Differentiation of communities of macroinvertebrates and cottoid fish associated with methane seeps of different bottom landscapes of Lake Baikal

The paper presents the results of the study of communities of macroinvertebrates and cottoid fish inhabiting methane seeps of Lake Baikal. For the analysis, we used video surveillance and collection of animals carried out with the help of "Mir" deep-water submersible, as well as NIOZ-type box-corer samplers from the board of a research vessel. Posolskaya Bank and Saint Petersburg methane seeps are located in different basins (southern and middle) and at different depths (300–500 m and ~ 1400 m), characterized by the different underwater landscapes (slope of underwater upland and hills formed by gas hydrates), by the structure of gas hydrates and their depth location in sediments, as well as the composition of microbial mats and communities of microorganisms of bottom sediments. Both seeps are characterized by bubble discharge of methane gas and the formation of highly productive communities of large invertebrates and cottoid fish on seep habitats. Seep animal communities consisted of species-depleted invertebrates and fish of the surrounding deep-water benthal of the Lake. We showed the similarities and differences in the composition of the faunas of two seeps, as well as the quantitative characteristics of taxonomic groups of macroinvertebrates and cottoid fishes. Obligate species have not been revealed on the methane seep Posolskaya Bank. For the methane seep Saint Petersburg, the gastropod species Kobeltocochlea tamarae Sitnikova, Teterina et Maximova, 2021 (Caenogastropoda: Benedictiidae) was designated as an obligate species; among bottom cottoid fishes, Neocottus werestschagini (Taliev, 1953) (Cottoidei: Abyssocottidae) had possible a transitional state to obligate. We presented the data on the assimilation by seep animals of mixed photo- and chemosynthetic food with different proportions of methane-derived carbon. A hypothesis has been substantiated that deep-water seep areas could serve as refugium for the preservation of endemic fauna during the Pliocene-Pleistocene glaciations of Lake Baikal.

A new approach to processing and imaging multibeam water column echosounder data: application to a complex methane seep on the southern Cascadia margin

Acoustic echosounding systems are increasingly used to image water-column backscatter in addition to mapping the seafloor. We have imaged an acoustic flare generated by methane bubbles emanating from a vent sourced at 1840 m water depth offshore northern California using a shipboard Kongsberg EM122. Data include five transits over the flare and approximately 11 h of continuous observation when the ship held station. Shipboard observations showed a strong flare splitting into multiple smaller, intermittent flares at a water depth of 800–1200 m and pronounced temporal variability. We introduce a new approach to processing the data in which we correct the backscatter data for ship motion and bin the data into voxels with dimensions of 20 m in X and Y and 40 m in Z for a transit over the flare and into vertical slices with dimensions of 15 m in X and Z and 4 min in time when the ship was stationary. The processed data indicate that the signal is dominated by bubbles emanating from a source region with a diameter of approximately 40 m located on the southern edge of what is likely a ring of sources with a diameter of approximately 600 m. When the ship was stationary, we were able to track an individual pulse rising at a rate of 8–10 m/min. Our results illustrate the limitations of monitoring temporal variation in gas flux using multibeam echosounders because of the trade-off between imaging the entire flare by averaging over tens of minutes to hours and observing a slice through the flare to capture short-lived pulses of gas expulsion. Nevertheless, because echosounders are widely available, they can continue to provide valuable data on the spatial and temporal distribution of gas emissions on continental margins that can be used to frame hypotheses and plan more comprehensive follow-up experiments.

