Cold Methane Seeps Research Articles

Gigantidas niobengalensis sp. nov. (Bivalvia: Mytiloidea) from a methane seep in the Krishna–Godavari Basin, Bay of Bengal

A new species of a deep-sea Bathymodiolinae mussel is described as Gigantidas niobengalensis sp. nov. based on both molecular and morphological data. It was collected at a cold methane seep environment in the Bay of Bengal at 1750 m water depth. It is compared with other species of Gigantidas, especially the related G. childressi, G. mauritanicus, G. platifrons, and the G. haimaensis group.

Elemental speciation of the sediments from cold methane seeps on the Laptev Sea shelf

Relevance. The lack of elemental speciation data in the bottom sediments of the Laptev Sea, as well as the absence of its quantitative changes in occurrence forms in response to methane emissions. Aim. To assess changes in the elemental speciation in areas where methane-containing fluids are discharged. Objects. Nine samples of bottom sediments taken from three horizons of one “cold seep” and two background stations of the outer shelf-continental slope of the Laptev Sea during the AMK-82 expedition aboard R/V “Academician M.V. Keldysh” in autumn 2020. Methods. Sequential extraction BCR (the European Community Bureau of Reference) was performed to determine the elemental speciation in surface sediments; the residual fraction was determined by dissolving in nitric acid. The obtained fraction solutions were analyzed by ICP-MS (NexIon 300D, Perkin Elmer, Waltham, MA, USA), Pyrolysis (Rock-Eval 6 Turbo, Vinci Technologies). Results. The trend towards an increase in element mobility has been identified as a result of a shift in geochemical conditions that is presumably caused by the authigenic carbonate and sulfide formation. Moreover, the increase in the recoverable fraction (associated only with Fe oxides/hydroxides) is highlighted. We assume that the process of unloading methane-containing fluids plays a significant role in enrichment of authigenic carbonates U, Ni, Zn Co, oxides Fe – Sn and V, authigenic sulfides – Co, Mn and W. We also assume the potential formation of chalcopyrite as an authigenic sulfide in methane discharge areas.

Stable carbon, nitrogen, and sulfur isotope composition of Oligobrachia haakonmosbiensis Smirnov, 2000 (Annelida: Siboglinidae) from the shallow-water Laptev Sea shelf

Cold methane seeps create suitable habitat conditions for chemosymbiotic frenulate tubeworms, one of the groups of the family Siboglinidae Caullery, 1914 that can form extensive aggregations at seeps at the shelf of the Arctic seas. The present study provides the first data on the ratio of natural stable C, N, and S isotopes in tissues of the siboglinid tubeworm Oligobrachia haakonmosbiensis found in methane seep fields on the Laptev Sea shelf. Values of δ13C, δ15N, and δ34S for various body parts, such as cephalic and abdominal regions, tissues, whole worms with their tubes, and also worms and tubes separated, fixed in 96% ethanol, have been obtained for O. haakonmosbiensis. The lowest isotopic composition values were recorded from bodies of O. haakonmosbiensis with their tubes collected in the coastal zone at a depth of 25m (δ13C = –72.7±1.9‰; δ15N = –10.9±1.2‰; and δ34S = –28.1±1.9‰). Such low values are suggested to be associated mainly with large amounts of dissolved organic matter near the Lena River estuary. Our analysis of stable C, N, and S isotopes of O. haakonmosbiensis from the Laptev Sea has shown that the major energy source for symbiotic bacteria of this tubeworm species is hydrogen sulfide.

Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs

Systematic documentation of pteriomorph bivalves of the late Eocene–early Oligocene Keasey Formation in western Oregon follows previous monographic treatments of the anomalodesmatans and heteroconchs. It includes material from coeval Cascadia Margin strata in southwestern Washington in the context of major molluscan faunal turnover during the dramatic doubthouse climate transition from an ice-free tropical marine environment to the establishment of permanent polar ice caps and a cold temperate marine climate. The families represented are Crenellidae, Parallelodontidae, Glycymerididae, Limopsidae, Isognomonidae, Pectinidae, Propeamussiidae, and Limidae. New taxa include the genus Bathyisognomon and five new species: Limopsis squiresi, Bathyisognomon smithwickensis, Delectopecten kieli, Delectopecten keaseyorum, and Propeamussium (Parvamussium) mistensis. Small to minute mud pectens and glass scallops are among the most abundant and frequent taxa in the Keasey fauna, although they are poorly preserved and have been overlooked heretofore. Pteriomorphs are prominent elements of recurring associations of species in mudstone and fine-grained siltstone facies adjacent to three Keasey cold methane seeps. Previously described Keasey anomalodesmatans and heteroconchs in these seep-adjacent strata have living relatives adapted to dysoxia and toxic geochemistry. Because the unique macrofaunal associations in these settings are neither seep fauna nor part of the background fauna, this paper introduces and defines the concept of a transitional peri-seep biotope. Relationships, monophyly, systematic resolution within major pteriomorph groups, and global biogeographic distributions are reviewed in each systematic treatment. Recurring patterns include occurrences in active margin settings, inferred oscillatory deepening of the calcium carbonate compensation depth, late Eocene origin of two-layered oceans following establishment of the Antarctic Circumpolar Current, and biogeographic gateway changes that disrupted earlier Paleogene larval dispersal patterns.

Genesis and preservation of authigenic magnetite and greigite in the cold seep sediments, Bay of Bengal

We report a new mechanism of authigenic greigite and magnetite formation and preservation in an anomalous “sulfide-rich” interval in the cold (methane) seep sediments from the Bay of Bengal. Rock magnetic proxies (SIRM/χlf, ARM/SIRM) coupled with microscopic (TEM, SEM) observations provided compelling evidence for the presence of nanoscale authigenic magnetite and greigite particles in the methanic sediments. Multiple peaks of SIRM/χlf coupled with high Hcr (>53 mT) and DJH values suggest significant contribution from authigenic greigite particles to the bulk sediment magnetism. High ARM/SIRM in the sulfide-rich intervals within methanic zone suggest that nanoscale authigenic magnetite formation is closely linked with the microbial iron reduction process fuelled by anaerobic oxidation of methane (AOM). We proposed that enrichment of reactive secondary iron (oxyhydr) oxides through selective leaching of Fe2+ during diagenetic dissolution and maghemitization of detrital magnetic particles and iron-sulfide mineral oxidation at the paleo-SMTZ fronts, hindered the pyritization processes and favoured microbial iron reduction which led to the formation and preservation of authigenic greigite and magnetite in methanic sediments.

Are ancient seep environments distinguishable by benthic foraminiferal assemblages? A case study of the Cretaceous Western Interior Seaway

Methane cold seeps are food-rich seafloor habitats that served as oases for macrofaunal communities in the Cretaceous Western Interior Seaway (WIS). Although seep-obligate taxa occur at modern seeps, macrofaunal inhabitants were not endemic to seep environments in the WIS. Here we investigate benthic foraminiferal assemblages across the WIS and test whether benthic foraminifera from seeps are distinct from non-seep environments. We compile abundance data from Campanian to Maastrichtian seep, nearshore, and offshore environments and use constrained multivariate ordination to center our analysis on differences in assemblage composition as it relates to environment setting. Genus-level analysis reveals that offshore settings have more calcareous taxa than nearshore settings, which, in contrast, have representation from various agglutinated genera. Analyses of guilds and morphotypes disregard test-wall type and still find compositional differences among environments that indicate a nearshore–offshore gradient in organic carbon supply. We also find that seep assemblages are generally more similar to offshore assemblages than nearshore ones, reflecting their position near the central axis of the WIS. However, there are no genera, guilds, or morphotypes unique to seeps. We further compare between assemblages from immature, predominantly soft-ground seeps and mature, predominantly hard-ground seeps. We find compositional differences consistent with abated chemical stressors and a well-developed physical substrate associated with seep maturity. Thus, although seep environments draw their benthic foraminiferal faunas from the surrounding metacommunity, assemblages reflect the environmental differences present at seeps and could be used as an additional proxy for seep settings and seep maturity in the WIS.

