Die Einschichtung der Rhone im Genfersee: Ergebnisse von Strömungsmessungen im August 1983

Results obtained by means of an ultrasonic current meter in the plume of the Rhone river are summarized as follows:

1. Currents of Rhone river water entering the lake were clearly discernible up to a distance of about 1 km from the river mouth. The interflow was observed at depth of 10 to 30 m.
2. Interflow velocities decreased with increasing distance from the river mouth: from 40cm/s at a distance of 350 m to about 15 cm/s at 1 km.
3. Short-term variations of current velocities and directions documented the highly turbulent nature of the interflow.
4. The entering river water (inflow direction to the NNW) interfered with a persisting northeastward current of the lake water. At a distance of 1–2 km from the mouth the interflow gradually assumed the same direction, possibly due to deflection by Coriolis forces.
5. Current velocities showed considerable variations within a time scale in the order of hours at the same measuring position. The reasons for these fluctuations remain unclear. Possible causes may be lateral oscillations of the entering river water or its deviation by river mouth bars during periods of reduced river discharge. Variations of the discharge alone cannot explain these current fluctuations.

     

Chemistry of submarine hydrothermal solutions at Guaymas Basin,Gulf of California

Reconnaisance ALVIN dives in the sediment-filled southern trough of the Guaymas Basin found active hot springs with temperatures ranging up to 315°C. High temperature activity is generally restricted to the crests of large mounds that rise out of the flat-lying basin sediments. The chemistry of the hydrothermal waters is distinctly different from that characteristic of sediment-starved, open ocean ridge axes in that the solutions are alkaline, contain ammonium as a major ion and are strongly depleted in the “ore-forming” metals. These compositions are interpreted as the result of reaction of a primary solution, similar in composition to those as 21°N, EPR, with the biogenous sediments overlying the intrusion zone. The pH of this fluid is raised both by the dissolution of carbonate and the addition of ammonium from thermocatalytic cracking of immature planktonic carbon. Metal sulfides are consequently precipitated at depth in the sediment column. The Guaymas Basin is thus the site of active formation of a sediment-hosted massive sulfide mineral deposit; the exiting waters are the “spent” ore-forming fluid. The ammonium data demonstrate that organic carbon (black shale) is, by itself, a sufficient source of alkalinity to induce the precipitation of sulfides from ascending solutions. Since ammonium does not participate directly in these reactions but does form secondary aluminosilicate minerals these latter should constitute a valuable exploration tool in the search for shale hosted deposits.     

Zur Ökologie des Brachsen (Abramis brama (L.)) im Bodensee

The reactions of the bream,Abramis brama, to the ecological changes due to man-made eutrophication of Lake Constance were investigated. It could be demonstrated that the bream adapted only to a negligible amount. Likely due to its largeness, structure of the body and population composition the bream is more conservative than perch, roach and whitefish. The only reactions to the changed environment are modificated vertical and horizontal distribution and partly plancton feeding in summer.      

A comparative study of observed and GCM-simulated turbulent surface fluxes at the positions of Atlantic weatherships

If oceanic models are to be driven with transient atmospheric input, data from standard daily analyses or from an atmospheric GCM simulation can be used. The question arises whether these “FF-data” (FF = field forcing) are appropriate to be used as a realistic oceanic forcing in the form of wind stress and turbulent heat fluxes.A series of different FF-data sets is compared with respective “LF-data” (LF = local forcing) derived from in situ weathership observations. We believe these LF data to be the most accurate and reliable long year maritime time series. The study is restricted to 8 Atlantic weatherships and January conditions and to fields obtained with the Hamburg University GCM or derived from analyses of the German Weather Service (DWD).It turns out that DWD based FF data sets are suitable only if long year mean values are required. In general, the interannual and synoptic scale variability is too small for all FF data sets. With respect to the windstress, the empirical formulae to obtain the surface wind (from the sea level pressure field) together with the usage of a windspread dependent drag coefficient yield the best though still unsatisfying results. The approach using generalized similarity theory gives worse results with respect to the synoptic scale and interannual variability.The GCM simulated data set is systematically biased over wide regions which is partly due to a shift in the model's quasistationary Icelandic Low and an increased temperature at the model's lowest level. The transients are simulated at some positions even poorer than those analysed by the DWD, but at other positions superior though still weaker than the LF data's.     

