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The distribution of diatoms, radiolarians, planktonic and benthic foraminifers, and sediment components in the fraction >0.125 mm was analyzed in the core obtained from the central Sea of Okhotsk within the frameworks of the Russian-German KOMEX project. The core section characterizes the period 190–350 ka, which corresponds to marine-isotopic stages (MIS) 7 to 10. During glacial MIS 10 and MIS 8, the basin accumulated terrigenous material lacking microfossils or containing them in low abundance, which reflects, along with their composition, heavy sea-ice conditions, suppressed bioproductivity, and bottom environments aggressive toward calcium carbonate. Interglacial MIS 9 was characterized by elevated bioproductivity with accumulation of diatomaceous ooze during the climatic optimum (328 to 320 ka). The water exchange with the Pacific was maximal from 328 to 324 ka ago. Environments became moderate and close to the present-day ones at the end of the optimum exhibiting the possible existence of a dichothermal layer with substantial amounts of the surface Pacific water still flowing into the basin. Similar to interglacial MIS 5e and MIS 1, the “old” Pacific water determined near-bottom environments in the central Sea of Okhotsk during that period, although the influx of terrigenous material was higher, probably reflecting a more humid climate of the region. Slight warming marked the terminal MIS 8 (approximately 260 ka ago). The paleoceanographic situation during interglacial MIS 7 was highly variable: from warm-water to almost glacial. The main climatic optimum of MIS 7 occurred within 220–210 ka, when the subsurface stratification increased and the dichothermal layer developed. Bottom environments during the studied time interval, except for the optimum of interglacial MIS 9, resembled those characteristic of glacial periods: the actively formed “young” Okhotsk water displaced the “old” Pacific deep water.
ISSN 0001-4370, Oceanology, 2006, Vol. 46, No. 4, pp. 501–512. © Pleiades Publishing, Inc., 2006.
Original Russian Text © M.S. Barash, A.G. Matul, G.Kh. Kazarina, T.A. Khusid, A. Abelmann, N. Biebow, D. Nürnberg, R. Tiedemann, 2006, published in Okeanologiya, 2006,
Vol. 46, No. 4, pp. 537–549.
The Sea of Okhotsk now represents the most impor-
tant element of the climatic system in northeastern Asia
and the Northwest Pacific, which affects the atmo-
spheric and oceanic circulation and the properties of the
Pacific water masses. The basin played a similar role in
the geological past as well. The high sedimentation
rates characteristic of the Sea of Okhotsk and various
microfossils contained in its sediments offer an oppor-
tunity for detailed paleoceanographic reconstructions.
Until recently, micropaleontological studies and rele-
vant inferences on the paleoceanographic history of this
basin concerned its young stages covering the last sev-
eral tens of thousand years. The cores of the Quaternary
sediments obtained within the frameworks of the Rus-
sian–German KOMEX project provide a record of a
longer time interval corresponding to several last gla-
cial cycles. Based on radiocarbon dates, oxygen-iso-
tope curves, data on magnetic susceptibility of sedi-
ments, their lithology, and biostratigraphic interpreta-
tions, age models were elaborated for these cores with
defining intervals corresponding to marine isotopic
stages (MIS), i.e., to the cycles in the development of
the Quaternary continental glaciation [14, 32]. Some
cores of the KOMEX project from different areas of the
Sea of Okhotsk were studied by micropaleontological
menthols. The data obtained made it possible to outline
the main features of the paleoceanographic evolution of
this basin during the last several hundred thousand
years with the main attention given to the last 220 ka
(MIS 7–MIS 1). In this article, we present the results of
micropaleontological studies of the preceding Middle
Quaternary stage from 350 to 190 ka ago or from the
end of MIS 10 to the end of MIS 7.
The vertical hydrological structure of the water col-
umn in the present-day Sea of Okhotsk is of the Subarc-
tic type. The thin (0 to 40 m) layer with high seasonal
Paleoceanography of the Central Sea of Okhotsk during
the Middle Pleistocene (350
190 ka)
As Inferred from Micropaleontological Data
M. S. Barash
, A. G. Matul
, G. Kh. Kazarina
, T. A. Khusid
, A. Abelmann
, N. Biebow
D. Nürnberg
, and R. Tiedemann
Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia
Wegener Institute for Polar and Marine Research, Bremershafen, Germany
Leibnitz Institut für Marine Forschung, Kiel, Germany
Received January 24, 2006
—The distribution of diatoms, radiolarians, planktonic and benthic foraminifers, and sediment com-
ponents in the fraction >0.125 mm was analyzed in the core obtained from the central Sea of Okhotsk within
the frameworks of the Russian–German KOMEX project. The core section characterizes the period
350 ka, which corresponds to marine–isotopic stages (MIS) 7 to 10. During glacial MIS 10 and MIS 8, the
basin accumulated terrigenous material lacking microfossils or containing them in low abundance, which
reflects, along with their composition, heavy sea-ice conditions, suppressed bioproductivity, and bottom envi-
ronments aggressive toward calcium carbonate. Interglacial MIS 9 was characterized by elevated bioproductiv-
ity with accumulation of diatomaceous ooze during the climatic optimum (328 to 320 ka). The water exchange
with the Pacific was maximal from 328 to 324 ka ago. Environments became moderate and close to the present-
day ones at the end of the optimum exhibiting the possible existence of a dichothermal layer with substantial
amounts of the surface Pacific water still flowing into the basin. Similar to interglacial MIS 5e and MIS 1, the
“old” Pacific water determined near-bottom environments in the central Sea of Okhotsk during that period,
although the influx of terrigenous material was higher, probably reflecting a more humid climate of the region.
Slight warming marked the terminal MIS 8 (approximately 260 ka ago). The paleoceanographic situation dur-
ing interglacial MIS 7 was highly variable: from warm-water to almost glacial. The main climatic optimum of
MIS 7 occurred within 220–210 ka, when the subsurface stratification increased and the dichothermal layer
developed. Bottom environments during the studied time interval, except for the optimum of interglacial MIS 9,
resembled those characteristic of glacial periods: the actively formed “young” Okhotsk water displaced the
“old” Pacific deep water.
Vol. 46
No. 4
BARASH et al.
variability is underlain by a layer with temperatures
close to the freezing point—the so-called dichothermal
layer occupying the depth interval from 40–50 to
200 m. Downward up to depths of 600–1000 m,
there is the young Okhotsk intermediate water mass
(OIWM) with a temperature of approximately
, a
high oxygen content (2.5 to 6.5 ml/l), and a salinity of
‰. The OIWM results from the
mixing of very cold freshened and oxygen-saturated
shelf-derived Okhotsk water with warmer and saline
intermediate Pacific water. Leaving the Sea of Okhotsk
and mixing with water of the Subarctic Oyashio Cur-
rent, this Okhotsk water contributes much to the venti-
lation of the North Pacific. Depths of 600–1350 m are
occupied by the transformed “old” deep Pacific water
with higher temperature and salinity, which penetrates
into the Sea of Okhotsk via the deepest Kuril straits.
