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In the Quaternary, the Ryukyu Islands evolved from a continental margin arc to an island arc by backarc spreading of the Okinawa Trough, accompanied by subsidence and isolation of the islands, a process that has continued to the present. Trough-parallel half grabens were filled with marine siltstone. Similar sediments filling orthogonal fault-controlled and west-draining non-tectonic valleys record island separation. New Quaternary nannofossil biostratigraphic data date the deposition of the marine siltstone at 1.552 ± 0.154 Ma. At that time, the entire 1000 km-long island chain comprising the Ryukyu Islands separated from the Asian continent by rifting extending from the Okinawa Trough to the Tsushima Strait. The Tokara, Kerama, and Yonaguni gaps, branched or transverse rifts of the Okinawa Trough, separate the island chain into subgroups of the Osumi, Amami, Okinawa, and Yaeyama islands, and Taiwan. The shallow Taiwan Strait separated Taiwan from the Chinese mainland. The Kuroshio warm current that previously ran offshore of the continental margin arc began to enter the opening backarc basin through the Yonaguni gap and to exit through the Tokara gap, flowing along the axis of the Okinawa Trough. Under influence of the warm current and because of entrapment of continentally sourced detrital sediments by the Okinawa Trough, coral reefs formed around each island. These reefs make up a unit called the Ryukyu limestone. Subsidence continued through the deposition of this limestone, resulting in further isolation of each island. Some islands did not separate from the mainland but emerged above sea level later as a result of volcanic edifice construction or forearc uplift. Following initial isolation, the Japanese islands and Taiwan may have been connected to the mainland by land bridges during some sea level low stands related to glacial periods, whereas the other islands remained isolated. Based on ages of isolation of each island, a Quaternary palaeogeographic map and ‘phylogenetic tree’ of the islands can be drawn showing the separation time of each island from the mainland and from each other. This information should be useful for phylogenetic molecular biologists studying evolution of Ryukyu endemic species and vicariant speciation and could facilitate analysis of the DNA substitution rate. [ABSTRACT FROM AUTHOR]

Abstract: Several identified ‘active faults’ located in the southwest Ryukyu area may be an origin of M5-class earthquakes taking place frequently every year, sometimes of M7-class earthquakes which cause a severe geohazard in this area. ‘The East Ishigaki Fault’, one of the ‘officially identified’ active faults, was studied precisely by multibeam echo sounding and the in-situ ROV observation. The survey expedition figured the topographic feature of the fault clearly and more precisely than that mapped by the Hydrographic Department of Japan. The survey also identified the locations of northwestern and southeastern termini of the fault, which revealed the total length to be 44 km: corresponding to a M7.4 earthquake. The new topographic map shows that the fault is composed of four or five segments with maximum vertical offset of ∼80 m in the southernmost segment; not all of the segment can slip at once. A reconnaissance survey of three of these segments by the ROV Hyper-Dolphin dives during the same expedition revealed that the headwall scarp is an outcrop made up of the Ryukyu Limestone of the Middle Pleistocene and that the lower scarp and the trough are covered with mostly talus apparently from the headwall. At least two of these segments look fresher in fault activity rather than the others, though all the observations show very recent activities along the fault. Based on these results, the East Ishigaki Fault, at least some of the segments, might cause the 1771 tsunami and also future tsunamis with the similar magnitude. [Copyright &y& Elsevier]

Provides information on a study which discussed geochemical variation within the northern Ryukyu arc. Tectonic and geological setting and samples; Analytical methods; Major and trace element compositions; Details on lead, strontium and neodymium isotopic compositions.

The Aoso volcano is a member of the newly defined volcanic front of Northeast Japan, characterized by the occurrence of low-K and hornblende andesites. Its activity can be divided into three stages: the early, caldera-forming, and late stages. While petrographic and geochemical data show all products underwent magma mixing or co-mingling, Sr and Nd isotopic ratios indicate that all are consanguineous. The end-member magmas are basaltic and andesitic in the early stage, but basaltic and dacitic in the late stage. In the caldera-forming stage, hornblende-free and hornblende-rich andesites co-mingled, which triggered an explosive eruption leading to caldera formation. Hornblende occurs also in the dacite from the early part of the late stage. These hornblende andesites and dacites are lower in magmatic temperature compared to hornblende-free andesites. The estimated basaltic end-member is low-K and high in magmatic temperature, and can be derived by a high degree of partial melting of mantle under the volcanic front. The estimated andesitic and dacitic end-members cannot be derived from the basaltic end-member magma through fractional crystallization, but can be derived from partial re-melting of the solidified low-K basalt, leaving amphibolitic and gabbroic residues, respectively. [ABSTRACT FROM AUTHOR]

