Origin of stratified basal ice in outlet glaciers of Vatnajökull and Öræfajökull,Iceland

Larson, G.J., Lawson, D.E., Evenson, E.B., Knudsen, Ó., Alley, R.B. & Phanikumar, M.S. 2010: Origin of stratified basal ice in outlet glaciers of Vatnajökull and Öræfajökull, Iceland. Boreas, Vol. 39, pp. 457–470. 10.1111/j.1502‐3885.2009.00134.x. ISSN 0300‐9483. During the period 2000–2005, we collected samples of englacial ice, vent water, frazil/anchor ice and stratified basal ice from warm‐based outlet glaciers of Vatnajökull and Öræfajökull, Iceland, and analysed them for ³H, ¹⁸O and D. Results of ³H analyses show that the stratified basal ice contains ³H from atmospheric thermonuclear testing and is younger than the englacial ice. Results of the ¹⁸O and D analyses show that frazil/anchor and stratified basal ice are both enriched by an average of 2.4‰ in ¹⁸O and 11‰ in D relative to vent water. These values are consistent with fractionation during partial freezing of supercooled subglacial water in an open system, one in which the remaining water is continuously removed and replenished by water of similar composition. The isotopic data and field observations do not support either a regelation or a thermal ad‐freeze‐on origin for the stratified basal ice.     

Landsystems of Morsárjökull,Skaftafellsjökull and Svínafellsjökull,outlet glaciers of the Vatnajökull Ice Cap,Iceland

This study utilizes a landsystem approach to analyse the landforms and sediments exposed on the forefields of three closely spaced outlet glaciers of the Vatnajökull Ice Cap, southeast Iceland; Morsárjökull, Skaftafellsjökull and Svínafellsjökull, in order to determine how individual glacier and environmental characteristics influence landscape development. Analysis of satellite imagery and fieldwork were used in conjunction to examine the geomorphology and sedimentology of the forefields, and to define the characteristic landsystems of each of the glaciers. Morsárjökull and Skaftafellsjökull have similar proglacial fields, with similarities in the distribution and scale of the landforms, and their characteristics conform to the established active temperate landsystem. Svínafellsjökull differs significantly from the other glaciers having a proglacial field that more closely resembles an early stage debris‐charged landsystem. Variations between the glaciers in terms of their ice distribution (hypsometry, equilibrium line altitude), bedrock type, topography and debris content are important factors that contribute to the landsystem variability evident in their proglacial fields. The forefields of these three glaciers may be used as analogues to enhance understanding of palaeoenvironmental conditions that existed along the southern margin of Pleistocene glaciers that covered much of northern North America and Europe in the past.     

闵江“98·6”特大暴雨洪水分析

就1998年6月闽江发生的本世纪内最大一场暴雨洪水的天气成因、发展过程及特性作了简要的介绍与分析."98·6"洪水是由历时17天的暴雨过程形成的,雨峰多且相互衔接,从而形成这场峰高、量大、多峰的特大洪水过程.     

Late Quaternary palæoecology and palæoclimates of the eastern Sahara

Latest field research and palæoenvironmental reconstructions have revealed that within less than 6000 years the eastern Sahara experienced a dramatic climatic change similar to that in the western Sahara, passing from hyperaridity to semi-aridity (dry savanna) to its present hyperarid state. Groundwater levels started to rise about 9300 years before present (¹⁴C years BP), leading to the formation of a mosaic of freshwater lakes and swamps. Within a few decades, the aquifers were loaded and the palæopiezometric surface was as much as 25 m higher than it is today. The uplands generated up to 800 km long fluvial systems, which put an end to the endorheic drainage of the region and functioned as migration paths for large savanna mammals. These wetter conditions persisted in Western Nubia during the Holocene until ca 5000 years BP The climatic deterioration began around 5700 years BP as shown by evaporitic sediments. Reversal events prior to aridification during the Late Holocene were not recorded systematically in the sediments of the eastern Sahara because of the stability of the ecosystems. Changes in land-surface conditions such as palæolakes, swamps and vegetation created water vapour sources that generated local rainfall and buffered short dry spells. Radiocarbon-dated charcoal indicates that Neolithic human occupation culminated during this Early Holocene wet phase and ended ca 2000 years after the fading of the wet phase at about 3000 years BP, when the shallow aquifers were exhausted.     

