Exploring a 2011 eruption and collecting worms

August 29, 2014

The day was mostly cloudy with light winds and some rain showers. Velella velella, commonly called “by-the-wind-sailor” are siphonophores related to Portugese man o’ war. They have been silently drifting past the ship by the millions. They have a small “sail” that is oriented at an angle to their float, which helps propel them much like a sail on a sailboat.

Today’s dive explored the 2011 eruption on the upper south rift zone of the volcano. The dive began on a fracture that was not in our pre-2011 eruption AUV bathymetry but is in our AUV data collected two weeks ago. It continued north past a small pillow mound that also appears in this new map but not in the old, which we confirmed was recently emplaced and is the southernmost 2011 lava that erupted on the upper south rift. Eighteen kilometers further south on the deep part of the rift is a large pillow ridge that also erupted in 2011, five kilometers long and 120 meters high, which we dove on last summer. Further north are the large sheet flows of the 2011 eruption that we explored on Wednesday’s dive and on two previous cruises.

The lava on the seafloor where our dive began was old and . The jumbled sheet flow had been turbulently emplaced, close to the fissure and channel system of a much earlier eruption. There was no fresh lava visible down in the new fracture but bacterial mat on the walls of the fracture indicates hot water is seeping up as the dike below is cooling. This observation confirms the hypothesis that the dike that fed the flows uprift during this eruption extended farther downrift, but here remained subsurface. This case also is an example of how fracturing can occur on older cones and flows, which we are using here and elsewhere on the mid-ocean ridge system to make interpretations about relative ages of the flows based on the concept that the more fractured flows are probably older.

The dive continued north to find contacts between 2011 eruption lavas, 1998 eruption lavas, and several older cones and sheet flows that had not yet been sampled and dated. A highlight was a descent into a deep collapse pit in an older inflated lobate flow, to find drainback shelves and lava veneer the entire way down the 20 meter high wall.

fracture in old jumbled lava flow

Fracture in old jumbled lava flow was caused by intrusion of a dike during the 2011 eruption. Hydrothermal staining indicates hot water is seeping from the fracture as the new lava underneath is cooling.

Fracture in thick sediments on an older flow further along the 2011 dike.

Fracture in thick sediments on an older flow further along the 2011 dike.

thick rim remaining of collapsed roof of a drained lobate flow

Thick rim remaining of the collapsed roof of a drained lobate flow is overhanging the floor of the pit by about 20 meters. Hollow lobate pillows also have drainage shelves and the lower edge (lower center) is populated by dense colonies of small pink zoanthid soft corals. Field of view is about three meters across.

drainage veneers and shelves on collapse pit walls

Drainage veneers and shelves on collapse pit walls were left behind as the molten lava level dropped in the pit and left chilled lava rinds behind on the massive lava pillars that are supporting the roof. Field of view is about five meters across.

Lava whorl solidified in a sheet flow.

Lava whorl solidified in a sheet flow.

—Jenny Paduan

This is my first time on a deep-sea research trip and my first time visiting a mid-ocean ridge. I am a biologist who has come along for the ride to photograph and collect deep-sea worms from the class Enteropneusta. Enteropneusts, commonly known as acorn worms, are closer relatives to vertebrates (that’s us!) than one would think. Despite their gelatinous appearance they have three of the five anatomical characteristics that define the phylum chordate: a dorsal hollow nerve cord, notochord and pharyngeal gill pores. In 2005 a new family, the Torquaratoridae, was described. This family of enteropneusts live on the deep seafloor and some, maybe all of them, are capable of floating in the water column from one foraging site to the next.

Several new species have been seen on previous MBARI expeditions in the Axial area. Since their discovery, many have been recorded on video but few specimens have been collected for morphological and molecular study. They are very delicate organisms and often disintegrate during collection. We don’t fully understand how these animals choose where to feed, although we do know that they eat sediment. New lavas aren’t covered in sediment for a while, so we tend to find them in areas where eruptions took place long ago. Enteropneusts found at Axial Seamount are primarily translucent with light pigmentation to their neck or intestinal region. They blend in with their surroundings very well and often the best way to see them is by the fecal trail left behind. Thankfully, we have two experts aboard who have trained eyes to spot these small, elusive worms.

After the collected animals make their long ascent to the surface, they are quickly removed from the ROV and put in the cold room. Collection containers are checked for symbiotic or parasitic organisms. Then the worm is gently transferred so that it can be photographed. After the animal has been photographed, tissue samples are removed for genetic analysis and then the organism is preserved. So far, nine specimens have been collected. Hopefully, digital imagery and molecular information can help elucidate what species they are, their various behaviors, and how they are distantly related to us.

Enteropneust crawling on lava pillow

Enteropneust (Torquaratoridae) crawling on lightly sedimented, breadcrust-like textured surface of a lava pillow. Two brittle stars are also visible. Red laser dots are 29 centimeters apart for scale.

lava pillow resembles a small bird

Lava pillow overhanging the edge of a fracture resembles a small bird perching over the precipice.

—Jessie Whelpley