A double bottom simulating reflector in the western Ross Sea, Antarctica

We present seismic evidence of the presence of gas hydrates and free gas in Victoria Land Basin (western Ross Sea, Antarctica). The occurrence of gas hydrates is inferred from a bottom simulating reflector (BSR), the first identified in the Ross Sea area. The gas presence was analyzed through targeted reprocessing of the multichannel seismic reflection data (3000 m streamer, 120 channels, 60-fold) acquired in 1990 by the Italian R/V OGS-Explora. The seismic data were reprocessed in order to increase the signal-to-noise ratio by adopting a true-amplitude approach, allowing a subsequent amplitude variations with offset (AVO) analysis to extract information on both P and S wave reflectivity. Along a profile from the deeper basin to an intrabasin structural high, the BSR evolves into crosscutting reflectors (CCRs) and enhanced amplitude reflectors (EARs). Locally, another seismic event is interpreted to be a second BSR (BSR0), identified about 150 m below the first BSR and is possibly related to the presence of a mixture of gases comprising methane and hydrocarbons with heavier molecular weight and therefore associated with a deeper hydrate stability zone. The presence of faults controls the upward gas migration, connecting the free gas zone below the BSR and the mud volcanoes and pockmarks on the seafloor. The modeling of the hydrate equilibrium zone shows that the observed BSR/BSR0 could be stable at the present pressure-temperature conditions.