Introductory perspective on the COREF Project

Island Arc (2006) 15, 393–406

© 2006 The AuthorsJournal compilation © 2006 Blackwell Publishing Asia Pty Ltd

doi:10.1111/j.1440-1738.2006.00537.x

Blackwell Publishing AsiaMelbourne, AustraliaIARIsland Arc1038-48712006 Blackwell Publishing Asia Pty LtdSeptember 2006154393406Thematic ArticleIntroductory perspective on the COREF ProjectY. Iryu

et al.

*Correspondence.

Received 1 December 2005; accepted for publication 11 April 2006.

Thematic ArticleIntroductory perspective on the COREF Project

YASUFUMI IRYU,1,* HIROKI MATSUDA,2 HIDEAKI MACHIYAMA,3 WERNER E. PILLER,4 TERRENCE M. QUINN5 AND MARIA MUTTI6

1Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Aobayama, Aoba-ku, Sendai 980-8578, Japan (email: [email protected]), 2Department of Earth Sciences, Faculty of Science, Kumamoto

University, Kurokami 2-39-1, Kumamoto 860-8555, Japan, 3Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science & Technology (JAMSTEC), Monobe-otsu 200, Nangoku, Kochi 783-8502, Japan, 4Institute for Earth Science (Geology and Paleontology), University of Graz, Heinrichstrasse 26, Graz 8010, Austria, 5John A.

and Katherine G. Jackson School of Geosciences, Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin TX 78712-0254, USA, 6Institut für Geowissenschaften, Universität

Postdam, PF 601553, D-14415 Potsdam, Germany

Abstract Coral reefs are tropic to subtropic, coastal ecosystems comprising very diverseorganisms. Late Quaternary reef deposits are fossil archives of environmental, tectonicand eustatic variations that can be used to reconstruct the paleoclimatic and paleoceano-graphic history of the tropic surface oceans. Reefs located at the latitudinal limits of coral-reef ecosystems (i.e. those at coral-reef fronts) are particularly sensitive to environmentalchanges – especially those associated with glacial–interglacial changes in climate andsealevel. We propose a land and ocean scientific drilling campaign in the Ryukyu Islands(the Ryukyus) in the northwestern Pacific Ocean to investigate the dynamic response ofthe corals and coral-reef ecosystems in this region to Late Quaternary climate and sealevelchange. Such a drilling campaign, which we call the COREF (coral-reef front) Project, willallow the following three major questions to be evaluated: (i) What are the nature, mag-nitude and driving mechanisms of coral-reef front migration in the Ryukyus? (ii) What isthe ecosystem response of coral reefs in the Ryukyus to Quaternary climate changes? (iii)What is the role of coral reefs in the global carbon cycle? Subsidiary objectives include (i)the timing of coral-reef initiation in the Ryukyus and its causes; (ii) the position of theKuroshio current during glacial periods and its effects on coral-reef formation; and (iii)early carbonate diagenetic responses as a function of compounded variations in climate,eustacy and depositional mineralogies (subtropic aragonitic to warm-temperate calcitic).The geographic, climatic and oceanographic settings of the Ryukyu Islands provide anideal natural laboratory to address each of these research questions.

Key words: coral, Integrated Ocean Drilling Program, International Continental ScientificDrilling Program, limestone, Quaternary, reef, Ryukyu Group, Ryukyu Islands, sealevel.

INTRODUCTION

Coral reefs are tropic to subtropic coastal ecosys-tems in which very diverse organisms coexist.Their community structures and geographic andlocal distributions are highly controlled by various

environmental factors, such as sea-surface tem-perature (SST), salinity, nutrient level and ter-rigenous input. Thus, their ancient counterparts,reef deposits, provide important, high-resolutionrecords of geoscientific events in tropic to sub-tropic shallow waters, such as vertical and lateraltectonic movements, sealevel fluctuations, paleocli-matic changes and paleoceanographic variations.The reefs at relatively higher latitudes within reefprovinces are particularly worth investigating

394 Y. Iryu et al.


because they are thought to have been more sen-sitive to the environmental changes than those inlow-latitude areas.

The Quaternary coral-reef deposits of theRyukyu Islands (the Ryukyus) formed at the limitof coral-reef formation, the ‘coral-reef front’ in thepresent paper, of the northwestern Pacific. As aresult, these rocks are ideal for the testing of somekey hypotheses regarding the evolution of coral-reef fronts in response to Quaternary climatic andoceanographic changes. The COREF (coral-reeffront) Project is designed to perform ocean andland drilling into these Quaternary reef depositsin different settings in the Ryukyus. The majorscientific objectives are to examine the followingquestions.1. If coral-reef fronts migrated to higher and

lower latitudes in response to Quaternary glo-bal warming and cooling and associated rapid,cyclic changes in climate, sealevel and oceano-graphic conditions, then how do those variousdrivers interact to determine the nature andmagnitude of the front’s movement?

2. Examinations of Pleistocene coral communitiesin the low latitude parts of the coral-reef prov-ince indicate that the taxonomic compositionremained constant throughout multiple epi-sodes of Quaternary climate change. In theCentral Ryukyus, however, some corals charac-teristic of warm-temperate regions of mainlandJapan are found in limestones deposited duringPleistocene glacial episodes. Does this meanthat the coral-reef ecosystem can in fact exhibita varied response to millennial and glacial–interglacial climatic changes?

