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Effect of Pleistocene Glaciation Upon Oceanographic Characteristics of the North Pacific Ocean and Bering Sea

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Effect of Pleistocene Glaciation Upon Oceanographic Characteristics of the North Pacific Ocean and Bering Sea Deep-Sea Research, Vol. 30, No. 8A, pp. 851 to 869, 1983. 0198-0149/83 $3.00 + 0.00 Printed in Great Britain. (c) 1983 Pergamon Press Ltd. Effect of Pleistocene glaciation upon oceanographic characteristics of the North Pacific Ocean and Bering Sea CONSTANCE SANCETTA* (Received 23 October 1982; in revisedform 28 January 1983; accepted 28 April 1983) Abstract--During intervals of Pleistocene glaciation, insolation of the high-latitude northern hemisphere was lower than today, particularly during summer. Growth of continental ice sheets resulted in a lowering of sea level by more than 100 m in the Bering Sea. As a result, the Bering Strait was closed and most of the Bering continental shelf exposed. A proposed model predicts that (1) sea-ice formation would occur along the (modern) outer continental shelf, (2)advection would transport the sea ice over the deep basin, and (3) brine would flow into the basin at some inter- mediate depth to enhance the halocline. The result would be a low-salinity surface layer with a cold, thick halocline and reduced vertical mixing. Diatom microfossils and lithologic changes in sediment cores from the North Pacific Ocean and Bering Sea support the model and suggest that the proposed oceanographic conditions extended into the North Pacific, where the cold low-salinity laycr was enhanced by meltwater from continentally derived icebergs. INTRODUCTION IN THE last decade, marine geologists have begun to make observations and interpretations bearing on the physical oceanography and climate of the earth in the past (e.g., CLIMAP, 1976; Coauss, 1979; KEIGWIN, BENDER and KENNETT, 1979). In most cases, the papers emphasize the geological techniques used, often in the form of paleotemperature estimates, and discuss the oceanographic processes involved only briefly. Exceptions were the study by SACHS (1976), who used geologically generated data to test the predictions of a model developed by WEYL (1968), and RUDDIMAN and MCINTYRE (198 I), who proposed a model to explain their published results from the North Atlantic. The period of the last glacial advance (approximately 20,000 years ago) has been the subject of many studies, primarily because (1) it represents a climatic extreme quite different from the present, but (2) it is close enough in time that much material is available, and data sources such as fossil organisms may be assumed not to have undergone significant evolution. Because the greatest growth of ice sheets occurred in the northern hemisphere (FLINT, 1971), high northern latitudes seem a logical place to examine the effects of glaciation upon the ocean system. RUDDIMAN and MCINTYRE (1976, 1981) presented data and models discussing the effect of glaciation on the North Atlantic in great detail, but less work has been done on the North Pacific Ocean and practically none on the Bering Sea. In this paper I review certain aspects of modern Bering Sea hydrography, discuss the possible effects of Pleistocene glaciation upon the system, and indicate the degree to which the available geologic data support the model. * Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964. U.S.A. 851 852 t~ONS rANCE SANC["I-rA MODERN HYDROGRAPHY OF THE NORTHERN NORTH PACIFIC OCEAN AND BERING SEA This paper presents only a basic review of modern hydrography in the region; more detailed reports can be found in DODIMEAD, FAVORITE and HmANO (1963), REID (1965, 1973), HOOD and KELLEY (1974), COACHMAN, AAGAARD and TglPP (1975), SAYLES, AAGAARD and COACHMAN (1979), and HOOD and CALDER ( 1981). Physiography and circulation The Bering Sea floor consists of two physiographic provinces--a very broad and flat con- tinental shelf, which occupies the eastern half of the sea and extends north into the Chukchi Sea, and a deep basinal region, intersected by two ridges and separated from the North Pacific to some degree by the Aleutian arc (Fig. 1), Circulation in the subarctic Pacific is a cyclonic gyre with secondary gyres in the marginal Bering and Okhotsk seas. The gyre is bounded to the south by the Subarctic Front, a zone of steep isotherms and isohalines. The subarctic gyre is characterized by a surface salinity minimum with a permanent haiocline and seasonal summer thermocline, in contrast to the subtropic gyre with a surface salinity maximum and permanent thermocline. The Bering and Okhotsk seas are sources for Sub- arctic Water created by vertical and horizontal mixing, which lowers salinities and temperatures and increases oxygen and nutrients. The ,greatest volume of exchange between the North