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Both environmental and physiological factors cause carbonate ion structural disorder in biogenic Mg-calcites. A major component of this disorder is driven by the incorporation of Mg through environmental forcing and growth rate kinetics although non-Mg factors (e.g., other cation/anion impurities, organic molecules) also contribute. Understanding the drivers of Mg content in biogenic calcite and its effects on disorder has implications for octocoral Mg paleo-proxies and the stability and diagenetic alteration of their calcitic skeletons. However, prior studies of biogenic Mg-calcites have often been complicated by sampling inconsistencies over space and time and potential intra-sample Mg variability. This study aims to analyze the relative contributing factors of octocoral Mg-calcite structural disorder along gradients of both depth and growth rate. Calcitic octocorals (Coralliidae and Keratoisididae, N = 28) were collected from 221–823 m depths across a natural gradient in biogeochemical parameters (pH: 7.4–7.9, T: 5–16 °C) off the Kona coast of Hawai‘i Island and were analyzed using Raman spectroscopy. Samples were collected during the same month, controlling for potential seasonal variability. Raman spectral parameters from the ν1 peak quantified total carbonate ion structural disorder (full-width at half maximum height [FWHM] of ν1) and Mg content (ν1 position, Raman shift). The total structural disorder was then partitioned into Mg-driven and non-Mg driven components (residual ν1 FWHM). The total structural disorder and Mg content decreased significantly with increasing depth, correlating with temperature and carbonate system parameters. The Mg-temperature relationships from this study were also consistent with prior studies. Non-Mg structural disorder did not correlate to any environmental parameters. When measured across an intra-sample gradient of ontogenetic growth rate, total structural disorder, Mg content, and non-Mg structural disorder increased with growth rate for all but one taxon, demonstrating the kinetic effect of growth rate as well as potential taxon-specific physiological effects. These results provide insight into how environmental and growth rate kinetic effects independently affect different components of carbonate ion structural disorder (Mg content and non-Mg factors). These findings also suggest that Raman spectroscopy may be helpful in quantifying solubility within biogenic calcites.

Understanding the mechanism of selective extinction is important in predicting the impact of anthropogenic environmental changes on current ecosystems. The selective extinction of externally shelled cephalopods at the Cretaceous-Paleogene (K-Pg) mass extinction event (am­monoids versus nautiloids) is often studied, but its mechanism is still debated. We investigate the differences in metabolic rate between these two groups to further explore the causes of selective extinction. We use a novel metabolic proxy—the fraction of metabolic carbon in the stable carbon isotope ratio of shell material (Cmeta)—to determine metabolic rate. Using this approach, we document significant differences in Cmeta among modern cephalopod taxa (Nautilus spp., Argonauta argo, Dosidicus gigas, Sepia officinalis, and Spirula spirula). Our results are consistent with estimates based on oxygen consumption, suggesting that this proxy is a reliable indicator of metabolic rate. We then use this approach to determine the metabolic rates of ammonoids and nautiloids that lived at the end of the Cretaceous (Maastrichtian). Our results show that the nautiloid Eutrephoceras, which survived the K-Pg mass extinction event, possessed a lower metabolic rate than co-occurring ammonoids (Baculites, Eubaculites, Discoscaphites, and Hoploscaphites). We conclude that the lower metabolic rate in nautiloids was an advantage during a time of environmental deterioration (surface-water acidification and resulting decrease in plankton) following the Chicxulub asteroid impact.

[Aim]Quantifying the importance of habitat areas for conservation of highly migratory marine species with complex life histories can be challenging. For example loggerhead turtles (Caretta caretta) nesting in Japan forage both oceanically and neritically after their reproductive period. Here, we aimed to quantify the proportions of turtles using these two contrasting habitats (foraging dichotomy) to suggest priority conservation areas. [Location]North Pacific Ocean. [Methods]We examined the occurrence of foraging dichotomy at three nesting sites (Ishigaki, Okinoerabu Islands and Ichinomiya) based on stable isotope analysis of the egg yolks for 82 turtles and satellite tracking of post-nesting migration for 12 turtles. Moreover, we used the data of three other sites from previous studies (Yakushima Island, Minabe and Omaezaki). [Results]Two neritic foraging grounds (East China Sea and the coastal area of the Japanese archipelago), and an oceanic ground (North Pacific Ocean) were identified. We found a latitudinal cline with respect to the occurrence of foraging dichotomy; >84% of the females nesting at southern sites (Ishigaki and Okinoerabu Islands), 73% at middle sites (Yakushima Island and Minabe) and, Special Issue: Biological traits, geographic distributions and species conservation in aquatic ecosystems

