Numerical Solution of Lock-Release Gravity Current with Viscous Self-Similar Regime

Lock-release gravity currents with a viscous self-similar regime are simulated by use of the renormalization group(RNG) k - ε model for Reynolds-stress closure. Besides the turbulent regime with initially a slumping phase of a conslant current front speed and later an inviseid self-similar phase of front speed decreasing as t^-1/3(where t is the time measured from release), the viseous self-similar regime is satisfactorily reproduced with front speed decreasing as t^-4/5,consistent with well known experimental observations.     

Multi-proxy evidence of long-term changes in ecosystem structure in a Danish marine estuary, linked to increased nutrient loading

This paper presents a study of changes in eutrophication over the past 100 years in a fertile estuary. The Danish estuary Mariager Fjord is a long, narrow sill-fjord with a permanently anoxic basin. In 1997 anoxia spread from the basin to the entire inner estuary, killing almost all eukaryotes and prompting debate on the causes. This paper reports a multi-proxy survey of ²¹⁰Pb-dated sediment cores from the anoxic basin. Analyses of diatoms, dinoflagellates, pigments and geochemical proxies were used to determine changes in ecosystem structure over the past 100 years. The aim was to establish ‘base-line conditions’, for management purposes, of the biological structure prior to 1900, and to examine possible causes of changes observed. Geochemical proxies total nitrogen (TN), total carbon (TC) and biogenic silica (BSi) were consistently high throughout the sediment record. Increased concentrations of pigments and natural isotopes (δ¹³C, δ¹⁵N) suggested increasing production and nutrient loading. The main changes in the biological proxies occurred between 1915 and the 1940s, and indicated that the estuary has been somewhat eutrophic since 1900, but that the eutrophication process increased over the past 100 years. A reconstruction of TN concentration by a diatom-based transfer function supports this interpretation, with inferred TN ca. 1900 around 60 μmol l⁻¹, and an increase in TN concentration over the past century to ca. 130 μmol l⁻¹ by 1995. Inferred TN decreased to ca. 100 μmol l⁻¹ by 2001, similar to present day monitoring data.     

Oil-generation kinetics for organic facies with Type-II and -IIS kerogen in the Menilite Shales of the Polish Carpathians

The Menilite Shales (Oligocene) of the Polish Carpathians are the source of low-sulfur oils in the thrust belt and some high-sulfur oils in the Carpathian Foredeep. These oil occurrences indicate that the high-sulfur oils in the Foredeep were generated and expelled before major thrusting and the low-sulfur oils in the thrust belt were generated and expelled during or after major thrusting. Two distinct organic facies have been observed in the Menilite Shales. One organic facies has a high clastic sediment input and contains Type-II kerogen. The other organic facies has a lower clastic sediment input and contains Type-IIS kerogen. Representative samples of both organic facies were used to determine kinetic parameters for immiscible oil generation by isothermal hydrous pyrolysis and S₂ generation by non-isothermal open-system pyrolysis. The derived kinetic parameters showed that timing of S₂ generation was not as different between the Type-IIS and -II kerogen based on open-system pyrolysis as compared with immiscible oil generation based on hydrous pyrolysis. Applying these kinetic parameters to a burial history in the Skole unit showed that some expelled oil would have been generated from the organic facies with Type-IIS kerogen before major thrusting with the hydrous-pyrolysis kinetic parameters but not with the open-system pyrolysis kinetic parameters. The inability of open-system pyrolysis to determine earlier petroleum generation from Type-IIS kerogen is attributed to the large polar-rich bitumen component in S₂ generation, rapid loss of sulfur free-radical initiators in the open system, and diminished radical selectivity and rate constant differences at higher temperatures. Hydrous-pyrolysis kinetic parameters are determined in the presence of water at lower temperatures in a closed system, which allows differentiation of bitumen and oil generation, interaction of free-radical initiators, greater radical selectivity, and more distinguishable rate constants as would occur during natural maturation. Kinetic parameters derived from hydrous pyrolysis show good correlations with one another (compensation effect) and kerogen organic-sulfur contents. These correlations allow for indirect determination of hydrous-pyrolysis kinetic parameters on the basis of the organic-sulfur mole fraction of an immature Type-II or -IIS kerogen.     

Investigating the scale of structural controls on chlorinated hydrocarbon distributions in the fractured-porous unsaturated zone of a sandstone aquifer in the UK

Contaminant migration behaviour in the unsaturated zone of a fractured porous aquifer is discussed in the context of a study site in Cheshire, UK. The site is situated on gently dipping sandstones, adjacent to a linear lagoon historically used to dispose of industrial wastes containing chlorinated solvents. Two cores of more than 100 m length were recovered and measurements of chlorinated hydrocarbons (CHCs), inorganic chemistry, lithology, fracturing and aquifer properties were made. The results show that selecting an appropriate vertical sampling density is crucial both to providing an understanding of contaminant pathways and distinguishing whether CHCs are present in the aqueous or non-aqueous phase. The spacing of such sampling should be on a similar scale to the heterogeneity that controls water and contaminant movement. For some sections of the Permo-Triassic aquifer, significant changes in lithology and permeability occur over vertical distances of less than 1 m and samples need to be collected at this interval, otherwise considerable resolution is lost, potentially leading to erroneous interpretation of data. At this site, although CHC concentrations were high, the consistent ratio of the two main components of the plume (tetrachloroethene and trichloroethene) provided evidence of movement in the aqueous phase rather than in dense non-aqueous phase liquid (DNAPL).     

