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GEOLOGY speeding top science to online platform

14 November 2012 Geological Society of America, The

Between 19 October and 13 November, The Geological Society of America’s top geoscience journal, GEOLOGY, posted 35 new studies online ahead of print. A selection of those studies is highlighted here and includes a broad spectrum of geoscience disciplines, such as volcanology, glaciology, paleoclimatology, paleontology, and mineralogy, with locations like the Society Islands, the U.S. Basin and Range Province, the Mojave Desert, Timor, Nicaragua, New Zealand's Southern Alps, and the Arctic Ocean Beaufort Sea margin.

To view all GEOLOGY articles published ahead of print, go to http://geology.gsapubs.org/content/early/recent.

GEOLOGY is online http://geology.gsapubs.org/. All abstracts are open-access; representatives of the media may obtain complimentary GEOLOGY articles by contacting Kea Giles.

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

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

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Silver Creek caldera -- The tectonically dismembered source of the Peach Spring Tuff
Charles A. Ferguson et al., Arizona Geological Survey, 416 West Congress, Suite 100, Tucson, Arizona 85719, USA. Posted online 19 October 2012; doi: 10.1130/G33551.1.

Charles Ferguson and colleagues describe the discovery of a volcano in the Mojave Desert of the southwestern United States that has implications for understanding the mechanism for super-eruptions, a class of volcanic eruption that taps very large (greater than 100 cubic kilometers) amounts of magma in a very short period of time (hours to days). Their paper includes new geochronological data confirming that densely welded volcanic ash within the volcano is the same age as a widespread outflow sheet of the ash that extends across over 14,000 square kilometers of the Mojave Desert. In addition, they identify a fragment of the volcano that was tectonically dismembered and translated more than 30 kilometers to the southwest between approximately 19 and 15 million years ago. The direction of extension is compatible with previous estimates derived from detailed structural analysis of the region.

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A reduced relevance of vegetation change for alluvial aggradation in arid zones
Jose Luis Antinao, Division of Earth and Ecosystem Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada 89512, USA; and Eric McDonald. Posted online 19 October 2012; doi: 10.1130/G33623.1.

This study by Jose Luis Antinao and Eric McDonald assesses the hypothesis that a climatically induced decrease in vegetation density on arid region hillslopes is a major factor behind erosion, sediment transport, and aggradation (sedimentation) downstream. This linkage is often used to explain sedimentation in alluvial fans during the Late Pleistocene Holocene period (8 to 14-thousand years ago) in the U.S. Southwest deserts. Antinao and McDonald compiled paleo-botanical and alluvial fan sedimentation histories during this period in the Mojave and northern Sonoran deserts. Timing of actual downstream aggradation was compared to timing of vegetation change in altitudinal zones susceptible to generate sediment, assuming that compositional changes in vegetation indicate changes in plant density or canopy cover. Their results reveal that onset of extensive alluvial fan deposition developed well before a decrease in catchment vegetative cover in all regions. Fan sedimentation could be linked, unexpectedly, to increased cover in one region. These ambiguous linkages indicate that vegetation change probably has a reduced role in aggradation. Antinao and McDonald conclude that the analysis of hillslope response to climate change should therefore incorporate other factors like local storm intensity or sediment redistribution in hillslopes, given the importance of coupled hillslope/alluvial system evolution in arid region ecosystem functions

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Parallel volcano trends and geochemical asymmetry of the Society Islands hotspot track
Jarod A. Payne et al. (Matthew G. Jackson, corresponding), Dept. of Earth Sciences, Boston University, 675 Commonwealth Avenue, Boston, Massachusetts 02215, USA. Posted online 19 October 2012; doi: 10.1130/G33273.1.

Plumes of hot, buoyant material are thought to rise from the deepest mantle, near the core-mantle boundary. In the shallow mantle, the plumes partially melt, and the melt is erupted at the surface at hotspot volcanoes. Several hotspot volcanoes, including those in Hawaii, Samoa, and the Marquesas, exhibit two parallel volcanic lineaments that are geochemically distinct. This geochemical separation is thought to result from the plumes being sourced from the northern side of a large, enriched geochemical and seismological anomaly in the deepest mantle. The south side of each plume then entrains the geochemically enriched material, which is reflected in the geochemical zonation of hotspot volcanoes at the surface. To test this hypothesis, Jarod Payne and colleagues examine volcanic lineaments in the Society Islands hotspot, which is located to the south of a seismologically anomalous region in the deep mantle. They find that the northern volcanic trend in the Society Islands is enriched relative to the southern volcanic trend. This observation is consistent with plume zonation arising from an enriched geochemical anomaly in the deepest mantle, and the relative geometry of the plume and the geochemical anomaly governs the sense of zonation in Pacific hotspots.

