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Home , Sea of Japan, Tsugaru Strait, Tsushima Strait

QUATERNARY RESEARCH 37, 346-360 (1992)

Sea Level, Surface of the , and the Younger Dryas Event in the Northwestern Pacific

L. D. KEIGWIN Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

AND

S. A. GORBARENKO

Pacific Oceanological Institute, Far Eastern Branch, USSR Academy of Sciences, 7 Radio Street, , 690032 USSR Received January 28, 1991

The Japan Sea was profoundly different during glacial times than today. Available &180evidence indicates that sea surface salinity was lower by several per mil. This probably increased the stability of the water column and caused anoxic sedimentary conditions in the deep sea, as shown by the absence of benthic microfossils and the presence of laminated sediment. These changes are likely related to the effects of late Quaternary sea-level change on the shallow sills (ca. 130 m) across which the Japan Sea exchanges with the open ocean. The Hwang He (Yellow River) has previously been implicated as the source of fresh water to the Japan Sea during glaciation, but the possible roles of the Amur River and excess precipitation over evaporation must also be consid- ered. Ambiguous radiocarbon chronologies for the latest Quaternary of Japan Sea cores do not adequately constrain the timing of salinity lowering. Previous studies have suggested that lowest sea surface salinity was achieved 27,000 to 20,000 14C yr B.P. However, if global sea-level fall restricted exchange with the open ocean circulation, then lowest salinity in the Japan Sea may have occurred as recently as 15,000 to 20,000 yr ago when sea level was lowest. If this alternative is correct, then as sea level abruptly rose about 12,000 yr ago, relatively fresh water must have been discharged to the open Pacific. This might have affected the dynamics of outflow, local faunal and floral expression of the polar front, and stable isotope ratios in foraminifera. These environmental changes could be misinterpreted as evidence for the cooling of Younger Dryas age, which has not been identified in nearby terrestrial records. 'iJ1992Universityof Washinglon.

INTRODUCTION deep-water production (Reid, 1979). Al- though no deep water is produced in the Study of nearshore marine paleoclimatic North at present, hydro- records is especially important because graphic conditions in the Japan Sea and the they reflect both terrestrial and marine pro- Okhotsk sea may be critical to the produc- cesses, and through their study the two can tion of Pacific Intermediate Water (Reid, be linked. Where nearshore locations are 1973; Riser, 1990; Talley, 1991). somewhat isolated from the open ocean, as Recently, Younger Dryas-type climatic in marginal , local climate influences oscillations have been reported in marine the local hydrography, geology, and biol- sediments near Japan. Using foraminiferal ogy. Whether or not local hydrographic and stable isotopic techniques, Chinzei et changes in marginal seas become important at. (1987) and Kallel et at. (1988) claim to on the larger (ocean basin) scale depends have identified a cooling episode that oc- on the particular geographical situation. curred 10,500 yr B.P., but in some of the For example, the modern outflow of saline very same sediment cores Heusser and water from the may Morley (1990) failed to find pollen evidence help precondition the North Atlantic for for a climatic reversal. It is important to 346 0033-5894/92 $5.00 Copyright 'iJ 1992 by Ihe University of Washington. All rights of reproduction in any form reserved. JAPAN SEA PALEOCEANOGRAPHY 347 identify and interpret deglacial events out- sills. The Soya Strait (Laperuza St. in Rus- side of the North Atlantic , where sian), between and , has they are best known, in order to determine a sill depth of 55 m and the Tartarsky Strait whether they were regional or global phe- (Mamiya St. in Japanese), between Sakha- nomena (Rind et aI., 1986). lin and , has a sill of only 12 m. The Japan Sea experienced large envi- Minato and Kimura (1980) modeled the ronmental changes from glacial to intergla- dynamic interaction between a marginal sea cial time at least in part due to the silled and a western boundary current to deter- straits through which its waters exchange mine why the net flux of surface waters in with the open Pacific. Stable isotopic evi- the Japan Sea is from south to north. They dence for these changes was first published found that the flow is from the Tsushima in the Japanese (Oba et al., 1980) and So- Strait to the due to the pres- viet (Gorbarenko, 1983) literature, but sure difference between the two straits those (and subsequent) contributions have caused by the wind-driven circulation. not yet seen wide exposure in the English Thus, in essence, accumulation of water in language literature. In this paper we review the East Sea by the subtropical gyre some of the latest Quaternary paleoenvi- drives the Tsushima Warm Current. ronmental changes in the Japan Sea, and Since only near-surface waters exchange discuss possible mechanisms linking sea- between the Japan Sea and other water level change driven primarily by North At- bodies, the sea develops its own peculiar lantic events and a Younger Dryas-type os- hydrographic properties below the upper cillation in the northwest Pacific off the few hundred meters (Yasui et al., 1967; coast of Japan. Moriyasu, 1972). Waters below the surface layer display little variability, with low tem- PHYSICAL SETTING perature (0-1°C), low salinity (34.05%0),and high oxygen content (5.0-5.5 ml/liter). The Japan Sea water exchanges with water well-mixed cold, fresh, oxygenated deep from adjacent marginal seas and the open waters are a result of a wintertime convec- Pacific Ocean through four straits (Fig. 1). tion in the northern part of the basin and a A good review of the general circulation and water masses of this sea can be found in southward return flow along the coast of (Vasiliev and Makashin, 1991). Moriyasu (1972). The broad Tsushima Only near-surface waters exhibit significant Strait, with a sill depth of about 130 m, sep- seasonal and spatial variability, mostly re- arates the Japanese from and flecting conditions in their is the principal conduit for marine water en- source. As first noted by Niino et al. (1969; tering the sea (Fig. 2A). The Tsugaru Strait see also Ujiie and Ichikura, 1973), the un- (Sangarsky St. in Russian nomenclature), usual hydrography of the Japan Sea is re- between and Hokkaido, has a sill flected in the unusually low sedimentary depth of 130 m, and is the main outlet from content of organic carbon, and an unusually the Japan Sea (Fig. 2B). The Tsushima shallow calcite compensation depth. Warm Current originates as a branch of the and flows in through the METHODS and out through the Tsu- garu Strait as the Tsugaru Warm Current. Even though the analytical results dis- Geostrophic estimates suggest that each of cussed below have been reported else- these currents accounts for a net flow of where (aba, 1984; Oba et al., 1991; Gor- about 1 or 2 Sv (1 Sv = 106 m3 sec -1, Yi, barenko, 1983, 1987), it is important to re- 1966; Toba et al., 1982; Shuto, 1982). Flow view the methods prior to discussing the through the other two straits is small by data. Stable isotope data from Japanese comparison because of much shallower cores represent a group effort, having been 348 KEIGWIN AND GORBARENKO

