A numerical study of the relative importance of wind and topographic forcing on oceanic eddy shedding by tall, deep water islands

A process-oriented, quasi-geostrophic, barotropic model has been developed with the aim of studying the relative importance of wind and topographic forcing on oceanic eddy generation by tall, deep water islands. As a case study, we chose the island of Gran Canaria. Topographic forcing was established using different intensities (weak, medium, strong, and very strong) for the oceanic current incident to the island. Wind forcing was introduced to simulate the mean wind curl observed in atmospheric tall island wakes. As observed from in situ data, the resulting wind curl consists of two cells of opposite sign which become a complementary source of vorticity at the island lee. The intensity and the shape of the two cells depend on the strength of the incident wind against the obstacle. The oceanic model was forced at three different wind (trade winds) speeds which correspond to weak, medium and strong wind intensities. Results from several numerical experiments show that in those periods where the incident wind is in the medium–strong range and the incident current speed is low (low Reynolds number), the wind forcing is the trigger mechanism for oceanic eddy generation. Eddies are spun off from the island for a lower Reynolds number (Re)/intensity of the oceanic flow (Re = 20) when compared with only topographic forcing (Re > 60). However, when the current speed is strong (high Reynolds number), the vorticity input by the wind is quickly advected by the oceanic flow and does not contribute to oceanic eddy generation. When only wind forcing is considered, only two stationary eddies are generated in the island wake. In this case, eddies of opposite sign are not sequentially spun off by the island and a Von-Kármán-like eddy street is not developed downstream of the island. Therefore, the main mechanism responsible for the development of an eddy street is the topographic perturbation of the oceanic flow by the island flanks. The wind over the island wake acts only as an additional source of vorticity, promoting the generation of an eddy street at a lower intensity of the incident oceanic flow, but not being capable of generating an eddy street without the topographic forcing.     

Chrono‐stratigraphy of the Upper Pleistocene and Holocene archaeological sequence in Cova Gran (south‐eastern Pre‐Pyrenees,Iberian Peninsula)

The Cova Gran de Santa Linya (Lleida, Spain) is a recently discovered site, with a broad chrono‐cultural sequence that contains archaeological levels dating to the Middle Palaeolithic, Early Upper Palaeolithic, Late Upper Palaeolithic, Neolithic and Early Bronze Age. We present the chronometric and stratigraphic context of these occupations, which were dated using ¹⁴C accelerator mass spectrometry and thermoluminescence. The sequence provides important indicators that aid in the reconstruction of the history of human occupation on the southern slopes of the Spanish Pyrenees over the past 50,000 years. Copyright © 2011 John Wiley & Sons, Ltd.     

Source and genesis of sulphate and phosphate–sulphate minerals in a quartz‐sandstone cave environment

Gypsum (CaSO₄·2H₂O), alunite (KAl₃(SO₄)₂(OH)₆), and rare phosphate–sulphate sanjuanite Al₂(PO₄)(SO₄)(OH) 9(H₂O) and rossiantonite (Al₃(PO₄)(SO₄) 2(OH)₂(H₂O)₁₄) have recently been identified as secondary mineral deposits in different quartz‐sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz‐sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to perhaps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteorological, hydrological and geographical data are used to investigate the origin of sulphates and phospho–sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz‐sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sulphate source, with potential contributions of marine non‐sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H₂S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sulphates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co‐occur with silica speleothems of biological origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz‐sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments.     

Towards a simple dynamic process conceptualization in rainfall–runoff models using multi‐criteria calibration and tracers in temperate,upland catchments