3. It has been suggested that Quaternary coralreefs may have contributed to abrupt climaticchanges through a positive feedback mecha-nism. That is, carbonate production rates ofreefs may have been accelerated during trans-gression because atmospheric CO2 concentra-tions increased as a result of the carbonateprecipitation in the sea, resulting in a rise inatmospheric temperature. Quaternary carbon-ate accumulation rates in coral reefs are a goodmeans to test this hypothesis, with ratesderived from the coral-reef front previouslyunexplored.The Ryukyu Islands, because of their geo-

graphic, climatic and oceanographic setting, areone of the best locations to test these hypotheses.To clarify the stratigraphic succession and litho-facies distribution, it is crucial to sample bothhighstand and lowstand reefs ranging from

approximately 200 m in elevation on islands downto approximately 150 m (or more) in depth onshelves and shelf slopes. It is necessary to combineland drilling (International Continental ScientificDrilling Program, ICDP) and ocean drilling (Inte-grated Ocean Drilling Program, IODP) to com-plete these scientific objectives.

This paper gives an introductory perspectiveon the COREF project, organized in two parts.The geological setting of the Ryukyu Islands isexplained in the first part and the scientific objec-tives of the combined ocean and land drilling aredescribed in the second part.

GEOLOGICAL SETTING OF THE RYUKYU ISLANDS

GENERAL SETTING

The Ryukyu Islands are situated to the southwestof mainland Japan. They encompass several tensof islands and islets, extending from Tane-ga-shima (30°44′N, 131°0′E) in the northeast to Yona-guni-jima (24°27′N, 123°0′E) in the southwest(Fig. 1). These islands are arranged in a curvedrow called the Ryukyu Arc, bounded by the EastChina Sea to the northwest and by the PacificOcean to the southeast. It is an active island arcgenerated by subduction of the Philippine SeaPlate along the Ryukyu Trench beneath the Eur-asian Plate. The Ryukyu Islands are geographi-cally divided into three regions (the North, Centraland South Ryukyus) by two major left-lateralfaults underlying the Tokara Gap and the KeramaGap, respectively (Fig. 1). The Okinawa Trough isa back-arc basin, which separates the arc from theEast China Sea Shelf.

The frontal arc consists of sedimentary andmetamorphic rocks ranging in age from Paleozoicto early Pleistocene; the volcanic arc is locatedbehind it. The sedimentary and metamorphicrocks in the Ryukyus are characterized by a par-allel arrangement of tectonic zones, each correlat-ing to tectonic/geological belts in mainland Japan(Konishi 1965). They are, from back-arc basin totrench: the Koshikijima Belt consisting of Creta-ceous to Paleocene shallow marine and terrestrialdeposits; the Ishigaki Belt of metamorphic rocks;the Motobu Belt of Jurassic to Early Cretaceousaccretionary complex; the Kunigami Belt of Cre-taceous to Eocene trench fill deposits; the Shima-jiri Belt of upper Miocene to lowest Pleistoceneslope to forearc basin deposits; and the KumageBelt of Eocene to Oligocene trench fill depositsoverlain by Miocene shallow-water sediments.

Introductory perspective on the COREF Project 395


The Neogene strata of the Ryukyus comprisethe Yaeyama and Shimajiri Groups, the ChinenFormation and the Ryukyu Group, in ascendingorder. The Yaeyama Group is Miocene in age andconsists mainly of sandstone associated with shale,limestone conglomerate and much less commoncoal beds. The group is exposed on Yonaguni-jima,Iriomote-jima and Kohama-jima, and distributedbeneath the shelf off Miyako-jima (Tsuburaya &Sato 1985). Analysis of the heavy mineral compo-nents indicates that the terrigenous material of theYaeyama Group was derived from the EurasianContinent (Saitoh & Masuda 2004). The ShimajiriGroup ranges in age from late Miocene to earliestPleistocene and is composed mainly of siltstoneand sandstone accompanied by tuff and much lesscommon volcanic/volcaniclastic rocks. It crops outalong the eastern periphery of the Ryukyu Arc(Kikai-jima, southern Okinawa-jima and Miyako-jima) and is considered to be slope to forearc basindeposits. However, the sediments that correlate tothis group are distributed extensively in the Oki-nawa Trough and on the eastern margin of theEast China Sea Shelf as well as around the RyukyuIslands.

The Chinen Formation is composed of a lowercalcareous siltstone and an upper sandy limestone(Nakagawa et al. 2001) that correlates to the tran-sitional lithofacies between siliciclastic slope to

forearc basin deposits (the Shimajiri Group) andcoral-reef carbonates (the Ryukyu Group). Thisformation is found on the periphery of southernOkinawa-jima (Chinen and the Katsuren Penin-sula) and adjacent small islets, such as Yabuchi-jima, Henza-jima, Miyagi-jima, Hamahiga-jima,Ikei-jima and Tsuken-jima. Calcareous nannofossilbiostratigraphy indicates that the Chinen Forma-tion correlates to the Late Pliocene to Early Pleis-tocene between 1.21 and 2.09 Ma (Nakagawa et al.2001; Sato et al. 2004; Odawara et al. 2005a).

