Publications

The mechanisms underlying the current greenhouse gas (GHG) forced decline in Mediterranean rainfall remain a matter of debate. To inform our understanding of the current and projected drying, we examined extended arid intervals in the late Quaternary, Eastern Mediterranean (EM) Levant indicated by substantial salt deposits in a Dead Sea sediment core covering the past 220 kyr. These arid events occurred during interglacials, when the Earth was at perihelion to the sun in boreal fall and during glacial-interglacial transitions, associated with icesheet melting. Climate models forced with realistic late Quaternary insolation variations show that when the Earth is closest to the Sun in boreal fall, the North Atlantic latitudinal surface temperature gradient in the winter intensifies. In response, the overlying midlatitude North Atlantic jet stream and the extratropical storm track move poleward while sea-level pressure rises in the subtropics. These changes bring about a weakening of the Mediterranean storm track and a decline in rainfall over the entire basin. During glacial-interglacial transitions, meltwater from continental icesheets forced abrupt subpolar North Atlantic cooling. This also strengthened the latitudinal surface temperature gradient, likely causing similar atmospheric response and aridity in the Mediterranean. There is a strong resemblance between this paleoclimate scenario and the climatic changes corresponding to the present and projected GHG drying of the EM. Hence, the late Quaternary palaeohydrology of the Dead Sea indicates an important North Atlantic centered response to external forcing, which leads to Mediterranean drying and is relevant in the present.

Northern Hemisphere insolation intensity is roughly in phase with Southern Hemisphere climate proxies, leading to a common conclusion that northern insolation forces southern climate during the Late Quaternary. However, mid-latitude Southern Hemisphere records place the advance of Patagonian and New Zealand glaciers before the Last Glacial Maximum (29,00018,000 years ago) by several millennia. To resolve the cause(s) of nearly synchronous global climate change requires continuous archives of mid-latitude glacial activity for the last glacial cycle. Here we assess the position of the Patagonian Ice Sheets marine-terminating margin over the last ~89,000 years using a sedimentary-beryllium-isotope record from the Chilean margin to track the proximity of local glaciers. We find that glaciations and deglaciations are synchronous with or precede Northern Hemisphere ice sheets by thousands of years. Glacial expansion was driven by equatorward migration and strengthening of the southern westerly winds, linked to global cooling and a steeper meridional temperature gradient. Glacial terminations occurred when global warming coincided with increasing obliquity and dramatic Northern Hemisphere cooling. Our results suggest that, on orbital timescales, a complex interaction between mean global climate, obliquity and interhemispheric teleconnections could have led to near-synchronous global ice sheet evolution through displacements of the southern westerlies.

Submarine groundwater discharge (SGD) is a globally important process supplying nutrients and trace elements to the coastal environment, thus playing a pivotal role in sustaining marine primary productivity. Along with nutrients, groundwater also contains allochthonous microbes that are discharged from the terrestrial subsurface into the sea. Currently, little is known about the interactions between groundwater-borne and coastal seawater microbial populations, and groundwater microbes' role upon introduction to coastal seawater populations. Here, we investigated seawater microbial abundance, activity and diversity in a site strongly influenced by SGD. In addition, through laboratory-controlled bottle incubations, we mimicked different mixing scenarios between groundwater and seawater. Our results demonstrate that the addition of 0.1 μm filtered groundwater stimulated heterotrophic activity and increased microbial abundance compared to control coastal seawater, whereas 0.22 μm filtration treatments induced primary productivity and Synechococcus growth. 16S rRNA gene sequencing showed a strong shift from a SAR11-rich community in the control samples to Rhodobacteraceae dominance in the

