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Find Your Science at GSA
25 January 2011
GSA Release No. 11-05
Contact:
Christa Stratton
Director - GSA Communications & Marketing
+1-303-357-1093
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February 2011 Geology cover
February 2011 GSA Today cover

Geology and GSA Today

February 2011

Boulder, CO, USA - GEOLOGY articles cover Patagonian glaciations, the Younger Dryas cold period, paleodiversity, submarine gullies, the Transantarctic Mountain micrometeorite collection, the "fastest glacier on Earth," salt diapirs in the Nordkapp Basin, reinterpretation of James Hutton's historic discovery on the Isle of Arran, a new tool to directly date dinosaur-bone fossils, ancient megalakes in Australia, Egypt's Kamil Crater, and more. GSA TODAY examines seismic activity to gain insights into the Rio Grande Rift.

Keywords: Ammonoids, Patagonia, Younger Dryas, map area, Hikurangi subduction margin, Great Oxidation Event, Transantarctic Mountain micrometeorite collection, shear zones, Jakobshavn Isbrę, Nordkapp Basin, Merida Andes, bedrock terraces, phreatic calcrete hardpan, James Hutton, North Atlantic drainage basin, Western Europe, San Juan Basin, mammal faunas, Australia, Arkansas River basin, Last Glacial Maximum, Siletzia accretion, Kamil Crater, Bothnian Bay, seismic activity, Rio Grand rift

Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.

GSA TODAY articles are open access. Access the GSA TODAY science article by clicking on the issue cover icon at http://www.geosociety.org/pubs/.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in articles published. Contact Christa Stratton for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, .

Geology


Ammonoid diversity and disparity track episodes of chaotic carbon cycling during the early Mesozoic
Jessica H. Whiteside, Dept. of Geological Sciences, Brown University, Box 1846, Providence, Rhode Island 02912, USA; and Peter D. Ward. Pages 99-102; doi:10.1130/G31401.1.

Diversity loss may have wider-reaching effects than conventionally believed, potentially driving long-term instability in marine food webs. Jessica H. Whiteside of Brown University and Peter Ward of the University of Washington draw together carbon isotope data from sedimentary rocks and fossils of ammonoids (carnivorous, octopus-like creatures with a coiled shell) across two of life's most severe mass extinction events (the end-Permian about 250 million years ago and the end-Triassic about 200 million years ago) to demonstrate that in the wake of mass extinctions, major instability in the carbon cycle lasts for millions of years, and that ecosystems do not recover from extinction until after carbon cycle recovery. The post-extinction ecosystems have unstable food webs, which cause boom/bust cycles that are reflected in the fluctuation of the carbon cycle. Whiteside and Ward's study provides an important perspective on current global change: Ecosystems may suffer detrimental effects even long after humans cease carbon emissions and ecosystem alteration.

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Exposure dating outwash gravels to determine the age of the greatest Patagonian glaciations
Andrew S. Hein et al., School of Geosciences, University of Edinburgh, Drummond Street, Edinburgh, Scotland EH8 9XP, UK. Pages 103-106, doi:10.1130/G31215.1.

Andrew S. Hein of the University of Edinburgh and colleagues discuss application of the cosmogenic nuclide dating technique to the dating of Quaternary glacial deposits in southernmost South America. Specifically, Hein and colleagues demonstrate that broad river terraces, which formed during previous ice ages when glaciers in Patagonia were much larger than they are today, have remained nearly unchanged since their formation (in some cases over one million years earlier). This exceptional long-term stability means that rocks on glacial surfaces can be used to determine the timing of these ancient ice ages by deriving "exposure ages," which are based on the concentration of rare cosmogenic nuclides that have accumulated in the rock through exposure to the atmosphere. Here, Hein et al. derive 10Be and 26Al surface exposure ages from sediment on two outwash terraces that give ages of around 600 thousand years and 1.2 million years, suggesting the glacial advance that formed the terraces occurred near to this time. The results of this study strongly suggest that well-preserved and stable glacial outwash terraces can be directly dated to determine the timing of past fluctuations of the Patagonian Ice Sheet over the past 1.2 million years; this approach should improve reconstructions of past climate changes at this key location in the southern mid-latitudes.

