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  • ItemOpen AccessAccepted version Peer-reviewed
    Persistent growth of a young andesite lava cone: Bagana volcano, Papua New Guinea
    (Elsevier, 2018-05-01) Edmonds, M; Wadge, Geoff; McCormick Kilbride, Brendan; JOhnson, Wally; Edmonds, Marie [0000-0003-1243-137X]
    Bagana, an andesite lava cone on Bougainville Island, Papua New Guinea, is thought to be a very young central volcano. We have tested this idea by estimating the volumes of lava extruded over different time intervals (1-, 2-, 3-, 9-, 15-, 70- years) using digital elevation models (DEMs), mainly created from satellite data. Our results show that the long-term extrusion rate at Bagana, measured over years to decades, has remained at about 1.0 m3s-1. We present models of the total edifice volume, and show that, if our measured extrusion rates are representative, the volcano could have been built in only ~300 years. It could also possibly have been built at a slower rate during a longer, earlier period of growth. Six kilometres NNW of Bagana, an andesite-dacite volcano, Billy Mitchell, had a large, caldera-forming plinian eruption 437 years ago. We consider the possibility that, as a result of this eruption, the magma supply was diverted from Billy Mitchell to Bagana. It seems that Bagana is a rare example of a very youthful, polygenetic, andesite volcano. The characteristics of such a volcano, based on the example of Bagana, are: a preponderance of lava products over pyroclastic products, a high rate of lava extrusion maintained for decades, a very high rate of SO2 emission, evidence of magma batch fractionation and location in a trans-tensional setting at the end of an arc segment above a very steeply dipping and rapidly converging subduction zone.
  • ItemOpen AccessPublished version Peer-reviewed
    Melting of peridotites through to granites: A simple thermodynamic model in the system KNCFMASHTOCr
    (Oxford University Press (OUP), 2018) Holland, TJB; Green, ECR; Powell, R; Holland, Tim [0000-0003-2852-9515]
    A new set of thermodynamic models is presented for calculating phase relations in bulk compositions extending from peridotite to granite, from 0.001 to 70 kbar and from 650 °C to peridotite liquidus temperatures, in the system K 2 O–Na 2 O–CaO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 –Cr 2 O 3 (KNCFMASHTOCr). The models may be used to calculate phase equilibria in partial melting of a large range of mantle and crustal compositions. They provide a good fit to experimental phase relation topologies and melt compositions across the compositional range of the model. Compared with the preliminary model of Jennings & Holland (2015) for peridotite–basalt melting relations, the inclusion of K 2 O and TiO 2 allows for better modelling of small melt fractions in peridotite melting, and in reproducing rutile-bearing eclogite melting at high pressures. An improved order–disorder model for spinel is now incorporated. Above 10 kbar pressure, wet partial melting relations may be significantly affected by the dissolution of silicates in aqueous fluid, so the set of models includes an aqueous low-density silicate-bearing fluid in addition to a high-density H 2 O-bearing silicate melt. Oxygen fugacity may be readily calculated for the whole range of bulk compositions investigated, and the effect of water content on melt fO 2 is assessed.
  • ItemOpen AccessAccepted version Peer-reviewed
    Controllably Hollow AgAu Nanoparticles via Nonaqueous, Reduction Agent-Assisted Galvanic Replacement
    (Wiley, 2018) Rehn, SM; Ringe, E; Rehn, SM [0000-0001-7640-4528]; Ringe, E [0000-0003-3743-9204]
    AbstractThe galvanic replacement reaction is a robust tool for the controlled synthesis of hollow and semihollow bimetallic nanostructures, which have applications in a range of science, engineering, and medical fields due to the tunability of their localized surface plasmon resonances (LSPRs) and surface chemistry. Here, a controllable galvanic replacement of Ag by Au coupled with coreduction is described, performed in nonaqueous solvents including methanol, ethanol, and an N,N‐dimethylformamide:toluene mixture and yielding hollow and semihollow alloyed nanoparticles. Structural control, from semihollow to nanoshell, and plasmon tunability are demonstrated via control of the Au:Ag stoichiometry. The high structural dependence on temperature is shown, with striking changes in nanoparticle surface smoothness and pinhole density, and reveals the optimal reaction temperature to be 65 °C in alcohols. Through optimizing this reaction, smooth closed shell AgAu alloy nanoparticles with LSPRs tunable from 494 to 567 nm are obtained. This work provides a framework for galvanic replacement of large anisotropic Ag nanoparticles with Au in nonaqueous media, which can be extended to other solvent systems suitable for air‐sensitive metals and precursors.
