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

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 intermediate 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 continental 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

60" ~d

ASeutw3n Basin O (Ira

F," Y

,50" 2. ALOSWOn Streom o~ ~oo0~ ~(~ Suborct~c Front

f I I I 40 I Fig. 1. Physiography and generalized surface circulation of the northern North Pacific and marginalseas. Bathymetryin 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). (a) Komandorsky Basin, (b) Bowers Basin. Effect of Pleistocene glaciation upon oceanographic characteristics 8 5 5 less rapidly. Thus, a positive feedback system allows ice to advance over most of the shelf and, in some years, over the northern part of the Aleutian and Komandorsky basins as well. The fate of brine from sea-ice formation is not known; AAGAARD, COACHMAN and CARMACK (1981) suggest that some of it may stay on the shelf--in areas where circulation is weak so that it is slowly admixed into the shelf water--while that produced in the northern Bering Sea passes through Bering Strait into the Chukchi Sea where it contributes to the Arctic Ocean halocline. The Sea of Okhotsk, which has less exchange with the Pacific, is completely covered by sea ice for 2 to 3 months and has significant ice cover for 5 months. Sea ice forms sooner in the year and lasts longer in the Sea of Okhotsk, so that vertical structure and circulation are destroyed and re-established each year. The greater volume of sea ice over the deep basin results in the intensification of the temperature minimum following spring melting.

LATE PLEISTOCENE MODEL

During the last glacial advance (late Wisconsin or Weichselian) about 20,000 years ago, high-latitude northern hemisphere winter insolation was about the same as that of today, while summer insolation was considerably less (BERGER, 1978). Moisture from the warmer subtropical regions was deposited on the northern land masses as ice and snow which, due to the decrease in summer radiation, was retained throughout the year, resulting in progressive accumulation of the great ice sheets [see RUDDIMAN and MCINTYRE (1981) for a review of mechanisms]. The growth of continental ice sheets caused a global fall in sea level of at least 100 m (FLINT, 1971; WILLIAMS, MOORE and FILLON, 1981 and references therein). CREAGER and MCMANUS (1967) estimated a sea-level fall of about 120 m from data in the Chukchi Sea; a similar estimate for the mid-shelf of the Bering Sea was given by KNEBEL, CREAGER and ECHOLS (1974). Due to the unusual physiography of the Bering Sea, the combination of lowered sea level and insolation would have had a profound effect on the oceanographic regime. Most of the shelf area was eliminated, and Bering Strait was part of a steppe tundra, far from any sea (Fig. 3). The Bering Sea thus became a deep back-arc basin surrounded by the continental area of Beringia, with a continental shelf 150 km wide or less. If we assume winters at least as severe as those of today and summers considerably more so, then the 'winter' season would have been extended. The growth season for sea ice would therefore have been prolonged, and the spring-summer decay period shortened. Sea-ice formation would have occurred on the ancient continental shelf, i.e., along the Siberian margin (as today) and along the eastern margin of Beringia, today the outer continental shelf of Alaska. Although warmer and more saline waters of the North Pacific would still have entered the Bering Sea through Near Strait, the shallow eastern and central passes would have been eliminated or restricted, so that the eastern Bering Sea, in particular, would have been isolated from the moderating effect of the North Pacific. The actual volume of sea ice that could be formed under the above conditions cannot be estimated accurately, due to the number of assumptions that would be involved. In the present descriptive model, we will assume that lower insolation, resulting in a longer winter season and longer duration of northeasterly winds, would result in approximately the same volume of ice production as today, the locus of production being shifted on the Alaskan side of the Bering Sea. Brine produced by ice freezing would collect on the outer continental shelf and flow down- 14(7' 130°W 130oE 140o 150* 160 ° 170 = 180 ° 170° 160° 150° I I I I ',_ I ,~ .... {I ~ I I I

