The sediments contained 181 diatom species and an intraspecific taxon in 64 diatom genera. The following genera included the most species: Pinnularia (18), Navicula (14), Chaetoceros (10), Diploneis (10), Thalassiosira (9), Aulacoseira (7), Eunotia (6), Nitzschia (6), Coscinodiscus (5), and Gomphonema (5). There were 88 marine, 19 brackish water, 57 freshwater, and 13 extinct diatoms along with three others of indeterminate ecology (Table 2).
Arctic Ocean
In the sediments of the Arctic Ocean (stations 77-25–77-30; 83-11, 83-12; depth 977–2,570 m), the diatom concentrations were very low (1–17,000 valves/g) except at station 77-25 (depth 297.7 m) where the diatom content was 1.609 × 106 valves/g (Fig. 2).
In the sediments of several stations (77-26–77-30; depth 133–2,569.8 m), the diatoms appeared as single valves or their fragments. One to seven species of different ecological affiliations were identified including the marine planktonic Coscinodiscus asteromphalus, Rhizosolenia hebetata, Chaetoceros diadema, and C. mitra, the brackish water planktonic-benthic Thalassiosira baltica, T. hyperborea, Melosira arctica, and Paralia sulcata, the freshwater planktonic Aulacoseira granulata, and the extinct marine Neogene Eupyxidicula zabelinae.
In the sediments of station 83-11 (depth 976.6 m) and station 83-12 (depth 1,268.7 m), the diatom assemblages had low species richness (9–10) but abundant marine species (80–94.2%) (Fig. 3).
The benthic-planktonic (tychopelagic) neritic species Paralia sulcata and especially the variety P. sulcata var. biseriata predominated (46–59%) (Fig. 4a). By contrast, the planktonic arctic-boreal oceanic species Actinocyclus curvatulus subdominated (16–32%) (Fig. 4b).
The assemblages also included the brackish water species Melosira arctica (2–18%), Thalassiosira hyperborea (2–10%), Navicula peregrina (6%), and the stenohaline eurythermal species Coscinodiscus asteromphalus (Table 2). Redeposited fragments of extinct Cretaceous-Paleogene Hemiaulus frigidus, Hemiaulus spp., and Eupyxidicula spp. were sourced in deposits from this age and were common at the bottom of the Arctic Ocean, the Arctic Seas, and their adjacent land (Strelnikova 1974, 1992; Tapia and Harwood 2002; Kim and Glezer 2007; Obrezkova et al. 2019).
In the sediments at station 77-25 (depth 297.7 m) in the northern East Siberian Sea, we detected a diatom assemblage consisting of 23 species. It was dominated by the marine bipolar species Thalassiosira antarctica (37.4%) mainly in spore form and several cryophiles (Fossulaphycus arcticus, Fragilariopsis cylindrus, F. oceanica, and F. reginae-jahniae) (33.2%) inhabiting the lower surfaces and ice edges and destroying the ice cover (Quillfeldt 1997; Quillfeldt et al. 2003; Polyakova 1997). Bacterosira bathyomphala (9.6%) and Chaetoceros sp. (8%) constituted a noticeable proportion of the total. Brackish water species such as Thalassiosira hyperborea, Melosira arctica, M. lineata, and M. moniliformis var. octagona were recorded only in small numbers.
The diatom assemblages in the deep sea sediments of the open Arctic Ocean comprise oceanic, marine, brackish water planktonic, planktonic-benthic, benthic, and freshwater species. Most of these are common in the sediments of the coastal areas of the Laptev and East Siberian Seas. This composition reflects transport by the Transpolar Drift Current which carries fresh water, ice, and the diatoms they bear from the coastal regions of the seas of Eastern Siberia. It also reflects the influence of the warm North Atlantic Current passing through this region. Dominance of the siliceous marine planktonic-benthic species Paralia sulcata typical of freshened coastal waters was probably the result of the transport of terrigenous material, perennial ice, and shelf diatoms from the continental shelf to the open sea (Wang and Wang 2008). Lateral transport by wind action may also contribute to the significant abundance of neritic Paralia sulcata in the deep-water sediments of the open sea (Witon et al. 2006). The high relative abundance of Paralia sulcata in the Transpolar Drift Current and selective silica dissolution might be explained by the deficiency of dissolved silicic acid in the water column and sediment pore water (Polyakova 1997; März et al. 2015; Polyakova et al. 2019).
