Keiichi Sasaki

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.

Cold water coral mounds revealed

The discovery of mounds and reefs hosting cold-water coral ecosystems along the northeastern Atlantic continental margins has propelled a vigorous effort over the past decade to study the distribution of the mounds, surface sediments, the ecosystems they host, and their environments [Hovland et al., 1994; Freiwald and Roberts, 2005].This effort has involved swath bathymetry, remotely operated vehicle deployments, shallow coring, and seismic surveys. Global coverage is difficult to gauge, but studies indicate that cold-water corals may cover as large an area as the better known warm-water corals that form shallow reefs (284,300 square kilometers) [Freiwald et al., 2005]. Cold-water corals occur in a variety of forms and settings, from small isolated colonies or patch reefs to giant mound structures such as those found west of Ireland.

Temporal and vertical distributions of anthropogenic 236U in the Japan Sea using a coral core and seawater samples

The input history of 236U to the surface water of the Japan Sea was reconstructed through measurement of the 236U/238U atom ratio in annual bands of a coral skeleton which was collected at Iki Island in the Tsushima Strait, the main entrance to the Japan Sea. The 236U/238U atom ratios and concentrations of U isotopes were measured for the period 1935–2010 using AMS and ICP-MS. The 236U/238U atom ratios revealed three prominent peaks: 4.51 × 10−9 in 1955, 6.15 × 10−9 in 1959 and 4.14 × 10−9 in 1963; thereafter the isotope ratios gradually decreased over the next several decades, attaining a value of ca.1.3 × 10−9 for the present day. A simplified depth profile model for 236U in the Japan Sea, using the reconstructed 236U value for the surface water together with observed depth profiles for 236U in the water column in 2010, yielded diffusion coefficients of 3.4–5.6 cm2/s for 6 sampling points. The diffusion coefficient values obtained for the northern stations were relatively large, and fitting uncertainty was also larger for stations in the northern region. It may be presumed that the distribution of 236U in the water columns have been influenced not only by diffusion but also by subduction of the surface water in the Japan Sea.

Radium geochemistry in Na-Cl type groundwater in Niigata Prefecture, Japan

Radium isotopes in 23 Na-Cl type groundwater sampled mainly from deep wells in Niigata Prefecture, which is the site of the largest oil- and gas-fields in Japan, were measured along with U isotopes, chemical components and hydrogen and oxygen isotope ratios to elucidate the distribution and behavior of Ra in a brackish environment underground. Also analyzed were U and Th isotopes in 38 rock samples collected from outcrops at 17 locations. Ra-226 concentrations (8.86-1637 mBq kg(-1)) of groundwater samples roughly correlated with total dissolved solid (TDS) concentrations and other alkaline earth contents. Their (228)Ra/(226)Ra activity ratios (0.32-5.2) were similar to or higher than the (232)Th/(238)U activity ratios (0.6-1.7) in the rocks. The most likely transport mechanism of Ra isotopes into groundwater was due to their alpha-recoil from the solid phase, probably from the water-rock interface where Th isotopes had accumulated, and adsorption/desorption reaction based on the increase in (226)Ra contents with TDS.

Anomalously high 234U/238U activity ratios of Tatsunokuchi hot spring waters, Ishikawa Prefecture, Japan

Uranium concentration and the 234U/238U activity ratio have been measured for the Tatsunokuchi hot spring waters of Ishikawa Prefecture in Japan, collected periodically over a long period (1977-2000). The concentration of 238U varied drastically between 0.045 and 1.02 mBq/l (a factor of about 20), while the 234U concentration was almost unchanged, ranging from 2.30 to 3.07 mBq/l. Resultant 234U/238U activity ratios showed a wide range from 2.7 to 51. Equilibrium calculation by using the geochemical code showed that U for one end-member representing low uranium contents and very high 234U/238U ratios was expected to exist as UO2(CO3)22-. By using the U isotopic and 14C dating methods, the age of this water was roughly estimated to be in the range of 104-105 years.

Low-level measurement of the cosmogenic 22Na radionuclide in fresh water by ultra low-background gamma-ray spectrometry after simple radiochemical separation

A simple analytical method was developed for the determination of cosmogenic 22Na radionuclide in fresh water samples by ultra low-background g-ray spectrometry after radiochemical separation. The method consists in pre-concentration of 22Na(Na) by a ion exchange resin column from a large volume of ca. 500-liter water samples, elution with 4M HCl from the resin, and removal of the main metal components such as Ca and Mg as carbonate precipitates and of 40K(K) as potassium tetraphenylborate K[B(C6H5)4]. The obtained 22Na fraction was successfully measured by an ultra low-background Ge detector installed in the Ogoya Underground Laboratory.

Reconstruction of anthropogenic 129I temporal variation in the Japan Sea using a coral core sample

The anthropogenic long-lived radionuclide 129I is receiving increased attraction as a new oceanic tracer in addition to usage as a fingerprint of radionuclide contamination of the marine environment. To demonstrate the robustness of 129I as an oceanic tracer in the Northwest Pacific area, specifically in the Japan Sea, the input history of 129I to surface seawater was reconstructed using a hermatypic coral core sample from Iki Island in the Tsushima strait. Iodine isotopes in each annual band were measured using AMS and ICP-MS after appropriate pre-treatments of small amounts of coral powder. The 129I/127I ratios in the 1940s were almost at background levels (<1 × 10-11) and increased abruptly in the early 1950s. Thereafter, the ratios continuously increased with some fluctuations; the maximum ratio, 7.13 ± 0.72 × 10-11, being found in the late 1990s. After that period, the ratios remained nearly constant until the present time (2011). The 129I originated mainly from the nuclear weapons testings of the 1950s and the early 1960s, and from airborne releasing by nuclear reprocessing facilities. The dataset obtained here was used to construct a simple model to estimate the diffusion coefficient of 129I in the Japan Sea. The 129I/236U ratios over the observation period were also reconstructed to help constraining sources of 129I to the marine environment. Based on the results, the 129I/236U ratio obtained here could be an endmember of the water mass in the Kuroshio Current area of the Northwest Pacific Ocean.