Titanite dates crystallization: Slow Pb diffusion during super‐solidus re‐equilibration

Titanite can be found in rocks of wide compositional range, is reactive, growing or regrowing during metamorphic and hydrothermal events, and is generally amenable to U–Pb geochronology. Experimental evidence suggest that titanite has a closure temperature for Pb ranging from 550 to 650°C, and thus titanite dates are commonly interpreted as cooling ages. However, this view has been challenged in recent years by evidence from natural titanite which suggests the closure temperature may be significantly higher (up to 800°C). Here, we investigate titanite in an enclave of migmatitic gneiss included within a granite intrusion. The titanite crystals exhibit textural features characteristic of fluid‐mediated mass transfer processes on length scales of <100 µm. These textural features are associated with variation in both Pb concentrations and distinct U–Pb isotopic compositions. Zr‐in‐titanite thermometry indicates that modification of the titanite occurred at temperatures in excess of 840°C, in the presence of a high‐T silicate melt. The Pb concentration gradients preserved in these titanite crystals are used to determine the diffusivity of Pb in titanite under high‐T conditions. We estimate diffusivities ranging from 2 × 10^?22 to 5 × 10^?25 m²/s. These results are significantly lower than experimental data predict yet are consistent with other empirical data on natural titanites, suggesting that Pb diffusivity is similar to that of Sr. Thus our data challenge the wide‐held assumption that U–Pb titanite dates only reflect cooling ages.     

Electronic and magnetic structure of pyroxenes I. Hedenbergite

The electronic and magnetic structure of the chain silicate hedenbergite (CaFe²⁺Si₂O₆) has been investigated by a number of experimental methods (neutron diffraction, Mössbauer spectroscopy, low temperature magnetic measurements), as well as by electronic structure calculations for clusters of different size in the local spin density approximation. The calculated size-converged spectroscopic data (d-d excitation energies, hyperfine parameters) are in quantitative agreement with the respective experimental values. The calculated magnetic coupling constants are about +25 cm^?1 and ?4 cm^?1 for intra-chain and inter-chain coupling, respectively. The latter value shows that weak superexchange via edges of silicon tetrahedra is well reproduced by the calculations, and it is in qualitative agreement with an observed metamagnetic transition at 4.2 K in an external magnetic field with an onset around 4 T but saturation is not achieved in fields up to 14.5 T. The large ferromagnetic intra-chain coupling is attributed to a nearly degenerate ground state. The ratio between the two magnetic coupling constants agrees with earlier estimates on similar compounds. Finally, it is demonstrated how the detailed discussion of the various exchange pathways contributes to an improved understanding of the connection between magnetic properties and the geometrical structure.     

Sea level,climate change and coastal evolution in Morar,northwest Scotland

Analyses of geomorphologically contrasting sites in Morar, NW Scotland, describe the forcing mechanisms of coastal change. Isolation basins (i.e. basins behind rock sills and now isolated from the sea following isostatic uplift) accumulated continuous marine and freshwater sediments from c.12 to 2 ka BP. Raised dune, marsh and wetland sites register breaching, migration and stability of dunes from c. 9 to 2 ka BP. High-resolution methods designed to address issues of macroscale and microscale sea-level changes and patterns of storminess include 1-mm sampling for pollen, dinocyst and diatom analyses, infra-red photography, X-ray photography and thin-section analysis. The data enhance the record of relative sea-level change for the area. Major phases of landward migration of the coast occurred during the period of low sea-level rise in the mid-Holocene as the rate of rise decreased from c. 3 to < 1 mm/year. Relative sea-level change controls the broad pattern of coastal evolution at each site; local site-specific factors contribute to short-term process change. There is no record of extreme events such as tsunami. Within a system of dynamic metastable equilibrium, the Holocene records show that site-specific factors determine the exact timing of system breakdown, e.g. dune breaching, superimposed on regional sea-level rise. The global average sea-level rise of 3 to 6 mm/yr by AD 2050 predicted by IPCC would only partly be offset in the Morar area by isostatic uplift of about 1 mm/yr. A change from relative sea-level fall to sea-level rise, in areas where the regional rate of uplift no longer offsets global processes, is a critical factor in the management of coastal resources.     

Simulation of the ice age atmosphere — January and July means

A simulation of the atmospheric state under ice age conditions (18,000 years before present) is presented. The T21 Atmospheric General Circulation Model (AGCM), originally developed at the European Centre for Medium-Range Weather Forecasts, was used for the calculation of six annual cycles. Results of the near-surface climatology (2 m-temperature, 10 m-wind, and precipitation), averaged over the last five model years, are presented. The presentation is restricted to January and July means in order to demonstrate the glacial changes in summer and winter. The model's response to ice age boundary conditions was quite consistent with paleogeological data on land and with AGCM experiments of other studies. Although the differences between the mean climate states of the model atmosphere under glacial and modern boundary conditions were statistically significant, the basic structure of the simulated atmospheric circulation was not altered substantially.