  • ItemOpen AccessAccepted version Peer-reviewed
    Exploring Scientific Ideas in Informal Settings: Activities for Individuals with Visual Impairments
    (American Chemical Society (ACS), 2018) Kumar, A; McCarthy, LA; Rehn, SM; Swearer, DF; Newell, RN; Gereta, S; Villarreal, E; Yazdi, S; Ringe, E; Ringe, E [0000-0003-3743-9204]
    Four new modules for the scientific education of individuals with visual disabilities are reported. Designed for informal settings, the modules are safe and inexpensive and last 10–15 min each. The module entitled The Sound and Feel of Data represents axes and data points with pipe cleaners and coins, respectively, allowing for a discussion of data presentation and trends without reliance on visual signals. The Conductivity of Materials module tactilely introduces concepts of electron flow in metallic, semiconducting, and insulating materials using marbles, followed by a sound volume representation of the conductivity of everyday objects. The Optical Lenses module uses rope segments secured to a corkboard to trace tactile ray diagrams representing the concept of focusing, diverging, and aberration in optical lenses. Finally, the Chemistry and Physics of Heat module uses the dissolution of ammonium nitrate and sodium sulfate in sealed plastic tubes as well as the crystallization of sodium acetate to describe endothermic and exothermic processes. The modules were tested during an event attended by 38 participants with visual impairments; answers to learning assessment questions as well as pre- and post-module surveys suggest that we were successful at introducing scientific concepts and generating interest in science. This work provides new ways to introduce science that are applicable to all learners, visually impaired or not.
  • ItemOpen AccessPublished version Peer-reviewed
    The electronic structure and the nature of the chemical bond in CeO2.
    (Royal Society of Chemistry (RSC), 2018-06-13) Maslakov, Konstantin I; Teterin, Yury A; Ryzhkov, Mikhail V; Popel, Aleksej J; Teterin, Anton Yu; Ivanov, Kirill E; Kalmykov, Stepan N; Petrov, Vladimir G; Petrov, Peter K; Farnan, Ian; Popel, Aleksej [0000-0003-4436-9961]; Farnan, Ian [0000-0001-7844-5112]
    The X-ray photoelectron spectral structure of CeO2 valence electrons in the binding energy range of 0 to ∼50 eV was analyzed. The core-electron spectral structure parameters and the results of relativistic discrete-variational calculations of CeO8 and Ce63O216 clusters were taken into account. Comparison of the valence and the core-electron spectral structures showed that the formation of the inner (IVMO) and the outer (OVMO) valence molecular orbitals contributes to the spectral structure more than the many-body processes. The Ce 4f electrons were established to participate directly in chemical bond formation in CeO2 losing partially their f character. They were found to be localized mostly within the outer valence band. The Ce 5p atomic orbitals were shown to participate in the formation of both the inner and the outer valence molecular orbitals (MOs). A large part in the IVMO formation is taken by the filled Ce 5p1/2, 5p3/2 and O 2s atomic shells, while the Ce 5s electrons participate weakly in the chemical bond formation. The composition and the sequent order of the molecular orbitals in the binding energy range of 0 to ∼50 eV were established. A quantitative scheme for the molecular orbitals of CeO2 was built. This scheme is fundamental for understanding the nature of chemical bonding and also for the interpretation of other X-ray spectra of CeO2. Evaluations revealed that the IVMO electrons weaken the chemical bond formed by the OVMO electrons by 37%.
