Using UAV acquired photography and structure from motion techniques for studying glacier landforms: application to the glacial flutes at Isfallsglaciären

Glacier and ice sheet retreat exposes freshly deglaciated terrain which often contains small‐scale fragile geomorphological features which could provide insight into subglacial or submarginal processes. Subaerial exposure results in potentially rapid landscape modification or even disappearance of the minor‐relief landforms as wind, weather, water and vegetation impact on the newly exposed surface. Ongoing retreat of many ice masses means there is a growing opportunity to obtain high resolution geospatial data from glacier forelands to aid in the understanding of recent subglacial and submarginal processes. Here we used an unmanned aerial vehicle to capture close‐range aerial photography of the foreland of Isfallsglaciären, a small polythermal glacier situated in Swedish Lapland. An orthophoto and a digital elevation model with ~2 cm horizontal resolution were created from this photography using structure from motion software. These geospatial data was used to create a geomorphological map of the foreland, documenting moraines, fans, channels and flutes. The unprecedented resolution of the data enabled us to derive morphological metrics (length, width and relief) of the smallest flutes, which is not possible with other data products normally used for glacial landform metrics mapping. The map and flute metrics compare well with previous studies, highlighting the potential of this technique for rapidly documenting glacier foreland geomorphology at an unprecedented scale and resolution. The vast majority of flutes were found to have an associated stoss‐side boulder, with the remainder having a likely explanation for boulder absence (burial or erosion). Furthermore, the size of this boulder was found to strongly correlate with the width and relief of the lee‐side flute. This is consistent with the lee‐side cavity infill model of flute formation. Whether this model is applicable to all flutes, or multiple mechanisms are required, awaits further study. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Biogeomorphology of the Mediterranean Posidonia oceanica seagrass meadows

Here we review the multiple interactions between the endemic Mediterranean seagrass, Posidonia oceanica, and coastal geomorphologic processes as an outstanding example of biogeomorphology, taking into account recent advances in the field. Seagrass meadows are among the most important elements for the functioning of marine coastal ecosystems, and represent a major focus for research and conservation. Being considered a priority habitat, P. oceanica meadows are protected by several European Union directives and national laws. In this paper we examine: the role of sedimentary features in controlling the development of the meadows; the interplay between P. oceanica leaf litter (i.e. beached necromass) cast ashore and erosional‐depositional processes on the beaches; the interactions between meadows and nearshore hydrodynamics, and; possible linkages between geomorphological features of the seafloor and the architecture of meadows. Finally, we provide perspectives for future research on P. oceanica and other Mediterranean seagrass meadows in a biogeomorphological context with specific reference to climate change. Copyright © 2016 John Wiley & Sons, Ltd.     

The dynamics of mountain erosion: Cirque growth slows as landscapes age

Glacial cirques are widely used palaeoenvironmental indicators, and are key to understanding the role of glaciers in shaping mountain topography. However, notable uncertainty persists regarding the rate and timing of cirque erosion. In order to address this uncertainty, we analyse the dimensions of 2208 cirques in Britain and Ireland and model ice accumulation to investigate the degree of coupling between glacier occupation times and cirque growth. Results indicate that during the last ~120 ka, cirques were glacier-free for an average of 52.0 ± 21.2 ka (43 ± 18%); occupied by small (largely cirque-confined) glaciers for 16.2 ± 9.9 ka (14 ± 8%); and occupied by large glaciers, including ice sheets, for 51.8 ± 18.6 ka (43 ± 16%). Over the entire Quaternary (i.e. 2.6 Ma), we estimate that cirques were glacier-free for 1.1 ± 0.5 Ma; occupied by small glaciers for 0.3 ± 0.2 Ma; and occupied by large glaciers for 1.1 ± 0.4 Ma. Comparing occupation times to cirque depths, and calculating required erosion rates, reveals that continuous cirque growth during glacier occupation is unlikely. Instead, we propose that cirques attained much of their size during the first occupation of a non-glacially sculpted landscape (perhaps during the timeframe of a single glacial cycle). During subsequent glacier occupations, cirque growth may have slowed considerably, with the highest rates of subglacial erosion focused during periods of marginal (small glacier) glaciation. We propose comparatively slow rates of growth following initial cirque development because a ‘least resistance’ shape is formed, and as cirques deepen, sediment becomes trapped subglacially, partly protecting the bedrock from subsequent erosion. In support of the idea of rapid cirque growth, we present evidence from northern British Columbia, where cirques of comparable size to those in Britain and Ireland developed in less than 140 ka. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

