M. Stiller

Changes in the thermo-haline structure of the Dead Sea: 1979–1984

Abstract From 1979 to 1984, the overall water balance of the Dead Sea was characterized by a water deficit. However, an excess of freshwater inflow during the 1979/80 rainy season resulted in a 3-year-long meromictic phase. This was followed by three consecutive overturns of the water column in December 1982, 1983 and 1984. The buildup and dissipation of the seasonal thermocline and halocline is followed throughout this period which covers a wide range of water balance situations. The gravitational stabilities of the summer pycnoclines, measured in terms of N 2 ∼ 3×10 −2 s −2 , are at least one order of magnitude greater than the values reported in freshwater lakes and oceans. The contributions of temperature and salinity to N 2 and to the integrated stability W are examined separately, and their interdependence is pointed out. Two irreversible effects in the evolution of the properties of the water masses are identified: (1) a monotonic increase in the density of the deep waters; and (2) a monotonic shift of the NaCl saturation curve towards higher salinities.

Water transport through Lake Kinneret sediments traced by tritium

The transport of water through the sediments of Lake Kinneret was estimated using tritium of fallout origin as a tracer. The calculation model takes into account the diffusion of the tracer and its decay, and allows for the advective flux through the sediment pores as a free parameter. The dependence of the interstitial diffusion constant on the porosity was assumed identical to that for chloride ions. Tritium profiles in nine cores were used for estimating the advective fluxes and velocities, by finding the best fit between data and model. Water advective fluxes of 3.4 g cm−2 yr−1 into the lake were found. This water flux is negligible in the hydrological balance of Lake Kinneret; but it carries along 7% of the unaccounted-for chloride input to the lake.

Variations of stable hydrogen isotopes in plankton from a freshwater lake

Abstract The isotopic composition of the organically bonded hydrogen in micro plankton and zoo-plankton samples collected monthly from Lake Kinneret during 1972 and 1973 ranges between −90 and −130%. The temporal variations which have been observed in the deuterium content of the plankton samples, seem to be controlled by metabolic processes and not by variations in the temperature and in the deuterium content of the lake water. The organic material of the Zooplankton is consistently enriched in deuterium, by about 20%., as compared to that of the phytoplankton.

Nutrients in pore waters from Dead Sea sediments

Pore waters were separated from 50 cm-long cores of Dead Sea sediments raised from waters depths of 25, 30 and 318 m. The salinity of the pore water is close to that of the overlying water at 225–230 g l−1 chloride. The titration alkalinity of the pore water is about 60 % of the overlying water, and sulfate is also depleted. Ammonia and phosphate concentrations are higher than those of the water column with up to 50 mg l−1 N-NH3 (ten times increase) and 350 µg l−1 P-POinf4sup3−(four to eight times increase). Early diagenetic reactions are a result of decomposition of organic matter and of water-sediment interactions, resulting in aragonite precipitation, phosphate removal to the sediments, probably by absorption on iron-oxyhydroxides followed by remobilization, reduction of sulfate and formation of iron sulfides and accumulation of ammonia. Mass balance calculations show that pore water contribute about 80% of the ammonia and 30% of the phosphate input into the Dead Sea water column. On the other hand, the sediments act as a sink for carbonate and sulfate.

GEOCHEMICAL INVESTIGATION OF PHOSPHORUS AND NITROGEN IN THE HYPERSALINE DEAD SEA

Abstract Dissolved and particulate phosphorus, ammonia, and nitrate concentrations were measured in the Dead Sea water column (330 m depth, salinity ca. 340 g/L), in the Lower Jordan River and in springs along its western coast. Dissolved phosphate in the water column is fairly homogeneous, at about 35 μg P/L. Particulate P shows larger variability, 30–50 μg/L. The main inputs of dissolved phosphate into the lake are diffusion from bottom sediments (58%) and the Jordan River inflow (41%). Input from springs is negligible (1%). Biological uptake is a minor removal pathway because in the present Dead Sea, primary production takes place only when major floods occur and dilute the upper layer, about once in 10 years. When this happens, only about 200 ton P, out of a whole-lake reservoir of ca. 5000 ton P, are removed from the biologically active upper layer of about 7 m. Chemical removal pathways, not yet investigated, such as coprecipitation with authigenic aragonite, could be of importance. The average ammonia concentration in the water column has gradually increased from 5.9 mg/L in 1960 to 8.9 mg/L, in 1991. Diffusion from the bottom sediments is a major source of ammonia to the Dead Sea. The annual input from freshwater inflows has been of lesser importance in the 1960s and 1970s. A pollution-derived increase in the ammonia concentration of the Jordan River in the 1980s may partially explain the concomitant rise in the ammonia load of the lake. It is also suggested that following the 1979 overturn, and the yearly turnovers of the 1980s, ammonia might have been produced within the oxygenated water column by mineralization. Nitrate concentration was very low, 20 μg N/L, in the 1960s and increased to 0.2 to 0.5 mg/L in 1981. This increase is shown to be due to human pollution of the Jordan River. We conclude that nutrient concentration in the Dead Sea water column is controlled by physical and chemical factors, whereas biological effects are minimal.