The sudden eruption of submerged calderas can potentially pose imminent danger to nearby communities. Hence, it is crucial to closely monitor the changes in volatile effluents in these environments. Unlike subaerial volcanoes, underwater volcanic activity is difficult to observe. Wakamiko, a submerged caldera in southern Japan shares a magma source with the active subaerial volcano, Mt. Sakurajima . Following years of inactivity, the volcanic eruptions in Mt. Sakurajima became more frequent in 2009; however, it is undetermined how the neighboring Wakamiko caldera has behaved in relation to this upsurge in activity. In this study, we assess the state of hydrothermal venting inside Wakamiko following recent disturbances in Mt. Sakurajima. Based on gas bubbles, seawater and sediment pore water measurements of samples obtained in 2015, it appears that acidic fluids enriched in 3He continue to be discharged to the seafloor. Excess 3He in the water column shows no apparent change since 2010; however, the helium isotopic ratio of the magmatic source continues to be elevated at ∼7.2 Ra. This ratio for the magmatic component is higher than previously observed in 1986 (∼6 Ra). The difference could be related to changes in the degassing activity as well as the mantle composition of the magmatic source. Using the new data from sediment pore water, we estimate the diffusive fluxes of 3He and 4He through the seafloor at 16.05 atoms cm-2s-1 and 1.52 x 106 atoms cm-2s-1, respectively. We also calculated the vent 3He flux at 3.38 x104 atoms cm-2s-1. With the use of the 3He flux, we calculated the diffusive CO2 flux from Wakamiko at 1.41 x 106 mol yr-1 and the vent flux at 3.72 x 108 mol yr-1. The available data that we have are still limited and a more frequent and longer observation is necessary to deduce the temporal changes in Wakamiko and its relation to the volcanic activity of Mt. Sakurajima.

Tsunamis are huge disasters that can significantly damage benthic organisms and the sea-bottom environment in coastal areas. It is of great ecological importance to understand how benthic ecosystems respond to such destructive forces and how individual species are affected. Investigating the effect of such disasters on animals that are seldom caught alive is particularly difficult. Bivalve mollusks are especially suitable for investigating how a tsunami affects coastal benthic species because they preserve an environmental record in their shells that can be extended back in time by crossdating the records of multiple individuals. Here we studied dead shells of Mercenaria stimpsoni, a long-lived clam, and precisely determined the time of death by using nuclear bomb–induced radiocarbon (bomb-14C) and by counting annual growth increments. First, a quasi-continuous, regional bomb-14C record was created by analyzing the shells of 6 live-caught M. stimpsoni individuals. Then 27 dead shells collected from the seafloor of Funakoshi Bay were 14C-dated and analyzed. The results showed that the huge tsunami that struck northeastern Japan on 11 March 2011 caused mass mortality of this bivalve in Funakoshi Bay. Nine of the 27 clams died during the March 2011 tsunami, probably by starvation after burial by tsunami deposits or exposure above the seafloor as a result of sediment liquefaction during the earthquake. The dating method used in this study can help us understand how long-lived marine organisms with low population density are affected by huge natural disasters such as a tsunami.

We report here hourly variations of Mg/Ca, Sr/Ca, and Ba/Ca ratios in a Mediterranean mussel shell (Mytilus galloprovincialis) collected at the Otsuchi bay, on the Pacific coast of northeastern Japan. This bivalve was living in the intertidal zone, where such organisms are known to form a daily or bidaily growth line comprised of abundant organic matter. Mg/Ca ratios of the inner surface of the outer shell layer, corresponding to the most recent date, show cyclic changes at 25–90 μm intervals, while no interpretable variations are observed in Sr/Ca and Ba/Ca ratios. High Mg/Ca ratios were probably established by (1) cessation of the external supply of Ca and organic layer forming when the shell is closed at low tide, and (2) the strong binding of Mg to the organic layer, but not of Sr and Ba. Immediately following the great tsunami induced by the 2011 Tohoku earthquake, Mg/Ca enrichment occurred, up to 10 times that of normal low tide, while apparent Ba/Ca enrichment was observed for only a few days following the event, therefore serving a proxy of the past tsunami. Following the tsunami, periodic peaks and troughs in Mg/Ca continued, perhaps due to a biological memory effect as an endogenous clock.