QUATERNARY REEF COMPLEX

Pleistocene reef deposits and their associatedmarine and non-marine siliciclastic sediments aredistributed over most of the islands of the Centraland South Ryukyus, where the deposits reach upto approximately 200 m in elevation (Fig. 2). Thesedeposits have been called the Riukiu Limestone(Yabe & Hanzawa 1930) or the Ryukyu Group(MacNeil 1960). Stratigraphic and sedimentologi-cal studies of the Ryukyu Group since the middleof the 1970s have shown that the group can besubdivided into several units, each of which com-prises reef-complex deposits that formed during asingle sealevel change (Nakamori 1986; Nakamoriet al. 1995; Iryu et al. 1998). Each individual unitis defined as the sequence that initiated during

Fig. 1 Schematic figure showing pale-oceanographic conditions during glacial peri-ods (low stand periods) in the Ryukyu Islands.Extensive areas of the present East China SeaShelf emerged (light gray), which causedincreased runoff (dark blue open arrows) thatin turn generated a turbid and low-salinitymarine environment around the Ryukyus. It isalso possible that the Kuroshio current, whichcurrently flows to the west of the RyukyuIslands (pink pattern), may have changed itsstreamline to the east (large light blue arrow).These environmental changes should haveresulted in a southward migration of the ‘coral-reef front’ (the northern limit of coral-reefformation).

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396 Y. Iryu et al.


the lowstand, developed through the subsequenttransgression and highstand, and terminated inthe regression. Thus, within one stratigraphiccolumn, the unit consists exclusively of corallimestone at proximal sites and of rhodolith,Cycloclypeus–Operculina, and poorly sorteddetrital limestones representing deeper waterenvironments at distal sites. Between the proximaland distal sites, a unit is composed of shallow-water coral limestone grading upward into deeper-water limestones, which are in turn overlain bycoral limestone. The uppermost coral limestone isnot observed in most cases. Spatially, conglomer-ate and coral, rhodolith, and poorly sorted detritallimestones are arranged from proximal (inland) todistal (coastal) parts of the islands.

Coral reefs started forming at 1.45–1.65 Ma inthe Ryukyus. Reef-complex deposits accompaniedby siliciclastics, which accumulated in a periodbetween 0.85 and 1.45–1.65 Ma, extend on theMotobu Peninsula, northern Okinawa-jima, andneighboring Ie-jima (Unit 1 of the Kourijima For-mation, Yamamoto et al. 2003, 2005; Takeuchi et al.2006). The coeval carbonate deposits are distrib-

uted on southern Okinawa-jima; these include theReddish Limestone (Kaneko 1994) and the lowerpart of the Naha Formation. The former correlatesto the Early Pleistocene between 1.21 and 1.65 Maand the latter is between 1.21 and 1.45 Ma (Kaneko& Ito 1995; Odawara et al. 2005a). The lower partof the Ryukyu Group on Irabu-jima ranges in agefrom 0.90 to 1.45 Ma (Sagawa et al. 2001).Although their geological ages have not beendetermined, the diagenetically altered limestonesthat underlie the main body of the Ryukyu Groupin the Yomitan area, Okinawa-jima (Iramina For-mation), on Yoron-jima (Mugiya and Ugachi For-mations), and on Kume-jima (Oha Limestone andNakandakari Formation) may be coeval with thereef-complex deposits described above (Odawara& Iryu 1999; Ehara et al. 2001; Odawara et al.2005b).

In contrast to the limited occurrence of the olderreef-complex deposits, those formed in latestEarly to Middle Pleistocene time between 0.41 and0.85 Ma occur on many islands of the Central andSouth Ryukyus. These deposits are thicker (up to∼100 m) than the older limestones, reach up to

Fig. 2 Lithostratigraphic succession of Plio-Pleistocene deposits on the Ryukyu Islands. The number of stratigraphic units developed during lowstand-to-highstand cycles is noted for each section as well as the nature of any available age data (Huang 1966; Nakagawa 1969; Koba 1980; Nakamori 1982;Omura 1982, 1984, 1988; Koba et al. 1985; Omura et al. 1994; Kaneko & Ito 1995; Odawara & Iryu 1999; Ehara et al. 2001; Nakagawa et al. 2001; Sagawaet al. 2001; Matsui et al. 2002; Oshimizu & Iryu 2002; Yamada & Matsuda 2001, 2002a, 2002b; Jiju 2003; Yamada et al. 2003, 2004; Mizota & Matsuda2004; Sato et al. 2004; Muraoka et al. 2005; Odawara et al. 2005a, 2005b; Yamamoto et al. 2005).

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-jima26–28Toku-no-shima24,25

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1) Omura et al. (1994)2) Yamada and Matsuda (2001)3) Omura (1984)4) Koba (1980)5) Koba et al. (1985)6) Yamada and Matsuda (2002a)7) Yamada et al. (2004)8) Sagawa et al. (2001)9) Nakamori (1982)10) Yamada and Matsuda (2002b)11) Matsui et al. (2002)12) Mizota and Matsuda (2004)13) Kaneko and Ito (1995)14) Jiju (2003)15) Odawara et al. (2005a)16) Sato et al. (2004)17) Nakagawa et al. (2001)18) Oshimizu and Iryu (2002)19) Odawara et al. (2005b)20) Muraoka et al. (2005)21) Yamamoto et al. (2005)22) Ehara et al. (2001)23) Odawara and Iryu (1999)24) Omura (1982)25) Yamada et al. (2003)26) Huang (1966)27) Nakagawa (1969)28) Omura (1988)

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200 m in elevation, and constitute the main body ofthe Ryukyu Group. The number of individualstratigraphic units within the main bodies variesfrom island to island and from area to area withinOkinawa-jima, which may reflect differences inlocal tectonic movements and/or differences in thetiming of coral-reef formation between the islands/areas.

