Riparian alder fens — source or sink for nutrients and dissolved organic carbon? — 2. Major sources and sinks

Natural riparian forest wetlands are known to be effective in their ability to remove nitrate by denitrification and sediments with attached phosphorus via sedimentation. On the other hand, litter input and decomposition is a process of crucial importance in cycling of nitrogen and phosphorus in a forest ecosystem.In this study we investigated the amount of nitrogen and phosphorus entering the alder fen ecosystem through leaf litter and its decomposition and the removal capacity of nitrogen and phosphorus by measuring denitrification and sedimentation in the alder fen.We found an average input of leaf litter during fall 1998 of 226 g m⁻² yr⁻¹ DW with nutrient concentration of 0.17% P and 1.6% N. This means a yearly input of 0.4 g m⁻² yr⁻¹ P and 3.6 g m⁻² yr⁻¹ N. The decomposition of leaf litter using litter bags with small and large mesh size resulted in bags with macroinvertebrates (large mesh size) and without macroinvertebrates (small mesh size). After 57 days the litter bags with macroinvertebrates had a decomposition rate of 79%.Denitrification was measured in May and June of 1997 using the acetylene inhibition technique on intact soil cores and slurry-experiments. The average annual denitrification rate was 0.2 g m⁻² yr⁻¹ N using data from the core experiments. The denitrification rate was higher after addition of nitrate, indicating that denitrification in the riparian alder fen is mainly controlled by nitrate supply.The sedimentation rate in the investigated alder fen ranged from 0.47 kg m⁻² yr⁻¹ DW to 4.46 kg m⁻² yr⁻¹ DW in 1998 depending on the study site and method we used. Sedimentation rates were lower in newly designed plate traps than in cylinder traps. The alder fen also showed lower rates than the adjacent creek Briese. Average phosphorus removal rate was 0.33 g m⁻² yr⁻¹ P.Input sources for the surface water of the alder fen are sediment mineralization and decomposition of leaf litter; output sources are sedimentation and denitrification. This study showed that a nutrient input of 24.58 kg ha⁻¹ yr⁻¹ N, 8.8 kg ha⁻¹ yr⁻¹ P and 419 kg ha⁻¹ yr⁻¹ DOC into the surface water of the alder fen is possible. Alder fens cannot improve water quality of an adjacent river system. This is only true for a nearly pristine alder fen with the hydrology of 10 months flooded conditions and 2 months non-flooding conditions a year.     

Paleolatitudes of the Kerguelen hotspot: new paleomagnetic results and dynamic modeling

The Kerguelen Plateau, a Large Igneous Province in the southern Indian Ocean, was formed as a product of the Kerguelen hotspot in several eruptive phases during the last 120 Myr. We obtained new paleolatitudes for the central and northern Kerguelen Plateau from paleomagnetic investigations on basalts, which were drilled during ODP Leg 183 to the Kerguelen Plateau-Broken Ridge. The paleolatitudes coincide with paleolatitudes from previous investigations at the Kerguelen Plateau and Ninetyeast Ridge (the track of the Kerguelen hotspot) and indicate a difference between paleolatitudes and present position at 49°S of the Kerguelen hotspot. We show that true polar wander, the global motion between the mantle and the rotation axis, cannot explain this difference in latitudes. We present numerical model results of plume conduit motion in a large-scale mantle flow and the resulting surface hotspot motion. A large number of models all predict southward motion between 3° and 10° for the Kerguelen hotspot during the last 100 Myr, which is consistent with our paleomagnetic results.     

How to obtain alert velocity thresholds for large rockslides

A reliable forecast of the failure stage of large rockslides is difficult, because of non-linear time dependency of displacements and seasonal effects. Aim of this paper is to suggest a practical method to prepare alert thresholds for large rockslides, assessing critical values of velocity for carrying out civil protection actions using monitoring data. Adopted data concern the 20 Mm³ Ruinon rockslide (Valfurva, Central Alps, Italy), still evolving and suitable to originate a fast moving rock avalanche. Multitemporal analysis of aerial photos, LIDAR-ALTM laser topography, field survey and geomechanical analyses allowed to infer the rockslide kinematics and better understand data provided by a monitoring network including distometers, extensometers, GPS benchmarks and inclinometers. The analysis of displacement and rainfall data over five years (1997–2001) allowed to recognise three different evolutionary patterns of displacements, showing a continuously increasing rate since 1997. Data representing large-scale behaviour of the rock mass were fitted by power-law curves, according to the “accelerating creep” model by Voight, in order to evaluate a suitable failure time. This was hampered by the large seasonal deviations, which can significantly delay the occurrence of failure. Data were fitted using the Voight’s equation, expressed in terms of displacement, through non-linear estimation techniques, in order to find values of the controlling parameters (A, α and t_f) suitable to represent the mechanical behaviour of the rock mass approaching the failure. This allowed to compute velocity–time theoretical curves and to define different velocity threshold values (pre-alert, alert and emergency) to be used for emergency management.     