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Late Pleistocene tropical Pacific temperature sensitivity to radiative greenhouse gas forcing
Kelsey A. Dyez (corresponding) and A. Christina Ravelo, Dept. of Earth and Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California 95064, USA. Posted online 19 October 2012; doi: 10.1130/G33425.1.

Understanding how global temperature changes with increasing atmospheric greenhouse gas concentrations, or climate sensitivity, is of central importance to climate change research. Climate models provide sensitivity estimates that may not fully incorporate slow, long-term feedbacks such as those involving ice sheets and vegetation. Geological studies, on the other hand, can provide estimates that integrate long- and short-term climate feedbacks to radiative forcing. This study by Kelsey Dyez and Christina Ravelo reveals results that suggest that models may not yet adequately represent the long-term feedbacks related to ocean circulation, vegetation, and associated dust, or the cryosphere, and/or may underestimate the effects of tropical clouds or other short-term feedback processes.

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Grounding-line retreat of the West Antarctic Ice Sheet from inner Pine Island Bay
Claus-Dieter Hillenbrand et al., British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Posted online 19 October 2012; doi: 10.1130/G33469.1.

Over the last few decades, melting of West Antarctic glaciers has contributed significantly to global sea-level rise. Since 1992, two major outlet glaciers have experienced up to 25 km of landward retreat of their "grounding line," the position at which the glacier margin starts to float. Both glaciers rest on a bed far below sea level, so their grounding line may undergo further rapid retreat over coming decades. Until today, the long-term context of the "snapshot" of ice-sheet history recorded over the last 20 years was poorly understood. This context is crucial for predicting future ice loss and resulting sea-level rise. In this new study, scientists from the United Kingdom, Germany, and Norway have reconstructed the timing of grounding-line retreat since the end of the last ice age about 12,000 years ago. The research team analyzed marine sediment cores recovered from the seabed offshore from the two glaciers. Their results demonstrate that the grounding line has been located within 110 km of its modern position for the last 10,000 years. Currently, there is no evidence that the glaciers ever re-advanced. Thus, the data imply that the fast retreat observed today is exceptional, if not unprecedented, over the last ten millennia.

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Coral record of reduced El Niño activity in the early 15th to middle 17th centuries
Kelly A. Hereid et al., Dept. of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, Texas 78705, USA. Posted online 19 October 2012; doi: 10.1130/G33510.1.

The El Niño-Southern Oscillation (ENSO) drives many of the catastrophic climate events that occur from one year to the next: floods, droughts, wildfires, and hurricanes. However, climate scientists do not yet know how ENSO will respond to climate change. A new multi-century reconstruction of ENSO variability, based on fossil corals from Papua New Guinea, reveals a century-long decline in the number of El Niño events starting in the mid-1500s. It is the first time such a shift in activity has been documented in either modern observations or past reconstructions. This reduced activity coincided with the initiation of an unusually cool period in the Northern Hemisphere called the Little Ice Age (LIA), which continued on into the mid-1800s. The reconstruction gives scientists a better picture of how ENSO behaved during the coolest period in the last 1,000 years and provides a new baseline for natural ENSO variability to improve future climate change projections.

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Golden plumes: Substantial gold enrichment of oceanic crust during ridge-plume interaction
A.P. Webber et al., School of Ocean and Earth Science, University of Southampton, National Oceanography Centre, European Way, Southampton SO14 3ZH, UK. Posted online 19 October 2012; doi: 10.1130/G33301.1.
 
This article shows that mantle plumes -- hot, upwelling portions of the Earth's mantle -- create large quantities of gold-rich crust when they are melted. These gold-rich rocks contain up to 13 times the amount of gold in normal crust. This means that crust created from mantle plumes represent a rich source of gold and other metals which could be incorporated into mineral deposits. As such, mineral deposits created by sourcing metals from such rocks may be larger or occur more frequently than deposits formed from normal crust. It follows that areas of the Earth's surface formed from the melting of mantle plumes might be highly prospective for gold deposits.