_/..,- z~q

i 70°N

',; ~ "-...: ~ -.. ¢%f_./

b. A ,g. ~C

7 A g J ^ ,;;' r,a" i. ~ co,

~// -~

0 0 0

0 5~ • I

• " - ~ sooo__7

4C J I ! J

Fig. 3. Geographyof Beringia during the late Wisconsin(after HOPKINS,1972). Modern con- tinentaloutline shown by dashedline. Ancientcontinental margin drawn at modern100-m isobath; land-sea contact stippled in regions not covered by glaciers. Areas of extensive glaciation generalized;indicated by carats.River drainage is speculative.Shelf-slope break shown by solid line in marginalseas; Dots representcores withstrata of'ice-rafteddetritus. Effect of Pleistocene glaciation upon oceanographic characteristics 8 5 7 slope until it reached some intermediate depth of equal density, where it would flow outward (horizontally). AAGAARDet al. (1981) proposed that such a process occurs around the Arctic Ocean today and results in the formation of a colder, thicker halocline than would otherwise exist. The ice would be driven by northeasterly winds over the basin waters; the melting of floes would produce a cold, low-salinity edge favoring further extension of the ice (as occurs today on the continental shelf). Throughout each winter ice would extend over much of the Bering Sea Basin and might also be carried into the northwest Pacific through Kamchatka Strait. When the ice finally melted in 'spring', the result would be a water column with a strong halocline-pycnocline, reinforced from above by melt water and laterally from the shelf by the high-salinity water derived from brine (compare Fig. 2a). Coastal runoff would also contribute to the effect; in particular the Yukon River, which today feeds north through the Bering Strait (NELSON and CREAGER, 1977), would have flowed west and south across the Bering shelf, probably cutting through one of the canyons of the slope. In short, the effect of sea-level lowering and lower insolation of Beringia would be to produce an upper water column colder and less saline than it is today throughout the Bering Sea, with a strong temperature minimum reaching to a greater depth, much like the modern Sea of Okhotsk (REID, 1965, his Figs 28 and 29; YASUOKA, 1967, 1968) or the Komandorsky Basin (SAYLESet al., 1979; Fig. 2a). The intensified pycnocline gradients would inhibit vertical mixing, and positive feedback would favor extension of the system from year to year; cold, low-salinity water from a previous year would allow earlier freezing and ice would travel further before melting. If we assume that the Sea of Okhotsk had a similar structure (possibly more intense than today), the combined outflow from the two seas would affect much of the western subarctic Pacific. At present, winter wind stress in the northwest Pacific is from the northwest (Siberia). Assuming a Siberian high at least as strong as that of the present (and probably stronger, GATES, 1976) sea ice and icebergs from Siberian glaciers would be driven southeast, resulting in a southeastward extension of the low-salinity surface water from ice melting. The distribution and intensity of the effect would depend on the volume of ice, its duration, and the strength of the winds, variables difficult to estimate. Certainly the effect would be strongest in the northwest, decreasing to the south and east. Icebergs from the eastern (American) margin (Fig. 3) might also contribute cold, low-salinity surface water to the northeast Pacific, in which case a vertical structure similar to Fig. 2a would extend across much of the subarctic Pacific. Under the above conditions, with surface water of the subarctic gyre cooler and less saline, the contrast with the subtropical gyre (warm, saline surface water) would be enhanced. The Subarctic Front would be an even stronger barrier to exchange of water. Less heat would thus be transported into the subarctic, horizontal exchange being limited by the front and upward vertical diffusion limited by the halocline (as for warm North Atlantic water in the modern Arctic Ocean, AAGAARD et al., 1981). With a lower heat content (cooler surface water), evaporation from the North Pacific would be less, resulting in less continental precipitation than today. The increase in land area of Beringia would also isolate the internal regions from precipitation, so that continental climate would be cooler and drier than today.

GEOLOGIC EVIDENCE

The model can be tested against interpretations of sediment cores from the North Pacific and Bering Sea. Two types of data are useful for the purpose: the presence of ice-rafted 8 5 8 CONS'I'ANC[: SANCETTA

elastics and changes of abundance of microfossils associated with specific hydrographic conditions.

