Role of glaciohydraulic supercooling in the formation of stratified facies basal ice: Svínafellsjökull and Skaftafellsjökull, southeast Iceland

Cook, S. J., Robinson, Z. P., Fairchild, I. J., Knight, P. G., Waller, R. I. & Boomer, I. 2009: Role of glaciohydraulic supercooling in the formation of stratified facies basal ice: Svínafellsjökull and Skaftafellsjökull, southeast Iceland. Boreas, 10.1111/j.1502‐3885.2009.00112.x. ISSN 0300‐9483. There is need for a quantitative assessment of the importance of glaciohydraulic supercooling for basal ice formation and glacial sediment transfer. We assess the contribution of supercooling to stratified facies basal ice formation at Svínafellsjökull and Skaftafellsjökull, southeast Iceland, both of which experience supercooling. Five stratified basal ice subfacies have previously been identified at Svínafellsjökull, but their precise origins have not been determined. Analysis of stratified basal ice stable isotope compositions (δ¹⁸O and δD), spatial distribution and physical characteristics demonstrates that two subfacies present at both glaciers are consistent with supercooling. These ‘supercool’ subfacies account for 42% of stratified facies exposed at Svínafellsjökull, although estimates at Skaftafellsjökull are precluded by limited basal ice exposure. Owing to their high debris contents, supercooling‐related facies contribute a debris flux of 4.8 to 9.6 m³ m^?1 a^?1 at Svínafellsjökull (83% of the stratified facies debris flux). Other stratified subfacies, formed by non‐supercooling processes, account for 58% of the stratified basal ice at Svínafellsjökull, but only contribute a debris flux of 1.0 to 2.0 m³ m^?1 a^?1 (17% of the stratified facies debris flux). We conclude that supercooling has a significant role in glacial sediment transfer, although in stratified basal ice formation its role is less significant at these locations than has been reported elsewhere.     

A seasonally ''dry'' interglacial climate in eastern England during the early Middle Pleistocene: palaeopedological and stable isotopic evidence from Pakefield, UK

The pedology and the isotope geochemistry of the early Middle Pleistocene interglacial 'rootlet bed' at Pakefield, eastern England, record the climate when this unit accumulated. Horizons of nodular pedogenic calcrete indicate the existence of a strongly seasonal precipitation regime that produced a pronounced net soil moisture deficit during which secondary carbonate was precipitated. The isotopic composition of these nodules confirms the seasonality of annual precipitation, as this indicates that intense soil moisture evaporation was a major mechanism in secondary carbonate formation. The climatic interpretation is enhanced by the palaeoecology of the unit that indicates average summer temperatures significantly warmer than those of the present. The climate of this period was, therefore, different from that of the present day in terms of both temperature and the seasonality of precipitation. Although it has previously been shown that certain interglacial periods in northwest Europe were warmer than the Holocene, this is the first study to show that the precipitation regime of one such episode may also have been significantly different.     

IDENTIFICATION OF CLIMATE CONTROLS ON THE DYNAMIC BEHAVIOUR OF THE SUBARCTIC GLACIER SALAJEKNA, NORTHERN SCANDINAVIA

ABSTRACT. In this paper we describe the dynamic behaviour of Salajekna, a valley glacier, over the last 200 years using terrestrial observations, in situ measurements, remote sensing observations and glacier reconstructions. The response time of the glacier was calculated using analytical models and field measurements. We were subsequently able to attribute specific dynamic responses to climate trends in the available climate record. The glacier's historical maximum extension was reached around 1880–1910 and was the result of a more continental climate with multi-modal airflows in the late 18th and early 19th centuries. A transition to more maritime conditions in the mid-19th century resulted in a near-continuous 20th century retreat before the glacier adjusted to a near-steady state.     

Metamorphic P-T Paths from Cordillera Darwin, a Core Complex in Tierra del Fuego, Chile

P–T conditions and prograde P–T paths have beencalculated for amphibolite-grade pelites and amphibolites fromCordillera Darwin, Tierra del Fuego, Chile. Peak P–T conditionsare nearly all within the kyanite stability field; temperaturesgenerally show an increase with increasing grade, but pressureshave a less consistent trend, possibly increasing slightly fromgarnet to kyanite grade. P–T paths from pelites show heatingby 80–100C during loading of 0•2–3 kbar. Texturalanalysis and previous structural work indicate that this segmentof the path correlates with back-folding deformation. P–Tpaths from two Mg-rich garnet amphibolites suggest a decreasein pressure of as much as 3 kbar with 25–50C of heatingfrom the kyanite stability field to the sillimanite, and areconsistent with pervasive, minor development of fibrolitic sillimanitealong plagioclase grain boundaries. Together, the P–Tpath segments from pelites and amphibolites constitute a clockwiseP–T trajectory. The proposed clockwise P–T paths are consistent with theinterpretation that Cordillera Darwin represents an extensionallyexhumed metamorphic core complex, in which loading during garnetgrowth in the pelitic rocks was succeeded by differential upliftduring garnet growth in magnesian amphibolites. ^* Present address: Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706     

The hydrodynamics of particle clusters and sediment entrapment in coarse alluvial channels

ABSTRACT Particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field. This has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows. Lift and drag forces are determined on clustered hemispherical particles of varying size. In the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion. On the stoss side of the cluster, drag is uniform regardless of the separation of the component particles, but lift is shown to increase when particle separation is small, so affecting stability. This mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape. On average, 46% of clustered particles are entrained by flood flow compared to 87% of particles in open plane-beds. The influence of clusters is a major determinant of sedimentary sorting.     

