniques, and have been applied to the analyses of environmental water samples. These species-specific primer sets were also chosen to amplify in conjunction with a universal set of PCR primers chosen from highly conserved neighboring sequences in the same gene. These multiplex or competitive PCR procedures enable testing with an internal marker and/or the quantitative estimation of the relative proportion of the microbial community that any one of these species occupies. In addition, this universal PCR primer set amplified the same size amplicon from a wide spectrum of procaryotic and eucaryotic organisms and may have potential in earth biota analyses.

Fossil Biofilms and the Search for Life on Mars

Microbial biofilms and mats are documented as fossils in rocks throughout the 3.5 b.y.-old morphological fossil record of life on Earth (Westall et al., 2000). The polymer-rich biofilms are, per se, highly robust structures capable of great resistance and durability. Moreover, the abundance of active groups in the polymers which can chelate mineral ions in solution, assures their ready preservation in the rock record. These active groups include the carboxylate, hydroxyl, amine and phosphate groups (Geesey and Jang, 1989). Precipitation of minerals within a microbial biofilm can be influenced by microbial metabolic control of the microenvironment. Furthermore, the presence of an organic template with active nucleation sites also contributes towards bio-catalysed precipitation of minerals. Some of the best-known examples of mineralised biofilms in the geological record are calcified and silicified stromatolites (Krumbein, 1983). Mineralisation of organic templates can occur very rapidly (within a day, Toporski et al., 2001a). Experiments to silicify microorganisms also document the potential faithfulness of reproduction of the original organism, and the fineness of detail obtainable, by silica impregnation (Westall et al., 1995; Westall, 1999; Toporski et al., 2001a).

The Versatility of Microorganisms

Living organisms are ubiquitous; they are observed in almost every ecological niche, from the air to various habitats on land and deep in the oceans. The abiding presence of microorganisms has also a temporal dimension. Living microorganisms have been found from 100 year-old beer bottles to 40 million-year-old amber. Fossil evidence suggests that the Earth’s earliest organisms were themselves “extremophiles.” Extremophiles can be defined as those organisms observed in uncommon habitats (from our anthropocentric viewpoint). Kristjansson and Hreggvidsson (1995) define an extremophile as one whose optimal growth conditions are found outside of “normal” environments, with normal being those that have a temperature between 4 and 40°C, pH between 5 and 8.5, and with a salinity between that of freshwater and that of seawater. Extremophiles include not only bacteria and archaea but also some eukaryotic organisms, see chapters by Roberts, Elster, Chretiennol-Dinet (q.v.).

The Potential of Thelypteris palustris and Asparagus sprengeri in Phytoremediation of Arsenic Contamination

The potential of two plants, Thelypteris palustris (marsh fern) and Asparagus sprengeri (asparagus fern), for phytoremediation of arsenic contamination was evaluated. The plants were chosen for this study because of the discovery of the arsenic hyperaccumulating fern, Pteris vittata (Ma et al., 2001) and previous research indicating asparagus ferns ability to tolerate >1200 ppm soil arsenic. Objectives were (1) to assess if selected plants are arsenic hyperaccumulators; and (2) to assess changes in the species of arsenic upon accumulation in selected plants. Greenhouse hydroponic experiments arsenic treatment levels were established by adding potassium arsenate to solution. All plants were placed into the hydroponic experiments while still potted in their growth media. Marsh fern and Asparagus fern can both accumulate arsenic. Marsh fern bioaccumulation factors (>10) are in the range of known hyperaccumulator, Pteris vittata. Therefore, Thelypteris palustris is may be a good candidate for remediation of arsenic soil contamination levels of ≤500 μg/L arsenic. Total oxidation of As (III) to As (V) does not occur in asparagus fern. The asparagus fern is arsenic tolerant (bioaccumulation factors <10), but is not considered a good potential phytoremediation candidate.

Archean Biofilms Preserved in the Swaziland Supergroup, South Africa

The discovery of microfossils and stromatolites in Archean rocks of South Africa and Australia respectively (Awramik et al., 1983; Walsh and Lowe, 1985; Byerly et al., 1986; Walsh, 1992; Schopf, 1993; Westall and Gerneke, 1998; Hofmann et al., 1999; Westall et al., 2001) indicate that life was present on Earth as 3.5 billion years ago, and geochemical evidence suggests that it may have arisen as early as 3.8 billion years ago (Schidlowski, 1988; Mojzsis et al., 1996; Rosing, 1999). Preserved microfossils, however, are rare in Archean rocks; our information on occurrence of biota is from our examination of other evidence of life, including the presence of preserved microbial mats or biofilms. Fossilized biofilms are contained within carbonaceous cherts, rocks composed almost entirely of microcrystalline quartz that are black or dark gray in color because of the presence of kerogen, degraded organic matter. The carbonaceous cherts formed by the early silicification of biogenic deposits of organic matter (Lowe, 1999). Total organic carbon in the cherts examined in this study ranges from 0.10 to 14.6 mg C/ g (Hayes et al., 1983; Walsh and Lowe, 1999). This paper describes the results of a petrographic investigation of the carbonaceous cherts of the Barberton Greenstone Belt, South Africa. Over 400 petrographic thin sections were examined using light microscopy in an effort to characterize the mode and environment of formation of the sediment precursors to the carbonaceous cherts. Scanning electron microscopy has also been employed in examining some of the samples and the results reported in Westall et al. (2002—this volume).

Is There an Alternative Path in Eukaryogenesis

The transition from prokaryotic to eukaryotic cells (‘Eukaryogenesis’) is still a biological mystery. The present paper revisits the question of the origin of the eukaryotic cell and suggests that the biochemical, ultrastructural aspects and the renewed efforts in space missions in Solar System exploration will present us with an opportunity for answering the question: Is there an alternative path in Eukaryogenesis?

Microbial Mats on the Early Earth: The Archean Rock Record

Fossil microbial mats provide a convincing record of life on the early Earth. Although mat-like features may be produced abiologically, a careful examination of physical and chemical characteristics can be used to determine the origin of fine laminations within carbonaceous cherts with some confidence. Examples of probable microbial mats are found in the Hooggenoeg, Kromberg, and Fig Tree Formations of the Kaapvaal Craton of southern Africa and the Dixon Island Formation and the Farrel Quartzite of the Pilbara Craton in northwestern Australia.