The palaeomagnetism and geology of the Patreksfjördur-Arnarfjördur region of Northwest Iceland

The headlands between the fjords Arnarfjördur and Patreksfjördur in northwest Iceland, consisting mostly of Upper Tertiary plateau basalt lavas, have been geologically mapped in detail. Magnetic properties of samples from some 50 lava flows and dykes in the region have been measured; the mean palaeomagnetic pole position obtained from forty normally magnetized lavas is at 88°N113°E. The application of the present work to stratigraphic correlation and magnetic anomaly interpretation in northwest Iceland is discussed.     

Saalian to Weichselian stratigraphy and sedimentation along the Lågjæren – Høgjæren escarpment,southwest Norway

Quaternary sediments along a profile crossing the southern part of the Jæren escarpment, southwestern Norway, have been investigated with regard to their glacial history and sea-level variations. Deposits from at least three glaciations and two ice-free periods between Oxygen Isotope Stage 6 and the Late Weichselian have been identified. Subglacial till directly overlain by a glaciomarine regressional succession indicates a deglaciation, and amino acid ratios in Elphidium excavatum between 0.083 and 0.118 date this event to Oxygen Isotope Stage 6. Sea-level dropped from 130 to below 110 m a.s.l. Subsequently, a short-lived ice advance deposited a marginal moraine and a sandur locally on the escarpment. Stratigraphical position and luminescence dates around 148 ka BP suggest deposition during the final stage 6 deglaciation. A Late Weichselian till covers most of the surface of Jæren. In addition to a well documented westerly ice flow, glaciotectonic indications of ice flow towards the north have been found. Ice flow directions and a hiatus between Oxygen Isotope Stage 6 and the Weichselian indicate enhanced erosion along the escarpment and the influence of a Norwegian Channel ice-stream. Copyright © 1999 John Wiley & Sons, Ltd.     

Atomistic model of diopside-K-jadeite (CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-KAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript>) solid solution

Atomistic model was proposed to describe the thermodynamics of mixing in the diopside-K-jadeite solid solution (CaMgSi₂O6-KAlSi₂O₆). The simulations were based on minimization of the lattice energies of 800 structures within a 2 × 2 × 4 supercell of C2/c diopside with the compositions between CaMgSi₂O₆ and KAlSi₂O₆ and with variable degrees of order/disorder in the arrangement of Ca/K cations in M2 site and Mg/Al in Ml site. The energy minimization was performed with the help of a force-field model. The results of the calculations were used to define a generalized Ising model, which included 37 pair interaction parameters. Isotherms of the enthalpy of mixing within the range of 273–2023 K were calculated with a Monte Carlo algorithm, while the Gibbs free energies of mixing were obtained by thermodynamic integration of the enthalpies of mixing. The calculated T-X diagram for the system CaMgSi₂O₆-KAlSi₂O₆ at temperatures below 1000 K shows several miscibility gaps, which are separated by intervals of stability of intermediate ordered compounds. At temperatures above 1000 K a homogeneous solid solution is formed. The standard thermodynamic properties of K-jadeite (KAlSi₂O₆) evaluated from quantum mechanical calculations were used to determine location of several mineral reactions with the participation of the diopside-K-jadeite solid solution. The results of the simulations suggest that the low content of KalSi₂O₆ in natural clinopyroxenes is not related to crystal chemical factors preventing isomorphism, but is determined by relatively high standard enthalpy of this end member.     

LiFeSi<Subscript>2</Subscript>O<Subscript>6</Subscript> and NaFeSi<Subscript>2</Subscript>O<Subscript>6</Subscript> at low temperatures: an infrared spectroscopic study

 Synthetic aegirine LiFeSi₂O₆ and NaFeSi₂O₆ were characterized using infrared spectroscopy in the frequency range 50–2000 cm⁻¹, and at temperatures between 20 and 300 K. For the C2/c phase of LiFeSi₂O₆, 25 of the 27 predicted infrared bands and 26 of 30 predicted Raman bands are recorded at room temperature. NaFeSi₂O₆ (with symmetry C2/c) shows 25 infrared and 26 Raman bands. On cooling, the C2/c–P2₁/c structural phase transition of LiFeSi₂O₆ is characterized by the appearance of 13 additional recorded peaks. This observation indicates the enlargement of the unit cell at the transition point. The appearance of an extra band near 688 cm⁻¹ in the monoclinic P2₁/c phase, which is due to the Si–O–Si vibration in the Si₂O₆ chains, indicates that there are two non-equivalent Si sites with different Si–O bond lengths. Most significant spectral changes appear in the far-infrared region, where Li–O and Fe–O vibrations are mainly located. Infrared bands between 300 and 330 cm⁻¹ show unusually dramatic changes at temperatures far below the transition. Compared with the infrared data of NaFeSi₂O₆ measured at low temperatures, the change in LiFeSi₂O₆ is interpreted as the consequence of mode crossing in the frequency region. A generalized Landau theory was used to analyze the order parameter of the C2/c–P2₁/c phase transition, and the results suggest that the transition is close to tricritical. Received: 21 January 2002 / Accepted: 22 July 2002     