Ice rafting Generally speaking, the presence of coarse sand-sized elastic detritus and small pebbles in deep-sea sediments far from land can only be explained by ice rafting. This is particularly true in the northwest Pacific, which is shielded from bottom transport of continental material by the Aleutian and Kuril trenches. The elastics may be transported by glacier-derived icebergs, which erode great quantities of rock, or by sea ice if the elastics are the result of volcanic eruption or if the ice is temporarily grounded on the continental shelf. CONOLLY and EWING (1970) and K~NT, OPDYKE and EWING (1971) examined the occurrence of ice-rafted detritus in cores from the North Pacific; I have added several more cores from south of Alaska. Paleomagnetic measurements show that all of the cores include the late Pleistocene (Brunhes) interval; the time represented varies with sedimentation rate and length of core, but biostratigraphy (extinction of the diatom Rhizosolenia curvirostris and the radiolarian Stylatractus universus) indicates that at least several glacial-interglacial cycles are represented in each core. Surface (modern) sediments in the cores are diatom oozes and diatom clays, reflecting modern conditions of high productivity. Samples downcore at varying levels contain significant amounts of poorly-sorted ice-rafted detritus, ranging from pebbles to silt-sized grains. Abundance of detritus is generally greater closer to land sources, but it extends into the central North Pacific (Fig. 3). So great is the amount of detritus that the lithology of the sediments ranges from a sand to a silty clay with rare diatoms. Most of the grains are fresh volcanic rock (including much ash), but pebbles of weathered volcanics or sedimentary rocks also occur. CONOLLY and EWlNG (1970) and KENT et al. (1971) found several intervals of ice-rafted detritus throughout the late Pleistocene interval, alternating with diatom ooze, presumably corresponding to alternate glacial and interglacial conditions. Identification of ice rafting in the Bering Sea is less certain, because sand-sized particles can be transported into the basins by turbidity currents. Many cores from the Komandorsky and Aleutian basins contain such material, but pebbles are found only in a few cores from the Komandorsky Basin.

Fabk, 1. Locations and age control on cores used in the diatom analysis

Time control Modern water Core level Core Latitude Longitude depth (m) (cm) Age (y B.P.) Method

G 119 54°59'N 166°26'W 145 76 22,000 AA* 105 29,300 + 320 I4-C*

RC14-121 54°51'N 170°41'W 2532 50-70 2750+ 110 14-C'i" 330-340 23,340 + 260 14-C"I"

RC14-126 60°00'N 173°21'E 3085 640-660 15,390 + 110 14-C+

RC11~172 51°15'N 164°56'N 4808 40 18,000 Rad +

V20-123 46°15'N 157°55'E 4903 80 18,000 Rad§

* Mollusc shells (GARDNER, personal communication). t Bulk organic carbon (SANCET'rAand ROBINSON,1983). C. davisiana abundance peak (SACHS,personal communication). § C. davisiana abundance peak (ROBERTSON, 1975), Effect of Pleistoceneglaciation upon oceatiographiccharacteristics 859

