11. CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY OF LEG 127 IN THE JAPAN SEA 1. Atiur Rahman 2

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1 Pisciotto, K.., Ingle, J., Jr., von Breymann,. T., Barron, J., et al., 99 Proceedings of the Ocean Drilling Program, Scientific esults, Vol. 7/8, Pt.. LEOUS NNNOOSSIL BIOSTTIGPHY O LEG 7 IN THE JPN SE tiur ahman BSTT alcareous nannofossils were studied by light microscopy in Neogene sedimentary rocks recovered at four sites of the Ocean Drilling Program Leg 7 in the Japan Sea. Nannofossils occur sporadically at all sites, and allow recognition of seven zones and two subzones; four zones in the Holocene to the uppermost Pliocene, and three zones and two subzones in the middle to lower iocene. orty-eight nannofossil species are recognized in 95 of the 808 irregularly-spaced samples taken from all the sites. The nannofossil assemblages in the iocene are more diverse than those in the Holocene to Pliocene sedimentary interval. The greater diversity and the presence of warm-water taxa, such as Sphenolithus and discoasters in the upper lower iocene to lower middle iocene, suggest a relatively warm and stable surface-water condition, attributed to an increased supply of warm water from the subtropical western Pacific Ocean. Site 797 in the southern part of the Yamato Basin contains the most complete and the oldest nannofossil record so far reported from the Japan Sea. The lowermost nannofossil zone at this site, the Helicosphaera ampliaperta Zone ( a) gives a minimum age for the Yamato Basin. This age range predates rotation of southwest Japan, an event previously believed to be caused by the opening of the Japan Sea. INTODUTION The Japan Sea in the northwestern Pacific region is one of the most extensively studied marginal seas, consisting of several deep basins (Japan Basin, Yamato Basin, Tsushima Basin) separated by ridges of continental crust (Tamaki, 988). The opening of the Japan Sea is most often attributed to a rapid clockwise rotation of southwest Japan and a counterclockwise rotation of northeast Japan in the middle iocene (Otofuji et al., 985a, 985b; Otofuji and atsuda, 987; Tatsumi et al., 989). The magnetic lineations in the Japan Sea are poorly developed and do not allow testing of the age of spreading (Kobayashi, 985; Tamaki, 988). The different stages of tectonic, sedimentary, and Oceanographic development of the Japan Sea are recorded by a thick sequence of sedimentary rocks that had never been fully recovered prior to the recent Ocean Drilling Program (ODP) Leg 7. The Deep Sea Drilling Project (DSDP) Leg only recovered upper Neogene sediment, with nannofossils mostly from the Quaternary (Ellis, 975; Karig, Ingle, et al, 975). On ODP Leg 8 we cored the Neogene sequence and recovered nannofossils in the Holocene, Pliocene, and middle iocene (uza, this volume). Leg 7, one of the two ODP legs in the Japan Sea, drilled at two sites (794 and 797) in the Yamato Basin and two sites (795 and 796) in the Japan Basin (ig. ). The primary objectives of Leg 7 were to determine the nature and age of the basement rocks, the tectonic style of deformation, and to reveal the structural, sedimentary, and paleoceanographic history of the Japan Sea. Ten holes at four sites recovered a thick sequence of Holocene to lower iocene sedimentary rocks and penetrated an underlying acoustic basement of interbedded volcanic and sedimentary rocks. The sedimentary rocks include volcaniclastic and clastic sandstones, oozes, and hemipelagic muds, claystones, and silty claystones rich in siliceous microfossils (Tamaki, Pisciotto, llan, et al., 990). ost of these sedimentary rocks contain little calcium carbonate because of high terrigenous influx and carbonate dissolution resulting in a shallow calcite compensation depth (D), which is presently at 000 m (Ujiié and Ichikura, 97). Pisciotto, K.., Ingle, J., Jr., von Breymann,. T., Barron, J., et al., 99. Proc. ODP, Sci. esults, 7/8, Pt. : ollege Station, TX (Ocean Drilling Program). Department of Geology and Geophysics, University of Utah, Salt Lake ity, Utah 84-8, U.S.. Well to poorly preserved nannofossils occur sporadically in the Holocene to uppermost Pliocene, and in the middle to lower iocene of the sedimentary sequence cored by Leg 7. Nannofossils are the best documented of all microfossil groups below the opal-/opal-t Boundary in the middle and uppermost lower iocene of the Japan Sea and provide important age control to estimate the sedimentation rates and to constrain the age of basement rocks (Leg 7 Shipboard Scientific Party, 989; Tamaki, Pisciotto, llan, et al, 990). Nannofossil assemblages are more diverse in the lower middle iocene to upper lower iocene and less diverse in the Holocene to upper Pliocene. The most complete and the oldest record of nannofossils in the Japan Sea is found at Site 797 in the southern part of the Yamato Basin. This report discusses the nannofossil biostratigraphy in the sedimentary sequence of the Japan Sea, and states the major implications of the results. SPLES ND ETHODS Each core of sedimentary rock was sampled at irregular intervals, depending on the recovery, the potential for locating nannofossil events (O, first occurrence;, last occurrence), the presence of relatively undisturbed core intervals, and on the calcium carbonate content. Smear slides were prepared following the technique outlined in ahman and oth (989). Samples were prepared for observation under scanning electron microscope by a centrifuging method (Perch- Nielsen, 985). ost work was done by transmitted-light microscopy, but the scanning electron microscope was used to observe small taxa that are difficult to identify under the light microscope. fter counting 5-0 randomly selected fields of view under the light microscope (at 560 magnification), the total number of nannofossils was expressed by using one of the following semiquantitative categories: = abundant, containing 0 or more specimens per field of view; = common, containing -9 specimens per field of view; = many, containing less than specimen per field, but more than specimen in 0 fields of view; = few, containing specimen in 0 fields of view; = rare, containing specimen in more than 0 fields of view. Smear slides were called barren if no nannofossil was observed in about 00 fields of view. Each smear slide containing nannofossils was observed under light microscope in randomly selected traverses for -4 hr. The relative abundance of each nannofossil taxon was noted by using categories (,,,, ) just defined. Samples from each site were investigated for the presence and relative 7

2 . HN N 45' 40' KOE 0 E 5 igure. Bathymetric map of the Japan Sea showing the locations of Leg 7 sites ( ) as filled circles, Leg 8 sites ( ) as squares, and DSDP Leg sites (99-0) as triangles. ontour values are in meters. Toothed line represents Japan Trench. Site 794 was drilled during Legs 7 and 8. abundance of taxa as listed in ppendix, where taxonomic concepts used in this study are also briefly noted. The state of nannofossil preservation was based on light microscopic observation of all species, rather than relying heavily on delicate taxa, such as Syracosphaera, Pontosphaera, habdosphaera, Discosphaera, znáolitothus (oth, 974). This is because the delicate taxa prefer relatively warm surface water (oche et al., 975; oth and oulbourn, 98), and may have been environmentally excluded from the Japan Sea, where present surface water is cool. The following preservational categories were used: E = excellent outer margin of the placoliths are smooth without any crenulation, intact specimens of delicate species {Syracosphaera, Pontosphaera) may be present, no overgrowth. E-l = slightly etched indicated by broken specimens, crenulated margins, fragments of delicate species, and isolated shields of some coccoliths. E- = moderately etched most coccoliths show crenulated margins, delicate bridges and central structures are mostly dissolved, and broken specimens and separate shields are common. E- = strongly etched most nannofossils are fragmented, coccoliths show crenulated margins, resistant taxa (occolithus, Discoaster, eticulofenestrá) are differentially enriched. O- = slight overgrowth the shields or central structures of some coccoliths show slight overgrowth, and slight thickening of the arms and central knobs of discoasters. 0- = moderate overgrowth most of the coccoliths and discoasters are overgrown but still mostly recognizable. 