Due to the international GEOTRACES program, numbers of seawater REE concentration and Nd isotopic composition data have been drastically increasing. This indicates we can now easily compare those data obtained at the same oceanic regions at the different timings, i.e., years and seasons. In this presentation, we are going to discuss temporal and seasonal changes of REE abundances and Nd isotopic composition in the same oceanic regions of the North Pacific, North Atlantic, and South China Sea.

We analyzed arsenic in surface sediments and porewaters from Joetsu Knoll, which collected during the environmental survey for shallow-type methane hydrate. Arsenic concentration of sediments increased in the sulfate reduction zone at the reference site, but did not increase around the bacterial mats, despite its active sulfate reduction-anaerobic methane oxidation. Arsenic concentrations of porewaters generally varied 5-100 µg/L, and the depth of concentration peak increased with distance from the bacteria mat. The arsenic concentration of porewater was exceptionally high at the red snow crab aggregation site (105-187 µg/L), which may be attributed to the decomposition of organic matter in the deep sediments. Furthermore, speciation analysis of arsenic in the porewater detected unknown arsenic compounds, suggesting that leached arsenic may have become thioarsenic compounds in the presence of hydrogen sulfide. This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).

Traditionally, the oxygen consumption rates of organic particles in aquatic environments have remained largely unexplored due to measurement difficulties, and the factors controlling these rates are unknown. In this study, we aimed to elucidate the primary carriers of oxygen consumption by fractionating organic particles in water by size and culturing them under in-situ light and temperature conditions. Measurements conducted in Lake Biwa revealed that the fraction of particles larger than 1.2 µm exhibited respiration rates close to the total, whereas organic particles smaller than 1.2 µm contributed minimally to respiration. On the other hand, it is generally known that dissolved organic particles are more prevalent. These results suggest that dissolved particles are stable, and organic particles larger than 1.2 µm are the controlling factors for oxygen consumption in aquatic environments. Furthermore, it was found that particles larger than 1.2 µm have shorter lifespans, indicating that events causing their descent could directly lead to the formation of hypoxic water masses.

Submarine mud volcanoes (SMVs) are mounds formed when high pore pressure sediments at deep subsurface ascend as mud diapirs and erupt onto the seafloor. They emit deep subsurface materials into the ocean including methane. Recent studies suggest that dissolved organic carbon (DOC) is produced as a byproduct associated with anoxic methane oxidation in the surface sediments at methane seeps, thereby contributing to the ocean carbon cycle. Since SMVs are methane seeps, they may also release DOC to the ocean. However, there have been few reports on DOC release from SMVs to the ocean. Thus, the release amounts and origins of DOC from SMVs remain unclear. In this study, we investigated the release amounts and origins of DOC from SMVs by analyzing DOC concentrations in water samples taken above SMVs. Additionally, we explored the correlation between DOC and dissolved CH₄ released from SMVs by analyzing dissolved CH₄ concentrations in the same water samples.

Global warming has been reported to have a number of effects on material dynamics, including an increase in ocean surface water temperature, changes in land vegetation, the occurrence of linear precipitation belt, the occurrence of slope collapses due to heavy rain, and increased soil erosion. In addition, human activities have caused changes in the land surface environment throughout Japan, including an increase in abandoned farmland and forest management land due to the declining birthrate and aging population. Therefore, in order to evaluate the sustainability and resilience of coastal oceans, it is important to combine land and sea and evaluate them as a system including ecosystems from the perspective of material dynamics. In this presentation, the basic concept of land-sea coupled systems and observation examples from survey areas in the Noto Peninsula and eastern Hokkaido will be introduced, and the importance of research that connects land and sea and evaluates them as a single watershed will be mentioned.

