ALBERT

Description: Kawakatsu and Abe [2016] have highlighted the potential complicating effect of sediment reverberations on the analysis and interpretation of crust and mantle phases inferred from receiver functions analyzed from ocean-bottom seismograms. In their comment, they identify resonant peaks in the power spectrum at one of the stations, T06 , in the analysis of [ Olugboji et al ., 2016], and demonstrate with synthetic modeling how sediment-induced resonances can cause instability in the recovered receiver-function (RF) traces. They also request a detailed explanation of how LQT rotation is conducted, and why its use leads to stable receiver functions in the analysis of Olugboji et al . [2016]. We welcome this query as an opportunity to highlight certain technical aspects of the data-analysis procedures used in Olugboji et al [2016]. Our methods derive partly from methods recommended by previous studies of receiver functions estimated from seismic seafloor data [ Bostock and Trehu , 2012; Janiszewski and Abers , 2015; Audet , 2016], particularly the use of the modal wavefield decomposition [e.g., Reading et al , 2003]) (which we approximated by the LQT rotation) to suppress reverberation signals in the overlying water column [ Bostock and Trehu , 2012]. This article is protected by copyright. All rights reserved.

Description: Receiver-function observations in the oceanic upper mantle can test causal mechanisms for the depth, sharpness and age-dependence of the seismic wavespeed decrease thought to mark the lithosphere-asthenosphere boundary (LAB). We use a combination of frequency-dependent harmonic decomposition of receiver functions and synthetic forward-modeling to provide new seismological constraints on this “seismic LAB” from 17 ocean-bottom stations and 2 borehole stations in the Philippine Sea and northwest Pacific Ocean. Underneath young oceanic crust, the seismic LAB depth follows the ∼1300 K isotherm but a lower isotherm (∼1000 K) is suggested in the Daito ridge, the Izu-Bonin-Mariana trench and the northern Shikoku basin. Underneath old oceanic crust, the seismic LAB lies at a constant depth ∼70 km. The age-dependence of the seismic LAB depth is consistent with either a transition to partial-melt conditions or a sub-solidus rheological change as the causative factor. The age-dependence of interface sharpness provides critical information to distinguish these two models. Underneath young oceanic crust, the velocity gradient is gradational, while for old oceanic crust a sharper velocity gradient is suggested by the receiver functions. This behavior is consistent with the prediction of the sub-solidus model invoking anelastic relaxation mediated by temperature and water-content, but is not readily explained by a partial-melt model. The Ps conversions display negligible two-lobed or four-lobed back-azimuth dependence in harmonic stacks, suggesting that a sharp change in azimuthal anisotropy with depth is not responsible for them. We conclude that these ocean-bottom observations indicate a sub-solidus elastically-accommodated grain-boundary sliding (EAGBS) model for the seismic LAB. Because EAGBS does not facilitate long-term ductile deformation, the Seismic LAB may not coincide with the conventional transition from lithosphere to asthenosphere. This article is protected by copyright. All rights reserved.

Description: Nature Geoscience 8, 509 (2015). doi:10.1038/ngeo2462 Authors: Shun-ichiro Karato, Tolulope Olugboji & Jeffrey Park

Description: In the Japan subduction zone, a locally-depressed 660-discontinuity has been observed beneath northeast Asia, suggesting downwelling of materials from the mantle transition zone (MTZ). Vertical transport of water-rich MTZ-materials across the major mineral phase changes could lead to water release and to partial melting in surrounding mantle regions, causing seismic low-velocity anomalies. Melt layers implied by low-velocity zones (LVZs) above the 410-discontinuity have been detected in many regions, but seismic evidence for partial melting below the 660-discontinuity has been limited. High-frequency migrated Ps receiver-functions indicate LVZs below the depressed 660-discontinuity and above the 410-discontinuity in the deep Japan subduction zone, suggesting dehydration melting induced by water transport out of the MTZ. Our results provide insights into water circulation associated with dynamic interactions between the subducted slab and surrounding mantle.