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Copper, lead, and silver isotopes solve a major economic conundrum of Tudor and early Stuart Europe
Anne-Marie Desaulty and Francis Albarede (corresponding), Laboratoire de Géologie de Lyon, Ecole Normale Supérieure, Université de Lyon 1, and CNRS, 69007 Lyon, France; and Dept. of Earth Science, Rice University, Houston, Texas 77030, USA. Posted online 6 November 2012; doi: 10.1130/G33555.1.

The Price Revolution in Europe, the unrelenting inflation during the years 1515 to 1650, has been variously explained by the influx of silver from Mexico and Peru, growth of the European population, and the decline of silver market price. Analysis of silver, copper, and lead isotope abundances in 1550-1650 English coinage show the dominance of silver from Europe and Mexico, contrasting with a remarkably small contribution from Peru. This observation is not consistent with the official registration of metal production in the mines of the Spanish Americas. Hence the question: Where did Potosi silver go? This novel observation indicates that Mexico silver was exported eastward, whereas Potosi silver flowed westward. However aware of the Pacific route of silver trade, scholars never agreed upon the volumes transported. This work demonstrates the vigor of the Potosi-China commercial trade and its disconnection from the Mexico-Europe routes.

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Accelerated subglacial erosion in response to stick-slip motion
L.K. Zoet et al., Dept. of Geosciences, and Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA. Posted online 6 November 2012; doi: 10.1130/G33624.1.

The form of many of the world's mountain ranges has been brought about through the erosive capabilities of the glaciers that lay upon them. Glacial erosion is generally depicted as a steady process, occurring over long periods coupled with steady motion of the glaciers. It is becoming increasingly evident that glaciers do not always behave in a steady manner and that at times they can undergo rapid advancements over their base similar to the way an earthquake slips. The sudden slip generates seismicity that can be observed remotely. L.K. Zoet and colleagues use seismic observations of glaciers as a basis for postulating that these brief advancements, followed by periods of little to no motion, can modify traditional mechanisms of glacial erosion and result in an amplification of the glacier's ability to erode.

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Eocene extension in Idaho generated massive sediment floods into the Franciscan trench and into the Tyee, Great Valley, and Green River basins
Trevor A. Dumitru et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Posted online 6 November 2012; doi: 10.1130/G33746.1.

One important process by which a continent can grow over geologic time is when the erosion of high mountain belts in the continental interior sheds large volumes of rock debris, which is carried away by rivers and eventually fills in adjacent ocean basins. An unusually vigorous episode of such growth occurred from 53 to 40 million years ago in the northwestern United States. Over that time period, intense mountain uplifts and volcanic eruptions occurred in the Challis volcanic province in central Idaho. The Challis Mountains shed very large volumes of sediment into three large, ancient river systems, which then filled in parts of the Pacific Ocean to the west off coastal Oregon and California, as well as partially filling a major lake basin to the southeast in southwestern Wyoming. These sediments now form much of the modern Earth surface under an area of the Oregon Coast Range about 80 km wide by 300 km long from near Tillamook in the north to Coos Bay in the south and under an area of the California Coast Ranges about 50 km wide by 250 km long from near Eureka in the north to Santa Rosa in the south.

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Variable history of Quaternary ice-sheet advance across the Beaufort Sea margin, Arctic Ocean
C.L. Batchelor et al., Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK. Posted online 13 November 2012; doi: 10.1130/G33669.1.

This study uses high-resolution, two-dimensional seismic reflection data, collected by ION Geophysical Corporation as part of the BeaufortSPAN East survey, to examine the seismic stratigraphy and sedimentary architecture of a 1000-km-long section of the Beaufort Sea margin. Three cross-shelf troughs, representing locations of former ice streams draining the north-west sector of the Quaternary North American Ice Sheet, are examined: the Mackenzie, Amundsen Gulf, and M'Clure Strait systems. The objectives of this work by C.L. Batchelor and colleagues are to constrain the number of ice advances through each trough, to discuss the possible timing of these events, and to examine the impact of Quaternary glaciation on the continental shelf and adjacent slope. The results of this study demonstrate significant contrasts between the Mackenzie Trough on the western Beaufort Sea margin and the Amundsen Gulf and M'Clure Strait troughs to the east, both in terms of their glacial history and resulting architecture. There is evidence for only two ice advances through the Mackenzie Trough and at least nine through the Amundsen Gulf Trough. Whereas the slope beyond the Mackenzie Trough lacks a significant glacial-sedimentary depocenter, major trough-mouth fans are present beyond Amundsen Gulf and M'Clure Strait.

