||20 October 2006
GSA Release No. 06-45
Boulder, CO - Topics include: discovery of Neoproterozoic glacial cap carbonates in the British-Irish Caledonides; Kilauea volcano analog for formation of Martian rock coatings; human impacts on coral reefs near Papua, New Guinea; evidence of a Martian acidic ocean; discovery of Laurentian cratonic seaway fossil fauna in Iowa's St. Peter Formation; and impact of channelization on the Missouri River since the days of Lewis and Clark. The GSA TODAY science article describes new technology for analyzing iron isotopes.
Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Ann Cairns for additional information or other assistance.
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- Glacial trinity: Neoproterozoic Earth history within the British-Irish Caledonides
G.A. McCay, University of Edinburgh, School of Geoscience, Edinburgh EH9 3JW, UK;
A.R. Prave (corresponding author), University of St. Andrews, School of Geography and Geosciences, University of St. Andrews, St. Andrews, Fife KY16 9AL, UK; et al. Pages 909-912.
- Neoproterozoic Earth history records a number of hallmark events such as extreme climate change, evolution of animals, supercontinental tectonics, and large shifts in the isotopic composition of oceans. Geochronological work is now placing temporal constraints on these events, and a globally integrated framework for Neoproterozoic Earth history is obtaining a good level of resolution by using age constraints combined with synchronous (or nearly so) global environmental changes such as large carbon-isotopic excursions and glaciations. Documenting these key events is crucial to enhancing scientific understanding of Neoproterozoic Earth System behavior, and Earth System evolution in general. McCay et al. report the discovery of a new Neoproteorozic glacial-cap carbonate succession in the Dalradian Supergroup of the British-Irish Caledonides. This new finding, combined with previous work, shows that the Dalradian strata contain a record of three stratigraphically distinct glaciations. McCay et al. discuss the wider implications of this discovery and use their data to provide additional evidence for the oscillatory tempo of climate change during Neoproterozoic time.
- Acid-fog deposition at Kilauea volcano: A possible mechanism for the formation of siliceous-sulfate rock coatings on Mars
Peter Schiffman, University of California, Davis, Department of Geology, Davis, California 95616-8605, USA; et al. Pages 921-924.
- Schiffman et al. present an alternative model for the formation of the water-bearing mineral, jarosite, on the surface of Mars. Data collected at Meridiani Planum by the Mars Exploration Rover have been interpreted to suggest that jarosite formed in an evaporating, shallow lake environment, within layered sedimentary rocks. The authors present data that demonstrate that the presence of jarosite does not require the existence of standing bodies of water at some past time on the surface of Mars. On the summit of Kilauea volcano, Hawaii, jarosite is actively forming through a mechanism in which volcanic gases, which become incorporated into an acidic fog, dissolve volcanic glass within layered volcanic ash deposits, and ultimately precipitate jarosite on the surface exposures of these ash deposits.
- Magma transport through the crust via interconnected sill complexes
Joseph Cartwright, Cardiff University, School of Earth, Ocean and Planetary Sciences, Main Building, Park Place, Cardiff, Wales CF103YE, UK; and Dorthe Møller Hansen. Pages 929-932.
- Volcanoes and lava plateaus built from flood basalts are among the most dramatic features on Earth. The mechanism by which magma ascends through the crust to be extruded at the surface is not fully understood. The research of Cartwright and Hansen sheds light on this mechanism by challenging the widely held view that most of this magma is transported along near-vertical dikes. Dikes are sheet-like igneous bodies that represent former conduits for magma. Using technology developed to search for hydrocarbons (3-D seismic imaging), Cartwright and Hansen show a complete magma plumbing network from the surface down to a 12 kilometer depth in the crust. This network consists of inclined sheet-like conduits, not of dikes but of sills, that are interconnected. Because the connections between the sills represent a continuous flow path, Cartwright and Hansen argue that this network once acted as a giant magma conduit from mid-lower crustal depths, where the magma chambers were situated, to within a hundred meters of the surface. Cartwright and Hansen's work therefore suggests a totally unexpected mode of magma transport.