Description: ABSTRACT Crustal anisotropy beneath ocean islands can be attributed to preferentially aligned minerals, cracks, or dike structures. Stacked with harmonic weighting, receiver functions from permanent ocean-island stations display evidence of strong and distinct anisotropy parameters in the underlying crust and underplated layer. We analyze data for eleven IRIS-GSN stations in the Pacific Ocean. We observe the prevalence of two-lobed receiver function (RF) amplitude variations with back-azimuth, consistent with “slow” tilted-axis anisotropy. In most cases the anisotropy is accommodated in the underplated crust. Synthetic modeling of a representative station indicates that the strength of anisotropy of Vp=10% and Vs=5% is possible. The strike direction of the inferred symmetry axis tends to align with plate motion, with some scatter. At stations in the northwest Pacific i.e. KWAJ, TARA, and WAKE, the strike direction of the symmetry axis aligns with plate motion at the time of volcano emplacement. Beneath station POHA and the closest stations to the present-day Hawaiian hotspot, alignment of the symmetry axis is almost orthogonal to the plate motion. We attribute the crustal anisotropy to the preferred alignment of dike structures that transported asthenospheric magma toward the seafloor volcanic edifice. Our results suggest that the thermal-plume origin for ocean islands must be supplemented by tectonic-stress heterogeneities that allow magma to penetrate the lithosphere via fractures. Magma-transport fractures should align normal to the least-compressive direction, which are predicted by theoretical models to align approximately with plate motion at the time of emplacement. This article is protected by copyright. All rights reserved.

Description: Abstract High‐frequency harmonic regression (2.0‐4.0‐Hz cutoff) of receiver functions at two long‐running seismic observatories at mid‐Pacific hotspot islands confirms earlier detections of this underplated material with seismic velocities intermediate to crust and mantle, and reveals it to be multilayered and anisotropic within ~30 km of the surface. Magmatic underplating beneath the oceanic Moho has been proposed to accompany basaltic melt that erupts at the seafloor and (eventually) atop a subaerial volcano. An alternate hypothesis is “metasomatic underplating” whereby crustal fractures developed during magma ascent allow seawater to infiltrate and to serpentinize the sub‐Moho mantle partially. Metasomatic underplating would lower seismic wavespeeds, promote the buoyancy of the hotspot swell, and induce textural anisotropy as metamorphic expansion of olivine‐rich peridotite promotes a crack network along which serpentinization spreads. Differential expansion of mantle peridotite and crustal gabbro promotes cracks in the crust that offer new pathways for seawater to descend to the Moho, allowing metasomatic underplating to expand laterally and to contribute anisotropy to the underplated layer. Rare serpentinized mantle xenoliths confirm that crack textures can develop during serpentinization at depth. The discovery of iron‐oxidizing microbial mats on the seafloor flank of the Loihi volcano, and many locations of diffuse low‐temperature venting worldwide, is consistent with the circulation of metasomatic fluids with reducing chemistry, sourced from serpentinization at depth. Post‐eruptive uplift of Santa Maria Island (Azores), and asymmetry of the Hawaiian swell, suggests that underplating requires 2‐4 Myr to complete, suggesting that fluid infiltration is slow, subject to cycles of blockage and fresh fracturing.

Description: Abstract The “metasomatic underplating” hypothesis argues that crustal fractures develop during hotspot magma ascent to allow seawater to infiltrate and to serpentinize the sub‐Moho mantle. Published seismic velocities and layer‐thicknesses support estimates that 1‐km of seawater could be chemically bound within a underplated layer, which may require 2‐4 Myr to mature. If a serpentinized mantle layer underlies hotspot tracks and/or aseismic ridges, their buoyancy can induce flat‐slab behavior within subduction zones (e.g., beneath central Chile and Peru), weaken slab rheology, and foster slab tears. During serpentinization, metasomatic underplating would produce more serpentine and talc, and less brucite and magnetite, if seawater equilibrates with silica in the gabbroic oceanic crust as it descends to the Moho. The restricted solubility of carbonate with temperature may induce seawater CO2 to sequester in the crust as carbonate concretions or intergrowths. Consumption of seawater H2O by serpentinization raises its salinity so that Fe cations stabilize in chloride complexes and depart the open system. Alkali cations in seawater contribute to the sodic metasomatism of pyroxenes, analogous to alterations observed in abyssal peridotites.

