Yasufumi Iryu

Distribution of marine organisms and its geological significance in the modern reef complex of the Ryukyu Islands

Abstract Extensive investigations of biota in the reef complex around the Ryukyu Islands have revealed ecologic specificity of many benthic organisms and have shown that characteristic assemblages are found in each of the topographic zones and sub-areas. The moat is divisible into a nearshore seagrass bed and an offshore sand bottom. Both inner reef flat and outer reef flat are characterized by abundant occurrences of hermatypic corals and nonarticulated coralline algae. However, the former is dominated by branching and foliaceous forms of corals and various, large, fleshy, erect forms of algae, whereas the latter is dominated by encrusting and tabular forms of corals, lacking these algae. Corals and coralline algae are not present on the reef crest, which is covered by rubble and gravel, where algal turf and Sargassum are spreading. Encrusting and tabular forms of corals flourish on the shallower part of the reef slope, with high coverage, while, with increasing depth, the coverage decreases and the dominating coral forms change, with hemispherical and encrusting forms on the middle part of reef slope, and foliaceous and encrusting forms on the deeper part of reef slope. Nonarticulated coralline algae are distributed throughout the reef slope. The composition of coral and coralline algal assemblages changes dramatically with increasing depth. Foraminiferal-algal nodules, rhodoliths, are the most abundant constituent on the island shelf, commonly with Cycloclypeus carpenteri. There are likely to be two types of shelves in tropical to subtropical regions: nutrient-rich Halimeda-dominant and nutrient-poor rhodolith-dominant. Sediments abundant in bryozoan skeletons occur occasionally on the shelf.

Reef response to sea-level and environmental changes during the last deglaciation: Integrated Ocean Drilling Program Expedition 310, Tahiti Sea Level

The last deglaciation is characterized by a rapid sea-level rise and coeval abrupt environmental changes. The Barbados coral reef record suggests that this period has been punctuated by two brief intervals of accelerated melting (meltwater pulses, MWP), occurring at 14.08–13.61 ka and 11.4–11.1 ka (calendar years before present), that are superimposed on a smooth and continuous rise of sea level. Although their timing, magnitude, and even existence have been debated, those catastrophic sea-level rises are thought to have induced distinct reef drowning events. The reef response to sea-level and environmental changes during the last deglacial sea-level rise at Tahiti is reconstructed based on a chronological, sedimentological, and paleobiological study of cores drilled through the relict reef features on the modern forereef slopes during the Integrated Ocean Drilling Program Expedition 310, complemented by results on previous cores drilled through the Papeete reef. Reefs accreted continuously between 16 and 10 ka, mostly through aggradational processes, at growth rates averaging 10 mm yr−1. No cessation of reef growth, even temporary, has been evidenced during this period at Tahiti. Changes in the composition of coralgal assemblages coincide with abrupt variations in reef growth rates and characterize the response of the upward-growing reef pile to nonmonotonous sea-level rise and coeval environmental changes. The sea-level jump during MWP 1A, 16 ± 2 m of magnitude in ∼350 yr, induced the retrogradation of shallow-water coral assemblages, gradual deepening, and incipient reef drowning. The Tahiti reef record does not support the occurrence of an abrupt reef drowning event coinciding with a sea-level pulse of ∼15 m, and implies an apparent rise of 40 mm yr−1 during the time interval corresponding to MWP 1B at Barbados.