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North Atlantic Deep Water and climate variability during the Younger Dryas cold period
Aurora C. Elmore and James D. Wright, Dept. of Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, USA. Pages 107-110, 10.1130/G31376.1.

The Younger Dryas, the last large millennial-scale climate oscillation (12.9-11.6 thousand years ago), has been widely attributed to a massive meltwater discharge event that disrupted ocean circulation and plunged the circum-North Atlantic back into a near-glacial state. Low-resolution deep-water reconstructions indicate lower North Atlantic Deep Water (NADW) production during the Younger Dryas, though the delta-C14 record requires some deep-water production. Elmore and Wright reconstruct deep-water mass variations using a southern Gardar Drift sediment core with an expanded Younger Dryas section. Elmore and Wright show that southern-sourced water invaded the deep North Atlantic to start the Younger Dryas, but was replaced by NADW within 500 years. Southern-sourced waters briefly reappeared at the end of the Younger Dryas. These deepwater reorganizations to start and end the Younger Dryas suggest that increased meltwater fluxes were limited temporally and focused on regions where deep-water convection occurred during the deglaciation.

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Using remote sensing and a geographic information system to quantify rock exposure area in England and Wales: Implications for paleodiversity studies
Alexander M. Dunhill, Dept. of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK. Pages 111-114, doi:10.1130/G31503.1.

A key debate among paleontologists is about the quality of the fossil record: Can we use the fossils as a reasonable guide to the history of life, or is the record heavily biased by incomplete preservation of ancient rocks? In recent studies, many geologists have used map area as a measure of rock availability, reasoning that the overall area of rocks of different ages on the geological map will give a rough guide to the likely access and intensity of collecting. However, access to rocks depends on the rocks appearing at the surface, termed exposure, and whether they are located in a quarry or on a coastline. If the rocks are there, but buried deep, then they cannot yield any fossils. In this study, Alexander M. Dunhill of the University of Bristol uses remote sensing and a Geographic Information System to show that map area and exposure do not match up, and so the current use of "map area" as a measure of fossil sampling is probably not appropriate.

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A topographic signature of a hydrodynamic origin for submarine gullies
Aaron Micallef, University of Malta, Msida, MSD 2080, Malta; and Joshu J. Mountjoy, National Institute of Water and Atmospheric Research Ltd., Private Bag 14901, Wellington, New Zealand. Pages 115-118, 10.1130/G31475.1.

The dynamics of mesoscale seafloor erosional and depositional processes are potentially susceptible to varying oceanic regimes that may result from global climate change. Submarine gullies are a ubiquitous feature of continental margin morphology whose detailed form has only become apparent with the advent of high-resolution seafloor mapping systems, such as multibeam sonar. The origin and evolution of these seafloor features remains poorly understood. Aaron Micallef of the University of Malta and Joshu J. Mountjoy present evidence of a topographic signature of submarine gully erosion in the Cook Strait sector of the Hikurangi subduction margin, New Zealand. This signature indicates that gully initiation is a threshold process driven by unconfined, directionally stable fluid or sediment gravity flows accelerating downslope. Micallef and Mountjoy propose cascading dense water, a type of current that is driven by seawater density contrast, as the source of these flows. The sensitivity of such ephemeral hydrodynamic events to climate change raises questions regarding implications for future variation of the distribution and magnitude of a significant seafloor erosion process.

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Late Archean euxinic conditions before the rise of atmospheric oxygen
Clinton T. Scott et al., Dept. of Earth Science, University of California, Riverside, California 92521, USA. Pages 119-122, doi:10.1130/G31571.1.

Life on Earth is thought to have coevolved with the chemistry of the oceans and atmosphere, and the shift from an anoxic to an oxic world across the Archean-Proterozoic boundary represents a fundamental step in this process. In order to understand the relative influence of biological and geological factors on this transition, scientists must constrain key variables in seawater chemistry before the Great Oxidation Event (circa 2500 million years ago). Scott et al. present a multi-element (C-S-Fe-Mo) biogeochemical study of circa 2662-million-year-old shales from the Hamersley Province in Western Australia. Data obtained by Clinton T. Scott of the University of California-Riverside and colleagues reveal a sustained episode of iron-limited pyrite formation under an anoxic and sulfidic (euxinic) water column. This is the oldest known occurrence of euxinia in Earth's history and challenges the paradigm of persistently iron-rich Archean oceans. Bulk trace metal chemistry and preservation of strong mass-independent sulfur isotope fractionations in sedimentary pyrites indicate that ocean euxinia was possible prior to oxidative weathering, suggesting that sulfidic waters may have been common throughout the Achaean. C-S-Fe systematics suggest that oxygenic photosynthesis was the primary source of organic carbon in the basin, and the absence of Mo enrichments highlights a potential link between inefficient nitrogen fixation and the delayed arrival of the Great Oxidation Event.