- Magma chamber of the Campi Flegrei supervolcano at the time of eruption of the Campanian Ignimbrite
Paola Marianelli, Università degli Studi di Pisa, Dipartimento Scienze della Terra, Pisa, PI I-56126, Italy; et al. Pages 937-940.
- At roughly 39,000 years before present, a super eruption known as the Campanian Ignimbrite occurred in the Campi Flegrei area of Italy, with regional- and global-scale environmental impacts. Investigation of melt inclusions revealed that the eruption was fed by high-temperature (about 1000 °C), water-rich (6 wt%) trachytic magma from a relatively deep (6-9 kilometer) magma chamber. Prior to the eruption, the magma chamber underwent radical changes related to differential upward movement of magma.
- Controlled source nonlinear tomography: A powerful tool to constrain tectonic models of the Southern Apennines orogenic wedge, Italy
L. Improta, Istituto Nazionale di Geofisica e Vulcanologia, Roma 1, Rome, Italy 00143; and M. Corciulo. Pages 941-944.
- Improta and Corciulo contribute to the longstanding debate on the deep structure of the Southern Apennines accretionary wedge (Italy), a key area for understanding the geodynamic evolution of the central Mediterranean, as well as for earthquake research and hydrocarbon exploration. In spite of extensive geophysical investigations encouraged by the recent discovery of giant onshore oil fields, major unresolved questions remain about the deep crustal structure of the wedge. Conflicting thin-skinned and thick-skinned tectonic models are proposed in the recent literature, mainly because of the lack of reliable deep seismic constraints. By applying an innovative nonlinear tomographic technique to crustal refraction data, Improta and Corciulo yield new and more firmly constrained information on the structure of the wedge at greater depths than previous information obtained by hydrocarbon exploration. They imaged a well-defined high-velocity (Vp > 7 km/s) body in the mid-crust that cannot be explained by sedimentary rocks, and that implies the presence of relatively shallow crystalline slivers, i.e., the basement involvement in the wedge. This interpretation, which is supported by local earthquake tomographies and by Bouguer-magnetic anomaly maps, supports a thick-skinned tectonic model and has significant implications that should be considered in future models of the wedge evolution, seismotectonics, and hydrocarbon potential.
- Mass mortality following disturbance in Holocene coral reefs from Papua New Guinea
J.M. Pandolfi, University of Queensland, Centre for Marine Studies, St. Lucia, Queensland 4072, Australia; et al. Pages 949-952.
- The frequency and intensity of disturbances on coral reefs is a matter of grave concern. In the past several decades, living reefs have been subject to a variety of natural and human-caused impacts, resulting in a changed seascape. Pandolfi et al. show that historical disturbances on the Holocene (9000-6000 years before present) reefs of Papua New Guinea were significantly less frequent than those occurring today in a human-dominated world. The recognition of disturbance in the fossil record of coral reefs provides an important tool for placing modern changes in the context of past natural variability that occurred prior to human impacts.
- Evidence for an acidic ocean on Mars from phosphorus geochemistry of Martian soils and rocks
James P. Greenwood, Wesleyan University, Earth & Environmental Sciences, Middletown, Connecticut 06459, USA; and Ruth E. Blake. Pages 953-956.
- The important bio-element, phosphorus, is found to correlate with sulfur and chlorine concentration in Martian dust and soils at both Mars Exploration Rover landing sites. Greenwood and Blake illustrate that sulfur and chlorine in Martian soils and rocks are originally derived from volcanic degassing, while phosphorus originates via acid weathering of Martian rocks. The correlation of these three elements of different geochemical origin is consistent with a common source for these elements via precipitation from aqueous fluids. This is considered strong evidence for an episode of ancient global acidic weathering on Mars, with acid lakes, rain, or an ocean. The high phosphorus, sulfur, and chlorine contents of Martian rocks and soils suggests that Mars did not have a prodigious biosphere in its past.
