GEOMAR Helmholtz Centre for Ocean Research Kiel

Modelled pathways consistent with the Paris Agreement goals to mitigate warming typically include the large-scale application of Carbon Dioxide Removal (CDR), which can include both land- and marine-based CDR methods. However, the Earth system responses and feedbacks to scaling up and/or combining different CDR methods remain understudied. Here, these are assessed by employing two Earth System Models, with a multifactorial setup of 42 emission-driven simulations covering the whole spectrum of Afforestation/Reforestation (0-927 Mha) and of Ocean Alkalinity Enhancement (0-18 Pmol) over the 21st century. We show that global carbon flux responses scale linearly when different CDR methods are scaled up and/or combined, which suggests that the efficiency of CDR is insensitive to both the amount of CDR and the CDR portfolio composition. Therefore, combining CDR methods, which seems beneficial for diversifying risks and remaining below sustainability thresholds, does not compromise the efficiency of individual applications.

Accurately representing ocean dynamics across interacting scales remains a challenge in numerical modeling. This study examines mesoscale eddy parameterization in eddy‐permitting ocean models by incorporating novel stochastic perturbations and comparing them with a well‐tested dynamic kinetic energy backscatter scheme. Momentum dissipation through eddy viscosity, a key aspect at such model resolutions, causes excessive dissipation not only at the grid scale but across all scales, including energy‐containing ones. This necessitates methods like dynamic backscatter to counteract energy loss and restore variability. Stochastic perturbations provide an alternative by reinjecting energy and capturing small‐scale variability. Using a double‐gyre FESOM2 configuration, we assess two stochastic forcing schemes, applied with and without dynamic backscatter. The stochastic perturbations are generated using linear inverse modeling based on a high‐resolution reference simulation. Both stochastic methods improve simulated dynamics, particularly heat distribution and kinetic energy, though they are less effective at large scales than dynamic backscatter. Contrary to expectations, combining stochastic forcing with dynamic backscatter does not yield substantial improvements. Moreover, none of the schemes significantly enhances mean kinetic energy in the jet region, suggesting unresolved dynamics at this resolution despite increased eddy‐kinetic energy (EKE). A comprehensive scale analysis, including kinetic energy production, transfer, dissipation, and spectra, highlights distinct energy pathways. Energy injection by dynamic backscatter directly increases kinetic energy, while stochastic perturbations enhance potential energy conversion and subsequent transfer to EKE. These findings emphasize the need for carefully designed energy injection patterns aligned with flow dynamics to improve parameterizations at eddy‐permitting resolutions.

The Qingdao Cold Water Mass (QCWM), located in the offshore waters near the Shandong Peninsula, exhibits notable seasonal variability and plays a crucial role in shaping hydrological conditions due to its distinct temperature and salinity structures. This study investigates the evolution of the QCWM in 2014 using cruise observations and the hydrodynamic model FVCOM. Results reveal that the QCWM forms in April and dissipates by June. Momentum analysis indicates that its springtime formation and persistence are facilitated by an anticyclonic circulation and the weakness of the pressure gradient force. In summer, the emergence of the Yellow Sea Cold Water Mass (YSCWM), combined with the frontal circulation at its edges, enhances the westward baroclinic pressure gradient force and destabilizes the structure of the QCWM. Lagrangian particle tracking experiments suggest that the bottom water of the QCWM primarily originates from local cold waters off the southeastern coast of the Shandong Peninsula in winter and dissipates locally in summer. The QCWM’s persistence in the absence of tidal signals underscores its localized characteristics.

The analysis of fossil diatoms preserved in continental shelf sediments can provide detailed insights into the paleoecological and environmental history of marginal seas. In this study, we consider the development of the northern South China Sea based on a 302-m-long drilling core taken on the outer continental shelf. Diatoms were absent in more than half of the analyzed samples, a relatively low absolute abundance, so that a diatom-based biostratigraphy could not be established. The low absolute abundance suggests that diatom preservation was poor because of highly dynamic oceanographic conditions and significant biosilica dissolution. The dominance of coastal species and the intermittent presence of mostly tropical open-sea diatoms along the core reflects strong land-sea interactions and a current circulation influenced by sea-level fluctuations, following the glacial and interglacial cycles of the Quaternary period. This study provides clues concerning the evaluation of diatom fossils in micropaleontology, as well as their role in biogeochemical cycles within complex sedimentary processes across various marginal sea shelves.

