Kotaro Shirai

Microdistribution of Mg/Ca, Sr/Ca, and Ba/Ca ratios in Pulleniatina obliquiloculata test by using a NanoSIMS: Implication for the vital effect mechanism

The Mg/Ca ratio within foraminiferal calcareous tests (shells) is widely used to reconstruct past seawater temperature. However, recent studies reported that the organic components within a test affect the Mg/Ca distribution. In this study, we have measured the Mg/Ca, Sr/Ca, and Ba/Ca ratios within the planktonic foraminifera Pulleniatina obliquiloculata by using a NanoSIMS (secondary ion mass spectrometer (SIMS)), which has excellent spatial resolution (∼1 μm) and allows us to compare the distribution of chemical compositions with that of the organic components. Element compositions show banding distributions composed of alternately higher and lower values of those elemental ratios. The Mg/Ca ratios, previously considered to be mainly controlled by calcification temperature, show larger variations than the values expected from the seawater temperature at the habitat depth of P. obliquiloculata. Comparison of the elemental distribution with the test microstructure reveals that the bands of high Mg/Ca and Sr/Ca ratios correspond with layers of the organic components. Such coincidence suggests that the organic components strongly affect the Mg/Ca and Sr/Ca ratios within a test. In spite of the heterogeneous distribution, temperature estimated from the averaged Mg/Ca ratio within a test is consistent with seawater temperature at the habitat depth of P. obliquiloculata, indicating that whole Mg/Ca ratio of foraminiferal test may be useful as paleotemperature proxy. In contrast to the Mg/Ca ratio the heterogenity in Ba/Ca ratio, which previously has been considered to be mainly controlled by the ambient seawater composition, is not fully matched with the layers of the organic compositions. Although the organic components concentrate Ba, other unknown factors appear to also cause heterogenity in Ba incorporation.

Disturbance of Shallow Marine Soft-Bottom Environments and Megabenthos Assemblages by a Huge Tsunami Induced by the 2011 M9.0 Tohoku-Oki Earthquake

Huge tsunami waves associated with megathrust earthquakes have a severe impact on shallow marine ecosystems. We investigated the impact of a tsunami generated by the 2011 M9.0 Tohoku-Oki earthquake on the seafloor and large benthic animals in muddy and sandy ria coasts (Otsuchi and Funakoshi bays) in northeastern Japan. We conducted underwater field surveys using scuba equipment in water depths of <20 m before the tsunami (September 2010) and after the tsunami (September 2011 and September 2012). During the study period, episodic changes in topography and grain-size composition occurred on the seafloor of the study area. Megabenthos sampling revealed a distinct pattern of distribution succession for each benthic species. For example, the protobranch bivalve Yoldia notabilis (Bivalvia: Nuculanidae) and the heterodont bivalve Felaniella usta (Bivalvia: Ungulinidae) disappeared after the tsunami event, whereas the distribution of the venus clam Gomphina melanaegis (Bivalvia: Veneridae) remained unchanged. In addition, the patterns of succession for a single species, such as the giant button top shell Umbonium costatum (Gastropoda: Trochidae) and the heart urchin Echinocardium cordatum (Echinoidea: Loveniidae), varied between the two bays studied. Our data also show that reestablishment of some benthic animal populations began within 18 months of the tsunami disturbance.

Past daily light cycle recorded in the strontium/calcium ratios of giant clam shells

The historical record of daily light cycle in tropical and subtropical regions is short. Moreover, it remains difficult to extract this cycle in the past from natural archives such as biogenic marine carbonates. Here we describe the precise analysis of Sr/Ca, Mg/Ca, and Ba/Ca ratios in a cultivated giant clam shell, using a laterally high-resolution secondary ion mass spectrometer with 2 μm resolution. The Sr/Ca ratio exhibits striking diurnal variations, reflecting the daily light cycle. A clear seasonal variation in Sr/Ca is also observed in another longer set of measurements with 50 μm resolution. Light-enhanced calcification and elemental transportation processes, in giant clam and symbiotic algae, may explain these diurnal and annual variations. This opens the possibility to develop the Sr/Ca ratio from a giant clam shell as an effective proxy for parameters of the daily light cycle.

