Boulder, Colo., USA – Geology articles posted online ahead of print on 20 Feb. 2013 include several modeling and simulation studies as well as studies on the Exmouth Sub-basin, Australia; the West Kunlun Range, northern Tibetan Plateau; Krakenes Lake, Norway; the Azores islands; and the hot springs of Colorado. The 12 new papers cover a variety of topics:
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The influence of normal fault geometry on igneous sill emplacement and morphology
Craig Magee et al., Dept. of Earth Science and Engineering, Imperial College, London SW7 2BP, UK. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G33824.1.
On its route to the surface, magma will often try to take the easiest pathway. This pathway may be pre-existing cracks or faults in the Earth's crust that do not necessarily allow magma to be transported directly upwards. However, our understanding of how magma exploits these fractures is limited. This paper by Craig Magee and colleagues uses 3D seismic reflection data, similar to sonar image through the Earth's crust, to study the shape and distribution of areas where ancient magma (intruded approximately 140 million years ago) has flowed along preexisting faults. They focus on an area offshore the NW coast of Australia called the Exmouth Sub-basin. Their results show that the shape of the faults and the relative magma flow direction control which fractures are exploited. This is important because it means that ascending magma will be localized, potentially controlling the location of volcanic eruptions.
Soil mineral depletion drives early Holocene lake acidification
John Boyle et al., School of Environmental Science, University of Liverpool, Liverpool L69 7ZT, UK. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G33907.1.
In recent decades, explanations for post-glacial lake acidification have focused on changing climate and biotic factors. John Boyle and colleagues challenge this view by presenting a unique lake sediment data set combining diatom-inferred acidity reconstruction with detailed quantitative assessment of soil base dynamics. They show, at Krakenes Lake in Norway, that historical development of soil mineral depletion inferred from the lake sediment record is consistent with the extent and timing of early Holocene acidification. The lake-water acidification can be fully accounted for by abiotic soil mineral depletion, suggesting a lesser role for alternative acidifying mechanisms like direct climate impacts and successional changes in organic acid production. There are at present few comparable data sets, but those that exist suggest similar rates of soil base depletion. This acidification is not confined to water; abiotic mineral depletion is likely also to impact terrestrial ecosystems, and dynamic vegetation models that exclude irreversible mineral depletion will fail to capture an important element of global ecology.
Uplift of the West Kunlun Range, northern Tibetan Plateau, dominated by brittle thickening of the upper crust
Xiaodian Jiang et al., Dept. of Marine Geology, Ocean University of China, 238 Songling Road, Qingdao, China. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G33890.1.
The collision between Indian and Eurasia led to the formation of the Tibetan Plateau that has an average elevation of 5 km and a much thickened crust. However, the mechanisms responsible for both the topographic uplift and the crustal thickening remain controversial. Xiaodian Jiang and colleagues report high-resolution seismic reflection data from the West Kunlun Range Front at the northwestern margin of the Tibetan Plateau that show that crustal structures there are dominated by nappes of upper crustal rocks. Horizontal shortening in the upper crust by brittle folding and faulting correlates positively with crustal thickening, an increase in Moho depth, and the topography. This work thus suggests that upper crustal shortening is a chief factor for topographic uplift and crustal thickening at the northwestern margin of the Tibetan Plateau. This mechanism could have played a major role in crustal thickening and proto-plateau uplift during the early stage of continental and terrane collisions in Tibet, which laid the condition for other mechanisms, such as crustal flow, to kick in during late Cenozoic plateau propagation.
Anticorrelation between low δ13C of eclogitic diamonds and high δ18O of their coesite and garnet inclusions requires a subduction origin
Daniel J. Schulze et al., Dept. of Earth Sciences, University of Toronto, Mississauga, Ontario L5L 1C6, Canada. Posted online 20 Feb. 203; http://dx.doi.org/10.1130/G33839.1.
Daniel J. Schulze and colleagues analyze the isotopic composition of diamonds with tiny garnet and silica inclusions from three diamond mines (in Venezuela, Australia, and Botswana) and find that their compositions are unlike material typical of the upper mantle, but identical to material that would be predicted to occur on the ancient ocean floor. Although many other workers have explained similar values in diamonds as due to crystallization of the diamonds from mantle fluids, the carbon in the diamonds is also equivalent to carbon of biologic origin. The oxygen isotope composition of the tiny minerals in the diamonds is also unlike that from typical mantle material and has only been observed elsewhere in rocks that have been altered on the ocean floor. This pairing, which Schulze and colleagues have documented for the first time, is a fundamental observation that can only be explained by the formation of these diamonds through subduction of rocks altered on the sea floor, mixed with biologic carbon, beneath the continents into the field of diamond stability.