KAGRA引力波探测器中蓝宝石测试镜光学性质研究

低温制冷技术是下一代激光干涉仪引力波探测器的核心技术之一. 日本引力波探测器KAGRA (Kamioka Gravitational Wave Detector)作为该技术的前沿开拓者, 将运行在20K的超低温环境中, 并使用在低温下热噪声较低的单晶蓝宝石晶体作为测试镜. 然而, 高质量大尺寸低吸收率的蓝宝石晶体极难制备. 此外, 由于蓝宝石晶体存在晶格结构不均匀, 很容易导致不必要的双折射效应, 从而影响探测器的目标灵敏度. 基于上述问题, 开发了两套大尺寸光学测量系统, 首次系统研究了KAGRA低温蓝宝石测试镜的光学特性. 首先, 根据探测器对测试镜热噪声的要求, 开发了一套基于光热共光路干涉技术的光学测量系统, 该系统可对测试镜以及测试镜表面涂层的光学吸收进行有效的表征. 其次, 基于光学吸收测量系统, 开发了一套双折射效应测量系统, 该系统可以有效表征测试镜中双折射的均匀性. 目前两套测量系统的搭建与调试已完成, 对蓝宝石测试镜光学吸收的测量灵敏度达到了1.5ppm/cm, 双折射测量系统的空间分辨率小于0.3mm times 0.3mm. 该工作对降低大尺寸低温测试镜双折射效应及提高探测器灵敏度具有重要意义.     

Magnetic reconnection: flares and coronal heating in active galactic nuclei

A magnetically structured accretion disc corona, generated by buoyancy instability in the disc, can account for observations of flare-like events in active galactic nuclei. We examine how Petschek magnetic reconnection, associated with MHD turbulence, can result in a violent release of energy and heat the magnetically closed regions of the corona up to canonical X-ray emitting temperatures. X-ray magnetic flares, the after effect of the energy released in slow shocks, can account for the bulk of the X-ray luminosity from Seyfert galaxies and consistently explain the observed short-time-scale variability.     

Drivers of interannual and intra‐annual variability of dissolved organic carbon concentration in the River Thames between 1884 and 2013

The world's longest record of river water quality (River Thames—130 years) provides a unique opportunity to understand fluvial dissolved organic carbon (DOC) concentrations dynamics. Understanding riverine DOC variability through long‐term studies is crucial to capture patterns and drivers influencing sources of DOC at scales relevant for decision making. The Thames basin (United Kingdom) has undergone massive land‐use change, as well as increased urbanisation and population during the period considered. We aimed to investigate the drivers of intra‐annual to interannual DOC variability, assess the variability due to natural and anthropogenic factors, and understand the causes for the increased DOC variability over the period. Two approaches were used to achieve these aims. The first method was singular spectrum analysis, which was used to reconstruct the major oscillatory modes of DOC, hydroclimatic variables, and atmospheric circulation patterns and to visualise the interaction between these variables. The second approach used was generalised additive modelling, which was used to investigate other non‐natural drivers of DOC variability. Our study shows that DOC variability increased by 80% over the data period, with the greatest increase occurring from the beginning of World War II onwards. The primary driver of the increase in DOC variability was the increase in the average value of fluvial DOC over the period of record, which was itself linked to the increase in basin population and diffuse DOC sources to the river due to land‐use and land‐management changes. Seasonal DOC variability was linked to streamflow and temperature. Our study allows to identify drivers of fluvial intra‐annual and interannual DOC variability and therefore empowers actions to reduce high DOC concentrations.     

