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Origins of the Nitrate 15 N Depletion in the Mediterranean Sea
No full textPrevious studies have reported a nitrate 15 N depletion in the Mediterranean Sea compared to the global ocean, attributed to either N 2 fixation or atmospheric deposition of anthropogenic N. In this study, we report basin‐wide full‐depth profiles of nitrate δ 15 N (vs. Air) and δ 18 O (vs. Vienna Standard Mean Ocean Water, VSMOW) in the Mediterranean Sea. Our results confirm a consistent 15 N depletion across the entire Mediterranean Sea, with significantly lower nitrate δ 15 N values in the eastern basin (2.2 ± 0.2‰) than in the western basin (2.9 ± 0.1‰). In contrast, there is no significant difference in nitrate δ 18 O between the two basins (2.2 ± 0.3‰ and 2.1 ± 0.2‰, respectively). These observations point to a supply of low‐ δ 15 N N to the Mediterranean Sea, accumulating as regenerated nitrate, which is diluted by the nitrate in the Atlantic inflow, creating an west‐to‐east gradient in nitrate δ 15 N. A four‐box model reveals that, given a water residence time of 120–170 years in the Mediterranean, a modest input rate of 1–3 Tg N yr −1 —originating from N 2 fixation, atmospheric deposition, or their combination—is adequate to produce the observed low δ 15 N of Mediterranean nitrate. Additionally, partial degradation of dissolved organic nitrogen imported from the Atlantic may add low‐ δ 15 N nitrate to the Mediterranean, but it alone cannot explain the full isotopic signal. Distinguishing among these sources will be aided by the reconstruction of Mediterranean nitrate δ 15 N through time using either time‐series data of nitrate δ 15 N or calcareous fossil‐bound organic nitrogen isotope ratios.
Key Points
Basin‐wide depth profiles of nitrate δ 15 N and δ 18 O indicate a supply of low‐ δ 15 N N to the Mediterranean Sea
Nitrate δ 15 N can be explained by modest rates of N 2 fixation and/or anthropogenic N deposition, but only partly by breakdown of DON
The 15 N‐depleted N input is best expressed in the eastern basin due to isolation from the Atlantic nitrate inflo
Future Scenarios of Global Fisheries and Ocean Alkalinity Enhancement Under Socio-Economic and Climate Pathways
No full textAchieving global climate goals while ensuring food security in a changing climate presents significant challenges, particularly when relying solely on land-based solutions. Covering over 70% of the Earth's surface, the ocean remains an underutilized resource for climate mitigation. Ocean alkalinity enhancement (OAE) is one such strategy, designed to strengthen the ocean's natural carbon sink, reduce atmospheric CO2, and mitigate ocean acidification. However, its implications for fisheries, critical for food security and livelihoods, remain uncertain. This study examines the interplay between global fisheries, OAE, and different future socioeconomic and climatic conditions, using the Shared Socioeconomic Pathways (SSPs) and Representative Concentration Pathways framework. We explore how global fisheries and OAE could evolve under three combined scenarios: SSP1-2.6 (sustainability-focused), SSP3-7.0 (regional rivalry), and SSP5-8.5 (high fossil fuel dependency). By integrating ecological, economic, societal, and technological perspectives, we develop scenario narratives and quantify key bio-economic parameters, including technological progress, fishing costs, fisheries management, marine aquaculture, and ecosystem carrying capacity. High-emission (SSP5-8.5) and fragmented development (SSP3-7.0) scenarios present significant barriers to the coexistence of OAE and fisheries, whereas sustainability-focused pathways (SSP1-2.6) offer the most favorable conditions for their alignment. Successfully integrating OAE with fisheries management will likely depend on technological advancements, international cooperation, and socio-economic developments. These scenarios are aligned with those used in model-based scenario studies conducted under the frameworks of the Intergovernmental Panel on Climate Change and the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES), providing a shared foundation for future work.
Key Points:
- Our scenarios extend the Shared Socioeconomic Pathway/Representative Concentration Pathway framework, focusing on fisheries and ocean alkalinity enhancement sectors
- We integrate ecological, economic, societal, and technological aspects to develop narratives and quantify key bio-economic parameters
- Our scenarios align with Intergovernmental Panel on Climate Change and IPBES model-based studies, providing a shared foundation for future researc
1. Wochenbericht L25-10
No full textForschungsexpedition L2510 Helgoland – 5. bis 23. Juli 2025
Erster Wochenberich
Verfahren und Vorrichtung zum Erfassen von elektrisch geladenen Teilchen eines Teilchenstroms sowie System zur Analyse von ionisierten Komponenten eines Analyten / Method and device for detecting electrically charged particles of a particle stream and system for analysis of ionized components of an analyte
No full textThe present invention relates to a method and a device for detecting electrically charged particles of a particle stream extracted from a particle source and to a system for analyzing ionized components of an analyte using such a device.
