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Itaki, Takuya,Kim, Sunghan,Rella, Stephan F.,Uchida, Masao,Tada, Ryuji,Khim, Boo-Keun Elsevier 2012 Deep-sea research. Part II, Topical studies in oce Vol.61 No.-
<P><B>Abstract</B></P><P>A high-resolution record of the radiolarian assemblage from 60 to 10ka was investigated using a piston core (PC-23A) obtained from the northern slope of the Bering Sea. Faunal changes based on the 29 major radiolarian taxa demonstrated that the surface and deep water conditions in the Bering Sea were related to the orbital and millennial-scale climatic variations known as glacial–interglacial and Dansgaard–Oeschger (D–O) cycles, respectively. During interstadial periods of the D–O cycles, the assemblage was characterized by increases in the high-latitude coastal species <I>Rhizoplegma boreale</I> and the upper-intermediate water species <I>Cycladophora davisiana</I>, while the sea-ice related species <I>Actinomma boreale</I> and <I>A. leptodermum</I> and many deep-water species such as <I>Dictyophimus crisiae</I> and <I>D. hirundo</I> tended to be reduced. This trend was more apparent in two laminated intervals at 15–13.5 and 11.5–11ka, which were correlated with well-known ice-sheet collapse events that occurred during the last deglaciation: melt-water pulse (MWP)-1A and MWP-1B, respectively. The radiolarian faunal composition in these periods suggests that oceanic conditions were different from today: (1) surface water was affected by increased melt-water discharge from continental ice-sheet, occurring at the same time as an abrupt increase in atmospheric temperature, (2) upper-intermediate water (ca. 200–500m) was well-ventilated and organic-rich, and (3) lower-intermediate water (ca. 500–1000m) was oxygen-poor. Conversely, the sea-ice season might have been longer during stadial periods of the D–O cycles and the last glacial maximum (LGM) compared to the interstadial periods and the earliest Holocene. In these colder periods, deep-water species were very abundant, and this corresponded to increases in the oxygen isotope value of benthic foraminifera. Our findings suggest that the oxygen-rich water was present in the lower-intermediate layer resulting from intensified ventilation.</P>
베링해 중부 대륙사면 지역의 지난 65,000년 동안 탄산염 함량 변화와 Dansgaard-Oeschger 사건들
김성한,김부근,Takuya Itaki,신혜선 한국해양과학기술원 2008 Ocean and Polar Research Vol.30 No.3
A piston core (MR06-04 PC23A) collected from the northern continental slope in the central Bering Sea has recorded the high-resolution millennial-scale variation of calcium carbonate (CaCO3) content during the last 65 kyr. An estimation of the age of the core sediments was carried out by using the lithologic correlation of the deglacial laminated layers with a neighboring core (HLY02023JPC), complementing the last appearance datum of both Lychnocanoma nipponica sakaii (54 kyr) and Amphimelissa setosa (85 kyr). The probable age of core MR06-04 PC23A was approximately younger than 65 kyr. Two distinct events of a significant increase of CaCO3 in the deglacial laminated sediments clearly correspond to MWP1A and MWP1B in the Bering Sea (Gorbarenko et al. 2005) and to T1ANP and T1BNP in the North Pacific (Gorbarenko 1996). These pronounced peaks of CaCO3 contents result from the elevated carbonate production in the surface water and the subsequent weakened dilution due to terrestrial input, along with an enhanced oxygen minimum zone. The CaCO3 contents are low (~2%) during the last glacial period mainly because of a low carbonate production caused by an expanded sea-ice cover and an increased dilution by terrigenous particles due to their closer distance to the continent during the sea-level low stand. The occurrence of seven distinct CaCO3 peaks in core MR06-04 PC23A is remarkable during MIS 3 and MIS 4, and they most likely correlate to the short-term millennial Dansgaard-Oeschger events.
김부근,김성한,Masao Uchida,Takuya Itaki 한국해양과학기술원 2010 Ocean science journal Vol.45 No.4
High-resolution geochemical, isotope and elemental data from core PC23A in the northern margin of the Aleutian Basin (Bering Sea) were used to reconstruct distinct paleoceanographic features of the last deglaciation (pre-Boreal[PB], Bølling-Allerød[BA], Younger Dryas[YD]). The PB and BA intervals are characterized by increased siliceous (diatom) and calcareous (coccolithophores and foraminifers) productivity represented by high biogenic opal and CaCO3 contents, respectively. The enhanced productivity can plausibly be attributed to an elevated sea-surface nutrient supply from increased melt-water input and enhanced Alaskan Stream injection under warm, restricted sea-ice conditions. High Corg/N ratios and low δ13C values of sediment organic matter during the PB and BA intervals reflect the contribution of terrestrial organic matters. The PB and BA intervals were also identified by laminated sediment layers of core PC23A, characterized by high Mo/Al and Cd/Al ratios, indicating that the bottom water condition remained anoxic. High δ15N values during the same period were attributed mainly to the increased nutrient utilization and subsequent denitrification of seawater nitrate. Part of high δ15N values may also be due to incorporation of inorganic nitrogen in the clay minerals. It is worthy of note that high total organic carbon (TOC) deposition occurred approximately 3,000 years before onset of the last deglaciation. Simultaneous high Corg/N ratios and low δ13C values clearly suggest that the high TOC content should be related to terrestrial organic carbon input. Low δ15N values during the high TOC interval also confirm the contribution of terrigenous organic matter. Although abundant calcareous phytoplankton production under cold, nutrient-poor conditions represented by Baex data was reported for high TOC deposition preceding the last deglaciation in an earlier study of the Okhotsk Sea, the main reason for the enhanced TOC deposition in the Bering Sea is an increased terrigenous input from the submerged continental shelves (Beringia) with a sea-level rise; this is further supported by Al enrichment of bulk sediments during the high TOC deposition.
