Implications of decadal to century scale glacio‐hydrological change for water resources of the Hood River basin,OR, USA

In glacier‐fed rivers, melting of glacier ice sustains streamflow during the driest times of the year, especially during drought years. Anthropogenic and ecologic systems that rely on this glacial buffering of low flows are vulnerable to glacier recession as temperatures rise. We demonstrate the evolution of glacier melt contribution in watershed hydrology over the course of a 184‐year period from 1916 to 2099 through the application of a coupled hydrological and glacier dynamics model to the Hood River basin in Northwest Oregon, USA. We performed continuous simulations of glaciological processes (mass accumulation and ablation, lateral flow of ice and heat conduction through supra‐glacial debris), which are directly linked with seasonal snow dynamics as well as other key hydrologic processes (e.g. evapotranspiration and subsurface flow). Our simulations show that historically, the contribution of glacier melt to basin water supply was up to 79% at upland water management locations. We also show that supraglacial debris cover on the Hood River glaciers modulates the rate of glacier recession and progression of dry season flow at upland stream locations with debris‐covered glaciers. Our model results indicate that dry season (July to September) discharge sourced from glacier melt started to decline early in the 21st century following glacier recession that started early in the 20th century. Changes in climate over the course of the current century will lead to 14–63% (18–78%) reductions in dry season discharge across the basin for IPCC emission pathway RCP4.5 (RCP8.5). The largest losses will be at upland drainage locations of water diversions that were dominated historically by glacier melt and seasonal snowmelt. The contribution of glacier melt varies greatly not only in space but also in time. It displays a strong decadal scale fluctuations that are super‐imposed on the effects of a long‐term climatic warming trend. This decadal variability results in reversals in trends in glacier melt, which underscore the importance of long‐time series of glacio‐hydrologic analyses for evaluating the hydrological response to glacier recession. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of azimuthal multipath asymmetry on long GPS coordinate time series

Carrier phase multipath is currently one source of unmodeled signals that may bias GPS coordinate time series significantly. We investigate the effect of simulated carrier phase multipath on time series of several sites covering the period 2002.0–2008.0 and spanning a range of observation geometries. High-, mid-, and low-latitude IGS sites are investigated as well as sites with significant signal obstructions. We examine the effect of multipath in different sectors of the sky, considering time-constant, horizontal reflectors at each of 0.1, 0.2, and 1.5 m below the antenna. The differences between a horizontally uniform multipath source are analyzed, and it is shown that positioning errors are generally larger when unmodeled carrier phase multipath is azimuthally heterogeneous. Using the adopted multipath model, height biases reach ±1 mm in case of the symmetric multipath and ±5 mm for the asymmetric multipath but this increases to being ±10 mm in the worst case. In addition to mean bias, low-frequency variations in the bias also exist, including periodic signals and leading to velocity biases of up to ±0.1 mm/year in the symmetric case and ±1 mm/year in the asymmetric case over the considered period. In contrast to the generally slowly varying observation geometry that is typically experienced, we show the effects of an abrupt change in geometry due to receiver/antenna hardware changes; in the case considered, we see changed pattern of temporal variation in the bias in addition to an instantaneous offset.     

The effect of silica activity on the diffusion of Ni and Co in olivine

The diffusion of Ni and Co was measured at atmospheric pressure in synthetic monocrystalline forsterite (Mg₂SiO₄) from 1,200 to 1,500 °C at the oxygen fugacity of air, along [100], with the activities of SiO₂ and MgO defined by either forsterite + periclase (fo + per buffer) or forsterite + protoenstatite (fo + en buffer). Diffusion profiles were measured by three methods: laser-ablation inductively-coupled-plasma mass-spectrometry, nano-scale secondary ion mass spectrometry and electron microprobe, with good agreement between the methods. For both Ni and Co, the diffusion rates in protoenstatite-buffered experiments are an order of magnitude faster than in the periclase-buffered experiments at a given temperature. The diffusion coefficients D _M (M = Ni or Co) for the combined data set can be fitted to the equation:

$$\log \,D_{\text{M}} \,\left( {{\text{in}}\,{\text{m}}^{2} \,{\text{s}}^{ - 1} } \right) = - 6.77( \pm 0.33) + \Delta E_{\text{a}} (M)/RT + 2/3\log a_{{SiO_{2} }}$$

with E_a(Ni) = ? 284.3 kJ mol^?1 and E_a(Co) = ? 275.9 kJ mol^?1, with an uncertainty of ±10.2 kJ mol^?1. This equation fits the data (24 experiments) to ±0.1 in log D _M. The dependence of diffusion on \(a_{{{\text{SiO}}_{2} }}\) is in agreement with a point-defect model in which Mg-site vacancies are charge-balanced by Si interstitials. Comparative experiments with San Carlos olivine of composition Mg_1.8Fe_0.2SiO₄ at 1,300 °C give a slightly small dependence on \(a_{{{\text{SiO}}_{2} }}\), with D \(\propto\) (\(a_{{{\text{SiO}}_{2} }}^{0.5}\)), presumably because the Mg-site vacancies increase with incorporation of Fe³⁺ in the Fe-bearing olivines. However, the dependence on fO₂ is small, with D \(\propto\) (fO₂)^0.12±0.12. These results show the necessity of constraining the chemical potentials of all the stoichiometric components of a phase when designing diffusion experiments. Similarly, the chemical potentials of the major-element components must be taken into account when applying experimental data to natural minerals to constrain the rates of geological processes. For example, the diffusion of divalent elements in olivine from low SiO₂ magmas, such as kimberlites or carbonatites, will be an order of magnitude slower than in olivine from high SiO₂ magmas, such as tholeiitic basalts, at equal temperatures and fO₂.

