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Glacier song

New Geology articles posted online ahead of print for October 2014

Geological Society of America

Boulder, Colo., USA - Mountain glaciers represent one of the largest repositories of fresh water in alpine regions. However, little is known about the processes by which water moves through these systems. In this study published in Geology on 24 Oct. 2014, David S. Heeszel and colleagues use seismic recordings collected near Lake Gornersee in the Swiss Alps to look for signs of water moving through fractures near the glacier bed. Analysis of these recordings reveals, for the first time, that harmonic tremor occurs within mountain glaciers and that individual icequakes at the glacier base can exhibit harmonic properties.

These observations suggest that there is a complex network of fluid-induced fracture processes at the glacier base. Because glacial lake drainage events can occur with little or no warning, there is the potential for damaging floods in valleys below the glacier. Unfortunately, because the water moves under and through the glacier, surface observations alone cannot predict lake drainage events.

Modeling changes in the observed harmonic frequencies indicates that the spectral characteristics of seismic data can provide important information about hydraulic fracture geometry and fluid pressure at depth, leading to important insights into subglacial hydrologic processes. Future modeling of these processes may lead to improved glacial outburst flood hazard predictions.

FEATURED ARTICLE

Humming glaciers

David S. Heeszel et al., Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, MC 0225, La Jolla, California 92093, USA, Current Address: U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, USA. Published online ahead of print on 24 Oct. 2014; http://dx.doi.org/10.1130/G35994.1.

Other recently posted GEOLOGY articles (see below) cover such topics as

1. Ice on Mercury;

2. 1.7 billion-year-old microfossils and the Great Ocean Oxidation Event; and

3. How methane enters a carbon store and is frozen within the sediment.

GEOLOGY articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above.

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.

Images of surface volatiles in Mercury's polar craters acquired by the MESSENGER spacecraft

Nancy L. Chabot et al., The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA. Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G35916.1.

Images acquired by NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft provide the first views at visible wavelengths of ice and other volatiles frozen within permanently shadowed craters near Mercury's north pole. Two decades ago, Earth-based radar images of Mercury revealed polar deposits postulated to consist dominantly of water ice, a hypothesis later confirmed by MESSENGER through neutron spectrometry, thermal modeling, and infrared reflectometry. MESSENGER's latest images now reveal the morphology of the polar deposits. Although the volatile deposits are in permanent shadow, images were acquired at the very low levels of light scattered from sunlit crater walls. Those images show extensive regions with distinctive reflectance properties. A location interpreted as hosting widespread surface water ice exhibits a cratered texture indicating that the ice was emplaced more recently than any of the underlying craters. In other areas, water ice is present but covered by a thin layer of dark material inferred to consist of frozen organic-rich compounds. The dark deposits display sharp boundaries, indicating that they are geologically recent. Overall, the images indicate that Mercury's polar deposits either were delivered to the planet recently or are regularly restored at the surface through an ongoing process.

Fossil evidence of iron-oxidizing chemolithotrophy linked to phosphogenesis in the wake of the Great Oxidation Event

Chris H. Crosby et al., Dept. of Earth Sciences, University of Minnesota-Twin Cities, Minneapolis, Minnesota 55455, USA, http://dx.doi.org/10.1130/G35922.1. Published online ahead of print on 7 Oct. 2014.

1.7 billion-year-old microfossils may record locally changing energy conditions on Earth and a microbial adaptation possibly instrumental in concentrating phosphorus in early stromatolites. Stromatolites, geological structures formed as material is trapped by sticky microbial biofilms, are indicative of some of the earliest life on Earth: photosynthetic microbes that use light to extract energy from water, releasing oxygen as a by-product. Subsequent utilization of oxygen by other microbes would have induced alternating conditions -- daytime oxygenated, nighttime anoxic -- and promoted metabolic strategies for adapting to these fluctuations. Some microbes today respond to this challenge by polymerizing excess phosphate and when oxygenated conditions go anoxic, accessing phosphate bond energy and releasing phosphate ions, ultimately contributing to the formation of phosphatic mineral deposits. Some recently studied marine microbes that extract energy from oxidizing dissolved iron also exhibit this ability, and strongly resemble these microfossils. The microfossil-bearing rock is unusual: The phosphate content is unusually high, most stromatolites are dominated by calcium-containing minerals while these are dominated by phosphate minerals, and most phosphate deposits are not stromatolitic. The presence of these organisms may account for the anomalous concentration of phosphorus in these enigmatic early phosphatic stromatolites found at this pivotal time in Earth's history.

