The terminus’ “mixing zone” is where all the action occurs. Here, warm salty ocean waters are drawn towards the glacier face in the huge turbulent boils of rising, buoyant waters that emerge from beneath the glacier at near-freezing temperatures. It is also the place where chunks of ice the size of a football field break off and rise to the surface from beneath as “shooters.”
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.
View from one of our time-lapse cameras showing our research vessel (the Amber Anne) and ROSE (the Robotic Oceanographic Surface Explorer) headed towards the glacier face and subglacial discharge plume that is the prime objective for our sampling.
ROSE - the Robotic Oceanographic Surface Explorer - is a remotely-operated science work-horse. ROSE performed 14 missions to the glacier face, during which she collected data deep beneath the ocean surface that reveals (1) how heat flows to the glacier terminus, (2) how melt-water flows away from the terminus, and (3) how the deep ‘subglacial discharge’ rises buoyantly to the surface, simultaneously pulling warm offshore waters into the fjord.
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.
Clockwise from upper left: (1) recovery of ROSE, (2) ROSE on deck, (3) the Amber Anne, our mission control center, (4) Mooring recovery and zoom-in of the mooring’s acoustics buoy, and (5) A three-image sequence of ROSE during a mission along the terminus face.
The ‘business side’ of ROSE is what is beneath the surface: on the keel are optical sensors and CTDs that measure salinity, temperature and sediments, turbulence sensors that capture the mixing and energy in the billowing eddies, and a 5-beam Teledyne RDI ADCP that images the velocity field up to 400 ft beneath the surface. At the very stern is a profiling CTD that measures salinity and temperature from the surface to bottom.
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.
Above is a plot that shows the locations of shipboard measurements (small pink dots), moorings (large red dots), and ROSE profiles (mid-sized colored dots). The heavy black line is the terminus location; thin lines show the depth of bottom.
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.
Drone photo of ROSE towing a mooring to the glacier for deployment.
ROSE headed to the terminus for another mooring deployment. This is the view from our ship.
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.
The calving event that moved ABLE. At 16:35 on Sept 6, a huge piece of ice fractured from the terminus, sweeping up and past our bottom lander as it rose to the surface, pulling with it a massive quantity of warm, deep waters. Within seconds, signals were observed of this event were observed throughout the fjord. Was this something special? NO — not at all! Leconte Glacier flows at 20-30 m every day - which means every day or two there is an event of this size. The pictures at right are a sequence of images from the time-lapse cameras that Jason Amundson and Christian Keinholz set up on the fjord’s cliffs. The inset to the left shows the horizontal and vertical velocities that the deep ADCP mooring recorded during this time.