Late Quaternary sedimentary processes and variations in bottom-current activity in the Ulleung Interplain Gap, East Sea (Korea)

Marine Geology 217 (2005) 119 – 142 www.elsevier.com/locate/margeo

Late Quaternary sedimentary processes and variations in bottom-current activity in the Ulleung Interplain Gap, East Sea (Korea) J.J. Bahka,T, S.H. Leeb, H.S. Yoob, G.G. Backc, S.K. Choughd a

Petroleum and Marine Resources Division, Korea Institute of Geoscience and Mineral Resources, Yuseong P.O. Box 111, Daejeon 305-350, Korea b Marine Geoenvironment and Resources Research Division, Korea Ocean Research and Development Institute, Ansan P.O. Box 29, Seoul 425-600, Korea c National Oceanographic Research Institute, Incheon 400-037, Korea d School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Korea Received 30 October 2003; received in revised form 11 January 2005; accepted 18 February 2005

Abstract A detailed facies analysis of core sediments from the Ulleung Interplain Gap (UIG) reveals ten sedimentary facies which reflect a variety of sedimentary processes: volcaniclastic high-concentration sediment gravity flows, low-density turbidity currents, pelagic settling under either well or poorly oxygenated bottom-water conditions, marine fallout of tephra, hampered sedimentation by bottom currents, top-truncation of turbidites by bottom-current reworking, and Mn-carbonate precipitation under changing bottom-water oxygenation states. Distribution of the facies in the cores, integrated with echo characters of high-resolution subbottom profiles, delineates spatial and temporal changes in sedimentary processes, influenced by variations in bottom-current activity during the late Quaternary. The glacial (N ~15 ka) sediment units from the Ulleung Interplain Channel (UIC), an erosional axial channel system in the UIG, consist mainly of manganiferous and muddy contourites, interbedded with coarse-grained volcaniclastic turbidites or debrites and non-bioturbated pelagites. This indicates prevalent activity of bottom currents, rarely interrupted by volcaniclastic high-concentration sediment gravity flows and deep-water stagnations which resulted in anoxic bottom-water conditions. On the other hand, the glacial units of the southeastern margin of the UIC, where upslope migrating sediment waves developed, are generally dominated by alternating fine-grained turbidites and bioturbated or non-bioturbated pelagites. This suggests that frequent turbidity currents derived from upslope mass wasting were responsible for the generation of the sediment waves. The post-glacial (b ~15 ka) units from both the UIC and its southeastern margin consist mostly of muddy contourites or bioturbated pelagites with basal manganiferous contourites. They correspond to an acoustically transparent layer which occurs as an elongate mound parallel to the UIC in the southeastern margin. These features are suggestive of bottom-current-controlled sedimentation, hampered sedimentation in the UIC axis by stronger bottom-current activity and focused accumulation of

T Corresponding author. Fax: +82 42 862 7275. E-mail address: [email protected] (J.J. Bahk). 0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.02.031

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resuspended sediments in the southeastern margin. The post-glacial bottom-current activity was initiated by renewed circulation with open ocean at ~ 15 ka, probably more vigorous than that of the present. The bottom-current activity seems to have been augmented by enhanced bottom-water formation by colder sea-surface temperature. D 2005 Elsevier B.V. All rights reserved. Keywords: bottom current; turbidity current; paleoceanography; Ulleung Interplain Gap; East Sea (Sea of Japan)

1. Introduction The Ulleung Interplain Gap (UIG) is a deep narrow (ca. 2500 m deep and 75 km wide) passage between the Japan and Ulleung basins of the East Sea (Sea of Japan) (Fig. 1). It has served as a conduit for deepwater circulation between the two basins, which has been driven by sinking of surface water by winter cooling in the northern part off Vladivostok (Gamo et al., 1986; Kim et al., 1991, 2002; Kawamura and Wu, 1998; Chang et al., 2002; Senjyu et al., 2002). Because the East Sea is connected to the open ocean with shallow and narrow sills and straits (Fig. 1A), the deep-water circulation was very susceptible to sealevel changes during the late Quaternary (Oba et al., 1991; Tada et al., 1999). Influences of bottom currents in the UIG were postulated by the presence of an erosional channel system (Ulleung Interplain Channel, UIC) (Fig. 1B; Chough et al., 1985, 2000; Lee et al., 2004). The UIC is up to 13.6 km wide and 15–85 m deep, and cuts into the underlying stratified sequences. In addition to bottom currents, large areas of slope-failure morphology on the neighboring topographic highs highlight influence of mass flows (Lee et al., 2004). The UIG probably received significant amounts of volcaniclasts either by resedimentation through mass flows or by pyroclastic processes such as submarine fallouts from eruptions on the adjacent volcanic islands (Ulleung and Dok islands) (Fig. 1B). Thus, the UIG provides a good example for complex interplay among bottom currents, mass flows and volcanic processes, which has been subject to changing oceanographic regime.

Discrimination of such depositional processes based on lithologic features is regarded as troublesome. In spite of numerous studies on modern and ancient contourites, diagnostic lithologic features of contourites still remain problematic (Fauge`res and Stow, 1993; Stow et al., 1998). The problems mainly arise from the fact that most of contourites, particularly muddy contourites, are generally bioturbated, not preserving primary sedimentary structures. Even in cases where some primary features are observed, they may resemble those of deposits by other processes such as fine-grained turbidites (Stow et al., 1998). The interaction between downslope turbidity currents and alongslope contour currents, including reworking of turbidites and deflection or entrainment of dilute parts of turbidity currents, has been known to be frequent in deep-ocean continental margins (e.g., Locker and Laine, 1992; Masse´ et al., 1998). However, lithologic criteria for the interaction are still equivocal and under ongoing debates (Shanmugam et al., 1993; Stow et al., 1998). Deep-sea tephra beds can be also formed by a wide variety of depositional processes such as submarine fallouts, subaqueous pyroclastic flows, and resedimentation. In addition to sedimentary features in cores, discrimination of each process often requires detailed petrologic data, regional distribution of tephra beds, and correlation with subaerial counterparts if preserved (e.g., Carey and Sigurdsson, 1980; Sparks et al., 1983–1984). A regional mapping of high-resolution echo characters in the UIG and adjacent areas by Lee et al. (2004) revealed that the late Quaternary sedimentation in the deep passage was dominated by an

Fig. 1. (A) Major physiographic features of the East Sea. KP= Korea Plateau; OB = Oki Bank; UB = Ulleung Basin; UIG = Ulleung Interplain Gap; WT = Wonsan Trough; YR = Yamato Ridge. Note shallow sills and straits connecting the East Sea with the North Pacific. (B) Detailed physiographic features of the Ulleung Interplain Gap (UIG) and adjacent areas. Dashed lines represent the Ulleung Interplain Channel (UIC). Also shown are the location of sub-bottom profiles in Fig. 3 (bars), piston cores (triangles), and mooring site (cross). Is. = Island; NUIS = North Ulleung Interplain Seamount; SKP= South Korea Plateau; Smt. = Seamount. Bathymetry in meters. Contour interval is 100 m.

