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Demonstration of significant abiotic iron isotope fractionation in nature. Thomas D. Bullen, et al. Water Resources Division, U.S. Geological Survey, Menlo Park, California 94025, USA. Pages 699-702. Iron is an abundant, bioreactive element and thus iron isotopes have the potential to be extremely useful for detecting microbial processes in the geologic record. Previous workers have suggested that iron isotopes on their own can be used as a biosignature, based on the assumption that only biologic processes are capable of fractionating these isotopes in nature. However, based on field and laboratory studies of the formation of the common iron mineral ferrihydrite, the authors present the first definitive evidence that abiotic processes are capable of fractionating iron isotopes in nature to the same extent as microbial processes. The authors also propose a plausible isotope fractionation mechanism, which promotes the preservation of an abiotic iron isotope fractionation signal in the geologic record.
How many Pacific hotspots are fed by deep-mantle plumes? Valérie Clouard, Jeune Equipe Terre-Océan, Université de la Polynésie Française, B.P. 6570, Faaa, French Polynesia, and Alain Bonneville, UMR Géosciences Marines, Institut de Physique du Globe, 4 place Jussieu, 75005 Paris, France. Pages 695-698. The hotspot volcanism theory of the 1960s proposes that the linear intraplate volcanic chains that show an age progression in the direction of the plate motion took their origin from deep-mantle plumes. The heads of such mantle plumes interact with the lithosphere to form oceanic plateaus on the seafloor; the plume tails then produce linear volcanic chains as the plate drifts over them. The authors have carefully examined the relationship between oceanic plateaus, linear volcanic chains, and hotspots on the Pacific plate. Among the fourteen hotspots on the Pacific plate, only four might correspond to the classical theory, whereas the majority exhibit short tracks and don't show any evidence of links with oceanic plateaus. The authors conclude that deep-mantle plumes are unnecessary to explain most, if any, volcanic chains on the Pacific plate. An updated compilation of radiometric ages of Pacific seamounts and islands is provided as additional material.
Metal leaching and inorganic sulfate reduction in volcanic-hosted massive sulfide mineral systems: Evidence from the paleo-Archean Panorama 1district, Western Australia. David L. Huston, et al. Australian Geological Survey Organisation, GPO Box 378, Canberra, Australian Capital Territory 2601, Australia. Pages 687-690. Three-billion-year-old rocks in Western Australia are yielding new clues to the working of modern-day "black smokers" (hydrothermal vents on the seafloor) and the chemistry of ancient seawater. Geochemical studies of the Panorama district copper-zinc deposits indicate that many of the metals--including copper, zinc, lead, barium, molybdenum, and sulfur--were leached from volcanic rocks below the seafloor by heated seawater, and that the rocks were oxidized during the process. It seems that three-billion-year-old seawater was locally sulfate bearing--perhaps more like modern seawater than generally thought.
Paleoclimatic significance of Phanerozoic reefs. Wolfgang Kiessling, Department of Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637, USA. Pages 751-754.
Reefs are thought to be very sensitive to climate change. In this article the author demonstrates that long-term paleoclimatic change has affected the reef ecosystem, but not in the expected way. In the past 540 m.y., the tropical reef zone widened and contracted, apparently independent from paleoclimate. Long-term paleoclimatic change only modulated biotic reef attributes, usually by favoring algal growth during cool periods. Mass extinctions exerted an important control on global reef distribution because they had the potential to contract the tropical reef zone for an extended period of time. This is remarkable because most mass extinction events are now seen to have occurred independent from long-term climate change.
Low seismic-wave speeds and enhanced fluid pressure beneath the Southern Alps of New Zealand. Tim Stern, et al. School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. Pages 679-682. This paper describes evidence for a 20 x 50 km region in the crust beneath the Southern Alps of New Zealand where excess water under high pressure resides. Low seismic wave speeds and high electrical conductivity are the principal data presented as evidence. The source of the fluid is thought to be water released by greywacke-schist rocks as they are carried to great depth and thus squeezed in the collision zone. When fluid pressures in the crust approach the weight per unit area of the overlying rock (this is called lithostatic pressure), the overlying crust is effectively floating. In this circumstance faults can move under much less stress-difference than if there were no fluid pressure present. Thus, this study raises the question of how major faults in plate boundary zone--such as this one situated between the Australian and Pacific plates--move? Do they move because of large, relatively infrequent earthquakes, or do they move in some other manner?
New evidence for the geological origins of the ancient Delphic oracle (Greece). J.Z. de Boer, et al. Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, USA. Pages 707-710. Ancient tradition linked the Delphic oracle in Greece to specific geological phenomena, including a fissure in the bedrock, intoxicating gaseous emissions, and a spring. Many modern scholars have dismissed these traditional accounts as specious. This paper locates young faults that occur at the oracle site, corresponding well to ancient reports of fissures. Moreover, the work identifies the vaporous emissions supposedly associated with the prophecies as light hydrocarbon bases generated in the underlying strata of bituminous limestone. The research confirms the validity of the ancient sources in startlingly complete detail.
Rock Varnish: Record of desert wetness? Wallace S. Broecker and Tanzhuo Liu, Lamont-Doherty Earth Observatory, Columbia University. Rock varnish, a dark, shiny, and very thin (<200 microns) coating made up of a cocktail of oxides rich in manganese, iron, and clay minerals, forms on rock surfaces and is ubiquitous in desert regions. It has become the center of a contentious controversy revolving around its use in dating geomorphic surfaces and/or evaluating past climate conditions. This paper summarizes findings that support the view that careful sampling and analysis of varnish "microbasins" can potentially be useful in reconstructing past climates in deserts, an important missing element in evaluating hypotheses for global-scale patterns of planetary wetness. The authors have documented pronounced temporal variations in manganese and barium that can be correlated over large regions and that appear to be related to variations in paleo-wetness. They suspect, but cannot prove, that higher manganese content is related to wetter climates. The mode of formation of varnish remains uncertain, but this paper documents anthropogenic lead concentrated in outermost varnish layers, indicating its continued formation. They also report on experiments conducted at Biosphere 2 that used short half-life cosmogenic beryllium isotopes, suggesting that while precipitation is a primary control, dust, dew, and aerosols may also be important in delivering the ingredients for the growth of varnish. Unfortunately, varnish is not directly datable and hence the timing of the potential paleoclimate record is not yet well constrained. The authors suggest several steps that may lead to the rejuvenation of and future breakthrough in varnish studies.
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