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Indian monsoon precipitation isotopes linked with high level cloud cover at local and regional scales

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Di Wang, Lide Tian, Zhongyin Cai, Lili Shao, Xiaoyu Guo, Ran Tian, Yike Li, Yiliang Chen, Chuan Yuan

Abstract

Precipitation stable isotopes preserve historic changes of evaporation in the source regions and precipitation processes, therefore, they can be used to reveal regional hydrological cycle dynamics and paleoclimate reconstructions. In monsoon regions, strong inverse impacts of convection on precipitation isotope ratios, have created a debate regarding the interpretation of isotope records as local climate proxies. The proportions of stratiform to convective precipitation on water isotopes, together with the influence mechanisms on seasonal and interannual scales remain highly uncertain. To further address the influence of precipitation patterns on water isotopes, we used 10 yrs of precipitation isotope data from the southern Tibetan Plateau (TP) to explore the effects of large-scale cloud cover and local climate on precipitation isotopes. Correlation analysis performed between local precipitation δ18O values and different level cloud data, indicated significant negative correlations between precipitation isotopes and high level cloud cover on both seasonal and interannual time scales. This result suggests that high-level convection in the upper moisture transport stream is a main control on precipitation isotopes in the southern TP. The clear and coherent variations of precipitation isotopes with the Southern Oscillation Index and outgoing longwave radiation confirmed that strong convection activity in the moisture source region and during transport significantly depleted heavy isotopes in vapor, producing substantially decreased precipitation δ18O in the study region. These results agree with earlier findings of tree ring cellulose isotope records that correlate with cloud cover, but we emphasized the important role of larger-scale regional cloud cover. We also delineated different maximum correlation zones for seasonal and interannual time scales, likely due to different mechanisms. These findings further improve the interpretation of paleoisotope records from the Indian summer monsoon region.

Accuracy and consistency of different global ionospheric maps released by IGS ionosphere associate analysis centers

Publication date: Available online 4 October 2019

Source: Advances in Space Research

Author(s): Peng Chen, Hang Liu, Yongchao Ma, Naiquan Zheng

Abstract

Due to the differences of ionospheric modeling methods and selected tracking stations, the accuracy and consistency of Global Ionospheric Maps (GIMs) released by Ionosphere Associate Analysis Centers (IAACs) are different. In this study, we evaluate and analyze in detail the accuracy and consistency of GIMs final products provided by six IAACs from three different aspects. Firstly, the comparison of these GIMs shows that the mean bias (MEAN) is related to the modeling methods of various IAACs. The variation trend of the standard deviation (STD) is consistent with the solar activities, and accompanied by certain seasonal and annual periodic variations. The MEAN between IGS and each center is about -1.3∼1.0 TECU, and the STD is about 1.4∼2.5 TECU. Secondly, the validation with GPS TEC shows that the STD of CODE is the smallest at various latitudes, and the STD is about 0.7∼4.5 TECU. Thirdly, The validation with the Jason2 VTEC shows that the STD between Jason2 and IAACs is about 4.4∼5.2 TECU. In addition, the STD between Jason2 and six GIMs in the areas with more tracking stations is better than that of the regions with fewer tracking stations in different latitude regions. Regardless of whether the tracking stations are more or less, the MEAN and STD in high solar activity are larger than in low solar activity.

