626
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
sediment layers of the Cores 21 and 23 from the eastern part of the Chukchi Sea (Figure 1)
together with diatom assemblages of the Ecozone VI to the time interval ranging from 14 to 12
Ka. These assemblages are characterized by absence of the Bering Sea elements of diatom
flora, relatively high abundance of freshwater and marine sublittoral diatoms corresponding to
coastal shallow marine environments, and predominance of cryophillic and ice-neritic species
indicating more severe than present ice conditions in the Chukchi Sea.
Subsequent sea level rise resulted in opening of the Bering Strait 12 Ka and penetration of the
Bering Sea waters into the Chukchi Sea. This caused sharp changes of the quantitative and
qualitative composition of diatom assemblages (Ecozones V-I). If assuming sedimentation rates
in the Chukchi Sea to be uniform during Holocene, then it is possible to estimate the age of
sediments with diatom assemblages of different ecological zones. The Ecozone III is the
marking one for all studied core sequences. All Holocene ecological zones were established in
the 180 cm long Core 17 obtained at the depth of 40 m. If assuming accumulation of diatom
assemblages to start since the onset of intensive water exchange between the Chukchi and
Bering Seas, i.e. since 12-11 Ka, it is possible to estimate sedimentation rates in this region of
the sea to be 16.8-18.3 cm per 1000 years. The Ecozone III was established in the core interval
110-70 cm, hence the age of its accumulation corresponds to the time interval 6.5-4.2 (or 6.03.8) Ka. The Ecozone II was established in the core interval of 55-70 cm, and the age of its
accumulation corresponds to 4.2-3.3 (or 3.8-3.0) Ka. Similar results were obtained for the
Core 23, where six ecozones corresponding to the end of the Late Pleistocene and Holocene
were established. The age range of the Ecozone II accumulation was estimated at 1.1-2.7 Ka,
and that of the Ecozone III - at 2.7-5.4 Ka. At the same time insignificant thickness of the
subsurface layer (including the Ecozone I) in this core suggests it to be probably eroded. In the
Chukchi Sea the thickness of sediments including diatom assemblages of the Ecozone I usually
exceeds 50 cm, i.e. the calculated age estimations of the ecozones in this core might be slightly
rejuvenated.
Shelf deposits of the East Siberian Sea yield scarce absolute age datings. However, if
assuming sea level changes in the Eurasian Eastern Arctic Seas to be synchronous, and taking
into account the data on eustatic sea level rise during the Holocene transgression (Morner,
1987; Selivanov, 1996) and scarce radiocarbon datings of the East Siberian Sea deposits, it
becomes possible to estimate both: the age of the onset of marine sedimentation in separate
regions of the East Siberian Sea shelf and the age of the established diatom ecozones. In the
Chukchi Sea the pra-Hope deltaic deposits at the depths of 48-50 m yield age datings of 14.6
Ka. The Chukchi Sea level seems to stand below -20 m until 10 Ka (McManus and Creager,
1984; McManus et a!., 1983). The data available for the East Siberian Sea suggest stabilization
of sea level at -12 m to take place about 8.3 Ka (Badyukov and Kaplin, 1979).
The studied bottom sediment cores in the East Siberian Sea are restricted to shallow areas
with the depths ranging from 14 to 27 m. The basal layers of the Cores 319, 321, and 322
situated northward and north-westward of the Indigirka mouth (sea depths of 14-19 m) are
represented by deltaic sediments (according to the freshwater diatom assemblages). However,
at the same depths (15-19 m) in the region between the Kolyma and Indigirka mouths only
marine sediments have been found (Cores 296, 300). The onset of marine sedimentation in the
East Siberian Sea at these depths probably started 8-9 Ka, i.e. the age of marine sequence
should not exceed 8 thousand years. Successive changes in the structure of diatom associations
allow suggesting that there have been no considerable breaks in sedimentation in this region of
the sea. Calculation of sedimentation rates revealed them to be 5.3 cm/Kyr in the Core 296, and
11.3 cmlKyr in the Core 300. On this basis the time span corresponding to accumulation of the
sediments with diatom assemblages of the marking Ecozone III is 5.7-3.8 Ka in the Core 296
and 4.9-3.6 Ka in the Core 300 (in the last case the thickness of the third horizon has been
probably reduced). In the Core 296 the sediments including diatom assemblages corresponding
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
sediment layers of the Cores 21 and 23 from the eastern part of the Chukchi Sea (Figure 1)
together with diatom assemblages of the Ecozone VI to the time interval ranging from 14 to 12
Ka. These assemblages are characterized by absence of the Bering Sea elements of diatom
flora, relatively high abundance of freshwater and marine sublittoral diatoms corresponding to
coastal shallow marine environments, and predominance of cryophillic and ice-neritic species
indicating more severe than present ice conditions in the Chukchi Sea.
