Search

Bottom sediments at methane discharge sites of the Laptev Sea shelf were investigated. The rates of microbial methanogenesis and methane oxidation were measured, and the communities responsible for these processes were analyzed. Methane content in the sediments varied from 0.9 to 37 µmol CH4 dm−3. Methane carbon isotopic composition (δ13C-CH4) varied from −98.9 to −77.6‰, indicating its biogenic origin. The rates of hydrogenotrophic methanogenesis were low (0.4–5.0 nmol dm−3 day−1). Methane oxidation rates varied from 0.4 to 1.2 µmol dm−3 day−1 at the seep stations. Four lineages of anaerobic methanotrophic archaea (ANME) (1, 2a–2b, 2c, and 3) were found in the deeper sediments at the seep stations along with sulfate-reducing Desulfobacteriota. The ANME-2a-2b clade was predominant among ANME. Aerobic ammonium-oxidizing Crenarchaeota (family Nitrosopumilaceae) predominated in the upper sediments along with heterotrophic Actinobacteriota and Bacteroidota, and mehtanotrophs of the classes Alphaproteobacteria (Methyloceanibacter) and Gammaproteobacteria (families Methylophilaceae and Methylomonadaceae). Members of the genera Sulfurovum and Sulfurimonas occurred in the sediments of the seep stations. Mehtanotrophs of the classes Alphaproteobacteria (Methyloceanibacter) and Gammaproteobacteria (families Methylophilaceae and Methylomonadaceae) occurred in the sediments of all stations. The microbial community composition was similar to that of methane seep sediments from geographically remote areas of the global ocean. [ABSTRACT FROM AUTHOR]

Bottom sediments at methane discharge sites of the Laptev Sea shelf were investigated. The rates of microbial methanogenesis and methane oxidation were measured, and the communities responsible for these processes were analyzed. Methane content in the sediments varied from 0.9 to 37 µmol CH4 dm−3. Methane carbon isotopic composition (δ13C-CH4) varied from −98.9 to −77.6‰, indicating its biogenic origin. The rates of hydrogenotrophic methanogenesis were low (0.4–5.0 nmol dm−3 day−1). Methane oxidation rates varied from 0.4 to 1.2 µmol dm−3 day−1 at the seep stations. Four lineages of anaerobic methanotrophic archaea (ANME) (1, 2a–2b, 2c, and 3) were found in the deeper sediments at the seep stations along with sulfate-reducing Desulfobacteriota. The ANME-2a-2b clade was predominant among ANME. Aerobic ammonium-oxidizing Crenarchaeota (family Nitrosopumilaceae) predominated in the upper sediments along with heterotrophic Actinobacteriota and Bacteroidota, and mehtanotrophs of the classes Alphaproteobacteria (Methyloceanibacter) and Gammaproteobacteria (families Methylophilaceae and Methylomonadaceae). Members of the genera Sulfurovum and Sulfurimonas occurred in the sediments of the seep stations. Mehtanotrophs of the classes Alphaproteobacteria (Methyloceanibacter) and Gammaproteobacteria (families Methylophilaceae and Methylomonadaceae) occurred in the sediments of all stations. The microbial community composition was similar to that of methane seep sediments from geographically remote areas of the global ocean.

Abstract Bubble emission mechanisms from submerged large igneous provinces remains enigmatic. The Kerguelen Plateau, a large igneous province in the southern Indian Ocean, has a long sustained history of active volcanism and glacial/interglacial cycles of sedimentation, both of which may cause seafloor bubble production. We present the results of hydroacoustic flare observations around the underexplored volcanically active Heard Island and McDonald Islands on the Central Kerguelen Plateau. Flares were observed with a split‐beam echosounder and characterized using multifrequency decibel differencing. Deep‐tow camera footage, water properties, water column δ3He, subbottom profile, and sediment δ13C and δ34S data were analyzed to consider flare mechanisms. Excess δ3He near McDonald Islands seeps, indicating mantle‐derived input, suggests proximal hydrothermal activity; McDonald Islands flares may thus indicate CO2, methane, and other minor gas bubbles associated with shallow diffuse hydrothermal venting. The Heard Island seep environment, with subbottom acoustic blanking in thick sediment, muted 3He signal, and δ13C and δ34S fractionation factors, suggest that Heard Island seeps may either be methane gas (possibly both shallow biogenic methane and deeper‐sourced thermogenic methane related to geothermal heat from onshore volcanism) or a combination of methane and CO2, such as seen in sediment‐hosted geothermal systems. These data provide the first evidence of submarine gas escape on the Central Kerguelen Plateau and expand our understanding of seafloor processes and carbon cycling in the data‐poor southern Indian Ocean. Extensive sedimentation of the Kerguelen Plateau and additional zones of submarine volcanic activity mean additional seeps or vents may lie outside the small survey area proximal to the islands.

