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Abstract

In a 1116-cm-long sediment core from the Kamchatka continental slope (sea depth 684 m), the distribution of planktonic foraminifera and the proportions of the main components (terrigenous grains, volcanic ashes, siliceous microfossils (diatoms and radiolarians), planktonic and benthic foraminifera) in the grain-size fraction greater than 0.125 mm were studied. The section covers about 180 ky, from the end of the oxygen iso­ topic stage (OIS 6) up to present. Under the conditions of the penultimate (OIS 6) and the last (OIS 5d-2) con­ tinental glaciations, terrigenous sediments with coarse-grained ice-rafted matter, volcanic ashes, and insignifi­cant contents of microfossils were accumulated. The sedimentation rates were 2.3-5.0 cm/ky. The temperatures of the surface water and their seasonal variability were minimum. The almost complete absence of microfossils suggests low biological productivity caused by the heavy ice conditions and weak mixing of the subsurface waters. Under the conditions of interglacials (OIS 5e and 1), layers of diatomaceous ooze with radiolarians and foraminifers were accumulated. The higher sedimentation rates (12.2 and 20-26 cm/ky, respectively) reflect the high biological productivity. The most warm-water conditions were characteristic of stage OIS 5e. The early stages of both interglacials were characterized by rather low seasonal temperature contrasts, poorly developed stratification, lower biological productivity, and increased concentrations of foraminifera in the deposits. The signs of the last deglaciation (reduction in the terrigenous matter supply and increase in the microfossil con­ tents) are noted after oxygen isotope peak 2.2 (17 850 ky B.P.). About 6.7 ky B.P., with the growth in the sum­ mer temperatures and seasonal contrasts, a distinct vertical water structure was established, the thermocline sunk to greater depths, a dichothermal layer appeared, and the biological productivity increased. At OIS 5e, similar changes took place approximately 121 ky B.P. A certain cooling could probably have occurred 3.4 ky B.P.
Oceanology, Vol. 45, No. 2, 2005, pp. 257-2 68. Translated fro m Okea nologiya, Vol. 45, No. 2, 20 05, pp. 273-285.
Origin al Russian Text Copyright © 2005 by Barash, Chekhovsk aya, Biebow, Nurnberg, Tiedeman.
English Translation Copyright © 2005 b y Pleiades Publishing , Inc.
- .. = MARINE GEOLOGY =
On the Quaternary Paleoceanology of the Southeastern Part
of the Sea of Okhotsk from Lithology
and Planktonic Foraminifera
M. S. Barash1, M. P. Chekhovskaya1, N. Biebow2, D. Niirnberg2, and R. Tiedeman2
1 Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia
2 Leibnitz Institute fo r Marine Research, Kiel, Germany
Received April 14, 2004
Abstract—In a l l 16-cm-long sediment core from the Kamchatka continental slope (sea depth 684 m), the dis
tribution of planktonic foraminifera and the proportions of the main components (terrigenous grains, volcanic
ashes, siliceous microfossils (diatoms and radiolarians), planktonic and benthic foraminifera) in the grain-size
fraction greater than 0.125 mm were studied. The section covers about 180 ky, from the end of the oxygen iso
topic stage (OIS 6) up to present. Under the conditions of the penultimate (OIS 6) and the last (OIS 5d-2) con
tinental glaciations, terrigenous sediments with coarse-grained ice-rafted matter, volcanic ashes, and insignifi
cant contents of microfossils were accumulated. The sedimentation rates were 2.3-5.0 cm/ky. The temperatures
of the surface water and their seasonal variability were minimum. The almost complete absence of microfossils
suggests low biological productivity caused by the heavy ice conditions and weak mixing of the subsurface
waters. Under the conditions of interglacials (OIS 5e and 1), layers of diatomaceous ooze with radiolarians and
foraminifers were accumulated. The higher sedimentation rates (12.2 and 20-26 cm/ky, respectively) reflect the
high biological productivity. The most warm-water conditions were characteristic of stage OIS 5e. The early
stages of both interglacials were characterized by rather low seasonal temperature contrasts, poorly developed
stratification, lower biological productivity, and increased concentrations of foraminifera in the deposits. The
signs of the last deglaciation (reduction in the terrigenous matter supply and increase in the microfossil con
tents) are noted after oxygen isotope peak 2.2 (17 850 ky B.P.). About 6.7 ky B.P., with the growth in the sum
mer temperatures and seasonal contrasts, a distinct vertical water structure was established, the thermocline
sunk to greater depths, a dichothermal layer appeared, and the biological productivity increased. At OIS 5e,
similar changes took place approximately 121 ky B.P. A certain cooling could probably have occurred
3.4 ky B.P.
INTRODUCTION
The Sea of Okhotsk exerts a great effect upon the
climatic in the North Pacific. It was found that the
anomalies of the sea ice area in the Sea of Okhotsk
influence the large-scale circulation of the atmosphere
not only in the Sea of Okhotsk region but also in the
Bering Sea, Alaska, and North America [16]. The ven
tilation of the North Pacific waters at depths of 1000
2000 m is performed by the young oxygen-enriched
Okhotsk water mass formed over the shelves of the Sea
of Okhotsk. In order to learn the interrelations between
the paleogeographic parameters in this region in the
geological past, it is necessary to study the environmen
tal conditions of the Sea of Okhotsk during the global
climatic changes related to the glacial and interglacial
intervals of the Quaternary continental glaciation. The
high sedimentation rates and abundant microfossils in
the deposits of the Sea of Okhotsk provide possibilities
for this kind of research.
The general present-day surface circulation in the
Sea of Okhotsk is represented by a cyclonic gyre con
sisting of the northward West Kamchatka Current and
the southward East Sakhalin Current. The winter tem
peratures of the surface waters are close to the freezing
value. In the summer, the surface waters are heated up
to 10-14°C [4].
A strong interannual variability of the area covered
with sea ice is observed [28]. In selected years, sea ice
with a density of 60% covers almost the entire sea,
while, in other years, only the northern and western
near-shore areas are covered with ice. The Sea of
Okhotsk is one of the most productive regions of the
World Ocean due to the high nutrient concentrations
and the particular features of the vertical structure and
dynamics of the waters [9].
The present-day Sea of Okhotsk is characterized by
the subarctic type of hydrological water structure of the
upper part of the water column. Under a thin (up to
40 m) layer with a high seasonal variability, a low-tem-
perature layer with a temperature close to the freezing
value is located. This is the so-called dichothermal
layer. It is 100-150 m thick. In the spring, summer, and
fall, it represents itself as the cold intermediate layer at
depths of 40-150 m. In the region of the Kuril Islands,
it is characterized by a salinity of 32.9-31.0%© and a
temperature from -1.7 to +1°C [5]. The formation of
257
258 BARASH et al.
this layer is related to the extensive winter formation of
sea ice. It is replenished during the winter convection.
In the cold seasons, the melt waters form a very stable
surface layer that prevents the warm subsurface waters
and the cold waters of the dichothermal layer from mix
ing. Thus, the seasonal changes in the sea ice area play
a key role in the maintenance of this kind of vertical
structure [39].