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interaction between bottom currents and mass flows. This paper complements the previous work by providing detailed lithologic features of sediment cores taken from the southeastern margin of the UIG (Fig. 1B). Different types of sediment reflecting diverse sedimentary processes are characterized by means of sedimentary structures, texture, and mineralogical and chemical compositions. Examination of echo-character distributions correlated with groundtruth lithologic features provides a detailed insight into the spatial and temporal variability of bottomcurrent influences on the late Quaternary sedimentation in the UIG.

2. Geological and oceanographic setting The East Sea consists of three deep basins (Ulleung, Japan and Yamato basins) separated by the Korea Plateau, Oki Bank and Yamato Ridge (Fig. 1A). The Ulleung Basin gradually deepens northeastward and is connected to the deep Japan Basin through the Ulleung Interplain Gap (UIG) (Fig. 1A). The UIG is bounded in the southeast by the slopes of the Oki Bank and Dok Island, and in the northwest by an ENE–WSW-trending escarpment of the South Korea Plateau (Fig. 1B). The UIG is punctuated by the Ulleung Seamount and North Ulleung Interplain Seamount (NUIS) in the southwestern and northeastern parts, respectively (Fig. 1B). In the UIG, a deepsea channel system (Ulleung Interplain Channel, UIC) occurs along the base-of-slope of the South Korea Plateau (Chough et al., 1985, 2000; Lee et al., 2004). The UIC is 1.5 13.6 km wide and 15 85 m deep. It is asymmetric in cross section: steep on the northwestern side (South Korea Plateau) and gentle on the southeastern side (Oki Bank and Dok Island). Near the NUIS, the UIC is relatively shallow (b 20 m deep) and deepens southwestward, up to 85 m deep. Near the Ulleung Seamount, it abruptly narrows and splits into several branches immediately south of the Ulleung Seamount (Fig. 1B). These branches gradually shallow southward, but the easternmost branch extends to 36850VN along the western base-of-slope of the Dok Island (Fig. 1B). The East Sea is connected to the North Pacific and adjacent seas through four shallow and narrow straits (Fig. 1A). Because of the shallow depths of the straits,

water exchanges with adjacent seas are limited to the upper 200 m depth, where the Tsushima Current, a branch of the warm saline Kuroshio Current, enters the East Sea through the Korea Strait and flows out through the Tsugaru and Soya straits (Moriyasu, 1972). Below the surface layer is present a quite homogeneous water mass which has a nearly constant low temperature of 0–1 8C. The water mass, named the East Sea (Japan Sea) Proper Water (ESPW) after Uda (1934), is known to be rich in dissolved oxygen (5–7 ml/l), and has renewed quickly within a few hundred years by a wintertime sinking of cold, oxygen-rich surface seawater in the northern part of the East Sea off Vladivostok (Gamo and Horibe, 1983; Gamo et al., 1986; Watanabe et al., 1991; Seung and Yoon, 1995; Kawamura and Wu, 1998). It is certain that the ESPW of the Ulleung Basin has been brought into the basin from the north (i.e., Japan Basin) because the sea surface temperature does not fall below 8 8C in the Ulleung Basin (Kim et al., 1991). The UIG has been proposed as main conduit of the deep water entering the Ulleung Basin (Kim et al., 1991). Mooring data from the southern exit of the UIG from May 1999 to May 2000 (Fig. 1B) display that currents at 2360 m water depth dominantly flow southwestward with a mean speed of 2.3 cm/s (Chang et al., 2002). Recently achieved long-term current measurements suggest that the inflow from the northeast through the UIG forms a deep cyclonic gyre in the Ulleung Basin with a strong boundary current in the western margin and a weaker return flow in the south and southeastern margin (Chang et al., 2004). The deep-water circulation in the East Sea has experienced significant fluctuations. For example, the bottom-water formation in the East Sea has become less active for the last 40 years, which is supported by a significant decrease in oxygen concentrations in deep waters accompanied with deepening of the oxygen minimum depth by more than 1000 m (Gamo et al., 1986; Gamo, 1999; Kim et al., 2001). Recent observations, however, report a sudden initiation of bottom-water formation in the northwestern part of the sea after a severely cold winter in 2000–2001 (Kim et al., 2002; Senjyu et al., 2002). The new bottom-water formation contributes to the spin-up of deep thermohaline circulation in the East Sea (Senjyu et al., 2002). During the last deglaciation, the East Sea underwent a drastic change in bottom-water oxygen concentration.

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Geochemical and paleontological evidence from sediment cores indicates that an anoxic bottom-water condition prevailed during the last glacial maximum when sea level was lowered by as much as 120 m (Oba et al., 1991; Crusius et al., 1999). Based on oxygen isotope data, the anoxia was attributed to a decrease in surface-water salinity by freshwater input and consequently intensified water-column stratification in the nearly isolated East Sea (Oba et al., 1991; Keigwin and Gorbarenko, 1992; Gorbarenko and Southon, 2000). In core sediments of the Ulleung Basin, a few widespread fallout tephra layers have been used as time markers: Aira-Tn (AT) (24.7 ka) and Aso-4 (89 ka) from Aira and Aso volcanoes in the southern Kyushu, respectively and Ulleung-Oki (U-Oki) (9.3 ka) from Ulleung Island (Arai et al., 1981; Furuta et al., 1986; Chun et al., 1997, 1998; Machida, 1999; Gorbarenko and Southon, 2000). Petrography of the tephra deposits was detailed by Furuta et al. (1986) and Chun et al. (1997, 1998). The U-Oki tephra consists primarily of white trachytic pumice lapilli and ash with minor amounts of alkali feldspar and hornblende phenocrysts. The AT tephra is composed mainly of fine-grained rhyolitic bubble-wall glass shards and associated with phenocrysts of orthopyroxene, hornblende, zircon and quartz. The Aso-4 tephra mainly comprises rhyodacitic bubble-wall glass shards with pale brown color. Phenocrysts found in this tephra consist of plagioclase, orthopyroxene, brown hornblende and clinopyroxene.

