Earth’s Cryosphere, 2024, Vol. XXVIII, No. 2, p. 27-38.
SNOW COVER AND GLACIERS
BOTTOM SEDIMENTS AS A NATURAL RECORD OF DEGLACIATION IN THE BASIN OF LAKE SYLTRANKEL, ELBRUS REGION
M.M. Ivanov1,2,*, A.L. Gurinov1, V.N. Golosov1,2, N.V. Kuzmenkova1,3, M.Yu. Alexandrin1, M.I. Uspenskiy1,2, I.G. Shorkunov1, E.V. Garankina1,2
1 Institute of Geography, Russian Academy of Sciences, Staromonetnyi Lane 29/4, Moscow, 119017 Russia
2 Lomonosov Moscow State University, Faculty of Geography, Leninskie Gory 1, Moscow, 119991 Russia
3 Lomonosov Moscow State University, Faculty of Chemistry, Leninskie Gory 1/10, Moscow, 119991 Russia
*Corresponding author; e-mail: ivanovm@igras.ru
Deglaciation history of the Syltrankel high-mountain lake basin (Mt. Elbrus region, Northern Caucasus) from the end of the 19th to the end of the 20th centuries was reconstructed. In 2022, a comprehensive examination of the area was carried out, including sampling of the bottom sediments and their radioisotope dating, structural and textural analysis, and counting the number of varves. Simultaneously, a set of published sources; topographic maps; ground, satellite, and aerial photographs; and field observations were analyzed to determine the positions of the edges of glaciers at different times. In the formation of bottom sediments, four stages correlating with the state of mountain glaciation and changes in the configuration of the lake’s drainage area were identified. Converging results obtained on the basis of independent sources indicate the high methodological value of studying bottom sediments of mountain lakes as one of the few environmental archives that record glaciation changes in the dynamic conditions of high mountains.
Keywords: bottom sediments, mountain lakes, mountain glaciers, deglaciation, sediment yield, radioisotopes, varves, paleoarchives, Northern Caucasus.
Recommended citation: Ivanov M.M., Gurinov A.L., Golosov V.N., Kuzmenkova N.V., Alexandrin M.Yu., Uspenskiy M.I., Shorkunov I.G., Garankina E.V., 2024. Bottom sediments as a natural record of deglaciation in the basin of Lake Syltrankel, Elbrus region. Earth’s Cryosphere XXVIII (2), 27–38.
Full text.
INTRODUCTION
Intensive deglaciation of mountain areas can be considered one of the main manifestations of the transformation of the natural environment under the global climate change [Nesje and Dahl, 2000; Solomina et al., 2016 Brighenti et al., 2019]. Glaciers are highly sensitive to climatic fluctuations. In the Caucasus, intensive glacier degradation began in the 1840s with brief periods of growth and stabilization in the 1860s–1880s, 1910s–1920s, and 1970s–1980s (for a detailed review, see [Solomina et al., 2016]) and continues to this day, leading in some cases to catastrophic consequences [Chernomorets et al., 2018].
As it was noted in [Carrivick and Tweed, 2021], the change in sediment yield under the conditions of deglaciation remains poorly studied, despite the effort spent on identifying spatial and temporal variability in the sediment yield caused by melting of glaciers [Huss and Hock, 2018]. Conclusions about long-term changes in the sediment yield are hampered by the lack of time series of observations [Hodgkins et al., 2003]. To overcome the lack of such data, it is necessary to apply various indirect evidences of changes in the natural environment observed in natural environment archives. Bottom sediments of lakes are such a archive.
There are more than 1500 lakes of various origins in the Caucasus [Efremov, 1988]. A significant part of them are high-mountain catchments, where the water balance is determined by the runoff of glacial meltwater. Detailed studies of the sedimentation are relatively few, but their number has been steadily increasing in recent years [Daryin et al., 2015; Messager et al., 2013; Alexandrin et al., 2018; Kuzmenkova et al., 2020, 2022; Grachev et al., 2021; Chepurnaya et al., 2022].