Comment on Analyses of Seismic Deformation at the Kibyra Roman Stadium,Southwest Turkey by Volkan Karabacak, Önder Yönlü, Eray Dökü, Nafiye Günenç Kıyak,Erhan Altunel, Şükrü Özüdoğru,Cahit Çağlar Yalçıner,and Hüsnü Serdar Akyüz

The ancient city of Kibyra, which has been damaged by ancient earthquakes, is located on the middle part of Burdur‐Fethiye Fault Zone (SW Turkey). The existence of an active NNE‐SSW‐trending left‐lateral fault cutting the stadium of Kibyra has been previously discussed in many publications (Akyüz and Altunel, 1997 , 2001 ; Karabacak, 2011 ). Karabacak et al. ( 2013 ) also adhere to this view and restate it in the recent article published in Geoarchaeology. However, there is no evidence directly indicating a fault cutting the stadium and the earthquake damage to the stadium was most likely caused by ground shaking. In conclusion, the arguments, observations and interpretations presented by Karabacak et al. ( 2013 ) are misleading in key respects and must be reviewed.     

Type-II xenoliths and related metasomatism from the Nógrád-Gömör Volcanic Field, Carpathian-Pannonian region (northern Hungary–southern Slovakia)

Xenoliths from the upper mantle and lower crust are abundant in Plio–Pleistocene alkali basalts of the Nógrád-Gömör Volcanic Field (NGVF; northern Pannonian Basin, northern Hungary/southern Slovakia), representing a valuable ‘probe’ of lithospheric structures and processes. Ultramafic xenoliths have been divided into two groups: (1) Type-I, composed mostly of olivine with subsidiary orthopyroxene, clinopyroxene and spinel, and (2) Type-II, containing mostly Al- and Ti-rich clinopyroxene with subordinate olivine, spinel and plagioclase. Both types often contain amphibole and, to a lesser extent, mica. The refractory character of Type-I xenoliths suggests they represent mantle depleted by prior episodes of partial melting. In contrast, Type-II series (wehrlites, olivine clinopyroxenites, clinopyroxenites and plagioclase-bearing ultramafic lithologies), on the basis of their textural features, thermobarometric histories and major and trace element variation, appear to have formed as magmatic cumulates. Petrologic and geochemical studies of Type-II xenoliths from Nógrád-Gömör suggest they crystallized from basaltic melts emplaced within the lithospheric mantle and lower crust, prior to the onset of Plio–Pleistocene volcanic activity. After their consolidation, metasomatic agents reacted with the anhydrous cumulate phases producing amphiboles and micas at the expense of olivine and clinopyroxene. The metasomatic agents appear to have been adakitic rather than basaltic in composition, possibly linked to a retreating arc–forearc system. Large-scale contamination of the lithospheric mantle can therefore be attributed to fluid and melt fractions related to subduction beneath the outer Carpathian arc.     

On Fröhlich's solution for Boussinesq's problem

The concentration factor introduced by O. K. Fröhlich is visualized as a procedure for examining the pattern of load transfer from surface loads to the interior of a geomaterial. The historical details that led to the introduction of the concentration factor are scant although it is widely used in the area of soil mechanics problems associated with tillage‐induced soil compaction. The purpose of this note is to examine the concentration factor in terms of the geomechanics of an elastic continuum and to identify the precise conditions that are satisfied by the distribution of stresses and strains that accommodate the concentration factor. Copyright © 2013 John Wiley & Sons, Ltd.     

Magnetic order in acmite; NaFeSi2O6

The magnetic properties of two samples of acmite, one natural and the other synthetic, were determined using magnetization and susceptibility measurements, Mössbauer spectroscopy and neutron diffraction. Exchange interactions are quite strongly antiferromagnetic, the paramagnetic Curie temperature being -46 K for a purely ferric synthetic sample, but its Néel temperature is only 8 K. The principal magnetic mode has the periodicity of the crystallographic structure and is made of ferromagnetic chains, coupled antiferromagnetically. Moments are oriented in a direction close to the chain axis, c. The antiferromagnetic exchange between adjacent Fe³⁺ ions in the same chain is overcome by their coupling to a common Fe³⁺ neighbour in the next chain. This indicates that the whole (SiO₄) group can act as a superexchange ligand in silicates.     

