Direct 40Ar/39Ar dating of Late Ordovician and Silurian brittle faulting in the southwestern Norwegian Caledonides

Structural data as well as U–Pb zircon and ⁴⁰Ar/³⁹Ar biotite and muscovite ages were collected from the Rolvsnes granodiorite in western Norway. The granodiorite intruded at c. 466 Ma, cooled quickly and escaped later viscous deformation. Brittle top‐to‐the‐NNW thrust faults (Set I) and WNW–ESE striking dextral strike‐slip faults (Set II) formed in a NNW–SSE transpressional regime. ⁴⁰Ar/³⁹Ar dating of synkinematic mica from both sets reveals a c. 450 Ma (Late Ordovician) age of faulting, which constrains early‐Caledonian brittle deformation. Set I and II faults are overprinted by a set of lower‐grade, variably oriented chlorite‐ and epidote‐coated faults (Set III) constraining WNW–ESE shortening. A lamprophyric dyke oriented compatibly with this stress field intruded at c. 435 Ma (Silurian), indicating that Set III formed at the onset of the Scandian Baltica–Laurentia collision. The preservation of Caledonian brittle structures indicates that the Rolvsnes granodiorite occupied a high tectonic level throughout the Caledonian orogeny.     

ArcGIS to Unity: a design pipeline for creation of 3D terrain in serious egames for geology

One of the key challenges during the development of the Virtual World Project ‘egame environment’ discussed in this issue was the creation of the digital virtual landscape from an original 2002 base map of a field area. In this [online] paper we will walk through the design pipeline that was developed by the virtual worlds project team, and discuss some of the issues that were discovered when translating from a 2D map to a 3D environment using available software, for others interested in replicating this work.     

Defining drivers of nitrogen stable isotopes (δ15N) of surface sediments in temperate lakes

The nitrogen stable isotopic signature (δ¹⁵N) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments δ¹⁵N of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN),  %N in the sediments and lake morphometric variables were the best predictors of sedimentary δ¹⁵N, explaining 66 % of its variation. Significant relationships were also found between sediment δ¹⁵N and human-derived N load ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.23, p < 0.001), the latter being a strong predictor of TN ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional δ¹⁵N. Alberta lake sedimentary δ¹⁵N signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment δ¹⁵N was best explained by  %N in the sediments and lake volume. Overall, our findings support the use of δ¹⁵N in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment δ¹⁵N are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context.     

The weathering and element fluxes from active volcanoes to the oceans: a Montserrat case study

The eruptions of the Soufrière Hills volcano on Montserrat (Lesser Antilles) from 1995 to present have draped parts of the island in fresh volcaniclastic deposits. Volcanic islands such as Montserrat are an important component of global weathering fluxes, due to high relief and runoff and high chemical and physical weathering rates of fresh volcaniclastic material. We examine the impact of the recent volcanism on the geochemistry of pre-existing hydrological systems and demonstrate that the initial chemical weathering yield of fresh volcanic material is higher than that from older deposits within the Lesser Antilles arc. The silicate weathering may have consumed 1.3% of the early CO₂ emissions from the Soufrière Hills volcano. In contrast, extinct volcanic edifices such as the Centre Hills in central Montserrat are a net sink for atmospheric CO₂ due to continued elevated weathering rates relative to continental silicate rock weathering. The role of an arc volcano as a source or sink for atmospheric CO₂ is therefore critically dependent on the stage it occupies in its life cycle, changing from a net source to a net sink as the eruptive activity wanes. While the onset of the eruption has had a profound effect on the groundwater around the Soufrière Hills center, the geochemistry of springs in the Centre Hills 5 km to the north appear unaffected by the recent volcanism. This has implications for the potential risk, or lack thereof, of contamination of potable water supplies for the island’s inhabitants.     