The purpose of this work is to reconstruct the history of the sediment yields entering the basin of Lake Syltrankel during the intensive deglaciation in the 19th–20th centuries. The reconstruction was based on a column of the bottom sediments that were sampled and comprehensively analyzed. In addition, documentary data on the state of the glacial cover of the lake basin since the end of the 19th century was also collected and analyzed. The field studies allowed us to verify the data. The results obtained by independent methods were compared with each other.
MATERIALS AND METHODS
The lake basin and headwaters of the Syltran-Su River are a complexly arranged stepped glacial cirque surrounded by rocky spurs of the The Lateral Range. Syltrankel is the largest cirque lake in the Terek River basin in the Central Caucasus. It is located in the upper reaches of the Syltran-Su River at 3184 m a.s.l., 17 km east of Mount Elbrus (43.325758° N, 42.673793° E) (Fig. 1). At present, the area of the lake is about 147 000 m2, its maximum depth is 35 m with a water volume of about 2 115 000 m3. The chair-shaped cirque was excavated by the glacier in a massif of fine- and medium-grained granite. Runoff from the lake is carried out through two channels crossing a rocky sill on the northeastern and southeastern margins of the lake.
The dominant mountains in the vicinity of the lake are Mukal (3899 m a.s.l., on the western boundary of the catchment) and Syltrankel-Bashi (3827 m a.s.l., on the southern boundary) mounts. Their spurs and rocky crests create a system of local watersheds, which divide the lake catchment into several small valleys and cirques partially isolated from one another. Small glaciers are preserved on the northern rocky slopes of Mukal and Syltrankel-Bashi mounts. In the western and southwestern parts of the catchment, glacial runoff feeds the two most full-flowing streams, which open into the lake in wide deltas, being the main suppliers of sediments to the basin.
The research focuses on identifying the relationships between the sediment yield and the morpholithodynamic processes that accompanied deglaciation in the lake catchment. It includes three blocks: (1) collection and systematization of historical and archival data, including multitemporal maps, remote sensing data, and published works for the reconstruction of glaciation changes in the watershed over the last 150–200 years; (2) a set of field studies; and (3) analytical processing of the samples collected during the study.
Documentary sources on the state of glacial cover in the catchment of Lake Syltrankel can be divided into three categories: textual evidence, cartographic materials, and aerial and satellite images. The obtained materials made it possible to assess the state of glaciers for several time slices.
Field work was conducted in the lake watershed during two field seasons. In July–August 2022, bottom sediments were sampled from the lake basin. In August 2023, field routes included a visual survey of the state of glaciers in the most remote parts of the basin, where remote sensing data could not be unambiguously interpreted.
Before the sampling of the bottom sediment, the lake depth was measured using a Deeper DP4H10S102 echo sounder. The drilling points were selected on the basis of the online bathymetric map (Fig. 2). Bottom sediments were sampled using a piston sampler [Nesje, 1992] into a polyvinyl chloride pipe of 110 mm in diameter. The sampling mechanism involves the loss of some water-saturated portion of the near-surface sediment pack. Therefore, it was expected that no data on the several years preceding the sampling year would be obtained.
The lower limit of the sediment sampling was determined by the capacity of a sampler to penetrate sediment and then safely retrieve it. Because the length of the sampler cables was limited to 25 m, sediment cores could not be taken in the deepest part of the lake (about 36 m). Therefore, shallower flattened areas of the bottom were selected, where viscoplastic deformations of the water-saturated sediment should minimally affect the quality of sampling. One column of sediment was sampled at a depth of 15 m in the area of the largest watercourse entering from the west. Coarse clastic material stopped an advance of a drill bit and caused deformation of its cutting edge. The results obtained are most representative for the catchment of the largest western tributary. Judging from local geomorphic conditions. the transformation of the glacier cover in this area has been multistaged and has lasted for a relatively long time.
The sampled 38-cm-thick sediment column was transported to the laboratory, where it was cut in half along the long axis. The resulting flat surface was mechanically prepared for macrophotography and creation of a single high-resolution panorama for visual analysis of the column. The resulting image was used to visually isolate and count cyclically accumulating sediments (varves).
A rectangular metal profile was pressed into the central part of one of the obtained halves of the sediment column, where mechanical deformations of the sediment during the extraction were minimal. Then, the sample was cut layer-by-layer with a step of about 5 mm. The use of the metal profile was motivated by the need to obtain samples of the same geometry for further calculation of the stocks of radionuclides.
