Beschreibung: 31.05. - 10.07.2022, Mindelo - Pt. Delgada 2. Wochenbericht 13. - 19.06.2022

Beschreibung: 31.05. - 10.07.2022, Mindelo - Pt. Delgada 2. Wochenbericht 06. - 12.06.2022

Beschreibung: 31.05. - 10.07.2022, Mindelo - Pt. Delgada 4th Weekly Report 20. - 26.06.2022

Beschreibung: In 1948, Le Danois reported for the first time the occurrence of living cold-water coral reefs, the so-called “massifs coralliens”, along the European Atlantic continental margin. In 2008, a cruise with R/V Belgica was set out to re-investigate these cold-water corals in the Penmarc'h and Guilvinec Canyons along the Gascogne margin of the Bay of Biscay. During this cruise, an area of 560 km2 was studied using multibeam swath bathymetry, CTD casts, ROV observations and USBL-guided boxcoring. Based on the multibeam data and the ROV video imagery, two different cold-water coral reef settings were distinguished. In water depths ranging from 260 to 350 m, mini mounds up to 5 m high, covered by dead cold-water coral rubble, were observed. In between these mounds, soft sediment with a patchy distribution of gravel was recognised. The second setting (350–950 m) features hard substrates with cracks, spurs, cliffs and overhangs. In water depths of 700 to 950 m, both living and dead cold-water corals occur. Occasionally, they form dense coral patches with a diameter of about 10–60 m, characterised by mostly stacked dead coral rubble and a few living specimens. U/Th datings indicate a shift in cold-water coral growth after the Late Glacial Maximum (about 11.5 ka BP) from shallow to deep-water settings. The living cold-water corals from the deeper area occur in a water density (sigma–theta) of 27.35–27.55 kg m− 3, suggested to be a prerequisite for the growth and distribution of cold-water coral reefs along the northern Atlantic margin. In contrast, the dead cold-water coral fragments in the shallow area occur in a density range of 27.15–27.20 kg m− 3 which is slightly outside the density range where living cold-water corals normally occur. The presented data suggest that this prerequisite is also valid for coral growth in the deeper canyons (〉 350 m) in the Bay of Biscay.

Beschreibung: We evaluate different hypotheses concerning the formation of a peculiar, flat-topped ridge at Rock Garden, offshore of the North Island of New Zealand. The coincidence of the ridge bathymetry with the depth at which gas hydrate stability intersects the seafloor has been previously used to propose that processes at the top of gas hydrate stability may cause seafloor erosion, giving rise to the flat ridge morphology. Two mechanisms that lead to increased fluid pressure (and sediment weakening) have previously been proposed: (1) periodic formation (association) and dissociation of gas hydrates during seafloor temperature fluctuations; and (2) dissociation of gas hydrates at the base of gas hydrate stability during ridge uplift. We use numerical models to test these hypotheses, as well as to evaluate whether the ridge morphology can develop by tectonic deformation during subduction of a seamount, without any involvement from gas hydrates. We apply a commonly-used 1D approach to model gas hydrate formation and dissociation, and develop a 2D mechanical model to evaluate tectonic deformation. Our results indicate that: (1) Tectonics (subduction of a seamount) may cause a temporary flat ridge morphology to develop, but this evolves over time and is unlikely to provide the main explanation for the ridge morphology; (2) Where high methane flux overwhelms the anaerobic oxidation of methane via sulphate reduction near the seafloor, short-period temperature fluctuations (but on timescales of years, not months as proposed originally) in the bottom water can lead to periodic association and dissociation of a small percentage of gas hydrate in the top of the sediment column. However, the effect of this on sediment strength is likely to be small, as evidenced by the negligible change in computed effective pressure; (3) The most likely mechanism to cause sediment weakening, leading to seafloor erosion, results from the interaction of gas hydrate stability with tectonic uplift of the ridge, provided bulk permeability strongly decreases with increasing hydrate content. Rather than overpressure developing from dissociation of hydrates at the base of gas hydrate stability (as previously thought), we found that the weakening is caused by focusing of gas hydrate formation at shallow sediment levels. This creates large fluid pressures and can lead to negative effective pressures near the seafloor, reducing the sediment strength.

