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GEOLOGY
Life on land in the Proterozoic: Evidence from the Torridonian rocks of northwest Scotland. A. R. Prave, School of Geography and Geosciences, University of St. Andrews, Fife KY16 9AL, Scotland. Pages 811-814.
Life has existed in Earth's oceans for at least 3.5 billion years. Less certain is when life first adapted to land. Some chemical data imply the presence of microbes in nonmarine rocks 1.0-2.0 billion years old, but associated physical evidence has been mostly lacking. This paper reports finding physical features in ~1.2 billion year old rocks in Scotland that are attributable to microbial binding activity on subaerial surfaces in nonmarine environments. If correct, this supports the inference that Earth's biosphere had colonized land surfaces well over a billion years ago.
Mantle insulation beneath the West African craton during the Precambrian-Cambrian transition. Miguel Doblas, et al., Departamento de Geología, Museo Nacional de Ciencias Naturales, Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain. Pages 839-842.
At the time of the Precambrian-Cambrian transition, the West African craton underwent widespread tectonothermal activity giving rise to a ring of fire along the rim of this craton. These thermal phenomena were due to the progressive peripheral release of mantle heat that had built up beneath this craton because of strong insulating conditions of the lithospheric welt. Massive ice melting and outgassing of volcanic CO2 gave rise to a planet-scale sea-level rise, a greenhouse effect, and the end of the icehouse snowball Earth. These processes played an important role in the Phanerozoic explosion of life on Earth.
Land-plant diversity and the end-Permian mass extinction.
P. McAllister Rees, Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA. Pages 827-830.
The greatest of all mass extinctions occurred around the Permian-Triassic boundary (251 million years ago), although there is no consensus regarding the cause(s). Recent studies have suggested a meteor impact and worldwide die-off of vegetation on the basis of sparse local observations. However, new analyses of global Permian and Triassic plant records show that the scale and timing varied markedly between geographic regions and affected different plant groups. The patterns are best explained by differences in geography, climate, and fossil preservation, not by catastrophic events. Permian and Triassic land masses were amalgamated in essentially one supercontinent, Pangea, which migrated ~25° northward during these intervals. There was also a trend from maximum vegetation diversity at low latitudes in the Early Permian (the last remnants of the Carboniferous tropical rain forest biome) toward maximum diversity at mid to high latitudes (and near absence of vegetation at low latitudes) by the Late Triassic. This finding is consistent with the effects of a gradual transition from global icehouse to hothouse (or greenhouse) climates. Although these changes occurred over a span of ~90 million years, there are parallels with future climate change and consequences on far shorter time scales, given the similarities of our present-day global icehouse climate to the Carboniferous and Early Permian, and the potential effects of global warming leading to a hothouse world.
Potential of the International Space Station for imaging Earth: Lessons from MOMS-2P aboard Mir.
Robert J. Stern, Geosciences Department, University of Texas, Dallas, P.O. Box 830688, 2601 North Floyd Road, Richardson, Texas 75083-0688, USA; et al. Pages 851-854.
Digital remote sensing imagery data collected by two similar systems were compared to assess the potential of the International Space Station (ISS) as an Earth-observing platform. Folds and faults in north Ethiopia were imaged with the Modular Optoelectronic Multispectral Stereo Scanner (MOMS-2P) on the Russian space station
Mir and the free-flying Landsat Thematic Mapper (TM) system. The most important difference between MOMS-2P and Landsat TM is the much lower altitude of the former (380 km vs. 705 km); MOMS-2P can resolve details not apparent from TM imagery over the same area. Corresponding improvements in spatial resolution could be expected if any of the freeflying imaging systems (ASTER, SPOT, Hyperion), which fly at altitudes of 700 to 820 km, were mounted on the ISS, which orbits at 380 km. The ultralow orbit of the ISS presents an outstanding, but currently underutilized, opportunity for observing Earth.
GSA TODAY
"Strength of the continental lithosphere: Time to abandon the jelly sandwich?,"
James Jackson, University of Cambridge, Department of Earth Sciences, Bullard Laboratories, Madingley Road, Cambridge CB3 OEZ, UK.
Plate tectonic theory views plates as rigid blocks consisting of crust and upper mantle that move on a plastic asthenosphere. For the past two decades, a prevailing model has been that the continental plates on Earth are like a "jelly sandwich," with a weak lower crust lying between a strong upper crust and a strong uppermost mantle. This model has had an important influence on geodynamical thinking, because the details of the strength distribution are very important for understanding continental deformation and mountain building. The jelly sandwich model has been intimately tied to ideas about lithospheric delamination, lower crustal flow, and mantle seismicity. This paper discusses a recent reassessment of earthquake depth distributions and gravity anomalies on the continents and suggests that that the layer in which earthquakes occur (usually the upper crust, but sometimes the whole crust) may be the only significant source of strength in the continental lithosphere, and that the upper mantle beneath the continents is relatively weak. This change of view, if it is correct, has several implications for continental tectonics and mechanics. For example, patterns of surface faulting on the scale of a few hundred kilometers are likely to be controlled by the strength of the crustal blocks, not of the mantle, and transient flow within a crust may require melt and fluid input. Overall, this paper suggests that this popular jelly sandwich concept of the continental lithosphere and its corollary that the mantle is the strongest part of the lithosphere are both generally incorrect. Instead, the behavior of the continental lithosphere is dominated by the strength of its earthquake-generating layer.
To review the abstracts for these articles, go to www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org. To review the complete table of contents for the September issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-toc&issn=0091-7613&volume=030&issue=09