Coesite in a Muong Nong‐type tektite from Muong Phin,Laos: Description,formation, and survival

We examined 16 white opaque inclusions exposed on two polished slices of a Muong Nong‐type Australasian tektite from Muong Phin, Laos. The inclusions usually consist of a core, surrounded by a froth layer, and a quartz neoblast layer. The cores are composed primarily of a mixture of silica glass, coesite, and quartz in varying proportions. A thin (up to ~4 μm) layer of SiO₂‐poor glass enriched in FeO, MgO, CaO, Al₂O₃, and TiO₂ is observed as a bright halo in backscattered electron images around the quartz neoblasts and in places contains μm‐sized crystals, which may be Fe,Mg‐rich spinel. The distribution and textural relationships between the coesite‐bearing inclusions and the tektite matrix point to an in situ formation of the coesite due to an impact, rather than to infall, from a nearby impact, into tektite melt produced by the aerial burst of a bolide. The quartz neoblasts probably formed by crystallization of silica melt squeezed out of the inclusion core during the development of the froth layer. The bright halo may be the result of silica diffusing from the adjacent tektite melt into the growing quartz neoblasts. We propose that the survival of coesite was possible due to the froth layer that acted as a heat sink during bubble expansion and then as a thermal insulator.     

Local structural relaxation around Co2+ along the hardystonite–Co-?kermanite melilite solid solution

Six pure compounds belonging to the hardystonite (Ca₂ZnSi₂O₇)–Co-?kermanite (Ca₂CoSi₂O₇) solid solution were investigated by the combined application of X-ray powder diffraction and electronic absorption spectroscopy. Structural refinements of the XRPD data revealed a negative excess volume of mixing due to the single isovalent substitution of Co for Zn in the tetrahedral site. In agreement with the diffraction data, deconvolution of the optical spectra showed a progressive decreasing of the crystal field strength parameter 10Dq moving toward the Co-?kermanite end-member, meaning that the local cobalt–oxygen bond distance, $ \langle {\text{Co}}{-}{\text{O}}\rangle^{\text{local}} $ , increased along the join with the amount of cobalt. The calculated structural relaxation coefficient around the fourfold coordinated Co²⁺ in the Ca₂(Zn_1?x Co _x )Si₂O₇ join was ε?=?0.69, very far from the one predicted by the Vegard’s law (ε?=?0) and at variance with ε?=?0.47 previously found for tetrahedrally coordinated Co²⁺ in gahnite–Co-aluminate spinel solid solution. This difference is consistent with the largest constraints existing on the spinel structure, based on cubic closest packing, compared to the more flexible layered melilite structure.     

Melilite-type and melilite-related compounds: structural variations along the join Sr2?xBaxMgSi2O7 (0?≤?x?≤?2) and high-pressure behavior of the two end-members

The structural variations along the solid solution Sr_2−x Ba _x MgSi₂O₇ (0 ≤ x ≤ 2), combined to the high-pressure characterization of the two end-members, have been studied. A topological change from the tetragonal (melilite-type) to the monoclinic (melilite-related) structure along the join Sr₂MgSi₂O₇ (e.g., P[`4]2₁ m P\bar{4}2_{1} m )–Ba₂MgSi₂O₇ (e.g., C2/c) occurs with a Ba content higher than 1.6 apfu. Favored in the crystallization from a melt, the tetragonal form has a tetrahedral sheet topology exclusively based on five-membered rings, which provide a regular “4 up + 4 down” ligand arrangement. In contrast, the melilite-related structure, favored by solid-state reaction synthesis, is made by alternating six- and four-membered tetrahedral rings, which give an asymmetric arrangement of alternated “5 up + 3 down” and “3 up + 5 down” ligands around Sr or Ba. This latter configuration is characterized by an additional degree of freedom with Ba polyhedra hosted in the interlayer with a more irregular and compact coordination and longer Ba–O bond distances. Further insights into the relationships between the two melilite typologies were achieved by investigating the in situ high-pressure behavior of these systems. The synchrotron high-pressure experiments allowed to calculate the elastic moduli for the Sr melilite-type end-member and for the Ba monoclinic polymorph (Sr₂MgSi₂O₇: K _T0 = 107, K _a=b  = 121, and K _c  = 84 GPa; m-Ba₂MgSi₂O₇: K _T0 = 85, K _a  = 96, K _b  = 72, and K _c  = 117 GPa) and compare them with those reported in the literature for ?kermanite (Ca₂MgSi₂O₇). The results show that, although the volume of Ba polyhedron in tetragonal polymorphs is larger than in the monoclinic forms, the interlayer compressibility is significantly lower in the former structures due to the occurrence of very short Ba–O distances. This unfavored Ba environment also makes tetragonal Ba₂MgSi₂O₇ a metastable phase at room conditions, possibly favored by high pressure. However, no phase transition occurs from monoclinic to tetragonal form due to kinetic hindrance in reconstructing the sheet topology.