This study investigated dissolved and particulate metals (Cu, Zn, Cd, and Pb), together with Pb isotopic compositions, in coastal seawater from the Oehwang River estuary and Onsan Port, which have been reported as severely contaminated areas due to industrial complexes and harbor activities. A total of 53 surface seawater samples were collected during each of four sampling periods (April, July, August, and October 2025). Water quality parameters, including salinity, pH, and turbidity, were measured in situ, and dissolved and particulate metal concentrations as well as Pb isotopic ratios were analyzed.

Overall, metal concentrations within Onsan Port were consistently higher than those in the estuarine zone. Concentrations of Cu, Zn, and Pb in Onsan Port frequently exceeded the chronic toxicity water quality guidelines. In addition, three sites sampled in August, located in front of wharfs B and C, showed Zn concentrations exceeding the acute toxicity guideline.

Pb isotopic compositions plotted against the inverse of Pb concentrations for both dissolved and particulate phases revealed three distinct two–end-member mixing relationships involving four isotopic end members. The ²⁰⁷Pb/²⁰⁶Pb ratios of the dissolved phase (particulate phase in parentheses) were 0.915 (0.942), 0.892 (0.894), 0.888 (0.883), and 0.870 (0.865). These mixing relationships correspond to three zones: the area in front of wharfs B and C within Onsan Port, the inner and outer areas of Onsan Port, and the Oehwang River estuary.

Pb isotopic signatures in the hotspot area indicate that the sources of elevated Pb and Zn concentrations were zinc concentrates imported from Australia and unloaded at wharf B during the sampling period. In contrast, other isotopic end members with distinct Pb isotope ratios represent metal concentrates that were unloaded during previous periods.

The strong linear correlations observed between dissolved and particulate metal concentrations suggest that dissolved Cu, Zn, Cd, and Pb in this area primarily originate from the dissolution of imported metal concentrates during unloading activities at the wharfs, while particulate metals represent residual materials remaining after partial dissolution in seawater.

Temporal variations in both the spatial distribution patterns and concentration ranges of metals were pronounced and were mainly controlled by harbor-related inputs rather than freshwater discharge. Because local metal sources could be clearly identified using spatial distributions of dissolved and particulate metals together with Pb isotopic compositions, key characteristics of the mixing processes—such as source-receptor area, mixing time scale, and spatial extent—could be effectively constrained in this complex coastal environment.

How to cite: Choi, M. S., Choi, S., Joe, D., Yang, D., and Lee, M.: Dissolved and particulate metals, and Pb isotopes in Oehwang River estuary and Onsan port seawaters, Korea   , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2068, https://doi.org/10.5194/egusphere-egu26-2068, 2026.

After collecting sediment using a box corer in the northern East China Sea, sub-samples were collected from the sediment chamber and sediment incubation experiments were performed to calculate sediment oxygen consumption rate, hydrogen sulfide flux, methane flux and nutrient flux. Sediment incubation experiments were conducted at a total 5 sites, and the experiments were conducted for approximately 144 hours (6 days). Organic carbon regeneration rate was calculated as the sediment oxygen consumption rate. The sediments at all sites were muddy or muddy sand sediments, and the organic carbon content ranged from 0.25% to 0.94%. The C/N ratio is in the range of 3 to 7, and most of the organic matter in the sediment is assumed to be of marine origin. The sediment oxygen consumption rate ranged from 0.82 to 4.2 mmol/m²/day, a relatively low value compared to the sediment oxygen consumption rate in coastal area. The organic carbon regeneration rate ranged from 0.39 to 1.99 g/m²/year. The fluxes of hydrogen sulfide were 0.86 μmol/m²/day and 0.39 μmol/m²/day at the Z3-A03 site and Z3-D03 site, respectively. The fluxes of methane ranged from 0.022 to 4.838 μmol/m²/day. Methane fluxes showed a good correlation with the sediment oxygen consumption rate. Nitrate concentrations showed tended to little change or increase at sites with low oxygen consumption rates, whereas at sites with high oxygen consumption rates, concentrations tended to sharply decreased during the incubation. Ammonium concentration showed no change during the initial phase of incubation, but increased sharply from the 2 days after. The silicate concentrations continuously increased during the incubation, with the silicate flux being proportional to the oxygen consumption rate.

How to cite: Lee, T., Lee, K., Hong, J., and Kim, H. J.: Methane, hydrogen sulfide and nutrients fluxes in southern Yellow Sea sediments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6057, https://doi.org/10.5194/egusphere-egu26-6057, 2026.

