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Several acoustics systems for oceanography are presented. These include the Kongsberg 302 Multibeam Echosounder, the Knudsen Sub-bottom Profiler and Echosounder, and the Ultra-Short Baseline Navigation Systems.

Using deep-towed side-scan sonar (SSS) and sub-bottom profiler (SBP) data, we have investigated the hydrothermal activity around the Mienhua submarine volcano (MHV) at the northern margin of the southern Okinawa Trough. Our result reveals widespread acoustic transparent zones (TZs) in the shallow subsurface, which are generally interpreted as porous strata filled with hydrothermal fluid. These acoustic transparent zones exhibit lateral thinning and overpressure characteristics. Because the fluid channels are usually found beneath the thicker part of the transparent zones, a vertical migration of the hydrothermal fluid at depth and a horizontal migration or diffusion of the hydrothermal fluid in shallow high-porosity sediments are suggested. Our SBP profiles indicate multiple fluid channels vertically cutting through sedimentary layers and connecting with the upper TZs. The over-pressured hydrothermal fluid sometimes escapes from the seafloor and forms gas flares in the seawater column. The associated chimney structures (possibly black smokers) at the seafloor can be observed in the SSS imagery. The existence of the fluid channels, TZs, chimney features, and gas flares indicates an active and focused hydrothermal activity. However, the hydrothermal activity is vigorous in the eastern side of MHV. In contrast, the western and southern sides of the MHV display a waning or cessation of the hydrothermal activity, because of few chimney structures, seabed subsidence, and almost no gas flares. The hydrothermal activity in the MHV area reveals a temporal and spatial shift from west to east. By calculating the distribution of the mound-like or chimney structures from the SSS data, we estimate that the possibly hydrothermal mineralization area is approximately 2.2 km 2 . Our findings provide important insights into a submarine hydrothermal activity that is helpful for potential seabed mining in the MHV area. Highlights Upward migration of hydrothermal fluids through fluid channels around the MHV has caused acoustic transparent zones in high-porous sediments. The hydrothermal activity in the western MHV is no longer active and is linked to seabed subsidence. The hydrothermal activity is vigorously ongoing in the eastern MHV, as evidenced by a wide distribution of gas flares and related seafloor chimney-like structures.

This paper investigates the deep-sea morphology and sedimentary processes within the Santa Catarina Plateau and the Vema Channel, prominent features crucial for influencing bottom circulation in the western South Atlantic and regulating the exchange of deep and bottom waters between the Argentine and Brazil basins. The study leverages a unique dataset comprising multibeam data for generating detailed digital terrain models and sub-bottom profiling records, alongside numerical modeling of bottom current velocities (GLORYS12V1 reanalysis). Numerous sediment waves, predominantly cresting mudwaves and mass-wasting deposits associated with steeper sea floor gradients, were identified in the region, aligning with findings from the Argentinian and South Brazilian slopes. The investigation of these features contributes not only to the identification of contourites but also to the enhancement of understanding regarding deep-water circulation within the 3,500-to-4,500-meter range. This depth range serves as a critical transition zone between the Argentine and Brazil basins and plays a pivotal role in elucidating the northward distribution of Antarctic Bottom Water (AABW), an integral component of the Atlantic meridional circulation. In contrast to prior studies that focused on specific regions of the plateau, this research offers a comprehensive overview of the region’s morphology and sedimentary processes, thereby advancing comprehension of deep-water circulation dynamics between the Argentinian and Brazilian basins.

