Limnology

Dead Sea

Content Cover Image

Satellite view of the Dead Sea. Source: National Aeronautcis and Space Admnistration

The Dead Sea is a hypersaline lake that forms part of the border between Jordan from Israel, and that has no outlet and only one principal source, the Jordan River.

This is the deepest hypersaline lake in the world, and it is also the lowest point in absolute elevation on Earth.

The deep basin was formed from the continental rifting induced by plate separation at the margin of the Arabian Plate severing from the African continent.

While there are no fish or other higher lifeforms found in the Dead Sea, a number of halophilic bacterial species occur here as well as microscopic fungal organisms; a surge in biomass occurs in the rare years of unusually high precipitation within the basin; this biomass is a group of Dunaliella algae, which support carotenoid halobacteria (red-pigmented species).

Since ancient times the Dead Sea perimeter has been used by such figures as King David and Herod I as a health resort, because of its unique mineral waters and also the additional ultraviolet protection afforded to sunbathers at extremely low altitudes, due to the enhanced depth of atmosphere for a water body at minus 423 meters below sea level in elevation.

Although the Dead Sea waters are not hospitable to higher lifeforms, and the terrestrial margin is arid and harsh, there are a variety of terrestrial flora and fauna surrounding the Dead Sea, particularly in the Jordan River Valley to the north and the nearby Levantine Mountains.

Geography

Lying along the Great Rift Zone that stretches from Zambezi, to Turkey, the Dead Sea forms part of the border between Jordan from Israel. The Jordan River is the principal influent source of Dead Sea waters; however, ongoing overdrafting of groundwater in the Jordan River Valley, combined with direct diversion of Jordan River surface water is causing a decline in level to the Dead Sea. 

Geology

The shores of the Dead Sea hold vast stores of salt; for example on the Israel side at the southwest there is a massive salt ridge called Kashum Usdum reaching a height of almost fifty meters, with a length of about eight kilometers; moveover, this landform is covered in places with layers of chalky limestone.

Early thinking held that the basin is a morphotectonic depression along the Dead Sea rift zone , an extension of the Red Sea Rift. In any case the continental crust here was consolidated in the Late Proterozoic. This basin is a young intracontinental plate boundary created from the late Cenozoic breakup of the Arabo-African contintent. The Dead Sea basin is the largest of several basins along this rift, being about 150 kilometers (km) long by about 16 km wide. The Dead Sea is filled with sediment depths as great as ten km of Neogene to recent deposition.  

The kinematics of the plate separation can be understood by examining the lateral offsets in geologic formations on the east and west side of the Dead Sea, which amount to a movement parallel to the rift zone axis of roughly 105 kilometers, about forty percent of which occurred in the most recent five million years.

A more modern variant of the basic rift theory holds the Dead Sea basin to be an outcome of a step-over discontinuity along the Dead Sea Transform, creating an extension of the crust with resultant subsidence.

Around three million years ago, the Jordan River plain, Dead Sea, and  Wadi Arabah were repeatedly inundated by waters from the Levantine Sea, the eastern element of the Mediterranean Sea. The waters formed in a narrow, curving bay that was attached to the Levantine Sea through what is currently termed the Jezreel Valley. Flooding of the valley oscillated driven by long time scale climate periodicity. In the time frame one to three million years before present, orogenic processes elevated lands of Israel, providing enhanced topographic separation between the Dead Sea and the Levantine. The lake that occupied the Dead Sea Rift, sometimes termed the area of Biblical Sodom and Gomorrah, deposited very deep salt-beds.  

The original prehistoric lake was a freshwater or brackish lake that extended at least 80 km south of the southern end of the present Dead Sea and 100 km north, well above the present Hula Valley. As the climate became more arid, the ancient lake shrank and became more saline. The large predecessor of the Dead Sea is sometimes termed Lake Lisan.

In prehistoric times, great amounts of sediment collected on the floor of the ancient Dead Sea. The sediment was heavier than the salt deposits and squeezed the salt deposits upwards into what are now the Lisan Peninsula and Mount Sodom (on the southwest side of the lake). The effect can be described heuristically by the outcome of setting a massive flat stone into a pail of mud, causing the latter to creep up the side of the pail. When the floor of the Dead Sea dropped further due to tectonic forces, the salt mounts of Lisan and Mount Sodom stayed in place as high cliffs.

