Amon mud volcano
Concentric ridges and depressions around a central and seeping elevated dome
The seafloor morphology of Amon mud volcano exhibits numerous concentric ridges and depressions (Fig. 4a). These features have a mean wavelength of about ~30 m and show amplitudes which are ranging on average from 2 to 3 m (Fig. 6). Subcircular around the centre of the mud volcano, they are however, interfingering in some places. The central dome, with a diameter of 125 m, is slightly elevated, and rises ~3 m above the rest of the mud volcano surface. The detailed bathymetry displays a summit with a chaotic relief while the seafloor imagery is clearly marked by high backscatter (Fig. 5a). The central anomalous high backscatter, with amplitudes up to 3 dB, significantly contrasts with the low amplitude backscatter of the surrounding sediments. The average backscatter amplitude at the surface of Amon mud volcano is of −13 dB (Figs. 5a, 6). The low backscatter amplitudes are caused by a relatively homogeneous mud breccia (1) composed of fine clay particles and numerous millimetric rock clasts with occasionally bigger ones (a few centimetres in size), and (2) already covered with hemipelagic sediments (Dupré et al. 2007). The high backscatter signal is mostly produced by metre-scale mud breccia blocks that make up parts of the central rough area (Fig. 8a). The homogeneous and structureless mud breccia which corresponds to a soft and unconsolidated material containing essentially millimetric rock clasts (some are authigenic carbonate concretions, Dupré et al. 2007; Gontharet et al. 2007) does not backscatter as much as energy as mud breccia with numerous and larger rock clasts (e.g., Amsterdam mud volcano located in the Anaximander Mountains area, Zitter et al. 2005). These rough mud breccia blocks are interspersed with smoother mounds, probably already shaped by erosion, and depressions (Fig. 8a, b). Based on in situ observations with the Nautile submersible (NAUTINIL expedition 2003, Dupré et al. 2007) and the Quest ROV (BIONIL expedition 2006), and analysis of the high-resolution bathymetry and backscatter maps (Figs. 4a, 5a), there is no evidence for any other sites of mud eruption at the surface of Amon mud volcano. The centre of the mud volcano coincides with a hot spot associated with elevated in situ sediment temperatures (45°C recorded at 10 m below the seafloor in 2003, Dupré et al. 2007), high gas content (free gas bubbles in the sediments) and elevated methane turnover of >20 mmol m−2 d−1 (Felden, Lichtschlag, Wenzhöfer, Boetius; unpublished data).
A major linear depression separating two different areas of the mud volcano
The western part of Amon mud volcano is deformed by a NNW–SSE to N–S trending depression that separates the main body of the mud volcano from an area partly covered with authigenic carbonates. The bathymetry and backscatter AUV data clearly define distinct geological environments in the vicinity of this depression (Figs. 4a, 5a, 6).
East of the depression, the seabed is mostly characterized by mud breccias covered with thin hemipelagic sediments. Here, the seabed is disturbed by numerous bioturbation mounds (Fig. 8d; Dupré et al. 2007). At the time of the BIONIL expedition in 2006, this type of habitat covered the largest faction of the total mud volcano area, ~95% with 19.105 m2. However, few places east of this depression are characterized by outcropping carbonates, as previously reported (NAUTINIL expedition 2003, Dupré et al. 2007). West of the depression, the seafloor morphology drastically changes with exposure of methane-derived authigenic carbonates (Bayon et al. 2006; Gontharet et al. 2007) that are in places partly or completely draped with hemipelagic sediments. These carbonate constructions are well displayed by the new AUV data, in particular by the backscatter map (Fig. 5a) as a result of the high backscatter of the carbonates. Very high backscatter amplitudes were recorded, reaching 7 dB, the highest value measured at the surface of Amon mud volcano. This significantly contrasts with the surrounding sediments that absorb more energy (Fig. 6). Based on the spatial distribution of the high backscatter and the weak penetration of the 200 kHz signal, the area of outcropping carbonates is estimated to ~83.103 m2 (i.e., ~4% when compared with the surface of Amon). Carbonate outcrops are of various types, including metre-scale chimney-like constructions or fractured domes (Fig. 8c), or plates partly covered by hemipelagic sediments (Fig. 8e). Away from these outcropping carbonates, the amplitude of the signal, although high, decreases quite rapidly.
The depression identified on the western side of Amon mud volcano is 1-km-long and 6–10 m deep (Figs. 4a, 6). The width of this V-shaped depression is ~100 m. Within this depression and along its eastern flank, ROV observations showed a large bacterial mat (Fig. 8e) distributed along a N–S oriented strip of gassy, dark reduced sediments, with numerous chemosynthetic tubeworm and bivalve faunal communities (vestimentiferams and lammelibrachia) at its borders (Fig. 8f). The surface of these sediments that are affected by fluid flow extends 1–2 m in width and a few tens of m in length. This strip of reduced, sulfidic sediments (“sulfur band”) that marks parts of the V-shaped depression seems to correlate with a very low backscatter, distinctly identifiable on the AUV backscatter map (Figs. 5a, 6).