We examined variations in age at seaward migration and sea age for the anadromous form of red-spotted masu salmon (Oncorhynchus masou ishikawae) in two Japanese rivers. The anadromous form of red-spotted masu salmon expressed only two sea migration patterns in the two rivers: (a) the majority of the salmon (95%, n = 81) were of age-0, and age-1 migrants were rare (n = 4); and (b) all the salmon examined (n = 22) made a return migration within a year, with 23% of the salmon exhibiting potamodromy in the river. Owing to low variation in their sea migratory patterns, the anadromous form of red-spotted masu salmon is likely vulnerable to environmental fluctuations.

In several parts of the world, mytilid mussels, Mytilus spp., are infected with pathogenic, single-celled, photosynthetic algae belonging to the genus Coccomyxa. The posterior shell edge of heavily infected mussels becomes considerably thickened with an extra shell material. Also, the external shell surface is usually eroded as a result of the microboring activity of endolithic cyanobacteria. We compared the number of bioeroded shells, the bioerosion degree, and the number of badly eroded shells, in uninfected and Coccomyxa-infected Mytilus spp. from the Lower St. Lawrence Estuary, Quebec, Canada. The thickness of prismatic and nacreous layers was measured. The epibionts (pink calcareous algae, crustose brown algae, and barnacles) which encrusted surface of studied shells, were counted. Epibionts did not occur frequently and their possible relationship with the partners of a three-way symbiosis, Coccomyxa sp. – Mytilus spp. – endolithic cyanobacteria, has been neglected. We suggest that the mussel provides the alga Coccomyxa a protected space and metabolic carbon for photosynthesis. The alga stimulates shell thickening, and this protects mussel against ocean acidification and predators. The endolithic cyanobacteria remove black-colored periostracum providing the mussel and alga with an increased ability to survive during sunny days when exposed at low tide. The eroded shells become more translucent which encourages alga photosynthesis. However, shell degradation caused by endolithic cyanobacteria is a possible reason for the death of the Coccomyxa-infected mussels at the studied sites.

The larval period is a critical stage for many aquatic organisms, because larvae are small, fragile, and have high mortality rates. Amphidromous fish spend their larval stages in the sea, and larval durations vary, possibly reflecting their dispersal tendencies. The genus Rhinogobius (suborder Gobioidei) is widely distributed throughout Asia and exhibits a variety of migratory patterns. Despite some studies of their migratory histories, larval durations of Rhinogobius species are uncertain. This study used analyses of otolith microstructure and trace-elemental chemistry to investigate habitat use throughout the life history of R. nagoyae. Twenty-six adult gobies were collected from seven rivers on Sado Island, in northern Japan, and otolith Sr : Ca ratios were quantified using electron microprobe analysis to infer amphidromous movement. All individuals showed shifts in Sr : Ca ratio across the otolith transect, which indicated movement between freshwater and saltwater. Marine larval duration was estimated by otolith increment counting, combined with Sr : Ca ratio. Larval duration ranged from 29 to 45 days (mean, 35.3), and there were no significant differences among rivers on the island. The relatively short larval duration of R. nagoyae, compared with other amphidromous fish such as ayu and galaxids, may reflect environmental factors such as island size, cool weather or species characteristics.

