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Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms
Quaternary International xxx (2017) 1e12
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Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms Antonina Artemova a, *, Sergey Gorbarenko a, Yuriy Vasilenko a, Xuefa Shi b, c, Yanguang Liu b, c, Min-Te Chen d, ** a
V.I. Ii'ichev Pacific Oceanological Institute, FEB RAS, 43 Baltiyskaya Street, Vladivostok, 690041, Russia First Institute of Oceanography, SOA, 6 Xianxialing Road, Qingdao, 266061, China Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266061, China d Institute of Earth Sciences Dean of the College of Ocean Science and Resource, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, 20224, Taiwan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 December 2016 Received in revised form 2 October 2017 Accepted 3 October 2017 Available online xxx
Paleontological records of six sediment cores in the Okhotsk Sea (OS) marked the regional environmental changes over the last 140 kyr on orbital time scales. The diatom assemblages and content of diatom frustules in the sediments during Marine Isotope Stage (MIS) 6e1 indicate the dramatic climatic and environmental changes in the OS. A small abundance and low diatom species diversity as well as the high percentage of near-ice species indicate the cold surface environmental condition during glacial time (MIS 6, 4, 2) with low temperatures, cold climate conditions and extended sea ice cover. The presence of extinct redeposited species in the glacial assemblages indicates a low sea level during this time. The proportion of ice species enlarged and diatom abundance reduced due to increase of the influence of the sea ice, reflecting the sharp climatic cooling of adjacent land and regional environmental deterioration. The subsequent increase in diatoms productivity at 129.8e117.0 kyr BP and 8.3e5.5 kyr BP indicates the strong climate warming accompanied by decrease of sea ice coverage and surface water stratification (mixing of surface and intermediate water) during the warmest MIS in the Okhotsk Sea. The diatom abundance and high content of the oceanic and warm-water species reflect the warm surface environmental condition during MIS 5e and 1 since 8.3 kyr due to decrease of the sea ice influence. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Diatom assemblages Okhotsk Sea Environment Holocene Late Pleistocene
1. Introduction The aim of this work is to reconstruct the climate and paleoenvironment conditions of the Okhotsk Sea during the Late Pleistocene and Holocene by diatom abundance and species changes in the fossil assemblages from the deep sea sediments. Diatoms are the most informative microfossils for palaeoceanographic research , 1962). (Jouse The Okhotsk Sea, marginal sea of the North Pacific, plays important role in the Pacific hydrology of Northwestern Pacific Intermediate Water masses formation (Honjo, 1997). Relatively warm and saline Pacific waters flow northward as West Kamchatka Current, and then mixed on the northern shelf and flow to south.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A. Artemova), [email protected] (M.-T. Chen).
The Japan Sea warm saline water flows into the southwestern part of the Okhotsk Sea (Fig. 1). The high diatom productivity of the Okhotsk Sea is related to nutrients abundance. There are two main sources of nutrients: Amur River and Pacific water (Chen et al., 2004). Vertical mixing supplies these nutrients to a photic zone resulting in the increase of productivity. Data on the taxonomic structure of diatom assemblages and the distribution of most typical species in the Okhotsk Sea sediments , 1962; Sancetta, 1981, 1982; Shiga were studied from 1962 (Jouse and Koizumi, 2000, Gorbarenko et al., 2010, 2012, 2014; Pushkar and Cherepanova, 2001, 2008, Tsoy et al., 2009; Ren et al., 2014). These researches showed that diatoms abundance and species composition in their assemblages for both the sediment cores and surface sediments. The researchers connect the diatom assemblages with water masses hydrology, sea-ice condition and sedimentation at the main bottom topography. However, diatom assemblages changes related to the Okhotsk Sea environment changes during the Late Pleistocene, deglaciation and Holocene
https://doi.org/10.1016/j.quaint.2017.10.002 1040-6182/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
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A. Artemova et al. / Quaternary International xxx (2017) 1e12
Fig. 1. Location of the investigated cores PC 07-R, LV 27-2-4, LV 28-41-5, LV 28-40-5, GE 99-10-3, 934.
remain still poorly studied in spite of the important diatom role as sensitive proxy of a surface hydrology and sea ice coverage variability in the OS. Here we presented the generalized data of different diatoms assemblages for six earlier dated sediment cores from different parts of the Okhotsk Sea over the last 140 kyr. The obtained results of studying the cores allow us to identify six diatom assemblages reflecting the OS climate and environmental changes over the last 140 kyr and correlate them with MIS 6 - MIS 1 according to earlier obtained stratigraphy (Gorbarenko et al. 2010, 2014). 2. Materials and methods The basis for this work was formed by the results of the diatom analysis for the several sediment cores obtained in the OS (Fig. 1, Table 1). Six gravity cores were obtained during the expeditions
carried out by the V.I. Il'ichev Pacific Oceanological Institute from the central, north-western and south parts of the Okhotsk Sea (Fig. 1, Table 1). Age model of the studied cores was earlier constructed for the MIS with age boundaries according to Martinson et al. (1987) by using the oxygen isotope records of planktonic and benthic foraminifera, magnetic sediment susceptibility records, AMS 14C data and tephrochronology (Gorbarenko et al., 2010, 2014). The sea ice conditions for studied core locations were earlier reconstructed by means of the calculation of the content of terrigenous particles in sediment fraction of more than 150 m and less than 2000 m per g of dry sediment called the ice rafting debris (IRD) (Vasilenko et al., 2011). The sedimentological changes indicated by variability of the magnetic sediment susceptibility (MS) were applied to reconstruct the Okhotsk Sea orbital scale climate changes according to our data (Gorbarenko et al., 2012; 2010, 2014) The taxonomic analyses, as well as the quantitative content of
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
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Table 1 Catalogue of station. Area
Station
Latitude
Longitude
Depth, m
Length, m
References
North-Okhotsk Rise Institute of Oceanology Rise Derugina Depression Pegasus Rise Northern slope of the Kuril Basin Central Part
LV27-2-4 LV28-41-5 LV28-40-5 GE 99-10-3 934 PC 07-R
54 30.15' 51 38.908' 51 20.040' 48 18.7' 48 32.5' 51 16.87'
144 45.14' 149 03.203' 147 10.631' 146º 08' 150 40.9' 149 12.57'
1305 1114 1312 1335 2195 1256
738 710 803 750 510 1722
Gorbarenko Gorbarenko Gorbarenko Gorbarenko Gorbarenko Gorbarenko
diatom frustules per weight unit of dry sediment) and ecological preferences of diatoms were applied. Diatom samples were prepared according to the standard technique (Gleser et al., 1974) for all cores in order to calculate the number of the diatom frustules per 1 g of dry sediment and species composition in diatom assemblages with determination of dominant and subdominant species. The study of diatoms was conducted via a Mikmed-6 microscope (LOMO, Russia) at 900 and 1300 magnification. To determine the number of diatom frustules per grammeme of air-dried sediment, we calculated the number of frustules in several horizontal rows with a subsequent recalculation to the absolute number in 1 g of sediment. To determine the quantitative ratios of diatom taxa in assemblages, 300e350 frustules were identified for each slide. As a result, we identified in the OS diatom assemblages several ecological groups esea ice, near-Sea ice and oceanic ones. Group of oceanic diatoms including Actinocyclus ochotensis , A. curvatulus Janisch, Neodenticula seminae (Simonsen et Jouse Kanaya) Akiba et Yanagisawa, Thalassiosira latimarginata Makarova, T. eccentrica (Ehrenberg) Cleve, Shionodiscus oestrupii (Ostenfeld) A.J.Alverson, S.H.Kang & E.C.Theriot, T. lineata Jouse, Coscinodiscus marginatus Ehrenberg, C. oculus-iridis Ehrenberg, Thalassiothrix longissima Cleve et