A Multi-Frequency Acoustic Investigation of Seafloor Methane Seep Sites at Omakere Ridge, Hikurangi Margin, New Zealand

<p>Omakere Ridge is an anticlinal thrust ridge in water depths of 1100–1700mon the Hikurangi Margin, east of the North Island of New Zealand, and is an area of active seafloor methane seepage associated with an extensive gas hydrate province. Methane seep sites on the Hikurangi Margin are characterised by localised buildups of authigenic carbonate and chemosynthetic seep fauna that exist on a seafloor otherwise characterised by soft, muddy sediments and provide a unique window into the workings of the gas hydrate system. Seafloor methane seeps sites on Omakere Ridge have been successfully imaged using three newly-acquired acoustic datasets: a P-CableTM high-resolution 3D seismic reflection dataset (60 Hz); a multibeam sonar backscatter dataset (12 kHz); and a ParasoundTM subbottom profiler dataset (4 kHz). Seafloor seismic amplitude and similarity maps have been derived from a preliminary shipboard post-stack migrated data cube. A pronounced acquisition artifact is manifest in the seafloor horizon slice as high- and low-amplitude stripes that alternate periodically in the crossline direction. This artifact has been removed from the seafloor horizon slice using 2D spatial frequency filtering, followed by direct sampling and stochastic removal of the very-low-frequency components in the spatial domain. The seismic amplitude map has then been transformed into a calibrated seafloor reflection coefficient map. Sonar backscatter mosaics have been created after correcting for beam pattern effects and angular variation in backscatter after taking into account the bathymetry. Several backscatter mosaics were incorporated into a stacked mosaic over the study area to attenuate random noise. The ParasoundTM sub-bottom profiler data were processed to display instantaneous amplitude and separated into 43 lines over the study area. Comparison of 3D seismic attributes, multibeam backscatter intensity and shallow subsurface reflection characteristics provides new insights into the previously unknown extent of authigenic carbonate build-ups, methane migration pathways and seep initiation mechanisms at five seep sites on Omakere Ridge. Areas of high seafloor 3D seismic reflection coefficient and high multibeam backscatter intensity are interpreted as carbonate formations of at least 6–7 m thickness, while areas exhibiting low seismic reflection coefficient and moderate/high sonar backscatter intensity are interpreted as areas where the carbonates are less developed. Anomalous high-amplitude subsurface reflections beneath the seeps in the ParasoundTM data are interpreted as buried carbonates and may indicate a previously unknown earlier phase of seepage at Omakere Ridge, but could also be caused by gas or gas hydrates. The extent of authigenic carbonates is directly related to the duration of seepage and thus provides a new proxy for the chronology of seepage at Omakere Ridge, which has proved consistent with an existing hypothesis based on the abundance of deceased and live chemosynthetic fauna at the seep sites.</p>

A Multi-Frequency Acoustic Investigation of Seafloor Methane Seep Sites at Omakere Ridge, Hikurangi Margin, New Zealand

<p>Omakere Ridge is an anticlinal thrust ridge in water depths of 1100–1700mon the Hikurangi Margin, east of the North Island of New Zealand, and is an area of active seafloor methane seepage associated with an extensive gas hydrate province. Methane seep sites on the Hikurangi Margin are characterised by localised buildups of authigenic carbonate and chemosynthetic seep fauna that exist on a seafloor otherwise characterised by soft, muddy sediments and provide a unique window into the workings of the gas hydrate system. Seafloor methane seeps sites on Omakere Ridge have been successfully imaged using three newly-acquired acoustic datasets: a P-CableTM high-resolution 3D seismic reflection dataset (60 Hz); a multibeam sonar backscatter dataset (12 kHz); and a ParasoundTM subbottom profiler dataset (4 kHz). Seafloor seismic amplitude and similarity maps have been derived from a preliminary shipboard post-stack migrated data cube. A pronounced acquisition artifact is manifest in the seafloor horizon slice as high- and low-amplitude stripes that alternate periodically in the crossline direction. This artifact has been removed from the seafloor horizon slice using 2D spatial frequency filtering, followed by direct sampling and stochastic removal of the very-low-frequency components in the spatial domain. The seismic amplitude map has then been transformed into a calibrated seafloor reflection coefficient map. Sonar backscatter mosaics have been created after correcting for beam pattern effects and angular variation in backscatter after taking into account the bathymetry. Several backscatter mosaics were incorporated into a stacked mosaic over the study area to attenuate random noise. The ParasoundTM sub-bottom profiler data were processed to display instantaneous amplitude and separated into 43 lines over the study area. Comparison of 3D seismic attributes, multibeam backscatter intensity and shallow subsurface reflection characteristics provides new insights into the previously unknown extent of authigenic carbonate build-ups, methane migration pathways and seep initiation mechanisms at five seep sites on Omakere Ridge. Areas of high seafloor 3D seismic reflection coefficient and high multibeam backscatter intensity are interpreted as carbonate formations of at least 6–7 m thickness, while areas exhibiting low seismic reflection coefficient and moderate/high sonar backscatter intensity are interpreted as areas where the carbonates are less developed. Anomalous high-amplitude subsurface reflections beneath the seeps in the ParasoundTM data are interpreted as buried carbonates and may indicate a previously unknown earlier phase of seepage at Omakere Ridge, but could also be caused by gas or gas hydrates. The extent of authigenic carbonates is directly related to the duration of seepage and thus provides a new proxy for the chronology of seepage at Omakere Ridge, which has proved consistent with an existing hypothesis based on the abundance of deceased and live chemosynthetic fauna at the seep sites.</p>