Sardine Sardinops sagax is an ecologically and economically important Clupeid found off the entire South African coast that includes both coastal upwelling and western boundary current systems. Although the management of the sardine fisheries historically assumed a single, panmictic population, the existence of three, semi-discrete subpopulations has recently been hypothesized. We conducted otolith δ18O and microstructure analyses to investigate nursery habitat temperatures and early life growth rates, respectively, of sardine collected from three biogeographic regions around South Africa’s coast to test that hypothesis. Analyses indicated that for both summer- and winter-captured adults and summer-captured juveniles, fishes from the west coast grew significantly slower in water that was several degrees cooler than those from the south and east coasts. This suggests that mixing of sardines between regions, particularly the west and other coasts, is relatively limited and supports the hypothesis of semi-discrete subpopulations. However, the west-south differences disappeared in the results for winter-captured juveniles, suggesting that differences in early life conditions between regions may change seasonally, and/or that all or most winter-captured juveniles originated from the west coast. Further elucidating the interactions between South African sardine subpopulations and the mechanisms thereof is important for sustainable harvesting of this species.

We examined the sea-entry conditions (i.e., timing and body size) of juvenile chum salmon that improve their survival during their coastal residency. On 25 June 2013, we sampled 365 juvenile chum salmon [57.5–98.6 mm fork length (FL)] off Konbumori, eastern Hokkaido, ⁓ 20 km east of the Kushiro River mouth, which originated from a hatchery in the Kushiro River. Sea-entry conditions of these Konbumori juveniles back-calculated using otolith daily increment analysis were compared with data from 373 juveniles released from the same hatchery that were captured at the mouth of Kushiro River (i.e., just before sea entry) from April to July 2013. Most of the Konbumori fish were estimated to have entered the sea from 25 May to 5 June, when coastal surface temperatures constantly exceeded 5 °C, which is considered favorable for juveniles. The estimated FLs at sea entry of the Konbumori fish were larger than FLs of fish sampled at the river mouth during a comparable period, which suggests that size-selective mortality existed. Back-calculated post-sea-entry growth rates of fish with larger FL at sea entry, particularly those with FL > 65 mm, tended to be high enough for survival among the Konbumori fish. Assuming growth-dependent mortality, this case study suggests that the release of larger-sized juveniles under favorable coastal temperature conditions improves their survival.

We conducted stable isotope (δ13C and δ15N) and gut content analyses to understand habitat use of juvenile reef fishes (Lethrinus lentjan, Lutjanus russellii, and Epinephelus coioides), particularly the influence of spatial habitat structure along seagrass–mangrove continua. Sampling was conducted in Setiu Lagoon, located in the southern waters of the South China Sea (Malaysia). Gut content analysis indicated that the focal fishes preyed mainly on benthic invertebrates and other smaller fishes; these were used as potential prey items for isotopic tracers to track fish habitat use. Stable isotope analyses highlighted that the juvenile reef fishes were highly associated with seagrass beds as feeding grounds; however, site-specific analyses suggest differences in fish feeding habitats. The juvenile fishes, particularly large juveniles, preyed in both mangrove and seagrass areas from the central lagoon, whereas all juveniles showed preferential foraging within seagrass meadows in the nearby lagoon mouth. Furthermore, the mean stable isotope values of muscle tissue differed among fish size classes, indicating their ontogenetic habitat/diet shifts, especially for L. lentjan and E. coioides. This study provides important information to support current efforts in protecting and preserving mangroves and seagrasses as crucial nursery habitats for juvenile reef fishes from the anthropogenic activities influencing Setiu Lagoon.

The long-distance migrations by marine fishes are difficult to track by field observation. Here, we propose a new method to track such migrations using stable nitrogen isotopic composition at the base of the food web (δ15 NBase ), which can be estimated by using compound-specific isotope analysis. δ15 NBase exclusively reflects the δ15 N of nitrate in the ocean at a regional scale and is not affected by the trophic position of sampled organisms. In other words, δ15 NBase allows for direct comparison of isotope ratios between proxy organisms of the isoscape and the target migratory animal. We initially constructed a δ15 NBase isoscape in the northern North Pacific by bulk and compound-specific isotope analyses of copepods (n = 360 and 24, respectively), and then we determined retrospective δ15 NBase values of spawning chum salmon (Oncorhynchus keta) from their vertebral centra (10 sections from each of two salmon). We then estimated the migration routes of chum salmon during their skeletal growth by using a state-space model. Our isotope tracking method successfully reproduced a known chum salmon migration route between the Okhotsk and Bering seas, and our findings suggest the presence of a new migration route to the Bering Sea Shelf during a later growth stage.

