Eytan Sass

Strontium behavior in the aragonite-calcite transformation: An experimental study at 40–98°C

The behavior of strontium during the replacement of aragonite by calcite, in a closed system between 40°C and 98°C, has been experimentally investigated. The experiments were conducted in CaCl2 solutions, with and without NaCl. The distribution coefficient of strontium in calcite (λSr2+C) was found to be affected only slightly by temperature changes, and almost insignificantly by the presence of NaCl. λSr2+C values at 0.01 mCa2+ (its concentration in normal sea water) are: 0.055 at 40°C and 0.058 at 98°C. These results indicate that the low (around 500 ppm) concentration of strontium in ancient limestones could have been brought about by aragonite-to-calcite transformation in a system open to sea water, and are not necessarily indicative of replacement in fresh waters.

Carbon and oxygen isotope study of the active water-carbonate system in a karstic Mediterranean cave: Implications for paleoclimate research in semiarid regions

Abstract In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikarst zone exert a profound influence on the isotopic compositions of waters seeping into a cave. Consequently, the isotopic compositions of speleothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate studies. This is confirmed by a four-year study of the active carbonate-water system in the Soreq cave (Israel). The δ18O (SMOW) values of cave waters range from −6.3 to −3.5%.. The highest δ18O values occur at the end of the dry season in waters dripping from stalactites, and reflect evaporation processes in the epikarst zone, whereas the lowest values occur in rapidly dripping (fast-drip) waters at the peak of the rainy seasons. However, even fast-drip waters are about 1.5%. heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. δ13C (PDB) values of dissolved inorganic carbon (DIC) vary from −15.6 to −5.4%., with fast-drip waters having lower δ13C values (mostly −15.6 to −12%.) and higher DIC concentrations relative to pool and stalactite-drip water. The low δ13C values of fast-drip waters and their supersaturation with respect to calcium carbonate indicates that the seepage waters have dissolved both soil-CO2 derived from overlying C3-type vegetation and marine dolomite host rock. The δ18O (PDB) values of various types of present-day low-magnesium calcite (LMC) speleothems range from −6.5 to −4.3%. and δ13C values from −13 to −5.5%. and are not correlated with speleothem type. An analysis of δ18O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of δ13C and δ18O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The δ13C–δ18O range of contemporary LMC thus reflects the variations in temperatures and isotopic compositions of the presentday cave waters. The 10%. variation in the δ13C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope fractionation accompanying CO2-degassing and carbonate precipitation. These variations may obscure the differences in the carbon isotopic composition of speleothems that could arise when vegetation cover changes from C3 to C4-type plants. This consideration emphasizes that it is necessary to characterize the full range of δ13C values associated with contemporaneous speleothems in order to clarify the effects of degassing from those due to differing vegetation types. Isotopic studies of a number of different types of fossil LMC speleothems show many of them to exhibit isotopic trends that are similar to those of present-day LMC, but others show both higher and lower δ18O ranges. In particular, the higher δ18O range has been shown by independent age-measurements to be associated with a period of drier conditions. The results of the study thus indicate that it is necessary to work on a well calibrated cave system in semiarid climates and that the fossil speleothem record should be obtained from different types of contemporaneous deposit in order to fully characterize the δ18O–δ13C range representative of any given climatic period.

Rainfall-recharge relationships within a karstic terrain in the Eastern Mediterranean semi-arid region, Israel: δ 18O and δD characteristics

Annual rainfall variations and processes in the upper vadose zone exert a profound influence on the chemical and isotopic compositions of waters of carbonate aquifers in semi-arid climatic zones. In order to define these processes we have studied the surface temperatures during rainfall events, the isotopic composition of rain, infiltrating and groundwaters. This study was carried out within a karstic terrain (Soreq Cave), Israel, located in Cenomanian dolomitic rocks, approximately 40 km inland of the Mediterranean Sea, and 400 m above sea level. The climate is typical of the Eastern Mediterranean semi-arid conditions, with a rainy winter and dry summer and the average annual rainfall in the area is ∼500 mm. Close monitoring indicates that the δD and δ 18O values of individual rainstorm events decrease with increasing rainfall. Annual average isotopic values of years with rainfall of 500–600 mm do not vary systematically. Years with extreme rainfall values define a negative covariation between the δ 18O and rainfall. The δD-δ 18O relationship of all rain events of more than 20 mm fall on the Mediterranean Meteoric Water Line (MMWL) with a slope of ∼8 and d-excesses of 20–30%. These rain events occur when mid-winter surface temperatures are 5 to 10°C. Rainfall events of less than 20 mm, mainly occurring at above 10°C, have slopes of less than 8 and smaller d-excess as a consequence of evaporation processes beneath the clouds. Two main water-types infiltrating into the cave are recognized: slow- and fast-drip. Slow-drip occurs from the tips of stalactites and takes place throughout the year; these waters represent seepage water that remains in the upper vadose zone for up to several decades. Fast-drip emanates from fissures in the cave roof during the winter seasons; these waters represent vadose flow with a short residence time of less than 1 year. The infiltration of the fast-drip water into the cave depends on the fracture system of the rock cover and on the intensity of the rainstorms. In the inner parts of the cave, where the rock cover is thick, fast-drip starts only after several massive (>20 mm) rainstorms, whereas below a thin roof the response time of fast-drip is a few hours to a few days after intensive rainstorms. Both fast- and slow-drip water fall on the MMWL, indicating that they are derived mainly from the relatively intensive rainstorms that compose about 23 of the annual rainfall. Although fast-dri water represents massive rainstorms, their δ 18O and δD values are nonetheless higher by ∼1% and 10% respectively, relative to the average rainwater compisition. This indicates that the fast-drip water consists predominantly of water derived from intensive rainstorms admixed with a minor component of isotopically 18O- and D-enriched water, which is represented by the slow-drip waters. The fast-drip waters are derived mainly from massive rain events at surface temperatures of 5 to 10°C, which thermally equilibrate with the host rock and upon reaching the cave they have already acquired the mean temperature (∼20°C) of the cave.