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Constraining the terrestrial age of micrometeorites using their record of the Earth's magnetic field polarity
Clement Suavet et al., Museo Nazionale dell'Antartide, Universita di Siena, Via Laterina 8, 53100 Siena, Italy. Pages 123-126, 10.1130/G31655.1.

Micrometeorites acquire a magnetization when they cool down in the atmosphere. In polar regions, the local magnetic field is almost vertical, which makes it possible for micrometeorites to record Earth's magnetic field polarity at the time of their fall. Clement Suavet of the University of Siena and colleagues selected micrometeorites from the Transantarctic Mountains (Antarctica) for which the direction of fall could be inferred thanks to the presence of gas vesicles or iron-nickel droplets, and measured their magnetization. Half of the samples recorded a reverse polarity, which indicates that they fell on Earth before the last magnetic field inversion 0.78 million years ago. Suavet et al.'s results confirm that the Transantarctic Mountain micrometeorite collection comprises the oldest non-fossil micrometeorites available. It is the first time that the paleomagnetic record of flying objects has been retrieved.

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Lithospheric shear zones as constant stress experiments
J.P. Platt and W.M. Behr, Dept. of Earth Sciences, University of Southern California, Los Angeles, California, 90089-0742, USA. Pages 127-130, doi:10.1130/G31561.1.

On Earth's surface, plate boundaries commonly form discrete faults, such as the San Andreas fault in California. At depths greater than about 15 km, however, rocks are hot enough to deform in a ductile fashion, without breaking to form faults. J.P. Platt and W.M. Behr of the University of Southern California ask what controls the width of these zones of ductile deformation, which are known as ductile shear zones, and how deep they penetrate into the body of the Earth. They propose that the width is related to the strength of the rocks on either side, as the shear zones form by deformation and damage of those rocks. This means that at any given depth, the shear zones form at a constant level of stress, equal to the strength of the rocks around them. The stress controls the rate of deformation within them, and hence their width because the shear zones have to be wide enough to accommodate the relative motion of the bounding plates. Under these conditions, the process that causes the shear zones to form is likely to be the reduction in grain size caused by deformation, which weakens rocks and allows them to deform more rapidly.

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Response of Jakobshavn Isbræ, Greenland, to Holocene climate change
Nicolas E. Young et al., Dept. of Geological Sciences, University at Buffalo, Buffalo, New York 14260, USA. Pages 131-134, 10.1130/G31527.1.

Jakobshavn Isbræ, located on west Greenland, is considered the fastest glacier on Earth, and is the Greenland Ice Sheet's single largest contributor to present-day sea-level rise. Young et al. determine the history of ice margin changes over the past 10,000 years for the Greenland Ice Sheet's largest outlet glacier. This is the first time that fluctuations of a Greenland outlet glacier have been determined at such a fine spatial and temporal resolution from before the modern satellite-based record. Nicolas E. Young of the University of Buffalo and colleagues discovered that Jakobshavn Isbræ responded rapidly to relatively modest centennial-scale temperature changes - changes similar to those predicted to occur over the next century. Specifically, between 8,000 and 7,500 years ago, Jakobshavn Isbræ retreated through its fjord at the rate of 100 m per year, likely in response to increasing regional and local temperatures. The Jakobshavn Isbræ margin remained behind its current position for 7,000 years, during a past warm period that was about 2 degrees Celsius warmer than today. Thus, throughout the past 10,000 years, Jakobshavn Isbræ underwent large and rapid adjustments in response to relatively modest Holocene temperature changes, which may foreshadow the Greenland Ice Sheet response to future warming.

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Magnetic expression of salt diapir-related structures in the Nordkapp Basin, western Barents Sea
Laurent Gernigon et al., Geological Survey of Norway (NGU), Geophysics, Leiv Eirikssons vei 39, Trondheim, N-7491, Norway. Pages 135-138, doi:10.1130/G31431.1.

