Toru Nakamori

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.

The mechanism of production enhancement in coral reef carbonate systems: model and empirical results

Coral reefs are characterized by enormous carbonate production of the organisms. It is known that rapid calcification is linked to photosynthesis under control of the carbonate equilibrium in seawater. We have established a model simulating the coexisting states of photosynthesis and calcification in order to examine the effects of photosynthesis and calcification on the carbonate system in seawater. Supposing that the rates of photosynthesis and calcification are proportional to concentrations of their inorganic carbon source, the model calculations indicate that three kinds of unique interactions of the organic and inorganic carbon productions are expected. These are photosynthetic enhancement of calcification, calcification which benefits photosynthesis and carbonate dissolution induced by respiration. The first effect appears when the photosynthetic rate is more than approximately 1.2 larger than that of calcification. This effect is caused by the increase of CO32− content and carbonate saturation degree in seawater. If photosynthesis use molecular carbon dioxide, the second effect occurs when the calcification rate is more than approximately 1.6 times larger than that of photosynthesis. Time series model experiments indicate that photosynthesis and calcification potentially enhance each other and that organic and inorganic carbon is produced more efficiently in the coexisting system than in the isolated reactions. These coexisting effects on production enhancement of photosynthesis and calcification are expected to appear not only in the internal pool of organisms but also in a reef environment which is isolated from the outer ocean during low tide. According to the measurements on the fringing type Shiraho Reef in the Ryukyu Islands, the diurnal change of water properties (pH, total alkalinity, total carbon dioxide and carbonate saturation degree) were conspicuous. This environment offers an appropriate condition for the appearance of these coexisting effects. The photosynthetic enhancement of calcification and the respiratory inducement of decalcification were observed during day-time and night-time slack-water periods, respectively. These coexisting effects, especially the photosynthetic enhancement of calcification, appear to play important roles for fluorishing coral reef communities.

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.

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.

A geochemical model for coral reef formation

The conspicuous growth of a reef crest and the resulting differentiation of reef topography into a moat (shallow lagoon), crest and slope have long attracted the interest of scientists studying coral reefs. A geochemical model is here proposed for reef formation, taking into account diffusion-limited and light-enhanced calcification. First, to obtain data on net photosynthesis and calcification rates in the field, a typical coral community was cultured in situ on a reef flat. Using these data, equations including parameters for calcification were then developed and applied in computer simulations to model the development over time of reef profiles and the diffusion of carbon species. The reef topography simulated by the model was in general agreement with reef topography observed in nature. The process of reef growth as shown by the modeling was as follows. Increases in the shore-to-offshore gradients of the concentrations of carbonate species result from calcification by reef biota, giving a lower rate of growth on near-shore parts of the reef than on those further offshore. As a result, original topography is diversified into moat and reef crest for the first time. Reef growth on the reef crest is more rapid than in the inshore moat area, because more light is available at the crest. Reef growth on the near-shore side of the reef is further inhibited by damming of carbon-rich seawater on the seaward side of the reef by the reef crest. Over time, the topographic expression of the reef crest and moat becomes progressively more clearly defined by these geochemical processes.