Various state-of-the-art analysis methods utilize a stream of electrically charged particles extracted from a particle source. For example, inductively coupled plasma mass spectrometry (ICP-MS) is used to perform trace analysis. In this method, a plasma medium is generated in a plasma source by applying a high-frequency alternating field, into which a carrier medium is introduced. During measurement, the carrier medium carries an analyte, for example, particles of a solid generated by laser ablation. The components of the analyte, particularly individual atoms and/or their isotopes, are ionizable in the plasma and can be extracted from the plasma as an ion beam via pinhole diaphragms, so-called samplers or skimmer cones, and subsequently analyzed in a mass spectrometer.
The results of such a mass spectrometric analysis and/or their reliability depend on the plasma conditions in the plasma source. For example, it is known that the ratio of the number of detection signals of different trace elements to each other at a high plasma temperature is temperature independent, while at low plasma temperatures a temperature dependence of the relative ratios of the elements was observed..
Mode ofextension during the Xisha Trough rift in the South China Sea
No full textA seismic transect across the Xisha Trough failed rift provides information on processes active during the opening of the South China Sea (SCS). The rift basement gradually thins at the conjugate flanks from ∼25 to 15 km toward the central sector, where it abruptly thins to ∼10–6 km in thickness. The seismic velocity model supports that 6.5 km/s velocity separates an upper from a lower crust layer, where Vp reaches 7.1 km/s above Moho. The upper crust extends across the entire rift, but the lower crust layer does not occur in the thin crust of the central sector, which is underlain by mantle with anomalously low Vp, indicating serpentinization. Changes in tectonic structure mimic the Vp distribution. The rift flanks have comparatively small faults associated to gradual thinning, whereas the central sector has larger faults that possibly reach the mantle. The faulting in the central sector thus indicates whole-crust embrittlement, which possibly led to synrift mantle serpentinization. Despite the weaker rheology, serpentinized mantle did not result in the development of low-angle faulting or continental break-up. Furthermore, the rift overall thinning and faulting structure is asymmetric with respect to the rift center, supporting an initial widespread moderate extension creating the flanks. When extension thinned the crust to <15 km, the whole crust was brittle and deformation first focused and subsequently laterally migrated to create the central sector. The continental Xisha Trough rift evolution is similar to classical magma-poor margins, but synrift magmatism described in adjacent regions indicate abrupt segmentation of the rift system.
Key Points
Xisha Trough rift continental extension evolved from initially distributed pure shear to focused simple shear
Asymmetric flanks support initial moderate extension followed by deformation focusing in a central sector underlaid by serpentinized mantle
Xisha Trough evolution resembles magma-poor margins, but adjacent magmatism indicates abrupt rift segmentation
Plain Language Summary
Continental rifting, breakup, and subsequent seafloor spreading represent a continuous evolutionary sequence. The South China Sea (SCS) experienced al steps until seafloor spreading, with much of the existing research focused on the continent-ocean transition (COT) zone. However, there is limited understanding of the structures emerged during the continental rifting. The Xisha Trough, an abandoned rift of the SCS, preserves features formed during continental extension. In this study, we revealed the velocity structure, tectonic features, and density distribution of the profile across Xisha Trough using wide-angle seismic data, multi-channel seismic data, and gravity data. Our results revealed that crustal structure is asymmetric, with distributed small faults at the beginning, and the extension concentrated in the axial region with evolution, where crust experienced significant thinning with the absence of lower crust, and hydration of the upper-most mantle rocks. Our findings indicate that the extension pattern in the Xisha Trough closely resembles that of magma-poor margins. Furthermore, when combined with insights from previous studies in the SCS, our research implies that the rheological behavior throughout the evolution of the SCS exhibits substantial lateral variability
The seascape of VOC production and cycling in a tropical coral reef