Spatial and temporal variability of pore‐water pressures in residual soil slopes in a tropical climate

It is critical to understand and quantify the temporal and spatial variability in hillslope hydrological data in order to advance hillslope hydrological studies, evaluate distributed parameter hydrological models, analyse variability in hydrological response of slopes and design efficient field data sampling networks. The spatial and temporal variability of field‐measured pore‐water pressures in three residual soil slopes in Singapore was investigated using geostatistical methods. Parameters of the semivariograms, namely the range, sill and nugget effect, revealed interesting insights into the spatial structure of the temporal situation of pore‐water pressures in the slopes. While informative, mean estimates have been shown to be inadequate for modelling purposes, indicator semivariograms together with mean prediction by kriging provide a better form of model input. Results also indicate that significant temporal and spatial variability in pore‐water pressures exists in the slope profile and thereby induces variability in hydrological response of the slope. Spatial and temporal variability in pore‐water pressure decreases with increasing soil depth. The variability decreases during wet conditions as the slope approaches near saturation and the variability increases with high matric suction development following rainfall periods. Variability in pore‐water pressures is greatest at shallow depths and near the slope crest and is strongly influenced by the combined action of microclimate, vegetation and soil properties. Copyright © 2002 John Wiley & Sons, Ltd.     

电离层水平不均匀性对无线电掩星资料反演的影响

电离层水平不均匀结构是引起掩星资料反演的主要误差源.本文模拟不均匀结构对无线电掩星资料反演的影响.首先利用3-D全球射线追踪技术进行了模拟.结果指出:电离层电子密度20%的变化可能引起GPS的时延增加约12m.其次,将90%的掩星时延对应的路径长度,定义为电离层掩星测量的水平分辨率,其值为1200-2000 km.最后,计算了不同纬度电离层当地正午、子夜和日出时的电离层水平不对称指数.它表明:低纬夏季日出时,不对称指数达-78%,反演结果可能有较大误差.     

Production of selected cosmogenic radionuclides by muons: 2. Capture of negative muons

We have determined the production yields for radionuclides in Al₂O₃, SiO₂, S, Ar, K₂SO₄, CaCO₃, Fe, Ni and Cu targets, which were irradiated with slow negative muons at the Paul Scherrer Institute in Villigen (Switzerland). The fluences of the stopped negative muons were determined by measuring the muonic X-rays. The concentrations of the long-lived and short-lived radionuclides were measured with accelerator mass spectrometry (AMS) and γ-spectroscopy, respectively. Special emphasis was put on the radionuclides ¹⁰Be, ¹⁴C and ²⁶Al produced in quartz targets, ²⁶Al in Al₂O₃ and S targets, ³⁶Cl in K₂SO₄ and CaCO₃ targets, and ⁵³Mn in Fe₂O₃ targets. These targets were selected because they are also the naturally occurring target minerals for cosmic ray interactions in typical rocks. We also present results of calculations for depth-dependent production rates of radionuclides produced after cosmic ray μ⁻ capture, as well as cosmic ray-induced production rates of geologically relevant radionuclides produced by the nucleonic component, by μ⁻ capture, by fast muons and by neutron capture.     

Identification of Mining Blasts at Mid- to Far-regional Distances Using Low Frequency Seismic Signals