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Institute (NORI) using a Chirp sonar system (Lee et al., 2004). X-radiographs of 1-cm-thick slabs were taken from the halves of lengthwise-cut split cores to observe macroscopic sedimentary structures. Fine structures and compositions of laminae were observed with optical micrographs and backscattered electron images (BSEI) on polished thin sections of impregnated sediments (Bahk et al., 2001). Grain size analysis was conducted using standard sieves and a Micrometrics SediGraph 5100 for the sand and mud fractions, respectively. Mineralogic compositions of sand fractions from selected samples were semi-quantitatively estimated on thin sections in two fractions (N 2/ and 2–4/) after impregnation with Caldofix resin. Chemical compositions of volcanic glass, which commonly occurs as discrete or dispersed tephra layers in the cores, were determined on polished thin sections by EPMA at a condition of 5–10 nA current, 15 kV voltage, and 20 s counting time. The analytical data of volcanic glass have been recalculated to 100%, water free. Age controls for the core sediments are based on correlative tephra layers with known eruption ages and AMS 14C dates. AMS 14C datings were conducted in selected cores (MB99-3 and 96EBP-6) on planktonic foraminifera of mixed species or monospecies of Globigerina bulloides (Bahk et al., 2001; Lee et al., 2004).

4. High-resolution echo characters 3. Materials and methods This study analyzes five long (b 12 m) piston cores obtained from the main axis and the southeast margin of Ulleung Interplain Channel (UIC) during the 1996– 2001 cruises aboard R/V Onnuri (Fig. 1B). Core MB99-3 was collected from the main axis of the UIC and core 96EBP-5 near the UIC wall. Cores 96EBP-6, EEZ01-3, and EEZ01-4 were achieved on the southeast margin of the UIC. Along with the cores, highresolution (2–7 kHz) subbottom profiles crossing the core sites are used to reveal echo characters and their distributions correlated with lithologic features (Fig. 1B). The high-resolution subbottom profiles were collected by the National Oceanographic Research

A synthesis of high-resolution (2 7 kHz) subbottom profiles has revealed that slides and slumps occur on the entire slopes of topographic highs (Oki Bank, South Korea Plateau, and Ulleung Seamount and Island) around the UIG (Fig. 2; Lee et al., 2004). The UIG is characterized by a large-scale, axial bottom-current channel system (Ulleung Interplain Channel, UIC) with an asymmetric pattern of channel margins: (i) gentle, wide mound of massflow deposits (debrites, turbidites and mass-flow chutes/channels) on the southeast margin bounded upslope by extensive areas of large-scale slope failures and, (ii) steep, narrow flank of mass-flow deposits (debrites and turbidites) on the northwest margin associated upslope with relatively small areas

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Fig. 2. Distribution of sedimentary processes in the Ulleung Interplain Gap and adjacent areas, based on high-resolution echo characters. Triangles denote the location of piston cores. Modified from Lee et al. (2004).

of small-scale slope failures (Fig. 2; Lee et al., 2004). The UIC shows a large-scale, slightly to strongly channelized surface topography with various bottom and subbottom echoes. The channel floor consists of sharp to prolonged bottom echoes with either no or a few intermittent, prolonged subbottom echoes (Fig. 3A, B, C). The prolonged subbottom echoes in the channel floor are truncated by the present channelfloor surface (Fig. 3C). In the channel wall, distinct to indistinct subbottom echoes are generally truncated, showing slightly irregular topography (Fig. 3A, B, C). These internal reflectors commonly prograde towards the channel floor, downlapping onto the prolonged subbottom echoes (Fig. 3A, B, C). The southeastern margin of the UIC consists mostly of sediment waves with small-scale massflow chutes/channels (Figs. 2 and 3; Lee et al., 2003, 2004). In downslope-parallel section, the sediment

waves show slightly irregular to regular wavy bottom echoes with upslope migrating internal reflectors (Fig. 3D). On the other hand, in contourparallel section, the waves exhibit irregular and less wavy forms (Fig. 3A, E). The more distinct wavy form in downslope-parallel section indicates that the wave crests are aligned parallel to the regional slope (e.g., Wynn et al., 2000). Most of the waves are characterized by an asymmetrical morphology with steeper, thicker upslope and gentler, thinner downslope flanks, ranging in wavelengths between 0.3 and 2 km and in heights between 1 and 15 m (Fig. 3D). The heights and asymmetry of the waves gradually decrease downslope (Fig. 3D). Furthermore, internal layers of the waves show a progressive downslope decrease in thickness (Fig. 3D). The mass-flow chutes/channels are oriented N S and are ca. 1 6 km in width (Figs. 2 and 3). These chutes/ channels are partially filled with acoustically trans-

J.J. Bahk et al. / Marine Geology 217 (2005) 119–142 Fig. 3. High-resolution (2 7 kHz) subbottom profiles and a line drawing showing acoustic characters and geometry of the main axis and the southeastern margin of the Ulleung Interplain Channel (UIC). For location of each profile, see Fig. 1B. Open arrows in (C) indicate prolonged subbottom echoes in the channel floor. MFC = mass-flow chutes/channel; UTL= uppermost transparent layer. Modified from Lee et al. (2004).

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Fig. 3 (continued).

parent masses (Fig. 3A, C, E). The sediment waves and mass-flow chutes/channels are overlain by an uppermost transparent layer (UTL; Fig. 3A, D, E). The transparent layer is 3 6 m thick and gradually

decrease in thickness, forming a NE SW-trending elongate patch along the UIC (Fig. 2; Fig. 11 in Lee et al., 2004). It generally drapes the preexisting seafloor surface, partly discordant to the underlying

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sedimentary sequences in the mass-flow chutes/ channels (Fig. 3A, E).

applied to a mud facies that consists mostly of mixed Mn-carbonate crystallites.