Contaminated soil in urban residential areas is often overlooked as a source of childhood exposure to toxic levels of lead (Pb). We document mean Pb concentrations of 1200 ± 1000 mg/kg, three times the now outdated EPA soil hazard standard of 400 mg/kg, for 370 surface soils collected from 76 homes in the boroughs of Brooklyn and Manhattan of New York City. The mean Pb content of 250 ± 290 mg/kg Pb for 571 surface soils collected from tree pits and public parks was much lower. A subset of 22 surface samples analyzed by EPA Method 1340 extracted 86 ± 21 % (±1SD) of total soil Pb, indicating that it the Pb was highly bioavailable. To investigate the origin of backyard contamination, 49 cores were collected to an average depth of 30 cm from a subset of 27 homes. Twelve soil cores were analyzed for ²¹⁰Pb and ¹³⁷Cs to constrain processes that impact contaminant distribution and inventories (particle focusing, soil accumulation, loss, and mixing). Concentrations of Pb declined with depth in 60 % of the cores but usually did not reach background. Mean uncorrected Pb inventories of 340 ± 210 g/m² Pb (mean ± 1SD, n = 12) were more than five times higher than the radionuclide corrected inventory of 57 g/m² from Central Park soil cores. Average inventories of ²¹⁰Pb_xs (3.5 ± 0.9 kBq/m²) and ¹³⁷Cs (0.9 ± 0.6 kBq/m²) corresponded to 71 ± 19 % and 50 ± 30 % of the predicted atmospheric inventories. Elevated Pb concentrations were found both in the fine (1 mm) fractions, the latter suggesting a local non-atmospheric source. This was confirmed by individual grains containing up to 6 % Pb and visible pieces of coal, bricks, and ash. Regardless of the source of contamination in backyard soils, systematic testing is needed to identify contaminated areas and reduce child exposure.

Water availability in the Levant is predicted to decline due to global warming in the upcoming decades and is expected to substantially impact the region. Determining the long-term natural rainfall variability in this region is essential for understanding the regional hydroclimatic response to external climate forcings and for contextualizing future hydroclimate changes. The Dead Sea (DS), located in the southern Levant, is a closed-basin lake whose size varies as a function of water availability. Reconstructing DS lake-level variations through time provides a quantitative measure of the natural hydroclimate variability and can inform on the local hydroclimate response to changes in global climate. Here, we constructed an updated lake-level history of the Holocene DS by: 1) studying lake high-stands derived from a series of new cores collected in the DS southern basin, 2) re-dating of the two major Holocene high-stand exposures, and 3) compiling all previously published ages of Holocene DS lake-level markers (n = 296 radiocarbon ages). The results show that the early (106.1 kyr cal BP) and late Holocene (3.60 kyr cal BP) in the DS were predominantly wet albeit punctuated by dry intervals, whereas the middle Holocene (6.13.6 kyr cal BP) was most likely relatively dry. This pattern of two Holocene humid intervals is also evident in distillation records derived from Levant speleothem caves (which represent the integrated magnitude of rainout from the vapor source to the caves), indicating that rainfall intensity and total water availability were correlated throughout the Holocene. These two humid intervals occurred during high and low summer insolation conditions, suggesting that they were modulated by different climatic mechanisms. The predicted future drying in the Levant is of similar magnitude to the natural hydroclimate variability and thus, it is crucial to assess whether the anthropogenic drying is in- or out-of phase with the natural climate variability.

Submarine groundwater discharge is increasingly recognized as an important component of the oceanic geochemical budget, but knowledge of the distribution of this phenomenon is limited. To date, reports of meteoric inputs to marine sediments are typically limited to shallow shelf and coastal environments, whereas contributions of freshwater along deeper sections of tectonically active margins have generally been attributed to silicate diagenesis, mineral dehydration, or methane hydrate dissociation. Here, using geochemical fingerprinting of pore water data from Site J1003 recovered from the Chilean Margin during D/V JOIDES Resolution Expedition 379 T, we show that substantial offshore freshening reflects deep and focused contributions of meteorically modified geothermal groundwater, which is likely sourced from a reservoir ~2.8 km deep in the Aysén region of Patagonia and infiltrated marine sediments during or shortly after the last glacial period. Emplacement of fossil groundwaters reflects an apparently ubiquitous phenomenon in margin sediments globally, but our results now identify an unappreciated locus of deep submarine groundwater discharge along active margins with potential implications for coastal biogeochemical processes and tectonic instability.

