Journal of Asian Earth Sciences, Vol. 16, No. 1, pp. 49±58, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S0743-9547(97)00048-2 1367-9120/98 $19.00 + 0.00

The Bering and Okhotsk Seas: modern and past paleoceanographic changes and gateway impact

Kozo Takahashi Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Higashi-ku Fukuoka 812-81, Japan

(Received 22 April 1996; Accepted 7 October 1997)

AbstractÐThe high biological productivity and an ecient biological pumping in the subarctic Paci®c and adjacent seas make this region important to the modern carbon cycle and both mod- ern and the past climate of the Earth. Knowledge of the northern marginal seas of the Paci®c, however, is unsubstantial. The Bering Sea is located between the Paci®c Ocean and the Arctic Sea and plays an important role in ocean circulation, involving balances of heat, salt and various chemical properties. Thus, it is necessary to unravel the geologic history of the Bering Sea as a gateway to the Paci®c and the Arctic/Atlantic during the last 5 million years and beyond. The Okhotsk Sea is considered a locus of North Paci®c intermediate water formation today. The inter- mediate water formation is linked with seasonal sea-ice cover. Diatom records from the Okhotsk Sea demonstrate that sea-ice cover was distributed on the western side of the sea and the eastern part was open water during the last glacial maximum. This con®guration permitted a better venti- lation of the glacial Okhotsk Sea through increased quantity of intermediate water, presumably formed there at that time. # 1998 Published by Elsevier Science Ltd. All rights reserved

Introduction ginal seas and the Paci®c Ocean and/or the Arctic Sea are important to understanding material and heat bal- Subpolar regions, including marginal seas, play signi®- ances and climate change. Studies of paleoceano- cant roles in the global carbon cycle and, hence, are graphic changes recorded in these seas provide important to global climate change. This is because pertinent information concerning the evolution of surface waters in these regions have the potential to northern hemisphere glaciation in association with the absorb atmospheric CO2. There are three principal Milankovitch orbital cycles, and other high-frequency belts of high biological productivity in the world cycles such as Dansgaard±Oeschger cycles. The past oceans, including, from north to south, the subarctic climatic±paleoceanographic changes and the need for belt (both Paci®c and Atlantic Oceans), the equatorial further studies in these regions will be also discussed in upwelling belt (Paci®c, Atlantic and Indian Oceans), this paper. and the circumpolar subantarctic belt (Berger et al. 1987). Moreover, the high biological productivity in the upper ocean involves either emission or absorption The Bering Sea of atmospheric CO2. It is generally concluded that the equatorial belt is the largest natural source of atmos- Geomorphology pheric CO2 (Tans et al. 1990; Murray 1995). The remaining two subpolar belts are generally regarded as The Bering Sea has a surface area of 2.29  106 km2 behaving as CO2 sinks. Based on measured carbon and a volume of 3.75  106 km3 and is the third largest and opal particle ¯uxes using sediment traps, Wong et marginal sea in the world, only surpassed by the al. (1995) showed that the central subarctic Paci®c is Mediterranean and the South China seas (Hood 1983). also a CO2 sink with a fairly e€ective opal pump. There are three major rivers which empty into the Analogous information from the northern marginal Bering Sea: the Kuskokwin and Yukon draining cen- seas of the Paci®c region, such as the Bering and tral Alaska and the Anadyr draining western Siberia Okhotsk Seas, is unsubstantial. However, available evi- (Fig. 1). The Yukon is the longest and supplies the lar- dence suggests that these two seas play a large role in gest discharge into the Bering Sea. Its discharge has a the global material balance and, in turn, climate peak in August of 4  104 m3s-1 because of melt water, change. about equal to the Mississippi, and a mean ¯ow for This paper will review current knowledge and dis- the year of 5  103 m3s-1, about two thirds the annual cuss the importance of the Bering and Okhotsk Seas, ¯ow of the Columbia River (Hood 1983). two northern marginal seas of the North Paci®c. The Approximately one half of the Bering Sea is a shal- present-day high productivity in the marginal seas, low (0±200 m) neritic area (Fig. 1). The major part of based on biogenic particle ¯uxes will be presented, and the continental shelf lies on the eastern side, o€ compared with that in the pelagic regions. The pro- Alaska, ranging from the Bristol Bay in the south to cesses of water mass exchange between these two mar- the Bering Strait in the north. The northern continen-

