Kohki Sowa

Ministry of Japan

The seismic hazard related to megathrust earthquakes in the Ryukyus (southern Japan) is poorly constrained as no large earthquake has been reported there. The Meiwa tsunami impacted the coasts of the Yaeyama and Miyako islands in 1771 but its origin is still debated. Global Navigation Satellite Systems measurements indicate that strain is accumulat...

This study focuses on Okinawa and Yoron islands, in order to better understand tectonics in the Ryukyu Arc related to the subduction zone. We used coral microatolls—known for their centimetric accuracy in the record of relative sea‐level (RSL) changes—to reconstruct RSL changes over the last century from living microatolls. A fossil microatoll in Y...

Coral sclerochronology is a powerful tool for understanding environmental and ecological changes on coral reefs. Geochemical, isotopic, and skeletal density banding analyses along the major growth axis of massive coral skeletons from tropical shallow-water reefs have been used successfully to reconstruct decadal- to centennial-scale histories of cl...

Corals deposit calcium carbonate (CaCO3 : aragonite ) skeletons with paired high- and low-density bands that reflect past sea surface conditions during their growth. In addition, the elemental and isotopic compositions of a coral skeleton are affected by environmental factors during deposition, such as sea surface temperature (SST), sea surface sal...

Detailed observations of mineral phases in the skeleton of a massive reef-building coral were performed at nano-to micrometer scales using an analytical transmission electron microscope. The most notable result is the discovery of a nanosized halite (NaCl) phase coexisting with aragonite (CaCO 3) in the coral skeleton. The halite grains are scatter...

Detailed observations of mineral phases in the skeleton of a massive reef-building coral were performed at nano- to micrometer scales using an analytical transmission electron microscope. The most notable result is the discovery of a nanosized halite (NaCl) phase coexisting with aragonite (CaCO3) in the coral skeleton. The halite grains are scatter...

The response of Acropora digitifera to ocean acidification is determined using geochemical proxy measurements of the skeletal composition of A. digitifera cultured under a range of pH levels. We show that the chemical composition (δ11B, Sr/Ca, Mg/Ca, and Ba/Ca) of the coral skeletons can provide quantitative constraints on the effects of seawater p...

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...

Previous studies have not demonstrated the estimate errors in growth parameters (skeletal density, extension rate and calcification rate) for massive coral skeletons. In order to discuss the variability of coral skeletal growth, it is crucial for quantitative evaluation of the parameters with errors. We report the protocol of calculating errors as c...

The skeletal texture and crystal morphology of the massive reef-building coral Porites lobata were observed from the nano- to micrometer scale using an analytical transmission electron microscope (ATEM). The skeletal texture consists of centers of calcification (COCs) and fiber area. Fiber areas contain bundles of needle-like aragonite crystals tha...

The historical record of daily light cycle in tropical and subtropical regions is short. Moreover, it remains difficult to extract this cycle in the past from natural archives such as biogenic marine carbonates. Here we describe the precise analysis of Sr/Ca, Mg/Ca, and Ba/Ca ratios in a cultivated giant clam shell, using a laterally high-resolutio...

Massive coral skeletal growth parameters (skeletal density, extension and calcification rate) are one of the indexes of coral health and ecological response to the ambient environmental changes such as ocean acidification (OA) and global warming. To predict and evaluate the influence of the environment changes to the coral skeletal growth, the cora...

The El Niño/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3-5 million years ago) may have existed in a permanent El Niño state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean. This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide...

The skeleton of massive reef-building corals records the past tropical-temperate environmental conditions in which they grow. Coral skeletal luminescence bands and/or lines irradiated by ultraviolet light have been used to investigate past precipitation and river flood. In pioneer studies involving investigation of coral luminescent bands, it was f...

温帯域の造礁性サンゴは地球温暖化や海洋酸性化の影響を敏感に反映し，骨格に記録していることが期待される。温帯域に生息する造礁性サンゴ骨格を用いた古環境復元の可能性を検討するため，高知県土佐清水市竜串湾において塊状のPorites lutea（コブハマサンゴ）骨格のコア試料を採取し，骨格の酸素・炭素同位体比分析及び軟X線画像解析，蛍光バンド観察を行った。現場の水温変化と比較するとサンゴ骨格の酸素同位体比には低水温が反映されていなかった。また軟X線画像解析の結果，低水温時には高密度バンドを形成し，骨格伸長量及び石灰化量が高水温時に比べ大きく減衰することがわかった。以上の結果からも本研究試料のサンゴは低水温時に骨格成長速度が著しく低下していると考えられる。炭素同位体比の値は2001年から2008年に...

Chemical and isotopic compositions of the Acropora nobilis skeleton were analyzed at various spatial resolutions to investigate the mechanism by which elements are incorporated into the skeleton. Chemical and isotopic profiles along growth axes of axial and radial corallites did not show seasonal variation, with the exception of the δ18O profile of...

The variant of Sr/Ca ratio of coral skeletons is sensitive to the changes of sea surface temperature so that the Sr/Ca ratio is one of the archive for reconstructing paleo-seawater environment. To resolve the mechanism of variant of Sr/Ca ratio in coral skeletons, it should be useful to get mineral phase information corresponding to coral skeletal...

Various inforτnation on the geochemical characteristics of seawater is recorded in reefｭ building coral skeletons, which have been widely utilized for the high temporal resolution (one or two weeks) analysis of paleoenvironment in the tropical ocean. The stable oxygen/carbon isoｭ 旬pic ratios and trace element contents in coral skeletons 訂e generall...