Lake Asal, Djibouti

Lake Assal (Arabic: بحيرة عسل‎ Buḥayrah ʿAsal, literally 'honey lake' due to the high viscosity of its brines) is a water-filled crater lake in central-western Djibouti at the western end of Gulf of Tadjoura in the Tadjoura Region, touching Dikhil Region. The lake is situated some 110 kilometres west of the nation's capital, Djibouti City. The lake depression lies in the NW-SE oriented Asal – Ghoubbat al Kharab (Ardoukôba) rift at the top of the Great Rift Valley and is hydrologically linked to the Gulf of Aden via seepage through fractures in the Ardoukoba Lava Ridge.

Lake Asal, has a water surface some 155 m below sea level, making it the lowest point on land in Africa and the third-lowest point on Earth after the Sea of Galilee and the Dead Sea. It occupies the lowest part of the Afar rift. The lake is small (area 52 km2), endorheic and shallow, with a maximum water depth of 30 m and an average depth of 7.4 m. The adjacent Afar depression contains a series of lakes and salt pans at various depths below sea level: Lake Badda (−50 m), Lake Assale (−118 m), Lake Bakili (−120 m), Lake Afrera (−111 m), Lake Acori (−94 m), with the lowest being Lake Asal (−155 m).

Surface concentration of Asal lake waters is 276.5 g/L and this increases to 398 g/L at a 20 m depth . Hence, Lake Assal contains a large salt resource, exploited under four concessions awarded in 2002 at the southeast end of the lake; the major share of production (nearly 80%) is held by Société d'Exploitation du Lac and Société d'Exploitation du Salt Investment S.A, de Djibouti. 

Located in a hot desert (Köppen climate – BWh), the lake experiences summer temperatures as high as 52 °C from May to September. Average winter temperatures are not low, at 34 °C from October to April, with the coastal area experiencing ocassional rains. Water temperature of the lake is reported to reach 33–34 °C. However, in winter, when the wind velocity is low and evaporation also low, the temperature recorded was found to be 20 °C for surface water and above 25 °C at shallow depths of the lake (heliothermic).

Strong, dry, hot winds are part of the environment. The monthly temperature variation is reported to be 6 °C. During the summer season, the dry hot winds blow in two directions, namely, the Sabo winds from the southwest and the Khamsin winds from the north-west. Winds during October and April blow from the east, resulting in sporadic rains. The lowest rainfall of 23 mm (0.91 in) was recorded in 1996. Regionally, the area is extremely arid with average rainfall below 100 mm/year and evaporation of about 5 m/year. Locally, in the Asal area, mean annual rainfall in the area is 175–200 mm/year and evaporation exceeds 3.5 m/year.
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In the vicinity of Asal, the rift is subaerially exposed over a 12 km distance between Lake Asal and the Red Sea and is part of a tectonic system considered to be one of two examples where an oceanic ridge outcrops; the other being on the island of Iceland. This part of the rift extends southwest into the East African Rift and is at the marine-fed end of what is the type example (Afar triangle) of a spreading centre (triple junction) in an arid part of world's rift systems.

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Holocene sediment distribution in Lake Asal, Republic of Djibouti (surface geology after Stieljes, 1973). Marine inflow is via fractured basalts of the Kodda Soma. A3-6 indicate positions of geothermal test wells - A1 and A2 lie further to the southwest. Insets show water levels in past 10,000 years (after Gasse and Fontes, 1989) and an idealised cross section from the ocean to the lake depression (Landsat image courtesy of NASA).

Although small, Lake Asal is one of the few modern examples of a marine-fed rift basin that is actively accumulating bedded evaporites. As such, it is the only Quaternary analogue for ancient marine-fed rifts including the numerous Mesozoic evaporitic lacustrine basins that underlie passive margins on either side of the Atlantic.