No full textVolatile organic compounds (VOCs) have been proposed to indicate coral reef health, but little is known about their cycling processes in coral reefs and the roles of reef components. We studied the distribution and cycling of ocean-leaving VOCs (dimethylsulfide (DMS), carbonyl sulfide (COS), CS2, dimethyl disulfide, isoprene, CH3I, CH2ClI, and bromomethanes CH2Br2 and CHBr3) across a coral reef in Mo'orea (French Polynesia) that has a fast and unidirectional water flow. Repeated sampling of transects between the open ocean and the reef outflow channel, across the shelf, the reef crest, and the back-reef lagoon, showed that reef waters were depleted in dissolved organic carbon, chlorophyll, phytoplankton, and bacteria, and enriched in nutrients. All studied VOCs increased in concentration after oceanic waters crossed the reef crest, with bromomethanes showing the largest increase. Incubation experiments of back-reef waters around midday suggested that: (a) photochemical reactions were a major source for COS and major sink for DMS; (b) microbial plankton were the main daytime source for DMS, isoprene, and CH3I, and an important sink for COS; (c) seaweeds were the main source of CH2ClI, CHBr3, and CH2Br2; and (d) carbonate sediments were a major source for CS2 and CH2ClI, an important source for DMS and isoprene, and the main sink for COS. The dominant coral Pocilloporasp. was a source only for DMS and COS. Decomposing seaweed rafts were an important but unquantified source for all VOCs except CH3I. In April 2018, the reef was a net producer of VOCs compared to the ocean, with the anticipation that production would increase if corals were lost and replaced by seaweeds. Using VOCs and other chemical tracers of reef waters, we estimated that one third of the water entering the reef is recirculated water from the same reef, with implications for ecosystem self-recruitment and genetic maintenance
Identifying Bottlenecks to Energy Circulation in the Bay of Biscay Pelagic Food Web: Key Species Under the Spotlight
No full textUnderstanding the functioning and resilience of marine ecosystems requires identifying the main energy flow pathways. Key trophic groups occupy strategic positions in the trophic interactions network, acting as hubs that control the energy distribution across the ecosystem. This study examines the Bay of Biscay's pelagic food web using stable isotope analysis with stomach content data, creating a network of 38 trophic groups and 125 interactions. Both annual-weighted and seasonal (spring and late summer) networks were constructed. The analysis of unweighted and weighted annual networks found that low-trophic level epipelagic fish (European anchovy, Engraulis encrasicolus; sardine, Sardina pilchardus; and sprat, Sprattus sprattus) is a key trophic group displaying higher scores in many centrality indices. These forage fish play a central role in facilitating energy transfer across trophic levels, thus representing a critical link between the planktonic food web and higher trophic level predators and fisheries. Overall, annual networks showed that phytoplankton-dominated grazing chains support a higher diversity of predators compared to chains originating from particulate organic matter (POM). The analysis of weighted networks accounting for seasonal variations in trophic interactions revealed that, during late summer, predators occupy more vulnerable positions than in spring. Changes in feeding preferences cause blue whiting to shift from mostly depending on grazing chains during spring to occupying a position along POM-dominated chains in late summer. These findings highlight the need for fisheries management strategies to prioritize the conservation of key trophic groups supplying energy to predators while considering seasonal shifts in the structure of the energy flow network
Genomic characterisation of Actinospongicola halichondriae gen. nov., sp. nov., the first sponge-derived cultivated representative of a new genus within the class Acidimicrobiia
No full textA new strain (named Hal317(T)) of class Acidimicrobiia was isolated from the marine breadcrumb sponge Halichondria panicea sampled in the Kiel Fjord (Baltic Sea, Germany). Phylogenetic analysis of the 16S rRNA gene sequence revealed similarities from 84.86 to 91.54% to all known type strains of the class Acidimicrobiia. Genome comparisons of strain Hal317(T) with type strains of the family Iamiaceae exhibited average nucleotide identities (ANI) < 71.49% and percentage of conserved proteins (POCP) values < 50.30%. Strain Hal317(T) was determined to be microaerophilic. Optimal growth occurred at 30-35 degrees C, 1-2% sea salt, and pH 6-7. Major fatty acids identified were C-18:1 omega 7c, C-16:1 omega 7c, C-17:1 omega 8c, and C-16:0. The major respiratory menaquinone was MK-9(H-8). The G + C content of strain Hal317(T) was 68.70%. Based on the polyphasic approach of bacterial classification, strain Hal317(T) could be distinguished from known genera of the family Iamiaceae, and we propose the name Actinospongicola halichondriae gen. nov., sp. nov. (= DSM 114536(T) = LMG 32795(T)). Genomic features indicated that strain Hal317(T) was characteristic of a sponge-associated lifestyle, such as the potential to metabolise diverse sponge-derived carbohydrates (e.g. chitin, l-fucose) and sulfur compounds (e.g. taurine). Moreover, the genome of strain Hal317(T) contained stress response-related genes, including those involved in ectoine synthesis and toxin-antitoxin systems. Within the family Iamiaceae strain Hal317(T) revealed unique genomic features, including a non-phosphorylated l-fucose pathway. Our study sheds light on the Acidimicrobiia, a class typified by few cultured representatives. Additionally, the new sponge-derived isolate offers valuable insights into the host-bacteria association