The deep South Okhotsk Basin below 1350 m is filled
with the bottom water mass with a temperature of
and a salinity of
‰, which originates from
the deep water mass of the Northwest Pacific.
The micropaleontological study of planktonic and
benthic microfossils in Upper Quaternary sediments [1,
3, 4, 8, 9, 11, 19, 28, 29, and others] revealed additional
characteristics of the paleoceanographic evolution of
the basin under consideration, its vertical hydrological
structure included. It was established that, during the
penultimate and last continental glaciations, when the
central areas of the Sea of Okhotsk accumulated terrig-
enous mud with coarse-grained ice-rafted material and
volcanic ash, temperatures of the near-surface water
layer and their seasonal variability were minimal. The
low abundance of microfossils points to low bioproduc-
tivity determined by heavy sea-ice conditions and
insufficient mixing of the near-surface waters. The
water exchange with the ocean was reduced, which
resulted in an elevated production of the young
Okhotsk intermediate water mass and a deepening of its
boundary with the underlying deep Pacific water.
During interglacial optimums (MIS 5e, MIS 1), the
temperature in the near-surface water layer and its sea-
sonal variability increased, which was accompanied by
a growth in bioproductivity, intense near-surface water
exchange with the ocean and the Sea of Japan, intense
influx of the old deep Pacific water into the Sea of
Okhotsk, and by the rise of its high boundary by several
hundred meters. The later phases of these warm periods
were marked by the development of the dichothermal
structure in the upper water layer and accumulation of
diatomaceous oozes. Judging from the composition of
microfossils, interglacial optimum MIS 5e was the
warmest of the last 220 ka.
The examined core KOMEX LV28-42-4, 1084 cm
long, was taken from the southeastern slope of the Insti-
tuta Okeanologii Rise in the central Sea of Okhotsk
E, water depth 1041 m)
during cruise 28 of R/V
Akademik M.A. Lavrentiev
in 1998 [14]. The core is largely composed of terrige-
nous silt with pebbles and gravel. The uniform section
of bioturbated terrigenous sediments is intercalated by
thin interbeds of diatomaceous ooze with grain-size
from sandy to clayey silt and biogenic opal concentra-
tion up to 50%. Volcanic ash forms interbeds and is dis-
persed in variable amounts over the section.
The chronology of the core is based on oxygen-iso-
tope stratigraphy combined with radiocarbon dates cor-
rected for cyclic oscillations in the orbital parameters.
The marine isotopic stages (MIS) and events are estab-
lished by a graphic correlation of the oxygen-isotope
curves based on benthic foraminifers with standard
curves and identified in line with the standard nomen-
clature described in [32]. According to the accepted age
model, the examined core interval below the level of
630 cm corresponds to the period of 190 to 350 ka ago
or to MIS 7, MIS 8, MIS 9, and, partly, MIS 10. Thus,
it comprises two interglacial and two glacial oxygen-
isotope stages. The boundaries between the stages are
placed at the following levels: 770 cm for MIS 7/MIS 8
(242 ka ago), 910 cm for MIS 8/MIS 9 (301 ka ago),
and 1022 cm for MIS 9/MIS 10 (334 ka ago). No sharp
lithological changes are observed at these boundaries.
Four microfossil groups were used for micropaleon-
tological studies: planktonic and benthic foraminifers,
diatoms, and radiolarians. The samples were collected
with a step of 10 cm. The study of coarse-grained sedi-
ments (fractions >0.125 mm) was accompanied by the
calculation of the ratios between their main compo-
nents (terrigenous mineral grains, ash particles, plank-
tonic and benthic foraminiferal tests, and siliceous
(radiolarians, diatoms) microfossils).
The microfossil assemblages reflect relevant water
environments. Diatoms are confined to the upper
euphotic water layer. Planktonic foraminifers mostly
characterize depths of 50–250 m in the water column.
Radiolarians dwell in the present-day Sea of Okhotsk in
the upper water layer down to approximately 1000 m
with populations of dominant species occurring in the
depth interval 200–500 m. Benthic foraminiferal
assemblages reflect bottom environments.
The samples for the diatom analysis were treated
according to the standard procedure with disintegration
in Na polyphosphate and hydrogen peroxide solutions.
All the species observed under the microscope were
identified by counting 200–500 specimens, when their
abundance was sufficient. For the foraminiferal analy-
sis, the coarse-grained fraction (>0.125 mm) was quar-
tered up to obtaining a weight containing at least 300
tests. The remainder of the fraction was examined
under the microscope for to reveal rare species. The
entire fraction was examined in the case of low-abun-
dance assemblages. In each sample, the taxonomic
composition of foraminiferal assemblages was deter-
mined by the calculation of the species percentage and
the abundance of tests in 1 g of dry sediment. The lab-
Vol. 46
No. 4
oratory treatment and study of radiolarians were con-
ducted in line with the standard technique [12]. The
fraction of 40–500
m prepared for the radiolarian
analysis was studied under magnifications of 160 or
320 times. At least 300–400 specimens were counted in
each preparation.
distribution of biogenic components and micro-
fossil groups.
The consistent distribution of oxygen
isotopes and biogenic components (organic carbon,
biogenic opal, and calcium carbonate) registers glacial
stages MIS 10 and MIS 8 and particularly distinct inter-
glacial optimums MIS 9 and MIS 7 (Fig. 1). According
to the micropaleontological data, most simple, but cli-
matically indicative, are the variations in the total abun-
dance of diatoms, which form prominent maxima dur-
ing climatic optimums and negative steady glacial
peaks. The abundance of planktonic foraminifers
increased mainly during interglacial periods as well.
Their maximal abundance coincides with the MIS 9
optimum marked by the maximal contents of all the
examined biogenic components. The more complex
distribution patterns are characteristic of radiolarians
and benthic foraminifers, which could reflect environ-
mental changes at greater depths remote from the water
surface, in contrast to other microfossils. In addition to
the interglacial maximums, radiolarians and benthic
foraminifers demonstrate abundance peaks at other lev-
els: at glacial-to-interglacial transitions marking the
initial stages of warming periods and inside glacial
MIS 8 likely reflecting interstadial conditions. More
detailed information on the distribution of different
microfossil groups through Core LV18-42-2 is consid-
ered below in corresponding sections.
Composition of the coarse-grained sediment
fraction (>0.125 mm).
The terrigenous and volcanic
ash material constitutes over 90% of all the components
through almost the entire core section examined
(Fig. 2). In the interval MIS 10, the sharply dominant
terrigenous material is accompanied by subordinate
volcanic ash (12%), siliceous microplankton (up to
5%), planktonic foraminifers (up to 2%), and single
benthic foraminiferal tests.
Fig. 1.
Distribution and accumulation rates of biogenic components and microfossils in the examined interval of core LV28-42-4.
The oxygen-isotope curve and the position of marine isotopic stages (MIS) are also shown.
0 4 8 0 20 40 0 6001200 0 60 120 0 100200 300
0 6 12 0 600 1200 0 30 60 0 100 200
0 0.3 0.6 0.9
, %
, %
Age, ka
O, benthic foraminifers
Biogenic components
Planktonic foraminifers
Benthic foraminifers
Vol. 46
No. 4
BARASH et al.