0- = strong overgrowth discoasters are overgrown beyond recognition to the species level, other coccoliths are difficult to identify. Preservation was called excellent (E) if nannofossils showed no evidence of etching or overgrowth, good (G) if nannofossils were only slightly etched and/ or overgrown, moderate () if nannofossils were moderately etched and/ or overgrown, and poor (P) if nannofossils were strongly etched and/ or overgrown. LEOUS NNNOOSSIL EVENTS ND ZONES The nannofossil zones used in this study for the Holocene to the uppermost Pliocene are adopted from zonations suggested earlier (Hay et al, 967; Gartner, 969; artini, 97; Bukry, 97; ahman and oth, 989) with few modifications as shown in igure. This figure also compares Holocene to upper Pliocene nannofossil zones of different authors with the zones used in this study. The middle Pliocene to iocene zones are adopted from Okada and Bukry (980). Table shows all the Neogene nannofossil zones, subzones, and events and their ages used in this study. The ages of the nannofossil events are taken from Thierstein et al. (977), Backman and Shackleton (98), Barron et al. (985a, b), Berggren et al. (985), ahman and oth (989), and Backman et al. (990). The bases of the Pleistocene, Pliocene, upper iocene, and middle iocene are approximated by the O's of Gephyrocapsa caribbeanica (.68 a), eratolithus acutus (4.9 a), Discoaster hamatus (0.5 a), and alcidiscus macintyrei (5.7 a), respectively. The age estimates of these events most closely correspond to the respective epoch and stage boundaries of the time scale of Berggren et al. (985). Nannofossil zones and subzones in the Japan Sea are recognizable in two sedimentary intervals, one in the Holocene to uppermost Pliocene, and the other in the middle to lower iocene. The precise 7

3 LEOUS NNNOOSSIL BIOSTTIGPHY ge (a) Period/ Epoch Hay et al. (967) huxleyi Gartner (969). huxleyi artini (97) E. huxleyi Bukry (97) huxleyi Gartner (977) E. huxleyi acme and E. huxleyi Bukry (978) E. huxleyi ahman and oth (989) huxleyi This study huxleyi Gephyrocapsa G. oceanica G. oceanica. cristatus G. oceanica Gephyrocapsa.00- Quaternary G. oceaπica G. caribbeanica P. lacuπosa P. lacunosa G. oceanica G. caribbeanica E. annula P. lacunosa Small Gephyrocapsa H. sellii. macintyrei E. ovata G. caribbeanica E. annula U. sibogae. leptoporus G. caribbeanica -. macintyrei P. lacunosa U. sibogae G. caribbeanica P. lacunosa.00- D. brouweri D. e tensus D. brouweri D. brouweri. macintyrei D. brouweri. macintyrei D. brouweri D. brouweri Pliocene D. surculus D. pentaradiatus D. pentaradiatus D. pentaradiatus D. pentaradiatus D. pentaradiatus.00- S. amphora D. surculus D. surculus D. tamalis D. tamalis D. tamalis D. tamalis igure. alcareous nannofossil zones and subzones between the Holocene and upper Pliocene. Zonal and subzonal boundaries are based on ages presented in Table. ges of the events not presented in Table are taken from Gartner (977) and ahman and oth (989). Table. alcareous nannofossil zones and subzones for the Neogene. Period /Epoch Quaternary Pliocene iocene Zone /Subzone E. huxleyi (N5) Gephyrocapsa (N4b) U. sibogae (N4a) G. caribbeanica (Nb) P. lacunosa (N a) D. brouweri (N). pseudoumbilica (N). tricorniculatus (N0). rugosus (NlOc). acutus (fi\ob) T. rugosus (NlOa) D. quinqueramus (N9) D. neohamatus (N8) D. hamatus (N7). coalitus (N6) D. exilis (N5) D. kugleri (N5b). miopelagicus (N5a) 5. heteromorphus (N4) H. ampliaperta (N) 5. belemnos (N) T. carinatus (N) D.drugü (Nlc) D. deflandrei (Qi\b) Event 0 O 0 O O O O O 0 O 0 cme Species E. huxleyi P. lacunosa G. oceanica G. caribbeanica D. brouweri S. neoabies. primus. acutus. acutus D. quinqueramus D. berggrenii D. hamatus D. hamatus. coalitus. floridanus S. heteromorphus. macintyrei S. heteromorphus S. belemnos D. drugii. abisectus ge (a) eference Note: irst and last occurrences of marker species defining the base of the biostratigraphic units are indicated. The ages of the datum events are taken from different sources as indicated. ode numbers of zones and subzones from Okada and Bukry (980) are shown in parentheses. ", Thierstein et al. (977);, ahman and oth (989);, Backman and Shackleton (98); 4, Berggren et al. (985); 5, Backman et al. (990); 6, Barron et al. (985a). position of some events within these intervals are unknown because of sporadic occurrence, absence, or scarcity of marker species. Discussion of events, zones, and subzones in this study is therefore restricted to these two intervals. In this study, the nannofossil zones between the Holocene and the upper Pliocene (ig. ) utilized first and last occurrence events, all of them except the 0 of Gephyrocapsa oceanica dated at.5 a were widely used in previous studies. This age, based on interpolation between radiometrically dated tephra layers and a modified species concept (ahman and oth, 989), is also supported by nannofossil data at DSDP Site 5 in the tlantic Ocean (ahman, in press) Berggren et al. (985) reported the 0 of G. oceanica at.68 a. I follow ahman and oth (989) for the age estimate of the 0 and the modified species concept of G. oceanica. This allows the recognition of the interval between the O's of G. oceanica and G. caribbeanica, and also changes the duration of zones that use the 0 of G. oceanica as a marker. The early estimate of the age of the O of Gephyrocapsa oceanica (.68 a, Berggren et al., 985) is close to or similar to the age estimate for the O of Gephyrocapsa caribbeanica. Takayama and Sato (987) estimate the age of the 0 of G. caribbeanica at.66 a in the North tlantic Ocean. ahman and oth (989) estimate this event at.68 a in the Gulf of den. Nannofossil zones recognized in the middle and lower iocene are the Discoaster exilis Zone, the Sphenolithus heteromorphus Zone, and the Helicosphaera ampliaperta Zone. The D. exilis Zone is divided into the Discoaster kugleri and occolithus miopelagicus Subzones. Bukry (97) used the 0 of D. kugleri and the of yclicargolithus floridanus to define the base of the D. kugleri Subzone and the top of the. miopelagicus Subzone. In the Japan Sea, D. kugleri and. floridanus occur together for part of the middle iocene at Sites 794 and 797. This suggests that () the O of D. kugleri, or the of.floridanusis diachronous, () that both are diachronous, or () that specimens of. floridanus are reworked. The of.floridanuswas used in this study to differentiate the D. kugleri and. miopelagicus Subzones, because this species is more abundant and more easily identified than D. kugleri. The latter species occurs sporadically and is difficult to recognize because of overgrowth. The of Sphenolithus heteromorphus is dated at 4 a in the Pacific Ocean by calibrating nannofossil events with magnetostratigraphy (Barron et al., 985a). Backman et al. (990) used similar methods to derive an estimate at.6 a for the Indian Ocean. The former estimate is accepted in this study because the latter authors indicated a low rate of sedimentation close to the of S. heteromorphus, as also indicated by their placement of the of yclicargolithus floridanus either coeval or closely following the of S. heteromorphus. The age of the O of Sphenolithus heteromorphus is estimated at 7.5 a by Barron et al. (985b), based on indirect correlation of magnetostratigraphy and planktonic biostratigraphy of yan et al. (974). Berggren et al. (985) reported an age estimate at 7. a, 7

4 . HN which was based on data from the southwestern tlantic Ocean (Berggren et al., 98, p. 685). The original data for this estimate showed sporadic occurrences of S. heteromorphus below the accepted level of O. Thus the 0 of S. heteromorphus is older than the estimate reported in Berggren et al. (985). I use an age estimate at 8.4 a from the Indian Ocean (Backman et al., 990), based on correlation of quantitative nannofossil data from closely spaced samples and magnetostratigraphy. Okada and Bukry (980) use the acme of Discoaster deflandrei and the 0 of alcidiscus macintyrei to define the base of the Sphenolithus heteromorphus Zone. The latter event is used in this study because the rare and sporadic occurrence of D. deflandrei did not allow the recognition of its acme in the Japan Sea. The O of. macintyrei is dated from the Pacific Ocean at 5.7 a by Barron et al. (985a). This age is close to the age of the acme of D. deflandrei recently dated at 6. a in the Indian Ocean by io et al. (990). In the following discussion, each of the zonal names is followed by the author name(s), code of Okada and Bukry (980), definition, and remarks; the discussion is limited to the zones where some remarks are necessary. or the discussion of the rest of the zones and subzones the reader is referred to earlier studies (Hay et al., 967; Gartner, 969; artini, 97; Bukry, 97; Okada and Bukry, 980; ahman and oth, 989). Definition Umbilicosphaera sibogae Zone ahman and oth (989), N4a Interval from the of Pseudoemiliania lacunosa to the 0 of Gephyrocapsa oceanica. emarks Both of the datum levels of this zone have been used separately in previous zonal schemes (Hay et al., 967; Gartner, 969; artini, 97). The Umbilicosphaera sibogae Zone has a shorter duration than the Emiliania ovata Subzone (N4a) of Okada and Bukry (980), because of the new age estimate of the 0 of G. oceanica. Gephyrocapsa caribbeanica Zone