Abraham Starinsky

Coprecipitation and isotopic fractionation of boron in modern biogenic carbonates

Abstract The abundances and isotopic composition of boron in modern, biogenic calcareous skeletons from the Gulf of Elat, Israel, the Great Barrier Reef, Australia, and in deep-sea sediments have been examined by negative thermal-ionization mass spectrometry. The selected species (Foraminifera, Pteropoda, corals, Gastropoda, and Pelecypoda) yield large variations in boron concentration that range from 1 ppm in gastropod shells to 80 ppm in corals. The boron content of the biogenic skeletons is independent of mineralogical composition and is probably related to biological (vital) effects. The δ 11 B values of the carbonates range from 14.2 to 32.2%. (relative to NBS SRM 951) and overlap with the δ 11 B values of modern deep-sea carbonate sediments ( δ 11 B = 8.9 to 26.2%.). The variations of δ 11 B may be controlled by isotopic exchange of boron species in which 10 B is preferentially partitioned into the tetrahedral species, and coprecipitation of different proportions of trigonal and tetrahedral species in the calcium carbonates. Carbonates with low δ 11 B values (~ 15%.) may indicate preferential incorporation of tetrahedral species, whereas the higher δ 11 B values (~30%.) may indicate 1. (1) uptake of both boron species assuming equilibrium with seawater 2. (2) preferential incorporation of B(OH) 4 − from in situ high-pH internal fluids of organisms that are isolated from seawater. The B content and δ 11 B values of deep-sea sediments, Foraminifera tests, and corals are used to estimate the global oceanic sink of elemental boron by calcium carbonate deposition. As a result of enrichment of B in corals, a substantially higher biogenic sink of 6.4 ± 0.9 × 10 10 g/yr is calculated for carbonates. This is only slightly lower than the sink for desorbable B in marine sediments (10 × 10 10 g/yr) and approximately half that of altered oceanic crust (14 × 10 10 g/yr). Thus, carbonates are an important sink for B in the oceans being ~20% of the total sinks. The preferential incorporation of 10 B into calcium carbonate results in oceanic 11 B-enrichment, estimated as 1.2 ± 0.3 × 10 12 per mil · g/yr. The boron-isotope composition of authigenic, well-preserved carbonate skeletons may provide a useful tool to record secular boron-isotope variations in seawater at various times in the geological record. The potential use of boron-isotope geochemistry in skeletons as a tracer for palaeoenvironments is demonstrated in Ostracoda and Foraminifera from the Gulf of Carpentaria, Australia. The δ 11 B values of glacial-age, buried skeletons (4.0 and 4.9%., respectively) are lower than that of their modern equivalents (17.6 and 13.3%., respectively). This may reflect a “terrestrial” boron-isotope signature of the water in the gulf during the Late Quaternary when it was isolated from the ocean.

Boron isotope variations during fractional evaporation of sea water: New constraints on the marine vs. nonmarine debate

Examination of boron isotopes, elemental B, Br, and Li in brines, and coprecipitated salts during fractional evaporation of sea water shows that Br, Li, and B in the evaporated sea water have lower concentrations than expected, as determined from mass-balance calculations. The deficiency is found beyond a degree of evaporation of ∼30 and is associated with a gradual increase in the δ11B values of the evaporated sea water, from 39‰ to 54.7‰ (relative to standard NBS 951). The high δ11B values of the brines and the relatively lower δ11B values of the coexisting precipitates (MgSO4 and K-MgSO4 salts; δ11B = 11.4‰ to 36.0‰) suggest selective uptake of 10B by the salts. Applying Rayleigh distillation equations, the empirical fractionation factors for the depletion of the salts in 11B are estimated as 30‰ (α = 0.969) for the early stages of precipitation (gypsum and halite range) and 20‰ (α = 0.981) for the late stages (K-MgSO4 minerals). Coprecipitation of B(OH)4- species with the salts, and/or precipitation of Mg-borate minerals with a coordination number of 4 are the proposed mechanisms for boron isotope fractionation during fractional evaporation of sea water. The boron isotope composition of sea water (δ11B = 39‰) is significantly higher than that of continental water (δ11B = -3‰ ±5‰). Our study shows that salt deposits may be depleted in 11B by 20‰ to 30‰ relative to their parent brines. These variations suggest that boron isotopes can be used to determine the origin (marine vs. nonmarine) of brines and ancient evaporitic environments.

