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.     

It's good to talk: performing and recording the telephone interview

This paper explores the transition of talk to text in the process of performing and recording semi-structured interviews over the telephone. In particular, it considers the ways respondents and the materials enjoined in the process of interviewing exceed their role in the research process. Drawing on my experience of conducting telephone interviews in the UK, I position the interview as an uncertain encounter: one framed by earlier narrations as much as the aims and objectives of the research project; and an orientation towards 'talk' as much as the reduction of performance to text. Far from losing track of these uncertainties I argue that tape-recorders are well equipped to document these in/audible realms.     

Recycling and “re-hydration” of degassed magma inducing transient dissolution/crystallization events at Stromboli (Italy)

Intrusive degassing and recycling of degassed and dense magma at depth have been proposed for a long time at Stromboli. The brief explosive event that occurred at the summit craters on 9 January 2005 threw out bombs and lapilli that could be good candidates to illustrate recycling of shallow degassed magma at depth. We present an extensive data set on both the textures and the mineral, bulk rock and glassy matrix chemistry of the “9 Jan” products. The latter have the common shoshonitic–basaltic bulk composition of lavas and scoriae issued from typical strombolian activity. In contrast they differ by the heterogeneous chemistry of their matrix glasses and their crystal textures that testify to crystal dissolution event(s) just prior magma crystallization upon ascent and eruption. Comparison between mineral paragenesis of the natural products and experimental phase equilibria suggest water-induced magma re-equilibration. We propose that mineral dissolution is related to water enrichment of the recycled degassed magma, via differential gas bubble transfer and to some extents its physical mixing with volatile-rich magma blobs. However, all these features illustrate transient processes. Even though evidence of mineral dissolution is ubiquitous at Stromboli, its effect on the bulk magma chemistry is minor because of the subtle interplay between mineral dissolution and crystallization in magmas having comparable bulk chemistry.     

Modeling flocculation processes of fine-grained particles using a size-resolved method: Comparison with published laboratory experiments

The transport of fine-grained particles in estuarine and coastal waters is influenced by flocculation processes (aggregation and floc breakup). As a consequence, the particle size varies with time in the water column, and can be orders of magnitude larger than those of primary particles. In this study the variations in floc size is simulated using a size-resolved method, which approximates the real size distribution of particles by a range of size bins and solves a mass balance equation for each bin. To predict the size distribution both aggregation and breakup processes are included. The conventional rectilinear aggregation kernel is used which considers both turbulent shear and differential settling. The breakup kernel accounts for the fractal dimension of the flocs. A flocculation simulation is compared to the settling column lab experiments of Winterwerp [1998. A simple model for turbulence induced flocculation of cohesive sediment, Journal of Hydraulic Research, 36, 309–326], and a one-dimensional sediment transport model is verified with the observed variations in floc size and concentration over tidal cycles in a laboratory flume experiment of Bale et al. [2002. Direct observation of the formation and break-up of aggregates in an annular flume using laser reflectance particle sizing. In: Winterwerp, J.C., Kranenburg, C. (Eds.), Fine Sediment Dynamics in the Marine Environment. Elsevier, pp. 189–201]. The numerical simulations compare qualitatively and quantitatively well with the laboratory measurements, and the analysis of the two simulation results indicates that the median floc size can be correlated to the sediment concentration and Kolmogorov microscale. Sensitivity studies are conducted to explore the role of settling velocity and erosion rate. The results are not sensitive towards the formulation of settling velocity, but the parameterization of erosion flux is important. The studies show that for predicting the sediment deposition flux it is crucial to include flocculation processes.     

Growing new generations of critical zone scientists

Critical zone (CZ) science is entering its second decade. A new generation of scientists is emerging trained specifically in CZ science and are contributing to advances in environmental science across disciplines. Concurrently, the global scope of CZ science is being elevated as new countries invest in CZ observatories. Global CZ science has great potential to address a diverse array of questions beyond any single discipline. In this commentary we discuss a series of CZ science grand challenges that should be targeted by early‐career researchers: understanding water availability in the CZ; expanding CZ science into new environments; communicating the societal relevance of CZ science including earthcasting to the public; seamlessly integrating biological sciences within the CZ framework; and scaling CZ processes over large spatial and temporal gradients. Targeting these grand challenges will push CZ science well into the future. We also highlight mechanisms for increased leadership within the CZ community. Copyright © 2017 John Wiley & Sons, Ltd.     