Probable patterns of gas flow and hydrate accretion at the base of the hydrate stability zone Richard J. Davies et al., School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne NE1 7RU, UK. Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G36047.1.

Marine gas hydrate is a frozen compound of water and methane gas that can occur around the margins of Earth's continents. It is the largest carbon reservoir in the global organic carbon cycle, and since methane is a potent greenhouse gas, its release may have a role in past and future climate change. But surprisingly little is known of how methane enters this carbon store and is frozen within the sediment, critically preventing it from entering the water column and the atmosphere. In this study, we used exceptionally high quality three-dimensional pictures of the bottom of a marine gas hydrate from offshore Mauritania to establish how this happens. We found that gas flows up hydraulic fractures, known as chimneys, that are hundreds of meters tall. The gas exits the top of the chimneys and hits the layers of frozen water and methane. It flows below these layers forming remarkable teardrop-like patterns. These trails of methane gas then convert into methane hydrate and are locked into the carbon store. These are the first pictures revealing how methane enters marine hydrate rather than escaping into the ocean or atmosphere.

Late Pliocene-Pleistocene expansion of C4 vegetation in semiarid East Asia linked to increased burning

Bin Zhou et al., Key Laboratory of Surficial Geochemistry (Ministry of Education), School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China and State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China. Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G36110.1.

Researchers at Nanjing University and collaborating institutions provide evidence for the presence of C4 taxa (largely sub-tropical and tropical grasses) on the Chinese Loess Plateau from at least the late Miocene, rising to prominence about 2.6 million years ago following changes in climate and -- critically -- increased biomass burning. Results support a positive feedback between increased fire activity and expansion of C4 biomass, within the context of increased aridity associated with uplift of the Tibetan Plateau, initiation and intensification of Northern Hemisphere glaciation, and associated monsoonal variations. Following this rise to prominence, anthropogenic activity further affected fire regimes, and the relative abundances of C4 taxa, and may have disrupted the coupling between climate, fire, and vegetation. Based on a combination of proxy records and spectral analysis, variations in climate and fire activity, rather than pCO2, appear to have been the main environmental controls over the relative abundances of C3 and C4 plants in East Asia since the late Miocene. These results will prove of interest to paleoclimatologists, climate change-vegetation response modelers, evolutionary biogeographers, and environmental/earth scientists more generally.

A mechanism for construction of volcanic rifted margins during continental breakup

David G. Quirk et al., Maersk Oil, Esplanaden 50, 1263 Copenhagen K, Denmark. Published online ahead of print on 24 Oct. 2014; http://dx.doi.org/10.1130/G35974.1.

This paper by David G. Quirk and colleagues presents a model for the formation of oceanic basins, the North Atlantic. Deep seismic images through Earth's crust along East Greenland are used to resolve a controversial part of plate tectonic theory -- how the rifting and breakup of continents transforms into construction of new oceanic crust. The work focuses on an intriguing belt of lava flows, thousands of meters thick, which were erupted above sea level but are now found on the continental slope of many of the world's oceans. These were produced when faults and magma ruptured the crust at the time the continents began to break apart. The authors use evidence on how these developed to propose a significant modification to plate tectonic theory, specifically on how new plates are initiated.

Profile of a paleo-orogen: High topography across the present-day Basin and Range from 40 to 23 Ma

Elizabeth J. Cassel et al., Dept. of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA. Published online ahead of print on 7 Oct. 2014; http://dx.doi.org/10.1130/G35924.1.

Earth's surface topography responds directly to plate tectonic processes and controls river drainages, landscapes, and climate. Many tectonic reconstructions of the North American Cordillera suggest that a high elevation plateau, like the Andean Altiplano, once existed in the location of the modern Basin and Range province. Exactly where and for how long that plateau existed and what drove its collapse remain controversial. In this article, Elizabeth Cassel and colleagues use ancient rainwaters preserved in volcanic glass to quantify past elevations across the region. They combine these elevation measurements with drainage pattern and environmental reconstructions based on the sedimentary rock record to find that from 40 to 23 million years ago, a high, broad mountain range stretched across Nevada. This mountain range reached elevations of 3.5 km, with a distinct crest that divided a >300 km westward draining slope from an internally drained, high elevation plateau in eastern Nevada. Orogen collapse and lowering of surface elevations did not begin until after 23 Ma, likely during the transition to a transform plate boundary on the western margin of North America. Based on the locations of the highest paleoelevations, mid-crustal flow was required to accommodate surface lowering.