The Pampean loess is the most extensive continental paleo-record of aeolian material in the Southern Hemisphere, recording the deposition of dust transported by two major zonal wind systems: the southern westerly winds and the subtropical jets. In order to increase the understanding on paleo-atmospheric circulation over the Southern Hemisphere, we evaluate dust provenance through REE, Nd, Sr and Pb isotopes in three sections deposited during late Pleistocene-early Holocene across 700 km in the loess belt of the Pampean region in central Argentina. The isotopic comparison of loess from the three sections with southern South American (SSA) potential dust sources show that (1) sources from the southern Altiplano to latitudes of northern Patagonia supplied dust to the Pampas, (2) the slight Sr[sbnd]Nd isotopic difference between fine and coarse loess may be attributable to grain size effects rather than to differences in provenance, and (3) higher mass accumulation rates in the Pampas are associated with an increased presence of dust originated in the southern Altiplano and southern Puna during the spans of 4341 ka BP, 2018 ka BP, 14.612.6 ka BP and 11.48.9 ka BP. We associate these rises in continental dust fluxes with climatic transitions from wetter to drier periods in the Puna-Altiplano Plateau related to synchronous climatic shifts to humid conditions at the Pampean Plain, probably triggered by El Niño-like conditions. The isotopic comparison with modern SSA dust indicates similar provenance compared to paleo-dust records, suggesting almost constant dust sources from MIS 3 to modern times and/or modest changes in the geochemical signature under the activation/deactivation of the different dust sources. Moreover, contrasting the isotopic signature of the loess sections with more distal palaeoarchives (i.e., South Atlantic Ocean marine sediment cores and Antarctic ice cores), the new data suggest that contrary to previous ideas, the Pampean Loess was not an important source of dust to these regions. Also, a common dust provenance during cold periods (e.g., Last Glacial Maximum and Antarctic Cold Reversal) supports the idea that changes in atmospheric transport efficiency can better explain dust flux variations observed over glacial/interglacial periods in distant palaeoarchives than changes in provenance.

Chemical events involving deep carbon- and water-rich fluids impact the continental lithosphere over its history. Diamonds are a by-product of such episodic fluid infiltrations, and entrapment of these fluids as microinclusions in lithospheric diamonds provide unique opportunities to investigate their nature. However, until now, direct constraints on the timing of such events have not been available. Here we report three alteration events in the southwest Kaapvaal lithosphere using U-Th-He geochronology of fluid-bearing diamonds, and constrain the upper limit of He diffusivity (to D≈1.8×10cm2 s−1), thus providing a means to directly place both upper and lower age limits on these alteration episodes. The youngest, during the Cretaceous, involved highly saline fluids, indicating a relationship with late-Mesozoic kimberlite eruptions. Remnants of two preceding events, by a Paleozoic silicic fluid and a Proterozoic carbonatitic fluid, are also encapsulated in Kaapvaal diamonds and are likely coeval with major surface tectonic events (e.g. the Damara and Namaqua-Natal orogenies).

The geochemistry of groundwater surrounding the Dead Sea reflects mixing between different water bodies that occupied the Dead Sea basin and fresh groundwater flowing from the highlands to the basin. The distribution of various geochemical parameters is affected by changes in the lake level, including natural fluctuations and the present lake level drop. This chapter deals with different aspects of groundwater flow mechanisms in the coastal aquifer of the Dead Sea and their relationship with the groundwater geochemistry. Dead Sea water circulation in the coastal aquifer occurs during a steady-state lake level but also persists during lake level drop. This interaction between the Dead Sea water and the sediments removes Ba, U, and 226Ra and contributes Fe, Mn, and short-lived Ra isotopes to the Dead Sea. 14C and tritium values in the Dead Sea groundwaters are affected by mixing with fresh water and between brines that occupied the Dead Sea Rift during different times. In addition to the inorganic waterrock interaction between the Dead Sea groundwater and the sediments, DIC δ13C, and S and O isotopes in dissolved SO4 suggest that methane oxidation coupled with bacterial sulfate reduction is taking place. On longer time-scales (glacial-interglacial), lake water penetrates the aquifer sediments during high stands and discharge back to the Dead Sea during low stands. The Ein-Qedem thermal springs composition suggests that the brine penetrated to the aquifer during the last glacial when the lake level was high and is discharging now back into the Dead Sea from a depth of about 1000 m.

