Evolution and degradation of flat‐top mesas in the hyper‐arid Negev,Israel revealed from 10Be cosmogenic nuclides

Mesas are ubiquitous landforms in arid and semiarid regions and are often characterized by horizontal stratified erodible rocks capped by more resistant strata. The accepted conceptual model for mesa evolution and degradation considers reduction in the width of the mesa flat‐top plateau due to cliff retreat but ignores possible denudation of the mesa flat‐top and the rates and mechanism of erosion. In this study we examine mesas in the northeastern hyperarid Negev Desert where they appear in various sizes and morphologies and represent different stages of mesa evolution. The variety of mesas within a single climatic zone allows examination of the process of mesa evolution through time. Two of the four sites examined are characterized by a relatively wide (200–230 m) flat‐top and a thick caprock whereas the other two are characterized by a much narrower remnant flat‐top (several meters) and thinner caprock. We use the concentration of the cosmogenic nuclide ¹⁰Be for: (a) determining the chronology of the various geomorphic features associated with the mesa; and (b) understanding geomorphic processes forming the mesa. The ¹⁰Be data, combined with field observations, suggest a correlation between the width of flat‐top mesa and the denudation and cliff retreat rates. Our results demonstrate that: (a) cliff retreat rates decrease with decreasing width of the flat‐top mesa; (b) vertical denudation rates increase with decreasing width of the flat‐top mesa below a critical value (~60 m, for the Negev Desert); (c) the reduction in the width of the flat‐top mesa is driven mainly by cliff retreat accompanied by extremely slow vertical denudation rate which can persist for a very long time (>10⁶ Ma); and (d) when the width of the mesa decreases below a certain threshold, its rate of denudation increases dramatically and mesa degradation is completed in a short time. Copyright © 2014 John Wiley & Sons, Ltd.     

Analytical procedure for simultaneous use of seven fluorobenzoates in multitracer tests

An analytical procedure allowing simultaneous use of seven fluorobenzoate (FBA) isomers was developed and examined through column and field tracer experiments. The analytical method, based on high performance liquid chromatography (HPLC), included a reverse-phase separation method. This method was found to be very efficient for the FBA analysis, allowing accurate determination of seven FBAs in a single HPLC run, while avoiding analytical interference of other natural water constituents or pollutants. Consequently, this separation method allows the simultaneous use of seven isomers in multitracer tests in highly saline or contaminated environments. The mobility properties of the FBAs were examined in percolation experiments through a column filled with a mixture of crushed chalk and common fracture filling and coating material. The FBAs behaved conservatively as compared to bromid. All seven FBA isomers were successfully used in a multitracer test in a field percolation experiment designed to identify flowpath along a fracture plane.     

Thermal alteration experiments on organic matter in recent marine sediment—I. Pigments

Sediment from Tanner Basin, located at the outer continental shelf off southern California, was analyzed for photosynthetic pigments and their derivatives, namely carotenes and chlorins. Samples of the sediment were also exposed to raised temperatures (65°, 100°, 150°C) for various periods of time (1 week, 1 month, 2 months). Analysis of the heat-treated sediment revealed the presence of α-ionene and 2,6-dimethylnaphthalene, thermal-degradation products of β-carotene. Chlorins were converted to nickel porphyrins of both DPEP and etio series. Possible mechanisms and geochemical significance of these transformations are presented.     

Biological markers in bitumens and pyrolyzates of Upper Cretaceous bituminous chalks from the Ghareb Formation (Israel)

The sterane and triterpane distributions of three bituminous chalks from the Upper Cretaceous Ghareb Formation (Israel) were investigated both in the original extractable bitumens and in extracts obtained after pyrolysis of whole rock and isolated kerogen samples at 450°C. Pyrolysis was performed in a closed system under hydrous (whole rock) and anhydrous conditions (isolated kerogens). The carbon number distributions of steranes and triterpanes differ significantly between original bitumen and pyrolyzates. Unlike the bitumens in which diasteranes were not detected, the anhydrous pyrolyzates contain small amounts of diasteranes. The presence of water during pyrolysis leads to an increase of sterane isomerization, the abundant formation of diasteranes and an increase of the $\text{18α(}\text{H}\text{)-}\text{trisnorneohopane}\text{17α(}\text{H}\text{)-}\text{trisnorhopane}$ ratio. Sterane isomerization maturation parameters show a closer match between original bitumen and pyrolyzates after pyrolysis in a closed system when compared with an open system.     

