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Institute
- Institut für Geowissenschaften (130) (remove)
Volcanic tremor extraction and earthquake detection using music information retrieval algorithms
(2021)
Volcanic tremor signals are usually observed before or during volcanic eruptions and must be monitored to evaluate the volcanic activity. A challenge in studying seismic signals of volcanic origin is the coexistence of transient signal swarms and long-lasting volcanic tremor signals. Separating transient events from volcanic tremors can, therefore, contrib-ute to improving upon our understanding of the underlying physical processes. Exploiting the idea of harmonic-percussive separation in musical signal processing, we develop a method to extract the harmonic volcanic tremor signals and to detect tran-sient events from seismic recordings. Based on the similarity properties of spectrogram frames in the time-frequency domain, we decompose the signal into two separate spec-trograms representing repeating (harmonic) and nonrepeating (transient) patterns, which correspond to volcanic tremor signals and earthquake signals, respectively. We reconstruct the harmonic tremor signal in the time domain from the complex spectrogram of the repeating pattern by only considering the phase components for the frequency range in which the tremor amplitude spectrum is significantly contribut-ing to the energy of the signal. The reconstructed signal is, therefore, clean tremor signal without transient events. Furthermore, we derive a characteristic function suitable for the detection of tran-sient events (e.g., earthquakes) by integrating amplitudes of the nonrepeating spectro-gram over frequency at each time frame. Considering transient events like earthquakes, 78% of the events are detected for signal-to-noise ratio = 0.1 in our semisynthetic tests. In addition, we compared the number of detected earthquakes using our method for one month of continuous data recorded during the Holuhraun 2014-2015 eruption in Iceland with the bulletin presented in Agustsdottir et al. (2019). Our single station event detection algorithm identified 84% of the bulletin events. Moreover, we detected a total of 12,619 events, which is more than twice the number of the bulletin events.
Rheology describes the flow of matter under the influence of stress, and - related to solids- it investigates how solids subjected to stresses deform. As the deformation of the Earth’s outer layers, the lithosphere and the crust, is a major focus of rheological studies, rheology in the geosciences describes how strain evolves in rocks of variable composition and temperature under tectonic stresses. It is here where deformation processes shape the form of ocean basins and mountain belts that ultimately result from the complex interplay between lithospheric plate motion and the susceptibility of rocks to the influence of plate-tectonic forces. A rigorous study of the strength of the lithosphere and deformation phenomena thus requires in-depth studies of the rheological characteristics of the involved materials and the temporal framework of deformation processes.
This dissertation aims at analyzing the influence of the physical configuration of the lithosphere on the present-day thermal field and the overall rheological characteristics of the lithosphere to better understand variable expressions in the formation of passive continental margins and the behavior of strike-slip fault zones. The main methodological approach chosen is to estimate the present-day thermal field and the strength of the lithosphere by 3-D numerical modeling. The distribution of rock properties is provided by 3-D structural models, which are used as the basis for the thermal and rheological modeling. The structural models are based on geophysical and geological data integration, additionally constrained by 3-D density modeling. More specifically, to decipher the thermal and rheological characteristics of the lithosphere in both oceanic and continental domains, sedimentary basins in the Sea of Marmara (continental transform setting), the SW African passive margin (old oceanic crust), and the Norwegian passive margin (young oceanic crust) were selected for this study.
The Sea of Marmara, in northwestern Turkey, is located where the dextral North Anatolian Fault zone (NAFZ) accommodates the westward escape of the Anatolian Plate toward the Aegean. Geophysical observations indicate that the crust is heterogeneous beneath the Marmara basin, but a detailed characterization of the lateral crustal heterogeneities is presented for the first time in this study. Here, I use different gravity datasets and the general non-uniqueness in potential field modeling, to propose three possible end-member scenarios of crustal configuration. The models suggest that pronounced gravitational anomalies in the basin originate from significant density heterogeneities within the crust. The rheological modeling reveals that associated variations in lithospheric strength control the mechanical segmentation of the NAFZ. Importantly, a strong crust that is mechanically coupled to the upper mantle spatially correlates with aseismic patches where the fault bends and changes its strike in response to the presence of high-density lower crustal bodies. Between the bends, mechanically weaker crustal domains that are decoupled from the mantle are characterized by creep.