55

OKHOTSK SEA

50

40

0 500km L J

. 30 130 140 150 FIG. 1. Map of Japan Sea and surrounding showing core locations discussed in text. The solid contour is the 200-m isobath; the 50-m isobath is shown only in the region of the Tatarskiy (Mamiya) Strait to the north (dotted contour). Core locations marked as solid circles are from the work of Oba and colleagues, and cores marked as open circles have been studied by Gorbarenko and colleagues. Numbers next to the Japan Sea core locations are the minimum 1)180values at each location which reflect glacial maximum conditions at the sea surface. Core locations marked with triangles are from Chinzei et al. (1987), and the square between Honshu and Hokkaido marks the position of core CH84-14 (KalleI"et al., 1988). A bar symbol in the lower left of the figure (East China Sea) marks a seismic line which shows evidence for a former position of the Hwang He channel (Milliman et al. 1989). A dashed circle between Honshu and Hokkaido illustrates the position of the Tsugaru Warm Current when it is in the "gyre mode" (Conlon, 1982). analyzed in three laboratories, using VG pretreatment, on specimens which were ei- Micromass 903 and Finnegan MA T251 ther > 177 11mor >250 11m.Where possible, mass spectrometers (Oba et ai., 1991). analyses were on individual species, but in Sample dissolution was at 60°C, with no intervals of very low abundance mixed JAPAN SEA PALEOCEANOGRAPHY 349

140' 141'

129' 130' 41"

JAPAN SEA 35'

HONSHU 34' 40'

0 L A B

FIG. 2. (A) Map of the Tsushima Strait (after Ujiie, 1973)showing the IIO-m isobath (solid line) and the 120-m isobath (dashed line). With a sea-level fall of 120 m, flow through the Tsushima Strait would be restricted to a narrow passage west of Tsushima . A 130-m drop in sea level would eliminate the passage in the absence of any tectonic effects. (B) Map of the Tsugaru Strait (after Mogi, 1979 and a map published by the Japan Railway Construction Corp.) showing the 150-m isobath (solid contour) and the location, width, and depth of three "choke points." The dashed contour marks the 130-m isobath. Sea-level fall of 130 m would result in a passage only 3 km wide at the northernmost sill.

planktonic specimens were used. Gor- sentative of the stable isotopic history of barenko analyzed mixed specimens of the late Quaternary of this region. planktonic foraminifera on a MU 1309 mass Oba's (1984) results from a site on Oki spectrometer, after dissolution "off-line" Bank (37°04'N, 134°42.4'E, 935 m) are pre- at 50°C. His results are related to PDB sented in Figure 3. Cores KH79 L-3 and through analysis of SMOW and a labora- C-3 were taken from almost exactly the tory carbonate standard. Radiocarbon de- same location. Because L-3 had the higher terminations on both Soviet and Japanese latest Quaternary deposition rate and was sediment cores were made on carbonate radiocarbon-dated, its record is patched to carbon in bulk sediment. the longer record from C-3 at 215 cm using the distinctive lithologic changes that mark CORE RESULTS Japan Sea cores (henceforth, and in Fig. 1, the composite record of these two cores Of the several Japan Sea sediment cores will be referred to as C-3). These include that have been reported by Oba, Gor- three layers of volcanic ash that are dated barenko, and their colleagues (Fig. 1), we on land (Ah = 6300 yr; Oki = 9300 yr; and will describe results of one Japanese core AT = 21,000 to 22,000 yr B.P.; Machida and one Soviet core. These cores have the and Arai, 1983), and the top of a thick in- highest resolution paleoenvironmental terval of finely laminated sediment. records and are believed to be most repre- Maximum enrichment in 180 in plank- 350 KEIGWIN AND GORBARENKO