Bukry (97), Nb, Upgraded in this Paper Definition Interval between the O's of Gephyrocapsa oceanica and Gephyrocapsa caribbeanica. emarks The 0 events of Gephyrocapsa oceanica and Gephyrocapsa caribbeanica are useful when a broader definition of these two species is used (ahman and oth, 989; ahman, in press). The G. caribbeanica Subzone (Nb) of Bukry (97) and Okada and Bukry (980) is upgraded to a zone in this paper. Due to the new age estimate of the upper marker, the O of G. oceanica (.5 a; ahman and oth, 989), this zone has a longer duration than Nb. SITE SUIES orty-eight species of nannofossils are recognized in 95 samples through light microscopic investigation of 808 samples from 0 holes at four sites of Leg 7. Nannofossils occur in two sedimentary intervals and define four zones in the Holocene to uppermost Pliocene, and three zones (two subzones) in the middle and lower iocene (Tables -5). Nannofossil events recognized in Leg 7 are summarized in Table 6. Taxonomic concepts used in this study are given in ppendix. The barren samples are listed in ppendix B, so that future nannofossil workers may avoid them. Delicate taxa are either rare or absent, even in samples with well-preserved nannofossils, suggesting environmental conditions adverse for delicate taxa. eworked forms are rare which is in agreement with Ellis (975) from DSDP Site 0 in the Japan Sea. eticulofenestra pseudoumbilica, which became extinct at.56 a, were reworked into the Pleistocene, and eticulofenestra umbilica, Discoaster lodoensis, and Nannotetrina, all of Paleogene age, were reworked into the iocene. Site 794 This site is located in the northernmost part of the Yamato Basin at 40 ll'n and 8 5'E, at a water depth of 85 m. Three holes at this site (794, 794B, 794) recovered a sedimentary sequence composed of clay and silty clay, clayey diatomaceous ooze, and claystone. These sedimentary rocks are poor in calcium carbonate, except in some thin layers. Hole 794 encountered a dolerite sill complex at a depth of 54 meters below seafloor (mbsf). ODP Leg 8 drilled two holes (794D, 794E) at this site and recovered a layer of sedimentary rock (.6 m thick) containing microfossils from a depth of 64.5 mbsf at Hole 794D (Ingle, Suyehiro, von Breymann, et al, 990). Nannofossils are present in only 0 out of 6 samples studied in Holes 794 and 794B. Their relative abundance varies from abundant to rare, and the preservation varies from excellent to poor (Table ). Three nannofossil zones and two subzones are recognized at Site 794, one zone in the Pleistocene, two zones and two subzones in the middle iocene. The Emiliania huxleyi Zone is unrecognizable at Site 794. Samples H-, 7-74 cm, through -4H-, cm, are assigned to the Umbilicosphaera sibogae Zone of late Pleistocene age, based on the presence of Pseudoemiliania lacunosa and Gephyrocapsa oceanica. The precise position of the zonal boundaries is indeterminate because of barren sedimentary intervals, both above and below. Gephyrocapsa caribbeanica first occurs with Gephyrocapsa oceanica in Sample H-, cm, which is underlain by Sample H-, cm, containing only two taxa, occolithus pelagicus and small eticulofenestra. This suggests environmental control on the coeval O's of G. caribbeanica and G. oceanica. Sample H-, cm, and 7-794B--, could not be assigned to any zone because of the lack of marker species. The former sample is probably younger than.56 a (Berggren et al., 985; ahman and oth, 989), based on the absence of eticulofenestra pseudoumbilica. The presence of. pseudoumbilica in the latter sample suggests that it is older than.56 a. Sample 7-794B-- is followed by a thick barren interval. Samples 7-794B-7-,-4 cm, through -7-, are assigned to the Discoaster kugleri Subzone of the Discoaster exilis Zone. The top of this subzone was not recognizable because of the absence of the marker species, atinaster coalitus. The of yclicargolithus floridanus located between the Samples 7-794B-7-, and -8-, brackets the base of the D. kugleri Subzone. Samples 7-794B- 8-, through --, 6-64 cm, are assigned to the occolithus miopelagicus Subzone of the D. exilis Zone, based on the presence of.floridanus and the absence ofsphenolithus heteromorphus. The. miopelagicus Subzone contains the most diverse nannofossil assemblage at Site 794. Samples from 7-794B--, cm, to -- are assigned to the Sphenolithus heteromorphus Zone, based on the presence of alcidiscus macintyrei and S. heteromorphus. The O of. macintyrei at the bottom of the S. heteromorphus Zone was unrecognizable at Site 794 because of a barren interval underlying this zone. Questionable remains of Sphenolithus heteromorphus in Sample 7-794B-5-, 7-8 cm, were identified during shipboard study 74

5 Table. Stratigraphic distribution, relative abundance, and preservation of calcareous nannofossils at Site 794. Sample interval is in centimeters. - Hole 794 B - Hole 794B Sample -H-, H-7, 9- -H-7, H- -4H-,6-9 -4H-, H-, H-, buπdanc petching r Overgrowt c B L E P G G P G eworked species :- B. bigelow. floridan JS o to. macintyrei. mexicai snoiß Q. O <O. miopela occolithu. pelagici to. rotula. strecke i D. brouwe D. challen<. D. defland D. exilis D. kugleri S ci to Ii! I to i s si ci to D. variabili sp. G. caribbe anica to! o psa small c H. carteri H. perch-nielseniae P. distinctε E P. pacifica era sp. Pontospha. pseudo iβstra small S? orphus.to r S. abies S. heterorr S. moriforr to S. neoabie jaera sp. cò T. heimii T. rugosus Zone/ Subzone U. sibogae Period/Epoch 0 b B B- - B-7-,-4 B-7- B-8- B-9-, B-9- B- O-, 70-7 B--, 6-64 B--, B-- B-- B-- G G P G G P D. kugleri. miopelagicus S. heteromorphus Note: bbreviations used: for abundance, = abundant, = common, = many, = few, = rare; for preservation, E = excellent, G = good, = moderate, P = poor; see text for more details.? D c D U o > r n > TO m o c Λ > z o I r w

6 . HN Table. Stratigraphic distribution, relative abundance, and preservation of calcareous nannofossils at Site 795. See Table for explanation. - Hole 795 B - Hole 795B Sample -H-, H-4, -4 -H-, H-, H-6, H- -5H-, H-, H-4, H-, 9-40 bundance Etching Overgrowth Preservation eworked species 0 G G 0 0 E P 0 G G P P ß. bigelowii. floridanus LL LL G. leptoporus. macintyrei occolithus sp. oc oc G. pelagicus. streckeri D. exilis D. perplexus D. productus E. huxleyi G. caribbeanica G. oceanica Gephyrocapsa small H. carteri P. lacunosa P. pacifica Pontosphaera sp.. pseudoumbilica eticulofenestra small S. abies S. neoabies T. heimii T. rugosus T. saxea U. sibogae Zone/ Subzone E. hu leyi U. sibogae Period / Epoch Quaternary -8X- B-5-6, B-5- B-6-6, B-6- B-7- B-9-, 40-4 P P G G G G Ti G 0 G LL 0 O oc oc G. miopelagicus iocene Table 4. Stratigraphic distribution, relative abundance, and preservation of calcareous nannofossils at Site 796. See Table for explanation. - Hole 796 B - Hole 796B Sample -H-, H- -5H-4, H-5, H-6, -4-5H-7, H- -9X- -X-, 0- B-7- O T < TI TI :hing LU 'ergrowth O P P sservatio worked s bigelowii L G TJ T P 'o ΦQ. G O" * ci? sp. a pelagicui o huxleyi LU 8 r cσ D D D oceanica mall cσ» carteri lacunosa U pacifica ntosphae ri O a tc small Zone/ Subzone E. huxleyi Gephyrocapsa and U. sibogae? jriod/ Epoc Q. 9rna T σ (Tamaki, Pisciotto, llan, et al., 990). long search during the shorebased study, however, did not confirm even a single specimen of this species in ore 7-794B-5. The lowest sample that contains nannofossils is 7-794B-- (Table ). This sample is older than the of S. heteromorphus, dated at 4 a (Barron et al., 985a). Site 795 Site 795 is located at the northern end of the Japan Basin, just off the flank of what appears to be a buried remnant spreading ridge at 44 00'N, 8 5.7'E, at a water depth of 74 m. Two holes (795, 795B) drilled at this site recovered a sedimentary sequence similar to that of Site 794, but with a greater proportion of terrigenous sediments. The calcium carbonate content of the sediments is low overall. Hole 795B drilling encountered a basement complex of basaltic rocks at 685 mbsf (Tamaki, Pisciotto, llan, et al., 990). The patterns of nannofossil occurrences at this site are similar to those at the previous site. Nannofossils of Pleistocene, Pliocene, and middle iocene ages are abundant to rare, well to poorly preserved, and occur sporadically in 7 samples out of 8 samples studied (Table ). Nannofossil data allowed recognition of two zones in the Holocene to Pliocene and one subzone in the middle iocene. 76

