INTRODUCTION

For a long time, the coastal areas of the East Siberian Sea floor were the least studied regions of the Russian Arctic from the geological viewpoint. Based on surface sampling data acquired over a long time period (since the 1930s) by specialists from NIIGA-VNIIOkeangeologiya, a set of small-scale maps (1 : 2 000 000) was compiled, characterizing Holocene sedimentation processes ([1, 2], etc.). Studies of the mineral and chemical composition of bottom sediments were carried out by scientists from the Far Eastern Branch of the Russian Academy of Sciences (RAS) ([3, 4], etc.) and the Shirshov Institute of Oceanology RAS (IO RAS) ([5, 6], etc.).

In contrast, the New Siberian Islands have been studied quite well. In 1978, under the supervision of G.V. Trufanov, a geological survey on a scale of 1 : 200 000 was completed there [7], and large-scale paleogeographical studies of the Late Pleistocene and Holocene environmental changes were carried out in the 2000s ([8, 9], etc.)

In 2018, as part of VSEGEI studies on state geological mapping at a scale of 1 : 1 000 000 (sheets R-56–60), geological and geophysical expeditionary work was carried out in coastal shallow waters from the mouth of the Kolyma River to Wrangel Island. In 2020, similar studies were organized east of Novaya Sibir’ Island (sheets S-55, 56). The studies were carried out onboard hydrographic vessel Ivan Kireev (2018) and R/V Kapitan Voronin (2020). Based on the interpretation of these data, the upper part of the geological section was subdivided into five acoustic units (AUs), correlated with different stages of the regional Quaternary history. Their distribution in the mapped area was traced [10], a concept of paleogeographic development of the coastal shelf was developed [11, 12], and a hypothesis was proposed for the formation of submarine ridges along the periphery of Novaya Sibir’ Island and the Kolyma River paleodelta [13].

At the same time, a number of questions on the Late Pleistocene–Holocene evolution of the region and transformation of sedimentation environment have remained open. Problems in interpreting the data existed, in particular, for cores recovered in 2020 in the western East Siberian Sea (from the mouth of the Indigirka River to coastal shallow waters along the periphery of Novaya Sibir’ Island). The resulting lithological, geochemical, palynological, macro- and micropaleontological dataset has prevented a consistent concept of the paleogeographic development of this region. In addition, the results of radiocarbon dating of samples from the lower horizons of cores based on particular organic matter obtained at the first stage of research were questionable, ranging from 18.7 to 11.7 cal. ka BP (thousand calendar years before present)Footnote 1 and characterizing the end of the Upper Pleistocene [11]. In order to resolve the contradictions within the framework of this study for sediment core 20BCM-8 recovered in the paleovalley of the Indigirka River, a set of paleontological studies and radiocarbon dating of mollusk shells were carried out. The data obtained afforded significant progress in reconstructing the paleogeographical development and sedimentation conditions of the studied area of the coastal shelf in the Holocene.

MATERIALS AND METHODS

In 2020, under the state geological mapping program at a scale of 1 : 1 000 000, VSEGEI conducted geological and geophysical studies in the East Siberian Sea coastal shelf between the mouth of the Indigirka River and northern periphery of Novaya Sibir’ Island. Two acoustic-seismic profiles (20BCM09 and 20BCM08) with a total length of 318 km crossed the Indigirka paleovalley (Fig. 1).

Fig. 1.
figure 1

The study area of the East Siberian Sea by VSEGEI in 2020 (a). Acoustic-seismic profiles (ASP) and core sites (black, this study); Photos and lithological composition of sediment cores (b): (1) silty clays; (2) clayey silts; (3) clay; (4) unlayered mottled, heterogeneous (mixed); (5) low-plasticity; (6) large folds with marbled texture, formed as a result of sea-ice gouging of seabed by hummocks; (7) mollusk shell fragments.

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The survey, using the continuous acoustic-seismic profiling (ASP), was carried out with an Innomar SES-2000-light high-frequency parametric profiler. The studies were carried out with ultra-high-resolution seismic exploration (UHRS) with electric spark sources with a center frequency of about 550 Hz. To excite elastic vibrations, a SplitMultiSeis Source 2500 sparker with an electrical energy storage device was used; recording was done with a 16-channel streamer with a 30 m active part, which are part of the SplitMultiSeis multichannel acoustic-seismic software and hardware complex. Based on the results of analyzing the geophysical data, the locations for six sediment cores were selected (Table 1; Figs. 1, 2).