Trace metal biogeochemistry in methane seeps on the Koryak slope of the Bering Sea

Methane seeps are widely distributed in the ocean being represented at all depths and latitudes. Along with hydrocarbons, seeping fluids can supply many chemical elements, including trace metals which benthic communities are needed. Under the condition of climate warming, the methane diffuse flow in the ocean can affect chemical balance. In this study for the first time, in the methane seep ecosystem recently discovered on the Koryak slope of the Bering Sea, we examined the distribution of some essential and potentially toxic chemical elements. In the benthic organisms, biotope water, and surface sediments, concentrations of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Zr, Mo, Cd, Ba, W, Pb, and U were quantified. Dissolved methane in water, total carbon (TC), and total inorganic carbon (TIC) in organisms and sediments, as well as the mineral composition of sediments, were also determined. Our data showed that bioaccumulation of many trace metals in benthic organisms is related to high concentrations of these elements in near-bottom water of methane cold seep habitats. Significantly elevated concentrations of many trace elements, especially chalcophile, were recorded in the gills and to a lesser extent, in the rest of the soft body of chemosymbiothrophic bivalve mollusk Calyptogena pacifica. Together with the essential elements Cu, Zn, Co, Ni, Cr, Mo, and Cd, potentially toxic As, Pb, U, and W also exhibited noticeably higher accumulation in the soft tissues of organisms. In all cases, tissues and whole organisms which are enriched in TOC exhibited higher trace element concentrations and BCF values. Bioconcentration Factor (BCF) exhibited a trend to decrease from the gills of C. pacifica (n106), to whole bodies of suspension feeders (n104). Reducing conditions led to strong depletion in most elements of surface sediments with the enrichment factor (EF) being around 1. Exceptions were Mo and Ba whose EF values (up to 2.2) suggest detectable authigenic enrichment.

Late Ordovician-Early Silurian extension of the northern margin of the Upper Yangtze Platform (South China) and its impact on organic matter accumulation

Breakthroughs in shale gas exploration in the Upper Ordovician-Lower Silurian strata of the Upper Yangtze Platform have attracted interest in its sedimentary-tectonic evolution, but the tectonic background of the northern margin of the Upper Yangtze Platform remains unclear. In this paper, the Wufeng-Longmaxi formations on the northern margin of the Upper Yangtze Platform were investigated. Based on geochemical and mineralogical analyses of the tuffs/K-bentonites of the Wufeng Formation and the barite in the Longmaxi Formation, as well as previous research results, it was concluded that the northern margin of the Upper Yangtze Platform was in an extensional tectonic background during the Late Ordovician-Early Silurian. Detailed analysis revealed that, (1) the U–Pb zircon age of the tuff in the Bajiaokou section in South Qinling is 443.91 ± 0.92 Ma. The Zr/TiO2–Nb/Y diagram of the tuffs/K-bentonites indicates that their protoliths were alkaline-subalkaline basalt and andesite series rock. Based on the Th–Hf/3-Ta, Th–Tb*3-Ta*2, and TiO2–Nb/3-Th diagrams, there are undiscovered intraplate tension calc-alkaline basalts in the northern Yangtze Platform or the southern Qinling region, which provided volcanic clastic materials to the Ziyang, Lan'gao, Chengkou, Yichang and other regions. (2) Scanning electron microscopy revealed that the barite crystals in the Longmaxi Formation exhibit dissolution features and have a large particle size. Energy spectrum analysis of these barite crystals revealed that they have C, O, S, and Ba contents of 8.48 wt%, 22.98 wt%, 13.09 wt% and 55.44 wt%, so they are speculated to have been formed via cold methane seep genesis in a weak extensional tectonic setting. The 87Sr/86Sr ratios of the barite revealed that different types of barite were simultaneously formed in this area under the influences of hydrothermal and cold methane seeps. (3) The analysis of the heavy minerals in the Lower Silurian strata in the Bajiaokou section revealed that the provenance in the South Qinling area changed significantly during the late Early Silurian. Based on the above analyses, the northern margin of the Upper Yangtze Platform was in an extensional tectonic setting during the Late Ordovician-Early Silurian. The distribution of the total organic carbon content indicated that the extensional tectonic background provided good conditions for the enrichment and preservation of organic matter. The results of this study provide an understanding of the regional sedimentary-tectonic pattern and evolution of the Yangtze Platform during this period, as well as a reference for future shale gas exploration in this region.