8180 mostly on species of Uvigerina and on Cas- 4 3 2 I 0 sidulina norcrossi. Considering just the 0 I . I . I &180pattern of C. japonica, the signal am- + 0 'y plitude (about 1.6%0)is less than half that of e .. 00 + + + + the planktonic foraminifera from the same 0 ++ interval. Minimum values are found at 112 200 -I 0 . cm, above the planktonic minimum, and + + + the maximum occurs at 76 cm (Fig. 4A). + In general, the pattern of &180changes in ~e 400 . mixed planktonic foraminifera from core ~ 1670 (north slope ofYamato Rise; 39°48'N, ..c:- :+ Q., 133°27'E, 1105 m; Gorbarenko, 1987) is the II> + + Q 600 same as that found in Oba's cores (Fig. 4B). + + A few analyses of G. bulloides and N. e N. pachyderm a (R) m + G. umbilicata + pachyderma (sinistral) compare favorably + 800 + with the mixed planktonic data (Gor- + Mixed planktonics 0 barenko and Matunina, 1991). Maximum + + + &180is found at 70 cm and below about 200 e cm, with minimum values found not at the 1000 core top (as in core C-3) but at about 100 cm FIG. 3. Oxygen isotope values for latest Quaternary (Fig. 4B). Fine-scale lithological features, planktonic foraminifera from core C-3 (Oba, 1984). Al- such as tephra Ah, and sediment lamination though this is a composite record made of two species and mixed assemblages, N. pachyderma (right coiling) were not recognized because the core was records the signal from the maximum in 1)180to the not split before sampling. Radiocarbon core top, and G. umbilicata records the full range of analyses of core 1670 (Fig. 4B) were pub- 1)180below core top values. lished first by Derkachev et al. (1985). tonic foraminifera (mostly N. pachyderma DISCUSSION and G. umbilicata) in core C-3 is found at about 100 cm and below about 300 cm (Fig. Stratigraphy and Chronology of Japan 3). Lowest &t80 values for the entire core Sea Paleoceanographic-Events are found at about 150 cm, and values about Results on cores C-3 and 1670 (and many 0.5%0heavier are found throughout the up- others not discussed here) are so unlike per 50 cm. Although in the whole core there those found in typical deep sea cores from are only two pairs of stable isotope analy- the open ocean that the conventional &180 ses that allow direct comparison of N. stratigraphy cannot be applied. For exam- pachyderma and G. umbilicata, the latter ple, in open ocean cores the lowest &180in species records the full oxygen isotopic os- the latest Quaternary is usually found at or cillation from heavy to light to heavy values near the core top, but in Japan Sea cores it between 100 and 300 cm (Fig. 3). For sim- may be as deep as 100 to 200 cm within the plicity, G. umbilicata, N. pachyderma, and core. Nevertheless, creating a local stratig- the mixed planktonics are considered to- raphy for Japan Sea cores is simple using gether in Figure 4A. both the &180extrema and volcanic ash lay- Benthic foraminifera are common ers identified as AT and Oki (Fig. 6). throughout most of core C-3, excepting the A chronology is less certain because the interval 132 to 255 cm which includes the radiocarbon age determinations performed laminated sediment (Fig. 4A). Above 132 on these cores do not agree very well. In cm, only Cassidulina japonica were ana- core 1670 the &180minimum has a 14Cage lyzed, whereas deeper analyses were of about 15,000 yr (Fig. 4B), whereas the JAPAN SEA PALEOCEANOGRAPHY 351

8180 8180 5 4 3 2 1 0 4 3 2 0

1.9 Or! D I!I :i -I!I ki I!I 11.3 . 10.8 14.2 100 15.4 ~~~. - --- I%J ec21 Benthic 'I!I .; 22.2 c. Pl/ ..~ 200 ~ I I'" . I!I 32.4 34.6 .' . I!I j $:EJ... , ~ ~

300 LLlJ A B FIG. 4. Oxygen isotope results for cores C-3 (A) and 1670 (B). In each core the stippled pattern along the left axis shows the location of volcanic ash layers, the horizontal-ruled pattern shows the extent of laminated sediment (not noted in core 1670), and the numbers represent radiocarbon age determinations on bulk sediment (in l(}oo yr B.P.). In core C-3 planktonic species (Fig. 3) have been lumped. Benthic species above 132 cm are Cassidulinajaponica; and below 250 cm analyses are based on Uvigerina and Cassidulina spp. The cooccurrence of low 1)180 in planktonic foraminifera, the absence of benthic foraminifera, and the presence of laminated sediment indicates that during glacial times sea-surface salinity was reduced, deep-sea ventilation was reduced, and the benthic macrofauna was eliminated. Oxygen isotope results are for mixed planktonic assemblages in core 1670. same event in core C-3 is dated at about land (Machida and Arai, 1983). Oba et al. 20,000 yr B.P. (Fig. 4A). Identification of (1991) report that up to 30% of the cocco- volcanic ash beds by Oba, Gorbarenko, and liths in the upper laminated sediments are their co-workers indicates that these 0180 pre-Quaternary in age, supporting the pos- minima must correlate (Fig. 5A). One pos- sibility of detrital carbonate contamination sible source of the age differences could be of their 14Cages. In Oba's previously pub- the 14Canalysis of bulk sediment which is lished work and in Oba et al. (1991) it is usually required in conventional radiocar- assumed that the 22,000 yr 14Cage for the bon analysis of pelagic marine sediments. It 169 cm level is accurate because Kato is well known that 14C age determinations (1984) reported an age for ash AT of 26,000 on bulk sediment are frequently plagued by yr in a core from the East China Sea. Be- contamination from detrital carbonate that cause that age was determined by bulk ra- may be infinitely old with respect to 14C diocarbon dating of sediment above and be- (e.g., Keigwin et al., 1984; Jones et al., low the ash layer, contamination by detrital 1989). In core C-3, Oba (1983) reported a carbonate is possible at that location as 14Cage of 22,000 j: 200 yr B.P. for an in- well. Although core 1670was not examined terval centered on 169 cm, 64 cm above ash for detrital carbonate content, its basin cen- AT which has the same age, based on more ter location and lower rate of sedimentation than 20 14Cdates which bracket the ash on suggest that the younger (15,000 yr B.P.) 352 KEIGWIN AND GORBARENKO

