Late Quaternary environmental changes on the Southeastern slope of the Sea of Okhotsk inferred from benthic foraminifera

Abstract

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.

Full text

Oceanology, Vol 45, No. 3, 2005, pp. 413-4l9. Translated from Okeanologiya, Vol 45, No. 3, 2005, pp. 440-446.
Original Russian Text Copyright €> 2005 by Khusid, Barash, Biebow, Niirnberg, Tiedemann.
English Translation Copyright €> 2005 by Pleiades Publishing, Inc.
MARINE ________________
GEOLOGY
Late Quaternary Environmental Changes on the Southeastern
Slope of the Sea of Okhotsk Inferred from Benthic Foraminifera
T. A. Khusid*, M. S. Barash*, N. Biebow**, D. Nuernberg**, and R. Tiedemann**
* Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia
**Leibnitz Institute of Marine Research, Kiel, Germany
Received April 28, 2004
Abstract—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 cov-
ers the last 180 ky, from marine isotopic stage (MIS) 6 to the present time. The substantial quantitative and tax-
onomic changes in the assemblages of benthic foraminifera reflect the climatic and paleoceanographic varia-
tions. 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 (lux 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 signif-
icantly 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 pro-
duction and the rise of its lower boundary were less significant.
INTRODUCTION
The hydrological processes at deep levels of the
modern Sea of Okhotsk are of importance for the entire
northwestern Pacific. The oxygen-rich water produced
on the shelf of the Sea of Okhotsk provides ventilation
down to depths of 1000-2000 m. The high sedimenta-
tion rates in the Sea of Okhotsk open opportunities for
detailed paloeoceanographic reconstructions and for
the study of its contribution to the hydrological changes
in this region during the Quaternary with its sharp cli-
matic fluctuations.
The taxonomic composition of the benthic foramin-
iferal communities populating the bottom surface and
thin subsurface sediment layer, as well as their abun-
dance, depend on many factors such as, primarily, the
intensity of the organic flux to the bottom, the chemis-
try of the near-bottom waters, and the hydrodynamical
regime. The study of fossil benthic foraminiferal
assemblages and their comparison with their present-
day counterparts makes it possible to reconstruct the
bottom environments and their dynamics in the geolog-
ical past, particularly the changes in the vertical hydro-
logical structure.
In the modern Sea of Okhotsk, the thin (up to 40 m)
upper water layer characterized by high seasonal vari-
ability is underlain by a cold dichothermal layer 100-
150 m thick, which represents a relict of the winter
mixed layer. This water washes the shelf and the upper
part of the continental slope.
The dichotermal layer, in its turn, is underlain by
several water masses, which influence the composition
and distribution of benthic foraminifera. Immediately
below the dichothermal layer, there is the Okhotsk Sea
Intermediate Water (SOIW) mass with a characteristic
temperature of+1.5°C, a high oxygen content (up to
6.5 ml/1), and a salinity of approximately 33.7%o [6, 8, 33].
This cold and relatively saline water is mainly gener-
ated on the northwestern shelf of the Sea of Okhotsk
under the influence of sea ice, from where it flows down
the continental slope to occupy its typical depths
[6, 21]. Passing via the Kuril straits [5], the water is
transformed and contributes much to the ventilation of
the North Pacific [30]. The young Okhotsk Sea Inter-
mediate Water overlies old Pacific waters with higher
temperatures (+2.2-2.5°C) and salinities (34.3-34.5%o)
and lower oxygen contents (0.5-0.8 ml/1); they occupy
depth from 600 to 1300 m. This intermediate water
mass of the North Pacific enters the Sea of Okhotsk
through the Kuril straits.
Owing to the high nutrient concentrations in the
waters and their particular vertical structure and
dynamics, the Sea of Okhotsk is one of the most pro-
ductive basins of the World Ocean [9].
The Upper Quaternary sediments of the Sea of
Okhotsk are sufficiently well studied by lithological,
isotopic, micropaleontological, strati graphic, tephro-
chronological, and geochemical methods. These stud-
ies revealed the main particularities in the paleoceano-
graphic evolution of this basin. It was established that
413

