Polyakova: Holocene Diatom Stratigraphy and Paleoceanography
629
productivity and spreading of warm-water diatom species. This was probably related to global
changes in oceanic circulation patterns which expressed themselves in both North Atlantic and
North Pacific.
Surface temperature rise was marked in many regions of the World Ocean. but the age of this
event is different. In the northern regions of the Atlantic Ocean it occurred 7-6 Ka. in the Red
Sea - about 8 Ka (Barash et aI., 1989), in Subantarctic region - about 9 Ka (Hays, 1978).
Production of the benthic foraminiferal assemblages of the Norwegian and south-western
Barents Sea increased by an order of magnitude during the Atlantic and Subboreal periods of
Holocene (Matishov and Pavlova, 1990).
Sometimes it is rather difficult to determine the influence of transgressions upon the
paleogeographical conditions on the arctic coasts. History of the Holocene vegetation of the
Chukchi Sea coast is one of the most striking examples of such influence. Now nearly all
lowlands and highlands of Northern Chukotka are covered with northern hypoarctic tundra and
only a narrow coastal region between Shelagin cape and the Amguema River mouth (the belt of
dense fogs) is occupied by arctic tundra itself (Yurtsev, 1974).
"Holocene climatic optimum" at the arctic coast of Chukotka corresponds to the most
favorable climatic conditions for terrestrial vegetation. This was a period of intensive
accumulation of peat with shrub and tree remnants (\.5-2 and even 3-m thick). This remnants
have several radiocarbon datings (Ivanov et a!., 1984). Abundant paleobotanic and radiocarbon
data show that favorable conditions for intensive peat accumulation existed at the arctic coast of
Chukotka 11.5 Ka. Northward expansion of shrub and tree vegetation occurred 10-8 Ka.
During the subsequent period, which is restricted to the time interval of 7-5 Ka. shrub and tree
vegetation degraded, and modern vegetation of hypoarctic tundra occupied coastal lowlands.
The established history of the Holocene transgression of the Chukchi Sea shows that during
the period of peat accumulation and expansion of tree and shrub vegetation the coastline stayed
tens kilometers northward from its present position. Severe ice conditions of the sea in the early
Holocene epoch allowed for existence of continental climatic conditions on the northern
Chukotka coast. Growth of summer temperature at the beginning of Holocene caused
northward migration of shrub-tree vegetation, as it has been previously shown for the North
Western Siberia (Danilov and Polyakova, 1986).
At the same time "marine optimum" in the Chukchi Sea lasted since 5.5-6 to 3-2.5 Ka. By
this time the Chukchi sea level reached its present position or was even several meters higher.
Intensity of the Bering Sea current was greater than now. Surface water temperature exceeded
the modern one by 2-4- C. This was a period of degradation of shrub and tree vegetation at the
northern coastal lowlands resulting from southward shift of coastline and subsequent increase
of marine features of climate. It was caused by frequent fogs that "ate coastal shrub and tree
vegetation" on the arctic coasts (Yurtsev; 1974). Warmer sea and reduced ice cover favored
appearance of dense fogs.
So, "climatic optimum" of Holocene at the arctic coast of Chukotka was represented by
climatic conditions favoring vegetation growth. In the Chukchi Sea it coincided with surface
temperature rise and activated biological processes ("hydrobiological or marine optimum").
These two phenomena were not simultaneous.
At the same time in the Western Arctic these paleogeographical events correlate with each
other in time. At the northwestern edge of the Russian Plain in vicinity of the White Sea,
maximum Holocene atmospheric humidity and average annual temperature rise were related to
predominance of spruce in the forests during the first half of the Subboreal epoch (Devyatova,
1986; and others). In Karelia these events occurred at the end of the Atlantic - about 5.5 Ka
(Klimanov, 1989). Study of the Barents Sea bottom sediments revealed that optimal
hydrobiological conditions existed since 5.7-3.7 Ka (the end of the Atlantic· the first half of
Subboreal time). So, optimum conditions on land and in the Barents Sea were simultaneous.