5. Sedimentary facies

5.1. Massive pumiceous sandy gravel or gravelly sand (MPGS)

On the basis of sedimentary structures on Xradiographs, grain size, and mineralogic composition, the core sediments are classified into ten sedimentary facies (Table 1). dPumiceousT or dvitricT sediments contain more than 50% volcanic glass in their gravel or sand fractions, respectively. dMn-carbonateT is

5.1.1. Description These facies mainly consist of sandy gravel or gravelly sand with 1–52% gravel and less than 10% mud (Fig. 4). The gravel mainly comprises pumice lapilli and trachytic lithic fragments with long diameters up to a few centimeters (Fig. 5A). The

Table 1 Description and interpretation of sedimentary facies in core sediments Facies

Description

Interpretation

Massive pumiceous sandy gravel or gravelly sand (MPGS)

Usually medium- to thick-bedded; abundant pumice lapilli; variable amounts of intrabasinal clasts (2–30%) in sand fractions; a few floating mud balls in some units; massive to stratified; either ungraded or graded; sharp, often deformed lower boundary; either sharp or gradational upper boundary Usually thin-bedded; abundant intrabasinal clasts (up to 52%) in sand fractions; horizontally or cross-laminated; overall normal grading; horizontal or inclined sharp lower boundary; sharp or gradational upper boundary Thin-bedded; bimodal size distribution; abundant pumices or bubble-wall shards (58–98%) in sand fractions; variable degrees of bioturbation Thin-bedded; laminated with silt-clay couplets; normally graded; sharp lower boundary; gradational upper boundary Thin-bedded; bioturbated; diffuse upper and lower boundaries; partially preserved primary lamination

Coarse-grained volcaniclastic turbidites or debrites (Stow et al., 1996); submarine fallout tephra

Laminated sand or silt (LS)

Massive to bioturbated vitric muddy sand (MBVMS) Laminated silty mud (LM) Bioturbated silty mud (BSM)

Crudely laminated mud (CLM)

Homogeneous mud (HM) Indistinctly layered mud (ILM)

Bioturbated mud (BM)

Laminated Mn-carbonate mud (CaM) Modified from Lee et al. (2004).

Thin-bedded; randomly scattered foraminifera; crude laminae characterized by strings of silt and diatom aggregates; sharp upper and lower boundaries Thin- to medium-bedded; lack of visible primary structures; gradational lower boundaries with facies LM or LS Medium- to thick-bedded; an alternation of greenish gray mud and olive to light gray mud with indistinct irregular boundaries; no systematic vertical variations in layer thickness and frequency Variable in thickness; characterized by burrows and pyrite filaments; often associated with facies ILM

Thin-bedded; composed mostly of Mn-carbonate crystallites; underlain by facies CLM and overlain by facies ILM

Fine-grained turbidites: Bouma’s (1962) Tc or Td divisions

Submarine fallout teprha; bioturbation of fine-grained turbidites Fine-grained turbidites: Piper’s (1978) E1 division Bottom-current reworking and bioturbation of fine-grained turbidites (Masse´ et al., 1998) Pelagites formed under poorly oxygenated bottom-water condition (Bahk et al., 2000) Fine-grained turbidites: Piper’s (1978) E2 division Manganiferous contourites; metal enrichments associated with hampered sedimentation by bottom currents (Stow et al., 1998) Pelagites formed under well-oxygenated bottom-water condition (Bahk et al., 2000); muddy contourites (Stow et al., 1998) Chemogenic sediments formed under changing bottom-water conditions from anoxic to oxic (Bahk et al., 2001)

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Fig. 4. Typical grain size distributions of sedimentary facies. For facies codes, see Table 1.

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Fig. 5. Photographs (A and B) and X-radiographs (C, D, and E) of selected core sections. For facies codes on the right side, see Table 1.

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sand fractions are dominated by either pumice or crystal and lithic clasts and commonly include variable amounts of intrabasinal clasts (2–30%) which mostly comprise calcareous fossils such as shell fragments and foraminifera. Beds of these facies are variable in thickness from 1.5 to 135 cm, usually medium- to thick-bedded (Figs. 6 and 7). The beds are either massive or crudely stratified (Figs. 6 and 7). They are ungraded entirely or normally graded in the upper or lower parts with an upward decrease in pumice coarse lapilli content (Figs. 5A and 6). Some thick beds include a few mud balls (up to 4 cm in long diameter) floating in the middle or upper parts (Fig. 6). The beds are usually overlain by homogeneous mud or laminated silty mud with either sharp or gradational boundaries (Figs. 6 and 7). The lower bed boundaries are sharp, often irregular with load casts when underlain by mud (Fig. 6). 5.1.2. Interpretation The generally disorganized structure, coarsegrained texture, usual medium to thick bedding, and abundant pumice lapilli suggest that most beds of these facies were formed by volcaniclastic highconcentration sediment gravity flows such as highdensity turbidity currents and sandy debris flows. A variety of depositional mechanisms can be inferred from the facies: collapse fallout from a surge type high-density turbidity current based on disorganized structures, en masse freezing based on floating mud balls, and continuous aggradations beneath nearsteady high-density turbidity currents based on crude stratification (Stow et al., 1996). The commonly associated overlying homogeneous mud or laminated silty mud suggests deposition from residual lowdensity turbidity currents. The volcaniclastic highconcentration sediment gravity flows may have been generated either by slumping of unstable volcanic debris from the flanks of volcanic islands or by direct transition from subaqueous pyroclastic flows by ingestion of ambient water (Cas and Wright, 1987, 1991; Mandeville et al., 1996). The former is inferred for the polymictic beds including significant amounts of intrabasinal clasts, crystal and lithic clasts which may have been incorporated during reworking and resedimentation, whereas the latter for the monomictic beds which are predominated by pumice and devoid of intrabasinal clasts. Some thin beds of MPGS which

lack intrabasinal clasts can be also attributed to submarine fallouts. 5.2. Laminated sand or silt (LS) 5.2.1. Description This facies comprises muddy sand or sandy silt (Fig. 4), characterized by alternating relatively claypoor and clay-rich laminae (Fig. 8A). The sand fractions include abundant planktonic foraminifera (up to 52%) as well as crystals, volcanic glass, and lithic clasts (Fig. 8A). Beds of this facies are usually thin-bedded (1–15 cm thick) and horizontally or cross-laminated with some internal scour surfaces (Fig. 7). Thicker beds show overall normal grading from sandy mud at the base to sandy silt at the top with an upward decrease in lamina thickness. This facies is commonly overlain by either laminated silty mud or homogeneous mud with gradational or sharp boundaries (Figs. 7 and 8A). The lower boundaries are sharp, either horizontal or inclined. 5.2.2. Interpretation The horizontal or cross-lamination, overall normal grading, and usual association with the overlying laminated silty mud or homogeneous mud suggest that the laminated sand or silt is comparable with Bouma’s (1962) Tc or Td division. Abundant planktonic foraminifera together with volcanic glass reflect mixing of volcanic and pelagic sediments during resedimentation by turbidity currents. 5.3. Massive to bioturbated vitric muddy sand (MBVMS) 5.3.1. Description This facies consists of muddy sand or sandy mud which commonly show bimodal size distributions with 1.5–3/ coarser and 6.5–8.5/ finer modes (Fig. 4). The sand fractions include 58–98% of volcanic glass, mostly composed of either pumices or bubblewall shards. Intrabasinal clasts such as calcareous fossils and glauconites are usually absent or scarce (b 5%). Beds of this facies are less than 3 cm thick and show variable degree of biouturbation: massive units with sharp boundaries, slightly bioturbated units with partially penetrated boundaries by burrows, and fully bioturbated units with diffuse boundaries by

