Rock Salt Holds the Key to a Paradigm Shift

Boulder, Colo., USA - A team of international scientists from China, France, Scotland, United States and led by Canadian Professors Nigel Blamey and Uwe Brand of Brock University in southern Ontario made a scientific breakthrough by measuring the oxygen content of Earth's ancient atmosphere. They discovered that gases trapped by halite (rock salt) during crystallization may contain atmospheric gases, among them oxygen.

Oxygen is a key component in determining the origin and evolution of higher life forms that ultimately made Earth's land and sea their home. The gases in inclusion of halite represent direct measurements of the ancient atmosphere, and can be used to calculate the dissolved oxygen content of past seawater and lay out the requirements for the evolution of higher life forms in the shallow and deep ocean.

This discovery has applications beyond the origin of life, to evaluating salt units as depositories for hazardous waste material, to tracking atmospheric changes in carbon dioxide and methane with climate change, to pinpointing the genesis of economic metal deposits, and application of this important scientific discovery to the search for life on extraterrestrial bodies.

FEATURED ARTICLE
Paradigm shift in determining Neoproterozoic atmospheric oxygen
Nigel J.F. Blamey et al., Department of Earth Sciences, Brock University, 1812 Sir Isaac Brock Way, St Catharines, Ontario L2S 3A1, Canada. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/07/08/G37937.1.abstract.

Other open-access GEOLOGY articles cover
1. Fossil DNA
2. The 2015 Mw 7.8 Gorkha earthquake
3. Groovy faults
4. Earthquake propagation
5. The growth of the Andes

All recently released Geology articles are highlight below.

GEOLOGY articles are online http://geology.gsapubs.org/. Representatives of the media may obtain complimentary articles by contacting Kea Giles at the e-mail address above. Please discuss articles of interest with the authors before publishing stories on their work, and please refer to GEOLOGY in your articles. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

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Fossil DNA persistence and decay in marine sediment over hundred-thousand-year to million-year time scales
John B. Kirkpatrick et al., Graduate School of Oceanography, University of Rhode Island, 215 South Ferry Road, Narragansett, Rhode Island 02882, USA. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/07/08/G37933.1.abstract.

DNA in marine sediment contains both fossil sequences and sequences from organisms that live in the sediment. The demarcation between these two pools and their respective rates of turnover are generally unknown. We address these issues by comparing the total extractable DNA pool to the fraction of sequenced chloroplast DNA (cpDNA) in sediment from two sites in the Bering Sea. We assume that cpDNA is a tracer of non-reproducing fossil DNA. Given >150,000 sequence reads per sample, cpDNA is easily detectable in the shallowest samples but decays with depth, suggesting that sequencing-based richness assessments of communities in deep subseafloor sediment are relatively unaffected by fossil DNA. The initial decrease in cpDNA reads suggests that most cpDNA decays within 100-200 k.y. of deposition. However, cpDNA from a few phylotypes, including some that match fossil diatoms, are present throughout the cored sediment, ranging in age to 1.4 Ma. The relative fraction of total sequences composed by cpDNA decreases with increasing sediment age, suggesting that detectable cpDNA becomes more recalcitrant with age. This can be explained by biological activity decreasing with sediment age and/or by preferential long-term survival of only the most thoroughly protected DNA. The association of cpDNA reads with published records of siliceous microfossils, including diatom spores, at the same sites suggests that microfossils may help to preserve DNA. This DNA may be useful for studies of paleoenvironmental conditions and biological evolution on time scales that approach or exceed one million years.

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Structural segmentation controlled the 2015 Mw 7.8 Gorkha earthquake rupture in Nepal
Judith Hubbard et al., Asian School of the Environment, Nanyang Technological University, Singapore 637459. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/07/08/G38077.1.abstract.

The 2015 Mw7.8 Gorkha earthquake, Nepal, was the result of slip on the Main Himalayan Thrust: a major fault that accommodates most of the convergence between India and Eurasia. We develop a new, 3D model of the fault and show that the subsurface geometry of the fault controlled the size and shape of the Gorkha earthquake. This suggests that fault geometry may play an important role in controlling earthquake nucleation and propagation. Adequately constraining the subsurface fault geometry of megathrusts may help us to better assess the sizes and locations of future earthquakes.