The lake surround today is entirely continental with no surface connection to the ocean. Yet, the fact its −155 m asl water level has changed little in the last few centuries argues it is fed a steady flow of seawater through the highly fractured ten km-wide volcanic ridge that separates oceanic water at Ghoubbat al Kharab from the hypersaline waters of Lake Asal. It constitutes an excellent example of a drawndown marine seepage basin fed through a volcanic ridge).

Over the longer time frame of the past 10,000 years, lake levels have changed drastically). Some 10,000 years ago Lake Asal was almost dry in the late Pleistocene. A highstand lake level at 160 m asl existed in the early Holocene, between 8.6 and 6 ka, and the lake at that time drained to the Ghoubbat el Kharab, across a shallow sill at an elevation of some 4 m bsl, to the Gulf of Aden, both as surface and subsurface flows. After this time, the lake level dropped considerably to 150 m below sealevel enabling at the first time subsurface seawater infiltration from the sea to the lake across the fractured divide. Since then, the lake water salinity is controlled, mainly, by seawater input and high evaporation, responsible also for Ca-sulphate and halite deposition.

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Lake Asal - edge of gypsum salina  (from Wikipedia)

Salts in the perennial standing body of lake brine (up to 40 m deep, mostly 5-7 m deep) are mostly gypsum, which is accumulating with mounded and bottom-nucleated aligned-layer textures identical to those that characterise the South Australian salinas. Gypsum precipitation dominates in this the southeastern part of the lake depression, located next to the seeps that bring seawater to the surface. Fractures also intersect the subaqueous lake floor and seawater can be seen to rise along fractures cutting the lake bottom. Older layered and mounded gypsum beds crop out around the lake edge to as high as 55 metres above the present lake level (Stieltjes, 1973). Stacked halite crusts characterise the saline pan facies that covers much of the northeastern part of the lake where it forms a wide (≈60 km2) and thick (20 -80 metre) accumulation of pan halite. Before 1978 the top of this halite plain lay some 30 to 80 cm above the typical water surface of the perennial gypsum lake. Then in 1978 renewed volcano-tectonic activity in the region reopened fractures and faults leading to an increased influx of waters from the Ghoubbat al Kharab and a rise of lake waters which partially submerged the top of the salt pan (Ruegg et al., 1979). Once again, as in most other basins accumulating bedded evaporites, the changes in hydrological base level in this depressed pan are in large part driven by tectonics, not marine eustasy. The separation between areas of subaqueous gypsum and pan halite clearly shows how two mineral phases cannot accumulate simultaneously in a single holomictic brine lake, but can be adjacent facies controlled by separate hydrologies (salina versus mudflat).

Three independent geothermal systems have been identified, so far, namely; Gale le Goma, Fiale, and South of Lake. Six deep wells have been drilled in the region, the first two in 1975 and the others in 1987–88. Well A2 was damaged and wells A4 and A5 encountered impermeable yet very hot (340–365 °C) rocks. Wells A1, A2, A3 and A6 produce highly saline (120 g/L TDS) fluids leading to significant mineral scaling. The first two deep wells, Asal-1 had a final depth of 1154 m and Asal-2, a final depth of 1554 m. These thermal test holes were drilled at the southwest edge of the geothermal area in a region where there is no direct inflow from the sea. Asal-1 produced 22 kg/sec of high salinity water with a measured temperature at 1040 m of 255°C before flow testing, and 260°C during a flow test in 1981. Water produced by Asal-1 had a very high solids concentration (128 g/l), and during the 1981 flow test, rapid incrustation in the casing had plugged the well in less than three months. The other well, Asal-2, was plugged inside the casing, possibly by drilling mud, and is no longer producing steam. Well test data form all six wells indicate that the reservoir is producing from layered fractured and porous zones. The estimated permeability-thickness of the deep Gale le Goma reservoir is in the 3–9 darcy-meter range. Free-standing lake brines reach salinities of 350 ‰ and, like their marine spring feeds have seawater like proportions, dominated by Cl, Na, K, SO4 and Mg).

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