Multibeam (MB) and subbottom profile (SBP) data along the forearc slope off Miyagi, Japan, are combined to investigate structural-morphological and sedimentary structures along the forearc slope of the Japan Trench subduction zone. In addition to dip-oriented slope gullies, the MB data image a nearly dip-perpendicular slope trough bounded by a fault scarp landward of the trench-slope break. Seaward of the trench-slope break, the subbottom mostly contains normal faults that dip in opposite directions. The SBP data show not only unconformity and sliding surfaces, but also underfilled (forearc trough) and filled structures that may reflect the most recent forearc subsidence and basin filling. We propose these forearc trough and associated filling structures may indicate the earliest developments of modern isolated basins. Thus, a model of forearc basin development from slope gully to slope trough, isolated basins and forearc basin, transferred by active structures, could be proposed. Observed seafloor liquefaction in the SBP data may represent a near-surface seep structure of a plumbing system that enables hydrosphere-mantle fluid migration plausibly activated by earthquake cycles.

Temperature histories are critical for understanding and predicting ecological functioning in marine systems. Stable oxygen isotopes (δ18O) locked within calcified tissues can uncover experienced temperatures of marine animals, but have been little evaluated in the statoliths of cephalopods. As such, we investigated field applications of oxygen isotope ratios in statoliths (δ18Ostatoliths) for reconstructing the thermal histories of cuttlefish, octopus, and squid. In doing so, we collated measured (instrumental and modelled) temperature and salinity data, alongside δ18Ostatoliths data, of ocean-caught species from Taiwan and Australia. To navigate potential species-bias or “vital effects,” two aragonite-specific thermometry equations were compared, whilst simulation modelling evaluated variation and addressed uncertainties. Linear regressions identified environmental and biological influences on the differences between measured and reconstructed temperatures. Variable trends in temperature reconstructions were observed between taxa, which may be due to ecological traits. For squid, the relationship was highly aligned with no significant model predictors. For cuttlefish and octopus, differences between measured and reconstructed temperatures increased in warmer conditions, and may indicate thermoregulation behaviour. Here, we demonstrate that δ18Ostatoliths values can function as natural proxies of ocean temperature and are an invaluable tool for broadening the field of thermal ecology for the important, yet understudied, cephalopods.

Effective management decisions are vital for the declining Japanese eel Anguilla japonica. Elucidating the distribution range of wild eels (i.e., naturally recruited individuals) represents the first step in resource management, although stocking of cultured eels throughout the waters of Japan masks the original distribution range of wild eels. Herein, we applied a recently developed discrimination model based on otolith oxygen and carbon stable isotopic ratios (δ18O and δ13C) to determine the distribution range of wild eels throughout the Tone River watershed, which is the largest fisheries ground for this species. The model indicated that eels naturally recruit up to 200 km in the catchment area from the river mouth. Although natural recruitment was confirmed at all sampling sites, the percentages of wild eel occurrence at each site ranged from 56 to 100% of sampled individuals. The presence of wild individuals at sites above river-crossing structures indicates that some individuals successfully navigate through these structures. The operation of sluice gates and fish passes installed on artificial structures might facilitate the recruitment of individuals in areas above the structures and further upstream in the watershed. Difference in the percentages of wild eels among sampling sites might be a product of major variation in the abundance of cultured eels stocked at respective sampling sites and nearby areas.

Stable carbon isotope ratios (δ13C) of mussel soft tissues have been widely used to characterize baseline δ13C isoscapes and identify carbon sources at the base of coastal food webs. Extending soft tissue δ13C records back in time, however, is extremely challenging due to very limited sample availability. Here, we test if the stable carbon isotopic composition of periostracum (the outermost organic layer of the shell) in the Mediterranean mussel (Mytilus galloprovincialis) can be used as an environmental archive, similar to soft tissue records. In general, spatial and seasonal variations of periostracum δ13C values are comparable to those of soft tissues, but apparently the latter are more time-averaged and smoothed. Irrespective of such offset, there is a significant linear correlation between mussel periostracum and soft tissue δ13C values (R2 = 0.608, p