New isotopic and trace element data presented here imply a temporal change in magma sources and thermal conditions beneath the northern Fossa Magna of NE Japan from the Miocene to the Pliocene. Less radiogenic 176 Hf/ 177 Hf and 143 Nd/ 144 Nd, high Zr/Hf, and little or no Hf anomaly characterize the Early Miocene volcanism in the northern Fossa Magna region. The mantle wedge consisted of chemically heterogeneous mantle source. Based on out isotope proxies, we propose that during the onset of subduction, influx of hot asthenospheric mantle provided sufficient heat to partially melt newly subducting sediment. Geochemical modeling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. In the Middle Miocene, the injection of hot and depleted asthenospheric material replaced the mantle beneath the northern Fossa Magna region of NE Japan. This caused the isotopic signature of the rocks to change from enriched to depleted. Then, the mantle wedge was gradually cooled during the Middle Miocene to the Pliocene with back-arc opening ending in the Late Miocene. Slab surface temperatures were still high enough for sediments to melt but not too high (<∼780 °C) to lose zircon as a residual phase. The Late Miocene and Pliocene volcanism at the post stage of the back-arc opening is best explained by a partial melting of subducted metasediment saturated with trace quantities of zircon and rutile. [ABSTRACT FROM AUTHOR]

Seasonally resolved records of carbon and oxygen isotopic composition (δ18 O) in fossil shells of giant clams ( Tridacnidae) recovered from the Kogachibaru Shell Mound (26°26′ N, 127°49′ E) and the Second Aragusuku- Shichabaru Ruin (26°18′ N, 127°46′ E) in Okinawa-jima, southwestern Japan, were investigated to reconstruct subtropical coral reef environments of the past. The samples, mainly composed of aragonite shells with limited amounts of diagenetically-altered calcite cements, were selected for geochemical analyses. The radiocarbon dating of the samples for stable isotope analyses yields about 4000 yr cal. BP for the Kogachibaru Shell Mound and about 1900-1700 yr cal. BP for the Second Aragusuku- Shichabaru Ruin, respectively, that correspond to the early and middle Shell Mound periods in Okinawa-jima. These show good agreement with the respective ages inferred from excavation. The shell δ18 O values roughly show seasonal variations, coincident with the occurrence of annual growth bands. Applying corrections for the effect of diagenetic alteration, the averages of annual, summer, and winter δ18 O values of fossil shells were significantly lower (by 0.1-0.7‰) than those of aragonite theoretically precipitated in present-day coral reef water of Okinawa-jima. These results demonstrate that annual mean seawater temperature and salinity at the sites where the Tridacnida spp. lived were about 1-3°C higher and/or about 1-2 lower than at present. This variation cannot be fully explained by global climate change, sea level change, and topographic growth of coral reef. Therefore, it is likely that the giant clams lived in extremely small and/or closed coral-reef lagoons with less water circulation where seawater is highly susceptible to insolation-induced temperature increase and input of fresh water; the effect could be enhanced by the fisheries lifestyle that stonewalling would be constructed at shallow waters through the use of tidal variation during the early and middle Shell Mound period in Okinawa-jima. [ABSTRACT FROM AUTHOR]

Abstract: In order to elucidate the sources of metals in urbanized areas, metal contents and Pb isotope ratios were determined in road-side dust and sediments samples collected from Tokyo, Osaka and Kyoto. The samples were leached using 1% HNO3 for metal concentrations and Pb isotopes analysis. Metal (Pb, Cd, Zn, As, Cr and Ni) concentrations and Pb isotopes (206Pb, 207Pb and 208Pb) in the solutions were analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The extent of anthropogenic contribution to road-side dust was assessed by comparing the metal concentration values in road-side dust of the sample sites to those of Okinawa, an unpolluted road-side dust site due to low traffic density. Considering a Pb background of 21.0ppm measured in the Okinawan road-side dust, Pb in road-side dust of Tokyo was found to be about twelve times higher which may indicate Pb pollution in the streets of Tokyo. The contaminated road-side dust can be transported into pond and river sediments during runoff and rainfall events. The concentrations of anthropogenic Pb in road-side dust and sediment were compared to those of other metals (Cd, Zn, As, Cr and Ni). There was a positive correlation found between Pb with Cd, Ni and Zn concentrations in sediment which suggests a common origin. The range of the Pb isotope ratios in road-side dust agreed well to that in the sediments (207Pb/206Pb=0.8644–0.8688 and 208Pb/206Pb=2.1044–2.170) suggesting that sediments may be contributed and transported by mean of road-side dust. Moreover, Pb isotope ratios (208Pb/206Pb and 207Pb/206Pb) confirmed that Pb pollution in the road-side dust and sediments was contributed the combination of battery and solder products. [Copyright &y& Elsevier]