  • ItemOpen AccessAccepted version Peer-reviewed
    Neogene Uplift and Magmatism of Anatolia: Insights from Drainage Analysis and Basaltic Geochemistry
    (Wiley-Blackwell, 2018) McNab, F; Ball, Patrick; Hoggard, Mark; White, Nicky; McNab, Fergus [0000-0002-8358-1466]; Ball, Patrick [0000-0001-7054-807X]; Hoggard, Mark [0000-0003-4310-3862]
    It is generally agreed that mantle dynamics have played a significant role in generating and maintaining the elevated topography of Anatolia during Neogene times. However, there is much debate about the relative importance of subduction zone and asthenospheric processes. Key issues concern onset and cause of regional uplift, thickness of the lithospheric plate, and the presence or absence of temperature and/or compositional anomalies within the convecting mantle. Here, we tackle these interlinked issues by analyzing and modeling two disparate suites of observations. First, a drainage inventory of 1,844 longitudinal river profiles is assembled. This geomorphic database is inverted to calculate the variation of Neogene regional uplift through time and space by minimizing the misfit between observed and calculated river profiles subject to independent calibration. Our results suggest that regional uplift commenced in the east at 20 Ma and propagated westward. Secondly, we have assembled a database of geochemical analyses of basaltic rocks. Two different approaches have been used to quantitatively model this database with a view to determining the depth and degree of asthenospheric melting across Anatolia. Our results suggest that melting occurs at depths as shallow as 60 km in the presence of mantle potential temperatures as high as 1400°C. There is evidence that potential temperatures are higher in the east, consistent with the pattern of sub-plate shear wave velocity anomalies. Our combined results are consistent with isostatic and admittance analyses and suggest that elevated asthenospheric temperatures beneath thinned Anatolian lithosphere have played a first order role in generating and maintaining regional dynamic topography and basaltic magmatism.
  • ItemOpen AccessPublished version Peer-reviewed
    Clay mineralogy, strontium and neodymium isotope ratios in the sediments of two High Arctic catchments (Svalbard)
    (Copernicus GmbH, 2018) Hindshaw, RS; Tosca, NJ; Piotrowski, AM; Tipper, ET; Tipper, Edward [0000-0003-3540-3558]
    Abstract. The identification of sediment sources to the ocean is a prerequisite to using marine sediment cores to extract information on past climate and ocean circulation. Sr and Nd isotopes are classical tools with which to trace source provenance. Despite considerable interest in the Arctic Ocean, the circum-Arctic source regions are poorly characterised in terms of their Sr and Nd isotopic compositions. In this study we present Sr and Nd isotope data from the Paleogene Central Basin sediments of Svalbard, including the first published data of stream suspended sediments from Svalbard. The stream suspended sediments exhibit considerable isotopic variation (εNd = −20.6 to −13.4; 87Sr ∕ 86Sr = 0.73421 to 0.74704) which can be related to the depositional history of the sedimentary formations from which they are derived. In combination with analysis of the clay mineralogy of catchment rocks and sediments, we suggest that the Central Basin sedimentary rocks were derived from two sources. One source is Proterozoic sediments derived from Greenlandic basement rocks which are rich in illite and have high 87Sr ∕ 86Sr and low εNd values. The second source is Carboniferous to Jurassic sediments derived from Siberian basalts which are rich in smectite and have low 87Sr ∕ 86Sr and high εNd values. Due to a change in depositional conditions throughout the Paleogene (from deep sea to continental) the relative proportions of these two sources vary in the Central Basin formations. The modern stream suspended sediment isotopic composition is then controlled by modern processes, in particular glaciation, which determines the present-day exposure of the formations and therefore the relative contribution of each formation to the stream suspended sediment load. This study demonstrates that the Nd isotopic composition of stream suspended sediments exhibits seasonal variation, which likely mirrors longer-term hydrological changes, with implications for source provenance studies based on fixed end-members through time.
  • ItemOpen AccessPublished version Peer-reviewed
    A scaling law for distinct electrocaloric cooling performance in low-dimensional organic, relaxor and anti-ferroelectrics.