Submarine groundwater discharge (SGD) has been shown to be an important source of nutrients in coastal environments, especially nitrogen and silica, and thereby relive nutrient limitation to phytoplankton. Here, we followed autotrophic microbial biomass, activity, and community composition at a site strongly influenced by SGD and a nearby nutrients-poor reference site at the oligotrophic Israeli shallow rocky coast [southeastern Mediterranean Sea (SEMS)] between 2011 and 2019. The surface water at the SGD-affected area had significantly higher NO₃ + NO₂ (∼10-fold) and Si(OH)₄ (∼2-fold) levels compared to the reference site, while no significant differences were observed for PO₄ or NH₄. This resulted in a significant increase in algae biomass (∼3.5-fold), which was attributed to elevated Synechococcus (∼3.5-fold) and picoeukaryotes (∼2-fold) at the SGD-affected site, and in elevated primary production rates (∼2.5-fold). Contrary to most SGD-affected coastal areas, diatoms biomass remained unchanged between sites, despite the elevated N and Si, suggesting the dominance of picophytoplankton over microphytoplankton at the SEMS. DNA sequencing of the 16S and 18S rDNA supported these findings. These results highlight the influence of SGD on shallow-water microbial populations. Our observations are consistent with recent studies showing that phytoplankton along the Israeli coast are likely nitrogen + silica limited, and may have important ecological and regulatory implications for environmental policy and management of coastal aquifers.

Modern observations document increased drought frequency together with more intense precipitation and flooding in the world's semi-arid and arid regions as a consequence of the warming climate. Climate models predict that such conditions will intensify in the future, impacting millions of people. Paleoclimate studies can complement the short modern observational record and model projections by documenting climate changes in the past. Here we report major shifts in the geographic sources, intensity, and seasonality of Eastern Mediterranean precipitation during the unusually warm last interglacial period Marine Isotope Stage (MIS) 5e, reflecting global shifts in the rain and desert belts, based on ²³⁴U/²³⁸U-ratios in mineral precipitates in the Dead Sea, combined with evidence from climate model simulations. In the Dead Sea catchment ²³⁴U/²³⁸U ratios are indicators of water sources, where the Jordan River (flowing from the north) and the western catchments show high activity ratios between ∼1.51.7, and the eastern and southern catchments and flash floods (in the south-west, south and east) show lower ratios of 1.01.2. In Dead Sea water and precipitated minerals, ²³⁴U/²³⁸U is nearly always ∼1.451.55 during both glacials and interglacials. However, during the last interglacial MIS 5e insolation peak (∼127122 ka) its value decreased to 1.21.3, and then to ∼1.0 towards its end (∼122116 ka). During the insolation peak, the U-isotope data, combined with climate model runs forced with period orbital and greenhouse gas concentrations, indicate that rainfall associated with the African Summer Monsoon in the Dead Sea catchment accounted for ∼50% of the total annual rainfall, in stark contrast to present-day dry summers. The geochemical evidence indicates that following the insolation peak the region experienced an extremely dry period (although punctuated with wetter intervals), signifying expansion of the desert belt, similar to predicted effects of anthropogenic warming. This drying is partly supported by climate model runs forced with the appropriate changes in orbital parameters. The extreme drying during late MIS 5e between ∼122116 ka reflected a major weakening of Mediterranean storm systems, resulting in a major decline of the Jordan River flow (indicated by the low ²³⁴U/²³⁸U ratios in the Dead Sea) and a relative increase in precipitation associated with the African Monsoon, shifting towards autumn. The Jordan River flow is estimated to be ∼10% of the present-day (pre-1964, prior to major diversion of the Jordan River and its sources for human use). Such changes, if they occur in the future, have serious implications for future water availability in the politically sensitive Middle East.