New species of Ophryotrocha (Annelida: Dorvilleidae) associated with deep-sea reducing habitats in the Bering Sea, Northwest Pacific

Four species of Ophryotrocha (Annelida, Dorvilleidae) from deep-sea reducing habitats (methane seeps on the Koryak slope and hydrothermal vents of the submarine Piip Volcano) were collected in the western part of the Bering Sea, NW Pacific. Descriptions of two of these new Ophryotrocha species, namely O. beringiana sp. nov. and O. seepens sp. nov., confirmed with molecular analyses, are presented herein. The detailed descriptions with electron microscopy illustrations for the new species are presented and their differences from similar species are discussed. In addition to the morphological description, the phylogenetic relationships of the new dorvilleids are provided using two nuclear (18S RNA, histone H3) and two mitochondrial (16S RNA and cytochrome c oxidase subunit 1, COI) markers in an analysis containing 54 terminal taxa. Phylogenetic analyses using MrBayes and Maximum Likelihood analyses show that O. beringiana sp. nov. and O. seepens sp. nov. are sister species, and together with O. platykephale, O. globopalpata and O. flabella form a subclade of deep-sea Pacific species within a clade referred to here as ‘globopalpata–platykephale’. In our molecular analysis, the clade currently consists of ten species, nine of those (except for O. longidentata, the only shallow Atlantic species not reported in chemosynthetic habitats) have been described from Pacific deep-sea reducing environments (hydrothermal vents, cold methane seeps, whale-falls). The ecology and geographic distribution of the reported species are discussed.

Response of benthic foraminifera to environmental successions of cold seeps from Vestnesa Ridge, Svalbard: Implications for interpretations of paleo-seepage environments

This paper presents the results of a study on the response of living benthic foraminifera to progressing environmental successions in a cold-seep ecosystem. Sediment samples were collected from Vestnesa Ridge (79°N, Fram Strait) at ~1200 m water depth. The distribution of live (Rose Bengal-stained) foraminifera were analyzed in the upper sediment layers in relation to pore water biogeochemical data together with the distribution of sulfur-bacterial mats and Siboglinidae tubeworms. At methane cold seeps, the process of environmental succession is strongly connected to the duration and strength of methane seepage and the intensity of methane-related biological processes, e.g, aerobic and anaerobic oxidation of methane (MOx and AOM, respectively). The results show that the distribution patterns of benthic foraminifera change according to the progressing environmental succession. The benthic foraminifera seemed to thrive in sediments with a moderate activity of seepage, dominated by MOx, i.e, at an early stage of seepage or when seepage decreases at a late stage of the succession. Species composition of the foraminiferal fauna under these conditions was similar to the control sites (outside of pockmarks with no seepage); the dominant species being Melonis barleeanus and Cassidulina neoteretis. In sediments with strong seepage and high AOM activity, the hostile environmental conditions due to the presence of toxic sulfide caused a reduction in the foraminiferal population, and samples were almost barren of foraminifera. In environments of moderate methane seepage, the presence of chemosynthetic Siboglinidae tube worms potentially support communities of the epibenthic species Cibicidoides wuellerstorfi. Despite the very different environmental conditions, the foraminiferal assemblages were very similar (or nearly absent). Therefore, the foraminiferal faunas cannot be used as exclusive indicators of past strength of methane seepage in palaeoceanographic interpretations.

