The melting of glaciers is accelerating, caused by higher ocean temperatures and changing climate. The Greenland Ice Sheet is a major contributor to sea level rise produced by both surface melt and glacier flow into the ocean. 

Ocean robot equipped with CTD online sensor and 360° sonar
Ice shelf cavities have been unexplored due to the difficulty of accessing them. Researchers from University of California Irvine, NASA's Jet Propulsion Laboratory at California Institute of Technology, USA, and Access Arctic, France, explored the ice cavity of Petermann Glacier, Greenland, using a Grounding Line Remote Operated Vehicle (GROV). The GROV was equipped with Star-Oddi's CTD online sensor measuring conductivity, temperature and depth, as well as a 360° interferometric multibeam sonar capable of imaging both the top and bottom of the cavity simultaneously. The GROV was deployed through a borehole in the ice shelf to map the ice shelf base and fjord bathymetry, revealing critical insights into ice-ocean interactions. 

Seafloor detected deeper than expected
Key findings include the discovery of a flat seafloor at 820 m depth, 200 m deeper than anticipated, filled with sediments. The ice shelf base exhibits spatial variations in melt processes, including widespread ice terraces separated by 30–40 m ice cliffs and a smooth central melt channel. The terraces are associated with low melt rates due to negligible water plume entrainment, while the cliffs indicate high melt rates due to steep basal slopes. These features are not visible at the ice surface, highlighting the limitations of surface elevation data for estimating ice melt rates. 

Several water layers distinguished with CTD measurements
The Star-Oddi CTD online sensor measured wide conductivity range 3-68 mS/cm and revealed a sharp pycnocline at 75 m depth, transitioning from pure freshwater to salty Atlantic-origin water. Below 100 m, the CTD detected double-diffusive convection (DC) staircases, characterized by temperature and salinity jumps, which limit ice shelf melt compared to turbulent mixing. 

Central melt channel migrates 100m every year
The study found that the main melt channel deviates ±40 m from flotation, and melt rates are higher on the east side, causing the channel to move eastward at 100 meters each year.  Additionally, the base of the ice shelf varies a lot from the floating position in some areas. This makes it difficult to use surface height data to accurately measure melt rates in detail.

Overall, the GROV survey provides unprecedented insights into the complex ice-ocean interactions beneath Petermann Glacier, emphasizing the importance of in-situ observations for understanding glacier evolution and improving ice sheet models. The findings highlight the need for further exploration to refine the relationship between DC staircases, ice terraces, and melt processes. 

The study was published in Advancing Earth and Space Sciences (AGU) and can be found here. 

Picture: earth.com