The discovery of Borealis mud volcano: a natural sanctuary for threatened Arctic species

The discovery of Borealis mud volcano: a natural sanctuary for threatened Arctic species | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article The discovery of Borealis mud volcano: a natural sanctuary for threatened Arctic species Giuliana Panieri, Claudio Argentino, Alessandra Savini, Bénédicte Ferré, and 16 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4305053/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 27 Jan, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract This paper presents the recent discovery of Borealis , a submerged mud volcano systemlocated in the Polar North Atlantic, distinct from the numerous methane seepages previously identified in the region. In situ observations using a remotely operated vehicle (ROV) have captured the expulsion of warm (11.5°C) Neogene sediments and the eruption of methane-dominated fluids from a localised site within a ~500 m diameter crater. The seafloor around Borealis comprises laterally extensive carbonate deposits, suggesting long-lasting diffuse methane migration. Sampling and seafloor images reveal that Borealis hosts unique habitats thriving in the low-oxygen environments around methane seeps. Additionally, the irregualry shaped carbonate structures serve as a natural refuge from bottom trawling as well as a substratum for sessile fauna and function as nursing grounds for threatened fish species. This discovery in the Polar North Atlantic highlights the ecological value of cold seep ecosystems, which play a critical role in biodiversity by acting as sanctuaries for marine species, hence emphasising the importance of their conservation. Earth and environmental sciences/Biogeochemistry Biological sciences/Biochemistry Figures Figure 1 Figure 2 Figure 3 Introduction Over the past ten years, marine surveys in the Polar North Atlantic continental shelf and slope have consistently identified new methane seeps on the seafloor 1 (Fig. 1). These sites are of great interest due to their potential impact on the marine ecosystem and global climate. Methane, a potent greenhouse gas, has increased atmospheric concentrations since the start of the Industrial Revolution, accelerating climate change 2 . Estimates suggest that between 218 and 371 teragrams of methane per year (Tg CH₄ yr⁻¹) are emitted from natural sources within terrestrial and aquatic settings, as determined by top-down and bottom-up approaches, respectively 3 . Methane's influence on the environment extends beyond its well-known role as a potent greenhouse gas; it is also a critical component in forming complex ecosystems that emerge from the interactions between biological, geochemical and geological processes 4 . In marine environments, methane cold seeps contribute to regional biodiversity by supporting specialised microbial and faunal communities adapted to harsh conditions 4-6 . In seep-impacted sediments, the anaerobic oxidation of methane (AOM) supports high fluxes of dissolved sulfide (H 2 S) toward the surface, which is colonised by sulfur-oxidizing bacterial mats and chemosymbiotic organisms, such as clams, mussels and tubeworms 6,7,8 . Methane seeps are often associated with widespread carbonate deposits formed in situ because of the increased local alkalinity induced by AOM 9 . These deposits have a wide range of morphologies, from flat pavements to vertical pinnacle-like structures, and highly variable dimensions from a few mm-sized concretions to beds several hundreds of meters in lateral extent and several meters in thickness 10,11 . Seep carbonates provide hard substrata for sessile organisms 4. and resources for other species in adjacent areas, contributing to a broader ecological network 6 . They are also a record of geological processes of methane oxidation spanning millions of years and have been used in high-resolution paleo-reconstructions 12,13 . Despite numerous observations of methane emissions from the seafloor in Arctic regions, only five mud volcanoes have been discovered in the Canadian Beaufort Sea (Western Arctic) 14 , and three in Alaska 15 , and so far, the Håkon Mosby Mud Volcano (HMMV) was the only known structure in Norwegian waters 16 . Mud volcanoes are surface manifestations of focused fluid flow in hydrocarbon-rich sedimentary basins along passive margins and at convergent plates characterized by high sedimentation rates. Mud volcanoes collectively play a significant role in the atmospheric methane budget 17,18 by releasing an estimated 60 Tg CH 4 yr -1 ) 18 sourced from several kilometres below the seafloor towards the surface 19 . Such vast quantity has large uncertainties because the total number of mud volcanoes worldwide and their temporal variability concerning methane emission rates are not confidently known, especially when considering undiscovered offshore mud volcanoes. Therefore, our current understanding of their potential contribution to atmospheric methane emissions and impact on climate, ocean chemistry and ecosystems is still limited, and so are the mechanisms triggering their activity and their role in the survival of specialised fauna thriving at these sites 5,6 . Here, we present a comprehensive study of a newly discovered mud volcano in the Polar North Atlantic named Borealis mud volcano (MV) located in Outer Bjørnøyrenna (Bear Island Trough, 72° 26.304´N, 17° 40.626´E, ~ 390 m water depth) in the Barents Sea (Fig. 1). This discovery marks the second mud volcano of this kind identified in the region, among numerous methane seeps previously detected 20 (Fig. 1), thus opening a new chapter in our understanding of Arctic geology and related fields of research. The Borealis MV is characterized by a cluster of craters from ~70 m to ~400 m in diameter and an active gryphon (~ 7 m in diameter and 2 meters high) expelling warm fluids, gas and oil. Our observations also show that Borealis MV acts as sanctuaries for fauna vulnerable to anthropogenic perturbations, specifically seabed trawling, which has a major impact on benthic ecosystems in the region 21,22 . Geophysical and water column insights Borealis MV is located in the eastern parts of the Sørvestsnaget Basin, where a thick sequence of Cretaceous and Cenozoic sedimentary rocks is covered by a wedge of Pliocene to Pleistocene sediments 23,24 . In this area, industry 3D-seismic data revealed the presence of a 500-600 m wide seismic chimney in the subseafloor terminating at the seafloor into a complex network of crater-like depressions (Fig. 1_SI in Supporting Information). Shallow bright amplitude acoustic anomalies observed in the Quaternary sediments are interpreted as local free gas accumulations within a vertical chaotic zone. The chimney is rooted at around 300 meters below sea floor (m bsf), at the base of the Quaternary/URU (Upper Regional Unconformity) boundary (Fig. 1; Fig. 1_SI in Supporting Information). The detection of the chimney structure steered our hydroacoustic observations, which subsequently revealed persistent, concentrated and active gas flares originating from the Borealis MV. Within a confined area (~0.14 km 2 ) around an active gryphon, we have identified a total of 26 gas flares (Fig. 1). These individual flares exhibit a typical height of 160 m on average, potentially ascending up to 355 m into the water column. In some instances, flares even approach the sea surface, as illustrated in Figure 2_SI. CTD water samples (Fig. 1) collected from one of these plumes showed the presence of methane with concentrations that remain relatively high throughout the water column and preserve notable levels, up to 11,390 nmol L -1 close to the seafloor (Fig. 1; Table 3_SI). This suggests an intense advection of hydrocarbon-charged fluid migration. In comparison, background values of methane concentrations are around 0.9 nmol L -1 , making the observed values approximately 100 times higher than the ambient conditions. The subsequent ROV seafloor observation confirmed the persistent and ongoing methane venting and revealed a major depression (500-600 m in diameter), comprising four main craters (Fig. 1) that individually range in diameter from ~70 m to ~400 m. The two northernmost craters host extensive carbonate exposed at the seafloor as stacked slabs forming tens of meters wide large pavements covering several tens of square meters. The thickness of single slabs is on the order of tens of cm, but the total thickness of the deposits can reach ~ 5 m along the steep wall of the craters. While carbonate crusts have been observed in other cold seeps in the Barents Sea 11,12,25 , the carbonates at Borealis are remarkably larger and thick, emphasizing the exceptional nature of this site. The gryphon, on the southern flank of the deepest crater of the Borealis MV system, has a central conduit that serves as the epicentre from which warm fluids and sediments are actively expelled, as observed from gas bubbles and sediment flows extending over the seafloor. (Figs. 1 and 2). Origin of fluids and expelled mud We collected the expelled unconsolidated sediments from the gryphon using a push core manipulated by the ROV. They consist of medium-sized sand with minor clayey content, silt, and a few small sedimentary rock fragments and coal pebbles. The sediment expelled contains planktonic foraminiferal fauna correlated with the micropaleontological zonation for the Neogene on the Vøring Plateau 26 while the benthic foraminiferal fauna with the one for the Cenozoic of the North Sea 27 . Nearly all the individuals are extinct species typically associated with Pleistocene deposits on the Norwegian Shelf from ca 700-1000 m bsf 28,29 . The lack of benthic foraminifera typical for Holocene and recent sediments on the Norwegian continental shelf, including Trifarina angulosa and Uvigerina per e grina 30-32 , and the almost complete lack of warm water-dwelling planktonic foraminifera indicate that no Holocene sediments are present in the rising sediment. Geochemistry performed on the sediment from the push core indicates that the fluid emitted from Borealis gryphon has lower levels of organic carbon and nitrogen compared to the average southwestern Barents Sea surface sediments (i.e. TOC>0.5 %; TN>0.05 %) 33 with values of 0.33 wt.% and 0.02 wt.