Akira Ijiri

An exceptionally well-preserved fossil seep community from the Cretaceous Yezo Group in the Nakagawa area, Hokkaido, northern Japan

Abstract A well-preserved fossil seep community has been found in a carbonate lens in the Santonian to Campanian Omagari Formation, Upper Yezo Group in the Nakagawa region, Hokkaido, north Japan. The carbonate lens (roughly ellipsoidal in plan view with a diameter of 10 m × 6 m, and a thickness of about 5 m) is composed mainly of various types of high-Mg calcite containing several to 10 mol% magnesium and little iron or manganese. The carbonate lens is divided into an upper tube worm-dominated boundstone and a lower carbonate breccia facies. In the boundstone facies, concentric cements occur in the vestimentiferan tubes, indicating that the worm tubes were conduits for seepage. Layered to veinlike precipitates of high-Mg calcite occur in the boundstone facies. The carbonate breccia facies contains clast-supported carbonate breccia with sideritic, silty and tuffaceous matrices. Chemosynthetic bivalves occur in the upper zone of the carbonate breccia. The most common of these is the lucinid Miltha sp. Others include the lucinid Thyasira sp., and vesicomyid Calyptogena. Many small molluscs occur in the matrices of the carbonate breccia. The most common of these are trochid archaeogastropods; the others are two acmaeid limpets, mesogastropods and nuculacean bivalves. Small terebratulid brachiopods are also common. The carbonate lens, with its chemosynthetic bivalves and vestimentiferan worm tubes, may have been formed by bacterial sulfate reduction and anaerobic methane oxidation, as it shows extreme 13C-depletion (δ13C = −41 to −45‰). The Omagari community resembles the modern cold-seep communities along the landward slope of the subduction-zone complex off the Pacific coast of Japan.

The Tarama Knoll: Geochemical and Biological Profiles of Hydrothermal Activity

Tarama Knoll is located about 60 km north of Tarama Island, Sakishima Islands, southwestern Japan. The knoll has an almost conical shape, with foot and summit depths of 2,000 and 1,490 m (total relief = 510 m) from the sea surface, respectively. This area has been identified as a possible active submerged volcano called “Tarama Knoll” (Otani et al. 2004). However, there are actually two separate knolls in the area. This knoll is located northeast of the other, which is named Tarama Hill. During the KT05-26 cruise on the R/V Tanseimaru, a methane anomaly was detected near the seafloor around the area and was considered to be of possible hydrothermal origin. Based on visual observation of the seafloor and its bathymetry, this knoll is considered a pumice cone. Dense turbid water is often observed around summit of the knoll, and a methane anomaly was detected in the water. These observations suggest that the turbid water is a hydrothermal plume. An iron-rich, red-brown sediment-covered area was discovered at a depth of 1,510–1,540 m on the southwestern slope near the summit. At the red-brown sediment area, a weak shimmering of clear fluid could be observed, and the fluid temperature reached 20 °C. Sampled shimmering fluid showed a high silica concentration (≥1 mM), indicating an interaction between the fluid and the surrounding rock. These chemical data support the occurrence of active hydrothermal circulation at Tarama Knoll.

Distribution and Biogeochemical Properties of Hydrothermal Plumes in the Rodriguez Triple Junction

In 2010, we conducted seven surveys for the deep-sea hydrothermal plume through conductivity-temperature-depth profiler (CTD) “tow-yo” cast in the area of the Kairei field. We observed a turbidity anomaly with a maximum thickness of 120 m, the upper limit of which was at 2,150 m water depth, approximately 300 m above the Kairei field hydrothermal vents (~2,440 m). The depth of upper limit of turbidity anomaly around Kairei field was the same height as in previous reports. Because the maximum height of hydrothermal plumes are regulated by the density (temperature and salinity) of the end-member hydrothermal fluid and dilution by the ambient seawater, the height of the plume suggested that the hydrothermal activity of the Kairei field was also the same as 17 years ago. Deep sequencing of microbial 16S rRNA genes showed that the SUP05 phylotypes and Epsilonproteobacteria, which are known as the potential sulfur oxidizer and/or possibly hydrogen oxidizer, were propagated in the early stage of the hydrothermal plume and in the hydrothermal fluid–seawater mixing zone near the Kairei hydrothermal vents. Our exploration found a hydrothermal plume at 14 km north of the Kairei field, which had different H2/CH4 ratio expected from the end-member hydrothermal fluid of Kairei field and the ambient seawater mixing. The north plume had a lower H2, higher CH4 concentration, and higher microbial cell density than those in the hydrothermal plume around Kairei field. The north hydrothermal plume represented too oxic condition to harbor methane production by anaerobic methanogens. In addition, our microbial community structure analysis based on deep sequencing of 16S rRNA genes more than 10,000-amplicon reads per one sample showed no signal of methanogenic archaea. This suggests little in situ methanogenesis from H2 in the plume. It seems likely that high concentration of methane in the north plume is derived from another hydrothermal plume source rather than the Kairei hydrothermal fluids. Further studies will be needed to understand the cause of high methane concentration in the north plume.