Fig. 2.
Proportions of the components in the coarse-grained (>0.125 mm) fraction of the sediments.
The onset of interglacial MIS 9 (331–332 ka) is
marked by a short-term increase in the abundance of
large-sized siliceous plankton up to 7–9% with the total
content of biogenic opal amounting to 36.53% [32],
which reflects the positive pulse in the productivity of
siliceous microorganisms. The period of 327 to 321 ka
corresponded to a longer and steady phase of their high
productivity: the total content of opal in the sediments
increases up to 26–36% against the background of the
siliceous microplankton content in the coarse-grained
fraction amounting to 90%. This is a period of the
diatomaceous ooze accumulation during the optimum
of interglacial MIS 9. In the sediments of that period,
the examined fraction is characterized by elevated con-
tents of both planktonic (up to 47%) and benthic (up to
8%) foraminifers. The abundance peaks of planktonic
species anticipate those of benthic forms by 2 ka. After
320 ka, ash beds AL 9.24 and AL 9.22 were deposited
The fraction corresponding to glacial MIS 8 almost
entirely consists of terrigenous material and volcanic
ash. Signs of the subsequent warming and productivity
increase are noted after 260 ka, when relative abun-
dances of planktonic and benthic foraminifers are as
high as 4% and up to 3%, respectively. After 246 ka, the
content of siliceous microfossils increases as well (up
to 9%).
The onset of MIS 7 is marked by an insignificant
increase in the abundance of planktonic and benthic
foraminifers. The enhanced development of siliceous
plankton that commenced at the end of MIS 8 contin-
ued during MIS 7 to reach maximum (9%) at approxi-
mately 233 ka, i.e., 6 ka after the foraminiferal peak.
Later (230 to 226 ka), volcanic ash becomes prevalent
in the coarse-grained fraction (up to 80%, bed Al 7.4).
Against the background of the dominant terrigenous
material, there is the same, although more distinct, tem-
poral succession of the components observable in the
section, which was characteristic of MIS 9 and at the
beginning of MIS 7: benthic foraminifers–planktonic
foraminifers–large siliceous plankton–volcanic ash
(bed Al 7.2 after [23]) (table). According to the data
cited, the optimums of MIS 7 are less manifested as
compared to that of MIS 9.
Diatoms (Fig. 3).
Glacial sediments are usually bar-
ren of diatoms or have them in negligible amounts: a
few hundred valves per gram of natural dry sediment
(Fig. 1). In interglacial sediments, the abundance of
diatom frustules sharply increases. Glacial sediments
of MIS 10 contain rare diatoms (less then 500 valves/g),
which are represented only by the most dissolution-
resistant species. In the sediments of the interglacial
optimum of MIS 9 (328–324 ka), the diatom abundance
rapidly increases up to 9.5
valves/g to fall sharply
down to complete disappearance higher in the section.
Age, ka
MIS 10
0 40 80 0 40 80
020400 9
Diatoms and
Vol. 46
No. 4
The low diatom content is also observed through the
final phase of MIS 9 and most of the glacial period MIS
8. Approximately 5–8 ka prior to the end of MIS 8, the
diatom abundance gradually increases again up to
valves/g. During interglacial MIS 7, the
abundance of diatoms was highly variable: from their
absence in selected short time intervals to significant
and even anomalous amounts at levels of 237, 212.5,
and 195 ka. This stage was likely characterized by rel-
atively unstable hydrological environments in the Sea
of Okhotsk. In the period from 215 to 210 ka, the dia-
tom content in the sediments reaches values maximal
for the section under consideration (over
valves/g), which reflects the climatic optimum
of MIS 7.
The diatom assemblages examined are diverse, con-
sisting of over 50 species. Most of them are typical of
the present-day Sea of Okhotsk, being widespread in
the bottom sediments [7, 35]. Several species became
Proboscia (P.) curvirostris, P. barboi, Thalassi-
osira (Th.) nidulus, and Actinocyclus (A.) ochotensis
The following species occur in abundance or are
dominant in diatom assemblages:
Neodenticula (N.)
seminae, Thalassionema (T.) nitzschiodes, Rhizosole-
nia (Rh.) hebetata, Th. latimarginata (=Th. trifulta),
Th. excentrica, Th. gravida+Th. antartica, A. curvatu-
, and others. The known natural habitats of these
species [2] allow some paleoecological inferences.
The sediments of glacial MIS 10 (350 to 337 ka) are
practically barren of diatoms. Inasmuch as the diatom
Content of sand fraction components (%) in the sediments of the interglacial MIS 7 optimum
Age, ka Sample, cm Terrigenous
grains Volcanic ash Siliceous
plankton Planktonic
foraminifers Benthic foraminifers
201.3 656 0
00 0
204.05 661 24.4
3.9 2.1 0.4
208.09 671 62.6 20.2
0.4 0.4
210.68 681 85 9.5 4.5 0.5 0.5
213.26 691 75.5 10 5
215.85 701 78.5 11 4
218.92 711 65.5 31 1 2 0.5
222.72 721 76.8 15.1 3.3 0.7
Note: The peaks of the component contents are presented in bold.
Fig. 3.
Distribution of main diatom species.
Age, ka
Rare diatoms
0 20 0 20 40 60 0 6 0 20 0 40 0 30 0 20 0 4 0 8 0 10 0 6 0 4 0 2 0 6
N. seminae
T. nitshchioides
Rh. hebetata
Th. oestrupii
Th. latimarginata
Th. excentrica
A. curvatulus
Th. gravida + Th. antarctica
B. fragilis
Extinct species
Th. nidulus
Rh. curvirostris
Rh. barboi
A. ochotensis f. fosiilis
%% % % %
Vol. 46
No. 4
BARASH et al.
flora develops only in the euphotic surface water layer,
its absence implies that the sea at that time was proba-
bly entirely covered with ice. The forthcoming warm-
ing (“deglaciation”) at the end of MIS 10 and in the ini-
tial MIS 9 (337 to 331 ka) is reflected in the appearance
of rare dissolution-resistant robust diatom frustules in
relevant sediments and the presence of scanty, although
diverse, freshwater flora that was probably allochtho-
nous, i.e., transported by meltwater. Robust frustules
are identified as
Rh. hebetata, A. curvatulus, Th.
, and
Th. antarctica.
Extinct species
P. barboi,
P. curvirostris, A. ochotensis
, and
Th. nidulus
as well as representatives of the brackish- and freshwa-
ter genera
Aulacoseira, Navicula, Cymbella, Synedra,
Cocconeis, Cyclotella
, and others occur as single
According to the diatom distribution, the climatic
optimum of MIS 9 corresponded to the period from 331
to 317 ka and was most expressed at 328 to 324 ka. This
period is marked by the bloom of the thermophilic oce-
anic diatom flora. The diatom abundance increases up
to 9.5·10
valves/g. During this period, the share of dia-
tom species notably increases, indicating the stable
water exchange between the Sea of Okhotsk and the
Pacific. These are
N. seminae
, large representatives of
the mainly oceanic genus
Coscinodiscus (C. perfora-
tus, C. oculus-iridis
, and
C. asteromphalus), Rh. styli-
Th. oestrupii.