Laptev Sea
The maximum diatom concentration (≤ 2.4 × 106 valves/g) was detected northeast of the river delta. The Lena River (stations 83-24 and 83-25) presented with far higher valve densities than those previously recorded for this area (Cremer 1998, 1999; Matul et al. 2007; Obrezkova et al. 2014) (Fig. 1). At all stations in the zone of influence of the Lena River runoff, the sediment diatom concentrations were in the range of 0.731–0.989 × 106 valves/g. To the north of the river delta, the diatom concentration decreased. Its lowest range was 0.028–0.043 × 106 valves/g in the sediments of the continental slope at depths of 2,157.5–2,447.4 m (stations 83-8 and 83-9). In the sediments of Yana Bay (stations 83-31–83-34), the diatom concentrations were in the range of 0.301–0.688 × 106 valves/g. The lowest diatom concentration (0.027 × 106 valves/g) was recorded at station 83-4 in the southeastern part of the Laptev Sea near the entrance to the Dm. Laptev Strait. For the sediments obtained in this area in 1999, the diatom concentrations were only in the low range of 0.003–0.181 × 106 valves/g (Tsoy 2001). In other areas of the Laptev Sea, the diatom concentrations in the surface sediments corresponds to those previously reported (Cremer 1999).
The sediments of the Laptev Sea were dominated by brackish water planktonic diatom species (32–99%). However, there was also a noticeable admixture of freshwater species in the zone of influence of the Lena River runoff (Fig. 3).
Sediments from the continental slope (station 83-8, depth 2,447.4 m; station 83-9, depth 2,157.5 m) were characterized by low diatom concentrations (0.028–0.043 × 106 valves/g) and poor species composition (11–12). The marine Paralia sulcata and especially P. sulcata var. biseriata predominated (42%) (Fig. 4a). The planktonic oceanic arctoboreal species Actinocyclus curvatulus (20%) and Chaetoceros spp. (16%) subdominated (Fig. 4b). The single freshwater species Epithemia turgida and Pinnularia spp. and fragments of the extinct Cretaceous-Paleogene species Eupyxidicula spp. and Hemiaulus sp. were also detected. This assemblage compositionally resembled those described for the sediments of the Arctic Ocean.
In the sediments of the outer shelf of the Laptev Sea (station 83-7, depth 61.2 m; stations 83-14–83-17, depth 38.2–48.5 m), the ice-neritic species Melosira arctica predominated (22–58.7%) while the subdominants were Paralia sulcata (8–58.5%) and Thalassiosira hyperborea (9–21%). The Chaetoceros spp. content was high (2.3–21%). Coscinodiscus asteromphalus, Thalassiosira antarctica, and Navicula peregrina were also observed, but their numbers were not significant.
The diatom assemblages in the sediments of the southeastern part of the sea near the Lena and Yana River Deltas (stations 83-4-6, 83-22–83-25, and 83-28–83-34) were dominated by the brackish water planktonic species Thalassiosira hyperborea and especially T. hyperborea var. pelagica (41–75%) (Fig. 5a), Melosira arctica (13.3–38%), Chaetoceros species (≤ 12%), and single specimens of 49 freshwater species (Table 2).
A significant abundance of freshwater species (10.7–25.3%) was recorded for the sediments of the zone of direct influence of the Lena River runoff (stations 83-25 and 83-28). Diatom assemblages dominated by freshwater species are typical of the shelf sediments near the mouths of the rivers in the Laptev Sea (Tsoy 2001) and other Arctic seas (Abelmann 1992; Polyakova 1997; Polyakova et al. 2000). Thalassiosira hyperborea is relatively more common in areas affected by the large rivers of Siberia and Canada than it is in the open parts of the Arctic Ocean (Hasle and Lange 1989). Thalassiosira hyperborea abounds in waters with salinity = 2–30 psu (Cremer 1999). It actively vegetates during springtime sea ice melting (Syversten 1990) and predominated in both the ice communities and the subglacial water layers of the study area (Ilyash and Zhitina 2009). Melosira arctica is a brackish ice-neritic species characterized by ecological preferences and distributions resembling those of Thalassiosira hyperborea (Hasle and Lange 1989). Nevertheless, it forms massive aggregations on the lower surfaces of Arctic drift ice in the central part of the Arctic Basin (Melnikov and Bondarchuk 1987).
The sediments in the study region were characterized by a predominance of Thalassiosira hyperborea (Tsoy 2001). In certain samples, however, Thalassiosira baltica was both the dominant and subdominant species, and cryophiles were observed in small numbers. In the present study, though, cryophiles were not detected in the sediments, and only individual Thalassiosira baltica were found.