—?This paper reports results from two recent monitoring experiments in Wyoming. Broadband seismic recordings of kiloton class delay-fired cast blasts and instantaneous calibration shots in the Black Thunder coal mine were made at four azimuths at ranges from 1° to 2°. The primary focus of this experiment was to observe and to explain low-frequency signals that can be seen at all azimuths and should routinely propagate above noise to mid-regional distances where most events will be recorded by International Monitoring System (IMS) stations.¶The recordings clearly demonstrate that large millisecond delay-fired cast blasts routinely produce seismic signals that have significant spectral modulations below 10?Hz. These modulations are independent of time, the azimuth from the source and the orientation of the sensor. Low-frequency modulations below 5?Hz are seen beyond 9°. The modulations are not due to resonance as they are not produced by the calibration shots. Linear elastic modeling of the blasts that is guided by mine-blast reports fails to reproduce the fine detail of these modulations but clearly indicates that the enhanced “spectral roughness” is due to long interrow delays and source finiteness. The mismatch between the data and the synthetics is likely due to source processes, such as nonlinear interactions between shots, that are poorly understood and to other effects, such as variations of shot time and yield from planned values, that are known to be omnipresent but cannot be described accurately. A variant of the Automated Time-Frequency Discriminant (Hedlin, 1998b), which uses low-frequency spectral modulations, effectively separates these events from the calibration shots.¶The experiment also provided evidence that kiloton class cast blasts consistently yield energetic 2–10 second surface waves. The surface waves are strongly dependent on azimuth but are seen beyond 9°. Physical modeling of these events indicates that the surface waves are due mainly to the extended source duration and to a lesser extent to the slap-down of spalled material. The directionality is largely a path effect. A discriminant that is based on the partitioning of energy between surface and body waves routinely separates these events from the calibration shots.¶The Powder River Basin has essentially no natural seismic activity. How these mining events compare to earthquake observations remains to be determined.     

The Influence of 3-D Structure on the Propagation of Seismic Waves Away from Earthquakes

-- The seismic records from significant earthquakes are profoundly affected by 3-D variations in crustal structure both in the source zone itself and in propagation to some distance. Even in structurally complex zones such as Japan and Mexico relatively coherent arrivals are found associated with different classes of propagation paths. The presence of strong lateral variations can disrupt the arrivals, and impose significant variations in propagation characteristics for different directions from the source as illustrated by observations for the 1995 Kobe and 2000 Tottori-ken Seibu earthquakes in western Japan. Such effects can be modelled in 3 dimensions using a hybrid scheme with a pseudospectral representation for horizontal coordinates and finite differences in depth. This arrangement improves parallel implementation by minimising communication costs. For a realistic 3-D model for the structure in western Japan the 3-D simulations to frequencies close to 1 Hz provide a good representation of the observations from subduction zones events such as the 1946 Nankai earthquake and the 2000 Tottori-ken Seibu earthquake. The model can therefore be used to investigate the pattern of ground motion expected for future events e.g., in current seismic gaps.     

Moment Tensor Inversion of Regional Phases: Application to a Mine Collapse

—?The aim of our study consists of analyzing potentially non-double-couple seismic events recorded at regional distances. In order to define the nature of the seismic source, a moment tensor inversion is carried out as this method is general enough not to initially constrain the source mechanism. In this paper we present an application to a seismic event induced by a mine collapse which occurred near the town of Halle in Germany. Because of its induced nature, many parameters such as the location and geometry of this seismic source are known. This information allows us to test the influence of inadequate propagation modeling on the moment tensor obtained from the inversion. Green's functions have been computed with the reflectivity method in a flat layered medium, using the European model EurID (Du et? al., 1998; Dufumier et al., 1997). From the inversion of P-wave seismograms recorded by the German Regional Seismic Network will, we obtained a source time function which can be decomposed into two subevents. The first one has a large isotropic part and a deviatoric mechanism with near vertical nodal planes. No volume change is observed for the second subevent, but a deviatoric component opposite of the first one. The addition of S-waves does not change the results of the inversion which are stable. Surface waves were not used because of their poor dispersion curves. Based on the moment tensor obtained from these inversions, the physical process at the source is compatible with a large cavity collapse.     

"THREE DIMENSIONAL MATHEMATICAL MODEL OF SEDIMENT TRANSPORT IN ESTUARINE REGIONS-- A CASE STUDY OF THE HAIHE RIVER MOUTH" Yuchuan Bai,Huanting SHEN and Shixiong HU

The paper presents the 3D finite element simulation of tidal flow and Sediment transport in the estuarine region of the Haihe river. The proposed model adopts sigma-transformation of the hydrodynamic and sediment transport equations. The hydrodynamic and sediment transport models are verified in case of a simple test problem for which analytical solutions are available. Finally the models are applied to muddy Haihe river estuary of North China and it is claimed that hydrodynamic and sediment transport models give a reliable comparison with the observed field data. However, there are certain discrepancies, and some reasonable questions regarding the present state-of-art, in the modeling of three-dimensional multilevel hydrodynamics and sediment transport, which are provided below for answer.