@article{KaaSternemannAppeletal.2022, author = {Kaa, Johannes M. and Sternemann, Christian and Appel, Karen and Cerantola, Valerio and Preston, Thomas R. and Albers, Christian and Elbers, Mirko and Libon, Lelia and Makita, Mikako and Pelka, Alexander and Petitgirard, Sylvain and Pl{\"u}ckthun, Christian and Roddatis, Vladimir and Sahle, Christoph J. and Spiekermann, Georg and Schmidt, Christian and Schreiber, Anja and Sakrowski, Robin and Tolan, Metin and Wilke, Max and Zastrau, Ulf and Konopkova, Zuzana}, title = {Structural and electron spin state changes in an x-ray heated iron carbonate system at the Earth's lower mantle pressures}, series = {Physical review research}, volume = {4}, journal = {Physical review research}, number = {3}, publisher = {American Physical Society}, address = {College Park}, issn = {2643-1564}, doi = {10.1103/PhysRevResearch.4.033042}, pages = {9}, year = {2022}, abstract = {The determination of the spin state of iron-bearing compounds at high pressure and temperature is crucial for our understanding of chemical and physical properties of the deep Earth. Studies on the relationship between the coordination of iron and its electronic spin structure in iron-bearing oxides, silicates, carbonates, iron alloys, and other minerals found in the Earth's mantle and core are scarce because of the technical challenges to simultaneously probe the sample at high pressures and temperatures. We used the unique properties of a pulsed and highly brilliant x-ray free electron laser (XFEL) beam at the High Energy Density (HED) instrument of the European XFEL to x-ray heat and probe samples contained in a diamond anvil cell. We heated and probed with the same x-ray pulse train and simultaneously measured x-ray emission and x-ray diffraction of an FeCO3 sample at a pressure of 51 GPa with up to melting temperatures. We collected spin state sensitive Fe K beta(1,3) fluorescence spectra and detected the sample's structural changes via diffraction, observing the inverse volume collapse across the spin transition. During x-ray heating, the carbonate transforms into orthorhombic Fe4C3O12 and iron oxides. Incipient melting was also observed. This approach to collect information about the electronic state and structural changes from samples contained in a diamond anvil cell at melting temperatures and above will considerably improve our understanding of the structure and dynamics of planetary and exoplanetary interiors.}, language = {en} } @article{KlemmeFeldhausPotapkinetal.2021, author = {Klemme, Stephan and Feldhaus, Michael and Potapkin, Vasily and Wilke, Max and Borchert, Manuela and Louvel, Marion and Loges, Anselm and Rohrbach, Arno and Weitkamp, Petra and Welter, Edmund and Kokh, Maria A. and Schmidt, Christian and Testemale, Denis}, title = {A hydrothermal apparatus for x-ray absorption spectroscopy of hydrothermal fluids at DESY}, series = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques}, volume = {92}, journal = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques}, number = {6}, publisher = {AIP Publishing}, address = {Melville}, issn = {0034-6748}, doi = {10.1063/5.0044767}, pages = {6}, year = {2021}, abstract = {We present a new autoclave that enables in situ characterization of hydrothermal fluids at high pressures and high temperatures at synchrotron x-ray radiation sources. The autoclave has been specifically designed to enable x-ray absorption spectroscopy in fluids with applications to mineral solubility and element speciation analysis in hydrothermal fluids in complex compositions. However, other applications, such as Raman spectroscopy, in high-pressure fluids are also possible with the autoclave. First experiments were run at pressures between 100 and 600 bars and at temperatures between 25 degrees C and 550 degrees C, and preliminary results on scheelite dissolution in fluids of different compositions show that the autoclave is well suited to study the behavior of ore-forming metals at P-T conditions relevant to the Earth's crust.