For the passive margins of SW Africa and Norway, two previously published 3-D conductive and lithospheric-scale thermal models were analyzed. These 3-D models differentiate various sedimentary, crustal, and mantle units and integrate different geophysical data, such as seismic observations and the gravity field. Here, the rheological modeling suggests that the present-day lithospheric strength across the oceanic domain is ultimately affected by the age and past thermal and tectonic processes as well as the depth of the thermal lithosphere-asthenosphere boundary, while the configuration of the crystalline crust dominantly controls the rheological behavior of the lithosphere beneath the continental domains of both passive margins.
The thermal and rheological models show that the variations of lithospheric strength are fundamentally influenced by the temperature distribution within the lithosphere. Moreover, as the composition of the lithosphere significantly influences the present-day thermal field, it therefore also affects the rheological characteristics of the lithosphere. Overall my studies add to our understanding of regional tectonic deformation processes and the long-term behavior of sedimentary basins; they confirm other analyses that have pointed out that crustal heterogeneities in the continents result in diverse lithospheric thermal characteristics, which in turn results in higher complexity and variations of rheological behavior compared to oceanic domains with a thinner, more homogeneous crust.
Transient permeability in porous and fractured sandstones mediated by fluid-rock interactions
(2021)
Understanding the fluid transport properties of subsurface rocks is essential for a large number of geotechnical applications, such as hydrocarbon (oil/gas) exploitation, geological storage (CO2/fluids), and geothermal reservoir utilization. To date, the hydromechanically-dependent fluid flow patterns in porous media and single macroscopic rock fractures have received numerous investigations and are relatively well understood. In contrast, fluid-rock interactions, which may permanently affect rock permeability by reshaping the structure and changing connectivity of pore throats or fracture apertures, need to be further elaborated. This is of significant importance for improving the knowledge of the long-term evolution of rock transport properties and evaluating a reservoir’ sustainability. The thesis focuses on geothermal energy utilization, e.g., seasonal heat storage in aquifers and enhanced geothermal systems, where single fluid flow in porous rocks and rock fracture networks under various pressure and temperature conditions dominates.
In this experimental study, outcrop samples (i.e., Flechtinger sandstone, an illite-bearing Lower Permian rock, and Fontainebleau sandstone, consisting of pure quartz) were used for flow-through experiments under simulated hydrothermal conditions. The themes of the thesis are (1) the investigation of clay particle migration in intact Flechtinger sandstone and the coincident permeability damage upon cyclic temperature and fluid salinity variations; (2) the determination of hydro-mechanical properties of self-propping fractures in Flechtinger and Fontainebleau sandstones with different fracture features and contrasting mechanical properties; and (3) the investigation of the time-dependent fracture aperture evolution of Fontainebleau sandstone induced by fluid-rock interactions (i.e., predominantly pressure solution). Overall, the thesis aims to unravel the mechanisms of the instantaneous reduction (i.e., direct responses to thermo-hydro-mechanical-chemical (THMC) conditions) and progressively-cumulative changes (i.e., time-dependence) of rock transport properties.
Permeability of intact Flechtinger sandstone samples was measured under each constant condition, where temperature (room temperature up to 145 °C) and fluid salinity (NaCl: 0 ~ 2 mol/l) were stepwise changed. Mercury intrusion porosimetry (MIP), electron microprobe analysis (EMPA), and scanning electron microscopy (SEM) were performed to investigate the changes of local porosity, microstructures, and clay element contents before and after the experiments. The results indicate that the permeability of illite-bearing Flechtinger sandstones will be impaired by heating and exposure to low salinity pore fluids. The chemically induced permeability variations prove to be path-dependent concerning the applied succession of fluid salinity changes. The permeability decay induced by a temperature increase and a fluid salinity reduction operates by relatively independent mechanisms, i.e., thermo-mechanical and thermo-chemical effects.
Further, the hydro-mechanical investigations of single macroscopic fractures (aligned, mismatched tensile fractures, and smooth saw-cut fractures) illustrate that a relative fracture wall offset could significantly increase fracture aperture and permeability, but the degree of increase depends on fracture surface roughness. X-ray computed tomography (CT) demonstrates that the contact area ratio after the pressure cycles is inversely correlated to the fracture offset. Moreover, rock mechanical properties, determining the strength of contact asperities, are crucial so that relatively harder rock (i.e., Fontainebleau sandstone) would have a higher self-propping potential for sustainable permeability during pressurization. This implies that self-propping rough fractures with a sufficient displacement are efficient pathways for fluid flow if the rock matrix is mechanically strong.