    (Springer Science and Business Media LLC, 2017-09-11) Shi, Yuping; Huang, Limin; Soh, Ai Kah; Weng, George J; Liu, Shuangyi; Redfern, Simon AT; Redfern, Simon AT [0000-0001-9513-0147]
    Electrocaloric (EC) materials show promise in eco-friendly solid-state refrigeration and integrable on-chip thermal management. While direct measurement of EC thin-films still remains challenging, a generic theoretical framework for quantifying the cooling properties of rich EC materials including normal-, relaxor-, organic- and anti-ferroelectrics is imperative for exploiting new flexible and room-temperature cooling alternatives. Here, we present a versatile theory that combines Master equation with Maxwell relations and analytically relates the macroscopic cooling responses in EC materials with the intrinsic diffuseness of phase transitions and correlation characteristics. Under increased electric fields, both EC entropy and adiabatic temperature changes increase quadratically initially, followed by further linear growth and eventual gradual saturation. The upper bound of entropy change (∆Smax) is limited by distinct correlation volumes (V cr ) and transition diffuseness. The linearity between V cr and the transition diffuseness is emphasized, while ∆Smax = 300 kJ/(K.m3) is obtained for Pb0.8Ba0.2ZrO3. The ∆Smax in antiferroelectric Pb0.95Zr0.05TiO3, Pb0.8Ba0.2ZrO3 and polymeric ferroelectrics scales proportionally with V cr-2.2, owing to the one-dimensional structural constraint on lattice-scale depolarization dynamics; whereas ∆Smax in relaxor and normal ferroelectrics scales as ∆Smax ~ V cr-0.37, which tallies with a dipolar interaction exponent of 2/3 in EC materials and the well-proven fractional dimensionality of 2.5 for ferroelectric domain walls.
  • ItemOpen AccessAccepted version Peer-reviewed
    High-pressure phase transitions of nitinol NiTi to a semiconductor with an unusual topological structure
    (American Physical Society (APS), 2018) Liu, G; Liu, H; Feng, X; Redfern, SAT; Redfern, Simon [0000-0001-9513-0147]
    Systematic ab-initio structure simulations have been used to explore the high-pressure behavior of nitinol (NiTi) at zero temperature. Our crystal structure prediction and first principles calculations reveal that the known B19 phase is dynamically unstable, an orthorhombic structure (Pbcm) and a face centered cubic B32 structure (Fd-3m) become stable above ~4 and 29 GPa, respectively. The newly-predicted, highest-pressure, B32 phase is composed of two interpenetrating diamond structures, with structural topology that is quite distinct from that of the other phases of NiTi. Interestingly, the B32 phase shows unusual semiconducting characteristic as a result of its unique band structure and the nature of 3d orbitals localization, whose expected synthesis pressure is accessible to current experimental techniques.
  • ItemOpen AccessAccepted version Peer-reviewed
    MTfit: A Bayesian approach to seismic moment tensor inversion
    (Seismological Society of America (SSA), 2018) Pugh, DJ; White, RS; White, Robert [0000-0002-2972-397X]
    MTfit is a Python module for Bayesian moment tensor source inversion of earthquake seismic data using polarities, amplitudes or amplitude ratios. It can solve for double couple or full moment tensor solutions, taking into account uncertainties in polarities, take-off angles of the rays from the source to the receiver, and amplitudes. It provides an easily accessible and extendable approach to earthquake source inversion which is particularly useful for local and regional events.
  • ItemOpen AccessPublished version Peer-reviewed
    Greenland records of aerosol source and atmospheric lifetime changes from the Eemian to the Holocene.