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Inferring sites of subglacial erosion using the Pb isotopic composition of ice-rafted feldspar: Examples from the Weddell Sea, Antarctica
M.J. Flowerdew et al., British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Posted online 13 November 2012; doi: 10.1130/G33644.1.

The Antarctic ice sheets drain from the continent interior to the ocean via fast flowing ice streams. As the ice within these streams moves, it erodes and modifies the bed over which it travels. Some of the eroded material is trapped within the ice as mineral and rock fragments and transported into the ocean as icebergs. As the icebergs melt, the minerals and rock fragments sink to the ocean floor as ice rafted debris. This study uniquely links lead isotope geochemistry of recently deposited ice rafted feldspars with geophysical observations over the glacial catchments and ice streams from where the feldspars originated. The findings presented by M.J. Flowerdew and colleagues indicate that subglacial erosion of bedrock is restricted to regions where ice velocity, basal shear stresses, and bed roughness are high. Changes in the age and isotope composition of ice-rafted debris are commonly taken as evidence for the collapse or disintegration of parts of the ice sheet. This study shows that significant variations in the chemistry and age of the ice rafted materials can instead result from changing the loci of subglacial erosion and do not necessarily correspond with episodes of major ice sheet instability.

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Lacustrine stable isotope record of precipitation changes in Nicaragua during the Little Ice Age and Medieval Climate Anomaly
Nathan D. Stansell et al., Byrd Polar Research Center, Ohio State University, Scott Hall Room 108, 1090 Carmack Road, Columbus, Ohio 43210, USA. Posted online 13 November 2012; doi: 10.1130/G33736.1.
 
The climate of Central America is influenced by changing conditions in both the North Atlantic region and the tropical Pacific Ocean, and how these systems have varied in the past to affect precipitation patterns in the tropics is poorly understood. Our ability to predict future changes in water resource availability requires a longer-term perspective based on the geologic record. In this paper, Nathan D. Stansell and colleagues analyze lake sediments from Nicaragua for their oxygen isotopic composition in order to infer past changes precipitation during the last ~1,400 years. This record is the first of its kind from Nicaragua that documents abrupt environmental changes at a resolution sufficient for comparing with other regional paleoclimate archives. In this region of the northern tropics, the Medieval Climate Anomaly was consistently wet, and the Little Ice Age was relatively dry. Based on what is known about modern conditions, these findings further suggest that changes in mean-state conditions of the both ocean basins operated differently in the past to affect the climate of Central America. Consequently, climate models that evaluate changes in water resources for this region need to consider that the modern analogue might not be consistent with what has operated on longer time-scales in the past.

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Marine diversity in the geological record and its relationship to surviving bedrock area, lithofacies diversity, and original marine shelf area
Andrew B. Smith (corresponding) and Roger B.J. Benson, Dept. of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK. Posted online 13 November 2012; doi: 10.1130/G33773.1.

The diversity of fossils recorded by paleontologists from the geological record is the product of two factors -- original biodiversity and the completeness of the geological rock record that survives today to be sampled. Andrew B. Smith and Roger B.J. Benson show that species diversity of Cretaceous echinoids can be predicted with a high degree of precision from proxies for marine sedimentary bedrock area and habitat diversity that is captured by the rock record. However, they found no simple relationships to link surviving bedrock area to the original area over which marine sediments were deposited, or the surviving lithofacies diversity to the range of habitats that once existed. Rather, the completeness of the rock record varies markedly during large-scale sea level cycles. The ability to sample fairly across a broad range of habitats is as crucial as the ability to sample fairly within those habitats for the accurate estimation of paleodiversity, but large-scale sea level cycles clearly hinder the former.

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Recurrent Early Triassic ocean anoxia
S.E. Grasby et al., Geological Survey of Canada-Calgary, Natural Resources Canada, 3303 33rd Street N.W., Calgary, Alberta T2L 2A7, Canada. Posted online 13 Nov. 2012; doi: 10.1130/G33599.1.

