The ice at Leconte Glacier moves at 20-30 m per day towards the ocean. Most of this ice either calves off the terminus, or melts as it contacts the warm subsurface waters of the fjords, so that the terminus can remain in a relatively stable location. But this delicate balance can be disrupted if either melt of calving exceeds the supply of ice - causing retreat — or advance if the supply of ice were to outpace melt and calving.
A day in the life of ROSE: another journey to the glacier FACE
It took years of preparations and hard work from students and engineers to develop a robotic vessel capable of performing the tasks we usually do from a traditional research vessel (OSU Robotics student Nick McComb’s webpage gives some history of the technical side of ROSE & ROSS.) This September, ROSE performed flawlessly, pushing through the ice to collect data at a place where we humans will never venture.
The data ROSE collects
ROSE executed 14 missions to the terminus face, during which it collected almost 200 CTD profiles, many of which to 150-m or more. These data reveal the existence of meltwater intrusions, which are the first direct evidence of meltwater from a tidewater glacier. These also map out the patterns of the subglacial plume, the patterns and quantity of warm inflowing ocean waters, and the variability that arises in response to changes in ice structure, due to both melt and calving events.
Moorings deployed by Rose:
Oceanographic moorings are instrument clusters that rise above the ocean floor and are deployed for days to months to years. They generally consist of an anchor weight (often a 1000 lb railroad wheel), a buoyant float, scientific sensors (often placed along a mooring line (or wire), and an “acoustic release,” a device that we can talk to (from our ship) that breaks apart the mooring when we wish to bring it back. Moorings have never before been deployed along a glacier terminus because manned ships can’t get there.
Instead of using a manned research vessel to deploy our moorings, we used ROSE to pull a small raft to the glacier face, and then, using a command sent by radio, we released a pair of trap doors that let the mooring fall to the bottom. Two weeks later, we used an acoustic “pinger” to call to the moorings and tell them to come back to the surface.
Some of our moorings primarily had hydrophones that listen to the melting and calving events. Other moorings had ADCPs (Acoustic Doppler Current Profilers) on them (to image the velocity field above the mooring), and sensors to measure the ocean temperatures and salinities. Teledyne RD Instruments kindly supported this project by lending us one of their new 5-beam Sentinel V 300 kHz ADCPs (see photos below) that we deployed on one of our “ABLE” moorings.
When we recovered the moorings, they had significant build-up of sediments and small rocks on them, which presumably came from a combination of melting glacial ice, calving events, and strong near-bottom flows.
One of our ADCPs moorings was directly hit by a calving event (TWICE!), which was evident both from the acoustics signals (below) reflecting directly off of the newly formed iceberg (right at the bottom!), and the pressure record of the instrument, which indicate that the entire mooring dropped 4 m at the time of the calving event. Either it was pushed down a slope, or the moraine that the mooring was deployed on slumped.