Une méthode de quantification de la dégradation d'un escarpement de faille au cours des cycles climatiques du Quaternaire : la faille de Jobourg (Nord Cotentin,France)Quantification of a fault-scarp degradation through Quaternary: the Jobourg fault (North Cotentin,France)

The degradation of the Jobourg fault-scarp occurred by cryoclastic processes in a periglacial environment during a part of Quaternary time. An attempt of quantification indicates a bulk scarp erosion of about 39 m³ m^?2, while the head accumulated at the bottom of the fault scarp only represents 4.6 m³ m^?2. To cite this article: M. Font et al., C. R. Geoscience 334 (2002) 171–178.     

Tracking recent recovery from eutrophication in a high arctic lake (Meretta Lake, Cornwallis Island, Nunavut, Canada) using fossil diatom assemblages

Meretta Lake (Resolute Bay, Cornwallis Island, Nunavut, Canada) is a high arctic lake that received raw sewage for almost 50 years from the Canadian Department of Transport Base. The lake was sampled from 1968–72 during the International Biological Programme, as part of the Char Lake Project. As the number of users at the Transport Base declined throughout the 1990s, so too did the lake's nutrient levels, and Meretta Lake is now classified as oligotrophic. A previous diatom-based paleolimnological study revealed marked species assemblage shifts coincident with sewage inputs beginning in the late 1940s; however, because the core was taken at a time when nutrient levels were still relatively high (i.e., 1993), the diatom record did not yet track any signs of recovery. In this present study, we examined fossil diatom assemblages from a sediment core taken in 2001. Our results indicate a shift to the pre-impact diatom assemblages in the most recent sediments, indicating that the paleolimnological record is tracking the decreased nutrient inputs to this high arctic lake, and confirms that no significant lags exist in these largely ice-covered lakes.     

The measurement of soil gases and shallow temperature for determination of active faults in a geothermal area: a case study from Ömer–Gecek,Afyonkarahisar (West Anatolia)

Afyonkarahisar is a very important geothermal province of western Anatolia and has low and medium enthalpy geothermal areas. This study has been carried out for the preparation of distribution maps of soil gases (radon and carbon dioxide) and shallow soil temperature and the exploration of permeable tectonic regions associated with geothermal systems and reveal the origins of radon and carbon dioxide gases. The western district of the study area is characterized by the high radon concentration (168.30 kBq/m³), carbon dioxide ratio (0.30%), and soil temperature (21.0 °C) values. Fethibey and Demirçevre faults, which allow the circulation of geothermal fluids, have been detected in the distribution maps of radon, carbon dioxide, and shallow depth temperature and the directions of the curves in these maps correspond to the strikes of Demirçevre faults. The effect of the fault plays an important role in the change of carbon dioxide concentration along the W-E directional geological section prepared to determine the change of soil gas and shallow depth temperature values depending on lithological differences, fault existence, and geothermal reservoir depth. On the other hand, it was determined that Rn²²² concentration and soil temperature changed as a function of geothermal reservoir depth or lithological difference. Tuffs in Köprülü volcano-sedimentary units are the main source of radon due to their higher uranium contents. Besides, the carbon dioxide in Ömer–Gecek soils has geothermal origin because of the highest carbon dioxide content (99.3%) in non-condense gas. The similarities in patterns of soil temperature, radon, and carbon dioxide indicate that the variation in soil temperatures is related to radon and carbon dioxide emissions. It is concluded that soil gas and temperature measurements can be used to determine the active faults in the initial stage of geothermal exploration successfully.     

Examination of the structural characteristics arising in gypsums by the GPR and MASW methods (Sivas,Turkey)

This study was performed at an area of 50?×?48 m² being defined as a new settlement in the northeast of Sivas. In the study, the discontinuities that are not deep and their geophysical characteristics were examined by the GPR and MASW methods. For interpretation, GPR cross sections were prepared as 2D–3D, and MASW cross sections were prepared as 2D. As for geophysical cross sections, about 10 m depth was examined. It was understood that the reflections observed in the form of hyperbolas in GPR cross sections correspond to areas having low S wave velocity (V_s) in MASW cross sections. It was understood that the S wave velocities are lower than 653 m/s, that the seismic velocities in between 653 and 275 m/s indicate partially deteriorated areas and that the S wave velocities of unweathered gypsums are higher than 1275 m/s at these low-velocity zones. Thus, it was thought that the fill material that may arise in the fracture, crack and deterioration areas arises from intercalation and clastic gypsum units, and that it plays a role in having low value S wave velocities. In all the geophysical cross sections, it was understood that the structures with gypsum are intense at the initial 5 m. And a fracture at the south of the study area, that it was estimated might be longer than 40 m, was determined as the largest gypsum structure. It was understood that this fracture starts from a depth of about 5 m in the west and that it slopes down to 7 m depth in the east. According to these results, it was understood that the damage amount arising in time in the gypsum structures from the effect of water may increase, the study area was defined as risky, and the required importance should be attached to these structures especially in foundation engineering.