Lower, down to depths of 600-1000 m [8, 38], the
young Okhotsk intermediate water mass with a temper
ature of about +1.C, a high oxygen content (2.5
6.5 ml/1), and a salinity of approximately 33.7%o is
located [5]. These waters are generated over the north
western shelf. Due to the sea ice formation, cold rela
tively saline near-bottom water is formed; it descends
over the slope down to the intermediate water depths [8,
19]. Precisely this type of water is transformed while
passing via the Kuril straits [3] and provides a signifi
cant contribution to the ventilation processes in the
North Pacific [33]. Below, abyssal water masses are
located.
During the past decades, numerous studies on the
sedimentology, isotopic composition, micropaleontol
ogy, stratigraphy, tephrochronology, and geochemistry
of the sediments have been performed. Based on their
results, age models for the cores from different regions
of the sea were constructed, characteristic lithological
layers were distinguished, and the main features of the
paleoceanological evolution of the Sea of Okhotsk
were recognized. The latter mostly refers to the latest
stages of its geological history—intervals of the penul
timate and the last continental glaciations, the intergla
cial, and the Holocene.
It was stated that both the Holocene and the intergla
cial optimum corresponding to oxygen isotopic stages
(OIS) 1 and 5e are mostly represented by the biogenic
and primarily diatomaceous sediments with a minor
admixture of terrigenous and ash matter. The layers
accumulated during the glaciation epochs correspond
ing to OIS 6 and 5d-5a are represented by terrigenous
sediments including ice-rafted matter with an admix
ture of ashes and biogenic components.
The cores contain marking ash interlayers whose
dating allowed us to formulate the tephrochronology of
the Late Quaternary deposits of the Sea of Okhotsk
[14]. These authors established a sequence of litholog
ical units (LU) from LU 1 to LU 4, which correspond
to the transition from the present-day interglacial con
ditions to the paleoenvironmental conditions of the last
continental glaciation. They are diatomaceous ooze
(LU 1), the transitional biogenic layer (LU 2), the layer
with abundant foraminifera (LU 3), and the terrigenous
layer (LU 4). The peak of CaC 03 in LU 3 (12.4
8.3 ky B.P.) corresponds to the last termination 1A, to
the global Boelling-Allereud warming (12.5 ky B.P),
and to termination IB during the preboreal warming
[7]. The beginning of the extremely active diatom accu
mulation in LU 1 (from 5.5 ky B.P. up to the present),
in the opinion of the authors of [14], coincided with the
warmest interval in the middle of the Holocene.
In order to reconstruct the changes in the regimes of
the accumulation of marine and terrigenous organic
matter over the past 27 ky, we studied the biological
markers in the sediment core from the Sea of Okhotsk
[34]. The profiles of the biomarkers accounting for the
distribution of the coarse-grained matter showed that
the delivery of the terrigenous matter was mostly
caused by floating ice. However, in the interval from
14 ky B.P. up to the present, this mechanism was weak,
probably because of the strong reduction of the ice
cover and shelf erosion. The profiles of the land-origi
nated plant n-alkanes and of the accumulation rates
over the past 30 ky featured maximums during the
deglaciation [30]. In the western and central areas of
the Sea of Okhotsk, we recognized a two-step growth in
these parameters, which was seemingly confined to the
two desalination pulses (Meltwater Pulse Events 1A
(14.5-13.5 ky B.P.) and IB (about 10 ky B.P.)). The
authors concluded that, under the conditions of the glo
bal warming at the deglaciation, the sea level rise
should have caused an increase in the supply of the
land-derived organic matter from the flooded shelf.
The studies of diatoms [21, 31] made it possible to
outline the evolution of the ice conditions in the Sea of
Okhotsk. In the interval 6600-4650 у В.P., the condi
tions of an ice-covered sea surface were replaced by the
conditions of an open sea free from ice. Later on, the
conditions of the open and ice-covered sea surface
alternated; now, ice-free conditions are dominating.
Multidisciplinary micropaleontological studies of
the planktonic and benthic microfossils revealed new
characteristics of the paleoceanological evolution of the
Sea of Okhotsk, in particular, of the vertical structure of
the water column. According to the data on the diatoms,
the planktonic foraminifera, and the proportions of the
principal components of the sandy grain-size fraction
of the sediments from the core obtained on the Instituta
Okeanologii Rise (station LV28-42-5, sea depth
1045 m), both the interglacial optimum (OIS 5e) and
the Holocene (OIS 1) may be subdivided into two inter
vals [2]. During the earlier intervals, the most active
supply of the tests of planktonic foraminifera to the sed
iments took place, and, in the diatom assemblages, oce
anic species were abundant due to the most intensive
water exchange with the Pacific Ocean. The later inter
vals are characterized by a domination of the Okhotsk
diatom species and by a decrease in the concentration
of the tests of planktonic foraminifera; their assem
blages are almost completely composed of the species
Neogloboquadrina pachyderma sin. These conditions
were similar to the present-day conditions with a dicho
thermal hydrological structure and extremely high bio
logical productivity. Based on the above micropaleon
tological evidence, the authors concluded that the for
mation of the dichothermal layer reflects the qualitative
changes in the vertical structure of the upper water
OCEANOLOGY Vol. 45 No. 2 2005
N
ON THE QUATERNARY PALEOCEANOLOGY OF THE SOUTHEASTERN PART 259
Fig. 1. Station location in the Sea of Okhotsk.
layer and the corresponding changes in the general
hydrological and ecological settings.
Similar conclusions on the vertical structure of the
water column were made by Matul and Abelman from
the data of the studies of radiolarians in a sediment core
from the central part of the Sea of Okhotsk [6]. During
glaciations, the dichothermal layer was poorly
expressed. During the last interglacial optimum
(OIS 5e), the production of the intermediate Okhotsk
water mass reduced and, at intermediate depths, it was
replaced by the Pacific water mass. According to micro
fossil studies, the interglacial optimum OIS 5e was the
warmest interval over the past 220 ky [2, 7].
This paper is devoted to the reconstruction of the
paleoceanological evolution of the southeastern part of
the Sea of Okhotsk based on the proportions of the
components of the sandy grain-size fraction of the sed
iments and on the assemblages of planktonic foramin
ifera.
MATERIALS AND METHODS
Bottom sediment core LV28-44-3, which was
obtained within the frameworks of the EIIEO Russian-
German project from the continental slope of Kam
chatka at a point with coordinates 52°02.514' N,
153°05.949' E with a sea depth of 684 m in the zone of
the West Kamchatka Current, was examined (Fig. 1).
The core is 1116 cm long and is represented by an
alternation of biogenic diatomaceous oozes and
silty-clayey terrigenous oozes with an admixture of
diatoms. The following layers were distinguished
(adapted after [11]):
0-145 cmdiatomaceous ooze enriched with fora
minifera;
145-354 cm— clayey-sandy silt, slightly diatoma
ceous, with abundant foraminifera, with interlayers of
volcanic ashes (194-212 cm), foraminiferal sand (231—
235 cm), and coarse-grained ice-rafted matter (253
254 cm);
354^454 cm— sandy silt with coarse-grained mat
ter, lenses of black sand, and an interlayer of white vol
canic ashes (374-377 cm);
454-625 cm— clayey-sandy silt, with coarse
grained matter and lenses of black sand;
625-663 cm—clayey-sandy silt, very diatoma
ceous;
663-837 cm— sandy silt and clayey-sandy silt, sand
layers at 680-685, 800, and 825 cm;
837-932 cmdiatomaceous ooze with interlayers
of foraminiferal sand at 897 and 902 cm;
932-1112 cm— clayey-sandy silt, with coarse
grained ice-rafted matter and lenses and interlayers of
black sand.