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The minimum scale of grooving on faults
Thibault Candela and Emily E. Brodsky, Department of Earth and Planetary Sciences, University of California-Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/07/08/G37934.1.abstract.

As one side of a fault slips past the other, it generates grooves that record the wear and tear of earthquakes. Until now, it was thought that grooves were a universal feature of faults. We show that at sufficiently small scales, the grooves do not exist. Extremely high-resolution maps of fault surfaces show a minimum length of grooves of 4-500 microns. This discovery helps us understand the fundamental underpinnings of the friction resisting slip on faults. Grooves form when a hard indenter slides past a softer surface. At small-scales, bumps on the fault surface appear to squish and therefore do not generate grooves. Squished bumps are thought to play an important role in making one side of the fault stick to the other. The minimum scale of grooving is providing clear evidence that this adhesive (sticking) process is happening at sub-micron scales on natural earthquake faults.

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Dynamic weakening and amorphization in serpentinite during laboratory earthquakes
Nicolas Brantut et al., Rock & Ice Physics Laboratory and Seismological Laboratory, Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/07/08/G37932.1.abstract.

Earthquakes propagate because faults weaken as they slide. During earthquake propagation, fault slip is fast (around meters per second) and generates heat due to friction; this heat has been previously shown to induce a number of physical and chemical transformations (such as melting) in the fault rocks, which can produce dramatic frictional weakening and hence potentially ease rupture propagation. Until now, the link between physico-chemical transformations of rocks and rupture propagation along faults had only been inferred indirectly, mostly through modelling. Here, we bring direct experimental evidences that mineral reactions and melting during a simulated laboratory earthquake promote fast dynamic rupture and large slip. One key result is that the fault appears to be nearly frictionless during the early stages of slip, most likely due to "flash" dehydration and melting of surface asperities. These thermal weakening mechanisms are likely to play a key role in the propagation of earthquakes at intermediate depths (60 - 300 km) in subduction zones, where rocks tend to contain significant amounts of thermally unstable minerals.

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Sedimentary record of plate coupling and decoupling during growth of the Andes
Brian K. Horton, Institute for Geophysics and Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA; and Facundo Fuentes, YPF S.A., 515 Macacha Güemes, Buenos Aires, Argentina. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/06/28/G37918.1.abstract.

New results define the age, provenance, and accumulation history of sedimentary deposits from the long lived (>100 million year) basin system in the Andes Mountains of South America.  Uranium-lead (U-Pb) ages for detrital zircon minerals help define the duration and controls on enigmatic variations in crustal deformation and a significant hiatus in sediment accumulation in the Malargüe and Neuquén regions of western Argentina.  These results reveal shifts in exhumation and accumulation during unsteady Cretaceous-Neogene deformation. Fully developed mountain building and uplift was only achieved during separate periods of Late Cretaceous and Neogene shortening, contemporaneous with probable episodes of subduction zone coupling in which the South American plate advanced rapidly westward toward the Pacific ocean basin. Separating these two shortening episodes is a 20-40 million-year phase of reduced sedimentation and unconformity development, potentially signifying a neutral to extensional tectonic regime during plate decoupling along this segment of the margin. We propose that the Andes Mountains and the adjacent basin systems have always been sensitive to variations in subduction dynamics, such that shifts in convergence, slab buoyancy, and mechanical coupling along the plate boundary have governed fluctuating contractional, extensional, and neutral tectonic regimes.

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Asymmetric exhumation of the Mount Everest region: Implications for the tectono-topographic evolution of the Himalaya
B. Carrapa et al., Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. This article is online at http://geology.gsapubs.org/content/early/2016/06/28/G37756.1.abstract.