Table 1.   Characteristics of sediment core site locations in the Indigirka paleovalley
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Fig. 2.
figure 2

Stacked ASP profiles (left, 20BCM08; right, 20BCM09) crossing the Indigirka paleovalley. Top, high-frequency profiler; middle, low-frequency (sparker source); bottom, interpretation of profile. Legend: 1–5A, reflectors; acoustic units (AUs) (after [11, 16]): AU-1, Upper Pleistocene (Sartanian cooling)–Holocene (Post-Sartanian warming, thermokarst deposits; marine ingression, marine deposits of various facies); AU-2, Upper Pleistocene, marine sediments (Karginian warming); AU-3, Upper Pleistocene, marine sediments (Kazantsevo transgression), lacustrine sediments, or sedimentation hiatus (Zyryanian regression); AU-4, Lower (?)–Middle Pleistocene, lacustrine, alluvial and marine undifferentiated deposits; AU-5, Gelasian (?)–Lower Pleistocene, marine, lacustrine, and alluvial formations.

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For all cores, descriptions, photographs, measurements of undrained shear strength, and geochemical studies were produced (including determination of the Br content on a SPECTROSKAN-MAX-G X-ray scanning crystal-diffraction spectrometer to calculate the paleosalinity using A.G. Grigoriev’s method [15]), grain size analysis (per every 1 cm of core) using a laser particle size analyzer Microtrac MrB (at VSEGEI laboratories); and for cores 20BCM-4, 20BCM-8, 20BCM-11, determination of organic matter content by coulometry on AN-7529 express analyzer (in IO RAS). For two sediment cores (20BCM-8 and 20BCM-9), geochemical studies and magnetic susceptibility measurements were carried out at IO RAS using a Geotek MSCL-XYZ automated system for the integrated study of cores.

The reference sediment core 20BCM-8, with a length of 177 cm, was obtained in the Indigirka paleovalley from 35 m water depth. The upper oxidized layer, sampled with a box corer, was disturbed during sampling with a gravity corer.

Sediment 2 cm thick samples for paleofauna studies were continuously taken in core 20BCM-8, dried, weighed, washed over a 63 µm mesh size sieve, dried, and weighed again. Species composition and total abundance of macro- (mollusks) and microfauna (foraminifera, ostracods) were determined in the >125 µm fraction. The total number of microfossils is presented relative to 100 g of dry bulk sediment. Due to the paucity of microfossils in most samples examined (<100 specimens), the relative abundances of foraminiferal and ostracod species and ecological groups were not calculated; their occurrence is also shown as the number of specimens per 100 g dry bulk sediment. Microfossil species diversity is defined as the number of species per sample. A lithological analysis was also carried out related to preparation of samples for microfossil study, including determination of the weight percentage of the >63 µm fraction, as well as counting under binocular of terrigenous grains of rocks and minerals in the >500 µm fraction, the number of which is expressed relative to 100 g of dry bulk sediment.

Palynological analysis was performed for reference core 20BCM-8. Sampling for this analysis was performed with 5 cm interval. Samples were processed according to V.P. Grichuk’s method [17]. The carbonate-free 2 cm thick sediment sample was disintegrated with sodium pyrophosphate and centrifuged in a heavy liquid. The samples were examined with an Olympus CX31P microscope. The interpretation was based on the ecology of modern vegetation. For species identification, pollen identification guides were used [18, 19]. Pollen diagrams were constructed using the TILIA program (E. Grimm, v. 3.7). The percentage of each taxon was calculated from the total sum of pollen and spores.

Three datings on dispersed organic matter from the basal sediment layers of cores 20BCM-4, 20BCM-8, and 20BCM-12 were obtained earlier using accelerator mass spectrometry (AMS) at the Laboratory of Radiocarbon Dating and Electron Microscopy of the Institute of Geography, Russian Academy of Sciences [11]. At this stage of research, at the Leibniz Laboratory of Kiel University (Germany), five AMS14C-datings of mollusk shells from reference core 20BCM-8 were obtained, and radiocarbon ages were converted to calendar ages (Table 2; see Section 3.2 below). Sedimentation rates (cm/kyr) were determined between the dated levels.