High Arctic forests during the middle Eocene supported by moderate levels of atmospheric CO2

Daniel P. Maxbauer et al., Dept. of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, USA, and Dept. of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA. Published online ahead of print on 18 Oct. 2014; http://dx.doi.org/10.1130/G36014.1.

Fossils from ancient polar forests lie in stark contrast to the polar ecosystems we see today. These ancient forests provide compelling information about our Earth during globally-warm "greenhouse" climates. The Napartulik fossil forest site in the Canadian High Arctic is one of the best examples of an ecosystem that flourished during middle Eocene greenhouse climates. High Arctic fossil forest sites, including Napartulik, have been extensively studied; however, there are no direct constraints on the atmospheric carbon dioxide (CO2) concentrations for any ancient polar forest. We provide the first direct estimates of atmospheric carbon dioxide (CO2) concentration from an ancient polar forest by studying fossil leaves of dawn redwood (Metasequoia) from Napartulik. Our results suggest that during the time of forest development and growth, CO2 levels were only ~1.5 times higher than preindustrial levels. This implies that the long-term earth-system sensitivity to CO2 was at times high (greater than three degrees Celsius per CO2 doubling). Importantly, our results reinforce the long-standing idea that some climate feedbacks that act during greenhouse climates remain poorly understood.

Enhanced carbon dioxide outgassing from the eastern equatorial Atlantic during the last glacial

G.L. Foster, Ocean and Earth Science, National Oceanography Centre, Southampton, University of Southampton, Waterfront Campus, Southampton SO14 3ZH, UK, and P.F. Sexton, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. Published online ahead of print on 7 Oct. 2014; http://dx.doi.org/10.1130/G35806.1.

It is well accepted that variations in oceanic carbon storage had a vital role to play in driving orbital-scale changes in atmospheric CO2. One process that may be important in this regard is the biological pump of carbon from the surface ocean into the deep. Indeed, biological activity and carbon export is enhanced throughout the glacial-aged low latitude ocean when CO2 was low (~180 ppm). Here, we use the boron isotopic composition of the shells of foraminifera that lived in the sunlit mixed layer over the last 30 thousand years as a novel tracer of the CO2 content of surface water in the eastern equatorial Atlantic. We find that upwelling in the east increased by ~5-fold in response to stronger glacial winds and the associated increase in nutrient supply drove the observed increases in productivity. However, we also show that this caused no net increase in the strength of the biological pump in the region allowing us to conclude that the equatorial Atlantic likely exerted a minimal role in contributing to lower glacial-aged atmospheric CO2.

Organic-walled microfossil assemblages from glacial and interglacial Neoproterozoic units of Australia and Svalbard

Leigh Anne Riedman et al., Dept. of Earth Science, University of California, Santa Barbara, California 93106, USA. Published online ahead of print on 7 Oct. 2014; http://dx.doi.org/10.1130/G35901.1.

Between 720 and 635 million years ago, before the diversification of animals in the Cambrian period, Earth experienced at least two global glaciations during which ice was present at low latitudes at sea level. Although it has been suggested that these "Snowball Earth" events resulted in mass extinctions of the chiefly single-celled marine life present at the time, very little is known about the impact of these events upon life. New paleontological data from shales of Australia and Svalbard deposited during and after the first glaciation suggest that diversity was, indeed, reduced during this time. These fossil assemblages are dominated by bacteria and possible algal resting cells in contrast to much more diverse assemblages found in early Neoproterozoic rocks. Intriguingly, this diversity drop appears to have occurred ~30 million years before the onset of the first glaciation. An unexpectedly early loss of diversity and absence of recovery after the termination of the first glaciation could suggest the Snowball Earth events may not have been the sole drivers of this biotic crisis.

How is topographic simplicity maintained in ephemeral, dryland channels?