OCEANOLOGY Vol. 45 No. 2 2005
260 BARASH et al.
The stratigraphic identification and the age model
for the core were based on a combination of the data on
the lithostratigraphy, biostratigraphy, tephrochronol-
ogy, and the changes in the magnetic susceptibility. The
ages of the boundaries and the events of the global oxy
gen isotopic scale were accepted according to the
scheme of Martinson et al. [23]: boundary OIS 1/2
(12 050 yr)— 260 cm; event 2.2 (17 850 yr)— 352 cm;
boundary OIS 2/3 (2 4 110 yr)— 377 cm; event 3.1
(25 420 yr)—381 cm; event 3.3 (50 210 yr)— 468 cm;
boundary OIS 3/4 (58 960 yr)— 506 cm; boundary
OIS 4/5 (73 910 yr)581 cm; event 5.1 (79 250 yr)—
610 cm; event 5.2 (90 950 yr)—:657 cm; event 5.3
(99380 yr)—690.5 cm; event 5.4 (110 790 yr)—
737 cm; 119 event 5.5 (123 820 yr)— 895.5 cm; bound
ary OIS 5/6 (129 840 yr)— 967 cm.
Thus, the core covers a stratigraphic interval from
the middle of OIS 6 (the penultimate continental glaci
ation) to OIS 1 (the Holocene). The reconstruction of
the actual paleoceanological conditions in this region of
the Sea of Okhotsk expressed in the lithology of the
deposits, in the distribution of the principal components
of the sandy grain-size fraction, and in the assemblages
of planktonic foraminifera shows a rather complicated
evolution of the environmental parameters.
We studied the distribution of planktonic foramin
ifera and the proportions of the main components (ter
rigenous mineral grains, ash particles, tests of plank
tonic and benthic foraminifera, and siliceous microfos
sils—radiolarians and diatoms) in the sandy (greater
than 0.125 mm) fraction of the sediments. The samples
were examined with an interval of 10 cm. We registered
the changes of the above parameters over the core
length, which reflected the paleoceanological changes
in this region of the Sea of Okhotsk. For the sake of the
foraminiferal analysis, the sandy fraction of the sedi
ment was quartered to obtain a sample containing no
less than 300 tests. The rest of the fraction was
reviewed in order to reveal rare species. Samples with
low test contents were analyzed without quartering. In
each of the samples, the species composition of the
assemblages was determined together with the percent
age of the species and the contents of the tests per 1 g
of dry sediment.
RESULTS AND DISCUSSION
Composition o f the Sandy (>0.125 mm)
Fraction o f the Sediments
Layers accumulated under the conditions of conti
nental glaciations (OIS 6, 5d-a, 4-2) always contain
certain amounts of volcanic ash (Fig. 2). The distinct
peaks of the volcanic glass contents and the tephra lay
ers reflect the events of major volcanic eruptions on
Kamchatka and the Kuril Islands. The most clearly
expressed layers with high ash contents in the sandy frac
tion are noted at 180-200 cm (69% ash), 380-390 cm
(78-80% ash), 440 cm (64% ash), 730 cm (60% ash),
800 cm (65% ash), 940 cm (82% ash), and 1090 cm
(70% ash). Some of these layers are described in [14]
as marking layers of tephra, and, on the basis of petro-
graphic data, the sources of eruption are identified. The
age of the peak at 180-200 cm distinguished in core
LV28-44-3 is estimated at 8.2-8.6 ky B.P.; it corre
sponds to the tephra marker layer TR with an age of
about 8.0 ky B.P.; the age of the peak at 380-390 cm is
estimated at 26 ky B.R, it correspond to the tephra layer
K2. We also recognized the weakly expressed layer КЗ
within the OIS 4 zone (550 cm, about 68 ky B.R) and
layer K4 confined to isotopic event 5.4 (109.2 ky B.P.),
which was suggested by the cited scientists. The mark
ing significance of the other tephra layers distinguished
by us remains unclear.
Although the fraction greater than 0.125 mm gener
ally makes up only a small share of the sediments (com
monly not greater than a few per cent), the proportions
between its components clearly reflect the changes in
the environmental conditions caused by climatic
changes. The relations between the components of the
sandy fraction, as well as the lithological description of
the core, help us to recognize four principal layers: two
layers with the domination of terrigenous mineral
grains and volcanic ashes and two layers in which bio
genic components prevail.
The lower terrigenous-volcanic layer (1112-932 cm)
was accumulated during the late stage of the penulti
mate continental glaciation (OIS 6), approximately
within the interval from 174 to 130 ky B.P. The sedi
ment fraction greater than 0.125 mm is mostly com
posed of terrigenous matter of sandy to gravel grain
size; together with the particles of volcanic ash, it com
prises almost 100%. The content of other components
(diatoms and benthic foraminifera) is 0-6% . To a cer
tain extent, the delivery of the main clayey-silty part of
the terrigenous matter may be explained by the eolian
supply, since, during the continental glaciations, the
global climate contrasts increased [1] and the general
atmospheric circulation was enhanced. Over vast land
areas, in particular in East Asia, loess sequences were
accumulated. Corresponding interlayers were also
found in the deposits of the northwestern Pacific [17].
Meanwhile, coarse-grained matter could be trans
ported only by floating ice. Therefore, the ice condition
of the Sea of Okhotsk should have been quite different
from the present-day one. The present-day floating ice,
despite its wide development in the winter [28], almost
does not supply terrigenous matter of sandy grain size
to the sediments. It seems that, under the conditions of
a low sea level, lower water and air temperatures, and
the existence of local glaciers on the adjoining land, the
floating ice and, probably, icebergs were significantly
more intense than at present and they were capable of
transporting coarse-grained terrigenous matter to the
bottom sediments. They might also have carried large
ash particles, which represent the second main compo
nent of the layers under consideration. The almost
OCEANOLOGY Vol. 45 No. 2 2005
ON THE QUATERNARY PALEOCEANOLOGY OF THE SOUTHEASTERN PART 261
40 %
80 0 40 80 0 40 %
80 % % %
0 10 20 0 2 4 6 0 40 80
<D
&0
cd
О
Сн
о
с
0)
QO
X
о
Fig. 2. Proportions of the components of the coarse-grained (larger than 0.125 mm) fractions of the sediments in core LV28-44-3
and the relations between the tests of benthic (BF) and planktonic (PF) foraminifera (BF/BF + PF).
absolute absence of planktonic and benthic foramin
ifera (less than 0.1 %) and the low diatom content (0
5.8%) suggest a low biological productivity, which was
most probably caused by the heavy ice conditions and
weak water mixing in the surface layer. The average
sedimentation rate of this layer is 2.27 cm/ky (Fig. 3).
The layer of diatomaceous ooze at 932-837 cm
almost exactly corresponds to the optimum of the last
interglacial OIS 5e and covers the interval 127-117 ky
B.P. At the peak of this interval (895 cm, event 5.5,
123.8 ky B.P.), the diatom concentration reached its
maximum (96%). Thus, active biogenic sedimentation
in this region started 2-2.5 ky after boundary OIS 6/5.