Finally, two long-term flow-through experiments with Fontainebleau sandstone samples containing single fractures were conducted with an intermittent flow (~140 days) and continuous flow (~120 days), respectively. Permeability and fluid element concentrations were measured throughout the experiments. Permeability reduction occurred at the beginning stage when the stress was applied, while it converged at later stages, even under stressed conditions. Fluid chemistry and microstructure observations demonstrate that pressure solution governs the long-term fracture aperture deformation, with remarkable effects of the pore fluid (Si) concentration and the structure of contact grain boundaries. The retardation and the cessation of rock fracture deformation are mainly induced by the contact stress decrease due to contact area enlargement and a dissolved mass accumulation within the contact boundaries. This work implies that fracture closure under constant (pressure/stress and temperature) conditions is likely a spontaneous process, especially at the beginning stage after pressurization when the contact area is relatively small. In contrast, a contact area growth yields changes of fracture closure behavior due to the evolution of contact boundaries and concurrent changes in their diffusive properties. Fracture aperture and thus permeability will likely be sustainable in the long term if no other processes (e.g., mineral precipitations in the open void space) occur.
Mixed sand- and gravel-bed rivers record erosion, transport, and fining signals in their bedload size distributions. Thus, grain-size data are imperative for studying these processes. However, collecting hundreds to thousands of pebble measurements in steep and dynamic high-mountain river settings remains challenging. Using the recently published digital grain-sizing algorithm PebbleCounts, we were able to survey seven large (>= 1,000 m2) channel cross-sections and measure thousands to tens-of-thousands of grains per survey along a 100-km stretch of the trunk stream of the Toro Basin in Northwest Argentina. The study region traverses a steep topographic and environmental gradient on the eastern margin of the Central Andean Plateau. Careful counting and validation allows us to identify measurement errors and constrain percentile uncertainties using large sample sizes. In the coarse >= 2.5 cm fraction of bedload, only the uppermost size percentiles (>= 95th) vary significantly downstream, whereas the 50th and 84th percentiles show less variability. We note a relation between increases in these upper percentiles and along-channel junctions with large, steep tributaries. This signal is strongly influenced by lithology and geologic structures, and mixed with local hillslope input. In steep catchments like the Toro Basin, we suggest nonlinear relationships between geomorphic metrics and grain size, whereby the steepest parts of the landscape exert primary control on the upper grain-size percentiles. Thus, average or median metrics that do not apply weights or thresholds to steeper topography may be less predictive of grain-size distributions in such settings.
The Upper Indus Basin (UIB), which covers a wide range of climatic and topographic settings, provides an ideal venue to explore the relationship between climate and topography. While the distribution of snow and glaciers is spatially and temporally heterogeneous, there exist regions with similar elevation-snow relationships. In this work, we construct elevation-binned snow-cover statistics to analyze 3415 watersheds and 7357 glaciers in the UIB region. We group both glaciers and watersheds using a hierarchical clustering approach and find that (1) watershed clusters mirror large-scale moisture transport patterns and (2) are highly dependent on median watershed elevation. (3) Glacier clusters are spatially heterogeneous and are less strongly controlled by elevation, but rather by local topographic parameters that modify solar insolation. Our clustering approach allows us to clearly define self-similar snow-topographic regions. Eastern watersheds in the UIB show a steep snow cover-elevation relationship whereas watersheds in the central and western UIB have moderately sloped relationships, but cluster in distinct groups. We highlight this snow-cover-topographic transition zone and argue that these watersheds have different hydrologic responses than other regions. Our hierarchical clustering approach provides a potential new framework to use in defining climatic zones in the cyrosphere based on empirical data.