7 Table 5. Stratigraphic distribution, relative abundance, and preservation of calcareous nannofossils at Site 797. See Table for explanation. O «ε - Hole 797 '8 g S S d. 5 a ««"S.». e797b, fifiillliislii - i B Hol s «fe I «sl! Ifiiiii * c-hoie797c I o,lll lllài llll.»l4 fi«lls^is.ssl.li s 4fi?ll i II ti IIIII i? 8 f S j i I! If I Sample < w o oi tr O Ü Ü Ü Ü Ü Ü U Q Q ööcicici ÖQQQD e > DiiαjQjαj < Scccc wcocococo co K ^ Zone/Subzone α. -H-,6-7 0 P ~~ ~ B-H-,0-04 Gephyrocapsa B-H-,6-7 B-H-6, 0-05 G B-H-6, 5-6 G " ~ B-H-7, G B-H- P? B-4H-4,6-7 fr B-4H- G _ E B-5H-4,6-7 G ~ U. Sibogae ig B-5H-7,6-7 = B-5H- G B-8H-,7-8? B-8H-6, 7-8 B-8H- G B-9H-5,9-0 B-9H- G G. caribbeanica B-4H-,6-7 B-4H- P Q E? B-40X-,6-7 B-40X-,- B-40X-, miopβlagius B-40X- _ B-4X-,95-96 P B-4X-,0- B-4X- P B-46X- P B-47X-, P B-47X-,45-46 P B-47X-,4-5 B-47X-6,4-4 P g heteromorphus 5 B-47X-6,45-46 P o B-47X- [ _J ^ B-48X-,-4 P B-48X-,9-0 B-48X-,49-50 B-48X-,08-09 P B-48X-, B-5X-,5-6 P? B-5X-4, 00-0 B-5X-5,5-6 P B-5X- H. ampliaperta B-5X-, 0- _ -- --, , , 4-44 P? -5-, 6-7 [ P I ' I s

8 . HN Table 6. Summary of calcareous nannofossil events of Leg 7 (Sites ) in the Japan Sea. Leg 7 Species Event Site 794 Site 795 Site 796 Site 797 E. huxleyi P. lacunosa G. oceanica. floridanus S. heteromorphus. macintvrei O 0 O - - B-7-/ B-8- B--, 6-64/B--, H-, 87-88/ -4H-, 46^7 -H-, 87-88/ -4H-, 46^7 -H-, 40-4/ -H- B-H-6, 0-05/B-H-6, 5-6 B-8H-6, 7-8/ B-8H- B-4X-, 95-96/ B-4X-, 0- B-48X-, 08-09/B-48X-, Note: Sample intervals are in centimeters. and B with sample numbers designate the holes at a single site, i.e., at Site 794 represents Hole 794. O = first occurrence, = last occurrence. Samples from H-,06-07 cm, through-h-,87-8 cm, are assigned to the Emiliania huxleyi Zone, based on the presence of the nominal species. This zone contains few to rare Braarudosphaera bigelowü, a species which is more common in shelf seas than in the open ocean (Perch-Nielsen, 985). Several whole coccospheres of this species were observed, which suggests the influence of nearshore conditions. The Gephyrocapsa Zone that directly underlies the Emiliania huxleyi Zone was not recognizable because of a barren interval. Samples H-, cm, through -4H-, contain Pseudoemiliania lacunosa and Gephyrocapsa oceanica and thus belong to Umbilicosphaera sibogae Zone. s at Site 794, the precise position of the bottom of this zone is uncertain because G. oceanica and Gephyrocapsa caribbeanica first occur in the same sample (Table ). The coeval O's of these two species and a low diversity of the assemblage in the underlying sample ( H-, cm), once again, suggest environmental control on these two events in the lower Pleistocene. Samples from H-, cm, to -H-, 9^0 cm, belong to the lowermost Pleistocene and the upper Pliocene, based on the absence of eticulofenestra pseudoumbilica and Gephyrocapsa oceanica. Sample X-, may be anywhere from lower Pliocene to the uppermost middle iocene, as indicated by the presence of. pseudoumbilica and the absence of yclicargolithus floridanus. These samples are not assigned to any zone because of the lack of marker species. Samples 7-795B-5-6,49-50 cm, through-9-,40-4 cm, are assigned to the occolithus miopelagicus Subzone of the Discoaster exilis Zone, based on the presence of yclicargolithus floridanus and the absence of Sphenolithus heteromorphus. The top of this subzone was unrecognizable because of an overlying barren interval. The lowest nannofossil-bearing sample (7-795B-9-, 40-4 cm) occurs between the 's of. floridanus (. a) and S. heteromorphus (4.0 a). Site 796 Site 796 is located at 4 50'N, 9 5'E at a water depth of m on the Okushiri idge, a complex, thrust-faulted structure along the northeastern flank of the Japan Basin. Two holes (796, 796B) were drilled through a sedimentary sequence of silty clay, sand, claystone, clayey diatom ooze, diatomite, and siliceous claystone. The sedimentary sequence is poor in calcium carbonate. In contrast to the previous sites, the fine-grained clastic and hemipelagic sediments at this site are interbedded with sandstones, indicating a close proximity to the terrigenous sources. Nannofossils at Site 796 are less common, less diverse, and more sporadic than at Sites 794 and 795, occurring only in 0 out of 64 samples studied (Table 4). They are abundant to rare, and well- to poorly preserved. Three nannofossil zones are defined in the Holocene to the upper Pliocene. Sample H-, 40-4 cm, belongs to the Emiliania huxleyi Zone, based on the presence of the nominal species. This sample contains abundant well-preserved nannofossils, including coccospheres of Braarudosphaera bigelowii, indicating the influence of a nearshore environment. The samples from H-, to -6H-, probably belong to the undifferentiated Gephyrocapsa and Umbilicosphaera sibogae Zones of Pleistocene age, based on the absence of Emiliania huxleyi, and the presence of Gephyrocapsa oceanica in the latter sample. Pseudoemliania lacunosa is present only in one sample ( H-), which precluded the separation of these two zones. The position of the base of this combined Gephyrocapsa -U. sibogae Zone was unrecognizable because G. oceanica and Gephyrocapsa caribbeanica first occur in the same sample, as at Sites 794 and 795. Samples between X-, and -796B-7-, belong to the lowermost Pleistocene and upper Pliocene, based on the absence of Gephyrocapsa oceanica, Sphenolithus, and eticulofenestra pseudoumbilica. These samples are not assigned to any zone because of lack of marker species. The lowest sample containing nannofossils (7-796B-7-) belongs to the uppermost Pliocene, based on diatom biostratigraphy (Tamaki, Pisciotto, llan, et al., 990). It is unclear whether the lower part of Hole 796B reached the middle iocene. This issue remained unresolved in the shipboard study. The only age control available in the lower part of the sedimentary sequence of Hole 796B was the occurrence of planktonic foraminifer Globigerina bulloides in Samples 7-796B- -4,4-45 cm, and --5,4-5 cm (Tamaki, Pisciotto, llan, et al., 990). The O of this species is dated at 6 a (Berggren et al., 985), which brackets the maximum age of the samples. ll the other sites of Leg 7 except Site 796 contain nannofossils in middle iocene sedimentary rocks; some of the samples are even younger than the of Sphenolithus heteromorphus (4 a). t Site 794, nannofossils occur above the of yclicargolithus floridanus (. a). These nannofossil occurrences imply that the base of Hole 796B is younger than. a, provided that the temporal distribution of nannofossils in the Japan Sea is controlled only by the surface water conditions, and not by local diagenetic processes. Site 797 Site 797 is situated at 8 6.7'N, 4.6'E at a water depth of 945 m in the southern part of the Yamato Basin and east of the Yamato ise. Three holes (797, 797B, 797) were drilled and recovered a thick sedimentary sequence of silty clay, diatomaceous clay, clayey diatomaceous ooze, claystone, and tuff in the upper part, with interbedded sandstone, siltstone, and silty claystone in the lower part. Hole 797 encountered a basement complex of basalts and dolerites interlayered with coarse- to fine-grained clastic sedimentary rocks. ompared to the Sites 794 and 796, Site 797 is situated at a greater water depth. This site has the highest content of calcium carbonate in the sediments, the most complete nannofossil record, and the most diverse nannofossil assemblages. ll these observations can be attributed to warmer surface water. Nannofossils are abundant to rare, and 78

9 LEOUS NNNOOSSIL BIOSTTIGPHY well to poorly preserved in 48 of 00 samples studied (Table 5). Six zones are recognized at Site 797, three zones in the Pleistocene, three zones and one subzone in the middle to lower iocene. Samples H-, 6-7 cm, through -797B-H-6, 0-05 cm, are assigned to the Gephyrocapsa Zone, based on the absence of Emiliania huxleyi and Pseudoemiliania lacunosa. The latter species last occurs between Samples 7-797B-H-6, 0-05 cm, and -5-6 cm (Table 5). The Samples 7-797B-H-6,5-6 cm, through-8h-6,7-8 cm, belong to the Umbilicosphaera sibogae Zone, based on the presence of Gephyrocapsa oceanica and Pseudoemiliania lacunosa. This zone also contains rare to abundant small reticulofenestrids, small Gephyrocapsa, occolithus pelagicus, and rare to common Gephyrocapsa caribbeanica. The 0 of G. oceanica that marks the bottom of the U. sibogae Zone occurs between Samples 7-797B- 8H-6, 7-8 cm, and -8H-. Site 797 was the only site of Leg 7 where the O of G. oceanica was recognized. Samples 7-797B-8H-, through -4H-, belong to the Gephyrocapsa caribbeanica Zone of the lower Pleistocene, based on the presence of the nominal species and the absence of Gephyrocapsa oceanica. The bottom marker of this zone, the 0 of G. caribbeanica, was not precisely located because of an underlying barren interval. This barren interval was underlain by the occolithus miopelagicus Subzone of the Discoaster exilis Zone in the middle iocene between the Samples 7-797B-40X-, 6-7 cm, through -4X-, cm. This subzonal assignment was based on the presence of yclicargolithus floridanus and the absence of Sphenolithus heteromorphus. In contrast to Site 794, the precise depth of the of yclicargolithus floridanus is unknown at Site 797, which perhaps can be attributed to the lack of recovery of the ores 7-797B-8 to -40. The depth of this event is, however, constrained