Engineering topics
Publications and source records attributed to Walsh, M. M..
The re-examination by SEM of 3.4 Ga fossiliferous carbonaceous cherts reveals fungal contaminants in addition to indigenous microfossils. Weathered volcanic flows associated with fossiliferous chert layers offer a promising area for further study of early life on Earth.
Polymers of bacterial origin, either through cell secretion or the degraded product of cell lysis, form isolated mucoidal strands as well as well-developed biofilms on interfaces. Biofilms are structurally and compositionally complex and are readily distinguishable from abiogenic films. These structures range in size from micrometers to decimeters, the latter occurring as the well-known, mineralised biofilms called stromatolites. Compositionally bacterial polymers are greater than 90 % water, with while the majority of the macromolecules forming the framework of the polymers consisting of polysaccharides (with and some nucteic acids and proteins). These macromolecules contain a vaste amount of functional groups, such as carboxyls, hydroxyls, and phosphoryls which are implicated in cation-binding. It is the elevated metal- binding capacity which provides the bacterial polymer with structural support and also helps to preserves it for up to 3.5 b.y. in the terrestrial rock record. The macromolecules, thus, can become rapidly mineralised and trapped in a mineral matrix. Through early and late diagenesis (bacterial degradation, burial, heat, pressure and time) they break down, losing the functional groups and, gradually, their hydrogen atoms. The degraded product is known as "kerogen". With further diagenesis and metamorphism, all the hydrogen atoms are lost and the carbonaceous matter becomes graphite. until the remnant carbonaceous material become graphitised. This last sentence reads a bit as if ALL these macromolecules break down and end up as graphite., but since we find 441 this is not true for all of the macromolecules. We have traced fossilised polymer and biofilms in rocks from throughout Earth's history, to rocks as old as the oldest being 3.5 b.y.-old. Furthermore, Time of Flight Secondary Ion Mass Spectrometry has been able to identify individual macromolecules of bacterial origin, the identities of which are still being investigated, in all the samples containing fossil biofilm, including the 3.5 b.y..-old carbonaceous cherts from South Africa and Australia. As a result of the unique compositional, structural and "mineralisable" properties of bacterial polymer and biofilms, we conclude that bacterial polymers and biofilms constitute a robust and reliable biomarker for life on Earth and could be a potential biomarker for extraterrestrial life.
The rationale for looking for prokaryote fossils in Martian materials is based on our present understanding of the environmental evolution of that planet in comparison to the history of the terrestrial environments and the development and evolution of life on Earth. On Earth we have clear, albeit indirect, evidence of life in 3.8 b.y.-old rocks from Greenland and the first morphological fossils in 3.3-3.5 b.y.-old cherts from South Africa and Australia. In comparison, Mars, being smaller, probably cooled down after initial aggregation faster than the Earth. Consequently, there could have been liquid water on its surface earlier than on Earth. With a similar exogenous and endogenous input of organics and life-sustaining nutrients as is proposed for the Earth, life could have arisen on that planet, possibly slightly earlier dm it did on Earth. Whereas on Earth liquid water has remained at the surface of the planet since about 4.4 b.y. (with some possible interregnums caused by planet-sterilising impacts before 3.8. b.y. and perhaps a number of periods of a totally frozen Earth, this was not the case with Mars. Although it is not known exactly when surficial water disappeared from the surface, there would have been sufficient time for life to have developed into something similar to the terrestrial prokaryote stage. However, given the earlier environmental deterioration, it is unlikely that it evolved into the eukaryote stage and even evolution of oxygenic photosynthesis may not have been reached. Thus, the impetus of research is on single celled life simnilar to prokaryotes. We are investigating a number of methods of trace element analysis with respect to the Early Archaean microbial fossils. Preliminary neutron activation analysis of carbonaceous layers in the Early Archaean cherts from South Africa and Australia shows some partitioning of elements such as As, Sb, Cr with an especial enrichment of lanthanides in a carbonaceous-rich banded iron sediment . More significantly, preliminary TOF-SIMS investigations of organics in the cherts reveals the presence of a biomarker, which appears to be a derivative of bacterial polymer, in the carbonaceous parts of the rocks. We conclude that a combination of morphological, isotope and biogeochemical methods can be used to successfully identify signs of life in terrestrial material, and that these methods will be useful in searching for signs of life in extraterrestrial materials.
There is widespread textural evidence for microbial activity in the cherts of the Early Archean Onverwacht Group. Layers with fine carbonaceous laminations resembling fossil microbial mats are abundant in the cherty metasediments of the predominantly basaltic Hooggenoeg and Kromberg Formations. In rare cases, filamentous microfossils are associated with the laminae. The morphologies of the fossils, as well as the texture of the encompassing laminae suggest an affinity to modern mat-dwelling cyanobacteria or bacteria. A variety of spheroidal and ellipsoidal structures present in cherts of the Hooggenoeg and Kromberg Formations resemble modern coccoidal bacteria and bacterial structures, including spores. The development of spores may have enabled early microorganisms to survive the relatively harsh surficial conditions, including the effects of very large meteorite impacts on the young Earth.