Pleistocene reef development in the southwest Ryukyu Islands, Japan

Abstract Calcareous nannofossil and sedimentological studies of the Pleistocene Ryukyu Group, based on 13 cores and surface outcrops on Irabu-jima and Shimoji-jima, the Ryukyu Islands, southwestern Japan, suggest that these sediments were deposited at 1.5–0.3 Ma and unconformably overlie the Pliocene Shimajiri Group. The Ryukyu Group consists mainly of reef complex deposits, which accumulated in a wide range of depositional environments, from shallow reef flat to deep insular shelf. Carbonate lithofacies representing 10 depositional environments were delineated by comparison with the present-day marine sediments and biota around the Ryukyus. Five coral assemblages were defined by species composition and the morphology of fossil forms within the coral limestone, each indicating a particular environment. The Ryukyu Group in the study area is composed of 13 lithologic units. A complete succession of units commences with coral limestone (LST or TST) grading upward into rhodolith and larger foraminiferal limestones (TST) overlain by coral limestone (HST). Episodic subaerial exposure and subordinate karstification commonly occurred after deposition. The study area subsided during the deposition of units 1–12 and then was uplifted during deposition of unit 13. Reef development may have been dominated by obliquity or precessional cycles (sixth- and seventh-order cycles, respectively) for the first 0.5 million years (∼1.5–1 Ma), after which it may have responded to fifth-order (eccentricity) cycles.

Holocene sea-level change and tectonic uplift deduced from raised reef terraces, Kikai-jima, Ryukyu Islands, Japan

Abstract Kikai-jima (Kikai Island) is surrounded by four Holocene raised coral reef terraces, which are thought to be an offlapping sequence of reef deposits caused by combined effects of seismic uplift and Holocene sea-level change. Many studies in this region have investigated Holocene sea-level changes and reef growth, but there are relatively few in which reliable sea-level indicators are given. We have found that Pocillopora verrucosa , one of the most abundant coral species on the upper-reef slopes of fringing reefs in the Ryukyus, has its peak abundance at a depth of 1.5 m. Therefore, this species is considered ideal for the analysis of relative sea-level change and can be used as a dipstick for the Holocene reef deposits in this area. Based on the distribution of P. verrucosa on the four Holocene raised terraces, we calculate relative paleo-mean sea levels to be 10.8–11.1 and 8.5–8.9 m for Terrace I, 5.0–5.3 m for Terrace II, 4.0–4.3 m for Terrace III and 1.9–2.5 m for Terrace IV. These results, combined with hitherto known and newly measured radiometric dates (103 total), clearly show that the four terraces formed in response to repeated seismic uplifts at 6.3, 4.1, 3.1 and 1.4 ka, and that sea level was higher than present between 7.0 and 6.3 ka.

Carbon and oxygen isotopic composition of a Guam coral and their relationships to environmental variables in the western Pacific

We examine the high-resolution (~32 samples/year) carbon and oxygen isotopic composition (y13Ccoral and y 18Ocoral) in a coral core (Porites lobata) from Double Reef, Guam over the years 1980–2000. The yCcoral shows clear seasonal variations with mean seasonal amplitude of 1.89x, which roughly corresponds with seasonal variations in solar radiation. The seasonal amplitude of y18Ocoral variations are small (0.23–0.57x), but they are significantly correlated with sea surface temperature (SST) and salinity (SSS). The yOcoral and SST are more strongly correlated during El Niño/Southern Oscillation (ENSO) warm phases (r= 0.81, pb0.01) than during non-ENSO phases (r= 0.65, pb0.01) and ENSO cool phases (r= 0.48, pb0.01). These different relationships are due to differences in winter SST and in seawater yO (y18Osw) during ENSO warm phases (b27 8C and higher values of yOsw) compared with cool phases (N28 8C and lower values of y Osw) at Guam. These differences in oceanic parameters result from movements of the Western Pacific Warm Pool (WPWP) during the different phases of ENSO. Anomalies in y18Osw, inferred from the y 18Ocoral and instrumental SST, are consistent with SSS anomalies for the years 1980–2000. These yOsw anomalies may reflect changes in SSS and evaporation–precipitation due to movements of the WPWP. This detailed analysis of a coral from Guam suggests that it may contain an excellent archive of past ENSO events. D 2004 Elsevier B.V. All rights reserved.