Beschreibung: Methane concentrations and carbon stable isotopic ratios (δ13CCH4), oxygen concentrations and hydrographic parameters (CTD) were investigated in the water column of the north-western Black Sea during the summers of 2003 and 2004. Water samples were collected along a transect which crosses three methane seep areas in 90, 220, and 600 m water depth. These active seeps strongly influence methane distribution in the overlying anoxic and oxic water column. Methane concentration and stable isotope (δ13CCH4) patterns indicate that water column stratification and microbial methane oxidation efficiently hamper the transfer of methane to the sea surface. Only the shelf seep site in 90 m water depth acts as a direct source of atmospheric methane. Microbial methane oxidation and/or gas stripping seem to cause oxygen depletion above the two shallower seep areas. The methane flux from the 90 m site into the water column is estimated to have been 0.599 × 106 mol yr− 1 (9.6 t yr− 1) in 2003 and 0.347 × 106 mol yr− 1 (5.6 t yr− 1) in 2004. Comparison of results from the two years shows different water column methane inventories in the deep part of the transect, implying a variable methane source strength at the 600 m deep site. The flux from this area is estimated to have been 11.35 × 106 mol yr− 1 in 2003.

Beschreibung: The relation between acoustic sea-floor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single-beam echosounder. This study is further supported by side-scan sonar recordings, high-resolution 5 kHz seismic data, pore-water analysis, grain-size analysis and visual sea-floor observations. The datasets were acquired during the 2003 and 2004 expeditions of the EC-funded CRIMEA project in the Dnepr paleo-delta area, northwestern Black Sea. More than 600 active methane seeps were hydroacoustically detected within a small (3.96 km by 3.72 km) area on the continental shelf of the Dnepr paleo-delta in water depths ranging from − 72 m to − 156 m. Multibeam and side-scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area. Seeps generally occur within medium- to high-backscatter areas which often coincide with pockmarks. High-resolution seismic data reveals the presence of an undulating gas front, i.e. the top of the free gas in the subsurface, which domes up towards and intersects the sea floor at locations where gas seeps and medium- to high-backscatter values are detected. Pore-water analysis of 4 multi-cores, taken at different backscatter intensity sites, shows a clear correlation between backscatter intensity and dissolved methane fluxes. All analyzed chemical species indicate increasing anaerobic oxidation of methane (AOM) from medium- to high-backscatter locations. This is confirmed by visual sea-floor observations, showing bacterial mats and authigenic carbonates formed by AOM. Grain-size analysis of the 4 multi-cores only reveals negligible variations between the different backscatter sites. Integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane-derived authigenic carbonates (MDACs) caused by AOM. The carbonate formation also appears to lead to a gradual (self)-sealing of the seeps by cementing fluid pathways/horizons followed by a relocation of the bubble-releasing locations.

Beschreibung: An ancient hydrocarbon seep province of 14 isolated, authigenic carbonate deposits has been identified in fine-grained, deep-marine siliciclastic strata of the Miocene East Coast Basin, North Island, New Zealand. These forearc sediments have been uplifted and complexly deformed into accretionary ridges, adjacent to the still-active Hikurangi convergent margin. Older active and passive margin strata (mid-Cretaceous to Oligocene in age) underlie the Neogene sequence, and contain oil- and gas-prone source rocks. Older Mesozoic meta-sedimentary rocks constitute the backstop against which the current phase of subduction-related sedimentation has accumulated (~ 24 Ma–present). The seep-carbonates (up to 10 m thick, 200 m across) archive methane signatures in their depleted carbon isotopes (to δ13C –51.7‰ PDB), and contain chemosynthesis-based paleocommunities (e.g. worm tubes, bathymodioline mussels, and vesicomyid, lucinid and thyasirid bivalves) typical of other Cenozoic and modern seeps. Northern and southern sites are geographically separated, and exhibit distinct lithological and faunal differences. Structural settings are variable. Seep-associated lithologies also are varied, and suggest carbonate development in sub-seafloor, seafloor and physically reworked (diapiric expansion, gas explosion, gravity slide or debris flow) settings, similar to Italian Apennine seep deposits of overlapping ages. Peculiar attributes of the New Zealand Miocene seep deposits are several, including digitate thrombolites of clotted microbial micrite encased in thick, isopachous horizons and botryoids of aragonite. Seep plumbing features are also well-exposed at some sites, displaying probable gas-explosion breccias filled with aragonite, tubular concretions (fluid conduits), and carbonate-cemented, thin sandstone beds and burrows within otherwise impermeable mudstones. A few seeps were large enough to develop talus-debris piles on their flanks, which were populated by lucinid bivalves and terebratulid brachiopods. Firmgrounds and hardgrounds were common, as evidenced by trace fossil associations or caryophyllid coral thickets atop some seep-carbonate deposits. Thus, the New Zealand examples show strong evidence of formation in sediments at or just beneath the seafloor, but some were clearly exhumed by erosion to sustain later non-seep, epifaunal and boring paleocommunities.