The samples of the bottom sediments were dried and homogenized without disturbing their particle size distribution. Then, the content of natural (210Pb, 226Ra) and anthropogenic (137Cs) gamma-emitting radionuclides was determined in the samples for the radioisotope dating. Gamma-spectrometric study of the samples was performed in containers with fixed geometry with the use of an ORTEC GEM-C5060P4-B gamma-spectrometer equipped with a semiconductor detector made of ultrapure germanium (HPGe) with a beryllium window (relative efficiency of 20% with minimum exposure of 60 000 s). Gamma activity was determined to a depth, up to which the meaningful parameters of the activity of the desired radionuclides could be traced. The nonequilibrium lead (210Pbex) was calculated by subtracting the specific activity of 226Ra from the specific activity of 210Pb itself.
The particle size distribution of the samples was determined using a Malvern Mastersizer 3000 particle size analyzer for the upper interval (0–15 cm) of the core. The samples were pretreated with 4% sodium pyrophosphate solution to disperse the clay fraction. The representative sample of the prepared suspension was pipetted into a cuvette for the liquid dispersant. The sediment in the cuvette was treated for 100 s with 40 W ultrasound and was intensively stirred with a centrifugal pump at 2400 rpm. After turning off ultrasound, ten repeated measurements were performed, and the results were averaged using Mastersizer v.3.62 software.
RESULTS OF THE STUDY OF THE DOCUMENTARY AND REMOTE SENSING DATA
The first mention of the lake dates back to 1874, when the expedition of F.C. Grove, F. Gordiner, and H. Wacker visited Syltrankel. According to the available description, the lake was entirely covered with ice at the time of the visit in July 1874. The glacier, along which the scientists climbed to Mont Syltrankel-Bashi, descended directly into the lake [Grove, 1875]. V.Y. Teptsov [1892] wrote: «we have not noticed even a slightest meltwater groove on any glacier, which is usually cut in ice by streams «. The map, compiled by the Office of Military Topographers [Map of the Office of Military Topographers, 1887–1888 and 1913], also records position of the glacier edge directly in the lake. In 1881, ice-stone mass collapsed into the lake basin, which triggered a outburst flood. As a result, the lake level probably lowered by 1.5–2 m (2–3 arshin) [Teptsov, 1892]. A mudflow occurred along the valley of the Syltran-Su River, which destroyed Urusbievo village (now, Upper Baksan) [Seinova, Zolotarev, 2001]. Studies of current fluctuations of the lake level have not revealed a potential threat of new mudflows [Krylenko et al., 2008; Kidyaeva et al., 2013]. However, the mudflows have repeatedly occurred in the 20th century along the Syltran-Su River valley and the receiving watercourse of the Kyrtyk River [Dzhappuev and Gyaurgieva, 2015].
In 1910, A.L. Reingard recorded the glacier (no. 330 according to the catalog compiled by him) in the valley of the Syltran-Su River. The length of the glacier reached 2.03 km; the height of the lower edge, 3220 m a.s.l.; the area, 2.22 km2; and the average height, 3420 m a.s.l. The position of the snow line was determined by A.L. Reingard «according to the geographical method» (the author’s definition) at 3420 m a.s.l. [Reingard, 1916]; snow could remain even at the lower hypsometric positions for quite a long time during the warm season. From the above-mentioned fact, we can conclude that in 1910, the edge of the glacier was in close proximity to the lake or descended into it.
The next evidence is the description of the first Soviet ascent to Mount Syltrankel-Bashi on August 9, 1936 [https://www.ullutau.ru/routes/siltran/?id=374, 2023]. The photographic materials demonstrate that a quarter a century after Reingard’s observations the glacier edge no longer descended to the shoreline of the lake and, apparently, significantly retreated (Fig. 3). The deltas of the two main watercourses flowing into the lake were completely exposed.
The first available aerial survey of the lake basin was taken in 1945. Judging from the absence of ice on the lake, which could remain until mid-summer, the photograph was taken in the second half of summer or in the early fall. Due to the presence of snow on the slopes it is impossible to precisely identify the glacier boundary, but it is possible to determine the maximum possible boundary of glaciation. After the retreat, the glacier could be preserved only on the slopes of northern exposure: the northern slope of Mount Syltrankel-Bashi and a section of the eastern slope of Mount Mukal (Fig. 5a).