by the base of the diatom zone ouxia californica (7.0 a) in Sample 7-797B-7X- (Tamaki, Pisciotto, llan, et al., 990). This sample and the apparent of.floridanusidentified an interval of reduced sedimentation rate that may include a hiatus. Samples 7-797B-4X-,0- cm, through -48X-,08-09 cm, belong to the Sphenolithus heteromorphus Zone, based on the presence of S. heteromorphus and alcidiscus macintyrei. Nannofossil assemblages within the lower part of this zone are the most diverse of Site 797 and other sites of Leg 7. Discoasters are more frequent throughout this interval. Based on the presence of Sphenolithus heteromorphus and the absence of alcidiscus macintyrei, the samples between 7-797B- 48X-, cm, and , are assigned to the Helicosphaera ampliaperta Zone. The lowest occurrence of S. heteromorphus is in Sample It is unclear whether the occurrence of 5. heteromorphus in this sample represents the O of this species. Below Sample , nannofossils are present, but only in four samples. Nannofossil assemblages in these samples consist of a few taxa such as alcidiscus leptoporus, occolithus pelagicus, Dictyococcites perplexus, occolithus miopelagicus, and discoasters, which are unidentifiable to the species level because of strong overgrowth. Sample ,6-7cm,istheoldestnannofossil-bearing sample at Site 797. The minimum age of this sample is constrained by the 0 of alcidiscus macintyrei (5.7 a) in Sample 7-797B- 48X-, cm. Its age may be anywhere between 5.7 and 8.4 a, and may even be older if the 0 of Sphenolithus heteromorphus at Site 797 is not affected by diagenesis or by the presence of adverse surface-water conditions in the early iocene. DISUSSION Importance of Nannofossil Biostratigraphic esults The sedimentary sequence of Leg 7 has the most diverse and complete nannofossil record ever recovered from the Japan Sea. Despite their spotty distribution, nannofossils provided age controls in the middle and lower iocene. Other microfossil groups such as foraminifers, radiolarians, and diatoms do not provide more precise age information than nannofossils in this interval. urthermore, the problems of remagnetization, erratic magnetic directions, low intensity of remanent magnetization, and poor core recovery precluded the use of magnetostratigraphy in the iocene sedimentary rocks (Tamaki, Pisciotto, llan, et al., 990). The nannofossil biostratigraphic results of this study put constraints on the minimum age of the Japan Sea, and allow a check of one of the most cited models explaining the opening ofthe Japan Sea (Otofuji et al., 985a, 985b). The model of Otofuji et al. (985a) suggests a rapid clockwise rotation (-50 ) of southwest Japan in the middle iocene at about 5 a, based on change in the direction of paleomagnetic declinations recorded in the Japanese islands. similar study in the northern part of Japan proposed a counter-clockwise rotation of northeast Japan between and a (Otofuji et al., 985b). These rotations occurred simultaneously as the result ofthe oceanward migration ofthe Japanese islands that gave rise to the fan-like opening of the Japan Sea backarc basin (Otofuji et al., 985b; Kimura and Tamaki, 986; Otofuji and atsuda,987; Tatsumi et al., 989;Honkuraetal., 99). This model suggests that the Japanese islands were parallel to the Eurasian margin and that the Yamato Basin did not open before the bending ofthe Japanese islands. ccording to Otofuji et al. (985a), the rotational motion for southwest Japan lasted for a short time interval of less than m.y., thus requiring an extremely high rate of seafloor spreading that is not supported by the presently known spreading rates (elaya and cabe, 987). Nannofossil records at Site 797 show the existence of the southern Yamato Basin already in the early middle iocene, earlier than the 0 of alcidiscus macintyrei (5.7 a). This is consistent with data from Site 794 in the northern Yamato Basin, where the earliest nannofossil record in Hole 794 is older than the of Sphenolithus heteromorphus (4 a). The actual age ofthe basin at this site is perhaps much older than 4 a, because marine diatoms, radiolarians, and foraminifers occur within a sedimentary layer at the base of Hole 794D, and the benthic foraminifers indicate a deep-water environment (Ingle, Suyehiro, von Breymann, et al., 990). The age of Japan Basin at Site 795 is older than the of yclicargolithus floridanus (. a). This study shows that at least the Yamato Basin was formed before the rotation of southwest Japan, and also that the latter event cannot be directly attributed to the initial opening of the Japan Sea as previously claimed by some workers. The new results warrant a consideration of other event(s) or mechanism(s) (e.g., Lallemand and Jolivet, 986). ny kinematic model proposed in the future should be able to explain the age constraints based on nannofossil biostratigraphy of Leg 7. Nannofossil ssemblage and Paleoceanographic Inferences The nannofossil assemblages in the Holocene to Pliocene sedimentary rocks of the Japan Sea are dominated by Gephyrocapsa caribbeanica, Gephyrocapsa oceanica, occolithus pelagicus, small reticulofenestrids, and small Gephyrocapsa. The assemblages are diverse in the lower middle iocene to upper lower iocene and contain mainly. pelagicus, eticulofenestra pseudoumbilica, yclicargolithus floridanus, and small reticulofenestrids, with relatively rare Sphenolithus heteromorphus, Sphenolithus moriformis, Sphenolithus abies, Sphenolithus neoabies, Helicosphaera carteri, alcidiscus macintyrei, and discoasters. igure shows a summary of nannofossil diversity in the sedimentary sequence of Leg 7. This figure is a plot of the number of nannofossils species against zones/ subzones and age. The diversity varied from 0 to 7 taxa within the interval between Helicosphaera ampliaperta Zone (N) and occolithus miopelagicus Subzone (N5a). Diversity dropped sharply within the Discoaster kugleri Subzone (N5b), and varied from 6 to 4 taxa between Pseu- 79

10 . HN.00 )N4b JN4a )Nb Na.00-J.O 4.0 ge Number of Species (a) N co N N5b N5a N4 N igure. alcareous nannofossil diversity in the sedimentary sequence recovered during Leg 7. Number of taxa, from all sites, are plotted against zones and subzones (Okada and Bukry, 980; this paper) and age. doemiliania lacunosa Subzone (Na) and Emiliania huxleyi Zone (N5). The surface-water temperature in the northern part of the Japan Sea is low and that part of the sea shows large seasonal fluctuations of temperature and salinity (Hidaka, 966). Within the southeastern Japan Sea, surface water is warmer, a condition that is directly related to the flow of the warm Tsushima urrent (Hidaka, 966). This influx of warm water from the south is closely tied to the eustatic sea level, because of the shallow depth of the Tsushima Strait. Low sea level reduced or stopped the influx of warm water from the Pacific Ocean and substantially lowered the surface water temperature. This relation has been shown in several studies based on microfossils in piston cores that recovered the Quaternary sequence from the Japan Sea (Ujiié, 98; aiya et al., 976; Koizumi, 987). The present environmental conditions of the Japan Sea are not conducive for tropical nannofossil taxa such as Sphenolithus and discoasters (Haq and almgren, 98; Wei and Wise, 989), that require warm temperature and small seasonal fluctuation of temperature. Thus, the presence of these taxa in the lower middle iocene to upper lower iocene sediments of the Yamato Basin suggests a warm and stable surface-water condition, which agrees with the paleontologic record along the Japan Sea coasts of southwest and central Japan and the Korean Peninsula. Planktonic foraminifers, larger foraminifers, and mollusks suggest a tropical to subtropical climate from the latest early iocene to the early middle iocene (Saito, 96; Itoigawa, 978; Tsuchi, 98, 990; atoba, 984; hinzei, 986). This warm fauna was distributed along the Japan Sea coasts as far north as 44 N latitude off the Japanese Islands and 4 N off the Korean Peninsula, indicating a more equable environmental condition than at present. The warm as surface-water conditions of the Japan Sea from the late early iocene to early middle iocene suggest a greater influx of warm surface water from the tropical western Pacific Ocean. This is supported by atoba (984), whose conclusions, based on foraminifers, proposed a deep-water connection between the southern part of the Japan Sea and the Pacific Ocean during the middle iocene, and is also consistent with hough and Barg (987), who showed that the Tsushima Strait was deeper in the middle iocene and became shallower in the latest middle iocene to earliest late iocene. Beside a deeper Tsushima Strait, warm water from the Pacific Ocean probably entered the Japan Sea through epicontinental seaways (Tsuchi, 98; hinzei, 986; Iijima et al., 988), and was aided by a high eustatic sea