Pacific and the Bering Sea occurs through the two deep western passes. North Pacific water, including at least some water from the Alaskan Stream, enters the Bering Sea through Near Strait; volume estimates range from 25 x 106 m 3 s -t (HUGHES, COACHMAN and AAGAARD, 1974) to 12 x 106 m 3 s -x (FAVORITE, 1974). Bering Sea water returns to the Pacific through Kamchatka Strait as the East Kamchatka Current (<20 x 10 6 m 3 s -1, HUGHES et al., 1974; 15 x 106 m 3 S-1, FAVORITE, 1974). Some exchange also occurs through the shallow eastern passes, but the source of the water and net transport are not well known; FAVOgXTE (1974) estimated 4 x 10 6 m s s -j of Alaskan Stream water through the central Aleutian passes and no net transport through eastern passes, while HUGHESet al. (1974) calculated 5 to 10 x 106 m 3 s -t through all passes. SCHUMACHER, PEARSON and OVERLAND (1982) report about 0.05 x 106 m 3 s -~ through Unimak Pass, stating that the source water is a low-salinity coastal current; other eastern passes have not been restudied in the light of more recent findings. Surface circulation (to 1000 m) within the basinal part of the Bering Sea is basically a cyclonic gyre. Flow on most ofthe shelf is slow and generally northerly, with a small volume of water passing through Bering Strait into the Chukchi Sea (about 0.08 x 10 ~ m 3 s -j, COACHMAN and AAGAARD, 1981). Fresh water from coastal runoff (including the Yukon River) contributes to the volume of shelf water and to the flow through Bering Strait. The details of flow on the shelf and in the deeper part of the basin are still a subject of study. Exchange and circulation in the Sea of Okhotsk are not known in detail. Surface waters are derived from the warm, saline Tsushima Current from the Sea of Japan and from the cold low-salinity East Kamchatka Current, which enters through the northern Kuril passes (YASUOKA, 1968). Surface circulation within the sea is a cyclonic gyre; mixing of shelf waters along the course of the gyre results in progressive decrease of temperature and salinity. Outflow from the Sea of Okhotsk is mostly at intermediate depths (t500 m), although there is some outward flow of surface waters through the central island passes. The water joins waters of the East Kamchatka Current to form the Oyashio Current. 130=E 140 = 150" 160" 170" 180* 170" 160" 150" 140" 130°W ~ ,~. 70*N 1 BarinQ Strait BROOKS RANGE 2 Ne~" Strait 3 Komchotka Strait "0 E $0 ALASKA RANGE O 60" ~d ASeutw3n Basin O (Ira F," Y ,50" 2. ALOSWOn Streom o~ ~oo0~ ~(~ Suborct~c Front f I 40 I I I Fig. 1. Physiography and generalized surface circulation of the northern North Pacific and marginal seas. Bathymetry in meters. 854 CONS'lANCE SANC['TI-A Vertical structure In winter the upper water column of the Bering Sea Basin (to 150 to 200 m) is cooled until it is isothermal, with a sharp thermocline below, in which the temperature increases to 400 to 600 m and then slowly decreases to the bottom (Fig. 2). In summer the surface layer (0 to 100 m) is warmed by insolation, and a temperature minimum is formed between 100 and 150 m. Salinity increases with depth and shows a permanent halocline below the temperature minimum and a seasonal halocline above (SAYLESet al., 1979). Density is primarily controlled by salinity. The temperature minimum is least strongly developed in the area of Near Strait and in Bowers Basin (where moderating Pacific water enters), of intermediate strength in the Aleutian Basin, and most intense in Komandorsky Basin (Fig. 2a and b). The greater intensity is due to reinforcing by offshore flow from the narrow shelves along the western side of the basin. A similar structure occurs in the Sea of Okhotsk, where the temperature minimum is even more strongly pronounced and is primarily a result of summer melting of winter sea ice, creat ing a strong vertical contrast in salinity (YASUOKA, 1967). Sea ice In autumn the shallow waters of the Bering Shelf are cooled by northeasterly winds until the water column is vertically uniform and reaches freezing temperature (PEASE, 1981). Ice forms primarily on the lee (southern) side of coastlines and is carried south and southwest over the shelf (McNuyr, 1981). As the ice encounters warmer water it is broken up and melts; the resultant meltwater forms a cooler and fresher surface water, so that subsequent floes melt POTFNTIAL TEMPER&TLIRE I'C) POTeNTiAL Te,Pt,ATU,E ~'C~ -I 0 I 2 3 4 ~ ~ ] z ~ 4 ~ s 7 e 9 SALINITY ( xlO 3) SAUltliTY ( x 103 ) 330 334 33,8 34 2 346 O ~ o IGO 200tO01 - - - Summer 200 -- Winter _-- Winter 300 5OO 400 400 ! ', 500 600 600 r ~x ~ 7OO 700 ,?. uJ Q 800 BOO 900 900 I000 I000 I I00 1100 1200 ;I 13oo X I 1400 :E (bJ ( 0 ) 1500 / i, Fig. 2. Vertical distribution of potential temperature and salinity for two areas in thc Bering Sea (after SAYLES et at., 19"/9).
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