This study presents and evaluates the distribution patterns and deposition fluxes of both natural and artificial radionuclides, ²¹⁰Pb and ¹³⁷Cs, in the sediments of the Yellow Sea region. Higher sediment deposition fluxes of ²¹⁰Pb were observed in the muddy sediment area compared to the sandy area. In addition, the deposition fluxes of excess ²¹⁰Pb at the dumping site and surrounding areas showed higher values (663.1 ± 588.6 Bq/m² yr⁻¹) than those at the non-dumping sites (359.7 ± 373.3 Bq/m² yr⁻¹) within the muddy area. Thus, although the sedimentation rates in the study area calculated using excess ²¹⁰Pb showed an average value of 0.35 ± 0.23 cm yr⁻¹, which is comparable with those reported in previous studies of the Yellow Sea, this study indicates the significant influence of anthropogenic waste dumping on the accumulation of ²¹⁰Pb in the bottom sediments. The ¹³⁷Cs activities in most stations exhibited an exponential decrease from the surface sediments, similar to the pattern of ²¹⁰Pb, although one site showed peak values at depths of 9–13 cm. However, unlike the typical ¹³⁷Cs peaks that correspond to major events, such as the 1963 nuclear tests, the age of the sediments at peak depths in this study ranged widely, from the 1950s to the 1970s. This imprecision in dating is likely attributable to the high sediment turbulence in the Yellow Sea and the weaker particle-reactive nature of ¹³⁷Cs, which facilitates the remobilization of particulate ¹³⁷Cs into the dissolved phase. Moreover, our results indicate a much lower deposition flux of ¹³⁷Cs compared to previous studies, further demonstrating the limitations of using ¹³⁷Cs as a tracer in high-sediment-surface areas. Overall, this study highlights the impact of waste dumping on the accumulation of ²¹⁰Pb in marginal seas, even in dynamic sedimentary environments such as the Yellow Sea.

How to cite: Kim, I. and Chen, X.: Behaviours of 210Pb and 137Cs in Yellow Sea Sediments Under the Influence of Waste Dumping, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6179, https://doi.org/10.5194/egusphere-egu26-6179, 2026.

As part of the South Korean government's long-term national objectives for achieving greenhouse gas neutrality, we are developing a project to establish a systematic framework for assessing organic carbon storage in seafloor sediments across the coastal and offshore regions of the South Korean Peninsula's Exclusive Economic Zone (EEZ), utilising sediment core and surface sediment analyses. In Korean waters, existing sedimentary carbon data are mostly limited to the surface layers, which are highly vulnerable to physical and chemical disturbances, resulting in considerable uncertainty in carbon storage evaluations. Through this project, we aim to address these critical limitations.

For this study, we collected sediment core samples from 48 stations in Korean EEZ waters, and sediments were analysed for surface and core sediment properties, water content, total carbon (TC), total inorganic carbon (TIC), and total organic carbon (TOC). Water content was calculated from the weights of wet and dry sediment and used as a proxy for sediment porosity and structural characteristics. Contrary to general expectations, our results show that samples characterised by lower mean surface grain size values (φ) exhibit relatively high-water content (R² = 0.4658) and a weak negative relationship to TOC (R² = 0.3061), resulting in elevated TOC values in sediments that are texturally classified as sandy. Furthermore, TOC concentrations show a weak positive relationship with increasing water content (R² = 0.2326), suggesting possible mechanisms such as poorly sorted sands containing interstitial fine particles, enhanced intragranular porosity associated with biogenic or carbonate-rich sands, and rapid burial of organic matter under energetic depositional conditions that limit early diagenetic degradation.

Our findings highlight the limitations of using mean surface grain size as a key predictor of surface sedimentary organic carbon and emphasise the importance of incorporating water content and sediment physical properties when assessing carbon storage potential. Understanding these physical and biogeochemical controls is crucial for enhancing estimates of marine carbon burial and for the development of robust frameworks to assess the role of seafloor sediments in climate mitigation strategies.

How to cite: Kaluthotage, P. and Kim, H. J.: Assessment of Carbon Storage in Marine Sediments of the South Korean EEZ: An Insight into Surface Sediment Grain Size in the Retention of Organic Carbon, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6229, https://doi.org/10.5194/egusphere-egu26-6229, 2026.