Sedimentation modes in the reef-complexdeposits changed between approximately 0.8 and1.0 Ma in the Ryukyus. This is well documented bythe stratigraphic succession, configuration of thelithofacies, and coral assemblages of the RyukyuGroup on Ie-jima (Takeuchi et al. 2006) and Irabu-jima (Sagawa et al. 2001). At Ie-jima, a thick pileof carbonates and siliciclastics (Unit 1), display-ing a single deepening-upward stratigraphic suc-cession, formed during a relatively long periodbetween approximately 0.8 and 1.45–1.65 Ma,whereas Units 2–4 accumulated in a relativelyshorter period between 0.41 and 0.8 Ma (<400 kyr).This contrast indicates that different factors con-trolled sedimentation patterns in Unit 1 versusUnits 2–4. The Mid-Pleistocene Climate Tran-sition (MPT) at approximately 0.9 Ma is known asa change from a dominance of small-amplitude,41-kyr cycles of foraminiferal δ18O variations inthe early Quaternary to a dominance of large-amplitude, 100-kyr cycles of foraminiferal δ18Ovariations in the late Quaternary (Mudelsee &Schulz 1997). As the foraminiferal δ18O variationscorrelate to Quaternary sealevel changes (Chap-pell et al. 1996), Unit 1 is considered to have beendeposited during a period of sealevel changes withsmall amplitudes and short frequencies. No indi-cations, however, of such sealevel changes havebeen found in the lithology, succession and config-uration of this unit. Thus, it is highly probable thattectonic subsidence is the factor most responsiblefor the signal of a relative sealevel rise duringthe deposition of Unit 1. In contrast, the shorterperiod of deposition (<130 kyr/unit) and estimatedmaximum amplitudes of relative sealevel changes(110 m for Unit 2, 120 m for Unit 3 and 120 m forUnit 4) implies that these units formed in responseto glacioeustatic cycles.

This inference is also supported by the strati-graphic section at Irabu-jima, where the RyukyuGroup consists of 13 units grouped into lower, mid-dle and upper parts. The relative sealevel curvereconstructed from the succession and configura-tion of the lithofacies, coupled with paleobathyme-try inferred from lithology and coral assemblages,is characterized by high-frequency and lower

amplitude cycles for units 1–4 (∼1.0–1.45 Ma) andlow-frequency and greater amplitude cycles forunits 5–12 (∼0.4–1.0 Ma). For the first 0.5 millionyears (∼1.0–1.5 Ma) of reef development, obliquity(6th order) or precessional (7th order) cycles mayhave predominated. The reefs may have formed inresponse to 5th order cycles (∼100 ka, eccentricity)between 0.4 and 1.0 Ma. Consequently, it can beconcluded that the MPT in the Ryukyus is markedby increased amplitude of sealevel changes and asubsequent enhanced mode of coral-reef deposi-tion. The mechanistic link(s) between changes inthe amplitude and frequency of sealevel changesand reef deposition at the Ryukyus is uncertain atthis time. The main body of the Ryukyu Groupforms a stratigraphic architecture of aggradationand retrogradation, which implies that the islandsand their environs were subsiding duringdeposition.

Reef-complex deposits younger than 0.4 Ma arerather minor components of the Ryukyu Group inthe Central Ryukyus (Fig. 2). Such deposits occuron Kikai-jima and form a well-developed set ofraised terraces and are rimmed by Holoceneraised coral reefs (Sugihara et al. 2003). U-seriesages indicate that these reef-complex depositsformed at marine isotope stages (MIS) 3–7(Omura 1988; Sasaki et al. 2004; Inagaki et al.2005) and that this island has been elevated at ahigh uplift rate of approximately 1.8 m/kyr sincethe last interglacial period (Ota & Omura 1992).This uplift and the resultant multiple Late Pleis-tocene terraces are the result of subduction of theAmami Plateau beneath the Eurasian Plate off thisisland (Tokuyama et al. 1985). On other islandsin the Central Ryukyus, the younger depositsare minor components of the Ryukyu Group andofflap the main body. As their geological age isunclear, the precise timing of changes in tectonicmovements from subsidence (indicated by astratigraphic architecture of aggradation andretrogradation of the Ryukyu Group older than0.4 Ma) to uplift (indicated by an offlapping succes-sion of the younger limestones) in those islandsand their environs is uncertain at this time.

In contrast to the Central Ryukyus, youngerreef-complex deposits occur on islands in theSouth Ryukyus (Koba 1980; Omura 1984; Kobaet al. 1985; Omura et al. 1994; Yamada & Matsuda2001, 2002a; Fig. 2). No reef-complex depositsolder than 1.0 Ma have been found in the westernSouth Ryukyus (Ishigaki-jima, Yonaguni-jima andHateruma-jima). The relatively later timing ofcoral-reef initiation in these islands indicates that

398 Y. Iryu et al.


the opening of the western Okinawa Trough mayhave been later in the southern Ryukyus relativeto more northerly areas.