}, language = {en} } @article{SirbescuSchmidtVeksleretal.2017, author = {Sirbescu, Mona-Liza C. and Schmidt, Christian and Veksler, Ilya V. and Whittington, Alan G. and Wilke, Max}, title = {Experimental crystallization of undercooled felsic liquids}, series = {Journal of petrology}, volume = {58}, journal = {Journal of petrology}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0022-3530}, doi = {10.1093/petrology/egx027}, pages = {539 -- 568}, year = {2017}, abstract = {The crystallization kinetics of silicate liquids were studied experimentally in the system haplogranite-B-Li-H2O, at variable degrees of undercooling and variable water concentration. We investigated the kinetics of nucleation and crystallization of unseeded synthetic hydrous haplogranite with 1 wt \% Li2O + 2 center dot 3 wt \% B2O3 added (composition C1) and 2 wt \% Li2O + 4 center dot 6 wt \% B2O3 added (composition C2). Compositions C1 and C2 are simplified representative bulk compositions of Li-rich pegmatites and their highly differentiated cores, respectively. Starting water contents varied between 3 and 9 wt \%. With few exceptions, the system remained water-undersaturated. About 86 isothermal runs of 1-60 days duration, grouped in 25 time series of constant temperature and initial H2O content, were carried out at temperatures from 400 to 700A degrees C at 300 MPa, corresponding to variable degrees of undercooling between the liquidus and glass transition. Viscosity measurements indicate that the glass transition for both compositions is below 400A degrees C for 3 wt \% water and below 300A degrees C for 6 center dot 5 wt \% water. The melts remained virtually crystal free at 400A degrees C, about 100A degrees C and 120A degrees C above the glass transition for compositions C1 and C2, respectively, in experiments up to 30 days long. This result is consistent with the existence of low-temperature, undercooled melts in the crust. At lower values of undercooling the runs crystallized partially, up to about 70\% volume fraction. Undercooling and the amount of water are the main factors controlling nucleation and growth rates, and therefore textures. Minerals nucleate and grow sequentially according to mineral-specific nucleation delays. The mineral assemblage started with Li-Al stuffed quartz (in C1) and virgilite (in C2), solid-solutions between quartz and gamma-spodumene. The quartz-like phases were typically followed by spherulitic alkali feldspar-quartz intergrowths, euhedral petalite, and fine-grained muscovite. Nearly pure quartz formed as rims and replacement of metastable virgilite and stuffed quartz, in particular at the boron- and water-rich crystallization front of large feldspar or petalite. With the exception of muscovite, all minerals nucleated heterogeneously, on the capsule wall or on pre-existing minerals, and grew inwards, towards the capsule center. Experimental textures resembled the textures of zoned pegmatites, including skeletal, graphic, unidirectional, radiating, spherulitic, massive, and replacement textures. In some cases, when fluid saturation was reached, miarolitic cavities developed containing euhedral crystals. Although unidirectional growth rates appeared to slow down in time, volumetric rates for stable graphic alkali-feldspar quartz intergrowths and petalite remained constant for up to 60 days and similar to 70\% crystallization. Metastable stuffed quartz and virgilite diminished in their growth rates in runs of 30 days or longer, were resorbed in the melt, and were partially replaced by second-generation quartz. Unobstructed, self-sustained crystal growth in conditions of very low nucleation density appears to be the dominant mechanism to form giant pegmatitic crystals, although experimental growth rates are much slower than predicted in nature based on conductive-cooling models.}, language = {en} } @article{SahleNiskanenSchmidtetal.2017, author = {Sahle, Christoph J. and Niskanen, Johannes and Schmidt, Christian and Stefanski, Johannes and Gilmore, Keith and Forov, Yury and Jahn, Sandro and Wilke, Max and Sternemann, Christian}, title = {Cation Hydration in Supercritical NaOH and HCl Aqueous Solutions}, series = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, volume = {121}, journal = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, publisher = {American Chemical Society}, address = {Washington}, issn = {1520-6106}, doi = {10.1021/acs.jpcb.7b09688}, pages = {11383 -- 11389}, year = {2017}, abstract = {We present a study of the local atomic environment of the oxygen atoms in the aqueous solutions of NaOH and HCl under simultaneous high-temperature and high-pressure conditions. Experimental nonresonant X-ray Raman scattering core-level spectra at the oxygen K-edge show systematic changes as a function of temperature and pressure. These systematic