Khim, B.K.,Tada, R.,Itaki, T. Pergamon Press 2014 QUATERNARY INTERNATIONAL Vol.333 No.-
Two piston cores (PC-05 and PC-08) were collected on the Yamato Rise in the East Sea/Japan Sea during the KR07-12 cruise. Both cores showed typical alternations of light (high L*) and dark (low L*) layers, which are characteristic in the hemi-pelagic sediment deposited in the deep part of the East Sea/Japan Sea. A composite core was achieved with the successful replacement of almost half of the upper part of core PC-05 by the entirety of core PC-08 based on the co-equivalence of L* values and the dark layers, because an interval (170 cm-410 cm) of core PC-05 was considerably disturbed due to fluidization during the core execution. Chronostratigraphy of the composite core was constructed by the direct comparison of L* values to the well-dated core MD01-2407 that was obtained in the Oki Ridge. The lower-bottom of the composite core extended to Marine Isotope Stage (MIS) 14, based on the age estimate by LR04 stacks. Downcore opal variation of the composite core exhibited the distinct orbital-scale cyclic changes; high during the interglacial and low during the glacial periods. However, downcore CaCO<SUB>3</SUB> variation showed no corresponding orbital-scale cyclic change between glacial and interglacial periods. Some intervals of both periods were high in CaCO<SUB>3</SUB> content. Frequent and large fluctuations in CaCO<SUB>3</SUB> content seemed to be more related to the presence of dark layers containing thin lamination (TL) within the glacial and interglacial intervals. It is worthy to note that MIS 2 and MIS 12 are characterized by distinctly high CaCO<SUB>3</SUB> content, showing up to 18% and 73%, respectively, among the glacial periods. Furthermore, in terms of lithology, MIS 2 was characterized by a thick dark layer (low L* values) with TL, whereas MIS 12 preserved the distinctly light layer (high L* values) with parallel laminations. Another remarkable dissimilarity between MIS 2 and MIS 12 was the nature of their CaCO<SUB>3</SUB> constituent; the CaCO<SUB>3</SUB> constituent of MIS 2 consisted of mostly planktonic foraminifera, whereas that of MIS 12 was mostly dump of coccolithophorids, regardless the presence of planktonic foraminifera. The distinctness of the CaCO<SUB>3</SUB> constituents between MIS 2 and MIS 12 indicates that the preservation of CaCO<SUB>3</SUB> contents was different temporarily during the glacial periods in the East Sea/Japan Sea. Enhanced CaCO<SUB>3</SUB> preservation in MIS 2 is attributed primarily to less dissolution during the sinking through the water column or at the seafloor, but increased CaCO<SUB>3</SUB> preservation in MIS 12 is mainly due to the high primary production in the surface water. With respect to the different function of the biological pump which controls CO<SUB>2</SUB> cycles, the East Sea/Japan Sea clearly experienced carbonate-ocean-like state during MIS 12, despite normally silica-ocean-like state.
Land—sea linkage of Holocene paleoclimate on the Southern Bering Continental Shelf
Katsuki, Kota,Khim, Boo-Keun,Itaki, Takuya,Harada, Naomi,Sakai, Hideo,Ikeda, Tomonori,Takahashi, Kozo,Okazaki, Yusuki,Asahi, Hirofumi SAGE Publications 2009 The Holocene Vol.19 No.5
<P>Detailed diatom records within surface and core sediments from the Southern Bering Continental Shelf (SBCS) reveal that the Holocene evolution of sea-ice distribution is associated with low pressure patterns. Holocene sea-ice distribution over the SBCS was mainly controlled by the location of the Aleutian Low. The corresponding paleoceanographic and paleoclimate conditions can be divided into three stages: (1) the early Holocene (before 7000 cal. yr BP) was characterized by extensive sea-ice distribution under two low-pressure cells, which covered the western Bering Sea and the Gulf of Alaska, respectively. (2) Between 3000 and 7000 cal. yr BP, the low-pressure system over the Gulf of Alaska became weak, causing total sea-ice mass over the SBCS to retreat. (3) In the past 3000 years, prevailing southwesterly winds over the SBCS due to the developing Aleutian Low have reduced further sea-ice cover on the SBCS. These paleoclimatic changes were probably a response to ENSO variation. The timings of water mass exchanges on the SBCS coincided with sea-level change along the Alaskan Peninsula. As a result, subsequent morphologic alterations have also influenced the paleoceanographic condition of the SBCS. The effect of the surface coastal water and bottom marine water on the SBCS intensified about 6000 cal. yr BP when sea level increased.</P>