Abstract
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 drape. Elongated seafloor depressions linked to faulting and gas seepage develop at the base between adjacent sediment waves. The elongated depressions become progressively wider and deeper toward shallow water depths and culminate in some locations into clusters of pockmarks. MTDs cover larger regions and level out paleo-topography. Their surface morphology results in fault-like deformation patterns of the sediment drape on top of the MTDs that is locally utilized for gas migration. Erosional factors are seen along channels and canyons as well as slope failures, where gas discharge occurs along head-scarps and ridges. Sediment that was removed by slope failures cover larger regions down-slope. Those regions are devoid of gas seepage either by forming impermeable barriers to gas migration or by removal of the formerly gas-rich sediments. Deep-rooted tectonic control on gas migration is seen in the eastern study region with wide-spread normal faulting promoting gas migration. Overall, gas seepage is widespread along the margin. Gas migration appears more vigorous in shallow waters below ∼160 m water depth, but the number of flare sites is not necessarily an indicator of the total volume of gas released.
Highlights
Studies on Gas Hydrates and Gas Venting in the Black SeaIn 1974, sampling of natural gas hydrates has been described for the first time in the Black Sea (Yefremova and Zhizchenko, 1974)
The bottom-simulating reflector (BSR) is distributed across the Danube deep-sea fan in a non-uniform pattern
Combining data from several research expeditions, we identified >10,100 individual vent sites within the Romanian and Bulgarian margin linked to the Danube deep-sea fan and eastern extension to the Dnepr fan
Summary
Studies on Gas Hydrates and Gas Venting in the Black SeaIn 1974, sampling of natural gas hydrates has been described for the first time in the Black Sea (Yefremova and Zhizchenko, 1974). Depending on the depth of the seafloor and the temperature gradient in the sediments, gas hydrates are expected to be stable within 200 m–300 mbsf (Bialas et al, 2014; Zander et al, 2017; Riboulot et al, 2018), defining upper and lower limits of the bottom-simulating reflector (BSR) distribution as initially shown by Popescu et al (2006) As part of these investigations, several authors identified that gas seepage sites are not necessarily randomly distributed, but show some association to structural and seafloor morphological features, such as slump scars, canyon flanks, or ridges (Naudts et al, 2006; Naudts et al, 2009; Riboulot et al, 2017; Römer et al, 2019). All these studies highlighted the need of multibeam bathymetry and water column data for the identification of gas seepages
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