Salt diapirs are atypical geological features formed when a thick bed of evaporite minerals intrudes overlying sedimentary rocks. The importance of salt structures in the understanding of worldwide sedimentary basins is widely recognized and the knowledge of such geological structures has considerably increased over the past 20 years. Recent geophysical investigation of the Nordkapp Basin, Western Barents Sea, demonstrates the capability of modern, high-resolution aeromagnetic surveys to provide an efficient and promising tool for mapping shallow features related to salt diapirism. Authors Laurent Gernigon of the Geological Survey of Norway and colleagues point out that salt diapirs are clearly visible by small, low-amplitude negative round-to-ellipsoidal magnetic pattern. This pattern coincides with sedimentary layers deformed by the rising salt during active and passive diapirism.

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Asynchronous Miocene-Pliocene exhumation of the central Venezuelan Andes
Mauricio A. Bermudez et al., Institut des Sciences de la Terre CNRS, Universite Joseph Fourier, BP53, 38041 Grenoble, France. Pages 139-142, doi:10.1130/G31582.1.

The Merida Andes form the highest mountain belt in Venezuela, with elevations rising to nearly 5000 m at Pico Bolívar, the nation's highest point. The mountains were formed by the oblique collision of the South American and Caribbean plates. Their rise had a profound impact on river patterns and ecosystems in northern South America, as they confined the Amazon rainforest to the north and forced major rivers like the Orinoco to follow an easterly course. However, the timing of uplift and exhumation of the Venezuelan Andes have not been precisely constrained before. Mauricio A. Bermudez of Universite Joseph Fourier, France, and colleagues present new apatite fission-track data from a profile up Pico Bolívar that allow for the cooling of rock samples below about 120 degrees Celsius as they are exhumed toward the surface. The data show that this region was uplifted and exhumed between 10-4 million years ago, whereas similar data from the Sierra La Culata just to the north show much younger rapid uplift and exhumation. The two massifs are separated by the Bocono fault, a major continental strike-slip fault comparable to the San Andreas fault in California. The data presented by Bermudez and colleagues imply that this fault not only accommodated large-scale lateral motions of two crustal blocks, but also differential uplift.

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Episodic bedrock strath terrace formation due to meander migration and cutoff
Noah J. Finnegan and William E. Dietrich, Dept. of Earth and Planetary Science, University of California-Berkeley, 307 McCone Hall, Berkeley, California 94720, USA, 10.1130/G31716.1.

In many actively incising river canyons, gravel-capped bedrock terraces form topographic steps adjacent to rivers. Long recognized as recording the former river bed elevation, bedrock terraces, when dated, provide a direct means of quantifying rates of vertical bedrock river incision and are therefore essential to studies of geomorphology and active tectonics. Although it is widely hypothesized that bedrock terraces result from changes in tectonic uplift rates or in climate, little is actually known about mechanisms of terrace formation. In order to explore terrace formation processes, Noah J. Finnegan and William E. Dietrich of the University of California-Berkeley created a physically based numerical model that couples vertical incision and lateral channel motion, the latter driven by river meandering. A surprising result of the modeling is that bedrock terraces in river canyons, widely argued to form from climatic or tectonic perturbations to rivers, can be explained entirely from the dynamics of meandering.

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Reinterpretation of James Hutton's historic discovery on the Isle of Arran as a double unconformity masked by a phreatic calcrete hardpan
Pierre Jutras et al., Dept. of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada. Pages 147-150, doi:10.1130/G31490.1.

Because it is partly masked by a phreatic calcrete hardpan (PCH), a rare and poorly-known type of rock that can transgress stratigraphic boundaries, there has been ongoing controversy concerning the exact position of James Hutton's first discovered unconformity on the Isle of Arran in southwest Scotland. The unconformity separates folded Neoproterozoic-to-lower Paleozoic (Dalradian) metasedimentary rocks from upper Paleozoic redbeds. The massive PCH developed in Upper Devonian red conglomerate above the unconformity, but it also assimilated some of the underlying basement rocks, thus giving the false impression that the unconformity lies at a lower position, as both host materials are almost entirely replaced by calcrete. At Hutton's discovery site, only a small remnant of the deeply calcretized Upper Devonian conglomerate was preserved from erosion prior to being disconformably overlain by Lower Carboniferous red conglomerate and sandstone. Thus, according to Pierre Jutras of St. Mary's University and colleagues, there are two unconformities at Hutton's historical site, but the younger has previously gone unnoticed, and the two redbed successions on each side of the disconformity were previously thought to belong to the same unit.