%, respectively. The isotopic composition of organic matter δ 13 C = −27.11 ‰ and δ 15 N = 4.58 ‰, associated with the high C/N ratio of 20.2, are consistent with a high contribution of ancient organic sources that underwent prolonged microbial degradation in the deep subsurface 34,35 (Fig. 3). Organic biomarkers revealed the presence of detectable amounts of crude oil associated with immature bitumen showing virtually no biodegradation (Fig. 3). Recent studies reported the widespread and extensive methane and oil release from geological reservoirs to the Arctic Ocean 20 and, although evident oil seepage or oil slicks were not observed at Borealis MV, the system can be considered as the surface expression of a deeper sited active petroleum- system. The bulk δ 13 C composition of saturated and aromatic hydrocarbons, ranging from −31.2 ‰ to −29.3 ‰ and from −31.6 ‰ to −28.2 ‰, respectively, do not lead to the univocal interpretation of the origin of the oil (Fig. 3). However, the Dibenzothiophene/Thiophene ratios < 1 coupled with Prystane/Phytane ratios between 1 and 3 suggest a marine shale/lacustrine source rock 36 (Fig. 3). The age of the source rock is estimated as Jurassic or younger based on sterane C-number distribution 37 and nordiacholestane ratio 38 (Fig. 3). Overall, hopane and sterane isomerism suggest an early oil window maturity. The 30-minute ROV in situ observations at the gryphon site show frequent (every 5-10 minutes) eruptions of warm water-mud-gas mixture that rise along the seep conduit below (Fig. 1). The gas composition is dominated by methane, which, together with d 13 C values of −60.8 ‰ and dD of −222 ‰ (Fig. 3), points towards a microbial origin 39 . The temperature of the discharged fluid measured above the active gryphon by the CTD installed on the ROV was 11°C, and that of the T-probe was 11.42°C. This temperature is substantially higher than the reference ca. 4°C measurements away from Borealis . A unique oasis for faunal communities Borealis MV hosts a diverse array of faunal communities, including seep-associated and background fauna, together with species that have commercial value. Within the MV´s craters, we observe dense and extensive patches of microbial mats and tubeworm aggregations ( Oligobrachia sp.) akin to those documented at other Arctic cold seeps (see Figure 2) 8 . Microbial mats and tubeworms seem to be the main characteristic of high-latitudes compared to lower-latitudes cold seeps, which host clams, mussels and vestimentiferan worms 4 . The microbial mats, extending several square meters, form the foundation for a variety of microorganisms, including a multitude of foraminifera species. Our environmental DNA (eDNA) analyses have unveiled the presence of both hard-shelled ( Cibicidoides wuellerstorfi, Reophax dentaliniformis, Stainforthia sp.), monothalamids and soft-shelled foraminifera nestled within these microbial ecosystems (Table 1_ Supplementary). Although, until now, no endemic foraminifera species have been documented within cold seeps 40 , our preliminary eDNA analyses suggest the possibility of previously unidentified species potentially unique to Borealis . The megafauna diversity at Borealis MV is relatively low, and appears to be influenced by several environmental factors, including elevated methane concentrations, extensive carbonate crusts on the seafloor and the suspended sediment particles emitted by the gryphon that further contribute to the challenging conditions that may be impacting the larger faunal assemblages. The megafauna is dominated by dense clusters of anemones and serpulids anchored to carbonate substrates and hydrozoan colonies ( Tubularia sp.) that flourish on the crater slopes, while the presence of cladorhizid sponges and sea stars is more sporadic. In addition, our sample collections from various habitats within the Borealis MV have yielded taxa not detectable via ROV imagery, such as annelids, amphipods, gastropods, polyplacophorans, nemerteans ( Nipponemertes spp.), and ophiuroids. Morphological and molecular analyses of these samples are currently underway to further elucidate the composition of these faunal communities. The observed scarcity of megafauna suggests that the immediate environmental conditions surrounding the Borealis MV may be inhospitable for a broader range of organisms. Remarkably high methane concentrations (11,390 nmol L -1 near the seafloor) or toxic sulfide levels are factors known to significantly influence invertebrate community structures at seep sites 41 . These chemically- enriched habitats create a gradient of extreme environmental conditions that can be detrimental to many forms of marine life. The toxicity of methane and hydrogen sulfide poses a significant challenge to the survival of organisms that are not specifically adapted to such conditions, often resulting in reduced biodiversity. Additionally, the high volume of suspended sediment particles emitted by the gryphon may impact the diversity and density of filter- and suspension-feeding organisms, potentially by clogging their feeding apparatus 42 . Nevertheless, some taxa appear resilient to the presence of suspended sediment. Hydrozoans, for example, are abundant in the carbonate area covered with sediment particles, similar to what was also observed on the Koryak slope at upper bathyal depths (∼660 m) 43 . While some parts of the seafloor within Borealis MV seem to be inhospitable for many species, the extensive carbonates provide additional habitat and suitable hard substratum for erect epifauna and dense aggregations of several species of anemones, serpulids, demosponges, nudibranchs, and sparse octocoral colonies ( Primnoa resedaeformis ). P. resedaeformis was only located in the jagged carbonate area where lower or no sediment deposition from the plumes of the gryphon was evident. As observed earlier 4 , carbonate deposits are known to shelter a large variety of fauna and represent oases for numerous sessile organisms, as commonly observed at other offshore seepage sites 44 45,46 . The extensive carbonates also provided a “reef-effect” 47 for the fish living in the area, testifying to the Borealis MV's ecological vitality. Large schools of saithe ( Pollachius virens ) and various demersal species such as spotted wolffish ( Anarhichas minor ), cod ( Gadus morhua ), four-bearded rockling ( Enchelyopus cimbrius ), and redfish (mainly Sebastes norvegicus but a few individuals tentatively identified as Sebastes viviparus and Sebastes mentella ) clustering around the jagged carbonate formations (Figure 2) suggest that these structures serve as critical habitats, offering both shelter and abundant feeding opportunities, thereby playing a pivotal role in sustaining the local fish populations. The redfish, particularly Sebastes norvegicus , listed as endangered on the Norwegian Red List for Species and subject to a fishing moratorium 48 49 , seems to utilise these carbonate structures as nurseries. The irregular morphology of the carbonates provides protective shelters, and the site's elevated temperatures may enhance reproductive success by accelerating egg development 50 . During our expedition, we encountered lost fishing gear from bottom trawling snagged on the jagged carbonate rock around the perimeter of the craters (Fig. 2). These were heavily colonised by sessile fauna (e.g., anemones and hydrozoans), suggesting that the gear was lost several years ago. The current cessation of fishing activities in the Borealis area is confirmed by the Vessel Monitoring System (VMS) data, which shows no fishing activities around the mud volcano system. This information, in conjunction with the presence of red-listed species Sebastes norvegicus and taxa indicative of Vulnerable Marine Ecosystems (VMEs) as defined by FAO/ICES 51 , such as the octocorals, suggests the Borealis MV is a de facto sanctuary for these endangered species. The absence of bottom trawling and the natural habitat's protective qualities offer a refuge where these species can thrive despite significant seafloor impacts from fishing in the surrounding Barents Sea 21,22 . Glacial history and genesis of Borealis MV The discovery of Borealis MV is significant as it represents the second mud volcano identified in Norwegian waters since the finding of HMMV 16 . The latter is located 110 km southwest of Borealis at ~ 1,260 m water depth. Previous studies suggest that its activity started ~330 ka before present, when fluids were expelled from the periglacial units loaded by a a 3 km thick glacial deposit 52 . During the Last Glacial Maximum (LGM) ~23 ka before present, the expansion of the Eurasian Ice Sheet (EIS) dominated the Barents Sea landscape 14 . Well-defined ploughmarks surrounding Borealis MV give evidence of seafloor erosion following the LGM. However, no evidence of such ploughmarks intersecting the mud volcano craters indicates that the latter must have formed after the deglaciation phase (Fig. 4_SI). One possible formation scenario is consistent with those depicted for the large Troll pockmark field 53 or the nearby seep area Bjørnøyrenna 54 . As ice retreated, warming temperatures and a decrease in pressure destabilised methane hydrates once trapped within the sediments. The dissociation of these gas hydrates suddenly liberated large amounts of methane, forming the craters. The observed widespread carbonates contributed to cementing the sediments, reducing the porosity/permeability and ultimately acting as a buffer layer for the rising methane-rich fluids. Those carbonate barriers periodically inhibit the upward migration of hydrocarbons, allowing gas accumulation underneath the calcite pavements 55 45 and forcing gas to find alternative pathways to release. This deflection of the fluid flow might have contributed to the genesis of the four observed craters in a sequential and ongoing geological process in the Borealis MV system, possibly by accumulating pressure and causing abrupt dissipation via explosive events. Diffuse, long-lasting gas hydrate dissociation is still currently ongoing at northern latitudes, as observed in numerous sites in the Barents and Norwegian seas 54,56 . This underscores the enduring influence of deglaciation on contemporary geological and environmental processes at high latitudes. The temperature anomaly measured at Borealis clearly indicates that the MV plumbing system is connected to deeper and warmer strata from which fluids rapidly migrate towards the surface. This observation is also very relevant in explaining potential triggers for the eruption model of Borealis MV. In addition to the “deglaciation trigger model” proposed above, we also suggest a “hot fluids surge” scenario to explain the formation of Borealis MV craters (Fig. 4_SI). Similarly to what has been observed at numerous mud volcanoes in Lake Baikal 57,58 , we suggest that 1) batches of upwelling warmer fluids periodically dissociated gas hydrates deposits at shallower depth, resulting in 2) increased volume and pressure that entrained the sediments, then causing 3) multiple surface eruptions. Since significant methane content was measured close to the sea surface, we may also speculate ongoing atmospheric emissions under favourable oceanic setting conditions. However, while the present contribution of atmospheric carbon from marine geological sources is deemed relatively minor, the Borealis MV presents a noteworthy exception. A considerable uncertainty persists concerning future emission projections on a warming planet. This uncertainty underscores the critical need for enhanced monitoring and research to quantify marine carbon fluxes accurately and better predict their potential impact on global climate dynamics. As the Earth's temperature rises, we must address these knowledge gaps to inform policy decisions and mitigate the risks associated with climate change. A natural sanctuary for threatened Arctic species The Borealis MV system also plays a complex role in supporting local marine life. Not only does it provide a sanctuary for endangered fish species from human impacts, but it also may function as a crucial nursery site. The elevated temperatures around the seepage sites create optimal conditions for reproductive processes, leading to more effective breeding cycles. This thermal advantage can enhance the survival and growth of fish populations in the deep sea, and the Barents Sea, one of the world's most productive fishing grounds, represents a significant ecological asset. Furthermore, preserving such ecosystems is crucial for biodiversity conservation and a comprehensive understanding of the intricate