49 50 K. Takahashi

Fig. 1. Major topographic features of the Bering Sea and Aleutian Islands. Contours of 100, 200, 1000 and 3500 m are shown. (Basic map from U.S. GLOBEC 1996). tal shelf is seasonally covered by sea ice, while little ice the Bering Sea, this is the major strait where it ¯ows occurs over the deep south-west areas. The continental out, followed by a secondary one at the Commander± slope occupies only 13% of the total Bering Sea area Near Strait at 2000 m present-day depth. and generally has a slope of 4±58. As the largest semi-enclosed marginal sea of the Other than the shelf regions, there are two signi®- Paci®c rim, the Bering Sea's indisputable in¯uence has cant topographic highs which provide better calcium been recognized in various oceanographic processes. carbonate preservation than the basins (Creager et al. Although the amount is less than the water exchange 1973). The Shirshov Ridge extends south from the through the Aleutian channels, the out¯ux of the Kamchatka Peninsula along 1708E separating the Bering Sea surface water is important, since it ¯ows Aleutian Basin into eastern and the western parts. The one way into the Chukchi Sea in the Arctic. This Bowers Ridge (sometimes referred to as the North Rat amount is estimated to be 0.8 Sv, according to Island Ridge/Bank) extends 300 km north from the Coachman and Agaard (1981). This is the only Aleutian Island Arc (Fig. 1). The Aleutian Basin is a ``Paci®c'' origin water that eventually ¯ows into the vast plain lying at a depth of 3800±3900 m with oc- Atlantic through the Arctic Sea. The Bering Strait pro- casional gradual sloping hollows to depths of as much vides one of the highest biological productivities in the as 4151 m (Hood 1983). world, 324 g C m-2y-1 over a wide area (2.12  104 km2: Sambrotto et al. 1984). Much of the biological pro- Physical oceanography and the signi®cance of the gate- duction of organic matter and associated nutrients way to the Arctic Sea ¯owing into the Arctic Ocean today is due to this northerly current direction. The Alaskan Stream, which is an extension of the This may have a profound e€ect on the nature of Alaskan Current ¯owing westward along the Aleutian carbonate production in the Atlantic and opal pro- Islands, mainly enters through the Amchitka Pass with duction in the Paci®c (the carbonate ocean vs silica the remainder entering through the pass west of Attu ocean hypothesisÐHonjo 1990). Such one way ¯ow Island in the eastern Aleutian Islands (Fig. 2). A part into the Arctic Ocean, however, did not necessarily of the Subarctic Current also joins the northward ¯ow always operate in the past. Glaciation and perennial coming from the Alaskan Stream, resulting in a com- sea-ice cover can certainly block such a ¯ow. During bined volume transport of 11 Sv (Ohtani 1965). Much the glacial periods the Bering Strait, which is about of the Paci®c water masses entering the Bering Sea 50 m deep today, was aerially exposed, due to sea-level goes out through passes in the Aleutian Islands. The drop and, thus, the Bering±Arctic gateway was com- most signi®cant one is through the Kamchatka Strait, pletely shut. What was the impact on water circulation present maximum depth of which is 4420 m. If the gla- then? It is not hard to imagine that such a closure cial North Paci®c intermediate water mass is formed in caused a major change in global water mass circulation The Bering and Okhotsk Seas 51