A multidisciplinary approach to the study of collisional orogenic belts can improve our knowledge of their geodynamic evolution and may suggest new tectonic models, especially for (U)HP rocks inside the accretionary wedge. In the Western Alps, wherein nappes of different origin are stacked, having recorded different metamorphic peaks at different stages of the orogenic evolution. This study focuses on the External (EPZ) and Internal (IPZ) ophiolitic units of the Piedmont Zone (Susa Valley, Western Alps), which were deformed throughout four tectonometamorphic phases (D1 to D4), developing different foliations and cleavages (S1 to S4) at different metamorphic conditions. The IPZ and EPZ are separated by a shear zone (i.e. the Susa Shear Zone (SSZ)) during which a related mylonitic foliation (SM) developed. S1 developed at high pressure conditions (Epidote-eclogite vs. Lawsonite-blueschist facies conditions for IPZ and EPZ, respectively), as suggested by the composition of white mica (i.e. phengite), whereas S2 developed at low pressure conditions (Epidote-greenschist facies conditions in both IPZ and EPZ) and is defined by muscovite. White mica defining the SM mylonitic foliation (T1) is mostly defined by phengite, while the T2-related disjunctive cleavage is defined by fine-grained muscovite. The relative chronology inferred from meso-and micro-structural observations suggests that T1 was near-coeval with respect to the D2, while T2 developed during D4. A new set of radiometric ages of the main metamorphic foliations were obtained by in situ Ar/Ar dating on white mica. Different generations of white mica defining S1 and S2 foliations in both the IPZ and EPZ and SM in the SSZ, were dated and two main groups of ages were obtained. In both IPZ and EPZ, S1 foliation developed at-46-41 Ma, while S2 foliation developed at-40-36 Ma and was nearly coeval with the SM mylonitic foliation (-39-36 Ma). Comparison between structural, petrological and geochronological data allows to define time of coupling of the different units and consequently to infer new tectonic implications for the exhumation of meta-ophiolites of the Piedmont Zone within axial sector of the Western Alps.
The Central Andes region in South America is characterized by a complex and heterogeneous deformation system. Recorded seismic activity and mapped neotectonic structures indicate that most of the intraplate deformation is located along the margins of the orogen, in the transitions to the foreland and the forearc. Furthermore, the actively deforming provinces of the foreland exhibit distinct deformation styles that vary along strike, as well as characteristic distributions of seismicity with depth. The style of deformation transitions from thin-skinned in the north to thick-skinned in the south, and the thickness of the seismogenic layer increases to the south. Based on geological/geophysical observations and numerical modelling, the most commonly invoked causes for the observed heterogeneity are the variations in sediment thickness and composition, the presence of inherited structures, and changes in the dip of the subducting Nazca plate. However, there are still no comprehensive investigations on the relationship between the lithospheric composition of the Central Andes, its rheological state and the observed deformation processes. The central aim of this dissertation is therefore to explore the link between the nature of the lithosphere in the region and the location of active deformation. The study of the lithospheric composition by means of independent-data integration establishes a strong base to assess the thermal and rheological state of the Central Andes and its adjacent lowlands, which alternatively provide new foundations to understand the complex deformation of the region. In this line, the general workflow of the dissertation consists in the construction of a 3D data-derived and gravity-constrained density model of the Central Andean lithosphere, followed by the simulation of the steady-state conductive thermal field and the calculation of strength distribution. Additionally, the dynamic response of the orogen-foreland system to intraplate compression is evaluated by means of 3D geodynamic modelling.
The results of the modelling approach suggest that the inherited heterogeneous composition of the lithosphere controls the present-day thermal and rheological state of the Central Andes, which in turn influence the location and depth of active deformation processes. Most of the seismic activity and neo--tectonic structures are spatially correlated to regions of modelled high strength gradients, in the transition from the felsic, hot and weak orogenic lithosphere to the more mafic, cooler and stronger lithosphere beneath the forearc and the foreland. Moreover, the results of the dynamic simulation show a strong localization of deviatoric strain rate second invariants in the same region suggesting that shortening is accommodated at the transition zones between weak and strong domains. The vertical distribution of seismic activity appears to be influenced by the rheological state of the lithosphere as well. The depth at which the frequency distribution of hypocenters starts to decrease in the different morphotectonic units correlates with the position of the modelled brittle-ductile transitions; accordingly, a fraction of the seismic activity is located within the ductile part of the crust. An exhaustive analysis shows that practically all the seismicity in the region is restricted above the 600°C isotherm, in coincidence with the upper temperature limit for brittle behavior of olivine. Therefore, the occurrence of earthquakes below the modelled brittle-ductile could be explained by the presence of strong residual mafic rocks from past tectonic events. Another potential cause of deep earthquakes is the existence of inherited shear zones in which brittle behavior is favored through a decrease in the friction coefficient. This hypothesis is particularly suitable for the broken foreland provinces of the Santa Barbara System and the Pampean Ranges, where geological studies indicate successive reactivation of structures through time. Particularly in the Santa Barbara System, the results indicate that both mafic rocks and a reduction in friction are required to account for the observed deep seismic events.