MODERN REEF COMPLEX

The Kuroshio current (North Pacific current) flowsinto the East China Sea through the straitbetween Taiwan and Yonaguni-jima, passes north-eastward along the Islands, and then bifurcatessouthwest of Yaku-shima (Fig. 1). The main cur-rent changes its direction and exits to the Pacificthrough Tokara Strait (the strait between Amami-o-shima and Yaku-shima), while a subsidiary cur-rent flows northward along Kyushu. This warm,northward-flowing current transports heat andlarvae of corals from the tropics to the Ryukyusand the southern half of mainland Japan. It allowsthe development of reefs with a highly diversifiedhermatypic coral fauna throughout the Ryukyus,although the islands are located at relatively highlatitudes for a reef province. Most of the RyukyuIslands are rimmed by coral reefs (fringing reefs),but those reefs are now degraded by infestationsof Acanthaster planci, an influx of terrigenous redsoil, or coral bleaching.

Fringing reefs around the Ryukyu Islands canbe subdivided into two basic topographic zones:reef flat and reef slope. The reef flat can be subdi-vided into five topographic subareas according todepth and sea-bottom morphology. These are,from shore to offshore, shallow lagoon, inner reefflat, reef crest, outer reef flat and reef edge. Thefollowing description of topographic and biotic fea-tures of the coral reefs is based mainly on investi-gations of reef flat sites surrounding Ishigaki-jima,South Ryukyus (Fig. 3).

The shallow lagoon is a trough-like depressionbordering the shore. It differs from lagoons ofatolls or barrier reefs by its shallow depth. It isgenerally several hundred meters wide and lessthan 3 m deep. It slopes gently seawards withoccasional abrupt rises caused by coral knolls andgrades into the inner reef flat. Seagrass beds, gen-erally less than 300 m wide, are distributed on theshoreward side of the shallow lagoon. These grassbeds are characterized by abundant marine sper-matophytes growing densely to form thickets. Twospecies of seagrasses, Thalassia hemprichii andCymodocea rotundata, dominate the seagrassassemblage. The seagrass beds grade seawardinto a sand/gravel bottom, with less abundantbenthic macroorganisms. The sand bottom is gen-erally several hundred meters wide. Hermatypic

coral patches are scattered here, with Acroporaformosa, Montipora digitata, Porites austra-liensis and Porites cylindrica dominating theassemblages.

The inner reef flat is several hundreds of meterswide and 0.5–1.0 m deep, and is characterized byabundant hermatypic corals. The corals growdensely up to the low-tide level to form a more orless flat plane with a rough sea bottom. Bioclasticsediments (sand and gravel) occur in pocket-likeholes between corals. The dominant coral speciesare Acropora pulchra, Montipora digitata, Mon-tipora aequituberculata, Montipora informis, andPorites australiensis.

The reef crest is a slightly raised subareabetween the inner and outer reef flat. It is thehighest part of the reef flat and only about 0.5 mdeep. The distribution of marine organisms on thereef crest varies with depth. Where the crest isso deep that it is submerged even at low tide,hermatypic corals cover the substratum. The coralassemblage is dominated by Acropora pulchra,Acropora aspera, and Montipora digitata. In con-trast, Sargassum beds and algal turf cover thereef crest where it is emergent at low tide.

The outer reef flat is a more or less smooth planebetween reef crest and reef edge, with a depthrange of 0.5–1.0 m. The reef edge forms the bound-ary between the reef flat and the reef slope, wherewaves break. The outer reef flat consists of reefrock covered with branching, encrusting andtabular forms of corals such as Pocillopora verru-cosa, Acropora monticulosa, Acropora aspera,Acropora hyacinthus, Goniastrea retiformis andPlatygyra ryukyuensis. Abundant coralline algaeare found covering dead corals.

The reef slope is a steep escarpment beginningat the reef edge. Subtidal spurs and grooves occurmore or less perpendicularly to the reef edge,running down to depths of 10–60 m. The slopeterminates at depths ranging from 80 m at Hateru-ma-jima to 30 m at Yaku-shima (Hori 1983).

The reef slope is characterized by abundantoccurrences of hermatypic corals and nongenicu-late coralline algae, with very rare, large, erectforms of fleshy algae. The upper reef slope (<5 mdeep) is dominated by encrusting and tabularforms of corals tolerant to high-energy environ-ments, with a species composition very similar tothat on the outer reef flat and reef edge (Acroporahumilis, Acropora hyacinthus, Acropora valida,Favia pallida, Favites abdita and Goniastrea reti-formis). Coral coverage is highest in this depthrange. The middle reef slope (5–25 m deep) is



characterized by the predominance of hemispheri-cal and encrusting forms such as Pocilloporaverrucosa, Acropora cytherea, Favia stelligera,Echinophyllia aspera, Oxypora lacera and Myce-dium elephantotus. The corals are less abundanton the lower reef slope (>25 m deep), where folia-ceous and encrusting species such as Pachyserisspeciosa and Leptoseris scabra dominate.

The shelf around the Ryukyu Islands is a flatplane that gently slopes seaward, the seawardmargin (shelf edge) of which lies 90–170 m deep.The width of the shelf varies from place to place,ranging from 0 to 25 km (Hamamoto et al. 1979;Kato et al. 1982). Beyond the fringing reefs, her-

matypic corals are only a minor element of theshelf. In contrast, nongeniculate coralline algaeare dominant, together with encrusting foramini-fers, forming pebble- to cobble-sized rhodoliths(foraminiferal–algal macroids). The rhodolithsare found at depths from 50 to 135 m aroundOkinawa-jima and from 60 to 150 m around Miy-ako-jima (Tsuji 1993; Iryu et al. 1995). Thelarger foraminifer Cycloclypeus carpenteri isfound at depths from 65 to 135 m. This depthrange corresponds to that of the rhodoliths. Sed-iments with abundant bryozoan skeletons aredistributed on the shelf at depths from 60 to200 m (Tsuji 1993).

