Dynamics and Variability of Terra Nova Bay Polynya

Abstract. We present a process study on the dynamics and variability of the Terra Nova Bay polynya in the western sector of the Ross Sea. The air-sea heat exchange is known to be particularly large in polynya during the winter, when differences between air and sea temperatures are large. We apply a 1-D model (Pease, 1987; Van Woert, 1999a, 1999b), which is modified in the latent heat parameterisation in order to account for time-dependent relative humidity and cloud coverage. Furthermore, the Ice Collection Depth is correlated linearly with a variable wind speed. The model is forced with two different meteorological data sets: the operational analysis of the European Center for Medium Range Weather Forecasts atmospheric data set and the meteorological parameters measured by an Automatic Weather Station located on the coast of Terra Nova Bay. The results are compared in terms of polynya extension, ice, and High Salinity Shelf Water production. According to the two different wind velocities, the results obtained from the different data sets clearly differ. Qualitatively, however, the results are in good agreement.     

Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe,western Alps

High precision U–Pb geochronology of rutile from quartz–carbonate–white mica–rutile veins that are hosted within eclogite and schist of the Monte Rosa nappe, western Alps, Italy, indicate that the Monte Rosa nappe was at eclogite-facies metamorphic conditions at 42.6 ± 0.6 Ma. The sample area [Indren glacier, Furgg zone; Dal Piaz (2001) Geology of the Monte Rosa massif: historical review and personal comments. SMPM] consists of eclogite boudins that are exposed inside a south-plunging overturned synform within micaceous schist. Associated with the eclogite and schist are quartz–carbonate–white mica–rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins at about 42.6 Ma occurred at eclogite-facies metamorphic conditions (480–570°C, >1.3–1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. The timing of eclogite-facies metamorphism in the Monte Rosa nappe determined in this study is identical to that of the Gran Paradiso nappe [Meffan-Main et al. (2004) J Metamorphic Geol 22:261–281], confirming that these two units have shared the same Alpine metamorphic history. Furthermore, the Gran Paradiso and Monte Rosa nappes underwent eclogite-facies metamorphism within the same time interval as the structurally overlying Zermatt-Saas ophiolite [∼50–40 Ma; e.g., Amato et al. (1999) Earth Planet Sci Lett 171:425–438; Mayer et al. (1999) Eur Union Geosci 10:809 (abstract); Lapen et al. (2003) Earth Planet Sci Lett 215:57–72]. The nearly identical P–T–t histories of the Gran Paradiso, Monte Rosa, and Zermatt-Saas units suggest that these units shared a common Alpine tectonic and metamorphic history. The close spatial and temporal associations between high pressure (HP) ophiolite and continental crust during Alpine orogeny indicates that the HP internal basement nappes in the western Alps may have played a key role in exhumation and preservation of the ophiolitic rocks through buoyancy-driven uplift. Coupling of oceanic and continental crust may therefore be critical in preventing permanent loss of oceanic crust to the mantle.     

Mineralogical and geochemical investigation of layered chromitites from the Bracco–Gabbro complex, Ligurian ophiolite, Italy

The Bracco–Gabbro Complex (Internal Liguride ophiolite), that intruded subcontinental mantle peridotite, contains layers of chromitite that are associated with ultramafic differentiates. The chromitites and disseminated chromites in the ultramafics have Al contents similar to the Al-rich podiform chromitites [0.40 < Cr# = Cr/(Cr + Al) < 0.55]. TiO₂ contents of the chromitites are unusually high and range up to 0.82 wt%. The calculated Al₂O₃ and TiO₂ content of the parental melt suggest that the melt was a MORB type. Geothermobarometrical calculations on few preserved silicate inclusions revealed formation temperatures between 970 and 820 °C under a relatively high oxygen fugacity (ΔlogfO₂ at +2.0–2.4). Chromitites were altered during the post-magmatic tectono-metamorphic uplift and the final exposure at the seafloor, as evidenced by the formation of ferrian chromite. The PGE contents of the chromitites and associated ultramafics are unusually low (PGEmax 83 ppb). The chondrite-normalized PGE spidergrams show positive PGE patterns and to some extent similarities with the typical trend of stratiform chromitites. No specific PGM have been found but low concentrations of PPGE (Rh, Pt, and Pd) have been detected in the sulphides that occur interstitially to or enclosed in chromite. Recently, it has been shown that the Internal Liguride gabbroic intrusions have formed by relatively low degrees of partial melting of the asthenospheric mantle. We conclude that the low degree of partial melting might be the main factor to control the unusual low PGE contents and the rather unique PGE distribution in the Bracco chromitites.     

