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The Bering and Okhotsk Seas: Modern and Past Paleoceanographic Changes and Gateway Impact

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The Bering and Okhotsk Seas: Modern and Past Paleoceanographic Changes and Gateway Impact 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 ecient 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 Â 106 km2 behaving as CO2 sinks. Based on measured carbon and a volume of 3.75 Â 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 eective 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 Â 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 Â 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 Â 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 eect 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 aect 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.
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