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The Dead Sea shores: From a liability to an asset - הנגשת חופי ים המלח: ממטרד למשאב
Abstract
The Dead Sea is a unique deep hypersaline lake that actively deposits halite (salt) layers. It is located along a tectonic plate boundary, namely the Dead Sea Rift, at the lowest place on Earth. The lake’s level continuously declines at a rate higher than one meter per year, due to upstream water capture and local industrial activity. As a result, a series of geological processes have developed within extremely high rates, hence forming fascinating and unique landforms, including: the active formation of evaporate salt layers, incision of canyons in the exposed mud flats, rapid progradation of alluvial fans and deltas, exhumation of lake-beds, hundred kilometers long curvy coastline formed as a result of subaqueous landslides, and formation of deep sinkholes which can be hazardous. Although the sinkholes are attractive geomorphic feature, sinkholes hazard prevents development of tourism, cause damage to infrastructures and damage the agriculture in the region. Consequently, the Israeli authorities refer to the region between Road-90 and the lake coastline, as dangerous and the road signs indicate “no entrance”; nevertheless, the public visit this region but under unsafe conditions (parking unsafely along the main road and then crossing unstable sinkholes fields). Here we present a more vital and safe approach that will allow the public to hike, explore and enjoy these dynamic landscapes for fun, tourism, education and for research. In order to make this area accessible we suggest: 1) to arrange parking grounds near Rd-90, 2) to mark safe trails from the parking grounds eastward toward the coastline, crossing safely the sinkholes areas; these trails will be monitored by early warning system for sinkholes hazard. Most of the coastline, which is highly attractive for tourism, is less prone to the sinkholes hazard, but still requires monitoring. We propose a dozen of coastal trails along the western coast of the Dead Sea, each trail starts from a safe east-west route from the parking grounds.
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As the only deep hypersaline, halite-precipitating lake on Earth today, the Dead Sea is the single modern analog for investigating the mechanisms by which large-scale and thick salt deposits, known as ‘salt giants’ have accreted in the geological record. We directly measure the hydroclimatic forcing and the physical limnologic processes leading to halite sedimentation, the vertical thermohaline structure, and salt fluxes in the Dead Sea. We demonstrate that changes in these forcing lead to strong seasonal and regional variations in the stratification stability ratio, triggering corresponding spatiotemporal variations in thermohaline staircase formation and diapycnal salt flux, and finally control the thickness of the halite layer deposited. The observed staircase formation is consistent with the mean-field γ-instability, causing layering in double-diffusive convection. We show that double diffusion and thermohaline staircase formation drive the spatial variability of halite deposition in hypersaline water bodies, underlying and shaping 'salt giants' basin architecture.
Many halite sequences in the geological record accumulated in deep hypersaline basins. However, modern analogs of active halite deposition were studied in shallow hypersaline environments and then applied in interpreting halite sequences. Recently, halite deposition in the deep, hypersaline Dead Sea has been studied together with its coeval environmental and limnogeological forcing; which is the closest modern analog for the deep environments. Therefore, stratigraphy, sedimentology, and petrography of a well-dated, high-resolution modern Dead Sea halite sequence are explored. The sequence was deposited during a ~30-m lake-level decline since the onset of modern halite deposition in 1980, and was compared with sub-annual lake levels, precipitation, and flood records. The sedimentology of the sequence documents the trend of shallowing water depth, including individual, short-term events. The base of the sequence is composed of alternating bottom growth-cumulate halite annual couplets, typical of deep hypolimnetic water deposition. The annual couplets disappear up-sequence and the upper part of the sequence is composed of cumulate layers with dissolution features, typical of shallow epilimnetic water deposition. As a result, halite deposition rate is reduced by 60% at the shallow lakefloor compared with the deep lakefloor. The top of the sequence contains shoreline deposits, halolites (halite ooids) and polygonal surface cracks, indicating subaerial exposure. We show petrographic indicators for summer thermal dissolution )partially dissolved crystals), which are distinct from dissolution features by winter floods that generate a regional truncation surface. We also observed spatial variations in halite thickness and facies, indicate much thinner and spatially limited halite unit compared to the modeled halite unit based on mass balance considerations. Our observations provide criteria for, (a) recognizing water depths and shallowing lake level trend from halite sequences throughout the geological record, and (b) interpretating paleolimnology, water column structure and the relations between stratigraphic horizons and corresponding shorelines.
