Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska

In the Aleutian volcanic chain (USA), the 2,050±50 bp collapse of Okmok caldera generated pyroclasts that spread over 1,000 km² on Umnak Island. After expelling up to 0.25 km³ DRE of rhyodacitic Plinian air fall and 0.35 km³ DRE of andesitic phreatomagmatic tephra, the caldera collapsed and produced the 29 km³ DRE Okmok II scoria deposit, which is composed of valley-ponding, poorly sorted, massive facies and over-bank, stratified facies with planar and cross bedding. Geological and sedimentological data suggest that a single density current produced the Okmok II deposits by segregating into a highly concentrated base and an overriding dilute cloud. The dense base deposited massive facies, whereas the dilute cloud sedimented preferentially on hills as stratified deposits. The pyroclastic current spread around Okmok in an axisymmetric fashion, encountering topographic barriers on the southwest, reaching Unalaska Island across an 8-km strait on the east, and reaching the shoreline of Umnak in the other directions. A kinematic model (Burgisser and Bergantz, Earth Planet Sci Lett 202:405–418, 2002) was used to show how decoupling of the pyroclastic current was triggered by both sea entrance and interaction with the topography. In the former case, the dense part of the current and the lithics transported by the dilute cloud went underwater. In the latter case, topographical barriers noticeably decelerated both parts of the decoupled current and favored sedimentation by partial or complete blocking. The resulting unloading of the dilute current drastically reduced the runout distance by triggering an early buoyant lift-off.Editorial responsibility: A.W. Woods     

Mosaic surface storages of a small boreal catchment

Recent studies have suggested that the hydrologic connectivity of northern headwater catchments is likely controlled by antecedent moisture conditions and land cover patterns. A water storage model (EWS), based on water levels (WLs), specific yield (Sy) and surface elevation (SE) changes, was compared with a basic water budget of a small, boreal, patterned fen (13 ha) during the ice‐free period. Results showed that the EWS model reproduced well storage variations derived from the water budget. These results suggest that storage variations can be properly represented by the fluctuations of WLs when we consider the heterogeneous soil properties. However, storage deviations occurred at the daily scale and could be explained by a lack of information on water retention in unsaturated layers, canopy interceptions and preferential flows. Despite the significant impact of SE changes on the different peatland cover storage budgets (strings and lawns), using Sy mean values had a low impact on storage estimations. This can be explained by the large proportion of pools and high WLs throughout the fen. At the fen scale, high storage in the pools seemed to reduce the Sy difference between strings and lawns. The results of this study provide new insights about the complex hydrological behaviour of northern catchments and allow for conceiving new hydrological modelling perspectives. Copyright © 2014 John Wiley & Sons, Ltd.     

Sea transport of animal and vegetable oils and its environmental consequences

The increasing production––and therefore sea traffic––of vegetable oils has regularly led to spillages during the past 40 years. The accident of Allegra, on October,1st, 1997, in the English Channel gave rise to a spillage of 900 tonnes of palm nut oil. The drift of this solid vegetable oil was followed by aerial observations. Samples of oil were collected in order to analyse its chemical evolution. This study, associated with several bibliographic cases of pollution by non-petroleum oils, shows that drifting oils can mix with floating material to sink or form a crust. They can also be oxidized or disperse and/or be degraded by bacteria. They may also polymerise. The coating properties of vegetable oils act as crude oils to affect sea life, tourism and yachting. As a result, it is necessary to quickly collect the oil after a spillage, using usual equipment (booms and pumps).     

Organic petrology and geochemistry of the Carboniferous coal seams from the Central Asturian Coal Basin (NW Spain)

This paper presents for the first time a petrological and geochemical study of coals from the Central Asturian Coal Basin (North Spain) of Carboniferous (Pennsylvanian), mainly of Moscovian, age. A paleoenvironmental approach was used, taking into account both petrographic and organic geochemical studies. Vitrinite reflectance (R_r) ranges from 0.5% to 2.5%, which indicates a high volatile bituminous to semianthracite and anthracite coal rank. The coal samples selected for paleoenvironmental reconstruction are located inside the oil–gas-prone phase, corresponding to the interval between the onset of oil generation and first gas generation and efficient expulsion of oil. This phase is represented by coals that have retained their hydrocarbon potential and also preserved biomarker information. Paleodepositional reconstruction based on maceral and petrographic indices points to a swamp environment with vitrinite-rich coal facies and variable mineral matter content. The gelification index (GI) and groundwater influence index (GWI) indicate strong gelification and wet conditions. The biomarkers exhibit a high pristane/phytane ratio, suggesting an increase in this ratio from diagenetic processes, and a high diterpanes ratio. This, in turn, would seem to indicate a high swamp water table and a humid climate. The maximum point of coal accumulation occurred during the regressive part of the Late Moscovian sequence and in the most humid climate described for this period of time in the well-known coal basins of Europe and North America.     