Deep ocean circulation modulated glacial–interglacial climates through feedbacks to the carbon cycle and energy distribution. Past work has suggested that contraction of well-ventilated North Atlantic Deep Water during glacial times facilitated carbon storage in the deep ocean and drawdown of atmospheric CO2 levels. However, the spatial extent and properties of different water masses remain uncertain, in part due to conflicting palaeoceanographic proxy reconstructions. Here we combine five independent proxies to increase confidence and reconstruct Atlantic deep water distributions during the Last Glacial Maximum (around 21 thousand years ago) and the following Heinrich Stadial 1—a time when massive ice rafting in the North Atlantic interfered with deep water formation and caused global climate shifts. We find that North Atlantic Deep Water remained widespread in both periods, although its properties shifted from a cold, well-ventilated mode to a less-ventilated, possibly warmer, mode. This finding implies a remarkable persistence of deep water formation under these cold boundary conditions, sustained by compensation between the two formation modes. Our constraints provide an important benchmark for evaluating Earth system models, which can enhance confidence in future climate projections.

Plain Language Summary Extreme warm events in the far eastern tropical Pacific, so‐called Coastal Niño events, were observed in 2017 and 2023. While both events had strong direct impacts on local weather and ecosystems, they evolved differently. The 2023 event developed into a basin‐wide El Niño event, spreading throughout the entire equatorial Pacific, while cold conditions in the central Pacific followed the 2017 event. Observational data shows large differences between single events. Hence, the small amount of well‐observed events limits our understanding of what influences this divergent evolution. Using a global coupled climate model which simulates Coastal Niño events realistically and captures their main characteristics, controlling factors for the development can be investigated. Warmer conditions in the subtropical northeastern Pacific and equatorial western Pacific, weaker trade winds in the western tropical Pacific and stronger equatorial waves all increase the chances that a Coastal Niño develops into a basin‐wide El Niño. Also, a Coastal Niño enhances the probability of warm conditions in the central Pacific in the following boreal winter season.

Aragonite undersaturation (Ωar${\Omega}_{\mathrm{ar}}$ <$<$ 1) events are projected to rapidly increase in frequency and duration in the Antarctic Weddell Sea by 2050. Thecosome pteropods (pelagic snails) are bioindicators of ocean acidification (OA) because their aragonite shell dissolves easily at low Ωar${\Omega}_{\mathrm{ar}}$ saturation states. Here, we describe the shell dissolution state of the pteropod Limacina helicina antarctica in relation to the water column Ωar${\Omega}_{\mathrm{ar}}$ in the southern Weddell Sea during austral summer 2018 as benchmark for future monitoring of ongoing OA. Ωar${\Omega}_{\mathrm{ar}}$ depth profiles at the sampling sites were consistently close to or in the range of threshold levels (Ωar${\Omega}_{\mathrm{ar}}$ ∼$\sim$ 1.1–1.3) for pteropod shell dissolution. Pteropods contributed up to 69% of total mesozooplankton biomass, and their distribution correlated positively with Ωar${\Omega}_{\mathrm{ar}}$ and chlorophyll a concentration. When analyzed with scanning electron microscopy, 78% of the investigated shells exhibited dissolution, and 50–69% showed the more severe Type II dissolution exceeding current projections of pteropod shell dissolution for the Southern Ocean. But importantly, in our study, only two specimens had the most severe Type III dissolution. Dissolution often co‐occurred with and occurred in scratch marks of unclear origin supporting notions that an intact periostracum protects the shell from dissolution. Where dissolution occurred in the absence of scratches or absence of evidence of periostracum breaches, microscale/nanoscale breaches may have been an important pathway for dissolution commencement supporting recent findings of a reduction of the organic shell content caused by low Ωar${\Omega}_{\mathrm{ar}}$/low pH. The dissolution benchmark we provide here allows future application of pteropods as early‐warning indicators of presumably progressing OA in the Weddell Sea.