Coastal mangrove habitats are among the most efficient natural carbon sinks on the planet: their upper soil typically contains about 3–5 times more soil organic carbon than the global average and can sequester carbon at rates 2–4 times greater than tropical terrestrial forests. Nevertheless, degradation and hydrological disruption driven by climate change and anthropogenic perturbations jeopardize these carbon reservoirs, potentially transforming mangroves from net carbon sinks into carbon sources. We hypothesized that restoring hydrological connectivity and vegetation would enhance soil organic carbon accumulation and reactivate blue carbon cycling. To test this, we examined the Sundarbans—the largest contiguous mangrove forest in the world, extending across India and Bangladesh. Three sites under distinct ecosystem conditions in the Indian part of the Sundarbans were selected: a degraded mangrove site—where vegetation loss and hydrological disruption have promoted oxidation and loss of soil organic carbon; a restored mangrove forest maintained by a local NGO; and a mature old-growth mangrove. Soil core samples collected up to 35 cm depth at 3–5 cm intervals were analyzed for total organic carbon (TOC) and nitrogen (N), stable carbon isotopes (δ¹³C), n-alkane biomarkers, and radiocarbon abundance. Soils from the degraded mangrove site contained low TOC, high δ¹³C values, a high ratio of aquatic-to-terrestrial n-alkanes (P_aq), and a consistent decrease in terrestrial plant wax n-alkanes (P_wax) with depth, along with variably lower C/N ratios, signifying heterogeneous carbon inputs dominated by aquatic sources. In contrast,  soils from the restored mangrove site showed TOC and δ¹³C values comparable to mature systems, higher P_wax, longer n-alkane average chain lengths (ACL), and consistently elevated C/N ratios, indicating stable, plant-dominated carbon inputs under restored vegetation and hydrological connectivity. The relationship between C/N ratios and δ¹³C values further confirmed a shift from aquatic-derived carbon in the degraded site to terrestrial C3 plant carbon in the restored site, with the old-growth mangrove site exhibiting mixed-source signatures. Radiocarbon profiles revealed a gradual decrease with depth, reflecting the aging and stabilization of organic matter. Our results highlight the dual role of mangrove restoration in rapidly rebuilding carbon stocks while enhancing carbon turnover, underscoring its importance for climate mitigation strategies and blue carbon credit frameworks.

[Keywords: mangrove restoration, blue carbon, carbon sequestration, n-alkane biomarker, carbon isotopes]

How to cite: Nizam, S., Dutta, S., Sen, I. S., and Sachse, D.: Isotopic and Biomarker Constraints on Soil Carbon Dynamics Across Degraded, Restored, and Old-Growth Mangroves in the Sundarbans, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6454, https://doi.org/10.5194/egusphere-egu26-6454, 2026.

Tropical islands play a significant role in oligotrophic gyres by supplying macro- and micronutrients that support phytoplankton production in otherwise nutrient-poor waters. This so-called island mass effect remains poorly documented and is likely to be highly system-specific. Here, we compare the island mass effects of two contrasting tropical environments: a volcanic island in the Caribbean Sea (Guadeloupe, French West Indies) and an ultramafic island enriched in nickel, iron, and other trace metals within the lagoon system of New Caledonia (Coral Sea).

This study combines oceanic and terrestrial field campaigns, including river surveys, to identify and quantify nutrient sources surrounding each island, together with analyses of phytoplankton community distributions. Complementary laboratory experiments were conducted to assess the role of coastal sediments as a source of trace metals and their effects on phytoplankton.

Our results highlight marked differences in the island mass effect both within and between the two systems. These differences likely result from a combination of environmental and physical drivers, including soil composition, hydrological regime, wind exposure, land use, hydrothermal activity, island-scale circulation, and the presence of a continental shelf. The impacts of these contrasting nutrient sources on natural phytoplankton assemblages are discussed.

How to cite: Boye, M., Moreau, E., Pascal, P.-Y., Juillot, F., Dupouy, C., and Messié, M.: The comparative island mass effect in volcanic and ultramafic tropical islands, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6517, https://doi.org/10.5194/egusphere-egu26-6517, 2026.

 Aquaculture plays a crucial role in sustainable food production, but excessive organic matter discharge can lead to environmental issues such as eutrophication and algal blooms. This study investigated the dynamics of organic pollution in a land-based olive flounder (Paralichthys olivaceus) aquafarm on Jeju Island, Korea, comparing water quality during feeding and fasting periods. Two 24-hour monitoring campaigns were conducted to assess changes in chromophoric (CDOM) and fluorescent dissolved organic matter (FDOM), alongside conventional parameters such as chemical oxygen demand (COD), dissolved organic carbon (DOC), and total dissolved nitrogen (TDN). Feed elution experiments were conducted to examine temporal changes in dissolved organic matter following feed addition. Significant differences (p<0.01) were observed between influent and effluent during feeding, with DOC, TDN, and FDOM increasing by 10–200% within one hour. When the peak excretion rate occurred 8–9 hours after feeding, all water quality parameters except specific ultraviolet absorbance (SUVA₂₅₄) showed increasing trends. A gradual increase was observed 7–8 hours post-feeding for all water quality parameters except SUVA₂₅₄, and feed elution experiments also showed a similar trend over time, suggesting the influence of excretion and unconsumed feed. Robust correlations between DOC and optical parameters were observed in both influent and effluent (r² = 0.56–0.91, P<0.001), suggesting that CDOM and FDOM can be used as indicators of aquaculture-derived organic wastewater. A unique fluorescence peak, previously unreported and observed in both effluent and feed elution samples, may serve as a tracer for aquaculture feed. These findings demonstrate that optical analysis is effective for rapid monitoring and can aid in tracing organic pollutants, informing water quality management strategies to support sustainable aquaculture while minimizing environmental impacts.