The Himalayas form the southern edge of the Tibetan Plateau and include the largest and highest mountain on planet Earth, Mt. Everest. The Tibet-Himalaya Mountains affect global precipitation through orographic barrier precipitation. Thus understanding when and how the Tibet and the Himalaya grew through time is important for our understanding of the interaction between tectonics, erosion and climate. Popular tectonic models for the Mt. Everest region require a coupling between extrusion of middle crustal rocks to the surface and climate. Our study suggests that the Tibetan Plateau extended farther to the south in the Miocene (ca. 17 m.y. ago) and was later dissected by erosion in the Pliocene. The modern topography of Mt. Everest was thus largely acquired in last few millions of years. This also indicates that the modern orographic barrier is a recent feature established in the last few millions of years and that no coupling between climate and tectonics is required for exhumation of Mt. Everest.

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Crystallographic control on lithium isotope fractionation in Archean to Cenozoic lithium-cesium-tantalum pegmatites
Tomáš Magna et al., Universität Münster, Corrensstrasse 24, D-48149 Münster, Germany. This article is online at http://geology.gsapubs.org/content/early/2016/06/28/G37712.1.abstract.

Pegmatites are an important economic source of rare metals, yet their formation and crystallization processes remain under-investigated. Ratios of two Li isotopes, 7Li and 6Li, vary greatly among major mineral phases of large pegmatite deposits. Crystallographic parameters, such as the distance between Li and neighboring atoms (O, OH and F) appear to be the major impetus in driving the natural 7Li/6Li variations on the order of 1-2% in chemically evolved systems although they vary comparatively little among dominant mantle minerals. This ratio also varies greatly among the same mineral phases of individual deposits, requiring very different crustal sources, although these are always to be sought in granitic systems. Equally old pegmatite deposits thus could have tapped distinctly old parental granites with dissimilar 7Li/6Li ratios. This could provide evidence for secular evolution of the continental crust from the onset of crustal formation in the Archean toward the present. Utility of these analyses may be foreseen in characterization of gem-quality precious stones, such as beryl and tourmaline.

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Water loss and the origin of thick ultramylonites
Melanie A. Finch et al., School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3800, Australia. This article is online at http://geology.gsapubs.org/content/early/2016/06/28/G37972.1.abstract.

Water weakens rocks and high water content is an important factor in localizing deformation of the crust into shear zones, consistent with studies that found water content is highest in the most deformed rocks in shear zones. We investigated the anomalously wide El Pichao shear zone, Argentina. We used Fourier transform infrared spectroscopy to measure water content in quartz and feldspar, comparing the most deformed rocks to less deformed rocks. We found that minerals in the most deformed rocks contained less than half the water of weakly deformed rocks, contrary to previous studies. We propose that the shear zone widened gradually in a three-stage process: (1) deformation favored water-rich rocks and caused the release of water from within the grains to the grain boundaries. This promoted diffusion which increased the rate of deformation and led to highly deformed rocks; (2) high pressure in this zone of intense deformation caused expulsion of water on grain boundaries, which caused the zone to gradually dry up, becoming stronger and making it more difficult to deform; (3) the expelled water migrated outwards, weakening rocks yet to be deformed, and increasing deformation rates there, where the process is repeated ultimately producing this wide shear zone.

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Evidence for Cretaceous-Paleogene boundary bolide “impact winter” conditions from New Jersey, USA
Johan Vellekoop et al., Marine Palynology and Paleoceanography, Department of Earth Sciences, Faculty of Geosciences, and Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584CD Utrecht, Netherlands. This article is online at http://geology.gsapubs.org/content/early/2016/06/28/G37961.1.abstract.

An abrupt and short-lived "impact winter," a brief period of global darkness and strong cooling, has often been implicated as cause of the mass-extinction at the Cretaceous-Paleogene (K-Pg) boundary (~66 million years ago), marking the end of the reign of the dinosaurs. However, so far only limited evidence was available for such a climatic perturbation. Here we apply a paleothermometer based on fossil organic material on three shallow cores from the New Jersey, USA, to assess the impact-provoked climatic perturbations immediately following the K-Pg impact and to place these short-term events in the context of long-term climate evolution. We provide evidence of impact-provoked, severe climatic cooling immediately following the K-Pg impact. This so-called impact winter occurred superimposed on a long-term cooling trend that followed a warm phase in the last 500 thousand years before the impact.

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Obliquity-forced climate during the Early Triassic hothouse in China
Mingsong Li et al., State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China. This article is online at http://geology.gsapubs.org/content/early/2016/06/28/G37970.1.abstract.