Entrepreneurship in the periphery and local growth: the case of northern Israel

Entrepreneurship in the northern periphery in Israel should be viewed as a response to the crisis in rural agriculture during the 1980’s. Most entrepreneurs left their farms for salaried employment for a few years and they took professional courses in order to learn necessary skills before they opened their enterprises. They have developed new small entreprizes using local resources at times informally as means to reduce risks and they specialize mainly in internal tourism and construction related branches. While Jewish entrepreneurs develop mainly tourism activities oriented toward the national market, Arab entrepreneurs develop mainly construction related branches to local and home regional markets. Both represent two styles of peripheral activities. It seems that both styles has only limited potential to overcome their marginality.     

Non-monotone spectral projected gradient method applied to full waveform inversion

The seismic inversion problem is a highly non‐linear problem that can be reduced to the minimization of the least‐squares criterion between the observed and the modelled data. It has been solved using different classical optimization strategies that require a monotone descent of the objective function. We propose solving the full‐waveform inversion problem using the non‐monotone spectral projected gradient method: a low‐cost and low‐storage optimization technique that maintains the velocity values in a feasible convex region by frequently projecting them on this convex set. The new methodology uses the gradient direction with a particular spectral step length that allows the objective function to increase at some iterations, guarantees convergence to a stationary point starting from any initial iterate, and greatly speeds up the convergence of gradient methods. We combine the new optimization scheme as a solver of the full‐waveform inversion with a multiscale approach and apply it to a modified version of the Marmousi data set. The results of this application show that the proposed method performs better than the classical gradient method by reducing the number of function evaluations and the residual values.     

Retardation of organic contaminants in natural fractures in chalk

Transport of a conservative compound and two sorbing compounds through fractured chalk was studied using flow-through columns consisting of chalk cores with a single subvertical fracture. Two types of chalk matrix were compared, an oxidized white chalk with low organic carbon content (0.2%), and a gray chalk with a higher organic carbon content (1.3%). Initial rapid breakthrough followed by a delayed approach to a relative concentration of unity for the conservative compound (2,6–difluorobenzoic acid [DFBA]) was clear evidence for diffusion into the porous chalk matrix. Matrix diffusion of DFBA was apparently much greater in the gray chalk columns than in the white chalk columns. Breakthrough curves (BTCs) of the sorbing compounds (2,4,6–tri-bromophenol [TBP] and ametryn [AME]) were retarded in all cases as compared to the conservative compound. Sorption retardation was far greater in the gray chalk as compared with the white chalk, in good agreement with results from batch sorption experiments. BTCs for the conservative compound were relatively nonhysteretic for both white and gray chalk columns. In contrast, BTCs for the sorbing compounds were hysteretic in all cases, demonstrating that sorption was not at equilibrium before desorp-tion began. These experiments suggest that on a field scale, transport of contaminants through fractures in chalk and other fractured porous media will be attenuated by diffusion and sorption into the matrix.     

Gradational thresholds and landform singularity: Significance for Quaternary studies

Geomorphic threshold conditions have been identifed at which stream patterns change and gully initiation occurs. For both, the threshold conditions are defined by the parameter of “relative shear stress” which is a measure of the energy state of the system and is based on known values of stream slope and mean annual discharge (for patterns) or drainage area (for gullies). The probability of passing from a stream pattern to another, or from stable to gullied valley floors, is a smooth function of relative shear stress and so the thresholds separating the different states of the geomorphic systems are gradational. The singularity of landforms prevents the identification of a sharp threshold, and as a result landform sensitivity will differ within the same area and under the same conditions. Therefore, geomorphic predictions and postdictions will be uncertain, and Quaternary correlations will lack precision.