Toshihiro Miyajima

Elemental analysis with isotope ratio mass spectrometry (EA/IRMS) is a widely used technique for stable isotope ratio analysis of nitrogen and carbon. Although the narrower-diameter EA/IRMS method developed to date has effectively enhanced sensitivity and addressed issues associated with low organic content, it may result in the drawback of reduced analytical precision when analyzing samples with high sulfur matrix levels due to interference from sulfur oxides. To address the sulfur breakthrough issue, this study incorporated an additional desulfurization reactor into an existing EA system. The improved method achieved high sensitivity (approximately a fivefold increase over conventional methods) and high sulfur resistance (sulfur tolerance exceeding that of conventional methods by up to 580 mg S). In addition, we applied this method to the analysis of environmental samples with low organic and high sulfur (S/C ratio: 0.7–343.1) contents, including native sulfur-rich seafloor sediments, sulfide chimneys of submarine hydrothermal deposits, and subseafloor sediments from hydrothermal regions. Moreover, in this study, we investigated the types of interference that occurred during sulfur breakthrough in EA/IRMS analysis. Consequently, broad non-integer peaks (hypothesized to result from metastable ions and charge exchanges generated within the flight tube of the mass spectrometer) originating from SO ₂ molecular and fragment ions were observed, and the background levels of N ₂ and CO ₂ were affected. Here, we recommend applying this improvement to the NC analysis of samples with low organic and high S contents.

A significant quantity of soil organic carbon (SOC) is buried in mangrove ecosystems. However, recent research has revealed that substantial amounts of carbon are exported to the atmosphere as CO ₂ or to the ocean as dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). This carbon outflow highlights the need for a comprehensive understanding of carbon burial, outgassing, and outwelling to clarify the role of mangroves in net atmospheric CO ₂ removal. To elucidate the carbon cycle within a mangrove ecosystem, we quantified CO ₂ outgassing through high‐resolution measurements CO ₂ and porewater‐derived fluxes of total alkalinity, DIC, DOC, and POC using radium isotopes ( ²²⁴ Ra and ²²³ Ra) in an island mangrove ecosystem in Japan. Our findings showed that the dominant carbon flux was DIC outwelling, at 189 ± 18 mmol m ⁻² day ⁻¹ , approximately 2.3 times higher than the global median. This pronounced DIC outwelling likely reflects the presence of a vast reservoir of poorly stabilized SOC. A comparison with ecosystem‐scale CO ₂ emissions revealed that approximately 89% of the DIC was transported into the estuary without being emitted as CO ₂ . This high DIC transport appears to result from the efficient water exchange characteristic of island mangroves, with a creek residence time of ∼1 day. Surprisingly, the adjacent estuary acted as a net CO ₂ sink, surpassing CO ₂ outgassing from the mangrove creeks. These results suggest that efficient water exchange in island mangroves, coupled with high biological productivity at the adjacent estuary, promotes long‐term sequestration of mangrove‐derived DIC in the ocean.

A significant quantity of soil organic carbon (SOC) is buried in mangrove ecosystems. However, recent research has revealed that substantial amounts of carbon are exported to the atmosphere as CO2 or to the ocean as dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC). This carbon outflow highlights the need for a comprehensive understanding of carbon burial, outgassing, and outwelling to clarify the role of mangroves in net atmospheric CO2 removal. To elucidate the carbon cycle within a mangrove ecosystem, we quantified CO2 outgassing through high-resolution measurements CO2 and porewater-derived fluxes of total alkalinity, DIC, DOC, and POC using radium isotopes (224Ra and 223Ra) in an island mangrove ecosystem in Japan. Our findings showed that the dominant carbon flux was DIC outwelling, at 189 ± 18 mmol m-2 day -1, approximately 2.3 times higher than the global median. This pronounced DIC outwelling likely reflects the presence of a vast reservoir of poorly stabilized SOC. A comparison with ecosystem-scale CO2 emissions revealed that approximately 89% of the DIC was transported into the estuary without being emitted as CO2. This high DIC transport appears to result from the efficient water exchange characteristic of island mangroves, with a creek residence time of ~1 day. Surprisingly, the adjacent estuary acted as a net CO2 sink, surpassing CO2 outgassing from the mangrove creeks. These results suggest that efficient water exchange in island mangroves, coupled with high biological productivity at the adjacent estuary, promotes long-term sequestration of mangrove-derived DIC in the ocean.

Estimating the metabolic rates of fish is crucial for understanding their physiology and ecology. However, metabolic rate information derived from otolith geochemical data (M_oto) has mainly been obtained from mid to high latitudes, leaving a critical information gap for low latitude fishes suffering from climate variability. In this study, we successfully estimated M_oto of experimentally reared Banggai cardinalfish (Pterapogon kauderni), a species found exclusively in the Coral Triangle, using a novel method based on radiocarbon (Δ¹⁴C) analysis. M_oto values for this species were generally consistent with previously reported M_oto and temperature data, but were slightly higher, suggesting high metabolism. Furthermore, we found a new species‐specific carbon isotope fractionation in otoliths (ε_total), using a combination of radiocarbon and stable carbon isotopes. Our Δ¹⁴C results will improve the understanding of the temperature dependence of fish metabolic rates, which is crucial for assessing the species resilience to temperature variability.