---

Zusammenfassung Eine Simulation des Zustands der eiszeitlichen Atmosphäre (18000 Jahre vor heute) wird vorgestellt. Das T21-Modell (allgemeines Zirkulationsmodell der Atmosphäre), entwickelt am Europäischen Zentrum für Mittelfrist-Wettervorhersage, wurde zur Berechnung von sechs Jahresgängen verwendet. Die Ergebnisse der oberflächennahen Klimatologie (2 m-Temperatur, 10 m-Wind und Niederschlag) werden präsentiert als Mittel über die letzten fünf Modelljahre. Die Darstellung ist eingeschränkt auf Januar- und Julimittel, um die eiszeitlichen Änderungen im Sommer und Winter deutlich zu machen. Die Antwort des Modells auf die eiszeitlichen Randbedingungen stimmt recht gut überein mit paläogeologischen Landdaten und mit anderen Simulationsrechnungen. Obgleich die Klimaunterschiede in der eiszeitlichen und der heutigen Modellatmosphäre statistisch signifikant sind, wurde die Grundstruktur der atmosphärischen Zirkulation vom T21-Modell nur wenig verändert.

---

Résumé Cette note présente une simulation de l'état de l'atmosphère dans les conditions de l'âge glaciaire, il y a 18.000 ans. Le calcul de 6 cycles annuels a été effectué au moyen du modèle T 21 de la circulation atmosphérique générale développé au Centre Européen de prévision du temps à moyen terme. Les éléments du climat proche de la surface (température à 2 m, vent à 10 m, précipitations) sont présentés en moyenne des cinq dernières années du modèle. Ces éléments sont limités aux moyennes de janvier et de juillet, de manière à mettre en évidence les changements hiver/été. La réponse du modèle aux conditions aux limites de l'âge glaciaire est en bon accord avec les données paléontologiques de terrain ainsi qu'avec d'autres calculs de simulation. Bien qu'il existe d'importantes différences climatiques entre les modèles d'atmosphère de l'âge glaciaire et d'aujourd'hui, la structure de base de la circulation atmosphérique du modèle T 21 est peu modifiée.

---

, 18000 . 21 = , = , 6 . , — 2 10 , 5 . , , , . . , 21 .

     

Kinematics of throughgoing fractures in jointed rocks

Throughgoing fractures play a major role in subsurface fluid flow yet the kinematics of their formation, which directly impact rock flow properties, are often difficult to establish. We investigate throughgoing fractures in the Monterey Formation of California that developed by the coalescence of pre-existing joints. At Lompoc Landing, throughgoing fractures fall into three main groups: linked, linked with aperture, and breccia zones. The segmented nature of their walls provides numerous piercing points to firmly establish the sense of displacement. Analysis of displacement vectors derived from piercing points demonstrates that the NW–SE trending throughgoing fractures, often interpreted as strike–slip faults, are in fact extensional structures in origin. We suggest that this method may be applied to throughgoing fractures that form by the same mechanism in other geologic settings. Establishing kinematics of throughgoing fractures will lead to a better understanding of their contribution to subsurface fluid flow.     

Solar shield: forecasting and mitigating space weather effects on high-voltage power transmission systems

In this paper, central elements of the Solar Shield project, launched to design and establish an experimental system capable of forecasting the space weather effects on high-voltage power transmission system, are described. It will be shown how Sun–Earth system data and models hosted at the Community Coordinated Modeling Center (CCMC) are used to generate two-level magnetohydrodynamics-based forecasts providing 1–2 day and 30–60 min lead-times. The Electric Power Research Institute (EPRI) represents the end-user, the power transmission industry, in the project. EPRI integrates the forecast products to an online display tool providing information about space weather conditions to the member power utilities. EPRI also evaluates the economic impacts of severe storms on power transmission systems. The economic analysis will quantify the economic value of the generated forecasting system. The first version of the two-level forecasting system is currently running in real-time at CCMC. An initial analysis of the system’s capabilities has been completed, and further analysis is being carried out to optimize the performance of the system. Although the initial results are encouraging, definite conclusions about system’s performance can be given only after more extensive analysis, and implementation of an automatic evaluation process using forecasted and observed geomagnetically induced currents from different nodes of the North American power transmission system. The final output of the Solar Shield will be a recommendation for an optimal forecasting system that may be transitioned into space weather operations.     

