About the reason of abnormal distribution of N. pachyderma sin. in Quaternary sediments of the Okhotsk Sea as a subarctic basin of the northwest Pacific

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FORAMS 2002. Int. Symp. on Foraminifera. 4-8 Febr. 2002.
Perth, Australia. 2002. P. 82.
M.S.Barash, N.V.Bubenshchikova
(P.P.Shirshov Institute of oceanology, Moscow, Russia)
About the reason of abnormal distribution of N. pachyderma
sin. in Quaternary sediments of the Okhotsk Sea as a subarctic
basin of the northwest Pacific
In sediments of cold-water areas of the World Ocean the curves of N.
pachyderma sin. percentage are usually very expressive as a climatic
indicator: the increase of its concentration corresponds to lowering of
temperature.
However, in the Okhotsk Sea situated in the northwestern Pacific, just as
probable in other subpolar regions of the World Ocean, the relations are
more complex.
In the modern Okhotsk Sea upper water layer (about 0-40 m) is
characterized by a high seasonal variability. The summer temperatures reach
10-15 degrees. In winter the large part of a surface is covered by ice.
Beneath this layer there is a near-freezing cold layer with a thickness
ranging from 100 to 150 m (“dichothermal” layer). This layer is a remnant
of the mixed upper winter layer. In the spring, summer, and autumn it
appears as a cold intermediate layer on depths of 40-150 m. It is
characterized by salinity of 32.9-31.0 0/00 and on the greater part of the sea of
negative temperature from -1.7°C up to positive significance more + 1°C in
area of the Kurile Islands. Similar structure is characteristic for the subarctic
areas where under the top layer of water with strong seasonal variability
there is a layer of cold water as a relict of winter cooling.
In a sediment core LV28-42-5 from the central part of the Okhotsk Sea the
complex micropaleontological study as well as other stratigraphic
investigations were done. According to the accepted age model the
sediments were accumulated during about 190 Kyr and represented the
marine isotopic stages (MIS) 6-1, i.e. the period from the Penultimate
continental Glaciation to the Holocene and Recent. Ten taxa of planktonic
foraminifers with acknowledged climatic characteristics are revealed. The

most cold-water N. pachyderma sin. dominates all along the core comprising
60-90 % of the fauna. (FIGs)

During certain intervals of the Penultimate and Last Glaciations (MIS 6 and
4-2) N. pachyderma sin. makes up at least 80% in all samples. It reflects a
low water temperature. Above these intervals decreases in N. pachyderma
sin. reflect increases of temperature, which should be expected at transitions
to warmer conditions of the Interglaciation Optimum (MIS 5, substage “e”)
and Holocene (MIS 1). However during these warm intervals when
oceanographic conditions arise, which exist in the Okhotsk Sea today,
paradoxical increase in N. pachyderma sin. took place.
According to plankton and oxygen-isotope data three groups of species have
been distinguished on the basis of prevailing depths of their life habit:
“shallow-water species” dwelling predominately in the upper 50 m of the
water column; “intermediate-water” species living in the upper 100 m but
predominately in the interval 50-100 m; and “deep-water species” living in
the upper few hundred meters and whose adult stages occur predominately
below 100 m. Among the cold-water species characteristic of the Okhotsk
Sea, Gq. pachyderma is “deep-water”, G. bulloides and Gt. glutinata are
“intermediate-water” species, and G. quinqueloba is “shallow-water”.
Among the cold-water species characteristic of the Okhotsk Sea, N.
pachyderma is only “deep-water” species. We infer that the abnormal
oceanographic conditions (very high season variability) reflected by the
occurrence of the dichothermal water structure suppress the development of
“shallow-water” (G. quinqueloba) and “intermediate-water” (G. bulloides

and Gt. glutinata) species and thus N. pachyderma sin. as a deep-water
dweller increases its relative abundance.
We infer that the special subarctic oceanographic conditions (very high
variability of upper water layer) suppress the development of shallow-water
and intermediate-water species and thus Gq. pachyderma sin. as a deep-
water dweller increases its frequency.
Such features of distribution of planktonic foraminifera in the Quaternary
sediments are necessary to take into account at paleoceanographic
interpretation of micropaleontological data.