White mica and tourmaline are the dominant hydrothermal alteration minerals at the world-class Panasqueira W-Sn-Cu deposit in Portugal. Thus, understanding the controls on their chemical composition helps to constrain ore formation processes at this deposit and determine their usefulness as pathfinder minerals for mineralization in general. We combine whole-rock geochemistry of altered and unaltered metasedimentary host rocks with in situ LA-ICP-MS measurements of tourmaline and white mica from the alteration halo. Principal component analysis (PCA) is used to better identify geochemical patterns and trends of hydrothermal alteration in the datasets. The hydrothermally altered metasediments are enriched in As, Sn, Cs, Li, W, F, Cu, Rb, Zn, Tl, and Pb relative to unaltered samples. In situ mineral analyses show that most of these elements preferentially partition into white mica over tourmaline (Li, Rb, Cs, Tl, W, and Sn), whereas Zn is enriched in tourmaline. White mica has distinct compositions in different settings within the deposit (greisen, vein selvages, wall rock alteration zone, late fault zone), indicating a compositional evolution with time. In contrast, tourmaline from different settings overlaps in composition, which is ascribed to a stronger dependence on host rock composition and also to the effects of chemical zoning and microinclusions affecting the LA-ICP-MS analyses. Hence, in this deposit, white mica is the better recorder of the fluid composition. The calculated trace-element contents of the Panasqueira mineralizing fluid based on the mica data and estimates of mica-fluid partition coefficients are in good agreement with previous fluid-inclusion analyses. A compilation of mica and tourmaline trace-element compositions from Panasqueira and other W-Sn deposits shows that white mica has good potential as a pathfinder mineral, with characteristically high Li, Cs, Rb, Sn, and W contents. The trace-element contents of hydrothermal tourmaline are more variable. Nevertheless, the compiled data suggest that high Sn and Li contents are distinctive for tourmaline from W-Sn deposits.
The Arctic is greatly impacted by climate change. The increase in air temperature drives the thawing of permafrost and an increase in coastal erosion and river discharge. This leads to a greater input of sediment and organic matter into coastal waters, which substantially impacts the ecosystems by reducing light transmission through the water column and altering the biogeochemistry, but also the subsistence economy of local people, and changes in climate because of the transformation of organic matter into greenhouse gases. Yet, the quantification of suspended sediment in Arctic coastal and nearshore waters remains unsatisfactory due to the absence of dedicated algorithms to resolve the high loads occurring in the close vicinity of the shoreline. In this study we present the Arctic Nearshore Turbidity Algorithm (ANTA), the first reflectance-turbidity relationship specifically targeted towards Arctic nearshore waters that is tuned with in-situ measurements from the nearshore waters of Herschel Island Qikiqtaruk in the western Canadian Arctic. A semi-empirical model was calibrated for several relevant sensors in ocean color remote sensing, including MODIS, Sentinel 3 (OLCI), Landsat 8 (OLI), and Sentinel 2 (MSI), as well as the older Landsat sensors TM and ETM+. The ANTA performed better with Landsat 8 than with Sentinel 2 and Sentinel 3. The application of the ANTA to Sentinel 2 imagery that matches in-situ turbidity samples taken in Adventfjorden, Svalbard, shows transferability to nearshore areas beyond Herschel Island Qikiqtaruk.