     

The accretion history of the universe with the SKA

In this paper, we investigate how the Square Kilometre Array (SKA) can aid in determining the evolutionary history of active galactic nuclei (AGN) from redshifts z = 0 → 6. Given the vast collecting area of the SKA, it will be sensitive to both ‘radio-loud’ AGN and the much more abundant ‘radio-quiet’ AGN, namely the radio-quiet quasars and their ‘Type-II’ counterparts, out to the highest redshifts. Not only will the SKA detect these sources but it will also often be able to measure their redshifts via the Hydrogen 21-cm line in emission and/or absorption. We construct a complete radio luminosity function (RLF) for AGN, combining the most recent determinations for powerful radio sources with an estimate of the RLF for radio-quiet objects using the hard X-ray luminosity function of [ApJ 598 (2003) 886], including both Type-I and Type-II AGN. We use this complete RLF to determine the optimal design of the SKA for investigating the accretion history of the Universe for which it is likely to be a uniquely powerful instrument.     

Nuclear obscuration in the high-ionization Seyfert 2 galaxy Tol 0109–383

We report on the BeppoSAX detection of a hard X-ray excess in the X-ray spectrum of the classical high-ionization Seyfert 2 galaxy Tol 0109–383. The X-ray emission of this source observed below 7 keV is dominated by reflection from both cold and ionized gas, as seen in the ASCA data. The excess hard X-ray emission is presumably caused by the central source absorbed by an optically thick obscuring torus with N _H∼2×10²⁴ cm⁻² . The strong cold X-ray reflection, if it is produced at the inner surface of the torus, is consistent with the picture where much of the inner nucleus of Tol 0109–383 is exposed to direct view, as indicated by optical and infrared properties. However, the X-ray absorption must occur at small radii in order to hide the central X-ray source but leave the optical high-ionization emission-line region unobscured. This may also be the case for objects such as the Seyfert 1 galaxy Mrk231.     

Holocene vegetation and climate change in southern New Zealand: linkages between forest composition and quantitative surface moisture reconstructions from an ombrogenous bog

This paper presents a Holocene pollen record from an ombrotrophic bog in Southland, New Zealand, together with multiproxy data (testate amoebae, peat humification and plant macrofossils) from the same core to establish an independent semiquantitative record of peatland surface moisture. Linkages between reconstructed peatland surface moisture and regional forest composition are investigated using redundancy analysis of the forest pollen data constrained with predicted bog water‐table depths. Over 32% of the pollen data variance can be explained by surface moisture changes in the bog, suggesting a common cause of water‐table and regional vegetation change. Water tables were higher during the early to mid‐Holocene when the forest was dominated by podocarp taxa. Water tables lowered after about 3300 cal. yr BP coevally with the expansion of Nothofagus species, culminating with the dominance of Nothofagus subgenus Fuscospora in the past 1200 cal. yr BP. This is in apparent opposition to the warm/dry to cool/wet trend suggested by subjective interpretation of pollen data alone, from this and other studies. We suggest that during the late Holocene, drier summers associated with shifts in solar insolation caused reduced surface wetness and summer humidity, which together with a trend to cooler winters, apparently favoured the regeneration of Nothofagus species. Copyright © 2002 John Wiley & Sons, Ltd.     

Rockfall risk management using a pre-failure deformation database

Terrestrial laser scanning (TLS) monitoring has been used to estimate the location, volume, and kinematics of a variety of small magnitude rockfalls before failure (1–1000 m³ range), and in some cases, potential failure time has been assessed through the application of inverse velocity methods. However, our current understanding of rock slope pre-failure behavior for this magnitude range and prediction ability is based on observations of a small number of failure case histories. In this study, a pre-failure deformation database was constructed for rockfall volumes exceeding 0.1 m³, observed over a 1252-day study interval at the Goldpan rock slope, British Columbia, Canada, in order to better understand the pre-failure behavior of rock slopes and provide an empirical means of estimating temporal failure ranges. Repeated TLS datasets were acquired at an average scanning interval of 2–3 months. A total of 90 rockfall events were recorded at this site, during this time period, of which 64 (71%) exhibited measurable deformation prior to failure. Classification of rockfalls by volume suggests that a scale dependency may exist, as deformation was detected for a greater proportion of rockfalls >?5 m³ (92%) than for smaller rockfalls in the range of 0.1–0.5 m³ (61%). A lower rate of pre-failure deformation detection was also reported for planar sliding failures as compared with wedge or toppling failures, suggesting that deformation was less easily detected for these failure types. This study proposes and implements a framework for rockfall assessment and forecasting that does not require continuous monitoring of deformation.