Grunow, Rhizosolenia hebetata f. hiemalis Bailey dominates in the open part of the present-day OS without sea ice influence. Neodenticula seminae dominated mostly in the OS sedi, 1962; Sancetta, 1982) ments during the interglacial periods (Jouse and may be indicator of increased input of the relative warm and saline North Pacific waters. Species of Attheya septentrionalis (Østrup) Crawford in Crawford, Gardner et Medlin, Fragilariopsis oceanica (Cleve) Hasle, Fragilariopsis cylindrus (Grunow) Kreiger, T. nordenskioeldii Cleve, T. hyalina (Grunow) Gran, T. kryophilia Jorgensen grow on the ice edge, beneath the ice and inside the ice (Polyakova, 1997) and are referred to the sea-ice group. Bacterosira bathyomphala (Cleve) Syversten et Hasle, T. gravida Cleve and T. antarctica Comber grow in the Okhotsk Sea in the vicinity of the sea ice edges, under sea ice influence and, therefore, they may be considered as near-ice species group.). Abundance of T. gravida shows a positive correlation with sea-ice cover and link with increase in IRD and drop stones (Gorbarenko et al., 2014). Thus, the relative abundance of this species can be used as the index of the of sea-ice coverage. Paralia sulcata (Ehrenberg) Cleve, tychopelagic species is indicator of the freshened surface water in the marginal seas (Zong, 1997). 3. Results and discussion The diatoms were investigated in the cores 934, LV 28-40-3, LV 27-2-3 and GE 99-10-3 are presented in the papers Gorbarenko et al. (2010, 2012, 2014) and combined in Table 2. The diatom abundance and dominated species for core LV 28-41-5 versus core depth are presented in Fig. 2. The identified diatom assemblages 1e6 according to main variations in the different ecological groups and diatom abundance in the all studied cores were correlated with
et al., 2010 and Goldberg., 2005 et al., 2010 and Goldberg., 2005 et al., 2014, 2010 et al., 2010, 2012, 2014
established earlier MIS boundaries. Earlier published results of diatom analyses for cores 934, LV 2840-3, LV 27-2-3 and GE 99-10-3 and new results for cores LV 28-415 and PC 07-R with established MIS boundaries allow us to consider spatial pattern variability of the oceanic, near-ice and sea ice groups in the OS versus time over the last 140 kyr (Table 2, Fig. 3). Sediment chronology for studied cores within each MIS was calculated by interpolation between the ages of MIS boundaries with regard to constant sedimentation rates for each MIS. As our results show, the identified diatom assemblages for each core are correlated quite well with similarly-named MIS. Photos of dominated species in diatom assemblages accumulated during Late Holocene (0e5 kyr) and MIS 2e6 are shown respectively in the Plate. Assemblage MIS 6 (140e129.8 kyr) (Table 2, Fig. 3) was found only in the core LV 28-41-5 from Institute of Oceanology Rise. The diatom abundance in the sediment during glaciation's MIS 6 was extremely small (Fig. 4). A remarkable characteristic of assemblage is the absence of warm species suggesting weak inflow of warm Pacific waters to central OS. Oceanic species Thalassiosira latimarginata, Coscinodiscus marginatus, Rhizosolenia hebetata f. hiemalis dominate in the diatom assemblage suggesting the summer ice free condition in the central OS. However the significant sub domination of tychopelagic species Paralia sulcate frequency (6e19%) shows the freshening of the surface water as a result of spring sea ice melting. The presence of near-ice species, particularly, Thalassiosira gravida (1e9%) and small numbers of sea-ice species in assemblage of MIS 6 characterize also the season sea ice cover in the OS. The composition of diatom taxa indicates a prevalence of the cold water masses in the OS and the decrease of inflow of warm Pacific waters (without Pacific endemic species). A small number and low diversity of diatoms species, as well as the high percentage of near-ice species, suggest the cold environmental conditions with extended seasonal sea ice cover and its summer melting. Assemblage MIS 5 (73.9e129.8 kyr) (Fig. 4, Table 2). The quantity of diatoms and the proportion of the dominant species throughout the MIS 5 vary which allows us to identify during MIS 5 several assemblages correlated with Substage 5e-5a reflecting the paleoenvironment and climatic variability. Assemblage MIS 5e (117e129.8 kyr). The diatom assemblage during this period shows similarity to the present-day assemblage. The diatom content (up to 33*106 cells g1) is highest for studied time, and more than that in the Holocene. The assemblage is dominated by oceanic group species (up to 100%) Thalassiosira latimarginata, Neodenticula seminae, Rhizosolenia hebetata f. hiemalis, and Thalassiothrix longissimi and content of the warm-waters species increasts up to 8.7% suggesting the warmest climate condition. This diatom complex and their abundance reflect probably the warmest climate and summer environmental condition. Increase in the frequency of species Neodenticula seminae and Thalassionema nitzschioides (Grunow) Mereschkowsky indicates the active input of Pacific and Japan Sea waters favorable for the diatom flora grow. Assemblage MIS 5d (110.7e117 kyr). The diatoms abundance
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
4
Northern slope of the Kuril Basin
Institute of Oceanology Rise
Pegasus Rise
Central Part between Institute of Oceanology Rise and Academy of Science Rice
Derugina Depression
North-Okhotsk Rise
934
LV28-41-5
GE 99-10-3
PC 07-R
LV28-40-5
LV27-2-4
Neodenticula seminae, T. latimarginata
T. latimarginata Actinocyclus curvatulus, Neodenticula seminae, Paralia sulcata
T. latimarginata, Neodenticula seminae, Actinocyclus curvatulus
T. latimarginata, Actinocyclus curvatulus, Neodenticula seminae
C. marginatus, T. latimarginata, R. hebetata f. hiemalis
R. hebetata f. hiemalis, Paralia sulcata, C. marginatus, T. gravida
R. hebetata f. hiemalis, T. gravida, Bacterosira fragilis, C. marginatus, Paralia sulcata T. latimarginata, C. marginatusm, R. hebetata f. hiemalis
R. hebetata f. hiemalis, T. gravida, Paralia sulcata.
Thalassiothrix longissima, R. hebetata f. hiemalis, Neodenticula seminae C. marginatus, Bacterosira fragilis, T. gravida
R. hebetata f. hiemalis, T. latimarginata, C. marginatus, Actinocyclus curvatulus
R. hebetata f. hiemalis, C. marginatus, T. latimarginata Rhizosolenia hebetata f. hiemalis, T. latimarginata, C. marginatus, Actinocyclus curvatulus R. hebetata f. hiemalis, C. marginatus
R. hebetata f. hiemalis, C. marginatus
R. hebetata f. hiemalis, T. latimarginata, C. marginatus
T. latimarginata, N. seminae R. hebetata f. hiemalis, Thalassiothrix longissima
R. hebetata f. hiemalis, C. marginatus T. latimarginata
R. hebetata f. hiemalis, Thalassiothrix longissima T. latimarginata, C. marginatus, R. hebetata f. hiemalis, Actinocyclus curvatulus C. marginatus, T. latimarginata, R. hebetata f. hiemalis, Actinocyclus curvatulus
R. hebetata f. hiemalis, T. latimarginata, C. marginatus, Actinocyclus curvatulus R. hebetata f. hiemalis, C. marginatus, T. latimarginata T. gravida T. latimarginata, R. hebetata f. hiemalis, Actinocyclus curvatulus, C. marginatus R. hebetata f. hiemalis, T. latimarginata, Actinocyclus curvatulus, C. marginatus T. latimarginata, R. hebetata f. hiemalis, Actinocyclus curvatulus, C. marginatus R. hebetata f. hiemalis, T. latimarginata, Actinocyclus curvatulus, C. marginatus N. seminae T. latimarginata, T. eccentrica, R. hebetata f. hiemalis, Thalassiothrix longissimi
C. marginatus, Thalassiothrix longissima, R. hebetata f. hiemalis, Paralia sulcata C. marginatus, T. longissima, R. hebetata f. hiemalis.
Time, kyr
MIS
Epoch
R. hebetata f. hiemalis, T. latimarginata, Neodenticula seminae
0e14.7
1
Holoc ene
C. marginatus, Paralia sulcata, A. curvatulus, Thalassiothrix longissima, T. gravida
14.7e28
2
Late Pleistocene
28e59.4
3
59.4e73.9
4
73.9e85
5а
85e95
5b
95e110
5c
110e117
5d
117e129.8
5e
>129.8
6
Middle Pleistoce ne
A. Artemova et al. / Quaternary International xxx (2017) 1e12
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Table 2 Changes in the diatoms assemblages through time synchronously with MIS 6 - MIS 1.
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Fig. 2. Changes of diatom abundance (# valves/g sediment*106) in the core LV 28-41-5, change of dominating species, in variation of ecological groups of species. The vertical lines indicate the boundaries of the diatom assemblages and the stratigraphical division of cores sediments for last 140 kyr, according of age model.