interactions between geology, geochemistry and biology in marine environments. The Arctic seafloor has become a vital asset, playing an important role in oil and gas exploitation activities and the emerging deep-sea mining industry. The responsible management of marine mineral and biological resources is paramount for sustainable development and environmental stewardship in the Arctic region. In the longer term, Norway has committed to the 30X30 target for spatial conservation measures of representative marine ecosystems, including in the deep sea 59 . Protecting large areas of the deep-sea floor along the Norwegian margin may result in seep refugia acting as source populations for wider recolonization and restoration of benthic biological communities. Methods 1. Multibeam echosounder Water Column Data Borealis MV' morphometric data were obtained from a 3x3 m resolution morphobathymetric map generated on board and obtained by processing multibeam echosounder data (i.e., depth measurements and backscatter) acquired using the hull-mounted Kongsberg MBES EM710, from which water column data were also recorded, documenting evident flares. Gas seep mapping at the study site was done using the QPS FMMidwater software. Gas seeps were detected as gas flares in the water column data caused by backscatter from the gas bubble streams. The acquired data from EM302 multibeam systems in *.all and *.wcd file formats were converted with FMMidwater to the generic water column format (*.gwc). *.gwc files were visualised in fan view and stacked view. The fan view allows for narrowing the opening beams to select individual flares in the stacked view and export them in an sd file for visualisation in Fledermaus. Only one flare was kept when multiple flares showed the same source in Fledermaus. Locations of individual gas flares were retrieved from the lowest point of the flare in the sd file. This identification is enabled by the significant differences in velocity and density between chains of gas bubbles and the water column, leading to pronounced contrasts evident in the acoustic signals. Flare locations were plotted on the maps and used during ROV-flying during the ROV dives to locate streams of gas bubbles. 2. CTD Temperature and salinity at specific depths in the water were obtained from a CTD (Conductivity, Temperature, Depth) mounted on a rosette, which is lowered in the water column from the hull of the vessel. Twelve 10-litre Niskin bottles were also mounted on the rosette, which we closed at specific depths to estimate dissolved methane. 3. ROV The Aurora work-class ROV is a 6000 m depth-rated ROV with a tethered management system (TMS) called Borealis and provides unique opportunities for science and filmmaking. Four dives (ROV13, ROV14, ROV15, and ROV16) were conducted along exploratory track lines crossing four craters (for a total of 14 hours and 4 minutes of diving time). Applying SfM (Structure from Motion), a photogrammetry technique 37 , selected ROV video frames provided 2D photorealistic orthomosaics of representative physical habitats and sedimentary facies (Fig. 2). 4. Fluid analyses 4.1 Temperature measurements The temperature (T) of the fluid emanating directly from the crater of the Borealis MV was measured using the CTD sensor and a temperature probe, the ISD400 Depth and Temperature Sensor from Impact Subsea (precision level of ±0.01% °C), both mounted on ROV Aurora. 4.2 Methane measurements Water samples collected from the CTD Niskin bottles were transferred into 120 mL glass bottles containing 5 mL of 1 M NaOH. The samples were stored in the dark at 4 ℃ until analyses with a Gas Chromatographer - FID (ThermoScientific Trace 1310). Before the analyses, we created a 5 mL headspace and let the samples equilibrate overnight. The dissolved methane was also continuously measured during the ROV dives using SAGE ( S ensor for A queous G ases in the E nvironment), a dissolved methane instrument designed and built in the Chemical Sensors Laboratory at the Woods Hole Oceanographic Institution (WHOI) 38 39 . SAGE has a detection range of 5-10,000 ppm CH 4 . SAGE uses a deep-sea membrane inlet to extract dissolved gas from seawater. Inside the instrument, the extracted gas fills a hollow core optical fibre. Laser spectroscopy measures the methane inside the optical fibre by coupling the light from a laser to the fibre. 4.3 Geochemistry 4.3.1 Sediment geochemistry We prepared 0.3 g of dry sediment from a surface sample collected at the seeping spot to measure its organic carbon, nitrogen content, and isotopic composition (d 13 C, d 15 N). The carbonate material was removed by acid addition using 6 N HCl. Analyses were conducted at the SIL (UiT) using a Thermo-Fisher MAT253 Isotope Ratio Mass Spectrometer (IRMS) coupled to a Flash HT Plus Elemental Analyzer. The d 13 C and d 15 N values were normalised to international standards, Vienna Pee Dee Belemnite (VPDB) (d 13 C) and Air-N 2 (d 15 N). Precision on d 13 C and d 15 N was better than 0.15 ‰ (1SD). The C/N atomic ratio was calculated using the atomic mass weighted ratio of TOC and TN as C/N =(TOC/12.011)/(TN/14.007). 4.3.2 Oil geochemistry Sediment samples for oil geochemistry were collected from sediment slices, wrapped in aluminium foil and stored at -20℃ for freeze-drying. All oil preparation and analysis procedures followed NIGOGA (Norwegian Industry Guide to Organic Geochemical Analysis), 4th Edition, and were conducted at Applied Petroleum Technology (APT, Oslo). Extractions were performed with a Soxtec Tecator unit and dried before deasphaltering. A small amount of dichloromethane (3 times the amount of EOM) is added. Pentane is added in excess (40 times the volume of EOM/oil and dichloromethane). The solution is stored for at least 12 hours in a dark place before centrifugation and the weight of the removal of asphaltenes. Quantifying saturates, aromatics A and polars (NSO-fraction) was done using two HPLC pumps, sample injector, sample collector and two packed columns. The pre-column is filled with Kieselgel 100 and heated at 600 °C for 2 hours to deactivate it. The main column, a LiChroprep Si60 column, is heated at 120 °C for 2 hours with a helium flow to make it water-free. Approximately 30 mg of deasphaltened oil or EOM diluted in 1 ml hexane is injected into a sample loop. The solvents used are hexane and dichloromethane. The stable carbon isotope composition of the different fractions was measured on a Delta V Plus Isotope Ratio Mass Spectrometer (IRMS) (Thermo Fisher Scientific) via Conflo IV. A standard (NGS NSO-1, topped oil) is analysed for each 12 th sample. The δ13C value obtained for this standard is –28.6‰ vPDB.The variation in the isotopic values for NSO-1 by repeated analysis over one year is ± 0.09‰. Age-specific biomarkers were measured via GC-MS/MS using a Thermo Scientific TSQ Quantum instrument. The column used is a 60 m CP-Sil-5 CB-MS with an i.d. of 0.25 mm and a film thickness of 0.25 µm. d4-27ααR was used as an internal standard. 4.3.3 Gas geochemistry Samples of seep gas were collected using a bubble catcher and stored in steel flasks. Aliquots of the samples were transferred to exetainers. 0.1-1ml were injected into an Agilent 7890 RGA GC equipped with Molsieve and Poraplot Q columns, a flame ionisation detector (FID) and 2 thermal conductivity detector (TCD). Hydrocarbons were measured by FID. The carbon isotopic composition of methane was determined via GC-C-IRMS. Aliquots were sampled with Triplus RSH autosampler and analysed on a Trace 1310 gas chromatograph (Thermo Fisher Scientific), equipped with a Poraplot Q column and PTV (Programable Temperature Vaporizing) injector. The GC is interfaced via GC-Isolink II and Conflo IV to Delta V Isotope Ratio Mass Spectrometer (IRMS) (Thermo Fisher Scientific). Repeated analyses of standards indicate that the reproducibility of δ13C values is better than 1 ‰ vPDB (2SD). The hydrogen isotopic composition of methane was determined by a GC-H-IRMS system. Aliquots were sampled with a Triplus RSH autosampler and analysed on a Trace 1310 gas chromatograph (Thermo Fisher Scientific) equipped with a Poraplot Q column and PTV (Programmable Temperature Vaporizing) injector. The GC is interfaced via GC-Isolink II and Conflo IV to Delta V Isotope Ratio Mass Spectrometer (IRMS) (Thermo Fisher Scientific). Repeated analyses of standards indicate that the reproducibility of δD values is better than 10 ‰ vSMOW (2SD). 5. Biological samples and observations 5.1 Mega/macrofauna The description of the fauna present and distribution across the main micro-habitats of the Borealis MV was obtained through the cataloguing and annotation of all the ROV Aurora imagery collected during the expedition. Additionally, experts collected and identified physical samples of both specimens and sediments to support the correct identification of the specimens observed in the videos. 5.2 Foraminifera (eDNA) The foraminiferal species list (Supp. X) was inferred from two sediment samples from a microbial mat. Briefly, after the DNA extraction using the DNeasy PowerLyzer PowerSoil kit, the specific foraminifera 37f hypervariable gene of18S rRNA gene was amplified 14F1-s15 primers 60 . A different combination of tagged primers was used per sample, and three PCR replicates were performed. The PCR products were then verified on agarose gel, pooled and added to a library. Sequencing libraries were prepared using TruSeq® DNA PCR-Free Library Preparation Kit (Illumina) and quantified by qPCR using Kapa Library Quantification Kit for Illumina Platforms (Kapa Biosystems). The libraries were paired-end sequenced on a MiSeq instrument using the kit v2 (300 cycles). The raw data was demultiplexed with a DTD module from SLIM 61 . We used then DADA2 62 to generate Amplicon Sequencing Variants (ASV), which were taxonomically assigned using VSEARCH (Rognes et al. 2016) with 90% min. similarity against a foraminifera database. Declarations Acknowledgements: This manuscript is dedicated to the memory of our beloved colleague and friend, Pär G. Jansson. The authors thank UiT, The Arctic University of Norway, the Norwegian Research Council through the projects AKMA, Advancing Knowledge of Methane in the Arctic (project number 287869), HOTMUD (project number 288299), and EMAN7 (project number 320100), the Norwegian Offshore Directorate, REV Ocean, Woods Hole Oceanographic Institution, and La Rochelle University for their financial support and facilitation of the research expeditions. TGS is thanked for permission to use the 3D-seismic data shown in Fig. 1_SI. We are grateful to Nicolas Straube and Ingvar Bykjedal at the Department of Natural History, University Museum of Bergen, for their help with the identification of fishes and to Alexandra Padilla for supporting the SAGE data analyses. The REV Ocean dive team's invaluable contributions and expertise in data collection are acknowledged with gratitude. SPR work was supported by FCT/MCTES in the scope of the CEEC contract (CEECIND/00758/2017) and funds attributed to CESAM (UIDP/50017/2020, UIDB/50017/2020 and LA/P/0094/2020). MHE was funded by the Trond Mohn Foundation through the Centre for Deep Sea Research (grant number TMS2020TMT13) and the Norwegian Biodiversity Information Centre (the Taxonomy Initiative) through the project “Fauna of hydrothermal vents and cold seeps in Norwegian waters” (project number 3-20-70184243). 