Millennial-scale variations of late Pleistocene radiolarian assemblages in the Bering Sea related to environments in shallow and deep waters Takuya Itaki a , Sunghan Kim b , Stephan F. Rella c , Masao Uchida d , Ryuji Tada c , Boo-Keun Khim b,n a Geological Survey of Japan, AIST, Tsukuba, Japan b Department of Oceanography, Pusan National University, Pusan, Republic of Korea c Department of Earth Planetary Science, University of Tokyo, Tokyo, Japan d National Institute for Environmental Studies, Tsukuba, Japan article info Available online 21 March 2011 Keywords: Paleoceanography Micropaleontology Dansgaard–Oeschger cycles Melt-water discharge Ventilation Productivity Radiolaria Bering Sea abstract A high-resolution record of the radiolarian assemblage from 60 to 10 ka 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 Rhizoplegma boreale and the upper-intermediate water species Cycladophora davisiana, while the sea-ice related species Actinomma boreale and A. leptodermum and many deep-water species such as Dictyophimus crisiae and D. hirundo tended to be reduced. This trend was more apparent in two laminated intervals at 15–13.5 and 11.5–11 ka, 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. 200–500 m) was well-ventilated and organic-rich, and (3) lower-intermediate water (ca. 500–1000 m) 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. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Regional climatic variations in the subarctic Pacific and its marginal seas are associated with millennial-scale climatic changes recognized in the Greenland ice-core record known as Dansgaard–Oeschger (D–O) cycles (Tada et al., 1999; Kiefer et al., 2001; Gorbarenko et al., 2005; Sakamoto et al., 2006). Intermedi- ate water ventilation in the North Pacific occurs via North Pacific Intermediate Water (NPIW) at depths of ca. 300–800 m in the present day (Ried, 1965), while NPIW apparently reached as deep as 2000 m during the last glacial period (Keigwin, 1998; Matsumoto et al., 2002). In addition, this water is interpreted to have extended as far as south to the California margin during stadials of the D–O cycles (Behl and Kennett, 1996; Tada et al., 2000). One major modern source area for NPIW formation is the Sea of Okhotsk (Yasuda, 1997), where the formation is closely related to dense water on the shelf, as a result of brine rejection occurring during winter sea-ice production (Kitani, 1973). Based on the changes in abundance of the intermediate water- dwelling radiolarian species Cycladophora davisiana, it has been hypothesized that the Bering Sea might be a source area for glacial NPIW formation (Ohkushi et al., 2003; Tanaka and Takahashi, 2005). Although Khusid et al. (2006) reported faunal abundance changes of benthic foraminiferal tests related to the D–O cycles in a well-dated core from the southern Bering Sea (GC-11), but the benthic foraminiferal record is only representa- tive of seafloor conditions, i.e., 3000 m at the GC-11 core. Polycyctine radiolarians (hereafter, radiolarians) belong to a planktonic protista group characterized by an opaline skeleton that is one of the major components in deep-water sediments from the subarctic Pacific region. The fossil record of radiolarians is a potential paleoceanographic indicator for surface- and deep-water Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.03.002 n Corresponding author. Tel.: þ82 51 510 2212; fax: þ82 51 581 2963. E-mail address: bkkhim@pusan.ac.kr (B.-K. Khim). Deep-Sea Research II 61–64 (2012) 127–144