Michael Bliss Singer, Dept. of Earth and Environmental Sciences, University of St Andrews, Irvine Building, North Street, St Andrews KY16 9AL, UK; and Katerina Michaelides, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK, (both at: Earth Research Institute, University of California-Santa Barbara, Santa Barbara, California 91306, USA). Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G36267.1.

The forces that shape desert streams are fundamentally different from those that sculpt perennially flowing rivers. Desert streams only flow during infrequent but intense rainstorms, and when they do, only parts of the channel contain water. So the flow along a desert stream bed is irregular and erratic. One rainstorm may erode sediment grains a short distance from one part of the channel, while another storm will move sediment in a different channel segment. Given this localized sediment movement during rainstorms, one might expect desert channels to contain mounds of sediment that undulate down the stream course reflecting the irregular flow. Paradoxically, though, desert streams have surprisingly simple topography with smooth, straight, and symmetrical form, which has until now defied explanation. This mystery has been resolved in this study by Michael Bliss Singer and Katerina Michaelides. The authors use field data in a numerical model of sediment movement to show that the simple shape of desert streams is maintained by the competing forces of different flow patterns over time. The researchers demonstrate that some configurations of streamflow accumulate sediments while other flows destroy these sediment deposits. They also show that rare, large floods completely reshape desert stream channels to keep them smooth and simple.

Rainfall conditions, typhoon frequency, and contemporary landslide erosion in Japan

H. Saito et al., College of Economics, Kanto Gakuin University, 1-50-1 Mutsuura-higashi, Yokohama, Kanagawa 236-8501, Japan, and Center for Spatial Information Science, The University of Tokyo, 5-1-5 Kashiwanoha Kashiwa, Chiba 277-8568, Japan. Published online ahead of print on 7 Oct. 2014; http://dx.doi.org/10.1130/G35680.1.

Dealing with predicted increases in extreme weather conditions due to climate change requires robust knowledge about controls on rainfall-triggered landslides. We explore relationships between rainfall and landslide size throughout the Japanese archipelago. We test whether the total volume of landslides can be predicted directly from rainfall totals, intensity, and duration using a nationwide inventory of 4744 rainfall-triggered landslides recorded from 2001 to 2011. We find that larger landslides were more abundant at the expense of smaller ones when total, maximum, and mean rainfall intensity exceeded ~250 mm per hour, ~35 mm per hour, and ~4 mm per hour, respectively. Frequency distributions of these rainfall parameters are peaked and heavily skewed. Yet neither the most frequent nor the most extreme values of these rainfall metrics coincide consistently with the maximum landslide volumes. A striking decrease of landslide volumes at both mean and maximum rainfall intensity, as well as duration, points to an exhaustion in hillslope geomorphic response. Our results underscore substantial offsets between the peaks of rainfall metrics and maximum associated landslide volumes, thus complicating straightforward estimates of geomorphic work from metrics of rainstorm magnitude or frequency. Only the rainfall total appears to be a suitable monotonic predictor of landslide volumes mobilized during typhoons and frontal storms.

A temperate former West Antarctic ice sheet suggested by an extensive zone of subglacial meltwater channels

Kathryn C. Rose et al., Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK. Published online ahead of print on 7 Oct. 2014; http://dx.doi.org/10.1130/G35980.1.

Here, we use satellite imagery and airborne geophysical techniques to examine the nature of the ice-sheet surface and the sub-ice landscape in the Weddell Sea Sector, West Antarctica. We discovered an extensive zone of major bedrock channels in the region between the Möller and Foundation Ice Streams. In order to form, these newly recognized channels required significant water to be routed along the base of the ice-sheet. Taking into account the present setting of the channels, we believe such water originated at the ice surface. Today, the Greenland Ice Sheet highlights how significant seasonal surface melt may be transferred to the subglacial environment through the ice sheet. For West Antarctica, the Pliocene (2.6 to 5.3 million years ago) represents the most recent sustained period when atmospheric temperatures were high enough to generate surface melt comparable to that observed on the Greenland Ice Sheet today. We propose, therefore, that the channels formed beneath a warm-based ice sheet that was present (at least periodically) in this location during the warm conditions of the Pliocene.

On the origin of recent intraplate volcanism in Australia

D. Rhodri Davies and Nicholas Rawlinson, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia and School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, Scotland, UK. Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G36093.1.