Large constellations assessment and optimization in LEO space debris environment

Publication date: Available online 4 October 2019

Source: Advances in Space Research

Author(s): Lorenzo Olivieri, Alessandro Francesconi

Abstract

Recent plans for large constellations in Low-Earth Orbit have opened the debate on both their vulnerability and their influence on the already hazardous space debris environment. In fact, given that large constellations normally employ satellites of small size, there might be situations in which cm-size debris could have enough energy to cause fragmentation of a significant part of these spacecraft upon impact, while smaller debris could affect the functionalities of critical subsystems, even compromising the success of disposal operations planned at end-of-life. In this context, this paper investigates: (1) collisions with large objects that could initiate the fragmentation of a significant part of the satellite, and (2) impacts with small debris that might perforate the spacecraft hull thus causing relevant performance/functionality degradation. These two points are merged in a simple statistical tool for risk assessment, which analyses the effects of the main parameters of the constellations on its vulnerability (i.e. operational life, number of satellites, spacecraft cross section, satellites reliability). In more details, the tool relates impact probability (for both small and large debris) to the ballistic response of spacecraft structures and protections, defining the critical configurations that might compromise the expected disposal operations. This method requires a limited knowledge of the spacecraft internal layout, as it is based on a statistical analysis of impact damage instead of a complete evaluation of the vulnerability of each subsystem. In parallel, non-debris related failures are also investigated and statistic models of spacecraft reliability characteristic are proposed. Among the results, it is shown that reducing the lifetime of individual satellites in a constellation might improve the success rate of post-mission disposal, thanks to the reduction of the spacecraft exposure to the space environment with the consequential degradation of its performance. On the other hand, reducing the lifetime would seriously affect the debris environment: the increase in traffic in the most crowded altitudes would be not counterbalanced by the higher post mission disposal success rate, causing an overall increase of the total number of uncontrolled resident objects.

Automatic extraction of lineaments based on wavelet edge detection and aided tracking by hillshade

Publication date: Available online 4 October 2019

Source: Advances in Space Research

Author(s): Junlong Xu, Xingping Wen, Haonan Zhang, Dayou Luo, Jinbo Li, Lianglong Xu, Min Yu

Abstract

Lineaments refer to the linear or curvilinear textures on remote sensing image, whose general spatial distribution characteristics are often the response of deep crustal structures at the surface. Firstly, we use wavelet modulus maxima transformation to detect the edges with 4 scale on Landsat - 8 OLI B5 image and analyze their multi-scale characteristics. As the result, it is determined that the optimal scale of edge detection is 4, and the outline that consist of the edge pixels is roughly corresponding to the geological structure of mine area. Thus the incomplete lineaments have been extracted by using the 2D otsu algorithm. Secondly, the hillshade map generated based on DEM is processed to generate binarized linear shadow. Finally, the linear shadow is superimposed on the lineaments preliminarily extracted to obtain the optimized lineaments. Experiment results show that, based on the method, there will be some deformation and displacement between the lineaments extracted and the actual geological structure, and it fail to effectively extract the Qilinchang Fault, but lineaments are in good correspondence with Kuangshanchang Fault, Dongtou Fault and Niulan River Fault, which are basically in accord with the geological structure framework of the mine area.

Astronomically forced climate evolution in a saline lake record of the middle Eocene to Oligocene, Jianghan Basin, China

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Chunju Huang, Linda Hinnov

Abstract

Following the early Eocene climatic optimum some 50 million years ago, the marine paleoclimate record indicates that a long interval of global cooling took place, punctuated by a series of short-term warming reversals and culminating with the glaciation of Antarctica at the Eocene-Oligocene transition. We investigate a saline lake record from the Qianjiang and Jinghezhen formations, Jianghan Basin, China for the corresponding continental climate response. A 20.25 Myr long astronomical time scale is constructed based on 405-kyr orbital eccentricity cycle tuning of gamma ray (GR) series measured from the Qianjiang and Jinghezhen formations. The halite-rich interval between 41-40.4 Ma correlates well with the Middle Eocene Climate Optimum recorded in the deep-sea δ18O record, confirming predominantly halite deposition during the warming. Silt-mudstone-rich intervals were deposited during ∼2.4 Myr orbital eccentricity minima. The evidence indicates a history of alternating fresh water (humid/cool) and saline water (dry/hot) lake cycles paced by ∼100 kyr orbital eccentricity cycles. Analysis of GR series from the deepest part of the lake indicates strong astronomical forcing of halite deposition throughout the Middle-Late Eocene, and a shift to siliciclastic deposition and cooler climates at ∼33.9 Ma. Early Oligocene wet/cool climate in the Jianghan Basin signals East Asian summer monsoon intensification, and a reduction from northwest winter monsoon in response to Tibetan Plateau uplift. Northern Hemisphere summer insolation minima in the Middle-Late Eocene with temperatures that are 4.6 °C higher than present may provide an analogy for near-future climate change in Jianghan Basin.