The sandy fraction of the lower biogenic layer is mostly
composed of the valves of diatoms and radiolarians
(46-96%); the contents of other components vary
within 0.1-10.0%. The concentrations of tests of plank
tonic foraminifera grow from 4% in the lower part of
the layer to 20% in its middle part; the concentrations
of benthic foraminifera reach 5-6%. The proportions of
microfossils reflect the gradual change in the environ
mental conditions— starting from 121 ky B.P., the con
centrations of foraminifera decrease, while the high
diatom productivity is retained. Therefore, the intergla
cial optimum interval may be subdivided into two parts:
the early stage (127-121 ky B.P.) characterized by high
foraminifera contents and the late stage with a predom
inance of siliceous microfossils (121-117 ky B.P.). A
similar situation related to an interglacial optimum
interval was registered in core LV28-42-5, which was
obtained in the central part of the Sea of Okhotsk on the
southeastern slope of the Akademii Nauk Rise (see Fig. 1).
According to the data reported by Kazarina [2], at the
early stage of this interval, oceanic forms of diatoms
dominated, while, at the later stage, the “Okhotsk
forms characteristic of the central water mass with its
dichothermal structure were dominant. Meanwhile, in
core LV28-42-5, the diatom concentrations were signif
icantly lower, which was probably explained by the
lower biological productivity. Despite the reduced ter
rigenous and volcanogenic matter supply in the region
under study during the interglacial optimum, the sedi
mentation rates grew more than fivefold and reached
12.2 cm/ky on the average, mostly due to the enhanced
biological productivity.
OCEANOLOGY Vol. 45 No. 2 2005
262 BARASH et al.
ky B.P.
0 40 80 120 160
cm
Fig* 3. Sedimentation rates in core LV28-44-3. On the right,
units of the oxygen isotope scale are presented. The loca
tions of peaks 2.2, 5.4, and 5.5 are shown.
Up the section (837-260 cm) is located a terrige-
nous-volcanogenic layer, which accumulated under the
conditions of a continental glaciation; in this region, the
glaciations occurred at the late substages OIS 5d-5a and
OIS 4-2. The composition of the sediment fraction
greater than 0.125 mm is close to that in the lower ter-
rigenous-volcanogenic layer. However, within inter
vals 5d-5a, interlayers enriched with valves of diatoms
(up to 22%) and foraminifera (up to 6%), i.e., short
term periods of enhanced productivity, happened here
during the late interglacial phases. The glacial environ
ment conditions proper were established here after oxy
gen isotope peak 5.4 (110790 yr, 737 cm), when the
sedimentation rates dropped down to characteristic val
ues of 5 cm/ky. This is approximately twice as high as
the rate of accumulation of the lower terrigenous-vol-
canogenic layer.
Meanwhile, at the end of the interval, above 354 cm,
i.e., after oxygen isotope event 2.2 (17.850 ky B.P.), the
rate of sedimentation sharply increases up to 20-
26 cm/ky and remains that high up to the present.
According to the lithological description, at 354 cm, the
accumulation of sandy silt with ice-rafted matter and
lenses of black sand was replaced by deposition of
sandy-clayey silts with diatoms and foraminifera with
interlayers of coarse-grained matter. These lithological
features indicate the beginning of deglaciation—bio
logical productivity is enhanced at high rates of accu
mulation of terrigenous matter. Only starting from this
time do the number of tests of planktonic foraminifera
begin to overrate those of benthic foraminifera. How
ever, at the end of the interval considered, even above
the OIS 1/2 boundary, the heavy ice conditions were
still manifested (an interlayer of coarse-grained ice-
rafted matter is located at 253-254 cm).
From the level of 260 cm, which corresponds to the
OIS 1/2 boundary (12.050 ky B.P.), up to 145 cm(about
6.7-7.0 ky B.P.), the terrigenous matter supply gradu
ally decreases from 58 to less than 1%, while the pro
portion of microfossils increase (up to 99%). Precisely
in this interval, the maximum concentrations of the test
of planktonic (up to 23%) and benthic (up to 6%) fora
minifera are observed. This interval seems to corre
spond to lithological unit 3 distinguished by Gor-
barenko and Nürnberg [14], which includes the warm
ing stages in the interval 12.5-9.5 ky B.P. During this
period, the productivity and the Corg accumulation in
the sediments increased.
About 6.7 ky B.P., a sharp change in the type of sed
imentation occurred. From this moment up to the
present (145-0 cm), diatomaceous ooze with radiolari
ans and foraminifera was accumulated. This change
reflects the onset of the conditions of high productivity,
which correspond to the present-day oceanological set
ting. At the time of the interglacial optimum (OIS 5e),
the productivity of the diatoms also increased, though
to a smaller extent.
This sequence of environmental changes may be
interpreted in the following way. With the beginning of
OIS 1, the climatic conditions in the Sea of Okhotsk
became less severe. The reduction of the ice cover
favored a growth in productivity. The relatively small
difference between the winter and summer tempera
tures helped to maintain the weak vertical stratification of
the upper layers of the water column. The profiles of the alk-
enon contents in the cores from the Pechora Sea showed
that, at the beginning of the deglaciation (15 ky B.P.), the
summer surface temperatures were approximately 5°C
lower than their present-day values and subsequently
they increased [35]. Under these conditions, close to
the present-day arctic conditions, characterized by
strong ice cover, low values and weak seasonal varia
tions of the temperature, and desalination of the surface
layer due to the melt water supply, foraminifera and sil
iceous microplankton developed. The production of
diatoms was relatively low. This provided conditions
favorable for the enrichment of the sediments with for
aminiferal tests.
During the climatic optimum o f the Holocene
(about 6.7 ky B.P.), with the further growth of the sum
mer temperatures and decrease in the ice condition
severity, the vertical water structure radically changed
and acquired characteristic features of the subarctic
waters. Rather strong summer heatings alternated with
intensive winter coolings and sea ice formation. Inten-
OCEANOLOGY Vol. 45 No. 2 2005
264 BARASH et al.
mum concentrations of planktonic foraminifera in the
central part of the sea (station LV28-42-5) were
encountered in the initial parts of the intervals [2],
while, in its eastern part (station LV28-44-3), they are
confined to the middle parts of the intervals. In the east
ern part of the sea, the corresponding arctic type of con
ditions (low values and weak seasonal variations of the
water temperature in the surface layers, weak vertical
stratification, and reduced production of diatoms)
existed over a longer period than in its central part. This
is probably related to the influence of the cold West
Kamchatka Current.
The species composition of the planktonic foramin-
ifera is rather poor; it involves only six relatively cold-
water species (see Fig. 4), which have a certain relation
to the climatic conditions [1]. The sequence ordered
from the cold-water to the warm-water species looks
the following way: the subarctic Neogloboquadrina
(N.) pachyderma sin. (Ehrenberg); the boreal Globige-
rina (G.) quinqueloba Natland, G. bulloides dOrbigny,
N. pachyderma dex. (Ehrenberg), and Gt. uvula
(=bradyi) Wiesner; and the subtropical Globorotalia
(Gr.) scitula Brady. In core LV28-42-5 from the central
part of the Sea of Okhotsk, ten species of planktonic
foraminifera were encountered [2], including more
warm-water species. The poorer and more cold-water
composition of the assemblage from core LV28-44-3
may be explained by the influence of the cold West
Kamchatka Current.