Empirically based understanding of streamflow generation dynamics in a montane headwater catchment formed the basis for the development of simple, low‐parameterized, rainfall–runoff models. This study was based in the Girnock catchment in the Cairngorm Mountains of Scotland, where runoff generation is dominated by overland flow from peaty soils in valley bottom areas that are characterized by dynamic expansion and contraction of saturation zones. A stepwise procedure was used to select the level of model complexity that could be supported by field data. This facilitated the assessment of the way the dynamic process representation improved model performance. Model performance was evaluated using a multi‐criteria calibration procedure which applied a time series of hydrochemical tracers as an additional objective function. Flow simulations comparing a static against the dynamic saturation area model (SAM) substantially improved several evaluation criteria. Multi‐criteria evaluation using ensembles of performance measures provided a much more comprehensive assessment of the model performance than single efficiency statistics, which alone, could be misleading. Simulation of conservative source area tracers (Gran alkalinity) as part of the calibration procedure showed that a simple two‐storage model is the minimum complexity needed to capture the dominant processes governing catchment response. Additionally, calibration was improved by the integration of tracers into the flow model, which constrained model uncertainty and improved the hydrodynamics of simulations in a way that plausibly captured the contribution of different source areas to streamflow. This approach contributes to the quest for low‐parameter models that can achieve process‐based simulation of hydrological response. Copyright © 2009 John Wiley & Sons, Ltd.     

High pressure-low temperature metamorphism in the Gran Paradiso basement, Western Alps

The Gran Paradiso basement complex of the French and Italian Alps is composed of metasediments, termed the gneiss minuti, and metabasic rocks, both of which are intruded by a late Hercynian granite. The Bonneval gneiss, which crops out at the western edge of the complex, is composed of highly deformed metasediments, volcanics and volcaniclastic rocks. Eclogites, now highly altered, occur in the metabasic rocks. Kyanite and blue-green amphibole are locally present in the gneiss minuti and aegirine plus riebeckite occur in the Bonneval gneiss. A moderately high pressure - low temperature metamorphic event of probable Alpine age occurred in the basement complex. This metamorphic event differs from that in the overlying Sesia unit and ophiolites of the Schistes lustrés nappe in being at lower pressures (below the ab = jd₁₀₀+ qz transition) and post-dating the major (D2_A) deformation. The origin of the metamorphism is discussed and interpreted as a probable consequence of the overlying nappe pile which was emplaced during the D2_A event. Subsequent greenschist facies metamorphism in the basement complex is a consequence of thermal relaxation during uplift.     

Insights into TT-OSL signal stability from single-grain analyses of known-age deposits at Atapuerca,Spain

Single-grain thermally transferred optically stimulated luminescence (TT-OSL) dating has recently been applied to a number of the Atapuerca palaeoanthropological sites. As yet, however, there have not been any direct assessments of TT-OSL signal stabilities for specific grains used for dating. In this study, we undertake a series of TT-OSL suitability assessments on known-age samples from Gran Dolina and Sima del Elefante. Our results suggest that the Atapuerca samples contain populations or sub-populations of grains with suitably stable TT-OSL signals for dating over late-Early and Middle Pleistocene timescales. Equivalent dose (D_e) distribution analysis in combination with pulse-annealing assessments provides a useful means of identifying inter-grain and inter-sample differences in TT-OSL signal stabilities. We also show that obtaining D_e values using different preheat conditions may help to identify potentially problematic TT-OSL behaviours. Analyses of multi-grain aliquot TL curves for these samples reveal that 'bulk' TL signal loss experiments may provide limited insights into TT-OSL source trap lifetimes due to averaging effects, the dominance of grain populations that do not produce TT-OSL, and interference from slowly bleaching OSL components. Our results improve the robustness and precision of existing TT-OSL chronologies for units TD6-3 and TE16-TE17 at Gran Dolina and Sima del Elefante, and support the broader suitability of the single-grain TT-OSL approach at the Atapuerca sites.     

Ignimbrites of the Roque Nublo group, Gran Canaria, Canary Islands

 Non-welded, lithic-rich ignimbrites, hereintermed the Roque Nublo ignimbrites, are the most distinctive deposits of the Pliocene Roque Nublo group, which forms the products of second magmatic cycle on Gran Canaria. They are very heterogeneous, with 35–55% volume lithic fragments, 15-30% mildly vesiculated pumice, 5–7% crystals and 20–30% ash matrix. The vitric components (pumice fragments and ash matrix) are largely altered and transformed into zeolites and subordinate smectites. The Roque Nublo ignimbrites originated from hydrovolcanic eruptions that caused rapid and significant erosion of vents thus incorporating a high proportion of lithic clasts into the eruption columns. These columns rapidly became too dense to be sustained as vertical eruption columns and were transformed into tephra fountains which fed high-density pyroclastic flows. The deposits from these flows were mainly confined to palaeovalleys and topographic depressions. In distal areas close to the coast line, where these palaeovalleys widened, most of the pyroclastic flows expanded laterally and formed numerous thin flow units. The combined effect of the magma–water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement temperature of the Roque Nublo ignimbrites. This fact also explains the transition from pyroclastic flows into lahar deposits observed in distal facies of the Roque Nublo ignimbrites. The existence of hydrovolcanic eruptions generating high-density pyroclastic flows, unable to efficiently separate the water vapour from the vitric components during transport, also accounts for the intense zeolitic alteration in these deposits. Received: 5 November 1996 / Accepted: 3 March 1997     

