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Volume 111, Issue 10
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Review
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Akihiro Kano, IODP Expedition 307 shipboard-scientists2005 Volume 111 Issue 10 Pages 571-580
Published: 2005
Released on J-STAGE: February 01, 2006
JOURNAL FREE ACCESSCoral reefs are commonly considered to have developed in tropical-subtropical shallow-water regions with typical carbonate depositional environments. However, discoveries of deep-water coral reefs in the last decade have overthrown this traditional overview. The deep-water reefs are ubiquitous at least in the NE Atlantic, and distributed in all major oceans. Cold-water corals, the major constructors of the deep-water reefs, differ from tropical-subtropical reef-building corals, in terms of their heterotrophic habitat independent of light. They prefer to settle on hard and stable substrates, such as boulders and seamounts, and construct conic carbonate bodies under the oceanic conditions of high nutrient content and low sediment influx.
The Porcupine Seabight on Ireland offshore is one of the most intensively studied provinces. There, thousands of the Pleistocene-recent coral reefs occur as carbonate bodies up to 2 km in width and 200 m in thickness. Depth of dense reef development ranges from 600 to 900 m, where the bottom current is enhanced by the internal tidal wave at the boundary between the Mediterranean Outflow Water (MOW) and Eastern North Atlantic Water (ENAW). Sediments recovered in IODP Expedition 307 revealed that a reef body consists of fragments of Lopheria pertusa enclosed in fine-grained matrix of clay, bioclasts, and calcareous nannofossils. The conic reef body have been kept growing due to sediment buffering by branching colonies of L. pertusa, which were fragmented by intensive bioerosion before the burial.
The deep-water coral reefs share fine-grained sediment composition and conic geometry with the Phanerozoic carbonate mud mounds, for which several different hypotheses have been proposed to explain their origins. They also resemble to the uppermost Jurassic-lower Cretaceous limestones in Japanese Islands in abrupt appearances within stratified non-carbonate sediments. Further researches are expected to provide significant insights for understanding environmental settings and processes of these ancient deposits, as well as the deep-water reefs themselves.
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Articles
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Yoshihiro Ganzawa, Norio Kito, Seiji Yanai, Noboru Sadakata2005 Volume 111 Issue 10 Pages 581-589
Published: 2005
Released on J-STAGE: February 01, 2006
JOURNAL FREE ACCESSThe early volcanic eruptive products, five tephra layers, of Hokkaido-Komagatake volcano located in southwest Hokkaido were found beneath Ko-h(17 ka). All tephras are predominantly abundant in plagioclase and spongy glass, and contain orthopyroxene and clinopyroxene. The refractive index of orthopyroxene (γ) ranges from 1.706 to 1.713. These mineral characteristics indicate that the tephras were supplied from Hokkaido-Komagatake volcano. The age of the earliest volcanic activity of E-x, situated beneath Toya ash, was estimated to be a middle stage of OIS5d, according to a pollen analysis of the peat layer intercalated beneath the E-x. An interval of the early stage volcanic activities from E-x to Ko-h was assessed between 12 ka and 32 ka judging from tephra ages and the thickness of loess deposits interbedded between tephra layers.
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Kuniyuki Furukawa, Hiroki Kamata2005 Volume 111 Issue 10 Pages 590-598
Published: 2005
Released on J-STAGE: February 01, 2006
JOURNAL FREE ACCESSThis study describes volcanic structures of four drilling cores obtained from the Takanoobane rhyolite lava distributed in Aso caldera, Japan and discuss on development of the structures of the lava. The Takanoobane lava is divided into three parts: Alternation of the pumiceous layers and the obsidian layers (the upper part), the crystalline rhyolite layer (the central part), and the obsidian layer (the lower part). This variation of internal structures is interpreted to have been formed as a result of cooling and degassing processes during flowage. The central crystalline rhyolite layer contains many minute cavity parts defined as crystalline with many minute cavities. The minute cavity parts were formed immediately before or after the extrusion. At distal drilling site, the minute cavity parts were flattened and show flow layering. The flow layering shows high tilt angle in upper part of the crystalline rhyolite layer, suggesting a ramp structure.
