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The present proposal offers an in-depth conceptual representation of the life cycle of methane seep ecosystems, showcasing their ecological development and biogeochemical processes from beginning to end. It combines current knowledge about microbial and metazoan communities, merging environmental and geological factors into a theoretical structure. A thorough review of literature on geology, biogeochemistry, and ecology supports the conceptual representation. It details five stages: birth, youth, maturity, senescence, and extinction, with each stage explaining the respective communities and their metabolic functions. It blends ideas from various fields, providing a comprehensive perspective on methane seep ecosystems and their intricate interactions. Additionally, the manuscript points out gaps in existing knowledge and proposes future research directions, highlighting the need for quantitative models and on-site investigations to validate the theoretical model. [ABSTRACT FROM AUTHOR]

This study describes an assemblage of Dzieduszyckia brachiopods preserved in sedimentary barite deposits from the Famennian, Upper Devonian of Nevada, USA. The brachiopods lived in a methane-seep environment like those described from Sonora, Mexico, and the Western Meseta, Morocco. Although the original carbonate fabrics are not preserved, pockets of limestone within the brachiopod barite are depleted in with δ13C values ranging from -31.1 to -27.3‰ VPDB. The fossils are preserved as internal and external molds only but record enough detail to demonstrate key internal features of Dzieduszyckia such as a pronounced dorsal septum, septalium, and vertical dental plates. The lack of common bifurcation or trifurcation of the costae as well as larger size of the shells recommends against inclusion within D. sonora as previously published. Based on the number of costae per width of shell, it is possible that there are two distinct species in Nevada, like other localities in Morocco, the Urals, and South China. These new data expand the body of knowledge of these earliest dimerelloids and it is hypothesized that this seep-dwelling macrofauna arose from depauperate-successful lineages between the Famennian biotic crises. As nearly all global Dzieduszyckia deposits are associated with active tectonic margins, it is also suggested that tectonically-driven methane expulsion in low nutrient waters spurred the evolution of dimerelloids.

This study describes an assemblage of Dzieduszyckia brachiopods preserved in sedimentary barite deposits from the Famennian, Upper Devonian of Nevada, USA. The brachiopods lived in a methane-seep environment like those described from Sonora, Mexico, and the Western Meseta, Morocco. Although the original carbonate fabrics are not preserved, pockets of limestone within the brachiopod barite are depleted in with δ13C values ranging from -31.1 to -27.3‰ VPDB. The fossils are preserved as internal and external molds only but record enough detail to demonstrate key internal features of Dzieduszyckia such as a pronounced dorsal septum, septalium, and vertical dental plates. The lack of common bifurcation or trifurcation of the costae as well as larger size of the shells recommends against inclusion within D. sonora as previously published. Based on the number of costae per width of shell, it is possible that there are two distinct species in Nevada, like other localities in Morocco, the Urals, and South China. These new data expand the body of knowledge of these earliest dimerelloids and it is hypothesized that this seep-dwelling macrofauna arose from depauperate-successful lineages between the Famennian biotic crises. As nearly all global Dzieduszyckia deposits are associated with active tectonic margins, it is also suggested that tectonically-driven methane expulsion in low nutrient waters spurred the evolution of dimerelloids. [ABSTRACT FROM AUTHOR]

We investigated fluid seepage within the Nyegga pockmark field (600–900 m water depths) off mid-Norway from Remotely Operated Vehicle dives at the so-called CNE sites (CNE01 to CNE17). The seafloor morphology of some of these features corresponds to pockmarks and adjacent ridges, with the latter being the focus of present seepage activity. These structures are underlain by chimneys above a gas-charged zone with, in some cases, a substantial body of hydrate-invaded sediment (down to 1.3 s in two-way travel time at CNE03). Present-day methane-rich fluid seepage through the seabed is indicated by chemosynthetic fauna, in particular Siboglinidae polychaetes (Oligobrachia haakonmobiensis webbi and Sclerolinum contortum), microbial mats and associated Rissoidae gastropod (Alvania sp.) grazers, and confirmed by measured in situ bottom-water methane anomalies, up to 2,130 nL/L. No free-gas bubble emissions were observed or acoustically identified. The presence of authigenic carbonates reveals past seepage with very low δ13C values (down to −58‰) indicating that the major source of carbon was methane carried by the venting fluids. The ages of major periods of methane venting are provided by vesicomyid bivalve shells (Isorropodon nyeggaensis) present in two sedimentary layers, 14,930 and 15,500 14C yr BP (ca. 17,238 and 17,952 cal yr BP), respectively, corresponding to the time of Melt Water Pulse IA. The seafloor morphology and pattern of seepage -chemosynthetic fauna and microbial mat distribution and dissolved methane concentration-are remarkably heterogeneous. Pore-water chemistry profiles in a gravity core taken only 40 m from major seepage sites indicate no seepage and anaerobic methane oxidation at a sub-bottom depth of about 2 m. Present-day seepage from the studied pockmark-chimney fluid-flow system charged with gas hydrate is dominated by the advection of methane solution in pore water. Some of this methane could result from the dissolution of hydrate in the chimney, most of which would have formed during an earlier period (post-LGM times) of history of the chimney, when it was venting free gas. However, the presence of free gas beneath this chimney is probably why the water entering the chimney is already saturated with methane and the process of hydrate formation in the chimney continues today.