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Orbital, ice sheet, and possible solar controls on Holocene moisture trends in the North Atlantic drainage basin
Bryan Shuman and Colin Plank, Dept. of Geology and Geophysics, University of Wyoming, Dept. 3006, 1000 University Avenue, Laramie, Wyoming 82071, USA. Pages 151-154, doi:10.1130/G31387.1.

The sensitivity of water resources to past climate changes need to be understood to provide a context for potential future changes. This survey by Bryan Shuman and Colin Plank (University of Wyoming) of the geological evidence for past changes in the water levels in eastern North America and western Europe shows that recent centuries were likely wetter than any time in the past 12,000 years. The increase in fresh water probably resulted from slow changes in Earth's orbit and the extent of continental ice sheets, as well as possible changes in solar activity. The changes may have been important for influencing riverflow into the North Atlantic Ocean and could in this way have also led to additional climate changes.

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Lithospheric delamination in the core of Pangea: Sm-Nd insights from the Iberian mantle
Gabriel Gutierrez-Alonso et al., Departamento de Geología, Universidad de Salamanca, Salamanca 37008, Spain. Pages 155-158, doi:10.1130/G31468.1.

The forces that continuously shape and reshape Earth's surface are vivid evidence of our dynamic planet. However, fundamental processes also take place in Earth's interior that are hidden from direct observation, the effects of which can only be observed if the overlying crust is removed by uplift and erosion. Recent evidence has shown that in the late stages of mountain building, a significant volume of the solid Earth can founder into the underlying mantle, causing a short-lived, but extensive transfer of heat from the mantle into the crust. Such a transfer results in widespread melting and magma generation. Gabriel Gutierrez-Alonso of the University of Salamanca and colleagues report an example of this process, which occurred in Western Europe about 300 million years ago in the core of Earth's latest supercontinent, Pangea. Radiogenic isotopes of the rare elements samarium and neodymium were used, which are sensitive to the removal of old and the creation of new mantle. Data presented here show mantle replacement occurred at the same time as buckling of the upper crust during continental collisions and Pangea formation. Gutierrez-Alonso and colleagues propose a cause-effect relationship between mountain-belt bending and mantle replacement, providing a robust explanation of many previously unexplained features that characterize Western Europe geology.

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Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico
James E. Fassett et al., 552 Los Nidos Drive, Santa Fe, New Mexico 87501, USA. Pages 159-162, doi:10.1130/G31466.1.

Dinosaur fossils are relatively rare throughout the world, although high concentrations do occur in a few localities. Moreover, the endemic nature of dinosaurs, in even closely spaced localities, has hindered the ability of vertebrate paleontologists to confidently determine the biogeographic diversity, evolution, and radiation of these animals. These problems have been exacerbated by the fact that precise age determinations of dinosaur-bearing rocks have generally not been possible, due to a lack of precisely dateable rock layers, such as altered volcanic ash beds, in dinosaur-bone bearing strata. The San Juan Basin (SWB), of northwest New Mexico and southwest Colorado is one of the few places where a series of radiometric ages through Upper Cretaceous strata provides precise age constraints for the abundant and diverse dinosaur fossils found in these rocks. In addition, the ages of dinosaurs from Paleocene strata in the SWB have been tightly bracketed by fossil pollen and paleomagnetic data. James E. Fassett and colleagues present data that, for the first time, directly date SWB Cretaceous and Paleocene dinosaur-bone samples themselves based on laser-ablation, U-Pb methodology. The use of this new tool to directly date dinosaur-bone fossils may well revolutionize our understanding of the worldwide evolution and radiation of these wondrous animals.

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Flat latitudinal gradient in Paleocene mammal richness suggests decoupling of climate and biodiversity
Peter J. Rose et al., Dept. of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA. Pages 163-166, oi:10.1130/G31099.1.

