At the end of the last ice age, geological fingerprints recorded in ocean sediments suggests glacier retreat of 50 meters a day, 18 km a year. For comparison the fastest retreating glaciers in Antarctica today are the Pope, and the Smith glacier system, retreating about 2.9 km a year.

The study data was obtained in the Larsen region of the Antarctic Peninsula, during the Weddell Sea Expedition 2019.

Researchers from the Scott Polar Research Institute in Cambridge deployed autonomous underwater vehicles with the mapping capability to examine the sediments of the seafloor in the western Weddell Sea.

The robots discovered a clear pattern of ridges that looked like a series of ladders where each rung was about 1.5 meter tall and spaced roughly 20-25 meter apart.

The interpretation of these ridges suggests that they are generated at the ice grounding zone, the point of a marine terminating glacier where the ice flowing off Antarctica into the ocean becomes buoyant and starts to float. The rungs are created as the ice at this location repeatedly pats the sediments as the tides rise and fall.

These topographic fingerprints of past glacier advances can only be preserved from retreating glaciers, advancing grounding lines would destroy the ridges.

The marks are dated to be around 12,000 years old, and if the theory holds, point to substantial rapid changes affecting sea levels around the globe.

Since past sea levels rose sometimes quickly, several meters a century, the findings add to the growing understanding that our climate induced environmental changes can happen much faster than one would think.

The authors also simulated a 100 km per year glacier retreat, the result implies even higher rates of grounding-line retreat, as the ice would have to lift off from its bed near instantaneously in order to preserve the delicate sea floor features.

One thing we know, not all glaciers retreat with the same speed, in Antarctica it basically comes down to the ocean water temperatures, where the glaciers, ocean water, and bedrock meet – the grounding line.

For instance in West Antarctica, the Pine Island Bay, part of the Amundsen Sea, water samples obtained in 2020 from the base of Thwaites Glacier, was 2 degree Celsius above the freezing temperature.
Since Thwaites bedrock topography goes further downhill farther inland, warmer waters can further erode the glacier grounding line, a scenario called Marine Ice Sheet Instability.
Another similar mechanism for rapid glacier retreat is called Marine Ice Cliff Instability, the physical characteristics of ice cliffs exceeding about 90 meters in height to likely collapse under their own weight.

Perhaps there are episodes of near instantaneous retreat.

At the oceanfront the Thwaites Glacier is about 600 meters tall, becomes thicker farther inland, together the West Antarctic Ice Sheets hold 3.3 meters of potential sea level rise.

Climate State, researching and reporting climate change.

References

• Delicate seafloor landforms reveal past Antarctic grounding-line retreat of kilometers per year https://www.repository.cam.ac.uk/handle/1810/306113 and https://science.sciencemag.org/content/368/6494/1020
• Climate change: ‘Stunning’ seafloor ridges record Antarctic retreat https://www.bbc.com/news/science-environment-52845990
• Glaciology in the Weddell Sea – Julian Dowdeswell https://www.youtube.com/watch?v=pbyrHZqvuv8
• Larsen C Ice Sheet Breaks Off. What Next? https://www.youtube.com/watch?v=jDXr98pt0bg
• Melt at grounding line controls observed and future retreat of Smith, Pope, and Kohler glaciers https://tc.copernicus.org/articles/13/2817/2019/
• Unprecedented data confirms that Antarctica’s most dangerous glacier is melting from below https://www.washingtonpost.com/climate-environment/2020/01/30/unprecedented-data-confirm-that-antarcticas-most-dangerous-glacier-is-melting-below/
• Antarctic & Greenland Melt Accelerates Sea Level Rise (Eric Rignot October 2019) https://www.youtube.com/watch?v=snMhPLEyoHo
• Cambridge Ideas – This Icy World https://www.youtube.com/watch?v=M5nXSC1woLU
• ScienceCasts: No Turning Back – West Antarctic Glaciers in Irreversible Decline https://www.youtube.com/watch?v=W2pYHMx5bN8
• Animation – Loss of West Antarctic Glaciers Appears Unstoppable https://www.youtube.com/watch?v=Adh86ma3oxw
• ITGC: Is Thwaites Glacier contributing to sea level rise? https://youtu.be/l4CCZrBPzyY
• BedMachine: A high-precision map of Antarctic ice sheet bed topography https://svs.gsfc.nasa.gov/4773
• On Thin Ice: Why Ice Loss Matters https://www.youtube.com/watch?v=Nywnud-ifWc

Related

• Huge cavity in Antarctic glacier signals rapid decay https://sealevel.nasa.gov/news/152/huge-cavity-in-antarctic-glacier-signals-rapid-decay
• Weddell Sea Expedition 2019 https://weddellseaexpedition.org
• Rift and Calving at Larsen Ice Shelf https://earthobservatory.nasa.gov/images/event/90556/rift-and-calving-at-larsen-ice-shelf
• Thwaites Glacier Transformed https://earthobservatory.nasa.gov/images/146247/thwaites-glacier-transformed
• The International Thwaites Glacier Collaboration https://thwaitesglacier.org
• The marine geological imprint of Antarctic ice shelves https://www.nature.com/articles/s41467-019-13496-5