In this work, we integrate five case studies harboring vulnerable deep-sea benthic habitats in different geological settings from mid latitude NE Atlantic Ocean (24–42° N). Data and images of specific deep-sea habitats were acquired with Remoted Operated Vehicle (ROV) sensors (temperature, salinity, potential density, O2, CO2, and CH4). Besides documenting some key vulnerable deep-sea habitats, this study shows that the distribution of some deep-sea coral aggregations (including scleractinians, gorgonians, and antipatharians), deep-sea sponge aggregations and other deep-sea habitats are influenced by water masses’ properties. Our data support that the distribution of scleractinian reefs and aggregations of other deep-sea corals, from subtropical to north Atlantic could be dependent of the latitudinal extents of the Antarctic Intermediate Waters (AAIW) and the Mediterranean Outflow Waters (MOW). Otherwise, the distribution of some vulnerable deep-sea habitats is influenced, at the local scale, by active hydrocarbon seeps (Gulf of Cádiz) and hydrothermal vents (El Hierro, Canary Island). The co-occurrence of deep-sea corals and chemosynthesis-based communities has been identified in methane seeps of the Gulf of Cádiz. Extensive beds of living deep-sea mussels (Bathymodiolus mauritanicus) and other chemosymbiotic bivalves occur closely to deep-sea coral aggregations (e.g., gorgonians, black corals) that colonize methane-derived authigenic carbonates.

Methane seep communities on the Koryak slope in the Bering Sea were discovered in 2018 during the 82nd cruise of the RV Akademik M.A. Lavrentyev using ROV Comanche 18. Methane seeps found between 400 m and 695 m depth are the northernmost chemosynthesis-based habitats (∼61°N) known to date in the Pacific. Macrofauna species composition and community structure of methane seep and background benthic communities were investigated at three depth horizons: 400–402 m, 417–429 m and 647–695 m. A total of 335 macrofaunal species were identified. At the depth 647–695 m the species richness in the methane seep communities was 265 species and 72 species in the background. The deepest seep communities were characterized by the highest species richness, relatively high evenness and strong variations in the composition and structure. At shallower depths the species richness was low: 33–34 species at methane seeps and 66–71 in the background. The methane seep communities at shallow depths showed relatively low evenness and low diversity of community structure. The highest species richness at the greatest depth can be related in part to increased habitat heterogeneity associated with carbonate crusts and vesicomyid clam beds occurring only at deeper methane seeps. The deepest methane seep communities were dominated by the chemosymbiotrophic pliocardiine Calyptogena pacifica ; dominant in the background was the ophiuroid Ophiophthalmus normani. At shallower methane seeps only single specimens of C. pacifica were found. The shallower methane seeps were dominated by the background species despite numerous extensive bacterial mats and high potential toxicity of the environment. The seep community at 417–429 m was dominated by the echinoid Brisaster latifrons with population density on the bacterial mat four times higher than in the background. Four chemosymbiotrophic species were recorded at 647–695 m and only one was found at shallower depths. Potentially obligate to reducing habitats polychaetes of the genus Neosabellides and Ophryotrocha occurred in methane seep communities at all depths. Further geochemical, geological and microbiological studies are required to better characterize differences of methane seeps at different depths and reasons of these differences. • Methane seeps were discovered on the Koryak slope (Bering Sea) at 400–700m depth. • Composition and structure of seep communities differed at depths 400–402m, 417–429m and 647–695m. • Background species, echinoids and actiniarians, dominated the shallower seeps. • Chemosymbiotrophic pliocardiine Calyptogena pacifica dominated the deepest seeps. • The deepest seeps have the highest species richness and vary in community structure. [ABSTRACT FROM AUTHOR]