A Soft-bottom Survey

September 23, 2008

William (Bill) Shedd
Geophysicist
Minerals Management Service

Today we explored site GB201, a large (~ 6.5 kilometer (km) by 5.5 km, 250 meters (m) high) sea-floor structure supported by a shallow feature called a salt diapir. After examining a 3-D seismic map of the sea floor, the northwestern flank and several discreet areas on and near the crest of the site that appeared to be hardground, and, thus, showed the most promise for coral habitat and were chosen for today’s dive.

One characteristic of the site is the abundance of “pockmarks,” circular depressions on the sea floor that are hypothesized to be strong flows of gas that excavate sediment from the area, as opposed to mud volcanoes and flows that add new sediments. Our hypothesis was that the sediments being ejected from areas near the pockmarks might inhibit the development of healthy communities of filter feeders such as corals, while areas away from pockmarks may support corals.

We launched the remotely operated vehicle (ROV) early in the morning and it reached the bottom at 520 m approximately 15 minutes after launch. The area consisted of soft bottom with occasional colonial soft coral or hydroid colonies. The ROV proceeded east, up-slope towards the primary target. The entire transit was over soft bottom, with partially consolidated sediments and thin carbonates and occasional boulders at the local topographic high.

Eventually, the ROV encountered bacterial mats and methane bubbling from the sea floor. Immediately adjacent to this were some tubeworms and anemones covering an unidentified structure. The ROV then turned its course and ran over an area of thin, friable carbonate pavement covered by a layer of mud and occasional sponge and zoanthid anemone aggregations.

During this entire transit, we saw very few epifaunal (bottom-dwelling) organisms and occasional pockmarks of varying sizes. In the area of the lowest seismic return, we did encounter a large field of pockmarks on the sea floor. This is near to the area of high, large-scale pockmarks observed on the multibeam bathymetry at 315 m depth. We hypothesized that the high frequency of blow-out events creating the pockmarks could generate additional sediment input to the entire area, potentially sufficient to prevent coral growth.

The ROV continued to climb, heading to the local peak of the mound, reaching approximately 300 m depth. There were more pockmarks in this area, despite our hopes that there would be exposed carbonate rock from the bright reflectivity apparent from the seismic data. The only biological features at the top of the mound were sea whips and one isolated gorgonian colony with two basket stars on the sediment next to them. We saw no other apparent communities or exposed hardgrounds at the top of the mound, so the ROV proceeded south down into one of the large pockmarks visible on the multibeam data. The steep slope of the pockmark showed no exposed hardground and few organisms other than fish and occasional sea whips.