Dead Sea, Middle East
The Dead Sea water surface defines what is the deepest continental position (−420 m asl) on the earth’s current terrestrial surface. It is our only modern example where bedded evaporitic sediments are accumulating on the floor of a deep brine body, where water depths are measured in hundreds of meters. Overall, this salt-encrusted depression is 80 km long and 20 km wide, has an area of 810 km2, is covered by a brine volume of 147 km3 and occupies the lowest part of a drainage basin with a catchment area of 40,650 km3.
However, falling water levels in the past few decades mean the permanent water mass now only occupies the northern part of the lake, while saline anthropogenic pans occupy the southern basin so that the current perennial “Sea” is now only some 50 km long. Rainfall in the region is 45–90 mm, evaporation around 1,500 mm, and air temperatures between 11 and 21°C in winter and 18–40°C in summer, with a recorded maximum of 51°C. The subsiding basin is surrounded by mountain ranges to the east and west, producing an orographic rain shadow that further emphasises the aridity of the adjacent desert.
So, until recently, the basin floor was physiographically divisible into two adjacent and permanent brine-covered water bodies, the Northern and Southern basins, separated mainly by a diapir-cored shallow sill, the Lisan Ridge and joined at the Lisan Straits. Top of Miocene salt is 100–120 m below the surface in the Lisan Peninsula and a few meters below the surface at Mt. Sedom. The land surface of the Lisan Peninsula is slowly rising and being karstified, at a rise rate of a few mm per year, driven by salt flow. Widespread at-surface evidence of this rise is swamped by the changing of the depositional and hydrological system, driven by fluctuating lake water levels. Continuing falling water levels since the 1930s means that today the North Basin is the only permanent natural water mass, with waters some 320 m deep.
Dead Sea
Geology of the Dead Sea basin showing the distribution of the various depositional settings in plan view and as a block diagram of the Northern Dead Sea basin. The River Jordan forms a fluviodeltaic wedge in the northern part of the basin. Active transtension faulting and block rotation has exposed older lake deposits of the Lisan Formation at higher elevations as well as an apron of alluvial fan sediments with a number of “hung” fan deltas tied to older higher water levels in the lake. The saline pan in the southern basin is dominated by halite, artificial pans fed with deep brines have been constructed in this area to produce sylvite and other potash products.
The Southern Basin is today a saline pan/mudflat, which would be a subaerially exposed plain, except that brine-filled saltworks now cover much of the former Southern Basin lake. Perennial brine sheets are artificially maintained in these pans by continually pumping of brines from the Northern Basin on both the Israeli and Jordanian sides of the Dead Sea (for more detail refer to Warren, 2016, Chapter 4).
So, until recently, the basin floor was physiographically divisible into two adjacent and permanent brine-covered water bodies, the Northern and Southern basins, separated mainly by a diapir-cored shallow sill, the Lisan Ridge and joined at the Lisan Straits. Top of Miocene salt is 100–120 m below the surface in the Lisan Peninsula and a few meters below the surface at Mt. Sedom. The land surface of the Lisan Peninsula is slowly rising and being karstified, at a rise rate of a few mm per year, driven by salt flow. Widespread at-surface evidence of this rise is swamped by the changing of the depositional and hydrological system, driven by fluctuating lake water levels. Continuing falling water levels since the 1930s means that today the North Basin is the only permanent natural water mass, with waters some 320 m deep. The Southern Basin is today a saline pan/mudflat, which would be a subaerially exposed plain, except that brine-filled saltworks now cover much of the former Southern Basin lake. Perennial brine sheets are artificially maintained in these pans by continually pumping of brines from the Northern Basin on both the Israeli and Jordanian sides of the Dead Sea (for more detail refer to Warren, 2016, Chapter 4).
Since the beginning of the 20th century, the water budget of the Dead Sea has been negative, leading to a continuous decrease in the water level. The extensive evaporation in the absence of major fresher water input led to an increase in the density of the upper water layer, which caused the lake to overturn in 1979 (Warren, 2016 for a summary of the hydrochemical evolution).
