Polyakova: Holocene Diatom Stratigraphy and Paleoceanography
627
to the Ecozone II were accumulated 3.8-2.9 Ka. This is in a good correlation with the data
obtained in the Chukchi Sea.
To the east in the region between the Chauna Bay and the Kolyma mouth (Cores 27, 70, 80)
the sea depths in the region of drilling vary from 14.5 to 22 m. This suggests the onset of
marine sedimentation in this region to start after 9 Ka. However, according to the results of
diatom analysis, the marine Holocene sequence has not been recovered completely even in the
longest cores (about 2 m). To the east of the Chauna Bay the sea depths increase up to 21-27 m
(Cores 298 and 299). So, marine sedimentation in this region started about 10-9 Ka. The
section of the Core 299 displays changes in the structure of diatom assemblages best of all.
Diatom assemblages suggest the basal core layers to be of shallow marine origin (sea depths of
not more than 5 m). On this basis, the calculated sedimentation rates in the Core 299 were about
8 cmlKyr. Hence, the sediments of the Ecozone III were accumulated 5.7-2.5 Ka, and those of
the Ecozone II - 2.5-1.3 Ka.
The radiocarbon datings of the bottom sediments of the Barents Sea and adjacent regions of
the North Atlantic and Norwegian-Greenland basin allow for estimating the age of the
established ecozones. In the North Atlantic transition from the cold Late Pleistocene
environments to the warm Holocene ones was rapid, and the boundary between these two
paleogeographical events was diachronous. In the North Atlantic the epoch of warming started
10.5 Ka, while in the Norwegian-Greenland basin - 9.8-9.3 Ka, i.e. during the PreborealBoreal periods of Holocene (Kellog et aI., 1978; Koc et aI., 1993). The age of this boundary in
the Barents Sea is 8.7-8.0 Ka (Bjorlykke et aI., 1978; Elverhoi and Bomstad, 1980). Warm
waters reached Svalbard between 8.7 and 7.7 Ka (Salvigsen et aI., 1992). Influx of warm
North Atlantic waters into the Barents Sea caused north-eastward shift of the Polar front, thus
sharply increasing the water productivity. We consider the sediments including diatom
assemblages of the Ecozone IV (Borehole 26) to be accumulated since this epoch. The lower
boundary of the Ecozone IV (7.5 m below the sea floor) corresponds to the general increase of
diatom abundance in sediments and considerable changes in the structure of diatom
assemblages. If assuming sedimentation rate to be uniform (88.2 cm/Kyr in the Borehole 26),
then the age of the sediments including diatom assemblages of the Ecozone III (depth range
3.2-4.3 m) will be 5.7-3.7 Ka, i.e. the end of the Atlantic - beginning of the Subboreal periods
of Holocene. Similar results were obtained for the core drilled to east of Kolguev Island
(Okuneva and Stelle, 1986), where the age of the sediments with the most warm-water
foraminiferal assemblages was determined according to radiocarbon datings of 4630( 120 (Ri309) and 5440(130 (Ri-309a) years.
Discussion
Results of the diatom analysis of the bottom sediments of the Eurasian Arctic seas allowed us to
for interprete the paleoceanological variations in this region during Holocene and the end of
Late Pleistocene. In the Eastern Arctic seas the most significant changes of paleoceanological
conditions resulted from the opening of the Bering Strait (12-11 Ka) and influx of relatively
warm high productive Pacific waters into the Chukchi and, partly, the East Siberian Seas. They
caused sharp increase of diatom productivity and introduction of relatively warm-water species
into diatom assemblages. The latter are uncommon for the arctic water masses. Gradual sea
level rise up to its present position during the first half of Holocene (11-6 (5.5) Ka, ecozones V
and IV) resulted in flooding of shallow Eastern Arctic seas and establishment of modern
hydrological (temperature, circulation patterns, distribution of water masses) and
hydrobiological conditions.