Graphical abstract

Importance of permeability and deep channel network on the distribution of melt, fractionation of REE in abyssal peridotites, and U-series disequilibria in basalts beneath mid-ocean ridges: A numerical study using a 2D double-porosity model

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Boda Liu, Yan Liang

Abstract

A plethora of observations have been made at mid-ocean ridges and mantle sections of ophiolites: presence of tabular replacive dunites, highly depleted LREE in residual abyssal peridotites, U-series disequilibria in fresh basalts, crustal thickness, and highly attenuated seismic and magnetotelluric structures beneath spreading centers. These independent observations can become more powerful if they are jointly analyzed in a self-consistent model. The difficulty for such a model is to resolve and evaluate the effect of fine scale petrologic feature such as dunite channels in a tectonic scale geodynamic model. Here we present a two-dimensional double-porosity ridge model that consists of low-porosity residual lherzolite and harzburgite matrix and high-porosity interconnected channel network. The geodynamic simulation features a spatially distributed channel network and anisotropic permeability that gradually develops as the upwelling mantle is deformed. We compute the porosity, melt and solid flow fields for several choices of channel distribution and permeability model. We use the calculated porosity distribution and velocity fields to model the variations of REE in residual mantle and U-series disequilibria in melts.

The present study underscores the importance of deep channel networks and permeability model to the interpretation of first order geophysical and geochemical observations at mid-ocean ridge spreading centers. The anisotropic permeability of channels can enhance melt focusing by 60%, resulting in thicker crust. The attenuated seismic and magnetotelluric structures require a channel network starting from 60 km depth beneath the ridge axis. The depleted REE patterns in clinopyroxenes in residual abyssal peridotites are also consistent with melt extraction into channels starting from 60 km depth. Although deep channels are conducive to producing U-series disequilibria in eruptible melts, the present model still cannot explain the full ranges of the observed U series disequilibria data in MORB samples. Additional factors, processes, and models are discussed. And finally, we found that, within the uncertainty of the permeability, the porosity varies by three folds and the excess of 230Th varies by up to two folds.

Graphical abstract

Synchrotron X-ray imaging in 4D: Multiscale failure and compaction localization in triaxially compressed porous limestone

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Lingcao Huang, Patrick Baud, Benoit Cordonnier, François Renard, Lin Liu, Teng-fong Wong

Abstract

Understanding failure and strain localization in porous rock is of fundamental importance in rock physics. Confined compaction experiments on porous rocks have revealed a broad spectrum of failure modes. Techniques such as acoustic emission location and velocity tomography provide kinematic information on the partitioning of damage and localization of strain. Complementary observations on deformed samples using microscopy and microcomputed tomography (μCT) can also be used to image microscale damage and its distribution. Only by synthesizing such measurements on multiple scales could one infer the multiscale dynamics of compaction localization and similar rock failure phenomena. Located at the European Synchrotron Radiation Facility, the HADES rig allows direct in situ 3D imaging of the whole rock sample as it is triaxially compressed. The μCT data provide an integrated perspective of the spatiotemporal evolution of damage and strain localization on scales ranging from grain to continuum. We conducted an experiment on Leitha limestone (initial porosity of ∼22%) at a confining pressure of 20 MPa. With increasing differential stress, the sample strain hardened and two distinct yield points were identified in the stress-strain curve. The spatiotemporal evolution of local porosity and damage were analyzed at multiple scales. At a mesoscopic scale of 10 voxels (65 μm), the time-lapse μCT images reveal the strain partitioning associated with the first yield point and development of strain localization with the second. The latter development of five discrete compaction bands is the first unambiguous observation of such a bifurcation phenomenon in a porous carbonate rock, with geometric attributes comparable to compactions bands observed in porous sandstones. The μCT data on the voxel-scale elucidate in refined details the nucleation and propagation of discrete compaction bands under quasi-static loading, as well as the micromechanical processes, which in the past could only be inferred from a synthesis of kinematic observations of acoustic emissions activity and post-mortem observations of microstructure and damage.