The most cold-water N. pachyderma sin. dominates
in the samples over the entire length of the core; com
monly, it comprises 60-90% of the overall fauna of
planktonic foraminifera. Together with the second
dominant species, G. bulloides, it comprises up to 90%
in all the samples, while, together with two other cold-
water forms, G. quinqueloba and N. pachyderma dex.,
it makes up almost 100% in all of the assemblages. Sin
gle specimens of Gt. uvula were encountered in the
warm-water layers referring to the interglacial opti
mum (OIS 5e), to the Boelling-Allerö d warming, and
to the Holocene optimum. The more warm-water spe
cies Gr. scitula was encountered only in layer OIS 5e.
The assemblages of glacial intervals (OIS 6, 5d-2),
regardless of the test contents in the sample, are mostly
represented by N. pachyderma sin. and contain notice
able concentrations of G. bulloides, which, by the ends
of the intervals, reached 30-50%. The abundances of
N. pachyderma dex. and G. quinqueloba are small. We
suggest that this kind of composition of the assem
blages corresponds to low temperatures of the upper
water layers free from significant seasonal variations.
The increase in the concentrations of G. bulloides by
the end of the glacial interval seems to reflect a gradual
temperature growth. Meanwhile, after the short-term
Boelling-Allereud warming, at the beginning of OIS 1
and later, a paradoxical growth in the concentrations of
N. pachyderma sin. up to 90% and more is observed.
This phenomenon might be explained by the Late
Dryassic cooling. However, high concentrations of this
species in the assemblages are observed in the overly
ing layers, as well as along with the growth of the total
content of the tests of planktonic foraminifera in the
sediment. The latter fact points to the continuing warm
ing and to the absence of significant dissolution of tha-
natocoenoses.
The variations in the proportions between N. pachy
derma sin. and G. bulloides evidently reflect the
changes in the dwelling conditions of these species
rather than variations in the solubility of their tests
within the sediments. Among the present-day plank
tonic foraminifera, these species are the most resistant
against dissolution [37] and are common in the surface
sediments of subpolar regions. The paradoxical growth
in the concentrations of N. pachyderma sin. under the
conditions of the Holocene warming may be explained
by the ecological features of this form. According to the
plankton studies and oxygen isotope data, three groups
of species of planktonic foraminifera may be distin
guished with respect to the dominating depths of their
dwelling [10]: shallow-water species dwelling mostly
in the upper 50 m of the water column; medium-depth
species dwelling in the upper 100 m of the water column
but mostly concentrated in the interval of 50-100 m; and
deep-water species dwelling in the upper few hundred
meters, whose mature individuals are mostly located
deeper than 100 m. Among the cold-water species char
acteristic of the Sea of Okhotsk, N. pachyderma is a
deep-water species, G. bulloides is a medium-water
species, and G. quinqueloba is a shallow-water species.
The suggestion that N. pachyderma is a species form
ing its greatest concentrations below the thermocline
was proved by numerous studies in different regions of
the World Ocean [13, 15, 18, 20, 27, 29, 32, 36].
A dependence of the depths of the highest concen
trations and major calcification of the tests of N. pachy
derma sin. on the structure of the upper water layer and
the depth of the thermocline was revealed in the Norwe-
gian-Greenland Basin [32]. Off Norway, where the
supply of Atlantic waters causes a thermal stratification
of the water column, the isotopic characteristics of
N. pachyderma sin. correspond to the water masses
located close to or below the pycnocline at depths of
70-250 m. Meanwhile, the calcification of the tests of
this species occurs closer to the surface (20-50 m) in
the Arctic waters of the western part of the Norwegian-
Greenland Basin.
One can suppose that, in the Sea of Okhotsk, during
glaciations accompanied by the arctic type of vertical
water structure (weakly expressed shallow ther
mocline), N. pachyderma sin. dwelled close to the sur
face and the low temperatures allowed its domination
over the composition of the assemblages. Probably,
during the deglaciation, a desalination of the surface
waters occurred because of the increase in the runoff of
the Amur River, the thawing of glaciers, and the degra
dation of the permafrost. This kind of phenomenon
OCEANOLOGY Vol. 45 No. 2 2005
ON THE QUATERNARY PALEOCEANOLOGY OF THE SOUTHEASTERN PART 265
took place in the Bering Sea. In the Bering Sea, the
ratios between the stable isotopes of carbon and nitro
gen in the organic matter showed the presence of an
anomaly during the last deglaciation. They were
explained by the suppression of the vertical mixing and
by the low nutrient contents in the surface waters due to
the supply of the melt water from the mountain glaciers
of the adjacent land. The seasonal temperature differ
ences were insignificant [26].
The enhancement of the thermal stratification in the
course of warming took place in the Sea of Okhotsk as
well due to the better heating of the surface water layer.
With the growth of the summer heating and the increase
in the thickness of the upper mixed layer, the seasonal
temperature contrasts in it increased. The thermocline
descended to greater depths; correspondingly, the depth
of the maximum of the concentrations of N. pachy-
derma sin also increased. Despite the certain growth of
the mean annual temperature in the surface layer, the
concentration of N. pachyderma sin. in the assemblages
remained very high, since this species, dwelling in the
region of the thermocline and below it, was not sub
jected to the temperature oscillations in the surface
waters. Meanwhile, the strong seasonal variations in
the upper mixed layer suppressed the development of
other (shallow-water) species [2].
The features of the distribution of the radiolarians in
the Sea of Okhotsk were explained in the same way.
Morley and Hays [24] showed that the physical charac
teristics of the subsurface waters influence the abun
dance and activity of shallow-water flora and fauna; in
so doing, the stable dwelling conditions of more deep-
water fauna are retained. According to the data of
plankton studies, the major part of the radiolarians
C. davisiana dwell below 200 m.
As it has been shown above, about 6.7 ky B.P., a
sharp change in the type of sedimentation occurred.
Here, starting from this moment and up to the present
(145-0 cm), diatoms oozes containing radiolarian and
foraminiferal tests, in addition to the valves of diatoms,
began to accumulate. The decrease in the production of
planktonic foraminifera in the overall plankton produc
tion and the increase in the production of diatoms
reflect the onset of the conditions of high productivity.
N. pachyderma sin. stayed dominate over the assem
blages at a certain increase in the proportions of the
other species. Though specimens with well-preserved
tests are encountered along with those featuring signs
of dissolution, the proportions of the species do not
change. Therefore, the high concentrations of N. pachy
derma sin. cannot be explained by selective dissolution.
The influence of the vertical stratification and dicho
thermal layer on the development of the species of
planktonic foraminifera is strongly confirmed by the
test morphology. The deep-water N. pachyderma sin.
develops under stable and favorable conditions; its tests
are well developed, covered with acrystallinecrust,
and feature a typical Arctic appearance. Other species
that developed under relatively unfavorable conditions
above the dichothermal layer, are usually represented
by suppressed individuals with unclear species proper
ties.