level during the early middle iocene (Haq et al., 987). Iijima et al. (988) suggested the existence of a basin of upper to middle bathyal depth from Niigata to northeastern Noto Peninsula, probably connecting the Japan Sea with the Pacific Ocean. The environment of the Japan Sea changed in the late middle iocene (atoba, 984). During the rotation of the southwest and northeast Japan (Otofuji et al., 985a, 985b) when the sea level was relatively low (Haq et al., 987), the seaway to the south was closed, thus preventing the inflow of warm currents; the opening the Japan Sea to the north increased the influence of cold water (atoba, 984; hinzei, 986). The presence of a barren interval close to the of yclicargolithus floridanus (. a) precludes the confirmation of the change in surface-water condition in the late middle iocene based on nannofossils. The surface-water cooling, however, did not occur before. a, as indicated by the presence of Sphenolithus abies and Sphenolithus neoabies in two samples (7-794B-7-, -4 cm, and -7-) above the of. floridanus at Site 794. This age constraint for the change in water condition is supported by faunal and floral changes in the Japanese land sections (Saito, 96; Takayangi et al., 976). These authors reported a change from warm water biofacies to cool temperate and subarctic biofacies immediately above the Globorotalia fohsi fohsi Zone, a planktonic foraminifer zone, the top of which is dated at. a (Berggren et al., 985). The surface -water cooling in the Japan Sea was coeval to a global change in climate that is reflected in extensive growth of east ntarctic icesheets (Barker et al., 988). Sporadic occurrence of low-diversity nannofossil assemblages in the Holocene to uppermost Pliocene is consistent with strong environmental fluctuations that are based on variation of diatom and radiolarian assemblages, and alteration of dark and white color bands in the sediments (Tamaki, Pisciotto, llan, et al., 990; Ingle, Suyehiro, von Breymann, et al., 990; Tada et al., this volume). Sedimentary intervals containing nannofossils were probably deposited during warm surface-water conditions, when eustatic sea level was high. In the Pleistocene sediments of the northern part of the Japan Sea, the presence of Gephyrocapsa oceanica (Ellis, 975; this study), which is presently living within a temperature range of 8-9 (clntyre et al., 970; Honjo, 977), suggests a warmer surface -water temperature than at present. Within the present Oceanographic setting, a warming in the northern part of the Japan Sea could be obtained by a strong influx of warm water from the Tsushima urrent. ONLUSION. Nannofossils occur sporadically at all sites of Leg 7 in the Japan Sea and allow recognition of seven zones and two subzones in 80

11 LEOUS NNNOOSSIL BIOSTTIGPHY two sedimentary intervals, the upper one in the Holocene to uppermost Pliocene, and the lower one in upper lower iocene to lower middle iocene.. ssemblages are less diverse in the upper sedimentary interval, and more diverse with warm water nannofossil taxa in the lower sedimentary interval, indicating a warm and stable surface-water condition related to a greater influx of warm water into the Japan Sea from the Pacific Ocean.. Nannofossil assemblages are most diverse and best recorded at Site 797 in the Yamato Basin. This site also has the oldest nannofossil record of all sites, older than the O of alcidiscus macintyrei (5.7 a). 4. This study suggests that the Yamato Basin existed before the rotation of southwest Japan, and that the rotation was not related to the initial opening phase of the Japan Sea. KNOWLEDGENTS Peter H. oth, rancis H. Brown, ark V. ilewich, Wuchang Wei, John. Barron, John Welsh, and Hiromi Honda reviewed this paper. Their comments and criticisms are appreciated. ahima ahman and arge Gundry assisted with sample preparation. Samples were supplied by the U.S. National Science oundation through the assistance of the Ocean Drilling Program, Texas & University. This study was partly funded by U.S. Science dvisory ommittee. EEENES Backman, J., 980. iocene-pliocene nannofossils and sedimentation rates in the Hatton-ockall Basin, NE tlantic Ocean. Stockholm ontrib. Geol., 6:-9. Backman, J., Schneider, D.., io, D., and Okada, H., 990. Neogene low-latitude magnetostratigraphy from Site 70 and revised age estimates of iocene nannofossil events. In Duncan,.., Backman, J., Peterson, L., et al., Proc. ODP, Sci. esults, 5: ollege Station, TX (Ocean Drilling Program), Backman, J., and Shackleton, N. J., 98. Quantitative biochronology of Pliocene and early Pleistocene calcareous nannofossils from the tlantic, Indian and Pacific oceans. ar. icropaleontol., 8:4-70. Barker, P., Kennett, J. P., and Leg Shipboard Scientific Party, 988. Weddell Sea palaeoceanography: preliminary results of ODP Leg. Palaeogeogr., Palaeoclimatoi, Palaeocol, 67:75-0. Barron, J.., Keller, G., and Dunn, D.., 985a. multiple microfossil biochronology for the iocene. In Kennett, J. P. (Ed.), The iocene Ocean: Paleoceanography and Biogeography. em. Geol. Soc. m., 6:-6. Barron, J.., Nigrini,.., Pujos,., Saito, T., Theyer, E, Thomus, E., and Weinreich, N., 985b. Synthesis of biostratigraphy, central equatorial Pacific, Deep Sea Drilling Project Leg 85: refinement of Oligocene to Quaternary biochronology. In ayer, L., Theyer, E, Thomas, E., et al., Init. epts. DSDP, 85: Washington (U.S. Govt. Printing Office), Berggren, W.., ubry,. P., and Hamilton, N., 98. Neogene magnetobiostratigraphy of Deep Sea Drilling Project Site 56 (io Grande ise, South tlantic). In Barker, P. E, arlson,. L., and Johnson, D.. (Eds.), Init. epts. DSDP, 7: Washington (U.S. Govt. Printing Office), Berggren, W.., Kent, D. V, and Van ouvering, J.., 985. The Neogene: Part. Neogene geochronology and chronostratigraphy. In Snelling, N. J. (Ed.), The hronology of the Geological ecord. em. Geol. Soc. m., 0:-60. Bukry, D., 97. Discoaster evolutionary trends. icropaleontology, 7:4-5., 97. Low-latitude coccolith biostratigraphic zonation. In Edgar, N. T, Saunders, J. B., et al., Init. epts. DSDP, 5: Washington (U.S. Govt. Printing Office), , 978. Biostratigraphy of enozoic marine sediment by calcareous nannofossils. icropaleontology, 4: Burns, D.., 975. Distribution, abundance, and preservation of nannofossils in Eocene to ecent ntarctic sediments. N.Z. J. Geol. Geophys., 8: elaya,., and cabe,., 987. Kinematic model for the opening of the Sea of Japan and the bending of the Japanese islands. Geology, 5:5-57. hinzei, K., 986. aunal succession and geographic distribution of Neogene molluscan faunas in Japan. Paleontol. Soc. Jpn., 9:7-. hough, S. K., and Barg, E., 987. Tectonic history of Illeung basin margin, East Sea (Sea of Japan). Geology, 5:45^-8. Ellis,. H., 975. alcareous nannofossil biostratigraphy Leg, DSDP. In Karig, D. E., Ingle, J., Jr., et al., Init. epts. DSDP, : Washington (U.S. Govt. Printing Office), Gartner, S., 969. orrelation of Neogene planktonic foraminifer and calcareous nannofossils zones. Trans. Gulf oast ssoc. Geol. Soc, 9: , 977. alcareous nannofossil biostratigraphy and revised zonation of the Pleistocene. ar. icropaleontol., :-5. Haq, B. U., Hardenbol, J., and Vail, P.., 987. hronology of fluctuating sea levels since the Triassic. Science, 5: Haq, B. U., and almgren, B.., 98. Potential of calcareous nannoplankton in paleoenvironmental interpretations a case study of the iocene of the tlantic Ocean. Stockholm ontrib. Geol., 7: Hay, W. W., ohler, H., oth, P. H., Schmidt,.., and Boudreaux, J. E., 967. alcareous nannoplankton zonation of the enozoic of the Gulf oast and aribbean-ntillean area, and transoceanic correlation. Trans. Gulf oast ssoc. Geol. Soc, 7:48^-80. Hidaka, K., 966. Japan Sea. In airbridge,. W. (Ed.), The Encyclopedia of Oceanography. Encyclopedia of Earth Sci. Ser. (Vol. ): New York (einhold), 47^4. Honjo, S., 977. Biogeography and provincialism of living coccolithophorids in the Pacific Ocean. In amsey,.t.s. (Ed.), Oceanic icropaleontology: San Diego (cademic), Honkura, Y, Okubo, Y, Nagaya, K., akino,., and Oshima, S., 99. magnetic anomaly map in the Japanese egion with special reference to tectonic implications. / Geomagn. Geoelectr., 4:7-76. Iijima,., Tada,., and Watanabe, Y, 988. Developments of Neogene sedimentary basins in the northeastern Honshu rc with emphasis on iocene siliceous deposits. J. ac. Sci., Univ. Tokyo, :47^-46. Ingle, J., Jr., Suyehiro, K., von Breymann,. T, et al., 990. Proc. ODP, Init. epts., 8: ollege Station, TX (Ocean Drilling Program). Itoigawa, J., 978. Evidence of subtropical environments in the iocene of Japan. Bull. izunami ossil us., 5:7-. Karig, D. E., Ingle, J., Jr., et al., 975. Init. epts. DSDP, : Washington (U.S. Govt. Printing Office). Kimura, G., and Tamaki, K., 986. ollision, rotation, and back-arc spreading in the region of the Okhotsk and Japan seas. Tectonics, 5: Knüttel, S., 986. alcareous