Middle Holocene daily light cycle reconstructed from the strontium/calcium ratios of a fossil giant clam shell

Insolation is an important component of meteorological data because solar energy is the primary and direct driver of weather and climate. Previous analyses of cultivated giant clam shells revealed diurnal variation in the Sr/Ca ratio, which might reflect the influence of the daily light cycle. We applied proxy method to sample from prehistoric era, a fossil giant clam shell collected at Ishigaki Island in southern Japan. The specimen was alive during the middle Holocene and thus exposed to the warmest climate after the last glacial period. This bivalve species is known to form a growth line each day, as confirmed by the analysis of the Sr enrichment bands using EPMA and facilitated age-model. We analyzed the Sr/Ca, Mg/Ca and Ba/Ca ratios along the growth axis, measuring a 2-μm spot size at 2-μm interval using NanoSIMS. The Sr/Ca ratios in the winter layers are characterized by a striking diurnal cycle consisting of narrow growth lines with high Sr/Ca ratios and broad growth bands with low Sr/Ca ratios. These variations, which are consistent with those of the cultivated clam shell, indicate the potential for the reconstruction of the variation in solar insolation during the middle Holocene at a multi-hourly resolution.

Assessing the usefulness of otolith elemental compositions for evaluating the population structure of the Pacific herring Clupea pallasii in northern Japan

The present study examined whether the elemental compositions of whole otoliths and chronological transect profiles of otoliths covering the complete life history are useful for evaluating the population structure of Pacific herring. Eight elemental ratios, Li:Ca, Na:Ca, Mg:Ca, K:Ca, Mn:Ca, Cu:Ca, Sr:Ca, and Ba:Ca, were measured in whole otoliths and in a series of ablations across the life-history transects by laser ablation inductively coupled plasma mass spectrometry. We studied fish from three sample groups collected from Lake Obuchinuma, Miyako Bay, and Akkeshi Bay of northern Japan from March to May 2008. Significant differences were found in the Na:Ca, Mg:Ca, P:Ca, K:Ca, Mn:Ca, and Ba:Ca ratios of the whole otoliths and in the Li:Ca, Na:Ca, K:Ca, Mn:Ca, Sr:Ca, and Ba:Ca ratios of the otolith core among the three sample groups. Therefore, the present study showed clear differences in elemental composition among the three sample groups from different locations, suggesting that otolith elemental compositions reflect the habitats experienced by individual fish.

Discrimination of wild and cultured Japanese eels based on otolith stable isotope ratios

Discrimination of wild and cultured Japanese eels based on otolith stable isotope ratios Kenzo Kaifu*, Hikaru Itakura, Yosuke Amano, Kotaro Shirai, Kazuki Yokouchi, Ryoshiro Wakiya, Naoko Murakami-Sugihara, Izumi Washitani, and Takashi Yada Faculty of Law, Chuo University, 724-1 Higashinakano, Hachioji-shi, Tokyo 192-0393, Japan Graduate School of Science, Kobe University, 1-1, Rokkoudaichou, Nadaku, Kobe, Hyogo 657-8501, Japan Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3-27-5, Shinhama, Shiogama, Miyagi 985-0001, Japan Atmosphere and Ocean Research Institute, The University of Tokyo 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8564, Japan National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Yokosuka, Kanagawa 238-0316, Japan Research and Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Nikko, Tochigi 321-1661, Japan *Corresponding author: tel: þ81 42 674 3243; fax: þ81 42 674 3243; e-mail: kaifu@tamacc.chuo-u.ac.jp

PALEOECOLOGY OF ECHINODERMS IN COLD SEEP ENVIRONMENTS REVEALED BY ISOTOPE ANALYSIS IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