Table 2.   AMS14C datings of sediments of core 20BCM-8 and calibrated calendar ages converted with the use of MARINE20 and IntCal20 databases
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RESULTS AND DISCUSSION

Lithology and geochemistry of bottom sediments. All analyzed cores were recovered within the upper acoustic unit (AU-1), that was accumulated at the last stage of geological development, starting with the Post-Sartanian warming and marine transgression [10, 11]. Core 20BCM-4 was recovered 100 km northeast off the modern mouth of the Indigirka River (water depth 15.3 m). The core revealed 37 cm of dense, dry, low-plasticity clayey silt. Sand content averages 5.4% (2.9–8%), silt 59.8% (57.6–61.8%), and pelite 34.8% (30.5–39.5%). Corg varies from 0.6 to 1.16%. The estimated salinity is 3.5–3.9‰, increasing to 7.5‰ in the upper 5 cm of the core. Dense, low-plasticity silty clays and clays, characterized by low (about 5‰) calculated salinity, were revealed in the lower horizons of cores 20BCM-12 and 20BCM-13, recovered on the western slope of the Indigirka paleovalley (Figs. 1, 2) at depths of 27.1 and 21.1 m, respectively.

Sediments of core 20BCM-12 below 133 cm are represented by pure clays with a very high (91.4%) content of pelite; the admixture of silt does not exceed 9%, and sand is absent (Fig. 3). The 133–97 cm interval comprises mainly clayey silts. The average content of the pelitic fraction is 42.3% (21–76.1%); silt fraction, 52.3% (23.5–60.4%); sand fraction, on average, 5.5%, varying significantly over the interval (from 0.3 to 18.8%); the 100–103 cm interlayer is the most enriched in sand (11.7–18.8%). In general, the 133–103 cm interval is characterized by a constant admixture of sand material; in the 100–103 cm interval, a weakly pronounced erosion layer is distinguished. For sediments of the 124–125 cm interval, an age of 16 969 cal. yrs BP was obtained based on dispersed organic matter. Upcore from the 133–100 cm interval, the Al, Si, K, Ti, Ca, and Fe contents decrease, as well as most of the ratios (K/Ti, Si/Ti, Si/Al, Si/Fe, Fe/Mn, Ti/Mn). Sediments of the lower interval of core 20BCM-13 (119–100 cm) are represented by silty clays and clays, forming irregularly distributed layers with a thickness of 1 to 8 cm. The content of the pelitic fraction is 71.7% (54.6–96.0%); silt fraction, 27.9% (4.0–44.5%); sand fraction, 0.4% (0–2.7%).

Fig. 3.
figure 3

Core photo, lithological description, grain size distribution, and salinity for sediment cores 20BCM-11 and 20BCM-12. Legend: AU-1, seismic sequence 1; sediment consistency: LP, low-plasticity; FP, fluid-plastic; SP, soft-plastic; F, fluid; grain size distribution: Pl, pelites (clays); APl, aleuropelites (silty-clays). Results of radiocarbon dating of dispersed organic matter are shown.

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Upsection, the deposits of core 20BCM-12, as well as cores 20BCM-9, 20BCM-11 and 20BCM-13 as a whole, represent a gradual transition from soft-plastic to fluid-plastic and fluid silty clays and clays. In grain size distribution, all cores show a weak transgressive trend with an increased content of fine fractions; in cores 20BCM-11, 20BCM-12, and 20BCM-13 (Fig. 3), recovered farther from the axial part of the paleovalley, the grain size parameters are characterized by greater variability; in cores 20BCM-9 and 20BCM-8, they are more consistent. The calculated paleosalinity naturally increases upcore in all cores: in core 20BCM-9, from 11 to 29‰ (with the most contrasting increase above 40 cm); in core 20BCM-11, from 11.6 to 22.7‰; in core 20BCM-12, from 9 up to 21‰; and in core 20BCM-13, from 9.5 to 17.6‰. Corg in core 20BCM-11 decreases slightly upcore from 1.34 to 1.14% (Fig. 3).

The 177-cm-long core 20BCM-8, chosen as the reference one, was recovered within the field of modern nepheloid deposits, in the deepest part of the Indigirka paleovalley, where the thickness of AU-1, according to ASP data, is about 3 m (Fig. 2). The entire core is represented by homogeneous soft silts with clayey interlayers.

Core sediments are characterized by an exceptionally consistent grain size distribution along the section. With the exception of the upper part, interpreted as modern marine deposits, the weight % of sand is 3.6% (0.5–7.2%); silt, 43.4% (40.0–53.2%); pelite, 51.0% (40.4–57.8%); silty clays predominate (Fig. 4). The wt % of >63 µm fraction varies from 0.5 to 1.5 (Fig. 5).