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
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(10*106 cells g1) is less than in the previous assemblage. Oceanic species Rhizosolenia hebetata f. hiemalis (11.5e40.3%), Coscinodiscus marginatus (16e25%) and Thalassiosira latimarginata (24e34%) dominate in the assemblage. Frequency of near-ice species increases slightly in assemblage compared with the above one suggesting some environmental cooling. Increase in the content of Paralia sulcata (up to 4.5%) preferring the freshened waters which suggests the surface water freshening, probably, due to increased sea ice formation. Assemblage MIS 5c (95e110.7 kyr) is characterized by higher abundance of diatoms (11e21*106 cells g1). Domination of oceanic species (Rhizosolenia hebetata f. hiemalis (41e50.2%), Coscinodiscus marginatus 6.3e17.8%) and increase in diatom abundance, probably, reflect the warmer environmental condition comparable with previously period. Assemblage MIS 5b (85e95 kyr) was found in two cores. The diatom abundance increases up to 7.6*106 cells g1 in sediments from IOR (5e13*106 cells g1), and 0.56e2.5*106 cells g1 in the Sakhalin slope sediments. The diatom assemblage is dominated by oceanic species Rhizosolenia hebetata f. hiemalis, Thalassiosira latimarginata suggesting the sea ice free summer condition. Decrease in frequency of the ocean species and increase in percentage of near-ice Thalassiosira gravida (7%) in the Sakhalin slope sediments reflect probably a definite environmental cooling. However, similar trend in taxa variability does not observe in the central OS (compared with the IOR one). Assemblage MIS 5a (73.9e85 kyr) was found in three cores and was marked by remarkable higher content of diatoms, up to 7*106 cells g1, in the Sakhalin slope sediments and up to 21*106 cells g1 in the IOR sediments. The diatom assemblage was formed mostly by oceanic species: Thalassiosira latimarginata, Thalassiothrix longissima, Coscinodiscus marginatus, Rhizosolenia hebetata f. hiemalis and Actinocyclus curvatulus. Such diatom assemblage and increase in abundance show the warmer environmental condition during MIS 5a compared with previously one. Assemblage MIS 4 (73.9e59.4 kyr) (Table 2, Fig. 3) was identified in the sediments from IOR, the Sakhalin slope and the Deriugina Depression. The assemblages are characterized by lower diatom content (0.2e3*106 cells g1) (Fig. 2) and poor species diversity. The oceanic species Rhizosolenia hebetata f. hiemalis (27e49%), Coscinodiscus marginatus (14e23%), Thalassiothrix longissima (6.6e25.6%) still dominate in assemblages. However, increase in frequency of near-ice species, particularly, Thalassiosira gravida (1e9%) and high number of IRD suggest the enhanced influence of sea ice due to climate cooling and accompanied active sea ice formation. Assemblage MIS 3 (59.4e28 kyr) (Table 2). This assemblage is characterized by some increase in diatoms content in the sediments. In the sediments from the IOR, the diatoms abundance varies from 0.7 to 8*106 cells g1 while, on the East Sakhalin slope, it is 3.6 106 cells g1. Very low diatom number in Deriugina Basin sediments (up to 3000 cells* g1) was governed by the strong frustules dissolution. In the assemblage MIS 3, the frequency of oceanic species Rhizosolenia hebetata f. hiemalis, Thalassiosira latimarginata, Coscinodiscus marginatus and Actinocyclus curvatulus increases; similar trend was also observed for frequency of tychopelagic Paralia sulcata. Such a composition of diatoms taxa and abundance reflect, probably, a warming and desalting surface water due to melting of sea ice, but the surface water temperature was lower than today. Assemblage MIS 2 (28e14.7 kyr) (Table 2, Plate 2) was presented in all studied cores. The lowest content of diatoms was observed in the sediments from Deriugina Basin, (up to 2000 cells g1), abundance in Pegasus Rise and Institute of Oceanology Rise (IOR) drops down to 0.35*106 cells g1 and 0.4*106 cells g1
respectively. The diatom assemblages are presented by mixed complex with domination of near-ice Thalassiosira antarctica þ T. gravida (2.4e25%), Bacterosira bathyomphala (3e11%) and sea-ice species Thalassiosira nordenskioeldii, Thalassiosira hyalina, Thalassiosira kryophila, Fragilariopsis oceanica, Fragilariopsis cylindrus (up to 15%) which reflects a long season of ice-covered sea. Oceanic species are presented by Rhizosolenia hebetata f. hiemalis, Coscinodiscus marginatus, Thalassiosira latimarginata, Actinocyclus curvatulus and Thalassiothrix longissima suggesting the summer ice melting in the studied areas. Largest percentages of the near-ice group are mostly presented by species T. gravida, high content of the sea-ice species and IRD arguing that influence of the sea ice was strong. The decrease in IRD values and insignificant frequency of the sea ice species in core 934 point to considerably weaker influence of sea ice in the southern OS. However, occurrence of the sea-ice and near-ice species was extremely high in core LV 27-2-4 showing more severe sea ice and environment conditions in the northern OS. The presence of extinct redeposited species shows a low stand of sea level. 3.1. Assemblages MIS 1 Subassemblage 1a (Bølling/Allerod, 14.7e12.8 kyr) (Table 2, Plate 1). The diatom abundance is low. Oceanic species dominate in sediments simultaneously with high percentage of near-ice species while the content of sea-ice species decreases during this period testifying the considerable effect of the sea ice role on the surface environment. The observed pattern of diatom assemblages reflected the strongly pronounced seasonal contrast with significant formation and extension of sea ice during winter and more intensive sea ice melting during warmer summer resulted in the longer vegetative period consistently with Bølling/Allerod climate warming. Increast in the number of diatom P. sulcata in all cores (up to 40%) was accompanied by high IRD accumulation pointing to active summer sea ice melting and surface water desalinity (Fig. 4) (Gorbarenko et al., 2014). Strong peaks in productivity proxy (chlorine, CaCO3 and Ba-bio) show significant increase in foraminifera and coccolithophorida production in the OS during Bølling/ Allerod warming, while the opal content remains low (Gorbarenko et al., 2014, 2012; Seki et al., 2004). The low diatom abundance during Bølling/Allerod warming may be a result of the strong surface water stratification due to sea ice melting, unfavourable for diatom production (Gorbarenko et al., 2014). Subassemblage 1b (YD, 12.8e11.7 kyr). Diatom abundance in the sediments remained low. Occurrence of near-ice species remaines still high (nearly 20%), close to the glacial values, indicating the significant influence of the sea ice. Oceanic group decreased. However, sea ice conditions here were not such severe as glacial ones according to lower content of the sea ice diatom species (Fig. 4). Decrease in the species P. sulcata content and decrease in IRD in the sediments confirm a weakening of the sea ice melting. Therefore, formation of the subassemblage 1b is consistent with the YD cooling. Decrease in IRD values points to severe sea ice coverage in the northern areas with the weaker summer season ice melting. Subassemblage 1c (11.7e8.3 kyr) is characterized by an increase are diatom abundance. The share of the oceanic diatom species significantly increases and near-ice species content decreases gradually with time and content of sea-ice species becomes very low. This complex reflects the temperate environmental condition of Preboreal and Boreal warming. IRD values decrease significantly with time in all studied cores with exception of northern core. Such the changes in the diatom assemblages of all cores and IRD values point to gradual climate warming consistently with regional climate variability at the beginning of Holocene (Demske et al.,
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
Fig. 3. Changes of diatom abundance (# valves/g sediment*106) through time (in calendar kyr) for studied cores, black line - IRD (# of terrigenic grains/g of dry sediments, content of near-ice species, %; content of sea-ice species, content of Paralia sulcata, %, and content of Neodenticula seminae, %. The vertical lines indicate the boundaries of the diatom assemblages and the stratigraphical division of cores sediments for last 150 kyr, according of age model. Gray areas represent the cold periods YD.
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
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A. Artemova et al. / Quaternary International xxx (2017) 1e12
Fig. 4. Changes of a) diatom abundance (# valves/g sediment*106) through time (in calendar kyr) for studied cores, b) IRD (# of terrigenic grains/g of dry sediments), c) content of near-ice species, %; content of sea-ice species, % and Paralia sulcata, %, The vertical lines indicate the stratigraphical division of cores sediments for last 140 kyr, according of age model. Blue areas represent the cold periods.
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
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9
Plate 1. Diatom assemblages in the sediments from the central part of Okhotsk Sea, Late Holocene, 0e5 kyr. 1e2. Thalassiosira latimarginata Makarova. 3. Thalassiosira eccentrica . 7e8. Shionodiscus oestrupii (Ostenfeld) A.J.Alverson, S.H.Kang & E.C.Theriot. 9.Thalassiothrix (Ehrenberg) Cleve. 4e5. Actinocyclus curvatulus Janisch. 6. Actinocyclus ochotensis Jouse longissima Cleve et Grunow. 10e11. Neodenticula seminae (Simonsen et Kanaya) Akiba et Yanagisawa. 12, 16. Chaetoceros furcellatus Bailey. 13. Thalassionema nitzschioides (Grunow) Mereschkowsky. 14. Rhizosolenia hebetata f. hiemalis Bailey. 15. Stellarima microtrias (Ehrenberg) Hasle et Sims. 1e11, 13 e core 934, Kuril Basin, Late Holocene. (1e11 e interval 6e8 cm, 13 e interval 10e12 cm). 12 e core GE 99-10-3, Pegasus Rise, Late Holocene, interval 7e8 cm. 14e16 e core LV 28-41-5, Institute of Oceanology Rise, interval 18e20 cm.
2005) The content of biogenic opal slightly increased with time which is consistent with weakening of the sea ice influence and decrease in the surface water stratification. Subassemblage 1d (last 8.3 kyr) is characterized by dramatically increasing content of diatoms from 8.3 kyr up to 5.5 kyr and remaining highest during the last 5.5 kyr. The near-ice species continue decrease strongly during 8.3e5.5 kyr up to present-day
values and remain at the same level up to present time. The oceanic species Thalassiosira latimarginata, Actinocyclus curvatulus, Neodenticula seminae, Thalassiothrix longissima and Thalassionema nitzschioides dominated and the sea-ice species are almost absent. IRD values strongly decreased during 8.3e5.5 kyr and then remained to be close to the modern ones. Opal content changes consistently with variability of the diatom abundance and
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Plate 2. Diatom assemblages in the sediments from the central part of Okhotsk Sea, Last Glaciation, MIS 2, 14.7e28 kyr. 1.Coscinodiscus marginatus Ehrenberg. 2. Thalassiosira gravida . 5. 5e6. Bacterosira bathyomphala (Cleve) Syversten et Hasle. 7. Thalassiosira hyalina (Grunow) Gran. 8. Paralia Cleve. 3. Actinocyclus curvatulus Janisch. 4. Actinocyclus ochotensis Jouse sulcata (Ehrenberg) Cleve. 9. Thalassiosira nordenskioeldii Cleve. 10. Fragilariopsis oceanica (Cleve) Hasle. 11e12. Fragilariopsis cylindrus (Grunow) Kreiger. 13. Thalassiothrix longissima Cleve et Grunow. 14. Rhizosolenia hebetata f. hiemalis Bailey. 15. Thalassiosira latimarginata Makarova. 16. Cocconeis scutellum Ehrenberg. 17. Cocconeis costata Gregory. 18. Diploneis suborbicularis (Gregory) Cleve. 1e18 e core LV 28-41-5, Institute of Oceanology Rise, interval 131e133 cm, and 165e166 сm.
assemblages reach the maximum values during last 5.5 kyr. Appearance of the species N. seminae, endemic species in the North Pacific, since last 6 kyr may suggests the enhanced input of the Pacific surface water.