Fig. 3 Comparison of topography and coral fauna between the Kabira Reef at Ishigaki-jima, South Ryukyus, and the Hirota Reef at Tane-ga-shima, thenorthernmost reef in the Ryukyus.

Topography

Ishigaki-jima Tane-ga-shima

Turbinaria spp.Hydnophora bonsai

Encrusting faviids (e.g. Goniastrea spp.)Goniastrea aspera, Montipora digitata

A. pulchra, A. hyacinthusPorites cylindrica, Acropora formosa

Coral

The reef flat comprising five bathymetric subareas: shallow lagoon, inner reef flat, reef crest, outer reef flat, and reef edge

•

The depth of the inner reef flat and the outer reef flat corresponding to mean low sea level

•

The reef flat comprising three bathymetric subareas: shallow lagoon, seaward reef flat, and reef edge

•

The seaward reef flat not differentiated into bathymetric subareas

•

Depth of the seaward reef flat is ~1 m below mean low sea level

•

Dominated by taxa characterizing low-latitude, reefal communities

• Dominated by taxa characterizing high-latitude, non-reefal communities

•

Low diversity and low coverage • High diversity and high coverage •

Characteristic species • Characteristic species •

Reef flat ReefslopeReef

crestShallow lagoon Inner reef flat

MHWLMTLMLWL

0

-200

-400

0 100 200 300 400 500 600 700 800 900 1000Distance from shore (m)

Dep

th (

cm)

Kabira Reef

Reef edge

Outerreef flat

Reef flat ReefslopeShallow lagoon

Seawardreef flat

MHWLMTLMLWL

0

-200

-400

0 100 200 300 400 500 600 700 800 900 1000Distance from shore (m)

Dep

th (

cm)

Hirota Reef

Reef edge

MHWL : Mean high water levelMTL : Mean tide levelMLWL : Mean low water level

400 Y. Iryu et al.


SCIENTIFIC OBJECTIVES OF COMBINED OCEAN AND LAND DRILLING

MAIN OBJECTIVES

Coral-reef front migration

Ecological constraints limit the latitudinal distri-bution of modern coral-reef ecosystems to the trop-ics (30°N to 30°S). Coral-reef ecosystems presentlylocated at these latitudinal limits, called coral-reeffronts, are excellent targets to study responses ofthis ecosystem to Quaternary climate and sealevelchanges. The Ryukyu Islands are located on theboundary between the coral-reef and non-coral-reef regions in the present-day northwesternPacific. Thick reef carbonates accumulated in theQuaternary in the South and Central Ryukyus, butfew accumulated in the North Ryukyus. Thus,there has been a northern limit of reef formationwithin the Ryukyu Islands. It is inferred that thecoral-reef front may have migrated northward ininterglacial and southward in glacial periods(Fig. 1). The possible factors for this migrationinclude SST, salinity, nutrient level, and terrige-nous input, all of which are supposed to have variedfrom glacial to interglacial periods around theRyukyus. Extensive areas of the present EastChina Sea Shelf are thought to have been exposedin the glacial periods and provided much more riv-erine input rich in terrigenous material and nutri-ents to the Ryukyu Islands and their environs.The paleoceanographic conditions in glacial periodsmay be characterized by lower SST, lower salinity,higher nutrient level and more turbid environ-ments. It is obvious that coral reefs were degradedin such deteriorated environments especially nearthe northern limit of reef ecosystems.

In order to detect the position of past coral-reeffronts, and the driving mechanism(s) for theirchanges in position, we need to circumscribe thecharacteristic features of the topography, sedi-ments and biota of the coral reefs at the northernlimits (Fig. 3). Ikeda et al. (2006) show that theseaward reef flat of the Hirota Reef at Tane-ga-shima, the northernmost coral reef in the Ryukyus,cannot be differentiated into different bathymetricareas, because it contracts with obvious subdivi-sions of the seaward reef flat areas in the Centraland South Ryukyus (Fig. 3). This difference maybe attributed to a lower reef growth rate and/orlater reef formation in Holocene time in the NorthRyukyus than in the South Ryukyus. Ikede et al.also noted that the coral fauna on the Hirota Reef

is delineated by low diversity and characterizedby taxa of high-latitude – non-reefal communitiessuch as Acropora loripes, Hydonophora bonsai,Goniastrea spp. and Turbinaria spp. Sagawa(2002) pointed out that faviid corals may be usedto detect the past coral-reef front in the Ryukyusbecause the species composition of faviid corals andcalix sizes in the Central and South Ryukyus aredifferent from those in Kyushu (Amakusa) andHonshu (Tateyama), both located in a warm-temperate region in mainland Japan.

Considering the geographic position and ourknowledge of reefs and reef-forming organisms,we believe that the Ryukyu Islands are one of thebest locations to delineate the nature and magni-tude of coral-reef front migration in response torepeated climatic/oceanographic changes in theQuaternary, and to clarify major constraints forthe initiation, development and demise of coralreefs.