Slide scars at the edges of the mud volcano
The subcircular structure of Amon mud volcano is locally deformed at its edges by sedimentary slides. The scars of these slides are well imaged on the multibeam seafloor imageries, in particular on the 2 m high-resolution bathymetric maps (Figs. 4a, 7). The largest gravity induced sedimentary destabilizations are located northeast and south of Amon mud volcano where metre-scale mud breccia blocks are visible up to ~1 km away from the edge of the mud volcano. At the northeastern border, the high-resolution bathymetry maps reveal a 350–400 m wide corridor bounded by NE–SW lineations and along which mud breccia sediments appear to be destabilized. At the southern border, several sedimentary lobes are identified. The most western lobe is 500 m long with a surface of <1 km2. These lobes are interpreted as mass transport deposits from mud breccia destabilizations initiated at the edge of the mud volcano where the flanks are known to be fairly abrupt (Dupré et al. 2007).
Isis mud volcano
Overall seabed morphology
The Isis mud volcano covers an area of 10.1 km2 and is located in more than 990 m of water (Figs. 1, 3). One-third of the mud volcano was surveyed with the AsterX AUV. The newly produced bathymetry and acoustic imagery maps of the seafloor exhibit similar morphological and backscatter features as those seen at Amon mud volcano (Figs. 4, 5, 9, 10, 11). Numerous ridges and depressions compose its surface (Figs. 9, 10, 11, 13). The features are not regularly distributed around the centre of the mud volcano, contrasting with Amon where they are fairly concentric. However, when compared with Amon mud volcano, the wavelength of the ridge and depression distribution on Isis is shorter, 17 m on average (versus 30 m on Amon, Figs. 4a, 6), and the amplitudes of these features are smaller, generally not exceeding 1 m.
Two distinct domes
The new high-resolution bathymetric map clearly reveals the existence of two distinct slightly elevated areas at the surface of Isis mud volcano, one in the geometric centre of the edifice (dome A in Fig. 9b) and the other in an offset position to the NE of the centre (dome B in Fig. 9b). These domes are both 4–5 m high, with a diameter of 340 and 175 m, respectively (Figs. 11, 12). The largest one, in the centre, corresponds to the unique mud eruption site discovered at the surface of Isis mud volcano (dome A in Fig. 9b), and exhibits a similar seafloor morphology (Fig. 13a, b) comparable to that of the active centre of Amon (Fig. 8a, b) with a chaotic distribution of mud breccia blocks on the summit (Fig. 11). At the Isis central dome, high-resolution bathymetry data allow us to distinguish (1) numerous cracks that characterize highs and lows and (2) several topographic highs from which elongated bodies of mud breccia are radiating. At this centre, elevated temperatures were measured during the BIONIL expedition in 2006 (Feseker personal communication) and during the NAUTINIL (2003) and MIMES (2004) campaigns (at more than 40°C at 10 m below the seafloor, Dupré et al. 2007; Feseker et al. 2009). High methane concentrations in the water column and in the sediments characterize this central dome (see details in Mastalerz et al. 2007) with a seafloor disturbed by millimetric holes interpreted as resulting from gas expulsion (Fig. 13b). The second dome, located 340 m to the northeast of the main one (dome B in Figs. 9b, 11, 12), exhibits a disturbed seabed morphology with a major elongated and slightly curved ridge of 70 m in length, as well as elevated seafloor temperatures (however, twice as cool as the centre, Feseker personal communication). However, and in contrast to the central dome, ROV observations here showed no evidence for active seepage, such as gas bubbling spots, fresh mud breccia outcrops or chemosynthetic and microbial mats (see seafloor picture, Fig. 13e). Sediments there are only disturbed by numerous decimetre-scale bioturbation mounds (Fig. 13e) as observed for the vast majority of the mud volcano seabed (Dupré et al. 2007). The detailed bathymetry map displays a third slightly elevated area, located 425 m east of the second dome (Figs. 9b, 13f). This area, however, showed no sub-seafloor temperature anomaly (Feseker personal communication) and no visual indications of fluid and gas seepage during the ROV survey (BIONIL 2006; Fig. 13f) and the Nautile survey (NAUTINIL 2003; Dupré et al. 2007).
The backscatter amplitude spectrum of Isis mud volcano is narrow, when compared with Amon, with maximum recorded values of only 4 dB (Figs. 10, 12). The generally low and uniform backscatter of Isis appears to reflect the restricted diversity of the sedimentary texture (Fig. 13a–f). In the AsterX AUV surveyed areas (Fig. 9a), ROV observations did not find authigenic carbonates or bivalves and tubeworms, that would enhance the seabed backscatter if present. Lateral variations in the backscatter amplitude occur in association with (1) the numerous metre-scale ridges and depressions observed on the surface of Isis mud volcano, (2) the abrupt relief of the mud volcano edges (e.g., the eastern border in Fig. 10), and (3) the chaotic metre-scale mud breccia blocks. There is a relatively good correlation between the small-scale variation of the topography and the backscatter amplitude (see along the profile in Fig. 12). The newly acquired high-resolution backscatter map, covering one-third of the total surface of Isis, displays only one significantly higher backscatter area. Located on top of the central dome, this high backscatter patch covers a sub-circular area of <50 m diameter, and is comprised within a wider zone of <150 m diameter that corresponds to the disturbed and chaotic area.