**6.1 Pacific margin**

*Glaciers and the Polar Environment*

this part of Antarctica [23].

**Figure 8.**

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

*is supposed to be equal to 49°C/km (modified after [23]).*

which contributes to sediment consolidation at depth.

in proximity of the banks, the gas hydrate stability zone results display thickness less than 100 m. On the other hand, the thickness of the gas hydrate stability zone increases in proximity of the basins to values exceeding 400 m related to bathymetry increase and seafloor temperature decrease. Moreover, the existence and dynamics of the gas hydrate distribution is strictly related to the existence and evolution of the shallow geological and geomorphological features below the sea floor, as suggested in the past by several authors. So, the presence of some geological and geomorphological features are in agreement with the gas hydrate presence in

*Distribution of the base of the gas hydrate stability zone from the seafloor (in meters). The geothermal gradient* 

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]),

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

**12**

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, thanks to the ODP Leg 178 [74].

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].