Late Cretaceous upwelling system along the Southern Tethys Margin (Israel): Interrelationship between productivity, bottom water environments, and organic matter preservation

Organic-rich Upper Cretaceous sequences in Israel were deposited in an extensive, highly productive upwelling-linked system which prevailed along the southern Tethys margin, and lasted for ∼19 m.y. (Santonian to late Maastrichtian). An understanding of the spatial and temporal characteristics of this system was gained through detailed paleontological and geochemical analyses of subsurface sequences in two basins in Israel, representing an outer (Shefela) and an inner (Zin) facies belt. The nature of the upwelling system, and its effect on the sedimentary record, is related to two basic environmental parameters, namely paleoproductivity intensity and oxygen levels at the bottom. The assessment of these parameters and their interrelationship has been performed through the development of paleontological (foraminiferal) criteria, which are independent of the organic matter content. Following the establishment of these criteria, it is concluded that the productivity reached its maximum intensity during the late Campanian, which was also the time of most notable differentiation between the center of the upwelling system in the inner belt and the less intense conditions in the outer basin. This distribution is expressed in varied lithology (organic-rich carbonates, phosphorites, and siliceous rocks) at the core of upwelling and a uniform lithology (organic-rich carbonates) at the margin of this system. The uniform lithology of the Maastrichtian in both basins, composed of organic-rich carbonates, is ascribed to a gradual weakening of productivity. The bottom conditions in the inner belt during the late Campanian (the time of maximum surface productivity) were near anoxic, changing to more aerated (dysaerobic) conditions during the early Maastrichtian. In the outer belt a more aerated bottom (dysaerobic) prevailed throughout the late Campanian to late Maastrichtian. The elevated organic matter content in both basins reflects the overall environment of high productivity; its actual variations, however, seem to be unrelated to changes in surface productivity and bottom oxygen levels.

Diagenesis in live corals from the Gulf of Aqaba. I. The effect on paleo-oceanography tracers

Abstract The effect of early diagenesis on trace element abundance in coral skeleton was studied in live coral heads (Porites) from the Nature Reserve Reef (NRR), Elat, Gulf of Aqaba, northern Red Sea. Petrography of the corals shows diagenetic features of dissolution, recrystallization, and secondary aragonite precipitation (pore filling), which are most extensive in the oldest part of the coral. Coral porewaters were extracted with a special setup and were analyzed for chemical composition. The total alkalinity and Sr deficit in pore water as compared to reef water is consistent with both precipitation of secondary aragonite and recrystallization of primary skeleton. The present rate constant of pore filling by secondary aragonite was estimated by a water replacement experiment to be 0.0015 y−1, which equals to pore filling rate of 1.5 ± 0.3 kg aragonite per year. The corals show clear seasonal fluctuations in Sr/Ca ratios that are interpreted as reflecting changes in sea surface temperature (SST). Yet, the secondary aragonite is characterized by a significantly higher Sr/Ca ratio than the average ratio in primary aragonite. Thus, measuring a mixed sample of pristine and secondary aragonite may produce erroneous (about 2°C lower) SST estimates by the Sr/Ca thermometers. It appears that the Sr/Ca ratio, a major proxy for paleo-environmental marine studies, is sensitive to subtle pore-filling and replacement of the original coral matrix by secondary aragonite in the marine environment.

Oxygen-isotope composition of diagenetic calcite in organic-rich rocks: Evidence for 18O depletion in marine anaerobic pore water

Upper Cretaceous organic-rich carbonates in Israel contain benthonic foraminifera and varying amounts of early diagenetic infilling of sparry calcite. The sparry calcite has oxygen-isotope values (down to -9.5‰ PDB) that are significantly lower than those of the coexisting skeletal calcite (averaging -2‰ PDB). Low δ18O values are very common in carbonates that are associated with organic matter. These occurrences are conventionally explained as being due to carbonate formation under conditions of low salinity and/or elevated temperature. On the basis of petrographic data and local geologic evidence, however, none of the conventional explanations can be applied in this case. We advance an alternative model that relates the light oxygen compositions of the diagenetic calcite to depletion of 18O in pore water of normal salinity. The proposed operating mechanism is a direct consequence of organic matter decomposition in the sulfate-reducing zone. Being inherent to reducing marine environments, this model can be applied to many organic-rich rock associations.

