ARTICLE IN PRESS Deep-Sea Research II 52 (2005) 2302–2331 www.elsevier.com/locate/dsr2 Radiolarians in the Sea of Okhotsk and their ecological implication for paleoenvironmental reconstructions A. AbelmannÃ, A. Nimmergut Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, 27515 Bremerhaven, Germany Received 17 December 2003; accepted 27 July 2005 Available online 11 October 2005 Abstract To assess the relationship of radiolarian production, species distribution in water and surface sediment to water-mass characteristics, biological productivity and export regimes in the Sea of Okhotsk (SOk) we accomplished a quantitative analysis of radiolarian assemblages obtained from 35 surface-sediment samples and 115 plankton samples recording the radiolarian depth distribution in the upper 1000 m of the water column at 23 locations. This study augments the knowledge on the autecological demands of radiolarians dwelling in a specific hydrographic and biological environment, and extracts new information on the significance of radiolarians for the assessment of past oceanographic and climatic development in high latitudes. Highest radiolarian accumulation rates and seasonal radiolarian standing stocks are encountered in the western part of the SOk close to Sakhalin, marking the environmental conditions in this area as most favorable for radiolarian production. Maximum standing stocks occur during summer, indicating that the radiolarian signal preserved in the sediment record is mainly produced during this season when the mesopelagic biomass is at highest activity. We identified seven radiolarian species and groups related to specific water-mass characteristics, depth habitats, and productivity regimes. Of those, Dictyophimus hirundo and Cycladophora davisiana are most prominent in the Sea of Okhotsk Intermediate Water (200–1000 m), the latter representing an indicator of the occurrence of cold and well ventilated intermediate/deep water and enhanced export of organic matter from a highly productive ocean surface. While Antarctissa (?) sp. 1 is typically related to the cold-water Sea of Okhotsk Dicothermal Layer (SODL), ranging between 50 and 150 m water depth, the surface waters above the SODL affected by strong seasonal variability are inhabited predominantly by taxa belonging to the Spongodiscidae, having a broad environmental tolerance. Taxa only found in the sediment record show that the plankton study did not cover all assemblages occurring in the modern SOk. This accounts for an assemblage restricted to the western Kurile Basin and apparently related to environmental conditions influenced by North Pacific and Japan Sea waters. Other important taxa include species of the Plagoniidae group, representing the most prominent contributors to the SOk plankton and surface sediments. These radiolarians show a more opportunistic occurrence and are indicative of high nutrient supply in a hydrographic environment characterized by pronounced stratification enhancing heterotrophic activity and phytodetritus export. r 2005 Elsevier Ltd. All rights reserved. Keywords: Radiolaria; Plankton; Surface sediments; Intermediate water; Mesopelagic; Sea of Okhotsk ÃCorresponding author. Fax: +40 471 4831 149. E-mail address: [email protected] (A. Abelmann). 0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.07.009 ARTICLE IN PRESS A. Abelmann, A. Nimmergut / Deep-Sea Research II 52 (2005) 2302–2331 2303 1. Introduction 1997; Sorokin and Sorokin, 1999; Broerse et al., 2000). This leads to high biogenic opal accumula- The Sea of Okhotsk (SOk) (Fig. 1) a marginal sea tion, accounting for up to 40% and more of modern of the North Pacific is suggested to present a SOk sediments, composed of diatoms and radiolar- modern analog of the glacial high-latitude ocean. Its ians (Lisitzin, 1996; Biebow and Hu¨ tten, 1999). unique environmental and hydrographic situation Radiolarians are widely distributed in the world leads to the development and sedimentation of ocean and comprise surface and deep-living species ‘‘glacial-type’’ radiolarian assemblages in plankton that are adapted to specific water masses and water- and surface sediments, respectively (Morley and mass structures (Abelmann and Gowing, 1997; Hays, 1983; Nimmergut and Abelmann, 2002; Hays Kling and Boltovskoy, 1995; Nimmergut and and Morley, 2003; Okazaki et al., 2003). The Abelmann, 2002; Itaki, 2003; Itaki et al., 2003). productivity regime in the SOk is characterized by Considering their habitats, radiolarians represent a a pronounced yearly plankton succession that promising tool to explore the mesopelagic environ- results in (1) high diatom production during spring ment in the geological past. Processes taking place that extends also into summer, and (2) high in this environment play a significant role in the production of heterotrophic organisms during sequestration of organic carbon, and thus for global summer and early fall (Lapshina, 1996; Mordasova, climate development. Such exploration requires 142° 144° 146° 148° 150° 152° 154° 156° 156 ¡ LV27-4 Kamcha LV27-1 LV27-5 