Fig. 2. A map showing surface currents in the Bering Sea (from Arsen'ev 1967). during the glacial periods. The glacial Yukon River North Paci®c intermediate water (NPIW) (e.g. Talley discharge, for example, had to eventually come out of 1991). Talley (1991) demonstrates that oxygen-rich the Bering Sea into the North Paci®c, without any Okhotsk deep water (to avoid a possible confusion alternative outlet. hereafter we de®ne this water as ``intermediate water'') Such a unidirectional ¯ow of the Bering Sea water, ¯ows into the Paci®c Ocean and ventilates the Paci®c eventually ¯owing into the Atlantic, should a€ect not subpolar gyre. The intermediate water formation only the heat balance, but also the salt balance and, during the summer months might be associated with hence, the formation of deep-water masses. It is the in¯ow of saline waters from the Japan Sea through known that during glacial intervals, the Atlantic Ocean the Soya Strait. During the winter months, the for- became more Paci®c-like in circulation and the Paci®c mation of sea ice leads to dense, saline shelf waters. Ocean became more Atlantic-like, in terms of calcium Alfultis and Martin (1987) estimated that the rate of carbonate and siliceous microfossil preservation. This Okhotsk intermediate water formation, driven by the is known as the ``basin to basin fractionation model'' mixing of dense shelf water with Paci®c waters, is (Berger 1970), and provides one of the clues to past nearly 2 Sv, which would be enough to have an impact changes in water circulation. Glacial intervals are on NPIW formation. characterized by the better preservation of calcareous The extent of the Okhotsk intermediate water out- plankton in the Paci®c, but also the production of ¯ow as a source of the recent NPIW is not very well plankton shifted more towards an Atlantic type understood. Today, the total amount of intermediate (Berger 1970). Such a shift made the two great oceans water ¯owing from the Okhotsk Sea into the Paci®c is far more alike in the chemistry of their water masses about 17 Sv. The intermediate water ¯ow through the than they are today. These changes can be determined main passages (Bussol, Kruzenshterna) into the Paci®c by examining the microfossil record. Since the global is about 6 Sv (Kurashina et al. 1967). In comparison, circulation of watermasses signi®cantly a€ects the cli- the Okhotsk intermediate water out¯ow is less than mate of the earth, we need to investigate the paleocea- the sum of deep-water over¯ow from the Norwegian± nographic changes recorded in the Bering Sea, and Greenland Sea into the Atlantic with 9 Sv perhaps the Arctic Sea sediments, to resolve how this (Worthington 1976), but greater than each of com- excursion occurred and how quickly it shifted from ponents stated below. The Norwegian±Greenland Sea one state to the other. deep-water over¯ow consists two parts as those through the Denmark Strait with 5 Sv and Iceland± The physical oceanography of the Okhotsk Sea pertinent Scotland Passage with 4 Sv (Worthington 1976), sum to its paleoceanography of which make up a signi®cant portion of the North Atlantic Deep Water (NADW). Therefore, the Recent oceanographic investigations indicate the Okhotsk intermediate water out¯ow has a signi®cant Okhotsk Sea (Fig. 3) as a possible source for the potential as a deep-water source for the NPIW during 52 K. Takahashi

Fig. 3. Major topographic features of the Okhotsk Sea and Kuril Islands. Contours of 100, 200, 500, 1000, 2000, 2500 and 3000 m are shown (modi®ed from Gnibidenko and Khvedchuk 1982).

the last glacial interval. For comparison, the Amur Surface water interactions between the open Paci®c river discharge is about 2 Sv, which is a signi®cant and the enclosed Okhotsk Sea have a clear impact on amount compared to the above quoted values. Was the formation of the Oyashio Current. The surface dis- this Amur discharge from land signi®cantly reduced or charge from the Okhotsk Sea forms the core of the increased during the glacial periods? Perhaps it is more Oyashio Current, which merges with the southward important to decipher the seasonal change of the ¯owing East Kamchatka Current (Fig. 4). A part of Amur discharge during the glacial periods, because the Oyashio Current branches out in the region further sea-ice formation occurs during the winter today. Would this change signi®cantly a€ect sea-ice formation o€shore to the south, forming the Subarctic Current and hence extent of the Paci®c intermediate water for- which ¯ows back in a north easterly direction mation? The answers to these questions lie in the sedi- (Favorite et al. 1976). During these complicated water ments of the Okhotsk Sea, inviting us to develop a mass transfers the salinity of the surface waters program of sediment coring. changes signi®cantly. The Bering and Okhotsk Seas 53

Fig. 4. A map showing surface currents in the Okhotsk Sea and adjacent regions. Note that a part of the Kamchatka Current enters into the Okhotsk Sea and a part of the Okhotsk Gyre water exits from the Sea to the Paci®c to form the Oyashio Current just south of the Kuril Islands. The Okhotsk Sea circulation is compiled by R. Tiedemann, employing data obtained by Dodimead et al. (1963), Sancetta (1981), and Talley (1991).

Sources of Paci®c deep-water formationÐthe The Okhotsk Sea intermediate water exchange with the Bering and Okhotsk Seas Paci®c Ocean