4. Conclusion The results of diatom analysis for the six earlier dated sediment cores obtained in the different parts of the OS demonstrate the
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significant changes in the climate, surface conditions, productivity and sea ice cover during the last 140 kyr with definite regional peculiarities. The obtained data of diatom abundance and species distribution in sediments allow us to identify six diatom assemblages reflecting the OS climate and environmental changes over the last 140 kyr correlating with MIS 6 - MIS 1. The abundance and composition of diatom taxa in assemblage 6 correlates with MIS 6 and indicates a prevalence of the cold water masses in the OS and the decrease in inflow of warm Pacific waters. A small abundance, high percentage of near-ice species and high IRD values during this period, indicate cold environmental conditions with extended seasonal sea ice cover and its summer melting. Severe glacial climate conditions promoted the increase of winter sea ice formation but, during summer seasons, the sea ice has mostly melted with significant surface desalination. Distribution of the near-ice, oceanic and sea ice species groups in assemblage 5e along with high diatom abundance and low IRD values indicate considerable OS climate and environment warming and decrease in sea ice influence during the MIS 5e. During this time, the regional productivity greatly increased and diatom data suggested still significant sea ice degradation. Increase in frequency of oceanic species Neodenticula seminae and Thalassionema nitzschioides indicates the active input of Pacific and Japan Sea waters favorable for the diatom flora vegetation. The diatom assemblages 5d, 5c, 5b and 5a and their abundances indicate sequence of changes in the OS environment and sea ice influence since the last glacial condition. The diatom assemblage 4 coeval with MIS 4 is characterized by lower diatom abundance and poor species diversity. The oceanic species dominate still in assemblages but frequency of near-ice species increases and high values of IRD suggest the enhanced influence of sea ice due to climate cooling and accompany by its melting during the summer season. The assemblage 3 is characterized by some increase in diatoms abundance in the sediments. The frequency of oceanic species increased and was accompanied by similar trend in frequency of tychopelagic Paralia sulcata. Such a composition of diatoms taxa and abundance reflect probably a climate warming and desalting of the surface water due to active melting of sea ice, but the surface water temperature was lower than today. The assemblage 2 was characterized by lowest diatoms abundance in the OS sediments coeval with cold climate condition of MIS 2. The diatom assemblages are presented by mixed complex with domination of near-ice and sea-ice species groups reflecting a long season of ice-covered surface condition with strong sea ice influence. However, occurrence of oceanic species suggests the summer ice melting in the studied areas. The IRD values and frequency of the sea ice species in the southern and northern cores 934 and LV 27-2-4 point to considerable differences in the sea ice influence in the southern and northern parts of the OS. The diatom species distribution and abundance of assemblage 1 allow us to divide it into the subassemblages 1a, 1b, 1c and 1d coeval with B/A warming, YD cooling, Early Holocene warming and Middle-Late Holocene respectively. The oceanic species dominate in subassemblage 1a simultaneously with high percentage of nearice species while the content of sea-ice species decreases testifying on considerable sea ice role in surface environment. The observed pattern of diatom assemblages reflects the strongly pronounced seasonal contrast with significant formation and extension of sea ice during winter and intensive sea ice melting during summer resulting in longer vegetative period consistently with B/A warming while diatom abundance was low due to strong surface water stratification unfavorable for diatom production. In the Subassemblage 1b (12.8e11.7 ka ago), the frequency of near-ice species remains still high with some decrease in role of
11
oceanic group suggesting the significant influence of the sea ice forced, probably, by the environmental cooling coeval with global YD condition. However, sea ice conditions here were not such severe as glacial ones. In the subassemblage 1c (11.7e8.3 ka), diatom abundance increased coeval with significant increases in a share of the oceanic diatom and decrease in the near-ice species content. IRD values significantly decreased pointing to gradual climate warming consistently with regional climate variability at the beginning of Holocene. The diatom production and biogenic opal content increased slightly with time which is consistent with weakening of the sea ice influence and lesser surface water stratification. Subassemblage 1d (last 8.3 kyr) is characterized by dramatically increasing in diatoms abundance from 8.3 kyr up to 5.5 kyr and the abundance remains high during the last 5.5 kyr. The number of near-ice species continues decrease strongly during 8.3e5.5 kyr up to the present-day values and remains at the same level up to present time. The oceanic species dominate and IRD values decrease strongly during 8.3e5.5 kyr and then remain close to the present-day ones. Appearance of the species N. seminae since last 6 kyr indicates the enhanced input of the Pacific surface water consistently with onset of the present-day OS hydrology. Acknowledgments This work is done at the FIO-POI Joint Research Center of Ocean and Climate. This work was supported by the Russian Foundation for Basic Research (grant numbers 16-35-60019 mol_a_dk, 16-0500127 A, 16-55-53048 GFEN_a), and Russian State budget theme N 5 of POI FEB RAS and international Taiwan-FEB RAS and Project of Ministry of Science and Technology Taiwan 2017e2019 (grant numbers 14-HHC-002, 17-MHT-003), and the National Natural Science Foundation of China (grant numbers U1606401, 41476056), and the National Program on Global Change and Air-Sea Interaction (grant numbers GASI-GEOGE-03, GASI-GEOGE-04) and International cooperative projects in polar regions (grant number 201613). References Chen, C.-T.A., Andreev, A., Kim, K.-R., Yamamoto, M., 2004. Roles of continental shelves and marginal seas in the biogeochemical cycles of the North Pacific Ocean. J. Oceanogr. 60, 17e44. Demske, D., Heumann, G., Granoszewski, W., Bezrukova, E., Oberhansli, H., 2005. Late Glacial and Holocene vegetation and regional climate variability evidenced in high-resolution pollen records from Lake Baikal. Global Planet Change 46, 255e279. Gorbarenko, S.A., Psheneva, O.Y., Artemova, A.V., Matul, A.G., Tiedemann, R., Nürnberg, D., 2010. Paleoenvironment changes in the NW Okhotsk Sea for the last 18kyr determined with micropaleontological, geochemical, and lithological data. Deep-Sea Res. Part I Oceanogr. Res. Pap. 57 (6), 797e811. Gorbarenko, S.A., Goldberg, E.L., 2005. Assessment of variations of primary production in the Sea of Okhotsk, Bering Sea, and Northwestern Pacific over the last glaciation maximum and Holocene. Dokl. Earth Sci. 9, 1380e1383. Gorbarenko, S.A., Harada, N., Malakhov, M.I., Velivetskaya, T.A., Vasilenko, Y.P., Bosin, A.A., Derkachev, A.N., Goldberg, E.L., Ignatiev, A.V., 2012. Responses of the Okhotsk Sea environment and sedimentology to global climate changes at the orbital and millennial scale during the last 350kyr. Deep-Sea Res. II 61e64, 73e84. Gorbarenko, S.A., Artemova, A.V., Goldberg, E.L., Vasilenko, Y.P., 2014. The Response of the Okhotsk Sea Environment to the Orbital-millennium Global Climate Changes during the Last Glacial Maximum, Deglaciation and Holocene Global and Planetary Change, vol. 116, pp. 76e90. Honjo, S., 1997. The Northwestern Pacific Ocean, a crucial ocean region to understand global change: rational for new international collaborative investigations. In: Tsunogai, S. (Ed.), Biogeochemical Processes in the North Pacific. Japan Marine Science Foundation, Tokyo, pp. 233e248. , A.P., 1962. Stratigraphic and Paleogeographic Studies in the Northwestern Jouse Pacific. Nauka, Moscow, p. 259 (in Russian). Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: development of a highresolution 0 to 300,000-year chronostratigraphy. Quat. Res. 27, 1e29. Polyakova, YeI., 1997. The Eurasian Arctic Seas during the Late Cenozoic. Scientific World, Moscow.
