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The thermal evolution of planetesimals during accretion and differentiation: consequences for dynamo generation by thermally-driven convection.
Full text linkThe meteorite paleomagnetic record indicates that differentiated (and potentially, partially differentiated) planetesimals generated dynamo fields in the first 6-20 Myr after the formation of calcium-aluminium-rich inclusions (CAIs). This early period of dynamo activity has been attributed to thermal convection in the liquid cores of these planetesimals during an early period of magma ocean convection. To better understand the controls on thermal dynamo generation in planetesimals, we have developed a 1D model of the thermal evolution of planetesimals from accretion through to the shutoff of convection in their silicate magma oceans for a variety of accretionary scenarios. The heat source of these bodies is the short-lived radiogenic isotope, 26Al. During differentiation, 26Al partitions into the silicate portion of these bodies, causing their magmas ocean to heat up and introducing stable thermal stratifications to the tops of their cores, which inhibits dynamo generation. In 'instantaneously' accreting bodies, this effect causes a delay on the order of >10 Myr to whole core convection and dynamo generation while this stratification is eroded. However, gradual core formation in bodies that accrete over >0.1 Myr can minimise the development of this stratification, allowing dynamo generation from ~4 Myr after CAI formation. Our model also predicts partially differentiated planetesimals with a core and mantle overlain by a chondritic crust for accretion timescales >1.2 Myr, although none of these bodies generate a thermal dynamo field. We compare our results from thousands of model runs to the meteorite paleomagnetic record to constrain the physical properties of their parent bodies
Ediacaran life close to land: coastal and shoreface habitats of the Ediacaran macrobiota in the central and southern Flinders Ranges, South Australia
Full text linkThe Rawnsley Quartzite of South Australia hosts some of the world's most diverse Ediacaran macrofossil assemblages, with many of the constituent taxa interpreted as early representatives of metazoan clades. Globally, a link has been recognized between the taxonomic composition of individual Ediacaran bedding-plane assemblages and specific sedimentary facies. Thorough characterization of fossil-bearing facies is thus of fundamental importance for reconstructing the precise environments and ecosystems in which early animals thrived and radiated, and distinguishing between environmental and evolutionary controls on taxon distribution. This study refines the paleoenvironmental interpretations of the Rawnsley Quartzite (Ediacara Member and upper Rawnsley Quartzite). Our analysis suggests that previously inferred water depths for fossil bearing facies are overestimations. In the central regions of the outcrop belt, rather than shelf and submarine canyon environments below maximum (storm-weather) wave base, and offshore environments between effective (fair-weather) and maximum wave base, the succession is interpreted to reflect the vertical superposition and lateral juxtaposition of unfossiliferous non-marine environments with fossil-bearing coastal and shoreface settings. Facies comprise: 1 and 2) Amalgamated channelized and cross-bedded sandstone (major and minor tidally influenced river and estuarine channels, respectively); 3) Ripple cross-laminated heterolithic sandstone (intertidal mixed-flat); 4) Silty-sandstone (possible lagoon); 5) Planar-stratified sandstone (lower shoreface); 6) Oscillation-ripple facies (middle shoreface); 7) Multi-directed trough- and planar-cross-stratified sandstone (upper shoreface); 8) Ripple cross-laminated, planar-stratified rippled sandstone (foreshore); 9) Adhered sandstone (backshore); and 10) Planar-stratified and cross-stratified sandstone with ripple cross-lamination (distributary channels). Surface trace fossils in the foreshore facies represent the earliest known evidence of mobile organisms in intermittently emergent environments. All facies containing fossils of the Ediacaran macrobiota remain definitively marine. Our revised shoreface and coastal framework creates greater overlap between this classic �White Sea� biotic assemblage and those of younger, relatively depauperate �Nama�-type biotic assemblages located in Namibia. Such overlap lends support to the possibility that the apparent biotic turnover between these assemblages may reflect a genuine evolutionary signal, rather than the environmental exclusion of particular taxa
Vertical mixing and heat fluxes conditioned by a seismically imaged oceanic front