At the end of the Holocene, starting from
3.4 ky B.P., the concentrations of G. bulloides and
N. pachyderma dex. increased. Probably, this reflects a
certain cooling, a decrease in the surface temperature
contrasts, and an enhancement of the dwelling condi
tions of the shallow-water species above the dichother
mal layer. This agrees with the data suggesting that, in
the North Pacific, after the period with a climate
warmer and drier than the present-day climate, colder
and more humid climatic conditions developed. The
glaciation that started in selected mountain regions as
early as in the Middle Holocene reached its full devel
opment after 3000 yr B.P. (neoglaciation) [22].
The sequence of the changes in the proportions of
the two principal species of foraminifera in the deposits
of the interglacial optimum (OIS 5e) differed from the
Holocene sequence. During the greater part of the inter
glacial interval (129-123 ky B.P.) the concentration of
G. bulloides was high (47-17%). One can suppose the
existence of a normal (free from the dichothermal
layer) vertical structure of the water column. Later,
conditions similar to the Holocene conditions during
the optimum developed. The share of N. pachyderma
sin. increased at the maximum overall content of the
tests of planktonic foraminifera in the sediment. In
addition, the species Globorotalia scitula, which is also
deep-water [10], was encountered only in this zone.
This species is the most warm-water species encoun
tered in the core [1]. Probably, after 123 ky B.P., the
temperature values were higher than in the
Holocene, the stratification of the waters enhanced,
and a dichotherm al structure developed. Meanwhile,
the biological productivity was significantly smaller
than in the Holocene, and the sedimentation rates
were twice as low.
CONCLUSIONS
Planktonic foraminifera and the proportions of the
principal components in the sediment grain-size frac
tion larger than 0.125 mm (terrigenous grains, volcanic
ash, siliceous microfossils (diatoms and radiolarians),
and planktonic and benthic foraminifera) were studied
in a sediment core from the continental slope of Kam
chatka. The core, which is 1116 cm long (sea depth
684 m), is represented by alternation of biogenic diato-
maceous oozes and silty-clayey oozes with an admix
ture of diatoms.
The lower terrigenous-volcanogenic layer (1112
932 cm) was accumulated during the latest stage of the
penultimate continental glaciation, or oxygen isotope
stage (OIS) 6, in the interval from 174 up to 130 ky B.P.
The fractions of 0.125 mm are mostly composed of ter
rigenous matter from sandy to gravel grain size.
OCEANOLOGY Vol. 45 No. 2 2005
266 BARASH et al.
Together with the volcanic ash particles, they make up
almost 100%. The supply of the principal clayey-silty
part of the terrigenous matter may be explained by the
eolian transport at high monsoon activity, while the
coarse-grained matter could be delivered only with
floating ice. It seems that, under the conditions of a low
sea level, low water and air temperatures, and an exist
ence of local glaciers on the adjacent land, thick sea ice
and icebergs were formed. The almost absolute absence
of microfossils points to low biological productivity
related to the severe ice conditions and poor mixing in
the upper water layer. The average sedimentation rate
was 2.27 cm/ky.
The overlying biogenic layer (932-837 cm) gener
ally corresponds to the optimum of the last interglacial
OIS 5e (129-117 ky B.P.). This interval may be subdi
vided into two parts: the early stage (127-121 ky B.P.)
characterized by high foraminifera contents and the late
stage with a predominance of siliceous microfossils
(121-117 ky B.P.). Despite the reduced terrigenous and
volcanogenic matter supply in the region under study
during the interglacial optimum, the sedimentation
rates grew more than fivefold and reached 12.2 cm/ky
on the average, mostly due to the enhanced biological
productivity.
Upward in the section (837-260 cm), a terrigenous-
volcanogenic layer accumulated under the conditions
of a continental glaciation; in this region, the glacia
tions continued from OIS 5d to OIS 2. In this layer, the
composition of the coarse-grained fraction of the sedi
ment is close to that in layer OIS 6; the sedimentation
conditions were also similar. However, the elevated
microfossil contents in the sediments of the later phases
of the global interglacial period (5d-5a) showed the
existence of short-term periods of enhanced productiv
ity. This kind of environment is also noted at the end of
OIS 2. Within this layer, the average sedimentation
rates were about 5 cm/ky, which is approximately twice
as high as the rate of accumulation of the lower terrig-
enous-volcanogenic layer.
At the end of the interval considered, above 354 cm,
i.e., after oxygen isotope event 2.2 (17.850 ky B.P.), the
rate of sedimentation sharply increases up to 20
26 cm/ky and remains that high up to the present. The
dense sandy-silty sediments with gravel are replaced
by more clayey slightly diatomaceous deposits with
foraminiferal and gravel interlayers. Only starting from
this time do the number of tests of planktonic foramin-
ifera begin to surpass those of benthic foraminifera.
These lithological features indicate the beginning of
deglaciationbiological productivity is enhanced at
high rates of accumulation of terrigenous matter.
From the level of 260 cm corresponding to the OIS
1/2 boundary (12.050 ky B.P.) up to 145 cm (about 6.7
7.0 ky B.P.), the terrigenous matter supply gradually
decreases, while the proportion of microfossils
increase. Precisely in this interval, the maximum con
centrations of the test of planktonic and benthic fora
minifera are observed. With the beginning of OIS 1, the
climatic conditions in the Sea of Okhotsk became less
severe. The decrease in the ice condition severity
favored the productivity enhancement. The relatively
small differences between the summer and winter tem
peratures maintained the weak vertical stratification of
the upper water layers. These conditions, which are
close to the present-day Arctic conditions, provided the
development of foraminifera and siliceous microplank
ton. The desalination of the surface layer by melt waters
deteriorated the vertical mixing; therefore, the produc
tion of diatoms was relatively low.
About 6.7 ky B.P., a sharp change in the type of sed
imentation occurred. From this moment up to the
present, diatomaceous ooze was accumulated, which
reflects the onset of the conditions of high productivity.
At that time, the vertical structure of the water column
also radically changed. It acquired characteristic fea
tures of the subarctic waters. Rather strong summer
heatings alternated with intensive winter coolings and
sea ice formation. Intensive mixing of the upper layer
took place. Gyres and upwellings characteristic of the
present-day Sea of Okhotsk were formed. The so-called
dichothermal layer with a lowered temperature came
into existence separating the upper layer with strong
seasonal variations from the lower layer with more sta
ble conditions.
Both in the interglacial optimum layer (OIS 5e) and
in the Holocene, the maximum concentrations of plank
tonic foraminifera in the central part of the sea (station
LV28-42-5) were encountered in the initial parts of the
intervals, while, in its eastern part (station LV28-44-3),
they were confined to the middle parts of the intervals.
In the eastern part of the sea, the corresponding arctic
type of conditions existed over a longer period than in
its central part. This is probably related to the cooling
effect of the cold West Kamchatka Current.
The species composition of planktonic foraminifera
is rather poor; it involves only six relatively cold-water
species. The assemblages of the glacial intervals (OIS 6,
5d-2), regardless of the test contents in the sample, are
mostly represented by N. pachyderma sin. and contain
noticeable concentrations of G. bulloides, which, by the
ends of the intervals, reached 30-50%. The abundances
of N. pachyderma dex. and G. quinqueloba are small.