The ultimate summit eruption of Puy de Dôme volcano (Chaîne des Puys,French Massif Central) about 10,700 years ago

The Puy de Dôme volcano is a trachytic lava dome, about 11,000 y old. New pyroclastic layers originating from the volcano itself were discovered covering the summit and the flanks of the volcano. These pyroclastic layers do not fit with the previous interpretation, assuming a non-explosive dome-forming eruption. The tephra display pyroclastic surge features and exhibit fresh trachytic lapilli, basement lithics, allogeneous basaltic lava and clinker fragments requiring an open vent eruption. This ultimate eruption occurred after a period of rest, long enough for vegetation to develop on the volcano, as evidenced by carbonized plant fragments. Radiocarbon dating of some of these fragments gave an age of c.10,700 y also suggesting a significant rest duration.     

On persistent primary variables for numerical modeling of gas migration in a nuclear waste repository

Numerical simulation of gas migration driven by compressible two-phase partially miscible flow in porous media is of major importance for safety assessment of deep geological repositories for long-lived high-level nuclear waste. We present modeling of compositional liquid and gas flow for numerical simulations of hydrogen migration in deep geological radioactive waste repository based on persistent primary variables. Two-phase flow is considered, with incompressible liquid and compressible gas, which includes capillary effects, gas dissolution, and diffusivity. After discussing briefly the existing approaches to deal with phase appearance and disappearance problem, including a persistent set of variables already considered in a previous paper (Bourgeat et al., Comput Geosci 13(1):29–42, 2009), we focus on a new variant of the primary variables: dissolved hydrogen mass concentration and liquid pressure. This choice leads to a unique and consistent formulation in liquid saturated and unsaturated regions, which is well adapted to heterogeneous media. We use this new set of variable for numerical simulations and show computational evidences of its adequacy to simulate gas phase appearance and disappearance in different but typical situations for gas migration in an underground radioactive waste repository.     

Mineralogical and petrological features of the Cemilköy ignimbrite,Cappadocia, Turkey

The Cemilköy ignimbrite is one of the voluminous ignimbrite deposits in Cappadocia. The Cemilköy ignimbrite contains pumice and lithic clasts of volcanic and ophiolitic origin in an ash matrix. The unwelded Cemilköy ignimbrite is distinguished from other deposits in Cappadocia by flattened pumices, elongate vesicles and a slaty fabric. The mineral assemblage of the Cemilköy ignimbrite is plagioclase, biotite, quartz and oxides (magnetite and Ti-magnetite) and the matrix is glassy. Eutaxitic texture is dominant and all pumice clasts have a vitrophyric-porphyritic texture. The same textural properties were observed throughout the spatial distribution of Cemilköy ignimbrite. Microprobe studies reveal that plagioclase compositions range from albite through oligoclase-andesine. Estimated plagioclase-liquid temperatures (T) and pressures (P) are varying between 806 and 847?°C and 4.2–7.1 (kbar), and the H₂O content of the melt is estimated to have been 5?wt.% from the pumice clasts. Based on geochemical data, the Cemilköy ignimbrite is rhyolitic and calc-alkaline in character, and all pumice clasts are enriched in LIL and LRE elements relative to HFS elements. Negative Nb, Ta and Ti anomalies, ratios of Ba/Nb >28 (56–77), Ba/Ta ?450 (590–700) and Th/Yb vs. Ta/Yb are consistent with a subduction-related origin. According to the geochemical and mineralogical-petrographical data, the Cemilköy ignimbrite originated from partial melting of a mantle source which was enriched during previous subduction processes with variable degrees of assimilation fractional crystallization (AFC) through time and Cemilköy ignimbrite erupted from a crustal magma chamber at shallow to intermediate depth.     