Chemical controls of cathodoluminescence of natural dolomites and calcites: new data and review

ABSTRACT Calcites and dolomites from the late Precambrian Bonahaven Formation of Scotland exhibit luminescence when MnCO₃>0.03 wt % (dolomite) or >0.06 wt % (calcite) and FeCO₃<2-2-5 wt % (dolomite). Comparison with other published data shows close similarity in the position of Fe-quenching in dolomite, but more variability in the position of Mn-activation in calcites and dolomites and the onset of Fe-quenching in calcites. These anomalies are thought to relate to chemical controls subsidiary to the dominant factors of Fe and Mn concentration. It is not practical to make a useful comparison between operating conditions of different luminescence instruments because of differences in design and an inability to measure routinely the current actually on the specimen surface.     

The Klokken Gabbro-Syenite Complex, South Greenland: Quantitative Interpretation of Mineral Chemistry

The layered syenite series in the Klokken stock formed by continuousin situ fractionation of a trachytic magma in a chamber linedby gabbro with 3000 m of cover rocks. The following mineralsand reactions are assessed as geothermometers and barometers:two feldspars; hypersolvus ternary feldspars; ferrohedenbergite-ß-wollastonite;clinopyroxene-olivine Fe-Mg exchange; Fe-Ti oxides; sanidine-magnetite-annite;ferroedenite stability. Estimates of silica activity are obtainedfrom the silica-magnetite-fayalite assemblage. The gabbros ended magmatic crystallization at > 1000–1050°C.The less fractionated members of the syenite range probablycrystallized with P_H2O < P_total, at T > 870°C and,P_H2O 800 bars. In the more fractionated syenites P_H2O = P_totalduring intercumulus feldspar growth, and all magmatic phasescrystallized within the interval 940–830°C and P_H2O< 1100 bars. Magmatic f_O2 (bars) was 1 log unit below theQFM buffer. a_SIO2 in gabbros was slightly above the albite-nephelinebuffer, but rose suddenly to just <1 in the syenites, a jumpmirrored by minor elements in pyroxenes and opaque oxides. Biotitegrew subsolidus in most rocks, at f_O2 < QFM, except in intermediaterocks when f_O2 > QFM and was defined by the sanidine-magnetite-biotiteassemblage. In these rocks P_H2O of 450 bars at 760°C isobtained using existing experimental data, but application ofthis data to Fe-rich biotites in the layered series (where biotiteis an intercumulus phase) requires P > 10 kb at magmatictemperatures. High TiO₂ or F: OH probably accounts for increasedT stability of natural annites at low P. The syenitic liquid fractionated down a low temperature zonein a multicomponent system precipitating alk fsp + ol + cpx+ mt and the more fractionated members of the layered serieshad a negligible crystallization interval, a prerequisite forthe development of the unique Klokken-type inversely gradedmineral layering.     

An electromagnetic device for automatic detection of bedload motion and its field applications

A new sensor for the continuous and unmanned detection and recording of bedload motion is described. The sensor acts in the same fashion as a conventional metal detector but is elongate and installed permanently in the bed of an alluvial channel. Artificial clasts are labelled with short lengths of ferrite rod and are seeded upstream from the sensor, replacing bed particles. The entrainment of seeded clasts takes them over the sensor where they distort the magnetic field and produce a change in inductance that is detected and recorded. A field installation demonstrates the value of the sensor by revealing for the first time in coarse-grained alluvium the spasmodic nature of particle motion reminiscent of kinematic waves. It also illustrates the importance of pebble clusters in delaying particle entrainment.     

The oxygen isotopic properties of olivines in the Semarkona ordinary chondrite

Abstract— In order to explore the origin of chondrules and the chondrites, the O isotopic compositions of nine olivine grains in seven chondrules from the primitive Semarkona LL3.0 chondrite have been determined by ion microprobe. The data plot in the same general region of the three-isotope plot as whole-chondrule samples from ordinary chondrites previously measured by other techniques. There are no significant differences between the O isotopic properties of olivine in the various chondrule groups in the present study, but there is a slight indication that the data plot at the ¹⁶O-rich end of the ordinary chondrite field. This might suggest that the mesostasis contains isotopically heavy O. The olivines in the present study have O isotopic compositions unlike the ¹⁶O-rich olivine grains from the Julesburg ordinary chondrite. Even though olivines in group A chondrules have several properties in common with them, the ¹⁶O-rich Julesburg olivines previously reported are not simply olivines from group A chondrules.     

A nitrogen and argon stable isotope study of Allan Hills 84001: Implications for the evolution of the Martian atmosphere

Abstract— The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] ～ 0.34 ppm; δ¹⁵N ～ +23%); although during stepped combustion, ¹⁵N-enriched N (δ¹⁵N ～ +143%) is released in a narrow temperature interval between 700 °C and 800 °C (along with ¹³C-enriched C (δ¹³C ～ +19%) and ⁴⁰Ar). Cosmogenic species are found to be negligible at this temperature; thus, the iso-topically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH 84001. The N and Ar data show that ALH 84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess ⁴⁰Ar from K decay. Deconvolution of radiogenic ⁴⁰Ar from other Ar components, on the basis of end-member ³⁶Ar/¹⁴N and ⁴⁰Ar/³⁶Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5–4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmos-phere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: δ¹⁵N ≥ +200%, ⁴⁰Ar/³⁶Ar ≤ 300 and ³⁶Ar/¹⁴N ≥ 17 × 10^?5. In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.