Sponge-associated microbes play fundamental roles in regulating their hosts’ physiology, yet their contribution to sexual reproduction has been largely overlooked. Most studies have concentrated on the proportion of the microbiome transmitted from parents to offspring, providing little evidence of the putative microbial role during gametogenesis in sponges. Here, we use 16S rRNA gene analysis to assess whether the microbial composition of five gonochoristic sponge species differs between reproductive and non-reproductive individuals and correlate these changes with their gametogenic stages. In sponges with mature oocytes, reproductive status did not influence either beta or alpha microbial diversity. However, in two of the studied species, Geodia macandrewii and Petrosia ficiformis, which presented oocytes at the previtellogenic stage, significant microbial composition changes were detected between reproductive and non-reproductive individuals. These disparities were primarily driven by differentially abundant taxa affiliated with the Nitrososphaeria archaeal class in both species. We speculate that the previtellogenic stages are more energetically demanding, leading to microbial changes due to the phagocytosis of microbes to meet nutritional demands during this period. Supporting our hypothesis, we observed significant transcriptomic differences in G. macandrewii , mainly associated with the immune system, indicating potential changes in the sponge’s recognition system. Overall, we provide new insights into the possible roles of sponge microbiomes during reproductive periods, potentially uncovering critical interactions that support reproductive success. IMPORTANCE Our research explores the fascinating relationship between sponges and their resident microbes, focusing specifically on how these microbes might influence sponge reproduction. Sponges are marine animals known for their complex and beneficial partnerships with various microbes. While previous studies have mainly looked at how these microbes are passed from parent sponges to their offspring, our study is among the first to examine how microbial communities change during the different stages of sponge reproduction. By analyzing the microbial composition in five sponge species, we discovered that significant changes occur in species with premature oocytes, suggesting that microbes may play a crucial role in providing the necessary nutrients during early egg development. This work not only enhances our understanding of sponge biology but also opens up new avenues for studying how microbes support the reproductive success of their hosts in marine environments.

On 27 March 2021 a 3‐months lasting seismic sequence struck the Central Adriatic Basin that is part of the Adria plate, a relatively undeformed plate since recent times. Analyzing the waveform data acquired by the Italian and Croatian seismic networks, we computed the location parameters of 160 earthquakes and the focal mechanisms of the Mw 5.2 mainshock and the larger aftershocks. Most events align along a WNW‐ESE trending, 30 km long, narrow belt. The depth distribution of events indicates that the mainshock and a few aftershocks occurred within the upper 4 km, while most aftershocks were located below 5 km within the carbonate platform. We propose that the evolution of the 2021 earthquake distribution is primarily ruled by the top ductile salt layer. Moreover, the presence of a salt layer explains the relatively high VP/VS ratio of 1.83 in the sediment rocks surrounding the salt bodies, as also observed in similar tectonic settings. We suggest that the seismogenic fault likely responsible for the 2021 events is an inherited SW‐dipping normal fault, reactivated by reverse kinematics in response to the regional compressive stress. These results, and the understanding that salt deposits play a key role in focusing deformation and seismogenesis, represent a novel contribution to the long‐standing challenge of seismic hazard assessment of the Central Adriatic Basin, where moderate to large events could have devastating impacts along the densely populated coasts.