Groundwater flow modelling of the regional aquifer of the Pampa del Tamarugal,northern Chile

The Pampa del Tamarugal Aquifer (PTA) is an important source of groundwater in northern Chile. In this study, a groundwater flow model of this aquifer is developed and calibrated for the period 1983–2004. The model reproduces the observed flow-field and the water balance components reasonably well. Five scenarios are defined to evaluate the response to different pumping situations. These scenarios show that groundwater heads will continue to decrease with the present pumping discharge rates. To account for variations in the model results due to uncertainties in average recharge rates, randomly generated recharge realizations with different levels of uncertainty are simulated. Evaporation flow rates and groundwater flowing out of the modelled area seem unaffected by the recharge uncertainty, whereas the storage terms can vary considerably. For the most intensive pumping scenario under the generated random recharge rates, it is unlikely that the cumulative discharged volume from the aquifer, at the end of the simulation period, will be larger than 12% of the estimated groundwater reserve. Fluctuations in simulated groundwater heads due to uncertainties in the average recharge values are more noticeable in certain areas. These fluctuations could explain unusual behaviour in the observed groundwater heads in these areas.

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Resumen El Acuífero de la Pampa del Tamarugal (PTA) es una fuente importante de agua subterránea en el norte de Chile. En este estudio se desarrolla y calibra un modelo de flujo de agua subterránea para el periodo 1983–2004. El modelo reproduce razonablemente bien el campo de flujo observado y los componentes del balance hídrico. Se definen cinco escenarios para evaluar la respuesta a diferentes situaciones de bombeo. Estos escenarios muestran que con las tasas de descarga de bombeo actuales las presiones de agua subterránea continuarán en descenso. Para explicar las variaciones en los resultados del modelo debidas a incertidumbres en tasas de recarga promedio se han simulado realizaciones de recarga generadas aleatoriamente con diferentes niveles de incertidumbre. Las tasa de flujo por evaporación y el agua subterránea que fluye fuera del área modelizada parecen no haber sido afectadas por la incertidumbre en recarga mientras que los términos de almacenamiento pueden variar considerablemente. Para el escenario de bombeo más intenso, bajo las tasas de recarga generadas aleatoriamente, no es probable que el volumen descargado acumulado del acuífero al final del periodo de simulación sea mayor del 12% de la reserva de agua subterránea estimada. Las fluctuaciones en las presiones simuladas de agua subterránea debido a las incertidumbres en los valores de recarga promedio son más notorios en ciertas áreas. Las fluctuaciones podrían explicar el comportamiento inusual en las presiones de agua subterránea observadas en estas áreas.

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Résumé L’aquifère de la Pampa del Tamarugal (PTA) représente une importante source d’eau souterraine dans le Nord du Chili. Dans cette étude, un modèle d’écoulement souterrain de cet aquifère est réalisé et calibré pour la période 1983–2004. Celui-ci reproduit le champs d’écoulement observé et les composantes du bilan d’eau raisonnablement bien. Cinq scénarios sont définis pour évaluer la réponse du système à des conditions de pompage différentes. D’après ces scénarios, les niveaux piézomètriques vont continuer à baisser sous l’effet des taux de pompage actuels. Pour tenir compte de la variabilité des résultats du modèle due aux incertitudes sur les taux de recharge moyens, des épisodes de recharge, générés aléatoirement et présentant différents niveaux d’incertitude sont simulés. Les taux d’évaporation et l’eau souterraine s’écoulant hors de la zone modélisée semblent non affectés par l’incertitude sur la recharge, les termes d’emmagasinement en revanche peuvent varier considérablement. Dans le cas du scénario avec le pompage le plus important et des taux de recharge aléatoires, il est peu probable que le volume cumulé pompé hors de l’aquifère à la fin de la simulation atteigne une valeur supérieure à 12% de la réserve d’eau souterraine estimée. Les fluctuations des niveaux piézomètriques simulés liées aux incertitudes sur les recharges moyennes sont plus perceptibles dans certaines zones. Ces fluctuations pourraient expliquer un comportement inhabituel des niveaux piézomètriques observés dans ces zones.

     

Stratigraphic architecture of the Pyreneo-Languedocian submarine fan,Gulf of Lions,western Mediterranean Sea

The Pyreneo-Languedocian submarine sediment body, located in the western sector of the Gulf of Lions, is an example of a fan-like depositional system essentially controlled by salt tectonics. The area was subjected to a combined effect of overburden subsidence into the evacuated salt layer and a significant distal salt thickening, due to preferential basinward salt migration. This mode of salt migration impacted the Quaternary sea-bottom morphology by creating a large midslope topographic low, providing space accommodation for the Pyreneo-Languedocian fan. At gulf scale, the fan is a unique feature because unchannelized sedimentary environment in the area occurs at slope level, thus in minor water depth in relation to all other deep-water sedimentary systems offshore Gulf of Lions. To cite this article: A.T. dos Reis et al., C. R. Geoscience 336 (2004).