- Crystal molds on Mars: Melting of a possible new mineral species to create Martian chaotic terrain
Ronald C. Peterson, Queen's University, Kingston, Miller Hall, Kingston, Ontario K7L 3N6, Canada; and Ruiyao Wang. Pages 957-960.
- Images sent back by the Mars Exploration Rover Opportunity from the Meridiani Planum show plate-shaped voids in sulfate-rich rocks that are interpreted as crystal molds formed after a late-stage evaporite mineral has been removed. These shapes match the shape of crystals of the hydrate metal sulfate MgSO4•11H2O grown at low temperatures on Earth. This phase melts incongruently above 2 °C to a mixture of 70% epsom salts (MgSO4•7H2O) and 30% H2O by volume. The existence of ice, low surface temperatures, and the high sulfate content of surface rocks and soil on Mars makes MgSO4•11H2O a likely mineral species on the surface at high latitudes or elsewhere in the subsurface. If an evaporite layer consisted of this phase, incongruent melting would result in a rapid release of a large volume of water, and could explain some of the landform features on Mars that are interpreted as outflow channels. The confirmation of MgSO4•11H2O on the surface of Mars would allow the designation of a new mineral species on another planet. However, unless extraordinary precautions were taken, it would be difficult for a mission to return samples of MgSO4•11H2O to Earth for study without degradation.
- Tsunami-generated boulder ridges in Lake Tahoe, California-Nevada
James G. Moore, U.S. Geological Survey, Volcano Hazards, Menlo Park, California 94025, USA; et al. Pages 965-968.
- The McKinney Bay landslide in Lake Tahoe, California-Nevada, has an estimated volume of 10 cubic kilometers and is one of the largest young landslides on the North American continent. The apparent removal of glacial moraines by tsunamis generated by the landslide indicate that it the slide occurred 7000-15,000 years ago. New evidence from dredging, remotely acquired video images, and scuba diving indicate that the landslide resulted from the gravitational failure of lake beds uplifted by faulting to an unstable position on the west rim of the lake. An array of east-trending boulder and cobble ridges 1-2 meters high and up to 2 kilometers long occur on the Tahoe City shelf, a submerged wave-cut bench in the northwest sector of the lake adjacent to the landslide head. The ridges are composed of glacial-derived debris heaped into giant ripples by the strong currents or waves that were generated by rapid movement of the huge landslide. Similar damaging waves could be produced by future landsliding of the Tahoe City shelf or nearby areas.
- A new Lagerstätte from the Middle Ordovician St. Peter Formation in northeast Iowa, USA
Huaibao P. Liu, Iowa Geological Survey, Iowa Department of Natural Resources, Iowa City, Iowa 52242, USA; et al. Pages 969-972.
- Liu et al. briefly report on an exceptional Middle Ordovician fossil fauna that was discovered in a newly recognized shale unit within the St. Peter Formation in northeast Iowa, USA. This fauna comprises conodonts (teeth-like microfossils) and natural conodont assemblages, jawless fish, eurypterids, phyllocarid crustaceans, linguloid brachiopods, and various indeterminate fossil forms, many with soft body tissues or impressions. This diverse marine fauna opens a unique window to the community that inhabited the margins of the Laurentian cratonic seaway about 460 million years ago, and will substantially advance our understanding of the details of these extinct organisms, the paleoenvironments in which they lived and died, and the fossil preservation processes. This discovery is also unusual because the St. Peter Formation, noted for its pure quartz sandstone composition, has yielded few diagnostic fossils over its vast geographic extent in the Midwest. Paleontologist Stig Bergström of The Ohio State University has referred to this fauna as "a very unexpected but most sensational paleontologic discovery. It is clearly of exceptional paleobiologic interest and may well be considered the discovery of the decade in early Paleozoic paleontology."
- Enhanced stage and stage variability on the lower Missouri River benchmarked by Lewis and Clark
Bethany L. Ehlmann, University of Oxford, Environmental Change Institute, Department of Geography & Environment, Oxford, Oxfordshire OX1 3QY, UK; and Robert E. Criss. Pages 977-980.