An essential role for sulfur in sulfide-silicate melt partitioning of gold and magmatic gold transport at subduction settings

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Yuan Li, Lu Feng, Ekaterina S. Kiseeva, Zenghao Gao, Haihao Guo, Zhixue Du, Fangyue Wang, Lanlan Shi

Abstract

Sulfide-silicate melt partitioning controls the behavior of gold in magmas, which is critical for understanding the Earth's deep gold cycle and formation of gold deposits. However, the mechanisms that control the sulfide-silicate melt partitioning of gold remain largely unknown. Here we present constraints from laboratory experiments on the partition coefficient of gold between monosulfide-solid-solution (MSS) and silicate melt (DAuMSS/SM) under conditions relevant for magmatism at subduction settings. Thirty-five experiments were performed in Au capsules to determine DAuMSS/SM at 950-1050 °C, 0.5-3 GPa, oxygen fugacity (fO2) of ∼FMQ-1.7 to FMQ+2.7 (FMQ refers to the fayalite-magnetite-quartz buffer), and sulfur fugacity (fS2) of −2.2 to 2.1, using a piston cylinder apparatus. The silicate melt composition changes from dry to hydrous andesite to rhyolite. The results obtained from electron microprobe and laser-ablation ICP-MS analyses show that the gold solubility in silicate melts ranges from 0.01 to 55.3 ppm and is strongly correlated with the melt sulfur content [S]melt at fO2 of ∼FMQ-1.7 to FMQ+1.6, which can be explained by the formation of complex Au-S species in the silicate melts. The gold solubility in MSS ranges from 130 to 2800 ppm, which is mainly controlled by fS2. DAuMSS/SM ranges from 10 to 14000 at fO2 of ∼FMQ-1.7 to FMQ+1.6, the large variation of which can be fully explained by combined [S]melt and fS2. Therefore, all of the parameters that can directly affect [S]melt and fS2, such as alkali metals, water, FeO, and fO2, can indirectly affect DAuMSS/SM. The mechanisms that control the sulfide-silicate melt partitioning of gold and the other chalcophile elements, such as Ni, Re, and Mo, differ significantly. This is because gold is dissolved mainly as Au-S species in the silicate melts, while the other chalcophile elements are dissolved mainly as metal oxides in the silicate melts. Applying the correlation between DAuMSS/SM and [S]melt to slab melting and arc magmatic differentiation under different redox conditions, we find that ancient to modern slab melts carry negligible to less than 25% of the slab gold to the subarc mantle; however, gold-enrichment can occur in MSS-saturated arc magmas that have differentiated under moderately oxidized conditions with fO2 between FMQ and FMQ+1.6, in particular if the magmatic crystallization follows a fractional crystallization model. We conclude that moderately oxidized magmas with high contents of alkali metals, sulfur, and water, owing to their low DAuMSS/SM and efficient magma-to-fluid transfer of gold and sulfur, have a high potential to form gold deposits.

The redox budget of the Mariana subduction zone

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Maryjo Brounce, Elizabeth Cottrell, Katherine A. Kelley

Abstract

Assessing the efficiency of material recycling at convergent margins is critical to constraining the impact of plate tectonic processes on the composition of surface and deep mantle reservoirs on geologic timescales. In particular, oceanic lithosphere bearing oxidized phases such as Fe-oxy-hydroxides, Fe-oxides, serpentine, carbonate, and sulfate minerals, subduct at convergent margins and the infiltration of aqueous fluids and sediment melts from the subducting slab into the mantle beneath arc volcanoes may thus carry oxidized forms of multi-valent elements (e.g., S, Fe, C) and lead to the generation of primitive arc melts that record elevated oxygen fugacities relative to mid-ocean ridge primitive melts. It is unclear, however, how efficiently aqueous fluids and silicate melts transport the oxidized signatures of any given subducting slab into the mantle wedge in a single subduction zone, and how much, if any, of these oxidized phases may be recycled into the deeper mantle. We present a mass balance of Fe3+, S2−, S6+, and C4+, as well as the O2 associated with these species, through the Mariana subduction zone to assess the efficiency of recycling oxidized materials in an end-member type subduction zone, where old oceanic lithosphere and a thick sediment package is subducted. To do this, we report Fe3+/ΣFe ratios of bulk sediments and altered oceanic crust recovered from ODP Site 801 in the western Pacific in order to constrain the bulk Fe3+/ΣFe ratio of the Pacific plate prior to subduction in the Mariana convergent margin. Site 801 sediments have Fe3+/ΣFe ratios >0.69 and the altered oceanic crust (801 Super Composite) has Fe3+/ΣFe of 0.51. Bulk Fe3+/ΣFe ratios of altered oceanic crust at Site 801 increase from 0.14 (pristine Jurassic-aged MORB glass) to 0.78 with increasing extent of alteration. We find that 68–95% of the O2 added to the subducting crust by sedimentation, in situ alteration of basaltic crust on the seafloor, and serpentinization of the mantle lithosphere is not output by Mariana arc or back-arc magmas. This result demonstrates that significant amounts of oxidized materials from Earth's surface are transported into the deeper mantle beyond subduction zones, despite the production of oxidized arc and back-arc basalts, and may contribute to elevated oxygen fugacities recorded by ocean island lavas such as Iceland and Hawaii. This oxygen cycle is likely to have been operating at least for the past 400–800 million years, and potentially for the duration of plate tectonics.