Diatom microfossils I have reported elsewhere (SANcETrA, 1981, 1982) that certain species of diatoms show good correlation with distinctive hydrographic conditions and thus can be used as tracers of such conditions in the past. In particular, if a modern dominant species (or species complex) becomes minor downcore and a different species dominates the assemblage, we can draw some conclusions as to changes in environmental conditions that might cause such a shift. Five cores from the North Pacific and Bering Sea were examined for assemblage changes. The time control on all cores is independent of the diatom data and is based upon chemical measures or the change in abundance of a radiolarian known to have globally synchronous patterns (Table 1). The Holocene-Pleistocene boundary (10,000 y B.P.) can be located in each core based on interpolation of sedimentation rates. Perhaps the most dramatic change downcore is the increased Wisconsin abundance in Bering Sea cores of two Nitzschia species known to be common in sea ice and rarely found in the water column (SANCETrA, 1981, 1982, and references therein). The species winter over in the lower layer of the ice and are characteristic of the early spring bloom before ice break-up. In modern sediments they are common only on the northern part of the Bering Shelf (Fig. 4), where sea ice is most prevalent. During the Wisconsin Nitzschia are common in cores from the Aleutian Basin and dominate G 119 on the continental shelf (Fig. 5). The latter site, now at 145-m water depth, represented an inner shelf environment during the glacial low-stand of sea level. The increased abundance of the species in the Bering Sea during the Wisconsin implies increased sea ice during that time. Only rare individuals occur in the North Pacific cores. Denticulopsis seminae is common in the Gulf of Alaska today and is present throughout the subarctic gyre, becoming less common in the western Bering Sea and the Sea of Okhotsk (Fig. 6). The species is a marker of relatively warm water with moderate salinity and a shallow summer thermocline, characteristic of the northeast Pacific today (SANCETTA, 1981, 1982). During the Wisconsin interval the species shows a strong reduction in abundance throughout the region, particularly in the northwest Pacific (Fig. 7). In the Bering Sea it is rare throughout the late Wisconsin and essentially absent during the time of peak glacial advance (approximately 20,000y B.P.). The species increases in abundance during the latest Wisconsin and into the Holocene, during the time of continental glacial retreat, with the change being most abrupt in the northwest Pacific. This implies that the oceanographic conditions characteristic of the modern subarctic Pacific did not exist during the Wisconsin and were only established in the Holocene. On the other hand, Thalassiosira trifulta was more abundant in the Wisconsin than today. In modern sediments the species is only common in the Sea of Okhotsk (Fig. 8), a region characterized by winter ice cover and a summer vertical stratification similar to that of Fig. 2a. It also occurs in the Bering Sea, where this water type has been reported (TAKENOUTI and OHTANI, 1974; SAYLES et al., 1979). I have interpreted the species as representing the low- salinity layer of the upper water column, underlain by the steep halocline and temperature minimum (SANCETTA, 1981, 1982). 7". trifulta shows an increased abundance during the Wisconsin in the deep Bering Sea (Fig. 9). The species is absent from the Bering Shelf, where water depths would have been too shallow to sustain the vertical stratification. Wisconsin-age samples in the North Pacific cores are dominated by T. trifulta; the levels also contain ice- rafted detritus. The decline in abundance of the species is fairly abrupt and occurs in the latest Wisconsin in the eastern North Pacific (Fig. 9); in the northwest Pacific decline was more gradual and the species persisted into the early Holocene, suggesting that conditions of lowered surface salinity and strong halocline persisted longer in the northwest Pacific. 130 140 150 160 170 180 170 160 I$0 140 130 ,C>

70 70 N. cylindrus 8 N. grunowii 50-70% r77~ 30-50% 10-30% 60 60

4 • • • • , .. • .. ;~,- \, • ;... eel, • • • Q • e• 4,qr " • .rl I ¢ • • ," 50 50 • e ,~,

o• °o

40 , ~ i i ~ , , i L i , i i , i *0 130 140 |50 160 170 180 170 160 150 140 130 Fig, 4, Modern distributionof Nitzschia speciesin surface sediments,as percentageof total diatom assemblage. 1500£ 140" 150= 160* 170. 180" 170" 160" 150" 140" 130*w

70*N

60*

50*

40* Fig. 5. Abundance of Nitzschia species in cores spanning the past 40,000y. Holocene (H)-- Wisconsin(W) boundary at 10,000 y B.P. (Table 1). Horizontal axes show percentage abundance of taxa, increasingto right, with tic marks at 25 and 50%. 130 140 150 160 170 180 170 160 150 140 130

70 70 D. semmoe

B~ >70% 50-70% r7-/-A :50 -50 %

. 60 60 ,o

/ • °. //

50 50

40 ~0 130 140 150 160 ~70 180 170 IbO 150 140 130

Fig. 6. Modern distribution of D. seminae in surface sediments, as percentage of total diatom assemblage 130*E 140" 150" 160" 170" 180" 170" 160" 150" 140" 130*w J] I I I I I I I I !