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Fig. 6. Sedimentary log of core MB99-3. For facies codes on the right side of columns, see Table 1. M, S, and G denote mud, sand, and gravel, respectively.

admixing with adjacent muds (Figs. 5C, 6, and 7). The facies are always encased in bioturbated or indistinctly layered mud, except that some massive beds are overlain by laminated sand or silt (Figs. 5C, 6, and 7). On the basis of chemical composition and morphology, the constituent glasses of selected beds can be classified into four distinct groups (Fig. 9). The group 1 consists exclusively of white pumice shards with nearly spherical vesicles and is chemically comparable with U-Oki (Ulleung-Oki) tephra. The

groups 2 and 3 consist mainly of bubble-wall, platy shards with minor tube-like pumice shards and match AT (Aira-Tanzawa) and Aso-4 tephra in composition, respectively. The group 4 consists mostly of brown scoriaceous shards with low vesicularity which are different in chemical composition from those of known fallout tephras in the East Sea (Furuta et al., 1986; Chun et al., 1998). The glass from individual bed is chemically either polymictic (Fig. 9A–D) or monomictic (Fig. 9E, F).

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Fig. 7. Sedimentary log of core EEZ01-3. For facies codes on the right side of columns, see Table 1. M, S, and G denote mud, sand, and gravel, respectively.

5.3.2. Interpretation The predominance of volcanic glass, thin bedding and bioturbational modification are comparable with features of distal marine fallout tephra beds (Huang, 1980; Sparks et al., 1983–1984). Sparks et al. (1983– 1984) identified a wide range of bioturbation effect on marine fallout ash layers from entire dispersion to partial penetration by burrows. The bimodality in size distribution is ascribed partially to bioturbational admixing with adjacent muds, but more importantly

to primary textural features of fallout ash layers. Many distal fallout ash layers have a biomodal distribution by premature fallout of fine ash aggregates (Sparks and Huang, 1980; Carey and Sigurdsson, 1982). Thinbedded, deep-sea ash layers which are susceptible to bioturbational modification, however, can be also formed by volcaniclastic low-density turbidity currents (Cas and Wright, 1987). Some beds with polymictic glasses were most likely formed by this process because juvenile pyroclasts from different

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5.4. Laminated silty mud (LM) 5.4.1. Description This facies comprises silty mud with more than 60% of silt contents and modal peaks around medium to fine silt (Fig. 4). The sand fractions are usually less than a few percent and mainly composed of either planktonic foraminifera (up to 90%) or crystals (up to 64%). They also include significant amounts of volcanic glass (2–49%). This facies is characterized by silt-clay couplets that generally show an upward decrease in silt-lamina thickness and frequency (Fig. 5E). Beds of this facies are less than a few centimeters in thickness and sometimes occur as a solitary silt lamina (Figs. 5E and 7). This facies is commonly overlain by homogeneous mud with a sharp to gradational boundary (Figs. 5E and 7). 5.4.2. Interpretation The alternation of silt and clay with overall normal grading and common association with the overlying homogeneous mud mimics E1 (laminated mud) division of fine-grained turbidites defined by Piper (1978) and Chough et al. (1984). The origin of the silt-clay lamination is attributed to either depositional sorting by increased shear (Stow and Bowen, 1980) or shear sorting by cyclic bursting process (Hesse and Chough, 1980) in viscous sub-layers of low-density turbidity currents. 5.5. Bioturbated silty mud (BSM) Fig. 8. Backscattered electron images (BSEI) of laminated sand or silt (LS), crudely laminated mud (CLM), and homogeneous mud (HM). Resin-filled voids of planktonic foraminifera (f) and clay-rich zones appear darker than detrital sand and silt. (A) Upper HM and lower LS. Note a wide variety of sand grains, including abundant planktonic foraminifera (f) and volcanic glass shards (s), and alternation of relatively clay-rich, foraminiferal laminae and claypoor, detrital laminae in the LS. (B) Upper CLM and lower HM. Note randomly scattered planktonic foraminifera (f) and strings of irregular wavy elongate silt aggregates in the CLM.

volcanic sources can be mixed together only by reworking (Schneider et al., 2001). The beds with monomictic glasses may represent primary fallout tephra which can be correlated with the known fallout tephra layers in the East Sea according to their glass compositions.

5.5.1. Description This facies comprises silty mud of which the size distributions generally display medium to fine silt modes as in the laminated silty mud facies (Fig. 4). It is more clay-rich (up to 42%) and less well sorted than the laminated silty mud. The sand fractions are usually dominated by crystals (61– 66%), but also contain significant amounts of intrabasional clasts of glauconites (12–15%) and calcareous fossils (9–12%) and volcaniclasts of glass and rock fragments (9–13%). Beds of this facies are thin-bedded (b 2 cm) and entirely bioturbated with diffuse upper and lower boundaries. In some cases, partially preserved primary laminations are observed.

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Fig. 9. TiO2 vs. Al2O3 plots of glass compositions from selected units of massive to bioturbated vitric muddy sand (MBVMS). Filled symbols indicate average compositions of known fallout tephra (U-Oki, AT, and Aso-4) based on data of Furuta et al. (1986). Open symbols represent compositional groups of the analyzed glasses.

5.6. Crudely laminated mud (CLM)

(Fig. 5E). This facies is commonly underlain by homogeneous mud and overlain by laminated silty mud or homogeneous mud with sharp boundaries (Figs. 5E and 7). In back-scattered electron images (BSEI), beds of this facies comprise well-preserved foraminiferal sand and poorly sorted diatomaceous mud (Fig. 8B). Foraminifers are randomly scattered throughout the bed (Fig. 8B). Crude laminae are identified by strings of irregular silt aggregates within diatomaceous mud matrix (Fig. 8B). The silt aggregates are characterized by lensoid or elongated wavy shapes with long axis ranging from a few tens of micrometers to a few millimeters and consist mainly of silt-grade terrestrial materials (quartz, feldspar and mica).