Satellite imagery of the second half of the 20th century showed that by 1971, in the southern part of the catchment, there was no any glacier uncovered by moraine and/or colluvium for reliable identification from the remote sensing data. Only three compact zones of the glacier distribution in the western, southwestern, and southern parts of the catchment could be distinctly identified by photo
tone and texture of the image (Fig. 5b). No visible reduction of these zones was observed over the next six years (Fig. 5c), which is quite consistent with the brief period of the relative stabilization of the Caucasian glaciers in the 1970s–1980s [Solomina et al., 2016]. After 10 years, by 1986, the areas that could be interpreted on a satellite image with a much lower resolution (SPOT) as the glacier-covered areas remained relatively stable. However, taking into consideration the much lower resolution of the image, this estimation can be highly tentative (Fig. 5d). By August 2023, the remnants of the glacier were observed only on the slopes of the northern exposure of Mukal and Syltrankel-Bashi mounts. Glaciers obviously degrade and, taking into consideration the current trend of climate change, they will probably disappear soon.
Therefore, the runoff of glacial meltwater feeding the lake has entered the lake basin directly from under the glacier in the early twentieth century. Over a little more than 25 years, from 1910 to 1936, due to the retreat of the glacier edge, the sediment yield was no longer discharged directly into the lake. Surface water (including glacial meltwater) was forced to travel a longer distance on its way to the lake. Over time, this path tended to grow and become more complex, which should have affected the composition of clastic material entering the lake.
RESULTS OF THE INTEGRATED ANALYSIS OF THE BOTTOM SEDIMENT COLUMN
After opening and separation of the sediment column, it was found that the sampled part consists of a rhythmic series of layers, which vary in structure and texture. Distinct varves (Fig. 6a) presumably corresponding to the annual cyclic accumulation of sediments and layers of fine-grained material were distinguished. As a result of the visual study, 154 layers were distinguished and numbered from top to bottom of the core.
Depending on the particle size composition and thickness of elementary layers, the sequence was divided into four series corresponding to the stages with different sedimentation patterns (Fig. 7a). The description of the assumed stages is given below in a chronological order.
Stage 1 (layers 154–140). Mostly fine sandy material is accumulated. The lack of a distinct horizontal bedding of layers indicates viscoplastic flow of soil under the subaquatic conditions and probably disturbance during sampling. No cyclically accumulated series of layers were identified.
Stage 2 (layers 139–108). Mostly sandy material of varying grain size is accumulated; it may be due to sorting mechanisms during transport and deposition of material under more dynamic conditions. Sedimentation is rather episodic. Only seven layers have been identified, which could potentially be considered as annual; although, more likely, they reflect the event variability (possibly within one or more years).
Stage 3 (layers 107–62). A series of alternating varves and thicker interlayers of sand of various grain sizes. This can be explained by the transition to the conditions, under which it became possible to preserve successively accumulated annual pairs of layers, periodically interrupted by episodes of the higher-energy sedimentation. Therefore, this stage can be considered as the transition to a calmer regime. There are 37 varves in the given interval. In view of their possible destruction during transit and subsequent accumulation of coarser material, it should be considered that the duration of this stage was at least 37 years.
Stage 4 (layers 61–1). Layered loam uninterrupted by any random inputs of coarse-grained material is being consistently accumulated in the youngest stage (Fig. 7b). This predetermines the most favorable conditions for calculating the duration of the stage by counting the number of varves – 61 years.
The counting of the number of varves allows us to state that the total duration of the sediment accumulation in the sampled core lasted for at least 105 years.
The accumulation of layered sediments with a distinct gradient of the mechanical composition can be associated in some cases with the descent of micro-mudflows, as well as with the uncreased water discharge during the yield-forming precipitation. Such layering of sediments in the central parts of the bottom (remote from the shores of mountain glacial lakes), is commonly interpretated as the annual cyclicity [Leemann, Niessen, 1994]. To reliably establish the chronology, it is necessary to use the independent dating based on radioisotopes and correlation with the historically documented events [Zolitschka et al., 2015].