The Arctic is greatly impacted by climate change. The increase in air temperature drives the thawing of permafrost and an increase in coastal erosion and river discharge. This leads to a greater input of sediment and organic matter into coastal waters, which substantially impacts the ecosystems by reducing light transmission through the water column and altering the biogeochemistry, but also the subsistence economy of local people, and changes in climate because of the transformation of organic matter into greenhouse gases. Yet, the quantification of suspended sediment in Arctic coastal and nearshore waters remains unsatisfactory due to the absence of dedicated algorithms to resolve the high loads occurring in the close vicinity of the shoreline. In this study we present the Arctic Nearshore Turbidity Algorithm (ANTA), the first reflectance-turbidity relationship specifically targeted towards Arctic nearshore waters that is tuned with in-situ measurements from the nearshore waters of Herschel Island Qikiqtaruk in the western Canadian Arctic. A semi-empirical model was calibrated for several relevant sensors in ocean color remote sensing, including MODIS, Sentinel 3 (OLCI), Landsat 8 (OLI), and Sentinel 2 (MSI), as well as the older Landsat sensors TM and ETM+. The ANTA performed better with Landsat 8 than with Sentinel 2 and Sentinel 3. The application of the ANTA to Sentinel 2 imagery that matches in-situ turbidity samples taken in Adventfjorden, Svalbard, shows transferability to nearshore areas beyond Herschel Island Qikiqtaruk.
Holocene temperature proxy records are commonly used in quantitative synthesis and model-data comparisons. However, comparing correlations between time series from records collected in proximity to one another with the expected correlations based on climate model simulations indicates either regional or noisy climate signals in Holocene temperature proxy records. In this study, we evaluate the consistency of spatial correlations present in Holocene proxy records with those found in data from the Last Glacial Maximum (LGM). Specifically, we predict correlations expected in LGM proxy records if the only difference to Holocene correlations would be due to more time uncertainty and more climate variability in the LGM. We compare this simple prediction to the actual correlation structure in the LGM proxy records. We found that time series data of ice-core stable isotope records and planktonic foraminifera Mg/Ca ratios were consistent between the Holocene and LGM periods, while time series of Uk'37 proxy records were not as we found no correlation between nearby LGM records. Our results support the finding of highly regional or noisy marine proxy records in the compilation analysed here and suggest the need for further studies on the role of climate proxies and the processes of climate signal recording and preservation.
The present work gives a detailed analysis of the metamorphic and structural evolution of the back-arc portion of the Famatinian Orogen exposed in the southern Sierra de Aconquija (Cuesta de La Chilca segment) in the Sierras Pampeanas Orientales (Eastern Pampean Sierras). The Pampeanas Orientales include from north to south the Aconquija, Ambato and Ancasti mountains. They are mainly composed of middle to high grade metasedimentary units and magmatic rocks.
At the south end of the Sierra de Aconquija, along an east to west segment extending over nearly 10 km (Cuesta de La Chilca), large volumes of metasedimentary rocks crop out. The eastern metasediments were defined as members of the El Portezuelo Metamorphic-Igneous Complex (EPMIC) or Eastern block and the western ones relate to the Quebrada del Molle Metamorphic Complex (QMMC) or Western block. The two blocks are divided by the La Chilca Shear Zone, which is reactivated as the Rio Chanarito fault.
The EPMIC, forming the hanging wall, is composed of schists, gneisses and rare amphibolites, calc- silicate schists, marbles and migmatites. The rocks underwent multiple episodes of deformation and a late high strain-rate episode with gradually increasing mylonitization to the west. Metamorphism progrades from a M-1 phase to the peak M-3, characterized by the reactions: Qtz + Pl + Bt +/- Ms -> Grt + Bt(2) + Pl(2) +/- Sil +/- Kfs, Qtz + Bt + Sil -> Crd + Kfs and Qtz + Grt + Sil -> Crd. The M-3 assemblage is coeval with the dominant foliation related to a third deformational phase (D-3).
The QMMC, forming the foot wall, is made up of fine-grained banded quartz - biotite schists with quartz veins and quartz-feldspar-rich pegmatites. To the east, schists are also overprinted by mylonitization. The M-3 peak assemblage is quartz + biotite + plagioclase +/- garnet +/- sillimanite +/- muscovite +/- ilmenite +/- magnetite +/- apatite.