In the period 324 to 315 ka,
relatively warm environments gave way to slightly
more temperate conditions resembling the present-day
ones. This is evident from the increased share of species
typical of the present-day Sea of Okhotsk with its well-
developed central gyre and subsurface dichothermal
layer (
Th. latimarginata, Th. excentrica, A. curvatulus
Thus, according to the diatom distribution patterns, the
climatic optimum of MIS 9 was characterized by a two-
stage development of the diatom flora. The beginning
of the stage was marked by warmer environments and
more intense water exchange with the Pacific as com-
pared to the modern basin. This was followed by a
period with a hydrological situation close to the
present-day one. During the terminal phase of MIS 9
and most of glacial MIS 8, diatoms are missing from
the sediments, which points to severe glacial environ-
ments. At the very end of MIS 8 (approximately 260
ka), clear signs of the forthcoming warming (deglacia-
tion) became evident. The diatom content increased up
to 5–6·10
valves/g. They are usually represented by
cold- and dissolution-resistant species with large and
robust frustules:
C. marginatus, Rh. hebetata,
gravida + Th. antarctica
. At the same time, the
increased share of the species
Chaetoceros spp
. and
Chaetoceros spp
can be considered indicative of a
slightly enhanced productivity in the surface waters.
The sediments accumulated during the subsequent
interglacial MIS 7 are characterized by sharp variations
in the diatom abundance: from their absence during
short-term periods to values maximal for the entire sec-
tion. This probably reflects highly variable paleoceano-
graphic environments: from severe, almost glacial to
relatively warm-water. The beginning of MIS 7 (from
240 to 230 ka) was a period of warm environments and
intense water exchange with the Sea of Japan. This is
evident from the presence of
Th. oestrupii
and the nota-
bly increased role of the species
T. nitzschioides
, which
is abundant in the present-day Sea of Japan. Judging
from the distribution of diatoms, the climatic optimum
of this interglacial period occurred at 217–207 ka. The
diatom assemblages are highly diverse, which implies
interglacial environments. The substantial share of
B. fragilis
, and
Th. antarctica+Th.
is indicative of high productivity.
The extinct species are distributed through the sec-
tion in the following manner:
at 260 ka, its single finding in the sediments dated back
to 220 ka is probably redeposited;
ochotensis f. fos-
silis became extinct 295 ka; last findings of rare P. bar-
boi and P. curvirostris are confined to the level
275280 ka, i.e., to MIS 8. The positions of all these
disappearance levels are consistent with the informa-
tion available on their stratigraphic ranges[10].
Planktonic foraminifers. The vertical distribution
of planktonic foraminifers demonstrates regular pat-
terns (Fig. 4). In most of the glacial sediments, they are
scarce: from <1 to several tens of specimens per gram
of dry sediment. The sediments of glacial MIS 10 con-
tain single tests. In the sediments corresponding to the
interglacial optimum of MIS 9 (328 to 324 ka), the
abundance of planktonic foraminifers rapidly increases
up to 1340 specimens/g at the end of this period to drop
again subsequently. This value is the maximum for the
entire section examined. During the remainder of
MIS 9 and through most of glacial MIS 8, planktonic
foraminifers are rare. Their abundance increases up to
100 specimens/g and higher approximately 15 ka prior
to the end of MIS 8 (deglaciation pulse). In the sedi-
ments of interglacial MIS 7, the contents of foramini-
fers vary from a few tens to more than 200 specimens/g,
which reflects unstable environments. Their maximal
values of 100–200 specimens/g in the period from 215
to 204 ka indicate a climatic optimum at that time.
The planktonic foraminiferal assemblage consists
mainly of relatively cold-water species that form the
following (from cold-resistant to thermophilic [1]) suc-
cession: the subarctic Neogloboquadrina (N.) pachy-
derma sin.; the boreal Globigerina (G.) quinqueloba,
G. bulloides, N. pachyderma dex., Globigerinita
(Gt.) bradyi (=uvula), Gt. glutinata; the subtropical
Globorotalia (Gr.) scitula, Gr. inflata; and the tropical
Globigerinoides (Gs.) ruber, Globoquadrina
(Gq.) dutertrei. N. pachyderma sin. is dominant in all
the samples examined, usually constituting 60–90% of
the whole assemblage. Together with the other domi-
nant species G. bulloides, it provides over 90% of the
total foraminiferal abundance in all the samples.
The glacial sediments of MIS 10 contain only scarce
specimens of N. pachyderma sin. and, locally, G. bul-
OCEANOLOGY Vol. 46 No. 4 2006
loides pointing to severe climatic conditions. The
warming episode (“deglaciation”) at the end of MIS 10
(337 ka) is reflected in the lowered share of N. pachy-
derma sin. (81.2%) and the higher total diversity of the
assemblage (four species).
The environments most favorable for the develop-
ment of planktonic foraminifers existed in the period
from 328 to 324 ka during the climatic optimum of
interglacial MIS 9 (328–312 ka), when their diversity
increased to seven species. The composition and mor-
phology of foraminiferal tests imply a stable near-sur-
face water exchange with the Pacific. The high diatom
productivity, composition of planktonic foraminifers,
and their morphology allow an assumption that, during
the climatic optimum, the hydrological structure of the
Sea of Okhotsk was characterized by the development
of a dichothermal layer similar to the present-day situ-
ation [2, 3].
The remaining part of interglacial MIS 9 and most
of the subsequent glacial MIS 8 were characterized by
severe glacial environments unfavorable for planktonic
foraminifers, which occur in the corresponding sedi-
ments as single specimens (N. pachyderma and, some-
times, two to three of the other most cold-resistant spe-
cies). Distinct signs of warming are registered at
approximately 260 ka, when the foraminiferal assem-
blage consisted of eight species, subtropical forms
included. It can be assumed that single tests of these
species were transported to the Sea of Okhotsk by the
subsurface or intermediate waters from the Pacific. The
sediments deposited during subsequent interglacial
MIS 7 host relatively diverse assemblages of planktonic
foraminifers indicative of unstable paleoceanographic
environments: from moderately cold to polar. The
assemblages corresponding to the onset of MIS 7
(244 ka) and to the period of 215 to 191 ka are more
thermophilic. The test morphology and composition of
the assemblages suggest the development of a dicho-
thermal layer. The slightly manifested climatic opti-
mum of this interglacial period took place at 223 to 204
ka. The finds of small underdeveloped specimens of
tropical species are explainable by their transportation
from the Pacific.
Radiolarians. Almost 100 radiolarian species and
forms were identified in the sediments of the examined
core. Here, we consider the distribution of the species,
ecological and biogeographic interpretations which are
more or less reliably substantiated by the studies of this
microfossil group in water samples, sediment traps, and
bottom sediments in the Sea of Okhotsk and other areas
of the Subarctic Pacific (Fig. 5).