Boron isotope geochemistry of Australian salt lakes

Boron isotope geochemistry has been investigated in brines, groundwaters, and sediments from the modern Australian salt lakes of Victoria, South Australia, and Western Australia by applying negative thermal-ionization mass spectrometry techniques. The geochemical history of the brines has been reconstructed by using δ11B, BCl, and NaCl ratios. The Victorian volcanic-crater lakes of southeastern Australia have water salinities of up to 60 g/L, molar NaCl ratios (0.87) similar to the marine ratio, molar BCl ratios of 2.9 × 10−4 to 4.9 × 10−4, and δ11B values of 54%. to 59%. (relative to NBS 951). The depletion of total B and the high positive δ11B values relative to seawater (BClratio = 7.9 × 10−4; δ11B = 39%.) are attributed to a marine (cyclic) salt origin together with adsorption processes in closed systems with low water/sediment (WR) ratios. In contrast, salt lakes from South Australia and Western Australia which are large shallow playas associated with halite, gypsum, and detrital clay minerals have interstitial and surface brines characterized by salinities of 80 to 280 g/L, molar NaCl ratios of 0.85 to 1, molar BCl ratios of 4 × 10−6 to 4 × 10−4, and δ11 values of 25%. to 48%.. The δ11 values of these brines are different from those of groundwaters from the Great Artesian Basin (δ11 = −15.9%. to 2.2%.; with high molar BCl ratios of 1 × 10−3 to 3.8 × 10−2), country rocks (δ11B = −8.7%. to 6.8%.), and modern detrital sediments present in these salt lakes (δ11B = −3.2%. to 12.3%.). The δ11B values of these salt lakes overlap with those of surface and brackish waters (δ11B = 28%. to 35%.) and with the B isotopic composition of seawater (δ11B = 39%.). Both low molar NaCl ratios ( 39%) of some brines indicate interaction of the brines with detrital sediments within the salt lake systems, δ11 values < 39% suggest mixing of brines of marine origin from which B was partly removed by adsorption, with waters of terrestrial origin with low δ11 values. NaCl ratios are used as indicators of the origin of the salts as well as of halite dissolution-precipitation. The δ11 values and BCl ratios are sensitive to a marine or non-marine origin, adsorption of boron onto clays, and the effective water/sediment ratio. At low WR ratios, the preferential removal of 10B from the solution affects the bulk solution, whereas at high WR ratios, the δ11 value of a solution is not affected by adsorption. Although the δ11 value of borate minerals may be a discriminant of marine or non-marine origin, boron isotopes are less distinctive in evaporative environments where boron is not an abundant component and where water/sediment interaction occurs.