252 million years ago at the end of the Permian Period, after the Great Dying, the Earth was marked by 5 million years of repeated ecosystem upheavals, global warming and ocean anoxia. The causes of these catastrophes are unknown. Recent geochronology constrains the latest age of major eruptions of the Siberian Traps to the first 0.5 million years of the Early Triassic Epoch. Thus it is unlikely that volcanism drove the catastrophes, as currently widely thought. Now, Mingsong Li and colleagues provide an alternative view: the global catastrophes in the aftermath of the Great Dying are linked to Earth’s astronomical forcing. They evaluate two marine sedimentary successions in South China for astronomically forced paleoclimate change. In these sections, terrestrial weathering proxy data reveal enhanced obliquity-forced sedimentation over prolonged intervals, characterized by a 32,800-year periodicity and modulated by 1.2-million-year cycles. This suggests a 22-hour length-of-day, a 398-day year, and a 1.2-million-year interaction between the orbits of Earth and Mars. Comparing the 1.2-million-year cycling with Early Triassic records of global temperature, ocean redox state and biotic evolution suggests that long-term astronomical forcing influenced the repeated climatic and biotic upheavals that took place in the Early Triassic Epoch.

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Duration and nature of the end-Cryogenian (Marinoan) glaciation
Anthony R. Prave et al., Department of Earth and Environmental Sciences, University of St Andrews, St. Andrews KY16 9AL, UK. This article is online at http://geology.gsapubs.org/content/early/2016/06/28/G38089.1.abstract.

The Cryogenian Period (ca. 720-635 Ma) was a momentous one in Earth history, archiving the extreme environmental changes expressed in the Snowball Earth hypothesis and the advent of animals. Hindering testing of many of the ideas offered to explain those phenomena has been a dearth of age constraints. Now, new radiometric ages from Namibia provide, for the first time, confirmation of the central prediction of the Snowball Earth hypothesis, namely, a long-duration of the Marinoan ice age, at least 4 million years (from ca. 639 to 635 Ma). They reveal that the main phase of that glaciation was marked by dynamic ice sheets and associated sedimentation, and at least intermittent open water conditions, findings of importance for models that attempt to investigate the climatic dynamics of Snowball Earth. The results, combined with those delimiting the timing of the end of the older Cryogenian glaciation, the Sturtian, further corroborate that the duration of interglacial interlude during the Cryogenian Period was 20 million years, and likely less.

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Vigorous deep-sea currents cause global anomaly in sediment accumulation in the Southern Ocean
Adriana Dutkiewicz et al., EarthByte Group, School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia. This article is online at http://geology.gsapubs.org/content/early/2016/07/08/G38143.1.abstract.

We often think of the deep ocean as a place where sediment particles gently settle through the water column and accumulate undisturbed over thousands and millions of years into thick sedimentary sequences. However, there is much commotion in the deep ocean and the sedimentary record is often not as continuous as we would like it to be. Now Adriana Dutkiewicz and Dietmar Müller at the University of Sydney have teamed up with oceanography experts Andy Hogg at the Australian National University and Paul Spence at the University of NSW to show that a wide belt of rapid sedimentation along the Southeast Indian Ridge in the Southern Ocean is a global anomaly. By combining geological observations from the seafloor with a computer model of ocean circulation they show that vigorous ocean bottom currents have caused major disruptions in the sedimentary record of the Southeast Indian Ridge region. The persistent currents have moved vast amounts of sediment over the last 5 million years. This has resulted in a build-up of deep-sea sediment along an 8,000-km-long (the distance between LA and London) segment of ridge. The good news is that the sediment accumulation likely contains a high-resolution climate record of the region.

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Establishing criteria to distinguish oil-seep from methane-seep carbonates
D. Smrzka et al., Department für Geodynamik und Sedimentologie, Erdwissenschaftliches Zentrum, Universität Wien, 1090 Wien, Austria. This article is online at http://geology.gsapubs.org/content/early/2016/07/12/G38029.1.abstract.