How to cite: Ji, S., Notter, B. L., and Kim, J.: Assessment of effluent water quality according to aquaculture activities using absorption and fluorescence spectroscopy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6534, https://doi.org/10.5194/egusphere-egu26-6534, 2026.

Cadmium (Cd) is a highly toxic heavy metal released into marine environments through anthropogenic activities such as mining, smelting, and industrial waste. Tracking these pollution sources requires precise isotope analysis, yet marine sediments present significant challenges due to low Cd concentrations (<0.2 mg/kg) and complex matrix interferences. While conventional methods often utilize desolvation systems to enhance sensitivity, they are frequently limited by instrumental mass bias instability and significant memory effects.

In this study, we optimized a robust Cd stable isotope analytical procedure using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) equipped with a standard pneumatic nebulizer (SPN). To achieve high-precision data, a two-step ion-exchange chromatography process (AGMP-1M and TRU-SPEC resins) was established, ensuring >90% Cd recovery and effective removal of major matrix elements(Al, Fe, Ca) and isobaric/molecular interferents (Sn, Mo, Nb, Zr) even without the help of a desolvator.

Our results define the optimal analytical thresholds for reliable δ^114/110Cd measurements: a minimum Cd concentration of 50 ng/g (approx. 150 ng total) is required, making this protocol applicable to sediments with Cd levels as low as 0.15 mg/kg. Strict interference control limits were established, maintaining matrix-to-Cd ratios (M/Cd ≤ 5) and suppressing molecular interferences from Mo, Nb, and Zr below ratios of 0.01, 0.0005, and 0.0001, respectively.

The data quality of the method was extensively validated through a comprehensive assessment of isotope fractionation and comparison with reference values for standard materials. The entire dataset (n=733), encompassing 12 types of reference materials (RMs) (e.g., NIST SRM 2711a, NRCC PACS-3, USGS NOD-A-1), verification standards (BAM-I012), and the zero-delta standard (NIST 3108), exhibited exceptional mass fractionation linearity. The slope between δ^114/110Cd and δ^113/110Cd was 0.7544 (R²=0.996) and that between δ¹¹⁴Cd and δ¹¹¹Cd was 0.2545 (R²=0.944). The slope between δ¹¹⁴/¹¹⁰Cd and δ¹¹³/¹¹⁰Cd was 0.7544 (R² = 0.996), and that between δ¹¹⁴Cd and δ¹¹¹Cd was 0.2545 (R² = 0.944). These slopes agree with predicted values based on mass-dependent fractionation, demonstrating that potential isobaric and molecular interferences were effectively eliminated and ensuring the fundamental reliability of the data. The measured δ^114/110Cd values for the 12 RMs showed an average absolute deviation of only 0.079±0.086‰ compared to previously reported literature values. Furthermore, a rigorous inter-laboratory cross-validation was conducted between Chungnam National University (using the SPN and external mass-bias correction with Ag-doping) and Kyoto University (using the conventional desolvator and double-spike mass-bias correction). This comparison, involving 5 RMs and 5 surface sediment samples from Onsan Port collected in April 2025 (OS4, OS5, OS8, OS14, OS35), yielded a high correlation coefficient (R²=0.944) and a small absolute deviation (0.037±0.015).

These findings demonstrate that our optimized analytical framework ensures international-level reliability and precision. This protocol provides a powerful tool for environmental forensics, successfully differentiating natural background levels from anthropogenic inputs—such as zinc smelting and coal—in complex marine ecosystems.

How to cite: Lee, M., Choi, M.-S., Kim, J., Choi, M.-S., and Takano, S.: Optimization of Cadmium Isotope Analysis in Marine Sediments Using a Standard Pneumatic Nebulizer Coupled with MC-ICP-MS, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8538, https://doi.org/10.5194/egusphere-egu26-8538, 2026.