1670 depth (em) 0180 0 100 200 300 4 3 2 0 0

100

0180 minimum

200 ---e~ .c AT ash ....c. ~ "C f") 300 U

400

core C-3 core 1670 500 A B FIG. 5. (A) Correlation of volcanic ash layers and features in the oxygen isotope records between cores 1670 and C-3. Levels in core 1670 can be scaled to those in C-3 by multiplying by 1.44. (B) Composite 1)180record from cores C-3 and 1670, after scaling the latter. Results from the two cores are in excellent agreement at the oxygen isotopic maximum and minimum. Local hydrographic dif- ferences at the core sites and sampling differences probably account for differences between the isotopic records. age for the minimum in 1)180may be accu- Our proposed correlation of "events" in rate. Figure 5A allows scaling of depth in core In this paper we assume that the upper- 1670 to that of C-3 by multiplying the most laminated interval in Japan Sea cores, former by 1.44. It also indicates that there which contains the minimum 1)180in plank- may be about 20 em missing from the top of tonic foraminifera, is equivalent to the level core 1670, although this is not supported by of maximum latest Pleistocene glaciation a young 14Cage (1900 yr) near the core top. marked in open-ocean cores by maximum Scaling core 1670 to C-3 and plotting the 1)180. Thus, the greatest oceanographic two records on a common depth axis shows change in the Japan Sea may have coin- excellent agreement between the pro- cided with the period of harshest terrestrial nounced 1)180maximum and minimum with and atmospheric conditions, which per- important differences above and below the sisted from about 20,000 to 15,000 yr B.P. 1)180oscillation (Fig. 5B). Such differences This interpretation is consistent with the ra- might result from different sampling inten- diocarbon dating on core 1670, and is like- sities of the two cores in the deeper inter- wise consistent with the terrestrial age for val, the choice and mixture of species, and volcanic layer AT in core C-3. Firm estab- differences in Holocene properties of near- lishment of a Japan Sea chronology for the surface waters at these two locations (Fig. latest Quaternary must await accelerator 1). For example, Holocene planktonic fora- mass spectrometry (AMS) 14C dating of minifera at the basin-center location of core hand-picked foraminifera from these cores. 1670 are subject to cooler waters outside JAPAN SEA PALEOCEANOGRAPHY 353

