Trace metal dynamics in the water column and pore waters in a temperate tidal system: response to the fate of algae-derived organic matter

Tidal and seasonal behaviour of the redox-sensitive trace metals Mn, Fe, Mo, U, and V have been investigated in the open-water column and shallow pore waters of the backbarrier tidal flats of the island of Spiekeroog (Southern North Sea) in 2002 and 2007. The purpose was to study the response of trace metal cycles on algae blooms, which are assumed to cause significant changes in the redox state of the entire ecosystem. Trace metal data were complemented by measurements of nutrients and enumeration of algae cells in 2007. Generally, Mn and V show a tidal cyclicity in the water column with maximum values during low tide which is most pronounced in summer due to elevated microbial activity in the sediments. Mo and U behave almost conservatively throughout the year with slightly increasing levels towards high tide. Exceptions are observed for both metals after summer algae blooms. Thus, the seasonal behaviour of the trace metals appear to be significantly influenced by productivity in the water column as the occurrence of algae blooms is associated with an intense release of organic matter (e.g. transparent exopolymer particles, TEP) thereby forming larger organic-rich aggregates. Along with elevated temperatures in summer, the deposition of such aggregates favours microbial activity within the surface sediments and release of DOC, nutrients and trace metals (Mn, Mo and V) during the degradation of the aggregates. Additionally, pronounced reducing conditions lead to the reduction of Mn(IV)-oxides and Fe(III)-(oxihydr)oxides, thereby releasing formerly scavenged compounds as V and phosphate. Therefore, pore-water profiles show significant enrichments in trace metals especially from July to September. Finally, the trace metals are released to the open water column via draining pore waters (esp. Mo, Mn, and V) and/or fixed in the sediment as sulphides (Fe, Mo) and bound to organic matter (U). Non-conservative behaviour of Mo in oxygenated seawater, first observed in the investigation area by Dellwig et al. (Geochim Cosmochim Acta 71:2745–2761, 2007a), was shown to be a recurrent phenomenon which is closely coupled to bacterial activity after the breakdown of algae blooms. In addition to the postulated fixation of Mo in oxygen-depleted micro-zones of the aggregates or by freshly formed organic matter, a direct removal of Mo from the water column by reduced sediment surfaces may also play an important role.     

Seismo-engineering parameters for sites of interim storages for spent nuclear fuel at German nuclear power plants

Within the scope of the licensing procedure for 12 interim storage facilities for spent nuclear fuel at nuclear power plants (NPP) and one for the dismantling of a NPP in Germany, site-specific expert reports were carried out to determine seismo-engineering design parameters. Our work was based on the German nuclear safety standard KTA 2201 part 1 “Design of Nuclear Power Plants against Seismic Events” which requires the derivation of the design earthquake in terms of (macroseismic) intensity. The intensity of the particular design earthquake was evaluated by the deterministic method stipulated in KTA 2201 part 1. According to the current state-of-the-art of science and technology supplementary to the deterministic approach, probabilistic evaluations were done setting a probability of exceedance for the occurrence of the design earthquake of 10⁻⁵ year⁻¹. Geological development and neotectonic conditions of the site surrounding areas were included in the evaluations. For 13 interim storage facility sites, soil-dependent design spectra and corresponding strong-motion duration values were determined.For sites with low seismic hazard site-specific design spectra from the literature were used. In the case of sites with higher seismic hazard (design earthquakeVII 1/2 MSK) strong-motion registrations representative for the site were selected from world wide data and subsequently evaluated to derive site-specific design spectra.     

Application of Kriging Interpolation for Glacier Mass Balance Computations

In order to compute the specific mass balance of a glacier, point measurements of mass balance need to be integrated and thus interpolated over the entire glacier. In this study kriging was applied as interpolation technique to mass balance data of Storglaciren, a 3 km² valley glacier, focusing on the sensitivity of results to the choice of some kriging parameters. Although the spatial means varied only little in most cases, the spatial distribution of the mass balance quantities was sensitive to the assumption on kriging parameters, suggesting that the kriging parameters need to be carefully optimized for each case if the spatial distribution is of interest.     