The Bering Sea deep-water exchange with the Paci®c There is strong evidence that the Okhotsk Sea Ocean played a major role as a source area for glacial Paci®c deep-water formation (Duplessy et al. 1988; Talley There is one piece of geological evidence suggesting 1991; Zahn et al. 1991; Keigwin et al. 1992). Hence, that the bottom water of the North Paci®c Ocean may the Okhotsk Sea may have controlled the extent of the have been generated in the Bering Sea in the past. deep-water ventilation and nutrient distribution in the Mammerickx (1985) discussed the possibility that a North Paci®c during the glacial periods. However, 13 bottom thermohaline circulation is the cause of the results from Cd/Ca ratios and d C data from benthic Meiji sediment tongue (Ewing et al. 1968), whose fea- foraminifera are contradictory. Both are quantitative tures are similar to those of the North Atlantic drifts, indicators of the nutrient distribution in deep and in- especially in the general shape, length, and thickness of termediate waters. Cd/Ca pro®les and oxidized brown the various sediment bodies (Scholl et al. 1977). In this clays indicate a well-ventilated North Paci®c deep scenario, sediments in the Meiji sediment tongue were water below 3000 m during the last glacial episode supplied from the Bering Sea through the Kamchatka (Boyle 1992), while benthic d13C records from the Strait (sometimes called the ``Commandorsky Strait''), equatorial Paci®c suggest an enhanced formation of located at 4420 m depth today. This is the only deep fresher North Paci®c intermediate water (NPIW) strait where a deep-water mass can exit from the between 1000±3000 m water depth (Duplessy et al. Kamchatka Basin (Commander Basin) to the North 1988). On the basis of benthic d13C records (2900± Paci®c. The next deep strait is the Commander±Near 3700 m), Zahn et al. (1991) found that the equatorial Strait, which is 2000 m deep; the rest of the straits in Paci®c was better ventilated than the North Paci®c the Aleutian Islands are less than 1155 m deep. and the Southern Ocean during the glacial periods. Furthermore, Warner and Roden (1995) found This suggests that the ventilation of deep waters in the anthropogenic chloro¯uorocarbons in the bottom central Paci®c originated from northern sources, per- waters of the Aleutian Basin in the eastern Bering Sea. haps from marginal seas such as the Okhotsk Sea. They attributed the presence of these chemicals to con- Furthermore, benthic d13C records at 1200 m water tact between bottom waters and the atmosphere within depth from the Academy of the Sciences Rise, the past 40 years, demonstrating that ventilation of the Okhotsk Sea, clearly indicate well-ventilated Okhotsk deep Bering Sea is occurring today. intermediate water during the glacial period (Keigwin 54 K. Takahashi

Fig. 5. Five-year long total mass ¯uxes measured at sediment trap Station AB in the Aleutian Basin of the Bering Sea during August 1990 through July 1995 (from Takahashi et al. 1997).

1995). Elsewhere in pelagic sediments, rather signi®- as much total mass ¯ux as the pelagic open Paci®c cant shifts in d13C values from the glacial to intergla- station. This trend is also seen in diatom ¯ux (Fig. 6) cial are seen, from 0.5 to +0.2. However, in the (Takahashi et al., 1996 1997). Furthermore, based on Okhotsk Sea this shift has not been seen, or the shift species list and % contribution of each taxon of silic- is very small, indicating well-ventilated conditions eous and calcareous plankton ¯ux, it appears that bio- during both glacial and interglacial periods. A similar logical ecosystems operating in the two regions are trend is also observed in the Bering Sea and in the generally not much di€erent from each other. North Paci®c just outside the Bering Sea on the Therefore, it is fair to conclude that the Bering Sea is Detroit Seamount, suggesting a source of ventilation roughly twice as productive as the pelagic region of in the Bering Sea and/or Detroit Seamount region the adjacent Paci®c Ocean, without di€ering much in (Gorbarenko 1997). All of these d13C data are testi- the taxonomic constituents of the shell-bearing plank- mony to well-ventilated glacial waters in the region, ton ¯ux. clearly suggesting that the formation of intermediate Furthermore, other trap data are also available from and deep waters occurred in this area. the northern Aleutian Basin (Bering Sea) at sites located further north of Station AB. For example, Honjo et al. (1995) reported a total mass ¯ux of The Bering and Okhotsk Seas: the signi®cance 144 mg m-2 d-1 at their northern Aleutian Basin Station of high biological productivity today during 1991-1992. The total mass ¯ux measured at our Station AB was 193 mg m-2 d-1 during 1991±1992. Both the Okhotsk and Bering Seas are ranked as Hence, there is a di€erence of ¯ux of 1.34 times or areas of high biological productivity at the present 34%, indicating sizable geographic variability within time (Berger et al. 1987). The large quantity of ®sh the basin. In addition, Honjo and his colleagues annually caught in these regions is testimony to their deployed sediment traps at Shoyo Station in the north- high productivity. A time-series sediment trap has been ern Okhotsk Sea and measured a mean ¯ux of deployed at a ®xed station to continuously measure 129 mg m-2 d-1 during the 1990±1991 period (Honjo et particle ¯uxes, beginning in August 1990 (Fig. 1, al. 1995). Station AB in the Aleutian Basin of the Bering Sea: The particle ¯ux constituents described from the 53.58N, 1778W, 3800 m water depth, 3200 m trap Bering Sea are ideal for an e€ective biological pump depth). The 5 year record for 1990±1995 shows a mean with high opal and relatively low calcium carbonate daily total mass ¯ux or export production of contributions. The major reason is that the production 177 mg m-2 d-1 (Fig. 5) (Takahashi et al. 1997). This of opal is associated with organic carbon, which draws ®gure can be contrasted well with a pelagic counter- CO2 from the upper ocean and subsequently draws part station just outside the Bering Sea: Station SA CO2 from the atmosphere, without involving CO2 (498N, 1748W, water depth: 5400 m, trap depth: emission. Production of calcium carbonate shells, on 4800 m) in the central subarctic Paci®c. At Station SA the other hand, involves both CO2 withdrawal by or- -2 -1 a 5 year mean total mass ¯ux of 91 mg m d was ganic matter formation and CO2 emission by shell for- acquired (Takahashi et al. 1997). Thus, the Bering Sea mation, whose ratios depend on various conditions data reveal that the sea produces approximately twice such as taxon, season, and physiology. A ratio of or- The Bering and Okhotsk Seas 55