The Great Meteor seamounts are located in the eastern Atlantic Ocean, about 750 km south of the Azores. Conjugate to the Corner seamounts in the western Atlantic Ocean, it has been suggested they formed at the same hotspot that generated the New England Seamount chain. However, isotopic data suggest the Great Meteor seamounts are genetically linked to the Azores rather than to the New England hotspot. To test this, we have used seismic, gravity and bathymetry data acquired onboard M/V Meteor in 1990 to reassess the crustal structure, elastic thickness, Te, and tectonic setting of the seamounts. The most prominent is Great Meteor, the largest seamount in the Atlantic Ocean. We show that the guyot comprises a pelagic, limestone (2.0–4.5 km s⁻¹) and extrusive basaltic lava (5.0–6.0 km s⁻¹) drape that overlies a relatively high P‐wave velocity (6.0–6.5 km s⁻¹) intrusive “core” of mafic and possibly ultramafic rocks. The seismic structure has been verified by gravity modeling assuming a Gardner and Nafe‐Drake relationship between P‐wave velocity and density. The best fit between the observed and calculated gravity anomaly based on a plate flexure model is for an elastic thickness, Te, of ∼20 km which implies an edifice age of ∼43 Ma, assuming a 450°C controlling oceanic isotherm. While the edifice age is greater than the sample age (∼17 Ma), it explains the subsidence history of Great Meteor and is compatible with dynamic models of plume‐ridge interactions that predict the Azores hotspot has migrated north during the Cenozoic.

Organic molecules exuded into water column by marine organisms represent a significant portion of marine dissolved organic matter (DOM) that modulates biochemical interactions. Secreted allelochemicals have been suggested to be involved in regulation of pathogen abundance in seagrass meadows, however, seagrass exo¬ metabolome has remained unstudied. We aimed to identify seagrass exometabolites, within and outside meadows, and explore their potential involvement in pathogen suppression under varying environmental con¬ ditions. We collected seawater (SW) samples from eelgrass (Zostera marina)-vegetated (V) and non-vegetated (NV) areas across 5 locations spanning 270 km of coastline along the German Baltic Sea. Comparative LC-MS/ MS-based untargeted computational metabolomics combined with statistical analyses and machine learning tools were employed to pinpoint (exo)metabolomic signatures of eelgrass leaves. Simultaneously, we measured abiotic parameters and the abundance of three common pathogenic taxa in seawater, and investigated spatio¬ temporal variations. Here we show the correlation of pathogen biomass and eelgrass pathogen reduction effect with increasing seawater temperature, eutrophication and anthropogenic influences. Exometabolomics studies revealed that eelgrass exudates contributed significantly to overall seawater DOM at molecular level, while SW overlying eelgrass meadows contained many chemical features unique to the eelgrass leaf metabolome. We identified four flavone aglycones as key biomarkers distinguishing SW-V and SW-NV samples. Their drastically increased concentrations correlated with the lowest pathogen biomass, suggesting their role in pathogen regu¬ lation. These combined analytical and microbiological approaches indicate that flavones are defensive alle¬ lochemicals released into eelgrass meadows upon environmental stress and serve as potential bioindicators of eelgrass’ sanitation effect.

The St. Anna Trough (SAT) plays a critical role in Arctic Ocean circulation by facilitating heat and water mass exchange, influencing sea‐ice melt and thermohaline dynamics. However, ocean circulation in this key region remains understudied compared to other parts of the Arctic. To better understand water mass pathways, origins, and mixing processes in the SAT, this study analyzes anthropogenic radionuclides iodine‐129 (¹²⁹I) and uranium‐236 (²³⁶U), alongside neodymium isotopes (εNd). Seawater samples were primarily collected from the SAT and Kara Sea during the 2021 Arctic Century Expedition, with complementary data sets from independent sampling campaigns in the Fram Strait (2021) and Barents Sea (2018) providing broader regional context. Distinct ¹²⁹I signatures reveal the mixing of Atlantic Waters with Arctic shelf‐formed waters, contributing to the formation of Cold Deep Water, which integrates into the intermediate and deep Arctic Ocean. Elevated ²³⁶U concentrations in mid‐depth samples indicate the intrusion of Arctic‐Atlantic Waters into the SAT, underscoring the region's role in Arctic water recirculation and mixing complexity. The εNd data indicate a strong riverine signal from the Ob and Yenisei rivers in the southern Kara Sea and Voronin Trough, whereas SAT surface waters show greater influence from Barents Sea Atlantic Waters. Elevated surface radionuclide concentrations above the Voronin Trough highlight this area as a primary gateway for radionuclides entering the central Arctic. These findings provide new insights into Arctic Ocean circulation and demonstrate the complementary strengths of radionuclides and εNd in resolving water mass transformations and pathways.