Kinorhynchs (Kinorhyncha) from methane cold seeps in the Bering Sea, with biogeographical discussion and ecological notes

Several species of kinorhynchs, including representatives of Pycnophyes schornikovi Adrianov, 1999 in Adrianov et Malakhov, 1999 and Condyloderes shirleyi Neuhaus et Higgins, 2019 in Neuhaus et al.2019, previously known from other localities in the Pacific Ocean, were found in detritus washed off the material of bacterial mats collected at the cold methane seeping area on the Chukchi bathyal slope in the Bering Sea. These specimens were studied and illustrated using light (DIC) and electron (SEM) microscopy to analyze morphological variations within representatives of these species collected from rather distant localities in the Pacific Ocean. Redescription of P. schornikovi is provided with careful mapping of all sensory spots, setae and glandular cell outlets, not sufficiently illustrated in the original description. Current taxonomic contributions have clearly showed the taxonomic significance of these structures, becoming a standard element in taxonomic descriptions of kinorhynchs. P. schornikovi appears to be the first eurybathic species of the Pycnophyidae in the Pacific, being distributed from shallow water to bathyal depth. C. shirleyi, characterized by a marked variation of some diagnostic morphological features even in the type locality in the North-East Pacific, appears only the second representative of the genus Condyloderes with the transoceanic (amphipacific) distribution. Both these species, P. schornikovi and C. shirleyi, constitute the first representatives of these genera in the Bering Sea. In the Pacific Ocean, kinorhynchs still have never been reported from the extreme environments of cold seeps. P. schornikovi and C. shirleyi appear to be the first representative of the genus found in the methane seepage area. Deep-sea communities of this cold seepage area are described and illustrated with ROV facilities. Biogeography discussion for both species and probable explanation of the transoceanic (amphipacific) distribution of Condyloderes shirleyi are suggested and illustrated.

Discordance of genetic diversification between deep‐ and shallow‐water species of Kobeltocochlea Lindholm, 1909 (Caenogastropoda: Truncatelloidea: Benedictiidae) endemic to Lake Baikal with the description of a new species, review of the genus, and notes on its origin

We describe a new species, Kobeltocochlea tamarae Sitnikova, Teterina & Maximova sp. nov., from Lake Baikal discovered in the Saint Petersburg methane seep at a depth of approximately 1400 m. The uncorrected genetic distance was 4.26% of the COI gene between this new species and K. falsipumyla collected from the Academic Ridge at depths between 157 and 163 m. The two shallow-water species, K. martensiana and K. olchonensis, differed in substrate preference and geographic habitats, with a genetic distance of 0.97% for COI. Phylogenetic analysis of mitochondrial (COI, 16S) and nuclear (ITSI, 28S) sequences indicated that Kobeltocochlea was paraphyletic with respect to other benedictiid genera Benedictia and Yaroslawiella, formed well-supported monophyletic groups. We revealed the discordance of morphological and genetic data within Kobeltocochlea and other benedictiids, which reflected their complicated history. The current knowledge of benedictiids does not allow for significant changes in the taxonomy of the Benedictiidae, except for synonymizing K. lindholmiana with K. olchonensis and Pseudobenedictia with Benedictia that were also investigated. We speculate that three main lineages of benedictiids originated from three populations of a single ancient species approximately 3.2 Ma. The speciation of deep-water species was probably driven independently by bathymetric isolation in the cold methane seeps of different geomorphological structures. The diversification of the third lineage consisting of littoral (including K. martensiana/K. olchonensis), deep-water, and eurybathic benedictiids may be explained by rapid adaptive radiation to intralacustrine environments.

Methane-Derived Authigenic Carbonates on the Seafloor of the Laptev Sea Shelf

Seafloor authigenic carbonate crusts are widespread in various oceanic and marine settings, excluding high-latitude basins that are corrosive to carbonate precipitation. Newly formed carbonate formations are relatively rare in modern Arctic marine sediments. Although the first-order principles of seep carbonate formation are currently quite well constrained, little is known regarding the duration or mode of carbonate formation in the Siberian Arctic shelf. Large (massive slabs or blocks) and small crusts that were micrite cemented have been recently discovered on the seafloor of the Siberian Arctic seas within the area of known seep activity in the outer Laptev Sea shelf. Cold methane seeps were detected in the area due to the presence of an acoustic anomaly in the water column (gas flares). Microbial mats, methane gas bubbles, and carbonate crusts were observed using a towed camera platform. Here, we report new geochemical and mineralogical data on authigenic shallow Siberian Arctic cold-seep carbonate crusts to elucidate its genesis. The Laptev Sea carbonate crusts mainly consist of high-Mg calcite (up to 23 mol % MgCO3). The δ13C values in carbonates range significantly (from –40.1 to –25.9‰ VPDB), while the δ18O values vary in a narrow range (+4.4 ± 0.2‰ VPDB). The δ13C values of Corg that was determined from carbonates range from –40.2 to –31.1‰ VPDB. Using the isotope data and taking into account the geological setting, we consider that not only microbial but possibly thermogenic methane participated in the authigenic carbonate precipitation. Carbonate crust formation occurred below the water/sediment interface of the shallow Siberian Arctic shelf as a result of gas hydrate dissociation during Holocene warming events. The studied carbonate crusts were exhumated after precipitation into shallow subsurface shelf sediments.