70*N

% L). sem/noe

60*

I '

50*

, 40* Fig. 7. Abundance ofD. seminae in cores spanningthe past 40,000 y. Axes as in Fig. 5. 130 140 150 160 170 180 170 160 150 140 130

70 70

77. tr/fu~to 3O-5O% I0-30%

• . *j o• • ..'..~, 60 60 -

50 50 / .°

t Ilxfl ' I~ 4C 40 i I !il ,,,m,,, i iii i i i~ i i I i 130 140 150 160 170 180 170 160 150 140 130 Fig. 8. Modern distribution of T. trifultain surface sediments, as percentage of total diatom assemblage. 13(I~E 140" 150" 160" 170" 180" 170" 160" 150" 140" 130*W I I I I 70*N % 7:. trifulto

60'

i t f . .,~oo"i

J t 50*

40' I I I ~~. ! Fig. 9. Abundance of To trifulta in cores spanning the past 40,000 y. Axes as in Fig. 5. 866 (it tNSFANCI- SAN(TEl IA

Other microfossil data ROBERTSON (1975) examined the population distribution of radiolaria in northwest Pacific sediments. Using species assemblages defined by factor analysis, he derived regression equations to estimate surface water temperature from quantitative faunal populations. The proce dure assumes that any change in population reflects a change in temperature, and is admittedly over-simplified. Robertson's results are nonetheless important; he found that a species (Cycladophora davisiana) that is very common in modern Okhotsk sediments and extremely rare in the North Pacific, was the dominant species in the northwest Pacific during the late Wisconsin. Robertson's equations implied that Wisconsin surface waters were colder in winter than they are at present; he suggested that the increased abundance of C. davisiana reflected an intensification of the temperature minimum throughout the northwest Pacific as a result of severe winter cooling and incomplete summer warming. ROBERTSON (1975) and SANCETTA (1979) both found that cores just north of the Subarctic Front had radiolarian and diatom species characteristic of the modern Sea of Okhotsk, while core samples of the same age (late Wisconsin) less than 5 ° of latitude to the south had unchanged subtropical assemblages. Both authors interpreted their data as reflecting a southward advance of severe subarctic (Okhotsk-like) conditions, thus intensifying the gradient of the Subarctic Front. THOMPSON (1981) examined planktonic foraminifera from several cores near the modern Subarctic Front; calcite is poorly preserved in the subarctic Pacific, so that he could not extend his study northward. During the late Wisconsin he found that cores dominated by his subtropical assemblage showed little change, but cores underlying his 'transition zone' (presumed equivalent to the Subarctic Front) showed an increase of a subarctic species (Neogloboquadrina pachyderma sinistral). He interpreted the change as reflecting a southward shift of the Subarctic Front by 5° of latitude, together with a steepening of the gradient.

EVALUATION OF THE EVIDENCE AND MODEL

Wisconsin-age sediments from the deep Bering Sea have significantly more sea-ice diatoms than Holocene sediments. G 119 on the continental shelf has species abundances equivalent to those of modern sediments on the northern shelf where sea ice is formed today. The diatom evidence thus provides strong support for the suggestion that sea-ice formation did occur around the Wisconsin margins of the Bering Sea. The amount of ice produced must have been great enough to cover much of the Bering Sea surface, because JOUSE (1962) found Nitzschia species in many cores from the central part of the sea. The presence of ice-rafted detritus at discrete levels in Pleistocene-age cores demonstrates unequivocally that there have been periods during which large volumes of ice were present in the open North Pacific. In cores that have had closer time control applied (RoBERTSON, 1975: this paper) the most recent interval of ice-rafting corresponds to the late Wisconsin. At least some of the ice must have been in the form of icebergs, because sea ice cannot erode continental rocks. Such icebergs could have been derived from both southern Alaska and British Columbia and from Siberia. Sea ice from the marginal seas may have been present but cannot be positively demonstrated. Fresh volcanic material could have been transported by sea ice. and rare sea-ice diatoms are found in Wisconsin-age levels of the North Pacific cores (but not in the modern sediments). The volume of ice, regardless of origin, must have been substantial to have released detritus over such a large area. Meltwater from the ice would therefore have been widely distributed. Effect of Pleistoceneglaciation upon oceanographic characteristics 867