Since then, except after two rainy seasons in 1980 and 1992, the Dead Sea remained holomictic and has been characterised by a NaCl supersaturation and halite deposition on the lake bottom, with total dissolved salt concentrations reaching 347 g/l. Due to the continuous evaporation of the Dead Sea, Na+ precipitates out as halite, while Mg2+, whose salts are more soluble, is further concentrated and has become the dominant cation in the present holomictic water mass.
Falling water levels have also facilitated encroachment of brackish ground waters into lake floor salt beds, creating major problems with salt-karst induced collapse in many areas of the lake's margins on both the Israeli and Jordanian sides.
Changes in the Dead Sea water level at various time scales. A) Historical changes in the water level of the Dead Sea brine level since 1800, showing obvious acceleration in rate of fall since the 1960s and the “droughty” or falling trend since 1918. B) Detailed curve for the last four decades showing how rapid rises are tied to “wet” years and the consequent meromictic lake hydrology. C) Variations in the areas of the Northern and Southern Basins and the growing area of anthropogenic salt pans as it relates to changing brine levels. D) Changes in the Dead Sea level for the past 8,000 years (A and C replotted in part from Enzel et al., 2003; B replotted from Gertman and Hecht, 2002, data from http://isramar.ocean.org.il and more recent measures).
Evolution of Dead Sea stratification in the period 1959-1979 showing the loss of brine stratification via lessening of freshened inflow from Jordan River and entry of brine waste from south. This resulted in increased salinity of surface waters, a deepening of the halocline and final equalisation of densities of the two masses and their overturn in 1979.
In situ observations of bottom sediments in the Dead Sea by Sirota et al., 2017, within the current holomictic hydrology of the Dead Sea, link seasonal thermohaline stratification, halite saturation, and the textural character of actively-forming halite-rich bottom sediments. The spatiotemporal evolution of halite precipitation in the current holomictic stage of the Dead Sea is influenced by (1) lake thermohaline stratification (temperature, salinity, and density), (2) degree of halite saturation, and (3) textural evolution of the active halite deposits. Observed relationships by Sirota et al., tie the textural characteristics of layered subaqueous halite deposits (i.e., grain size, consolidation, and roughness) to the degree of saturation, which in turn reflects the limnology and hydroclimatology of the lake sump. The current halite-accumulating lake floor is divided into two main environments: 1) a deep, hypolimnetic (below thermocline) lake floor and, 2) a shallow, epilimnetic lake floor(above thermocline) (Salty Matters, Aug 31, 2018).
In the deeper hypolimnetic lake floor, halite, which is a prograde salt, continuously precipitates with seasonal variations so that : (a) During summer, consolidated coarse halite crystals under slight supersaturation form rough crystal surfaces on the deep lake floor. (2) During the cooler conditions of winter, unconsolidated, fine halite crystals form smooth lake-floor deposits under high supersaturation. These observations support interpretations of the seasonal alternation of halite crystallisation mechanisms. The shallow epilimnetic lake floor is highly influenced by the seasonal temperature variations, and by intensive summer dissolution of part of the previous year’s halite deposit, which results in thin sequences with annual unconformities. This emphasises the control of temperature seasonality on the characteristics of the precipitated halite layers. In addition, precipitation of halite on the hypolimnetic floor, at the expense of the dissolution of the epilimnetic floor, results in lateral focusing and thickening of halite deposits in the deeper part of the basin and thinning of the deposits in shallow marginal basins.