    (Springer Science and Business Media LLC, 2018-04-16) Schüpbach, S; Fischer, H; Bigler, M; Erhardt, T; Gfeller, G; Leuenberger, D; Mini, O; Mulvaney, R; Abram, NJ; Fleet, L; Frey, MM; Thomas, E; Svensson, A; Dahl-Jensen, D; Kettner, E; Kjaer, H; Seierstad, I; Steffensen, JP; Rasmussen, SO; Vallelonga, P; Winstrup, M; Wegner, A; Twarloh, B; Wolff, K; Schmidt, K; Goto-Azuma, K; Kuramoto, T; Hirabayashi, M; Uetake, J; Zheng, J; Bourgeois, J; Fisher, D; Zhiheng, D; Xiao, C; Legrand, M; Spolaor, A; Gabrieli, J; Barbante, C; Kang, J-H; Hur, SD; Hong, SB; Hwang, HJ; Hong, S; Hansson, M; Iizuka, Y; Oyabu, I; Muscheler, R; Adolphi, F; Maselli, O; McConnell, J; Wolff, EW; Fischer, H [0000-0002-2787-4221]; Mulvaney, R [0000-0002-5372-8148]; Abram, NJ [0000-0003-1246-2344]; Thomas, E [0000-0002-3010-6493]; Svensson, A [0000-0002-4364-6085]; Kjaer, H [0000-0002-3781-9509]; Vallelonga, P [0000-0003-1055-7235]; Goto-Azuma, K [0000-0003-0992-1079]; Spolaor, A [0000-0001-8635-9193]; Adolphi, F [0000-0003-0014-8753]; Wolff, EW [0000-0002-5914-8531]
    The Northern Hemisphere experienced dramatic changes during the last glacial, featuring vast ice sheets and abrupt climate events, while high northern latitudes during the last interglacial (Eemian) were warmer than today. Here we use high-resolution aerosol records from the Greenland NEEM ice core to reconstruct the environmental alterations in aerosol source regions accompanying these changes. Separating source and transport effects, we find strongly reduced terrestrial biogenic emissions during glacial times reflecting net loss of vegetated area in North America. Rapid climate changes during the glacial have little effect on terrestrial biogenic aerosol emissions. A strong increase in terrestrial dust emissions during the coldest intervals indicates higher aridity and dust storm activity in East Asian deserts. Glacial sea salt aerosol emissions in the North Atlantic region increase only moderately (50%), likely due to sea ice expansion. Lower aerosol concentrations in Eemian ice compared to the Holocene are mainly due to shortened atmospheric residence time, while emissions changed little.
  • ItemOpen AccessAccepted version Peer-reviewed
    Hamilton's principle and normal mode coupling in an aspherical planet with a fluid core
    (Oxford University Press (OUP), 2018) Al-Attar, D; Crawford, O; Valentine, AP; Trampert, J
    We apply Hamilton’s principle to obtain the exact equations of motion for an elastic planet that is rotating, self-gravitating, and comprises both fluid and solid regions. This variational problem is complicated by the occurrence of tangential slip at fluid-solid boundaries, but we show how this can be accommodated both directly and using the method of Lagrange multipliers. A novelty of our approach is that the planet’s motion is described relative to an arbitrary reference configuration, with this generality offering advantages for numerical calculations. In particular, aspherical topography on the free surface or internal boundaries of the planet’s equilibrium configuration can be converted exactly into effective volumetric heterogeneities within a geometrically spherical reference body by applying a suitable particle relabelling transformation. The theory is then specialised to consider the linearised motion of a planet about a steadily rotating equilibrium configuration, with these results having applications to normal mode coupling calculations used within studies of long period seismology, tidal deformation, and related fields. In particular, we explain how our new theory will, for the first time, allow aspherical boundary topography to be incorporated exactly within such coupling calculations.