The Dead Sea Deep Drilling Project drilled 456 meters into the deepest floor of the Dead Sea and recovered a record of the past similar to 220 kyr of the Levant hydroclimate history, that is, Marine Isotope Stages 1-7, including the last three interglacials and the last two glacials. We present an updated chronology of the core from DSDDP Hole 5017-1-A, from the Dead Sea's deepest basin, that refines our previous chronology (Torfstein et al. 2015) based on new information. The updated chronology uses the following approaches: (1) radiocarbon ages of Kitagawa et al. (2017); (2) correlation of specific layers in the core with U-Th-dated sediments on the Dead Sea margin, particularly during the interval of glacial Lake Lisan (MIS 2,3,4); (3) tuning of the delta O-18 data of DSDDP core aragonite to the U-Th dated oxygen isotopes of regional cave speleothems; and (4) tuning of the DSDDP aragonite delta O-18 data to summer insolation curves when the cave delta O-18 chronology is less clear. The updated chronology reveals a strong relationship between the sedimentary facies comprising the core and Northern Hemisphere summer insolation variations. It shows that sequences of sediments marking drier/wetter/drier climate conditions in the lake's watershed (e.g., salt/muds/salt, respectively) appear across the flank/peak/flank segments of several summer insolation peaks. In particular, the transition from drier to wetter sedimentary facies during mid-latitude insolation peaks coincides with the intervals of sapropel conditions in the Mediterranean, reflecting enhanced Nile flow due to intense African monsoonal conditions, and marking the impact of the tropical precession cycles on Eastern Mediterranean hydroclimate. This pattern was lost during MIS 2,3,4, when mostly sequences of primary aragonite are punctuated by gypsum precipitation during Heinrich events, marking the strong impact of the North Atlantic on the last glacial Levant hydroclimate. 

Permafrost in circum-polar regions has been recently undergoing thawing, with severe environmental consequences, including the release of greenhouse gases and amplification of global warming. Although highly important, direct methods of tracking thawing hardly exist. In a research study conducted at Adventdalen, Svalbard, we identified a permafrost radioisotope fingerprint, and show that it can be used to track thawing. Ratios of long- to the shorter-lived radium isotopes are higher in ground ice than in active layer water, which we attribute to the permafrost closed system and possibly to the long residence time of ground ice in the permafrost. Also, daughterparent ²²⁴Ra/ ²²⁸Ra ratios are lower in permafrost than in the active layer. These fingerprints were also identified in a local stream, confirming the applicability of this tool to tracing thawed permafrost in periglacial watersheds. A combination of radium isotope ratios and ³H allowed the identification of recent intra-permafrost segregation processes. The permafrost radium fingerprint should be applicable to other permafrost areas, which could assist in regional quantification of the extent of permafrost thawing and carbon emissions to the atmosphere.

The chemical composition and delta Cl-37 of pore fluids from the ICDP core drilled in the deepest floor of the terminal and hypersaline Dead Sea, and halites from the adjacent Mount Sedom salt diapir, are used to establish the dynamics of halite precipitation and dissolution during the last interglacial and glacial periods. Between similar to 132 and 116 thousand years ago (ka) halites precipitated in the lake resulting in the expulsion of Na+ and Cl- from the residual solution. Over 50% of the Cl- reservoir was removed, resulting in a decrease in the Na/Cl ratio from 0.57 to 0.19. This process was accompanied by a decrease in delta Cl-37 values in the precipitating halites and the associated residual Cl- in the lake. The observed decrease fits a Rayleigh distillation curve with a fractionation factor of Delta((NaCl-Dead Sea solution)) = +0.32 parts per thousand (+/- 0.12) determined in the present study. This behavior implies negligible contribution of external sources of Cl- to the lake during the main peak of the last interglacial, MIS5e. Subsequently, during the last glacial (ca. 117 to 17 ka) dissolution of halite took place, the Na+ and Cl- inventory were replenished, accompanied by an increase in Na/C1 from 0.21 to 0.55 and in the delta Cl-37 values from -0.46 parts per thousand to -0.12 parts per thousand. While the lake underwent significant dilution during that time, the decrease in salinity was somewhat suppressed by the dissolution of the halite which was mostly derived from Mount Sedom salt diapir. (C) 2018 Elsevier B.V. All rights reserve.