Abstract: A cold seep is an extreme environment characterized by a specialized group of organisms generally referred to as “chemosynthetic communities”. Until recently, echinoderms were thought to be rare in cold seep environments and had not been treated as a member of chemosynthetic communities, which otherwise are composed of a variety of taxa. One fossil echinoderm assemblage associated with a cold seep comes from the middle Campanian Pierre Shale Formation of the U.S. Western Interior Seaway. In this study, the taxonomy, morphology, and paleoecology of fossil echinoderms (crinoids and echinoids) from the Pierre Shale are discussed. Elemental chemical analyses of the fossil echinoderm skeletons and stable carbon isotopes were used to clarify the influence of seep hydrocarbons chemistry on the formation and diagenesis of echinoderm skeletons. We show that the crinoids Lakotacrinus brezinai from seep carbonates developed highly specialized morphologies and skeletons with low δ13C values, suggesting that it was adapted to cold seep environments and might be an obligate member of the chemosynthetic community. The echinoids from the Western Interior Seaway seep carbonates have morphologies and skeletal δ13C values not significantly different from those from non-seep environments, suggesting that the echinoids were not obligate members of the chemosynthetic community, but opportunists living in the periphery of the cold seep habitat.

Helium and methane sources and fluxes of shallow submarine hydrothermal plumes near the Tokara Islands, Southern Japan

Shallow submarine volcanoes have been newly discovered near the Tokara Islands, which are situated at the volcanic front of the northern Ryukyu Arc in southern Japan. Here, we report for the first time the volatile geochemistry of shallow hydrothermal plumes, which were sampled using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. These surveys were performed during the research cruise KS-14-10 of the R/V Shinsei Maru in a region stretching from the Wakamiko Crater to the Tokara Islands. The 3He flux and methane flux in the investigated area are estimated to be (0.99–2.6) × 104 atoms/cm2/sec and 6–60 t/yr, respectively. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. Methane at the Wakamiko Crater is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. This suggests that the Tokara Islands submarine volcanoes are a key feature of the transition zone between the volcanic front and the spreading back-arc basin.

Stimpson's hard clam Mercenaria stimpsoni; A multi-decadal climate recorder for the northwest Pacific coast

A sclerochronological and radiocarbon-based study of life history traits of Stimpsons hard clam (Mercenaria stimpsoni), collected alive from Funakoshi Bay, northeast Japan, showed the lifespan of the species to be at least 92 years (determined from annual growth line counts). Three M. stimpsoni specimens exhibited the following synchronous growth pattern, suggestive of environmental control; annual increment width increasing after 1955 to a maximum value between 1970 and 1980, subsequently decreasing gradually until 2000, and thereafter remaining constant or increasing slightly. Variations on annual growth patterns, as well as standardized growth indices chronology, were relatively closely linked to the Atlantic Multidecadal Oscillation (AMO), but less so to Pacific Decadal Oscillation (PDO). Carbonate samples collected from ontogenetically younger shell portions, estimated from growth line counts to have been deposited before 1950, contained no nuclear bomb-test radiocarbon, thereby supporting the accuracy of annual growth line counts (versus overcounting from ventral margin). Together with the synchronous annual increment width patterns, this indicated that age and annual growth rate estimations for M. stimpsoni based on growth line counts were reliable and applicable to high-resolution sclerochronological analyses, which should contribute to a deeper understanding of multi-decadal northwest Pacific climate variability.

Architecture of crossed-lamellar bivalve shells: The southern giant clam (Tridacna derasa, Röding, 1798)

Tridacna derasa shells show a crossed lamellar microstructure consisting of three hierarchical lamellar structural orders. The mineral part is intimately intergrown with 0.9 wt% organics, namely polysaccharides, glycosylated and unglycosylated proteins and lipids, identified by Fourier transform infrared spectrometry. Transmission electron microscopy shows nanometre-sized grains with irregular grain boundaries and abundant voids. Twinning is observed across all spatial scales and results in a spread of the crystal orientation angles. Electron backscatter diffraction analysis shows a strong fibre texture with the [001] axes of aragonite aligned radially to the shell surface. The aragonitic [100] and [010] axes are oriented randomly around [001]. The random orientation of anisotropic crystallographic directions in this plane reduces anisotropy of the Youngs modulus and adds to the optimization of mechanical properties of bivalve shells.