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Walter, M. R.

Publications and source records attributed to Walter, M. R..

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Diversification in the Archean Biosphere: Insight from NanoSIMS of Microstructures in the Farrel Quartzite of Australia

The nature of early life on Earth is difficult to assess because potential Early Archean biosignatures are commonly poorly preserved. Interpretations of such materials have been contested, and abiotic or epigenetic derivations have been proposed (summarized in [1]). Yet, an understanding of Archean life is of astrobiological importance, as knowledge of early evolutionary processes on Earth could provide insight to development of life on other planets. A recently-discovered assemblage of organic microstructures in approx.3 Ga charts of the Farrel Quartzite (FQ) of Australia [2-4] includes unusual spindle-like forms and a variety of spheroids. If biogenicity and syngeneity of these forms could be substantiated, the FQ assemblage would provide a new view of Archean life. Our work uses NanoSIMS to further assess the biogenicity and syngeneity of FQ microstructures. In prior NanoSIMS studies [5-6], we gained an understanding of nano-scale elemental distributions in undisputed microfossils from the Neoproterozoic Bitter Springs Formation of Australia. Those results provide a new tool with which to evaluate poorly preserved materials that we might find in Archean sediments and possibly in extraterrestrial materials. We have applied this tool to the FQ forms.

Oehler, D. Z.↗
NASA NTRS2008

"Nano" Scale Biosignatures and the Search for Extraterrestrial Life

A critical step in the search for remnants of potential life forms on other planets lies in our ability to recognize indigenous fragments of ancient microbes preserved in some of Earth's oldest rocks. To this end, we are building a database of nano-scale chemical and morphological characteristics of some of Earth's oldest organic microfossils. We are primarily using the new technology of Nano-Secondary ion mass spectrometry (NanoSIMS) which provides in-situ, nano-scale elemental analysis of trace quantities of organic residues. The initial step was to characterize element composition of well-preserved, organic microfossils from the late Proterozoic (0.8 Ga) Bitter Springs Formation of Australia. Results from that work provide morphologic detail and nitrogen/carbon ratios that appear to reflect the well-established biological origin of these 0.8 Ga fossils.

Oehler, D. Z.↗
NASA NTRS1999

Astrobiology: exploring the origins, evolution, and distribution of life in the Universe

The search for the origins of life and its presence beyond Earth is strengthened by new technology and by evidence that life tolerates extreme conditions and that planets are widespread. Astrobiologists learn how planets develop and maintain habitable conditions. They combine biological and information sciences to decipher the origins of life. They examine how biota, particularly microorganisms, evolve, at scales from the molecular to the biosphere level, including interactions with long-term planetary changes. Astrobiologists learn how to recognize the morphological, chemical, and spectroscopic signatures of life in order to explore both extraterrestrial samples and electromagnetic spectra reflected from extrasolar planets.

NASA Center ARC↗
NASA NTRS1996

Lithofacies and biofacies of mid-Paleozoic thermal spring deposits in the Drummond Basin, Queensland, Australia

The Devonian to Carboniferous sinters of the Drummond Basin, Australia, are among the oldest well established examples of fossil subaerial hot springs. Numerous subaerial and subaqueous spring deposits are known from the geological record as a result of the occurrence of economic mineral deposits in many of them. Some are reported to contain fossils, but very few have been studied by paleobiologists; they represent an untapped source of paleobiological information on the history of hydrothermal ecosystems. Such systems are of special interest, given the molecular biological evidence that thermophilic bacteria lie near the root of the tree of extant life. The Drummond Basin sinters are very closely comparable with modern examples in Yellowstone National Park and elsewhere. Thirteen microfacies are recognisable in the field, ranging from high temperature apparently abiotic geyserite through various forms of stromatolitic sinter probably of cyanobacterial origin to ambient temperature marsh deposits. Microfossils in the stromatolites are interpreted as cyanobacterial sheaths. Herbaceous lycopsids occur in the lower temperature deposits.

NASA Center ARC↗
NASA NTRS1996

The limits of palaeontological knowledge: finding the gold among the dross

Palaeontological interpretation rests on two interwoven sets of comparisons with the modern world. Palaeobiological interpretation relies on the placement of fossils within a phylogenetic and functional framework based primarily on the comparative biology of living organisms. Analogy to currently observable chemical, physical and taphonomic processes enables palaeoenvironmental inferences to be drawn from geological data. In older rocks, comparisons with the modern Earth can become tenuous, limiting palaeontological interpretation. The problem reaches its apogee in Archaean successions, yet pursuit of multiple lines of evidence establishes that complex microbial communities, fuelled by autotrophy and, likely, photoautotrophy, existed 3500 million years ago. Although Archaean palaeontology has to date focused on silicified coastal sediments, improved understanding of Earth's earliest biosphere may depend on the development of alternative environmental and taphonomic analogies. Spring precipitates and hydrothermal metal deposits are promising candidates. Terrestrial organisms may be of limited value in interpreting such fossils as may be found on Mars, although some points of comparison could prove general. Given limited opportunities for exploration, proper choice of environmental analogy is critical. Spring precipitates constitute excellent deposits for addressing questions of biology on another planet.

