5.1. Formation Mechanisms of the Aral Sea Ice-Gouging Topography
The morphology and distribution of the scours derived from satellite imagery and field analysis are in many ways similar to the ice-gouging topography of modern freezing seas and large lakes previously studied by side scan sonar (SSS) surveys [4
] etc., allowing us to suppose their creation by ice. To prove the origin of the Aral Sea bottom landforms, we compared them to well-studied ice gouges in other seas and lakes. These included the Baydaratskaya Bay of the Kara Sea [53
], because of its extensive coverage by SSS data during investigations for construction of an underwater pipeline crossing [4
], the northern Caspian Sea [12
], which is less studied but is proximate to the Aral Sea and has similar conditions, and Lake Erie [23
] because of its similar latitudes, water area and conditions.
Identically to ice gouges in these regions, all of the Aral Sea scours have specific morphology with a depression in the axis and parallel side berms, giving evidence of plowing of the bottom ground by ice formations. Relatively dense deposits in the depressions imply pressure of heavy sea ice formations, while looser sediments in the side berms suggest the effect of plowing of the bottom grounds. Both single scours and their combs can be encountered in all freezing seas and lakes ([3
], Figure 17
). Such combs appear when a grounded hummocky formation or stamukha plows the bottom with its multiple keels. These large ice formations are usually frozen into vast ice floes, increasing their weight and gouging force [54
]. The larger the ice hummock is, the more keels penetrate into the ground increasing the depth of the scours.
Another feature encountered in the Arctic Seas and in the Caspian and Aral Sea is that both the ice scours and their combs are often imposed (Figure 18
) as a result of their consequent formation [3
]. One single ice hummock can create numerous scours of different directions cutting each other, as it drifts along the winds and currents.
The scours and ice gouges in all freezing seas, including the former Aral Sea, have bends (Figure 19
) which can be both sharp or smooth depending on the rates of the wind direction changes and on the topography of the coastal zone. Stamukha pits, appearing when a large ice hummocky formation (stamukha) is grounded on a shoal and is too heavy for the wind currents to move it, are typical ice gouging landforms as well.
A feature directly evidencing the ice gouging origin of a scour is the presence of front mounds both at the ends of the scours and along their sides (Figure 20
). Such mounds, typical for ice gouges of the Caspian and Kara Seas were observed both in field (Figure 10
c,d and Figure 12
) and by remote sensing at the former Aral Sea bottom.
In addition, the distribution of the scours with their orientation correlating with the directions of the most frequent winds in winter [30
], (Figure 8
) makes it possible to reliably attribute the gouges of the Aral Sea to traces of ice impact on the bottom.
The morphometric parameters of the scours at the bottom the Aral Sea are also comparable to the dimensions of ice-gouging landforms in other modern freezing seas and lakes (Table 3
). The ice gouges of Baydaratskaya Bay are presumably the longest; the SSS surveys showed them to be at least 2 km long; however, parallel surveying lines allow us to suppose much more considerable gouges of several kilometers or even tens of kilometers [4
]. Values for the Caspian Sea, Lake Erie and the Aral Sea are comparable, making several kilometers. The Aral Sea ice gouges are wide in relation to other seas and lakes. They are also shallower than the ice gouges of the Caspian Sea, Kara Sea and Lake Erie. Firstly, all of the ice gouges at the Aral Sea were smoothened by waves during the water level decrease, while in all other seas, there are still deep water areas with little wave action and small sedimentation rates, where the scours remain well-preserved. Secondly, after the exposure, aeolian processes contributed to their further filling. Generally, the dimensions of the Aral Sea scours are of the same order as the ice gouging landforms of other freezing seas and lakes.
5.2. Ice-Gouging Intensity Patterns
As seen from Table 3
, the average water depths at which the ice scours form vary greatly in different freezing seas and large lakes. On the one hand, the intensity of ice gouging depends on a large array of parameters, being the climate, mechanical properties of ice, bottom topography, etc. On the other hand, conditions of preservation of the scours can be different. As a result, the degree of ice impact does not correlate directly with the number of ice gouges seen on the bottom [3
Areas close to the coast are usually occupied by fast ice, which moves little and does not form large hummocks; therefore, the forming gouges are small and shallow [1
]. They are usually destroyed by the first spring storm; therefore, the concentration of ice gouges in coastward regions is usually low. The area with the greatest intensity of ice gouging is the fast ice rim, along which the largest hummocks and ice floes usually drift [14
]. Deeper water areas are rarely affected, and can be plowed by the largest ice formations only, as their keels have to be very deep to reach the bottom. However, with no wave action and in the absence of currents, the ice scours can be preserved for many years, being repeatedly imposed. After a decade, this will result in high concentrations of gouges, while the ice impact on the bottom in fact occurs rarely.