Elasticity of akimotoite under the mantle conditions: Implications for multiple discontinuities and seismic anisotropies at the depth of ∼600–750 km in subduction zones

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Shangqin Hao, Wenzhong Wang, Wangsheng Qian, Zhongqing Wu

Abstract

The equation of state and elastic properties of akimotoite at simultaneously high pressures and high temperatures are obtained using first-principles calculations based on the density functional theory (DFT). The calculated results agree with the available experimental data. Combining our results with the elastic data of other minerals, we estimated the VP, VS, and density contrasts caused by the akimotoite-related transitions. The velocity contrasts between akimotoite and bridgmanite are 4.6% and 8.3% for VP and VS, respectively, which are only about half of those between majorite and akimotoite. Moreover, because both the akimotoite-bridgmanite and majorite-akimotoite transitions have broad phase boundaries, these two phase transitions may not contribute to multiple discontinuities around ∼660 km depth in subduction zones as detected by seismic studies. Instead, the decomposition of pyrope into bridgmanite and corundum, which would occur in cold subduction zones with a sharp phase boundary and a large impedance contrast due to the inhibition of the pyroxene-garnet transformation at relatively low temperatures, could be a more reasonable explanation for the discontinuity at ∼700–750 km in subduction zones. Furthermore, the transformation from high-pressure clinopyroxene to akimotoite at the depth of ∼600 km can increase the VP, VS, and density by 10.1%, 14.8%, and 9.9%, respectively, indicating that the phase transition may account for the local discontinuity at ∼600 km in some subduction zones. In addition, the anisotropies of akimotoite are significantly higher than those of other major minerals at the base of the mantle transition zone and could be the origin of the seismic anisotropies detected in some subduction zones.

Slip partitioning along an idealized subduction plate boundary at deep slow slip conditions

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Melodie E. French, Cailey B. Condit

Abstract

Below the base of many subduction seismogenic zones, the plate interface periodically slips at rates 1 to 2 orders of magnitude faster than tectonic plate velocities. A number of competing hypotheses exist to explain the mechanisms for these slow slip events (SSEs), but they remain incompletely tested because we do not know how deformation is partitioned across the lithologically complex plate boundary interface. We use the deepest exposure of the Arosa zone, a ∼520 m-thick exhumed subduction interface, as a case study to evaluate the partitioning of strain between lithologic units throughout the SSE cycle. We review and synthesize published constitutive relations for the five lithologic units present to express shear stress as a function of deformation rate. We use these results to predict (1) the shear stress across the plate boundary as a function of slip velocity and (2) the partitioning of deformation among the different lithologic units for SSE and aseismic creep velocities. We conduct this analysis for pore fluid pressures from hydrostatic to near-lithostatic. Our results show that, at pore fluid pressure close to hydrostatic, aseismic creep and SSE velocities occur by viscous deformation of calcareous and quartzose units. However, once the pore fluid pressure increases above 80% of lithostatic, plate boundary slip migrates from the calcareous and quartzose rocks during aseismic creep to frictional deformation of talc schist during slow slip. This result is insensitive to differences in the thicknesses of metasedimentary units that may be present along subduction plate boundaries and, therefore, may apply to subduction plate boundaries in general.