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Fujio Kumon, Kinuyo Kanamaru, Takaharu Tawara, Naoko Kakuta, Masamichi ...2005 Volume 111 Issue 10 Pages 599-609
Published: 2005
Released on J-STAGE: February 01, 2006
JOURNAL FREE ACCESSTotal organic carbon content (TOC) of lake sediments has been used recently as a useful paleoclimate proxy. But, no study has shown the genuine relationship between TOC content of lake sediments and climate parameters. Therefore, we try to examine modern relationships among TOC in lake sediments, biological productivity of lake water and weather factors using limnological observation data conducted monthly in Lake Kizaki since AD 1981.
The sediment cored on 11 December 2003 was cut into 0.5 cm interval, and apparent density and TOC content were measured. Apparent density peaks can be correlated with the known flood events. Sedimentation rates were determined precisely on the basis of flood ages, and were used for the age determination of the cored sediment. Then, annual TOC content can be determined from AD 1981 to 2002. Monthly measurements of chlorophyll a amount in water column were integrated to annual chlorophyll a amount which can be a representative of annual biological productivity. Meteorological data at Omachi City, 4 km south to the lake were used as weather factors during the same period. As a result, we have found a correlation (n = 21, r = 0.47) between winter temperature and annual chlorophyll a amount. The TOC of sediment also shows some concordance with chlorophyll a (n = 21, r = 0.50) and winter temperature (n = 23, r = 0.44). When we compare TOC content in sediments with 3-year moving average of winter temperature and annual chlorophyll a, the correlations between them become much better. These results suggest that organic carbon of lake sediment reflects the lake biological productivity which is controlled by winter temperature, namely, shortness of the coldest season. It is concluded that the TOC content of lake sediments can be used as a useful paleoclimate proxy when sedimentation rate is almost constant.
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Fumito Shiraishi, Yasutaka Hayasaka, Yoshio Takahashi, Masaharu Tanimi ...2005 Volume 111 Issue 10 Pages 610-623
Published: 2005
Released on J-STAGE: February 01, 2006
JOURNAL FREE ACCESSStrontium isotopic ratio provides high-resolution ages of the limestone of Yatsuji Formation (Torinosu Group) in Niyodo Village (Kochi Prefecture), on the basis of considering diagenetic effects. Results of cathodoluminescence and trace elements analysis revealed that brachiopod shell is retaining the original marine strontium isotopic value. Stromatoporoids and a calcified sponge Chatetopsis sp. tend to record higher values than the brachiopod shells collected from the same horizon. Their differences are attain to +0.00005, and which corresponds to 2.8 m.y. during late Jurassic to early Cretaceous periods. Therefore, we concluded that stromatoporoids and Chatetopsis sp. are not suitable for the precise age determination. Obtained strontium isotope data of brachiopod shells are converted to depositional age by fitting to recently proposed strontium isotopic curve (LOWESS Look-up Table Version 4). The depositional age of the limestone body of the lower Yatsuji Formation is 146.1-148.4 Ma (middle Tithonian). This age is consistent with microfossil ages, and has higher time resolution.
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Prompt Report
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Yujiro Ogawa, Katsuo Sashida2005 Volume 111 Issue 10 Pages 624-627
Published: 2005
Released on J-STAGE: February 01, 2006
JOURNAL FREE ACCESSA meter-scale, bedded red radiolarian chert block was found on the Yo-oka Beach at the eastern edge of the Mineoka Belt, Boso Peninsula, central Japan. The block is included in the area composed of fragments of the Mineoka ophiolite and other sedimentary and igneous rocks, including a Paleocene to Miocene pelagic limestone/chert sequence. The radiolarian fauna indicates an assemblage zone of middle to late Albian (Early Cretaceous), partly corresponding to the age of the Tethyan fauna. The tectonic significance of this Cretaceous radiolarian chert block is not clear: it could be from either the Mineoka ophiolite and related oceanic plate stratigraphy or from the Shimanto Supergroup.
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