Fig. 4.
figure 4

Core photo, lithological description, grain size distribution, salinity, and results of geochemical studies for sediment core 20BCM-8. See Fig. 3 for legend. Radiocarbon dating results of mollusk shells (black) and dispersed organic matter (red) are shown. Results of the measurements of chemical elements in cps (count per second).

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Fig. 5.
figure 5

Changes in lithological characteristics and composition of macro- and microfossils in core 20BCM-8. For gray and green shading—see explanations in the text.

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Terrigenous grains of rocks and minerals from the >500 µm fraction, represented mainly by quartz, are rare in core 20BCM-8, especially in its lower part (Fig. 5). This coarse-grained detritus in the Arctic marine sediments usually represent ice-rafted debris (IRD), which is transported from coastal areas by the ice formed during autumn storms [20–22]. If this newly formed IRD- rich ice is incorporated into the fast ice, it melts in place the following summer and ends up in sediments of the inner shelf, because fast ice is not carried out to the open sea. It melts in place on both sides, on the side of the polynya due to heating of its waters, and on the shore side due to the supply of warm river water during the flood. If young autumn ice is carried away from the coast to the seasonal drift ice boundary, the IRD then melts out on its driftway. For the region of the inner shelf where the studied core is located, it can be suggested that an increase in IRD content most likely indicates colder conditions that existed during autumn ice freeze-up, which led to quicker formation of fast ice with incorporated IRD-rich ice floes. In the studied core, a similar cooling can be reconstructed for sediments in its upper part (gray shading in Fig. 5).

The grain size distribution confirms the assumption of a quiescent depositional environment. The paleosalinity calculated from the bromine content in sediments increases upcore from 20 to 33‰. The organic matter content is quite high (average 1.32%, from 1.19 to 1.42%), with a weak tendency to decrease upcore. Based on the results of geochemical studies using the Geotek MSCL-XYZ system, the Ni, Pb, Mn, and S distributions in the upper part of the core mark the transition to typical marine sedimentation. The distributions of Rb content and Si/Al, Si/Ti ratios, as well as, to a large extent, magnetic susceptibility, are governed by the grain size factor and supply of terrigenous matter. An increase in the Zr/Rb ratio may also indicate an increase in the heavy mineral fraction in sediments. Sharp increases in Mn content indicate periodic changes in the redox potential. The increase in Ca/Al in the upper part of the core corresponds to increased dissolution of shell carbonate.

Age model and changes in sedimentation rates in core 20BCM-8. The age model of core 20BCM-8 is based on five AMS14C datings obtained from the shells of bivalve mollusks of the species Portlandia arctica (Table 2). Although P. arctica is a detritivore that can assimilate ancient carbon [23], we used shells of this species for dating because it is the only source of marine biogenic calcite in the sediments of the core. To convert radiocarbon dates to calendar ages using the CALIB 8.2 program [24], a reservoir correction of 370 years was used in accordance with the data obtained for the modern Laptev Sea [25] with environmental conditions similar to the East Siberian Sea. For calibration, two approaches were used, using the IntCal20 and MARINE20 databases, which gave similar results (Table 2). Even though the use of MARINE20 database is not recommended for high-latitude basins [26], we believe that in the case of the western East Siberian Sea, we have a proper regional correction ΔR (–180 years in accordance with [25]); therefore we can avoid the inaccuracies associated with ΔR. Accordingly, to create an age model using linear interpolation, we used calendar ages obtained with MARINE20 database (Table 2). The extrapolated age of the basal core sample (176–177 cm) is approximately 6.2 cal. ka BP.

Sedimentation rates in core 20BCM-8 gradually decrease upcore (Fig. 6) from 51 cm/kyr in the period 6.2–5.5 cal. ka BP to 13 cm/kyr in the last approximately 1.8 cal. kyrs. Undoubtedly, this reflects the gradual abrasion-induced retreat of the coastline from the core site under modern-like sea-level position.

Fig. 6.
figure 6

Changes in sedimentation rates in core 20BCM-8.

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Results of palynological analysis of core 20BCM-8. In core 20BCM-8, based on the results of spore–pollen analysis, five palynozones were identified (Fig. 7). In addition, when describing palynozones, finds of aquatic palynomorphs are noted represented by freshwater green algae and cysts of marine dinoflagellates.