Fig. 6. Four-year long ¯uxes of total diatoms, Neodenticula seminae, and Chaetoceros resting spores measured at sedi- ment trap Station AB in the Aleutian Basin of the Bering Sea during August 1990 through July 1994 (from Takahashi et al. 1996).

ganic carbon to inorganic carbon (Corg/Cinorg)isan marginal seas. Furthermore, Behl and Kennett (1996) important indicator to determine whether a CO2 sink recently found that a cyclic sedimentary sequence in or source situation persists (Berger and Kier 1984). the Santa Barbara Basin, analogous to the Japan Sea Corg/Cinorg ratios measured at Station AB were almost laminated and non-laminated sequence, was formed always greater than 1 throughout the 5 years of under the in¯uence of a northern source of oxygenated measurement, indicating that the Bering Sea is an at- Paci®c intermediate water. Today, North Paci®c inter- mospheric CO2 sink (Takahashi et al. 1997). Although mediate water (NPIW) is thought to be formed in the values are slightly less than those at Station AB, Bering Sea (Talley 1991). Since the Okhotsk Sea is a Station SA in the central subarctic Paci®c also rep- strong candidate for the locus of the past intermediate resents a CO2 sink. water formation, such a record in cyclicity may also be Considering the opal contributions cited above, both present in the Okhotsk Sea. If and when the Bering the Bering and Okhotsk Seas play signi®cant roles as and Okhotsk Seas are drilled with current technology, CO2 sinks. It is of interest to delineate how the Bering the results will bring a wealth of information to bear and Okhotsk Seas may have had behaved during gla- on these unanswered questions. cial and interglacial cycles in terms of the carbon One important factor in the formation of the NPIW cycle. There are many aspects to be dealt with: micro- is the presence and distribution of sea ice in the fossils, carbon isotopes, organic compounds and other Okhotsk Sea. Severe winter cooling takes place in the relevant proxies, that are preserved in the sediments of eastern Okhotsk Sea in open water, whereas sea-ice these regions. formation takes place in the western Okhotsk Sea o€ the Amur River, due to low salinity (Kitani 1973). The seasonal sea-ice formation results in production of The semi-closed marginal seas of the Northern saline cold water, which leads to formation of a basin Paci®c: high-resolution climatic records in the scale, 100 m thick, subzero dichothermal layer, charac- sediments terized by extremely cold water (less than 1.58C) at about 150 m depth below the euphotic layer (Honjo et The Bering, Okhotsk, and Japan Seas experienced al. 1996). Thus, either or both, severe cooling in open major excursions of climate during the late Pleistocene. water and the formation of sea ice, may be linked to For instance, a sequence of laminated and non-lami- the formation of intermediate water. Although no nated sediments in the Japan Sea testify that the direct evidence has yet been presented, Riser (1997), exchange of Japan Sea water with the open pelagic based on chloro¯uorocarbon content, argues that the Paci®c was restricted during periods of glacial low NPIW must have been relatively recently ventilated in stands (Tada et al. 1992). The laminated sequence can winter in the Okhotsk Sea. be correlated with the Dansgaard±Oeschger cycles that The production of intermediate water must have have been identi®ed in the Greenland ice-core record varied with time in the geologic past, according to (Dansgaard et al. 1993) and represents global events varying degree of climatic forcing. If so, how did the rather than regional phenomena (Broecker 1994; Tada distribution of sea ice vary with time? A recent study, et al. 1992). According to Broecker (1994), there are based on diatom ¯oral analysis, has demonstrated that seven locations (other than in the North Atlantic) that there has been a signi®cant change in seasonal sea-ice can be correlated on the basis of Dansgaard±Oeschger cover in the Sea of Okhotsk in the recent past. Shiga cycles, Heinrich events, or the Younger Dryas. The at- et al. (1994) showed that sea-ice cover changed during mospheric temperature recorded in the Greenland ice the last 22 ky, based on the analysis of 15 cores cores, as well as lake sediment records, can be tied in (Fig. 7). They have reconstructed patterns of areal with high-resolution marine sedimentary records in the expansion and contraction of open water, as well as 56 K. Takahashi