Crustal and mantle strengths in continental lithosphere: is the jelly sandwich model obsolete?

The relative importance of the contribution of the lower crust and of the lithospheric mantle to the total strength of the continental lithosphere is assessed systematically for realistic ranges of layer thickness, composition, and temperature. Results are presented as relative strength maps, giving the ratio of the lower crust to upper mantle contribution in terms of crustal thickness and surface heat flow. The lithosphere shows a “jelly sandwich” rheological layering for low surface heat flow, thin to average crustal thickness, and felsic or wet mafic lower crustal compositions. On the other hand, most of the total strength resides in the seismogenic crust in regions of high surface heat flow, crust of any thickness, and dry mafic lower crustal composition.     

Hydrochemistry and isotope geochemistry as tools for groundwater hydrodynamic investigation in multilayer aquifers: a case study from Lomellina, Po plain, South-Western Lombardy, Italy

A multicriteria approach in studying hydrodynamics of a multilayer aquifer system has been used in the Lomellina region (Northern Italy). It involves the reconstruction of the hydrogeological framework coupled to the definition of the hydrochemical and isotopic features of the aquifers. A shallow phreatic aquifer, reaching depths of about 60–80 m from the surface, and deeper aquifers containing confined groundwater, were distinguished. Groundwater generally shows mineralisation decreasing with depth; dissolved ions depict calcium-bicarbonate hydrochemical facies and stable isotopes define the recharge mechanisms, the origin of groundwater, and the hydraulic confinement of deep aquifers. The phreatic aquifer is fed by local infiltration and by streams and irrigation channels. Tritium and Carbon-14 groundwater dating indicate long residence times (on the order of thousands of years) for confined aquifers. The confined aquifers show essentially passive hydrodynamic conditions and maintain a higher piezometric level than the phreatic aquifer. This inhibits the possibility of recent water penetrating far below the surface. The hydrogeological setting of the Lomellina region displays features which are common to other sectors of the Po plain. As a consequence, the results of this study, although conducted on a restricted area, are highly illustrative of groundwater hydrodynamics in large sedimentary aquifers.     

Low-pH waters discharging from submarine vents at Panarea Island (Aeolian Islands,southern Italy) after the 2002 gas blast: Origin of hydrothermal fluids and implications for volcanic surveillance

A geochemical survey of thermal waters collected from submarine vents at Panarea Island (Aeolian Islands, southern Italy) was carried out from December 2002 to March 2007, in order to investigate (i) the geochemical processes controlling the chemical composition of the hydrothermal fluids and (ii) the possible relations between the chemical features of the hydrothermal reservoir and the activity of the magmatic system. Compositional data of the thermal water samples were integrated in a hydrological conceptual model, which describes the formation of the vent fluid by mixing of seawater, seawater concentrated by boiling, and a deep, highly-saline end-member, whose composition is regulated by water-rock interactions at relatively high temperature and shows clear clues of magmatic-related inputs. The chemical composition of concentrated seawater was assumed to be represented by that of the water sample having the highest Mg content. The composition of the deep end-member was instead calculated by extrapolation assuming a zero-Mg end-member. The Na–K–Ca geothermometer, when applied to the thermal end-member composition, indicated an equilibrium temperature of approximately 300 °C, a temperature in agreement with the results obtained by gas-geothermometry.