Responses of the coral-reef ecosystem to climatic changes on millennial to glacial–interglacial timescales

A reef-coral community is defined as a local groupof hermatypic corals with a particular combinationof coral species. Its distribution is closely relatedto reef topography. Examinations of Pleistocenecoral communities in Barbados and HuonPeninsula indicate that the taxonomic compositionof the communities has remained constant over along period of time (Jackson 1992; Pandolfi 1996).This implies that, despite experiencing numerouscycles of major perturbation such as sealevelchanges and possible SST fluctuations, tropic reefshave been able to re-establish themselves and pro-duce reef communities of similar composition.These results, however, are based on research inthe central parts of the coral-reef province, whereclimate-related deterioration in glacial periods isnot considered to have been so severe as in rela-tively high latitudes.

In this area of the northwestern Pacific therehas been only one study with implications for thecoral response at the coral-reef front to majorQuaternary perturbations. Sagawa (2002) demon-strated that some faviid species and morphotypeslimited to warm-temperate regions of mainlandJapan (Kyushu and Honshu) occur in limestonesdeposited in glacial periods in the CentralRyukyus. Consequently, there is a possibility thatthe composition of coral communities may havevaried in response to climatic changes on millen-nial to glacial–interglacial timescales.



Role of coral reefs in the carbon cycle

Reef biotas have played an important role in thecarbon cycle by fixing carbon in carbonatesthroughout the Phanerozoic. Quaternary coralreefs may have also contributed to abrupt climaticchanges through a positive feedback mechanism.Carbonate production rates of reefs may havebeen accelerated during a transgression, becausethe atmospheric CO2 concentration increased as aresult of the carbonate precipitation in the sea,resulting in a rise in atmospheric temperature.This process was called ‘the coral reef feedback’(Opdyke & Walker 1992), a shallow-water versionof the Alkalinity Hypothesis proposed bySarmiento et al. (1988). Quaternary carbonateaccumulation rates in coral reefs as a function ofsealevel changes are considered a good means totest this hypothesis. Consequently, it is inevitablyneeded to estimate the accumulation rates at manyreef sites including those at the coral front.

SUBSIDIARY OBJECTIVES

Initiation of the ‘coral sea’

The ‘coral sea’ is considered to have been initiatedin the Ryukyus after the opening of the OkinawaTrough and the subsequent influx of the Kuroshiocurrent into the back-arc basin. Because the Kuro-shio current is a source of warm oligotrophicwaters, its influx into the Okinawa Trough was alsoimportant for the evolution of modern oceano-graphic conditions and climate states in Far EastAsia. The opening of the Okinawa Trough began inEarly Pleistocene time (Park et al. 1998).

When coral-reef formation began in theRyukyus is controversial. Ujiié (1989, 1994) arguedthat coral reefs started forming in the Ryukyus atapproximately 0.6 Ma, when terrigenous materialfrom the Eurasian Continent was trapped in a pre-cursor of the Okinawa Trough. This argument isbased on the observation that Middle to UpperPleistocene reef complex deposits (Ryukyu Group)rest unconformably on the Upper Miocene to low-est Pleistocene siliciclastic rocks (Shimajiri Group)with a 1.2-Ma gap in sedimentation between∼1.8 Ma (latest age of the Shimajiri Group) andapproximately 0.6 Ma (earliest age of the RyukyuGroup). The Shimajiri Group emerged and waseroded, and antecedent edifices for the coral-reefformation were constructed during this period. Asimilar scenario was presented by Koba (1992). Hestated that the Yonaguni Depression formed at the

boundary between the Ryukyu Arc and Taiwanbecause of the collision of the Luzon Volcanic Arcwith Taiwan, which made the Kuroshio currentflow into the back-arc side, and that coral reefsbegan to flourish at 0.6–0.7 Ma.

In contrast, recent investigations (Nakagawaet al. 2001; Yamamoto et al. 2003; Odawara et al.2005a) reveal that the initiation of coral reefs datesback to earliest Pleistocene time (1.45–1.65 Ma)and that there are no large gaps within the Shima-jiri Group, the Chinen Formation and the RyukyuGroup. Molecular and evolutionary biological in-vestigations of amphibian and reptile lineages inthe present-day Central and South Ryukyus byOta (1998) indicate that the Central Ryukyus havebeen isolated from the Eurasian Continent and theSouth and North Ryukyus since Pliocene time.These new data suggest the interpretations ofUjiié (1989, 1994) and Koba (1992) are incorrect.As just described, the paleogeographic evolutionaround the Ryukyus has not been well understood.

In order to clarify when and how the ‘coral sea’formed in the Ryukyus, it is necessary to decipherenvironmental records in a Plio-Pleistocenesequence that extends from land to the back-arcbasin and the forearc trench (Ryukyu Trench).Drilling of shelf-to-basin transects and at the gate-way (strait between Taiwan and Yonaguni-jima) willprovide clues to delineate the paleoceanographicevolution from the ‘mud sea’ to the ‘coral sea’.