  • ItemOpen Access
    XPS study of ion irradiated and unirradiated CeO2 bulk and thin film samples
    (Elsevier, 2018-08-01) Maslakov; Teterin, Yury; Popel, AJ; Teterin, Anton; Ivanov, Kirill; Kalmykov, Stepan; Petrov, Vladimir; Petrov, Peter; Farnan, Ian; Popel, Aleksej [0000-0003-4436-9961]; Farnan, Ian [0000-0001-7844-5112]
    This work considers the effect of fission-energy ion irradiation on the electronic structure at the surface of bulk and thin film samples of CeO2 as a simulant for UO2 nuclear fuel. For this purpose, thin films of CeO2 grown on Si substrates and bulk CeO2 samples were irradiated by Xe ions (92 MeV, 4.8 × 1015 ions/cm2) to simulate the fission damage that occurs within nuclear fuels. The irradiated and unirradiated samples were characterized by X-ray photoelectron spectroscopy. A technique of the quantitative evaluation of cerium ionic composition on the surface of the samples has been successfully applied to the obtained XPS spectra. This technique is based on the intensity of only one of the reliably identifiable high-energy peak at 916.6 eV in the Ce 3d XPS spectra. The as-produced samples were found to contain mostly the Ce4+ ions with a small fraction of Ce3+ ions formed on the surface in the air or under X-rays. The core-electron XPS structure of CeO2 was associated with the complex final state with vacancies (holes) resulting from the photoemission of an inner electron. The Xe ion irradiation was found to increase the Ce3+ content in the samples of CeO2, with the thin films being more sensitive than the bulks samples.
  • ItemOpen AccessPublished version Peer-reviewed
    Melt segregation from silicic crystal mushes: a critical appraisal of possible mechanisms and their microstructural record.
    (Springer Science and Business Media LLC, 2018) Holness, Marian B; Holness, Marian B [0000-0001-9911-8292]
    One of the outstanding problems in understanding the behavior of intermediate-to-silicic magmatic systems is the mechanism(s) by which large volumes of crystal-poor rhyolite can be extracted from crystal-rich mushy storage zones in the mid-deep crust. The mechanisms commonly invoked are hindered settling, micro-settling, and compaction. The concept of micro-settling involves extraction of grains from a crystal framework during Ostwald ripening and has been shown to be non-viable in the metallic systems for which it was originally proposed. Micro-settling is also likely to be insignificant in silicic mushes, because ripening rates are slow for quartz and plagioclase, contact areas between grains in a crystal mush are likely to be large, and abundant low-angle grain boundaries promote grain coalescence rather than ripening. Published calculations of melt segregation rates by hindered settling (Stokes settling in a crystal-rich system) neglect all but fluid dynamical interactions between particles. Because tabular silicate minerals are likely to form open, mechanically coherent, frameworks at porosities as high as ~ 75%, settling of single crystals is only likely in very melt-rich systems. Gravitationally-driven viscous compaction requires deformation of crystals by either dissolution-reprecipitation or dislocation creep. There is, as yet, no reported microstructural evidence of extensive, syn-magmatic, internally-generated, viscous deformation in fully solidified silicic plutonic rocks. If subsequent directed searches do not reveal clear evidence for internally-generated buoyancy-driven melt segregation processes, it is likely that other factors, such as rejuvenation by magma replenishment, gas filter-pressing, or externally-imposed stress during regional deformation, are required to segregate large volumes of crystal-poor rhyolitic liquids from crustal mushy zones.
  • ItemOpen AccessPublished version Peer-reviewed
    Quantitative planktonic foraminifers taphonomy and palaeoceanographic implications over the last 1 My from IODP Sites U1436 and U1437
    (Informa UK Limited, 2018) Mleneck-Vautravers, Maryline J; Mleneck-Vautravers, Maryline J [0000-0003-2534-219X]
    International Ocean Discovery Program Sites U1437 and U1436, two intermediate water depth sites located near the Kuroshio Current were studied for planktonic proxies, shell weights, percent fragments, foraminifers concentrations and benthic/planktonic ratios. Over the last 1 My, the foraminifers assemblages responded to local temperature changes. The taphonomy of these assemblages limits their usefulness as palaeoclimatic records but greatly inform qualitatively of intermediate water-mass changes on each side of the Izu Rise. Carbonate dissolution (CD) is pervasive and always more intense during interglacials at the shallowest Site U1436 to the East. Carbonate preservation improved during glacials after 0.6 Ma at U1437B (West) likely signaling changes within the glacial Antarctic Intermediate Water and therefore, its source water the Glacial North Atlantic Intermediate Water from Marine Isotope Stage 16. The first occurrence of a very large Laurentide ice-sheet at the time is proposed as a potential indirect cause for this observation. Both the intensification of carbonate dissolution during each interglacials from MIS17 onwards and the better preservation during succeeding glacials after that are attributed to the increasing influence of the North Atlantic Deep Water and the Glacial North Atlantic Intermediate Water via the Antarctic Intermediate Water during Interglacial and Glacial intervals, respectively.