From 70,000 to 12,000 years before present, the lake level was 100 m  to 250 m higher than its present level. This lake, called Lake Lisan, fluctuated dramatically, rising to its highest level around 26,000 years ago, indicating a very wet climate in the Middle East toward the closing part of the last major ice age, the lake level dropped substantially, likely to levels even lower than today. During the last several millennia, the Dead Sea has fluctuated approximately 400 m. Current theories as to the cause of this dramatic drop in levels are inconsistent with volcanism, but the change could have been seismically driven.

Hydrology and Water Chemistry

caption Recent shrinkage of the Dead Sea. Source: Hoshana. 2007

This deepest hypersaline lake of the world attains a maximum depth of 330 meters. With a surface area of 40,650 square kilometers, the Dead Sea is the third largest lacustrine body in Asia, behind Russia's much larger Caspian Sea and the Naujan Sea in the Philippines.

The Dead Sea's chief salts are magnesium and potassium anions paired with chloride cations, at 52 and 37 percent of the total salinity respectively. Sodium occurs at quite low levels. Bromide cations are also present.

Potash was the first substance to be commercially produced from the Dead Sea, with the first plant contructed, according to Mizrahi, in the early twentieth century near the site of the Biblical city of Sodom. Other minerals harvested included sodium chloride and carnallite. Extraction was encouraged with overexploitation of Jordan River input, which almost exhausted riverine influent as long as fifty years ago. 

During the prior 70,000 years the Dead Sea depth has fluctuated dramatically over amounts of several hundred meters; moreover, in the past several millennia the Dead Sea has been declining in lake level, due to an arid climate in this period and reduced inflow from the Jordan River through the Hula marshes and Lake Kinesset.

Over the period 1960 to 2007 the water level of the Dead Sea has declined substantially due to water diversion and extraction in the Jordan River Valley. Some consider this shrinkage an ecological impact to the Dead Sea; however, the chief ecological damage is to the Jordan River Valley, where more biodiversity had existed, rather than the allready biologically depauperate Dead Sea. Rainfall levels in this era are generally in the range of less than 25 millimeters per annum. 

Aquatic ecology

Although no fish species or other higher level aquatic plants or animals are found in the Dead Sea, because of the hypersaline environment, certain bacteria and microbial fungi are present; these organisms especially flourish in the rare years where abundant rainfall occurs in the basin.

Pioneering work by Volcani in 1944 isolated the pleomorphic red cup-shaped bacteria Halobacterium marismortui. Subsequently another red cup-shaped taxon, Halobacterium volcanii, was isolated. In 1983 Oren discovered a new bacterium in the Dead Sea, a red rod-shaped taxon which he named Halobacterium sodomense. This taxon, first isolated eight kilometers east of Ein Gedi, requires high concentrations of divalent cations, clay minerals such as bentonite and synthesis of purple membrane at low oxygen tensions.

Both Jordan and Israel have designated certain perimeter waters and lands of the Dead Sea as nature reserves.

caption Panorama of northern Dead Sea from Israeli side.

caption Panorama of the Dead Sea from Mount Sodom. Source: Wikipedia

Terrestrial ecosystems

See main article: Eastern conifer-sclerophyllous-broadleaf forests

Much of the Dead Sea margin is rimmed by the Eastern conifer-sclerophyllous-broadleaf forests, an ecoregion which extends into the Jordanian plains and west to the Israel and Lebanon Levantine Sea coastline. Macrobotanical evidence indicates that deleterious effects of human activities became evident in the region as early as 3000 BC. Since that time, high temperatures, low atmospheric humidity, and poor soil conditions have impeded the plantlife from recovering after human disturbances.

One of the most widespread and important communities of this ecoregion is the Kermes oak (Quercus coccifera) community. Its high ecological tolerance and capacity to recover in degraded Pinus brutia forests make it one of the most dominant vegetation types.

The coastal plains of the upper Jordan Valley are dominated by Hyparrhenia birta grasslands, with scattered thorny shrubs and small trees such as Paliurus spina-christii, Rhamnus palaestinus, Rhus tripartita, and Anagyris foetida.

The eastern part of the ecoregion is poorer in terms of woody vegetation since it receives only 400-500 mm of annual precipitation. A line running through Viran?ehir delineates the boundary between the desert and the Mediterranean formation. One of the main factors linking this xeric eastern area to the Mediterranean part of the ecoregion is the existence of Olea europea and other shrub communities that incorporate Mediterranean elements

Populus euphratica and Salix triandra make up another tree community that forms gallery forests along the segments of the Jordan River. Amygdalus arabica, Cerasus microcarpa, C. mahalep, Cercis siliquastrum, Ficus carica, Acer monspessulanum, Crateagus aronica, Pyrus syriaca, Celtis tournefortii, Pistacia kinjuk, and P. vera are the main species forming shrub communities in the Jordan River Valley.