The species Dictyophimus hirundo and Semantrum
quadrifore (= Lophospyris sp. 1) are characteristic of
the deepwater assemblage from the lower part of the
Okhotsk Sea intermediate water mass (OSIW) and
deeper layers, which bear signatures of the Pacific deep
Fig. 4. Distribution of planktonic foraminiferal species.
Age, ka
0 20 400 480480480 6
N. pachyderma sin.
G. bulloides
N. pachyderma dex.
G. quinqueloba
Gt. glutinata
Gt. bradyi
MIS 10
Gt. scitula Gq. dutertrei +
Gr. inflata
Gr. scitula Gs. ruber +
Gs. sacculifer
Gr. scitula
OCEANOLOGY Vol. 46 No. 4 2006
BARASH et al.
waters poorly ventilated in the Sea of Okhotsk [31].
Cycladophora davisiana characterizes the uniform
well-ventilated upper part of the OSIW in the depth
interval 200–500 m [31]. The Plagoniidae spp. group is
a reliable indicator of the cold seasonally stratified (in
the summer) productive neritic waters of the Sea of
Okhotsk [13]. The rare Antarctissa ? sp. 1 is the only
species which is associated with the dichotermal layer
of the Sea of Okhotsk: its presence in the sediments
suggests a strong summer stratification of the near-sur-
face waters [13]. The occurrence of Amphimelissa set-
osa in the Quaternary sediments of the Sea of Okhotsk
coupled with the general elevated content of radiolari-
ans is indicative of the well-mixed cold (<2°C) near-
surface layer at depths up to 300 m [29].
According to the planktonic samples and sediment
trap data, Ceratospyris borealis dwells in the depth
interval from 50 to 500 m, populating cold waters both
with a well-developed dichothermal layer and without
one [13]. Being one of the subordinate species in the
radiolarian assemblage from the bottom sediments of
the central Sea of Okhotsk, C. borealis is very abundant
in the Holocene and recent sediments of the Bering Sea
and northern areas of the Subarcic Pacific [24, 34, 36].
We consider this species as a potential indicator of the
low-temperature subsurface waters that penetrate into
the Sea of Okhotsk from the Northwest Pacific.
C. borealis is a moderately cold-resistant (5°ë) spe-
cies indicative of the relatively warm near-surface
water layer that is developed during the summer in the
southeastern Sea of Okhotsk influenced by the Pacific
waters [31]. It is dominant in the recent bottom sedi-
ments on the continental slope of the Bering Sea [15]
and in the northwestern Subarctic Pacific [34]. Spongo-
discus sp. was characteristic of the Quaternary glacial
sediments in the Sea of Okhotsk (Matul and Abelman,
unpublished data) until its extinction in the North
Pacific at the end of oxygen-isotope event 8.5 [28]. This
species is reported from the Quaternary sediments of
the Bering Sea and the Emperor Seamounts in the
Northwest Pacific, where it forms mass accumulations
[25, 26]. Representatives of the family Spongodiscidae
in the present-day North Pacific are indicators of the
near-surface water mass [13] and prevail in the modern
assemblages of the Bering Sea [15]. By analogy, we
assume that Spongodiscus sp. could be a dweller of the
Subarctic Pacific (Bering Sea?) surface waters.
Variations in the total content of radiolarians over
the core examined are visually consistent with the oxy-
gen-isotope curve and changes in the contents of bio-
genic components, for example, of biogenic opal
(Fig. 1). The low radiolarian content in the sediments of
glacial MIS 10 and MIS 8 is determined by the decrease
in the abundance of A. setosa. The maxima in the radi-
olarian abundances during glacial-to-interglacial MIS
10/MIS 9 and MIS 8/MIS 7 transitions and subsequent
Fig. 5. Distribution of main radiolarian species.
60 20
Age, ka
A. setosa
C. davisiana
Plagoniidae spp.
Antarctissa ? sp.1
C. borealis
D. hirundo
S. quadrifore
Spongodiscus sp.
S. venustum
MIS 10
40 20 40 8 2 10 20 10 3 10 20 30 10 20
%% %
OCEANOLOGY Vol. 46 No. 4 2006
interglacial periods (optimum of MIS 9 and threefold
warming of MIS 7) were provided by the simultaneous
increase in the abundance of the following species:
A. setosa, indicating good near-surface water mixing;
Plagoniidae spp., pointing to the high productivity of
the bacterial and phytoplankton communities; and
C. davisiana, implying water ventilation and mixing at
intermediate depths.
At the MIS 10/MIS 9 transition and the beginning of
MIS 9, there were substantial short-term variations in
the radiolarian abundances and composition. The
period from 338 to 332 ka is marked by a sharp increase
in the share of the species indicating the influx of the
Pacific waters (D. hirundo, S. quadrifore, C. borealis,
Spongodiscus sp.) and by a decrease in the concentra-
tions of the typical Sea of Okhotsk species such as
C. davisiana and Plagoniidae spp. down to insignificant
values. The glacial–interglacial transition in the sea of
Okhotsk was likely accompanied by a wide develop-
ment of relatively cold waters that penetrated from the
neighboring Northwest Pacific at all the depth levels
occupied by radiolarians. The productivity growth was
largely provided by the Pacific invaders. Judging from
the sharp increase in the content of C. davisiana, the
formation of the OSIW mass recommenced in the
period from 332 to 328 ka to result probably in basin
stratification, as is evident from the appearance of
Antactissa ? sp. 1. The productivity of radiolarians was
highly variable, being minimal at approximately 330 ka
(corresponding to a minimum in the Plagoniidae spp.
content). The role of all the “Pacific” species was neg-
ligible. The most notable period during MIS 9 with
respect to the radiolarian productivity was that corre-
sponding to 328–320 ka, when the water column, the
subsurface and the upper OSIW layer included, was
intensely mixed, as is inferred from the high concentra-
tions of C. davisiana and A. setosa. The influence of the
Pacific waters was insignificant at that time. After
approximately 320 ka, their influx increased to result in
the near-surface stratification, which is evident from the
increased share of Antactissa ? sp. 1, Spongodiscus sp.,
and S. venustum.
At the transition from MIS 8 to MIS 7 and during the
optimums of interglacial MIS 7 (244–235 and 218212 ka),
the productivity of radiolarians sharply increased,
owing largely to the “Sea of Okhotsk” species and indi-
cating the development of the OSIW mass and the strat-
ification of the upper layer. Judging from the increased
contents of A. setosa, in the periods 235–230 and
212205 ka, water mixing above the OSIW layer could
have strengthened. A significant increase in the Pacific
water influx is registered in the period of 230 to 218 ka,
when the concentrations of C. davisiana dropped, while
the role of D. hirundo and C. borealis was enhanced.
Benthic foraminifers. In total, 68 species of
benthic foraminifers are identified in the samples exam-
ined. Of them, only 10–12 species, the ecological pref-
erences of which are well known, are used for recon-
structing environments (Fig. 6). In eutrophic settings,
when fluxes of organic matter and carbonate material to
the sediments are high, the following calcareous spe-
cies develop in abundance on the bottom: Uvigerina
(U.) peregrina, U. ‡uberiana, Bulimina (B.) truncana,
Valvulineria (V.) sadonica, and Alabaminella (A.) wed-
dellensis. The latter species feeds only on fresh readily
decomposed detritus. All others can assimilate trans-
formed organic matter of both marine and terrestrial
origins [18, 20, 37]. The agglutinating species Marti-
notiella (M.) communis and Karreriella (K.) baccata
develop in highly productive environments even under
insignificant influx of carbonate material. Cassidulina
(ë.) teretis, C. subglobosa, and Miliammina (M.) herzen-
steini are adopted to oligotrophic conditions.