The strength and rheology of commercial tungsten carbide cermets used in high-pressure apparatus

Uniaxial compressive stress-strain curves have been measured on a suite of 26 commercial grades of tungsten carbide cermets and three maraging steels of interest for use in high-pressure apparatus. Tests were conducted on cylindrical specimens with a length to diameter ratio of two. Load was applied to the specimens by tungsten carbide anvils padded by extrudable lead disks. Interference fit binding rings of maraging steel were pressed on to the ends of the specimens to inhibit premature corner fractures. Bonded resistance strain gages were used to measure both axial and tangential strains. Deformation was exremely uniform in the central, gauged portion of the specimens. Tests were conducted at a constant engineering strain rate of 1×10^–5s^–1. The composition of the specimens was principally WC/Co with minor amounts of other carbides in some cases. The Co weight fraction ranged from 2 to 15%. Observed compressive strengths ranged from about 4 to just above 8 GPa. Axial strain amplitude at failure varied from 1.5% to 9%. Representative stress-strain curves and a ranking of the grades in terms of yield strength and strain at failure are presented. A power law strain hardening relation and the Ramberg-Osgood stress-strain equation were fit to the data. Fits were very good for both functions to axial strain amplitudes of about 2%. The failure of these established functions is accompanied by an abrupt change in the trend of volumetric strain consistent with the onset of substantial microcrack volume.     

Complex exposure histories of chert clasts in the late Pleistocene shorelines of Lake Lisan,southern Israel

Activities of ²⁶Al and ¹⁰Be in five chert clasts sampled from two beach ridges of late Pleistocene Lake Lisan, precursor of the Dead Sea in southern Israel, indicate low rates of chert bedrock erosion and complex exposure, burial, and by inference, transport histories. The chert clasts were derived from the Senonian Mishash Formation, a chert‐bearing chalk, which is widely exposed in the Nahal Zin drainage basin, the drainage system that supplied most of the material to the beach ridges. Simple exposure ages, assuming only exposure at the beach ridge sampling sites, range from 35 to 354 ky; using the ratio ²⁶Al/¹⁰Be, total clast histories range from 0·46 to 4·3 My, unrelated to the clasts' current position and exposure period on the late Pleistocene beach ridges, 160–177 m below sea level. Optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20·0 ± 1·4 ka and 36·1 ± 3·3 ka. These ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of Lake Lisan, which suggest that the lake reached its highest stand around 27 000 cal. years BP . If the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production, then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50–1300 ky and 390–3130 ky respectively. Alternatively, the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock. Back calculating such rates of rock erosion suggests values between 0·4 and 12 m My^?1. The relatively long burial periods indicate extended sediment storage as colluvium on slopes and/or as alluvial deposits in river terraces. Some clasts may have been stored for long periods in abandoned Pliocene and early Pleistocene routes of Nahal Zin to the Mediterranean before being transported again back into the Nahal Zin drainage system and washed on to the shores of Lake Lisan during the late Pleistocene. Copyright © 2003 John Wiley & Sons, Ltd.     

Contrasting water management techniques in Tunisia: towards sustainable agricultural use

Tunisia is a marginal country hydrologically and it has adopted a number of distinctive methods of water management for agriculture. The central region supports modern dam irrigation, whilst traditional rainwater harvesting is practiced in the south. These contrasting techniques are described and evaluated in terms of sustainability using empirical field data and secondary literature for two study sites. Research focuses primarily on the physical environment, but socio-cultural and economic viability are also examined. Analysis indicates that traditional water management advantageously partitions the continuum dividing hazards and resources through subtle manipulation of the environment. A potentially hazardous environment is rendered secure by resourceful water management based on community action and cumulative knowledge. This practice minimizes community dependency and local economic imbalance. With dam irrigation, carrying capacity is established more forcibly by centralized control in order to place society within world markets. An almost total break from environmental variability is made in the short term, but this can lead to disequilibrium over longer durations. Additionally, the spatial and social distributions of development are uneven. In Tunisia, maintenance of traditional methods can reduce the negative impacts caused by modern programmes and support their positive characteristics. A mix of both methods offers a foundation to sustainable water supply in the new millennium.