Cold seeps are hotspots of biodiversity and can deeply impact the local sediment geochemistry in marine environments (e.g., promoting the formation of authigenic carbonate crusts) throughout all the oceans. Natural gas seepage can lead to changes in sediment properties and nutrient cycling supporting unique benthic fauna living in or near the substrate, eventually promoting the establishment of chemosynthetic biological communities. In this study, a relatively shallow water area offshore northern Svalbard (located at roughly 150m of water depth), where evidence of gas seepage has been observed, is investigated using optical, high-resolution seafloor imagery, and OBIA (Object-Based Image Analysis) techniques. Visual data consists of two photomosaics assembled from frames extracted from videos acquired by means of a work-class Remotely Operated Vehicle (i.e. the ROV ÆGIR 6000), and processed by applying underwater Structure from Motion (SfM) photogrammetry technique. The study aims to detect, classify, and count each single specimen representing benthic epifaunal communities at the seafloor and describe changes in seafloor substrates (i.e. sediment grain size and morphometric attributes) across all the photo-referenced datasets. ArcMap software and direct ROV-based video analysis were used to annotate all visible epibenthic fauna (more than 20,000 individuals), identified to the lowest possible taxonomic level based on discernible external morphological characteristics. In a further step, OBIA techniques (using Trimble eCognition^® software) were applied on seafloor geomorphological characteristics, to provide quantitative and repeatable classification of the substrate into four distinct classes. Finally, annotated benthic epifauna and seafloor substrate classes’ data were combined to quantify patterns of community diversity, abundance, and structure in relation to seafloor morphometric parameters. Cluster analysis revealed substrate class similarities, as well as colonization preferences exhibited by the fauna, especially where methane-derived authigenic carbonates (MDAC) occur at the seafloor. All the fauna and substrate classification outcomes are reported in a catalogue which can be used as a bionomic guide for future studies. This work comprises data collected during the CAGE 20-7 cruise conducted in November 2020 as part of the Centre of Excellence for Arctic Gas Hydrate, Environment and Climate (CAGE) at UiT – The Arctic University of Norway and within the framework of the INTPART-AKMA “Advancing Knowledge on Methane in the Arctic (AKMA)”.

How to cite: Hemmateenejad, F., Fallati, L., Panieri, G., Ribeiro, P. A., Fusca, C., Ferré, B., and Savini, A.: The role of substrate attributes as a driver for benthic epifaunal communities investigated applying OBIA techniques and image analysis on the Norskebanken cold seep site (Arctic Ocean), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-911, https://doi.org/10.5194/egusphere-egu24-911, 2024.

Hydrothermal vents (HVs) and cold seeps (CSs) are typical deep-sea extreme ecosystems with their own geochemical characteristics to supply the unique living conditions for local communities. Once the fluid vents stop emission, the dramatically environmental change would pose survival risks to deep-sea organisms and further shape the whole ecosystems. Up to now, limited knowledge was available to understand the biological responses and adaptive strategies to these extreme environments and their dual-state from active to extinct stage. In this study, bathymodiolin mussels, the dominant and successful species surviving in diverse deep-sea extreme ecosystems, were sampled from active and extinct HVs (Southwest Indian Ocean) or CSs (South China Sea) via two individual cruises. The transcriptomic analysis, determination of multiple biological indexes in stress defense and metabolic systems were conducted in both gill and digestive gland of mussels, together with the metagenomic analysis of symbionts in mussels. The results revealed the fluid-specific transcriptional regulation in mussels, addressing the autologous adaptations in successful antioxidant defense, varied energy utilization and key compounds (i.e. sulfur) metabolism due to distinction in different fluid environments. Coordinately, a heterologous adaptation, characterized by the functional compensation between symbionts and mussels in energy utilization, sulfur and carbon metabolism, was also evidenced by the bacterial metagenomic analysis in these chemosynthetic ecosystems. Taken together, a new insight was proposed that the dual-state of fluid vents drives symbiotic bathymodiolin mussels to develop an autologous and heterologous combined adaptation for successful survival.