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Supplementary Files FigureS1.png Fig. 1_SI: Seismic random line crossing the Borealis mud volcano from 3D-seismic dataset SPE16M01. Directly below the mud volcano, in the centre of the crater, an acoustic chimney is observed with several associated bright amplitude anomalies interpreted as shallow gas. FigureS2.png Fig. 2_SI: WCD (Water Coloum Data)-line 0341 across the Borealis mud volcano site (EM710). A) Fan view of gas flares in the water column at the site of Dive 13, 14, 15 and 16. Several acoustic gas flares, representing streams of gas bubbles, are imaged from the seafloor and up through the water column, some of them almost reaching the sea surface. The depression at the seafloor at the location of the largest flare observed with ROV dives is indicated with a yellow dotted oval, also in B. B) R-Stack along ship track crossing the Borealis mud volcano site showing biological noise in the water column most likely fish school also observed during the ROV visual survey. FigureS3.png Fig. 3_SI: The methane concentration (nmol L -1 ) measured with the SAGE sensor from each dive. The bubble plumes observed with the ROV camera are marked with white circles. FigureS4.pdf Fig. 4_SI: High-resolution bathymetric map (with a 4-meter grid) of Borealis MV. The map details the volcano's four distinct craters, which are located on the seafloor in correspondence with the conduit identified in the seismic random line in Fig 1_SI that serve as conduits for fluid expulsion. Adjacent to the craters, a notable ploughmark is visible, indicating a past event of glacial or sedimentary activity that has left a linear depression on the seafloor. Table1Samplesinformationsubmitted.docx Table 1_SI: Analyzed samples and type of analyses performed at the Borealis MV. Table2SIGeochemistrysubmitted.docx Table 2_SI: Results 2H, 18O, 34S and 87/86Sr isotope analyses of the fluid collected directly at the top of the active emitting gryphon crater. Table3SICTDsubmitted.xlsx Table 3_SI: The methane concentration (nmol L-1) measured in water samples collected with CTD during CTD 84 and 86. Table4SIAllDivesLatLonCH4.xlsx Table 4_SI: The methane concentration (nmol L-1) measured with the SAGE sensor from each dive. The text provides details on the sensor. Cite Share Download PDF Status: Published Journal Publication published 27 Jan, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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(a.) Map showing the location of \u003cem\u003eBorealis \u003c/em\u003eMV\u003cem\u003e \u003c/em\u003eand\u003cem\u003e \u003c/em\u003eother cold seeps in the area (from some of them, the origin of emitting methane is shown in Fig. 3)\u003cem\u003e \u003c/em\u003e\u0026nbsp;(b.) Compiled observations, including seabed topography from high-resolution multibeam fdata (5 m grid cell), a seismic cross-section from 3D-seismic dataset SPE16M01 (the complete seismic section is available at Fiog.1_SI), multi-beam echosounder data (320 kHz) tracing streams of gas bubbles (gas flares) in the water column, CTD data indicated as a vertical line of diamonds showing locations of the water samples and, by colors, concentrations of dissolved methane measured. The SAGE data showed as ROV tracks and by colors the concentrations of dissolved methane measured in real-time. Georeferenced ROV images show extensive microbial mats (showing an area of ca 2 m\u003csup\u003e2\u003c/sup\u003e), a carbonate pinnacle colonised by Octocorallia (pinnacle high ca 120 cm), and the active gryphon emitting fluids, gas and sediment (diameter ca 7 m).\u003c/p\u003e","description":"","filename":"Fig.1Panierietal.submitted.png","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/683594aa19c39c33948e8aef.png"},{"id":55341616,"identity":"07c69351-a1ad-40de-858d-2ec7d84da3c9","added_by":"auto","created_at":"2024-04-26 02:19:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1974953,"visible":true,"origin":"","legend":"\u003cp\u003eKey ROV observations displaying the complexity of the \u003cem\u003eBorealis\u003c/em\u003eMV, including a) mosaic of the \u003cem\u003eBorealis \u003c/em\u003eMV; b) tubeworm aggregations (\u003cem\u003eOligobrachia\u003c/em\u003e sp.), and c) the dense colonies of hydrozoans (\u003cem\u003eTubularia\u003c/em\u003e sp.) located in the slope area near the volcano crater; d) a mosaic of the carbonate structures colonized by sessile fauna and used by various fish species, such as e) redfish (e.g. \u003cem\u003eSebastes norvegicus\u003c/em\u003e)\u003cem\u003e \u003c/em\u003eas breading grounds and refuge areas; \u0026nbsp;f) old lost fishing gear stuck on the carbonates structures and colonized by the typical sessile fauna observed in the region.\u003c/p\u003e","description":"","filename":"Fig.2Panierietalcompressed.png","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/baeea1a0c7fedd22d6ca3f0e.png"},{"id":55341347,"identity":"4548d6d4-9572-454f-9dab-8f4702c8e89b","added_by":"auto","created_at":"2024-04-26 02:11:16","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1509964,"visible":true,"origin":"","legend":"\u003cp\u003eGeochemistry of the emitted fluid from the\u0026nbsp;\u003cem\u003eBorealis\u003c/em\u003e\u0026nbsp;mud volcano system, comprising\u0026nbsp;genetic diagrams for the gas and expelled mud source. (a) Stable carbon (δ\u003csup\u003e13\u003c/sup\u003eC) and hydrogen (δD) isotope composition of methane from headspace gas analysis. Sample data from Borealis MV are reported in yellow dots, and for comparison, other high-latitudes cold seeps are reported: from\u0026nbsp;Prins Karl Forland in orange (Panieri et al., 2024), from Leirdjupet Fault Complex in green (Argentino et al., 2021), from Håkon Mosby Mud Volcano in light blue (Lein et al., 1999) and from Vestnesa Ridge in white (Panieri et al., 2017). Genetic fields of hydrocarbons (CR -CO\u003csub\u003e2\u003c/sub\u003e\u0026nbsp; reduction, F—methyl-type fermentation, EMT—early mature thermogenic gas, OA—oil-associated thermogenic gas, LMT—late mature thermogenic gas) after Milkov and\u0026nbsp; Etiope 2018. (b) Plot of δ\u003csup\u003e13\u003c/sup\u003eC -CH\u003csub\u003e4\u003c/sub\u003e versus the composition of light hydrocarbon components (C1/(C2 + C3) ratio). Grey arrows indicate the main processes affecting the isotopic and molecular compositions of gases.\u0026nbsp;c) Stable C isotopic ratios for saturates (δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003esat\u003c/sub\u003e) and aromatics (δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003earo\u003c/sub\u003e) showing inferences for marine/terrigenous distinction (after Sofer 1974). d) Depositional environment inferred for sources of oils based on dibenzothiophene (DBT)/phenanthrene (P) and Pristane(Pr)/Phytane (Ph) ratios (after Hughes et al. 1995). (e) Age indications based on sterane C-number distribution and nordiacholestane ratio (NDR). NDR is defined as: βα(20R+20S) 24-nordiacholestanes / sum of βα(20R+20S) 24-nor + 27-nordiacholestanes. 28bb = 24-methyl-5α(H),14β (H),17β(H), 20R,S- cholestane; 29bb = 24-ethyl-5α(H),14β (H),17β(H), 20R,S- cholestane.(f) Maturity indication from isomerism of hopanes derived from GC-MS-SIR. Abbreviations: 30ab = 17α(H),21β(H)-30-hopane; 30ba = 17β(H),21α(H)-30-hopane; 31abS = 17α(H),21β(H)-22S-homohopane; 31abR = 17α(H),21β(H)-22R-homohopane.\u003c/p\u003e","description":"","filename":"Fig.3Panierietal.submitted.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/b52a411c98cbb7498008d34a.jpg"},{"id":74904977,"identity":"ee44e8de-ed34-40ce-aa8c-498541db98f5","added_by":"auto","created_at":"2025-01-28 08:07:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5428486,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/06f93e03-7f7c-4b37-bce9-06bb1c9cd583.pdf"},{"id":55341337,"identity":"64b67cd1-a930-4763-b1e5-7cfb322fc987","added_by":"auto","created_at":"2024-04-26 02:11:15","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":6037514,"visible":true,"origin":"","legend":"\u003cp\u003eFig. 1_SI: Seismic random line crossing the \u003cem\u003eBorealis\u003c/em\u003emud volcano from 3D-seismic dataset SPE16M01. Directly below the mud volcano, in the centre of the crater, an acoustic chimney is observed with several associated bright amplitude anomalies interpreted as shallow gas.\u003c/p\u003e","description":"","filename":"FigureS1.png","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/10c07150a4bc012e4498ea7a.png"},{"id":55341341,"identity":"231bc031-83be-405c-a063-78be81369a20","added_by":"auto","created_at":"2024-04-26 02:11:15","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":3289494,"visible":true,"origin":"","legend":"\u003cp\u003eFig. 2_SI: WCD (Water Coloum Data)-line 0341 across the Borealis mud volcano site (EM710). A) Fan view of gas flares in the water column at the site of Dive 13, 14, 15 and 16. Several acoustic gas flares, representing streams of gas bubbles, are imaged from the seafloor and up through the water column, some of them almost reaching the sea surface. The depression at the seafloor at the location of the largest flare observed with ROV dives is indicated with a yellow dotted oval, also in B. B) R-Stack along ship track crossing the \u003cem\u003eBorealis \u003c/em\u003emud volcano site showing biological noise in the water column most likely fish school also observed during the ROV visual survey.\u003c/p\u003e","description":"","filename":"FigureS2.png","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/0dafd6e63e06c90cb6aafa46.png"},{"id":55341340,"identity":"0307d75e-8a20-4e0f-b301-530017ff9915","added_by":"auto","created_at":"2024-04-26 02:11:15","extension":"png","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":2146029,"visible":true,"origin":"","legend":"\u003cp\u003eFig. 3_SI: The methane concentration (nmol L\u003csup\u003e-1\u003c/sup\u003e) measured with the SAGE sensor from each dive. The bubble plumes observed with the ROV camera are marked with white circles.\u003c/p\u003e","description":"","filename":"FigureS3.png","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/e71debf9f2d3b2442f0ece6f.png"},{"id":55341345,"identity":"a08c86de-b9f1-47ef-8168-4cfce1a81e53","added_by":"auto","created_at":"2024-04-26 02:11:16","extension":"pdf","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":9101654,"visible":true,"origin":"","legend":"\u003cp\u003eFig. 4_SI: High-resolution bathymetric map (with a 4-meter grid) of \u003cem\u003eBorealis\u003c/em\u003e MV. The map details the volcano's four distinct craters, which are located on the seafloor in correspondence with the conduit identified in the seismic random line in Fig 1_SI that serve as conduits for fluid expulsion. Adjacent to the craters, a notable ploughmark is visible, indicating a past event of glacial or sedimentary activity that has left a linear depression on the seafloor.\u003c/p\u003e","description":"","filename":"FigureS4.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/440f387cd18773d21bad7643.pdf"},{"id":55341617,"identity":"84518c49-a5b7-4925-a21f-dbdd7e1d307c","added_by":"auto","created_at":"2024-04-26 02:19:16","extension":"docx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":21341,"visible":true,"origin":"","legend":"\u003cp\u003eTable 1_SI: Analyzed samples and type of analyses performed at the \u003cem\u003eBorealis\u003c/em\u003e MV.