Debris structures in basal ice exposed at the margin of the Greenland ice sheet

The basal ice of many glaciers contains debris structures that reflect subglacial processes. Presented here is an unusually clear photograph of ice and debris in the lowest 2 m of the basal layer at the margin of the Greenland ice sheet. The photograph shows ice-debris relationships and deformation structures that reflect entrainment processes and flow history.     

Venus and the VRM mission

The most profound advances in the Earth sciences during the next decade will not be made on Earth. This paradoxical situation has come about because, while it is becoming more and more difficult to find new approaches to investigating the planet we are so familiar with, by the end of the decade we will have had the opportunity to study a second planet that is remarkably similar to the Earth, but one that has evolved quite differently: Venus. Such study may help us to learn why the Earth evolved in the way that it did, with the unique combination of surface conditions that allow life to be sustained.     

STORURDI: A LATE HOLOCENE ROCK-SLOPE FAILURE (STURZSTROM) IN THE JOTUNHEIMEN, SOUTHERN NORWAY

An extensive accumulation of rock debris at the mouth of Urdadalen in the Jotunheimen, southern Norway, is interpreted as the deposit of a rock-slope failure (sturzstrom). This is the first large-scale rock-slope failure to be recognised in the Jotunheimen but is unlikely to be the only one. The debris descended to the valley floor, passed across the valley axis and moved 40 m up the opposite side of the valley. Some debris passed over the lip of Urdadalen and now forms a boulder tongue that extends for c. 800 m into Utledalen. A maximum velocity of 80 m s⁻¹ for debris movement from crest of hillside to toe of boulder tongue has been estimated, and a minimum velocity of 28 m s⁻¹ was required for the debris to run up the opposite side of Urdadalen. The degree of boulder hardness/weathering as determined by Schmidt hammer suggests the sturzstrom occurred in the Late Holocene (1.825 ± 0.76 cal. ka bp ), during a period of climatic deterioration. The localised geomorphological impact of the failure event has been to dam the valley and create a small lake, to cause valley widening, to produce a north-facing hillside embayment at 1200–1400 m above sea level with the potential to collect and retain snow that might, in future, lead to niche glacier development and cirque initiation, and to provide a large volume of rock debris for entrainment and onward transport in future glacial cycles.     

Dust mobilization and transport in the northern Sahara during SAMUM 2006 – a meteorological overview

The SAMUM field campaign in southern Morocco in May/June 2006 provides valuable data to study the emission, and the horizontal and vertical transports of mineral dust in the Northern Sahara. Radiosonde and lidar observations show differential advection of air masses with different characteristics during stable nighttime conditions and up to 5-km deep vertical mixing in the strongly convective boundary layer during the day. Lagrangian and synoptic analyses of selected dust periods point to a topographic channel from western Tunisia to central Algeria as a dust source region. Significant emission events are related to cold surges from the Mediterranean in association with eastward passing upper-level waves and lee cyclogeneses south of the Atlas Mountains. Other relevant events are local emissions under a distinct cut-off low over northwestern Africa and gust fronts associated with dry thunderstorms over the Malian and Algerian Sahara. The latter are badly represented in analyses from the European Centre for Medium–Range Weather Forecasts and in a regional dust model, most likely due to problems with moist convective dynamics and a lack of observations in this region. This aspect needs further study. The meteorological source identification is consistent with estimates of optical and mineralogical properties of dust samples.     

NOTES ON THE NEW MAP OF THE ALETSCH GLACIER

ABSTRACT

The estimation of recharge and boundary flux is an important problem in deterministic groundwater modelling since these quantities are often difficult to measure directly in the field. A new method (inferred recharge) is proposed for this estimation, based upon water level and transmissivity observations; it is particularly applicable to those semiarid regions where a steady state formulation can be used and recharge is confined to certain known areas. The inferred recharge method which assumes that recharge occurs in these areas only is compared with an alternative method which does not use this information, and is found to be superior. In the example of the inferred recharge method applied to the Oman coastal plain, random normal errors have been added to both water levels and transmissivities in simulation experiments to assess the effect of errors in the data. The results of the simulations have been used to test the reliability of variance estimates derived from the theory. In addition, statistical tests have been used to examine the inferred recharge results.     

A Detailed Geochemical Study of Island Arc Crust: the Talkeetna Arc Section, South-Central Alaska

The Early to Middle Jurassic Talkeetna Arc section exposed inthe Chugach Mountains of south–central Alaska is 5–18km wide and extends for over 150 km. This accreted island arcincludes exposures of upper mantle to volcanic upper crust.The section comprises six lithological units, in order of decreasingdepth: (1) residual upper mantle harzburgite (with lesser proportionsof dunite); (2) pyroxenite; (3) basal gabbronorite; (4) lowercrustal gabbronorite; (5) mid-crustal plutonic rocks; (6) volcanicrocks. The pyroxenites overlie residual mantle peridotite, withsome interfingering of the two along the contact. The basalgabbronorite overlies pyroxenite, again with some interfingeringof the two units along their contact. Lower crustal gabbronorite(10 km thick) includes abundant rocks with well-developed modallayering. The mid-crustal plutonic rocks include a heterogeneousassemblage of gabbroic rocks, dioritic to tonalitic rocks (30–40%area), and concentrations of mafic dikes and chilled mafic inclusions.The volcanic rocks (7 km thick) range from basalt to rhyolite.Many of the evolved volcanic compositions are a result of fractionalcrystallization processes whose cumulate products are directlyobservable in the lower crustal gabbronorites. For example,Ti and Eu enrichments in lower crustal gabbronorites are mirroredby Ti and Eu depletions in evolved volcanic rocks. In addition,calculated parental liquids from ion microprobe analyses ofclinopyroxene in lower crustal gabbronorites indicate that theclinopyroxenes crystallized in equilibrium with liquids whosecompositions were the same as those of the volcanic rocks. Thecompositional variation of the main series of volcanic and chilledmafic rocks can be modeled through fractionation of observedphase compositions and phase proportions in lower crustal gabbronorite(i.e. cumulates). Primary, mantle-derived melts in the TalkeetnaArc underwent fractionation of pyroxenite at the base of thecrust. Our calculations suggest that more than 25 wt % of theprimary melts crystallized as pyroxenites at the base of thecrust. The discrepancy between the observed proportion of pyroxenites(less than 5% of the arc section) and the proportion requiredby crystal fractionation modeling (more than 25%) may be bestunderstood as the result of gravitational instability, withdense ultramafic cumulates, probably together with dense garnetgranulites, foundering into the underlying mantle during thetime when the Talkeetna Arc was magmatically active, or in theinitial phases of slow cooling (and sub-solidus garnet growth)immediately after the cessation of arc activity. KEY WORDS: island arc crust; layered gabbro; Alaska geology; island arc magmatism; lower crust