Evidence from both radiolaria and diatoms indicates that species common only in the Sea of Okhotsk today were widely distributed throughout the western subarctic Pacific during the Wisconsin. T. trifulta, the diatom, was also common in the Bering Sea; radiolarian data on the region are not published. The two species are believed to be indicators of the unusual vertical structure of the Okhotsk upper water column, with surface waters of low salinity (30 to 31 x 10 -3) and a strongly developed temperature minimum within the halocline. Such a structure in the Sea of Okhotsk is a result of prolonged winter ice cover and incomplete summer mixing. The microfossil data thus support a model in which sea ice in the marginal seas and icebergs in the North Pacific melted during the 'summer' season to produce the vertical stratification described. Modern subarctic conditions of warmer, moderately saline (32 to 33 x 10 -3) surface water with a shallow thermocline would therefore have been less or non-existent, reflected by the low abundances of D. seminae. The model suggests that the oceanographic regime would be most strongly affected in the northwest Pacific, due to the combined effect of the Bering and Okhotsk seas, along with Siberian icebergs, and less affected in the eastern region, subject only to input from North American icebergs. Wisconsin conditions might also be expected to last longer in the northwest Pacific. The records of D. seminae and T. trifulta support this interpretation. In the eastern core (RC 11-172) replacement of T. trifulta by D. seminae occurs during the latest Wisconsin (roughly corresponding to continental glacial retreat) while in the west (V20-123) T. trifulta persisted into the early Holocene, coinciding with the increase of D. seminae. Planktonic foraminifera, radiolaria, and diatom data from sediments near the modern Sub- arctic Front all imply an intensification of the front during the Wisconsin. Subtropical assemblages south of the front were not very different from those of today, but assemblages in sites underlying the front show substantial increases of subarctic species, so that the transition from 'subarctic' to 'subtropical' conditions was more dramatic, i.e., gradients across the front were steepened. Unfortunately, plankton organisms are sensitive to both temperature and salinity as well as other variables, so that it is not possible to separate the two parameters. The final step in the model suggests that, with a steepened front and a subarctic colder and less saline than at present, less evaporation would occur over the North Pacific, resulting in less precipitation in Beringia. Studies of Wisconsin climate of Beringia (HoPKtNS, 1972 and references therein) indicate that glaciers covered the Koryak, Brooks, and Alaska ranges, but that a continental ice sheet did not exist. Instead, results of soil, pollen, and fossil analyses suggest that low-land Beringia was a tundra or steppe-tundra (HoPKINs, 1972; CWYNAR and Rn-CmE, 1980) swept by cold dry winds. The low precipitation in Beringia was probably a result of several factors (HOPKINS, 1972)--increased land area resulting in isolation of inland Beringia from the maritime climate, cold dry winds from the Laurentide ice sheet to the east, and possibly also less moisture transport from the North Pacific, as suggested here. British Columbia, however, experienced significant mountain glaciation (FuNa-, 1971), so that evaporation from the eastern North Pacific must still have been sufficient to provide moisture to the glaciers. Insufficient data exist to determine whether precipitation in the region was as great as at present or not. This aspect of the model remains speculative. The evidence thus generally supports the model, although some aspects remain untested (e.g., the fate of brine produced by ice formation). It is also worth noting that there is no evidence that clearly contradicts the model. Microfossil species characteristic of warm or high-salinity waters are absent from Wisconsin sediments; diversity of radiolaria and diatoms was lower, suggesting less seasonal contrast, a more stressful environment, or both. The model is descriptive and speculative, but it is internally consistent and accords with the 868 CONSTANCESANCErrA geologic data available. It is hoped that this paper will stimulate the interest of climatologists and physical oceanographers and lead to fruitful exchanges for all disciplines.

Acknowledgements--I am grateful to many colleagues for their comments and suggestions, among them J. [MBRIf~ (Brown Univ.), T. KINDER(NORDA), B. WARREN(WHOI), A. GORDON(LDGO), and T. ROYER(IMS, Alaska). L. COACHMAN(U. Wash.) and W. RUDDIMAN(LDGO) reviewed the paper and made constructive comments. J. OURVAN and M. BRAUN provided drafting and photographic assistance. Research was funded by NSF Grant OCE80-07301. This is LDGO Contribution No. 3488.

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