Thick halite intervals recovered by the Dead Sea Deep Drilling Project cores show evidence for severely arid climatic conditions in the eastern Mediterranean during the last three interglacials. In particular, the core interval corresponding to the peak of the last interglacial (Marine Isotope Stage 5e or MIS 5e) contains similar to 30m of salt over 85 m of core length, making this the driest known period in that region during the late Quaternary. This study reconstructs Dead Sea lake levels during the salt deposition intervals, based on water and salt budgets derived from the Dead Sea brine composition and the amount of salt in the core. Modern water and salt budgets indicate that halite precipitates only during declining lake levels, while the amount of dissolved Na+ and Cl- accumulates during wetter intervals. Based on the compositions of Dead Sea brines from pore waters and halite fluid inclusions, we estimate that similar to 12-16cm of halite precipitated per meter of lake-level drop. During periods of halite precipitation, the Mg2+ concentration increases and the Na+/Cl- ratio decreases in the lake. Our calculations indicate major lake-level drops of similar to 170 m from lake levels of 320 and 310 m below sea level (mbsl) down to lake levels of similar to 490 and similar to 480 mbsl, during MIS 5e and the Holocene, respectively. These lake levels are much lower than typical interglacial lake levels of around 400 mbsl. These lake-level drops occurred as a result of major decreases in average fresh water runoff, to similar to 40% of the modern value (pre-1964, before major fresh water diversions), reflecting severe droughts during which annual precipitation in Jerusalem was lower than 350 mm/y, compared to similar to 600 mm/y today. Nevertheless, even during salt intervals, the changes in halite facies and the occurrence of alternating periods of halite and detritus in the Dead Sea core stratigraphy reflect fluctuations between drier and wetter conditions around our estimated average. The halite intervals include periods that are richer and poorer in halite, indicating (based on the sedimentation rate) that severe dry conditions with water availability as low as similar to 20% of the present day, continued for periods of decades to centuries, and fluctuated with wetter conditions that spanned centuries to millennia when water availability was similar to 50-100% of the present day. These conclusions have potential implications for the coming decades, as climate models predict greater aridity in the region. (C) 2017 Elsevier B.V. All rights reserved.

Most of the fossil corals in the elevated reef terraces along the Gulf of Aqaba (GOA) were extensively altered to calcite. This observation indicates extensive interaction with freshwater, possibly when the terraces passed through a coastal aquifer that existed along the shores of the GOA, implying a wetter climate during the time of recrystallization from aragonite to calcite. Thus, dating of the recrystallization events should yield the timing of past wetter conditions in the current hyperarid area of the GOA. In the present study, 18 aragonite and calcite corals were collected from several elevated coral reef terraces off the coast, south of the city of Aqaba. While aragonite corals were dated with the conventional closed system age equation (assuming zero initial Th), the dating of the calcite corals required the development of adequate equations to allow the calculation of both the initial formation age of the aragonite corals and the time of recrystallization to calcite. The two age calculations were based on the assumptions that each reef terrace went through a single and rapid recrystallization event and that the pristine aragonite corals were characterized by a rather uniform initial U concentration, typical for pristine modern corals. Two recrystallization events were identified at 104 +/- 6 ka and 124 +/- 8 ka. The ages coincide with the timing of sapropel events S4 and S5, respectively, when the African monsoon induced enhanced wetness in the desert area. Considering the age uncertainties, the times of formation of the two major reef terraces are estimated to be similar to 124 ka (reef terrace R-2) and similar to 130 ka (reef terrace R-3), matching the peaks in the global sea level during the last interglacial MIS 5e stage. Apparently, sea level of the GOA did not fluctuate a lot during the period between similar to 130 ka and similar to 104 ka and remained close to the Marine Isotopic stage (MIS) 5e highstand. The availability of freshwater (during the sapropel periods) and limited sea level fluctuations facilitated the recrystallization of the GOA reef corals to calcite. (C) 2016 Elsevier Ltd. All rights reserved.

A study of an International Continental Drilling Program core recovered from the middle of the modern Dead Sea has identified microbial traces within this subsurface hypersaline environment. A comparison with an active microbial mat exhibiting similar evaporative processes characterized iron-sulphur mineralization and exopolymeric substances resulting from microbial activity. Exopolymeric substances were identified in the drilled sediment but unlike other hypersaline environments, it appears that they have a limited effect on the precipitation of calcium carbonate in the sedimentary column. Sulphate reduction, however, plays a role in all types of evaporative facies, leading to the formation of diagenetic iron sulphides in glacial and interglacial intervals. Their synthesis seems to occur under progressive sulphidation that generally stops at greigite because of incomplete sulphate reduction. The latter may be caused by a lack of suitable organic matter in this hypersaline, hence energy-demanding, environment. Pyrite may be found in periods of high lake productivity, when more labile organic matter is available. The carbon and sulphur cycles are thus influenced by microbial activity in the Dead Sea environment and this influence results in diagenetic transformations in the deep sediment.