  • ItemOpen AccessAccepted version Peer-reviewed
    Thermal impact of magmatism in subduction zones
    (Elsevier BV, 2018) Rees Jones, DW; Katz, RF; Tian, M; Rudge, JF; Rees Jones, DW [0000-0001-8698-401X]; Katz, RF [0000-0001-8746-5430]; Rudge, JF [0000-0002-9399-7166]
    Magmatism in subduction zones builds continental crust and causes most of Earth's subaerial volcanism. The production rate and composition of magmas are controlled by the thermal structure of subduction zones. A range of geochemical and heat flow evidence has recently converged to indicate that subduction zones are hotter at lithospheric depths beneath the arc than predicted by canonical thermomechanical models, which neglect magmatism. We show that this discrepancy can be resolved by consideration of the heat transported by magma. In our one- and two-dimensional numerical models and scaling analysis, magmatic transport of sensible and latent heat locally alters the thermal structure of canonical models by $\sim$300 K, increasing predicted surface heat flow and mid-lithospheric temperatures to observed values. We find the advection of sensible heat to be larger than the deposition of latent heat. Based on these results we conclude that thermal transport by magma migration affects the chemistry and the location of arc volcanoes.
  • ItemOpen AccessPublished version Peer-reviewed
    Pteropods counter mechanical damage and dissolution through extensive shell repair.
    (Springer Science and Business Media LLC, 2018-01-17) Peck, Victoria L; Oakes, Rosie L; Harper, Elizabeth M; Manno, Clara; Tarling, Geraint A; Oakes, Rosie L [0000-0001-8380-2049]
    The dissolution of the delicate shells of sea butterflies, or pteropods, has epitomised discussions regarding ecosystem vulnerability to ocean acidification over the last decade. However, a recent demonstration that the organic coating of the shell, the periostracum, is effective in inhibiting dissolution suggests that pteropod shells may not be as susceptible to ocean acidification as previously thought. Here we use micro-CT technology to show how, despite losing the entire thickness of the original shell in localised areas, specimens of polar species Limacina helicina maintain shell integrity by thickening the inner shell wall. One specimen collected within Fram Strait with a history of mechanical and dissolution damage generated four times the thickness of the original shell in repair material. The ability of pteropods to repair and maintain their shells, despite progressive loss, demonstrates a further resilience of these organisms to ocean acidification but at a likely metabolic cost.
  • ItemOpen AccessPublished version Peer-reviewed
    The thickness of the crystal mush on the floor of the Bushveld magma chamber.
    (Springer Science and Business Media LLC, 2017) Holness, Marian B; Cawthorn, R Grant; Roberts, James; Roberts, James [0000-0002-2950-9949]
    The thickness of the crystal mush on magma chamber floors can be constrained using the offset between the step-change in the median value of dihedral angles formed at the junctions between two grains of plagioclase and a grain of another phase (typically clinopyroxene, but also orthopyroxene and olivine) and the first appearance or disappearance of the liquidus phase associated with the step-change in median dihedral angle. We determined the mush thickness in the Rustenburg Layered Suite of the Bushveld Complex at clinopyroxene-in (in Lower Main Zone) and magnetite-in (in Upper Zone). We also examined an intermittent appearance of cumulus apatite in Upper Zone, using both the appearance and disappearance of cumulus apatite. In all cases, the mush thickness does not exceed 4 m. These values are consistent with field observations of a mechanically rigid mush at the bases of both magnetitite and chromitite layers overlying anorthosite. Mush thickness of the order of a few metres suggests that neither gravitationally-driven compaction nor compositional convection within the mush layer is likely to have been important processes during solidification: adcumulates in the Bushveld are most likely to have formed at the top of the mush during primary crystallisation. Similarly, it is unlikely either that migration of reactive liquids occurs through large stretches of stratigraphy, or that layering is formed by mechanisms other than primary accumulation.