changes are distinct for the two different solutes and are described well by calculations within the Bethe- Salpeter formalism for snapshots from ab initio molecular dynamics simulations. The agreement between experimental and simulation results allows us to use the computations for a detailed fingerprinting analysis in an effort to elucidate the local atomic structure and hydrogen-bonding topology in these relevant solutions. We observe that both electrolytes, especially NaOH, enhance hydrogen bonding and tetrahedrality in the water structure at supercritical conditions, in particular in the vicinity of the hydration shells. This effect is accompanied with the association of the HCl and NaOH molecules at elevated temperatures.}, language = {en} } @article{MunzSchmidt2017, author = {Munz, Matthias and Schmidt, Christian}, title = {Estimation of vertical water fluxes from temperature time series by the inverse numerical computer program FLUX-BOT}, series = {Hydrological processes}, volume = {31}, journal = {Hydrological processes}, publisher = {Wiley}, address = {Hoboken}, issn = {0885-6087}, doi = {10.1002/hyp.11198}, pages = {2713 -- 2724}, year = {2017}, abstract = {The application of heat as a hydrological tracer has become a standard method for quantifying water fluxes between groundwater and surface water. The typical application is to estimate vertical water fluxes in the shallow subsurface beneath streams or lakes. For this purpose, time series of temperatures in the surface water and in the sediment are measured and evaluated by a vertical 1D representation of heat transport by advection and conduction. Several analytical solutions exist to calculate the vertical water flux from the measured temperatures. Although analytical solutions can be easily implemented, they are restricted to specific boundary conditions such as a sinusoidal upper temperature boundary. Numerical solutions offer higher flexibility in the selection of the boundary conditions. This, in turn, reduces the effort of data preprocessing, such as the extraction of the diurnal temperature variation from the raw data. Here, we present software to estimate water fluxes based on temperaturesFLUX-BOT. FLUX-BOT is a numerical code written in MATLAB that calculates vertical water fluxes in saturated sediments based on the inversion of measured temperature time series observed at multiple depths. FLUX-BOT applies a centred Crank-Nicolson implicit finite difference scheme to solve the one-dimensional heat advection-conduction equation. FLUX-BOT includes functions for the inverse numerical routines, functions for visualizing the results, and a function for performing uncertainty analysis. We present applications of FLUX-BOT to synthetic and to real temperature data to demonstrate its performance.}, language = {en} } @article{MunzOswaldSchmidt2017, author = {Munz, Matthias and Oswald, Sascha and Schmidt, Christian}, title = {Coupled Long-Term Simulation of Reach-Scale Water and Heat Fluxes Across the River-Groundwater Interface for Retrieving Hyporheic Residence Times and Temperature Dynamics}, series = {Water resources research}, volume = {53}, journal = {Water resources research}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0043-1397}, doi = {10.1002/2017WR020667}, pages = {8900 -- 8924}, year = {2017}, abstract = {Flow patterns in conjunction with seasonal and diurnal temperature variations control ecological and biogeochemical conditions in hyporheic sediments. In particular, hyporheic temperatures have a great impact on many temperature-sensitive microbial processes. In this study, we used 3-D coupled water flow and heat transport simulations applying the HydroGeoSphere code in combination with high-resolution observations of hydraulic heads and temperatures to quantify reach-scale water and heat flux across the river-groundwater interface and hyporheic temperature dynamics of a lowland gravel bed river. The model was calibrated in order to constrain estimates of the most sensitive model parameters. The magnitude and variations of the simulated temperatures matched the observed ones, with an average mean absolute error of 0.7 degrees C and an average Nash Sutcliffe efficiency of 0.87. Our results indicate that nonsubmerged streambed structures such as gravel bars cause substantial thermal heterogeneity within the saturated sediment at the reach scale. Individual hyporheic flow path temperatures