Cold hydrocarbon seeps and hydrothermal vents have been intensively studied in the last three decades to better constrain (1) the flow of energy in ecosystems that are based on the oxidation of chemical compounds derived from the Earth’s interior and (2) the adaptation mechanisms of biota that rely on such energy. The most complete record of Phanerozoic chemosynthesis-based life is that of hydrocarbon-seep deposits with their high abundance but low diversity assemblages of mollusks, tubeworms, and brachiopods. Yet, a problem that still hampers our understanding of macrofaunal evolutionary patterns at seeps is the inability to discriminate ancient oil seeps from methane seeps. This manuscript presents two new proxies, the contents of rare earth elements and yttrium as well as the ratio of molybdenum to uranium, which allow discerning the different types of seeps and provide a novel means for reconstructing the role of fluid composition for the evolution of seep-dwelling invertebrates throughout the Phanerozoic.

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Origin and time evolution of subduction polarity reversal from plate kinematics of Southeast Asia
Christoph von Hagke et al., Institute of Structural Geology, Tectonics, and Geomechanics, RWTH Aachen, Lochnerstrasse 4-20, 52056 Aachen, Germany. This article is online at http://geology.gsapubs.org/content/early/2016/07/08/G37821.1.abstract.

Extinct oceanic plate responsible for present day kinematics of Southeast Asia, Christoph von Hagke and colleagues found a now extinct ocean controls plate kinematics of Southeast Asia and architecture of the Taiwan mountain belt. Subduction of this old ocean more than 30 million years ago is responsible for Eurasia being overridden by the Philippine Sea Plate at present. This discovery is a prerequisite for correctly assessing the earthquake hazard potential of the region.

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Temporal and genetic link between incremental pluton assembly and pulsed porphyry Cu-Mo formation in accretionary orogens
Hervé Rezeau et al., Department of Earth Sciences, University of Geneva, 1205 Geneva, Switzerland. This article is online at http://geology.gsapubs.org/content/early/2016/07/08/G38088.1.abstract.

Economically important porphyry Cu-Mo deposits (PCDs) are generally hosted by upper-crustal plutons of variable chemical compositions related to distinct geodynamic settings. The absolute timing and duration of pluton assembly and PCD formation are critical to understanding the genetic relationship between these interrelated processes. Here, we present new comprehensive zircon U-Pb and molybdenite Re-Os ages that tightly constrain the timing and duration of pluton assembly and the age of mineralization in one of the largest ore-bearing plutons of the central Tethyan metallogenic belt, the Meghri-Ordubad pluton, southern Armenia and Nakhitchevan, Lesser Caucasus. This composite pluton was incrementally assembled during three compositionally distinct magmatic episodes over ~30 m.y., comprising Middle Eocene (48.9-43.1 Ma) calc-alkaline subduction-related magmatism lasting 5.8 +/- 0.8 m.y., followed by postsubduction Late Eocene-Middle Oligocene (37.8-28.1 Ma) shoshonitic magmatism over 9.7 +/- 0.9 m.y., and Late Oligocene-Early Miocene (26.6-21.2 Ma) adakitic magmatism consisting of shoshonitic dikes and high-K calc-alkaline granodioritic magmas emplaced over 5.4 +/- 0.4 m.y. Despite the distinct geodynamic settings and magma compositions, each intrusive suite culminated in the formation of variably sized PCDs, including the giant Oligocene Kadjaran porphyry Cu-Mo deposit associated with high-Sr/Y shoshonitic magmas. Complementary in situ zircon hafnium (eHfzircon = +8 to +11.3) and oxygen (δ18Ozircon = +4.6‰ to +6.0‰) isotope data support a mantle-dominated magma source with limited crustal contribution and/or cannibalization of young and juvenile lower-crustal cumulates. We conclude that, independent of geodynamic setting and magma composition, long-lived (5-10 m.y.) incremental mantle-derived magmatism is a prerequisite to form fertile magmatic-hydrothermal systems, and especially giant PCDs.

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The answers are blowin’ in the wind: Ultra-distal ashfall zircons, indicators of Cretaceous super-eruptions in eastern Gondwana
M. Barham et al., The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia. This article is online at http://geology.gsapubs.org/content/early/2016/07/08/G38000.1.abstract.

