GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 18, 1939, doi: /2003gl018070, 2003

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 18, 1939, doi: /2003gl018070, 2003 The production scheme of Cycladophora davisiana (Radiolaria) in the Okhotsk Sea and the northwestern North Pacific: implication for the paleoceanographic conditions during the glacials in the high latitude oceans Yusuke Okazaki, 1 Kozo Takahashi, 1 Takeshi Nakatsuka, 2 and Makio C. Honda 3 Received 29 June 2003; accepted 7 August 2003; published 20 September [1] The sediment trap study clearly unraveled that the distinct peaks of C. davisiana fluxes occurred during summer to autumn in both the Okhotsk Sea and the northwestern North Pacific. The production of C. davisiana is closely related to the microbial production in the intermediate water. In the Okhotsk Sea, the distinct microbial biomass in the intermediate water, which is associated with the seasonal sea-ice rejected brine water during winter, may support the high C. davisiana abundance. Therefore, the significantly high C. davisiana abundance during the Last Glacial Maximum (LGM) in the high latitude open oceans implies the distinct microbial biomass in the intermediate water caused by the seasonal sea-ice coverage analogous to the present Okhotsk Sea. INDEX TERMS: 3030 Marine Geology and Geophysics: Micropaleontology; 4855 Oceanography: Biological and Chemical: Plankton; 4267 Oceanography: General: Paleoceanography; 4803 Oceanography: Biological and Chemical: Bacteria. Citation: Okazaki, Y., K. Takahashi, T. Nakatsuka, and M. C. Honda, The production scheme of Cycladophora davisiana (Radiolaria) in the Okhotsk Sea and the northwestern North Pacific: implication for the paleoceanographic conditions during the glacials in the high latitude oceans, Geophys. Res. Lett., 30(18), 1939, doi: /2003gl018070, Introduction [2] The radiolarian species Cycladophora davisiana is a cosmopolitan species. The relative abundance of C. davisiana in surface sediment samples exceeds 20% only in the Okhotsk Sea, whereas zero to several % in the open oceans [Morley, 1980]. On the contrary, the relative abundance of C. davisiana from the glacial sediments in the high latitude open oceans showed significantly high values, often exceeding 20% [Morley, 1980]. The variation patterns of % C. davisiana during the last glacial cycle, which tends to co-vary with the changes in oxygen isotope ratios, have been used as a stratigraphic marker in carbonate-poor sediments from the high latitude open oceans [e.g., Morley et al., 1995]. Nimmergut and Abelmann [2002] reported that 1 Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, Fukuoka, Japan. 2 Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan. 3 Mutsu Institute for Oceanography, Japan Marine Science and Technology Center, Yokosuka, Japan. Copyright 2003 by the American Geophysical Union /03/2003GL OCE 3-1 the high C. davisiana density in the Okhotsk Sea occurred at intermediate depths such as m at 24 plankton net stations during the summer 1998 and the spring The Okhotsk Sea Intermediate Water (OSIW, ca m) is indicated as a plausible source for the North Pacific Intermediate Water (NPIW) [e.g., Talley, 1991]. A part of the NPIW is formed in the Okhotsk Sea and flows out mainly through the Bussol Strait into the North Pacific [e.g., Talley, 1991]. The OSIW is mostly formed by sinking of the brine water, which is the product of the sea-ice rejects, in the northwest shelf of the Okhotsk Sea [e.g., Wong et al., 1998]. Ganopolski et al. [1998] indicated that the NPIW formation was increased and extended to the low latitude during the LGM from the model study. Takahashi [1998] pointed out a possibility that the OSIW in large quantity flowed into North Pacific during the LGM with the increase of sea-ice. On the other hand, Ohkushi et al. [2003] suggested that the NPIW was formed in the Bering Sea during the glacials, rather than in the Okhotsk Sea based on the glacial distribution of microfossils including C. davisiana. [3] In this study, we present the modern C. davisiana flux variations in the Okhotsk Sea and the northwestern North Pacific, and discuss their seasonality and association with the environmental conditions. The purpose of this study is to clarify the mechanism of C. davisiana production in the Okhotsk Sea and the high latitude open oceans during the glacial cycle. 