Experimental evidence for wall-rock pulverization during dynamic rupture at ultra-high pressure conditions

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Sarah Incel, Alexandre Schubnel, Jörg Renner, Timm John, Loïc Labrousse, Nadège Hilairet, Helen Freeman, Yanbin Wang, François Renard, Bjørn Jamtveit

Abstract

The mechanisms triggering intermediate and deep earthquakes have puzzled geologists for several decades. There is still no consensus concerning whether such earthquakes are triggered by brittle or ductile mechanisms. We performed a deformation experiment on a synthetic lawsonite-bearing blueschist at a confining pressure of 3 GPa and temperatures from 583 to 1,073 K. After deformation, the recovered sample reveals conjugated shear fractures. Garnet crystals are dissected and displaced along these narrow faults and reveal micro- and nanostructures that resemble natural pulverization structures as well as partial amorphization. Formation of such structures at low confining pressures is known to require high tensile stresses and strain rates and is explained by the propagation of a dynamic shear rupture. The absence of shearing in the pulverized wall rock is taken as evidence that these structures pre-date the subsequent heat-producing frictional slip. In analogy to observations at low pressure we infer that the garnet structures in our experiment result from rapid propagation of a shear fracture even at the high pressure exerted on the sample and thus suggest that brittle deformation is possible at lower crustal to upper mantle depths.

Recycling reduced iron at the base of magmatic orogens

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Ming Tang, Cin-Ty A. Lee, Gelu Costin, Heidi E. Höfer

Abstract

The Earth's continental crust is thought to originate from melting of the mantle, but it is too felsic and depleted in Fe relative to a primary mantle melt. This depletion in Fe is also commonly found in continental arc magmas and is often attributed to magnetite crystallization. However, Fe depletion in arc magmas coincides with an enrichment in ferric Fe relative to ferrous Fe, which cannot be explained by removal of a ferric Fe-rich mineral like magnetite. Deep-seated garnet pyroxenite arc cumulates (arclogites) have Fe-rich compositions that complement the Fe-depleted nature of the continental crust and continental arc magmas, and are likely candidates for the “missing link” between basaltic mantle magmas and the felsic continental crust. To test this suggestion, we present high precision in-situ Fe valence data for garnets in arclogites and reconstruct whole rock Fe valence states. We show that arclogites have low bulk Fe3+/∑Fe due to the low Fe3+/∑Fe of garnets and the lack of magnetite. At high pressures, garnet crystallizes, but magnetite does not, the former causing preferential accumulation of ferrous Fe at the base of magmatic orogens. Arclogite fractionation thus leads to the formation of oxidized felsic residual liquids (Fe3+/∑Fe of 0.2-0.4). Such oxidation may profoundly influence the speciation of magmatic volatiles as well as the oxidative weathering capacity of the crust.

Where does subduction initiate and cease? A global scale perspective

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Martina M. Ulvrova, Nicolas Coltice, Simon Williams, Paul J. Tackley

Abstract

The thermo-mechanical evolution of the Earth's mantle is largely controlled by the dynamics of subduction zones, which connect the surface tectonic plates with the interior. However, little is known about the systematics of where subduction initiates and ceases within the framework of global plate motions and evolving continental configurations. Here, we investigate where new subduction zones preferentially form, and where they endure and cease using statistical analysis of large-scale simulations of mantle convection that feature self-consistent plate–like lithospheric behaviour and continental drift in the spherical annulus geometry. We juxtapose the results of numerical modelling with subduction histories retrieved from plate tectonic reconstruction models and from seismic tomography. Numerical models show that subduction initiation is largely controlled by the strength of the lithosphere and by the length of continental margins (for 2D models, the number of continental margins). Strong lithosphere favours subduction inception in the vicinity of the continents while for weak lithosphere the distribution of subduction initiation follows a random process distribution. Reconstructions suggest that subduction initiation and cessation on Earth is also not randomly distributed within the oceans, and more subduction zones cease in the vicinity of continental margins compared to subduction initiation. Our model results also suggest that intra-oceanic subduction initiation is more prevalent during times of supercontinent assembly (e.g. Pangea) compared to more recent continental dispersal, consistent with recent interpretations of relict slabs in seismic tomography.