the influence of the warm Tsushima Cur- out as a cause of the 8180 minimum in the rent. Summer sea-surface temperatures Japan Sea because of the lack of ice-rafted (SST) at the location of core 1670 are about sediment in the laminae of these cores. Oba 3°C lower than over core C-3, and at 100 m (1983) and Chinzei and Oba (1986) pro- this difference increases to 10°C (Yasui et posed that during glacial lowering of sea al. 1967). level the mouth of the Hwang He (Yellow River) moved seaward and discharged fresh Late Quaternary Environmental Changes water very close to or into the Tsushima in the Japan Sea Strait. However, Milliman et al. (1989) con- ducted a seismic survey of the East China Late Quaternary changes in sea level Sea and found what is probably an ancient must have had profound effects on the hy- channel of the Hwang He seaward of the drography of the Japan Sea since the sill deepest entrance to the Tsushima Strait depths of the Tsushima and Tsugaru Straits (Fig. 1). This indicates that the river may (about 130 m) are close to the 120-m fall of have bypassed the Tsushima Strait at some eustatic sea level 15,000 14C yr ago docu- time in the past, although it still could have mented by Fairbanks (1989). Circulation influenced the salinity of local waters in the through narrow straits no deeper than 10m East China Sea. Oxygen isotope results was probably restricted and may have been from a core in the East China Sea (core completely shut off if maximum sea-level 1595; Fig. 1), however, show a typical 8180 lowering was greater than 120 m or if there stratigraphy which Gorbarenko (in press) were any local tectonic effects at the cites as evidence of normal sea-surface sa- straits. As discussed previously by Oba et linity. al. (1980, and in subsequent papers) and Possible influence of the Amur River Gorbarenko (1987), the 8180 of planktonic (Fig. 1)on Japan Sea salinity should be con- foraminifera during glacial times in the Ja- sidered. The Amur River drains a substan- pan Sea was so low that salinity must have tial region of the Asian adjacent been lowered in near-surface waters. With- to the Japan Sea and has an annual dis- out the influence of salinity on 8180, unrea- charge of 325 km3 yr-l (Milliman and sonably high SSTs would be indicated. Meade, 1983). Today, the Amur River emp- Low salinity probably could have been ties into the middle of the Tartarsky Strait maintained only if the basin was in re- and its discharge flows toward the Okhotsk stricted communication with saltier waters Sea, but this pattern might have changed from the Pacific surface. Thus, we disagree during glacial times with lowered sea level. with the conclusion of Morley et al. (1986) If the Amur River influenced Japan Sea sa- that the Japan Sea must have been in open linity during the late Quaternary, then we communication with the Pacific during full would expect the interval of low 8180 to lowering of glacial sea level because of the have begun earlier and lasted longer be- presence of open sea radiolaria and forami- cause for most oflatest Quaternary time the nifera. An alternate interpretation is that shallow sill at Tartarsky Strait (12 m) would the Japan Sea was large enough to maintain have been flooded. Seismic studies should its own planktonic fauna and that the fauna be conducted to trace the former channel of was able to live in waters of reduced salin- this river and locate its fan, and additional ity. coring of the Japan Sea should be con- There are only two ways of lowering sur- ducted in higher latitudes to test for gradi- face ocean salinity: (1) through increased ents in 8180. precipitation relative to evaporation and (2) Gorbarenko (in press) recently asserted through dilution by iceberg or river dis- that lowered surface salinity in the Japan charge. Iceberg flux can probably be ruled Sea could have resulted from increased pre- 354 KEIGWIN AND GORBARENKO