2. Materials and Methods [4] The samples in this study were obtained from five time-series sediment trap sites, two trap sites in the Okhotsk Sea (Stations M4 and M6) and three traps in the northwestern North Pacific (Stations 50N, KNOT, and 40N) during December 1997 to June 2000 (Figure 1, Table 1). The methods for radiolarian analyses were described by Okazaki et al. [in press]. Aliquot sizes ranged from 1/100 to 1/132. All coarse-sized C. davisiana specimens (>63 mm) on a microslide were counted and computed to derive C. davisiana fluxes (No. radiolarians m 2 day 1 ) at each station. 3. Results [5] Temporal fluxes of C. davisiana at each station are shown in Figure 2, together with the total organic carbon (TOC) fluxes at each trap in the Okhotsk Sea [Nakatsuka et al., submitted] and in the northwestern North Pacific

2 OCE 3-2 OKAZAKI ET AL.: THE PRODUCTION SCHEME OF CYCLADOPHORA DAVISIANA calculated. C. davisiana fluxes during July to October at Stations M4, M6, 50N, KNOT, and 40N were 53.7%, 48.1%, 48.1%, 53.7% and 51.1%, respectively, of their annual fluxes (Figure 3). Figure 1. Map showing the locations of five sediment trap stations with general circulation patterns in the Okhotsk Sea and the northwestern North Pacific. Duration of sea-ice distribution area (in months) in the Okhotsk Sea is also shown [Lisitzin, 1972]. [Honda, 2001; Honda et al., 2002]. The means of C. davisiana fluxes at Stations M4, M6, 50N, KNOT and 40N were 448, 154, 167, 335 and 135 (radiolarians m 2 day 1 ), respectively, during 7 Aug to 30 Jan This sampled period was common to each trap, although there were periods of sample hiatuses depending on sites (see Figure 2). The mean TOC fluxes at Stations M4, M6, 50N, KNOT and 40N were 7.9, 5.8, 3.2, 6.3, and 4.4 (mg m 2 day 1 ), respectively, during 7 Aug to 30 Jan [Nakatsuka et al., submitted; Honda, 2001; Honda et al., 2002]. [6] C. davisiana was the most dominant taxon at both Stations M4 and M6 in the Okhotsk Sea, contributing 19.0% of total radiolarian assemblage at Station M4 and 21.2% at Station M6. On the other hand, in the northwestern North Pacific, C. davisiana was not a dominant taxon at three stations, only contributing 3.3% at Station 50N, 5.5% at Station KNOT, and 1.1% at Station 40N. [7] Significant peaks of C. davisiana fluxes were found during summer-autumn (July to October) at all the traps. The monthly mean values of radiolarian fluxes were 4. Discussion 4.1. What Control the Recent Production of Cycladophora davisiana? [8] In the Okhotsk Sea, C. davisiana was the most abundant taxon at both Stations M4 and M6. The distinct summer-autumn peaks of C. davisiana fluxes were found at both Stations M4 and M6. Nimmergut and Abelmann [2002] indicated that C. davisiana dwelled mainly in the OSIW and their high standing stocks were found in the Sakhalin shelf region based on the plankton tow observations. Seasonal variation of the vertical structures of temperature and salinity at each station are shown in Figure 3[Levitus and Boyer, 1994]. The hydrographic features of the OSIW, C. davisiana mainly dwells, are relatively low temperature ( C) and low salinity ( PSU), whose conditions are nearly stable throughout the year. Therefore, the seasonal variation of the physical parameters may not have