We analyzed the hydrogen and oxygen isotope compositions of interstitial water collected from Miocene–Quaternary carbonate platforms beneath the continental shelf margin off northwestern Australia during the International Ocean Discovery Program Expedition 356. By integrating isotope data with onboard salinity and chemical concentration measurements, we investigated the behavior of interstitial water in this continental shelf system. The interstitial water was classified into three groups based on salinity. Group 1 (salinity = 34−40) consisted of upper low-salinity water infiltrating from the modern seafloor, transitioning downward into highly saline water. This high-salinity water likely formed in situ during glacial periods. Group 3, characterized by extremely high salinity (salinity = 101−153), likely originated from the Late Miocene seawater modified by evaporation in an arid coastal environment. Group 2 (salinity = 43−98) represented a mixture of Groups 1 and 3. This study offers valuable insights into the hydrology of modern and ancient carbonate platforms, with implications for similar systems worldwide.

Seagrasses thrive in shallow lagoons between land and fringing coral reefs, serving as a buffer that mitigates the propagation of environmental stressors from land to reefs. However, mass grazing by megaherbivores poses a significant threat to seagrass loss, impacting the sustainability of tropical coastal habitats. Despite the urgency of this issue, few studies have investigated the possible consequences of grazing on the mitigating functions of seagrass beds. To explore the roles of seagrass beds in sediment accumulation and stabilization and assess the effects of herbivory on these functions, we conducted sediment trap experiments in a coral reef lagoon to examine the impact of seagrasses on sediment mobility. Hydrodynamic forces governing sediment mobility were quantified using a customized reef‐scale hydrodynamic model. The results indicated that sediment resuspension primarily driven by bottom shear stress was the primary factor influencing sediment mobility within the lagoon. Resuspension within healthy seagrass beds was generally low and exhibited minimal sensitivity to hydrodynamic forcing. Consequently, river‐borne mineral soil particles predominantly remained confined within the nearshore seagrass zone due to the trapping and stabilizing effects of seagrasses. Since 2019, seagrasses have been severely depleted due to intensive grazing by sea turtles. Simultaneously, sediment resuspension has intensified, and its dependence on bottom shear stress has become evident. Our findings demonstrate that reef seagrass can effectively suppress sediment resuspension by capturing river‐borne mineral soil. Furthermore, the loss of this function through mass grazing could potentially lead to sediment destabilization and erosion.

The outwelling of remineralized dissolved inorganic carbon (DIC) from the subsurface soil layer through tidal pumping has recently been noted for its importance in the blue carbon budget of mangroves. However, the age of soil organic carbon (SOC) that has been decomposed into DIC remains uncertain. In this study, two distinct models (a two endmembers model and three endmembers model) were utilized to verify the age of the decomposed SOC, employing radiocarbon (Δ¹⁴C) as a tracer. We conducted a 24‐h measurement of DIC and DIC isotopes (δ¹³C_DIC and Δ¹⁴C_DIC), in addition to vertical profiles of Δ¹⁴C of SOC, in a subtropical mangrove ecosystem in Japan. The Δ¹⁴C expected for the source SOC that decomposed into DIC exhibited a comparable trend for the two distinct models, with mean values ranging from −86.6‰ to −16.8‰. The Δ¹⁴C values were converted to calendar age using the OxCal program, with the calibration curve from IntCal20 being employed. This conversion revealed that DIC in the mangrove creek originated from SOC buried over a century ago. Given that mangroves are confined to the intertidal zone, it is likely that there are spatial constraints on the burial of SOC in mangrove ecosystems. Consequently, the transition from SOC to the DIC carbon pool in the ocean could be a key process in supporting long‐term carbon sequestration over millennia.

Prediction of the impact of anthropogenic disturbances and global change on organic carbon (OC) pools in mangrove soils requires a detailed understanding of the mechanisms underlying OC stabilization. Using density fractionation to physically separate OC fractions with varying degrees of mineral association and protection, this study aimed to assess distributions of these fractions and the geochemical factors influencing the most dominant and refractory mineral-associated, high-density fraction (HF) in mangrove soil. We conducted forest-wide soil sampling in the Gaburumata mangrove forest on Ishigaki Island, Japan, along three transects (upstream, midstream, downstream) and at five depths (until 100 cm). The OC in HF (OC _HF ) was the oldest (median Δ ¹⁴ C value of -13.81‰) and contributed most significantly to total soil OC (43%-63%) and nitrogen (64%-85%). Among the extractable metals analyzed (aluminum [Al], iron [Fe], calcium [Ca], and magnesium [Mg]) with different crystallinity, only organically complexed Al and Fe showed strong positive correlations with OC _HF . Together with high OC _HF :Fe ratios that surpassed the maximum sorptive capacity of Fe oxides, these results indicate that co-precipitation of OC and Fe was the dominant mode of organo-mineral associations. The low clay content reduced the importance of Ca and Mg on OC _HF , as these divalent cations typically facilitate OC stabilization through cation bridging between negatively charged clay surfaces and organic matter. Furthermore, the Δ ¹⁴ C–OC relationship suggested efficient incorporation of mangrove-derived modern C into HF, in addition to the pre-existing old C. Thus, mangrove expansion is likely to enhance stable soil OC pools in addition to increasing plant biomass and litter. Overall, this study proposes a biogeochemical mechanism for how stable mangrove OC is newly formed, as well as maintained, with ramifications for global mangrove expansion and plantation efforts.