Effect of soil crusting on the emission and transport of wind-eroded sediment: field measurements on loamy sandy soil

Field data are reported for the horizontal and vertical flux of wind-eroded sediment on an agricultural field in northern Germany. Measurements were made during a windstorm that hit the region on 18 May 1999. The magnitude of both fluxes was significantly affected by the presence of a surface crust covering the test field. Measuring the physical crust strength at 45 locations with a torvane, the relationships between crust strength (τ) and the horizontal (F_h) and vertical (F_v) sediment fluxes were investigated. Both fluxes decreased as the surface crust became stronger. The decay behaved as an exponential function for both types of flux. The horizontal sediment flux over a crusted surface can be accurately predicted by completing Marticorena and Bergametti's [Journal of Geophysical Research 100 (1995) 16415] erosion model with a crust function. The vertical particle flux over crusted soil can be calculated by adding a similar function to Alfaro and Gomes's [Journal of Geophysical Research 106D (2001) 18075] dust production model. The study also suggests that the gradual bombardment of a surface crust by impacting particles does not immediately result in a decay of the crust's protective effect, provided that the crust has a minimum thickness. However, once the crust becomes perforated, its protective effect disappears very quickly, leading to much higher horizontal and vertical sediment fluxes than predicted for undamaged crusted soil.     

Authigenic gypsum found in gas hydrate-associated sediments from Hydrate Ridge, the eastern North Pacific

Hydrate Ridge is located at the second accretion-ary ridge along the Cascadia margin of Oregon in the eastern North Pacific (fig. 1). The Bottom Simulating Reflector (BSR) underlies the entire Hydrate Ridge[1]. The Ocean Drilling Program (ODP) in 1992 at Site 892 and the TECFLUX99 and 2000 showed that the gas hydrate occurs just beneath the thin sediment- covered surface and at the horizon of around 64 meter below seafloor (mbsf) on Hydrate Ridge[25]. The col-lision of the Juan de …     

Thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions

The thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions was investigated. Members of these series were either coprecipitated or synthesized hydrothermally and were characterized by XRD, FTIR, electron microprobe analysis, ICP-MS, and thermal analysis. Partial alkali substitution and vacancies on the Fe/Al sites were observed in all cases, and the solids studied can be described by the general formula K_1-x-yNa_y(H₃O)_xFe_zAl_w(SO₄)₂(OH)_6-3(3-z-w)(H₂O)_3(3-z-w). A strong preferential incorporation of Fe over Al in the jarosite/alunite structure was observed. Heats of formation from the elements, ΔH°_f, were determined by high-temperature oxide melt solution calorimetry. The solid solutions deviate slightly from thermodynamic ideality by exhibiting positive enthalpies of mixing in the range 0 to +11 kJ/mol. The heats of formation of the end members of both solid solutions were derived. The values ΔH°_f = −3773.6 ± 9.4 kJ/mol, ΔH°_f = −4912.2 ± 24.2 kJ/mol, ΔH°_f = −3734.6 ± 9.7 kJ/mol and ΔH°_f = −4979.7 ± 7.5kJ/mol were found for K_0.85(H₃O)_0.15Fe_2.5(SO₄)₂(OH)_4.5(H₂O)_1.5, K_0.85(H₃O)_0.15Al_2.5(SO₄)₂(OH)_4.5(H₂O)_1.5, Na_0.7(H₃O)_0.3Fe_2.7(SO₄)₂(OH)_5.1(H₂O)_0.9, and Na_0.7(H₃O)_0.3Al_2.7(SO₄)₂(OH)_5.1(H₂O)_0.9 respectively. To our knowledge, this is the first experimentally-based report of ΔH°_f for such nonstoichiometric alunite and natroalunite samples. These thermodynamic data should prove helpful to study, under given conditions, the partitioning of Fe and Al between the solids and aqueous solution.     

A comparison of stygofauna communities inside and outside groundwater bores

Sampling stygofauna is both time consuming and labour intensive. The challenge is to get samples from as many bores as possible within a limited time. The essential assumption for this is that faunal communities inside bores are comparable and representative of the communities outside.To compare relative abundance, taxonomic richness and community composition of the fauna inside groundwater bores to the fauna of the surrounding aquifer, 20 monitoring bores in Palatinate, southwestern Germany, were sampled twice in 1 month. Initially, a sample of 4 l of water was collected from the bottom of each bore. A further sample of 51 l was collected from the groundwater surrounding the bore using a pneumatic piston pump with double packer sampler.Water chemistry inside and outside the bore was similar, but the relative amounts of sediments within the bores were higher compared to those from outside. Relative abundances of fauna inside the bores were higher than in the aquifer, but taxonomic composition was similar with the exception of the proportions of nematodes and amphipods, which were higher inside. As a result, the proportions of cyclopoids were lower inside. Higher nematode proportions are explained partially by the nearly complete extraction of bore sediment. A “habitat heterogeneity effect” states that in heterogeneous aquifers with few suitable habitats, faunal distribution is supposed to be extremely patchy. Thus, detritus accumulates in bores, attracting animals and providing “habitat islands” in the groundwater. This effect could explain the higher amphipode proportions inside the bores, which were generally more frequently populated than the surrounding groundwater. As a consequence, fauna is thought to be nearly absent from groundwater, where suitable habitats are lacking. In those sparsely populated aquifers, samples representative of the aquifer taxonomic richness and composition can only be collected by removing large volumes of water, or by sampling the bottom of bores. These findings also suggest that the use of unbaited colonisation chambers or traps in the groundwater, which are comparable with bores, would seem to be a promising approach.     