In this way, the depths of the greatest ice impact vary in different seas, as the zone with the most intense ice action is attributed to the fast ice rim, not to a certain water depth. For Baydaratskaya Bay, Kara Sea, this zone lies at depths between 12 and 26 m [4
]. The depths of ice gouging in the American Great Lakes (17–21 m, Table 3
) are comparable to the Arctic Seas [23
]. In the Caspian Sea, the zone of the most intense ice gouging was proved to appear in shallower water areas at depths of 2–5 m [29
It has been previously supposed [23
] that the depth of ice gouging is mainly controlled by accumulated freezing degree-days (AFDD). At Baydaratskaya Bay, the average air-freezing index calculated in the same way ranged from 3000 to 3500 [57
], being the greatest. At the Caspian Sea this value was from 300 to 1300 AFDD [58
], being greater than in the region of Lake Erie (285 to 582 AFDD, [23
]). At the same time, the depth of ice impact at Lake Erie reaches 25 m [7
], while the maximum depth of stamukha penetration in the Caspian Sea is 12 m [11
] with average values of 2–5 m [29
]. The values for the Aral Sea at Barsakelmes Island in its middle part reach 316–1415 AFDD [59
], being close to the Caspian Sea values.
The ice thickness does not correlate directly with the depths of impact either: it makes up to 1.2–1.4 m in Baydaratskaya Bay [60
], while the average values for Lake Erie and the Caspian Sea are comparable: up to 0.5 m on the average, and never exceeding 0.8 m in Lake Erie [23
] and not more than 0.6–0.7 m for drifting ice and 0.9–1.2 m for fast ice in the Northern Caspian Sea [11
]. The modern ice thickness varies greatly in the West and the North Aral Sea and depends notably on local conditions; because of the salinity increase, ice thickness in the past should have been greater.
A factor controlling the depth of the greatest ice-gouging intensity might be the bottom topography and inclination of the underwater slopes. Lake Erie and Baydaratskaya Bay are characterized by steeper underwater slopes than the flat Caspian Sea bottom. The floating fast ice has a limited width; at some point it cracks and forms a fissure, along which its rim forms. If, e.g., Lake Erie has steeper underwater slopes than the Caspian Sea, the fast ice rim forming at the same distance from the coast in these two lakes will form at different water depth intervals. Therefore, areas with the most intense ice impact along the rim will be attributed to different depths. The Aral Sea had a very flat bottom in its central and eastern parts, comparable to the Caspian Sea. Therefore, the Northern Caspian Sea can be a model of the past conditions in the Aral Sea. Its extent is comparable to the past area of the Aral Sea; the Southern and Middle Caspian do not freeze and are much deeper, which makes ice gouging impossible. Northerly and northeasterly wind directions prevail in both regions. Moreover, the Caspian Sea and the Aral Sea are situated in similar continental arid desert climate, contrary to the Great American lakes, and experienced considerable water level fluctuations. In this way, we suppose that the patterns of ice impact in the past at the Aral Sea were comparable to the modern Northern Caspian Sea, with depths of the most intense ice gouging along the fast ice rim at 2–5 m. In the nearshore zone with 1–2 m depths, fast ice should have been stable and hummocking was not intensive. At depths of more than 6 m, hummocky formations were presumably not thick enough for their keels to penetrate into the ground.
The preservation of the ice scours at freezing seas is generally controlled by the wave base, which, in its turn, is influenced by the wind fetch, the size of the lake or sea and wind direction. Wave action could not be great at the Aral Sea. The winds blow from land both in winter and in summer and rarely affect the northeastern coast. As the wave base is close to the sea depth, the waves lose their energy, not reaching the nearshore zone. In the conditions of water level decrease, the depth of wave impact was controlled by the vast shallows, limiting sediment transport. This confirms the absence of scours to the northeast from the Vozrozhdeniya Island, and abundant scours to the southwest from it, in the wind shadow.
At the same time, unlike the Caspian Sea and at the Arctic seas, the distribution of ice scours at the Aral Sea bottom was influenced by another factor, absent elsewhere: its dramatic and rapid water level drop. While it could not affect the density of the ice scours, it promoted their unprecedented preservation. In one year, the coastline could retreat by several kilometers, and therefore the wave action did not have time to destroy the ice scours. The density of the scours, in turn, was more influenced by the local bottom topography and the width of the water surface affecting the acceleration of ice floes and the formation of hummocks. In this way, while the intensity of ice scouring can be compared to the Arctic seas and to the Caspian Sea, the coverage of the bottom by scours is in some way a snapshot showing both old ice scours with good preservation and young scours which formed in one winter that would otherwise have been destroyed by waves.