Most volcanism takes place near plate boundaries, particularly where one tectonic plate descends beneath another in a process known as subduction. However, there are a relatively small but significant number of so-called intra-plate volcanos whose origins are unrelated to plate boundary processes. The cause of such volcanism is a subject of intense debate in the global earth sciences community. In this study, we combine 3-D seismic tomography and sophisticated geodynamic modelling to show that convective instabilities in the upper mantle can produce localized zones of upwelling beneath the lithosphere that may be responsible for recent intra-plate volcanism in Victoria, southeast Australia. The two crucial elements to our model are the presence of significant topography at the base of the lithosphere and plate motion; these can combine to produce a focused recirculation cell in the upper mantle which has vertical velocities of a magnitude sufficient to generate significant melt, which can then percolate through the crust to the surface. Our new model may be applicable to other intra-plate volcanoes around the world.

Paleogeographic record of Eocene Farallon slab rollback beneath western North America

M. Elliot Smith et al., School of Earth Science and Environmental Sustainability, Northern Arizona University, 602 S. Humphreys, Flagstaff, Arizona 86011, USA. Published online ahead of print on 15 Oct. 2014; http://dx.doi.org/10.1130/G36025.1.

This study utilizes the sensitive record of Earth's surface provided by ancient lake deposits to document a wave of topography that moved from NE to SW across the central Rocky Mountains from 53 to 47 million years ago. We apply geochronology of volcanic ash beds and proximal volcanic deposits to quantify this sweep of topography, associated volcanism, and secession of reverse faulting. We argue that the ultimate cause of southwestward migration of lakes in the Rocky Mountains was the progressive removal of low-angle subduction from the base of the North American lithosphere.

Fluid mixing from below in unconformity-related hydrothermal ore deposits

Paul D. Bons et al., Dept. of Geosciences, Eberhard Karls University Tübingen, Wilhelmstrasse 56, 72074 Tübingen, Germany. Published online ahead of print on 15 Oct. 2014; http://dx.doi.org/10.1130/G35708.1.

Hot, so-called "hydrothermal" fluids that flow through rocks can produce ore deposits, typically with gold, silver, lead, or zinc. Often, we find that these fluids are mixtures of fluids that came from the surface and from deep in the crust. In this study, we provide an answer to the outstanding question about how these different fluids can actually converge and mix. Rain and seawater first slowly infiltrate the crust from the surface. The oldest fluids reach the deepest and hottest levels where they dissolve the (precious) metals, while the latest fluids remain closer to the surface. Tectonic events trigger the sometimes violent release of these fluids through rapidly rising fractures. These fractures tap fluids from all levels that mix during their ascent through the crust. Our findings shed new light on the origin of a range of ore deposits, such as those in the German Schwarzwald.

Measuring the time and scale-dependency of subaerial rock weathering rates over geologic time scales with ground-based LiDAR

Amit Mushkin et al., Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel and University of Washington, 1410 NE Campus Parkway, Seattle, Washington 98195, USA. Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G35866.1.

Emerging field-based 3-D surveying technologies, such as LiDAR and/or automated photogrammetry, are significantly advancing our ability to readily acquire spatially extensive and/or repeated high-resolution roughness data for natural terrains. Here, we examine the spectral (i.e., scale-dependent) evolution of surface roughness through time as a new field-based approach for quantifying the rates of geomorphic surface processes. We focus on the subaerial weathering of rocks on late Quaternary (5,000 to 87,000-year-old) alluvial surfaces in the hyperarid Negev desert of Israel and utilize ground-based LiDAR data to characterize rock weathering mechanisms and their rates as a function of both length scale and time. Power Spectral Density (PSD) analysis of surface roughness revealed rock-weathering rates that increase with length-scale while decaying through time as an inverse power-law function from >20 mm per thousand years (kyr) at the initial stages, down to <1 mm/kyr within ~60 kyr. We propose that such field-based characterization of rock weathering mechanisms and time-variant rates can be used to better inform current models of landscape evolution, which commonly assume that rock weathering is a constant-rate process over geologic time-scales.

Linking rift propagation barriers to excess magmatism at volcanic rifted margins

Hannes Koopmann et al., Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30177 Hannover, Germany; and Gottfried Wilhelm Leibniz Universität Hannover, Institut für Mineralogie, Callinstrasse 3, 30167 Hannover, Germany. Published online ahead of print on 15 Oct. 2014; http://dx.doi.org/10.1130/G36085.1.