Food web manipulation by extreme enhancement of piscivory: an invertebrate predator compensates for the effects of planktivorous fish on a plankton community

By enhancing the stock of piscivorous fish in a whole-lake experiment in Gräfenhain (Germany) since 1981 to such an extent that almost all planktivorous fish were exterminated, we examined the concept of “over-biomanipulation”. This hypothesis predicts that (a) extremely strong piscivory will allow uncontrolled development of large invertebrate predators and (b) these invertebrates can exert the same strong predation pressure on large herbivorous zooplankton as planktivorous fish. The hypothesis is tested the first time by a cross-comparison of the long-term response of the plankton community structure in the experimental lake (Piscivore L.) with (1) the intermediate response in the same lake and with (2) that of the long-term state in a nearby reference lake (Planktivore L.) densely inhabited by planktivorous fish (Leucaspius delineatus, a small cyprinid). The intermediate (1989–1992) response in Piscivore L. revealed a strong increase of the abundance of the invertebrate predator Chaoborus flavicans. Large daphnids were able to coexist with C. flavicans so that edible phytoplankton were suppressed and water transparency increased.As part (a) of the hypothesis predicts, the long-term response in Piscivore L. was characterized by the immigration of the larger predator Chaoborus obscuripes which displaced the smaller C. flavicans completely. The results support also part (b) of the hypothesis of “over-biomanipulation”: C. obscuripes-dominated Piscivore L. showed not much difference in biomasses of daphnids and total and edible phytoplankton as well as Secchi depth compared with fish-dominated Planktivore Lake. On the other hand, C. obscuripes-dominated Piscivore L. was characterized by distinctly lower biomass of daphnids, mean body volume of all crustaceans and Secchi depth as well as by higher biomass of edible phytoplankton compared with C. flavicans-dominated Piscivore Lake. We conclude that long-lasting success of biomanipulation cannot be achieved by extremely high piscivory leading to the almost complete extermination of planktivorous fish.     

A stabilized extended finite element framework for hydraulic fracturing simulations

We present a stabilized extended finite element formulation to simulate the hydraulic fracturing process in an elasto‐plastic medium. The fracture propagation process is governed by a cohesive fracture model, where a trilinear traction‐separation law is used to describe normal contact, cohesion and strength softening on the fracture face. Fluid flow inside the fracture channel is governed by the lubrication equation, and the flow rate is related to the fluid pressure gradient by the ‘cubic’ law. Fluid leak off happens only in the normal direction and is assumed to be governed by the Carter's leak‐off model. We propose a ‘local’ U‐P (displacement‐pressure) formulation to discretize the fluid‐solid coupled system, where volume shape functions are used to interpolate the fluid pressure field on the fracture face. The ‘local’ U‐P approach is compatible with the extended finite element framework, and a separate mesh is not required to describe the fluid flow. The coupled system of equations is solved iteratively by the standard Newton‐Raphson method. We identify instability issues associated with the fluid flow inside the fracture channel, and use the polynomial pressure projection method to reduce the pressure oscillations resulting from the instability. Numerical examples demonstrate that the proposed framework is effective in modeling 3D hydraulic fracture propagation. Copyright © 2016 John Wiley & Sons, Ltd.     

Hydrating laterally extensive regions of continental lithosphere by flat subduction: A pilot study from the North American Cordillera

It has been suggested that much of the lithopheric mantle beneath the Colorado Plateau was hydrated by the dehydration of the Farallon plate when it was undergoing low angle subduction during the Laramide orogeny. If correct, low angle subduction could be a viable mechanism for weakening laterally extensive regions of continental lithosphere, allowing such lithosphere potentially to be recycled back into the Earth's interior and into the asthenospheric mantle wedge. To test this hypothesis, we model the release of water during prograde metamorphism of a flat-subducting Farallon plate by considering a thermal model for flat subduction and tracking open-system metamorphic phase equilibria. Our model indicates that significant amounts of water can be laterally transported ∼700 km inboard of the trench, close to the width of the North American Cordillera. The amount of water released is shown here to have been large enough to influence the rheology of the overriding North American lithosphere and the potential for melting at its base. Anomalously high S-velocities in the lithospheric mantle supports our modeled calculations of laterally extensive weakening at the base of the continental lithosphere.