Annotation of the results obtained in 2023
Our results have allowed us to preliminary reconstruct the topography of the Central Kamchatka Depression (CKD) that existed at the beginning of the period under study and to estimate the time of accumulation of lacustrine deposits at different altitudinal levels. Lake basins existed in the CKD repeatedly during the middle to late Pleistocene (Zelenin et al., 2022). Ancient lake sediments were described in the middle reaches of the Kamchatka River, i.e., relatively high above the flow threshold. Trends in the tectonic development of the CKD indicate that even during periods for which lake sediments are unknown, lake basins in some configuration were preserved in the lowest part of the CKD. Even now, more than 400 lakes with a total area of about 200 square kilometers are located in the lower reaches of the Kamchatka River. The history of the youngest paleolake started about 30 ka BP. Before that time, lake sedimentation occurred at the altitudes up to 200 m a.s.l. and formed the "Sand Island" - a large sand massif between the Kamchatka and Karakovaya rivers. Between 40 and 25 ka, the water body shrank sharply, which is reflected in the appearance of ~25 ka old river terraces. However, in the northern part of the CKD, lacustrine sedimentation continued from at least 30 ka BP - older sediments are unknown because they are overlain by lavas of the Kliuchevskoi volcanic group. Tephrochronological dating of the alluvial gravels in the Dlinny Yar does not allow us to combine downstream and upstream segments into a single water basin, as suggested in (Ponomareva et al., 2021). Thus, at 25 ka BP, the water level in the reservoir did not exceed 100 m. Then, until about 17 ka BP, the paleolake level rose for 120 m, leading to the accumulation of terrace material exposed in the Lamutskoy (Rybachok) outcrop. The subsequent stages of paleolake drainage were published earlier (Zelenin et al., 2023b). This year is the last one under this project. Despite the fact that the project tasks have been successfully completed and the RSF does not permit the extension, we continue to work on this topic and welcome the involvement of other specialists in Quaternary geology and paleogeography of Kamchatka. In this regard, all project results have been collected and placed in the AWS CodeCommit repository (mirrored on Yandex Cloud and local computers of the project implementers) with the Git version control system. In particular, all field journals were digitized and provided with a common table of contents, photographs of the studied objects were selected and catalogued, geospatial data were compiled into a single GIS, and the catalog of specimens was linked to the field diaries and GIS. Online access to the project data (https://git-codecommit.eu-north-1.amazonaws.com/v1/repos/GIS_Paleolake) was previously organized for the project participants and will be maintained in the future. Access can also be provided at the request of any interested specialist, but since the data discussed are not published, we cannot provide online open access.

 

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Annotation of the results obtained in 2021
During the first year of work on the project "History of development and degradation of a giant Late Pleistocene lake in Kamchatka" a number of remote and field works were performed. A scheme of interpretation and a geomorphological map of the bottom of the Central Kamchatka Depression (CKD) were created. The field works aimed at the study of the bottom facies of lacustrine deposits were carried out within the limits of the supposed lake. The sections in the upper reaches of the Kamchatka River and in the valleys of the left tributaries of Kamchatka, Bystraya, Andrianovka, and Vetlovaya rivers, were also described. Grainsize and diatom analysis of sediments from reference sections of Kliuchevskoe Lake and Andrianovka was started. Samples for spore-pollen analysis have already been processed and analyzed, and primary results are being prepared for publication. The Kliuchevskoi Lake section was sampled at 5-10 cm increments (at some depths up to 20–35 cm). More than 80 samples for spore-pollen analysis were obtained in total. The prepared material is unique in its completeness and age detail (Ponomareva, Pendea, Zelenin et al., 2021) as an archive of paleoclimatic conditions in the CKD for the last 28 thousand years. According to the results of the spore-pollen analysis, 8 local pollen zones were identified, reflecting the main stages of vegetation changes in the CKD in the interval between 28.4 and 7.9 thousand calibrated years ago (ka BP). The results show that phytoclimatic conditions at the Pleistocene-Holocene boundary on Kamchatka were changing very rapidly, and this was expressed in a dramatic change of vegetation patterns and corresponding pollen spectra. This is also confirmed by other researchers of Kamchatka (Pendea et al., 2017; Braitseva et al., 1968; Mukhametshina et al., 2021). At the same time, the pollen spectra changes during the transition from MIS3 to MIS2 were smoother. Apparently, the climate changed relatively slowly during the transition from the Late Pleistocene interstadial to the last glacial maximum. It is important to note the presence of pollen of aquatic plants in the section. In this case, no pollen of thermophilic aquatic plants was found in sediments younger than 25.3 ka BP, and in sediments younger than 11.6 ka BP, no pollen of aquatic plants was found. The section of Lake Azhabachy in the basin of the lower reaches of the Kamchatka River, the modern flow of the CKD, was a unique find. The sediments of about 10 m thick and containing 51 tephra layers accumulated here for at least 50 thousand years: tephra from the upper part of the section correlates well with our dated ashes from the Kliuchevskoe Lake key section, and below the Gor28 tephra we found T0 tephra identified in the sediments of Elgygytgyn Lake and dated to 58 ka BP (Nowaczyk et al., 2013). This is the first find of T0 tephra in Kamchatka, confirming its Kamchatka rather than Aleutian-Alaska origin (van den Bogaard et al., 2014). This transect requires further study because, to date, it is the only known paleoarchive of the natural environment in Kamchatka, spanning the time of lake origin in the CKD (~30 thousand BP). Pleistocene lake sediments in the Central Kamchatka Depression (CKD) have been described by many authors since 1940, but their age has remained a matter of debate. The lake sediments accumulated over long time intervals contain tephra layers. These tephras have never been systematically studied before except for several horizons in the lake sediments of the Pakhcha River and one tephra from the Polovinka site, for which mineralogical descriptions and bulk chemical analyses were made as early as in the early 1980s. The source volcanoes of the described tephra have not been identified. However, even beyond the capabilities of the radiocarbon method, tephra correlations with their distant facies in dated marine and terrestrial sediments can be used to determine the age of deposits. The greater the number of sections that contain any tephra, the greater the number of dates or estimates from age models can be obtained for this tephra, and the more accurate will be the age correlations of all sections containing this tephra. For the first time, we were able to reliably date lake deposits of the stages 320–300 and 180–30 thousand BP within the CKD. These dates and distribution of deposits prove that the depression was repeatedly filled with long-lived large lakes.