Large amount of 137Cs was released to marine environments by the Fukushima Dai-ichi Nuclear Power Plant accident. In this study, we investigated the surface seawater circulation in the Bering Sea and Arctic Oceans using 137Cs. Seawater samples were collected during Oshoro Maru and Mirai expeditions in 2022-2023. The concentration of 137Cs was measured using low background gamma-spectrometry via. AMP precipitation. The 137Cs concentration at the surface in the Bering Sea showed a decrease after 2022, following the increase from 2017. Significant variations in 137Cs concentration were also observed in the same latitude region in the Bering Sea, reflecting the complex seawater circulation. Comparing with 228Ra concentration, we clarify the circulation processes of 137Cs (and other dissolved components).

134-Cs is an artificial radionuclide released into the marine environment by the 2011Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, and is subsequentlytransported along with currents. From the short half-life (2.06 years) and solublecharacteristics, 134-Cs currently detected in seawater is derived from the FDNPP accident. Therefore, 134-Cs is a strong chemical tracer to study the seawater circulations. In this study, we examined the temporal variation of 134-Cs concentrations in the sea surface off the Doto and the southwestern Okhotsk Sea from 2018 to 2024 and discuss the subarctic current systems in the northwestern North Pacific Ocean, including water circulation timespans.

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident caused the release of radiocesium (¹³⁴Cs and ¹³⁷Cs) into the marine environment. From the short half-life (2.06 years) and soluble characteristics, ¹³⁴Cs currently detected in seawater is derived from the FDNPP accident. ¹³⁴Cs has partially transported from the FDNPP accident to southwestern Okhotsk Sea and off the Doto via. counterclockwise current in the North Pacific Ocean, the Kamchatka - Oyashio Current. We examined vertical profiles of ¹³⁴Cs concentrations in these regions during 2020-2024 and discuss current systems in the Okhotsk Sea and out flow pattern to the Pacific Ocean side.

In this study, oxygen isotope ratio (d¹⁸O_sw, n = 236) data, collected in the western Pacific Ocean in summer 2022 during the La Niña event, were used to elucidate the mechanism of determining d¹⁸O_sw distribution in the western Pacific Ocean and to examine the potential of d¹⁸O_sw data as a water mass tracer. In the 2022 La Niña event, the Intertropical Convergence Zone (ITCZ) existed between 5 and 10ºN latitude, and precipitation lowered d¹⁸O_sw. The salinity-d¹⁸O_sw relationship in the western equatorial Pacific south of 20ºN , including the ITCZ, has a larger slope and a smaller intercept (-9.56 permille) than that of the Global Seawater Oxygen-18 Database (LeGrande and Schmidt , 2006), which may reflect the atmospheric and oceanic conditions during the La Niña event. In the subtropical high-pressure zone around 20ºN and 160ºE in the western Pacific during the La Niña event, evaporation was preferential and d¹⁸O_sw was high, while the influence of high-latitude rivers was strong and d¹⁸O_sw was low in the region influenced by the Oyashio Current north of 30ºN. The d¹⁸O_sw data for the vertical samples showed that the North Pacific Intermediate Water (NPIW) is marked by d¹⁸O_sw values of -0.3 ~ -0.1 permille and can be traced to 23ºN.

Nitrogen and oxygen stable isotope ratios (D¹⁵N, D¹⁸O) are used as robust tracers for the nitrogen cycle in the ocean. Using a highly sensitive quantification method for stable isotopes allow us to determine the proportion of atmospheric nitrate in total oceanic nitrate from d (15,18) value. An extraction device capable of introducing the entire sample was constructed, which enabled the introduction of four times the amount of the conventional method, and the measurement accuracy was achieved at 0.3 µM, which is the same as the conventional method. Using this method, nitrate and their isotopes were measured every two months at seven layers up to 200 m depth in Suruga Bay. The results showed that d (15,18) values at less than 50 m were lower than those in the subsurface layer from February to June, and that atmospheric nitrate contributed 1-3% to the water depth of 100 -150 m during the same period.