cipitation relative to evaporation during Even if precipitation reduction of this glacial times rather than transport of low- amount was typical of the entire Japan Sea, salinity surface waters through the Tsu- there would still be sufficient excess P-E to shima Strait. Making assumptions about have substantially lowered the salinity of the mass balance, Gorbarenko (in press) es- surface waters during maximum glaciation. timates that salinity may have been 6 to 7%0 Thus, it is not necessary to call on the lower than today. In addition, he mapped Hwang He as a source of fresh water. minimum glacial 8180 to show its limited Substantial changes in the salinity of the geographic variability, with no evidence of Japan Sea surface during glacial time must lowest values nearest the Tsushima Strait. have affected the convection which venti- Combining the available Japanese and So- lates that basin today. As first noted by viet data on the 8180 minimum indicates a Ujiie and Ichikura (1973), at some depth be- range of only 0.7%0 with no evidence for low the seafloor Japan Sea sediments are lowest values closest to the Tsushima Strait laminated. Both Gorbarenko and Oba pro- (Fig. 1). Within the limits of the available posed that low-salinity surface waters must data, this indicates that either there was no have stratified the water column, prevent- one point source (i.e., river) of fresh water ing convection in the northern part of the discharge to the southern Japan Sea during sea and leading to deep anoxia. This hy- glacial maximum times, or that surface wa- pothesis accounts for the laminated sedi- ters were very well mixed. ments and the absence of benthic foraminif- Assuming restricted exchange between era in the interval marked by anomalously the Japan Sea and the open ocean, a simple low 8180 in core C-3 (Fig. 4A) and in many calculation shows that the modern excess other cores. Both Gorbarenko (1987) and of precipitation over evaporation (P-E) Oba et al. (1991) presented other sedimen- over the Japan Sea (about 30 mm yr-1; tary evidence for restricted convection dur- Baumgartner and Reichel, 1975) is more ing glaciation. Their evidence includes than sufficient to account for glacial lower- higher carbonate and authigenic pyrite con- ing of sea-surface salinity. Multiplying P-E tents in the low 8180 interval, which they by the area ofthe Japan Sea (0.9 x 106km2) attributed to better preservation during sul- gives a net flux of fresh water of 27 km3 fate reduction on the seafloor, and higher yr-1 which is comparable to the freshwater opal and organic C content during the de- discharge of the Hwang He (40 km3 yr - 1; glacial and Holocene intervals due to in- Milliman and Meader, 1983). If we assume creased surface water productivity. that the upper 100 m of the sea was fresh- Oba (1984) showed that benthic forami- ened, then the volume under consideration nifera reappear in C-3 at 132 cm, just above was 90,000 km3 (neglecting reduced surface the laminated sediment and the planktonic area due to lowered sea level), which could 8180 minimum (150 cm). Shortly after they be completely replaced by fresh water in reappeared, the 8180 of C. japonica de- about 3000 yr. Unfortunately, available cli- creased, at a level where 8180 of planktonic mate model results do not resolve P-E for species was increasing (Fig. 4A). Oba an area as small as the Japan Sea, but there (1984) interpreted this pattern of change to is some evidence for a slight increase over indicate that as sea level increased and ex- modern values in the annual zonal mean change with the open ocean resumed, deep during glacial times (Kutzbach and Guetter, convection was renewed and water of 1986). lower salinity (but not as low as at full- Local pollen data, however, indicate that glacial times) was carried to depth. effective precipitation in the coastal zone of Marine conditions in the Japan Sea be- the Japan Sea at about 36°N was reduced fore the deposition of the latest interval of by ~50% relative to today (Tsukada, 1986). laminated sediments (equivalent to 8180 JAPAN SEA PALEOCEANOGRAPHY 355 stage 3) must have been unlike those of ei- effect of excess continental ice volume. By ther the Holocene or the glacial maximum. 11,000 yr ago, deglaciation was only half Planktonic foraminifera had greater 0180 complete and open marine waters were still than in the Holocene, suggesting communi- enriched in 180 (by about 0.6%0).Low ol3C cation with a colder open ocean than today at that time was probably due to the sea- or higher local salinity, but benthic forami- water-reservoir effect (currently thought to nifera had 0180 values similar to the Ho- be a few tenths per mil; Duplessy et al., locene (Fig. 4A). Although global sea level 1988; Curry et al., 1988). is not known with the same precision be- fore 20,000 yr B.P. as it is since then, it Possible Influence of the Japan Sea on was probably not as low as that at glacial Surrounding Regions maximum times. Deep convection in the Ja- pan Sea must have been reduced, judging The following discussion assumes that from the scarcity of benthic foraminifera the interval of minimum 0180 in the Japan and the occasional presence of laminae. It Sea was coincident with maximum sea- is likely that salinity was lower in the region level lowering. Discussion of the sequence of deep convection because of the rela- of events in the "older" chronology (mini- tively low 0180 of benthic foraminifera. mum 0180 from about 27,000 to 20,000 yr This scenario would require a significant B.P.) can be found in Oba et al. (1991). latitudinal gradient in surface salinity for When the rate of sea-level rise increased pre-glacial maximum conditions or ventila- abruptly 12,000 yr ago (Fairbanks, 1989), tion of the basin at lower latitudes. low-salinity surface water must have been We propose that the abrupt maximum in flushed from the Japan Sea to the Pacific planktonic 0180 values following the glacial Ocean via the Tsugaru Strait. Sediment minimum in 0180 reflects the reestablish- cores close to the Tsugaru Strait outflow ment of normal marine conditions during might be expected to record some evidence latest glacial time. Previously, Oba (1983) of that event. Recently, such a core (CH84- proposed that maximum 0180 (with mini- 14) from the bight between Honshu and mum ol3C) and cold faunal elements were Hokkaido was described by Kallel et al. consistent with flushing of the basin from (1988; Fig. 1). AMS 14Cdates on planktonic the north with cold subpolar (Oyashio) wa- foraminifera from that core showed that a ters through the Tsugaru Strait. However, distinct event of low 0180 occurred about during the glacial maximum the sense of the 12,000 yr ago (Fig. 6). Kallel et al. (1988) pressure differential between the two found decreased abundance in the polar straits produced by the wind-driven ocean planktonic foraminiferal species N. pachy- circulation was probably the same as today: derma (sin.) at about the same time, and the North Equatorial Current would still linked that event indirectly with tempera- have caused higher pressure at the Tsu- ture by suggesting that the polar front must shima Strait and southerly advance of the have shifted northward. Kallel et al. de- polar front may have caused even lower scribed a read vance of the polar front at pressure at the Tsugaru Strait. Thus, build- Younger Dryas time based on increased ing on the model of Minato and Kimura abundance of N. pachyderma and in- (1980), we speculate that as soon as ex- creased 0180. change with the open ocean was renewed, Polar front movement was discussed pre- the flow must have been from south to viously by Chinzei et al. (1986), based on north as it is today. In this alternate sce- three sediment cores taken from the Pacific nario the maximum 0180 in the Japan Sea coast of Honshu (Fig. 1). Those authors re- resulted from renewed exchange with open ported faunal and floral evidence for a ocean waters still enriched in 180 by the southern advance of the polar front at 356 KEIGWIN AND GORBARENKO