Extensive volcanic activity accompanied the separation of continental plates along the southern and northern Atlantic. Examples from various locations along the margin, such as the island of Iceland or Giant's Causeway in Ireland, show a link between volcanic activity and transfer fault systems. Observational research results suggest that plate separation advancement was delayed in these locations. The model shows that this delay played an important role in the localization of volcanic provinces along the Atlantic continental margins. Just as water flows from one full basin to an empty one if a barrier between the two is removed, hot material flows parallel to the continental margin if the delay at one of the transfer fault systems is overcome. At the same time, there is increased productivity at the location of the delay, causing the localization of volcanic provinces at these points.

Temperature and leaf wax d2H records demonstrate seasonal and regional controls on Asian monsoon proxies

Elizabeth K. Thomas et al., Dept. of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island 02912, USA. Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G36289.1.

In monsoon regions, precipitation isotope records (delta-2H, delta-18O) have been interpreted as proxy records of precipitation amount during the monsoon season, the changing influence of isotopically distinct sources, or changes in upstream dynamics of vapor transport. In reality, a combination of these factors likely contributes to precipitation isotope variability. There is one published orbital-scale precipitation isotope record in East Asia, based on speleothem delta-18O in central China. Here, we generate an orbital-scale leaf wax hydrogen isotope (delta-2Hwax) record from southeast China. The timing of these two orbital-scale precipitation isotope records is different, indicating strong regional variability in the mechanisms that control East Asian precipitation isotopes. The differences in timing indicate that in southeast China, precipitation isotopes are strongly influenced by insolation, whereas precipitation isotopes in central China are influenced by global ice volume as well as insolation. These findings highlight the need to consider the entire suite of factors that influence precipitation isotopes, including regional variability, seasonality, temperature, and source area and transport history. Our findings suggest that glacial boundary conditions may play an important role in modulating these factors in higher-latitude regions, including in central China.

Probing large intraplate earthquakes at the west flank of the Andes

G. Vargas et al., Depto. de Geología, CEGA, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, 8370450 Santiago, Chile. Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G35741.1.

Estimating the potential for the occurrence of large earthquakes on slow-slip-rate faults in continental interiors, away from plate boundaries, is possible only if the long-term geological record of past events is available. However, our knowledge of strong earthquakes appears to be incomplete for thrust faults flanking large actively growing mountain ranges, such as those surrounding Tibet and the Andes Mountains. We present a paleoseismic study of a prominent fault scarp at the west flank of the Andes in Santiago, Chile. The evidence demonstrates recurrent faulting with displacement of ~5 m in each event. With two large earthquake ruptures within the past 17 to 19 thousand years, and the last event occurring ~8 thousand years ago, the fault appears to be ripe for another large earthquake (moment magnitude, Mw 7.5). These results emphasize the potential danger of intraplate continental faults, particularly those associated with youthful mountain fronts.

How is topographic simplicity maintained in ephemeral, dryland channels?

Michael Bliss Singer, Dept. of Earth and Environmental Sciences, University of St Andrews, Irvine Building, North Street, St Andrews KY16 9AL, UK; and Katerina Michaelides, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK, (both at: Earth Research Institute, University of California, Santa Barbara, California 91306, USA). Published online ahead of print on 17 Oct. 2014; http://dx.doi.org/10.1130/G36267.1.

The forces that shape desert streams are fundamentally different from those that sculpt perennially flowing rivers. Desert streams only flow during infrequent but intense rainstorms, and when they do, only parts of the channel contain water. So the flow along a desert stream bed is irregular and erratic. One rainstorm may erode sediment grains a short distance from one part of the channel, while another storm will move sediment in a different channel segment. Given this localized sediment movement during rainstorms, one might expect desert channels to contain mounds of sediment that undulate down the stream course reflecting the irregular flow. Paradoxically, though, desert streams have surprisingly simple topography with smooth, straight, and symmetrical form, which has until now defied explanation. This mystery has been resolved in this study by Michael Bliss Singer and Katerina Michaelides. The authors use field data in a numerical model of sediment movement to show that the simple shape of desert streams is maintained by the competing forces of different flow patterns over time. The researchers demonstrate that some configurations of streamflow accumulate sediments while other flows destroy these sediment deposits. They also show that rare, large floods completely reshape desert stream channels to keep them smooth and simple.

Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry

Matthijs A. Smit et al., Dept. of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, 2020-2207 Main Mall, Vancouver V6T 1Z4, Canada and Dept. of Earth Science, University of California-Santa Barbara, 1006 Webb Hall, Santa Barbara, California 93106-9630, USA. Published online ahead of print on 15 Oct. 2014; http://dx.doi.org/10.1130/G35878.1.

The Pamir-Tibet mountain belt includes Earth's highest mountains and most extensive high plateaus, and is a natural laboratory for studying all aspects of active mountain building. It is clear that the India-Asia collision (about 50 million years ago [Ma]) was the main event causing the crust to thicken, heat up, and support uplift. However, the "how and when" of these processes is not well constrained. To progress in this field, we investigated the history of Cenozoic deep-crustal rocks from the Pamir plateau. The analyses revealed that (1) heating of the deep crust was underway by 37-27 Ma, differing per locality; and (2) rocks that were heated earlier ended up having the highest peak temperatures. These results illustrate how heating spread throughout the deep crust of the Pamir and ultimately caused temperatures to rise beyond 750 °C on a regional scale. This effect is attributed to the break-off of the Indian slab at ca. 45 Ma and the subsequent invasion of hot mantle material beneath the region. Besides confirming a long-predicted thermal evolution for the Pamir, the history uncovered here provides a causal link between slab break-off and crustal heating for regions overlying a broken or retreating slab.

From continent to intra-oceanic arc: Zircon xenocrysts record the crustal evolution of the Solomon island arc

Simon Tapster et al., Dept. of Geology, University of Leicester, Leicester LE17 RH, UK and NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK. Published online ahead of print on 24 Oct. 2014; http://dx.doi.org/10.1130/G36033.1.

Significant expanses of present day continental crust initially formed in subduction-related oceanic-arc environments far away from areas of older continental crust. Thus, the well documented yet enigmatic presence of continentally-derived zircon crystals that occurs within these regions is a paradox. The ages of zircons shown to be recycled within the oceanic Solomon island arc (southwest Pacific) suggest that episodes of extension at the Australian continent margin from ca. 95 Ma onwards not only generated the oceanic setting in which the islands now lie, but also transported continental material thousands of kilometers and mixed it into the oceanic-arc setting ca. 40 Myr later. It is likely that the occurrences of similar phenomena observed within ancient examples relate to similar tectonic processes. The example of the Solomon island arc highlights that a spectrum of arcs exists between oceanic and continental end-members that must be considered when interpreting the observable features of arc rocks elsewhere.

Constraints on the recent rate of lunar ejecta breakdown and implications for crater ages

Rebecca R. Ghent et al., Dept. of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada and Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, Arizona 85719, USA. Published online ahead of print on 24 Oct. 2014; http://dx.doi.org/10.1130/G35926.1.

The rates of geological processes on Earth's Moon are important because they act as a reference for the ages of surface features on other planetary bodies. However, these rates have historically been difficult to determine. An example is the breakdown of large rocks, exposed in the ejecta blankets of impact craters, via bombardment by small impactors -- the process responsible for forming the lunar regolith. Here, we present a new method for measuring this rate. We use observations from the Lunar Reconnaissance Orbiter (LRO) Diviner thermal radiometer, which measures thermal infrared emission from the lunar surface. By exploiting the fact that bare rocks radiate at higher temperatures during the lunar night than fine regolith, it is possible to discriminate between rocks and regolith. We use this to measure the abundance of large rocks in fresh crater ejecta, and to investigate variations in this quantity with crater age. We find that rock abundance decreases rapidly with crater age, and implies shorter rock survival times than previously inferred. This also represents a new method for dating young lunar craters, which will provide the basis for future investigation of the rate of lunar bombardment by large objects over time.

Ocean acidification in the aftermath of the Marinoan glaciation

Frank Ohnemueller et al., Dept. of Geosciences and MARUM-Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, D-28359 Bremen, Germany. Published online ahead of print on 24 Oct. 2014; http://dx.doi.org/10.1130/G35937.1.