414
KHUSID el al.
т i i i i i i
Fig. 1. Location of the core examined in the Sea of Okhotsk.
the Holocene and the interglacial optimum correspond-
ing to MIS 1 and MIS 5e are largely characterized by
diatomaceous sediments with insignificant admixtures
of terrigenous and ash material. Terrigenous sediments,
icerafted included, with admixtures of ash and organ-
ogenic components accumulated during the glaciations
corresponding to MIS 6 and MIS 5(da)MIS 2.
The studies dedicated to the benthic foraminifera in
the Quaternary sediments of the Sea of Okhotsk [1,3,4,
1114, 18, and others] present comprehensive data on
their assemblages studied in different areas of the basin
at different depths. It is noted that the abundance of
benthic foraminifera in the sediments that accumulated
during interglacial periods is usually several times higher
than during glaciations, which is explained by the higher
biological productivity of the wanner basin. The study of
the foraminiferal taxonomic composition in the
Holocene, glacial, and interglacial sediments showed
that their assemblages were substantially diverse during
warm periods and rather uniform during glacial periods.
The foraminifera] assemblages of the warm periods
are mainly composed of Uvigerina (U.) peregrina, Val
vulineria (V.) sadonica, and Cassididina (C.j laevigata.
The latter species is dominant during the degiaciation
(beginning of MIS 1) and interglacial optimum (MIS
5e). In addition to the species listed, the assemblage of
MIS 5e contains abundant tests of the agglutinating
species Martinottiella communis. The assemblages of
glacial periods mostly consist of Uvigerina auberiana,
Angulogerina angulosa, and Cassididina teretis
(Islandiella norcrossi).
MATERIAL AND METHODS
Benthic foraminifera were studied in core LV28443,
which was 1112 cm long and obtained from the Kam-
chatka continental slope (52°02.514' N, 153°05.949' E)
at a sea depth of 684 m during cruise 28 of R/V Aka
demik M.A. Lavrent'ev, which was carried out in line
with the RussianGerman KOMEX project in the
southeastern part of the Sea of Okhotsk (Fig. 1). Station
LV2844 is located near the boundary between the
young Okhotsk Sea Intermediate and the Pacific Deep
water masses. At all the stations studied during this
cruise, a jump in the temperature and hydrochemical
parameters, i.e., the boundary between these water
masses, was registered at depths of approximately
600 m [15, Appendix 4, p. A58].
The following layers are defined in the core section
(after [15], simplified):
(0145 cm) diatomaceous ooze enriched in foramin-
ifera;
(145354 cm) clayeysandy silt, slightly diatoma-
ceous, with abundant foraminifera and interbeds of volca-
nic ash (194212 cm), foraminiferal sand (231235 cm),
and coarsegrained icerafted debris (253254 cm);
(354454 cm) sandy silt with coarsegrained mate-
rial, lenses of black sand, and with an interbed of white
volcanic ash (374377 cm);
(454625 cm) clayeysandy silt with an admixture
of coarsegrained matter and lenses of black sand;
(625663 cm) clayeysandy highly diatomaceous
silt;
(663837 cm) sandy and clayeysandy silt with
sandy beds in the interval of 680685 cm and at 800
and 825 cm;
(837932 cm) diatomaceous ooze with interbeds of
foraminiferal sand at 897 and 902 cm;
(9321112 cm) clayeysandy silt with admixture of
coarsegrained icerafted material (dropstones) and
lenses and interbeds of black sand.
The stratigraphic subdivision and age model for this
core are based on the lithostratigraphic, biostrati
graphic, tephrochronological, and magnetic suscepti-
bility data. The ages of the boundaries and events with
respect to the global oxygenisotope scale are accepted
after [25].
The upper layer of diatomaceous ooze 145 cm thick
(06.7 ky B.P.) accumulated in a setting similar to the
presentday one.
Sediments from the interval of 145260 cm up to the
MIS 1MIS 2 boundary (12.05 ky B.P.) characterize the
transition from the glacial environments to presentday
ones.
The terrigenousvolcanogenic sediments constitut-
ing the interval of 260837 cm accumulated under the
influence of the continental glaciation that existed dur-
ing MIS 2 to MIS 4 and the late stages of MIS 5 (da),
i.e., in the period from 117 to 12.05 ky B.P.
OCEANOLOGY Vol. 45 No. 3 2005