0180 G. bulloides (---8-) that the prevalence of either mode is gov- 5 4 3 2 erned by the Rossby radius of deformation, 8 R = (g(tlp/p)D)I/:J 1, where g is the accel- eration of gravity, tlp is the difference in density between the surface and deeper lay- ~ 10 ~ ers, D is the thickness of the surface layer, ...... andfis the Coriolis parameter. In addition, M 0 12 Conlon speculated that the presence of the .... '" gyre mode might effectively block the pen- OJ) etration of Oyashio waters into coastal Ja- < 14 pan south of Hokkaido, thus controlling the local expression of the polar front. 16 Since the important variables in Rare (1) 100 90 80 70 the thickness of the surface layer and (2) the % N. pachyderma (right) density contrast, and since these parame- total N. pachyderma ters must have changed seaward of the (--) Tsugaru Strait over the last 20,000 yr as sea FIG. 6. Late-glacial to early Holocene oxygen iso- level and the Japan Sea salinity changed, it tope results for core CH84-14 in the bight between Honshu and Hokkaido (after Kallel et al., 1988). The is important to estimate how R (and the lo- isotopic minimum and minimum in abundance of N. cal polar front) might have changed on de- pachyderma (left coiling), which are AMS 14C-dated to glaciation. Although Ichiye (1982) ques- about 12,000 yr B.P., may be a response to abrupt tioned the strict dependency of circulation sea-level increase and discharge of low-salinity surface mode on R because the modern pycnocline water from the Japan Sea at that time. If that hypoth- esis is correct, then the isotopic maximum and the is not very sharp seaward of the Tsugaru faunal maximum between 11,000 and 10,000 yr B.P. do Strait, the pycnocline must have been much not necessarily reflect cooling similar to the Younger sharper 12,000 yr ago due to the salinity Dryas event in the North Atlantic region. contrast between the Tsugaru outflow and the open Pacific waters. For the following analysis we make two Younger Dryas time in northern cores C-l important assumptions: (1) that sill depth in and C-6, but not in core C-4. A comparable the Tsugaru Strait approximates the sur- 0180 oscillation was found only in the mid- face-layer thickness (D) and (2) that tlp is dle core (C-6), although it might have been affected mostly by salinity (for our calcula- missed in the northern core (C-1) which is tions we assume a temperature of 10°C). only about 12,000 yr old at the bottom. Figure 7 illustrates the dependency of R on There is reason to believe that a link ex- layer thickness and the salinity contrast be- ists between faunal evidence for local tween outflowing low-salinity water from movement of the Kuroshio-Oyashio (polar) the Japan Sea and the underlying Pacific front, deglacial 0180 changes in planktonic water. In the modern situation, Conlon foraminifera, and discharge from the Japan (1982) found that the gyre mode dominated Sea. Conlon (1982) pointed out that the for conditions where R > 15 km, and the Tsugaru Warm Current exits the Japan Sea coastal mode was found for R < 10 km. in two modes. In the first mode, occurring Thus, for example, with outflowing surface during summertime, the outflow forms an waters less dense due to 5%0lower salinity, anticyclonic gyre about 100 km in diameter, the gyre mode would prevail for layer thick- that dominates the gulf between Honshu nesses greater than about 60 m (Fig. 7). and Hokkaido (Fig. 1). In the second mode, Following the late Quaternary maximum the flow turns abruptly southward along the lowering of sea level (120 m) and minimum coast of Honshu. Conlon (1982) proposed sill depth in the Tsugaru Strait (10 m), sea JAPAN SEA PALEOCEANOGRAPHY 357

80 at 12,000 yr ago, close to the time of the E deglacial maximum in seasonality, may 1 :lacial2,500maximumyr / (fresh)35 %0 6 60 have been warmer than those of the open g Pacific, enhancing the density contrast in '" ::I the Tsugaru outflow. In addition, we have :e 40-I: : :/ -- 10%0 .. probably underestimated the thickness of .... .0 the outflowing surface layer by substituting '" 0 20 V: :: Gvre mode :0 5%0yr the sill depth. Today, the average layer ..:: thickness seaward of the Tsugaru Strait is 0 about 200 m, 70 m greater than the sill depth 0 50 100 150 200 250 300 (Conlon, 1982). Finally, the glacial depres- Surface layer thickness (m) sion of sea-surface salinity probably ex- FIG. 7. Rossby radius of deformation (R) as a func- ceeded 5%0(Gorbarenko, in press). tion of surface layer thickness and salinity (density) contrast in the Tsugaru Strait outflow for various times The northward movement of the polar in the past. For reference, the surface layer thickness front along coastal Japan 12,000 yr ago today (0 yr) is 200 m, the density contrast is small, and (Chinzei et al., 1987; Kallel et al., 1988) for R > 15 Tsugaru Strait outflow is in the "gyre possibly was a response to Tsugaru outflow mode" (Conlon, 1982). This situation is typical of sum- which must have intensified at that time as mer months, and the gyre blocks cold Oyashio waters from the bight between Honshu and Hokkaido (Fig. 1). sea level rose abruptly. Thus, since 8180 In contrast, Conlon (1982) found the "coastal mode" evidence for a Younger Dryas-aged oscilla- of circulation prevailed in winter months (R < 10), tion is as much a function of low ratios with cold waters invading the bight. As sea level rose 12,000 yr ago as high ratios between 11,000 abruptly about 12,000 yr ago during deglaciation, Tsugaru outflow would have changed from the and 10,000 yr ago, the oxygen isotopic rec- "coastal mode" to the "gyre mode," assuming a 5%0 ord in the northwestern Pacific may be sig- salinity contrast between Japan Sea surface waters nificantly influenced by events in the Japan and Pacific surface waters and a layer thickness the Sea. Although we have provided an alter- same as sill depth. This model could account for the nate interpretation for the 8180 record of apparent Younger Dryas age temperature oscillation between about 12,000 and 10,000 yr B.P. found by the Younger Dryas in the northwestern Pa- Chinzei et al. (1987) and Kallel et al. (1988). cific, faunal and floral oscillations like those presented by Chinzei et al. (1987) and Kallel et al. (1988) are still important evi- level slowly increased by 20 m until 12,500 dence for brief events of local cooling. yr ago (from Fairbanks' 1989 curve). As- Elsewhere in the Pacific Ocean, AMS ]4C- suming for that time a 5%0salinity contrast dated cores from the (Linsley and and a layer thickness of only 30 m, the Thunell, 1990) and the Tsugaru outflow still would have been in (Keigwin and Jones, 1990)contain Younger the coastal mode (Fig. 7). In the ensuing Dryas-aged oscillations that most likely re- millennium, however, when sea level rap- flect widespread climatic forcing. Our main idly rose an additional 24 m, the circulation point in this paper is that caution should be regime between Honshu and Hokkaido exercised in interpreting parallel records of would have clearly left the coastal mode, faunal and isotopic change as evidence for approaching the modern threshold for the paleotemperature change where there is the gyre mode. R may have actually exceeded possibility of significant salinity overprint- the modern threshold for the gyre mode, ing of a 8]80 record. Our argument is anal- considering that our assumptions are con- ogous to that made by Fairbanks (1989) servative. For example, by using only sa- concerning interpretation of the Younger linity in calculating Llp,we may have under- Dryas in the North Atlantic, although the estimated the density contrast. Surface mechanism is different. In the North Atlan- temperatures in a well-stratified Japan Sea tic, low 8]80 may have been a direct re- 358 KEIGWIN AND GORBARENKO

