Multiple bottom-simulating reflections in the Black Sea: Potential proxies of past climate conditions (original) (raw)
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On the origin of multiple BSRs in the Danube deep-sea fan, Black Sea
Earth and Planetary Science Letters
High-resolution 2D seismic data reveal the character and distribution of up to four stacked bottom simulating reflectors (BSR) within the channel-levee systems of the Danube deep-sea fan. The theoretical base of the gas hydrate stability zone (GHSZ) calculated from regional geothermal gradients and salinity data is in agreement with the shallowest BSR. For the deeper BSRs, BSR formation due to overpressure compartments can be excluded because the necessary gas column would exceed the vertical distance between two overlying BSRs. We show instead that the deeper BSRs are likely paleo BSRs caused by a change in pressure and temperature conditions during different limnic phases of the Black Sea. This is supported by the observation that the BSRs correspond to paleo seafloor horizons located in a layer between a buried channel-levee system and the levee deposits of the Danube channel. The good match of the observed BSRs and the BSRs predicted from deposition of these sediment layers indicates that the multiple BSRs reflect stages of stable sealevel lowstands possibly during glacial times. The observation of sharp BSRs several 10,000 of years but possibly up to 300,000 years after they have left the GHSZ demonstrates that either hydrate dissociation does not take place within this time frame or that only small amounts of gas are released that can be transported by diffusion. The gas underneath the previous GHSZ does not start to migrate for several thousands of years.
Seismic expression of gas and gas hydrates across the western Black Sea
Geo-marine Letters, 2007
This study is a synthesis of gas-related features in recent sediments across the western Black Sea basin. The investigation is based on an extensive seismic dataset, and integrates published information from previous local studies. Our data reveal widespread occurrences of seismic facies indicating free gas in sediments and gas escape in the water column. The presence of gas hydrates is inferred from bottom-simulating reflections (BSRs). The distribution of the gas facies shows (1) major gas accumulations close to the seafloor in the coastal area and along the shelfbreak, (2) ubiquitous gas migration from the deeper subsurface on the shelf and (3) gas hydrate occurrences on the lower slope (below 750 m water depth). The coastal and shelfbreak shallow gas areas correspond to the highstand and lowstand depocentres, respectively. Gas in these areas most likely results from in situ degradation of biogenic methane, probably with a contribution of deep gas in the shelfbreak accumulation. On the western shelf, vertical gas migration appears to originate from a source of Eocene age or older and, in some cases, it is clearly related to known deep oil and gas fields. Gas release at the seafloor is abundant at water depths shallower than 725 m, which corresponds to the minimum theoretical depth for methane hydrate stability, but occurs only exceptionally at water depths where hydrates can form. As such, gas entering the hydrate stability field appears to form hydrates, acting as a buffer for gas migration towards the seafloor and subsequent escape.
The Danube Deep-Sea Fan (NW Black Sea) is one of the most developed deep-sea sediment depositional systems in Europe. Although the morphology and the architecture have been widely described in the past years, little is known about the stratigraphy of this depositional system. For the late Quaternary, this results from the lack of significant stratigraphic markers, the scarcity of radiocarbon ages and the difficulty in constraining reservoir ages. Recent robust quantification of reservoir ages has allowed the construction of a new stratigraphic framework for the Black Sea from the end of the last glacial period to the Holocene, thus giving the opportunity to correlate sedimentological and geochemical features previously described on the NW Black Sea margin with climatic events identified in the Northern
Gas migration pathways and slope failures in the Danube Fan, Black Sea
Marine and Petroleum Geology, 2018
Highlights ► Identify 3 groups of gas migration structures in seismic data from the Danube Fan. ► Migration structures related to shallow gas migration and flares at the seafloor. ► Gas migration is controlled by lithological heterogeneity and sediment deformation. ► Mass transport deposits play a role in controlling vertical migration occurrence.