Diagenetic Controls on the Formation of the Anarraaq Clastic-Dominated Zn-Pb-Ag Deposit, Red Dog District, Alaska

AbstractThe Anarraaq clastic-dominated (CD) Zn-Pb-Ag deposit (Red Dog district, Alaska, USA) has an inferred mineral resource of 19.4 Mt at 14.4% Zn, 4.2% Pb, and 73 g/t Ag and is spatially associated with a separate ~1 Gt barite body. This study presents new cross sections and petrographic evidence from the Anarraaq area. The barite body, previously shown to have formed in a shallow subsurface environment akin to a methane cold seep, contains multiple generations of barite with locally abundant calcite masses, which are discordant to sedimentary laminae, and is underlain by an interval of massive pyrite containing abundant framboids and radiolarians. Calcite and pyrite are interpreted to have formed by methane-driven diagenetic alteration of host sediment at the sulfate-methane transition (SMT). The sulfide deposit contains two zones of Zn-Pb mineralization bounded by faults of unknown displacement. The dominant hydrothermal minerals are marcasite, pyrite, sphalerite, quartz, and galena. The presence of hydrothermal pseudomorphs after barite, early pyrite resembling diagenetic pyrite associated with the barite body, and hydrothermal quartz and sphalerite filling voids formed by dissolution of carbonate all suggest that host sediment composition and origin was similar to that of the barite body prior to hydrothermal mineralization. Rhenium-osmium isochron ages of Ikalukrok mudstone (339.1 ± 8.3 Ma), diagenetic pyrite (333.0 ± 7.4 Ma), and hydrothermal pyrite (334.4 ± 5.3 Ma) at Anarraaq are all within uncertainty of one another and of an existing isochron age (~338 Ma) for the Main deposit in the Red Dog district. This indicates that the Anarraaq deposit formed soon after sedimentation and that hydrothermal activity was approximately synchronous in the district. The initial Os composition of the Anarraaq isochrons (0.375 ± 0.019–0.432 ± 0.025) is consistent with contemporaneous seawater, indicating that a mantle source was not involved in the hydrothermal system. This study highlights the underappreciated but important role of early, methane-driven diagenetic processes in the paragenesis of some CD deposits and has important implications for mineral exploration.

Microbial Communities Associated with Bentic Invertebrates of Lake Baikal.

The first results of a study into the microbiomes of benthic invertebrates found in sites with seeps (containing methane, oil, or a combination of methane and mud) and an underwater low-temperature vent of Lake Baikal are presented. Microorganisms were detected in the intestine of an oligochaete from the cold methane seep using microscopy. Analysis of 16S rRNA gene libraries revealed that the highest diversity of microorganisms was found in the nematode microbiomes where the members of 11 phyla were identified. Some of the detected prokaryotes are methanogens, nitrifiers, and nitrogen fixators, while some are involved in the sulfur cycle. Methanotrophs were detected in the microbiomes of oligochaetes and chironomids. The microbiomes of nematodes, chironomids, and bathynellids are composed of members of the Bacteroidetes and Firmicutes phyla, which are related to the symbiotic bacteria found in insects and animals from other ecotopes. Microorganisms typically found in the water and sediments of Lake Baikal were also detected in the invertebrates microbiomes.