Super-eruptions, capable of spreading tera-tonnes of volcanic material over thousands of kilometers and affecting global climate systems, are well known from the relatively recent past (e.g. Toba, Indonesia) and have even been implicated in the evolution of our species. However, the incomplete nature of geological sequences means that recognition of these earth-shattering volcanic events is difficult in deeper geological time. Sand-sized crystals found in Western Australia are presented as the products of volcanic airfall (using geochemistry, grain-shape characteristics and correlation with fossil age indicators), despite being 2300 km away from remnants of their source volcanoes in eastern Australia. Such distal projection of a unique volcanic mineral population demonstrates that super-eruptions were occurring in eastern Australia approximately 106 million years ago during the break-up of the supercontinent Gondwana. The arrangement of landmasses and atmospheric circulation at the time indicates that the recorded eruptions occurred during the southern hemisphere winter, when strong winds from the east would have pushed volcanic ejecta towards the west.

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Precipitation changes in the western tropical Pacific over the past millennium
J.N. Richey and J.P. Sachs, School of Oceanography, University of Washington, Seattle, Washington 98195, USA; U.S. Geological Survey, St. Petersburg, Florida 33701, USA. This article is online at http://geology.gsapubs.org/content/early/2016/07/12/G37822.1.abstract.

The intense band of thunderstorms that circles the globe near the equator, where the northeast and southeast trade winds come together, is known as the Intertropical Convergence Zone (or ITCZ). It moves northward in the Northern Hemisphere summer, and southward during the Southern Hemisphere summer, causing many locations in the tropics to experience distinct rainy seasons and dry seasons. In addition to migrating north and south on a seasonal basis, it is hypothesized that the ITCZ can change its mean position on longer timescales (e.g., decades to centuries), causing a redistribution of rainfall across the tropics, and profound socioeconomic and ecological impacts in regions dependent on predictable rainfall patterns. Researchers, Julie Richey and Julian Sachs, from the University of Washington, School of Oceanography, have generated a record of relative rainfall variability in Palau spanning the past millennium. Palau is an island nation situated in the western tropical Pacific Ocean, at the northern extent of the modern ITCZ. Their findings, combined with other rainfall reconstructions spanning the tropical Pacific, suggest that the ITCZ shifted southward in response to cooling of the Northern Hemisphere during the Little Ice Age (~1400-1850 AD).

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Subducted lithosphere controls halogen enrichments in the Iceland mantle plume source
Sæmundur A. Halldórsson et al., Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, 107 Reykjavík, Iceland. This article is online at http://geology.gsapubs.org/content/early/2016/07/12/G37924.1.abstract.

Mantle-derived chlorine can be highly susceptible to contamination by surficial and seawater-derived chlorine in shallow-level crustal environments. This is particularly true for chlorine, as well as other halogens (F, Br and I), emitted to the surface via submarine volcanic systems. To overcome this problem, a new study utilized subglacial volcanic glasses from Iceland, where the mid-ocean ridge system emerges above sea level and is free of seawater. Chlorine abundance and isotope data for the Iceland volcanic glasses, show no evidence for contamination by surficial and/or seawater-derived chlorine. In contrast, the data are explained by the presence of recycled, but mantle-chlorine in the Icelandic mantle. This deep chlorine, was once at the Earth’s surface but later became an integral part of its mantle following subduction at convergent plate boundaries.

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Aridity-induced Miocene canyon incision in the Central Andes
F.J. Cooper et al., School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. This article is online at http://geology.gsapubs.org/content/early/2016/07/12/G38254.1.abstract.

The Atacama Desert is one of the driest places on Earth, partly because it lies in the shadow of the Andes, which block moisture traveling west from the Amazon Basin. However, the temporal relationship between rise of the Andes and the onset of aridity is unclear. It has been suggested, for example, that the climate was dry long before the Andes reached high elevations, and that low rainfall boosted uplift by reducing erosion. Deep river canyons carved into the Andean flanks offer a way to resolve this because they record changes in the balance between uplift and erosion. We record some of the oldest canyon incision on the western Andean margin in northern Chile by tracking the downward response of the local water table. By combining analysis of a river profile in one of these deep canyons with dating of hematite (iron oxide) precipitation above the water table over the last ~30 million years, we find that (1) canyon incision began at circa 16 Ma in response to drying of the climate, and (2) the Central Andes have only risen ~700 m since incision began. We thus conclude that the onset of aridity occurred after the main phase of Andean uplift.