Controls on Gas Emission Distribution on the Continental Slope of the Western Black Sea
Frontiers in Earth Science, 2021
The continental slopes of the Black Sea show abundant manifestations of gas seepage in water depth of <720 m, but underlying controls are still not fully understood. Here, we investigate gas seepage along the Bulgarian and Romanian Black Sea margin using acoustic multibeam water column, bathymetry, backscatter, and sub-bottom profiler data to determine linkages between sub-seafloor structures, seafloor gas seeps, and gas discharge into the water column. More than 10,000 seepage sites over an area of ∼3,000 km2were identified. The maximum water depth of gas seepage is controlled by the onset of the structure I gas hydrate stability zone in ∼720 m depth. However, gas seepage is not randomly distributed elsewhere. We classify three factors controlling on gas seepage locations into depositional, erosional, and tectonic factors. Depositional factors are associated with regionally occurring sediment waves forming focusing effects and mass-transport deposits (MTDs) with limited sediment...
Turkish Journal of Earth Sciences
Some strong reflections about 3 to 5 m thick were observed at depths of 25-60 metres below the sea floor using deeptowed, 5 kHz subbottom profiler data in the Turkish shelf and upper slope of the Eastern Black Sea at water depths of 250 to 700 m. Strong reflections of this kind are generally attributed to shallow and localized gas accumulations. We, however, observed that the reflection polarity of these strong reflections was positive, suggesting that they do not correspond to reflections from the upper boundary of a possible gas front. In this study, we evaluate these reflections to determine if they represent hydrogen sulphide-rich shallow gas hydrate layers, which would be an unusual gas hydrate occurrence in a shallow marine environment. The existence of gas hydrate formations in thermobaric conditions, as in our study area (shallower water depths and relatively higher temperatures), depends completely on the gas composition in the hydrate structure; it is possible for gas hydrates to be stable only if they are formed by a certain amount of hydrogen sulfide together with methane. We closely examined the hydrogen sulphide potential of the area and found that the maximum total hydrogen sulfide concentration in the surficial sediments in the area was, at 5550 ppm, enough to produce hydrogen sulphide-rich gas hydrates. Using geoacoustic and geochemical data, we propose a conceptual model for the formation of shallow hydrogen sulfide-rich gas hydrates. According to this model, conjectural gas hydrate layers in the area should be formed along the boundary between a sulphate-reducing zone and an underlying carbonate reducing zone, where methane comes into contact with hydrogen sulphide. We also propose that the strongly reflective appearance of these layers on the subbottom profiler data indicates that the hydrate zone consists of a number of gas hydrate sheets with a decreasing thickness towards the seabed, interbedded with non-hydrate-bearing sediments.
Seismic identification of gas hydrates: A case study from Sakarya Canyon, western Black Sea
TURKISH JOURNAL OF EARTH SCIENCES, 2019
Multichannel seismic, 3.5-kHz Chirp subbottom profiler and multibeam bathymetric data were collected along the western Black Sea margin, offshore Sakarya River, to investigate the bottom-simulating reflections (BSRs), free gas accumulations, and mud volcanoes. Geometries from the seismic data indicate widespread BSRs along the continental rise between 750 and 1950 m water depths, 70 to 350 ms below the seafloor. Seismic attribute analyses have been applied to the seismic data to reveal the acoustic properties of the gas hydrates. According to the results from such analyses, we conclude that there are acoustically transparent zones beneath most of the BSRs in the area, which are interpreted as free gas accumulations, and the gas hydrate-bearing sediments are acting as seals for the free gas in the underlying sediments. Stability analysis of the gas hydrates from different BSR zones in the area suggests that the gas composition in the gas hydrates may change locally. As we do not have ground truth data from BSR zones, the exact composition of the gas forming the gas hydrates is unknown. However, hydrocarbon productivity of the area, chromatography results of the shallow sediment samples nearby, and stability analysis of the gas hydrates indicate the possible existence of a thermogenic gas component in the gas hydrate composition, resulting in a mixture of gas hydrates.