LATE QUATERNARY ENVIRONMENTAL CHANGES ON THE SOUTHEASTERN SLOPE 415
The layer of diatomaceous ooze (the interval of
837-932 cm) corresponds to the optimum of the last
interglaciation (MIS 5e) and is 117 to 127 ky old
according to the accepted model.
The lower terrigenous-volcanogenic layer (interval
of 932-1112 cm) accumulated during the terminal part
of the penultimate continental glaciation (MIS 6) in the
period from 174 to 127 ky B.P.
The distribution of the main coarse-grained (frac-
tion >0.125 mm) components and planktonic foramin-
ifera over this core and the relevant paleoceanographic
reconstructions are described in [2].
In total, 117 samples taken with a step of 10 cm
(locally 5 cm) were studied. Benthic foraminifera were
examined in coarse-grained (>0.125 mm) fractions. For
the foraminiferal analysis, residues were quartered to
obtain weighted samples containing at least 300 tests
each. The remaining parts of the residues were exam-
ined under a microscope to identify rare species. Sam-
ples with low contents of tests were studied without
quartering. The taxonomic composition of the foramin-
iferal assemblages, the percentages of individual spe-
cies, and the abundances (individuals per gram of dry
sediment) were determined for each sample.
RESULTS AND DISCUSSION
Benthic foraminifera are present in all the samples
examined (Fig. 2). The abundance of foraminifera shows
significant variations throughout the core. Their highest
abundances are characteristic of the diatomaceous sedi-
ments in the intervals of 0-140 and 850-930 cm that accu-
mulated in the Late Holocene and during the interglacial
optimum. The foraminifera abundance varies from 100 to
450 indVg in the Holocene sediments and from 200 to
900 ind./g in the sediments of the interglacial optimum. In
the sediments corresponding to its early stage, the abun-
dance of foraminiferal tests amounts to 1400 ind./g. In the
Holocene, the sedimentation rate was as high as 20-
25 cm/ky, which is almost two times higher as compared
with that during MIS 5e. Taking into consideration the fact
that the foraminiferal abundance in the Holocene was two
times lower and the sediments corresponding to these peri-
ods are similar, one can conclude that the productivities of
the foraminifera was also similar.
In the sediments accumulated during deglaciation
(MIS 1), the foraminiferal abundance is lower than in
the diatomaceous oozes ranging from 70 to 160 ind./g.
Inasmuch as their density is 1.5 times higher as com-
pared with that of the diatomaceous oozes, while the
sedimentation rates were almost similar, it can be
assumed that, since the deglaciation time until the
present, i.e., during the last 12 ky, the productivity of
benthic foraminifera was uniformly high.
In the glacial period (MIS2-MIS5a-d), the sedi-
mentation rates were 4-5 times lower as compared with
their values in the Holocene. The foraminiferal abun-
dance in the glacial sediments does not exceed 50 ind./g
often decreasing down to 10 ind./g and, sometimes, to
a few individuals per gram of dry sediment. Only in the
interval of 650-660 cm with a notable admixture of dia-
toms does their abundance increase up to 70-100 ind./g
of sediment. Taking into consideration the lithology
and sedimentation rates, we can suggest that the abso-
lute accumulation rates of benthic foraminifera during
the glacial period were approximately one-two orders
of magnitude higher than during the Holocene.
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. The dif-
ference is in the amplitude of the changes. In the assem-
blages of MIS 1, benthic forms constitute from 3 to
60% of all the foraminiferal tests with their minimal
share of 3-6% during the deglaciation and with a max-
imal proportion up to 40-60% in the Late Holocene.
The share of benthic forms in the assemblages of MIS 5e
is less than 20%. The highest proportion of benthic spe-
cies is observed in the glacial foraminiferal assem-
blages, where it is as high as 60 to 99-100%, which is
explained, in our opinion, by the extremely low produc-
tivity of planktonic foraminifera.
In total, 65 benthic species were found in the exam-
ined core. The species composition of the assemblages
is highly variable reflecting the changes in the bottom
environments. The main features of the bottom environ-
ments can be reconstructed by comparing the fossil
assemblages of the benthic foraminifera with their
present-day counterparts, whose distribution was con-
sidered in [10 and others]. Only a few species occur
throughout the entire section: Cassidulina (C.) teretis,
Miliammina (M.) herzensteini, Uvigerina (U.) pereg-
rina, Alabaminella (A.) weddellensis, and Valvulinera
(V.) sadonica. The distribution of first three species cor-
responds to the trend of climatic changes. For instance,
Cassidulina teretis is most abundant (40-60%) in gla-
cial assemblages, while, in the warm periods, its abun-
dance decreases down to 10-15%. M. herzensteini
becomes abundant (30-33%) also during glaciations
and is highly subordinate (up to 1-2%) in the warm
periods. In contrast, U. peregrina shows maximal abun-
dance (30-40%) in the Holocene and during the opti-
mum of the last interglaciaton, while, in glacial assem-
blages, its share decreases down to 1-2% and is some-
times (MIS 2 and 5a) as high as 10-30%. The content of
V. sadonica is almost uniform throughout the entire col-
umn. The A. weddellensis share varies from 1-2 to 30-
40% regardless of the climatic changes.
Of highest interest are the less abundant species, the
distribution of which depends on certain climatic con-
ditions. For instance, Bolivina (B.) seminuda, Cassid-
ulina obtusa, C. laevigata, Elphidium (E.) batialis,
Nonionella (N.) labradorica, and N. digitata occur
almost exclusively during warm periods. At least, they
OCEANOLOGY Vol. 45 No. 3 2005