The plot (Fig. 7c) demonstrates that a peak of 137Cs activity can be attributed to layers 20–28 due to the global deposition of the bomb origin. The beginning of the 137Cs deposition from the atmosphere, which dates back to 1954, can be correlated with layers 36 to 41. There is a small peak of activity in layers 3–7, which can be very conditionally attributed to the 137Cs deposition of the Chernobyl origin in 1986. The latter was considered only as an additional marker owing to its poor manifestation. Detection of nominal values of 137Cs activity, the origin of which is entirely anthropogenic, at depths of more than 70 mm (below layer 41) can be considered the result of cross-contamination of samples during core opening. Assuming the one-year formation of the separated layers, only two variants are possible, when the differences between the layer numbers and the assumed ages coincide in only two possible variants of dating (Table 1.)
Table 1. Variants of the absolute dating of layers.
Therefore, the youngest sediment in the sampled core (layer 1) is dated to 1989–1990, and the loss of data in the records concerns the last 32–33 years. This is generally expected with the used technique of the sampling. Such large losses significantly complicate the use of 210Pb of the atmospheric origin for dating sediments, because they do not allow us to reliably determine its modern concentration in sediments. In addition, the observed distribution differs significantly from the ideal exponential profile formed by the uniform accumulation and subsequent decay of 210Pb coming from the atmosphere, on the basis of which it could be reliably estimated [Sanchez-Cabeza and Ruiz-Fernández, 2012]. The uniform profile of the distribution is broken at a depth of 140–160 mm (layer 97, Fig. 6d), which corresponds to a thick interlayer of coarse clastic material (Figs. 7a, 7c). The absence of 210Pbex in it indicates that the material has not been exposed at the surface and was deposited in one event. 210Pbex is fixed up to a depth of 225 mm (layers 114–115), indicating that they are younger than necessary for its disappearance due to radioactive decay (≤150 years). Consequently, the sediments of Stage 3, starting from layer 106, were accumulated after the specified time, i.e., later than 1872. The absence of 210Pbex in the older sediments may also be related to the fact that the lake was covered with ice blocking lead entry into the bottom sediments. Based on the comparison of the radiocesium dating and varve counts and considering the distribution of 210Pbex, the approximate and internally consistent chronological frames were estimated for the selected stages (Table 2).
Table 2. Chronological frames of the identified stages of sedimentation.
Based on the obtained chronological frames, it is likely that the sampled sediments were formed after the 1881 event, and debris blocking advance of the sampler may be a product of the ice-rock mass collapse, although it is impossible to determine its exact origin from the available data.
If we assume that the sampled sediments in the lower part are older than 1881, then such a large event should have been reflected in the structure of the bottom sediments, presumably in the form of some distinctive interlayer, expectedly enriched in coarser debris than the rest interlayers. There are two layers in the studied sediments that can pretend to such a role.
The first candidate is layer 97 (140–165 mm) (Fig. 6d). This version is supported by the largest size of sediment grains among all selected layers and the absence of the visible sorting and internal structure. Regardless of whether this layer is associated with the 1881 collapse, it at least indicates the single and powerful event in the history of the lake. Against this evidence is the presence of 210Pbex excess at greater depths, up to 225 mm, which could theoretically remain for 150 years; however, such ancient dates obtained by standard radiolead models are very unreliable [Binford, 1990].
The second candidate is layer 135 (295–320 mm) (Fig. 6c). It falls within the estimated time frame of the beginning and end of the sedimentation stage. Layer 61 with a relatively reliable dating (1928–1929) is separated from it by the time interval of at least 37 years. If we use layer 135 as the chronological marker of 1881 and date layer 61 by counting varves up the core, its formation date will be ≥ 1919. This value does not seem to be contradictory at first glance, but the arithmetic does not take into consideration a significant number of sandy layers, the deposition of which under the higher energy conditions could have disturbed or even destroyed previously accumulated thinner sedimentary layers (varves). Such sandy interlayers cannot be accepted as annuals. In addition, they include layer 97, which, as mentioned earlier, obviously stands out from the general scenario of sedimentation. Layer 135 is characterized by internal sorting, which does not agree with colluvial and chaotic input of clastic material.
