Hironobu Kan

A 60-year isotopic record from a mid-Holocene fossil giant clam (Tridacna gigas) in the Ryukyu Islands: physiological and paleoclimatic implications

We have constructed a 60-year stable isotope record from a C-dated fossil giant clam, Tridacna gigas (6216 years BP), at its northernmost latitudinal limit in the geological past, on Kume Island, Central Ryukyu Archipelago, Japan. Stable oxygen (δO) and carbon (δC) isotopic analyses are combined with observations of growth lines seen on the inner shell layer. Sixty pairs of summer/winter growth lines which preserve daily growth increments were observed in the inner shell layer. Two growth phases, characterized by a growth curve and isotopic profiles, are clearly recognized throughout the growth history of this specimen. No significant shifts in average values of the two isotopic ratios were detected during its growth history, although the growth rate varied widely from 1 mm to 15 mm per year over 60 years, including after the onset of sexual maturity. Spectral analysis of the fossil Tridacna δO

The evolution of narrow reef flats at high-latitude in the Ryukyu Islands

This paper describes a Holocene reef structure observed from a cutting in a modern reef on Okierabu Island in the Ryukyu Islands and proposes a process for the initiation and formation of narrow, lagoon-less fringing reef flats. Reef formation began around 7050 yBP at 11 m below present sea level. The reef was constructed by a uniform facies of in situ tabular corals and kept up with sea level rise. Geomorphological zonation has been restricted by the lack of a cross-reef energy gradient during reef formation. Sediment trapped by limestone caves abutting the shore has contributed to this characteristic. The slope break around-10 m and steep scarp of the shore create a narrow substrate that is responsible for the development of an equally narrow reef flat.

Influence of Land Development on Holocene Porites Coral Calcification at Nagura Bay, Ishigaki Island, Japan

To evaluate the relationships between coral calcification, thermal stress, and sedimentation and eutrophication linked to human impact (hereafter referred to as “land development”) by river discharge, we analyzed growth characteristics in the context of a paleoenvironment that was reconstructed from geochemical signals in modern and fossil (1.2 cal kyr BP and 3.5 cal kyr BP, respectively) massive Porites corals from Nagura Bay (“Nagura”) and from modern Porites corals from the estuary of the Todoroki River, Shiraho Reef (“Todoroki”). Both sites are on Ishigaki Island, Japan, and Nagura is located approximately 12 km west of Todoroki. At Nagura, the individual corals provide time windows of 13 (modern), 10 (1.2 cal kyr BP), and 38 yr in length (3.5 cal kyr BP). Here, we present the coral annual calcification for Nagura and Todoroki, and (bi) monthly resolved records of Sr/Ca (a proxy of sea surface temperature (SST)) and Ba/Ca (a proxy of sedimentation and nutrients related to land development) for Nagura. At Nagura, the winter SST was cooler by 2.8°C in the 1.2 cal kyr BP, and the annual and winter SSTs in the 3.5 cal kyr BP were cooler by 2.6°C and 4.6°C, respectively. The annual periodicity of Ba/Ca in modern coral is linked to river discharge and is associated with land development including sugar cane cultivation. Modern coral calcification also has declined with SST warming and increasing Ba/Ca peaks in winter. However, calcification of fossil corals does not appear to have been influenced by variations in Sr/Ca and Ba/Ca. Modern coral growth characteristics at Nagura and Todoroki indicate that coral growth is both spatially and temporally influenced by river discharge and land development. At Nagura, our findings suggest that land development induces negative thermal sensitivity for calcification in winter due to sugar cane harvest, which is a specifically modern phenomenon.

Formation of a coral reef-front spur

Theseawardmarginofcoralreefs,wherelivingorganismsare concentrated, is the site of the most persistent waveattack, and is the place where construction must takeplace if the reef is to sourish and expand (Ladd 1961).Spurs and grooves are a signiÞcant topographic constitu-entofmanyreef-edges.Althoughtheroleoferosionversusconstruction in the formation of spurs and grooves hasbeen controversial (e.g. Cloud 1959; Goreau 1959), fewstudies have focused on the processes generating thesemicro-topographic features. Shinn et al. (1977; 1981) de-termined the vertical growth of spurs by precise drilling.Planimetrically, the orientation of spurs and grooves isorthogonal to the direction of refracted waves, and spac-ing is related to wave energy (Munk and Sargent 1948;Emery et al. 1949; Weydert 1979; Sneh and Friedman1980; Blanchon and Jones 1995). However, no researchhas been carried out on the planimetric as opposed tovertical growth of spurs.The authors present an accretion process for spur ex-pansionbased on exposures of a cutting excavated acrossa modern reef-edge on the eastern coast of Cape Agari-henna (125

