Seafloor mounds, craters and depressions linked to seismic chimneys breaching fossilized diagenetic bottom simulating reflectors in the central and southern Scotia Sea, Antarctica

Based on an extensive dataset including swath bathymetry, chirp sub-bottom profiler (TOPAS) and multichannel seismic reflection profiles obtained during four cruises in the Scotia Sea aboard the R/V Hespérides, we report a variety of seismic and morphological structures related to focused fluid flow in the Scan Basin (southern Scotia Sea) and the central Scotia Sea (Antarctica). We show that both positive-relief (mounds) and negative-relief (craters and elongated depressions) seafloor morphologies are associated with deep seismic chimneys that link the deep source zone to the subsurface structures through a network of fractures that progressively breach sub-horizontal bands of anomalously high-amplitude reflections. Based on the recognition that these bands of reflections generally mimic the seafloor topography and locally cross-cut the stratigraphic seismic reflections, we recognize three different bottom simulating reflectors (BSRs). According to the theoretical model for hydrate and silica diagenesis stability conditions in the central and southern Scotia Sea and the calculations of temperature and seismic polarity for the three BSRs, we infer that BSR-2 and BSR-3 are reflections caused by the transformation between Opal-A/Opal-CT and Opal-CT/Quartz, respectively. We thus postulate that the successive diagenetic fronts were caused by significantly high geothermal gradients during the early-middle Miocene. In contrast, the low temperatures calculated for the depth of the BSR-1 event rule out its diagenetic origin but delineate the base of the gas hydrate stability zone (GHSZ).An evolutionary model is proposed to explain the plumbing system and chimney structures that help the focused flow of gas-rich fluids to migrate into the subsurface. Firstly, the formation of silica transformation zones may have acted as reservoir traps during Neogene times. Secondly, the progressive decrease of heat flow during the late Pliocene and Quaternary favored the development of the networks of polygonal faults forming collapses and downward tapering chimneys. Finally, seafloor mounds are formed as a result of the continuous injection of gas-enriched fluids through these networks of fractures; they are transformed into gas hydrates above the present base of the GHSZ and move upwards by buoyancy drive as they lose density and increase their volume. We present these structures as type cases that might represent highly concentrated hydrates around local seafloor fluid venting structures. Furthermore, they may be one of the most important conduits into the ocean-atmosphere system for deep methane in the Antarctic seafloor. The breach of BSRs influenced by global warming may induce the catastrophic release of greenhouse gases to the ocean-atmosphere system and, in turn, impact on the Earth's evolution.