Mobilisation and transport of dissolved organic carbon and iron in peat catchments—Insights from the Lehstenbach stream in Germany using generalised additive models

During the last decades, increasing exports of both dissolved organic carbon (DOC) and iron were observed from peat catchments in North America and Europe with potential consequences for water quality of streamwater and carbon storages of soils. As mobilisation and transport processes of DOC and iron in peat catchments are only partly understood, the purpose of this study was to elucidate these processes in an intensively monitored and studied system. Specifically, it was hypothesised that dissimilatory iron reduction in riparian peatland soils mobilises DOC initially adsorbed to iron minerals. During stormflow conditions, both DOC and iron will be transported into the stream network. Ferrous iron may be reoxidised at redox interfaces on its way to the stream, and subsequently, ferric iron could be transported together with DOC as complexes. To test these hypotheses, generalised additive models (GAMs) were applied to 14 years of weekly time series of discharge and concentrations of selected solutes measured in a German headwater stream called Lehstenbach. This stream drains a 4.19‐km² forested mountain catchment; one third of which is covered by riparian peatland soils. We interpreted results of different types of GAM in the way that (a) iron reduction drove the mobilisation of DOC from peatland soils and that (b) both iron and DOC were transported as complexes after their joint mobilisation to and within the steam. It was speculated that low nitrate availability in the uppermost wetland soil layer, particularly during the growing season, promoted iron reduction and thus the mobilisation of DOC. However, the influence of nitrate on the DOC mobilisation remains relatively uncertain. This influence could be further investigated using methods similar to the GAM analysis conducted here for other catchments with long‐term data as well as detailed measurements of the relevant species in riparian wetland soils and the adjacent stream network.     

A note on momentum transfer in the bottom mixed layer of the West European Basin

Summary About 20 benthic storms, originated by deep reaching eddies, have been observed between September '83 and May '86 in the deep West European Basin. First results show, that these eddies influence the dispersion of tracers in the near-bottom layers significantly, e.g. due to deformation by shear. This paper is an attempt to estimate the eddy viscosity tensor in the bottom mixed layer. It will be examined as in to how far benthic storms and vortices result in changes of momentum transfer within this layer. The results indicate that the influence of these phenomena is mainly restricted on meso-scale processes.

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Über den Impulsaustausch in der durchmischten Bodenschicht des Westeuropäischen Beckens

Zusammenfassung Zwischen September 83 und Mai 86 wurden in der Tiefsee des Westeuropäischen Beckens 20 benthische Stürme beobachtet, die durch tiefreichende Wirbel verursacht wurden. Erste Ergebnisse zeigen, daß diese Wirbel die Ausbreitung von Stoffen in den bodennahen Schichten erheblich beeinflussen können, z. B. durch Deformation von Wasserkörpern infolge von Scherung. In dieser Arbeit wird versucht, den Impulsaustauschtensor in der durchmischten Bodenschicht abzuschätzen. Ferner wird untersucht, inwieweit benthische Stürme oder Wirbel den Impulsaustausch in dieser Schicht beeinflussen. Die Ergebnisse zeigen, daß sich ihr Einfluß im wesentlichen auf mesoskalige Prozesse beschränkt.

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Contribution à l'étude des transports d'énergie dans la couche profonde du bassin ouest-européen de l'Atlantique Nord

Résumé Une vingtaine de tempêtes benthiques provoquées par des tourbillons à fort développement vertical ont été observées de septembre 1983 à mai 1986 dans les profondeurs du bassin ouest-européen de l'Atlantique Nord. Les premiers résultats montrent que ces tourbillons exercent une influence significative sur la dispersion des traceurs disposés dans les couches proches du fond, par suite des déformations dues au cisaillment. Une tentative d'estimation du tenseur de viscosité du tourbillon dans la couche de mélange profonde est proposée dans la présente contribution, qui étudie dans quelle mesure les tempêtes benthiques et les systèmes tourbillonnaires engendrent des modifications du transfert d'énergie cinétique au sein de cette couche. Les résultats semblent indiquer que l'influence de ces phénomènes n'est surtout sensible que pour les processus méso-échelle.

     

Determination of groundwater discharge rates and water residence time of groundwater‐fed lakes by stable isotopes of water (18O, 2H) and radon (222Rn) mass balances

Lacustrine groundwater discharge (LGD) and the related water residence time are crucial parameters for quantifying lake matter budgets and assessing its vulnerability to contaminant input. Our approach utilizes the stable isotopes of water (δ¹⁸O, δ²H) and the radioisotope radon (²²²Rn) for determining long‐term average and short‐term snapshots in LGD. We conducted isotope balances for the 0.5‐km² Lake Ammelshainer See (Germany) based on measurements of lake isotope inventories and groundwater composition accompanied by good quality and comprehensive long‐term meteorological and isotopic data (precipitation) from nearby monitoring stations. The results from the steady‐state annual isotope balances that rely on only two sampling campaigns are consistent for both δ¹⁸O and δ²H and suggested an overall long‐term average LGD rate that was used to infer the water residence time of the lake. These findings were supported by the good agreement of the simulated LGD‐driven annual cycles of δ¹⁸O and δ²H lake inventories with the observed lake isotope inventories. However, radon mass balances revealed lower values that might be the result of seasonal LGD variability. For obtaining further insights into possible seasonal variability of groundwater–lake interaction, stable water isotope and radon mass balances could be conducted more frequently (e.g., monthly) in order to use the derived groundwater discharge rates as input for time‐variant isotope balances.     