Description: For almost 30 years, the Soil and Water Assessment Tool (SWAT) has been successfully implemented to address issues around various scientific subjects in the world. On the other hand, it has been reaching to the limit of potential flexibility in further development by the current structure. The new generation SWAT, dubbed SWAT+, was released recently with entirely new coding features. SWAT+ is designed to have far more advanced functions and capacities to handle challenging watershed modeling tasks for hydrologic and water quality processes. However, it is still inevitable to conduct model calibration before the SWAT+ model is applied to engineering projects and research programs. The primary goal of this study is to develop an open-source, easy-to-operate automatic calibration tool for SWAT+, dubbed IPEAT+ (Integrated Parameter Estimation and Uncertainty Analysis Tool Plus). There are four major advantages: (i) Open-source code to general users; (ii) compiled and integrated directly with SWAT+ source code as a single executable; (iii) supported by the SWAT developer group; and, (iv) built with efficient optimization technique. The coupling work between IPEAT+ and SWAT+ is fairly simple, which can be conducted by users with minor efforts. IPEAT+ will be regularly updated with the latest SWAT+ revision. If users would like to integrate IPEAT+ with various versions of SWAT+, only few lines in the SWAT+ source code are required to be updated. IPEAT+ is the first automatic calibration tool integrated with SWAT+ source code. Users can take advantage of the tool to pursue more cutting-edge and forward-thinking scientific questions.

Description: The traditional practice to assess accuracy in lidar data involves calculating RMSEz (root mean square error of the vertical component). Accuracy assessment of lidar point clouds in full 3D (three dimension) is not routinely performed. The main challenge in assessing accuracy in full 3D is how to identify a conjugate point of a ground-surveyed checkpoint in the lidar point cloud with the smallest possible uncertainty value. Relatively coarse point-spacing in airborne lidar data makes it challenging to determine a conjugate point accurately. As a result, a substantial unwanted error is added to the inherent positional uncertainty of the lidar data. Unless we keep this additional error small enough, the 3D accuracy assessment result will not properly represent the inherent uncertainty. We call this added error “external uncertainty,” which is associated with conjugate point identification. This research developed a general external uncertainty model using three-plane intersections and accounts for several factors (sensor precision, feature dimension, and point density). This method can be used for lidar point cloud data from a wide range of sensor qualities, point densities, and sizes of the features of interest. The external uncertainty model was derived as a semi-analytical function that takes the number of points on a plane as an input. It is a normalized general function that can be scaled by smooth surface precision (SSP) of a lidar system. This general uncertainty model provides a quantitative guideline on the required conditions for the conjugate point based on the geometric features. Applications of the external uncertainty model were demonstrated using various lidar point cloud data from the U.S. Geological Survey (USGS) 3D Elevation Program (3DEP) library to determine the valid conditions for a conjugate point from three-plane modeling.

Description: Density is a key property controlling the chemical state of Earth's interior. Our knowledge about the density of relevant melt compositions is currently poor at deep-mantle conditions. Here we report results from first-principles molecular-dynamics simulations of Fe-bearing MgSiO3 liquids considering different valence and spin states of iron over the whole mantle pressure conditions. Our simulations predict the high-spin to low-spin transition in both ferrous and ferric iron in the silicate liquid to occur gradually at pressures around 100 GPa. The calculated iron-induced changes in the melt density (about 8% increase for 25% iron content) are primarily due to the difference in atomic mass between Mg and Fe, with smaller contributions (〈2%) from the valence and spin states. A comparison of the predicted density of mixtures of (Mg,Fe)(Si,Fe)O3 and (Mg,Fe)O liquids with the mantle density indicates that the density contrast between the melt and residual-solid depends strongly on pressure (depth): in the shallow lower mantle (depths 〈 1,000 km), the melt is lighter than the solids, whereas in the deep lower mantle (e.g., the D″ layer), the melt density exceeds the mantle density when iron content is relatively high and/or melt is enriched with Fe-rich ferropericlase. ©2018. American Geophysical Union. All Rights Reserved.