\u003c/p\u003e","description":"","filename":"Table1Samplesinformationsubmitted.docx","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/458b19ce214902dcba989a3b.docx"},{"id":55341342,"identity":"e4cb3f9b-6b9e-4466-8212-23724a871454","added_by":"auto","created_at":"2024-04-26 02:11:15","extension":"docx","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":16438,"visible":true,"origin":"","legend":"\u003cp\u003eTable 2_SI: Results 2H, 18O, 34S and 87/86Sr isotope analyses of the fluid collected directly at the top of the active emitting gryphon crater.\u003c/p\u003e","description":"","filename":"Table2SIGeochemistrysubmitted.docx","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/c515f0d3e48f5927c5113d25.docx"},{"id":55341343,"identity":"f994d02f-f78a-4941-a574-2de23d2fe19a","added_by":"auto","created_at":"2024-04-26 02:11:15","extension":"xlsx","order_by":7,"title":"","display":"","copyAsset":false,"role":"supplement","size":11187,"visible":true,"origin":"","legend":"\u003cp\u003eTable 3_SI: The methane concentration (nmol L-1) measured in water samples collected with CTD during CTD 84 and 86.\u003c/p\u003e","description":"","filename":"Table3SICTDsubmitted.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/48516bf0153e6b2058e2747e.xlsx"},{"id":55341344,"identity":"188d026d-4a6d-4c44-b720-70d52e4f94e9","added_by":"auto","created_at":"2024-04-26 02:11:16","extension":"xlsx","order_by":8,"title":"","display":"","copyAsset":false,"role":"supplement","size":1563718,"visible":true,"origin":"","legend":"\u003cp\u003eTable 4_SI: The methane concentration (nmol L-1) measured with the SAGE sensor from each dive. The text provides details on the sensor.\u003c/p\u003e","description":"","filename":"Table4SIAllDivesLatLonCH4.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4305053/v1/ad0965c6cde897622ee96432.xlsx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"The discovery of Borealis mud volcano: a natural sanctuary for threatened Arctic species","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOver the past ten years, marine surveys in the Polar North Atlantic continental shelf and slope have consistently identified new methane seeps on the seafloor \u003csup\u003e1\u003c/sup\u003e (Fig. 1). These sites are of great interest due to their potential impact on the marine ecosystem and global climate. Methane, a potent greenhouse gas, has increased atmospheric concentrations since the start of the Industrial Revolution, accelerating climate change\u003csup\u003e2\u003c/sup\u003e. Estimates suggest that between 218 and 371 teragrams of methane per year (Tg CH₄\u0026nbsp;yr⁻¹) are emitted from natural sources within terrestrial and aquatic settings, as determined by top-down and bottom-up approaches, respectively\u003csup\u003e3\u003c/sup\u003e. Methane's influence on the environment extends beyond its well-known role as a potent greenhouse gas; it is also a critical component in forming complex ecosystems that emerge from the interactions between biological, geochemical and geological processes\u003csup\u003e4\u003c/sup\u003e. In marine environments, methane cold seeps contribute to regional biodiversity by supporting specialised microbial and faunal communities adapted to harsh conditions\u003csup\u003e4-6\u003c/sup\u003e. In seep-impacted sediments, the anaerobic oxidation of methane (AOM) supports high fluxes of dissolved sulfide (H\u003csub\u003e2\u003c/sub\u003eS) toward the surface, which is colonised by sulfur-oxidizing bacterial mats and chemosymbiotic organisms, such as clams, mussels and tubeworms \u003csup\u003e6,7,8\u003c/sup\u003e. Methane seeps are often associated with widespread carbonate deposits formed in situ because of the increased local alkalinity induced by AOM \u003csup\u003e9\u003c/sup\u003e. These deposits have a wide range of morphologies, from flat pavements to vertical pinnacle-like structures, and highly variable dimensions from a few mm-sized concretions to beds several hundreds of meters in lateral extent and several meters in thickness \u003csup\u003e10,11\u003c/sup\u003e. Seep carbonates provide hard substrata for sessile organisms \u003csup\u003e4.\u0026nbsp;\u003c/sup\u003e and resources for other species in adjacent areas, contributing to a broader ecological network \u003csup\u003e6\u003c/sup\u003e. They are also a record of geological processes of methane oxidation spanning millions of years and have been used in high-resolution paleo-reconstructions \u003csup\u003e12,13\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDespite numerous observations of methane emissions from the seafloor in Arctic regions, only five mud volcanoes have been discovered in the Canadian Beaufort Sea (Western\u0026nbsp;Arctic)\u003csup\u003e14\u003c/sup\u003e , and three in Alaska\u003csup\u003e15\u003c/sup\u003e, and so far,\u0026nbsp;the Håkon Mosby Mud Volcano (HMMV) was the only known structure in\u0026nbsp;Norwegian waters\u003csup\u003e16\u003c/sup\u003e. Mud volcanoes are surface manifestations of focused fluid flow in hydrocarbon-rich sedimentary basins along passive margins and at convergent plates characterized by high sedimentation rates. \u0026nbsp;Mud volcanoes collectively play a significant role in the atmospheric methane budget\u003csup\u003e17,18\u003c/sup\u003e by releasing an estimated 60 Tg CH\u003csub\u003e4\u003c/sub\u003e yr\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e18\u003c/sup\u003e sourced from several kilometres below the seafloor towards the surface\u003csup\u003e19\u003c/sup\u003e. Such vast quantity has large uncertainties because the total number of mud volcanoes worldwide and their temporal variability concerning methane emission rates are not confidently known, especially when considering undiscovered offshore mud volcanoes. Therefore, our current understanding of their potential contribution to atmospheric methane emissions and impact on climate, ocean chemistry and ecosystems is still limited, and so are the mechanisms triggering their activity and their role in the survival of specialised fauna thriving at these sites \u003csup\u003e5,6\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eHere, we present a comprehensive study of a newly discovered mud volcano in the Polar North Atlantic named \u003cem\u003eBorealis\u0026nbsp;\u003c/em\u003emud volcano (MV) located in Outer Bjørnøyrenna (Bear Island Trough, 72° 26.304´N, 17°\u0026nbsp;40.626´E,\u0026nbsp;~ 390 m water depth) in the Barents Sea (Fig. 1).\u0026nbsp;This discovery marks the second mud volcano of this kind identified in the region, among numerous methane seeps previously detected\u003csup\u003e20\u003c/sup\u003e (Fig. 1), thus opening a new chapter in our understanding of Arctic geology and related fields of research. The\u003cem\u003e\u0026nbsp;Borealis\u003c/em\u003e MV is characterized by a cluster of craters from ~70 m to ~400 m in diameter and an active gryphon (~ 7 m in diameter and 2 meters high) expelling warm fluids, gas and oil. Our observations also show that \u003cem\u003eBorealis\u003c/em\u003e MV acts as sanctuaries for fauna vulnerable to anthropogenic perturbations, specifically seabed trawling, which has a major impact on benthic ecosystems in the region \u003csup\u003e21,22\u003c/sup\u003e.\u003c/p\u003e\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n"},{"header":"Geophysical and water column insights ","content":"\u003cp\u003e\u003cem\u003eBorealis\u003c/em\u003e MV is located in the eastern parts of the Sørvestsnaget Basin, where a\u0026nbsp;thick\u0026nbsp;sequence of Cretaceous and Cenozoic sedimentary rocks is covered by a wedge of Pliocene to Pleistocene sediments\u003csup\u003e23,24\u003c/sup\u003e. In this area,\u0026nbsp;industry 3D-seismic data revealed the presence of\u0026nbsp;a 500-600\u0026nbsp;m\u0026nbsp;wide\u0026nbsp;seismic chimney in the subseafloor terminating at the seafloor into a complex network of crater-like depressions (Fig.\u0026nbsp;1_SI in Supporting Information).\u0026nbsp;Shallow\u0026nbsp;bright amplitude acoustic anomalies observed\u0026nbsp;in the Quaternary\u0026nbsp;sediments are interpreted as local free gas accumulations within a vertical chaotic zone. The chimney is rooted at around 300 meters below sea floor (m bsf), at\u0026nbsp;the base of the Quaternary/URU (Upper Regional Unconformity) boundary\u0026nbsp;(Fig. 1; Fig.\u0026nbsp;1_SI in Supporting Information).\u0026nbsp;\u003c/p\u003e\u003cp\u003eThe detection of the chimney structure steered our hydroacoustic observations, which subsequently revealed persistent, concentrated and active gas flares originating from the \u003cem\u003eBorealis\u0026nbsp;\u003c/em\u003eMV.\u0026nbsp;Within a confined area (~0.14 km\u003csup\u003e2\u003c/sup\u003e) around an active gryphon, we have identified a total of 26 gas flares (Fig. 1). These individual flares exhibit a typical height of 160 m on average, potentially ascending up to 355 m into the water column. In some instances, flares even approach the sea surface, as illustrated in Figure 2_SI. CTD water samples (Fig. 1) collected from one of these plumes\u0026nbsp;showed\u0026nbsp;the presence of methane with concentrations that\u0026nbsp;remain relatively high throughout\u0026nbsp;the water column\u0026nbsp;and preserve\u0026nbsp;notable levels, up to 11,390\u0026nbsp;nmol L\u003csup\u003e-1\u003c/sup\u003e close to the seafloor (Fig. 1; Table\u0026nbsp;3_SI). This suggests an intense advection of hydrocarbon-charged fluid migration. In comparison, background values of methane concentrations are around 0.9\u0026nbsp;nmol L\u003csup\u003e-1\u003c/sup\u003e, making the observed values approximately 100 times higher than the ambient conditions.\u003c/p\u003e\u003cp\u003eThe subsequent ROV seafloor observation confirmed the persistent and ongoing methane venting and revealed a major depression (500-600 m in diameter), comprising four main craters (Fig. 1) that individually range in diameter from ~70 m to ~400 m. The two northernmost craters host extensive carbonate exposed at the seafloor as stacked slabs forming tens of meters wide large\u0026nbsp;pavements covering several tens of square meters. The thickness of single slabs is on the order of tens of cm, but the total thickness of the deposits can reach ~ 5 m along the steep wall of the craters. While carbonate crusts have been observed in other cold seeps in the Barents Sea\u003csup\u003e11,12,25\u003c/sup\u003e, the carbonates at \u003cem\u003eBorealis\u003c/em\u003e are remarkably larger and thick, emphasizing the exceptional nature of this site. The gryphon, on the southern flank of the deepest crater of the \u003cem\u003eBorealis\u003c/em\u003e MV system, has a central conduit that serves as the epicentre from which warm fluids and sediments are actively expelled, as observed from gas bubbles and sediment flows extending over\u0026nbsp;the seafloor. (Figs. 1 and 2).\u003c/p\u003e"},{"header":"Origin of fluids and expelled mud ","content":"\u003cp\u003eWe collected the expelled unconsolidated sediments from the gryphon using a push core manipulated by the ROV. They consist of medium-sized sand with minor clayey content, silt, and a few small sedimentary rock fragments and coal pebbles. The sediment expelled contains planktonic foraminiferal fauna correlated with the micropaleontological zonation for the Neogene on the Vøring Plateau\u003csup\u003e26\u003c/sup\u003e while the benthic foraminiferal fauna with the one for the Cenozoic of the North Sea\u003csup\u003e27\u003c/sup\u003e. Nearly all the individuals are\u0026nbsp;extinct\u0026nbsp;species typically associated with Pleistocene deposits on the Norwegian Shelf from ca 700-1000 m bsf\u003csup\u003e28,29\u003c/sup\u003e. The lack of benthic foraminifera typical for Holocene and recent sediments on the Norwegian continental shelf, including \u003cem\u003eTrifarina angulosa\u003c/em\u003e and \u003cem\u003eUvigerina per\u003c/em\u003e\u003cem\u003ee\u003c/em\u003e\u003cem\u003egrina\u003csup\u003e30-32\u003c/sup\u003e\u003c/em\u003e, and the almost complete lack of warm water-dwelling planktonic foraminifera indicate that no Holocene sediments are present in the rising sediment.