Comparison of polynomial chaos and Gaussian process surrogates for uncertainty quantification and correlation estimation of spatially distributed open-channel steady flows

Data assimilation is widely used to improve flood forecasting capability, especially through parameter inference requiring statistical information on the uncertain input parameters (upstream discharge, friction coefficient) as well as on the variability of the water level and its sensitivity with respect to the inputs. For particle filter or ensemble Kalman filter, stochastically estimating probability density function and covariance matrices from a Monte Carlo random sampling requires a large ensemble of model evaluations, limiting their use in real-time application. To tackle this issue, fast surrogate models based on polynomial chaos and Gaussian process can be used to represent the spatially distributed water level in place of solving the shallow water equations. This study investigates the use of these surrogates to estimate probability density functions and covariance matrices at a reduced computational cost and without the loss of accuracy, in the perspective of ensemble-based data assimilation. This study focuses on 1-D steady state flow simulated with MASCARET over the Garonne River (South-West France). Results show that both surrogates feature similar performance to the Monte-Carlo random sampling, but for a much smaller computational budget; a few MASCARET simulations (on the order of 10–100) are sufficient to accurately retrieve covariance matrices and probability density functions all along the river, even where the flow dynamic is more complex due to heterogeneous bathymetry. This paves the way for the design of surrogate strategies suitable for representing unsteady open-channel flows in data assimilation.     

Connecting Coronal Mass Ejections to Their Solar Active Region Sources: Combining Results from the HELCATS and FLARECAST Projects

Coronal mass ejections (CMEs) and other solar eruptive phenomena can be physically linked by combining data from a multitude of ground-based and space-based instruments alongside models; however, this can be challenging for automated operational systems. The EU Framework Package 7 HELCATS project provides catalogues of CME observations and properties from the Heliospheric Imagers on board the two NASA/STEREO spacecraft in order to track the evolution of CMEs in the inner heliosphere. From the main HICAT catalogue of over 2,000 CME detections, an automated algorithm has been developed to connect the CMEs observed by STEREO to any corresponding solar flares and active-region (AR) sources on the solar surface. CME kinematic properties, such as speed and angular width, are compared with AR magnetic field properties, such as magnetic flux, area, and neutral line characteristics. The resulting LOWCAT catalogue is also compared to the extensive AR property database created by the EU Horizon 2020 FLARECAST project, which provides more complex magnetic field parameters derived from vector magnetograms. Initial statistical analysis has been undertaken on the new data to provide insight into the link between flare and CME events, and characteristics of eruptive ARs. Warning thresholds determined from analysis of the evolution of these parameters is shown to be a useful output for operational space weather purposes. Parameters of particular interest for further analysis include total unsigned flux, vertical current, and current helicity. The automated method developed to create the LOWCAT catalogue may also be useful for future efforts to develop operational CME forecasting.     

Mg-rich harzburgites from Vesta: Mantle residua or cumulates from planetary differentiation?

We describe petrographic, electron microprobe, and laser ablation ICP-MS analyses of Mg-rich harzburgite clasts in the Dominion Range 2010 howardites, and conclude that they are xenolithic samples of the vestan mantle. Key chemical and petrologic characteristics of these rocks provide tests for differentiation models. Our results indicate the mantle of Vesta formed through variable degrees of partial melting, which left behind a harzburgite and possibly dunite residuum. The Mg-rich clasts are composed of orthopyroxene and olivine, with minor clinopyroxene, FeNi metal, and distinctive pyroxene–chromite symplectites. We use mineral chemistry to demonstrate the absence of a genetic link between diogenites and the Mg-rich harzburgites. We propose a secondary origin for the formation of symplectites: interaction of silicate and metallic melts during primordial differentiation and core formation. The occurrence of FeNi metal containing ~1.5 wt% Cr within the assemblage indicates a very reducing environment during mantle differentiation (≪IW). Our study suggests that Vesta did not experience complete melting early in its history, and instead supports the formation of a shallow magma ocean.