The sediments of the study region presented with single fragments of the extinct Paleogene species Pyxilla gracilis and P. oligocaenica var. tenuis. Redeposited Cenozoic species were previously recorded for the study area (Polyakova 1997; Tsoy 2001) and were associated with the erosion of Cenozoic deposits on the adjacent land and shelf.
The diatom assemblages in the sediments of the underwater valley of the western Lena (stations 83-19–83-21, depth 28.9–30.8 m) were characterized by a predominance of Thalassiosira hyperborea (24.5–33.5%) and T. antarctica (22–32%), a subdominance of Melosira arctica (13–18%), and noticeable amounts of the cryophiles Fossulaphycus arcticus (0.8–9%) and Chaetoceros spp. (3–6.4%). The single benthic species Pseudogomphonema kamtchaticum, Diploneis elliptica, D. didyma, D. smithii, and D. subcincta were also detected. The freshwater species Aulacoseira subarctica, Stauroneis phoenicenteron, Gomphonema affine, and others constituted 2.5–5.5% of the total.
The maximum numbers of total species (25–38) and freshwater species were noted for the samples from the zone of influence of the Lena River runoff (stations 83-21, 83-22, 83-25, and 83-28). The minimum number of species (3–8) was determined mainly for the samples from the coastal areas of the Laptev Sea (stations Lv83-31 and Lv83-32).
East Siberian Sea
The diatom concentrations were low in the sediments of the ESS (average 0.094 × 106 valves/g). The lowest concentrations (0.004–0.066 × 106 valves/g) were noted in the western part of the sea north of the mouth of the Indigirka River. In the eastern part of the sea, the diatom concentrations gradually increased and reached a maximum of 0.748–1.02 × 106 valves/g in the Long Strait. The diatom concentrations increased in the sediments from east to west (Polyakova 1997; Obrezkova et al. 2014; Tsoy and Obrezkova 2017). Similar distribution patterns were observed for phytoplankton primary production according to chlorophyll levels measured by satellite (Romankevich and Vetrov 2001). Elevated diatom content in the sediments of the eastern part of the ESS was confirmed from the high Cd content in these sediments (Sattarova et al. 2021). Cd accumulates in diatoms transported by the highly productive waters of the Bering Sea. The ecological structures of the diatom assemblages in the sediments of the ESS varied widely from west to east. Brackish-water diatoms predominated in the western part (52–100%) while marine diatoms predominated in the eastern part (66.4–95.7%).
In the sediments of the western part of the sea (station 83-10, depth 77.2 m), Paralia sulcata predominated (38.8%), Melosira arctica subdominated (15.3%), and the marine planktonic species Thalassiosira antarctica (8%) and Chaetoceros spp. (12%) and the brackish-water species Thalassiosira hyperborea (8%) and Navicula peregrina (6.5%) were abundant. Single extinct Pyxilla gracilis and Eupyxidicula spp. and freshwater Stauroneis gracilis were also detected.
In the sediments of the zone of influence of the Indigirka River runoff (station 83-2, depth 15 m; station 83-3, depth 14 m), the diatom assemblages were dominated by Melosira arctica (59–94%), Thalassiosira hyperborean and especially T. hyperborea var. pelagica (2–16%), and the brackish, benthic Navicula peregrina (4%). The freshwater Pinnularia brevicostata, P. major, P. stomatophora, P. borealis, P. neorabenhorstii, and others constituted 5% of the total assemblage.
The sediments of the ESS north of the Indigirka River mouths were dominated by the brackish-water planktonic species Thalassiosira hyperborea (18–78%) (stations 77-36–77-40, 77-42, 77-45, 83-35, and 83-36) (Astakhov et al. 2022) and Melosira arctica (36–94%) (stations 83-2, 83-3, and 83-37–83-39) and subdominated by the benthic species Navicula peregrina (10.5–57.3%) (Fig. 5b) as well as the marine planktonic Chaetoceros spp. (12–69.3%) (Fig. 6b) occurring mainly as resting spores. The latter predominates in the plankton to the north of the Indigirka River region (Sukhanova et al. 2021) and in those of all Arctic seas in general (Gogorev and Samsonov 2016). The abundances of the cryophiles Fossulaphycus arcticus, Fragilariopsis oceanica, F. cylindrus, and F. reginae-jahniae were high (7.3–13.0%) (stations 77-34, 77-36, and 77-40) (Fig. 6a) as were those of Paralia sulcata (stations 77-45 and 77-36).