Bubble emission mechanisms from submerged large igneous provinces remains enigmatic. The Kerguelen Plateau, a large igneous province in the southern Indian Ocean, has a long sustained history of active volcanism and glacial/interglacial cycles of sedimentation, both of which may cause seafloor bubble production. We present the results of hydroacoustic flare observations around the underexplored volcanically active Heard Island and McDonald Islands on the Central Kerguelen Plateau. Flares were observed with a split‐beam echosounder and characterized using multifrequency decibel differencing. Deep‐tow camera footage, water properties, water column δ3He, subbottom profile, and sediment δ13C and δ34S data were analyzed to consider flare mechanisms. Excess δ3He near McDonald Islands seeps, indicating mantle‐derived input, suggests proximal hydrothermal activity; McDonald Islands flares may thus indicate CO2, methane, and other minor gas bubbles associated with shallow diffuse hydrothermal venting. The Heard Island seep environment, with subbottom acoustic blanking in thick sediment, muted 3He signal, and δ13C and δ34S fractionation factors, suggest that Heard Island seeps may either be methane gas (possibly both shallow biogenic methane and deeper‐sourced thermogenic methane related to geothermal heat from onshore volcanism) or a combination of methane and CO2, such as seen in sediment‐hosted geothermal systems. These data provide the first evidence of submarine gas escape on the Central Kerguelen Plateau and expand our understanding of seafloor processes and carbon cycling in the data‐poor southern Indian Ocean. Extensive sedimentation of the Kerguelen Plateau and additional zones of submarine volcanic activity mean additional seeps or vents may lie outside the small survey area proximal to the islands. Plain Language Summary: Bubbles are constantly escaping the seafloor, either from thick sediments with gas‐producing bacteria or from volcanic gases escaping rocks deep beneath the seafloor. We use sonar to observe bubbles that appear as "hydroacoustic flares" as they travel from the seafloor into the ocean. Our paper shows the results of the 2016 RV Investigator voyage to Heard and McDonald Islands, two of Australia's most remote islands and home to its only active volcanoes. We observed hundreds of flares around the two islands and were able to examine bubbles in some of our camera footage. By examining the seafloor, the shallow subseafloor, and the chemistry of sediments and the water column, we suggest the bubbles may be composed of different gases. Around Heard Island, bubbles were associated with thick sediment that are likely rich with bacteria that produce methane gas; near the McDonald Islands, bubbles were not associated with sediments and possibly indicate shallow volcanic activity emitting gases, like CO2, originating from the mantle. Knowing where bubbles are escaping is crucial for understanding how much gas is contributed from the seafloor into the ocean and atmosphere. Key Points: Active submarine gas emission around Heard and McDonald Islands on the Central Kerguelen Plateau in the southern Indian OceanGas release near the McDonald Islands possibly shallow diffuse hydrothermal venting, uncommon on an intraplate large igneous provinceGas release near Heard Island either or both a biogenic orthermogenic methane seep, the second reported in the Southern Ocean to date [ABSTRACT FROM AUTHOR]

An area of cold methane seeps at the bottom of the Laptev Sea was investigated. High rates of methane oxidation were revealed in the sediments and in the water column. Anaerobic methane oxidation carried out by the ANME-2 a/b consortium was coupled to sulfate reduction. Bacteria of the genera Sulfurovum and Arcobacter were the agents of the sulfur cycle. Methane unconsumed in the sediments diffused into the near-bottom water, where it was oxidized by methanotrophic bacteria. Methanotrophic activity was essential for development of symbiotrophic tubeworms of the upper sediment layers, which were responsible for the process of bioturbation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]