The DSDP holes 26/353 have been drilled in the central part of the Vema Fracture Zone (VFZ) to investigate the sediment infilling, however, the upper 96 m of the sediments were not sampled. Therefore, the age and character of the uppermost sediments at a water depth of > 5 km remain unknown. During the cruises 60 and 65 of the RV Akademik Ioffe in 2021 and 2023, respectively, the high-resolution sub-bottom profiling (SBP) and coring of the uppermost sediment cover at DSDP site 26/353 location have been conducted to address the sediment age and origin, as well as the mean sedimentation rates. The results of sub-bottom profiling demonstrate an alternation of acoustically transparent and stratified units reflecting cyclic changes in sedimentation mode. Grain size and microfossil data from three sediment cores, supplemented by the AMS-14C dating support an assumption on the contourite and turbidite-contourite origin of the upper transparent sediment layer, represented by terrigenous mud and underlying the postglacial mainly pelagic calcareous cap. Our results suggest the eastward transport of terrigenous, commonly turbiditic, sediments by the Antarctic Bottom Water (AABW) and their transformation into contourites along the Vema valley.

In 2021, E/V Nautilus expeditions covered broad areas of the northern and eastern Pacific, mapping the seafloor with the ship's multibeam sonar and sub-bottom profiler. Mapping provides the foundation for expeditions by modeling seafloor geomorphology in advance of exploratory ROV dives. New mapping data also contribute to a primary goal of the National Strategy for Mapping, Exploring, and Characterizing the United States Exclusive Economic Zone (NOMEC, June 2020), add to the Global Multi-Resolution Topography (GMRT) data synthesis, and support the broader global goals of producing a complete global seabed model under the Nippon FoundationGEBCO Seabed 2030 project (SB2030). The 2021 Nautilus mapping resulted in a total mapped area of 107,246 km2 of seafloor, including 80,829 km2 within the US Exclusive Economic Zone (EEZ), along 22,904 km of trackline.

Reservoir sediments sequester significant amounts of organic carbon (OC), but at the same time, high amounts of methane (CH4) can be produced and emitted during the degradation of sediment OC. While the greenhouse gas emission of reservoirs has received a lot of attention, there is a lack of studies focusing on OC burial. In particular, there are no studies on reservoir OC burial in the Amazon, even though hydropower is expanding in the basin. Here we present results from the first investigation of OC burial and CH4 concentrations in the sediments of an Amazonian hydroelectric reservoir. We performed sub-bottom profiling, sediment coring and sediment pore water analysis in the Curuá Una (CUN) reservoir (Amazon, Brazil) during rising- and falling-water periods. The spatially resolved average sediment accumulation rate was 0.6 cm yr−1, and the average OC burial rate was 91 g C m−2 yr−1. This is the highest OC burial rate on record for low-latitude hydroelectric reservoirs. Such a high rate probably results from a high OC deposition onto the sediment, which compensates the high OC mineralization at a 28–30 ∘C water temperature. Elevated OC burial was found near the dam and close to major river inflow areas. C:N ratios between 10.3 and 17 (average ± SD: 12.9±2.1) suggest that both land-derived and aquatic OC accumulate in CUN sediments. About 23 % of the sediment pore water samples had dissolved CH4 above the saturation concentration. This represents a higher share than in other hydroelectric reservoirs, indicating a high potential for CH4 ebullition, particularly in river inflow areas.

This study applies three classification methods exploiting the angular dependence of acoustic seafloor backscatter along with high resolution sub-bottom profiling for seafloor sediment characterization in the Eckernförde Bay, Baltic Sea Germany. This area is well suited for acoustic backscatter studies due to its shallowness, its smooth bathymetry and the presence of a wide range of sediment types. Backscatter data were acquired using a Seabeam1180 (180 kHz) multibeam echosounder and sub-bottom profiler data were recorded using a SES-2000 parametric sonar transmitting 6 and 12 kHz. The high density of seafloor soundings allowed extracting backscatter layers for five beam angles over a large part of the surveyed area. A Bayesian probability method was employed for sediment classification based on the backscatter variability at a single incidence angle, whereas Maximum Likelihood Classification (MLC) and Principal Components Analysis (PCA) were applied to the multi-angle layers. The Bayesian approach was used for identifying the optimum number of acoustic classes because cluster validation is carried out prior to class assignment and class outputs are ordinal categorical values. The method is based on the principle that backscatter values from a single incidence angle express a normal distribution for a particular sediment type. The resulting Bayesian classes were well correlated to median grain sizes and the percentage of coarse material. The MLC method uses angular response information from five layers of training areas extracted from the Bayesian classification map. The subsequent PCA analysis is based on the transformation of these five layers into two principal components that comprise most of the data variability. These principal components were clustered in five classes after running an external cluster validation test. In general both methods MLC and PCA, separated the various sediment types effectively, showing good agreement (kappa >0.7) with the Bayesian approach which also correlates well with ground truth data (r2 > 0.7). In addition, sub-bottom data were used in conjunction with the Bayesian classification results to characterize acoustic classes with respect to their geological and stratigraphic interpretation. The joined interpretation of seafloor and sub-seafloor data sets proved to be an efficient approach for a better understanding of seafloor backscatter patchiness and to discriminate acoustically similar classes in different geological/bathymetric settings.