  • ItemOpen AccessPublished version Peer-reviewed
    Atlantic deep water provenance decoupled from atmospheric CO2 concentration during the lukewarm interglacials.
    (Springer Science and Business Media LLC, 2017-12-08) Howe, Jacob NW; Piotrowski, Alexander M; Howe, Jacob NW [0000-0002-9310-9632]
    Ice core records show that atmospheric CO2 concentrations and Antarctic temperature were lower during the 'lukewarm interglacials' from 800 to 430 ka than the subsequent five interglacials. These different interglacial 'strengths' have been hypothesised to be controlled by Antarctic overturning circulation. How these variations in Antarctic overturning relate to Northern Atlantic overturning circulation, a major driver of Northern Hemisphere climate, is uncertain. Here we present a high-resolution record of authigenic neodymium isotopes-a water mass tracer that is independent of biological processes-and use it to reconstruct Atlantic overturning circulation during the last 800 kyr. This record reveals a similar proportion of North Atlantic Deep Water during the 'lukewarm interglacials' and the more recent interglacials. This observation suggests that the provenance of deep water in the Atlantic Ocean can be decoupled from ventilation state of the Southern Ocean and consequently the atmospheric concentration of carbon dioxide.
  • ItemOpen AccessPublished version Peer-reviewed
    Microstructural and paleomagnetic insight into the cooling history of the IAB parent body
    (Elsevier BV, 2018) Nichols, CIO; Krakow, R; Herrero-Albillos, J; Kronast, F; Northwood-Smith, G; Harrison, RJ; Nichols, Claire [0000-0003-2947-5694]; Krakow, Robert [0000-0003-3371-5662]; Harrison, Richard [0000-0003-3469-762X]
    The IABs represent one of only two groups of iron meteorites that did not form by fractional crystallization of liquid Fe-Ni in the core of a differentiated planetesimal. Instead, they are believed to originate from a partially differentiated body that was severely disrupted by one or more impacts during its early history. We present a detailed microstructural and paleomagnetic study of the Odessa and Toluca IAB meteorites, with a view to further constraining the complex history of the IAB parent body. X-ray photoemission electron microscopy and energy dispersive spectroscopy were used to generate high-resolution Ni/Fe maps. The crystallographic architecture of Odessa was analysed using electron backscatter diffraction. Paleomagnetic signals and the magnetic properties of several microstructures were also assessed using X-ray magnetic circular dichroism. Odessa exhibits a complex series of microstructures, requiring an unusual evolution during slow cooling. A conventional Widmanstätten microstructure, consisting of multiple generations of kamacite lamellae surrounded by M-shaped diffusion profiles, developed via continuous precipitation to temperatures below ∼400°C. Multiple generations of pearlitic plessite nucleated from kamacite/taenite (T > 400°C) and tetrataenite rim/taenite interfaces (T < 400°C), via a process of discontinuous precipitation. Rounded rafts of Ni-rich taenite, observed within some regions of pearlitic plessite, are shown to have the same crystallographic orientation as the parental taenite, and a non-standard orientation relationship with the enclosing kamacite. Contrary to current theories, these rafts cannot have formed by coarsening of pre-existing pearlitic plessite. A new bowing mechanism is proposed, whereby rafts of Ni-enriched taenite form between advancing lobes of an irregular reaction front during discontinuous precipitation. Subsequent coarsening leads to the growth of the taenite rafts, and the partial or complete removal of pearlite lamellae, resulting in spheroidised plessite with a crystallographic architechture matching the experimental observations. We find no evidence for a strong magnetic field on the IAB parent body, suggesting it did not have an active core dynamo at the time of cloudy zone formation. This supports the prediction that the IAB parent body was unable to form a significant core due to the redistribution of metal during an earlier impact event.