Search

Summary The mineralogical composition of clays (< 2μm) in representative profiles of all soil types of Israel was investigated. The soils were classified according to their clay mineral assemblages into three groups. I. Montmorillonitic soils. Montmorillonite is the dominant mineral and exceeds 65 per cent of the total minerals found; each of the other minerals comprises less than 15 per cent. 2. Montmorillonitic-kaolinitic soils. The soil clay fractions contain 50-60 per cent montmorillonite and 15-25 per cent kaolinite, generally adding up to more than 75 per cent of the clay fraction. 3. Montmorillonitic-calcitic soils. The clays contain more than 10 per cent calcite. Montmorillonite is the dominant clay mineral (except for one soil type, mountain rendzina, where calcite is dominant). The first and second assemblages are typical of the soils of the Mediterranean zone, whereas the soils of the desert zone are characterized by the third assemblage. The origin of montmorillonite, kaolinite, and illite, the three main clay minerals, was found to be detritic, as was the origin of palygorskite which was mainly found in the calcite rich soils of the desert zone. The cation exchange capacity of montmorillonite seems to be higher under higher precipitation. Montmorillonite content and cation exchange capacity of the clays were found to be highly correlated. The carbonate content of the clay fraction and the amount of carbonate in the soil were also highly correlated.

Summary Nubian sandstone exposures in sub-humid, semi-arid, and arid environments have given rise to red, sandy but in other respects very different soils. Soils have a fairly well developed profile only in the sub-humid zone, including a textural B horizon and are free of soluble salts and carbonates. In the semi-arid and arid zones, profile differentiation is weak or non-existent. Soils are shallow and contain carbonates, and in the arid zone also soluble salts, including gypsum. Kaolinite is the only clay mineral which is common to all the nubian sandstone parent materials. It is the major clay mineral in the sub-humid zone soil. In the semi-arid soils smectite is a second major clay component. In the arid zone both smectite and palygorskite, in minor amounts, accompany kaolinite. Both smectite and palygorskite are probably pedogenic neoformation products. Material of aeolian origin has probably been introduced into the silt and fine sand fractions of both the semi-arid and arid soils. Some contamination of the clay fractions may have also occurred.

The objective of this study is to locate as closely as possible the sites of strontium, magnesium, sodium, and potassium in modern aragonitic corals, specifically whether these cations are adsorbed, or are substituted in the carbonate lattice or are incorporated in organic components. In addition to locating the sites of each of these four elements we wanted to find out quantitatively how much of each element occurs at each site. The experiments in this study are based on the dissolution rate of aragonite in distilled water and on the substitution of strontium and magnesium by calcium and sodium. Special attention has been given to the occurrence of strontium, magnesium, sodium, and potassium in the organic components of the corals. The main site for strontium in the corals is in the aragonite lattice. Twenty-five per cent of the total magnesium occurs in adsorbed sites and in organic compounds. The rest of the magnesium may be located in the aragonite lattice, but it is easily removed by repeated leaching or by replacement with calcium ions. Another possibility is that magnesium may occur in a dispersed mineral phase more soluble than aragonite because magnesium was released at a higher Mg to Ca ratio than is found in the solid coral; also because no local concentration of magnesium could be detected with an electron microprobe. About 12% of the total sodium is in adsorbed sites and is included in the organic compounds. The rest of the sodium might be in the lattice replacing calcium, but the low total exchange capacity is not enough to provide the needed charge balance. Another possibility is that sodium is located in a proposed mineral phase. Potassium is in adsorbed sites and incorporated in the organic compounds to an extent greater than all the other elements studied (30% of the total potassium), but again the evidence suggests that the remaining potassium is in a proposed mineral phase. Calcite is detected on the surfaces of aragonite corals after 5 months in the substitution experiment. The change of argonite to calcite took place after the inhibitor magnesium was exchanged from the surface sites and replaced by calcium. The organic compounds in corals contain small amounts of strontium, magnesium, sodium and potassium. Strontium is preferentially enriched in the organic compounds over magnesium.

Uranium occurs in corals at three sites: 1, in organic matter; 2, adsorbed on the surfaces of skeletal aragonite; and 3, in the aragonite lattice. Organic matter incorporates from sea water by chelation 40–70 ppm uranium; skeletal aragonite incorporates only 3 ppm. However, as the organic fraction is low (0·1%), its high concentration of uranium does not significantly affect the total concentration of uranium in the coral. A negligible concentration of uranium, 40–60 ppb, is adsorbed on skeletal aragonite from which it is readily leached or exchanged. This low concentration of adsorbed uranium (

The present study has three objectives: (1) to show that cements form within centimeters of depth below surfaces of reefs and eliminate pore space almost entirely within less than 60 cm below the surfaces of reefs, (2) to describe and illustrate the fabrics and the kinds of minerals of marine cements in Red Sea reefs, and (3) to infer a possible mechanism of precipitation of carbonate cement which is based on observations of strong etching, corrosion, and partial replacement by calcium-carbonate cement of quartz particles in Red Sea reefs. Although seawater is generally undersaturated with respect to silica corrosion and embayment as a result of dissolution, such as observed in the Red Sea reefs, are not normally prevalent in quartz particles lodged on the sea bottom. Thus a mechanism other than undersaturation of silica must explain the partial dissolution of quartz; as carbonate cement replaced quartz particles, carbonate precipitation and quartz solution must proceed simultaneously. These reactions are under the control of pH: a pH level exceeding 9 and even 10 appears to be a necessary condition for the dissolution of quartz and the precipitation of carbonate. Photosynthesis and respiration of the biomass in the reef cause a shift in the carbonate buffer system of seawater with the uptake of CO2. Although pH values in excess of 9 can be measured in the waters of reefs, microlevels of pH of 10 and even 10.5 may likely be maintained in thin gellike films or monomolecular layers ("skin effect") that cling to the surfaces of the framework builders of reefs. Such high pH levels may trigger the precipitation of carbonate cement in reefs. The crucial evidence in this study is petrographic (the replacement of quartz particles by carbonate cement); the pH control is inferred.