In the sediments corresponding to the early MIS 10,
the abundance of benthic foraminifers is as high as 1 to
10 specimens/g of dry sediment. In the period 337 to
334 ka, their abundance and accumulation rates
increase by an order of magnitude (Fig. 1). The assem-
blage consists of the calcareous U. peregrina, A. wed-
dellensis, C. teretis, and V. sadonica. The maximal
share (40–85%) in the assemblage belongs to U. pere-
grin, which is characteristic of highly productive areas
[16, 17, 27, 30]. Poor preservation of the tests suggests
an intense dissolution.
The benthic foraminiferal assemblage occurring in
the sediments of MIS 9 is highly variable in composi-
tion and abundance. In the sediments enriched in volca-
nic ash and almost barren of CaCO3 that accumulated
in the period from 333 to 331 ka, benthic foraminifers
are relatively abundant, being represented only by
agglutinating forms (83–92%) with the dominant
M. communis. The latter species is absent in the
present-day Sea of Okhotsk. In the Pacific, it occurs in
highly productive areas [21]. It is recorded in abun-
dance in the sediments corresponding to the late cli-
matic optimum of interglacial MIS 5e [2, 5, 6]. Tests of
this species are highly resistant with respect to dissolu-
The assemblage occurring in diatomaceous ooze
dated back to 327–324.8 ka is impoverished in both
diversity and abundance. The unique feature of this
assemblage is the prevalence of B. truncana, which is
missing from the present-day Sea of Okhotsk. It is
recorded in abundance below 1000 m in the productive
area of the southwestern Pacific washed by the oxygen-
deficient Antarctic intermediate water [20]. Represen-
tatives of the genus Bulimina prefer environments with
a stable high productivity [33, 37]. B. truncana is
accompanied by other calcareous species occurring in
notable quantities: C. subglobosa, U. peregrina,
V. sadonica, and A. weddellensis. All of them occur in
the present-day Sea of Okhotsk. The diverse and abun-
dant assemblage corresponding to the period
327324.8 ka points to environments favorable for the
development of benthic foraminifers: maximal influx
of organic matter and carbonate material to the bottom
OCEANOLOGY Vol. 46 No. 4 2006
BARASH et al.
sediments. It is similar to the assemblages characteriz-
ing the climatic optimums of MIS 5e and MIS 1 [2, 11,
19], although the latter are dominated by other species.
None of the younger assemblages include B. truncana,
the dominant role of which in the assemblage of MIS 9
is probably explained by the high influx of terrestrial
organic matter.
In the sediments of the terminal part of MIS 9 accu-
mulated after 322 ka, the foraminiferal assemblage is
highly impoverished and poorly preserved. The share
of agglutinating species increases up to 31–64%. The
assemblage is dominated by K. baccata, M. herzensteini,
and calcareous U. auberiana. The latter species belongs
to the infauna [16, 17, 22], representatives of which are
known to be usual in environments with low-energy
bottom hydrodynamics.
In the sediments of MIS 8, the foraminiferal assem-
blage is characterized by a low abundance that notably
increases in the upper part of the corresponding inter-
val. The fauna is largely represented by calcareous spe-
cies (over 90%) with Ä. weddellensis, U. ‡uberiana, and
ë. teretis (in the upper part) being dominant. The high
share of the sestonophagous species Ä. weddellensis
points to strengthened bottom hydrodynamics.
The abundance and taxonomic composition of
benthic foraminifers in the sediments of MIS 7 are
notably variable, with a prevalence of the calcareous
Ä. weddellensis and U. ‡uberiana. In the upper part
(213–183 ka), a significant role also belongs to C. tere-
tis and M. herzensteini. The bottom environments were
similar to those during glacial periods.
The sediments corresponding to glacial MIS 10 are
almost completely composed of terrigenous material
practically barren of diatoms and planktonic foramini-
fers. The low concentrations of other microfossils
reflect heavy sea-ice conditions, low bioproductivity,
and bottom environments aggressive toward calcium
carbonate. The end of this period is marked by a warm-
ing, which is evident from the appearance of diatoms,
brackish- and freshwater species brought by meltwater
The onset of interglacial MIS 9 is associated with
the productivity growth and change in the taxonomic
composition of all the microfossil groups, which
reflects the climatic optimum of 328–320 ka. Two
phases are recognizable in the near-surface environ-
ments: from 328 to 324 ka, when a stable water
exchange with the Pacific existed, and from 324 to
320 ka, when the environments were milder, resem-
bling the present-day ones. The composition of the dia-
tom assemblages implies the development of a dicho-
thermal layer, although no radiolarian species charac-
teristic of such environments are observed. The coarse-
grained fraction of the climatic optimum is composed
of biogenic material, which is subsequently replaced by
volcanic ash. Thus, during the entire interglacial MIS 9,
the influx of terrigenous material and, correspondingly,
the influence of sea ice were minimal. The radiolarian
composition after 320 ka suggests an enhanced inflow
of the Pacific waters and a strengthened stratification of
the surface layer.
Fig. 6. Distribution of main benthic foraminiferal species.
Age, ka
M. communis
MIS 10
60 50 20 40 40 80 20 40 80 30 60 30 60 40
%% %
K. baccata
B. truncana
C. subglobosa
U. peregrina
V. sadonica
U. auberiana
A. weddellensis
C. tereis
M. herzensteini
OCEANOLOGY Vol. 46 No. 4 2006
In the sediments accumulated during the terminal
part of interglacial MIS 9 and most of glacial MIS 8,
diatoms and foraminifers are almost missing, which
indicates their insignificant productivity. Simulta-
neously, similar to MIS 10, terrigenous sedimentation
was determined by ice rafting that increased during
MIS 8. At the end of MIS 8 (approximately 260 ka),
there are signs of warming: the abundance and diversity
of all microfossils increase. Benthic foraminifers are
the first to demonstrate maximal concentrations, indi-
cating that glacial conditions were first replaced by
interglacial ones in the bottom layer influenced by the
Pacific waters. At that time and during interglacial
MIS 7, the Pacific intermediate waters brought rare
tests of tropical and subtropical planktonic foramini-
Paleoceanographic environments of interglacial
MIS 7 were highly variable: from severe and almost
glacial to relatively warm. At the MIS 8/MIS 7 transi-
tion and during the interglacial maximums of MIS 2
(244–235 and 218–212 ka), radiolarians imply the
existence of the Sea of Okhotsk intermediate water and
stratification in the upper layer. The increased influence
of the Pacific waters is recorded in the period from 230
to 218 ka. As is evident from the elevated productivity
and diversity of all the microfossils, the main climatic
optimum took place from 220 to 210 ka and was
accompanied by a strengthened stratification and for-
mation of the dichothermal layer. The coarse-grained
fraction of the core interval corresponding to the period
from 223 to 201 ka demonstrates against the back-
ground of the dominant accumulation of terrigenous
material the same, but even more distinct, succession,
like that in the sediments of MIS 9 and initial MIS 7:
benthic foraminifers–planktonic foraminifers–sili-
ceous plankton–volcanic ash.