The southern East China Sea (ECS) is influenced by four major water masses—China Coastal Water, Taiwan Warm Current (TWC), upwelling water, and Kuroshio Water—each characterized by distinct hydrographic and biogeochemical properties. Our results show pronounced spatial contrasts in nutrient availability and planktonic processes among these water masses. Nitrate concentrations were highest in China Coastal Water, followed by upwelling water and TWC, and lowest in Kuroshio Water. Correspondingly, chlorophyll a and particulate organic carbon (POC) exhibited decreasing gradients from coastal to offshore waters. Bacterial production and plankton community respiration (CR) closely mirrored these patterns, with the highest rates observed in China Coastal Water, intermediate values in upwelling and TWC regions, and minimal rates in Kuroshio Water. Mean plankton CR integrated over the upper 40 m ranged from ~129 mg C m⁻³ d⁻¹ in coastal waters to ~11 mg C m⁻³ d⁻¹ in Kuroshio waters. Significant positive relationships were identified between plankton CR and chlorophyll a, POC, and bacterial production, indicating tight coupling among phytoplankton biomass, microbial activity, and organic carbon consumption. These findings highlight the dominant role of nutrient-rich coastal and upwelling waters in driving organic carbon remineralization on the southern ECS shelf and underscore the importance of water-mass mixing in regulating carbon cycling and ecosystem functioning across this dynamic marginal sea.

How to cite: Chen, C.-C. and Shiah, F.-K.: Impact of water masses on plankton community respiration in the southern East China Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8568, https://doi.org/10.5194/egusphere-egu26-8568, 2026.

Nitrogen is one of the key nutrients limiting marine primary productivity and plays a central role in controlling phytoplankton growth and ecosystem structure. In this study, we focus on the Tonegawa River estuary along the Pacific coast of Japan. In addition to direct nutrient inputs from the Tonegawa River, this region is influenced by the interaction between the Kuroshio and Oyashio currents, leading to strong variability in water masses and elevated biological productivity. Owing to these characteristics, the area has long been recognized as one of Japan’s major fishing grounds.

In the estuary and adjacent coastal waters, the sediment–water interface also represents an important site for inorganic nitrogen regeneration. Organic nitrogen deposited on the seafloor can be transformed into dissolved inorganic nitrogen through mineralization and nitrification processes and subsequently released into the overlying water column. Using bottom-water observations together with simulations from a sediment biogeochemical model, we show that sedimentary processes provide a substantial source of inorganic nitrogen to the water column in the study area. More than 80% of the regenerated nitrogen is released as ammonium, while approximately 16% is returned as nitrate, highlighting the dominant role of ammonium regeneration in sustaining nitrogen availability in the estuarine system.

Previous studies using hydrodynamic models (MRI.COM) coupled with ecosystem models (NPZD), without explicitly including sediment processes, have emphasized river inputs as the primary nitrogen source in this region. Furthermore, our results indicate that benthic biogeochemical processes at the sediment–water interface make an important contribution to the nitrogen budget of the Tonegawa estuary. These findings highlight the need to explicitly include sedimentary nitrogen regeneration when studying nitrogen cycling and ecosystem dynamics in marginal seas and estuarine environments.

How to cite: Wang, Y., Nakajima, T., Urakawa, S., Matsumura, Y., and Itoh, S.: Role of Sedimentary Nitrogen Regeneration in the Tonegawa River Estuary, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14179, https://doi.org/10.5194/egusphere-egu26-14179, 2026.

Arctic fjord ecosystems are sensitive to environmental changes such as accelerating warming and ocean acidification. In northwestern Greenland, fjords show strong contrasts in temperature, salinity, and pH due to differences in bathymetric sill depth and ocean circulation. Measurements conducted during the 2019 Ryder expedition onboard I/B Oden revealed a warm (<4 °C) and acidic (pH down to ~7.5) surface ocean from the outer part of Sherard Osborn Fjord. The surface ocean of Petermann Fjord, on the other hand, was colder (~0 °C) and less acidic (pH>8). These hydrographic differences could potentially affect baseline carbonate chemistry and sediment–water interactions. Here, we examine whether natural ocean acidification as a result of meltwater input can be buffered by carbonate weathering on the seafloor of northwestern Greenland, using controlled high-pressure flow-through incubations to simulate in-situ seafloor conditions.