Effectiveness of Fluorescent Viability Assays in Studies of Arctic Cold Seep Foraminifera

Highly negative δ13C values in fossil foraminifera from methane cold seeps have been proposed to reflect episodes of methane release from gas hydrate dissociation or free gas reservoirs triggered by climatic changes in the past. Because most studies on live foraminifera are based on the presence of Rose Bengal staining, that colors the cytoplasm of both live and recently dead individuals it remains unclear if, and to what extent live foraminifera incorporate methane-derived carbon during biomineralization, or whether the isotopic signature is mostly affected by authigenic overgrowth. In this paper, modern foraminiferal assemblages from a gas hydrate province Vestnesa Ridge (∼1,200 m water depth, northeastern Fram Strait) and from Storfjordrenna (∼400 m water depth in the western Barents Sea) is presented. By using the fluorescent viability assays CellTrackerTM Green (CTG) CMFDA and CellHunt Green (CHG) together with conventional Rose Bengal, it was possible to examine live and recently dead foraminifera separately. Metabolically active foraminifera were shown to inhabit methane-enriched sediments at both investigated locations. The benthic foraminiferal faunas were dominated by common Arctic species such as Melonis barleeanus, Cassidulina neoteretis, and Nonionellina labradorica. The combined usage of the fluorescence probe and Rose Bengal revealed only minor shifts in species compositions and differences in ratios between live and recently dead foraminifera from Storfjordrenna. There was no clear evidence that methane significantly affected the δ13C signature of the calcite of living specimens.

Octopodoidea as predators near the end of the Mesozoic Marine Revolution

Abstract Octopodoidea are a highly versatile and diverse group of marine predators comprising > 200 species today; however, their diversity and ecology in deep time are virtually unknown. Because these soft-bodied cephalopods have a low preservation potential, only a single body fossil species has been documented. Unlike other modern cephalopods, octopodoids leave behind a characteristic drill hole on their molluscan and crustacean prey. These traces provide a means to track their presence and behaviour in deep time. Although severely understudied, some of such holes have been documented from the Eocene–Pleistocene fossil record. We document the oldest recognized drill holes attributed to octopodoids, found in lucinid bivalves from the Late Cretaceous (Campanian) of South Dakota, USA. These observations demonstrate that the drilling habit of these animals evolved early in the evolutionary history of Octopodoidea, ~25 Myr earlier than was previously known. The drilled lucinids lived in cold methane seeps in the Western Interior Seaway. These predation traces have never been found in fossil seeps previously, thus adding a new predator to the food web of cold seeps. Finally, our results provide direct evidence that Octopodoidea were an integral component of the rise of shell-destroying predators during the Mesozoic Marine Revolution.

Methane Derived Authigenic Carbonate (MDAC) Aragonite Cemented Quaternary Hardground from a Methane Cold Seep, Rathlin Basin, Northern Ireland: δ13C and δ18O Isotopes, Environment, Porosity and Permeability

A block of sandstone retrieved by divers from near Rathlin Island, Co. Antrim, Northern Ireland, represents an aragonite cemented sand formed during the Quaternary. Strongly negative δ13C of the aragonite cement (−50 to −60‰ δ13C) indicates that the hardground was formed by the anaerobic oxidation of methane (AOM), resulting in the formation of a methane-derived authigenic carbonate (MDAC) hardground. Such hardgrounds have previously been recorded as forming extensive pavements in deeper waters in the mid Irish Sea (e.g., Croker Carbonate Slabs), although the latter also contains high-magnesium calcite. Sand was initially deposited as part of a storm lag deposit, with a reworked bivalve and gastropod fauna. This sand was then colonised by a probable crustacean fauna, producing horizontal open dwelling burrows (Thalassinoides). After aragonite cementation, the hardground was colonised by boring bivalves, with slightly negatively elevated levels of δ13C. Finally, the hardground was colonised by an encrusting fauna (bryozoans, calcareous algae and serpulids), by then in warmer seas. Continued depleted levels of δ13C present within the encrusting fauna (−1 to −5‰ δ13C) indicate continued methane generation and seepage, which may still be active to the present day, and to the possibility of shallow gas reserves. The δ18O values change between macro-infauna vs. encrusters, indicating a warming in water temperature, reflecting glacial and post-glacial environments. The aragonite cemented sandstone has a highly variable porosity, with large vugs (open burrows and borings), smaller mouldic porosity within gastropods and bivalves and complex micro-porosity associated with acicular aragonite cements. Overall permeability was recorded at the 2.5 to 23 Darcies level, reflecting the highly variable vuggy porosity, although matrix permeability was around 100 mD and controlled by the MDAC fabric. Actual permeability will likely be controlled by the extent to which larger pores are interconnected. The sea around the Rathlin Island area contains a diverse fauna, which is worthy of future study in the context of cold seep and MDAC pavement formation.