Ocean acidification (decreased pH) and rising temperatures caused by global warming may weaken the bonds that bind toxic heavy metals to sediment particles, potentially increasing their solubility in seawater and posing greater ecological risks. This study aims to understand the current behavior and potential solubility of toxic heavy metals, particularly lead (Pb), in a semi-closed coastal area (West Nanao Bay). Initial results of two cores show significant increases, at least by a factor of 1.5, in detrital Pb since the 1950s for NW-11 and NW-7 cores. Most elements showed minor enrichment factors (EF ＜ 3), reflecting an anthropogenic input. Mining, fossil fuel combustion, maritime transport, and land use changes were associated with the temporal changes in sediments. High concentrations of toxic metals within the uppermost surface sediments (0-10 cm) pose a potential risk due to their high solubility rates. They could potentially accumulate in higher predators through the food chain. Furthermore, additional Nanao Bay sediments are needed to estimate the solubility of toxic metals under different pH and temperature conditions.

Platinum (Pt) has a wide range of applications and anthropogenic impacts in the aquatic environment are a concern. Pt accumulation in marine organisms begins with primary producers through the food chain, but there are few examples of Pt studies in marine macroalgae, which are producers in the food chain. The objective of this study was to evaluate Pt contamination in the coastal environment by measuring Pt concentrations in wakame seaweed collected in Otsuchi Bay, Iwate Prefecture. Since there was no established method for Pt analysis in seaweeds, we established analytical conditions: total dissolution of seaweeds with HF 0.24 ml/g and HCl 7 ml/g, and solid-phase extraction with an anion exchange resin, which enabled us to measure the concentration. The established method was used to measure Pt concentration in wakame seaweed. The distribution of Pt in the stem and mekabu decreased from the mouth to the back of the bay, while the distribution at the root showed the opposite trend. The results suggest that there are different sources of Pt in the upper and lower parts of the wakame.

In this study, we focused on the relationship between DON and NO₃- in the subtropical South and North Pacific and attempted to quantitatively compare d¹⁵N between DON and NO₃-. The results showed that DON concentrations from the surface to 150 m depth in the subtropical North Pacific were 4 to 5 uM at most points. The d¹⁵N of DON were +0 to +6 which were relatively lower than those of NO₃-. Whereas the DON concentrations from the surface to 100 m depth in the subtropical South Pacific were similar to those in the North Pacific subtropical region, the d¹⁵N of NO₃- up to 200 m depth was +5 ~ +11, and that of DON at the same depth was +5 ~ +9, both higher than in the North Pacific subtropical region.

To understand material cycles on surface Earth, it is important yet challenging to estimate energy flow through ecosystems. To address this issue, stable isotopic compositions of carbon and nitrogen and radiocarbon contents of organic compounds in organisms are useful. I and my collaborators are working on the following studies:1) Amino acid nitrogen isotopic compositions2) Amino acid radiocarbon contents3) Chlorophyll a radiocarbon contentsWe aim to find a simple and universal rule hidden behind apparently complex ecosystems by means of compound-specific isotope analyses that are governed by physicochemical rules.

Sulfur isotopes have been used to understand the sulfur cycle throughout the Earth's history. The apparent isotopic fractionation between sulfides and sulfates is believed to have increased in response to global oxygenation due in part to changes in the fractionation factors for microbial sulfate reduction. However, accurate estimates of the fractionation factors are difficult due to the lack of sulfate in the Precambrian period, and there are few examples of fractionation factors for 33S and 36S using Proterozoic sedimentary rocks. In this study, we established an analytical method to estimate fractionation factors for microbial sulfate reduction (34epsilon), mass-dependent exponents (33lambda and 36lambda), and initial d34S for sulfate (d34Si) using only pyrite isotopic ratios without sulfate analysis. This method was validated by using isotopic ratios of modern sedimentary sulfides. The estimated d34Si are consistent with measured sulfate values. Re-evaluation of multiple sulfur isotope records of the Proterozoic using this method suggests that the fractionation factors in the Proterozoic are significantly smaller than that in the Phanerozoic and increased from the Ediacaran.