How to cite: Zhao, R., Xu, J., and Di, Y.: Systemic comparisons of the adaptations in symbiotic bathymodiolin mussels from diverse stages of hydrothermal vents and cold seeps , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5051, https://doi.org/10.5194/egusphere-egu24-5051, 2024.

The presence of submarine springs and seepages within the hypersaline Dead Sea appears to be a common feature. Hydrothermal fluid escape was first proposed in the mid-1980s based on temperature anomalies measured in the lake, and acoustic blanking observed on high-resolution seismic reflection data. However, the actual existence of such springs was not verified until recently, since the phenomenon only became the focus of dedicated scientific studies during the last decade. As a result of combined anthropogenic intervention and climate change, lake levels have been dropping since the 1960s by over 1 m per year. This has led to large expanses of the lake floor becoming dry land and for submarine springs and other venting features that were previously in deeper water to become shallower. As a consequence, such features are now accessible for direct study either by skilled scuba divers or even along the coast where some have become exposed. Underwater observations include pockmark-like structures, fast and slow-flowing springs, and even salt chimneys formed when brines with different ionic composition than Dead Sea water escape from these vents and come in contact with chlorine-saturated hypersaline background brine leading to the precipitation of halite and other minerals. Diverse microbial communities seem to thrive at these venting locations. Here we will discuss the different types of features, their connection to regional tectonics, and their evolution and development from water to land.

How to cite: Lazar, M., Ionescu, D., and Siebert, C.: The unique geomorphology of submarine venting features as revealed by dropping lake levels in the Dead Sea , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5523, https://doi.org/10.5194/egusphere-egu24-5523, 2024.

By means of Digital Terrain Models, produced based on multibeam echosounding data and underwater photography, spectacular sea floor geomorphologies and features have been discovered and mapped.

In some parts of the NW Black Sea, the submarine geomorphology is characterized by the presence of fluid escape features. These very specific features are present in the flexure area and the upper continental slope. There are prevailing the so-called pockmarks (large depressions on the sea floor) and carbonate chimnies - positive small morphological items on top of the sea bottom, micro-biogeochemically build as the result of the fluid escapes from the sea floor. These kind of submarine geomorphologies are the result of the high dynamics of the fluid escapes that occur in areas with high sedimentation rates, both of sediments and organic matter. The sediments are of Quaternary age and are subject of consolidation processes, that means expulsion of pore water, sometimes accompanied by important amounts of gases, mainly biogenic methane. These submarine sea bottom elements are the best testimonies for the high dynamics of fluids in the pile of young sediments and point out that subjacent to these underwater morphologies could be located hot spots of organic matter accumulations.

The pockmarks could be isolated or clustered in groups of scattered elements or linear patterns. Often, mostly the linear clusters of pockmarks, are associated to the local highs of the sea floor geomorphology. The carbonate chimnies cannot be detected by means of multibeam technologies, but in some upper parts of the Danube and Dnieper deep sea fans such structures can by observed by means of underwater photography.

How to cite: Ion, G., Popa, A., Lazăr, C., Apotrosaei, V. A., and Duțu, F.: Fluid escape submarine geomorphological features in the NW Black Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8298, https://doi.org/10.5194/egusphere-egu24-8298, 2024.