5.6.1. Description This facies is represented by sub-centimeter to afew-cm thick, dark olive gray mud (Fig. 5B, E). On X-radiographs, individual laminae are generally poorly defined and there is no systematic vertical change in both clarity and thickness of the laminae

5.6.2. Interpretation The poorly sorted diatomaceous mud matrix and the randomly scattered foraminiferal sand as well as the absence of systematic vertical variation in texture and lamina thickness indicate that this facies was mainly formed by pelagic sedimentation.

5.5.2. Interpretation The mixed sand compositions, thin bedding, partially preserved primary laminations, and the similar size distribution with the laminated silty mud collectively suggest that this facies represents bioturbated equivalents of E1 divisions (laminated mud) of fine-grained turbidites (Piper, 1978; Chough et al., 1984). The pervasive bioturbation is ascribed to absence of overlying non-bioturbated divisions (homogeneous mud) of fine-grained turbidites which may have been removed by bottom-current reworking during or shortly after deposition (Masse´ et al., 1998).

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The preservation of the crude laminae also suggests that periodic variations in biogenic and detrital influx played an important role under a poorly oxygenated bottom-water condition where bioturbating macrofauna was suppressed (Bahk et al., 2000, 2001). The strings of irregular silt aggregates may have resulted from vertically restricted redistribution of primarily continuous detrital laminae by micro-bioturbation. This kind of ichnofabric has been interpreted to represent bioturbation by epifaunal meiofauna disrupting only the most superficial sediment layer (Brodie and Kemp, 1994). 5.7. Homogeneous mud (HM) 5.7.1. Description This facies is represented by poorly sorted, light olive gray mud with over 60% of clay (Fig. 4). On X-radiographs, the facies usually lacks visible primary structures, but occasionally shows very indistinct bedding (Fig. 5E). When the upper boundaries are not bioturbated, the uppermost parts of the individual units are often graded by concentration of diatom frustules and appear light-colored on X-radiographs because of high X-ray transmissivity (Fig. 5E). Beds of this facies are various in thickness from sub-centimeters to a few tens of centimeters (Figs. 5E, 6 and 7). This facies is usually underlain by laminated silty mud or laminated sand or silt with sharp or gradational boundaries (Figs. 6 and 7). It is overlain by either bioturbated mud or crudely laminated mud with gradational and sharp boundaries, respectively (Figs. 5E, 6 and 7). 5.7.2. Interpretation The general absence of primary sedimentary structures and bioturbation indicates rapid deposition from suspension. The gradational boundary with the underlying laminated mud and the diatom concentration at the top of the beds are comparable to those of Piper’s (1978) E2 (graded mud) division of fine-grained turbidites. Since biogenic siliceous materials have low tendency to form flocs, they may have been deposited from residual sediment cloud at the waning stage of turbidity currents.

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5.8. Indistinctly layered mud (ILM) 5.8.1. Description This facies consists of poorly sorted mud with up to 70% clay content (Fig. 4) and is characterized by an alternation of greenish gray mud and olive to light gray mud with indistinct irregular boundaries (Fig. 5B–D). The greenish gray mud layers are stiffer than the adjacent muds and often microbrecciated. On X-radiographs, the layers are lightcolored with higher X-ray transmissivity and sometimes diffusely laminated. They are usually less than 2 cm thick and repeat themselves with no systematic vertical variations. Bioturbation with minute pyrite filaments is common but generally sparse compared to the bioturbated mud. Beds of this facies are medium to thick-bedded and dominantly occur in cores from the UIC. 5.8.2. Interpretation Discontinuous and diffuse layers without systematic vertical variations resemble characteristics of muddy contourites which were ascribed to intermittent bottom-current activity (Chough et al., 1984). The alternation of layers that differ in color implies that its formation was closely linked with changes in redox conditions of the mud during or after the deposition. The greenish gray mud layers most likely represent metal enrichments associated with hampered sedimentation by bottom currents as in dmaganiferous contouritesT (Fauge`res and Stow, 1993; Me´zerais et al., 1993; Stow et al., 1998). The higher strength and common microbrecciation of the layers are also suggestive of significant induration during nondeposition periods. 5.9. Bioturbated mud (BM) 5.9.1. Description This facies is represented by burrowed mud of various thickness (Figs. 6 and 7). It consists mostly of clay (over 80%) and silt, but also includes a few percent of sand (Fig. 4). Beds of this facies are generally olive to light gray in color, but reddish brown and dark gray units are also observed. The beds are characterized by circular, oval and tube-shaped burrows, commonly filled with pyrite filaments (Figs. 5C, 6 and 7). They

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often alternate with indistinctly layered muds in the UIC (Figs. 5C and 6). 5.9.2. Interpretation The bioturbated mud was most likely formed by pelagic settling under generally oxygenated bottomwater conditions in which the bioturbation kept pace with the deposition. The worm-tube-shaped pyrite filaments may have been formed in localized anoxic micro-environments in which oxygen was depleted by oxic degradation of tube worm remains (Berner, 1970). The common association with manganiferous contourites (facies ILM) in the cores of the UIC also suggests that much of the bioturbated mud beds are most likely influenced by bottom currents, hence, can be interpreted as muddy contourites. Because muddy contourites of modern oceanic drifts are usually structureless with intense bioturbational mottling, they are hardly distinguished from true pelagites (Fauge`res and Stow, 1993; Stow et al., 1998). 5.10. Laminated Mn-carbonate mud (CaM) 5.10.1. Description This facies comprises 2–4 cm thick, yellowish brown mud with dark brown bands (Fig. 5B). Beds of this facies are underlain by crudely laminated mud and, in turn, usually overlain by indistinctly layered mud with either sharp or gradational boundaries (Figs. 5B, 6 and 7). Microscopic observations and electron microprobe analyses of polished thin sections of the beds revealed that they mostly comprise Mn-carbonate (or Ca-rhodochrosite) crystallites with an average composition of (Mn64Ca32Mg4)CO3 (Bahk et al., 2001). 5.10.2. Interpretation The laminated Mn-carbonate mud which consists mostly of Mn-carbonate crystallites is suggestive of authigenic formation in anoxic sediments (Sternbeck and Sohlenius, 1997; Lepland and Stevens, 1998). Calvert and Pedersen (1993, 1996) have suggested that such authigenic Mn-carbonate formation in anoxic sediments requires initial surface accumulation of Mn-oxides by oxic inflows and release of Mn (II) ions by burial, because concentrations of dissolved Mn(II) in bottom waters of anoxic basins do not reach such levels as to precipitate Mn-carbonates. The

unique association with lower crudely laminated mud and upper indistinctly layered mud also indicates that the formation of the laminated Mn-carbonate mud was closely related with an abrupt change in bottomwater oxygentation from anoxic to oxic states (Bahk et al., 2001).