"Moist MORB" axial magmatism in the Oman ophiolite: The evidence against a mid-ocean ridge origin
Christopher J. MacLeod et al., School of Earth & Ocean Sciences, Cardiff University, Cardiff CF10 3AT, Wales, UK. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G33904.1.
The Oman ophiolite has axial volcanics and sheeted dikes similar in composition to modern mid-ocean ridge basalt (MORB). It formed at a fast spreading rate and is regarded by many as being directly comparable to ocean lithosphere from the East Pacific Rise. Oman has accordingly assumed great significance in guiding conceptual models for fast-spreading ridge processes. However, there has long been controversy over the geodynamic setting in which the ophiolite formed and the extent to which the analogy can be drawn, because later volcanics in Oman are similar to those found at convergent margins. To some this implies that the entire ophiolite formed above a subduction zone; others, however, maintain that it formed at a true open-ocean mid-ocean ridge, and that the later magmatism documents the initiation of the thrusting that led to ophiolite emplacement. In this paper, Christopher MacLeod and colleagues reexamine the MORB-like affinity of the axial volcanics in Oman and show that they fractionated in the presence of water at concentrations significantly higher than any open-ocean MORB. Instead, trends are identical to those of arc volcanics. Open-ocean models for the Oman ophiolite are therefore ruled out; rather, it may have formed by seafloor spreading above a newly-initiated subduction zone.
Man-made versus natural CO2 leakage: A 400 k.y. history of an analogue for engineered geological storage of CO2
Neil M. Burnside et al., School of Geosciences, University of Edinburgh, Edinburgh EH9 3JW, UK. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G33738.1.
Major obstacles to deploying carbon capture and storage (CCS) are demonstration of safe geological CO2 storage and determining the risk of leakage. Neil M. Burnside and colleagues present a unique field site where three types of leakage, diffuse and fault focused natural flow and abandoned wells, can be compared in a single geological setting. Coupling U-Th dating and volume measurement of travertine we have determined a 400,000 year CO2 leakage history for the site. The two faults that penetrate the natural CO2 accumulation produce two different types of natural leakage. Where low permeability layers are present, leakage is focused along faults; where these layers are absent a diffuse pattern dominates and leakage extends 500 m from the fault. Comparison of total CO2 emission shows that fault focused leakage is twice as severe as diffuse leakage. An actively leaking abandoned exploration well allows for comparison of man-made and natural leakage. Calculating total CO2 emissions from this well shows that, in this instance, man-made leakage is 13 times greater than natural fault focused leakage. This study demonstrates that style and volume of leakage is dependent on the underlying geology and that man-made leakage poses a far greater risk than natural leakage to the success of safe geological storage of CO2.
Morphodynamic diversity of the world's largest rivers
Andrew Nicholas, Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G34016.1.
Large alluvial rivers transport globally significant quantities of water, sediment and nutrients to the oceans, temporarily storing and cycling this material within the bars, islands and floodplains that define their morphology. The world's largest rivers are characterized by remarkable variety in form and behavior (e.g., width, depth, number of channels, bar and island stability), which remain poorly understood. This study by Andrew Nicholas applies a new numerical model of water flow and sediment transport to show how the morphology of large sand-bed rivers is influenced by bed sediment mobility, bank erodibility, and rate of floodplain development. Simulations demonstrate that a wide range of river styles, including meandering, anabranching, and braiding, can occur over a relatively narrow range of environmental conditions. Results highlight the suspension of bed material as a key control on river morphology, which promotes an inverse relationship between bed sediment mobility and the degree of channel branching. Moreover, high mobility of bed and bank sediments is hypothesized to favor contrasting river styles, although both may be promoted by increasing river gradient. These results explain the inability of existing theory to predict the morphology of the world's largest rivers, and highlight the potential for investigating river-floodplain co-evolution using physics-based simulation models.
The fossil record of insect color illuminated by maturation experiments
Maria E. McNamara et al., School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G33836.1.
Structural coloration underpins communication strategies in many insects alive today, but its evolution is poorly understood. This stems, in part, from limited data on how color alters during fossilization. Maria E. McNamara and colleagues resolve this by using elevated pressures and temperatures to simulate the effects of burial on structurally colored cuticles of modern beetles. These experiments show that the color generated by multilayer reflectors changes due to alteration of the refractive index and periodicity of the cuticle layers. Three-dimensional photonic crystals are equally resistant to degradation and thus their absence in fossil insects is not a function of limited preservation potential but implies that these color-producing nanostructures evolved recently. Structural colors alter directly to black above a threshold temperature in experiments, identifying burial temperature as the primary control on their preservation in fossils. Color-producing nanostructures can, however, survive in experimentally treated and fossil cuticles that now are black. An extensive cryptic record is thus available in fossil insects to illuminate the evolution of structural color.