The anatomy of uppermost mantle shear-wave speed anomalies in the western U.S. from surface-wave amplification

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Lewis Schardong, Ana M.G. Ferreira, Andrea Berbellini, William Sturgeon

Abstract

We build SWUS-amp, a three-dimensional shear-wave speed model of the uppermost mantle of the western U.S. using Rayleigh wave amplification measurements in the period range of 35–125 s from teleseismic earthquakes. This represents the first-ever attempt to invert for velocity structures using Rayleigh wave amplification data alone. We use over 350,000 Rayleigh wave amplitude measurements, which are inverted using a Monte Carlo technique including uncertainty quantification. Being a local seismic observable, Rayleigh wave amplification is little affected by path-averaged effects and in principle has stronger depth resolution than classical seismic observables, such as surface wave dispersion data. SWUS-amp confirms shallow mantle heterogeneities found in previous models. In the top 100 km of the mantle, we observe low-velocity anomalies associated with Yellowstone and the Basin & Range province, as well as a fast-velocity anomaly underneath the Colorado Plateau, where a strong velocity gradient at its edges shows a drastic contrast with its surroundings. SWUS-amp also gives additional insights into the current state of the uppermost mantle in the region. We image a high-velocity anomaly beneath the high-topography Wyoming province with a maximum depth extent of about 150–170 km, which is shallower than in previous tomographic models, and resolves previous inconsistencies with geological information. Beneath the Snake River Plain, a finger-like low-velocity anomaly dips to the west, suggesting lateral flow in the region. Below about 150 km depth, SWUS-amp shows a north-south dichotomy in shear-wave speed structure, with the northern region showing mostly high-velocity anomalies, whereas the southern region shows low-velocity anomalies. This is consistent with the continuous subduction history of the western U.S. and with the recent extension and uplift of the southern region.

Author Index

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s):

List of Referees

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s):

On the difference between real-time and research simulations with CTIPe

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s): Isabel Fernandez-Gomez, Mariangel Fedrizzi, Mihail V. Codrescu, Claudia Borries, Martin Fillion, Timothy J. Fuller-Rowell

Abstract

Understanding the thermosphere and ionosphere conditions is crucial for spacecraft operations and many applications using radio signal transmission (e.g. in communication and navigation). In this sense, physics based modelling plays an important role, since it can adequately reproduce the complex coupling mechanisms in the magnetosphere-ionosphere-thermosphere (MIT) system. The accuracy of the physics based model results does not only depend on the appropriate implementation of the physical processes, but also on the quality of the input data (forcing). In this study, we analyze the impact of input data uncertainties on the model results. We use the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics model (CTIPe), which requires satellite based solar wind, interplanetary field and hemispheric power data from ACE and TIROS/NOAA missions. To identify the impact of the forcing uncertainties, two model runs are compared against each other. The first run uses the input data that were available in real-time (operational) and the second run uses the best estimate obtained in post-processing (research or historical run). The analysis is performed in a case study on the 20th November 2003 extreme geomagnetic storm, that caused significant perturbations in the MIT system. This paper validates the thermosphere and ionosphere response to this storm over Europe comparing both CTIPe model runs with measurements of Total Electron Content (TEC) and thermosphere neutral density. In general, CTIPe results show a good agreement with measurements. However, the deviations between the model and observations are larger in the ionosphere than in the thermosphere. The comparison of the two model runs reveals that the deviations between model results and measurements are larger for the operational run than the research run. It is evident for the storm analyzed here, that data gaps in the input data are impacting considerably the model performance. The consistency between simulation and measurements allows the interpretation of the physical mechanisms behind the ionosphere perturbations and the changes in neutral composition during this event. Joule heating in the Auroral region, generating meridional winds and large scale surges, is suggested to be the main driver of the positive ionospheric storm over central Europe. In the polar cap and Auroral region, convection processes dominate the thermosphere-ionosphere conditions. This study does not only illustrate the importance of working with a good estimate of the model forcing, but also indicates the necessity of using measurements and models, to get a better understanding of the most likely responsible processes for the observed storm effects.

