New insights into geology and geochemistry of the Kerch seep area in the 1

27 High-resolution 3D seismic data in combination with deep-towed sidescan sonar data 28 and porewater analysis give insights into the seafloor expression and the plumbing system of 29 the actively gas emitting Kerch seep area, which is located in the northeastern Black Sea in 30 around 900 m water depth, i.e. well within the gas hydrate stability zone (GHSZ). Our 31 analysis shows that the Kerch seep consists of three closely spaced but individual seeps above 32 a paleo-channel-levee system of the Don Kuban deep-sea fan. We show that mounded seep 33 morphology results from sediment up-doming due to gas overpressure. Each of the seeps 34 hosts its own gas pocket underneath the domes which are fed with methane of predominantly 35 microbial origin along narrow pipes through the GHSZ. Methane transport occurs dominantly 36 in the form of gas bubbles decoupled from fluid advection. Elevated sediment temperatures of 37 up to 0.3 °C above background values are most likely the result of gas hydrate formation 38 within the uppermost 10 meters of the sediment column. Compared to other seeps occurring 39 within the GHSZ in the Black Sea overall only scarce gas indications are present in 40 geoacoustic and geophysical data. Transport-reaction modeling suggests that the Kerch seep 41 is a young seep far from steady state and probably not more than 500 years old. 42 43

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