NASA Discipline Exobiology↗
NASA NTRS1995

Thermal Springs and the Search for Past Life on Mars

Ancient thermal spring sites have several features which make them significant targets in a search for past life. Chemical (including redox) reactions in hydrothermal systems possibly played a role in the origin of life on Earth and elsewhere. Spring waters frequently contain reduced species (sulfur compounds, Fe(sup +2), etc.) which can provide chemical energy for organic synthesis. Relatively cool hydrothermal systems can sustain abundant microbial life (on Earth, at temperatures greater than 110 C). A spring site on Mars perhaps might even have maintained liquid water for periods sufficiently long to sustain surface-dwelling biota had they existed. On Earth, a variety of microbial mat communities can be sampled along the wide range of temperatures surrounding the spring, thus offering an opportunity to sample a broad biological diversity. Thermal spring waters frequently deposit minerals (carbonates, silica, etc.) which can entomb and preserve both fluid inclusions and microbial communities. These deposits can be highly fossiliferous and preserve biological inclusions for geologically long periods of time. Such deposits can cover several square km on Earth, and their distinctive mineralogy (e.g., silica- and/or carbonate-rich) can contrast sharply with that of the surrounding region. As with Martian volcanoes, Martian thermal spring complexes and their deposits might typically be much larger than their counterparts on Earth. Thus Martian spring deposits are perhaps readily detectable and even accessible. Elysium Planitia is an example of a promising region where hydrothermal activity very likely remobilized ground ice and sustained springs.

DesMarais, D. J.↗
NASA NTRS1989

Fossil life on Mars

Three major problems beset paleontologists searching for morphological evidence of life on early Earth: selecting a prospective site; finding biogenic structures; and distinguishing biogenic from abiogenic structures. The same problems arise on Mars. Terrestrial experience suggests that, with the techniques that can be employed remotely, ancient springs, including hot springs, are more prospective than lake deposits. If, on the other hand, the search is for chemical evidence, the strategy can be very different, and lake deposits are attractive targets. Lakes and springs frequenly occur in close proximity, and therefore a strategy that combines the two would seem to maximize the chance of success. The strategy for a search for stromatolite on Mars is discussed.

Walter, M. R.↗
NASA NTRS1986

Isotopic, petrologic and biogeochemical investigations of banded iron-formations

It is recognized that the first occurrence of banded iron-formations (BIFs) clearly predates biological oxygenation of the atmosphere-hydrosphere system and that their last occurrences extend beyond plausible dates of pervasive biological oxygenation. For this reason, and because enormous quantities of oxidizing power have been sequestered in them, it is widely thought that these massive, but enigmatic, sediments must encode information about the mechanism and timing of the rise of atmospheric O2. By coupling isotopic analyses of iron-formation carbonates with biogeochemical and petrologic investigations, we are studying (1) the mechanism of initial sedimentation of iron; (2) the role of iron in microbially mediated diagenetic processes in fresh iron-formation sediments; and (3) the logical integration of mechanisms of deposition with observed levels of banding. Thus far, it has been shown that (1) carbonates in BIFs of the Hamersley Group of Western Australia are isotopically inhomogenous; (2) the nature and pattern of isotopic ordering is not consistent with a metamorphic origin for the overall depletion of C-13 observed in the carbonates; (3) if biological, the origin of the C-13 depleted carbonate could be either respiratory or fermentative; (4) iron may have been precipitate d as Fe(3+), then reduced to Fe(2+) within the sediment; and (5) sedimentary biogeochemical systems may have been at least partially closed to mass transport of carbonate species.

Hayes, J. M.↗
NASA NTRS1985

Millimeter-scale variations of stable isotope abundances in carbonates from banded iron-formations in the Hamersley Group of Western Australia

Several diamond drill cores from formations within the Hamersley Group of Western Australia have been studied for evidence of short-range variations in the isotopic compositions of the carbonates. For a set of 32 adjacent microbands analyzed in a specimen from the Marra Mamba Iron Formation, carbon isotope compositions of individual microbands ranged from -2.8 to -19.8 per mil compared to PDB and oxygen isotope compositions ranged from 10.2 to 20.8 per mil compared to SMOW. A pattern of alternating abundances was present, with the average isotopic contrasts between adjacent microbands being 3.0 per mil for carbon and 3.1 per mil for oxygen. Similar results were obtained for a suite of 34 microbands (in four groups) from the Bruno's Band unit of the Mount Sylvia Formation. Difficulties were experienced in preparing samples of single microbands from the Dales Gorge Member of the Brockman Iron Formation, but overall isotopic compositions were in good agreement with values reported by previous authors. Chemical analyses showed that isotopically light carbon and oxygen were correlated with increased concentrations of iron. The preservation of these millimeter-scale variations in isotopic abundances is interpreted as inconsistent with a metamorphic origin for the isotopically light carbon in the BIF carbonates. A biological origin is favored for the correlated variations in 13C and Fe, and it is suggested that the 13C-depleted carbonates may derive either from fermentative metabolism or from anaerobic respiration. A model is presented in which these processes occur near the sediment-water interface and are coupled with an initial oxidative precipitation of the iron.

NASA Program Exobiology↗
NASA NTRS1981

Microfossil-like objects from the Archaean of Greenland - A cautionary note

Recent reports have described 'yeast-like microfossils' (Isuasphaera isua Pflug) in 3,800-million year old metaquartzites from the Isua supracrustal belt of south-west Greenland. A biogenic interpretation of these objects is inconsistent with the tectonic history of the Isua region, with the petrology of the metaquartzites, and with the morphology of the microstructures themselves. The putative microfossils are indistinguishable from limonite-stained fluid inclusions: microstructures which are demonstrably inorganic and post-depositional in origin. As such, it is contended that these objects should not be regarded as evidence of early Archaean life forms.

Bridgwater, D.↗