Full text linkThe southwest Atlantic gyre connects several distinct water masses, which means that this oceanic region is characterized by a complex frontal system and enhanced water mass modification. Despite its significance, the distribution and variability of vertical mixing rates have yet to be determined for this system. Specifically, potential conditioning of mixing rates by frontal structures, in this location and elsewhere, is poorly understood. Here, we analyze vertical seismic (i.e., acoustic) sections from a three-dimensional survey that straddles a major front along the northern portion of the Brazil-Falkland Confluence. Hydrographic analyses constrain the structure and properties of water masses. By spectrally analyzing seismic reflectivity, we calculate spatial and temporal distributions of the dissipation rate of turbulent kinetic energy, ε, of diapycnal mixing rate, K, and of vertical diffusive heat flux, FH. We show that estimates of ε, K, and FH are elevated compared to regional and global mean values. Notably, cross-sectional mean estimates vary little over a 6 week period whilst smaller scale thermohaline structures appear to have a spatially localized effect upon ε, K, and FH. In contrast, a mesoscale front modifies ε and K to a depth of 1 km, across a region of O(100) km. This front clearly enhances mixing rates, both adjacent to its surface outcrop and beneath the mixed layer, whilst also locally suppressing ε and K to a depth of 1 km. As a result, estimates of FH increase by a factor of two in the vicinity of the surface outcrop of the front. Our results yield estimates of ε, K and FH that can be attributed to identifiable thermohaline structures and they show that fronts can play a significant role in water mass modification to depths of 1 km
Size Control in the Colloidal Synthesis of Plasmonic Magnesium Nanoparticles
No full textNanoparticles of plasmonic materials can sustain oscillations of their free electron density, called localized surface plasmon resonances (LSPRs), giving them a broad range of potential applications. Mg is an earth-abundant plasmonic material attracting growing attention owing to its ability to sustain LSPRs across the ultraviolet, visible, and near-infrared wavelength range. Tuning the LSPR frequency of plasmonic nanoparticles requires precise control over their size and shape; for Mg, this control has previously been achieved using top-down fabrication or gas-phase methods, but these are slow and expensive. Here, we systematically probe the effects of reaction parameters on the nucleation and growth of Mg nanoparticles using a facile and inexpensive colloidal synthesis. Small NPs of 80 nm were synthesized using a low reaction time of 1 min and ∼100 nm NPs were synthesized by decreasing the overall reaction concentration, replacing the naphthalene electron carrier with biphenyl or using metal salt additives of FeCl3 or NiCl2 at longer reaction times of 17 h. Intermediate sizes up to 400 nm were further selected via the overall reaction concentration or using other metal salt additives with different reduction potentials. Significantly larger particles of over a micrometer were produced by reducing the reaction temperature and, thus, the nucleation rate. We showed that increasing the solvent coordination reduced Mg NP sizes, while scaling up the reaction reduced the mixing efficiency and produced larger NPs. Surprisingly, varying the relative amounts of Mg precursor and electron carrier had little impact on the final NP sizes. These results pave the way for the large-scale use of Mg as a low-cost and sustainable plasmonic material
Global trends in novel stable isotopes in basalts: Theory and observations
No full textThe geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological and temperature heterogeneities originating in both the upper and lower mantle. Recently, non-traditional stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity (e.g., major and trace elements, radiogenic isotopes), because mineral- and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. Here, we investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle temperature and/or lithological heterogeneity. We use a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites), responding to changes in mantle mineralogy, oxygen fugacity, temperature and pressure. We find that, given current analytical precision, the stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of 10% MORB-like pyroxenite in the mantle source, given current analytical precision. Magnesium and Ca stable isotopes show most sensitivity to a garnet-bearing source lithology, and Fe and Cr stable isotopes are potentially sensitive to the presence of MORB-like pyroxenite in the mantle source, although the behaviour of Cr isotopes is comparatively under-constrained and requires further work to be applied with confidence to mantle melts. When comparing the magnitude and direction of predicted equilibrium isotopic fractionation of peridotite and pyroxenite melts to natural MORB and OIB data, we find that aspects of the natural data (including the mean Mg-Ca-Fe-V isotopic composition of MORB, the range of Mg-Ca isotopic compositions seen in MORB data, the mean Mg-Ca-Cr isotopic composition of OIB, and the range of Mg-V-Cr isotopic compositions in OIB data) can be matched by equilibrium isotope fractionation during partial melting of peridotite and pyroxenite sources – with pyroxenite required even for some MORB data. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. We conclude that the five stable isotope systems considered here have potential to be powerful complementary tracers to other geochemical tracers of the source lithology of erupted basalts. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these novel stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts
Large-Scale Tectonic Forcing of the African Landscape
Full text linkAbstract
Successful inverse modeling of observed longitudinal river profiles suggests that fluvial landscapes are responsive to continent-wide tectonic forcing. However, inversion algorithms make simplifying assumptions about landscape erodibility and drainage planform stability that require careful justification. For example, precipitation rate and drainage catchment area are usually assumed to be invariant. Here, we exploit a closed-loop modeling strategy by inverting drainage networks generated by dynamic landscape simulations in order to investigate the validity of these assumptions. First, we invert 4,018 African river profiles to determine an uplift history that is independently calibrated, and subsequently validated, using separate suites of geologic observations. Second, we use this tectonic forcing to drive landscape simulations that permit divide migration, interfluvial erosion and changes in catchment size. These simulations reproduce large-scale features of the African landscape, including growth of deltaic deposits. Third, the influence of variable precipitation is investigated by carrying out a series of increasingly severe tests. Inverse modeling of drainage inventories extracted from simulated landscapes can largely recover tectonic forcing. Our closed-loop modeling strategy suggests that large-scale tectonic forcing plays the primary role in landscape evolution. One corollary of the integrative solution of the stream-power equation is that precipitation rate becomes influential only if it varies on time scales longer than ∼1 Ma. We conclude that calibrated inverse modeling of river profiles is a fruitful method for investigating landscape evolution and for testing source-to-sink models.
Plain Language Summary
There is excellent geologic evidence that large portions of the African landscape were lifted up above sea level over the last 30 million years by upward flow of hot mantle rocks beneath the tectonic plate. The strongest evidence comes from marine deposits which contain fossil fish and sea snakes that are now perched at elevations of hundreds of meters in the middle of the North African desert. Mantle processes gave rise to an egg-carton pattern of gigantic swells and depressions that characterizes much of the continent. As the landscape evolved, it was sculpted and eroded by the action of massive rivers such as the Niger, the Nile and the Congo. Height along the length of each of these rivers varies and appears to preserve a memory of landscape growth. In that sense, rivers appear to act as tape recorders of tectonic processes such as mantle flow. Here, we use computer simulations of an evolving landscape to test the idea that rivers contain mantle memories. These simulations, which include complexities such as variable rainfall, allow rivers to develop naturally as landscapes grow. Our results suggest that the African landscape and its drainage patterns contain valuable information about deep Earth processes
Multiple Avalanche Processes in Acoustic Emission Spectroscopy: Multibranching of the Energy−Amplitude Scaling
Full text linkSeveral physical processes can conspire to generate avalanches in materials. Such processes include avalanche mechanisms like dislocation movements, friction processes by pinning magnetic domain walls, moving dislocation tangles, hole collapse in porous materials, collisions of ferroelectric and ferroelastic domain boundaries, kinks in interfaces, and many more. Known methods to distinguish between these species which allow the physical identification of multiavalanche processes are reviewed. A new approach where the scaling relationship between the avalanche energies E and amplitudes A is considered is then described. Avalanches with single mechanisms scale experimentally as E = SiAi2. The energy E reflects the duration D of the avalanche and A(t), the temporal amplitude. The scaling prefactor S depends explicitly on the duration of the avalanche and on details of the avalanche profiles. It is reported that S is not a universal constant but assumes different values depending on the avalanche mechanism. If avalanches coincide, they can still show multivalued scaling between E and A with different S-values for each branch. Examples for this multibranching effect in low-Ni 316L stainless steel, 316L stainless steel, polycrystalline Ni, TC21 titanium alloy, and a Fe40Mn40Co10Cr10 high-entropy alloy are shown
Dislocation interactions during low-temperature plasticity of olivine and their impact on the evolution of lithospheric strength.