Development of coral reefs of the Ryukyu Islands (southwest Japan, East China Sea) during Pleistocene sea-level change

Abstract The Pleistocene coral reef deposits called the Ryukyu Group are widely distributed through the Ryukyu Islands. Lithofacies are represented by conglomerate, calcareous sandstone and limestones. The limestones can be divided into six lithofacies on the basis of macro-benthos and large foraminifers. They are coral, rhodolith, Cycloclypeus-Operculina, Halimeda, and poorly- to well-sorted detrital limestones. Their depositional environments are reconstructed referring to the depth range of the Recent coral reef biota around the Ryukyu Islands. Depositional history of the Ryukyu Group is recorded in a stratigraphic cross-section on Toku-no-shima. Sequence stratigraphical units are recognized in the vertical section of the limestones. Units 1 and 3 are considered to have been formed when the relative sea-level was high, while Units 2 and 4 were deposited during the low stands of sea-level. It is noteworthy that the coral limestones of the Units 2 and 4 were accumulated during glacial stages. The generalized sequence of the limestones is deduced from a transition probability on Miyako-jima. It begins with the Cycloclypeus-Operculina limestone and is followed by rhodolith limestone and ends with coral limestone. These facies indicate a shallowing upward succession.

Water circulation and carbon flux on Shiraho coral reef of the Ryukyu Islands, Japan

Abstract Water circulation and its relation to carbon production are studied in a preliminary investigation on the fringing reef of the Ryukyu Islands. Arrangement of the coral patches on the reef flat reflects the water circulation. The time series of current velocities indicates that water flux is 2.24 × 10 6 m 3 day −1 , which is about twice as much as the volume on the reef flat. Tentative biogeochemical studies suggest the net production rate of organic matter and carbonates of the whole reef ecosystem. It appears that about 3% of ΣCO 2 in the sea water could be consumed while the water mass moved through Shiraho system.

The Pliocene to recent history of the Kuroshio and Tsushima Currents: a multi-proxy approach

The Kuroshio Current is a major western boundary current controlled by the North Pacific Gyre. It brings warm subtropical waters from the Indo-Pacific Warm Pool to Japan exerting a major control on Asian climate. The Tsushima Current is a Kuroshio offshoot transporting warm water into the Japan Sea. Various proxies are used to determine the paleohistory of these currents. Sedimentological proxies such as reefs, bedforms, sediment source and sorting reveal paleocurrent strength and latitude. Proxies such as coral and mollusc assemblages reveal past shelfal current activity. Microfossil assemblages and organic/inorganic geochemical analyses determine paleo- sea surface temperature and salinity histories. Transportation of tropical palynomorphs and migrations of Indo-Pacific species to Japanese waters also reveal paleocurrent activity. The stratigraphic distribution of these proxies suggests the Kuroshio Current reached its present latitude (35 °N) by ~3 Ma when temperatures were 1 to 2 °C lower than present. At this time a weak Tsushima Current broke through Tsushima Strait entering the Japan Sea. Similar oceanic conditions persisted until ~2 Ma when crustal stretching deepened the Tsushima Strait allowing inflow during every interglacial. The onset of stronger interglacial/glacial cycles ~1 Ma was associated with increased North Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion when reefs reached their present latitude (~31 °N), thereafter the reef front advanced (~31 °N) and retreated (~25 °N) with each cycle. Foraminiferal proxy data suggests eastward deflection of the Kuroshio Current from its present path at 24 °N into the Pacific Ocean due to East Taiwan Channel restriction during the Last Glacial Maximum. Subsequently Kuroshio flow resumed its present trajectory during the Holocene. Ocean modeling and geochemical proxies show that the Kuroshio Current path may have been similar during glacials and interglacials, however the glacial mode of this current remains controversial. Paleohistorical studies form important analogues for current behavior with future climate change, however, there are insufficient studies at present in the region that may be used for this purpose. Modeling of the response of the Kuroshio Current to future global warming reveals that current velocity may increase by up to 0.3 m/sec associated with a northward migration of the Kuroshio Extension.