The Yotvata Sabkha, a continental sabkha having an area 45 sq km, fills the entire Arava Valley, whose width is about 10 km. The climate is one of the most arid in the world. Short, ephemeral streams (wadis), prevented by alluvial fans from reaching the Gulf of Aqaba, provide water that seeps into the soil, thus keeping the water table close to the surface. The capillary fringe above this water table marks the floor of the sabkha. The sediments of the sabkha interfinger with alluvial fans that extend from the scarps which control the valley. The sediments of the sabkha consist of sand, silt, and clay brought in by the wadis. Three distinct zones exist in the sabkha: (1) the central barren zone, (2) the transitional zone, and (3) the outer vegetated zone with halophytic vegetation. The central barren zone forms the core of the sabkha. Depths to groundwater range from 90 cm in the central barren zone to more than 3 m in the outer vegetated zone. Chlorinity values in the groundwater are highest in the central barren zone (up to >= 200^pmil) and decrease toward the transitional and outer vegetated zones (

Seven cores from deep water were taken along a transect following the northwest-southeast axis of Baffin Bay between Greenland and the Canadian Archipelago. The sediments in the cores consist of sandy and silty muds. Pore water was squeezed from the sediments and analyzed for chlorinity, sulfate, calcium, magnesium, sodium, potassium, strontium, rubidium, and lithium. Chlorinity and sulfate in the interstitial waters are similar to those of Copenhagen seawater. Apparently bacterial reduction of sulfates in pore waters does not occur. The Ca/Cl ratio in interstitial waters decreases slightly relative to Copenhagen seawater, whereas the trend for the Mg/Cl ratio is one of increase. Increase in magnesium is considered to be related to its release by chlorite, which is abundant in the Bar in Bay sediments. Values for the Na/Cl and K/Cl ratios in the interstitial waters cluster about those for Copenhagen seawater. The ST/Cl ratio of interstitial waters tends to decrease relative to Copenhagen seawater, although in the pore waters of some cores this ratio overlaps with and even increases relative to Copenhagen seawater. The Rb/Cl ratio is higher in interstitial water than in normal seawater. The expected parallelism between the changes in the K/Cl and Rb/Cl ratios is not apparent. Despite some exceptions, the Li/Cl ratio in interstitial waters increases relative to seawater. Many of the changes in the cation/Cl ratios reported in this paper differ from those of other studies of interstitial waters and cannot be explained by simple cation exchange.

Hydrocarbon reservoirs are present in algal-laminated carbonate rocks, especially in those that have become dolomitized. Analogous modern algal-mat environments are rare; an example is a hypersaline pool along the shores of the Gulf of Aqaba. This pool should provide a better understanding of the environments in which algal-laminated limestones and dolostones formed in the geologic past. The study of this pool is concerned with the formation of various kinds of particles as well as with the formation of laminites which ultimately would give rise to algal-laminated limestones and dolostones and interlaminated calcite and dolomite. The hypersaline pool is isolated from the Gulf waters by a gravel bar. Finely laminated algal mats carpet the shallow "shelf" of the pool and gypsum floors the slope and bottom. Algae precipitate pellets, ooids, oncolites, grapestones, flakes, and carbonate laminites. The ooids have a radial texture; hence, contrary to statements in the literature, ooids with a radial texture are formed in the depositional environment. Some laminites consist of fibrous aragonite, others are cryptocrystalline high-magnesian calcite; the latter enclose abundant dispersed organic matter. Cryptocrystalline laminites mimic the micrite of the rock record including its pelletlike fabric; even after the organic matter has disappeared these laminites can preserve the morphology of the mats. Scanning electron m crographs show that cryptocrystalline laminites consist of a mosaic of micron-size rhombohedrons which during diagenesis would stabilize to low-magnesian calcite. Hence, the origin of some ancient stromatolitic limestones (pelmicrites) may be explained in terms of precipitation of cryptocrystalline high-magnesian calcite laminites. These laminites are lithified within the algal mats and do not require postdepositional cementation. These laminites prove that algal precipitation can be a potent force in lithification. This conclusion may supersede the concept that all micrites result from neomorphic replacement of aragonite. The algae create a microenvironment in which magnesium becomes enriched in organic matter and in which high-magnesian calcite with up to 40 percent molecular MgCO3 is precipitated. The total molecular percent MgCO3 between the magnesium organic complex and high-magnesian calcite may reach 60. This preferential concentration of magnesium may explain the high level of dolomitization of stromatolitic rocks in the geologic record. Alternation of aragonite and high-magnesian calcite laminites would yield interlaminated calcite and dolomite in the rock record. As part of the biologic system amino acids devoid of sulfur, especially aspartic acid, may exert considerable influence on the precipitation of the carbonate laminites and particles.