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Pushkar, V.S., Cherepanova, M.V., 2001. Diatoms of Pliocene and Anthropogene of the North Pacific (Stratigraphy and Paleoecology). Dalnauka, Vladivostok. Pushkar, V.S., Cherepanova, M.V., 2008. Diatom Assemblages and Quaternary Deposits Correlation of the North Western Pacific. Dalnauka, Vladivostok. Ren, J., Gersonde, R., Esper, O., Sancetta, C., 2014. Diatom distributions in northern North Pacific surface sediments and their relationship to modern environmental variables. Palaeogeogr. Palaeoclimatol. Palaeoecol. 402, 81e103. Sancetta, C., 1981. Oceanographic and ecological significance of diatoms in surface sediments of the Bering and Okhotsk seas. Deep-Sea Res. 28A, 789e817. Sancetta, C., 1982. Distribution of diatom species in surface sediments of the Bering and Okhotsk seas. Micropaleontology 28, 221e257. Seki, O., Ikehara, M., Kawamure, K., Nakatsuka, T., Ohnishi, K., Wakatsuchi, M.,
Narita, H., Sakamoto, T., 2004. Reconstruction of paleoproductivity in the Sea of Okhotsk over the last 30 kyr. Paleoceanography 19. PA 1016. Shiga, K., Koizumi, I., 2000. Latest Quaternary oceanographic changes in the Okhotsk Sea based on diatom records. Mar. Micropaleontol. 38, 91e117. Tsoy, I.B., Obrezkova, M.S., Artemova, A.V., 2009. Diatoms in surface sediments of the sea of Okhotsk and the northwest Pacific Ocean. Oceanology 49, 130e139. Vasilenko, Yu.P., Gorbarenko, S.A., Zou, J., 2011. Ice cover of the sea of Okhotsk during late Pleistocene glaciation and Holocene. Bull. Far East. Branch Russ. Acad. Sci. 2, 70e77 (in Russian). Zong, Y., 1997. Implications of Paralia sulcata abundance in Scottish isolation basins. Diatom Res 12, 125e150.
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Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms Antonina Artemova a, *, Sergey Gorbarenko a, Yuriy Vasilenko a, Xuefa Shi b, c, Yanguang Liu b, c, Min-Te Chen d, ** a
V.I. Ii'ichev Pacific Oceanological Institute, FEB RAS, 43 Baltiyskaya Street, Vladivostok, 690041, Russia First Institute of Oceanography, SOA, 6 Xianxialing Road, Qingdao, 266061, China Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266061, China d Institute of Earth Sciences Dean of the College of Ocean Science and Resource, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, 20224, Taiwan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 December 2016 Received in revised form 2 October 2017 Accepted 3 October 2017 Available online xxx
Paleontological records of six sediment cores in the Okhotsk Sea (OS) marked the regional environmental changes over the last 140 kyr on orbital time scales. The diatom assemblages and content of diatom frustules in the sediments during Marine Isotope Stage (MIS) 6e1 indicate the dramatic climatic and environmental changes in the OS. A small abundance and low diatom species diversity as well as the high percentage of near-ice species indicate the cold surface environmental condition during glacial time (MIS 6, 4, 2) with low temperatures, cold climate conditions and extended sea ice cover. The presence of extinct redeposited species in the glacial assemblages indicates a low sea level during this time. The proportion of ice species enlarged and diatom abundance reduced due to increase of the influence of the sea ice, reflecting the sharp climatic cooling of adjacent land and regional environmental deterioration. The subsequent increase in diatoms productivity at 129.8e117.0 kyr BP and 8.3e5.5 kyr BP indicates the strong climate warming accompanied by decrease of sea ice coverage and surface water stratification (mixing of surface and intermediate water) during the warmest MIS in the Okhotsk Sea. The diatom abundance and high content of the oceanic and warm-water species reflect the warm surface environmental condition during MIS 5e and 1 since 8.3 kyr due to decrease of the sea ice influence. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Diatom assemblages Okhotsk Sea Environment Holocene Late Pleistocene
1. Introduction The aim of this work is to reconstruct the climate and paleoenvironment conditions of the Okhotsk Sea during the Late Pleistocene and Holocene by diatom abundance and species changes in the fossil assemblages from the deep sea sediments. Diatoms are the most informative microfossils for palaeoceanographic research , 1962). (Jouse The Okhotsk Sea, marginal sea of the North Pacific, plays important role in the Pacific hydrology of Northwestern Pacific Intermediate Water masses formation (Honjo, 1997). Relatively warm and saline Pacific waters flow northward as West Kamchatka Current, and then mixed on the northern shelf and flow to south.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A. Artemova), [email protected] (M.-T. Chen).
The Japan Sea warm saline water flows into the southwestern part of the Okhotsk Sea (Fig. 1). The high diatom productivity of the Okhotsk Sea is related to nutrients abundance. There are two main sources of nutrients: Amur River and Pacific water (Chen et al., 2004). Vertical mixing supplies these nutrients to a photic zone resulting in the increase of productivity. Data on the taxonomic structure of diatom assemblages and the distribution of most typical species in the Okhotsk Sea sediments , 1962; Sancetta, 1981, 1982; Shiga were studied from 1962 (Jouse and Koizumi, 2000, Gorbarenko et al., 2010, 2012, 2014; Pushkar and Cherepanova, 2001, 2008, Tsoy et al., 2009; Ren et al., 2014). These researches showed that diatoms abundance and species composition in their assemblages for both the sediment cores and surface sediments. The researchers connect the diatom assemblages with water masses hydrology, sea-ice condition and sedimentation at the main bottom topography. However, diatom assemblages changes related to the Okhotsk Sea environment changes during the Late Pleistocene, deglaciation and Holocene
https://doi.org/10.1016/j.quaint.2017.10.002 1040-6182/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Fig. 1. Location of the investigated cores PC 07-R, LV 27-2-4, LV 28-41-5, LV 28-40-5, GE 99-10-3, 934.
remain still poorly studied in spite of the important diatom role as sensitive proxy of a surface hydrology and sea ice coverage variability in the OS. Here we presented the generalized data of different diatoms assemblages for six earlier dated sediment cores from different parts of the Okhotsk Sea over the last 140 kyr. The obtained results of studying the cores allow us to identify six diatom assemblages reflecting the OS climate and environmental changes over the last 140 kyr and correlate them with MIS 6 - MIS 1 according to earlier obtained stratigraphy (Gorbarenko et al. 2010, 2014). 2. Materials and methods The basis for this work was formed by the results of the diatom analysis for the several sediment cores obtained in the OS (Fig. 1, Table 1). Six gravity cores were obtained during the expeditions
carried out by the V.I. Il'ichev Pacific Oceanological Institute from the central, north-western and south parts of the Okhotsk Sea (Fig. 1, Table 1). Age model of the studied cores was earlier constructed for the MIS with age boundaries according to Martinson et al. (1987) by using the oxygen isotope records of planktonic and benthic foraminifera, magnetic sediment susceptibility records, AMS 14C data and tephrochronology (Gorbarenko et al., 2010, 2014). The sea ice conditions for studied core locations were earlier reconstructed by means of the calculation of the content of terrigenous particles in sediment fraction of more than 150 m and less than 2000 m per g of dry sediment called the ice rafting debris (IRD) (Vasilenko et al., 2011). The sedimentological changes indicated by variability of the magnetic sediment susceptibility (MS) were applied to reconstruct the Okhotsk Sea orbital scale climate changes according to our data (Gorbarenko et al., 2012; 2010, 2014) The taxonomic analyses, as well as the quantitative content of
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Table 1 Catalogue of station. Area
Station
Latitude
Longitude
Depth, m
Length, m
References
North-Okhotsk Rise Institute of Oceanology Rise Derugina Depression Pegasus Rise Northern slope of the Kuril Basin Central Part
LV27-2-4 LV28-41-5 LV28-40-5 GE 99-10-3 934 PC 07-R
54 30.15' 51 38.908' 51 20.040' 48 18.7' 48 32.5' 51 16.87'
144 45.14' 149 03.203' 147 10.631' 146º 08' 150 40.9' 149 12.57'
1305 1114 1312 1335 2195 1256
738 710 803 750 510 1722
Gorbarenko Gorbarenko Gorbarenko Gorbarenko Gorbarenko Gorbarenko