Beschreibung: Extensive ROV-based sampling and exploration of the seafloor was conducted along an eroded transform-parallel fault scarp on the northeastern side of the Guaymas Basin in the Gulf of California to observe the nature of fluids venting from the seafloor, measure the record left by methane-venting on the carbonates from this area, and determine the association with gas hydrate. One gas vent vigorous enough to generate a water-column gas plume traceable for over 800 m above the seafloor was found to emanate from a ∼10-cm-wide orifice on the eroded scarp face. Sediment temperature measurements and topography on a sub-bottom reflector recorded in a transform-parallel seismic reflection profile identified a subsurface thermal anomaly beneath the gas vent. Active chemosynthetic biological communities (CBCs) and extensive authigenic carbonates that coalesce into distinct chemoherm structures were encountered elsewhere along the eroded transform-parallel scarp. The carbon isotopic composition of methane bubbles flowing vigorously from the gas vent (−53.6±0.8‰ PDB) is comparable to methane found in sediment cores taken within the CBCs distributed along the scarp (−51.9±8.1‰ PDB). However, the δ13C value of the CO2 in the vent gas (+12.4±1.1‰ PDB) is very distinct from those for dissolved inorganic carbon (DIC) (−35.8‰ to −2.9‰ PDB) found elsewhere along the scarp, including underneath CBCs. The δ13C values of the carbonate-rich sediments and rocks exposed on the seafloor today also span an unusually large range (−40.9‰ to +12.9‰ PDB) and suggest two distinct populations of authigenic carbonate materials were sampled. Unconsolidated sediments and some carbonate rocks, which have lithologic evidence for near-seafloor formation, have negative δ13C values, while carbonate rocks that clearly formed in the subsurface have positive δ13C values (up to +23.0‰) close to that measured for CO2 in the vent gas. There appears to be two carbon sources for the authigenic carbonates: (1) deeply-sourced, isotopically heavy CO2 (∼+12‰); and (2) isotopically light DIC derived from local anaerobic oxidation of methane at the sulfate–methane interface in the shallow subsurface. Addition of isotopically light methane-derived carbon at the seafloor may completely mask the isotopically heavy CO2 signature (+12.4‰) in the underlying sediments. Thus, the authigenic carbonates may have formed from the same methane- and carbon dioxide-bearing fluid, but under different migration and alteration conditions, depending on how it migrated through the sediment column.

Beschreibung: Analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the Kodiak Trench, Hydrate Ridge, Monterey Bay Clam Flats, and the Eel River Basin, coupled with the studies of the chemistry of the pore fluids, have shown that these carbonates have grown within the sediment column. Geochemical profiles of pore fluids show that, in deep water seeps (Kodiak Trench—4450 m; Monterey Bay—1000 m; Hydrate Ridge—650 m), δ13C (DIC) values are low (isotopically light), whereas in the Eel River area (∼ 350–500 m), δ13C (DIC) values are much higher (isotopically heavier). In all cases, the δ13C values indicate that processes of methane oxidation, associated with sulfate reduction, are dominant in the shallow sediments. Data on the isotopic composition of authigenic carbonates found at sites in Kodiak Trench, Eel River Basin South, and Eel River Basin North indicate a variable composition and origin in different geochemical environments. Some of the authigenic carbonates from the study sites show a trend in their δ13C values similar to those of the pore fluids obtained in their vicinity, suggesting formation at relatively shallow depths, but others indicate formation at greater sediment depths. The latter usually consist of high magnesium calcite or dolomite, which, from their high values of δ13C (up to 23‰;) and δ18O (up to 7.5‰), suggest formation in the deeper horizons of the sediments, in the zone of methanogenesis. These observations are in agreement with observations by other workers at Hydrate Ridge, in Monterey Bay, and in the Eel River Basin.