Experiments were conducted at ca. 102 (+/- 1) bar and 6 (+/- 0.5) °C, hypothetical conditions simulating the intrusion of warm water. A custom-built high-pressure system consisting of three incubators, each containing paired upper and lower sand-packs (6 sand-packs in total), was used. The system was equipped with sapphire-window visual cells and fiber-optic pH sensors, enabling continuous monitoring of pH, temperature, pressure, and flow rate. Four sediment sand-packs (incubators 1 and 2) were filled with mixed sediments collected from the Petermann Fjord collected during the 2024 GEOEO North of Greenland Expedition onboard I/B Oden, spanning depths from the surface to 53 cm below seafloor, and incubated with IAPSO standard seawater. Sediment-free IAPSO seawater was incubated under the same conditions in incubator 3 as a control. During the three-month long experiments, inlet seawater was gradually acidified from pH 8.3 to 7.5 using 37% high purity HCl. Fluids were automatically collected six consecutive days per week using an auto-sampler throughout the experiments. Fluids were analyzed for major cations and anions by dual-channel ion chromatography (IC-dual). System performance and integrity were verified prior to the actual experiments by using silica gel and IAPSO seawater.

Preliminary results indicate a systematic increase in the Ca/Cl ratio responding to lower pH values in sediment-containing incubators compared to the controlled incubation, suggesting enhanced carbonate dissolution. Temporal variability in solute concentrations suggests dynamic fluid–sediment interactions and potentially coupled dissolution–precipitation processes during acidification. These findings provide experimental evidence that ongoing ocean acidification may promote carbonate dissolution in northwestern Greenland fjord sediments, with implications for coastal carbon cycling under ongoing ocean acidification.

How to cite: Amiri, N., Hong, W.-L., O'Regan, M., Kirchner, N., and Jakobsson, M.: Can seafloor carbonate weathering buffer ocean acidification in northwestern Greenland?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14382, https://doi.org/10.5194/egusphere-egu26-14382, 2026.

Coastal regions are recognized as areas of significant biogeochemical activities, primarily due to the influx of nutrients from rivers. However, under the influence of climatic changes, these riverine fluxes are diminishing, significantly altering the biogeochemistry of coastal regions. Such changes could also impact carbon chemistry in these regions, affecting the health of ecosystems. Therefore, it is imperative to conduct long-term studies to monitor the impact of these hydrological changes on the carbon chemistry of coastal systems. In this study, we conducted an analysis of a decadal time series data on dissolved inorganic carbon (DIC) and total alkalinity (TA), along with physicochemical parameters (including pH, Temperature, Salinity), in the Mission-Aransas estuary, located in the northwestern Gulf of Mexico. These samples were collected at five stations from April 2014 to August 2025. Our focus was on characterizing the sources and sinks of DIC and TA and evaluating the hydrological impacts on carbonate parameters. The results show both non-conservative removal and gain of DIC and TA from the mixing line between river water and seawater. Gain of DIC and TA is mainly observed <~25 salinity, whereas removal is observed at higher salinities. Additional supply of DIC and TA can be attributed to the fluxes at the sediment-water interface and the dissolution of carbonates. The most significant increase is observed during periods of high-water flow, which suggests that carbonate dissolution is facilitated by a decrease in pH resulting from the mixing of low pH and high pCO₂ water. Additionally, the remineralization of organic matter transported by rivers contributes to this process by lowering the pH. Removal of DIC and TA is due to the biological activities, for example calcification by oysters, which are abundant in this estuary. Although these water samples predominantly exhibit supersaturation with respect to both calcite and aragonite, this suggests the potential for abiotic precipitation of carbonate minerals. Furthermore, the oxidation of reducing agents such as sedimentary sulfide, introduced through the sediment-water interface because of significant resuspension activities, may potentially decrease the TA concentration. This study suggests that DIC and TA cycling in coastal regions is greatly influenced by hydrological changes, and more global studies are needed to predict carbon behaviors in these biogeochemically active regions to accurately quantify the impact on net export fluxes of DIC and TA to the coastal ocean.

How to cite: Danish, M. and Hu, X.: Hydrological controls on the distribution of dissolved inorganic carbon and total alkalinity in a northwestern Gulf of Mexico estuary, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15906, https://doi.org/10.5194/egusphere-egu26-15906, 2026.