Performance analysis of quiet and disturbed time ionospheric TEC responses from GPS-based observations, IGS-GIM, IRI-2016 and SPIM/IRI-Plas 2017 models over the low latitude Indian region

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s): Kanaka Durga Reddybattula, Sampad Kumar Panda

Abstract

This paper focuses on the variability of ionospheric Total Electron Content (TEC) over low latitude Indian sub-continental region during the geomagnetically quiet and disturbed periods under the high solar activity phase (years 2012–2015) of the 24th solar cycle. The study is carried out with Global Positioning System (GPS) observations from four International Global Navigation Satellite System Service (IGS) stations, aligned in a longitudinal fashion from magnetic equatorial location to beyond Equatorial Ionization Anomaly (EIA) crest latitude across Indian longitude. The corresponding values are extracted by four-point Inverse Distance Weighted (IDW) interpolation of TEC from IGS-Global Ionosphere Maps (IGS-GIMs) and by running the globally acceptable empirical ionosphere models: International Reference Ionosphere (IRI-2016) and Standard Plasmasphere Ionosphere Model (SPIM/IRI-Plas 2017). Apart from the manifestations of broader daytime diurnal peak TEC, its double-humped structure, EIA, winter anomaly/seasonal anomaly, and storm-time alterations, the study tried to probe the model performances with respect to the GPS based TEC observations. While GIM-TEC portrays a systematically varying magnitude for a particular location, the underestimation (equatorial latitude) and overestimation (higher latitude) is being witnessed by IRI model in the diurnal and seasonal variations. However, SPIM-2017 manifests an all-time overestimated TEC irrespective of time and season though the variation pattern replicates the GPS-TEC observations. Concerning storm-time response, the sub-models embedded in IRI/SPIM hardly reveal a remarkable alteration in the regular TEC variation except for trivial divergences in the IRI outcomes. In spite of compelling timely improvements in climatological representations, the limited storm-time response in IRI/SPIM models attracts the attention for further improvements in the existing primitive storm model specifications. Moreover, the intentional ingestion of external GPS-TEC into the SPIM-2017 model resulted in an exceptional improvement (about 75%) in the model prediction with improved correlation coefficients. The results from this study would complement a realistic global climatological representation of ionosphere for a faithful estimation of ionospheric delay error in radio signal applications. Finally, our results emphasize the flexibility of scopes in SPIM-2017 model towards external ingestions of observed ionospheric parameters. The ingestion of optional inputs convolved with rescaling and optimization would significantly improve the model performances for global climatological as well as storm-time ionospheric representation.

A description of the Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE)

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s): R. Gardiner-Garden, M. Cervera, R. Debnam, T. Harris, A. Heitmann, D. Holdsworth, D. Netherway, B. Northey, L. Pederick, J. Praschifka, A. Quinn, M. Turley, A. Unewisse, B. Ward, G. Warne

Abstract

The Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE) was an extensive experiment undertaken by the Defence Science and Technology (DST) Group that focused on collecting ionospheric sensor data from multiple overlapping ionospheric paths in the Australian region in order to improve understanding of the characteristics of ionospheric variability and its effect on HF radio propagation. The experiment ran from July to October 2015 and included a period of three weeks of increased sample density and three days of dedicated over-the-horizon (OTH) radar operations. It was anticipated that ELOISE would sample a wide range of environmental conditions and present an opportunity to characterise periods of “normal variability” and periods of “exceptional variability” in the ionosphere.

This report is a general description of the aims of this experiment and the types of data collected. Particular interest focused on observing and measuring variability in ionospheric electron density gradients and their effect on oblique HF propagation. To this end, ELOISE established a pair of two dimensional HF receiver arrays to directly measure the oblique angle of arrival (AoA) on many overlapping oblique paths. ELOISE also established a dense sub-network of spatially separated quasi-vertical incidence soundings in the vicinity of Alice Springs in central Australia. This enabled a comparison of gradients observed in a dense network of vertical sounders with gradient effects observed in oblique propagation passing overhead. Several additional ionospheric observing systems were also used to give complementary pictures of the fine scale characteristics of ionospheric variability in the region.

Plain Language Summary

This paper is an overview of the 2015 Elevation sensitive Oblique Incidence Sounder Experiment (ELOISE), an experiment designed to observe and characterise mid-latitude ionospheric disturbances in the Australian region and understand their impact on high frequency (HF) signal propagation.

ELOISE involved the simultaneous operation of a large collection of ionospheric sounders enabling ionospheric variability to be characterised on a finely sampled large scale.

Particular efforts were made to provide direct high fidelity measurements of the angle of arrival (AoA) on many oblique HF propagation paths. These direct AoA measurements imply horizontal electron density gradients that can be compared to ionospheric gradients estimated from conventional models of the ionosphere derived from the spatial network of sounder sites.

The size and scope of the experiment are detailed in this paper and some preliminary results are presented.

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