Kara-Bogaz-Gol, Turkmenistan

Kara-Bogaz-Gol (Garabogazköl Aylagy) lies in the cool arid steppes of Turkmenistan at the edge of the Caspian Sea with a brine surface some 29 meters below sea level. The waters of Karabogazgol are derived from the Caspian Sea via a narrow inlet that gives the Gulf its name (in the Turkmen language it means lake of the black or mighty strait or throat. In the past 100 years, water levels in Karabogazgol have been a metre or so less than the adjacent Caspian water surface, which has fluctuated between 25 and 29 meters below sea level.

For much of the Holocene, Karabogazgol (Bay) has been connected via a narrow 500m-wide inlet to the Caspian Sea, which flows water from the Caspian to Karabogazgol at a rate of 25-44 meters/minute. Natural inflow from the Caspian replaces water lost by evaporation from the Bay, and when this happens, the drawndown water level in the Gulf is consistently 0.5-3 meters below that of the Caspian Sea. This makes it one of the few modern examples of a drawdown basin with a highly restricted hydrographic connection between one isolated seaway and another.

caspian%20and%20the garabogazkol%20Earth.com
Caspian Sea and Karabogazgol (from Earth.com)
Karabogazgol%20reduced

Kara-bogaz-gol today, with Caspian connection restored

1280px Kara-Bogaz-Gol_inlet_from_the_Caspian_STS111

The “black throat” with Caspian Sea on the left

Karabogazgol, with a salinity in excess of 340‰, is sufficiently restricted to allow gypsarenite to accumulate and remain on the brine-covered lake bottom. Water levels in the Caspian Sea have varied considerably over the 10,000 years, and so Karabogazgol repeatedly lost its hydrographic connection with the Caspian whenever water levels fell below the entrance sill. In earlier times of colder temperatures that allowed winter freezing of a perennial water body, beds some 3 to 8 metre-thick containing cryogenic sodium sulphate formed and are today composed of varying amounts of calcite-hydromagnesite-gypsum-halite-epsomite-mirabilite-glauberite.

From 1929 until 1940 there was a 1.8 fall in the level of the Caspian Sea, and by 1948 the Caspian level was -27.87 m asl. The volume of inflow to the Gulf fell to 12-14 km3/yr by the late 1940s and 8 km3/yr in 1956. Differences in water level between Caspian and the Gulf increased to 3.17 m in 1947 and 3.80 m in 1955. By 1957-1959 the water-covered area in Karabogazgol had fallen to around 13,000 km2. By 1977 water level in the Caspian was -29.02 m asl, it had fallen by more than 3 meters since 1929. The 1977 water surface in Karabogazgol was 4.5 meters lower than the Caspian, and the strandline of southern Karabogazgol had retreated by 50-60 km in 50 years. This ongoing drop in water level in Karabogazgol since 1929 meant fisheries were threatened on the Caspian side and numerous salt works had developed in the increasingly exposed and accessible brine flats in the Karabogazgol depression.

There is an interesting anthropogenic aspect to the natural loss of water from the Caspian to the Gulf and the periodic salting of Karabogazgol. Early last century (before 1929) the Caspian sealevel was around -26 masl and Karabogazgol was almost entirely water-filled (as it is today). The water-covered surface area was approximately 18,000 km2, and its maximum water depth was around 9 meters. The annual volume of water flowing through the inlet was high, somewhere between 18 to 26 km3 and the difference in level between waters of the Caspian and Karabogazgol was around 0.45 meters.

The ongoing fall of the Caspian Sea level from 1929 and well into the 1970s, was thought by the Russian Government to be related to the continuous loss of water from the Caspian to Karabogazgol. After all, the climate in the Karabogazgol was arid, and the annual evaporation rate was 1000-1500 mm and yearly rainfall around 70 mm. 

Russian government scientists argued that damming the entryway would stop the Caspian spilling into the Gulf and so arrest the fall in the Caspian water level and the threat to fisheries. This notion of interfering in a natural saline system was opposed by many Turkmeni scientists who argued that the rise and fall of the Caspian and the variable filling in Karabogazgol were natural processes and had been noted in the texts of ancient Islamic scholars living in the region. 

Karabogazgol%20levels

Karabogazgol and the Caspian Sea. A) Variation in the level of the Caspian Sea with the low in the period 1960-1990 driving the hydrographic isolation of Karabogazgol. B) Desiccated Karabogazgol in 1985 after damming of the natural connection between the waters of the Caspian and the brine lake of Karabogazgol. C) Water filled Karabogazgol in 2001, soon after hydrographic connection to Caspian Sea was restored.

Ignoring the argument that rise and fall of brine level in isolated saline waters bodies, such as the Caspian, was the usual situation, a decision was made in 1977 by the central administration in Moscow that the entryway would be damned. Central administration scientists had concluded that sufficient water would remain in Karabogazgol after damming to allow the salt industry to continue its operations. The dam was not constructed for another three years. Ironically, the natural fall in the level of the Caspian had reversed in 1977 and had begun to rise once more, with a corresponding rise in waters levels in the Gulf. From 1977 until 1995 the Caspian level rose by the same 3 meters it had dropped since the 30s. However a decision had been made back in 1977 to dam the entryway, so in March 1980, even though the Caspian water level was rising once more, Russian engineers dammed the entry strait between the Caspian and Karabogazgol.

This complete loss of hydrographic connection to the Caspian did not have the desired effect, which was by then an irrelevancy, but egos and the authority of the Russian Empire were involved. Instead, the damming of the inflow resulted in rapid desiccation of Karabogazgol, so that by 1983 only a small brine-covered area remained in the lowest part of the gulf depression. Much of the former brine lake floor had turned into a saline dust bowl, blowing high levels of sodium salts onto downwind irrigated pasture lands.