Weakening and strain localization produced by syn-deformational reaction of plagioclase

There are many observations in naturally deformed rocks on the effects of mineral reactions on deformation, but few experimental data. In order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase + H₂OM more albitic plagioclase + zoisite + kyanite + quartz. We utilized fine-grained (2-6 µm) plagioclase aggregates of two compositions (An54 and An60), both dried and with 0.1-0.4 wt% H₂O present, in shear deformation experiments at two sets of conditions: 900 °C, 1.0 GPa (in the plagioclase stability field) and 750 °C, 1.5 GPa (in the zoisite stability field). Dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep, but samples with 0.1 to 0.4 wt% water deformed in the zoisite stability field showed extreme strain localization into very narrow (~1-3 µm) shear bands after low shear strain. In these samples the microstructures of reaction products in the matrix differ from those in the shear bands. In the matrix, large (up to 400 µm) zoisite crystals grew in the direction of finite extension, and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic. In the shear bands, the reaction products albitic plagioclase, zoisite, white mica, and traces of kyanite form polyphase aggregates of very fine-grained (<0.1 µm) dislocation-free grains. Most of the sample strain after % ~2 has occurred within the shear bands, within which the dominant deformation mechanism is inferred to be diffusion-accommodated grain boundary sliding (DAGBS). The switch from dislocation creep in dry samples deformed without reaction to DAGBS in reacted samples is associated with a decrease in flow stress from ~800 to <200 MPa. These experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine-grained polyphase assemblage, leading to a switch in deformation mechanism and significant weakening. Thus, localization of deformation in polyphase rocks may occur on any pressure (P),temperature (T)-path where the equilibrium composition of the constituent minerals changes.     

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.     

Modification of fluid inclusions in quartz by deviatoric stress. II: experimentally induced changes in inclusion volume and composition

Fluid inclusions in quartz are known to modify their densities during shear deformation. Modifications of chemical composition are also suspected. However, such changes have not been experimentally demonstrated, their mechanisms remain unexplained, and no criteria are available to assess whether deformed inclusions preserve information on paleofluid properties. To address these issues, quartz crystals containing natural CO₂–H₂O–NaCl fluid inclusions have been experimentally subjected to compressive deviatoric stresses of 90–250 MPa at 700°C and ~600 MPa confining pressure. The resulting microcracking of the inclusions leads to expansion by up to 20%, producing low fluid densities that bear no relation to physical conditions outside the sample. Nevertheless, the chemical composition of the precursor inclusions is preserved. With time the microcracks heal and form swarms of tiny satellite inclusions with a wide range of densities, the highest reflecting the value of the maximum principle stress, σ ₁. These new inclusions lose H₂O via diffusion, thereby passively increasing their salt and gas contents, and triggering plastic deformation of the surrounding quartz via H₂O-weakening. Using microstructural criteria to identify the characteristic types of modified inclusions, both the pre-deformation fluid composition and syn-deformation maximum stress on the host mineral can be derived from microthermometric analysis and thermodynamic modelling.     

Diffusion,degradation or on-site stabilisation – Identifying causes of kinetic processes involved in metal–humate complexation

The applicability of equilibrium models for humic-bound transport of toxic or radioactive metals is affected by kinetic processes leading to an increasing inertness of metal–humic complexes. The chemical background is not yet understood. It is widely believed that bound metals undergo an in-diffusion process within the humic colloids, changing from weaker to stronger binding sites. This work is focussed on the competition effect of Al(III) on complexation of Tb(III) or Eu(III) as analogues of trivalent actinides. By using ion exchange and spectroscopic methods, their bound fractions were determined for solutions of Al and humic acid that had been pre-equilibrated for different periods of time. Whilst the amount of bound Al remained unchanged, its blocking effect was found to increase over a time frame of 2 days, which corresponds to the kinetics of the increase in complex inertness reported in most pertinent studies. Thus, the derived “diffusion theory” turned out to be inapplicable, since it cannot explain an increase in competition for the “initial” sites. A delayed degradation of polynuclear species (as found for Fe) does not occur. Consequently, the temporal changes must be based on structural rearrangements in the vicinity of bound Al, complicating the exchange or access. Time-dependent studies by laser fluorescence spectroscopy (steady-state and time-resolved) yielded evidence of substantial alterations, which were, however, immediately induced and did not show any significant trend on the time scale of interest, suggesting that the stabilisation process is based on comparatively moderate changes.