Autonomous methane seep site monitoring offshore Western Svalbard: Hourly to seasonal variability and associated oceanographic parameters

Improved quantification techniques of natural sources is needed to explain variations in atmospheric methane. In polar regions, high uncertainties in current estimates of methane release from the seabed remain. We present two unique 10 and 3 months long time-series of bottom water measurements of physical and chemical parameters from two autonomous ocean observatories deployed at separate intense seabed methane seep sites (91 and 246 m depth) offshore Western Svalbard from 2015 to 2016. Results show high short term (100–1000 nmol L-1 within hours) and seasonal variation, as well as higher (2–7 times) methane concentrations compared to previous measurements. Rapid variability is explained by uneven distribution of seepage and changing ocean current directions. No overt influence of tidal hydrostatic pressure or water temperature variations on methane concentration was observed, but an observed negative correlation with temperature at the 246 site fits with hypothesized seasonal blocking of lateral methane pathways in the sediments. Negative correlation between bottom water methane concentration/variability and wind forcing, concomitant with signs of weaker water column stratification, indicates increased potential for methane release to the atmosphere in fall/winter. We highlight uncertainties in methane inventory estimates based on discrete water sampling and present new information about short- and long-term methane variability which can help constrain future estimates of seabed methane seepage.

Relationships between biodiversity and ecosystem functioning proxies strengthen when approaching chemosynthetic deep-sea methane seeps

As biodiversity loss accelerates globally, understanding environmental influence over biodiversity–ecosystem functioning (BEF) relationships becomes crucial for ecosystem management. Theory suggests that resource supply affects the shape of BEF relationships, but this awaits detailed investigation in marine ecosystems. Here, we use deep-sea chemosynthetic methane seeps and surrounding sediments as natural laboratories in which to contrast relationships between BEF proxies along with a gradient of trophic resource availability (higher resource methane seep, to lower resource photosynthetically fuelled deep-sea habitats). We determined sediment fauna taxonomic and functional trait biodiversity, and quantified bioturbation potential (BPc), calcification degree, standing stock and density as ecosystem functioning proxies. Relationships were strongly unimodal in chemosynthetic seep habitats, but were undetectable in transitional ‘chemotone’ habitats and photosynthetically dependent deep-sea habitats. In seep habitats, ecosystem functioning proxies peaked below maximum biodiversity, perhaps suggesting that a small number of specialized species are important in shaping this relationship. This suggests that absolute biodiversity is not a good metric of ecosystem ‘value’ at methane seeps, and that these deep-sea environments may require special management to maintain ecosystem functioning under human disturbance. We promote further investigation of BEF relationships in non-traditional resource environments and emphasize that deep-sea conservation should consider ‘functioning hotspots' alongside biodiversity hotspots.