By 1984 the villages surrounding Karabogazgol had turned into ghost towns. In 1984, in an attempt to restore the ecosystem, eleven large diameter pipes were laid across the dam so that Caspian water could be pumped back into Karabogaz at a controlled rate. In June 1992, after the fall of the Soviet Empire, this approach was abandoned, and the dam was entirely demolished by order of the president of the newly independent Turkmenistan. As it refilled from 1992 to 1995, the gulf water level rose by some 6 meters, and since then its water level has once again matched that of the Caspian Sea and it is once again fulfilling its role as a natural desalinator of the waters of the Caspian. 

With the entryway connection to the Caspian restored, the water chemistry of the main open water body in the centre of the Gulf is a Na-Mg-Cl brine, with a density of 1.2 g/cm3, and pH values that range between 7.2 and 9 and salinites in excess pf 340‰. Surface water temperatures range from 4°C in December to 25°C in July. These temperature fluctuations are seen in the precipitation of different mineral phases during the year. Calcite, aragonite and perhaps hydromagnesite usually precipitate from lake waters in spring, gypsum in summer (via solar evaporation), and rafts, blown shoreward, into stacked crusts of cryogenic mirabilite and halite accumulate on the shores and mudflats of the embayment edge throughout the winter. Gypsum (as lenticular sand-sized grains) is the only permanent sulphate phase seen in cores sampling surface sediments from beneath today’s central embayment water body. Below this modern hydrology, NaSO4 salts occur in 4 main beds (Salt members 1-4), dominated by glauberite/astrakanite, with alteration textures suggesting a mirabilite precursor). Three separate glauberite layers are present, a fourth lowermost layer is dominated by bloedite: the upper layer is at a depth of 0 to 0.3 m and is up to 3 m thick; the middle layer is at a depth of 5 to 8 m; and the lower layer is at a depth of 14 to 18 m. Gypsiferous-carbonate muds separate the salt layers by 2 to 5 m. Depending on the water salinity in Kara Bogaz, the following succession of various salts have precipitated in response to increasing salinity and temperature fluctuations: carbonates–gypsum–mirabilite–glauberite–halite (in the summertime) and mirabilite (in the winter) -epsomite–astrakanite. 

Karabogazgol_strat

Kara Bogaz Gol, Kazakhstan. A) Plan view showing distribution of second salt member zones with higher NaSO4,which tend to occur about bay margin (after Strakhov, 1970). B) The Landsat image insert shows detail of the narrow inlet allowing surface entry of Caspian Seawater into bay. C) Typical lake stratigraphy showing thickness, relative water levels and Holocene ages of the four salt members that define the saline successions in the bay (in part after Karpychev, 2007).

Today, although mirabilite precipitates each winter across the Gulf, much of it is redissolved with the passage into warmer weather and drying of much of the Gulf floor. Hence, across the central parts of the Gulf, mirabilite is not currently contributing large volumes of precipitates to the at-surface gypsum bed. Each of the four underlying NaSO4 beds is a likely remnant from cooler climatic periods over the last 10,000 years (Karpychev, 2007). Then copious amounts of mirabilite formed each winter, much like today but, unlike today, the cooler more humid glacial climate meant the bay was not as subject to summer desiccation. Dense residual bottom brines were perennially ponded and so preserved a summer-halite sealing bed. This allowed the underlying mirabilite/epsomite winter precipitates to be preserved across the lake floor. During the following winter the process was repeated as mirabilite/epsomite/halite beds stacked one atop the other to create a future ore horizon. In time, the combination of groundwater and exposure, especially nearer the Gulf’s strandzone, converted most of the mirabilite, along with epsomite, to astrakanite, and then both phases to glauberite in the upper three beds. This explains the association of the richer glauberite zones with the lake edges (Strakhov, 1970).

Water of crystallization is created by the mirabilite to thenardite conversion, then slightly diluted any strong residual brine, so facilitating a high sodium-sulphate mineral and brine composition across the bay (Kurilenko et al., 1988).Even today, a portion of the air-brine winter mirabilite precipitates do not sink, but form crystal rafts that are blown by prevailing winds to stack into crusts fringing the lake shore. In contrast to the current complete loss of mirabilite from the lake floor each summer, cooler earlier Holocene conditions favoured retention of bottom-cumulates much like winter salting in saline lakes in Canada today. Today’s gypsum/glauberite/thenardite fringes form during the summer transformation of the mirabilite, epsomite and bloedite that accumulated as windblown fringes each previous winter about the edges Karabogazgol. That is, because the climate and bay hydrology has changed over the last few thousand years, the present is not a direct key to immediate past textures and extents.

Winter salt harvest 

At a brine temperature of 5.5° C, a solid, crystalline precipitate of pure mirabilite precipitates from the gulf waters. All other salts tend to remain dissolved. Mirabilite crystals first form as rafts, some settle on the bottom and are moved by waves to the shore of the bay. Other rafts are blown directly into the lake strandzone. Each year, from November to March, some 6 million tons of mirabilite precipitate. In the same winter months, the strongest storms move crystals throw large amounts from the shallow lake floor onto shore-lining crystal mounds where millions of tons can be collected directly from the frozen ground.

kara bogaz-gol_mirabilite%20dunes%20winter%20copy

Harvesting mirabilite from winter buildups of near 100% pure mirabilite, Karabogazgol strandline.

1280px Garabogazköl_%28right%29_visible_from_a_bridge_on_a_barrier_separating_it_from_the_Caspian_Sea_%28left%29

Road to the “throat", Caspian on the right

© 2016-2024 Saltwork Consultants Pty Ltd