This unique snapshot setting raises the interest in complementary simultaneous studies of the Northern Caspian and Aral Seas. Because of their comparable climate, the mechanisms and patterns of sea ice effect should have been similar in the past. Today, the Northern Caspian still represents conditions typical for the Aral Sea several decades ago. Its ice gouging landforms are seen on the remotely sensed images only immediately after the water area becomes clear of ice (Figure 21
). Then, they are eroded by the first spring storms. As the studies of parameters and distribution of such forms are constrained by their short lifetime, good preservation of ice gouges at the silty Aral Sea bottom ground after the level drop allows us to investigate similar landforms and extend the results to the Northern Caspian.
At the same time, while today conditions in the Aral Sea are not favorable for ice gouging, there is a possibility to reconstruct such past conditions by observing modern sea ice and analyzing different ice phenomena in the Caspian Sea (Figure 22
). Similarly to the modern Northern Caspian, in the Aral Sea, multiple ice floes collided under the wind force, forming ice ridges that could affect the bottom. Stamukhas were breaking off the ice cover and contributing to the new hummocking. In this way, the Caspian Sea provides an opportunity for investigations of ice conditions, mechanisms and processes of ice gouging, while at the Aral Sea, the results of such processes can be documented. Further comprehensive studies of the two seas could add to our understanding of the ice-gouging processes in the mid-latitude climates.
In this way, the distribution of the scours in the Aral Sea and their density patterns (Figure 16
) are a result of both the varying ice-gouging intensity and the different degree of their preservation. The spatially non-uniform intensity of ice impact resulted in lower concentrations of ice scours in the coastward parts, while in the central part, there were more ice gouges, just as in Baydaratskaya Bay [4
] and the Caspian Sea [63
]. The largest coverage of the central part of the Eastern Aral Sea by scours was also provided by their long-term accumulation when the water level was at 2–5 m above vast flat bottom plains in its center. Moreover, a fast water level drop promoted the preservation of bottom fragments with high ice scour concentrations even in relatively shallow areas. Despite the northerly and northeasterly winds, which pushed the ice to the south in the Caspian and Aral Sea, most of the Aral Sea ice gouges are concentrated in its flat central part. On the one hand, the southern coasts were protected by the fast ice. On the other hand, the coastline retreated faster in the north and east, while in the south, water remained until the 2000s. Therefore, old ice gouges from the 1990s remained in the central and eastern part, and were destroyed by waves in the south. Younger ice gouges from the 2000s were less abundant as the air and water temperatures increased, along with the salinity; the ice formations became smaller and could execute less impact on the bottom.
At the North Aral Sea, the small size of the water area, insufficient to speed up drifting ice and hummocks, and its complete freezing every year limited ice gouging. The West Aral Sea could not provide favorable conditions for ice gouging because of its high salinity and steep nearshore bottom slopes. Therefore, there are no ice gouges neither at the North Aral Sea nor near the western coast of the West Aral Sea.
5.3. Temporal Evolution of the Aral Sea Ice-Gouging Topography
Knowing the position of the retracting Aral Sea shoreline in space and time [40
] (Figure 16
), and assuming that the most intense ice impact is typical for depths of 2–5 m, similarly to the Northern Caspian, we were able to reconstruct the history of the ice-gouging topography at the Aral Sea bottom. Based on the known rates of lake level fluctuation, the formation time of separate scours can be estimated. We suggest that most of them formed along with the rapid sea-level fall of 1980–mid-1990s when the depth interval of 2–5 m at the East Aral generally shifted westwards along with the coastline. During this whole level fall, the zone of intense ice impact moved from depths of about 15–18 m to 22–25 m in relation to the 53-m base elevation. The rate of water level drop (reaching 70 cm per year from the mid-1970s to the early 1990s) was so high that the ice scours could not be filled with the bottom sediments. In one year, several kilometers of the former bottom surface became exposed, providing an unprecedented degree of ice-gouging topography preservation.
In the mid-1990s and 2000s, the shallowing slowed down, and extensive shoals formed. At that time, vast areas were in conditions favorable for ice gouging (2–5 m depths). At the same time, the wave action on the east coast was almost absent due to its flat topography, small depths and prevalence of storm winds blowing from the northeast. In the late 2000s, the waters of the East Aral Sea became hypersaline, and the ice formation diminished. The surface area of the sea reduced to such an extent that rare ice could not get enough acceleration for the hummocking. The ice-gouging processes, therefore, largely ceased.
Today, ice gouging is almost absent at the Aral Sea. The East Aral Sea, which used to be the area with the most intense ice impact, has now entirely dried out. In the West Aral Sea, the water is hypersaline, and ice forms at extremely low temperatures; it is thin and incapable of plowing the bottom. On the North Aral Sea, ice gouging is limited, as it always was. Today, no significant regional climate or anthropogenic drivers can cause an increase of the Aral Sea level [64
], so it is unlikely that the ice effect on the bottom will intensify in the nearest future.