Modelling detrital cooling-age populations: insights from two Himalayan catchments

The distribution of detrital mineral cooling ages in river sediment provides a proxy record for the erosional history of mountain ranges. We have developed a numerical model that predicts detrital mineral age distributions for individual catchments in which particle paths move vertically toward the surface. Despite a restrictive set of assumptions, the model permits theoretical exploration of the effects of thermal structure, erosion rate, and topography on cooling ages. Hypsometry of the source‐area catchment is shown to exert a fundamental control on the frequency distribution of bedrock and detrital ages. We illustrate this approach by generating synthetic ⁴⁰Ar/³⁹Ar muscovite age distributions for two catchments with contrasting erosion rates in central Nepal and then by comparing actual measured cooling‐age distributions with the synthetic ones. Monte Carlo sampling is used to assess the mismatch between observed and synthetic age distributions and to explore the dependence of that mismatch on the complexity of the synthetic age signal and on the number of grains analysed. Observed detrital cooling ages are well matched by predicted ages for a more slowly eroding Himalayan catchment. A poorer match for a rapidly eroding catchment may result from some combination of large analytical uncertainties in the detrital ages and inhomogeneous erosion rates within the basin. Such mismatches emphasize the need for more accurate thermal and kinematic models and for sampling strategies that are adapted to catchment‐specific geologic and geomorphic conditions.     

Luminescence chronology of incision and channel pattern changes in the River Ganga, India

The River Ganga in the central Gangetic plain shows the incision of 20 m of Late Quaternary sediments that form a vast upland terrace (T₂). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T₁) and the present-day flood plain (terrace-T₀). Terrace-T₁ shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T₀ and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided.Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T₂ show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka.     

The Effect of Alongshore Protection Structures on Shore Dynamics

A mathematical model of the seashore dynamics is considered. The model describes the seashore protection against destruction by alongshore through structures, underwater trenches, and artificial islands. The results of model calculations are presented and correlated with the data of measurements made on hydraulic models.     

Effect of Sliding Rate on the Activity of Acoustic Emission During Stable Sliding

--The mechanical and statistical characteristics of acoustic emission (AE) events during stable sliding are investigated through a laboratory experiment using a granite specimen with a pre-cut fault. Numerous AE events are found to be generated on the pre-cut fault, indicating that microscopically unstable fracture occurs during macroscopically stable sliding. The composite focal mechanism solution of AE events is determined from the first motion directions of P-waves. The determined mechanism is consistent with the double-couple one expected for the slip on the pre-cut fault. The source radii of large AE events are estimated to be about 10 mm from the widths of the first P-wave pulses. These indicate that the AE events are generated by shear fracture whose faulting area is a part of the pre-cut fault plane. The occurrence of AE events as a stochastic process approximately obeys the Poisson process, if the effect of mutually dependent events consti tuting clusters is corrected. The observed amplitude-frequency relation of AE events approximately follows a power law for a limited amplitude range. As the macroscopic sliding rate increases, the number of AE events per unit sliding distance decreases. This rate dependence of the AE activity is qualitatively consistent with the observation that the real area of contact between sliding surfaces decreases with an increase in the sliding rate as reported in the literature.     

Evaluation of Sea Water Quality in the Coastal Zone of Primor'e Using the Method of Microbial Indication

The response of marine microorganism community to variations in the concentrations of heavy metals in the environment is investigated for the coastal waters of Peter the Great Bay and the northern part of Primor'e. The fact of the existence and sensitivity of the individual response of plankton community microorganisms to variations in the concentrations of metals in the water under the conditions of water contamination with a complex of metals are confirmed on the basis of microbiological data and the results of their comparison to the data of chemical analysis. Microbial indication is shown to serve as an operative method of monitoring and short-term forecasting of changes in the environmental conditions of sea coastal waters. This method allows obtaining preliminary information at the stage of selecting environmental quality observation stations.     

Optimal Model for Geoid Determination from Airborne Gravity

Two different approaches for transformation of airborne gravity disturbances, derived from gravity observations at low-elevation flying platforms, into geoidal undulations are formulated, tested and discussed in this contribution. Their mathematical models are based on Green's integral equations. They are in these two approaches defined at two different levels and also applied in a mutually reversed order. While one of these approaches corresponds to the classical method commonly applied in processing of ground gravity data, the other approach represents a new method for processing of gravity data in geoid determination that is unique to airborne gravimetry. Although theoretically equivalent in the continuous sense, both approaches are tested numerically for possible numerical advantages, especially due to the inverse of discretized Fredholm's integral equation of the first kind applied on different data. High-frequency synthetic gravity data burdened by the 2-mGal random noise, that are expected from current airborne gravity systems, are used for numerical testing. The results show that both approaches can deliver for the given data a comparable cm-level accuracy of the geoidal undulations. The new approach has, however, significantly higher computational efficiency. It would be thus recommended for real life geoid computations. Additional errors related to regularization of gravity data and the geoid, and to accuracy of the reference field, that would further deteriorate the quality of estimated geoidal undulations, are not considered in this study.     

Extracting flow structures from tracer data

A method of visualizing structures in closed chaotic flows out of homogenous particle distributions is presented in the example of models of a meandering jet. To this end, the system will be leaked or opened up by defining a region of the flow, so that a particle is considered to be escaped if it leaves this region. By applying this method to an ensemble of nonescaped tracers, we are able to characterize mixing processes by visualizing the converging and stretching filamentations (stable and unstable manifolds) in the flow without using additional mathematical tools. The possibility of applying the algorithm to analyze buoy data, and a comparison with the finite time manifolds are discussed.     

Atmospheric nitrogen inputs into the North Sea: effect on productivity

The ANICE (Atmospheric Nitrogen Inputs into the Coastal Ecosystem) project addressed the atmospheric deposition of nitrogen to the North Sea, with emphasis on coastal effects. ANICE focused on quantifying the deposition of inorganic nitrogen compounds to the North Sea and the governing processes. An overview of the results from modelling and experimental efforts is presented. They serve to identify the role of the atmosphere as a source of biologically essential chemical species to the marine biota. Data from the Weybourne Atmospheric Observatory (UK) are used to evaluate the effect of short episodes with very high atmospheric nitrogen concentrations. One such episode resulted in an average deposition of 0.8 mmol N m⁻² day⁻¹, which has the potential to promote primary productivity of 5.3 mmol C m⁻² day⁻¹. This value is compared to long-term effects determined from model results. The total calculated atmospheric deposition to the North Sea in 1999 is 948 kg N km⁻¹, i.e. 0.19 mmol N m⁻² day⁻¹ which has the potential to promote primary productivity of 1.2 mmol C m⁻² day⁻¹. Detailed results for August 1999 show strong gradients across the North Sea due to adjacent areas where emissions of NO_x and NH₃ are among the highest in Europe. The average atmospheric deposition to the southern part of the North Sea in August 1999 could potentially promote primary production of 2.0 mmol C m⁻² day⁻¹, i.e. 5.5% of the total production at this time of the year in this area of the North Sea. For the entire study area the atmospheric contribution to the primary production per m² is about two-third of this value. Most of the deposition occurs during short periods with high atmospheric concentrations. This atmospheric nitrogen is almost entirely anthropogenic in origin and thus represents a human-induced perturbation of the ecosystem.