 

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Annotation of the results obtained in 2022
Within the limits of the supposed lake, field works were carried out to study lake deposits; in addition, a field geomorphological survey was carried out at the northern boundary of the modern Kamchatka River basin. The sections in the coastal bluffs of the middle reaches of the Kamchatka River and its tributaries - the Malaya Kimitina, Karakovaya (Kozyrevka), Pakhcha, and Yelovka Rivers - were described and sampled. Pits were dug and boreholes from 2 to 10 m deep were drilled on landforms without natural outcrops. Sampling was conducted at 10-20 cm intervals, and at 40-50 cm intervals for outcrops with homogeneous composition. A total of 866 samples were taken for analysis. In addition to describing and sampling of sediments, we measured the absolute height of the layers using a geodetic GNSS receiver in RTK mode, as well as UAV surveying. Geochemical composition of key tephra horizons was analyzed. Judging by the composition of volcanic glass, the studied tephra from lake sediments originated from all volcanic areas of Kamchatka: the Eastern Front, the Rear-Arc, the Central Kamchatka Depression (CKD), and the Sredinny Range. A variety of compositions facilitates the identification and correlation of newly described ashes with previously dated ones. Tephrochronological studies outlined lake deposits younger than 30 ka that prevail at the foot of Shiveluch volcano, as well as along the western margin of the Kliuchevskoi group of volcanoes. The age of the sediments was estimated by comparing the newly described and previously studied marker tephra layers (Ponomareva et al., 2021). Southward, the sediments of the so-called "sand island", which is a lake delta (Kuprina, 1970), older ashes appear. This fact confirms the long-term formation of the "sand island" deposits and does not allow us to consider them the result of catastrophic discharge of a dammed lake, as suggested by Pevzner et al. (2020). Further to the south, older sediments are exposed along the CKD floor. Middle Pleistocene lake sediments are exposed at the base of the highest so-called great bluffs of the CKD (Braitseva et al., 1968). The Middle Pleistocene age of these deposits (about 300 thousand years) is also confirmed by our studies (Ponomareva et al., 2022). A draft of the geomorphological map of the Central Kamchatka Depression at a scale of 1 : 600 000 (in 1 cm 6 km) was created. The map creation methods have been presented as a manuscript for submission to the Journal of Maps. Information on volcanic landforms was collected from published sources and generalized to the scale of the map. Exogenous landforms were mapped following the results of our field work and interpretation of remote data. Objects with ambiguous interpretation of its age or genesis are highlighted. Similar cartographic materials were presented only in a number of works published half a century ago and had considerably less detail (Braitseva et al., 1968; "Kamchatka, Kuril,...", 1974). A model of the CKD hydrographic network evolution since 30 ka was developed. The scale and chronology of the last glacial maximum along the modern drain of the Kamchatka River was reviewed, closely related to the volcanic history of Shiveluch, the largest source of glaciation in this part of the CKD. The lake descent was probably gradual and began about 19 thousand years ago within the LGM, so it is not a direct consequence of climatic changes. As the glacial accumulation zone is located in the summit of the Shiveluch volcano, for which large sectoral collapses are known in the Late Pleistocene, changes in the ice dam height may be caused by the destruction or growth of the volcanic edifice. The tendency of tectonic submergence of the CKD prevents complete drainage of the lake basin - even now in the wide floodplain of the Kamchatka River there is a lake system with a total water surface area comparable to the largest lakes of the peninsula.

 

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