Thick sequences of salt (halite) have been recovered in a 456-m-long core drilled at the deepest floor of the Dead Sea by the Dead Sea Deep Drilling Project and extending similar to 200 k.y. back in time. The salt sequences were precipitated in the ancient lake that occupied the Dead Sea Basin during the last three interglacials during intervals of extreme aridity in the lake's watershed. The salt layers alternate with "mud" layers that indicate wetter periods in the watershed, when floods transported fine detritus matter to the lake. The salt sources include brine discharge and freshwater runoff that dissolved halite units. Dissolved salts accumulated in the lake during glacials and relatively wet periods when the lake expanded, and precipitated during interglacials when the lake levels dropped. This study establishes for the first time the evaporite facies and sedimentological features of the deep Dead Sea brine during interglacial periods, by using the modern precipitation of halite in the Dead Sea as an analogue for past halite depositional periods as recorded in the drill core. The halite intervals provide a record of facies characterizing a deep-water evaporitic environment. The halite layers consist mainly of two types of crystals: small cumulate crystals containing halite rafts, which indicate precipitation from the surface brine of the lake (epilimnion), and bottom-growth (usually large) halite crystals that precipitated on the lake floor (hypolimnion). The layers of small halite crystals formed during drier periods as compared to the bottom-growth crystals. The bottom-growth halite crystals contain variable quantities of detritus and show mild dissolution structures at the contact between the mud and the halite crystals. These two main types of halite, in combination with "muds" and gypsum, comprise seven categories of salt facies that reflect the hydrological conditions (dry-to-wet), and that display a cyclic (decadal to millennial) pattern along the sampled core intervals. Frequent alternation of these two salt crystal types suggests seasonal changes, whereby the small cumulate crystals were formed during the summer, and the bottom-growth crystals were formed during the winter, when the surface temperatures of the lake were low, and the surface water was less saline and less likely to be saturated with respect to halite. Comparison of the last interglacial halite with the modern halite facies, together with the absence of significant dissolution features within the halite and the cyclic nature of the facies, indicates that the lake was continuously deep (>100 m) during the last 200 k.y.

This study presents the behavior of radon and radium isotopes and their application to groundwater age and flow dynamics. The research was conducted in the complex Dead Sea groundwater system, which includes a large variety of sediments, groundwater salinities, flow mechanisms and groundwater ages. Groundwater around the Dead Sea contains high activities of radon (up to tens of thousands dpm/L) and radium (up to hundreds dpm/L). Adsorption of radium, which is partially salinity controlled, is an important source of unsupported Rn-222, which is used for estimating the adsorption partition coefficient of radium. In addition to salinity, the concentration of Mn and Fe oxides and aquifer heterogeneity are important factors controlling the adsorption partition coefficient. The different nature of the rocks on both sides of the Dead Sea transform, with lower Th/U ratios in the carbonate rocks on the western catchment of the Dead Sea compared to higher ratios in the sandstone aquifer east of the Dead Sea, is reflected in a higher Ra-228/Ra-226 activity ratio in the eastern compared with the western groundwaters (averages of 0.76 and 0.15, respectively). The different groundwater groups around the Dead Sea contain secular or non-secular equilibrium ratios, which depend on the age of the groundwater (the time since the groundwater entered the aquifer) or whether the groundwater system is in a steady state (the age of the groundwater system). Young groundwater, such as the Dead Sea water that circulates in the aquifer or freshwater springs, is depleted in the long-lived radium isotopes compared to the short-lived isotopes, whereas old groundwater contains relatively high activity of Ra-226 (similar to 500 dpm/L) and the radium activity ratios are close to secular equilibrium. The common secular equilibrium ratios between all four radium isotopes in the Dead Sea groundwaters suggest that many of the groundwater flow paths did not change significantly during the past 8000 years. (C) 2015 Elsevier B.V. All rights reserved.