\u0026nbsp;\u003c/p\u003e\u003cp\u003eGeochemistry performed on the sediment from the push core indicates that the fluid emitted from \u003cem\u003eBorealis\u003c/em\u003e gryphon has\u0026nbsp;lower\u0026nbsp;levels of organic carbon and nitrogen compared to the average southwestern Barents Sea surface sediments\u0026nbsp;(i.e. TOC\u0026gt;0.5 %; TN\u0026gt;0.05 %) \u003csup\u003e33\u003c/sup\u003e with values of\u0026nbsp;0.33 wt.% and 0.02 wt.%, respectively. The isotopic composition of organic matter\u0026nbsp;δ\u003csup\u003e13\u003c/sup\u003eC = −27.11 ‰ and\u0026nbsp;δ\u003csup\u003e\u0026nbsp;15\u003c/sup\u003eN = 4.58 ‰, associated with the high C/N ratio of 20.2, are\u0026nbsp;consistent\u0026nbsp;with a high contribution of ancient organic\u0026nbsp;sources\u0026nbsp;that underwent prolonged microbial degradation in the deep subsurface \u003csup\u003e34,35\u003c/sup\u003e (Fig.\u0026nbsp;3). Organic biomarkers revealed the presence of\u0026nbsp;detectable amounts of crude oil associated with immature bitumen showing virtually no biodegradation\u0026nbsp;(Fig.\u0026nbsp;3).\u0026nbsp;Recent studies\u0026nbsp;reported\u0026nbsp;the\u0026nbsp;widespread and extensive methane and oil release from geological reservoirs to the Arctic Ocean\u003csup\u003e20\u003c/sup\u003e and,\u0026nbsp;\u0026nbsp;although\u0026nbsp;evident oil seepage or oil slicks\u0026nbsp;were not observed at \u003cem\u003eBorealis\u0026nbsp;\u003c/em\u003eMV,\u0026nbsp;the\u0026nbsp;system can be\u0026nbsp;considered as the surface expression of a deeper sited active\u0026nbsp;petroleum-\u0026nbsp;system.\u0026nbsp;The bulk\u0026nbsp;δ\u003csup\u003e13\u003c/sup\u003eC composition of\u0026nbsp;saturated\u0026nbsp;and aromatic hydrocarbons, ranging from −31.2 ‰ to −29.3 ‰ and from −31.6 ‰ to −28.2 ‰, respectively, do not lead to the univocal interpretation of the origin of the oil (Fig.\u0026nbsp;3). However, the Dibenzothiophene/Thiophene ratios \u0026lt; 1 coupled with Prystane/Phytane ratios between 1 and 3 suggest a marine shale/lacustrine source rock \u003csup\u003e36\u003c/sup\u003e (Fig.\u0026nbsp;3). The age of the source rock is estimated as Jurassic or younger based on sterane C-number distribution\u003csup\u003e37\u003c/sup\u003e and nordiacholestane ratio\u003csup\u003e38\u003c/sup\u003e (Fig.\u0026nbsp;3). Overall, hopane and sterane isomerism suggest an early oil window maturity.\u003c/p\u003e\u003cp\u003eThe 30-minute ROV in situ observations at the gryphon site show frequent (every 5-10 minutes) eruptions of warm water-mud-gas mixture that rise along the seep conduit below (Fig. 1). The gas composition is dominated by methane, which, together with\u0026nbsp;d\u003csup\u003e13\u003c/sup\u003eC values of −60.8 ‰ and dD of −222 ‰ (Fig. 3), points towards a microbial origin \u003csup\u003e39\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\u003cp\u003eThe temperature of the discharged fluid measured above the active gryphon by the CTD installed on the ROV was 11°C, and that of the T-probe was 11.42°C. This temperature is substantially higher than the reference ca. 4°C measurements away from \u003cem\u003eBorealis\u003c/em\u003e.\u0026nbsp;\u003c/p\u003e"},{"header":"A unique oasis for faunal communities ","content":"\u003cp\u003e\u003cem\u003eBorealis\u003c/em\u003e MV hosts a diverse array of faunal communities, including seep-associated and background fauna, together with species that have commercial value. Within the MV´s craters, we observe dense and extensive patches of microbial mats and tubeworm aggregations (\u003cem\u003eOligobrachia\u003c/em\u003e sp.) akin to those documented at other Arctic cold seeps (see Figure 2)\u003csup\u003e8\u003c/sup\u003e. Microbial mats and tubeworms seem to be the main characteristic of high-latitudes compared to lower-latitudes cold seeps, which host clams, mussels and vestimentiferan worms \u003csup\u003e4\u003c/sup\u003e. The microbial mats, extending several square meters, form the foundation for a variety of microorganisms, including a multitude of foraminifera species. Our environmental DNA (eDNA) analyses have unveiled the presence of both hard-shelled (\u003cem\u003eCibicidoides wuellerstorfi, Reophax dentaliniformis, Stainforthia\u003c/em\u003e sp.), monothalamids and soft-shelled foraminifera nestled within these microbial ecosystems (Table 1_ Supplementary). Although, until now, no endemic foraminifera species have been documented within cold seeps\u003csup\u003e40\u003c/sup\u003e, our preliminary eDNA analyses suggest the possibility of previously unidentified species potentially unique to \u003cem\u003eBorealis\u003c/em\u003e.\u003c/p\u003e\u003cp\u003eThe megafauna diversity at \u003cem\u003eBorealis\u003c/em\u003e MV is relatively low, and appears to be influenced by several environmental factors, including elevated methane concentrations, extensive carbonate crusts on the seafloor and the suspended sediment particles emitted by the gryphon that further contribute to the challenging conditions that may be impacting the larger faunal assemblages. The megafauna is dominated by dense clusters of anemones and serpulids anchored to carbonate substrates and hydrozoan colonies (\u003cem\u003eTubularia\u003c/em\u003e sp.) that flourish on the crater slopes, while the presence of cladorhizid sponges and sea stars is more sporadic. In addition, our sample collections from various habitats within the \u003cem\u003eBorealis\u003c/em\u003e MV have yielded taxa not detectable via ROV imagery, such as annelids, amphipods, gastropods, polyplacophorans, nemerteans (\u003cem\u003eNipponemertes\u003c/em\u003e spp.), and ophiuroids. Morphological and molecular analyses of these samples are currently underway to further elucidate the composition of these faunal communities. The observed scarcity of megafauna suggests that the immediate environmental conditions surrounding the \u003cem\u003eBorealis\u003c/em\u003e MV may be inhospitable for a broader range of organisms. Remarkably high methane concentrations (11,390 nmol L\u003csup\u003e-1\u003c/sup\u003e near the seafloor) or toxic sulfide levels are factors known to significantly influence invertebrate community structures at seep sites\u003csup\u003e41\u003c/sup\u003e. These chemically- enriched habitats create a gradient of extreme environmental conditions that can be detrimental to many forms of marine life. The toxicity of methane and hydrogen sulfide poses a significant challenge to the survival of organisms that are not specifically adapted to such conditions, often resulting in reduced biodiversity. Additionally, the high volume of suspended sediment particles emitted by the gryphon may impact the diversity and density of filter- and suspension-feeding organisms, potentially by clogging their feeding apparatus\u003csup\u003e42\u003c/sup\u003e. Nevertheless, some taxa appear resilient to the presence of suspended sediment. Hydrozoans, for example, are abundant in the carbonate area covered with sediment particles, similar to what was also observed on the Koryak slope at upper bathyal depths (∼660 m)\u0026nbsp;\u003csup\u003e43\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\u003cp\u003eWhile some parts of the seafloor within \u003cem\u003eBorealis\u003c/em\u003e MV seem to be inhospitable for many species, the extensive carbonates provide additional habitat and suitable hard substratum for erect epifauna and dense aggregations of several species of anemones, serpulids, demosponges, nudibranchs, and sparse octocoral colonies (\u003cem\u003ePrimnoa resedaeformis\u003c/em\u003e). \u003cem\u003eP. resedaeformis\u003c/em\u003e was only located in the jagged carbonate area where lower or no sediment deposition from the plumes of the gryphon was evident. As observed earlier \u003csup\u003e4\u003c/sup\u003e, carbonate deposits are known to shelter a large variety of fauna and represent oases for numerous sessile organisms, as commonly observed at other offshore seepage sites \u003csup\u003e44\u003c/sup\u003e \u003csup\u003e45,46\u003c/sup\u003e. The extensive carbonates also provided a “reef-effect” \u003csup\u003e47\u003c/sup\u003e for the fish living in the area, testifying to the \u003cem\u003eBorealis\u003c/em\u003e MV's ecological vitality. Large schools of saithe (\u003cem\u003ePollachius virens\u003c/em\u003e) and various demersal species such as spotted wolffish (\u003cem\u003eAnarhichas minor\u003c/em\u003e), cod (\u003cem\u003eGadus morhua\u003c/em\u003e), four-bearded rockling (\u003cem\u003eEnchelyopus cimbrius\u003c/em\u003e), and redfish (mainly \u003cem\u003eSebastes\u003c/em\u003e \u003cem\u003enorvegicus\u003c/em\u003e but a few individuals tentatively identified as \u003cem\u003eSebastes viviparus\u003c/em\u003e and \u003cem\u003eSebastes mentella\u003c/em\u003e ) clustering around the jagged carbonate formations (Figure 2) suggest that these structures serve as critical habitats, offering both shelter and abundant feeding opportunities, thereby playing a pivotal role in sustaining the local fish populations. The redfish, particularly \u003cem\u003eSebastes norvegicus\u003c/em\u003e, listed as endangered on the Norwegian Red List for Species and subject to a fishing moratorium \u003csup\u003e48\u003c/sup\u003e \u003csup\u003e49\u003c/sup\u003e, seems to utilise these carbonate structures as nurseries. The irregular morphology of the carbonates provides protective shelters, and the site's elevated temperatures may enhance reproductive success by accelerating egg development\u003csup\u003e50\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\u003cp\u003eDuring our expedition, we encountered lost fishing gear from bottom trawling snagged on the jagged carbonate rock around the perimeter of the craters (Fig. 2). These were heavily colonised by sessile fauna (e.g., anemones and hydrozoans), suggesting that the gear was lost several years ago. The current cessation of fishing activities in the \u003cem\u003eBorealis\u003c/em\u003e area is confirmed by the Vessel Monitoring System (VMS) data, which shows no fishing activities around the mud volcano system. This information, in conjunction with the presence of red-listed species \u003cem\u003eSebastes norvegicus\u003c/em\u003e and taxa indicative of Vulnerable Marine Ecosystems (VMEs) as defined by FAO/ICES \u003csup\u003e51\u003c/sup\u003e, such as the octocorals, suggests the \u003cem\u003eBorealis\u003c/em\u003e MV is a \u003cem\u003ede facto\u003c/em\u003e sanctuary for these endangered species. The absence of bottom trawling and the natural habitat's protective qualities offer a refuge where these species can thrive despite significant seafloor impacts from fishing in the surrounding Barents Sea \u003csup\u003e21,22\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Glacial history and genesis of Borealis MV","content":"\u003cp\u003eThe discovery of \u003cem\u003eBorealis\u003c/em\u003e MV is significant as it represents the second mud volcano identified in Norwegian waters since the finding of HMMV \u003csup\u003e16\u003c/sup\u003e.