A new particulate Mn-Fe-P-shuttle at the redoxcline of anoxic basins

Pelagic redoxclines of anoxic basins and deeps form the suboxic transition between oxygenated surface and anoxic or even sulfidic bottom waters. Intense element cycling, favoured by elevated microbial activity, causes steep gradients of physico-chemical parameters, nutrients and redox-sensitive trace metals. This study presents a conceptual model for authigenic particle formation at pelagic redoxclines, which is based on the tight coupling of Mn, Fe, and P cycles. Besides the well-known occurrence of Mn-oxides, textural (SEM-EDX) and geochemical (ICP-OES, ICP-MS) analyses of particles from the redoxclines of the Black Sea and the Baltic Sea (Gotland Basin, Landsort Deep) evidence the existence of earlier postulated Fe-oxyhydroxo-phosphates and emphasize mixed phases consisting of Mn-oxides and Fe-oxyhydroxo-phosphates as a new solid species. Most of the analyzed particles are star-shaped, of about 5 μm in size, and occur as single particles or aggregates without any morphological differences between Mn-oxides, Fe-oxyhydroxo-phosphates, and mixed phases. Throughout the redoxcline, these minerals show a general succession with maximum abundance of Mn-oxides above the redoxcline followed by mixed phases and almost pure Fe-phosphates within and below the redoxcline, respectively. Molar Fe/P ratios of single particles argue against the formation of known pure Fe-phosphates like vivianite or strengite at the lower end of the redox transition zone, but are consistent with recent experimental findings for colloidal P-bearing hydrous ferric oxides. Moreover, morphological similarities suggest the formation of irregular Fe-oxyhydroxo coatings due to oxidation of upward diffusing Fe²⁺ by oxygen and stepwise replacement of Mn(IV) by Fe(III) on sinking MnO_x particles followed by immediate adsorption or even co-precipitation of phosphate. Batch-type experiments using biogenic MnO_x particles demonstrate the efficient potential of Fe²⁺ oxidation by sinking MnO_x particles. When entering sulfidic waters MnO_x particles are progressively reduced leading to an increasing relative abundance of Fe- and P-rich particles. In deeper parts of the water column these particles are also reductively dissolved, thereby releasing Fe²⁺ and phosphate to the water column. This Mn-Fe-P-shuttle likely affects phosphate transport throughout the water column and thus impacts primary production at least over longer time scales. Furthermore, the particulate Mn-Fe-P-shuttle must have played an important role for the cycling of P and certain trace metals in ancient ocean basins, e.g., during certain periods of Cretaceous black shale formation and should be considered in future mass balances and modeling approaches dealing with oxic/anoxic interfaces of aquatic ecosystems.     

Toward critical spatial thinking in the social sciences and humanities

The integration of geographically referenced information into the conceptual frameworks and applied uses of the social sciences and humanities has been an ongoing process over the past few centuries. It has gained momentum in recent decades with advances in technologies for computation and visualization and with the arrival of new data sources. This article begins with an overview of this transition, and argues that the spatial integration of information resources and the cross-disciplinary sharing of analysis and representation methodologies are important forces for the integration of scientific and artistic expression, and that they draw on core concepts in spatial (and spatio-temporal) thinking. We do not suggest that this is akin to prior concepts of unified knowledge systems, but we do maintain that the boundaries to knowledge transfer are disintegrating and that our abilities in problem solving for purposes of artistic expression and scientific development are enhanced through spatial perspectives. Moreover, approaches to education at all levels must recognize the need to impart proficiency in the critical and efficient application of these fundamental spatial concepts, if students and researchers are to make use of expanding access to a broadening range of spatialized information and data processing technologies.     

Petrologic, isotopic, and radiometric age constraints on the origin and tectonic history of the Malino Metamorphic Complex, NW Sulawesi, Indonesia

The Malino Metamorphic Complex (MMC) is located at the western end of the north arm of Sulawesi. It consists of mica schists and gneisses (derived from proximal turbidite and granitoid protoliths), with intercalations of greenschist, amphibolite, marble, and quartzite, forming an E-W elongated dome-like structure bounded on all sides by faults. The age of the MMC is constrained between Devonian and Early Carboniferous. This Paleozoic age, the presence of Archean and Proterozoic inherited zircons, and the isotopic signature of the mica schists and gneisses indicate that the terrane was derived from the New Guinea-Australian margin of Gondwana. Similarities with basement rocks in the Bird’s Head suggests a common origin. Greenschists forming a discontinuous selvage (metamorphic carapace) around the complex were derived from adjacent autochthonous Paleogene formations. The rocks of the MMC show a Barrovian-type progression from greenschist through epidote-amphibolite to amphibolite facies. P–T estimations suggest a depth of burial of up to 27–30 km. K/Ar and 40Ar/39Ar cooling ages of 23–11 Ma, and a 7 Ma age for unconformably overlying volcanic rocks, indicate that the complex was exhumed during the Miocene. Two tectonic scenarios are considered: 1. the continental fragment docked with Sulawesi during the Mesozoic and was exhumed as a metamorphic core complex during the Miocene; 2. it was subducted beneath the north arm during the late Oligocene and then rapidly returned back to the surface.