The Cryogenian-Ediacaran transition was a decisive time in Earth history. At that time the Earth experienced a severe ice age known as "Snowball Earth" leading to dramatic climate changes and modifications in the ocean-atmosphere-continent interaction. Among others, the aftermath of the so-called Marinoan glaciation (~635 million years ago) was characterized by high atmospheric pCO2 levels. Ohnemueller et al. now reveal that during the deglacial phase a global ocean acidification event by oceanic CO2 uptake took place. Boron isotopes of cap carbonates from South China (Yangtze Platform), Namibia (Congo craton) and Kazakhstan (Karatau microcontinent) show similar acidification patterns with local to regional differences in the duration and magnitude of the acidification event. It appears that platform facies record a faster return to more alkaline pH states than shelf-break sections which is most likely a consequence of the higher continental alkalinity influx to the ocean and associated buffer capacities at more proximal settings.

How was the Iapetus Ocean infected with subduction?

John W.F. Waldron et al., Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. Published online ahead of print on 24 Oct. 2014; http://dx.doi.org/10.1130/G36194.1.

The supercontinent Pangea came together about 400 million years ago as a result of the collision of several large continental blocks. The Iapetus Ocean had opened about 150 million years previously by rifting between three of these continents. However, the mechanism by which an ocean transitions from opening to closing is an unsolved problem in tectonics. In this paper, we suggest that the Iapetus Ocean started to close when a zone of volcanic activity and plate convergence (or subduction) entered the Iapetus from the east, causing deformation of rocks along the margins of the adjacent continents. Once "infected" with subduction, the floor of the Iapetus Ocean was progressively consumed, leading to collisions between the surrounding continents that built a mountain chain now preserved in the Appalachians of North America and the Caledonides of Europe, as Pangea was assembled. The future fate of the modern Atlantic may be similar, if subduction zones around the Caribbean and Scotia plates continue to consume Atlantic Ocean floor; eventually the continents around the Atlantic may collide to form a new supercontinent.

Microdiamond discovered in the Seve Nappe (Scandinavian Caledonides) and its exhumation by the "vacuum-cleaner" mechanism

Jarosław Majka et al. (corresponding author: Iwona Klonowska), Uppsala University, Dept. of Earth Sciences, CEMPEG, Villavägen 16, 752-36 Uppsala, Sweden. Published online ahead of print on 24 Oct. 2014; http://dx.doi.org/G36108.1.

When a continent collides with an island arc or other continent, continental crust of the subducted continent may be buried to depths exceeding 100 kilometers and exposed to pressures which can cause formation of diamond and coesite (high pressure analogue of SiO2). This process leads to substantial density increase in SiO2-rich rocks and, in turn, to a reduction of the buoyancy of the subducted material, which should inhibit its exhumation back to the Earth surface. However, we do observe diamond- and coesite-bearing rocks at the surface. Also, such SiO2-rich rocks containing diamond have been found for the first time in Sweden (Snasahögarna Mountains, Jämtland) by researchers and students Uppsala University (Sweden) and their colleagues from Slovakia, Germany, Poland and Japan. This discovery calls for general reconsideration of existing exhumation models of deeply buried continental crust. Dr. Jaroslaw Majka and the group have proposed that exhumation can be facilitated by local reduction of horizontal compressive stress to a level below the lithostatic pressure, resulting from the downward extraction of the Earth's mantle wedge above the subducting continental crust. This mechanism, novel in geology, works more or less like a mega-scale vacuum-cleaner, which sucks heavy diamond-bearing rocks back to the surface.

Multi-speleothem record reveals tightly coupled climate between central Europe and Greenland during Marine Isotope Stage 3

Gina E. Moseley et al., Institute of Geology, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria. Published online ahead of print on 15 Oct. 2014; http://dx.doi.org/10.1130/G36063.1.

From the abstract: The last glacial period was punctuated by abrupt, millennial-scale climate changes that contain useful information about the rate at which the climate can change from one state to another. Improvement in our knowledge of the temporal and spatial character of these rapid climate changes is important for understanding their causes and effects, and provides essential observational information for modeling studies. Here, we expand the coverage of terrestrial climate records during the last glacial period, and present a series of high-resolution stalagmite records from a cave in the northern Alps (central Europe) covering parts of the period 35,000 to 65,000 years ago (before A.D. 1950).

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