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Formation of continental fragments: The Tamayo Bank, Gulf of California, Mexico
R. Abera et al., Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA. This article is online at http://geology.gsapubs.org/content/early/2016/06/28/G38123.1.abstract.

It has been known for decades that oceanic plates carry small slivers of continental lithosphere, called microcontinents. How is this possible? We developed a model that explains the formation of such microcontinents.  When continents break up, an oceanic spreading ridge forms and new oceanic crust is being created. We found that this oceanic spreading ridge is sometimes, after it has formed, not very robust, lacking sufficient magmatism to keep the spreading ridge going. The spreading ridge then becomes inactive, and spreading starts elsewhere (in the continental lithosphere), breaking off a small sliver of continent: a microcontinent. We found a microcontinent in the southern Gulf of California that had not previously been detected. The model explains how this microcontinent formed. Insights from this study are of importance to the petroleum industry; the microcontinents are bordered by thick sediment packages where petroleum may have formed and accumulated.

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Empirical constraints of shock features in monazite using shocked zircon inclusions
Timmons M. Erickson et al., Department of Applied Geology, Curtin University, Perth, GPO Box U1987, Western Australia 6845, Australia. This article is online at http://geology.gsapubs.org/content/early/2016/07/08/G37979.1.abstract.

This paper reports the first quantification of shock deformation features within monazite. Deformation microstructures in monazite derived from the Vredefort Dome in South Africa were analyzed by electron backscatter diffraction, and reveal a combination of low-angle grain boundaries, deformation twins and dynamically recrystallized neoblasts. Many of the deformation twins reported in this study have never been described previously, and represent new deformation microstructures in monazite. In addition, the analyzed monazites all contain shocked zircon inclusions, which have either planar deformation bands or shock twins. Therefore, the new deformation twins documented herein are interpreted to represent diagnostic shock features indicative of hyper-velocity impacts. These results establish monazite as a mineral that develops diagnostic shock features that can be used to confirm impact structures. Because monazite is a commonly used geochronomter these results suggest that monazite not only preserves diagnostic impact evidence but can also be used to impact craters, which is often challenging.

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Voluminous plutonism during volcanic quiescence revealed by thermochemical modeling of zircon
Casey R. Tierney et al., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA. This article is online at http://geology.gsapubs.org/content/early/2016/07/12/G37968.1.abstract.

While some large accumulations of silicic magma in the upper-crust erupt catastrophically in devastating supereruptions, not all do. However, the exact conditions determining whether magma accumulates or erupts remains uncertain. Tierney et al. (2016) investigated five lava domes located in the Altiplano-Puna Volcanic Complex of the Central Andes distributed over ~2000 km2, equivalent to the footprint of a large caldera. Despite their distinct identities, the domes show synchronous zircon crystallization lasting hundreds of thousands of years prior to eruption. The zircon record reveals continuous magma presence underneath each of these volcanoes where magma recharge from deeper sources maintained the sub-volcanic magma reservoirs at conditions required for zircon crystallization. Rates of recharge during this interval, however, were significantly diminished compared to those during earlier stages of widespread caldera formation in the region. Reduced recharge thus prevented conditions for supereruptions being reached, and instead led to amalgamation of a sub-volcanic plutonic body of supervolcanic proportions. This study empirically defines a threshold of magma recharge where superuptions are triggered or not. A consequence of this work is that volcanic history should not always be considered a perfect proxy for magmatic history, and that plutons may continue to grow during times of volcanic quiescence.

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14 July 2016
GSA Release No. 16-42

Kea Giles Managing Editor,
GSA Communications
+1-303-357-1057
kgiles@geosociety.org

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Halite from the Wieliczka Salt Mine, UNESCO World Heritage Site, Wieliczka, Malopolskie, Poland. Photo courtesy Didier Descouens, CC BY-SA.