Glacier song

(Press-News.org) 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.

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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 END