The formation of natural cryogenic brines

Abstract The source of salts in the Ca-chloridic, hypersaline brines (up to 190 g Cl L −1 ) occurring in crystalline basement rocks in the Canadian, Fennoscandian and Bohemian Shields and their evolution have been investigated and reported. The Cl-Br-Na relationship indicates that these waters have been concentrated from seawater, by freezing during glacial times. The Na/Cl ratio (0.25 to 0.35) in the more saline fluids is compatible with cooling down to −30°C, where the most saline waters have been concentrated by a factor of 25 to 30 relative to the parent seawater. The brines formed from seawater within cryogenic troughs, along the subarctic continental margins, around ice sheets. The depressions within which the brines formed are the cryogenic analogues of the classic, evaporitic lagoon. One million years suffice to saturate with brine a 2000km-radius by 1km-depth rock volume at an H 2 O removal rate of only 2.8 mm/yr. Density-induced brine migration on a continental scale takes place via fissures below the ice. Our calculations, that were performed on a hypothetical ice sheet with dimensions compatible with the Laurentide ice sheet, demonstrate that during 1m.y., a 60m thick cryogenic sediment section could have formed. However, the precipitated minerals (mirabilite and hydrohalite) are repeatedly dispersed by the advance and retreat of the ice sheet, dissolved by melt water-seawater mixtures, and eroded during postglacial uplift, leaving almost no trace in the geological record. The cryogenic brines formed intermittently during and between glacial periods. The repeating advance and retreat of the ice sheets exerted a major control on the direction and intensity of brine flow. The cryogenic concentration of seawater and the migration of brine towards the center of the glaciostatic depression occurred mainly during the build up of the ice sheet, while reversal of the water flow from the center of the cryogenic basin outwards happened upon deglaciation. The flow of the waters in the subsurface was, inevitably, accompanied by significant dilution with melt water from the ice sheets. Using a “granitic” U concentration of 4 ppm and a (Ca-Mg mass balance based) rock/water ratio anywhere between 3.4 and 6.8 kg L −1 , a few hundred thousand years of brine-rock interaction are sufficient for the growth of 129 I in the most saline Canadian Shield brine to its present concentration (3.4×10 8 atoms 129 I L −1 ). Hence, both the formation of the saline fluids and their emplacement in their present sites occurred most likely within the Pleistocene. The young age calculated for cryogenic brines in crystalline shields and the dynamic water flow therein should raise concern about the planning and construction of high-grade nuclear waste repositories in such rocks, which are already under way.

Boron isotope and geochemical evidence for the origin of Urania and Bannock brines at the eastern Mediterranean: effect of water-rock interactions

The origin of hypersaline brines from Urania and Bannock deep anoxic basins in the eastern Mediterranean Sea has been investigated by integrating geochemical data and boron isotopic ratios. Bottom brines from Urania basin have chloride contents up to 4200 mmole/kg H2O and a marine Na/Cl ratio (0.87). All the other ionic ratios are different from the marine ratios and show a relative enrichment in Ca, K, Br, and B and depletion in Mg and SO4, as normalized to the chloride ion. The δ11B values of the Urania brines (δ11B = 29.8 ± 2.9‰; n = 7) are lower than that of Mediterranean seawater (39‰). The concentrations of Cl and Na, which make up 95% of the total dissolved ions (in molal units), suggest that the Urania brines were derived from eightfold evaporated seawater. The relative enrichment of Ca and depletion of Mg and SO4 reflect dolomitization, gypsum precipitation, and sulfate reduction processes which modified the original evaporated seawater while the brines were entrapped as interstitial waters in the sedimentary section of the Mediterranean. The relative enrichments of Br, B, and K, and the low δ11B value of the Urania brines suggest high-temperatures interactions of the evaporated sea water with sediments. Mass-balance calculations suggest that desorption of exchangeable B from the sediments (δ11B ∼ 20‰) modified the marine B isotopic composition of the original eightfold evaporated seawater. Potassium was also leached from clay minerals whereas Br was contributed from degradation of organic matter in the sediments. This is consistent with a thermal anomaly (up to 45°C) recorded at depth in the region of Urania basin. In contrast, bottom brines and shallow interstitial fluids from Bannock basin with low temperatures (15°C) show marine δ11B (δ11B = 39.6 ± 2.8‰; n = 5; 38.5 ± 2.2‰; n = 5, respectively) and B/Cl ratios (7 × 10−4). The B isotope data confirm that the Bannock brines were derived from twelvefold evaporated seawater. We argue that the brines from both basins are relics of fossil evaporated seawater that was entrapped in Late-Miocene sediments and accumulated in the deep basins of the Mediterranean seafloor.