This paper shows for the first time a field-based estimation of the volume of dispersive density-driven long-term seawater circulation in coastal aquifers, which is crucial to the understanding of water-rock interaction and to the assessment of its potential impact on elemental mass balances in the sea. The Dead Sea is an ideal place for studying this type of circulation due to the absence of tides and the accessibility of the shallow fresh-saline transition zone. The unique antithetical behavior of Ra-226 and Ra-228 during seawater circulation in the Dead Sea aquifer, where Ra-228 is added and Ra-226 is removed, provides a robust new method for quantifying aquifer circulation. Here we estimate water circulation through the Dead Sea aquifer to be 400 million m(3)/yr (similar to 2.5 million m(3)/yr per 1 km of shoreline), which is similar to 20% of the fresh water inflow prior to the 1960s. This large volume can affect trace element concentrations in the Dead Sea, e.g. it is a sink for Ra-226, Ba and U and a source for Ra-228 and Fe. These results suggest that dispersive density-driven seawater circulation in aquifers may play an important role in mass balances in other lacustrine and oceanic settings.

The Dead Sea system provides a unique opportunity to study flow velocities and reaction rates during seawater circulation in the aquifer. We present here a novel application of calculating groundwater age and velocity along the flow path of the hypersaline water from the Dead Sea into the aquifer using the buildup rate of Ra-228 in this water. The calculated circulation velocities are 1-10 m/y, which is in agreement with estimates based on the Na/Cl ratios in this water (1.5-4 m/y). The latter is unique to the Dead Sea-aquifer system, where the Na/Cl ratio has been decreasing during the past 50 years due to the precipitation of halite in the lake. The velocity estimates facilitated the calculation of the rates of water rock interactions in the Dead Sea aquifer.SO4 is removed relatively fast (k = 0.8 y(-1)) due to gypsum precipitation while barite or celestine precipitation removes Ra-226 and Ba in a time scale of years (k = 0.22 y(-1)). Similar rates were found for redox-driven reactions, such as U removal (k = 0.4 y(-1)) and Fe and Mn contribution due to the dissolution of oxides (k = 0.15 y(-1)). In the fresh-saline groundwater transition zone, gypsum which precipitated from hypersaline water in higher lake stands, is now being dissolved and enrich the water with SO4. (C) 2013 Elsevier Ltd. All rights reserved.

We present a new approach to studying the behavior of radium isotopes in a coastal aquifer. In order to simulate radium isotope distributions in the dynamic flow field of the Dead Sea aquifer, a multi-species density dependent flow model (SUTRA-MS) was used. Field data show that the activity of Ra-226 decreases from 140 to 60 dpm/L upon entering the aquifer from the Dead Sea, and then further decreases linearly due to mixing with Ra-poor fresh water. On the other hand, an increase is observed in the activity of the shorter-lived isotopes (up to 52 dpm/L Ra-224 and 31 dpm/L Ra-223), which are relatively low in Dead Sea water (up to 2.5 dpm/L Ra-224 and 0.5 dpm/L Ra-223). The activities of the short lived radium isotopes also decrease with decreasing salinity, which is due to the effect of salinity on the adsorption of radium. The relationship between Ra-224 and salinity suggests that the adsorption partition coefficient (K) is linearly related to salinity. Simulations of the steady-state conditions, show that the distance where equilibrium activity is attained for each radium isotope is affected by the isotope half-life, K and the groundwater velocity, resulting in a longer distance for the long-lived radium isotopes. K affects the radium distribution in transient conditions, especially that of the long-lived radium isotopes. The transient conditions in the Dead Sea system, with a 1 m/yr lake level drop, together with the radium field data, constrains K to be relatively low (

The present study examines the response of groundwater systems to expected changes in the Mediterranean Sea (rise of

Radium isotopes in the Dead Sea Lake and the surrounding aquifer were studied in order to define the processes controlling their activities in the lake and in the groundwater. ²²⁶Ra activities in the groundwater show a significant removal of ²²⁶Ra from the lake water as they enter the aquifer. Short-lived radium isotopes show a nonlinear relation with salinity which is caused by the effect of salinity on the adsorption of radium. Simulations of radium distribution in the aquifer, using the code of SUTRA-MS, were done in order to estimate the adsorption distribution coefficient of radium in the aquifer. ²²⁸Ra activities in the Dead Sea water are much higher than the expected activities according to the radium sources to the Dead Sea. These observations together with the removal of ²²⁶Ra in the Dead Sea groundwater may indicate a large-scale saline water circulation in the aquifer.

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.