sponse to the stratification changes occur- movement of the polar front at that time. ring during meltwater events, whereas in Thus, Younger Dryas-type oscillations in the northwest Pacific off Japan low 8180 sediment cores off Japan may contain an may be a direct response to sea-level rise. important component of local oceano- graphic change that is independent of global CONCLUSIONS cooling. Sometime after 12,000 yr B.P., but before melting of ice A review of Japanese and Soviet paleoc sheets was complete, the Japan Sea was ceanographic results for cores from the Ja- probably in full exchange with the open pan Sea, which are not widely available in ocean, which was still partially enriched the English language literature, has resulted in 180. in some new interpretations of paleoenvi- ronmental change occurring in that basin ACKNOWLEDGMENTS and surrounding waters during the latest We thank R. Dunbar, D. Oppo, and L. Peterson for Quaternary. critical reviews of the manuscript and L. Pratt for As recognized previously, lowered sea helpful discussions on the dynamics of flow through level during glaciation isolated the Japan straits. We are especially indebted to T. Oba, who Sea because of its shallow sills, leading to despite disagreeing with some of our interpretations of his data, generously provided his unpublished data, lowered sea-surface salinity. With sills in helpful discussions, and comments on the text. S. the Tsushima and Tsugaru Straits at 130 m Honjo and T. Tanaka helped with translating Japa- and sea-level lowering of 120 m, exchange nese. This research was supported by NSF Grant with the open ocean may have been much OCE90-00434. less than the 1-2 Sv estimated for today. REFERENCES Low salinity of surface waters could have been maintained by a small excess of re- Baumgartner, A., and Reichel, E. (1975). "The World gional precipitation over evaporation, or by Water Balance." Elsevier, Amsterdam. Chinzei, K., Fujioka, K., Kitazato, H., Koizuma, I., discharge of the Amur River and/or the Oba, T., Oda, M., and Okada, H., et al. (1987). Hwang He into the isolated basin. Either of Postglacial environmental change of the Pacific these mechanisms could have quickly led to Ocean off the coasts of central Japan. Marine Mi- stable stratification of surface waters and cropaleontology 11, 273-291. deep anoxia. These hypotheses should be Chinzei, K., and Oba, T. (1986). Oxygen isotope stud- ies of the deep sea sediments around Japan. Recent tested by further coring. Our chronology Progress of Natural Sciences in Japan 11, 35-43. [in for latest Quaternary events is based pri- Japanese] marily on knowledge of the timing of global Conlon, D. M. (1982). On the outflow modes of the sea-level change (Fairbanks, 1989) and in- Tsugaru Warm Current. La Mer 20, 60-64. terpretation of how dated sea-level changes Curry, W. B., Duplessy, J. C., Labeyrie, L. D., and Shackleton, N. J. (1988). Changes in the distribution might have affected the Japan Sea hydrog- of S13Cof deep water lCO2 between the last glaci- raphy. This chronology is a testable alter- ation and the Holocene. Paleoceanography 3, 317- native to the interpretations presented by 341. Oba (1984) and Oba et al. (1991). Our as- Derkachev, N. N., Utkin, I. V., and Gorbarenko, sertions should be checked by AMS 14C S. A., et al. (1985). Correlation and accumulation rate of sediments of the Japan Sea during and after dating of foraminifera from the Japan Sea. late-glacial time. Pacific Ocean Geology N4, 22-29. By 12,000 yr ago, when the rate of sea- [in Russian] level rise increased rapidly, the Japan Sea Duplessy, J.-C., Shackleton, N. J., Fairbanks, R. G., was probably flushed of its low-salinity lid. Labeyrie, L., Oppo, D., and Kallel, N. (1988). This process may account for low 8180 at Deepwater source variations during the last climatic that time in cores from the Pacific side of cycle and their impact on the global deepwater cir- culation. Paleoceanography 3, 343-360. Japan, and dynamics of the Tsugaru out- Fairbanks, R. G. (1989). A 17,OOO-yearglacio-eustatic flow may account for evidence for local sea level record: Influence of glacial melting rates on JAPAN SEA PALEOCEANOGRAPHY 359

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