Ra isotopes and Rn in brines and ground waters of the Jordan-Dead Sea Rift Valley: Enrichment, retardation, and mixing

Abstract Thirty-six springs and wells from the Dead Sea Rift Valley were periodically sampled and analyzed. The latter included full chemical analyses, 222 Rn and 226 Ra (by α-counting emanometry), as well as 228 Ra/ 226 Ra and 224 Ra/ 228 Ra activity ratios (γ spectrometry). Sampling stretched over almost 2 yr. Several hundred Rn-Ra analyses and close to a hundred isotopic ratios were measured. Most waters in the Rift Valley have both elevated Ra (several to 750 dpm/L) and Rn (a few hundred to almost 60,000 dpm/L) content. In practically all samples 222 Rn activity considerably exceeds that of its parent 226 Ra. The Ra content is the result of all Dead Sea Rift Valley waters being mixtures of fresh water with saline brines. Ra is efficiently extracted from surrounding rocks into the brine end member. 228 Ra/ 226 Ra ratios are exceptionally low −0.07 to 0.9, mostly less than 0.2. This apparently reflects the U over Th enrichment in the source rocks that contribute the Ra. Ra enrichment (both 228 Ra and 226 Ra) is locally correlated throughout the Rift Valley with water salinity. This correlation can be used to constrain the age of the brine-freshwater mixing process. In one of the hydrologic subsystems studied (the Fuliya block), the mixing of the shallow ground water must have occurred in less than 200 to 300 yr ago, probably before no more than some 30 yr. High radon activities in surface waters along the Dead Sea Rift Valley result from the formation of radium-enriched linings on the aquifer rock surface. The mixing of the radium-extracting brines with fresh water leads to continual adsorption of radium as water salinity decreases. The unique combination of fast upward flow and continual mixing in the Dead Sea Rift Valley accounts for a constant replenishment of radium in the waters. This causes a gradual buildup of a radium lining on the aquifer walls until eventually a steady-state surface activity is established.

The behavior of lithium and its isotopes in oilfield brines: evidence from the Heletz-Kokhav field, Israel

Twenty-four brine samples from the Heletz-Kokhav oilfield, Israel, have been analyzed for chemical composition and Li isotope ratios. The chemical composition of the brines, together with geological evidence, suggests derivation from (Messinian) seawater by evaporation that proceeded well into the gypsum stability field but failed to reach the stage of halite crystallization. The present salinity of the samples (18 - 47 g Cl/L) was achieved by dilution of the original evaporitic brine by local fresh waters. Like brines from other sedimentary basins, the Li/Cl ratios in the Heletz-Kokhav samples show a prominent Li enrichment (five-fold to eight-fold) relative to modern seawater. The isotopic ratios of Li, expressed in the 6 Li notation, vary from 26.3 to 17.9‰, all values being significantly higher than that of modern seawater ( 32‰) irrespective of their corresponding Li concentration (1.0 -2.3 mg/L). The isotopic composition of Li and the Li/Cl ratio in the oilfield brines were acquired in two stages: (a) The original evaporated seawater gained isotopically light Li during the diagenetic interaction between the interstitial Messinian brine and the basin sediments. A parent brine with an elevated Li/Cl ratio was formed. The brine was later diluted in the oilfields. (b) The 6 Li values of the final brines were determined during epigenetic interaction with the Heletz-Kokhav aquifer rocks. At the same time, the Li/Cl ratio inherited from stage (a) remained largely unchanged. This work represents the first use of lithium isotopic composition to elucidate the origin and evolution of formation waters in sedimentary basins. Copyright