The studied segment suffered multiphase deformation and metamorphism. Some of these phases can be correlated between both blocks. D-1 is locally preserved in scarce outcrops in the EPMIC but is the dominant in the QMMC, where S-1 is nearly parallel to S-0. In the EPMIC, D-2 is represented by the S-2 foliation, related to the F-2 folding that overprints S-1, with dominant strike NNW - SSE and high angles dip to the E. D-3 in the EPMIC have F-3 folds with axis oblique to S-2; the S-3 foliation has striking NW - SE dipping steeply to the E or W and develops interference patterns. In the QMMC, S-2 (D-2) is a discontinuous cleavage oblique to S-1 and transposed by S-3 (D-3), subparallel to S-1. Such structures in the QMMC developed at subsolidus conditions and could be correlated to those of the EPMIC, which formed under higher P-T conditions. The penetrative deformation D-2 in the EPMIC occurred during a prograde path with syntectonic growth of garnet reaching P-T conditions of 640 degrees C and 0.54 GPa in the EPMIC. This stage was followed by a penetrative deformation D-3 with syn-kinematic growth of garnet, cordierite and plagioclase. Peak P-T conditions calculated for M-3 are 710 degrees C and 0.60 GPa, preserved in the western part of the EPMIC, west of the unnamed fault.
The schists from the QMMC suffered the early low grade M-1 metamorphism with minimum PT conditions of ca 400 degrees C and 0.35 GPa, comparable to the fine schists (M-1) outcropping to the east. The D-2 deformation is associated with the prograde M-2 metamorphism. The penetrative D-3 stage is related to a medium grade metamorphism M-3, with peak conditions at ca 590 degrees C and 0.55 GPa.
The superimposed stages of deformation and metamorphism reaching high P-T conditions followed by isothermal decompression, defining a clockwise orogenic P-T path. During the Lower Paleozoic, folds were superimposed and recrystallization as well as partial melting at peak conditions occurred. Similar characteristics were described from the basement from other Famatinian-dominated locations of the Sierra de Aconquija and other ranges of the Sierras Pampeanas Orientales.
Multi-component (MC) diffusion simulations enable a process based and more precise approach to calculate transport and sorption compared to the commonly used single-component (SC) models following Fick's law. The MC approach takes into account the interaction of chemical species in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. We studied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominate diffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SC models due to anion exclusion from the DDL. This hampers diffusion of the predominant species CaUO2(CO3)32-. On the one side, species concentrations and ionic strengths of the porewater and on the other side surface charge of the clay minerals control the composition and behaviour of the DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow to transfer computationally intensive MC simulations to SC models via calibrated effective diffusion and distribution coefficients. Simulations for one million years depict maximum uranium diffusion lengths between 10 m and 35 m. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock for nuclear waste repositories.
Near-surface supergene ores of the Merensky Reef in the Bushveld Complex, South Africa, contain economic grades of platinum-group elements, however, these are currently uneconomic due to low recovery rates. This is the first study that investigates the variation in platinum-group elements in pristine and supergene samples of the Merensky Reef from five drill cores from the eastern Bushveld. The samples from the Richmond and Twickenham farms show different degrees of weathering. The whole-rock platinum-group element distribution was studied by inductively coupled plasma-mass spectrometry and the platinum-group minerals were investigated by reflected-light microscopy, scanning electron microscopy, and electron microprobe analysis. <br /> In pristine ("fresh") Merensky Reef samples, platinum-group elements occur mainly as discrete platinum-group minerals, such as platinum-group element-sulfides (cooperite-braggite) and laurite as well as subordinate platinum-group elementbismuthotellurides and platinum-group element-arsenides, and also in solid solution in sulfides (especially Pd in pentlandite). During weathering, Pd and S were removed, resulting in a platinum-group mineral mineralogy in the supergene Merensky Reef that mainly consists of relict platinum-group minerals, Pt-Fe alloys, and Pt-oxides/hydroxides. Additional proportions of platinum-group elements are hosted by Fe-hydroxides and secondary hydrosilicates (e.g., serpentine group minerals and chlorite). <br /> In supergene ores, only low recovery rates (ca. 40%) are achieved due to the polymodal and complex platinum-group element distribution. To achieve higher recovery rates for the platinum-group elements, hydrometallurgical or pyrometallurgical processing of the bulk ore would be required, which is not economically viable with existing technology.