Brackish-water and marine benthic Diploneis smithii, D. littoralis var. clathrata, Entomoneis kjelmanii, Navicula directa, Nitzschia hybrida, and others were sporadically found. The ice species Craspedopleura kryophila is endemic to the marine Arctic (Poulin 1993) and was detected in the sediments of the ESS. The freshwater Cyclotella meneghiana, Cymbella arctica, Aulacoseira subarctica, and Tryblionella hungarica and the extinct Eupyxidicula zabelinaei were also seen. Moreover, T. hyperborea was nearly monodominant in this area (Polyakova 1997). Chaetoceros diadema (Fig. 5c), C. mitra (Fig. 5d), and other indeterminate spores of this genus (≤ 67% of the total) increased in abundance with distance from the coast.
In the eastern (stations 77-10–77-20) and northern (stations 77-21-77-23, 77-31, 77-77-33) parts of the ESS, the cold-water neritic species Thalassiosira antarctica (6.8–47.3%) (Fig. 4a), Chaetoceros spp. (≤ 44.7%), and various cryophiles (≤ 46.3%) predominated (Fig. 6a). Thalassiosira heperborea, T. nordenskioeldii, Bacterosira bathyomphala, and Paralia sulcata were abundant while Melosira arctica and oceanic Rhizosolenia hebetata were constantly detected albeit in non-significant numbers. The diatom distributions observed here generally corresponded to those previously identified (Polyakova 1997; Obrezkova et al. 2014).
In the sediments at stations 77-24, 77-32, 77-35, 77-41, 77-43, and 77-44, the diatoms were represented by single valves of the marine and brackish-water species Thalassiosira hyperborea, Paralia sulcata, Melosira arctica, and Navicula peregrina which are characteristic of this region. Freshwater Aulacoseira granulata and Eupyxidicula zabelinae extinct in the Neogene were also observed.
Chukchi Sea
The diatom concentrations in the surface sediments of the Chukchi Sea were higher than those in the Laptev Sea, ESS, and Arctic Ocean (average 2.991 × 106 valves/g). The maximum concentration (5.642–7.998 × 106 valves/g) was recorded in the southern part of the sea coinciding with the zone of influence of the highly productive Bering Sea waters. High diatom content is typical of the recent sediments in this region of the Chukchi Sea (Polyakova 1997; Obrezkova 2012; Obrezkova et al. 2014; Astakhov et al. 2015; Tsoy et al. 2017; Obrezkova and Pospelova 2019). In the southwestern part of the Chukchi Sea, there were 1.338–2.72 × 106 valves/g. In the remainder of the Chukchi Sea, however, there were 0.95–3.13 × 106 valves/g. Overall, these data were consistent with those previously published (Polyakova 1997; Obrezkova et al. 2014; Tsoy and Obrezkova 2017; Sattarova et al. 2022; Obrezkova et al. in reviews).
In the sediments of the Chukchi Sea, the diatom assemblages were dominated by marine planktonic and planktonic-benthic neritic species (72.6–96.7%) and significant amounts of oceanic species (≤ 10%). In the western part of the Chukchi Sea (stations 77-8 and 77-9) at the zone of influence of the Siberian coastal current, Thalassiosira antarctica (27.3–32%) and Chaetoceros spp. (11.9–32.4%) predominated. However, Thalassiosira nordenskoeldii (5.6–13.5%) (Fig. 6c) and cryophiles (8.6–12.7%) were also relatively abundant. The sediments of the southernmost part of the Chukchi Sea (stations 77-1 and 83-1) were dominated by the arctoboreal-tropical species Thalassiosira nordenskoeldii (17.3–38.8%) (Shevchenko et al. 2020) which also occurs in large aggregates in the waters and sediments of the western part of the Bering Sea (Semina 1981; Sancetta 1982; Ran et al. 2013) and indicates the presence of Bering Sea waters in the Chukchi Sea. Paralia sulcata (12.7–15.3%), Bacterosira bathyomphala (8.3–12.7%), Chaetoceros spp. (8.3–15.6%), Thalassiosira antarctica (12%), and various cryophiles (4.8–14.7%) were also abundant. The sediments of the zone of influence of the Bering Sea waters (stations 77-2–77-7) were dominated by Chaetoceros species (13–49.8%) and Paralia sulcata (11–23.6%) and subdominated by various cryophiles (2.7–21.8%), Thalassiosira antarctica (7.3–18.3%), Bacterosira bathyomphala (1.8–9.1%), Thalassiosira nordenskioeldii (0–6%), and Pauliella taeniata (~1%). Permanent constituents of the diatom assemblages in the sediments of the Chukchi Sea included Stephanopyxis nipponica, Thalassiosira hyalina, Actinoptychus senarius, Odontella aurita, Thalassionema nitzchioides, and Thalassiothrix longissima.