629
productivity and spreading of warm-water diatom species. This was probably related to global
changes in oceanic circulation patterns which expressed themselves in both North Atlantic and
North Pacific.
Surface temperature rise was marked in many regions of the World Ocean. but the age of this
event is different. In the northern regions of the Atlantic Ocean it occurred 7-6 Ka. in the Red
Sea - about 8 Ka (Barash et aI., 1989), in Subantarctic region - about 9 Ka (Hays, 1978).
Production of the benthic foraminiferal assemblages of the Norwegian and south-western
Barents Sea increased by an order of magnitude during the Atlantic and Subboreal periods of
Holocene (Matishov and Pavlova, 1990).
Sometimes it is rather difficult to determine the influence of transgressions upon the
paleogeographical conditions on the arctic coasts. History of the Holocene vegetation of the
Chukchi Sea coast is one of the most striking examples of such influence. Now nearly all
lowlands and highlands of Northern Chukotka are covered with northern hypoarctic tundra and
only a narrow coastal region between Shelagin cape and the Amguema River mouth (the belt of
dense fogs) is occupied by arctic tundra itself (Yurtsev, 1974).
"Holocene climatic optimum" at the arctic coast of Chukotka corresponds to the most
favorable climatic conditions for terrestrial vegetation. This was a period of intensive
accumulation of peat with shrub and tree remnants (\.5-2 and even 3-m thick). This remnants
have several radiocarbon datings (Ivanov et a!., 1984). Abundant paleobotanic and radiocarbon
data show that favorable conditions for intensive peat accumulation existed at the arctic coast of
Chukotka 11.5 Ka. Northward expansion of shrub and tree vegetation occurred 10-8 Ka.
During the subsequent period, which is restricted to the time interval of 7-5 Ka. shrub and tree
vegetation degraded, and modern vegetation of hypoarctic tundra occupied coastal lowlands.
The established history of the Holocene transgression of the Chukchi Sea shows that during
the period of peat accumulation and expansion of tree and shrub vegetation the coastline stayed
tens kilometers northward from its present position. Severe ice conditions of the sea in the early
Holocene epoch allowed for existence of continental climatic conditions on the northern
Chukotka coast. Growth of summer temperature at the beginning of Holocene caused
northward migration of shrub-tree vegetation, as it has been previously shown for the North
Western Siberia (Danilov and Polyakova, 1986).
At the same time "marine optimum" in the Chukchi Sea lasted since 5.5-6 to 3-2.5 Ka. By
this time the Chukchi sea level reached its present position or was even several meters higher.
Intensity of the Bering Sea current was greater than now. Surface water temperature exceeded
the modern one by 2-4- C. This was a period of degradation of shrub and tree vegetation at the
northern coastal lowlands resulting from southward shift of coastline and subsequent increase
of marine features of climate. It was caused by frequent fogs that "ate coastal shrub and tree
vegetation" on the arctic coasts (Yurtsev; 1974). Warmer sea and reduced ice cover favored
appearance of dense fogs.
So, "climatic optimum" of Holocene at the arctic coast of Chukotka was represented by
climatic conditions favoring vegetation growth. In the Chukchi Sea it coincided with surface
temperature rise and activated biological processes ("hydrobiological or marine optimum").
These two phenomena were not simultaneous.
At the same time in the Western Arctic these paleogeographical events correlate with each
other in time. At the northwestern edge of the Russian Plain in vicinity of the White Sea,
maximum Holocene atmospheric humidity and average annual temperature rise were related to
predominance of spruce in the forests during the first half of the Subboreal epoch (Devyatova,
1986; and others). In Karelia these events occurred at the end of the Atlantic - about 5.5 Ka
(Klimanov, 1989). Study of the Barents Sea bottom sediments revealed that optimal
hydrobiological conditions existed since 5.7-3.7 Ka (the end of the Atlantic· the first half of
Subboreal time). So, optimum conditions on land and in the Barents Sea were simultaneous.