nannofossil biostratigraphy of the East Pacific ise, Deep Sea Drilling Project Leg 9: evidence for downslope transport of sediments. In Leinen,., ea, D. K., et al., Init. epts. DSDP, 9: Washington (U.S. Govt. Printing Office), Kobayashi, K., 985. Sea of Japan and Okinawa Trough. In Nairn,.E.., Stehli,. G., and Uyeda, S. (Eds.), The Ocean Basins and argins (Vol. 7): The Pacific Ocean: New York (Plenum), 49-^50. Koizumi, I., 987. Pulses of Tsushima urrent during the Holocene. Quat. es. Tokyo, 6:-5. (in Japanese with English abstract) Lallemand, S., and Jolivet, L., 986. Japan Sea: a pull-apart basin? Earth Planet. Sci. Lett., 76: Leg 7 Shipboard Scientific Party, 989. Exploring the Japan Sea. Geotimes, 4:9-. aiya, S., Saito, T, and Sato, T, 976. Late enozoic planktonic biostratigraphy of northwest Pacific sedimentary sequences. In Takayanagi, Y, and Saito, T. (Eds.), Progress in icropaleontology: New York (icropaleontology Press), 95^. artini, E., 97. Standard Tertiary calcareous nannoplankton zonation. In arinacci,. (Ed.), Proc. nd Planktonic onf. oma, ome (Ed. Tecnosci.), : atoba, Y, 984. Paleoenvironment of the Sea of Japan. In Oertli, H. J. (Ed.), Benthos '8, nd Int. Symp. Benthic oraminifera, clntyre,., Be, W. H., and oche,. B., 970. odern Pacific occolithophorida: apaleontological thermometer. Trans. N.Y.cad. Sci., :70-7. Okada, H., and Bukry, D., 980. Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (Bukry, 97; 975). ar. icropaleontol., 5:-5. Otofuji, Y, Hayashida,., and Torii,., 985. When was the Japan Sea opened? Paleomagnetic evidence from Southwest Japan. In Nasu, N., Uyeda, S., Kushiro, I., Kobayashi, K., and Kagami, H. (Eds.), ormations of ctive Ocean argins: Tokyo (Terra Publ.), Otofuji, Y, and atsuda, T, 987. mount of clockwise rotation of Southwest Japan fan shape opening of the southwestern part of the Japan Sea. Earth Planet. Sci. Lett., 85:89-0. Otofuji, Y, atsuda, T, and Nohda, S., 985. Paleomagnetic evidence for the iocene counter-clockwise rotation of Northeast Japan rifting process of the Japan rc. Earth Planet. Sci. Lett., 75:

12 . HN Perch-Nielsen, K., 985. enozoic calcareous nannofossils. In Bolli, H.., Saunders, J. B., and Perch-Nielsen, K. (Eds.), Plankton Stratigraphy: ambridge (ambridge Univ. Press), ahman,., in press. Late Neogene calcareous nannofossil biostratigraphy of the Blake Outer idge, DSDP Site 5, northwestern tlantic Ocean. N. Jb. Geol. Palaont. ahman,., and oth, P. H., 989. Late Neogene calcareous nannofossil biostratigraphy of the Gulf of den region. ar. icropaleontol, 5:-7. io, D., ornaciari, E., and affi, I., 990. Late Oligocene through early Pleistocene calcareous nannofossils from western equatorial Indian Ocean (ODP Leg 5). In Duncan,.., Backman, J., Peterson, L., et al., Proc. ODP, Sci. esults, 5: ollege Station, TX (Ocean Drilling Program), oche,. B., clntyre,., and Imbrie, J., 975. Quantitative paleoceanography of the Late Pleistocene-Holocene North tlantic: coccolith evidence. In Saito, T., and Burckle, L. H. (Eds.), Late Neogene Epoch Boundaries: New York (icropaleontology Press), oth, P. H., 974. alcareous nannofossils from the northwestern Indian Ocean, Leg 4, Deep Sea Drilling Project. In isher,. L., Bunce, E. T, et al., Init. epts. DSDP, 4: Washington (U.S. Govt. Printing Office), oth, P. H., and oulbourn, W. T., 98. loral and solution patterns of coccoliths in surface sediments of the North Pacific. ar. icropaleontol, 7:-5. yan, W.B.., ita,. B., awson,. D., Burckle, L. H., and Saito, T., 974. paleomagnetic assignment of Neogene stage boundaries and the development of isochronous datum planes between the editerranean, the Pacific and Indian oceans in order to investigate the response of the world ocean to the editerranean "Salinity risis." iv. Ital. Paleontol, 80: Saito, T., 96. iocene planktonic foraminifera from Honshu, Japan. 5c/. ep. Tohoko Univ., Ser., 5:-8. Takayama, T., and Sato, T., 987. occolith biostratigraphy of the North tlantic Ocean, Deep Sea Drilling Project Leg 94. In uddiman, W.., Kidd,. B., et al., Init. epts. DSDP, 94: Washington (U.S. Govt. Printing Office), Takayanagi, Y, Takayama, T, Sakai, T, Oda,., and Kitazato, H., 976. icrobiostratigraphy of some iddle iocene sequences in northern Japan. In Takayanagi, Y, and Saito, T. (Eds.), Progress in icropaleontology: New York (icropaleontology Press), Tamaki, K., 988. Geological structure of the Japan Sea and its tectonic implications. hishitsu hosasho Geppo, 9: Tamaki, K., Pisciotto, K., llan, J., et al., 990. Proc. ODP, Init. epts., 7: ollege Station, TX (Ocean Drilling Program). Tatsumi, Y, Otofuji, Y, atsuda, T., and Nohda, S., 989. Opening of the Japan back-arc basin by asthenospheric injection. Tectonophysics, 66:7-9. Theodoridis, S., 984. alcareous nannofossil biozonation of the iocene and revision of the helicoliths and discoasters. Utrecht icropaleontol. Bull,. Thierstein, H.., Geitzenauer, K.., olfino, B., and Shackleton, N. J., 977. Global synchroneity of late Quaternary coccolith datum levels: validation by oxygen isotopes. Geology, 5: Tsuchi,. (Ed.), 98. Neogene of Japan Its Biostratigraphy and hronology: Shizuoka, Japan (Kurofune Printing)., 990. Neogene events in Japan and the Pacific. Palaeogeogr., Palaeoclimatol, Palaeoecol, 77: Ujiié, H., 98. Geological history of the Sea of Japan: problems from standpoints of sediments and microfossils. In Hoshino,., and Shibasaki, T. (Eds.), Geology of Japan Sea: Japan (Tokai Univ. Press), 77^-8. (in Japanese) Ujiié, H., and Ichikura,., 97. Holocene to uppermost Pleistocene planktonic foraminifers in a piston core from off San'in district, Sea of Japan. Trans. Proc. Palaeontol Soc. Jpn., 9:7-50. Wei, W, and Wise, S. W., Jr., 989. Paleogene calcareous nannofossil magnetobiochronology: results from South tlantic DSDP Site 56. ar. icropaleontol, 4:9-5. Date of initial receipt: 8 arch 99 Date of acceptance: July 99 s 7/8B- PPENDIX Nannofossil taxa recognized in the present study are arranged below, alphabetically by the genus and species epithets. Each species epithet is followed by the original author(s), by possible subsequent authors who proposed the preferred combinations, and by some remarks if necessary. or taxonomic references of the listed species see Perch-Nielsen (985) and ahman and oth (989). Braarudosphaera bigelowii (Gran and Braarud, 95, p. 89, ig. 67) Deflandre (947, p. 49, igs. -5). alcidiscus leptoporus (urray and Blackmann, 898, p. 40-4, 49) Loeblich and Tappan (978, p. 9). emarks: Specimens larger than 8 µm in size of this species are observed in the lower Pleistocene. They differ from alcidiscus macintyrei by having a lower number (< 0) of elements. alcidiscus macintyrei (Bukry and Bramlette, 969, p. -, PI., igs. -) Loeblich and Tappan (978, p. 9). emarks: This group includes subelliptical forms of alcidiscus macintyrei (= alcidiscus premacintyrei Theodoridis, 984, p. 8-8, PI., igs. -), few specimens of which were observed in the iocene. In the original description of. macintyrei the number of elements was given as about 40 and the size range of 8- µm. The size cutoff alone cannot differentiate. macintyrei from alcidiscus leptoporus because large specimens of the latter species (some exceeding 9.5 µm) were present near the of. macintyrei in the Gulf of den area (ahman and oth, 989). Thus, the number of elements has to be considered when distinguishing the large variety of. leptoporus from. macintyrei. The central opening of. macintyrei is larger in the iocene than in the Pliocene forms. atinaster mexicanus Bukry (97, p. 50, PI., igs. 7-9). occolithus pelagicus (Wallich, 877, p. 48, igs. -, 5, -) Schiller (90, p. 46, igs. -4). emarks: There are variations in the overall coccolith size, and relative size and structure of the central area. The central area may have a small opening, be spanned by a bridge, or be filled. occolithus miopelagicus Bukry (97, p. 0, PI., igs. 6-7). occolithus sp. emarks: This species has a rim similar to occolithus pelagicus and has a granular cross at the center. The bars of the cross are very short and are parallel to the major axes of the coccolith. occolithus streckeri Takayama and Sato (987, p. 690, PI., igs. 4a-4b, PI., igs. -0, PL 8, ig. ). emark: This species differs from occolithus pelagicus by having a large central opening, which is traversed by a delicate bridge paralleling the short axis of the coccolith. yclicargolithus floridanus (oth and Hay in Hay et al., 967, p. 445, PI. 6, igs. -4) Bukry (97, p. -). emark: yclicargolithus floridanus varies in outline from circular to broadly elliptical. yclolithela rotula (Kamptner, 956, p. 7) Haq and Berggren (978, p. 9, PI. l,igs. 5-6). Dictyococcites perplexus Burns (975, p. 594, igs., 9, 0). emarks: Elliptical placolith with a closed central area that appears very bright under crossed nicols. This species shows a straight extinction band, which occupies at least one half of the length of the elliptical central area, parallel to the major axis. Dictyococcites antarcticus Haq (976, p. 56, PI., igs. -, 7-8) is very similar to Dictyococcites perplexus, and is considered a junior synonym. Dictyococcites productus (Kamptner, 96, p. 7, PI. 8, igs. 4, 44) Backman (980, p. 49, PI. 4, igs. -, 6-7). emarks: Small placolith (usually range between and 4 µm) with a closed or nearly closed elliptical central area, and a median furrow. This species differs from Dictyococcites perplexus by having a smaller size and a less well-developed central furrow. Discoaster exilis artini and Bramlette (96, p. 85, PI. 04, igs. -). emarks: This species is characterized by slender rays with faint ridges slightly off-centered from the median of the rays, and a constriction below the bifurcations that gives rise to two short branches. oth (974) found this species to be restricted to the middle iocene in the northwestern Indian Ocean. 8

