**6. Opal-A/opal-CT phase boundary**

Silica diagenesis consists of precipitation from an initial amorphous phase (opal A) to an intermediate phase (opal CT) and finally to the final form with quartz crystallization. The presence of large amounts of biogenic silica in marine sediments can affect their physical properties [68]. In fact, diagenetic alteration of biogenic opal-A to opal-CT causes a drastic reduction of porosity (about 20 vol% according to [69]), which contributes to sediment consolidation at depth.

It is possible to recognize the passage from one phase to another on seismic profiles because of the presence of a high amplitude reflector, produced by a positive impedance contrast between the overlying silica-rich sediments and the underlying sediments in which biogenic silica is dissolved. This reflector simulates the seafloor morphology, so it is still called BSR. This BSR is different from the hydrate-related BSR, as well documented in literature (i.e., "in [70]"). The positive polarity, the depth, no noticeable drop of frequency, and compressional velocity below the

**13**

*Gas Hydrates in Antarctica*

**6.1 Pacific margin**

**7. Conclusions**

in-depth analysis.

**Acknowledgements**

presence.

*DOI: http://dx.doi.org/10.5772/intechopen.94306*

earlier opal transition at greater pressures.

thanks to the ODP Leg 178 [74].

BSR suggest the absence of the gas hydrate and the diagenetic-related origin of the observed BSR. Moreover, diagenesis-related BSR have a constant depth below seafloor or even decreasing sub-bottom depth with increasing water depth due to

In the Pacific margin of the Antarctic Peninsula, several seismic lines where acquired with the main purpose to study sediment drift presented in the northwest part. There lines were analyzed in detail in order to extract information about the petrophysical properties relevant to seismic stratigraphy studies in the continental shelf and rise [71, 72]. In particular, a seismic line showed the presence of an anomalous reflector, interpreted as a BSR [73]. Borehole data are also available,

In order to understand the nature of the observed BSR, [73] performed a detailed study concluding that the BSR observed in the seismic line is due to opal-A/opal-CT phase boundary and not to the gas hydrate presence. Moreover, they attempted a quantitative estimation of biogenic silica content within marine sediments using seismic reflection and physical properties data across the silica diagenesis-induced BSR. The estimated biogenic silica content increases with depth and reaches a maximum of 23.3 wt % above the BSR. Such quantifications are of prime importance for submarine slope stability assessment as the deep seated transformation of biogenic silica from opal-A to opal-CT is able to trigger slope instability not only at local scale but also at regional scale, as previously shown by [69, 75].

The most important area from gas hydrate point of view in Antarctica is the South Shetland Margin where a huge hydrate reservoir is present; it is very well documented in literature thanks to several geophysical acquisition legs performed. The analyses of geophysical data allows concluding that the accumulation of fluids within sediments is strictly related with tectonics features, such as faults and folds, revealing a close relationship between gas hydrate accumulation and geological features. Moreover, the hydrocarbons trapped and detected in the sediment cores may indicate the existence of deeper reserves, confirming that the BSR should be considered as an indicator of conventional deep reservoir. Finally, due to the warming measured in this part of Antarctica, a monitoring of the evolution of the gas hydrate reservoirs offshore Antarctic Peninsula is required for an environmental

The main seismic indicator of the gas hydrate presence, the BSR, was recorded in few parts of Antarctica (Pacific and Atlantic margin of Antarctic Peninsula, Wilkes Land margin), but it was associated to opal A/CT transition. The other potential areas for gas hydrate presence (Ross Sea, Weddell Sea and Wilkes Land Margin) needs further measurements in order to confirm or refuse the hypothesis of their

We wish to thank all colleagues that contributed to improve the knowledge of the gas hydrate in Antarctica. In particular, in alphabetic order: Flavio Accaino (OGS, Italy), Daniela Accettella (OGS, Italy), Angelo Camerlenghi (OGS, Italy),

BSR suggest the absence of the gas hydrate and the diagenetic-related origin of the observed BSR. Moreover, diagenesis-related BSR have a constant depth below seafloor or even decreasing sub-bottom depth with increasing water depth due to earlier opal transition at greater pressures.