Full text linkThe strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, models of lithospheric flexure reveal differences in lithospheric strength that are difficult to reconcile based on such flow laws. Recent rheological data from low-temperature deformation experiments on olivine suggest that this discrepancy may be largely explained by strain hardening. Details of the mechanical data, specifically the effects of temperature-independent back stresses stored in the samples, indicate that strain hardening in olivine occurs primarily due to long-range elastic interactions between dislocations. These interpretations provided the basis for a new flow law that incorporates hardening by development of back stress. Here, we test this hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of ≤ 4.3 GPa or in a nanoindenter at applied contact stresses of ≥ 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10 14 m-2 and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned along slip bands and interacting with dislocations of other types that act as obstacles. The stress heterogeneities and accumulations of dislocations along their slip planes are consistent with strain hardening resulting from long-range back-stresses acting between dislocations. These results corroborate the mechanical data in supporting the form of the new flow law for low-temperature plasticity and provide new microstructural criteria for identifying the operation of this deformation mechanism in natural samples. Furthermore, similarities in the structure and stress fields of slip bands formed in single crystals deformed at low temperatures and those formed at high temperatures suggest that similar hardening processes occur in both regimes, providing a new constraint for models of transient creep at high temperatures
Shear dispersion in a porous medium. Part 1. Fixed-extent immiscible intrusion.
Full text linkThe dispersion of tracer through a liquid injected into a confined aquifer with vertically
varying permeability is studied theoretically. The injected fluid is buoyant and of high
viscosity relative to the original fluid in the aquifer, which causes the nose region of the
flow, where the thickness of the injected fluid is less than the thickness of the aquifer, to
advance with constant velocity and fixed shape. Behind the nose, tracer is sheared at early
times owing to the vertically varying permeability. At later times, cross-aquifer diffusion
homogenises the tracer distribution, which becomes independent of depth but spreads
longitudinally in this shear dispersion regime, which leads to much faster spreading
than by diffusion alone. As tracer diffuses symmetrically in the longitudinal direction, it
eventually reaches the nose. Subsequently, the nose acts as a no-flux boundary and the
concentration profile transitions to a half-Gaussian, with the maximum concentration at
the front. The centre of mass of the tracer spreads backwards relative to the fixed nose at
a rate proportional to [D(T −T 0 )] 1/2 where D is the dispersion coefficient and T 0 is a time
offset owing to the adjustment to shear dispersion and the interaction with the nose. The
initial release of tracer may not be vertically uniform owing to the heterogeneity and we
show that this can lead to the centre of mass of tracer initially advancing faster than the
mean flow. We consider the implications of our results on tracer migration in sedimentary
layers
Barium isotopes in cold-water corals
No full textRecent studies have introduced stable Ba isotopes (delta(138)/Ba-134) as a novel tracer for ocean processes. Ba isotopes could potentially provide insight into the oceanic Ba cycle, the ocean's biological pump, water-mass provenance in the deep ocean, changes in activity of hydrothermal vents, and land-sea interactions including tracing riverine inputs. Here, we show that aragonite skeletons of various colonial and solitary cold-water coral (CWC) taxa record the seawater (SW) Ba isotope composition. Thirty-six corals of eight different taxa from three oceanic regions were analysed and compared to delta(138)/Ba-134 measurements of co-located seawater samples. Sites were chosen to cover a wide range of temperature, salinity, Ba concentrations and Ba isotope compositions. Seawater samples at the three sites exhibit the well-established anti-correlation between Ba concentration and delta(138)/Ba-134. Furthermore, our data set suggests that Ba/Ca values in CWCs are linearly correlated with dissolved Ba] in ambient seawater, with an average partition coefficient of D-CWC/SW = 1.8 +/- 0.4 (2SD). The mean isotope fractionation of Ba between seawater and CWCs Delta(138)/Ba-134(CWC-SW) is -0.21 +/- 0.08 parts per thousand (2SD), indicating that CWC aragonite preferentially incorporates the lighter isotopes. This fractionation likely does not depend on temperature or other environmental variables, suggesting that aragonite CWCs could be used to trace the Ba isotope composition in ambient seawater. Coupled Ba] and delta(138)/Ba-134 analysis on fossil CWCs has the potential to provide new information about past changes in the local and global relationship between Ba] and delta(138)/Ba-134 and hence about the operation of the past global oceanic Ba cycle in different climate regimes. (C) 2018 Elsevier B.V. All rights reserved