diatom frustules per weight unit of dry sediment) and ecological preferences of diatoms were applied. Diatom samples were prepared according to the standard technique (Gleser et al., 1974) for all cores in order to calculate the number of the diatom frustules per 1 g of dry sediment and species composition in diatom assemblages with determination of dominant and subdominant species. The study of diatoms was conducted via a Mikmed-6 microscope (LOMO, Russia) at 900 and 1300 magnification. To determine the number of diatom frustules per grammeme of air-dried sediment, we calculated the number of frustules in several horizontal rows with a subsequent recalculation to the absolute number in 1 g of sediment. To determine the quantitative ratios of diatom taxa in assemblages, 300e350 frustules were identified for each slide. As a result, we identified in the OS diatom assemblages several ecological groups esea ice, near-Sea ice and oceanic ones. Group of oceanic diatoms including Actinocyclus ochotensis , A. curvatulus Janisch, Neodenticula seminae (Simonsen et Jouse Kanaya) Akiba et Yanagisawa, Thalassiosira latimarginata Makarova, T. eccentrica (Ehrenberg) Cleve, Shionodiscus oestrupii (Ostenfeld) A.J.Alverson, S.H.Kang & E.C.Theriot, T. lineata Jouse, Coscinodiscus marginatus Ehrenberg, C. oculus-iridis Ehrenberg, Thalassiothrix longissima Cleve et Grunow, Rhizosolenia hebetata f. hiemalis Bailey dominates in the open part of the present-day OS without sea ice influence. Neodenticula seminae dominated mostly in the OS sedi, 1962; Sancetta, 1982) ments during the interglacial periods (Jouse and may be indicator of increased input of the relative warm and saline North Pacific waters. Species of Attheya septentrionalis (Østrup) Crawford in Crawford, Gardner et Medlin, Fragilariopsis oceanica (Cleve) Hasle, Fragilariopsis cylindrus (Grunow) Kreiger, T. nordenskioeldii Cleve, T. hyalina (Grunow) Gran, T. kryophilia Jorgensen grow on the ice edge, beneath the ice and inside the ice (Polyakova, 1997) and are referred to the sea-ice group. Bacterosira bathyomphala (Cleve) Syversten et Hasle, T. gravida Cleve and T. antarctica Comber grow in the Okhotsk Sea in the vicinity of the sea ice edges, under sea ice influence and, therefore, they may be considered as near-ice species group.). Abundance of T. gravida shows a positive correlation with sea-ice cover and link with increase in IRD and drop stones (Gorbarenko et al., 2014). Thus, the relative abundance of this species can be used as the index of the of sea-ice coverage. Paralia sulcata (Ehrenberg) Cleve, tychopelagic species is indicator of the freshened surface water in the marginal seas (Zong, 1997). 3. Results and discussion The diatoms were investigated in the cores 934, LV 28-40-3, LV 27-2-3 and GE 99-10-3 are presented in the papers Gorbarenko et al. (2010, 2012, 2014) and combined in Table 2. The diatom abundance and dominated species for core LV 28-41-5 versus core depth are presented in Fig. 2. The identified diatom assemblages 1e6 according to main variations in the different ecological groups and diatom abundance in the all studied cores were correlated with
et al., 2010 and Goldberg., 2005 et al., 2010 and Goldberg., 2005 et al., 2014, 2010 et al., 2010, 2012, 2014
established earlier MIS boundaries. Earlier published results of diatom analyses for cores 934, LV 2840-3, LV 27-2-3 and GE 99-10-3 and new results for cores LV 28-415 and PC 07-R with established MIS boundaries allow us to consider spatial pattern variability of the oceanic, near-ice and sea ice groups in the OS versus time over the last 140 kyr (Table 2, Fig. 3). Sediment chronology for studied cores within each MIS was calculated by interpolation between the ages of MIS boundaries with regard to constant sedimentation rates for each MIS. As our results show, the identified diatom assemblages for each core are correlated quite well with similarly-named MIS. Photos of dominated species in diatom assemblages accumulated during Late Holocene (0e5 kyr) and MIS 2e6 are shown respectively in the Plate. Assemblage MIS 6 (140e129.8 kyr) (Table 2, Fig. 3) was found only in the core LV 28-41-5 from Institute of Oceanology Rise. The diatom abundance in the sediment during glaciation's MIS 6 was extremely small (Fig. 4). A remarkable characteristic of assemblage is the absence of warm species suggesting weak inflow of warm Pacific waters to central OS. Oceanic species Thalassiosira latimarginata, Coscinodiscus marginatus, Rhizosolenia hebetata f. hiemalis dominate in the diatom assemblage suggesting the summer ice free condition in the central OS. However the significant sub domination of tychopelagic species Paralia sulcate frequency (6e19%) shows the freshening of the surface water as a result of spring sea ice melting. The presence of near-ice species, particularly, Thalassiosira gravida (1e9%) and small numbers of sea-ice species in assemblage of MIS 6 characterize also the season sea ice cover in the OS. The composition of diatom taxa indicates a prevalence of the cold water masses in the OS and the decrease of inflow of warm Pacific waters (without Pacific endemic species). A small number and low diversity of diatoms species, as well as the high percentage of near-ice species, suggest the cold environmental conditions with extended seasonal sea ice cover and its summer melting. Assemblage MIS 5 (73.9e129.8 kyr) (Fig. 4, Table 2). The quantity of diatoms and the proportion of the dominant species throughout the MIS 5 vary which allows us to identify during MIS 5 several assemblages correlated with Substage 5e-5a reflecting the paleoenvironment and climatic variability. Assemblage MIS 5e (117e129.8 kyr). The diatom assemblage during this period shows similarity to the present-day assemblage. The diatom content (up to 33*106 cells g1) is highest for studied time, and more than that in the Holocene. The assemblage is dominated by oceanic group species (up to 100%) Thalassiosira latimarginata, Neodenticula seminae, Rhizosolenia hebetata f. hiemalis, and Thalassiothrix longissimi and content of the warm-waters species increasts up to 8.7% suggesting the warmest climate condition. This diatom complex and their abundance reflect probably the warmest climate and summer environmental condition. Increase in the frequency of species Neodenticula seminae and Thalassionema nitzschioides (Grunow) Mereschkowsky indicates the active input of Pacific and Japan Sea waters favorable for the diatom flora grow. Assemblage MIS 5d (110.7e117 kyr). The diatoms abundance
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
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Northern slope of the Kuril Basin
Institute of Oceanology Rise
Pegasus Rise
Central Part between Institute of Oceanology Rise and Academy of Science Rice
Derugina Depression
North-Okhotsk Rise
934
LV28-41-5
GE 99-10-3
PC 07-R
LV28-40-5
LV27-2-4
Neodenticula seminae, T. latimarginata
T. latimarginata Actinocyclus curvatulus, Neodenticula seminae, Paralia sulcata
T. latimarginata, Neodenticula seminae, Actinocyclus curvatulus
T. latimarginata, Actinocyclus curvatulus, Neodenticula seminae
C. marginatus, T. latimarginata, R. hebetata f. hiemalis
R. hebetata f. hiemalis, Paralia sulcata, C. marginatus, T. gravida
R. hebetata f. hiemalis, T. gravida, Bacterosira fragilis, C. marginatus, Paralia sulcata T. latimarginata, C. marginatusm, R. hebetata f. hiemalis
R. hebetata f. hiemalis, T. gravida, Paralia sulcata.
Thalassiothrix longissima, R. hebetata f. hiemalis, Neodenticula seminae C. marginatus, Bacterosira fragilis, T. gravida
R. hebetata f. hiemalis, T. latimarginata, C. marginatus, Actinocyclus curvatulus
R. hebetata f. hiemalis, C. marginatus, T. latimarginata Rhizosolenia hebetata f. hiemalis, T. latimarginata, C. marginatus, Actinocyclus curvatulus R. hebetata f. hiemalis, C. marginatus
R. hebetata f. hiemalis, C. marginatus
R. hebetata f. hiemalis, T. latimarginata, C. marginatus
T. latimarginata, N. seminae R. hebetata f. hiemalis, Thalassiothrix longissima
R. hebetata f. hiemalis, C. marginatus T. latimarginata
R. hebetata f. hiemalis, Thalassiothrix longissima T. latimarginata, C. marginatus, R. hebetata f. hiemalis, Actinocyclus curvatulus C. marginatus, T. latimarginata, R. hebetata f. hiemalis, Actinocyclus curvatulus
R. hebetata f. hiemalis, T. latimarginata, C. marginatus, Actinocyclus curvatulus R. hebetata f. hiemalis, C. marginatus, T. latimarginata T. gravida T. latimarginata, R. hebetata f. hiemalis, Actinocyclus curvatulus, C. marginatus R. hebetata f. hiemalis, T. latimarginata, Actinocyclus curvatulus, C. marginatus T. latimarginata, R. hebetata f. hiemalis, Actinocyclus curvatulus, C. marginatus R. hebetata f. hiemalis, T. latimarginata, Actinocyclus curvatulus, C. marginatus N. seminae T. latimarginata, T. eccentrica, R. hebetata f. hiemalis, Thalassiothrix longissimi
C. marginatus, Thalassiothrix longissima, R. hebetata f. hiemalis, Paralia sulcata C. marginatus, T. longissima, R. hebetata f. hiemalis.
Time, kyr
MIS
Epoch
R. hebetata f. hiemalis, T. latimarginata, Neodenticula seminae
0e14.7
1
Holoc ene
C. marginatus, Paralia sulcata, A. curvatulus, Thalassiothrix longissima, T. gravida
14.7e28
2
Late Pleistocene
28e59.4
3
59.4e73.9
4
73.9e85
5а
85e95
5b
95e110
5c
110e117
5d
117e129.8
5e
>129.8
6
Middle Pleistoce ne
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Table 2 Changes in the diatoms assemblages through time synchronously with MIS 6 - MIS 1.
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Fig. 2. Changes of diatom abundance (# valves/g sediment*106) in the core LV 28-41-5, change of dominating species, in variation of ecological groups of species. The vertical lines indicate the boundaries of the diatom assemblages and the stratigraphical division of cores sediments for last 140 kyr, according of age model.
