Natural oil seepage at Kobuleti Ridge, eastern Black Sea

• Satellite images reveal persistent oil seepage from two sites in the Black Sea.
• Oil-coated gas bubbles reach the sea surface from depth >1100 m.
• Oil coatings in combination with gas hydrate formation enhance bubble lifetime.
• Combined oil and gas seepage is suggested to be a source of atmospheric methane.

Analysis of Advanced Synthetic Aperture Radar satellite images in combination with water column and seafloor investigations documented natural oil seepage from Pechori Mound and Colkheti Seep in 1000–1200 m water depth in the eastern Black Sea offshore Georgia. Hydroacoustic imaging of the water column using multibeam echosounder evidenced numerous gas emissions from both structures. Gas bubbles rose as high as 45 m below sea surface. It is proposed that oil coatings around gas bubbles hamper their dissolution allowing them to reach the sea surface where widespread oil slicks are formed. Slow rise velocities (14 ± 1 cm s−1) of bubbles with radii of 2.6 ± 0.3 mm (n = 101) derived from video data obtained with the remotely operated vehicle MARUM QUEST4000, corroborate the assumption that bubbles are oil-coated. High resolution seafloor mapping with the autonomous underwater vehicle MARUM SEAL5000 at Colkheti Seep revealed a crater-strewn morphology whose formation is explained by frequent rafting of shallow gas hydrate deposits. Satellite imaging of oil slicks on the sea surface above both sites indicates that oil seepage is rather persistent since 2003. An order-of-magnitude estimation of minimum oil seepage rates suggests discharge rates of ∼40 liters per hour from both sites. The data presented are the first comprehensive description of oil seepage in the Black Sea.