Quantifying the ocean’s ability to sequester atmospheric carbon is essential in a climate change context. Measurements of gravitational carbon export to the mesopelagic seldom balance the carbon demand or the oxygen consumption there, suggesting the potential presence of other mechanisms of carbon export. We deployed a biogeochemical Argo float in a cyclone in the Benguela upwelling system for five months, and estimated vertical carbon export and respiration in the eddy via particle imagery with an underwater vision profiler 6 in a quasi Lagrangian way. A sensitivity analysis shows that, under certain assumptions, oxygen consumption rates could match the carbon supply and carbon demand. We furthermore identified a mechanism of vertical particulate carbon export, the full eddy core submergence pump. Our analysis suggests that at 450 m depth, within this eddy, this pump exports about one fourth to half of the total carbon compared to the biological gravitational pump.

Marine carbon dioxide removal (mCDR) options could potentially play an important role in future CDR policy portfolios. They include, for example, ocean alkalinity enhancement, blue carbon projects such as mangrove cultivation, as well as sub-seabed storage of captured atmospheric CO2. In this paper we present a novel assessment framework designed for mCDR options. The framework provides important conceptual advancements to existing frameworks currently used to assess climate options: It clearly distinguishes between and allows for the assessment of both the feasibility and desirability of mCDR options, it makes explicit the evaluative standards upon which the assessment is based and it separates the descriptive listing of information from the evaluation of said information. The assessment framework aims to advance the debate on what role mCDR can and should play in responding to the climate crisis by providing a tool for both policymakers and stakeholders to assess mCDR options in a transparent and comprehensive way.

The arange and biomass distribution of marine fish species offer insights into their underlying niches. Quantitative data are rare compared to occurrences and remain underused in species distribution models (SDMs) to explore realized niches—the actual space occupied by a species shaped by abiotic and biotic factors. Local densities drive differences in species contributions to ecological processes and ecosystem function rather than through presence alone. If a species growth rate is strongly controlled by macro‐environmental conditions, then predicting geographical abundance or densities should be possible. We collated 20 years (2001–2020) of standardized scientific bottom trawl data to fit several versions of hierarchical generalized additive models using biomass (kg km⁻²) of four dominant demersal species (Common dab, European flounder, European plaice, Atlantic cod) within yearly and seasonal (winter and autumn) time windows. Covariates were represented with trawl‐level geographic information (position, depth) and high‐resolution oceanographic features. This work illustrates species‐specific spatiotemporal biomass patterns across two decades and demonstrates superior predictive performance with seasonally variable smoothing terms, revealing seasonally different responses to oceanographic predictors. Firstly, we find relative stasis in Common dab biomass which is linked to the macro‐environmental salinity gradient in the western Baltic Sea but with different temperature responses across seasons. Secondly, we show both European flounder and plaice have increased in biomass in the western Baltic Sea with different seasonal relationships to bottom temperature, and that flounder switches between salinity conditions based on season during spawning/feeding periods. Lastly, both juvenile and adult Atlantic cod life stages are shown to have declined most significantly in the Bornholm Deeps and the Gdańsk Deeps. For cod, we conclude that biomass was less reliably predicted in comparison to the other major Baltic demersals studied here, warranting dynamic fishing covariates as a formerly major commercial fishing target. These models approach more dynamic species distribution models and are increasingly valuable to constrain uncertainties in biogeographic forecasting which often rely on annually‐averaged response curves, occurrence data, and suitability maps which rarely discriminate between areas of high and low biomass areas in space and time.