\u0026nbsp;The latter is located 110 km southwest of \u003cem\u003eBorealis\u003c/em\u003e at ~ 1,260 m water depth. Previous studies suggest\u0026nbsp;that its activity\u0026nbsp;started ~330 ka before present, when fluids were expelled from the periglacial units loaded by a a\u0026nbsp;3 km thick glacial\u0026nbsp;deposit\u003csup\u003e52\u003c/sup\u003e. During the Last Glacial Maximum (LGM) ~23 ka before present, the expansion of the Eurasian Ice Sheet (EIS) dominated the Barents Sea landscape\u003csup\u003e14\u003c/sup\u003e. Well-defined ploughmarks surrounding \u003cem\u003eBorealis\u0026nbsp;\u003c/em\u003eMV give evidence of seafloor erosion following the\u0026nbsp;LGM. However, no evidence of such ploughmarks intersecting the mud volcano craters indicates that the latter must have formed after the deglaciation phase (Fig. 4_SI). One possible formation scenario is consistent with those depicted\u0026nbsp;for\u0026nbsp;the large Troll pockmark field\u003csup\u003e53\u003c/sup\u003e or\u0026nbsp;the\u0026nbsp;nearby seep area\u0026nbsp;Bjørnøyrenna\u0026nbsp;\u003csup\u003e54\u003c/sup\u003e.\u0026nbsp;As ice retreated, warming temperatures and a decrease in pressure\u0026nbsp;destabilised\u0026nbsp;methane hydrates once trapped within the sediments. The dissociation of these gas hydrates suddenly liberated large amounts of methane, forming the craters.\u0026nbsp;The observed widespread carbonates contributed to cementing the sediments, reducing the porosity/permeability and ultimately acting as a buffer layer for the rising methane-rich fluids. Those carbonate barriers periodically inhibit the upward migration of hydrocarbons, allowing gas accumulation underneath the calcite pavements\u003csup\u003e55\u003c/sup\u003e \u003csup\u003e45\u003c/sup\u003e and forcing gas to find alternative pathways to release. This deflection of the fluid flow might have contributed to the genesis of the four observed craters in a sequential and ongoing geological process in the \u003cem\u003eBorealis\u003c/em\u003e MV system, possibly by accumulating pressure and causing abrupt dissipation via explosive events. Diffuse, long-lasting gas hydrate dissociation is still currently ongoing at northern latitudes, as observed in numerous sites in the Barents and Norwegian seas \u003csup\u003e54,56\u003c/sup\u003e. This underscores the enduring influence of deglaciation on contemporary geological and environmental processes at high latitudes.\u003c/p\u003e\u003cp\u003eThe temperature anomaly measured at \u003cem\u003eBorealis\u003c/em\u003e clearly indicates that the MV plumbing system is connected to deeper and warmer strata from which fluids rapidly migrate towards the surface. This observation is also very relevant in explaining potential triggers for the eruption model of \u003cem\u003eBorealis\u003c/em\u003e MV. In addition to the “deglaciation trigger model” proposed above, we also suggest a “hot fluids surge” scenario to explain the formation of \u003cem\u003eBorealis\u003c/em\u003e MV craters (Fig. 4_SI). Similarly to what has been observed at numerous mud volcanoes in Lake Baikal \u003csup\u003e57,58\u003c/sup\u003e, we suggest that 1) batches of upwelling warmer fluids periodically dissociated gas hydrates deposits at shallower depth, resulting in 2) increased volume and pressure that entrained the sediments, then causing 3) multiple surface eruptions.\u0026nbsp;\u003c/p\u003e\u003cp\u003eSince significant methane content was measured close to the sea surface, we may also speculate ongoing atmospheric emissions under favourable oceanic setting conditions. However, while the present contribution of atmospheric carbon from marine geological sources is deemed relatively minor, the \u003cem\u003eBorealis\u003c/em\u003e MV presents a noteworthy exception. A considerable uncertainty persists concerning future emission projections on a warming planet. This uncertainty underscores the critical need for enhanced monitoring and research to quantify marine carbon fluxes accurately and better predict their potential impact on global climate dynamics. As the Earth's temperature rises, we must address these knowledge gaps to inform policy decisions and mitigate the risks associated with climate change.\u003c/p\u003e"},{"header":"A natural sanctuary for threatened Arctic species","content":"\u003cp\u003eThe \u003cem\u003eBorealis\u003c/em\u003e MV system also plays a complex role in supporting local marine life. Not only does it provide a sanctuary for endangered fish species from human impacts, but it also may function as a crucial\u0026nbsp;nursery site. The elevated temperatures around the seepage sites create optimal conditions for reproductive processes, leading to more effective breeding cycles. This thermal advantage can enhance the survival and growth of fish populations in the deep sea, and the Barents Sea, one of the world's most productive fishing grounds, represents a significant ecological asset. Furthermore, preserving such ecosystems is crucial for biodiversity conservation and a comprehensive understanding of the intricate interactions between geology, geochemistry and biology in marine environments. The Arctic seafloor has become a vital asset, playing an important role in oil and gas exploitation activities and the emerging deep-sea mining industry. The responsible management of\u0026nbsp;marine mineral and biological\u0026nbsp;resources is paramount for sustainable development and environmental stewardship in the Arctic region.\u0026nbsp;In the longer term, Norway has committed to the 30X30 \u0026nbsp;target for spatial conservation measures of representative marine ecosystems, including in the deep sea \u003csup\u003e59\u003c/sup\u003e. Protecting large areas of the deep-sea floor along the Norwegian margin may result in seep refugia acting as source populations for wider recolonization and restoration of benthic biological communities.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003e1. \u0026nbsp; Multibeam echosounder Water Column Data\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eBorealis\u003c/em\u003e MV\u0026apos; morphometric data were obtained from a 3x3 m resolution morphobathymetric map generated on board and obtained by processing multibeam echosounder data (i.e., depth measurements and backscatter) acquired using the hull-mounted Kongsberg MBES EM710, from which water column data were also recorded, documenting evident flares.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eGas seep mapping at the study site was done using the QPS FMMidwater software. Gas seeps were detected as gas flares in the water column data caused by backscatter from the gas bubble streams. The acquired data from EM302 multibeam systems in *.all and *.wcd file formats were converted with FMMidwater to the generic water column format (*.gwc). *.gwc files were visualised in fan view and stacked view. The fan view allows for narrowing the opening beams to select individual flares in the stacked view and export them in an sd file for visualisation in Fledermaus. Only one flare was kept when multiple flares showed the same source in Fledermaus. Locations of individual gas flares were retrieved from the lowest point of the flare in the sd file. This identification is enabled by the significant differences in velocity and density between chains of gas bubbles and the water column, leading to pronounced contrasts evident in the acoustic signals. Flare locations were plotted on the maps and used during ROV-flying during the ROV dives to locate streams of gas bubbles.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2. \u0026nbsp;CTD\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTemperature and salinity at specific depths in the water were obtained from a CTD (Conductivity, Temperature, Depth) mounted on a rosette, which is lowered in the water column from the hull of the vessel. Twelve 10-litre Niskin bottles were also mounted on the rosette, which we closed at specific depths to estimate dissolved methane.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3. \u0026nbsp;ROV\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Aurora work-class ROV is a 6000 m depth-rated ROV with a tethered management system (TMS) called Borealis and provides unique opportunities for science and filmmaking. Four dives (ROV13, ROV14, ROV15, and ROV16) were conducted along exploratory track lines crossing four craters (for a total of 14 hours and 4 minutes of diving time). Applying SfM (Structure from Motion), a photogrammetry technique \u003csup\u003e37\u003c/sup\u003e, selected ROV video frames provided 2D photorealistic orthomosaics of representative physical habitats and sedimentary facies (Fig. 2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4. \u0026nbsp;Fluid analyses\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.1 Temperature measurements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe temperature (T) of the fluid emanating directly from the crater of the \u003cem\u003eBorealis\u003c/em\u003e MV was measured using the CTD sensor and a temperature probe, the ISD400 Depth and Temperature Sensor from Impact Subsea (precision level of \u0026plusmn;0.01% \u0026deg;C), both mounted on ROV Aurora.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.2 Methane measurements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWater samples collected from the CTD Niskin bottles were transferred into 120 mL glass bottles containing 5 mL of 1 M NaOH. The samples were stored in the dark at 4 ℃ until analyses with a Gas Chromatographer - FID (ThermoScientific Trace 1310). Before the analyses, we created a 5 mL headspace and let the samples equilibrate overnight.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe dissolved methane was also continuously measured during the ROV dives using SAGE (\u003cu\u003eS\u003c/u\u003eensor for \u003cu\u003eA\u003c/u\u003equeous \u003cu\u003eG\u003c/u\u003eases in the \u003cu\u003eE\u003c/u\u003environment), a dissolved methane instrument designed and built in the Chemical Sensors Laboratory at the Woods Hole Oceanographic Institution (WHOI) \u003csup\u003e38\u003c/sup\u003e \u003csup\u003e39\u003c/sup\u003e. SAGE has a detection range of 5-10,000 ppm CH\u003csub\u003e4\u003c/sub\u003e. SAGE uses a deep-sea membrane inlet to extract dissolved gas from seawater. Inside the instrument, the extracted gas fills a hollow core optical fibre. Laser spectroscopy measures the methane inside the optical fibre by coupling the light from a laser to the fibre.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.3\u0026nbsp;Geochemistry\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.3.1 Sediment geochemistry\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe prepared 0.3\u0026nbsp;g of dry sediment from a surface sample collected at the seeping spot to measure its organic carbon, nitrogen content, and isotopic composition (d\u003csup\u003e13\u003c/sup\u003eC, d\u003csup\u003e\u0026nbsp;15\u003c/sup\u003eN). The carbonate material was removed by acid addition using 6 N HCl. Analyses were conducted at the SIL (UiT) using a Thermo-Fisher MAT253 Isotope Ratio Mass Spectrometer (IRMS) coupled to a Flash HT Plus Elemental Analyzer. The d\u003csup\u003e\u0026nbsp;13\u003c/sup\u003eC and d\u003csup\u003e\u0026nbsp;15\u003c/sup\u003eN values were normalised to international standards, Vienna Pee Dee Belemnite (VPDB) (d\u003csup\u003e\u0026nbsp;13\u003c/sup\u003eC) and Air-N\u003csub\u003e2\u003c/sub\u003e (d\u003csup\u003e\u0026nbsp;15\u003c/sup\u003eN). Precision on d\u003csup\u003e\u0026nbsp;13\u003c/sup\u003eC and d\u003csup\u003e\u0026nbsp;15\u003c/sup\u003eN was better than 0.15 \u0026permil; (1SD). The C/N atomic ratio was calculated using the atomic mass weighted ratio of TOC and TN as C/N =(TOC/12.011)/(TN/14.007).