We suggest that this kind of composition of the assem
blages corresponds to low temperatures of the upper
water layers free from significant seasonal variations.
The increase in the concentrations of G. bulloides in
layer OIS 2 reflects a gradual temperature growth.
Meanwhile, after the short-term Boelling-Allereud
warming, at the beginning of OIS 1 and later, a para
doxical growth in the concentrations of N. pachyderma
sin. up to 90% and more is observed. Probably, with the
growth of the summer heating and increase in the thick
ness of the upper mixed layer, the seasonal temperature
contrasts in it increased. The thermocline descended to
greater depths. N. pachyderma sin. being a deep-water
OCEANOLOGY Vol. 45 No. 2 2005
ON THE QUATERNARY PALEOCEANOLOGY OF THE SOUTHEASTERN PART 267
species dwelling in the region of the thermocline and
below it was not subjected to the sharp seasonal varia
tions in the surface waters, which suppressed the devel
opment of other (shallow-water) species.
At the end of the Holocene, starting from 3.4 ky
B.P., the concentrations of G. bulloides and N. pachy
derma dex. increased. Probably, this reflects a certain
cooling (known in the continental rimming of the North
Pacific as neoglaciation), a decrease in the surface tem
perature contrasts, and an enhancement of the dwelling
conditions of the shallow-water species above the
dichothermal layer.
During the greater part of the interglacial interval
OIS 5e (129-123 ky B.P.), the concentration of G. bul
loides was high (47-17%). One can suppose the exist
ence of a normal vertical structure of the upper part of
the water column. However, later, conditions similar to
the Holocene conditions during the optimum devel
oped.
ACKNOWLEDGMENTS
This study was supported by the German Federal
Ministry of Science and Education and by the Russian
Foundation for Basic Research, project nos. 01-05
64263 and 04-05-64567a.
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OCEANOLOGY Vol. 45 No. 2 2005
... In the past few decades, the paleoproductivity of the Sea of Okhotsk has been studied using both micropaleontological [2,3,13,15,17,42,43] and geochemical methods [1,5,8,28,38]. The periods of maximal productivity during the Holocene and the optimum of the last interglacial (MIS 1 and 5e, respectively) were identified, and during the glacial periods (MIS 5d-2 and 6), productivity decreased due to the more severe ice conditions. ...
... Productivity remains very high until the end of substage 5e. When studying the southeastern part of the Sea of Okhotsk, Barash et al. [3] divided the optimum of the last interglacial (substage 5e) into two parts: early (127000-121000 years ago) with a large foraminifera content and late (121000-117000 years ago) with dominant siliceous microfossils. The authors also noted that increased biogenic sedimentation began 2000-2500 years later than the onset of MIS 5 [2,3]. ...
... When studying the southeastern part of the Sea of Okhotsk, Barash et al. [3] divided the optimum of the last interglacial (substage 5e) into two parts: early (127000-121000 years ago) with a large foraminifera content and late (121000-117000 years ago) with dominant siliceous microfossils. The authors also noted that increased biogenic sedimentation began 2000-2500 years later than the onset of MIS 5 [2,3]. In accordance with our data, it was exactly this time period that showed the first productivity peak. ...
Article
Variations in the content of chlorin (a derivate of chlorophyll a) in 11 cores of bottom sediments from different parts of the Sea of Okhotsk are studied. The data show variations in paleoproductivity of this sea for the past 160000 years from the end of marine isotope stage (MIS) 6 until recently. A common pattern of the variation in paleoproductivity is established for the entire Sea of Okhotsk. During the interglacials (MIS 5e and 1), productivity increased, and in glacial periods, it decreased, probably due to the longer lasting marine ice cover throughout the year. The features of variations in productivity through time are recorded in the eastern part of the sea, which is more prone to the influence of inflowing Pacific waters.
... In some cases, we use a single value from a certain interval due to lack of data [for example, 30,34]. If the quantitative data were inaccessible, [1,2,5,13,18,19,40], we used the aver age proxy value taken form the published plot for each interval. The present day productivity was estimated from the uppermost layer of the sediment core, which is no older than 1 kyr BP. ...
... The low BF abundance also indicates a reduced flux of organic matter to the seafloor, although the dilution of the sediment by ter rigenous material cannot be ruled out. We think that dissolution is not the main cause for the low abun dance of carbonate microfossils, as indicated by the absence of corroded BF tests (for example, [1]). ...
... Our paper presents (a) new results on the radiolarians (data on the total abundances and C. davisiana) in the core LV28-44-3, (b) new interpretation of the available information on the benthic and planktonic foraminifers in the core, previously obtained by Barash et al. (2005), and Khusid et al. (2005). Fig. 3 shows the down-core variations of the total microfossil content per 1 g of dry sediment. ...
... Special interest to study the sediment core LV28-44-3 has several reasons: (1) the depth of the core location 684 m is within SOIW just above the oxygen minimum zone (OMZ), so that we can directly recognize the changes in the oxygen conditions of paleo-SOIW, (2) the content of the biogenic matter in the core sediments is high enough to allow the quantitative microfossil analysis, (3) the stratigraphic range of the core spans over two interglacials and two long glacials, which is good for the comparative studies of the different paleoclimatic intervals. Barash et al. (2005), and Khusid et al. (2005) presented a detailed description of the distribution of the indicative foraminiferal species in the core LV28-44-3. From these studies, the general characteristics of the foraminiferal assemblages are associated with the glacial/interglacial changes. ...
... The distribution of the main coarse-grained (fraction >0.125 mm) components and planktonic foraminifera over this core and the relevant paleoceanographic reconstructions are described in [2]. ...
... The main trends in the quantitative variations of the benthic and planktonic foraminifera are similar throughout the entire section [2]. The increase in the abundance of both planktonic and benthic foraminifera is observed in the sediments accumulated during MIS 1 and the optimum of the last interglacial period. ...
Article
Full-text available
Benthic foraminifera were studied in 117 sediment samples from a 1112-cm-long core obtained from the Kamchatka continental slope (52°02.514′ N, 153°05.949′ E) at a sea depth of 684 m. The section covers the last 180 ky, from marine isotopic stage (MIS) 6 to the present time. The substantial quantitative and taxonomic changes in the assemblages of benthic foraminifera reflect the climatic and paleoceanographic variations. The insignificant contents of foraminiferal tests in the sediments that accumulated during glaciations (MIS 6, MIS 5(d-a)-MIS 2) suggest a minimal organic flux to the sea bottom. During deglaciation and in the Holocene (MIS 1) and, particularly, in the interglacial optimum (MIS 5e), the organic flux to the bottom significantly increased. Sestonophagous species prevailed in the foraminiferal assemblages of glacial periods, when the production of the young Sea of Okhotsk Intermediate Water (SOIW) increased. The assemblages of warm periods (MIS 1 and 5e) are mainly composed of detritophagous species. Now, conditions favorable for these species exist in the bottom areas influenced by the old Pacific waters. During the warm interglacial optimum (MIS 5e), when the SOIW production decreased, its thickness became reduced and the boundary with the Pacific water mass substantially rose (probably by 200-400 m). During MIS 1, the decrease in the SOIW production and the rise of its lower boundary were less significant.