According to Peter J. Rose of the University of Minnesota and colleagues, mammal faunas from western North America, during a very warm climatic interval of Earth's history about 60 million years ago, exhibit no predictable change in numbers of species with latitude. In contrast, mammal faunas in the same region today show a strong latitudinal gradient in numbers of species. Proxy indicators of past climate suggest that the latitudinal temperature gradient during the time interval of interest was similar to the modern one. The flat species diversity gradient in the past indicates either different responses to climatic gradients by faunas dominated by extinct mammals or distinct ecological processes during the diversification of mammals following a mass extinction event about 65 million years ago.

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Continental aridification and the vanishing of Australia's megalakes
Tim J. Cohen et al., Dept. of Environment and Geography, Macquarie University, Sydney, NSW 2109, Australia. Pages 167-170, doi:10.1130/G31518.1.

On the Australian continent, humans have been shown to have arrived between 60 and 51 thousand years ago, coincidental with the last mass extinction of Australia's megafauna. The debate surrounding this extinction has been hampered by the lack of a climate record for much of the continental interior. Tim J. Cohen of Macquarie University and colleagues show that Australia's megalake system experienced a dramatic change (the biggest recorded change over 100 thousand years) where Lake Mega-Eyre became separate from Lake Mega-Frome. Cohen and colleagues demonstrate that Australia has become increasingly dry since 45 thousand years ago, with megalakes becoming continually smaller through to the arid present. The authors suggest varying moisture sources for the Australian continent over the last glacial cycle.

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Assessing climatic and nonclimatic forcing of Pinedale glaciation and deglaciation in the western United States
Nicolas E. Young et al., Dept. of Geological Sciences, University at Buffalo, Buffalo, New York 14260, USA. Pages 171-174, doi:10.1130/G31527.1.

In the western United States, it has been suggested that the timing of when glaciers achieved, and retreated from, their maximum positions at the termination of the last ice age (about 20,000 years ago) differs from region to region. These differences in timing are routinely used to decipher patterns of regional climate change during the Last Glacial Maximum. Nicolas E. Young of the University of Buffalo and colleagues reconstruct a history of glaciation, deglaciation, and the catastrophic draining of an ice-dammed lake that occurred 19,000 and 17,000 years ago in the upper Arkansas River basin located in central Colorado. These data, when combined with previously published histories of glaciation, indicate that the timing of when glaciers retreated from their maximum positions at the end of the Last Glacial Maximum is generally synchronous across the western United States. This study suggests that the near-synchronous demise of western U.S. glaciers was the result of the first major Northern Hemisphere warming beginning 15,000 years ago.

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Seismically imaged relict slab from the 55 Ma Siletzia accretion to the northwest United States
Brandon Schmandt and Eugene Humphreys, Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. Pages 175-178, doi:10.1130/G31558.1.

Generally, when oceanic and continental plates collide, the oceanic plate is more dense and gets thrust beneath the continent, where it sinks deep into Earth's mantle. This process is referred to as subduction. New seismic images of the mantle beneath the northwest United States reveal a large, curtain-shaped body extending vertically from about 100 km to as much as 600 km depth beneath an area thought to be just inland of the west coast of North America prior to about 55 million years ago. The geometry and seismic properties of this curtain are consistent with that expected for the ocean plate that was subducting at this time, and a short-lived volcanic trend lies directly above the imaged curtain. Brandon Schmandt and Eugene Humphreys of the University of Oregon suggest that subduction stalled 55 million years ago when an ocean seamount chain was accreted near the modern coast and that the crust of the stalled slab was quickly melted, accounting for the overlying volcanic episode and leaving the remaining slab curtain sufficiently buoyant to avoid its sinking deep into the mantle. Other recent research suggests such circumstances may not be uncommon, thus a more diverse range of subducted slab behavior needs to be considered.

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Kamil Crater (Egypt): Ground truth for small-scale meteorite impacts on Earth
Luigi Folco et al., Museo Nazionale dell'Antartide, Universita di Siena, Via Laterina 8, 53100 Siena, Italy. Pages 179-182, doi:10.1130/G31624.1.