The period lasting from the end of Atlantic to the beginning of Subboreal was the most
627
to the Ecozone II were accumulated 3.8-2.9 Ka. This is in a good correlation with the data
obtained in the Chukchi Sea.
To the east in the region between the Chauna Bay and the Kolyma mouth (Cores 27, 70, 80)
the sea depths in the region of drilling vary from 14.5 to 22 m. This suggests the onset of
marine sedimentation in this region to start after 9 Ka. However, according to the results of
diatom analysis, the marine Holocene sequence has not been recovered completely even in the
longest cores (about 2 m). To the east of the Chauna Bay the sea depths increase up to 21-27 m
(Cores 298 and 299). So, marine sedimentation in this region started about 10-9 Ka. The
section of the Core 299 displays changes in the structure of diatom assemblages best of all.
Diatom assemblages suggest the basal core layers to be of shallow marine origin (sea depths of
not more than 5 m). On this basis, the calculated sedimentation rates in the Core 299 were about
8 cmlKyr. Hence, the sediments of the Ecozone III were accumulated 5.7-2.5 Ka, and those of
the Ecozone II - 2.5-1.3 Ka.
The radiocarbon datings of the bottom sediments of the Barents Sea and adjacent regions of
the North Atlantic and Norwegian-Greenland basin allow for estimating the age of the
established ecozones. In the North Atlantic transition from the cold Late Pleistocene
environments to the warm Holocene ones was rapid, and the boundary between these two
paleogeographical events was diachronous. In the North Atlantic the epoch of warming started
10.5 Ka, while in the Norwegian-Greenland basin - 9.8-9.3 Ka, i.e. during the PreborealBoreal periods of Holocene (Kellog et aI., 1978; Koc et aI., 1993). The age of this boundary in
the Barents Sea is 8.7-8.0 Ka (Bjorlykke et aI., 1978; Elverhoi and Bomstad, 1980). Warm
waters reached Svalbard between 8.7 and 7.7 Ka (Salvigsen et aI., 1992). Influx of warm
North Atlantic waters into the Barents Sea caused north-eastward shift of the Polar front, thus
sharply increasing the water productivity. We consider the sediments including diatom
assemblages of the Ecozone IV (Borehole 26) to be accumulated since this epoch. The lower
boundary of the Ecozone IV (7.5 m below the sea floor) corresponds to the general increase of
diatom abundance in sediments and considerable changes in the structure of diatom
assemblages. If assuming sedimentation rate to be uniform (88.2 cm/Kyr in the Borehole 26),
then the age of the sediments including diatom assemblages of the Ecozone III (depth range
3.2-4.3 m) will be 5.7-3.7 Ka, i.e. the end of the Atlantic - beginning of the Subboreal periods
of Holocene. Similar results were obtained for the core drilled to east of Kolguev Island
(Okuneva and Stelle, 1986), where the age of the sediments with the most warm-water
foraminiferal assemblages was determined according to radiocarbon datings of 4630( 120 (Ri309) and 5440(130 (Ri-309a) years.
Discussion
Results of the diatom analysis of the bottom sediments of the Eurasian Arctic seas allowed us to
for interprete the paleoceanological variations in this region during Holocene and the end of
Late Pleistocene. In the Eastern Arctic seas the most significant changes of paleoceanological
conditions resulted from the opening of the Bering Strait (12-11 Ka) and influx of relatively
warm high productive Pacific waters into the Chukchi and, partly, the East Siberian Seas. They
caused sharp increase of diatom productivity and introduction of relatively warm-water species
into diatom assemblages. The latter are uncommon for the arctic water masses. Gradual sea
level rise up to its present position during the first half of Holocene (11-6 (5.5) Ka, ecozones V
and IV) resulted in flooding of shallow Eastern Arctic seas and establishment of modern
hydrological (temperature, circulation patterns, distribution of water masses) and
hydrobiological conditions.
The period lasting from the end of Atlantic to the beginning of Subboreal was the most