Methane seeps on the outer shelf of the Laptev Sea: characteristic features, structural control, and benthic fauna

Two areas with cold methane seeps on the outer shelf of the Laptev Sea were studied by two interdisciplinary expeditions onboard the RV Akademik Mstislav Keldysh in August–September of 2017 and 2018. These fields lie in water between depths of 63 and 73 m, and in a region of growing interest to the international community. Characteristic features of the methane seeps were obtained, which include their distribution and appearance on the seabed based on acoustic anomalies and seafloor observations. The cold seeps are part of a domain striking in a SW–NE direction along the Laptev Sea Rift System, Khatanga–Lomonosov Fracture Zone, and the Gakkel Ridge junction, and its structure was determined by shallow faults on the outer shelf. These faults are related to subsidence of the outer shelf cutting the caprock formed by permafrost and gas hydrates. Faults serve as conduits for an intense bubble methane discharge at the seabed. Shallow-water methane seep fauna were described for the first time in the Siberian Arctic. The frenulate siboglinid tubeworm Oligobrachia haakonmosbiensis was among the dominant species of the methane seep communities. A newly discovered gastropod species Frigidalvania sp. was also found in abundance at the seeps as well as an ophiuroid Ophiocten sericeum. Significant differences were observed between benthic communities of the two seep fields and background fauna including integral community parameters and the presence/absence of certain species. Development of shallow methane seep communities in the Laptev Sea apparently is related to extremely oligotrophic conditions in this area.

Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments

Cold seep environments are characterized by methane-rich fluid migration and discharge at the seafloor. These environments are also intimately linked to microbial communities, which oxidize methane anaerobically, increase alkalinity and promote authigenic carbonate precipitation. We have analyzed a suite of methane-derived authigenic carbonate (MDAC) crusts from the North and Barents Sea using stable and clumped isotopes (δ13C, δ18O, δ44Ca, and Δ47) to characterize the sources of fluids as well as the environment of carbonate authigenesis. We additionally assess the potential of MDACs as a Δ47-based paleotemperature archive.The MDACs occur as three main textural-mineralogic types: micritic Mg-calcite cements, micritic aragonite cements and cavity filling aragonite cements. We find that micritic Mg-calcite cements have low δ13CVPDB values (−30 to −47‰), high δ44CaSW values (−0.4 to −0.8‰), and Δ47-temperatures (0–6 °C) consistent with shallow sub-seafloor precipitation in isotopic equilibrium. Micritic aragonite cements and cavity filling aragonite cements both have a wider range in δ13CVPDB values (−18 to −58‰), lower δ44CaSW values (−0.8 to −1.6‰) and a larger range in Δ47-based apparent temperatures (–2 – 25 °C) with samples displaying equilibrium and disequilibrium clumped isotope values.The range in apparent temperatures as well as δ44CaSW values seen in the aragonite MDACs suggest two kinetic processes: a kinetic isotope effect (KIE) due to the incomplete equilibration of carbon and oxygen isotopes among DIC species from the different sources of DIC (i.e., seawater, methane-sourced DIC and DIC residual to CO2 degassing or diffusion) and a KIE due to a fast, irreversible precipitation affecting the cations, particularly Ca, bound to carbonate mineral. Our results improve the understanding of kinetic effects on clumped isotope temperatures in MDACs and demonstrate how the multi-isotopic approach combined with textural-mineralogic criteria can be used to identify MDACs for accurate paleotemperature reconstructions.