Chemical tracing of salinity sources in Lake Kinneret (Sea of Galilee), Israel

Lake Kinneret is a freshwater lake in northern Israel that receives a major part of its salt input from unmonitored springs that discharge through the lake’s bottom. We attempt to characterize the nature of these springs by estimating their chemical composition. While the springs around Lake Kinneret are subject to wide spatial and temporal variations in their ionic concentrations, specific sodium (Na), potassium, magnesium (Mg), strontium, bromine, and lithium to chlorine (Cl) ion ratios are almost constant within individual springs and spring groups. The radium : Cl ratio and the d18O–Cl relationship confirm the notion that the spring waters result from recent mixing between saline brines and freshwater. Available compositional data from past years along with new analyses of the lake and its known springs allow identification of the salinity source that causes the observed deficit in the lake’s salt budget (e.g., 91–93% chloride). The relative contributions from these saline springs are different for different ions; this contribution is highest for bromide (95%), decreases to 84% for Na, and is less than 50% for Mg. Two independent approaches have been used for balancing the salts in the lake, and they are as follows: (1) an annual mass balance between salt removal and supply of the different ions, assuming a steady-state lake; and (2) simulation of the lake’s evolution from 1964 (the beginning of salt removal from the lake via the Salinity Diversion Channel) until the present. Both methods predict very similar ionic ratios for the (yet unknown) average saline spring(s), testifying to the reliability of both approaches. The ionic ratios so obtained closely resemble Fuliya (6 Tabgha)-type waters, excluding the Tiberias and eastern shore springs as significant salt sources. This inferred composition of the average unmonitored springs depends strongly on present-day diversion of saline springs (this diversion thus prevents their flow into the lake). The different ionic ratios that identify the various spring groups reflect the respective compositions of the brine pockets that feed them. Our simulation also shows that the layered structure of Lake Kinneret enhanced the evolution rate of the lake after the implementation of the salt diversion program in 1964. Lake Kinneret (LK; Lake Tiberias or the Sea of Galilee) is a lake of immense historical and religious significance. It is also the only freshwater lake in Israel and the main water reservoir for the National Water Carrier (NWC)—the water supply system that transfers freshwater to central and southern Israel. About 30% of the country’s water supply was pumped from LK in 1995. Furthermore, with the water shortage in the Middle East becoming more and more acute, LK is also a waterbody of great political significance. The lake is monomictic; it is stratified between mid-March and December. The mixing of a fully stratified to a fully mixed lake takes about 4–5 months. Though it is a flowthrough lake, it is significantly saltier than the Jordan River, which is its major water source (Table 1). The composition 1 Present address: Department of Environmental Sciences, Weizmann Institute of Science, 76100, Rehovot, Israel.

Time response of the water table and saltwater transition zone to a base level drop

[1] This paper investigates the effect of a drainage base level drop on the groundwater system in its vicinity, using theoretical analysis, simulations, and field data. We present a simple and novel method for analyzing the effect of a base level drop by defining two characteristic times that describe the response of the water table and the transition zone between the fresh and saline water. The Dead Sea was chosen as a case study for this process because of the lake’s rapid level drop rate. During a continuous lake level drop, the discharge attains a constant value and the hydraulic gradient remains constant. We describe this new dynamic equilibrium and support it by theoretical analysis, simulation, and field data. Using theoretical analysis and sensitivity tests, we demonstrate how different hydrological parameters control the response rate of the transition zone to the base level drop. In some cases, the response of the transition zone may be very rapid and in equilibrium with the water table or, alternatively, it can be much slower than the water table response, as is the case in the study area.

Relics of evaporated sea water in deep basins of the Eastern Mediterranean

Abstract Reexamination of data on hypersaline bottom brines from the deep anoxic Tyro and Bannock basins in the Eastern Mediterranean reveals that despite their similar chlorinity (6.0 mole/kg H 2 O) their chemical composition is significantly different. The brine in the Tyro basin has a Na-chloride composition ( Na Cl ∼ 1 ) with conspicuously low ratios of conservative elements to chloride ( Br Cl = 2.4 × 10 −4 , Li Cl = 1.4 × 10 −5 , B Cl = 1.6 × 10 −4 ), indicating dissolution of halite. In contrast, brines from the Bannock basin are characterized by an Mg-chloride signature ( Na Cl = 0.78 ) with relatively high ratios of conservative elements to chloride, ( Br Cl = 1.7 × 10 −3 , Li Cl = 5.2 × 10 −5 , B Cl = 8.4 × 10 −4 ). The contents of Na, Mg, K, Li, Cl, Br, and B in the deep Bannock brines (brine II) are identical to those in evaporated sea water of a degree of evaporation of 12–13, whereas Ca is enriched and SO 4 is depleted. In contrast to previous studies it is suggested that brine II from the Bannock basin is a relic of ancient evaporated sea water that was slightly modified by sulphate reduction and gypsum dissolution. The interstitial ancient evaporated sea water was liberated and accumulated on the sea floor, probably during the formation of the deep basins in the Bannock area.