Tsutomu Yamada

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.

Movement of radiocaesium fallout released by the 2011 Fukushima nuclear accident

The moment magnitude (Mw) 9.0 Tohoku-Oki Earthquake occurred on March 11, 2011, generating an unusually large tsunami. The seismic shocks and tsunami inundation severely damaged the Fukushima Daiichi Nuclear Power Plant. Radionuclide emission due to reactor breakdown contaminated wide areas of Fukushima and its surroundings. Heavy rainfall causes runoff across surface soil, and fine soil particles are susceptible to uptake by the flowing water. The high radioactivity of grains suspended in floodwater indicates that radioactive fallout was streamed into rivers in particulate form and transported downstream under high-flow conditions. Here, we investigated the diachronic mode of 134Cs and 137Cs in central Fukushima, through which the contaminated air mass drifted and caused wet deposition of radionuclides. Stratigraphic measurements of radioactivity in sediment cores is the method employed in this study to determine the basin-wide movement of 134Cs and 137Cs, to evaluate the significance of the erosion–transportation–accumulation processes on natural decontamination in terrain characterized by steep slopes and high precipitation. Stratigraphic results illustrate the process of fluvial sediment discharge, and the massive deposition of radiocaesium suggests basin-wide movement of fallout during concentrated rainfall. Grain suspension in torrential currents is an important pathway for transportation of radionuclides from land to sea, and the appearance of hotspots on floodplains and the offshore sea floor is the consequence of erosion and transportation under seasonal heavy precipitation. Radioactive horizons occur in offshore sediment columns and thus radiocaesium discharged from the estuary will persist forever under the sea floor if no artificial disturbance occurs.

Aftereffects of Subduction-Zone Earthquakes: Potential Tsunami Hazards along the Japan Sea Coast

The 2011 Tohoku-Oki Earthquake is a typical subduction-zone earthquake and is the 4th largest earthquake after the beginning of instrumental observation of earthquakes in the 19th century. In fact, the 2011 Tohoku-Oki Earthquake displaced the northeast Japan island arc horizontally and vertically. The displacement largely changed the tectonic situation of the arc from compressive to tensile. The 9th century in Japan was a period of natural hazards caused by frequent large-scale earthquakes. The aseismic tsunamis that inflicted damage on the Japan Sea coast in the 11th century were related to the occurrence of massive earthquakes that represented the final stage of a period of high seismic activity. Anti-compressive tectonics triggered by the subduction-zone earthquakes induced gravitational instability, which resulted in the generation of tsunamis caused by slope failing at the arc-back-arc boundary. The crustal displacement after the 2011 earthquake infers an increased risk of unexpected local tsunami flooding in the Japan Sea coastal areas.

Controlling factors in stalagmite oxygen isotopic composition and the paleoprecipitation record for the last 1,100 years in Northeast Japan

Maboroshi Cave in Hiroshima Prefecture in western Japan, and they compared their data with data from Chinese stalagmites and Greenland ice cores. Sone et al. (2013) reconstructed the EAWM intensity and winter precipitation over the past 10,000 years. They analyzed a stalagmite from the Fukugaguchi Cave in Niigata Prefecture on the Japan Sea side of central Japan, a location that is subjected to heavy winter snowfall. However, the influences of EAM and EAWM in Northeast Japan have not been sufficiently clarified. Climate reconstructions using stalagmites have not been pursued, despite the presence of many limestone caverns along the Pacific side of Northeast Japan. This study was conducted to identify the major controlling factors that determine stalagmite oxygen isotopic compositions in Northeast Japan, through comparison of a high-resolution stalagmite d18O profile and meteorological observational data for the last several decades. The history of paleoclimatic change and its influence on human activity in this region over the last 1,100 years were reconstructed from a stalagmite d18O profile.