Dead Sea Sediment Textures. A) Compiled columnar section and primary sedimentary facies (after Torfstein et al., 2015). The main types of sediments comprising the DSDDP core are: (1) Alternating aragonite and detritus laminae (aad facies; brown-white lamination), characterizing periods of enhanced freshwater input; (2) Laminated silts (ld facies; brown), characterizing floods occurring during periods of relative aridity; (3) Halite (gray) and gypsum layers (red) marking lake level drops. A pebble unit ca. 235 mblf (yellow) marks the catastrophic low point of water elevations during MIS5 B) Schematic of textures tied to hydrological evolution of the Dead Sea brine lake
Historical (anthropological and religious) context
The Dead Sea has attracted visitors from around the Mediterranean Basin for thousands of years. It was one of the world's first health resorts (perhaps even for Herod the Great), and it retains that reputation today. It has been the long-term supplier of a wide variety of products, ranging from asphalt for Egyptian mummification to potash.toady. Its muds have putative therapeutic and cosmetic properties. The name for the Dead Sea comes from the Arabic where it is known Al-Baḥr Al-Mayyit ("Sea of Death"), and in Hebrew, it is called Yam HaMelaẖ ("the Salt Sea"). Aristotle wrote about the remarkable salty waters. Millenia ago Nabateans and others discovered the value of the globs of natural asphalt that constantly floated to the surface where they could be harvested with nets. The Egyptians were steady customers, as they used asphalt in the embalming process that created mummies. The Ancient Romans knew the Dead Sea by its Greek name "Palus Asphaltites" (Asphalt Lake). A cargo boat on the Dead Sea can be seen on the Madaba Map (a mosaic see No. 3), from the 6th century AD.
The Dead Sea was an important trade route with ships carrying salt, asphalt and agricultural produce. Many anchorages existed on both sides of the sea, including in Ein Gedi, Khirbet Mazin (where the ruins of a Hasmonean-era dry dock are located), Numeira and near Masada. King Herod the Great built or rebuilt several fortresses and palaces on the western bank of the Dead Sea. The most famous was Masada, where in 70 CE a small group of Jewish zealots fled after the fall of the destruction of the Second Temple. The zealots survived until 73 CE, when a siege by the Roman X Legion ended in the deaths by suicide of its 960 inhabitants. Another historically significant fortress was Machaeruson the eastern bank of the Dead Sea, where, according to Josephus, John the Baptist was imprisoned by Herod Antipas and died.
Lot’s Wife, an erosional carapace of a salt diapir, or the mortified remains of a woman who had dared disobey a genocidal and homophobic stone-age deity?
Josephus (Yosef ben Matityahu; 37-100 CE) identified the Dead Sea in geographic proximity to the ancient Biblical city of Sodom. However, in his writings he referred to the lake by its Greek name, Asphaltites as at the time it was a major supplier of asphalt to Egypt for mummification. In Arabic the Dead Sea is also known as Birket Lut, (lit. "Lake/Sea of Lot). To believer’s in the Old Testament version of the supernatural (Genesis 19:26) Lot’s wife was turned into a pillar of salt by a vengeful “God” as she and her traumatised co-believers were fleeing the aftermath of the genocidal deity's actions. She dared to look back at the destruction wrought by the indignant sky faery on the city of Sodom.
A structure known as “Lot’s Wife” was noted in the journals of Fulcher of Chartres (Chaplain to King Baldwin) who accompanied the crusader Baldwin I across the Dead Sea valley in December 1100 AD. In reality, this apophenic feature is a 12 m-high column of salt lying at the foot of the much larger Mt Sedom (Usdum) on the edge of the Dead Sea. It is one of a number of dissolutional remnants along the gypsum-capped cavernous edge of an outcropping diapir composed of Miocene salt, which also makes up Mount Sedom.
In Roman times, various sects of Jews settled in caves overlooking the Dead Sea. The best known of these are the Essenes of Qumran; Pliny the Elder identifies their location with the words, "on the west side of the Dead Sea, away from the coast ... [above] the town of Engeda.” It is therefore a popular but contested hypothesis today, that these Essenes are identical with the settlers at Qumran and that "the Dead Sea Scrolls" discovered during the 20th century in the nearby caves had been their library. The town of Ein Gedi, mentioned many times in the Mishna, produced persimmon for the temple's fragrance and for export, using a secret recipe. "Sodomite salt" was an essential mineral for the temple's holy incense, but believers said it was dangerous for home use and could cause blindness.