416 KHUSID et al.
Oxygen isotope stages
Abundance, ind./g
1500
1000-
500
0 100 200 300 400 500 600 700 800 900 1000 1100
cm
Fig. 2. Distribution and percentage of benthic foraminiferal species in core LV28-44-3.
OCEANOLOGY Vol. 45 No. 3 2005

LATE QUATERNARY ENVIRONMENTAL CHANGES ON THE SOUTHEASTERN SLOPE 417
become abundant in these periods. В seminuda and
C. obtusa are abundant in the Holocene: the former in
the Late Holocene and the latter during the deglaciation
and Early Holocene. In the assemblages of the intergla
cial optimum, their share is minimal. During this
period, C. laevigata, which is missing in the Holocene,
is abundant. The other abovementioned thermophilic
species E. batialis, N. labradorica, and N. digitata are
abundant during both the Holocene and the MIS 5e
stage. Species belonging to the genera Cibicides
(C. lobatulus and C. kullenbergi), Angulogerina (A)
angulosa, together with C. teretis, are the main constit-
uents of the coldwater assemblages characteristic
of the MIS 2MIS 4, MIS 5ad, and MIS 6 glacial
periods.
The ecological preferences of the species that pre-
vail in either cold or warm periods are different. The
feature in common for the coldresistant species is their
habitat mode: they live at the bottom surface. Some of
them attach to the substrate such as Cibicides lobatulus
and others are characterized by a free way of life.
C. lobatulus, C. kullenbergi, and A. angulosa are
sestonophagous forms: they are suspension feeders. For
these forms, intense bottom hydrodynamics are favor-
able [16, 17, 22, 23, 32]. C. teretis is an eurybiont spe-
cies, which is characterized by high variability with
respect to its feeding and dwelling modes [32].
A compositionally close assemblage, dominated by
the species of the Cibicides genus, Angulogerina angu-
losa, C. teretis, and M. herzensteini, is found in the
recent sediments on the northern shelf and slope of the
Sea of Okhotsk, in the Kashevarova Bank region, north
of Iona Island, and near the Kuril island arc [10]. These
areas are washed by the cold oxygenrich (up to 5 ml/1)
nearbottom water. The area near the Kuril arc is char-
acterized by strong nearbottom currents with veloci-
ties up to 1540 cm/s [6].
The prevalence of these species during the cold peri-
ods (MIS 2MIS 4, MIS 5ad, and MIS 6) implies that
the area sampled was washed by the Sea of Okhotsk
Intermediate Water, the lower boundary of which low-
ered to deeper levels at those times. This assumption is
consistent with the concept of higher water productivity
during these periods [18, 20].
Almost all the species that form mass populations
during the warm epochs are detritophagous: they con-
sume organic detritus from the sediments and are capa-
ble of burying themselves deep into sediments to
escape anaerobic environments, which can develop at
the bottom in highly productive areas [16, 26, 28, 29].
Some abundant species characteristic of the warm
periods demonstrate specific features. For example,
Elphidium batialis is now abundant on the slope of the
KurilKamchatka Trench in the Pacific Ocean and on
the slopes of the Shirshov Ridge [10], the most produc-
tive area of the Bering Sea, where the Corg content in the
sediments exceeds 2% [7]. This species shows maximal
concentrations during the deglaciation at the beginning
of MIS 1 and during MIS 5e. B. seminuda, which is
characteristic of highly productive areas of the World
Ocean, can survive extreme oxygen deficiency down to
<0.1 ml/1 [19, 27, 28]. This explains the abundance of
this species in the Upper Holocene sediments during
the last 6 ky, when the biological productivity and the
organic matter flux to the bottom were maximal. Pre-
cisely this period demonstrates an increased share of
benthic forms in the total quantity of foraminifera (up
to 60%). The bottom waters during this period were
likely deficient in dissolved CaC03, which resulted in
the partial dissolution of the carbonate tests, especially
those of planktonic forms.
Based on the quantitative changes in the benthic for
aminiferal assemblages, their taxonomic composition,
and the lithology of the sediments, it can be assumed
that two main types of bottom hydrological regime
were characteristic of the last 200 ky. During the long
glacial periods corresponding to MIS 2MIS 4, MIS
5ad, and MIS 6, the bottom environments were char-
acterized by high hydrodynamic activity. The biologi-
cal productivity at those times was significantly
reduced, and the benthic communities were dominated
by sestonophagous organisms. The high proportions of
С. teretis against the background of the low total abun-
dance of benthic foraminifera are explained by the high
resistance of this species to the unfavorable conditions
of the glacial period.
The foraminiferal assemblages from the diatoma
ceous ooze accumulated during MIS 5e and the
Holocene include many species in common: Non
ionella labradorica, Uvigerina peregrina, and Bolivina
seminuda. Most similar are the assemblages character-
istic of MIS 5e and of the deglaciation period (12
9 ky B.P ago) with the main component represented by
Elphidium batialis (approximately 40%), which is rare
in the Holocene diatomaceous oozes. The feature in
common for these assemblages is also the relatively
low share of benthic forms (1012%) in the total fora-
miniferal community, which exceeds 20% and can be
as high as 4060% in the diatomaceous oozes. Thus,
during the optimum MIS 5e, the bottom environments
were most similar to those at the beginning of MIS 1.
As was noted, Cassidulina laevigata, which is miss-
ing in the sediments of MIS 1, was abundant during
MIS 5e. This species dominates under bottom water
temperatures exceeding 2°C [24]. In the modern Sea of
Okhotsk, it occurs at depths from 900 to 1100 m within
the Pacific water mass with temperatures exceeding
2.02.5°C [10]. The core V28443 examined is sam-
pled from a sea depth of 684 m near the upper boundary
of the Pacific Deep Water (temperature of 2°C). Now,
these depths are occupied by a mixed assemblage lack-
ing dominant species: Uvigerina peregrina, Elphidium
batialis, E. incertum, Globobulimina auriculata, Cas-
sidulina teretis, and Angulogerina angulosa are equally
represented
OCEANOLOGY Vol. 45 No. 3 2005