Fig. 7. Temporal changes of sea-ice cover in the Sea of Okhotsk during the past 22 ky based on 15 cores (adopted from Shiga et al. 1994). seasonal and perennial sea-ice covers. They show that these marginal seas is ecient not only in their high open water has been generally con®ned to the eastern opal content, but also in quantity in terms of high part of the Sea of Okhotsk, o€ the Kamchatka ¯uxes. It is concluded here that both the Bering Sea Peninsula. Perennial ice cover occurred in the western and Okhotsk Sea act as CO2 sinks. part of the sea, including much of the Kuril Basin The Bering Sea, the largest marginal sea of the (Fig. 7). Although there is a need for studies of more Paci®c, is located between the Paci®c Ocean and the detailed past sea-ice distribution as well as longer geo- Arctic Sea, and mediates the transport of Paci®c water logic record in the Okhotsk Sea, the above data by into the Arctic Ocean. During past glacial low stands Shiga et al. (1994) clearly demonstrate the importance the modern deep sill of the Bering Strait (50 m) was of the sea-ice distribution in formation of the NPIW. aerially exposed and the gateway closed. This must Furthermore, Keigwin (1995) demonstrated that the have had a major impact on water mass exchange Okhotsk Sea intermediate water was better ventilated between the Atlantic and the Paci®c. Detailed study of during the glacial period than at present, based on the Pleistocene paleoceanography of the Bering Sea is d13C of benthic foraminifera and seawater samples. needed. It would also be desirable to investigate the Bering Sea connection with the Arctic over a longer time scale. Available fossil evidence suggests that the Conclusions Bering land bridge and the Bering Strait gateway have operated since Late Miocene time. The Bering and Okhotsk Seas are located in the sub- The Okhotsk Sea is considered to be a locus of polar region of the Paci®c and play signi®cant roles in Paci®c intermediate water formation today. Formation global carbon cycle. This mainly stems from the highly of this water is thought to be linked with seasonal sea ecient biological pump operating in these regions, ice, which is formed in the winter, and the permanent, re¯ected in the production of greater amounts of opal 100 m thick, subzero, dichothermal layer which lies (diatoms) than calcium carbonate. Measured biogenic below the euphotic layer. Thus, the temporal and geo- particle ¯ux data indicate that these two seas experi- graphic distribution of sea ice in this sea are important ence twice the annual production of adjacent pelagic in delineating the past history of Paci®c intermediate open areas. Therefore, the biological pump manifest in waters. Based on ice algae and planktonic diatoms, the The Bering and Okhotsk Seas 57 maximum extent of open water in the Okhotsk Sea by the Ministry of Education, Science and Culture (Grant- during the last glacial maximum was in the eastern in-Aid for scienti®c Research C project no. 07680562). sea, allowing a vigorous heat exchange through the surface layer. This, in turn, lead to the production of intermediate water, which must have exited through REFERENCES the Bussol Strait (2300 m deep today) to the North Paci®c, probably in greater quantity than at the pre- Alfultis, M. A. and Martin, S. (1987) Satellite passive microwave stu- sent time. dies of Sea of Okhotsk ice cover and its relation to oceanic pro- cesses, 1978±1982. Journal of Geophysical Research 92, 13013± 13028. Arsen'ev, V. S. (1967) Currents and water masses of the Bering Sea. Future Work Nauk. Moscow, 135 pp. (in Russian with English summary). [Transl., 1968. 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