directly affected the seasonal flux changes of C. davisiana. The food availability is also important for the radiolarian production. In the OSIW near Sakhalin, significantly high microbial production (50 mg m 3 day 1 at 400 m) was reported [Sorokin and Sorokin, 1999]. This microbial production was much higher than that in the subarctic Pacific ( mg m 2 day 1 in m) [Nagata et al., 2000]. Nimmergut and Abelmann [2002] suggested that the radiolarian production including C. davisiana in the Okhotsk Sea was closely related to the specific food chains with bacteria and detritus because the highest radiolarian standing stock was found in the subsurface and intermediate depths during the period of maximum heterotrophic activity in summer, when bacterioplankton biomass reaches maximum. Nakatsuka et al. [2002] indicated that the flux of particulate organic carbon (POC) from the NW continental shelf, where sea-ice had rejected a large amount of brine water in winter into the OSIW, substantially exceeded that of direct sinking POC from the surface water, and suggested that this transport supported the biological production in the OSIW. Our TOC results are the highest during times with peak occurrence of C. davisiana. We assume that a significant part of our TOC material is in the form of microbial and detrital material as reported earlier by Sorokin and Sorokin [1999], thus being a major food source for C. davisiana. Nimmergut and Abelmann [2002] showed that the radiolarian standing stocks were significantly low in the surface layer of the Okhotsk Sea. Besides, Okazaki et al. [in press] showed that the mean values of radiolarian flux at lower depths Table 1. Summary information for the sediment trap samples used in this work Trap Station Latitude Longitude Water depth (m) Mooring depths (m) Sampling duration M N E Aug Jun. 00 M N E Aug Jun N N E Dec May 00 KNOT N E Dec May 00 40N N E Dec Jan. 00

3 OKAZAKI ET AL.: THE PRODUCTION SCHEME OF CYCLADOPHORA DAVISIANA OCE 3-3 Figure 2. Temporal fluxes of C. davisiana and TOC at five stations in the Okhotsk Sea and the northwestern North Pacific during (M4: 1550 m, M6: 700 m) were much higher than that of upper depths (300 m) at both stations in the Okhotsk Sea (ca. 6.4 times at M4; ca. 1.9 times at M6). These results also suggest that the radiolarian production in the OSIW is dependent on the organic matter transported from the continental shelf as well as from the surface waters. We can summarize the scheme of distinct summer-autumn peaks of C. davisiana flux in the Okhotsk Sea as follows: (1) the microbial biomass in the OSIW is primarily responsible for the C. davisiana production; (2) such a high microbial biomass is supported by the POC rich water flowing into the OSIW from the NW continental shelf; and (3) the production of the POC rich water is closely related to the sea-ice rejected brine water in winter. The mean value of C. davisiana flux at Station M4, whose location is near the shelf region, was ca. 2.9 times as much as that at Station M6. This suggests that C. davisiana fluxes at both stations reflect the difference in distance from the NW continental shelf. On the contrary of the OSIW, the radiolarian standing stock in the surface water was extremely low because of the strong seasonal changes in the hydrography including the formation of sea-ice during winter [Nimmergut and Abelmann, 2002]. This is one of the reasons why C. davisiana is the most dominant taxon in the Okhotsk Sea. [9] In the northwestern North Pacific, the significant summer-autumn peaks of C. davisiana fluxes were also Figure 3. Monthly mean values of C. davisiana flux and seasonal variations of the vertical structures of temperature and salinity at five stations in the Okhotsk Sea and the northwestern North Pacific (Contours drawn by Ocean Data View [Schlitzer, 2002]).