Abstract Widespread indications of free and hydrate gas accumulations and mud volcanoes were imaged using multichannel seismic reflection, chirp sub-bottom profiler, multibeam bathymetry, and deep-towed side-scan sonar data collected along the Southwestern Black Sea margin, offshore Akçakoca. These indications are typically associated with sedimentary ridges along the continental slope and rise, particularly with the “Ereğli Plateau” (850–1350 m water depth), where 20 mud volcanoes were found. Two types of bottom-simulating reflections (BSR) were identified, both mimicking the seafloor relief: Type-1 crosscuts the sedimentary reflections with amplitudes similar to the surrounding strata, while Type-2 shows higher amplitudes that terminate against the base of the gas hydrate stability zone. These types were observed over large portions of the continental rise, indicating the base of gas hydrate accumulations. Analyses of the BSR depth indicate that thermogenic gas is possibly present in the gas hydrates. A fault-driven hypothetical model was then developed to describe the formation of gas hydrate and mud volcanoes and the effect of submarine fluid flow in the area. According to this model, Type-1 BSRs form through biogenic gas accumulations. The presence of fault regions with Type-2 BSRs suggests active fluid transfer between permeable and impermeable units, where the fault surfaces act as possible conduits for thermogenic gases produced in the deeper sediments and transported into the shallower subsurface where then thermogenic biogenic gases coexist.

Acoustic mapping enables an understanding of the surface distribution of shallow gas hydrate (GH) and related products. Acoustically characteristic materials such as fluid-seepage-related methane-derived authigenic carbonate and/or shallow GHs, may be widely distributed beneath the shallow seafloor of the Sakata Knoll. High-amplitude reflectors over the knoll are the top of gas-bearing permeable layers and connect to the reverse fault at the foot of the knoll. Shallow GH and bacterial mats were observed at the high-amplitude layer cut by depression and/or the locally disturbed seafloor. Acoustic blanking zones observed on the sub-bottom profiler sections are current gas migration routes from the depth to the seafloor. Optical observations indicate that fluid seepage is not active in the current seafloor, and it is not necessarily observed above the acoustic blanking zones or shallow faults reaching the seafloor. In the Sakata Knoll, the tectonically formed reverse fault and gas-bearing permeable layers play more important roles in fluid migration from depth to the summit area of the knoll compared to acoustic blanking and shallow faults. The depression at the summit area of the Sakata Knoll was formed by the dissociation of a shallow GH at around the last glacial maximum. Limited fluid seepage is currently witnessed within and around the depression and it is less extensive than that in the past. Such knolls, with tectonically formed large faults and an anticline are abundant in the area and they can be good reservoirs for shallow GH along the eastern margin of the Sea of Japan.

This paper provides information on the integrated geological, geophysical and hydrobiological investigations, as well as on passing observations on cetaceans in the Eastern Atlantic (cruise 66 of the R/V  Akademik Ioffe ) in June–July 2024. The preliminary scientific results are discussed.