Recent dynamical models of Solar System evolution and isotope studies of rock-forming elements in meteorites have suggested that volatile-rich asteroids formed in the outer Solar System beyond Jupiter's orbit, despite being currently located in the main asteroid belt. The ambient temperature under which asteroids formed is a crucial diagnostic to pinpoint the original location of asteroids and is potentially determined by the abundance of volatiles they contain. In particular, abundances and 13C/12C ratios of carbonates in meteorites record the abundances of carbon-bearing volatile species in their parent asteroids. However, the sources of carbon for these carbonates remain poorly understood. Here we show that the Tagish Lake meteorite contains abundant carbonates with consistently high 13C/12C ratios. The high abundance of 13C-rich carbonates in Tagish Lake excludes organic matter as their main carbon source. Therefore, the Tagish Lake parent body, presumably a D-type asteroid, must have accreted a large amount of 13C-rich CO2 ice. The estimated 13C/12C and CO2/H2O ratios of ice in Tagish Lake are similar to those of cometary ice. Thus, we infer that at least some D-type asteroids formed in the cold outer Solar System and were subsequently transported into the inner Solar System owing to an orbital instability of the giant planets., 21 pages, 3 figures, revised version where only the reference numbering is modified

Shaking of the ground caused by earthquakes and wave loading on the seafloor during storms can liquefy seabed sediments, highlighting that seabed ecosystems can be substantially affected by modification of the substrate. To investigate exogenous-forced behavior of organisms in liquefied substrates we measured the body density of benthic organisms including bivalves, gastropods, annelids, and echinoderms collected from coastal areas of Japan, and compared them with the liquefied substrate density. The measured density for most benthic individuals was less than that of the liquefied substrate, suggesting that these species would float upwards in liquefied substrates. In contrast, a few individuals of three species including the corbicula clam (Corbicula japonica), Stimpson's hard clam (Mercenaria stimpsoni), and the sand dollar (Scaphechinus mirabilis) had higher body densities than the density of the liquefied substrate. These heavier benthic invertebrates would be expected to move downward during liquefaction of the substrate. These findings indicate that body density may determine the response of each benthic species to seabed liquefaction, forcing them to move upward or downward depending on their body density.

Coastal areas are socioeconomically important but are susceptible to disturbances by both natural and man-made events. On March 11, 2011, the eastern coast of Japan was seriously inundated by a mas...

Sea surface salinity (SSS) is a key parameter to understand and predict many physical, chemical and biological processes in dynamic coastal environments. Yet, in many regions, instrumental measurements are spatially sparse and insufficiently long, hindering our ability to document changes, causes, and consequences of SSS across different time scales. Therefore, there is an need to develop a robust proxy to extend SSS records back in time. Here, we test whether SSS can be reconstructed reliably and quantitatively from shell oxygen isotopic ratios (δ18Oshell) of the mussel Mytilus galloprovincialis (Lamarck, 1819) in Otsuchi Bay, Northern Japan. δ18Oshell ratios vary spatially and temporally and exhibit strong linear correlations with both sea surface temperature (SST) and SSS measurements, indicating that the composite signal recorded by δ18Oshell measurably responds to variations in both parameters. By combining contemporaneous variations of SST and δ18Oshell, SSS records encoded into mussel shells are deconvolved that significantly correlate with in situ SSS values. To further validate the robustness of δ18Oshell as a quantitative SSS proxy, high-resolution and temporally aligned time-series of δ18Oshell-derived SSS are reconstructed that are highly synchronous with the instrumental records. In particular, two lowered SSS scenarios occur concomitantly with periods of the summer monsoon and typhoon events. δ18Oshell-derived SSS time-series are also comparable to those from numerical modeling. In conclusion, our findings demonstrate that mussel δ18Oshell signatures can be used as a useful tool to construct high-resolution records of SSS in the coastal regions.

