Figure F4. A–D. Seismic slices from the 3-D seismic data that illustrate the characteristics of reflection A. E–H. Slices that illustrate reflections B, B', and C. I. The locations of these profiles are shown on the map, which has topographic contours overlain on deep-towed side-scan data (Johnson and Goldfinger, unpubl. data). Note the bright spot southwest of the summit. It probably defines the extent of a buried apron of carbonates that surrounds the carbonate "pinnacle" that was discovered during Alvin dives in 1999. This carbonate structure sits in a moat and rises ~50 m above the surrounding seafloor (C). Its shadow appears in the side-scan data as a black spot within the bright spot. Dives show abundant vent fauna in cracks on the pinnacle, indicating aqueous fluid flow, but no bubbles have been observed. In the seismic data, this feature is characterized by blanking of underlying seismic reflections. A "tongue" of intermediate-strength seafloor reflectivity northeast of the pinnacle probably delimits the region of massive hydrate at the seafloor and appears in the seismic data as a region of bright, chaotic reflectivity (B). All bubbles observed on southern Hydrate Ridge via submersible or 12-kHz echo sounder emanate from this summit region. Reflection pair B, B', and C are associated with anticline A (Fig. F5) on the eastern flank but are not associated with any reflectivity anomalies on the seafloor. J.Predicted and observed BSR depths along line 230. Fine lines show predicted bottom-simulating reflector (BSR) depths for different assumed temperature gradients. The lower of the two solid lines is for an assumed seafloor temperature of 4°C; the upper fine line is for a seafloor temperature that decreases linearly from 4° to 3°C as depth increases from 800 to 1200 m, as indicated by hydrographic data (Trehu et al., 1995). The thick dashed line is calculated from line 230 assuming a constant average velocity between the seafloor and the BSR of 1.6 km/s. A temperature gradient of ~0.7°C/km is suggested for the saddle between the axis of the ridge and anticline A; a gradient of 0.055°C/km is suggested for the slope basin. Open symbols show effects of uncertainties in average velocity. A velocity of 1.8 km/s above the BSR removes the apparent thermal anomaly at a water depth of 860–900 m; a velocity of 1.5 km/s removes the anomaly for water depths >1020 m. This figure illustrates the difficulty of resolving small differences in apparent temperature gradient from the BSR data in this region.