13 LEOUS NNNOOSSIL BIOSTTIGPHY Discoaster adamanteus Bramlette and Wilcoxon (967, p. 08, PL 7, ig. 6). Discoaster brouweri Tan (97, p. 0, text-igs. 8a-8b) emend. Bramlette and iedel (954, p. 40, PL 9, ig., text-igs. a-b). Discoaster challenged Bramlette and iedel (954, p. 40, PL 9, ig. 0). Discoaster deflandrei Bramlette and iedel (954, p. 99, PL 9, ig. 6, text-igs, la-lc). Discoaster kugleri artini and Bramlette (96, p. 85, PL 0, igs. -). Discoaster sp. emark: This category includes all the discoasters that are not identifiable to the species level due to strong etching, fragmentation, or overgrowth. Discoaster stellulus Gartner (967, p., PL 4, igs. -). Discoaster variabilis artini and Bramlette (96, p. 854, PL 04, igs. 4-9). Emiliania huxleyi (Lohman, 90, p. 9-0, PL 4, igs. -9, PL 6, ig. 69) Hay and ohler in Hay et al. (967). Gephyrocapsa caribbeanica Boudreaux and Hay in Hay et al. (967, p. 447, PL -, igs. -4). emarks: species of Gephyrocapsa about µm or larger in size, with a bridge angle ranging from 50 to 85 with the short axis of the coccolith. This species has a smaller central opening than Gephyrocapsa oceanica. The following species, only recognizable in the electron microscope were most likely included in G. caribbeanica: Gephyrocapsa margerelii Bréhéret, Gephyrocapsa muellerae Bréhéret, Gephyrocapsa lumina Bukry, and some Gephyrocapsa rota Samtleben. Gephyrocapsa oceanica Kamptner (94, p. 4-49). emarks: species of Gephyrocapsa about µm or larger in size, with a bridge angle ranging from 5 to 45 with the short axis of the coccolith, and a relatively large central opening. The following species that can only be identified in the electron microscope were probably included in this taxon: Gephyrocapsa omega Bukry (=Gephyrocapsa parallela Hay and Beaudry). Gephyrocapsa small emarks: ny Gephyrocapsa species smaller than µm. small percentage of minute Gephyrocapsa oceanica and Gephyrocapsa caribbeanica may have been put into the small Gephyrocapsa group because it is not possible to consistently determine the angle of the bridge in specimens less than µm in size due to limitation of resolution in light microscopy. Helicosphaera ampliaperta Bramlette and Wilcoxon (967, p. 05, PL 6, igs. -4). emarks: This species has a broadly elliptical shape with a biconvex central opening. It differs from Helicosphaera carteri, Helicosphaera recta and Helicosphaera perch-nielseniae by the lack of a bridge, which divides the central opening into two. Helicosphaera carteri (Wallich, 877, p. 48, PL 7, igs. -4, 6-7, 7) Kamptner (954, p., 7, igs. 7-9). Helicosphaera perch-nielseniae (Haq, 97, p. 6, PL 0, igs. 5-7) Jafar and artini (975, p. 9). emarks: The terminal flange of this species forms an acute angle in axial view. Helicosphaera perch-nielseniae differs from Helicosphaera recta and Helicosphaera obliqua by the lack of openings in the central area and the acute angle of the terminal flange. The latter two species have a terminal flange with a rectangular outline. Pontosphaera discopora Schiller (95, p., PL, ig. 4) emend. Burns (97, p. 5, PL, ig. 6). Pontosphaera distincta (Bramlette and Sullivan, 96, p. 4, PL, igs. 8a-8b, 9a-9c) Burns (97, p. 5, PL, igs. -). Pontosphaera multipora (Kamptner, 948, p. 5, PL, ig. 9) oth (970, p. 860) emend. Burns (97, p. 5-5). Pontosphaera pacifica Burns (97, p. 50-5, PL, igs. 4-5) Pontosphaera sp. emark: This category includes all pontosphaerid rims without central structure. Pseudoemiliania lacunosa (Kamptner, 96, p. 7, PL 9, ig. 50) Gartner (969, p. 598, PL, igs. 9-0). emarks: Bukry (97) split Pseudoemiliania lacunosa into two species, Emiliania annula and Emiliania ovata, having a circular shape and oval shape, respectively. These two varieties were not separated because they have similar stratigraphic ranges, and a considerable intergradation in morphology (ahman and oth, 989). ollowing Gartner (977) and Backman (980), P. lacunosa is considered a valid name and the two varieties, E. annula and E. ovata, are considered conspecific and subjective junior synonyms of P. lacunosa. eticulofenestra pseudoumbilica (Gartner, 967, p. 4, PL 6) Gartner (969, p. 598, PL, ig. 4). emarks: species of eticulofenestra larger than 6 µm. The differentiation of this species under the light microscope from other eticulofenestra in the Pliocene is based on size. This criterion is difficult to use because of specimens that are transitional in size. eticulofenestra pseudoumbilica typically consists of forms ranging in size from 5 to 0 µm, with the majority of forms ranging from 6 to 8 µm (Backman and Shackleton, 98). eticulofenestra small emarks: Backman (980) showed that it is impossible to distinguish Neogene reticulofenestrids at the species level even with multivariate statistical techniques. However, he clearly showed two size groups, one larger and the other smaller than 5 µm. The latter group includes such species as eticulofenestra haqii Backman, eticulofenestra minutula (Gartner) Haq and Berggren, eticulofenestra minuta oth, and is here referred to as "eticulofenestra small." The reticulofenestrids that are equal to or larger than 6 µm are assigned to eticulofenestra pseudoumbilica. Sphenolithus abies Deflandre in Deflandre and ert (954, p. 64, PL 0, igs. -4). Sphenolithus compactus Backman (980, p , PL, igs. 0-). emarks: Sphenolithus compactus differs from Sphenolithus neoabies, Sphenolithus abies, and Sphenolithus moriformis by having parallel sides in side view, and the lack of a long apical spine. S. compactus has a smooth distal outline under the light microscope. This species is comparable in size to S. neoabies, but it is much smaller than 5. abies and S. moriformis. Sphenolithus heteromorphus Deflandre in Deflandre and ert (954, p , igs. -); Bramlette and Wilcoxon (967, p. -4, PL, igs. 6-9). emarks: Sphenolithus heteromorphus is characterized by a relatively long robust apical spine that is strongly birefringent at the 45 position and dark at the parallel position under crossed nicols. It differs from Sphenolithus abies and Sphenolithus moriformis by a long robust apical spine and from Sphenolithus belemnos Bramlette and Wilcoxon by the taller and less conical proximal column and a narrower apical spine. S. heteromorphus is a solution-resistant species, and is often identifiable even in poorly preserved assemblages. Sphenolithus moriformis (Brönnimann and Stradner, 960, p. 68, igs. -6) Bramlette and Wilcoxon (967a, p. 4-6, PL, igs. -6). emarks: Sphenolithus moriformis is more robust, less conical, and more birefringent than Sphenolithus abies. It differs from Sphenolithus neoabies by having a larger size and stronger birefringence. Sphenolithus neoabies Bukry and Bramlette (969, p. 40, PL, igs. 9-). emarks: This species of Sphenolithus is relatively small, and lacks the prominent apical spine typical of most species of the genus. Sphenolithus umbrellus (Bukry, 97, p. 50, PL, igs. 0-) ubry and Knüttel in Knüttel (986, p. 79, PL 5, igs. -, 5-0). emarks: Sphenolithus umbrellus was provisionally assigned to the genus atinaster, which is unacceptable for the reasons stated in Knüttel (986). This species is similar to other species of Sphenolithus in terms of morphological and optical characteristics, and differs from them only by the lack of an apical spine. S. umbrellus ranges from the lowermost part of the upper iocene (ahman and oth, 989) to upper Oligocene (Bukry, 97; Knüttel, 986). Syracosphaera histrica Kamptner (94, p. 84, 04, PL 6, igs ). emark: This central area of Syracosphaera histrica can be distinguished from all other species of Syracosphaera by the presence of a characteristic hourglass-shaped extinction pattern. Thoracosphaera heimii (Lohmann, 99, p. 7, ig. 9) Kamptner (954, p. 40-4, igs. 4^). Thoracosphaera saxea Stradner (96, p. 84, ig. 7). Triquetrorhabdulus rugosus Bramlette and Wilcoxon (967, p. 8, PL 9, igs. 7-8). Umbilicosphaera sibogae (Weber van Bosse, 90, p. 7, 40, PL 7, igs. -) Gaarder (970, p. -6, igs. 9c-9d). 8