Synsedimentary faults and amalgamated unconformities: Insights from 3D-seismic and core analysis of the Lower Triassic Middle Buntsandstein, Ems Trough, north-western Germany

The Late Permian/Early Triassic succession of the Central European Basin (CEB) was repeatedly affected by the tectonic pulses associated with the earliest phases of Tethyan and Arctic–North Atlantic rifting. Effects of the differential tectonic subsidence are particularly well recorded by unconformities, which form widespread sequence boundaries. Such unconformities are most obvious in areas occupied by fault-controlled intra-basinal highs (swells). In that areas, stratigraphic loss may comprise entire Lower and Middle Buntsandstein formations and in places remnant Middle Buntsandstein successions directly rest on Permian strata. Analysis of 3D-seismic data and well logs combined with high-resolution sedimentological logging of drillcores at the western margin of the Ems Trough (NW Germany) reveals details of synsedimentary tectonic control on sequence development. Early Triassic extensional faulting of basement blocks provided stepwise addition of accommodation space for continental sequences by growth faulting along north–south oriented fault zones blocks on the flanks of the East Netherlands High. This process is most evident during the development of the Hardegsen Unconformity, which is characterised by an amalgamation of succeeding unconformity surfaces in areas of structurally controlled intrabasinal highs.     

Seismic velocities from the Yaquina forearc basin off Peru: evidence for free gas within the gas hydrate stability zone

Multichannel seismic (MCS) data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Lateral variations of the reflection pattern at the assumed base of the gas hydrate stability zone in terms of continuity, amplitude, and signal attenuation underneath are observed, as well as the possible occurrence of paleo-bottom simulating reflectors (BSRs). Phase reversed reflections above the bottom simulating reflector point to free gas within the gas hydrate stability zone (GHSZ). To constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the MCS line from high-resolution ocean bottom hydrophone recordings with two independent methods. Heat flux values estimated on the basis of the velocity-depth functions increase with decreasing amplitude of the BSR and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is trapped under parts of the BSR, and within the GHSZ, particularly under the seafloor and under an erosional unconformity. The hypothesis of a paleo-BSR that reflects the uplift of the base of the hydrate stability zone caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values.     

The Mesozoic–Cenozoic structural framework of the Bay of Kiel area, western Baltic Sea

A dense grid of multichannel high-resolution seismic sections from the Bay of Kiel in the western Baltic Sea has been interpreted in order to reveal the Mesozoic and Cenozoic geological evolution of the northern part of the North German Basin. The overall geological evolution of the study area can be separated into four distinct periods. During the Triassic and the Early Jurassic, E–W extension and the deposition of clastic sediments initiated the movement of the underlying Zechstein evaporites. The deposition ceased during the Middle Jurassic, when the entire area was uplifted as a result of the Mid North Sea Doming. The uplift resulted in a pronounced erosion of Upper Triassic and Lower Jurassic strata. This event is marked by a clear angular unconformity on all the seismic sections. The region remained an area of non-deposition until the end of the Early Cretaceous, when the sedimentation resumed in the area. Throughout the Late Cretaceous the sedimentation took place under tectonic quiescence. Reactivated salt movement is observed at the Cretaceous Cenozoic transition as a result of the change from an extensional to compressional regional stress field. The vertical salt movement influenced the Cenozoic sedimentation and resulted in thin-skinned faulting.     

Do clay mineral dissolution rates reach steady state?

The temporal evolution of natural illite du Puy dissolution rates was measured from Si release rates in single-pass flow-through experiments lasting at least 100 days at 25°C and pH ranging from 2 to 12. Si release rates decreased by a factor of five and three at pH 12 and 2, respectively, during the experiments. These observations are interpreted to stem from changes in illite du Puy reactive surface area during these experiments. As the edges of clay minerals dissolve faster than the basal planes, dissolution tends to change clay mineral morphology decreasing the percentage of reactive edge sites. This continuously changing morphology prevents illite dissolution rates from attaining steady state during laboratory experiments lasting 100 to 200 days. A similar temporal decrease in dissolution rates is evident for many different sets of clay mineral dissolution rate data available in the literature. It seems reasonable, therefore, to expect that clay mineral dissolution does not attain steady state in nature, but rather their dissolution rates decrease continuously during their dissolution.     

Efficient finite-difference modelling of seismic waves using locally adjustable time steps

Conventional finite-difference modelling algorithms for seismic forward modelling are based on a time-stepping scheme with a constant (global) time step. Large contrasts in the velocity model or in the spatial sampling rate cause oversampling in time for some regions of the model. The use of locally adjustable time steps can save large amounts of computation time for certain modelling configurations. The computation of spatial derivatives across the transition zone between regions of the model with different temporal sampling requires the definition of the wavefield at corresponding time levels on both sides of the transition zone. This condition can be obtained by extrapolation in time, which is inaccurate, or by multiple time integration in the transition zone. The error in the latter solution is of the same order as the conventional time-stepping scheme because both methods are based on the same iteration formula. The technique of multiple time integration simply requires the use of different sizes of time step. It is applicable only for certain factors of variation of the time step.