Small impact craters (more than 300 m in diameter) are rare on Earth and deeply eroded, so that knowledge of their formation mechanism, and the hazard small impactors constitute to human populations, is largely based on physical models. Luigi Folco of the University of Siena and colleagues report on the geophysical investigation of the Kamil Crater they recently discovered in southern Egypt. The Kamil Crater is a less-than-5000-year-old impact crater 45 m in diameter with a pristine rayed structure. Such well-preserved structures have been previously observed only on extraterrestrial rocky or icy planetary bodies. This feature and the association with an iron meteorite impactor and shock metamorphism provide a unique picture of small-scale hypervelocity impacts on the Earth's crust. Contrary to current models, ground data indicate that iron meteorites with masses on the order of tens of tons can penetrate the atmosphere without substantial fragmentation.

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Zincian dolomite: A peculiar dedolomitization case?
Maria Boni et al., Dipartimento di Scienze della Terra, Universita di Napoli "Federico II," Via Mezzocannone 8, 80134 Naples, Italy. Pages 183-186, doi:10.1130/G31486.1.

According to Maria Boni and colleagues from the University of Naples, newly formed zinc-dolomite can be produced during weathering of zinc sulfide ores. A peculiar dedolomitization phenomenon is associated with the supergene alteration of Zn-Pb sulfide ores, resulting in the precipitation of newly formed carbonate phases. In addition to the deposition of calcite and several metal carbonates, this phenomenon results in a widespread replacement of host-rock dolomites by zincian dolomite phases. This phase may lead to an incorrect evaluation of the metallic resources contained in the supergene nonsulfide deposits.

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Late Holocene freshening of the Baltic Sea derived from high-resolution strontium isotope analyses of mollusk shells
Anders Widerlund and Per S. Andersson, Division of Geosciences, Lulea University of Technology, SE-971 87 Lulea, Sweden. Pages 187-190, doi:10.1130/G31524.1.

Strontium isotopic composition in up-to-7000-year-old mollusk shells occurring in raised-beach sediments was used by Widerlund and Andersson to measure postglacial salinity variations of the brackish Baltic Sea. The salinity data show that the largest surface-water freshening during the past 3000 years, from ten to eleven parts per mil down to one to three parts per mil occurred in Bothnian Bay. The decrease in salinity was caused by rapid postglacial land uplift restricting the inflow of saline waters into this northernmost subbasin of the Baltic Sea. The pronounced freshening of Bothnian Bay is consistent with the absence of a permanent salinity stratification resulting in well-oxygenated surface sediments acting as an efficient phosphorus trap in the present-day Bothnian Bay, where primary production is low and phosphorus limited. A predicted future increase in precipitation of 10%-20% in the Baltic catchment could lead to accelerated freshening of the Baltic Sea, resembling what has already occurred in Bothnian Bay. Studies of environmental changes in Bothnian Bay, involving historical salinity data, may be crucial to improving our understanding of the possible effects of any future climate-induced freshening of the Baltic Sea.


GSA Today Science Article

Alternative perspectives of crustal and upper mantle phenomena along the Rio Grande rift
Marshall Reiter and Richard M. Chamberlin, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. Pages 4–9; doi: 10.1130/GSATG79AR.

Naturally occurring seismic activity can provide important insights into Earth's structure. Here, Marshall Reiter and Richard M. Chamberlin of the New Mexico Bureau of Geology and Mineral Resources have used naturally occurring seismic activity to attempt to gain insights into the Rio Grande Rift, a north-trending zone of lithospheric extension expressed in the upper crust by a series of north-south-trending aligned Cenozoic basins that extend more than 1000 km from Colorado through New Mexico into west Texas. The velocity of measured seismic waves gives insight into the amount of heat flow in the mantle and lithosphere, itself related to the tectonic activity of the rift. To gain insight into the nature of the rift, Reiter and Chamberlin acquired data from La Ristra, a linear series of seismic velocity measurements taken at approximately 45 degrees to the orientation of the Rio Grande Rift. The data reveal that the cold, strong crust and upper mantle of the Great Plains has resisted extension, as focused in the Rio Grande rift, while the high-viscosity mantle under the Great Plains tends to restrain convection. Upward advection occurs in regions of extension and lithosphere thinning as along the Rio Grande rift. The oblique orientation of the La Ristra data provides a unique perspective on the nature and morphology of the Rio Grande Rift demonstrating, among other qualities, that apparent broadening of the transition boundary between the Rio Grande rift and the Great Plains along La Ristra is only a matter of perspective; the tectonically active Rift and the stable craton to the east remain sharply demarcated.

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