... In some cases, we use a single value from a certain interval due to lack of data [for example, 30,34]. If the quantitative data were inaccessible, [1,2,5,13,18,19,40], we used the aver age proxy value taken form the published plot for each interval. The present day productivity was estimated from the uppermost layer of the sediment core, which is no older than 1 kyr BP. ...
... The low BF abundance also indicates a reduced flux of organic matter to the seafloor, although the dilution of the sediment by ter rigenous material cannot be ruled out. We think that dissolution is not the main cause for the low abun dance of carbonate microfossils, as indicated by the absence of corroded BF tests (for example, [1]). ...
Article
Full-text available
The se-�surface bioproductivity changes over the last 25 kyr were inferred from published data on 30 sediment cores from the open Northwest Pacific (NWP), Sea of Okhotsk, Bering Sea and Sea of Japan accounting for the glacioeustatic se-�level changes. A novel method was developed to compare the variations of several independent productivity proxies relative to the present-day values. During the Last Glacial Maximum, the bioproductivity in the Sea of Okhotsk and the western Bering Sea (BS) was lower than at present, whereas the southern and southeastern Bering Sea and the open NWP are characterized by enhanced bioproductivity. During the early deglacial stage, an increase in bioproductivity was estimated only for the southeastern Bering Sea. High and fairly high bioproductivity was estimated for Heinrich 1 in the open NWP, above the Umnak Plateau and on the Shirshov and Bowers Ridges in the Bering Sea. The high productivity in the Bering Sea, Sea of Okhotsk and NWP during the Bølling/Allerød was caused by the global warming and enhanced nutrient supply by meltwater from the continent. During the Early Holocene, high productivity was estimated for almost the entire NWP. The Late Holocene sea�surface bioproductivity was generally lower than that of the Early Holocene. Proposed factors that have controlled the sea�surface bioproductivity during the last 25 kyr include: the location of the sea ice margin, the river runoff, gradual flooding of the Bering Sea and the Sea of Okhotsk shelf areas, the water mass exchange between the marginal seas and the open NWP, the eolian supply and the deep vertical mixing of the water column.
... For other intervals, especially in MIS 5.5, low IRD accumulation rates suggest that seasonal sea ice gave way to ice-free conditions with sporadic ice transported in winter. High CaCO 3 contents together with abundances of diatom and foraminifera (Barash et al., 2005;Khusid et al., 2005;Wang and Wang, 2008) support this finding. ...
... kyr) (Kaiser, 2001) 8.2e8.6 (cal. kyr) (Barash et al., 2005) 7.5-7.98 ( 14 C) kyr-8.0-8.4 (cal. kyr) (Braitseva et al., 1995(Braitseva et al., , 1997Melekestsev et al., 1991;Ponomareva et al., 2004Ponomareva et al., , 2007Zaretskaya et al., 2001) 7.43 ( 14 C) (Hasegawa et al., 2011) Kurile (Gorbarenko et al., 1998(Gorbarenko et al., , 2002 8.0e8.05 ( 14 C) (Kaiser, 2001) 8.0 cal kyr 8.54 ( 14 C) kyr- (Hasegawa et al., 2011) (Sakamoto et al., 2006) 39.43e40.12 ...
Article
The fullest summary on composition, age and distribution of 23 tephra layers detected and investigated in the Okhotsk Sea Pleistocene-Holocene deposits is presented. Seven tephra layers are surely identified with powerful explosive eruptions of volcanoes of Kamchatka, Kurile and Japanese Islands. For them, the areas of ash falls including which weren't revealed earlier on the land are specified and established. It is estimated that explosive eruptions of volcanoes of the Kamchatka Sredinny Range were the sources for three tephra layers. Complex investigations of morphological, mineralogical and chemical composition of tephras including composition of rare and earth-rare elements (electron microprobe analysis and laser ablation method - LA ICP MS) have been made for all studied layers. They were a basis for tephrostratigraphic correlation of the regional deposits promoting to specification of stages of volcanic explosive activity in this region.
... Calculations show that now we have a peak of CO 2 growth. This is in agreement with other studies, e.g., the analysis of remains of planktonic organisms in bot tom sediments [1,19]. After some time, when unre strained rise in the level of atmospheric CO 2 and land surface temperature was anticipated, the ever growing number of researchers come to the conclusion that in the nearest decades (or centuries), we are to expect cli mate cooling and a decrease in CO 2 in the atmosphere and hydrosphere. ...
Article
Full-text available
Changes in the climate conditions in the recent decade arouse the heightened interest to the problem of the greenhouse effect and consequently to studying the dynamics of CO2 concentration in the ocean-atmosphere system. The modern changes in CO2 concentration and temperature can result both from the anthropogenic influence and from the rhythms of natural processes. The results of modelling carbon equilibrium in the World Ocean water for the Quaternary suggest that the modern climate change is a part of natural climate variations having taken place for at least more than 400 thousand years.
... (2) open-ocean conditions free of sea ice are characterized by the dominance of open-ocean assemblages (Barash et al. 2001(Barash et al. , 2005(Barash et al. , 2006. In this scenario, the warm current from the north Pacific gyre frees the Okhotsk Sea of sea ice and brings with oceanic assemblages; (3) seasonal sea ice conditions are characterized by a lower proportion of sea-ice assemblages and a higher proportion of high-productivity assemblages. ...
Article
Full-text available
This study provides insight into changes in sea ice conditions and the oceanographic environment over the past 500 kyr through analysis of the diatom record. Based on the relative abundance of 13 diatoms species in piston core MD012414, four types of environmental conditions in the central Okhotsk Sea over the last 330 ka BP have been distinguished: (1) open-ocean alternating with seasonal sea-ice cover in Stages 9, 5, and 1; (2) almost open-ocean free of sea-ice cover in Stages 7 and 3; (3) perennial sea-ice cover in Stages 6, 4, and 2; and (4) a warm ice-age dominated by open ocean assemblages in Stage 8. The littoral diatom species,Paralia sulcata, showed a sudden increase from the glacial period to the interglacial period over the last 330 ka BP, except during Stage 8. Such a result implies that melting sea-ice transported terrigenous materials from the north Okhotsk Sea continental shelves to the central ocean during deglaciation. From Stage 13 to Stage 10, however, cold and warm marine conditions unexpectedly occurred in the late interglacial periods and the glacial periods, respectively. One possible reason for this is a lack of age control points from Stage 13 to Stage 10, and the different sediment accumulation rates between glacial and interglacial periods. This study suggests not only the process by which oceanographic variation of sea ice occurred, but also new significance for Paralia sulcata as an indicator in the diatom record of the Okhotsk Sea.
Article
The analysis of foraminiferal assemblages in sediments that were deposited during the last 30 kyr revealed similar patterns in their distribution in the central and marginal parts of the Deryugin Basin. The similar composition of foraminifers through the entire basin implies similarity in natural environments within its limits. The absence of benthic foraminifers or extreme impoverishment of the assemblages during the maximum of the last glaciation could result from a combination of several factors: drastic decrease in bioproductivity due to general cooling, development of bottom anoxia, and presumably unfavorable influence of seeps on geochemical parameters of bottom waters. The weak activity of barite-methane seeps in the central part of the basin during the Holocene is evident from some variations in the structure of benthic foraminiferal assemblages against the background of their similar taxonomic compositions.