Fig. 7.
figure 7

Spore–pollen diagram of sediment core 20BCM-8.

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Pollen assemblage zone (PAZ) 1 (136–177 cm, age 5.4–6.2 cal. ka BP) is characterized by a very low concentration of pollen and spores with good preservation of grains. Single fungal spores were noted; no dinoflagellates or coal particles were found. Throughout the zone, herbs dominate (up to 89.1%), predominated by Poaceae, accounting up to 50.0% of spectra, as well as Asteraceae (up to 9.1%) and Caryophyllaceae (up to 5.5%). At the interval of 136–150 cm, there is a sharp increase of Cyperaceae to 24%, and single Aquatic also appear; Ranunculaceae reach 2.5%. At the same time, trees do not exceed 8.1% due to Alnaster (up to 6.5%), Salix (up to 4.6%) and Betula nana (up to 3.7%). Spores do not exceed 15.3%, with Bryales being the most noticeable (up to 12.3%). These spectra indicate cold and harsh climatic conditions, similar to modern conditions in the Arctic deserts. Most likely, such spectra reflect the active supply of pollen and spores from the sediments of coastal cliffs, composed of yedoma sediments and located in close proximity to the core site. A sharp increase in climate humidity occurred during the period of sediment accumulation.

PAZ 2 (92–136 cm, age 4.3–5.4 cal. ka BP) is characterized by an increase in the concentration of pollen and spores, with trees reaching 35.4%. Up to 20.7% come from Alnaster; up to 14.1% from Betula nana; Salix does not exceed 3.7%. Conifers are noted: single Larix and Abies, as well as Picea (up to 4.6%), Pinus (up to 3.9%), and Pinus sibirica (up to 2.6%). The appearance of coniferous pollen is likely associated with an increasing role of river runoff in pollen transport and a decreasing role of coastal abrasion. Among herbs (up to 76.8%), Poaceae (up to 37.8%), Cyperaceae (up to 13.2%), Rosaceae (up to 10.7%), and Saxifragaceae (up to 8.0%) are distinguished. Bryales spores completely vanish from the spectra, unlike Polypodiaceae (up to 5.3%), Selaginella (up to 2.2%), and Lycopodium. Single dinoflagellates and several peaks in the content of small coal particles were recorded. The data obtained indicate warmer and humid climatic conditions.

PAZ 3 (28–92 cm, age 1.8–4.3 cal. ka BP) shows generally similar proportion of herbs, but Pinus increases up to 52.9%. Up to 4.8% of Abies is also noted. Conifers are well preserved. Spores do not exceed 2.9%; Polypodiaceae are present. Some samples are enriched in large coal particles, and dinoflagellates are also noted throughout this palynozone.

In the pollen spectra of PAZ 4 (10–28 cm, age 0.6–1.8 cal. ka BP) Pinus decreases to 22.5%, while Pinus sibirica reaches it maximum of 17.7%. Larix and Picea were observed sporadically. The total content of arboreal pollen does not exceed 46.8%. Among grasses (up to 59.0%), Ericaceae (up to 21.3%), as well as Polygonaceae (up to 9.2%) and Saxifragaceae (up to 8.9%), predominate. Bryales spores reappear, reaching 14.8%. The concentration of pollen and spores in the samples is low, and the preservation is generally good. Palynomorphs are almost completely absent except for cysts of colonial algae. No dinoflagellates were recorded. It is supposed that the PAZ 3 reflects direct aeolian transport of pollen into sediments, while PAZ 4, at the same time, records certain climate warming and the approach of the northern forest boundary to the coasts of the continent promoting the possibility of pollen grains being incorporated into coastal sediments of the New Siberian Islands.

PAZ 5 (0–10 cm, age 0–0.6 cal. ka BP) corresponds to the modern marine sediments. Trees predominate (up to 61.7%), among which Pinus reaches up to 50%. Among grasses (up to 49.4%), Poaceae (up to 24.1%) is distinguished. Dinoflagellates and coal particles are present.