The Kuroshio current during glacial periods and its effects on coral-reef formation

As noted above, the flow path of the Kuroshio cur-rent is very decisive for coral-reef building (e.g.Veron 1992). Some studies have shown that thepathway during the last glacial period was differ-ent from the present because of the presence ofthe land bridge connecting the Central and SouthRyukyus and Taiwan which prevented the mainbranch of the Kuroshio current from flowing intothe back-arc side of the Ryukyu Arc (Ujiié et al.1991; Ujiié 1998; Ujiié & Ujiié 1999). It wasthought that the current flowed eastward to thesouth of the Ryukyus (Fig. 1) and that this shiftcaused a significant cooling in the northwesternPacific. In contrast, Xu and Oda (1999) illustratedthat the Kuroshio current flowed into the back-arctrough during the last glacial periods. Thus, thequestions arise: (i) did the Kuroshio current flowinto the back-arc side in glacial periods? and (ii) towhat extent were the paleoceanographic condi-tions in glacial periods different from the intergla-

402 Y. Iryu et al.


cial periods? Limited amounts of quantitativeinformation, such as past SST and its latitudinalgradient and nutrient levels, are available toaddress these questions (e.g. Thompson 1981; Oba& Yasuda 1992; Oda & Takemoto 1992; Takemoto& Oda 1997). Corals reefs formed in the last glacialperiod (MIS2; Sasaki et al. 2006) and in a latePleistocene glacial period (MIS4; Inagaki et al.2005) were reported from off Miyako-jima andKikai-jima, respectively. These indicate that coralreefs could persist in glacial periods in the south-ern part of the Ryukyus and that the coral-reeffront remained in the northern part of the islands.

Multiple drilling is indispensable to determinepaleoceanographic conditions in the Ryukyus inglacial periods. The drilling should be conductedon 3–5 transects across the Ryukyu Arc (from theOkinawa Trough to the Ryukyu Trench) which cov-ers most latitudes of the Ryukyus.

Early carbonate diagenesis in subtropic to warm-temperate regions

The pattern of marine and freshwater diagenesisis markedly different between subtropic and tem-perate carbonates. Subtropic carbonates are com-posed mainly of biocomponents derived fromchlorozoan biota and are aragonite-dominated. Incontrast, the warm-temperate carbonates aremainly of foramol composition and are calcite-dominated (Lees 1975; Nelson 1988; Rao 1996).The differences in carbonate mineralogy, watertemperature and chemistry between the two cli-matic zones result in different patterns of marineand freshwater diagenesis. In the Ryukyus, bothtypes of shallow-water carbonates are present andwere subject to repeated, episodic, subaerial expo-sures because of the Quaternary sealevel changes.Thus, the temporal and spatial distribution ofdiagenetic products within the carbonate sedi-ments of the Ryukyus should reveal latitudinalmigrations of the diagenetic regimes at the bound-ary between subtropic and warm-temperateregions. Globally, diagenetic responses in arago-nite-dominated and calcite-dominated sedimentsare well characterized, but little research hasfocused on sites where the two end-members arestratigraphically commingled.

Modern dolomites, although very limited, occurin association with marine hardgrounds and formin open, normal marine conditions in the Ryukyus(Matsuda et al. 1994). The chemical composition ofinterstitial water beneath insular shelves displaysstrong magnesium depletion (Matsuda et al. 1995).

This implies that the dolomitization is proceedingin places in these sedimentological and geochemi-cal settings, although significant intervals of dolo-mites have not been found from the RyukyuGroup. If dolomite formation is wide spread in theQuaternary carbonates of the Ryukyus, then theycould potentially be the largest example of contem-porary marine dolomitization and may provideunique insights to the ‘dolomite problem’ (e.g.Budd 1997).

A hydrogeological study will be needed to clarifythe spatial distribution of fluid geochemistry anddiagenetic products and to depict groundwater cir-culation within the carbonate sequences. In situmeasurements will be planned on flow rate, flowdirection, geothermal gradient and chemical com-position of interstitial water. These will providefundamental insights into the diagenesis of sub-tropic to warm-temperate reef and shelf carbon-ates. The resultant knowledge will be applicable tocomparable carbonate platforms through geologi-cal time.

CONCLUSIONS

To accomplish the objectives mentioned above, itis crucial to clarify the stratigraphic successionand configuration of lithofacies in the Quaternaryreef-complex deposits at the coral-reef front in theRyukyu Islands. The carbonates are distributed inextensive areas ranging from approximately 200 min elevation on islands down to approximately −150 m (or more) in depth on shelves to shelf slopes.Surface exposure is limited by the luxuriantgrowth of tropic rainforests and intensive land useby Ryukyu people (∼1.5 million). Consequently,land drilling (ICDP) combined with ocean drilling(IODP) is the only way to obtain the necessaryinformation. The COREF Project will provide thecores needed and will help elucidate the answersto the following three key questions: (i) What arethe nature, magnitude and driving mechanism(s)of coral-reef front migration in the Ryukyus? (ii)What is the ecosystem response of coral reefs atthe northern coral-reef front to Quaternary cli-mate changes? (iii) What is the role of coral reefsin the global carbon cycle?

ACKNOWLEDGEMENTS

We are grateful to collaborators on the COREFProject for their valuable discussion and critical



comments on this research plan. They are Y. Tsuji(Japan Oil, Gas and Metals National Corporation),T. Nakamori (Tohoku University), T. Yamada(Tohoku University), K. Arai (National Institute ofAdvanced Industrial Science and Technology), K.Sasaki (Kanazawa Gakuin University) and S. Sakai(Japan Agency for Marine-Earth Science andTechnology). The constructive comments and sug-gestions from Drs. D. Budd (University of Colo-rado, Boulder), K. Mori (Tohoku University) andT. Sato (Akita University) were helpful for improv-ing this paper.

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