The concentration of sulfur that can be dissolved in a silicate liquid is of fundamental importance because it is closely associated with several major Earth-related processes. Considerable effort has been made to understand the interplay between the effects of silicate melt composition and its capac-ity to retain sulfur, but the dependence on pressure and temperature is mostly based on experiments performed at pressures and temperatures below 6 GPa and 2073 K. Here we present a study of the effects of pressure and temperature on sulfur content at sulfide saturation of a peridotitic liquid. We performed 14 multi-anvil experiments using a peridotitic starting composition, and we produced 25 new measurements at conditions ranging from 7 to 23 GPa and 2173 to 2623 K. We analyzed the recovered samples using both electron microprobe and laser ablation ICP-MS. We compiled our data together with previously published data that were obtained at lower P-T conditions and with various silicate melt compositions. We present a new model based on this combined data set that encompasses the entire range of upper mantle pressure-temperature conditions, along with the effect of a wide range of silicate melt compositions. Our findings are consistent with earlier work based on extrapolation from lower-pressure and lower-temperature experiments and show a decrease of sulfur content at sulfide saturation (SCSS) with increasing pressure and an increase of SCSS with increasing temperature. We have extrapolated our results to pressure-temperature conditions of the Earth's primitive magma ocean, and show that FeS will exsolve from the molten silicate and can effectively be extracted to the core by a process that has been termed the "Hadean Matte." We also discuss briefly the implications of our results for the lunar magma ocean.
Assessing volcanic hazard in regions of distributed volcanism is challenging because of the uncertain location of future vents. A statistical-mechanical strategy to forecast such locations was recently proposed: here, we further develop and test it with analog models. We stress a gelatin block laterally and with surface excavations, and observe air-filled crack trajectories. We use the observed surface arrivals to sample the distributions of parameters describing the stress state of the gelatin block, combining deterministic crack trajectory simulations with a Monte Carlo approach. While the individual stress parameters remain unconstrained, we effectively retrieve their ratio and successfully forecast the arrival points of subsequent cracks.
Spectral analysis is a technique of time-series analysis that decomposes signals into linear combinations of harmonic components. Rooted in the 19th century, spectral analysis gained popularity in palaeoclimatology since the early 1980s. This was partly due to the availability of long time series of past climates, but also the development of new, partly adapted methods and the increasing spread of affordable personal computers. This paper reviews the most important methods of spectral analysis for palaeoclimate time series and discusses the prerequisites for their application as well as advantages and disadvantages. The paper also offers an overview of suitable software, as well as computer code for using the methods on synthetic examples.
Cyanobacteria are important primary producers in temperate freshwater ecosystems. However, studies on the seasonal and spatial distribution of cyanobacteria in deep lakes based on high-throughput DNA sequencing are still rare. In this study, we combined monthly water sampling and monitoring in 2019, amplicon sequence variants analysis (ASVs; a proxy for different species) and quantitative PCR targeting overall cyanobacteria abundance to describe the seasonal and spatial dynamics of cyanobacteria in the deep hard-water oligo-mesotrophic Lake Tiefer See, NE Germany. We observed significant seasonal variation in the cyanobacterial community composition (p < 0.05) in the epi- and metalimnion layers, but not in the hypolimnion. In winter-when the water column is mixed-picocyanobacteria (Synechococcus and Cyanobium) were dominant. With the onset of stratification in late spring, we observed potential niche specialization and coexistence among the cyanobacteria taxa driven mainly by light and nutrient dynamics. Specifically, ASVs assigned to picocyanobacteria and the genus Planktothrix were the main contributors to the formation of deep chlorophyll maxima along a light gradient. While Synechococcus and different Cyanobium ASVs were abundant in the epilimnion up to the base of the euphotic zone from spring to fall, Planktothrix mainly occurred in the metalimnetic layer below the euphotic zone where also overall cyanobacteria abundance was highest in summer. Our data revealed two potentially psychrotolerant (cold-adapted) Cyanobium species that appear to cope well under conditions of lower hypolimnetic water temperature and light as well as increasing sediment-released phosphate in the deeper waters in summer. The potential cold-adapted Cyanobium species were also dominant throughout the water column in fall and winter. Furthermore, Snowella and Microcystis-related ASVs were abundant in the water column during the onset of fall turnover. Altogether, these findings suggest previously unascertained and considerable spatiotemporal changes in the community of cyanobacteria on the species level especially within the genus Cyanobium in deep hard-water temperate lakes.