620
Land-Ocean SYstems in the Siberian Arctic: Dynamics and History
32%), thus reflecting deepening of the sea basin. Abundance of the sea-ice species remains
relatively high (up to 30%). There have been found several relatively warm-water species
(Proboscia alata, Pyxidicula nipponica) uncommon for the arctic water masses.
Diatom abundance, as well as species composition and correlation between different
ecological and biogeographical groups of diatom assemblages of the Ecozone IV are very close
to the mordern ones of this region of the Chukchi Sea, thus indicating similar hydrological and
hydrobiological conditions during accumulation of sediments of this ecozone.
Diatom assemblages of the Ecozone III are characterized by the maximum number of diatom
valves (up to 9,3 million valves/g) and sharp increase of the total taxonomic diversity. The
widely distributed species (Thalassionema nitzschioides, Actinoptychus undulatus and others)
as well as the cold-water (Thalassiosira nordenskioeldii, T.antarctica, T.gravida and others)
and even the sea-ice ones (Nitzschia grunowii, Chaetoceros septentrionalis, Navicula
vanhoeffenii and others) increase in number. However, the percentage of sea-ice species
remains the same. The characteristic feature of the Ecozone III diatom assemblages is
appearance of both: diverse relatively warm-water species brought by the Bering Sea current
(Coscinodiscus asteromphalus, C.perforatus, C. radiatus, Proboscia alata, Neodenticula
seminae and others), and even boreal-tropical (Chaetoceros dydimus). This indicates
considerable northward displacement of planktonic warm-water species caused by
intensification of the Bering Sea current.
The subsequent short-time sedimentation period (Ecozone II, Figure 2) is characterized by
low total diatom number (down to 3,8 million valves/g) and poor taxonomic composition of
diatom assemblages in which relatively warm-water species are eliminated. Extremely coldwater diatom flora and very low number of relatively warm-water species give evidence for
reduction of intensity of the Bering Sea current probably due to the sea level fall. Subsurface
sediments include diatom assemblages similar to the mordern ones reflecting the present
hydrological and sedimentological conditions.
Core 17. Though the total number of diatoms in sediments of this core is even 5 times as
high as that of the above described Core 23, similar trends in variations of the total diatom
abundance and correlations between different ecological and biogeographical diatom groups
have been established along the core sequence (Figure 3), thus reflecting the same
paleoenvironmental changes in the Chukchi Sea during the Holocene. Nevertheless, five
Holocene diatom ecological zones distinguished in this core have several peculiarities
comparable with those of the Core 23. Like in the case with the Core 23, diatom assemblages
from the lowermost core layers (Ecozone V) are characterized by maximum abundance of
sublittoral diatoms (up to 34%). However, sea-ice species are extremely abundant in
assemblages of the Ecozone IV (up to 51 %), thus reflecting more severe sea-ice conditions in
this region. The Ecozone III is the marking one for this core as well, since it is characterized by
the maximum number of diatoms (up to 48,1 million valves/g) and high taxonomical diversity.
Occurrence of warm-water species with maximum total abundance up to 0,5 million valves/g
gives evidence for their northward migration (up to 72( N and even more) during this time. The
most characteristic feature of diatom assemblages of the Ecozone II is sharp decrease of both:
the total diatom number (down to 2,9 million valves/g) and their taxonomic diversity (by 3
times). Against the background of extremely cold-water diatom flora, increase of the sea-ice
species number (up to 74%) indicates severe sea-ice conditions and lowering of surface
temperatures. The Ecozone I in this core is subdivided into two subzones. The lower subzone
(Ib) is characterized by higher total diatom abundance due to the presence of ice-neritic and seaice species (up to 5 I %). Diatom assemblages of the upper subzone are similar to morden ones
of this region thus reflecting similar hydrobiological conditions.
Land-Ocean SYstems in the Siberian Arctic: Dynamics and History
32%), thus reflecting deepening of the sea basin. Abundance of the sea-ice species remains
relatively high (up to 30%). There have been found several relatively warm-water species
(Proboscia alata, Pyxidicula nipponica) uncommon for the arctic water masses.
Diatom abundance, as well as species composition and correlation between different
ecological and biogeographical groups of diatom assemblages of the Ecozone IV are very close
to the mordern ones of this region of the Chukchi Sea, thus indicating similar hydrological and
hydrobiological conditions during accumulation of sediments of this ecozone.
Diatom assemblages of the Ecozone III are characterized by the maximum number of diatom
valves (up to 9,3 million valves/g) and sharp increase of the total taxonomic diversity. The
widely distributed species (Thalassionema nitzschioides, Actinoptychus undulatus and others)
as well as the cold-water (Thalassiosira nordenskioeldii, T.antarctica, T.gravida and others)
and even the sea-ice ones (Nitzschia grunowii, Chaetoceros septentrionalis, Navicula
vanhoeffenii and others) increase in number. However, the percentage of sea-ice species
remains the same. The characteristic feature of the Ecozone III diatom assemblages is
appearance of both: diverse relatively warm-water species brought by the Bering Sea current
(Coscinodiscus asteromphalus, C.perforatus, C. radiatus, Proboscia alata, Neodenticula
seminae and others), and even boreal-tropical (Chaetoceros dydimus). This indicates
considerable northward displacement of planktonic warm-water species caused by
intensification of the Bering Sea current.
The subsequent short-time sedimentation period (Ecozone II, Figure 2) is characterized by
low total diatom number (down to 3,8 million valves/g) and poor taxonomic composition of
diatom assemblages in which relatively warm-water species are eliminated. Extremely coldwater diatom flora and very low number of relatively warm-water species give evidence for
reduction of intensity of the Bering Sea current probably due to the sea level fall. Subsurface
sediments include diatom assemblages similar to the mordern ones reflecting the present
hydrological and sedimentological conditions.
Core 17. Though the total number of diatoms in sediments of this core is even 5 times as
high as that of the above described Core 23, similar trends in variations of the total diatom
abundance and correlations between different ecological and biogeographical diatom groups
have been established along the core sequence (Figure 3), thus reflecting the same
paleoenvironmental changes in the Chukchi Sea during the Holocene. Nevertheless, five
Holocene diatom ecological zones distinguished in this core have several peculiarities
comparable with those of the Core 23. Like in the case with the Core 23, diatom assemblages
from the lowermost core layers (Ecozone V) are characterized by maximum abundance of
sublittoral diatoms (up to 34%). However, sea-ice species are extremely abundant in
assemblages of the Ecozone IV (up to 51 %), thus reflecting more severe sea-ice conditions in
this region. The Ecozone III is the marking one for this core as well, since it is characterized by
the maximum number of diatoms (up to 48,1 million valves/g) and high taxonomical diversity.
Occurrence of warm-water species with maximum total abundance up to 0,5 million valves/g
gives evidence for their northward migration (up to 72( N and even more) during this time. The
most characteristic feature of diatom assemblages of the Ecozone II is sharp decrease of both:
the total diatom number (down to 2,9 million valves/g) and their taxonomic diversity (by 3
times). Against the background of extremely cold-water diatom flora, increase of the sea-ice
species number (up to 74%) indicates severe sea-ice conditions and lowering of surface
temperatures. The Ecozone I in this core is subdivided into two subzones. The lower subzone
(Ib) is characterized by higher total diatom abundance due to the presence of ice-neritic and seaice species (up to 5 I %). Diatom assemblages of the upper subzone are similar to morden ones
of this region thus reflecting similar hydrobiological conditions.