Macro- and microfossils in sediments of core 20BCM-8. Fossil mollusks are present throughout the core section. Basically, these are shells of the bivalve species Portlandia arctica, a typical inhabitant of shallow waters of Arctic shelf seas. This detritivore prefers soft sediments rich in organic matter and tolerates freshening, high sedimentation rates, and turbid waters characteristic of coastal environments [27– 31]. In surface sediments of the Laptev Sea, P. arctica is the most abundant in samples from water depths of about 20 m in the area near the Lena River delta with significantly freshened surface waters, abundant input of plant detritus, and fast ice cover for most of the year [32]. In addition to the dominant species, the core sediments contain bivalves of the following species: Axinopsida orbiculata, Lyonsia arenosa and gastropods of the genus Cylichna sp., which usually feed on P. arctica.

Benthic foraminifers and, in particular, ostracods are generally sparse in the core, but there are two intervals that are particularly poor in microfossils: 4–4.5 and 2.1–3 cal. ka BP (green shading in Fig. 5). Two ecological groups are dominant throughout the core section (Fig. 5): so-called “river-proximal” benthic foraminifera [33], represented by the dominant species Haynesina orbiculare in combination with the species Elphidium bartletti, E. incertum, Elphidiella groenlandica, Buccella frigida, Polymorphina spp., as well as euryhaline ostracod species Paracyprideis pseudopunсtillata (dominant) and Heterocyprideis sorbyana (Fig. 5). These species prefer the same habitat conditions as P. arctica and are found in surface sediments of the shallow areas of Eurasian arctic shelf seas, experiencing freshening by river runoff [32–37]. The same environmental conditions are favorable for both Elphidium clavatum, an opportunistic benthic foraminiferal species which occurs in various parts of the Arctic seas, but reaches maximum abundance in coastal areas [33], and the brackish-water ostracod species Cytheromorpha macchesneyi [36, 37]. Species that prefer distal offshore areas away from the coast are also present in the core, but are more abundant in its upper part. These are the so-called “river-intermediate” benthic foraminifers [33], mainly, Cassidulina reniforme together with Elphidium subarcticum and milliolids (Pyrgo williamsoni, Quinqueloculina sp.), as well as relatively deep-water ostracod species (Rabilimis mirabilis, Cytheropteron spp., Polycope sp.). There are also ostracods that prefer mid-shelf marine environments, such as Cluthia cluthae, Roundstonia globulifera, Semicytherura complanata, Cytheropteron elaeni, C. suzdalskyi. It is interesting that single shells of benthic foraminifers of the species Nonion labradoricum are found only in older sediments units (Fig. 5). This species is associated with zones of high seasonal productivity near sea-ice margin [33], suggesting less severe ice conditions during the time period prior to 4.5 cal. ka BP.

Paleogeographical reconstructions. Based on existing knowledge about sea-level changes [25] based on bottom topography models, analysis of the sediment thickness of the upper seismostratigraphic sequence, and the age model of 20BCM-8 sediment core, paleogeographical models were constructed to illustrate the development of marine transgression in the studied shelf area (Fig. 8).

Fig. 8.
figure 8

Paleogeographical reconstructions of the Indigirka paleovalley area for various time slices (from 11 to 6.5 ka BP): (a) subaerial conditions (1, core site 20BCM-8; 2, isolines of the buried subaerial topography); (b) onset of basin sedimentation in the paleo-bay; (c) gradual separation of Zhokhov Island from mainland; (d) formation of New Siberian Islands (dates show latest terrigenous faunal and archaeological finds on islands) [8, 9, 38]; (e) degradation of island territories and formation of local paleo-straits around Novaya Sibir’ Island.

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Overall, the reconstructions agree well with the geoarchaeological research results. At the first stage of the marine transgression, the rate of sea-level rise was high, both according to the study of the Laptev Sea sediment cores [25] and coastal sections on islands. The radiocarbon date of a mammoth tusk found on Bennett Island (12 590 ± 60 year BP (LU-2096)), located 130 km northeast of Zhokhov Island, shows that the territory between these islands was flooded in a relatively short time period of about 2000 years [38]. About 11 ka BP, almost the entire study area was exposed; the paleo-Indigirka flowed into the sea in the area of the modern -50-m isobath. However, already ca. 10 ka BP, as a result of marine ingression, a bay protruding deeply into the land formed, and coastal-marine sedimentation started at the site of core 20BCM-8. By 9 ka BP, the bay expanded significantly; however, based on analysis of acoustic-seismic profiles, sedimentation was localized within the paleovalley, and its slopes were the sources of sediment material, comprising Upper Pleistocene deposits that undergone a subaerial stage.