According to benthic foraminifers, the bottom envi-
ronments during the entire period characterized by the
examined core, except the optimum of MIS 9, were
similar, “glacial,” although slightly variable. Intergla-
cial MIS 7 is almost unrecognizable in the benthic for-
aminiferal record. At the same time, the species diver-
sity was higher as compared to that observed in the sub-
sequent glacial periods. The bottom water layer was
probably better aerated owing to the high-energy
hydrodynamics. This period was marked by an intense
formation of the Sea of Okhotsk intermediate water,
which displaced the Pacific waters.
The composition of benthic foraminifers during the
optimum of MIS 9 reflects the high primary production
(influx of fresh phytoplankton to the bottom sediments)
and the oxygen deficiency. Similar to subsequent inter-
glacial periods, the old Pacific water mass was wide-
spread at that time. Meanwhile, the influx of terrestrial
organic matter was more intense, probably indicating
the development of a humid climate.
This work was supported by the Russian Foundation
for Basic Research (project no. 04-05-64567a) and
Deutsches Bundesministerium für Bildung und For-
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the Bottom Up,” Paleoceanography 10, 545–562 (1995).
... In the process of the tephrostratigraphical studies, data on age of deposits and results of identification of tephra interlayers with known occurrences of explosive volcanism on adjacent land are of a great importance. We used the published materials for a number of standard sediment cores from the Sea of Okhotsk for which the complex of methods of deposits correlation was involved as a basis of stratigraphic position of the tephra layers under study (Gorbarenko, 1991;Ivanova and Gorbarenko, 2001;Barash et al., 2001Barash et al., , 2006Derkachev et al., 2004;Levitan et al., 2007;Gorbarenko et al., 1998Gorbarenko et al., , 2002Gorbarenko et al., , 2004Gorbarenko et al., , 2007Gorbarenko et al., , 2010Gorbarenko et al., , 2012Gorbarenko et al., , 2014Greinert et al., 2002;Kaiser, 2001;Nürnberg and Tiedemann, 2004;Okazaki et al., 2005;Sakamoto et al., 2005Sakamoto et al., , 2006. In addition, data of age and chemical composition of known layers of tephra in the Pleistocene-Holocene soil-pyroclastic cover of the adjacent land (Kamchatka, Kurile and Japanese Islands) were used (Bazanova et al., 2005;Melekestsev et al., 1991Melekestsev et al., , 1996Melekestsev et al., , 1997Machida and Arai, 2003;Machida, 1999;Aoki, 2008;Aoki and Arai, 2000;Arai et al., 1986;Braitseva et al., 1993Braitseva et al., , 1995Braitseva et al., , 1997Hasegawa et al., 2008Hasegawa et al., , 2009Hasegawa et al., , 2011Hasegawa et al., , 2012Hunt and Najman, 2003;Katoh et al., 1995;Kyle et al., 2011;Kishimoto et al., 2009;Machida and Arai, 2003;Matsumoto, 1996;Nakagawa et al., 2008;Okumura, 1991;Pevzner, 2015;Ponomareva et al., 2004Ponomareva et al., , 2007Zaretskaya et al., 2001Zaretskaya et al., , 2007Yamamoto et al., 2010 and others). ...
... ( 14 C) kyr (Katoh et al., 1995) 39.5e40.1 (cal. kyr) (Aoki and Arai, 2000;Hunt and Najman, 2003) (Okazaki et al., 2005;Sakamoto et al., 2006) 100-130 kyr (Machida and Arai, 2003) 115-120 kyr e 2011;Yamamoto et al., 2010) (Barash et al., 2006;Levitan et al., 2007) Magadan city (Uptar mine) - (Melekestsev et al., 1991) volcanism. This applies especially to identifying the tephra layers of the Pleistocene age as their traces on the adjacent land were not preserved due to denudation processes during Pleistocene-Holocene periods. ...
... In the rocks (especially, with increased silica content) of the backarc of island arcs, 90e310 kyr -MIS 9.1e9.2 (Barash et al., 2006) 307 kyr -MIS 9.1e9.2 (Gorbarenko et al., 2010(Gorbarenko et al., , 2014 90e900 kyr - (Nürnberg and Tiedemann, 2004); MIS 23? (Levitan et al., 2007) unknown unknown ...
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.
... Recent studies provide evidence for significant glacial to interglacial variations of the Okhotsk Sea seasonal ice cover, terrestrial organic matter inflow, marine productivity and circulation (Gorbarenko et al., 2002a;Gorbarenko et al., 2010;Iwasaki et al., 2012;Nürnberg and Tiedemann, 2004;Seki et al., 2004;Seki et al., 2009;Seki et al., 2012). In particular, benthic foraminifera-based studies suggest increased productivity, enhanced inflow of Old Pacific Water (DPW here) and weakened outflow of the OSIW during interglacial marine isotope stage (MIS) 1, 5e and 9 compared to glacial MIS 2-5d, 6-8 and 10 (Barash et al., 2001, Barash et al., 2006Khusid et al., 2005). In high-resolution sediment cores throughout the Okhotsk Sea, benthic foraminiferal assemblages dominated by bolivinids were found during termination (T) I, pointing to increased productivity, reduced oxygenation of bottom waters and intensification of the OMZ (Bubenshchikova et al., 2010;Gorbarenko et al., 2002a. ...
... In the Okhotsk Sea, assemblages with abundant B. spissa have been described in TI sediments (Bubenshchikova et al., 2010;Gorbarenko et al., 2002a;Gorbarenko et al., 2010) but were found neither in recent sediments nor in TII, TIII or TIV sediments (Saidova, 1961(Saidova, , 1997Barash et al., 2001Barash et al., , 2006Khusid et al., 2005; our unpublished data for core MD01-2415). The results of the present study reveal that the B. spissa assemblage was distributed during both TV and TI (Fig. 6a, c). ...
... In Stage 9, high productivity assemblages are dominant and diatom abundance is always above average, with the exception of a period 310 ka BP, when the open-ocean assemblages show an increase with large fluctuations. Two short open ocean events between 300 -340 ka were examined in recent research on the central Okhotsk Sea (Barash et al. 2006). Four cooling events were evidenced by corresponding increases in sea-ice assemblages and smaller numbers of diatom valves. ...
... Stage 8 was a very warm ice age, quite distinct from the other glacial periods. In support of our observations , this anomalous stage was previously examined by Barash et al. (2006) and Lui et al. (2006). In addition, previously determined alkenone-SST in the PC2 (Seki et al. 2004a, b) suggest a warm event in Stage 5a and a cold period in early Stage 3; both these events are recognized in this study. ...
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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.