Mountainous rims of the Dead Sea offer habitat for a number of mammals including hyrax, camel, ibex, jackas, fox and Arabian leopard. The Dead Sea area is a key resting area for migrating birds, who fly over the Bosphorus rather than navigate the wider parts of the Mediterranean Sea.

Human history

caption Qumran ancient settlement, associated with the caves
that yielded the Dead Sea Scrolls. Source: Mark A.Wilson

The account of the Greek historian Strabo (64/63 BC – sometime after AD 23) is consistent with Biblical accounts that the cities of Sodom and Gomorrah, situated along the ancient rim of the Dead Sea, were destroyed by volcanic (brimstone) activity. This cataclysmic event has also been associated with a known meteor impact in 3123 BC to the north, whose back-plume location would have been consistent with a major incendiary event in the Dead Sea basin.

King David (1040–970 BC) was known to utilize the shores of the Dead Sea as a retreat. In that era and as late as the time of Christ, the Jordan Delta was documented to be a lush forest of palms by the Romano-Jewish historian Flavius Jospehus. King Herod I, in the first century BC, used the Dead Sea area as a health resort.

In the modern period 1947-1956 a collection of 972 texts from the Hebrew Bible along with extra-biblical documents were recovered in archaeological excavations along the Dead Sea's northwest shoreline. This locale is known as Khirbet Qumran, an ancient Jewish settlement and associated cave system.

The texts, known as the Dead Sea Scrolls, are of great religious and historical significance, representing the earliest surviving Biblical and extra-biblical documents; the texts dating from circa 150 BC to 70 AD, are scribed in Hebrew, Aramaic and Greek, chiefly on parchment.

In modern times, the Dead Sea area is a destination not only for extensive tourism, but as a health resort for treatment of psoriasis, rhinosinusitus and osteoarthritis. Because of the unusual low altitude, sunbathers are protected from ultraviolet sunlight to a great degree, because of the thickness of the atmosphere at this deepest depth of Earth's atmosphere.

References

  • Gustav Bischof. 1854. Elements of chemical and physical geology, Volume 1 (Google eBook) Printed for the Cavendish Society
  • L.Boulos, A.G.Miller, and R.R. Mill. 1994. Southwest Asia and the Middle East. Pages 293-349 in S.D.Davis, V.H.Heywood, and A.C.Hamilton, editors. Centers of Plant Diversity. Information Press, Oxford, England.ISBN 1-2831701988
  • Gene E. Likens. 2010. Lake ecosystem ecology: a global perspective: a derivative of Encyclopedia of inland waters (Google eBook) Academic Press. 463 pages
  • Eva Kaptijn. 2009. Life on the Watershed. Reconstructing Subsistence in a Steppe Region Using Archaeological Survey: A Diachronic Perspective on Habitation in the Jordan Valley. Sidestone Press. ISBN 9789088900297.
  • Joseph Mizrahi. 2002. Developing an industrial chemical process: an integrated approach (Google eBook) CRC Press. 229 pages
  • Aharon Oren. 1983. Halobacterium sodomense sp. novum, a Dead Sea Halobacterium with an Extremely High Magnesium Requirement. International Journal of Systematic Bacteriology. Apr. '83, pp 381-86
  • John K. Warren. 2006. Evaporites: sediments, resources and hydrocarbons. Birkhäuser. ISBN 9783540260110
  • Tina M.Niemi, Zvi Ben-Avraham & Joel Gat. 1997. The Dead Sea: the lake and its setting. Oxford University Press. 286 pages
  • Peter Saundry. 2011. Seas of the world. Topic ed. C.Michael Hogan. Ed.-in-chief Cutler J.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC
  • B.E.Volcani. 1944. The micro-organisms of the Dead Sea. In Papers collected to commemorate the 70th anniversary of Dr. Chaim Weizmann. Collective volume. Daniel Sieff Research Institute, Rehovoth, Israel pp 71-81
  • M.Zohary. 1973. Geobotanical Foundations of the Middle East. 2 vols. Fischer, Sttutgart, and Sweets and Zeitlinger, Amsterdam. 739 pp.
Glossary

Citation

Hogan, C. (2013). Dead Sea. Retrieved from http://www.eoearth.org/view/article/151663

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