Tectonically controlled fluid flow and water-assisted melting in the middle crust: An example from the Central Alps

Melting triggered by influx of a free aqueous fluid in the continental crust has commonly been inferred, but the source of water in such contexts remains a matter of debate. We focus on the Tertiary migmatites in the Southern Steep Belt of the Central Alps (Switzerland) to discuss the petrology, structures and geodynamic setting of water-assisted melting. These migmatites comprise various structural types (e.g. metatexites, diatexites, melt in shear zones), which reflect variable leucosome fractions. The melting event itself as well as the variable melt fractions are related to the amount of aqueous fluids. At a given P and T, melt-fractions in rocks of minimum melt composition correlate with the amount of infiltrated aqueous fluids. In more granodioritic systems the water distributes between melt and newly crystallizing hydrous phases such as amphibole, such that the melt fraction correlates with the contents of H₂O, Al, and Ca in the system. Phase-equilibrium modelling indicates that the stabilization of amphibole leads to slightly lower melt fractions than in a granitic system at the same P, T and bulk water content. Phase-equilibrium models further indicate that in the Alpine migmatite belt: (1) several wt.% water (fluid:rock ratio of  1:30) are necessary to produce the inferred melt fraction; (2) the activity of H₂O in the fluid is high; and (3) spatially associated metapelites are unlikely as a source for the required aqueous fluids.

We present a tectonic scenario for the southern margin of the Central Alps, to which these migmatites are confined, and we propose that water was produced from dehydration reactions in metapelites in the Southern Alps. We model fluid production rates at the time of melting and demonstrate that the resulting fluid flow pattern is mainly controlled by the differences in permeability between the fluid source region and melting region. The proposed model requires strong gradients in temperature and permeability for the two tectonic blocks. This is consistent with the scenario involving indenter tectonics at the boundary between the Central and the Southern Alps in Oligocene times.     

From subduction to collision: Thermal overprint of HP/LT meta-sediments in the north-eastern Lepontine Dome (Swiss Alps) and consequences regarding the tectono-metamorphic evolution of the Alpine orogenic wedge

The Cenozoic-age metamorphic structure of the Alps consists of a throughgoing pressure-dominated belt (blueschists and eclogites) that strikes parallel to the orogen and was later truncated by two thermal domes characterised by Barrow-type metamorphism (Lepontine dome and Tauern window). This study documents for the first time that relics of Fe-Mg carpholite occur also within meta-sedimentary units that are part of the north-eastern Lepontine structural and metamorphic dome, where so far exclusively Barrovian assemblages were found. They occur in meta-sediments of both Valais Oceanderived Lower Penninic Bündnerschiefer and structurally lower Europe-derived Sub-Penninic cover nappes and slices. These high-pressure units were subsequently overprinted by a thermal event, as is documented by the growth of new minerals typical for Barrovian metamorphism.We present evidence for a two-stage metamorphic evolution in the northern part of the Lepontine dome: (1) Early subduction-related syn-D1 (Safien phase) HP/LT metamorphism under blueschist facies conditions (350–400 °C and 1.2–1.4 GPa) was immediately followed by “cold” isothermal (or cooling) decompression during D2 nappe-stacking (Ferrera phase). (2) Collisionrelated Barrovian overprint (500–570 °C and 0.5–0.8 GPa) postdates the D3 nappe-refolding event (Domleschg phase) and represents a late heating pulse, separated by D2 and D3 from the D1 high-pressure event. It occurred before and/or during the initial stages of D4 (Chiéra phase) representing a second nappe-refolding event.In discussing possible heat sources for the late Barrow-type heating pulse it is argued that heat release from radioactive decay of accreted material may play an important role in contributing much to heat production. Based on the field evidence, we conclude that heat transfer was essentially conductive during these latest stages of the thermal evolution.     

Salinity‐induced increase of the hydraulic conductivity in the hyporheic zone of coastal wetlands

In coastal zones globally, salinization is rapidly taking place due to the combined effects of sea level rise, land subsidence, altered hydrology, and climate change. Although increased salinity levels are known to have a great impact on both biogeochemical and hydrological processes in aquatic sediments, only few studies have included both types of processes and their potential interactions. In the present paper, we used a controlled 3‐year experimental mesocosm approach to test salinity induced interactions and discuss mechanisms explaining the observed hydrological changes. Surface water salinity was experimentally increased from 14 to 140 mmol Cl per L (0.9 and 9 PSU) by adding sea salt which increased pore water salinity but also increased sulfate reduction rates, leading to higher sulfide, and lower methane concentrations. By analyzing slug test data with different slug test analysis methods, we were able to show that hydraulic conductivity of the hyporheic zone increased 2.8 times by salinization. Based on our hydrological and biogeochemical measurements, we conclude that the combination of pore dilation and decreased methane production rates were major controls on the observed increase in hydraulic conductivity. The slug test analysis method comparison allowed to conclude that the adjusted Bouwer and Rice method results in the most reliable estimate of the hydraulic conductivity for hyporheic zones. Our work shows that both physical and biogeochemical processes are vital to explain and predict hydrological changes related to the salinization of hyporheic zones in coastal wetlands and provides a robust methodological approach for doing so.     