A recently acquired multidisciplinary dataset comprising acoustic surveys (high-resolution sub-bottom profiles, multi-beam bathymetry, and broad band mid-water echo sounder), geochemistry (gas chemical and isotopic composition, porewater chemistry), and sedimentology (core lithology and X-ray CT) in the area of the Landsort deep (450 m of depth), south of Stockholm Archipelago, revealed the existence of an extensive (20 km²) region of the seafloor where massive gas release is occurring in the form of multiple bubble streams. This new discovery represents a major seafloor methane release site in Europe and is comparable in area to other large sites worldwide such as the ones in Svalbard and in the South Atlantic Ocean associated with gas hydrate provinces. The gas is formed mostly by methane of microbial origin. Surprisingly, bubbles rise 100’s of meters above the seafloor and reach surface waters above the halocline/oxycline at around 80 m of depth. Some bubbles appear to reach the sea-air interface and their potential methane contribution to the atmosphere is under investigation. Another surprising observation is the absence of major seafloor features like pockmarks in the gas release area. The reasons for the seafloor methane release in the Landsort deep are still not entirely clear, but our preliminary acoustic and sedimentological data suggest that bottom currents may have acted to facilitate the accumulation of organic-rich sediments in a thick drift deposit during the Holocene and the modern warm period (latest 100 years). Our data further suggest that the high sedimentation rate in the drift deposit continuously supplies fresh organic matter that is quickly buried below a thin sulphate reduction zone, fueling vigorous methanogenesis and abundant methane formation. Similar methane release sites might be discovered in other known large drift deposits in the Baltic Sea.

How to cite: Ketzer, M., Stranne, C., Chang, C., Owari, S., Yu, C., Migeon, S., O'Regan, M., and Jakobsson, M.: Discovery of a major seafloor methane release site in Europe: The Landsort deep, Baltic Sea., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10370, https://doi.org/10.5194/egusphere-egu24-10370, 2024.

In this research benthic foraminiferal response to shallow water methane (CH₄) emissions located in the area around Scoglio d’Africa (Tuscan Archipelago, Northern Tyrrhenian Sea were used as proxy for the individuation of) was investigated. The site is located in the southernmost part of the Elba-Pianosa Ridge, a mainly submarine, north-south elongated morpho-structural high separating the Tuscany Shelf to the east from the Corsica Basin to the west. In the study area, submarine methane emissions have been studied since the 1960s and they are linked to the combined action of two processes: biogenic (microbial process called methanogenesis) and thermogenic origin. The aim of this study is to verify the use of foraminifera as a proxy for detecting the presence of methane emissions and elaborate a microfaunal pattern distribution to apply in recent, future and fossil record. Methane (CH₄) is an important greenhouse gas, with a global warming potential about 20 times as large as carbon dioxide (CO₂) on a 100-year horizon. In the marine environment, coastal areas represent methane hotspots highly exceeding emissions from the open ocean. In this view, Scoglio d’Africa provides a much-promising study site for multidisciplinary marine research like carbon capture and storage, geochemistry of hydrothermal fluids and ocean acidification vs. benthic and pelagic organisms. The microfaunal analyses were carried out from sediment samples coming from 11-16 m depth are shown. The samples were collected by grab and scuba during two sampling surveys in 2021 and 2022. The preliminary results of this research highlighted a very patch distribution and variability in density and biodiversity probably linked to the irregular distribution of the venting activity on the ground floor. The complexity of the interaction of the ecological factors characterizing extreme environments such as shallow hydrothermal vents did not allow us to carry out a real pattern of biota responses in situ. However, some significant considerations can be highlighted. Firstly, a strong loss of biodiversity and collapse in faunal density are recorded due to the combined effects by the CH₄ emissions and the mud flow setting. Secondarily, the rare living specimens are represented by agglutinated species like Lepidodeuterammina ochracea and Ammodiscus sp., miliolid taxa like Quinqueloculina stelligera and Siphonaperta agglutinans, and among hyaline species, Rosalinids and H. depressula resulted the more resilient taxa. Moreover, the research provides new constrain on the ecological behaviour of some foraminiferal species in response to extreme conditions due to methane release.

How to cite: Di Bella, L., Casalbore, D., Conte, A. M., Conti, A., Cornacchia, I., D’Ambrosi, A., Gaglianone, G., Ingrassia, M., Spatola, D., Pierdomenico, M., Provenzani, C., Ruspandini, T., and Chiocci, F. L.: Are the foraminiferal assemblages useful proxy for detecting methane emissions in shallow water environments? the case of Scoglio d’Africa (Tuscan Archipelago, Northern Tyrrhenian Sea) ?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15075, https://doi.org/10.5194/egusphere-egu24-15075, 2024.

