Southern Hydrate Ridge and Weather

Thursday, August 03, 2017
Hummocky Methane Seeps at Hydrate Ridge

The weather gods have been playing with us this past few days. Winds were above 20-25 knots with angry seas, which prevented us from diving after the dive in the International District Hydrothermal Field at the summit of Axial Seamount. Once the dive at Axial Seamount was completed, during which time we crossed the April 2011 lava flow seeing spectacular scenery, the R/V Revelle transited ~ 1 hr to the Axial Base site. Here, weather prevented us from diving so a CTD vertical cast was completed in the morning to verify instruments on the Shallow Profiler Mooring. Once that was finished, the ship transited ~ 18 hrs back across the Juan de Fuca Plate to the Slope Base Site.

The transit was a rolling ride due to the waves – the seas were beautiful, but the ride was not. Chairs migrated across the floor, most folks did awkward “dances” walking around the ship moving from side to side, and many wedged themselves into their bunks against the cabin walls to prevent rolling back and forth with every wave the ship encountered. For those of us who do not get seasick, watching the waves crest was exhilarating and beautiful.

During the day, folks used the down time to catch up on sleep and work (there is a lot of documentation that is required for such complex cruises, from equipment inventories to recording of test results, to writing up dive reports). Some members of the science party watched movies, and there was a knot tying “class” for the students.

Early in the morning of July 2 (~0400), we arrived onsite at Slope Base, but the seastate was deemed by the Jason Expedition Leader to not be calm enough to dive in. Watching the weather closely, Deb and Skip decided to transit to Southern Hydrate Ridge about a 2 hr steam towards shore where it looked like the seas would be calmer. This site is a fascinating place marked by hummocky mounds from which methane gas escapes from beneath the seafloor. Here, lenses of methane ice in the subsurface release methane into the sediments that vent onto the seafloor, sometimes explosively. Each of the active seep sites host dense bacterial mats rimmed by clams that have symbiotic microbes in their guts. This area has not been fished extensively because there is a >30 m-tall carbonate pinnacle adjacent to the seep sites. Hence, the sedimented and carbonate-cobble seafloor hosts abundant rockfish, flounder, hag fish, soft corals, pout eels, and starfish, as well as other animals.

Jason dove to a depth of ~800 m near the Einsteins’ Grotto site, the main instrumented area for the Cabled Array. In 2014, explosions of methane bubbles emanated from a pit at the base of a steep sided hummock in the seep field. The bubbles entrained sediment from inside the hummock. Similar to past years, dive observations yesterday showed that once again, this area and undergone extreme changes. The pit has grown several meters in length, a large part of the western side of the Einstein mound has slumped off, and bacterial mat distribution has changed dramatically. The area around the mound has become significantly more hummocky – in concert, these observations show how dynamic this environment is over short-periods. The cabled digital still camera at this site has documented this transition over this past year. There was also a short visit to the Smokey Taverns site to the north, which has also changed significantly since 2010. Here, a very large collapsed zone with vertical walls and exposed hydrated in 2015, is now collapsed and dominated by hummocks inside the subdued depression. At the end of the dive, a brief visit was made to an area that was a Neptunea (very very large snails) nursery in 2015. Now, only relict egg casings remain.

With weather improving, the R/V Revelle returned to the Slope Base site to turn instrumented packages on the Shallow Profiler Mooring. Those dives are now underway.