strongly depend on the flow path residence time, flow path depth, river, and groundwater temperature. Variations in individual hyporheic flow path temperatures were up to 7.9 degrees C, significantly higher than the daily average (2.8 degrees C), but still lower than the average seasonal hyporheic temperature difference (19.2 degrees C). The distribution between flow path temperatures and residence times follows a power law relationship with exponent of about 0.37. Based on this empirical relation, we further estimated the influence of hyporheic flow path residence time and temperature on oxygen consumption which was found to partly increase by up to 29\% in simulations.}, language = {en} } @article{GruenebergerSchmidtJahnetal.2016, author = {Gr{\"u}neberger, Anja Maria and Schmidt, Christian and Jahn, Sandro and Rhede, Dieter and Loges, Anselm and Wilke, Max}, title = {Interpretation of Raman spectra of the zircon-hafnon solid solution}, series = {European journal of mineralogy}, volume = {28}, journal = {European journal of mineralogy}, publisher = {Schweizerbart}, address = {Stuttgart}, issn = {0935-1221}, doi = {10.1127/ejm/2016/0028-2551}, pages = {721 -- 733}, year = {2016}, abstract = {Zircon (ZrSiO4), hafnon (HfSiO4) and five intermediate compositions were synthesized from a Pb silicate melt. The resulting crystals were 20-300 mu m in size and displayed sector and growth zoning. Raman spectra were acquired at locations in the sample for which preceding electron microprobe (EMP) analyses revealed sufficient compositional homogeneity. The dataset documents shifts of Raman bands with changing composition. In this study, bands that have previously not been reported were found for the intermediate compositions and for pure hafnon, in particular at wavenumbers less than 200 cm(-1). For these external modes, the dataset provides new insight into the compositional dependence of their frequencies. Density-functional theory calculations support the observations and are used for a detailed interpretation of the spectra. The pitfalls of the EMP analysis along the zircon-hafnon join are highlighted.}, language = {en} } @article{MusolffSchmidtRodeetal.2016, author = {Musolff, Andreas and Schmidt, Christian and Rode, Michael and Lischeid, Gunnar and Weise, Stephan M. and Fleckenstein, Jan H.}, title = {Groundwater head controls nitrate export from an agricultural lowland catchment}, series = {Advances in water resources}, volume = {96}, journal = {Advances in water resources}, publisher = {Elsevier}, address = {Oxford}, issn = {0309-1708}, doi = {10.1016/j.advwatres.2016.07.003}, pages = {95 -- 107}, year = {2016}, abstract = {Solute concentration variability is of fundamental importance for the chemical and ecological state of streams. It is often closely related to discharge variability and can be characterized in terms of a solute export regime. Previous studies, especially in lowland catchments, report that nitrate is often exported with an accretion pattern of increasing concentrations with increasing discharge. Several modeling approaches exist to predict the export regime of solutes from the spatial relationship of discharge generating zones with solute availability in the catchment. For a small agriculturally managed lowland catchment in central Germany, we show that this relationship is controlled by the depth to groundwater table and its temporal dynamics. Principal component analysis of groundwater level time series from wells distributed throughout the catchment allowed derivation of a representative groundwater level time series that explained most of the discharge variability. Groundwater sampling revealed consistently decreasing nitrate concentrations with an increasing thickness of the unsaturated zone. The relationships of depth to groundwater table to discharge and to nitrate concentration were parameterized and integrated to successfully model catchment discharge and nitrate export on the basis of groundwater level variations alone. This study shows that intensive and uniform agricultural land use likely results in a clear and consistent concentration-depth relationship of nitrate, which can be utilized in simple approaches to predict stream nitrate export dynamics at the catchment scale. (C) 2016 Elsevier Ltd. All rights reserved.