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.3.2 Oil geochemistry\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSediment samples for oil geochemistry were collected from sediment slices, wrapped in aluminium foil and stored\u0026nbsp;at -20℃\u0026nbsp;for freeze-drying. All oil preparation and analysis procedures followed NIGOGA (Norwegian Industry Guide to Organic Geochemical Analysis), 4th Edition, and were conducted at Applied Petroleum Technology (APT, Oslo). Extractions were performed with a Soxtec Tecator unit and dried before deasphaltering. A small amount of dichloromethane (3 times the amount of EOM) is added. Pentane is added in excess (40 times the volume of EOM/oil and dichloromethane). The solution is stored for at least 12 hours in a dark place before centrifugation and the weight of the removal of asphaltenes. Quantifying saturates, aromatics A and polars (NSO-fraction) was done using two HPLC pumps, sample injector, sample collector and two packed columns. The pre-column is filled with Kieselgel 100 and heated at 600 \u0026deg;C for 2 hours to deactivate it. The main column, a LiChroprep Si60 column, is heated at 120 \u0026deg;C for 2 hours with a helium flow to make it water-free. Approximately 30 mg of deasphaltened oil or EOM diluted in 1 ml hexane is injected into a sample loop. The solvents used are hexane and dichloromethane. The stable carbon isotope composition of the different fractions was measured on a Delta V Plus Isotope Ratio Mass Spectrometer (IRMS) (Thermo Fisher Scientific) via Conflo IV. A standard (NGS NSO-1, topped oil) is analysed for each 12\u003csup\u003eth\u0026nbsp;\u003c/sup\u003esample. The \u0026delta;13C value obtained for this standard is \u0026ndash;28.6\u0026permil; vPDB.The variation in the isotopic values for NSO-1 by repeated analysis over one year is \u0026plusmn; 0.09\u0026permil;. Age-specific biomarkers were measured via GC-MS/MS using a Thermo Scientific TSQ Quantum instrument. The column used is a 60 m CP-Sil-5 CB-MS with an i.d. of 0.25 mm and a film thickness of 0.25 \u0026micro;m. d4-27\u0026alpha;\u0026alpha;R was used as an internal standard.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e4.3.3 Gas geochemistry\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSamples of seep gas were collected using a bubble catcher and stored in steel flasks. Aliquots of the samples were transferred to exetainers. 0.1-1ml were injected into an Agilent 7890 RGA GC equipped with Molsieve and Poraplot Q columns, a flame ionisation detector (FID) and 2 thermal conductivity detector (TCD). Hydrocarbons were measured by FID. The carbon isotopic composition of methane was determined via GC-C-IRMS. Aliquots were sampled with Triplus RSH autosampler and analysed on a Trace 1310 gas chromatograph (Thermo Fisher Scientific), equipped with a Poraplot Q column and PTV (Programable Temperature Vaporizing) injector. The GC is interfaced via GC-Isolink II and Conflo IV to Delta V Isotope Ratio Mass Spectrometer (IRMS) (Thermo Fisher Scientific). Repeated analyses of standards indicate that the reproducibility of \u0026delta;13C values is better than 1 \u0026permil; vPDB (2SD). The hydrogen isotopic composition of methane was determined by a GC-H-IRMS system. Aliquots were sampled with a Triplus RSH autosampler and analysed on a Trace 1310 gas chromatograph (Thermo Fisher Scientific) equipped with a Poraplot Q column and PTV (Programmable Temperature Vaporizing) injector. The GC is interfaced via GC-Isolink II and Conflo IV to Delta V Isotope Ratio Mass Spectrometer (IRMS) (Thermo Fisher Scientific). Repeated analyses of standards indicate that the reproducibility of \u0026delta;D values is better than 10 \u0026permil; vSMOW (2SD). \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e5. Biological samples and observations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e5.1 Mega/macrofauna\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe description of the fauna present and distribution across the main micro-habitats of the \u003cem\u003eBorealis\u003c/em\u003e MV was obtained through the cataloguing and annotation of all the ROV Aurora imagery collected during the expedition. Additionally, experts collected and identified physical samples of both specimens and sediments to support the correct identification of the specimens observed in the videos.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e5.2 Foraminifera (eDNA)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe foraminiferal species list (Supp. X) was inferred from two sediment samples from a microbial mat. Briefly, after the DNA extraction using the DNeasy PowerLyzer PowerSoil kit, the specific foraminifera 37f hypervariable gene of18S rRNA gene was amplified 14F1-s15 primers \u003csup\u003e60\u003c/sup\u003e. A different combination of tagged primers was used per sample, and three PCR replicates were performed. The PCR products were then verified on agarose gel, pooled and added to a library. Sequencing libraries were prepared using TruSeq\u0026reg; DNA PCR-Free Library Preparation Kit (Illumina) and quantified by qPCR using Kapa Library Quantification Kit for Illumina Platforms (Kapa Biosystems). The libraries were paired-end sequenced on a MiSeq instrument using the kit v2 (300 cycles). The raw data was demultiplexed with a DTD module from SLIM \u003csup\u003e61\u003c/sup\u003e. We used then DADA2 \u003csup\u003e62\u003c/sup\u003e to generate Amplicon Sequencing Variants (ASV), which were taxonomically assigned using VSEARCH (Rognes et al. 2016) with 90% min. similarity against a foraminifera database.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements:\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis manuscript is dedicated to the memory of our beloved colleague and friend, P\u0026auml;r G. Jansson. The authors thank UiT, The Arctic University of Norway, the Norwegian Research Council through the projects AKMA, Advancing Knowledge of Methane in the Arctic (project number 287869), HOTMUD (project number 288299), and EMAN7 (project number 320100), the Norwegian Offshore Directorate, REV Ocean, Woods Hole Oceanographic Institution, and La Rochelle University for their financial support and facilitation of the research expeditions. TGS is thanked for permission to use the 3D-seismic data shown in Fig. 1_SI. We are grateful to Nicolas Straube and Ingvar Bykjedal at the Department of Natural History, University Museum of Bergen, for their help with the identification of fishes and to Alexandra Padilla for supporting the SAGE data analyses. The REV Ocean dive team\u0026apos;s invaluable contributions and expertise in data collection are acknowledged with gratitude. SPR work was supported by FCT/MCTES in the scope of the CEEC contract (CEECIND/00758/2017) and funds attributed to CESAM (UIDP/50017/2020, UIDB/50017/2020 and LA/P/0094/2020). MHE was funded by the Trond Mohn Foundation through the Centre for Deep Sea Research (grant number TMS2020TMT13) and the Norwegian Biodiversity Information Centre (the Taxonomy Initiative) through the project \u0026ldquo;Fauna of hydrothermal vents and cold seeps in Norwegian waters\u0026rdquo; (project number 3-20-70184243). The Nippon Foundation is acknowledged for its support of the Ocean Census programme, whose scientists participated in this expedition and the subsequent identification of fauna.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAndreassen, K., da Silveira Ramos Esteves, M., Sarti, F. \u0026amp; Sancak Sert, Z. CAGE Final Report 2017\u0026ndash;2023. \u003cem\u003eCAGE \u0026ndash; Centre for Arctic Gas Hydrate, Environment and Climate Report Series\u003c/em\u003e \u003cstrong\u003e10\u003c/strong\u003e, doi:10.7557/cage.7044 (2023).\u003c/li\u003e\n\u003cli\u003eKirschke, S.\u003cem\u003e et al.\u003c/em\u003e Three decades of global methane sources and sinks. \u003cem\u003eNature Geoscience\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 813-823, doi:10.1038/ngeo1955 (2013).\u003c/li\u003e\n\u003cli\u003eSaunois, M.\u003cem\u003e et al.\u003c/em\u003e The Global Methane Budget 2000\u0026ndash;2017. \u003cem\u003eEarth Syst. Sci. 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J.\u003cem\u003e et al.\u003c/em\u003e DADA2: High-resolution sample inference from Illumina amplicon data. \u003cem\u003eNature Methods\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 581-583, doi:10.1038/nmeth.3869 (2016).\u003cstrong\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4305053/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4305053/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis paper presents the recent discovery of \u003cem\u003eBorealis\u003c/em\u003e, a submerged mud volcano systemlocated in the Polar North Atlantic, distinct from the numerous methane seepages previously identified in the region. In situ observations using a remotely operated vehicle (ROV) have captured the expulsion of warm (11.5°C) Neogene sediments and the eruption of methane-dominated fluids from a localised site within a ~500 m diameter crater. The seafloor around \u003cem\u003eBorealis\u003c/em\u003e comprises laterally extensive carbonate deposits, suggesting long-lasting diffuse methane migration. Sampling and seafloor images reveal that \u003cem\u003eBorealis\u003c/em\u003e hosts unique habitats thriving in the low-oxygen environments around methane seeps. Additionally, the irregualry shaped carbonate structures serve as a natural refuge from bottom trawling as well as a substratum for sessile fauna and function as nursing grounds for threatened fish species. This discovery in the Polar North Atlantic highlights the ecological value of cold seep ecosystems, which play a critical role in biodiversity by acting as sanctuaries for marine species, hence emphasising the importance of their conservation.\u003c/p\u003e","manuscriptTitle":"The discovery of Borealis mud volcano: a natural sanctuary for threatened Arctic species","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-26 02:11:10","doi":"10.21203/rs.3.rs-4305053/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"9c324463-3f59-4b9a-8396-3bab53460c67","owner":[],"postedDate":"April 26th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":31119335,"name":"Earth and environmental sciences/Biogeochemistry"},{"id":31119336,"name":"Biological sciences/Biochemistry"}],"tags":[],"updatedAt":"2025-01-28T08:07:03+00:00","versionOfRecord":{"articleIdentity":"rs-4305053","link":"https://doi.org/10.1038/s41467-024-55712-x","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-01-27 05:00:00","publishedOnDateReadable":"January 27th, 2025"},"versionCreatedAt":"2024-04-26 02:11:10","video":"","vorDoi":"10.1038/s41467-024-55712-x","vorDoiUrl":"https://doi.org/10.1038/s41467-024-55712-x","workflowStages":[]},"version":"v1","identity":"rs-4305053","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4305053","identity":"rs-4305053","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}