MAPRIDGES: Geometry of global mid-ocean ridge plate boundaries, and the role of transform faults and non-transform offsets
No full textThe global mid-ocean ridge system produces the oceanic lithosphere accounting for ~70% of the Earth’s surface, while hosting active processes (tectonic, volcanic, hydrothermal circulation). The ridge system is segmented by both transform faults and non-transform offsets, and their geometry can be now re-evaluated with existing multibeam bathymetry (with a resolution of ~100 m or better), both from publicly accessible datasets (e.g., GMRT, NCEI, Pangaea, AWI, among others) and available through published studies. This high-resolution bathymetry is now available for ~25% of the ocean seafloor, but covers a significant proportion of the global mid-ocean ridge system (>70%) and is thus suitable to refine and finely define its geometry.
The MAPRIDGES database (https://doi.org/10.17882/99981) provides a global dataset that includes the newly-defined geometry of individual mid-ocean ridge segments, the most complete catalog to date of transform faults, and identifies non-transform offsets (NTOs). This effort is linked to the World 5M project by CGMW (Commission for the Geological Map of the World). We calculate the lateral offset associated with these NTOs, and determine if they correspond to overlaps of adjacent segments or if they are associated with a gap (underlap). Two different plate models (MORVEL and GSRM) are used to estimate the length of overlaps, underlaps and their links to variations in spreading direction.
Our new database, gives a global, detailed view of the global mid-ocean ridge geometry, and provides the first evaluation of the overall lengths of ridges and associated lateral offsets, both transform and non-transform. Mid ocean ridge segments (1471) show a cumulative length of ~71200 km, with and along-axis distance of ~4800 km of overlapping segments, and ~1700 km of underlap; taking these offsets into account this yields a total length of along-axis segments of ~75300 km. We have also digitized the traces of 262 transform faults to obtain the most complete catalogue to date of these structures. Transform faults account for a cumulative lateral offset of ridges of ~27000 km. We report a first estimate of the lateral offset of 1058 identified NTOs at ~10400 km, accounting for >30% of the cumulative transform fault length. The resulting cumulative lateral offset from both transform and non-tranform segments is thus ~37400 km, and is ~50% of the total ridge length. As in the case of transform faults, these NTOs are associated with deformation of a significant volume of the recently accreted oceanic lithosphere, and thus likely facilitating hydrothermal circulation and alteration of the lithosphere. This study will facilitate the quantification of these processes and provides a basis to better understand their implications on local and global environments (e.g., chemical fluxes associated with alteration at all offsets)
Impact of seamounts on the hydration of subducting plates
No full textThe subduction of seamounts greatly affects arc volcanism, earthquakes, and tectonic deformation of the overriding plate, but the role of seamounts during bending and hydration of the incoming plate at subduction zones is poorly understood. We present seismic tomographic results along three profiles from the Middle America Trench offshore northern Costa Rica. The crustal and upper mantle P-wave velocities decrease toward the trench, with the onset of velocity reduction at ∼70 km from the trench axis, indicating bend-faulting, alteration, and hydration of the incoming plate. The most prominent low-velocity anomaly of 7.6−7.8 km/s in the upper mantle occurs beneath a seamount at the outer rise, indicating enhanced hydration with ∼2.4 wt% water content, compared to ∼1.1−1.2 wt% in the subducting plate away from the seamount. Near the seamount, extremely low heat flow (<10 mW/m2) supports vigorous hydrothermal recharge of seawater. Our results reveal that subducting seamounts efficiently increase the permeability of the oceanic crust prior to subduction, facilitate the transport of seawater into the mantle, exert control on widespread serpentinization, and potentially promote water recycling back into Earth’s interior