Subsequent sea level rise resulted in opening of the Bering Strait 12 Ka and penetration of the
Bering Sea waters into the Chukchi Sea. This caused sharp changes of the quantitative and
qualitative composition of diatom assemblages (Ecozones V-I). If assuming sedimentation rates
in the Chukchi Sea to be uniform during Holocene, then it is possible to estimate the age of
sediments with diatom assemblages of different ecological zones. The Ecozone III is the
marking one for all studied core sequences. All Holocene ecological zones were established in
the 180 cm long Core 17 obtained at the depth of 40 m. If assuming accumulation of diatom
assemblages to start since the onset of intensive water exchange between the Chukchi and
Bering Seas, i.e. since 12-11 Ka, it is possible to estimate sedimentation rates in this region of
the sea to be 16.8-18.3 cm per 1000 years. The Ecozone III was established in the core interval
110-70 cm, hence the age of its accumulation corresponds to the time interval 6.5-4.2 (or 6.03.8) Ka. The Ecozone II was established in the core interval of 55-70 cm, and the age of its
accumulation corresponds to 4.2-3.3 (or 3.8-3.0) Ka. Similar results were obtained for the
Core 23, where six ecozones corresponding to the end of the Late Pleistocene and Holocene
were established. The age range of the Ecozone II accumulation was estimated at 1.1-2.7 Ka,
and that of the Ecozone III - at 2.7-5.4 Ka. At the same time insignificant thickness of the
subsurface layer (including the Ecozone I) in this core suggests it to be probably eroded. In the
Chukchi Sea the thickness of sediments including diatom assemblages of the Ecozone I usually
exceeds 50 cm, i.e. the calculated age estimations of the ecozones in this core might be slightly
rejuvenated.
Shelf deposits of the East Siberian Sea yield scarce absolute age datings. However, if
assuming sea level changes in the Eurasian Eastern Arctic Seas to be synchronous, and taking
into account the data on eustatic sea level rise during the Holocene transgression (Morner,
1987; Selivanov, 1996) and scarce radiocarbon datings of the East Siberian Sea deposits, it
becomes possible to estimate both: the age of the onset of marine sedimentation in separate
regions of the East Siberian Sea shelf and the age of the established diatom ecozones. In the
Chukchi Sea the pra-Hope deltaic deposits at the depths of 48-50 m yield age datings of 14.6
Ka. The Chukchi Sea level seems to stand below -20 m until 10 Ka (McManus and Creager,
1984; McManus et a!., 1983). The data available for the East Siberian Sea suggest stabilization
of sea level at -12 m to take place about 8.3 Ka (Badyukov and Kaplin, 1979).
The studied bottom sediment cores in the East Siberian Sea are restricted to shallow areas
with the depths ranging from 14 to 27 m. The basal layers of the Cores 319, 321, and 322
situated northward and north-westward of the Indigirka mouth (sea depths of 14-19 m) are
represented by deltaic sediments (according to the freshwater diatom assemblages). However,
at the same depths (15-19 m) in the region between the Kolyma and Indigirka mouths only
marine sediments have been found (Cores 296, 300). The onset of marine sedimentation in the
East Siberian Sea at these depths probably started 8-9 Ka, i.e. the age of marine sequence
should not exceed 8 thousand years. Successive changes in the structure of diatom associations
allow suggesting that there have been no considerable breaks in sedimentation in this region of
the sea. Calculation of sedimentation rates revealed them to be 5.3 cm/Kyr in the Core 296, and
11.3 cmlKyr in the Core 300. On this basis the time span corresponding to accumulation of the
sediments with diatom assemblages of the marking Ecozone III is 5.7-3.8 Ka in the Core 296
and 4.9-3.6 Ka in the Core 300 (in the last case the thickness of the third horizon has been
probably reduced). In the Core 296 the sediments including diatom assemblages corresponding