During the past three decades, the Dead Sea (DS) water level has dropped at an average rate of ~1 m/year, resulting in the formation of thousands of sinkholes along its coastline that severely affect the economy and infrastructure of the region. The sinkholes are associated with gradual land subsidence, preceding their collapse by periods ranging from a few days to about five years. We present the results of over six years of systematic high temporal and spatial resolution interferometric synthetic aperture radar (InSAR) observations, incorporated with and refined by detailed Light Detection and Ranging (LiDAR) measurements. The combined data enable the utilization of interferometric pairs with a wide range of spatial baselines to detect minute precursory subsidence before the catastrophic collapse of the sinkholes and to map zones susceptible to future sinkhole formation. We present here four case studies that illustrate the timelines and effectiveness of our methodology as well as its limitations and complementary methodologies used for sinkhole monitoring and hazard assessment. Today, InSAR-derived subsidence maps have become fundamental for sinkhole early warning and mitigation along the DS coast in Israel and are incorporated in all sinkhole potential maps which are mandatory for the planning and licensing of new infrastructure.
Worldwide stratigraphic records present thick halite layers accreted in deep hypersaline basins under dry climate conditions. The thickness and distribution of these halite units are used in basin analyses and paleoenvironmental reconstructions. Recent studies have raised doubts regarding the assumption that a given halite layer's thickness is directly related to net evaporation of the overlying water, where the areas of both water surface and deposited halite are similar. Here we present halite focusing, a limno-sedimentological model for halite accumulation based on observations from the Dead Sea. The model accounts for a halite-saturated hypersaline basin under negative water balance and a stratified water column. Under such stratification, double-diffusive flux transfers dissolved salt from the epilimnion (upper water layer in a stratified lake) down to the hypolimnion, resulting in an undersaturated epilimnion and continuous halite focusing; i.e., large amplification (increasing thickness of halite layers) of its accretion in the depocenter, at the expense of dissolution from the shallow basin margins. Halite focusing can almost triple the thickness calculated by uniform precipitation, meaning that a given halite unit may have accumulated faster, during shorter, less-arid intervals than previously proposed. Halite focusing explains (1) extremely high deposition rates, (2) accretion of exceptionally thick halite sequences in deep basins, and (3) a marginal basin that is fully or partially devoid of halite, with coeval thick sequence deposition in the deep basin.
Seismically disturbed sedimentary sequences (seismites) in the last glacial (70-14 ka) Lisan Formation are exposed in the marginal terraces of the Dead Sea and recovered from sedimentary core drilled by the International Continental Scientific Drilling Programs at the depocenter of the lake at water depth of 300 m. The core reveals various types of centimeter- to meter-scale disturbed lake sediments: turbidites, homogenites, slumps, and other deformations, interspersed with undisturbed lamination. The transported sediments comprise a main source of the thickness tripling of the Lisan Formation at the depocenter of the lake compared to the margins. Excluding (mass transport deposits MTDs) from chronology yields a normal, event-free age-depth model for core. Moreover, time intervals of units missing at the exposed sections of the lake margin are synchronous with intervals of mass transport deposits (MTDs) at the deepest lake floors. In the deep core, the recurrence interval range of MTD with thickness > 1 cm is ~ 100-300 years, while that of the thick MTD (>50 cm) is ~ 2,500 years, twice the recurrence interval of the seismites at the lake's margins. The ~1,000 year recurrence at the lake's margins lies within the ranges that have been calculated for ~M6.5-7 earthquakes. The significantly higher figure shown by depocenter core suggests activity on various faults and the response of the lake sediments in the entire Dead Sea basin. Overall, an unprecedented chronology of seismic activity was achieved for the late Pleistocene Lisan Formation providing a framework of earthquake activity in the vicinity of the Dead Sea basin during the period of the last high lake stand.
Meandering channels and valleys are dominant landscape features on Earth. Their morphology and remnants potentially indicate past base‐level fluctuations and changing regional slopes. The prevailing presence of meandering segments in low‐slope areas somewhat confuses the physically‐based relationships between slope and channel meandering. This relationship underlies a fundamental debate: do incised sinuous channels actively develop during steepening of a regional slope, or do they inherit the planform of a preexisting sinuous channel through vertical incision? This question was previously explored through reconstructed evolution of meandering rivers, numerical simulations, and controlled, scaled‐down laboratory experiments. Here, we study a rare, field‐scale set of a dozen adjacent perennial channels, evolving in recent decades in a homogeneous erodible substrate in response to the Dead Sea level fall (>30 m over 40 years). These channels are fed by perennial springs and have no drainage basin and no previous fluvial history; they initiated straight and transformed into incising meandering channels following the emergence of the preexisting lake bathymetry, which resulted in increased channel lengths and regional slopes, at different rates for each channel. This field setting allows testing the impact of changing regional slope on the sinuosity of a stream in the following cases: (a) relatively long and low‐gradient shelf‐like margins, (b) a sharp increase in the basinward gradient at the shelf‐slope transition, and (c) gradually steepening slopes. Under a stable and low valley slope, the channels mainly incise vertically, inheriting a preexisting sinuous pattern. When the regional slope steepens, the channels start to meander, accompanying the vertical incision. The highest sinuosity evolved in the steepest channel, which also developed the deepest and widest valley. These results emphasize the amplifying impact of steepening regional slope on sinuosity. This holds when the flow is confined and chute cutoffs are scarce.