418 KHUSID
Judging from the distribution of C. laevigata, the
thickness of the Okhotsk Sea Intermediate Water mass
decreased in the periods of its reduced production on
the shelf; its boundary with the underlying Pacific
waters substantially rose to take its position above the
station LV28443 site. If the core of the Pacific water
mass actually rose to this depth, the amplitude of its
boundary can be estimated at 200400 m. The absence
of С laevigata in the Holocene sediments indicates that
the reduction in the Okhotsk Sea water production and
the rise of its lower boundary were less significant at
that time.
CONCLUSIONS
(1) Substantial quantitative and qualitative changes
in the assemblages of benthic foraminifera reflect cli-
matic and oceanographic changes. The insignificant
content of foraminiferal tests in the sediments accumu-
lated during the glacial periods (MIS 6, MIS 5da, MIS 2)
indicates the low organic matter flux to the bottom at
those times. During the deglaciation period and the
Holocene (MIS 1), particularly during the interglacial
optimum (MIS 5e), the organic flux to the bottom sig-
nificantly increased. Indirect calculations show that the
accumulation rates of benthic foraminifera in the gla-
cial sediments were approximately onetwo orders of
magnitude lower as compared with those in the inter-
glacial periods.
(2) The foraminiferal assemblages of the glacial
periods (MIS 6, MIS 5(da), and MIS 2) are mainly
composed of sestonophagous species, which are sus-
pension feeders and prefer highenergy settings. The
prevalent development of sestonophagous species is
characteristic of the glacial periods with a more intense
production in the waters on the northern shelf, which,
flowing down over the continental slope, formed the
young Okhotsk Sea Intermediate Water mass.
(3) The foraminiferal assemblages characterizing
the warm epochs of MIS 1 and MIS 5e are mainly com-
posed of detritophagous species, which consume
organic matter from the sediments and are capable of
penetrating deep into the sediments to survive the
strong oxygen deficiency. The development of detri-
tophagous species is favored by the lowenergy near
bottom hydrodynamics and by the high flux of organic
matter produced by phytoplankton to the bottom sedi-
ments. Such environments are now characteristic of the
areas washed by the old Pacific waters below the depth
where the core examined was obtained.
(4) The reduced production in the Okhotsk Sea
waters during the warm period corresponding to the
optimum of MIS 5e resulted in a decrease of its thick-
ness and in a substantial rise of its boundary with the
Pacific water mass by approximately 200400 m. Dur-
ing the Holocene, the reduction in the Okhotsk Sea
water production and the rise of its lower boundary
were less significant.
et al.
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64567a.
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