Oxygen isotopes in the Azores islands: Crustal assimilation recorded in olivine
Felix S. Genske et al., GeoZentrum Nordbayern, Friedrich-Alexander-Universität Erlangen-Nürnberg, Schlossgarten 5, 91054 Erlangen, Germany; and Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Macquarie University, Sydney NSW 2109, Australia. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G33911.1.
The variability of the oxygen isotopic composition in magmatic systems is useful for the interpretation of the nature of ocean island basalts (OIB), especially since these volcanic rocks provide a clear view into the upper and possibly even lower mantle. The role of recycled material in Earth's mantle may therefore be constrained by geochemical studies of OIB. In detail, the source-sensitive isotope tracers have great potential to decipher individual contributions from subducted, recycled oceanic lithosphere and its components to the heterogeneous mantle. However, Felix S. Genske and colleagues show that recent assimilation of hydrothermally altered material during magma ascent through the overlying crust can strongly imprint on the chemical signature of OIB. In particular, the isotopic composition of the major element oxygen, which is commonly measured on olivine minerals, records such processes. Genske and colleagues demonstrate that the variability observed in the oxygen isotope data is strongly correlated with the chemical evolution of the minerals, hence excluding lower ratios to be representative of their mantle source. Instead, they suggest that most of the oxygen isotope variability recorded in olivine from the Azores is due to recent contamination processes, and this may be true for OIB in general.
Mantle 3He and CO2 degassing in carbonic and geothermal springs of Colorado and implications for neotectonics of the Rocky Mountains
Karl E. Karlstrom et al., Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G34007.1.
Geochemical analyses of many of the famous hot springs of Colorado show that these waters carry helium and carbon dioxide gases that originated in Earth's mantle more than 40 km (ten miles) below the surface. Mantle degassing is unexpectedly widespread across much of the Colorado Rockies but is highest in regions underlain by warm, buoyant mantle. This paper by Karl E. Karlstrom and colleagues shows that hot springs, carbonic springs, and CO2-rich regional gas fields are surface vent regions for neotectonic degassing of mantle volatiles through continental regions that are undergoing active tectonism and epeirogenic uplift. The total CO2 flux through these springs constitutes a small but persistent contribution to the CO2 budget and is important to understand for studies of carbon sequestration and natural leakage.
Two opposing effects of sulfate reduction on carbonate precipitation in normal marine, hypersaline, and alkaline environments
Patrick Meister, Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28205 Bremen, Germany. Posted online 20 Feb. 2013; http://dx.doi.org/10.1130/G34185.1.
Bacteria living at the bottom of shallow lagoons or lakes are thought to be responsible for the formation of hard, layered limestone structures, so-called stromatolites. Stromatolites occur in some of the oldest rocks on Earth and may thus represent the earliest signature of life. A longstanding controversy is which type of bacteria is responsible for the precipitation of the limestone, the building material of the stromatolites. Some scientists suggest cyanobacteria, which are living at the very surface of the stromatolites and produce oxygen like plants. Others suggest that bacteria below the millimeter thick cyanobacterial layer capable of breathing sulfate in absence of oxygen are responsible for the precipitation of limestone. Patrick Meister of the Max Planck Institute, Bremen, demonstrates by a model calculation that if small amounts of sulfate are consumed, sulfate metabolism promotes limestone dissolution. Only if the available sulfate is almost completely consumed, which is not commonly the case in stromatolites, sulfate metabolism promotes precipitation. If we assume that the ocean was more oversaturated with limestone than the modern ocean during several times in Earth history, he writes, then sulfate metabolism would not have contributed to limestone precipitation at all. Thus, stromatolites may either have formed by other groups than sulfate reducing bacteria, or the bacteria were just passively encrusted by limestone.
Reconstructing magma failure and the degassing network of dome-building eruptions
Yan Lavallée et al., Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool L69 3GP, UK. Posted online 20 Feb. 2013, http://dx.doi.org/10.1130/G33948.1.
Volcanic eruptions are regulated by the rheology of magmas and their ability to degas. Both detail the evolution of stresses within ascending subvolcanic magma. But as magma is forced through the ductile-brittle transition, new pathways emerge as cracks nucleate, propagate, and coalesce, constructing a permeable network. Current analyses of magma dynamics center on models of the glass transition, neglecting important aspects such as incremental strain accommodation and (the key monitoring tool of) seismicity. Here, in a combined-methods study, Yan Lavallée and colleagues report the first high-resolution neutron-computed tomography and microseismic monitoring of magma failure under controlled experimental conditions. The data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain-to-failure and importantly, the geometry and efficiency of the permeable fracture network that regulates degassing events. Extrapolation of their findings yields magma ascent via strain localization along conduit margins, thereby providing an explanation for gas-and-ash explosions along arcuate fractures at active lava domes. They conclude that a coupled deformation-seismicity analysis holds a derivation of fracture mechanisms and network, and thus holds potential application in forecasting technologies.
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