Shallow marine hydrothermal systems with natural CO₂ emissions generate localized chemical gradients and perturbations in seawater carbonate chemistry, offering opportunities to investigate coastal biogeochemical processes under elevated CO₂ conditions. We present a multidisciplinary characterization of the Calent Mound hydrothermal field (Columbretes Islands, Western Mediterranean) based on two oceanographic surveys conducted in 2020 and 2021. Hydrographic measurements revealed pronounced pH anomalies, with reductions of up to 1.12 units relative to reference conditions localized above active venting areas. Water-column temperature and salinity anomalies were minimal, whereas subsurface sediments exhibited thermal anomalies up to +5.7 °C below the seafloor. CO₂ emission-frequency analysis revealed heterogeneous degassing patterns, from sporadic to continuous, producing an estimated flux of ~3.3 kt yr^-1 over an active area of 17,000 m². Dissolved inorganic nutrient concentrations in vent fluids were markedly enriched relative to open-water reference values, particularly for phosphate, nitrate + nitrite, and silicate. Sequencing of microbial mats revealed distinct prokaryotic and eukaryotic communities associated with hydrothermal influence, including sulfur-, iron-, and ammonia-oxidizing taxa. Interannual variability was evident, although several key microbial taxa were consistently detected across both surveys. These observations characterize the chemical processes governing natural CO₂ venting effects on local and regional biogeochemistry and highlight the influence of hydrothermal inputs on carbonate chemistry, nutrient dynamics, and microbial community structure in a shallow marine environment.

How to cite: Martín-Díaz, J. P., González-Vega, A., Pérez-Barrancos, C., Arrieta, J. M., Palomino, D., Baena, I., Mallol, S., R. de la Ballina, N., Díez, I., Vázquez, J. T., Díaz-Viñolas, D., and Fraile-Nuez, E.: Hydrothermal CO2 venting, pH anomalies, and biogeochemical consequences in a shallow hydrothermal system (Calent Mound, Western Mediterranean), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18905, https://doi.org/10.5194/egusphere-egu26-18905, 2026.

Urbanized and turbid reef environments pose challenges for coral-based paleoclimate reconstructions, highlighting the need to broaden the range of target taxa beyond conventional massive Porites and to develop additional archives suitable for climate proxy applications. In this study, we present monthly-resolved δ¹⁸O and δ¹³C records covering the past 20 years, from two Psammocora digitata corals (SG1, SG2) and one Porites core (SG3) collected from a turbid nearshore reef in Singapore. All three records show clear seasonal cycles, with the Psammocora exhibiting higher intra-coral reproducibility. SG1 and SG2 share nearly identical δ¹⁸O and δ¹³C means, variances, and seasonal amplitudes, consistent with similar hydrographic conditions at ~3 m depth. The shallower Porites core (SG3, ~2 m) is offset toward lower δ¹⁸O and δ¹³C values and displays reduced δ¹⁸O seasonal amplitude, perhaps reflecting species-specific baselines. Seasonal δ¹⁸O variability is primarily driven by salinity changes (~60%), with temperature exerting a smaller influence (~40%). SG1 and SG2 show δ¹⁸O amplitudes of ~1.8‰ and closely matched interannual patterns, while SG3 records a smaller amplitude (~1.2‰) consistent with its lower variance. δ¹³C exhibits larger overall variability (standard deviations 0.65–0.71‰; seasonal amplitudes ~3.0–3.9‰), responding to changes in underwater light, turbidity, colored dissolved organic matter, ambient δ¹³C_DIC, and minor colony-level physiological effects. In each coral, δ¹⁸O and δ¹³C show significant positive correlations (r = 0.59–0.68), reflecting shared environmental drivers—such as freshwater input or seasonal hydrological variability—rather than colony-specific metabolic effects. Taken together, these results show that Psammocora can provide robust, high-resolution isotopic records suitable for reconstructing hydroclimate and biogeochemical variability in sediment-rich coastal environments.

How to cite: Lin, K., Zhang, Y. Z., Han, T., Morgan, K., Martin, P., Wu, M.-H., and Wang, X.: Evaluating coral proxy fidelity at monthly resolution in urban reefs: A 20-year δ18O and δ13C comparison of Psammocora and Porites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20271, https://doi.org/10.5194/egusphere-egu26-20271, 2026.

Since its eruption in 2011, the submarine volcano Tagoro (El Hierro, Canary Islands) has been continuously monitored by the Spanish Institute of Oceanography (IEO-CSIC), as it is one of the few submarine volcanoes studied from the onset of its formation. This study analyzes data from the VULCANA-0421 and VULCANA-0125 cruises, focusing on key inorganic nutrients essential for marine productivity: silicate, phosphate, ammonium, nitrite, and nitrate. The results reveal that, more than a decade after the eruption, intense hydrothermal activity persists, continuing to alter the chemical composition of the water column. Elevated and spatially variable concentrations of all nutrients were observed, with enrichment levels reaching up to 57.00-fold for silicate and 3.96-fold for phosphate in the vicinity of the Tagoro volcano. Nitrogen species exhibited an increasing oxidation gradient with distance, suggesting active nitrification processes. In addition, the effect of sample freezing on silicate measurements was evaluated, showing underestimations at concentrations above 40 μmol/kg. As a methodological improvement, a thermal treatment at 50 °C is proposed to enhance analytical accuracy. This study underscores the importance of long-term monitoring and contributes valuable insights into nutrient dynamics in volcanically disturbed marine environments.