Geochemistry, petrogenesis, and geodynamic typification of metavolcanics of the Tunka terrane (Baikal–Hövsgöl region)

The geochemical composition of metavolcanics (metabasalts and metaandesites) of the Tunka terrane is considered. They are differently enriched in incompatible elements relative to N-MORB. The geochemical features of the basalts (LILE and LREE enrichment, Zr and Hf negative anomalies) point to the suprasubductional nature of synsedimentary volcanism; at the same time, the rocks lack negative Nb, Ta, and Ti anomalies. Comparison of the Tunka metavolcanics with those of modern geodynamic settings shows their analogy to back-arc basin basalts.     

Sedimentation and stratigraphy at Eyjabakkajökull—An Icelandic surging glacier

A model for sedimentation by surging glaciers is developed from analysis of the debris load, sedimentary processes, and proglacial stratigraphy observed at the Icelandic surging glacier, Eyjabakkajökull. Three aspects of the behavior of surging glaciers explain the distinctive landformsediment associations which they may produce: (a) sudden loading of proglacial sediments during rapid glacier advances results in the buildup of excess pore pressures, failure, and glacitectonic deformation of the overridden sediments; (b) reactivation of stagnant marginal ice by the downglacier propagation of surges is associated with large longitudinal compressive stresses. These induce intense folding and thrusting during which basal debris-rich ice is elevated into an englacial position in a narrow marginal zone. As the terminal area of the glacier stagnates between surges, debris from this ice is released supraglacially and deposited by meltout and sediment flows; (c) local variations in overburden pressure beneath stagnant, crevassed ice cause subglacial lodgement tills, which are sheared during surges, to flow into open crevasses and form “crevasse-fill” ridges.     

Thermodynamics of mixing in diopside–jadeite,CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>–NaAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript>, solid solution from static lattice energy calculations

Static lattice energy calculations (SLEC), based on empirical interatomic potentials, have been performed for a set of 800 different structures in a 2 × 2 × 4 supercell of C2/c diopside with compositions between diopside and jadeite, and with different states of order of the exchangeable Na/Ca and Mg/Al cations. Excess static energies of these structures have been cluster expanded in a basis set of 37 pair-interaction parameters. These parameters have been used to constrain Monte Carlo simulations of temperature-dependent properties in the range of 273–2,023 K and to calculate a temperature–composition phase diagram. The simulations predict the order–disorder transition in omphacite at 1,150 ± 20°C in good agreement with the experimental data of Carpenter (Mineral Petrol 78:433–440, 1981). The stronger ordering of Mg/Al within the M1 site than of Ca/Na in the M2 site is attributed to the shorter M1–M1 nearest-neighbor distance, and, consequently, the stronger ordering force. The comparison of the simulated relationship between the order parameters corresponding to M1 and M2 sites with the X-ray refinement data on natural omphacites (Boffa Ballaran et al. in Am Mineral 83:419–433, 1998) suggests that the cation ordering becomes kinetically ineffective at about 600°C.     

Blueschist facies metamorphism in Nordenskiöld Land of west‐central Svalbard

Recent fieldwork in Nordenskiöld Land, Svalbard's Southwestern Basement Province, has established the presence of high‐pressure (HP) lithologies. They are strongly retrogressed blueschists consisting mainly of garnet and Ca‐amphibole with remnants of ferroglaucophane and phengite. The pressure–temperature (P–T) conditions were estimated using phase equilibrium modelling in the NCKFMMnASHTO system. P–T estimates based on the garnet, phengite and ferroglaucophane compositional isopleths and modelled paragenetic assemblage indicate peak metamorphism at 470–490 °C and 14–18 kbar. These data fall close to the 7–8 °C km^?1 geotherm, which is similar to that from Motalafjella, the only previously known occurrence of blueschists in Svalbard's Caledonides. The newly discovered blueschists could have formed during the early stage of the Caledonian Orogeny and may represent a vestige of missing marginal basins of the western Iapetus developed at the onset of subduction. The likely counterpart to Svalbard's blueschists is the ophiolitic sequence in the Pearya Terrane of northern Ellesmere Island.