}, language = {en} } @article{MunzOswaldSchmidt2016, author = {Munz, Matthias and Oswald, Sascha and Schmidt, Christian}, title = {Analysis of riverbed temperatures to determine the geometry of subsurface water flow around in-stream geomorphological structures}, series = {Journal of hydrology}, volume = {539}, journal = {Journal of hydrology}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0022-1694}, doi = {10.1016/j.jhydrol.2016.05.012}, pages = {74 -- 87}, year = {2016}, abstract = {The analytical evaluation of diurnal temperature variation in riverbed sediments provides detailed information on exchange fluxes between rivers and groundwater. The underlying assumption of the stationary, one-dimensional vertical flow field is frequently violated in natural systems where subsurface water flow often has a significant horizontal component. In this paper, we present a new methodology for identifying the geometry of the subsurface flow field using vertical temperature profiles. The statistical analyses are based on model optimisation and selection and are used to evaluate the shape of vertical amplitude ratio profiles. The method was applied to multiple profiles measured around in-stream geomorphological structures in a losing reach of a gravel bed river. The predominant subsurface flow field was systematically categorised in purely vertical and horizontal (hyporheic, parafluvial) components. The results highlight that river groundwater exchange flux at the head, crest and tail of geomorphological structures significantly deviated from the one-dimensional vertical flow, due to a significant horizontal component. The geometry of the subsurface water flow depended on the position around the geomorphological structures and on the river level. The methodology presented in this paper features great potential for characterising the spatial patterns and temporal dynamics of complex subsurface flow geometries by using measured temperature time series in vertical profiles. (C) 2016 Elsevier B.V. All rights reserved.}, language = {en} } @article{MusolffSchmidtSelleetal.2015, author = {Musolff, Andreas and Schmidt, Christian and Selle, Benny and Fleckenstein, Jan H.}, title = {Catchment controls on solute export}, series = {Advances in water resources}, volume = {86}, journal = {Advances in water resources}, publisher = {Elsevier}, address = {Oxford}, issn = {0309-1708}, doi = {10.1016/j.advwatres.2015.09.026}, pages = {133 -- 146}, year = {2015}, abstract = {Dynamics of solute export from catchments can be classified in terms of chemostatic and chemodynamic export regimes by an analysis of concentration-discharge relationships. Previous studies hypothesized that distinct export regimes emerge from the presence of solute mass stores within the catchment and their connectivity to the stream. However, so far a direct link of solute export to identifiable catchment characteristics is missing. Here we investigate long-term time series of stream water quality and quantity of nine neighboring catchments in Central Germany ranging from relatively pristine mountain catchments to agriculturally dominated lowland catchments, spanning large gradients in land use, geology, and climatic conditions. Given the strong collinearity of catchment characteristics we used partial least square regression analysis to quantify the predictive power of these characteristics for median concentrations and the metrics of export regime. We can show that median concentrations and metrics of the export regimes of major ions and nutrients can indeed be inferred from catchment characteristics. Strongest predictors for median concentrations were the share of arable land, discharge per area, runoff coefficient and available water capacity in the root zone of the catchments. The available water capacity in the root zone, the share of arable land being artificially drained and the topographic gradient were found to be the most relevant predictors for the metrics of export regime. These catchment characteristics can represent the size of solute mass store such as the fraction of arable land being a measure for the store of nitrate. On the other hand, catchment characteristics can be a measure for the connectivity of these solute stores to the stream such as the fraction of tile drained land in the catchments. This study demonstrates the potential of data-driven, top down analyses using simple metrics to classify and better understand dominant controls of solute export from catchments. (C) 2015 Elsevier Ltd. All rights reserved.}, language = {en} }