Genesis and evolution of a curved mountain front: paleomagnetic and geological evidence from the Gran Sasso range (central Apennines, Italy)

The Gran Sasso range is a striking salient formed by two roughly rectilinear E–W and N–S limbs. In the past 90° counterclockwise (CCW) rotations from the eastern Gran Sasso were reported [Tectonophysics 215 (1992) 335], suggesting west–east increase of rotation-related northward shortening along the E–W limb. In this paper, we report on paleomagnetic data from Meso-Cenozoic sedimentary dykes and strata cropping out at Corno Grande (central part of the E–W Gran Sasso limb), the highest summit of the Apennine belt. Predominant northwestward paleomagnetic declinations (in the normal polarity state) from both sedimentary dykes and strata are observed. When compared to the expected declination values for the Adriatic foreland, our data document no thrusting-related rotation at Corno Grande. The overall paleomagnetic data set coupled with the available geological information shows that the Gran Sasso arc is in fact a composite structure, formed by an unrotated-low shortening western (E–W trending) limb and a strongly CCW rotated eastern salient. Late Messinian and post-early Pliocene shortening episodes documented along the Gran Sasso front indicate that belt building and arc formation occurred during two distinct episodes. We suggest that the southern part of a late Messinian N–S front was reactivated during early–middle Pliocene time, forming a tight range salient due to CCW rotations and differential along-front shortening rates. The formation of a northward displacing bulge in an overall NW–SE chain is likely a consequence of the collision between the Latium-Abruzzi and Apulian carbonate platforms during northeastward propagation of the Apennine wedge, inducing lateral northward extrusion of Latium-Abruzzi carbonates towards ductile basinal sediment areas.     

Eruption and emplacement of a basaltic welded ignimbrite during caldera formation on Gran Canaria

The 14.1 Ma old composite ignimbrite cooling unit P1 (45 km³) on Gran Canaria comprises a lower mixed rhyolite-trachyte tuff, a central rhyolite-basalt mixed tuff, and a slightly rhyolite-contaminated basaltic tuff at the top. The basaltic tuff is compositionally zoned with (a) an upward change in basalt composition to higher MgO content (4.3–5.2 wt.%), (b) variably admixed rhyolite or trachyte (commonly <5 wt.%), and (c) an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments. The basaltic tuff is divided into three structural units: (I) the welded basaltic ignimbrite, which forms the thickest part (c. 95 vol.%) and is the main subject of the present paper; (II) poorly consolidated massive, bomb- and block-rich beds interpreted as phreatomagmatic pyroclastic flow deposits; and (III) various facies of reworked basaltic tuff. Tuff unit I is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias, radial sheet-like distribution around the central Tejeda caldera, thickening in valleys but also covering higher ground, and local erosion of the underlying P1 ash. A gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding; the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature. A similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections. In proximal sections the base is dense where it was emplaced on hot felsic P1 tuff. The intensity of welding, especially at the base, and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets. The flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse. Model calculations suggest eruption through low pyroclastic fountains (<1000 m high) with limited cooling during eruption and turbulent flow from an initial temperature of 1160°C. The large volume of 26 km³ of erupted basalt compared with only 16 km³ of the evolved P1 magmas, and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption. It is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic P1 magma chamber and its ring-fissure conduits. Evolution of the entire P1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude. The basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma, but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c. 10 km diameter roof block, which followed a decrease in magma chamber pressure during low viscosity basaltic outflow.