Seismic evidence of shallow gas in the sediment on the shelf off Trabzon, southeastern Black Sea

High-resolution seismic surveys carried out on the shelf off Trabzon (south eastern Black Sea) show that the sub-bottom stratigraphy consists of two ma in depositional sequences (A and B) one of which contains zones of acoustic turbidity. Of these, the upper depositional sequence (A) is thought to com prise Holocene sediments, while the lower depositional sequence (B) is inte rpreted as approximating to the Pleistocene. The boundary between these seq uences is defined by a reflector (R), which is interpreted as the pre-Holoc ene erosional surface. The acoustic turbidity observed on seismic profiles is interpreted as representing gas accumulations in the sediments of the up per depositional sequence. Previous geochemical investigations in the area indicate that acoustic turbidity implies biogenic methane gases in bubble f orm. On the basis of published hypotheses on the generation of the methane gases, it is suggested that the upper boundary of the zone of acoustic turb idity on seismic profiles in this study corresponds to the boundary between a biogenic sulfate-reducing zone and the underlying carbonate-reducing zon e. Apart from acoustic turbidity, some bright spot anomalies which are also interpreted to arise from the presence of methane gases within the upper d epositional sequence are observed on the seismic records collected from the shelf off Trabzon. (C) 1999 Elsevier Science Ltd. All rights reserved.