The greatest extinction event in Earth's history, the Latest Permian Extinction, saw a loss of more than 90% of the planet's species. Second to the mystery of what caused this extinction is why life on Earth took a prolonged five-million years to recover (referred to as the Early Triassic period). Previous work has suggested that the world's oceans had remained anoxic and uninhabitable during this time (no dissolved oxygen in sea water for life). However, this new work by S.E. Grasby and colleagues demonstrates that Earth's oceans quickly returned to habitable conditions and were then driven back to anoxia several times during the Early Triassic. This caused several failed attempts by life to recover until the age of the dinosaurs finally began. The cause of these repeated stresses remains uncertain, but Grasby and colleagues point to evidence for a series of major volcanic eruptions during that time. They suggest that emissions of volcanic gases in addition to nutrients and toxins could have had significant and repeated impacts on the global ecosystem.

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Rapid Pliocene uplift of Timor
N. Nguyen et al. (J. Shulmeister, corresponding), School of Geography, Planning and Environmental Management, University of Queensland, St Lucia, 4072 QLD, Australia. Posted online 13 November 2012; doi: 10.1130/G33420.1.

Changes in vegetation from coastal mangroves to montane forest are used to record the emergence of Timor from the sea and to track the uplift of the island from sea level to over 2000 m above sea level. The island started to emerge soon after 4.5 million years ago, but the uplift dramatically accelerated after 3.1 million years ago to rates of 2-5 mm per year. The emergence of Timor is earlier than Indonesia to the west, but follows the same pattern. The uplift of Timor is closely associated with the constriction of the Indonesian through flow. This study helps constrain both the nature of tectonic activity in this region and also reinforces understanding of the causes of climate change in the last three million years, including the timing of development of a modern El Niño Southern Oscillation (ENSO) system.

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Drainage capture and discharge variations driven by glaciation in the Southern Alps, New Zealand
Ann V. Rowan et al., School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. Posted online 13 November 2012; doi: 10.1130/G33829.1.

Mountain glaciers respond to climate change by rapidly advancing or receding as temperatures vary, and this change in glacier extent controls hydrology, sediment transport, and deposition in rivers downstream. The sedimentary record in glaciated catchments therefore represents an important archive with which to unravel past climate change. However, unlike rivers, whose flow is controlled entirely by land surface topography, glaciers are able to flow uphill and across ridgelines -- a process called "transfluence." Glacier advance and recession can result in drainage capture by transfluent ice flow, and so change catchment drainage areas and hydrological budgets. Ann Rowan and colleagues use a 3-D glacier model of ice flow during the last glaciation in adjacent catchments in the Southern Alps of New Zealand to find that glacial-drainage capture is a more important control on hydrology and the sedimentary record than climate change alone. These results from New Zealand demonstrate how scientists can use the sedimentary record to investigate past climate change both in this mid-latitude Southern Hemisphere location and in glaciated settings worldwide.

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Reappraisal of the relationship between the northern Nevada rift and Miocene extension in the northern Basin and Range Province
Joseph P. Colgan et al., U.S. Geological Survey, Menlo Park, California 94025, USA. Posted online 13 November 2012; doi: 10.1130/G33512.1.

For the past 17 million years and perhaps longer, motion of Earth's tectonic plates has moved California west, away from Utah and Arizona. As the area in between grew wider, Earth's crust cracked along major faults to form the mountains and valleys of the Basin and Range Province that today cover Nevada and western Utah. In the early stages of this process, large amounts of molten basalt intruded Earth's crust in northern Nevada, forming a linear, near-vertical dike swarm over 500 km long. This structure -- named the "northern Nevada rift" -- was interpreted to record roughly southwest-oriented stretching during early growth of the Basin and Range, which changed to a more northwesterly direction around 10 million years ago. This paper by Joseph P. Colgan and colleagues presents new and existing data that instead suggest that Basin and Range stretching has always been oriented west or slightly northwest. This direction is consistent with formation of the Basin and Range by "collapse" of a formerly high plateau -- somewhat like northern Chile today -- that once occupied what is now the state of Nevada, while the orientation of the northern Nevada rift may have been controlled by unusual, short-lived conditions associated with the beginning of the Yellowstone hotspot.

http://www.geosociety.org/news/pr/12-92.htm

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