The remarkable Re–Os chronometer in molybdenite: how and why it works

The Re–Os (rhenium–osmium) chronometer applied to molybdenite (MoS₂) is now demonstrated to be remarkably robust, surviving intense deformation and high‐grade thermal metamorphism. Successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite, unique to each sample, such that recognized spatial decoupling of ¹⁸⁷Re parent and ¹⁸⁷Os daughter within individual molybdenite crystals is overcome. Highly precise, accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry (ID‐NTIMS). Spatial decoupling of parent–daughter precludes use of the laser ablation ICP‐MS microanalytical technique for Re–Os dating of molybdenite. The use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons. The Rb–Sr, K–Ar and ⁴⁰Ar/³⁹Ar chronometers are susceptible to chemical and thermal disturbance, particularly in terranes that have experienced subsequent episodes of hydrothermal/magmatic activity, and therefore should not be used as a basis for establishing accuracy in Re–Os dating of molybdenite, as has been done in the past. Re–Os ages for molybdenite are almost always in agreement with observed geological relationships and, when available, with zircon and titanite U–Pb ages. For terranes experiencing multiple episodes of metamorphism and deformation, molybdenite is not complicated by overgrowths as is common for some minerals used in U–Pb dating (e.g. zircon, monazite, xenotime), nor are Re and Os mobilized beyond the margins of individual crystals during solid‐state recrystallization. Moreover, inheritance of older molybdenite cores, incorporation of common Os, and radiogenic Os loss are exceedingly rare, whereas inheritance, common Pb and Pb loss are common complications in U–Pb dating techniques. Therefore, molybdenite ages may serve as point‐in‐time markers for age comparisons.     

Study and suppression of anomalous fast events in inorganic scintillators for dark matter searches

The status of dark matter searches with inorganic scintillator detectors at Boulby mine is reviewed and the results of tests with a CsI(Tl) crystal are presented. The objectives of the latter experiment were to study anomalous fast events previously observed and to identify ways to remove this background. Clear indications were found that these events were due to surface contamination of crystals by alphas, probably from radon decay. A new array of unencapsulated NaI(Tl) crystals immersed either in liquid paraffin or pure nitrogen atmosphere is under construction at Boulby. Such an approach allows complete control of the surface of the crystals and the ability to remove any surface contamination. First data from the unencapsulated NaI(Tl) do not show the presence of anomalous fast events.     

ASEAN navies: new missions and old problems

For the Association of Southeast Asian Nations (ASEAN) — Indonesia, Malaysia, the Philippines, Singapore and Thailand — the classical naval missions, protecting the country from seaborne attack and keeping vital sea lanes open, are less important than functions related to new concepts in the law of the sea, coupled with problems which have plagued South-east Asians for hundreds of years. Each of the ASEAN countries now is entitled to claim vastly extended exclusive economic zones (EEZs) and continental shelves, and all but Singapore have been able to extend their maritime territories greatly. However, the old problems of combating piracy, smuggling and illegal barter trade still exist. To these must be added the new patrol functions necessary to protect offshore resources and enforce extended maritime jurisdictional claims.     

Experimental investigation of the morphodynamic response of riffles and pools to unsteady flow and increased sediment supply

Gravel-bed rivers characteristically exhibit shallow riffles in wide sections and deeper pools where the channel becomes constricted and narrow. While rivers can adjust to changing flow and sediment supply through some combination of adjustments of channel slope, bed-surface sorting, and channel shape, the degree to which riffle-pools may adopt these changes in response to changing flows and sediment supplies remains unclear. This article presents results from a flume experiment investigating how constant- and variable-width channels adjust their morphology in response to changing flow and increased sediment supply. Two flume geometries were used: (1) constant-width and (2) variable-width, characterized by a sinusoidal pattern with a mean width equal to that of the first channel. The variable-width channel developed bed undulations in phase with the width, representing riffle-pools. The experiment consisted of three phases for each flume geometry: (1) steady flow, constant sediment supply; (2) unsteady flow, constant sediment supply; and (3) unsteady flow, doubled sediment supply. Unsteady flow was implemented in the form of repeated symmetrical stepped hydrographs, with a mean discharge equal to that in the steady flow phase. In all phases the bed and sediment supply were composed of a sand/gravel mixture ranging from 1 to 8 mm. In both the straight and variable-width channels, transitioning from steady flow to repeated hydrographs did not result in significant changes in bed morphology. The two channel geometries had different responses to increased sediment supply: the slope of the straight channel increased nearly 40%, while the variable-width channel reduced the relief between bars and pools and decreased the variability in cross-sectional elevation with a slight slope increase. Bar-pool relief varied with repeat discharge hydrographs. Pool elevation changed twice the distance of bar elevations, emphasizing the relevance of pool scour for riffle-pool self-maintenance in channels with width variations.