The carbonate system in hypersaline solutions: dead sea brines

Abstract Various investigators reported a decrease in pH as seawater is concentrated. A similar phenomenon was reported for Dead Sea waters which are about ten times more saline than seawater. The reasons for the low pH values of Dead Sea waters (pH 5.9–6.5), which precipitate CaCO 3 , were investigated by determining the apparent dissociation constants of carbonic acid in these brines. A new method, based on alkalinity titration and least-squares fitting, was used to estimate the proton activity coefficient (γ H +) and the first and second dissociation constants of carbonic acid ( K 1 ′, K 2 ′) in natural and artificial Dead Sea waters. It was found that as the salt content increases, p K ′ 1 and p K ′ 2 values progressively decrease whereas γ H + sharply increase. At the highest salinity investigated (TDS = 330 gl −1 ) γ H + p K ′ 1 and p K ′ 2 values are 24.5, 5.09 and 6.23, respectively, as compared to about 0.8, 5.9, 9.1 respectively for normal seawater (19‰ chlorinity) at the same temperature (30°C). The implication of the results of this study regarding solubility of CaCO 3 and the general behavior of the carbonate system in hypersaline solutions is discussed.

The Cretaceous carbonate platform in Israel

Abstract A huge carbonate platform extended between the open Tethys Sea in the north-west and the Arabo Nubian Massif in the south-east during the Cretaceous. The edge of the platform was situated close to and parallel to the present day coastline of Israel and Sinai. Deposition on the platform was controlled by two main factors: (1) rudist barrier-reefs which to a certain extent isolated the platform from the open sea and (2) local shelf basins which developed within the platform. Because the effects of these factors on the depositional regime changed both in time and degree on a local scale, different lithostratigraphic sequences evolved in geographically close regions. Volcanic activity occurring contemporaneously with sedimentation over part of the platform was another factor contributing to local differences in the sedimentary sequences. Dolomites were by far the dominant deposit which accumulated on the platform, indicating above-normal salinities. In the slightly deeper shelf basins, close to normal salinities prevailed resulting in accumulation of limestones, chalks and marls with some pelagic elements. The Carmel lithostratigraphic sequence is characteristic of the platform edge environment whereas the Judean Hills and Galilee sequences are typical of the platform-environment proper.

Behaviour of strontium in subsurface calcium chloride brines: Southern Israel and Dead Sea rift valley

Calcium chloride brines are, as a rule, relatively rich in strontium, but the enrichment is usually limited and is found to be related to the concentration of calcium. The limiting mechanisms were evaluated as a model which comprises simple interactions between minerals and solutions. Based on the known ranges of strontium concentration in minerals, mineral solubilities and partition coefficients of strontium (both poorly known in certain cases), six fields of SrCa molar ratios were defined in terms of participating minerals and processes: (a) 0.38−1.56 × 10− 3 by dolomitization of calcite; (b) 1.5−2.2 × 10− 2 due to dolomitization of aragonite; (c) 0.4−1.4 × 10− 2 as a result of solution-reprecipitation of calcite; (d)0.12−0.20 through transformation of aragonite to calcite; (e)0.10−0.60 through equilibrium of the pair calcite-strontianite; and (f)0.01−0.08 by equilibrium with gypsum and celestite. The model was applied to the analysis of two groups of brines from southern Israel which are originated in the coastal plain (group C) and in the rift valley (group R). The low MgCa ratios of both water groups point to dolomitization as the main subsurface modifying process. SrCa ratios of brines belonging to group C are consistent with dolomitization of aragonitic surface sediments at the beginning of their evolution. Brines of group R bear evidence to a similar pathway at the beginning of their evolution, but most of them were further affected by interaction with limestone.

Stable isotopes, chemistry and petrology of carbonate concretions (Mishash formation, Israel)

Abstract Carbonate concretions of variable sizes occur in the upper member of the Senonian (Upper Cretaceous) Mishash formation in Israel. Eight concretions and their surrounding country rocks were examined in the field, in thin sections and by X-rays. The isotopic composition of carbon and oxygen in the carbonates, the amount of acid insoluble residue, as well as the concentrations of P 2 O 5 , Ca, Mg, Sr and Fe were determined. Other concretions and country rocks were analyzed for oxygen and carbon isotopes only. The concretions are composed of almost pure, microsparitic calcite, whereas the country rocks are porcellanitic-phosphoritic chalks. Compared with the surrounding rocks, the concretions are strongly enriched in 12 C and are depeleted in insoluble residue, P 2 O 5 , SiO 2 , Fe, Sr and Mg. It is postulated that the concretions were formed by addition of CaCO 3 to sites of anaerobic decomposition of organic matter, while CaCO 3 was mobilized in the surrounding sediments, in which aerobic decomposition of organic matter prevailed. Several consequences of this model are considered, concerning the quantitative volume changes, as well as the distribution of SiO 2 and Mg between concretions and country rocks.