The results of detailed geoarchaeological studies of Zhokhov Island convincingly prove that before the period of about 7.8 ka BP, the island was connected to the mainland. Numerous bones of terrestrial fauna were identified and dated here, and human activity at the Zhokhov site was studied in detail, falling within the time interval of 8.0–7.9 ka BP [39]. Analysis of an ASP profile made in 2020 east of Zhokhov Island suggests that the island was separated from the mainland 9–7 ka BP, and during the formation of a strait south of the island, partial erosion of previously formed deposits took place.

The paleoreconstructions also confirm the time of isolation of Vilkitsky Island, located 45 km southwest of Zhokhov island. Based on the dating of a horse humerus bone from Vilkitsky Island, it can be suggested that the island remained connected to the mainland 7900 ± 40 yrs BP (Beta-191 338) [38], whereas by 7 ka BP, the island was already separated by a strait. By this time, a shallow strait had formed between the present-day Novaya Sibir’ Island and the mainland coast, but the land area was even much larger than today.

By 6.5 ka BP, stable marine sedimentation conditions were established throughout almost the entire study area. The evolution of the sedimentation conditions at this stage consisted of gradual distancing from the provenance areas and an increase in salinity in the basin. The evolutionary history of the western part of the inner East Siberian shelf over the past 6.2 cal. kyrs BP, as follows from the records of core 20BCM-8 from the Indigirka paleovalley, is primarily characterized by the gradual inland retreat of the coastline. At the modern like sea-level position [25], this retreat was caused by active abrasion of shores consisting of yedoma. The distance of the coast from the location of the core is evident from the decrease in sedimentation rates, as well as from the gradual increase in the abundance of river-intermediate species among benthic foraminifers and marine relatively deep-water species among ostracods. However, the dominance of shallow-water microfossil species characteristic of the inner shelf of the Arctic seas strongly affected by river runoff indicates modern like environments. At the same time, a slight increase in IRD, combined with the composition of microfossils, suggests the existence of slightly shallower environments with less severe sea-ice cover prior to 3.5–4.5 cal. ka BP, after which slightly more marine environments were established under cooler climate conditions.

According to the study of the New Siberian Islands [8, 38], a number of authors put forward hypotheses about recurring transgressive–regressive cycles in the Mid–Late Holocene. Based on the study of lagoonal sediments on Zhokhov Island, it was suggested that there were periods of cooling with a maximum around 8.5 ka BP, which followed the Early Holocene warming, and subsequent warming around 8 ka BP [38]. The maximum level of transgression corresponds to 7.8–7.4 ka BP, after which the lagoons of Zhokhov Island again became separated from the sea until 6.5 ka BP [38]. According to the study and dating of terraces of Novaya Sibir’ Island [8], it was suggested that at the last stage of paleogeographical development, the height of the sea-level rise relative to the modern one was 6–8 and 4–6 m at 4.9–4 and 1.2 ka BP, respectively.

According to our research, as well as the earlier study of the Laptev Sea sediment cores [25], these oscillations could not be established. In other regions (e.g., the Baltic Sea [40]), relative decreases in sea level, even of small amplitude, are recorded in sediment cores by the presence of erosion layers, or alternating regressive and transgressive trends in the changes in grain size distribution. In this case, we found no such evidence. It is possible, however, that this is due to the low sedimentation rates in the East Siberian Sea during the Holocene.

CONCLUSIONS

According to geological and geophysical studies of the coastal shelf areas of the East Siberian Sea, it was established that the thickness of sediments formed at the last stage of its paleogeographical evolution during the Post-Sartanian marine transgression within the Indigirka paleovalley reaches 5–7 m.

On the slopess of the paleovalley, according to acoustic-seismic data, sediments that passed the subaerial stage during the Sartanian regression are exposed. Marine sediment cores record a transgressive trend with gradual distancing from provenance areas and an increase in paleosalinity.

The dominance of shallow-water microfossil species characteristic of the river-affected Arctic inner shelf areas indicates modern like conditions over the last 6.2 cal. kyrs. At the same time, a slight upward increase in the amount of IRD in the core from the Indigirka paleovalley, combined with an increase in the share of normal marine microfossil species, suggests the existence of slightly shallower environments with less severe sea-ice cover prior to 3.5–4.5 cal. ka BP, followed by establishment by more marine environments under cooler climate conditions.

Sedimentation rates varied from 51 cm/kyr during the period 6.2–5.5 cal. ka BP to 13 cm/ka in the last approximately 1.8 cal. kyrs.