The Long Drop

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Jason in the waters above Axial Seamount. Credit: M. Elend, University of Washington; V19.

It’s one thing to know on an intellectual level that the Pacific Ocean is very deep, but it really sinks in (no pun intended) when you lift a piece of equipment over the side of the ship, lower it into the water, let it sink out of sight, and then send a robotic submersible after it. It takes almost 2 hours for Jason to descend 2600 meters (8530 feet, or about three times the height of the Burj Khalifa building in Dubai), watching the water turn from clear turquoise blue to deeper and deeper shades until it finally goes entirely black, and the sub has to turn on the lights. This was the process for most of our activities today, after we left the ASHES vent field atop Axial Seamount.

The 14.3 ft tall multibeam sonar system ‘COVIS’ comes onboard the R/V Atlantis subsequent to being installed at the ASHES hydrothermal field. Credit: M. Elend, University of Washington. V19.

Before we left ASHES, Jason snagged the COVIS instrument package, which is a NSF funded project led by Dr. Karen Bemis at Rutgers University. Although this is not a core OOI instrument, one of the exciting features of the OOI Regional Cabled Array is the expandability factor: the plentiful power and bandwidth provided by the electro-optical cable, and the additional ports on all the junction boxes, means that researchers can write proposals to deploy their equipment near the Array or plug directly into it, allowing them to get their data back in real- time. This adds additional data streams to what OOI delivers to the public and allows scientists to use OOI data from collocated instruments to bolster their own research or validate their data.

COVIS (Cabled Observatory Vent Imaging Sonar), was designed to measure fluid flow issuing from underwater hot springs. The end result is to use these measurements to calculate heat flux, something that has been very difficult do quantitatively in the past.  But this knowledge is critical to understand cooling of underwater volcanoes and impact on the overlying ocean. The active sonar sits on a 4 m-high tripod frame, with motors to adjust the leveling and direction of the acoustic signal. It strikes an imposing figure looming out of the darkness when Jason approaches.

The design of COVIS allows it to measure fluid flow high above the seafloor, but it also means that it won’t fit neatly into the undervator, nor can it be carried in the mechanical arms of the ROV. For this recovery, Jason pilots used the equivalent of an automobile tow cable to hook a float on top of COVIS and carry it back up to the surface slung underneath the vehicle. Once at the surface, the load was transferred to the ship’s crane, and the massive instrument was brought aboard.

The HPIES instrument installed in 2018 is recovered underneath Jason. Credit: M. Elend, University of Washington;V19.

We then moved back to the Axial Base site, where an HPIES (Horizontal Electrometer Pressure Inverted Echosounder) instrument was assembled, tested, and then dropped in a freefall to the seafloor. The instrument hosted a beacon that allowed tracking of the package as it descended through the deep ocean from the ship using a specialized navigation system. Jason followed, and the instrument was quickly located and then plugged into the cable. HPIES is a unique instrument made up of a pressure sensor, a 12-kHz inverted echo-sounder, and a horizontal electrometer. These measure electrical fields, overlying ocean water pressure, and time it takes for sound to travel from the seafloor to the sea surface, to learn more about the physical properties of the water column, like water currents and temperature-related changes in sea surface height. The HPIES instrument installed in 2018 was brought back to the ship using Jasons’ underbelly winch.

We also deployed a CTD and optical attenuation and absorption instrument on a mini tripod, which required another long ascent to the surface to pick up the instrument, and then another long drop back to the seafloor. One of the highlights of today’s long dives for the students and marine biologists onboard was the sight of a small, purple Graneledone octopus hiding behind one of the legs of the HPIES, as well as a pom-pom anemone that was swept past in the current.

A Pom Pom anemone on sedimented seafloor 8530 ft beneath the oceans’ surface. Credit: UW/NSF-OOI/WHOI; V19.

Finally, after all of the instruments were plugged in, and Jason was back on the ship, we took some water samples (with the invaluable help of the VISIONS students) and started heading for our next deployment site: back to Slope Base. Deep water means long dives, but there’s always something new to see!