6. Facies distribution, process interaction, and paleoceanographic implications Facies analysis of the core sediments reveals a wide variety of sedimentary processes in the UIG: bottom currents, turbidity currents, debris flows, pelagic settling under well or poorly oxygenated bottom-water conditions, and submarine fallout of tephra. Vertical and lateral distributions of the facies, integrated with echo characters of high-resolution subbottom profiles, highlight spatial and temporal variations in sedimentary processes and their interactions. AMS 14C dates, tephra layers of age-known eruptions, and lithologic correlations provide age controls for the cores and reveal paleoceanographic implications of the facies distribution. 6.1. Southeastern margin of the Ulleung Interplain Channel Based on vertical distribution of the facies, the cores of the southeastern margin of the UIC (EEZ013, EEZ01-4, and 96EBP-6) can be generally divided into two lithologic units: Units I and II in descending order (Fig. 10). Unit I is dominated by pelagic muds (or muddy contourites) (facies BM) with basal manganiferous contourites (facies ILM). It is interpreted to represent a period dominated by pelagic settling and bottom currents under a generally well oxygenated bottom-water condition. Unit II generally consists of alternating fine-grained turbidites (facies LS, LM, and HM) and the overlying pelagic muds (faices BM and CLM). In the upper part of Unit II, pelagic muds are non-bioturbated (facies CLM), whereas generally bioturbated (facies BM) in the lower parts. This lithologic change from the lower to the upper parts of Unit II is attributed to a variation in bottom-water oxygenation from oxic to anoxic states. In core 96EBP-6, manganiferous contourites (facies ILM) with thin bioturbated turbidite layers (facies

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Fig. 10. Summary of sedimentary logs and correlation of cores. For facies codes in the legend, see Table 1. Solid triangles indicate tephra layers of known eruption ages. Open triangles represent AMS 14C dates. Asterisks denote the layers of laminated Mn-carbonate mud (facies CaM).

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MBVMS and BSM) are frequent in the lower part of Unit II (Fig. 10). This suggests an increased influence of bottom currents which may have caused hampered sedimentation and reworking of turbidites (Fig. 10). The laminated Mn-carbonate mud (facies CaM) layers between Units I and II indicate authigenic Mncarbonate formation by an abrupt change in bottomwater oxygenation from anoxic to oxic states. Units I and II are correlated with the uppermost transparent layer (UTL) and upper part of the wavy stratified internal reflectors in the subbottom profiles, respectively (Figs. 3 and 10). The abundance of turbidites in Unit II, together with the wave crests aligned parallel to the regional slope, suggests that the upslope-migrating sediment waves were most likely formed by turbidity currents derived from the slopes of the Oki Bank and Dok Island (Wynn et al., 2000; Wynn and Stow, 2002; Lee et al., 2003). The overall decrease in proportion of turbidite beds in Unit II of cores EEZ01-3 to 96EBP-6 (Fig. 10) matches with the progressive downslope decrease in thickness of the internal layers of the waves (Fig. 3D). It also supports that the wave sequence was sourced from turbidity currents upslope (Wynn et al., 2000; Wynn and Stow, 2002). In contrast, the UTL, which occurs as an elongate mound parallel to the UIC and consists mainly of pelagic muds and manganiferous contourites (Figs. 2, 3, and 10), is suggestive of formation by bottom-current-controlled sedimentation: hampered sedimentation near the bottom-current channel (UIC) and focused accumulation of resuspended sediments on the channel flank. The manganiferous contourites in the basal part of Unit I (Figs. 3 and 10) further indicate that the formation of the UTL was initiated by relatively strong bottom currents which resulted in metal enrichments by hampered sedimentation on the entire southeastern margin of the UIC. 6.2. Ulleung Interplain Channel Lithoglogic characteristics of the cores from the channel floor (MB99-3) and wall (96EBP-5) of the UIC are significantly different from those of the southeastern margin upslope (Fig. 10). In core MB993, medium- to thick-bedded coarse-grained volcaniclastic turbidites or debrites (facies MPGS) are predominant and interbedded with muddy contourites (or pelagites) and manganiferous contourites (facies

BM and ILM) (Figs. 6 and 10). The prolonged subbottom echoes in the core site (Fig. 3B) are ascribed to the predominance of the coarse-grained deposits. The MPGS beds represent deposition by a variety of volcaniclastic high-concentration sediment gravity flows which were initiated either by slumping on the slopes of nearby volcanic islands and seamounts, or by transition from subaqueous pyroclastic flows. The interbedded muddy countourites and manganiferous contourites are suggestive of bottomcurrent activity during quiescent times of mass wasting and/or volcanic activity. The undulating, downlapping internal reflectors in the channel wall generally display similar acoustic and geometric characters to the upslope sediment waves, suggesting prograding mass-flow deposits towards the channel floor (Fig. 3A, B, C; Lee et al., 2004). The common truncation of both the internal reflectors in the channel wall and the prolonged subbottom echoes in the floor suggests a reworking of the mass-flow deposits by the bottom currents along the UIC (Fig. 3A, B, C; Lee et al., 2004). The interaction of the downslope mass flows and alongslope bottom currents is corroborated by the core 96EBP-5 from the channel wall, which consists mainly of muddy contourites (or pelagites) and mangiferous contourites with intermittent bioturbated thin turbidite layers (facies MBVMS and BSM) (Fig. 10). The common occurrence of bioturbated thin turbidite layers suggests that bottom currents may have been sufficiently active to remove dilute parts of turbiditiy currents and to prevent the rapid deposition of the homogenenous muddy top (facies HM) (Masse´ et al., 1998). In cores of the UIC, Mn-carbonate mud (facies CaM) layers occur once (96EBP-5) or twice (MB993) below the unit boundary (Fig. 10). The recurrent CaM layers most likely represent anoxic-to-oxic changes in bottom-water conditions which were not recorded in the cores from the southeastern margin. In addition, the Unit I, which are correlated with the uppermost transparent layer in the subbottom profiles, greatly decreases in thickness in the UIC (up to ca. 80 cm in MB99-3, if the near instantaneous event beds of MPGS are removed) (Fig. 10). These lithologic characteristics imply that in spite of the dominance of volcaniclastic turbidites or debrites, overall sedimentation rates in the UIC were significantly reduced compared to the southeastern margin by bottom-