Coral records of the 1990s in the tropical northwest Pacific: ENSO, mass coral bleaching, and global warming

The decade of the 1990s was very likely the warmest of the second millennium. The 1990s was also characterized by one of the strongest El Nino-Southern Oscillation (ENSO) events of the twentieth century, which occurred in 1997–1998 and was followed by worldwide mass coral bleaching. In this study, we examined the signal-recording ability of Porites spp. corals by comparing high-resolution coral oxygen isotope (δ18O) records from the northwestern Pacific with instrumental records of these profound events of the 1990s. Temporal changes in coral skeletal δ18O records from Ishigaki Island, Japan, showed good agreement with instrumental records of sea-surface temperature (SST) because the effect of seasonal and interannual variations in salinity on the δ18O of seawater in that region was relatively small. In the northwestern equatorial Pacific, the cooler SST and relative drought-characterizing El Nino events were particularly well recorded by corals from the Philippines. These conditions were also faithfully recorded as distinct positive anomalies in coral δ18O records from Chuuk Atoll and Pohnpei Island in Micronesia. Bleached Porites spp. corals from Ishigaki Island, as well as corals from Pandora Reef of the Great Barrier Reef, Australia, showed a growth gap, indicating a substantial decrease in skeletogenesis during the 1997–1998 mass bleaching event. At Ishigaki Island and the Pacific side of the Philippine Islands, a decreasing trend in δ18Oc was found even over the relatively short period between 1980 and 2000 which may indicate an SST rise and/or surface-water freshening related to global warming. Our results demonstrate the potential of coral isotope records to serve as indicators of climatic change in the tropical and subtropical waters of the northwestern Pacific, where the number of coral records is still limited.

Response of the Great Barrier Reef to sea-level and environmental changes over the past 30,000 years

Previous drilling through submerged fossil coral reefs has greatly improved our understanding of the general pattern of sea-level change since the Last Glacial Maximum, however, how reefs responded to these changes remains uncertain. Here we document the evolution of the Great Barrier Reef (GBR), the world’s largest reef system, to major, abrupt environmental changes over the past 30 thousand years based on comprehensive sedimentological, biological and geochronological records from fossil reef cores. We show that reefs migrated seaward as sea level fell to its lowest level during the most recent glaciation (~20.5–20.7 thousand years ago (ka)), then landward as the shelf flooded and ocean temperatures increased during the subsequent deglacial period (~20–10 ka). Growth was interrupted by five reef-death events caused by subaerial exposure or sea-level rise outpacing reef growth. Around 10 ka, the reef drowned as the sea level continued to rise, flooding more of the shelf and causing a higher sediment flux. The GBR’s capacity for rapid lateral migration at rates of 0.2–1.5 m yr−1 (and the ability to recruit locally) suggest that, as an ecosystem, the GBR has been more resilient to past sea-level and temperature fluctuations than previously thought, but it has been highly sensitive to increased sediment input over centennial–millennial timescales.The Great Barrier Reef has migrated rapidly in response to sea-level changes since the last glacial period, suggesting resilience to environmental stress over this interval, according to a reconstruction of reef accretion.

Discovery of Iron Grapnel Anchors in Early Modern Ryukyu and Management of Underwater Cultural Heritage in Okinawa, Japan

The Yarabuoki underwater site contains seven iron grapnel anchors and Early Modern Okinawan ceramic jars and is dated to the 16th–19th centuries. The site lies at a depth of 12–32 m off the western coast of Ishigaki Island in Okinawa, Japan. Based on underwater archaeological and broadband multibeam surveys, as well as historical research of the artefacts and Early Modern Ryukyuan shipping, we discuss the possible anchor and vessel types in Ryukyu and Eastern Asia in Early Modern times. We also discuss the efficacy of low-cost ROV for assisting surveys in shallow-water environments and the value of educational programmes for promoting the management and conservation of underwater cultural heritage.