... л.н. (Q 2 5 -Q 2 2 ), который включает изотопно-кислородные стадии (ИКС) 7-10 [ Barash et al., 2006]. Для холодного интервала ИКС 10 (350-337 тыс. ...
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The analysis of the neotectonic structure of the region is usually accompanied by the compilation of neotectonic maps and schemes. Models of the summit surface serve as the initial material for compiling different-scale neotectonic maps and schemes [Ufimtsev, 1984]. The summit surface is one of the main properties of the Earth's recent topography (including the land surface and sea bottom) and represents an ideal surface connecting the maximal heights of the present-day relief of different geomorphologic levels. The universal development of the summit surface allows in to be used for revealing and studying the neotectonic structural elements both on land and seas. When compiling the structural - neotectonic map of the Sea of Okhotsk (Fig. 2), we accepted the polygenetic polychronous "summit" surface of the sea bottom shown by the isobaths relative to the present-day sea level as the primary ("structural") one. The map is largely based on data from the bathymetric maps and represents, in fast, a static model of neotectonics. The structural-neotectonic map served as a basis for compiling the scheme of the principal neotectonic structural elements of the studied region (Fig. 3). For clarifying the formation history of the neotectonic structural elements, we compared their present-day spatial position relative paleogeographical schemes of the lithophysical complexes (LC), which are united into four regional seismostratigraphic complexes (RSSC) corresponding to the following time intervals: RSSC I to K2-P1-2; RSSC II to P3-N11; RSSC III to N11-2; RSSC IV to N13-N2 [Sergeyev, 2006], besides showed general characteristic of the paleogeographical settings that controlled the accumulation of different lithophysical complexes (Fig. 4).
... Earlier, based on the geochemical and micro-paleontological studies of planktonic and benthic microfossils in the sediments of the OS, the variations in the sea paleoenvironment on the orbital scale were traced and assumptions of the Quaternary paleoceanography of the OS during glacial periods and interglacial optima were made (Basov et al., 2001;Barash et al., 2001Barash et al., , 2006Matul and Abelmann, 2001;Gorbarenko et al., 2002;Okazaki et al., 2003aOkazaki et al., , 2003bOkazaki et al., , 2005Itaki et al., 2008;Matul et al., 2009Matul et al., , 2013Bubenshchikova et al., 2010;and others). ...
... The nucleotide substitu tion rate between the M. stelleri groups from these localities, assessed from the cytb based calibration [30] is thought to have a paleoclimatic interpretation within an interval of 0.23 to 0.45 million years ago. According to the fossil data from the sediments of the Okhotsk Sea, that was the period of cyclic glacialinterglacial changes in climatic variations of Late Pleistocene [31,32]. In this regard, the genetic vari ability of M. stelleri from the Sea of Japan and Okhotsk Sea, as assessed from the mtDNA encoded data, may be associated with isolation during glacial optima. ...
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Abstract—The phylogenetic relationships among five species of sculpins, including Myoxocephalus stelleri, M. brantii, M. jaok, M. ochotensis, and Megalocottus platycephalus, were estimated from the sequence vari� ability of the mtDNA cytochrome b (cytb) and cytochrome oxidase 1 (CO1) genes. Analysis of the topologies of combined phylogenetic trees showed that all of the morphologically described species from different gen� era represented monophyletic groups with high support of branch robustness. Haplotypes with different kary� otypes from different geographical localities with an intragroup nucleotide diversity of 0.42% were combined into the M. stelleri clade. All of the species (except for M. stelleri from the Sea of Japan and the Okhotsk Sea coast of the Hokkaido Island) were characterized by relatively low values of the intragroup variation, along with high values of interspecific variation of mtDNA�encoded markers. Studies of the M. stelleri karyotypes using Ag�banding showed that karyotypes of the individuals from the Sea of Japan and Okhotsk differed not only in the number of chromosomes (2n) but also in the number of active nucleolus organizers (NO) and stained NO blocks, calling into question whether they belong to a single species. The observed discrepancy between the phylogenetic topologies and karyological data is discussed in relation to the informative capacity of mtDNA fragments and the phenomenon of intraspecific chromosomal polymorphism.
The response of benthic foraminifera (BF) and radiolarian production in the central Okhotsk Sea (OS) to orbital and millennial changes of climate and environment over the last 136 kyr have been studied on the basis of the sediment core PC-7R, with available productivity, geochemical and lithological records and a modified age model. We used the calculated accumulation rate of microfossils (AR, fluxes) of the main ecological groups and species indicative of radiolarians and of BF as quantity responses in their production, which is related to environmental and hydrological changes of the water column and surface sediment in the central OS and to global and regional climate changes. The responses of the opportunistic BF Alabaminella weddellensis and the suboxic group during the last 136 kyr were mostly related to changes in organic fluxes from the surface, sea ice cover and surface sediment oxygenation at both orbital and millennial time scales. The AR of radiolarians from the surface, dichothermal and mesopelagic groups significantly differed from the AR of BF. The AR has a maximum during the middle and late Marine Isotope Stage 5e and lower values during the earlier Holocene, while the minimum happened during cold Marine Isotope Stages 6, 4 and 2 with maximal ventilation of OS intermediate water. Production of the mesopelagic radiolarian Cycladophora davisiana in the OS was mostly related to the lateral delivery of terrigenous material and OM into the water column from the northwestern shelf and/or from the western subarctic Pacific. Due to the sensitivity of marine organisms to environmental conditions, the production of radiolarians and BF sharply changed with the extinction of Amphimelissa setosa, since Heinrich Stadial 8 is related to the termination of global environment cooling and the hydrology changes of the early last glacial.
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.
The study of assemblages of benthic and planktonic foraminifers showed that during most of the recent 60 Ka the temperature of surface and deep waters in the area of the USSR Academy of Sciences Rise, Sea of Okhotsk, was lower than the current temperature. Warming of surface and deep waters occurred 50000, 30000, and 13280 years ago.
A combined examination of the siliceous microfossils (diatoms and radiolarians) and the lithophysical parameters (magnetic susceptibility and contents of the coarse-grained sediment fraction and the ice-rafted detritus) revealed the stratigraphy of the sediment core from the deep northern shelf of the Sea of Okhotsk in the scale of the oxygen-isotope stages for the last two glacial cycles of the Pleistocene and allowed us to make a number of paleoceanographic conclusions. The data on the ice-rafted detritus exhibit strong variability, and a similar amplitude of such changes existed in the ice formation on the northern shelf of the sea both during the glacial period and during the interglacial period. Based on the data on the assemblages of siliceous microfossils, we can conclude that the interglacial optimum of the oxygen-isotope substage 5e was the warmest interval for the last 220 ky, which reflected the increased influence of the North Pacific water masses on the paleoenvironment of the Sea of Okhotsk.
Diatoms are the most abundant microfossils in seafloor sediments of the northern North Pacific area, but confusion over taxonomy has impaired their use in biostratigraphy and paleoecology. Analysis of more than 200 surface sediment samples reveals several distinct diatom assemblages which appear to be related to environmental regimes. Referring to original diagnoses, these assemblages can be sorted into 36 species, of which one, Delphineis kippae, is new.