The activities of early life and its environment remain uncertain. The Strelley Pool Formation (SPF) in the Pilbara Craton, Western Australia is one of the oldest sedimentary rocks containing microfossils, making it a suitable sample for elucidating early life activities and its environment. In this study, small spherical pyrite with a concentric growth structure with about 20µm in diameter was discovered coexisting with microfossils. Using a microscope, field emission scanning electron microscope (FE-SEM), and secondary ion mass spectrometry (NanoSIMS), we performed morphology, microstructure, and micron-scale carbon and sulfur isotope analysis on this spherical pyrite. These results showed that the spherical pyrite formed in the early diagenetic stage of metamorphism of the organic-rich chert layer, at least before the solidification of the sediments. Furthermore, the negative sulfur isotope value and its variation indicated the contribution of biological activity to the pyrite formation.

The Upper Cretaceous Kem Kem Group in Morocco, where the depositional environment is at the boundary between terrestrial and submerged, saltwater and freshwater, produces vertebrate fossils that may have been adapted to multiple environments, including dinosaurs, reptiles and fish. The cooccurrence of these fossils may be related to differences in habitat. Strontium isotope ratios (87Sr/86Sr), which reflect the isotope ratios of the geological species in which they grow, are useful for studying the habitats of extinct animals. In this study, we reconstructed the habitat differences of the fossil vertebrates of the Kem Kem Group, using Sr isotope analysis. The results of the elemental composition analysis showed the influence of diagenetic effects was smaller in enamel-like tissue than in the dentin. The results of 87Sr/86Sr analysis of the enamel-like tissues of the tooth fossils showed that the values for terrestrial to semi-aquatic reptiles were above 0.7090, whereas those for fish ranged from 0.7077 to 0.7085. The 87Sr/86Sr values for dentin tended to converge around 0.7090, suggesting that they were influenced by sediment-derived values.

Ribose is a sugar that constitutes RNA and thus is an essential molecule for the origin of life. Ribose is known to be formed by the formose reaction, a condensation reaction of formaldehyde, but the environment in which ribose accumulates is unclear. Borate stabilizes ribose, and it also inhibits ribose formation. In this study, we investigate the effects of borate on the formose-type reaction by quantifying the transitions of various sugar amounts produced by the response. Ribose was quickly formed and reduced its amounts in all the reactions. The higher the concentration of borate, the lower the maximum concentration of ribose. However, the rate of ribose decrease was substantially declined by the effect of borate, resulting in the amount of remaining ribose in a borate-rich experiment being more than ten times higher than that in a borate-free experiment after 72 hrs. These results suggest that borate is effective in facilitating the accumulation of ribose over geological time scales on early Earth.

The oceans from the Archean to the Paleoproterozoic were significantly different from their present counterparts, hosting reducing environments rich in dissolved iron. Understanding the biogeochemical cycles in these ancient marine environments can be aided by studying modern analogous lakes. Lake Matano in Indonesia is one such modern analogue characterized by its stratification throughout the year and its ferruginous deep water. We aim to elucidate the biogeochemical cycles in the Archean to Proterozoic oceans using numerical modeling, and we validate our model using data from the modern analogues. In this presentation, we show that our vertical one-dimensional model can reproduce many of the biogeochemical observational data from Lake Matano. Furthermore, we discuss the limiting factors of primary productivity in the lake using our model.

Lipids are a ubiquitous class of biomolecules whose evolution and diagenesis in sedimentary rocks has been extensively studied, particularly from plant, bacterial and algal sources. However, the diagenetic pathways that operate on steroids from bone tissue have yet to be elucidated. In this study, we perform biomarker analysis on three marine mammal bone fossils from the Oligocene Morawan Formation (Ashoro, Hokkaido) - two desmostylian and one whale bone. We find that desmostylian bones yielded a spectrum of C27-C29 steranes and diasteranes, while the whale bone was distinctive in displaying purely cholestane. However, the whale bone displayed a remarkable preservation of several polycyclic aromatic hydrocarbons (PAHs) that were indicative of early diagenesis. These results indicate that while the cholesterol in desmostylian bones could possibly have had a microbial input and had mostly altered to PAHs, those in the whale bone were likely indigenous, still in early diagenesis, with diagenetic continuum preserved. The whale bone, thus, displays an as-yet undetermined preservation mechanism, or is possibly allochthonous to the locality of the desmostylians.