Typification and reassessment of seventeen species of coralline red algae (Corallinales and Sporolithales, Rhodophyta) described by W. Ishijima during 1954–1978

Type specimens of 17 species of non-geniculate coralline red algae belonging to the Corallinales and Sporolithales (Corallinophycidae, Rhodophyta) and established between 1954 and 1978 by Wataru Ishijima have been re-examined, focusing on characters relevant to modern coralline taxonomy. The species are from the Early Cretaceous, Eocene, Oligocene, Miocene and Pleistocene rocks of southern and central Japan, Indonesia, Malaysia, Indian Ocean, Philippines, Pakistan and the Galapagos Archipelago, and were attributed by Ishijima to Lithothamnion (as Lithothamnium), Mesophyllum, Lithophyllum and Archaeolithothamnion (as Archaeolithothamnium). With two exceptions, all species are reassigned to different genera or are of uncertain generic placement within particular subfamilies, families or orders. The type specimen of Lithophyllum oboraensis is retained in Lithophyllum and the type specimen of Mesophyllum izuensis is retained in Mesophyllum. The type specimens of Lithophyllum izuensis (Ishijima) comb nov. (= Lithothamnion izuensis), Phymatolithon tokumanensis (Ishijima) comb. nov. (= Mesophyllum tokumanensis), Sporolithon borneoensis (Ishijima) comb. nov. (= Archaeolithothamnium borneoense) and Sporolithon galapagosensis (Ishijima) comb. nov. (= Archaeolithothamnion galapagosensis) possess features justifying their assignment to various genera within a particular family and subfamily. A specimen ascribed by Ishijima to Archaeolithothamnion galapagosensis (as Archaeolithothamnium) in the protologue of that species is a misidentified specimen of Sporolithon ptychoides Heydrich. The type specimens of Lithophyllum minoensis and Mesophyllum indicum belong to the Corallinaceae, subfamily Mastophoroideae but are of uncertain generic placement. The type specimens of Lithothamnion makinogoensis, L. nodai, L. tenuicrustatum, Mesophyllum niinoi and M. yuyashimaensis belong to the Hapalidiaceae (Melobesioideae) but are of uncertain generic placement. The type specimens of Lithophyllum hashimotoi, Lithophyllum minae, Lithothamnion huseinii and Lithothamnion tokiensis lacked sufficient data to allow for order, family, subfamily or genus placement within the Corallinophycidae. Lectotype specimens have been designated as necessary, and relevant historical, repository, geological, morphological and other data are presented.

Correlative relationships between carbon- and oxygen-isotope records in two cool-temperate brachiopod species off Otsuchi Bay, northeastern Japan

Abstract. We present high-resolution, three-dimensional carbon (&dgr;13C)- and oxygen (&dgr;O)-isotope compositions from calcite shells of two modern brachiopod species (Terebratulina crossei and Terebratalia coreanica) and their correlative relationships. &dgr;13C and &dgr;18O values from the secondary shell layer, which constitutes the main body of a brachiopod shell, are in and/or out of the range of &dgr;13C and &dgr;18O values of calcite precipitated in isotopic equilibrium with ambient seawater (equilibrium calcite). The &dgr;C and &dgr;18O values of samples from the outermost part of the secondary shell layer show positive correlations. The values of high-growth-rate portions are less than those of low-growth-rate portions; these results are ascribed to a kinetic isotope fractionation effect. Metabolic influences are identified in the isotopic compositions of the low-growth-rate portions for T. coreanica, resulting in decreases in &dgr;13C values compared with those of equilibrium calcite. We illustrate the effects of kinetic isotope fractionation and metabolism on the isotopic compositions of brachiopod shell calcite, which vary among shell portions within a single shell, as well as between the two species. However, appropriate selection of brachiopod taxa and shell portions that reflect the isotopic composition of ambient seawater enables their use as a reliable paleoenvironmental proxy.