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current activity during the quiescent times of mass wasting and/or volcanic activity. 6.3. Paleoceanographic implications On the basis of the AMS 14C dates, the upper boundary of Unit II represents ~15 ka. The upper part of Unit II, which consists of repetitive fine-grained turbidites (facies LS, LM, and HM) and nonbioturbated pelagic muds (facies CLM), most likely represents deposition by turbidity currents from frequent slope failures and normal pelagic sedimentation under poorly oxygenated bottom-water conditions during the last glacial maximum (LGM). In this period, deep-water ventilation of the East Sea is believed to be prohibited by a decrease in surface water salinity (Oba et al., 1991; Gorbarenko and Southon, 2000). The decrease in surface water salinity is ascribed to a freshwater input or excess precipitation with reduced flux from open ocean by the sealevel fall near to the sill depths (Oba et al., 1991; Gorbarenko and Southon, 2000). Unit I suggests bottom-current-controlled sedimentation associated with renewed deep-water ventilation during the post-glacial period (b ~15 ka). The Mn-carbonate layer (facies CaM) in the boundary between Units I and II implies that oxygenation of the anoxic bottom water by the renewed deep-water ventilation was abrupt enough to accumulate sufficient amounts of Mn-oxides on the sea floor, which facilitated Mn-carbonate precipitation during burial. The occurrence of manganiferous contourites (facies ILM) at the basal part of Unit I indicates that bottom current activity was probably stronger at the onset of deep-water ventilation. The present deep-water ventilation in the East Sea is known to be derived by a wintertime sinking of cold surface water in the northern part of the East Sea and has become less active since 1960’s or earlier (Gamo et al., 1986; Gamo, 1999; Kim et al., 2001). The onset of deep-water ventilation at ~15 ka has been ascribed to the inflow of the cold Oyashio Current into the East Sea through the Tsugaru Strait, which was probably enabled by the southward shift of the Pacific Polar Front during the LGM (Oba et al., 1991; Ishiwatari et al., 1999; Gorbarenko and Southon, 2000). Bottom-water formation at that times may have

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been much more facilitated by colder sea-surface temperature and resulted in more vigorous bottomcurrent activity in the UIG. Prior to the LGM, bottom-current activity may have been generally prevalent in the UIC, as suggested by significantly reduced sedimentation rates of the muddy and manganiferous contourites (facies BM and ILM) in Unit II of the UIC cores (Fig. 10). Relatively short-lived bottom-water anoxia and cessations of bottom-current activity are also indicated by occasional thin beds of non-bioturbated pelagic muds (facies CLM) (Fig. 10). Some of such bottom-water anoxia were probably as profound as the anoxic event in the LGM and led to the deposition of Mn-carbonate layers (facies CaM). The relations of these anoxic events with paleoceanographic changes in the East Sea, however, warrant further timing controls.

7. Conclusions A detailed facies analysis of sediment cores and echo characters in high-resolution subbottom profiles from the Ulleung Interplain Gap reveals a spatial and temporal variation in diverse sedimentary processes which was controlled by drastic changes in deepwater circulation during the late Quaternary. During the glacial period prior to ~15 ka, turbidity currents derived from mass wasting on the slopes of the Oki Bank and Dok Island were frequent and led to the formation of upslope migrating sediment waves in the southeastern margin of the Ulleung Inerplain Channel (UIC). Bottom currents were generally prevalent on the floors and walls of the UIC and formed sequences of muddy and manganiferous contourites with significantly reduced sedimentation rates. The bottomcurrent sedimentation was seldom interrupted by volcaniclastic high-concentration sediment gravity flows and deep-water stagnations. The deep-water stagnations resulted in anoxic bottom-water conditions where bioturbation by benthic fauna was inhibited. Redox changes in bottom waters caused by renewed deep-water ventilation were sometimes great enough to precipitate Mn-carbonates by enhanced redox-cycling of manganese. One of such significant redox changes in bottom waters is recognized during the last deglaciation (~15 ka), when

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stagnant bottom waters caused by lowered sea level during the last glacial maximum were ventilated by renewed circulation with open ocean as sea level rose. During the post-glacial period (b ~15 ka), sedimentation was generally controlled by bottom currents in both areas of the UIC and its southeastern margin: hampered sedimentation in the UIC axis by stronger bottom-current activity and focused accumulation of resuspended sediments in the southeastern margin. This bottom-current-controlled sedimentation formed the post-glacial sequence of dominant pelagic muds (or muddy contourites) with basal manganiferous contourites, which occurs as an elongate mound parallel to the UIC. The bottom-current activity at the onset of deep-water ventilation at ~15 ka seems to have been much more vigorous than that of the present, probably because of enhanced bottom-water formation due to colder sea-surface temperature. Acknowledgements This work was supported by Korea Ocean R and D Institute under grant PE87500 and EEZ project of the Ministry of Marine Affairs and Fishery (Korea). Chough was supported by the BK21 project of the Ministry of Education (Korea). We are grateful to Drs. J.C. Fauge`res, S. Berne´, and D.J.W. Piper for useful and constructive comments on the manuscript. References Arai, F., Oba, T., Kitazato, H., Horibe, Y., Machida, H., 1981. Late Quaternary tephrochronology and paleo-oceanography of the sediments of the Japan Sea. Quat. Res. Jpn. 20, 209 – 230 (in Japanese with English abstract). Bahk, J.J., Chough, S.K., Han, S.J., 2000. Origins and paleoceanographic significance of laminated muds from the Ulleung Basin, East Sea (Sea of Japan). Mar. Geol. 162, 459 – 477. Bahk, J.J., Chough, S.K., Jeong, K.S., Han, S.J., 2001. Sedimentary records of paleoenvironmental changes during the last deglaciation in the Ulleung Interplain Gap, East Sea (Sea of Japan). Glob. Planet. Change 28, 241 – 253. Berner, R.A., 1970. Sedimentary pyrite formation. Am. J. Sci. 268, 1 – 23. Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits. Elsevier, Amsterdam. Brodie, I., Kemp, A.E.S., 1994. Variation in biogenic and detrital fluxes and formation of laminae in late Quaternary sediments

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