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(10*106 cells g1) is less than in the previous assemblage. Oceanic species Rhizosolenia hebetata f. hiemalis (11.5e40.3%), Coscinodiscus marginatus (16e25%) and Thalassiosira latimarginata (24e34%) dominate in the assemblage. Frequency of near-ice species increases slightly in assemblage compared with the above one suggesting some environmental cooling. Increase in the content of Paralia sulcata (up to 4.5%) preferring the freshened waters which suggests the surface water freshening, probably, due to increased sea ice formation. Assemblage MIS 5c (95e110.7 kyr) is characterized by higher abundance of diatoms (11e21*106 cells g1). Domination of oceanic species (Rhizosolenia hebetata f. hiemalis (41e50.2%), Coscinodiscus marginatus 6.3e17.8%) and increase in diatom abundance, probably, reflect the warmer environmental condition comparable with previously period. Assemblage MIS 5b (85e95 kyr) was found in two cores. The diatom abundance increases up to 7.6*106 cells g1 in sediments from IOR (5e13*106 cells g1), and 0.56e2.5*106 cells g1 in the Sakhalin slope sediments. The diatom assemblage is dominated by oceanic species Rhizosolenia hebetata f. hiemalis, Thalassiosira latimarginata suggesting the sea ice free summer condition. Decrease in frequency of the ocean species and increase in percentage of near-ice Thalassiosira gravida (7%) in the Sakhalin slope sediments reflect probably a definite environmental cooling. However, similar trend in taxa variability does not observe in the central OS (compared with the IOR one). Assemblage MIS 5a (73.9e85 kyr) was found in three cores and was marked by remarkable higher content of diatoms, up to 7*106 cells g1, in the Sakhalin slope sediments and up to 21*106 cells g1 in the IOR sediments. The diatom assemblage was formed mostly by oceanic species: Thalassiosira latimarginata, Thalassiothrix longissima, Coscinodiscus marginatus, Rhizosolenia hebetata f. hiemalis and Actinocyclus curvatulus. Such diatom assemblage and increase in abundance show the warmer environmental condition during MIS 5a compared with previously one. Assemblage MIS 4 (73.9e59.4 kyr) (Table 2, Fig. 3) was identified in the sediments from IOR, the Sakhalin slope and the Deriugina Depression. The assemblages are characterized by lower diatom content (0.2e3*106 cells g1) (Fig. 2) and poor species diversity. The oceanic species Rhizosolenia hebetata f. hiemalis (27e49%), Coscinodiscus marginatus (14e23%), Thalassiothrix longissima (6.6e25.6%) still dominate in assemblages. However, increase in frequency of near-ice species, particularly, Thalassiosira gravida (1e9%) and high number of IRD suggest the enhanced influence of sea ice due to climate cooling and accompanied active sea ice formation. Assemblage MIS 3 (59.4e28 kyr) (Table 2). This assemblage is characterized by some increase in diatoms content in the sediments. In the sediments from the IOR, the diatoms abundance varies from 0.7 to 8*106 cells g1 while, on the East Sakhalin slope, it is 3.6 106 cells g1. Very low diatom number in Deriugina Basin sediments (up to 3000 cells* g1) was governed by the strong frustules dissolution. In the assemblage MIS 3, the frequency of oceanic species Rhizosolenia hebetata f. hiemalis, Thalassiosira latimarginata, Coscinodiscus marginatus and Actinocyclus curvatulus increases; similar trend was also observed for frequency of tychopelagic Paralia sulcata. Such a composition of diatoms taxa and abundance reflect, probably, a warming and desalting surface water due to melting of sea ice, but the surface water temperature was lower than today. Assemblage MIS 2 (28e14.7 kyr) (Table 2, Plate 2) was presented in all studied cores. The lowest content of diatoms was observed in the sediments from Deriugina Basin, (up to 2000 cells g1), abundance in Pegasus Rise and Institute of Oceanology Rise (IOR) drops down to 0.35*106 cells g1 and 0.4*106 cells g1
respectively. The diatom assemblages are presented by mixed complex with domination of near-ice Thalassiosira antarctica þ T. gravida (2.4e25%), Bacterosira bathyomphala (3e11%) and sea-ice species Thalassiosira nordenskioeldii, Thalassiosira hyalina, Thalassiosira kryophila, Fragilariopsis oceanica, Fragilariopsis cylindrus (up to 15%) which reflects a long season of ice-covered sea. Oceanic species are presented by Rhizosolenia hebetata f. hiemalis, Coscinodiscus marginatus, Thalassiosira latimarginata, Actinocyclus curvatulus and Thalassiothrix longissima suggesting the summer ice melting in the studied areas. Largest percentages of the near-ice group are mostly presented by species T. gravida, high content of the sea-ice species and IRD arguing that influence of the sea ice was strong. The decrease in IRD values and insignificant frequency of the sea ice species in core 934 point to considerably weaker influence of sea ice in the southern OS. However, occurrence of the sea-ice and near-ice species was extremely high in core LV 27-2-4 showing more severe sea ice and environment conditions in the northern OS. The presence of extinct redeposited species shows a low stand of sea level. 3.1. Assemblages MIS 1 Subassemblage 1a (Bølling/Allerod, 14.7e12.8 kyr) (Table 2, Plate 1). The diatom abundance is low. Oceanic species dominate in sediments simultaneously with high percentage of near-ice species while the content of sea-ice species decreases during this period testifying the considerable effect of the sea ice role on the surface environment. The observed pattern of diatom assemblages reflected the strongly pronounced seasonal contrast with significant formation and extension of sea ice during winter and more intensive sea ice melting during warmer summer resulted in the longer vegetative period consistently with Bølling/Allerod climate warming. Increast in the number of diatom P. sulcata in all cores (up to 40%) was accompanied by high IRD accumulation pointing to active summer sea ice melting and surface water desalinity (Fig. 4) (Gorbarenko et al., 2014). Strong peaks in productivity proxy (chlorine, CaCO3 and Ba-bio) show significant increase in foraminifera and coccolithophorida production in the OS during Bølling/ Allerod warming, while the opal content remains low (Gorbarenko et al., 2014, 2012; Seki et al., 2004). The low diatom abundance during Bølling/Allerod warming may be a result of the strong surface water stratification due to sea ice melting, unfavourable for diatom production (Gorbarenko et al., 2014). Subassemblage 1b (YD, 12.8e11.7 kyr). Diatom abundance in the sediments remained low. Occurrence of near-ice species remaines still high (nearly 20%), close to the glacial values, indicating the significant influence of the sea ice. Oceanic group decreased. However, sea ice conditions here were not such severe as glacial ones according to lower content of the sea ice diatom species (Fig. 4). Decrease in the species P. sulcata content and decrease in IRD in the sediments confirm a weakening of the sea ice melting. Therefore, formation of the subassemblage 1b is consistent with the YD cooling. Decrease in IRD values points to severe sea ice coverage in the northern areas with the weaker summer season ice melting. Subassemblage 1c (11.7e8.3 kyr) is characterized by an increase are diatom abundance. The share of the oceanic diatom species significantly increases and near-ice species content decreases gradually with time and content of sea-ice species becomes very low. This complex reflects the temperate environmental condition of Preboreal and Boreal warming. IRD values decrease significantly with time in all studied cores with exception of northern core. Such the changes in the diatom assemblages of all cores and IRD values point to gradual climate warming consistently with regional climate variability at the beginning of Holocene (Demske et al.,
Please cite this article in press as: Artemova, A., et al., Palaeoceanography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms, Quaternary International (2017), https://doi.org/10.1016/j.quaint.2017.10.002
Fig. 3. Changes of diatom abundance (# valves/g sediment*106) through time (in calendar kyr) for studied cores, black line - IRD (# of terrigenic grains/g of dry sediments, content of near-ice species, %; content of sea-ice species, content of Paralia sulcata, %, and content of Neodenticula seminae, %. The vertical lines indicate the boundaries of the diatom assemblages and the stratigraphical division of cores sediments for last 150 kyr, according of age model. Gray areas represent the cold periods YD.
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Fig. 4. Changes of a) diatom abundance (# valves/g sediment*106) through time (in calendar kyr) for studied cores, b) IRD (# of terrigenic grains/g of dry sediments), c) content of near-ice species, %; content of sea-ice species, % and Paralia sulcata, %, The vertical lines indicate the stratigraphical division of cores sediments for last 140 kyr, according of age model. Blue areas represent the cold periods.
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Plate 1. Diatom assemblages in the sediments from the central part of Okhotsk Sea, Late Holocene, 0e5 kyr. 1e2. Thalassiosira latimarginata Makarova. 3. Thalassiosira eccentrica . 7e8. Shionodiscus oestrupii (Ostenfeld) A.J.Alverson, S.H.Kang & E.C.Theriot. 9.Thalassiothrix (Ehrenberg) Cleve. 4e5. Actinocyclus curvatulus Janisch. 6. Actinocyclus ochotensis Jouse longissima Cleve et Grunow. 10e11. Neodenticula seminae (Simonsen et Kanaya) Akiba et Yanagisawa. 12, 16. Chaetoceros furcellatus Bailey. 13. Thalassionema nitzschioides (Grunow) Mereschkowsky. 14. Rhizosolenia hebetata f. hiemalis Bailey. 15. Stellarima microtrias (Ehrenberg) Hasle et Sims. 1e11, 13 e core 934, Kuril Basin, Late Holocene. (1e11 e interval 6e8 cm, 13 e interval 10e12 cm). 12 e core GE 99-10-3, Pegasus Rise, Late Holocene, interval 7e8 cm. 14e16 e core LV 28-41-5, Institute of Oceanology Rise, interval 18e20 cm.