How to cite: Ayuso-Candal, S., González-Vega, A., Martín-Díaz, J. P., Clemente Martín, S., Escánez-Pérez, J., Pereira, C., and Fraile-Nuez, E.: Study of the Distribution of Inorganic Nutrients in the Natural Fertilization Induced by the Submarine Volcano Tagoro (El Hierro) and the Effect of Sample Freezing., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20272, https://doi.org/10.5194/egusphere-egu26-20272, 2026.

Traditionally, Blue Carbon habitats included coastal and intertidal settings such as salt marshes, seagrass meadows and mangrove forests. However, the sub-tidal sediments of continental shelves are now also recognised as a globally significant carbon sink due to the sheer volume and expanse, with increasing focus placed on qualifying organic carbon quality (or reactivity) and quantifying organic carbon stock. The ultimate sink for Blue Carbon in the marine environment is in seafloor sediments, however, once deposited, the fate of this carbon is variable. Many environmental factors are at play in the stabilisation, transformation, and/or liberation of carbon from sediments. Clay minerals and Fe-(hydr)oxides have been shown to play instrumental roles in OC preservation and transformation in marine sediments. Building on previous carbon-stock evaluations performed in the area, an investigation into the role that mineralogy plays in Blue Carbon storage and stability in Irish offshore continental sediments is outlined.

Focus is placed on vibrocores to a depth of >5m below the seafloor from mud-dominant, depositional areas, to assess how carbon storage, source and stability has changed throughout modern sediments and the Holocene. Bulk physical and geochemical characterisation of sediment are determined, including particle size analysis (PSA), % total OC (TOC), total nitrogen (TN) and elemental concentrations generated by X-ray fluorescence (XRF). The relationship between PSA, TOC, and total specific surface area (TSSA) is examined to help elucidate the role of mineralogy in carbon storage. From selected depths, the <2μm sediment fraction is separated from the bulk by centrifugation. Clay mineral identification is conducted by X-ray diffraction (XRD) on the <2μm fraction. Organic matter characterisation through pyrolysis-gas chromatography-mass spectrometry is conducted on the same depths as those selected for clay mineral XRD. Sedimentation rates for the uppermost horizons are estimated using gamma spectrometry (e.g. ²¹⁰Pb). Radionuclide profiles suggest minimal downcore sediment mixing has occurred at the study site, with sedimentation rates remaining relatively stable in the upper 1m if the sediment column. TOC concentrations range from 0.8-1.4w t%, with values stabilising below 2m and remaining relatively consistent to 5.5m, indicating sustained sediment OC storage throughout the sediment profile. C:N molar ratios suggest organic matter is primarily of marine source, however increased C:N ratios are observed in the upper 30cm of the sediment profile, coinciding with increased heavy metal concentrations (Zn, Pb, Cu). Preliminary Bulk XRD results indicate clay-rich mineral assemblage, consistent with enhanced organic matter preservation in fine-grained sediments.

These results highlight the strong control that sedimentary environment and mineralogical composition exert on the capacity of continental shelf sediments to store and stabilise organic carbon. The predominance of marine-derived organic matter suggests that offshore shelf sediments play a significant role in recycling and preserving oceanic primary production, while the elevated C:N ratios and metal concentrations in surface sediments point to recent anthropogenic influences on carbon inputs and sediment geochemistry. As such, the preservation of mud-dominated shelf environments should be considered in marine spatial planning and climate mitigation strategies, given their potential to function as stable, long-term carbon sinks under future environmental change.

How to cite: Ní Mhaoldomhnaigh, C., Grey, A., Chatting, M., Walsh, P., Smeaton, C., Coughlan, M., and Kelleher, B.: The Influence of Clay Mineralogy on Carbon Stabilisation in Ireland’s Shelf Sediments: Past and Present, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21487, https://doi.org/10.5194/egusphere-egu26-21487, 2026.

How to cite: Dong, J., Feng, R., Yao, Z., Wang, J., Wang, Y., Wang, H., Yan, D., Cui, Y., Xie, H., Du, Y., and Xia, X.: Layer-specific mechanisms of perfluoroalkyl acid (PFAA) transport and partition in estuarine environments: Unveiling thedepth-dependent differences, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16572, https://doi.org/10.5194/egusphere-egu26-16572, 2026.