Dynamos with a flat α-effect distribution

Abstract

In order to obtain a better insight into the excitation conditions of magnetic fields in flat objects, such as galaxies, we have calculated critical dynamo numbers of different magnetic field modes for spherical dynamos with a flat α-effect distribution. A simple but realistic approximation formula for the rotation curve is employed. In most cases investigated a stationary quadrupole-type solution is preferred. This is a consequence of the flat distribution of the α-effect. Non-axisymmetric fields are in all cases harder to excite than axisymmetric ones. This seems to be the case particularly for flat objects in combination with a realistic rotation curve for galaxies. The question of whether non-axisymmetric (bisymmetric) fields, which are observed in some galaxies, can be explained as dynamos generated by an axisymmetric αω-effect is therefore still open.     

How well developed is the dynamo theory of flat objects?

Abstract

The existing models of disk dynamos are reviewed and an attempt is made to assess the reliability of their predictions. We analyze key questions such as whether the infinitely extended layer is a proper approximation of the finite disk, or whether it overlooks the possibility of ‘‘forgotten modes'', i.e. whether there is a guarantee that, when the model is used in applications, the mode it predicts as the most easily excited is truly such.

What we find is that all existing models of disk dynamos have shortcomings in these respects, so that the analysis of improved models is worth the effort.     

Tritium balance in macro‐scale river basins analysed through distributed hydrological modelling

The recession of bomb tritium in river discharge of large basins indicates a contribution of slowly moving water. For an appropriate interpretation it is necessary to consider different runoff components (e.g. direct runoff and ground water components) and varying residence times of tritium in these components. The spatially distributed catchment model (tracer aided catchment model, distributed; TAC^D) and a tritium balance model (TRIBIL) were combined to model process‐based tritium balances in a large German river basin (Weser 46 240 km²) and seven embedded sub‐basins. The hydrological model (monthly time step, 2 × 2 km²) estimated the three major runoff components: direct runoff, fast‐moving and slow‐moving ground water for the period of 1950 to 1999. The model incorporated topography, land use, geomorphology, geology and hydro‐meteorological data. The results for the different basins indicated a contribution of direct runoff of 30–50% and varying amounts for fast and slow ground water components. Combining these results with the TRIBIL model allowed us to estimate the residence time of the components. Mean residence times of 8 to 14 years were found for the fast ground water component, 21 to 93 years for the slow ground water component and 14 to 50 years for an overall mean residence time within these basins. Balance calculations for the Weser basin indicate an over‐estimation of loss of tritium through evapotranspiration (more than 60%) and decay (10%). About 28% were carried in stream‐flow where direct runoff contributed about 12% and ground water runoff 13% in relation to precipitation input over the studied 50‐year period. Neighbouring basins and nuclear power plants contributed about 1% each over this time period. Copyright © 2007 John Wiley & Sons, Ltd.     

Biogeochemical responses to late-winter storms in the Sargasso Sea,III—Estimates of export production using 234Th:238U disequilibria and sediment traps

Direct measurements of new production and carbon export in the subtropical North Atlantic Ocean appear to be too low when compared to geochemical-based estimates. It has been hypothesized that episodic inputs of new nutrients into surface water via the passage of mesoscale eddies or winter storms may resolve at least some of this discrepancy. Here, we investigated particulate organic carbon (POC), particulate organic nitrogen (PON), and biogenic silica (BSiO₂) export using a combination of water column ²³⁴Th:²³⁸U disequilibria and free-floating sediment traps during and immediately following two weather systems encountered in February and March 2004. While these storms resulted in a 2–4-fold increase in mixed layer NO₃ inventories, total chlorophyll a and an increase in diatom biomass, the systems were dominated by generally low ²³⁴Th:²³⁸U disequilibria, suggesting limited particle export. Several ²³⁴Th models were tested, with only those including non-steady state and vertical upwelling processes able to describe the observed ²³⁴Th activities. Although upwelling velocities were not measured directly in this study, the ²³⁴Th model suggests reasonable rates of 2.2–3.7 m d^?1.Given the uncertainties associated with ²³⁴Th derived particle export rates and sediment traps, both were used to provide a range in sinking particle fluxes from the upper ocean during the study. ²³⁴Th particle fluxes were determined applying the more commonly used steady state, one-dimensional model with element/²³⁴Th ratios measured in sediment traps. Export fluxes at 200 m ranged from 1.91±0.20 to 4.92±1.22 mmol C m^?2 d^?1, 0.25±0.08 to 0.54±0.09 mmol N m^?2 d^?1, and 0.22±0.04 to 0.50±0.06 mmol Si m^?2 d^?1. POC export efficiencies (Primary Production/Export) were not significantly different from the annual average or from time periods without storms, although absolute POC fluxes were elevated by 1–11%. This increase was not sufficient, however, to resolve the discrepancy between our observations and geochemical-based estimates of particle export. Comparison of PON export rates with simultaneous measurements of NO₃^? uptake derived new production rates suggest that only a fraction, <35%, of new production was exported as particles to deep waters during these events. Measured bSiO₂ export rates were more than a factor of two higher (p<0.01) than the annual average, with storm events contributing as much as 50% of annual bSiO₂ export in the Sargasso Sea. Furthermore it appears that 65–95% (average 86±14%) of the total POC export measured in this study was due to diatoms.Combined these results suggest that winter storms do not significantly increase POC and PON export to depth. Rather, these storms may play a role in the export of bSiO₂ to deep waters. Given the slower remineralization rates of bSiO₂ relative to POC and PON, this transport may, over time, slowly decrease water column silicate inventories, and further drive the Sargasso Sea towards increasing silica limitation. These storm events may further affect the quality of the POC and PON exported, given the large association of this material with diatoms during these periods.