Therefore, we can conclude that there is no reliable evidence directly indicating the exact correspondence of any of the sand layers in the sediment cores to the 1881 event (collapse of ice-rock masses). The available assumptions have either indirect substantiation or arguments against them. Therefore, unfortunately, it is not possible to use this event as an additional reliable and accurate chronological marker.
DISCUSSION
The conditions of the ice cover formation can be considered similar to the glaciers of the eastern slope of Mount Elbrus. These glaciers are located in the least favorable microclimatic conditions for their development. Among all other glaciers, they are characterized by the highest average rates of retreat [Panov, 1993]. The available data on the changes in their boundaries for 1887–1933, 1933–1957 and 1957–1987 indicate that in the middle of the 20th century, the retreat rates were generally increased or remained relatively stable (Table 3). In many respects, the similar scenario was observed for the Terskol Glacier, where the intensive degradation occurred from the late 19th century to at least the late 1950s [Panov et al. 2008; Bushuyeva et al., 2016].
Lake Syltrankel and its basin are located at the elevations very close to the position of the snow boundary, which, considering the local climatic features and the redistribution of matter within the glaciers, closely correlates with the lower boundary of mountain glaciation. During the period from 1881/1910 to 1965/1976, the average height of the lower boundary of glaciers increased by 110 m in the Kuban River basin and 120 m in the Terek River basin. In the Baksan River valley, this increase was up to 210 m [Panov, 1993].
Therefore, it is expected that any climatic fluctuations and associated changes in the ice cover had the significant impact on the sediment yield and were a driver of its transformation. Bottom sediments did not directly reflect the position of the glacier margin; however, the sedimentation was strongly affected by the change in the path of sediment migration with the meltwater runoff during glacier retreat expressed in the appearance of local zones of the intra-basin accumulation.
The distinguished stages most likely reflect the changing role of the glacier in the formation of the sediment yield. The sediments entered the lake no later than the end of the 19th century and continued to be supplied until the end of the 20th century. Stages 1 and 2 are supposedly represented by fluvioglacial sediments that entered the lake directly from beneath the glacier bed during deglaciation up to the very end of the 19th century. The change in the sedimentation regime at the beginning of the 20th century (Stage 3) was reflected in the gradual increase in the share of fine-grained material with well-defined layering. This indicates a change in the hydrodynamic conditions of sedimentation and the appearance of mechanisms for size sorting the material on the way of its transportation to the lake basin. The gradual transition to the accumulation of entirely rhythmically layered fine-grained sediments (Stage 4) from 1928–1929 indicates that coarser clastic material either could no longer be transported by water flows or that barriers arose in the form of intra-basin accumulation zones impassible for coarse particles. The presented reconstruction is in good agreement with the results of analysis of the documentary data and with the general scenario of deglaciation in the Caucasus in the late 19th–20th centuries.
CONCLUSIONS
Over the last 140 years, changes in the ice cover have been the main factor in the transformation of the sediment yield entering the Syltrankel Lake basin. The integrated study of the sampled column of the bottom sediment and the analysis of a number of documentary sources allowed us to identify and to date the four stages of sedimentation for the period of no earlier than 1881. As new catchment areas were released from glaciers, the distance of transport, the intensity of mechanical sorting, and the intra-basin accumulation of clastic material increased. The modern stage, which presumably began in the late 1920s, is characterized by the successive accumulation of only fine-grained layered sediments. At the earlier stages, coarse-grained particles could enter the lake together with the runoff of glacial meltwater due to the proximity of the glacier margin to the lake; and the accumulation of these particles was mostly episodic. The obtained results point to a great potential for study of the changes in the sedimentation regime as an additional chronological marker of deglaciation in the basins of several high-mountain lakes.
ACKNOWLEDGMENTS
The work was supported by the Russian Science Foundation, project no. 19-17-00181 (sampling and analysis of bottom sediments). Reconstruction of the glaciation history based on documentary sources and remote sensing data were performed in agreement with the State Assignment of the Institute of Geography of the Russian Academy of Sciences no. FMWS-2024-0005.
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URL: https://www.ullutau.ru/routes/siltran/?id=374 (accessed August 24, 2023).
Received October 24, 2023
Revised January 30, 2024
Accepted February 5, 2024
Translated by V. Krutikova