Geographical traceability of marine bivalves is critical to guarantee their quality and safeguard the interest of both consumers and producers. The neodymium isotopic ratio (143Nd/144Nd) of the coastal water mainly reflects the geology of its neighboring watershed, displaying the distinct and systematic variability at high level of geographical detail and thereby shedding light on its potential as a geochemical tracer. For the first time, the present study investigated the utility and robustness of 143Nd/144Nd archived in mytilid mussel shells for geographical traceability purposes. The reproducibility of 143Nd/144Nd ratios maintained in mussels shells from the same cohort demonstrates that the Nd isotopic ratio meets the major requirement for an ideal geochemical tracer, i.e., the biologically induced variation should be rather minimal. The distribution and variability of mussel shell 143Nd/144Nd patterns were subsequently mapped along the Japanese and Chinese coastal waters. Neodymium isotopes of mussel shells record 143Nd/144Nd variations among local regions and between the two countries, which are rather compatible with the ages and lithology of the continental bedrocks. These findings highlight the great potential of 143Nd/144Nd for tracing the geographical origin of marine bivalves.

Since magnesium concentration (Mg/Ca) in biogenic calcite is considered to reflect water temperature during precipitation, the magnesium-to‑calcium ratio has been examined as a proxy for water temperature in paleoclimate research, although factors other than temperature may also influence Mg/Ca in biogenic calcite, thereby introducing a potential bias in the relationship between Mg/Ca and temperature observed in inorganic systems. To better understand factors controlling Mg incorporation into the calcitic shells of bivalves, the distribution of Mg in the Mediterranean mussel Mytilus galloprovincialis was studied, being compared with ambient sea surface temperature (SST), shell growth rate and the distribution of organic matter. Although a positive relationship between Mg/Ca and SST was observed, Mg/Ca had been influenced by additional factors, including the enhancement of Mg incorporation by organic matter, evidenced by the growth line being more Mg-enriched than the growth increments. Furthermore, Mg/Ca was relatively enriched in the undulating (higher curvature) shell portion, being linked to neither SST, growth rate or organic matter. Zoning of Mg2+/Ca2+ within extrapallial fluid at the time of undulating shell portion formation was hypothesized, and heterogeneous Mg distribution in contemporaneously formed shell portions concluded as limiting the usability of Mytilus shell Mg/Ca as a proxy for water temperature.

The widely distributed East Asian Japanese eel Anguilla japonica constitutes a single genetically homogeneous population with a single spawning area near the West Mariana Ridge. Otolith 87Sr/86Sr ratios of adults (categorized as “river“, “estuarine” or “sea” eels, according to their habitat use history determined from otolith Sr:Ca ratio analysis) collected from the spawning area in 2008 and 2009 were examined in an attempt to determine their juvenile growth areas. In addition, 87Sr/86Sr ratios of water samples from rivers in China, Taiwan, Korea, and Japan were determined. Otolith 87Sr/86Sr ratios of the “river”, “estuarine” and “sea” eels were 0.707793, 0.708580-0.709944 and 0.709068, respectively, and water sample ratios from China, Korea, Taiwan, and Japan were 0.7104320-0.7141010, 0.7190826-0.7227976, 0.7115523-0.7146914 and 0.706191-0.712484, respectively. “River” and “estuarine” eels, which had otolith 87Sr/86Sr ratios less than 0.7092 (seawater ratio), appeared to have inhabited Japanese rivers and/or estuaries because similarly low isotope ratios were recorded only from Japan. However, the juvenile growth areas of other eels were unknown, as their origins could not be determined from otolith 87Sr/86Sr ratios alone, and required further information regarding otolith elemental and isotope compositions.

The helium isotope ratio is an important tracer of mantle-derived fluids. Different reservoirs in nature have distinct helium isotope signatures that make it possible to identify various sources and their mixing. At 5 years after the 2011 Tohoku-Oki earthquake, we continue to investigate possible fluid venting near the epicenter by measuring helium isotopes. We collected seawater and sediment samples in 2016 at the landward slope of the Japan Trench onboard R/V Shinsei Maru. Sediments were collected using a multiple corer. Seawater samples were obtained from different depths using a CTD rosette system. Pore water was extracted from sediments via centrifugation. Gas from each sample was then exsolved from solution and introduced into the in-house purification system equipped with charcoal traps and Ti getter. Helium isotopes were measured using a noble gas mass spectrometer. Our data show less excess 3He in bottom seawater samples than indicated by helium isotope ratios measured after the earthquake, which...