Seabed fluid flows (SFF) refer to the movement of fluids (gases and liquids) from sediments to seawater. SFF has broad implications for (i) human activity in the ocean, which is often associated with geohazards, (ii) global climate, and (iii) benthic ecology. BLUEL project aimed to long-term monitor the submarine active pockmark field in the Gulf of Patras, Greece, and investigate its relationship to seismic activity, examining the occurrence of changes in their fluid flow behavior during local earthquakes towards evaluating their potential for use as earthquake precursors. The pockmark field extends to an area of 2.4 km², in water depths of 17 to 45 meters and consists of 115 pockmarks of which 92 are visible and 23 are buried under the infrastructure of the recently constructed South Port of Patras. The formation and activity of the field appears to be controlled by tectonics (faults) while methane fluxes and fluid escapes into the water column were recorded in the past, increased after strong earthquakes.

A high-resolution mapping and monitoring of the Patras Gulf pockmark filed (PGPF) was carried out through high resolution acoustic mapping techniques, including swath bathymetry, sidescan sonar backscatter and sub-bottom profiling, revealing pockmarks morphological evolution through time and assessing the spatial patterns of bubble flares after major seismic events. Results showed that the main mechanisms for the development of the field are local tectonism and internal characteristics of gas-charged sedimentary layers. Sediment and water samples were collected and in-situ measurements of CH₄ concentration were performed using a methane sensor. The chemical composition and origin of the fluids in the seawater and the sediments were assessed and implications about the volume of greenhouse gases escaping to the atmosphere were made. The geochemical analysis showed that heavy metal concentrations are always higher in sediments collected inside the pockmarks than those collected outside the sites. Isotopic analysis also revealed that CH₄ of microbial origin is the dominant component of the released gas. The annual emissions of methane from the pockmark field wider area to the atmosphere have been also estimated between 7.6 to 8.4 tons per year.

A 200m long submarine optical fiber was installed inside a selected active pockmark to measure the water temperature through a Distributed Temperature Sensing (DTS) system, acquiring data over 1.5 years. Spectral analysis methods were applied to fill missing data, reconstruct the temperature time series along the cable length and reveal any underlying periodicities or anomalous events. The above measurements were supported by meteorological and tidal data collected in the area, as well as by a microseismic network to record the seismic activity over the corresponding period. Comparisons were performed between the above datasets, revealing significant relationships between anomalous thermal events and local seismicity.

How to cite: Papatheodorou, G., Geraga, M., Christodoulou, D., Fakiris, E., Sokos, E., Roumelioti, Z., Etiope, G., kokkalas, S., Giannopoulos, N., Dimas, X., Georgiou, N., Giannakopoulos, V., and Ferentinos, G.: Insights into Seabed Fluid flows: Pockmark dynamics mapping and monitoring in Patras Gulf, Greece, Unveil Correlations to local tectonics and Earthquakes. The BLUEL project., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16419, https://doi.org/10.5194/egusphere-egu24-16419, 2024.

On the Greek island Milos and in shallow water at its coast, many spots with hydrothermal activity have been found and studied in the past. The M192 cruise in August 2023 with the German research vessel METEOR followed the idea that these systems may continue along a tran­sect from shal­low, nearshore, photic to the deeper, off­shore, aphotic zone around the island, accompanied by changes in terms of environmental parameters. 

Volcanism along the Hellenic volcanic arc started during the Early to Middle Pliocene, while the last eruption occurred in 1950 (Nea Kammeni volcano). The intense seismic activity in the area is associated with important geothermal gas venting, with the major systems being found in relatively shallow waters (1–500m depth) at Methana, Milos, Santorini (Kolumbo submarine volcano), Kos and Nisyros.