4 OCE 3-4 OKAZAKI ET AL.: THE PRODUCTION SCHEME OF CYCLADOPHORA DAVISIANA found at all of three Stations 50N, KNOT, and 40N. The vertical distribution patterns of plankton organisms were observed during August 1998 at the same location as Station KNOT (44 N, 155 E) and their biomass was quantified [Yamaguchi et al., 2002]. They indicated that the close relation was observed between biomass of bacteria and protozooplankton, including Radiolaria except for the 0 40 m data. Besides, the time-series heterotrophic bacteria abundance was studied at Station KNOT during June 1998 to June 2000 [Liu et al., 2002]. The periods with high heterotrophic bacteria abundance tend to be coincident with the high C. davisiana flux at Station KNOT. These facts suggest that the microbial biomass such as bacteria is substantially responsible for the C. davisiana production also in the northwestern NPIW. The mean value of C. davisiana flux at Station KNOT was the highest among three open ocean stations, representing more than twice of other stations. The mean value of total mass fluxes at Station KNOT exhibited much higher value than those of other two stations during December 1997 to January 2000 (Stations 50N, KNOT, and 40N: 110.0, 163.8, and 94.6 (mg m 2 day 1 ), respectively) [Honda et al., 2002]. Station KNOT belongs to the Oyashio region which is characterized by high oxygen and nutrient contents. This is because that the oxygen rich Okhotsk Sea water is substantially involved in formation of the Oyashio water [e.g., Talley, 1991]. Further, Onodera et al. [submitted] indicated the significant influence of coastal waters at Station KNOT based on the diatom taxa, especially Chaetoceros resting spores which increased during summer to autumn. The mean flux values of Chaetoceros resting spores at Stations 50N, KNOT, and 40N were 0.48, and 0.01 (10 6 valves m 2 day 1 ), respectively. They suggested that the high diatom fluxes at Station KNOT were caused by the supply of coastal waters during summer. The high C. davisiana flux at Station KNOT can be considered as a result of high primary production in the Oyashio water and the supply of lithogenic materials during summer-autumn The Fossil Record of C. davisiana: Can It Tell Anything About the Okhotsk Sea and High Latitude Open Ocean Environments During the Glacial Periods? [10] During the LGM, the relative abundance of C. davisiana in the high latitude open oceans (>40 ) in both hemispheres showed significantly high values, often exceeding 20%, which are analogous to the present values in the Okhotsk Sea [Morley, 1980]. Sancetta [1992] indicated that environmental conditions in the North Atlantic and North Pacific Oceans were very similar during the glacial periods, suggesting the high biological production resulting from high nutrient contents caused by melting of sea-ice. In the Emperor Sea Mount sediments around 45 N in the North Pacific, the ice rafted detritus (IRD) particles were found during the glacial ages (Marine Isotope Stages 2 and 4) [Narita et al., 2002]. It implies that the microbial production in the intermediate waters in the glacial open oceans must have been increased as a result of seasonal sea-ice expansion. On the other hand, the radiolarian production of the surface dwellers in the surface waters might have been decreased by the severe surface seaice conditions like that of the present Okhotsk Sea [Morley et al., 1995]. We assumed that the microbial production is fluctuating according to the OSIW production and thus it increased during the glacial periods. Provided that the microbial biomass is an important food supply for radiolarians, the occurrence of C. davisiana can become an important tool in modeling the flow of shelf produced intermediate water into the North Pacific Ocean. [11] Acknowledgments. We thank captains, crew, and scientists on board R/V Khromov and R/V Mirai, and Drs. K. I. Ohshima, Y. Fukamachi, G. Mizuta, and Ms. C. Yoshikawa at Hokkaido University for their efforts in deployments, recoveries, and sample splitting of the sediment trap materials. We thank Drs. K. R. Bjørklund and S. A. Gorbarenko for providing constructive reviews in improving this paper. This study was funded by the CREST program (M. Wakatsuchi, PI), Japan Science and Technology Corporation and MEXT Grants-in-Aid-for Scientific Research B2 Project No , B1 Project No and GCMAPS program (H. Kawahata, PI). YO received a partial fund from Prof. Tatsuro Matsumoto Scholarship Fund. References Ganopolski, A., S. Rahmstorf, V. Petoukhov, and M. Claussen, Simulation of modern and glacial climates with a coupled global model of intermediate complexity, Nature, 391, , Honda, M. 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