- Since Meriwether Lewis and William Clark first traversed it in their 1803-06 expedition, the lower Missouri River of North America has changed profoundly in response to channelization and regulation of water levels by release from upstream reservoirs. Decisions over how to revise river management to restore ecosystems and provide better protection against flooding are hindered by a lack of information on the natural hydrology of the river because the first management activities only began as early as 1820. Ehlmann and Criss address this problem by examining 200 years of stage (water level) data on the Missouri River using measurements recorded in the journal of Lewis and Clark, along with stage records kept since the 1870's by various government agencies. Their findings show a doubling in daily stage fluctuations from the nineteenth century to 2005. Annual maximum stages have, at some sites, become more extreme, and seasonality is more variable. These changes increase flood risk and, moreover, have a detrimental impact on river ecosystems. Timing of the changes in stage variability, beginning as early as 1900, suggests channelization is the major cause. Ehlmann and Criss's results imply that efforts to restore the natural hydrology of the Missouri River must focus on channel size and shape in addition to changing reservoir water release. Ehlmann and Criss suggest that efforts to remove channelization structures in select locations in order to reconnect the river to its floodplain, which began after the 1993 floods, should continue and be expanded.
- Benthic foraminiferal Li/Ca: Insights into Cenozoic seawater carbonate saturation state
Caroline H. Lear, Cardiff University, School of Earth, Ocean and Planetary Sciences, Cardiff, S. Glamorgan CF10 3YE, UK; and Yair Rosenthal. Pages 985-988.
- The floors of most oceans are covered with carbonate sediments, comprising tiny fossils made of calcium carbonate (chalk). Two things are required for these sediments to form: (1) a supply of dead carbonate plankton must sink through the water column and (2) the carbonate shells must not dissolve in the corrosive bottom waters of the deep oceans. Many factors influence the corrosivity of ocean bottom waters, including the amount of carbon dissolved within them (a key part of the global carbon cycle, linked to Earth's climate). An increase in dissolved carbon makes seawater more acidic (corrosive). Thus far, it has been impossible to reconstruct changes in the corrosivity of ocean waters on long time scales (millions of years), during which time Earth's climate has changed considerably. Lear and Rosenthal's research proposes a new proxy for changes in the corrosivity of ocean waters — it appears that tiny animals called benthic foraminifera (that live on the seafloor) take less lithium into their carbonate shell if they are living in more corrosive water. This research presents a record of the amount of lithium incorporation into benthic foraminifera for the last 35 million years. The surprising discovery was that the extent of the lithium incorporation (which should track ocean corrosivity) seems to match well with the record of global sea-level changes. Lear and Rosenthal explain this correspondence by invoking the shelf-basin fractionation hypothesis, which holds that a sea-level fall will reduce the areas of shallow-water carbonate production (e.g., by aerial exposure of coral reefs, etc.). However, rivers would still provide the ingredients for calcium carbonate production to the oceans. With a reduction in the uptake of these ingredients in the highly productive shallow-water 'carbonate factory,' the effect would be to reduce the corrosivity of the deep oceans. Therefore, it appears that sea-level changes resulting from melting or expanding ice sheets can have far reaching impacts on the corrosivity of the oceans and the geology of the deep sea environment.
- Fe isotopes: An emerging technique for understanding modern and ancient biogeochemical cycles
Clark M. Johnson and Brian L. Beard, Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA.
- The New Iron Age: Iron is one of the most abundant elements on Earth (6.7%) and it is involved in a broad range of chemical processes from formation of giant iron ore deposits more than two billion years ago to acting as a critical nutrient in the oceans. Clark Johnson and Brian Beard (University of Wisconsin) are at the forefront of a scientific vanguard that has developed the technology to analyze isotopes of iron, a formerly inaccessible element on the periodic table. In a recent GSA Today article, Johnson and Beard provide an overview of new advances in this young field. In particular, iron isotopes are sensitive trackers of bacterial activity that has left an isotopic fingerprint in rocks at least 2.9 billion years old. Research in iron isotopes holds tremendous potential for deciphering the role of bacterial processes in shaping the evolution of the oceans and atmosphere over Earth's history.
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