The Dead Sea Deep Drilling Project drilled 456 meters into the deepest floor of the Dead Sea and recovered a record of the past ∼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 δ¹⁸O data of DSDDP core aragonite to the U–Th dated oxygen isotopes of regional cave speleothems; and (4) tuning of the DSDDP aragonite δ¹⁸O data to summer insolation curves when the cave δ¹⁸O 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.
Global eustatic lowstands can expose vast areas of continental shelves, and occasionally the shelf edge and the continental slope. The degree of fluvial connectivity to receding shores influences the redistribution of sediments across these emerging landscapes. Shelf and slope emergence in the Dead Sea since the middle of the 20th century, offers a rare opportunity to examine evolution of stream connectivity in response to continuous base-level decline. We characterize the connectivity evolution of two streams, using high-resolution time series of aerial imagery and elevation models, field mapping, and grain-size analyses. Our rich spatiotemporal dataset of evolving channel geomorphology, sediment transport conditions, and sediment redistribution, allows calculating potential coarse sediment mobility in response to base level decline. Following shelf emergence, alluvial fans first prograde onto the low-gradient shelf under unfavourable conditions for transporting coarse sediment to the regressing shoreline. Then, with shelf and slope emergence, the two adjacent streams evolved differently. The smaller, more arid watershed still maintains its highstand delta progradation on the shelf and is practically disconnected from the receding lake. The larger catchment, heading in wetter environments and having a narrower shelf, has incised the shelf and renewed and gradually intensified the sediment transport from the highstand to the lowstand delta. Sediment mobilization to lowstand shorelines is controlled by the evolution of the channel profile and by the average speed of gravel transport (10s-100s m yr-1). These findings from the Dead Sea are relevant to fluvial processes operating on continental shelves during glacial maxima. Streams would have commonly stored high proportions of their coarse sediment on the continental shelves rather than efficiently connecting with the lowstand level. Additionally, differences in sediment routing patterns should exist among nearby streams, primarily due to continental margin geometry and watershed hydrology.
The geomorphic evolution of the Jordan River in recent decades indicates that interaction between incision and high‐magnitude floods controls sinuosity changes under increasing mouth gradients during base‐level fall. The evolution of the river was analyzed based on digital elevation models, remotely sensed imagery, hydrometric data, and a hydraulic model. The response varies along the river. Near the river mouth, where incision rate is high and a deep channel forms, overbank flooding is less likely; large floods exert high shear stress within the confined channel, increasing sinuosity. Upstream, near the migrating knickzone channel gradients also increase, incision is more moderate and floods continue to overtop the banks, favoring meander chute cutoffs. The resulting channel has a downstream well‐confined meandering segment and an upstream low‐sinuosity segment. These new insights regarding spatial differences along an incising channel can improve interpretations of the evolution of ancient planforms and floodplains that responded to base‐level decline.
During the past three decades, thousands of sinkholes were formed along the Dead Sea (DS) shorelines in Israel and Jordan, due to dissolution of subsurface salt by undersaturated groundwater. The sinkholes are associated with gradual subsidence preceding their collapse by periods ranging from a few days to almost 5 years. To determine the factors controlling this precursory subsidence, we examine tens of subsidence-sinkhole sequences along the DS shorelines in Israel. The duration and magnitude of the precursory subsidence are determined by Interferometric Synthetic Aperture Radar (InSAR) measurements and simulated by viscoelastic damage rheology models. Longer periods of precursory subsidence are found in the cemented alluvial fans and in simulations of higher-viscosity sediments. While surface subsidence accelerates during the precursory period, the widths of the subsiding areas remain uniform, suggesting that during this period upward propagation of damage from the subsurface cavity is not accompanied by upward migration of the actual cavity. Our observations and simulations are used to constrain the viscosity of the sediments along the DS and to reduce sinkhole hazards by assessing the precursory times of future sinkholes in the different sedimentary environments.