GAS HYDRATE IN THE SOUTH SHETLAND MARGIN, OFFSHORE ANTARCTIC PENINSULA: A REVIEW OF TWO DECADES OF STUDIES

Global climate change is particularly amplified in transition zones, such as the peri-Antarctic regions. For this reason, the gas hydrate reservoir located offshore Antarctic Peninsula was studied in the last 20 years acquiring a quite extensive geophysical dataset. The presence of a diffused and discontinuous BSR was discovered during the Italian Antarctic cruises of 1989-1990 and 1996-1997, onboard the R/V OGS Explora. During these expeditions, a strong and continuous bottom simulating reflector was identified between ca. 500 and 3000 m water depth (mwd). BSR has been associated with the presence of a gas reservoir below the hydrate and with diagenetic alteration of biogenic silica Opal-A/Opal-CT transition as suggested by Ocean Drilling Program Leg 178 results near the area. Along this margin, the extent of the BSR was mapped based on about 1,000 km of seismic lines. OBSs deployed during the 1996-1997 cruise provided energy arrivals from the BSR and the refraction and the converted waves from the base of the free-gas zone, the so-called base of the free gas reflector or BGR. During the austral summer 2003-2004, additional data were acquired in the same area: multibeam bathymetry, seismic profiles, chirp, and sediment gravity cores. Seismic velocities obtained from advanced analysis of seismic data were analyzed to determine gas hydrate and free-gas distributions and to estimate the methane volumetric fraction trapped in the sediments. The jointly interpolation of the 2D models allowed obtaining a 3D model of gas hydrate concentration from the seafloor to the BSR. The average concentration of hydrate is 6,9% which assuming a porosity of 41% results in a total volume of hydrate, for the area where the interpolation is reliable (600 km 2 ), of about 16 × 10 9 m 3 . The estimated amount of gas hydrate can vary in a range of 12 × 10 9 - 20 × 10 9 m 3 . Moreover, considering that 1 m 3 of gas hydrate corresponds to about 140 m 3 of free gas in standard pressure temperature conditions, the total free gas trapped in this reservoir ranges between 1.68 × 10 12 and 2.8 × 10 12 m 3 . During the past five decades, the Antarctic Peninsula has been warming up faster than any other part of the Southern Hemisphere, and long term ocean warming could affect the stability of this hydrate reservoir at shallow waters. Extending to 2100 yr the seabed ocean warming trend observed over the past three decades of 0.023 °C/y, our model predicts that emissions could start in 2028 at 375 mwd and extend to 442 mwd at an average rate of about 0.91 mwd/y, releasing ca. 1.13x10 3 mol/y of methane per metre along the margin by 2100. These emissions originate from dissociation at the top of the hydrate layer, a physical process that steady-state modeling cannot represent. Our results are speculative on account of the lack of direct evidence of a shallow water hydrate reservoir, but they illustrate that the South Shetland Margin is a key area to observe the effects of ocean warming-induced hydrate dissociation in the coming decades.