LV28-14 UT99-19 LV28-29 UT99-20 LV27-2 UT99-15 tka Amur river LV27-3 LV27-6 ° 54° delta UT99-16 54 LV28-34 LV28-7 UT99-13 UT99-21 LV27-7 GE99-12 LV28-41 LV28-43 UT99-22 52° LV28-44 52° Kamchatka LV28-42 Current n LV28-40 LV27-8 LV28-4 UT99-5 Sakhali ° 50° LV27-11 50 Sakhalin Current GE99-38 GE99-10 LV27-12 LV28-2 LV28-55 UT99-4 LV28-3 LV28-61 ° Oyashio 48° 48 LV28-64 UT99-23 GE99-4 Current LV28-66 GE99-9 UT99-24 GE99-2 Kuril Basin Sibiria 154¡ 156¡ E 46° 46° Soya Current GE99-1 Japan 142° 144° 146° 148° 150° 152° 154° 156° Fig. 1. Locations of surface-sediment and plankton samples presented in this study. Plankton stations are indicated by dots and surface- sediment samples by triangles. Stations where both plankton and surface sediments are taken are marked with an asterisk. Surface-water circulation in the Sea of Okhotsk (SOk) is marked by gray arrows. ARTICLE IN PRESS 2304 A. Abelmann, A. Nimmergut / Deep-Sea Research II 52 (2005) 2302–2331 detailed ecological information about the relation of radiolarian accumulation rates (RARs) that will be specific radiolarian key species to water-mass compared to the latitudinal pattern of the spring properties, feeding behavior, and rivalry strategies. and summer radiolarian standing stocks extracted The SOk, which is one of the most productive areas from recently accomplished plankton data (Nim- of the world ocean, is dominated by mesopelagic mergut and Abelmann, 2002). radiolarians (Nimmergut and Abelmann, 2002) and thus presents an ideal site to understand the 2. Hydrographic setting ecological constraints that affect the distribution pattern of specific radiolarian species. We combined The specific hydrographic conditions in the SOk surface-sediment and plankton data gathered from are strongly related to the seasonal variation of the the SOk in order to define radiolarian key species relatively stable atmospheric pressure cells and the and their ecological requirements to reconstruct polar jet stream (Fig. 2). During winter, the move- water-mass structures and productivity regimes in ment of polar air masses over the SOk causes the geological past. In addition, we compare our extensive formation of sea ice covering most of the results with recently accomplished seasonal flux SOk (Alfultis and Martin, 1987; Talley and Nagata, studies of radiolarians from sediment traps located 1995). The only ice-free region (except during very in the northeastern and western part of the SOk strong winters) is the Kamchatka Current area in (Hays and Morley, 2003; Okazaki et al., 2003). the eastern SOk, which is influenced by the inflow of Previous studies of radiolarians in SOk surface ‘‘warmer’’ North Pacific water via the Oyashio sediments are based on the relative abundance Current. During summer, warm and moisture-laden pattern of one single radiolarian species (Cyclado- southwesterly winds lead to sea-ice melt and to a phora davisiana) or on a qualitative description of strong warming in the uppermost layer of the SOk the radiolarian taxa (Ling, 1974; Kruglikova, 1975; waters reaching more than 14 1C(Fig. 2). The warm Morley and Hays, 1983). The present study shows and thin surface layer (upper 50 m) is underlain by a the first quantitative analysis of radiolarians in SOk near-freezing temperature minimum (remnant of surface sediments and comprises the estimation of winter water), which leads to the development of the Spring / Summer Winter sea-surface warming sea ice formation high biomass production T = ~1.7 - 14°C S = ~30 - 32.7 ~ 50 m mean winter sea ice distribution ~ 200 m T°C -2 0 4 8 12 16 0 SW T = ~1 - 1.7°C S = ~32.7 - 33.3 T minimum SODL Stratification T = ~1 - 2°C ~ 500 m S = ~33.3 - 34 upper SOIW 500 ~ 1,000 m T = ~2 - 2.3°C water depth (m) S = ~34 - 34.6 lower SOIW 1,000 32 33 34 35 S Deep Water Fig. 2. Schematic presentation of the water-mass structure in the upper 1000 m and sea-ice distribution in the SOk. Indicated are temperature (black line) and salinity (gray line). SW, surface water; SODL, Sea of Okhotsk Dicothermal Layer; SOIW, Sea of Okhotsk Intermediate Water. ARTICLE IN PRESS A. Abelmann, A. Nimmergut / Deep-Sea Research II 52 (2005) 2302–2331 2305 Sea of Okhotsk Dicothermal Layer (SODL), result- (Fig. 1). The studied surface-sediment samples ing in a pronounced stratification of the upper generally comprise the topmost 0.5 cm of the 200 m during summer (Kitani, 1973; Yang and sediment, carefully removed from one or two Honjo, 1996; Freeland et al., 1998). The relatively MUC or MIC tubes (diameter: 6 cm). Altogether stable Sea of Okhotsk Intermediate Water (SOIW) 115 plankton samples were collected at 23 locations occurs below the SODL (Fig. 2). The SOIW is with a multiple opening/closing sampling net device suggested to be formed by isopycnal mixing of dense from five depth intervals over the upper 1000 m of shelf-derived water with inflowing North Pacific the water column during spring (May 22–June 11, water in the interior of the sea (Kitani, 1973; 1999) and summer (August 8–September 12, 1998) Freeland et al., 1998; Wong et al., 1998).