Systematic bathymetry and water column acoustic survey work with METEOR's multibeam with the autonomous underwater vehicle (AUV) MARUM-SEAL on the M192 cruise revealed several previously uncharted hydrothermal vent fields offshore Milos. They are located in the southeast extending from the bays Kiriaki to Paleochori and Thiorychia, as well as in an area northwest of Milos, offshore the bay of Vani. The distribution of the hydrothermal vents seems to be tectonically controlled and follow the prominent faults that have been mapped on Milos.

The areal extents of venting were identified by echosounding using the acoustic anomaly the presence of gas bubbles causes in the water column. But selected hydrothermal vents were furthermore visually observed and sampled using the remotely operated vehicle (ROV) MARUM-SQUID. These individual vents revealed pronounced differences; whereas the shallower vents (around 100 m water depth) were noticed as white patches (of sulfur-oxidizing bacteria) on the sandy seafloor with diffuse venting comparable to the shallow vents close to the coast, the deeper vents (around 200 m water depth) featured remarkable chimney structures sometimes several meters in height that are covered with white biofilms and vent fluids reaching temperatures up to 180 °C. Sampled fluids showed mildly reducing and slightly acidic (pH between 5.0 and 7.9) conditions and were rich in dissolved hydrogen sulfide and dissolved metals. These signals extended up to about 10 m into the water column, as recorded by CTD-rosette water sampler stations.

To date, shal­low-wa­ter and deep-sea hy­dro­thermal sys­tems have been treated as in­de­pend­ent, seem­ingly un­re­lated en­tit­ies; the results of the M192 expedition presented here are the first foray into re­mov­ing this ar­bit­rary bound­ary.

How to cite: Bühring, S., Koschinsky, A., Bach, W., Elvert, M., Kleint, C., Kumawat, P., Maak, J., Meckel, E.-M., Nomikou, P., Röttgen, C., and Schefuß, E.: Discovery of a unique submarine hydrothermal system between shallow photic and deep dark sites around the Greek island of Milos (Aegean Sea, Greece), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17022, https://doi.org/10.5194/egusphere-egu24-17022, 2024.

Fluid expulsion and sediment mobilization are typical processes in accretionary prisms, where sediments are scraped off the subducting plate and piled up and squeezed to originate a tectonic prism, resulting in fluid venting, mud volcanoes and mud diapirs. The Mediterranean region is characterized by subduction zones where residual portions of the Tethyan oceans have survived the Aftica-Eurasia continental collision. Among these subduction zones, the Calabrian accretionary prism is known to be populated by mud volcanoes. The Ionian offshore of the Crotone promontory offers examples where the expressions of fluid expulsion and sediment mobilization are visible both in the subsurface and at the seafloor. The analysis of a proprietary 3D seismic cube allows to characterize patterns of pockmarks, which are direct expression of fluid expulsion at the seafloor, and to identify a mud diapir which appears at the seafloor as a large mud pool, ca. 1200 m in diameter. The high resolution 3D seismic profiles also allow to infer differences in the  mechanisms of fluid focussing at very shallow depth. Small, closely spaced normal faults, produced by outer arc extension, and dilation in the shallow unconsolidated sediments, due to sharp slope gradient increase, both favour fluid focussing. In some instances it can be shown that fluid venting also contributed to destabilize the uppermost sedimentary strata, triggering small landslides along the slope. A Pliocene extensional system has  developed within a mobile shale domain. The diapir that surfaces as a mud pool has been mobilized along a recent extensional fault, which tapped into the mobile shale domain. Furthermore, a fossil mud pool has also been recognized in the study area. This fossil mud pool is sealed by undeformed sedimentary strata which allow to constrain a minimum age for fluid and sediment mobilization in the accretionary prism. Seismic reflections amplitude suggests that the fossil conduit still acts as a preferential fluid seepage pathway, contributing to destibilize the overlaying slope sediments.

How to cite: Argnani, A. and Rovere, M.: Submarine Morphology Offshore Crotone (Calabrian Accretionary Prism, Central Mediterranean): Pockmark Fields and a Mud Diapir in a Mobile Shale Domain, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21390, https://doi.org/10.5194/egusphere-egu24-21390, 2024.