Tritium in the Dead Sea

Abstract Tritium data in the Dead Sea for the period 1960–1979 are given. Tritium levels have increased until 1965 in the upper layers of the Dead Sea reaching a level of 170 TU, in response to the atmospheric buildup of tritium from thermonuclear testing. The levels have been decreasing ever since, both because of rapidly declining atmospheric concentrations of tritium and because of mixing of the surface layers with tritium deficient, deeper water masses. The depth of penetration of the tracer delineated the depth of meromictic stratification and successfully monitored the deepening of the pycnocline, until the overturn in 1979 homogenised the entire tritium profile. Modelling the changing tritium inventory over this period showed the predominance of the direct exchange across the air/sea interface, both in the buildup of tritium in the lake and also in its subsequent removal from it. The good fit between calculated and measured tritium inventories confirmed the evaporation estimate of 1.46 m/yr (the mean value for the period) with a precision unattained by other methods.

Iron in the Dead Sea

Abstract The distributions of dissolved and of particulate iron in the Dead Sea during the period which preceeded its overturn and thereafter (1977–1980) are reported. During 1977–1978, the vertical profiles of the iron phases revealed facets of the mixing pattern: the progressive deepening of the pycnocline, restricted mixing within the upper water mass and penetration of surface waters into the deepest layer. The inventories of particulate iron suggest resuspension of bottom sediments in November 1978 and after the overturn the gradual disappearance from the water column of iron sulfides and iron oxy-hydroxides. Fluxes of iron from and to the lake in the undisturbed meromictic Dead Sea have been estimated: it appears that diffusion of divalent iron from bottom sediments was the major source for the standing crop of iron in the lower water mass. Low settling velocities of solid particles in the dense and viscous Dead Sea is one of the causes for the relatively large concentrations of particulate iron. The rate constant for oxidation of divalent iron in Dead Sea sediment interstitial waters is larger by two orders of magnitude than in other natural waters.

Cl-37 in the Dead Sea system-preliminary results

Abstract This study presents the first set of δ 37 Cl measurements in the Dead Sea environment. δ 37 Cl values for the meromictic (long term stratified) Dead Sea water column prior to its complete overturn in 1979 were −0.47‰ SMOC for the UWM (Upper Water Mass) and +0.55‰ SMOC for the LWM (Lower Water Mass). The δ 37 Cl values for the pre-overturn Dead Sea cannot be explained by the prevailing model on the evolution of the Dead Sea during the last few centuries and require corroboration by more measurements. The 1979 overturn wiped out almost completely the isotopic differences between the UWM and LWM. Even so, Cl isotope data could be used to decipher physical processes related to the overturn such as incomplete homogenization of the deep water mass. Inputs into the lake, comprising freshwaters (springs and the Jordan River) and saline springs gave a range of −0.37‰ to +1.0‰ with the freshwater sources being more enriched in δ 37 Cl. Based on the δ 37 Cl measurements of the End-Brine (the effluent from Dead Sea evaporation ponds) and of recent Dead Sea halite, the Cl isotopic composition of the originating brines have been estimated. They gave a narrow isotopic spread, +0.01‰ and +0.07‰ and fall within the same range with Dead Sea pore water (+0.13‰) and with the post-overturn Dead Sea (−0.03‰ and +0.16‰). Rock salt from Mount Sdom gave a value of −0.59‰ indicating its formation at the last stages of halite deposition from evaporating sea water. The hypersaline En Ashlag spring gave a depleted δ 37 Cl value of −0.32‰, corresponding to a residual brine formed in the very latest stages (including bishofite deposition) of seawater evaporation.

210Pb and210Po during the destruction of stratification in the Dead Sea

Abstract The progressive weakening and final disappearance (in 1979) of the long-term meromictic structure of the Dead Sea are clearly reflected in the depth profiles of 210 Pb and 210 Po. In 1977/78, prior to overturn, dissolved 210 Pb (35–50 dpm kg −1 ) predominated over particulate 210 Pb (1–2 dpm kg −1 ) in the oxic upper waters, whereas the reverse was true in the anoxic deep waters (16–20 dpm kg −1 particulate vs. 2–5 dpm kg −1 dissolved). The exact extent of the disequilibrium between 210 Pb and 226 Ra is hard to evaluate in the upper oxic layers, because the progressive deepenings resulted in mixing with deep waters. By contrast, one can estimate the residence time of dissolved 210 Pb in the unperturbed anoxic deepest layers, because these remained isolated, at about 3 years. Following the overturn of 1979, dissolved 210 Pb exceeded particulate 210 Pb at all depths. The 210 Po profiles of the stratified lake resembled in shape those of its grandparent 210 Pb, but with distinct characteristics of their own in the oxic upper waters where particulate 210 Po (8–12 dpm kg −1 ) was greatly in excess over particulate 210 Pb, while dissolved 210 Po (25–40 dpm kg −1 ) was slightly deficient. Immediately following the overturn, dissolved and particulate 210 Po were similar (about 15 dpm kg −1 ), at all depths. The destruction of the lakes meromictic structure was accompanied by a reduction of its 210 Pb inventory, while that of 210 Po was almost unaffected. Thus, at overturn a transient state was created with the inventory of 210 Po exceeding that of 210 Pb.

Recent climatic changes recorded by the salinity of pore waters in the Dead Sea sediments

The salinity of lakes is subject to variations imposed by climatic changes. These variations are recorded in the salinity profile of pore waters. Meromictic lakes, such as the Dead Sea, are a special case where waters which underlie the mixolimnion reflect salinity variations. In a sediment core from Dead Sea shallow waters, the salinity profile exhibited a minimum at about 1.9 m depth. It is shown by a diffusion model that this minimum can be attributed to lower salinities which prevailed at the sediment water interface for several decades around the turn of this century. No such minimum was observed in a sediment core from the deepest part of the lake where, during the last two centuries, the overlying brines had a constant salinity.