2005) The content of biogenic opal slightly increased with time which is consistent with weakening of the sea ice influence and decrease in the surface water stratification. Subassemblage 1d (last 8.3 kyr) is characterized by dramatically increasing content of diatoms from 8.3 kyr up to 5.5 kyr and remaining highest during the last 5.5 kyr. The near-ice species continue decrease strongly during 8.3e5.5 kyr up to present-day
values and remain at the same level up to present time. The oceanic species Thalassiosira latimarginata, Actinocyclus curvatulus, Neodenticula seminae, Thalassiothrix longissima and Thalassionema nitzschioides dominated and the sea-ice species are almost absent. IRD values strongly decreased during 8.3e5.5 kyr and then remained to be close to the modern ones. Opal content changes consistently with variability of the diatom abundance and
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Plate 2. Diatom assemblages in the sediments from the central part of Okhotsk Sea, Last Glaciation, MIS 2, 14.7e28 kyr. 1.Coscinodiscus marginatus Ehrenberg. 2. Thalassiosira gravida . 5. 5e6. Bacterosira bathyomphala (Cleve) Syversten et Hasle. 7. Thalassiosira hyalina (Grunow) Gran. 8. Paralia Cleve. 3. Actinocyclus curvatulus Janisch. 4. Actinocyclus ochotensis Jouse sulcata (Ehrenberg) Cleve. 9. Thalassiosira nordenskioeldii Cleve. 10. Fragilariopsis oceanica (Cleve) Hasle. 11e12. Fragilariopsis cylindrus (Grunow) Kreiger. 13. Thalassiothrix longissima Cleve et Grunow. 14. Rhizosolenia hebetata f. hiemalis Bailey. 15. Thalassiosira latimarginata Makarova. 16. Cocconeis scutellum Ehrenberg. 17. Cocconeis costata Gregory. 18. Diploneis suborbicularis (Gregory) Cleve. 1e18 e core LV 28-41-5, Institute of Oceanology Rise, interval 131e133 cm, and 165e166 сm.
assemblages reach the maximum values during last 5.5 kyr. Appearance of the species N. seminae, endemic species in the North Pacific, since last 6 kyr may suggests the enhanced input of the Pacific surface water.
4. Conclusion The results of diatom analysis for the six earlier dated sediment cores obtained in the different parts of the OS demonstrate the
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significant changes in the climate, surface conditions, productivity and sea ice cover during the last 140 kyr with definite regional peculiarities. The obtained data of diatom abundance and species distribution in sediments allow us to identify six diatom assemblages reflecting the OS climate and environmental changes over the last 140 kyr correlating with MIS 6 - MIS 1. The abundance and composition of diatom taxa in assemblage 6 correlates with MIS 6 and indicates a prevalence of the cold water masses in the OS and the decrease in inflow of warm Pacific waters. A small abundance, high percentage of near-ice species and high IRD values during this period, indicate cold environmental conditions with extended seasonal sea ice cover and its summer melting. Severe glacial climate conditions promoted the increase of winter sea ice formation but, during summer seasons, the sea ice has mostly melted with significant surface desalination. Distribution of the near-ice, oceanic and sea ice species groups in assemblage 5e along with high diatom abundance and low IRD values indicate considerable OS climate and environment warming and decrease in sea ice influence during the MIS 5e. During this time, the regional productivity greatly increased and diatom data suggested still significant sea ice degradation. Increase in frequency of oceanic species Neodenticula seminae and Thalassionema nitzschioides indicates the active input of Pacific and Japan Sea waters favorable for the diatom flora vegetation. The diatom assemblages 5d, 5c, 5b and 5a and their abundances indicate sequence of changes in the OS environment and sea ice influence since the last glacial condition. The diatom assemblage 4 coeval with MIS 4 is characterized by lower diatom abundance and poor species diversity. The oceanic species dominate still in assemblages but frequency of near-ice species increases and high values of IRD suggest the enhanced influence of sea ice due to climate cooling and accompany by its melting during the summer season. The assemblage 3 is characterized by some increase in diatoms abundance in the sediments. The frequency of oceanic species increased and was accompanied by similar trend in frequency of tychopelagic Paralia sulcata. Such a composition of diatoms taxa and abundance reflect probably a climate warming and desalting of the surface water due to active melting of sea ice, but the surface water temperature was lower than today. The assemblage 2 was characterized by lowest diatoms abundance in the OS sediments coeval with cold climate condition of MIS 2. The diatom assemblages are presented by mixed complex with domination of near-ice and sea-ice species groups reflecting a long season of ice-covered surface condition with strong sea ice influence. However, occurrence of oceanic species suggests the summer ice melting in the studied areas. The IRD values and frequency of the sea ice species in the southern and northern cores 934 and LV 27-2-4 point to considerable differences in the sea ice influence in the southern and northern parts of the OS. The diatom species distribution and abundance of assemblage 1 allow us to divide it into the subassemblages 1a, 1b, 1c and 1d coeval with B/A warming, YD cooling, Early Holocene warming and Middle-Late Holocene respectively. The oceanic species dominate in subassemblage 1a simultaneously with high percentage of nearice species while the content of sea-ice species decreases testifying on considerable sea ice role in surface environment. The observed pattern of diatom assemblages reflects the strongly pronounced seasonal contrast with significant formation and extension of sea ice during winter and intensive sea ice melting during summer resulting in longer vegetative period consistently with B/A warming while diatom abundance was low due to strong surface water stratification unfavorable for diatom production. In the Subassemblage 1b (12.8e11.7 ka ago), the frequency of near-ice species remains still high with some decrease in role of
11
oceanic group suggesting the significant influence of the sea ice forced, probably, by the environmental cooling coeval with global YD condition. However, sea ice conditions here were not such severe as glacial ones. In the subassemblage 1c (11.7e8.3 ka), diatom abundance increased coeval with significant increases in a share of the oceanic diatom and decrease in the near-ice species content. IRD values significantly decreased pointing to gradual climate warming consistently with regional climate variability at the beginning of Holocene. The diatom production and biogenic opal content increased slightly with time which is consistent with weakening of the sea ice influence and lesser surface water stratification. Subassemblage 1d (last 8.3 kyr) is characterized by dramatically increasing in diatoms abundance from 8.3 kyr up to 5.5 kyr and the abundance remains high during the last 5.5 kyr. The number of near-ice species continues decrease strongly during 8.3e5.5 kyr up to the present-day values and remains at the same level up to present time. The oceanic species dominate and IRD values decrease strongly during 8.3e5.5 kyr and then remain close to the present-day ones. Appearance of the species N. seminae since last 6 kyr indicates the enhanced input of the Pacific surface water consistently with onset of the present-day OS hydrology. Acknowledgments This work is done at the FIO-POI Joint Research Center of Ocean and Climate. This work was supported by the Russian Foundation for Basic Research (grant numbers 16-35-60019 mol_a_dk, 16-0500127 A, 16-55-53048 GFEN_a), and Russian State budget theme N 5 of POI FEB RAS and international Taiwan-FEB RAS and Project of Ministry of Science and Technology Taiwan 2017e2019 (grant numbers 14-HHC-002, 17-MHT-003), and the National Natural Science Foundation of China (grant numbers U1606401, 41476056), and the National Program on Global Change and Air-Sea Interaction (grant numbers GASI-GEOGE-03, GASI-GEOGE-04) and International cooperative projects in polar regions (grant number 201613). References Chen, C.-T.A., Andreev, A., Kim, K.-R., Yamamoto, M., 2004. Roles of continental shelves and marginal seas in the biogeochemical cycles of the North Pacific Ocean. J. Oceanogr. 60, 17e44. Demske, D., Heumann, G., Granoszewski, W., Bezrukova, E., Oberhansli, H., 2005. Late Glacial and Holocene vegetation and regional climate variability evidenced in high-resolution pollen records from Lake Baikal. Global Planet Change 46, 255e279. Gorbarenko, S.A., Psheneva, O.Y., Artemova, A.V., Matul, A.G., Tiedemann, R., Nürnberg, D., 2010. Paleoenvironment changes in the NW Okhotsk Sea for the last 18kyr determined with micropaleontological, geochemical, and lithological data. Deep-Sea Res. Part I Oceanogr. Res. Pap. 57 (6), 797e811. Gorbarenko, S.A., Goldberg, E.L., 2005. Assessment of variations of primary production in the Sea of Okhotsk, Bering Sea, and Northwestern Pacific over the last glaciation maximum and Holocene. Dokl. Earth Sci. 9, 1380e1383. Gorbarenko, S.A., Harada, N., Malakhov, M.I., Velivetskaya, T.A., Vasilenko, Y.P., Bosin, A.A., Derkachev, A.N., Goldberg, E.L., Ignatiev, A.V., 2012. Responses of the Okhotsk Sea environment and sedimentology to global climate changes at the orbital and millennial scale during the last 350kyr. Deep-Sea Res. II 61e64, 73e84. Gorbarenko, S.A., Artemova, A.V., Goldberg, E.L., Vasilenko, Y.P., 2014. The Response of the Okhotsk Sea Environment to the Orbital-millennium Global Climate Changes during the Last Glacial Maximum, Deglaciation and Holocene Global and Planetary Change, vol. 116, pp. 76e90. Honjo, S., 1997. The Northwestern Pacific Ocean, a crucial ocean region to understand global change: rational for new international collaborative investigations. In: Tsunogai, S. (Ed.), Biogeochemical Processes in the North Pacific. Japan Marine Science Foundation, Tokyo, pp. 233e248. , A.P., 1962. Stratigraphic and Paleogeographic Studies in the Northwestern Jouse Pacific. Nauka, Moscow, p. 259 (in Russian). Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: development of a highresolution 0 to 300,000-year chronostratigraphy. Quat. Res. 27, 1e29. Polyakova, YeI., 1997. The Eurasian Arctic Seas during the Late Cenozoic. Scientific World, Moscow.
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