Description: Hydrocarbon seeps harbor copious chemosynthesis-dependent life, the traces of which are preserved in the fossil record within authigenic carbonates. These environments are mostly characterized by seepage of methane-rich fluids, yet numerous crude oil–dominated seeps have been discovered in recent years. Oil seepage has a profound influence on the local fauna, but recognizing such seeps in the rock record remains elusive. This study presents new geochemical data that will allow for a more confident identification of ancient oil-seep deposits. Geochemical data from modern and ancient seep limestones reveal that oil-dominated seep carbonates are enriched in rare earth elements and uranium compared to their methane-dominated counterparts. These trace element patterns have the potential to serve as a basis for an improved understanding of the adaptation of chemosynthetic life to oil seepage, and to better constrain the marine carbon cycle in the geologic past.

Description: Pockmarks are seafloor depressions commonly associated with fluid escape from the seabed and are believed to contribute noticeably to the transfer of methane into the ocean and ultimately into the atmosphere. They occur in many different areas and geological contexts, and vary greatly in size and shape. Nevertheless, the mechanisms of pockmark growth are still largely unclear. Still, seabed methane emissions contribute to the global carbon budget, and understanding such processes is critical to constrain future quantifications of seabed methane release at local and global scales. The giant Regab pockmark (9°42.6' E, 5°47.8' S), located at 3160 m water depth near the Congo deep-sea channel (offshore southwestern Africa), was investigated with state-of-the-art mapping devices mounted on IFREMER’s (French Research Institute for Exploitation of the Sea) remotely operated vehicle (ROV) Victor 6000 . ROV-borne micro-bathymetry and backscatter data of the entire structure, a high-resolution photo-mosaic covering 105,000 m 2 of the most active area, sidescan mapping of gas emissions, and maps of faunal distribution as well as of carbonate crust occurrence are combined to provide an unprecedented detailed view of a giant pockmark. All data sets suggest that the pockmark is composed of two very distinctive zones in terms of seepage intensity. We postulate that these zones are the surface expression of two fluid flow regimes in the subsurface: focused flow through a fractured medium and diffuse flow through a porous medium. We conclude that the growth of giant pockmarks is controlled by self-sealing processes and lateral spreading of rising fluids. In particular, partial redirection of fluids through fractures in the sediments can drive the pockmark growth in preferential directions.