Thwaites Gets Ribbed

Thwaites Glacier and surrounding landmarks, via NASA

“Thwaites,” in this instance, is Thwaites Glacier and Ice Shelf, located in Antarctica, west and a little south of the Antarctic Peninsula and Ellsworth Land, it dumps into the Amundsen Sea. “Ribbed” is, well we will get to that in a minute.

Thwaites has gotten a lot of press. Rolling Stone, back in 2017, called Thwaites Glacier the “Doomsday Glacier,” because its rate of retreat and sheer volume of ice has the potential to raise world ocean levels in this century by more than a meter. It is retreating, in fits and starts, by a kilometer or more a year. A study was recently published examining field data from 2019. The study reveals a great deal about the complex dynamics of the ice shelf and the glacier feeding it. But for now, WC wants to focus on the very strange geology under the snout of Thwaites on the Amundsen Sea floor.

The 2019 expedition deployed an autonomous submersible to explore the sea floor as close as under the towering ice shelf. What it found on the sea bottom was comparatively warm waters melting the underside of the ice shelf. That was more or less expected. But what was extraordinary was the geomorphology of that sea bottom.

Sea bottom formations, Thwaites Ice Shelf front, Alastair G.C. Graham et al., “Rapid Retreat of Thwaites Glacier in the pre-satellite era,” Nature, Sep. 5, 2022

These remarkable sea floor images were made from an autonomous submersible using high-resolution (sub-meter) multibeam bathymetry and acoustic imagery from sidescan sonar, “flying’ at an altitude of 50–90 meters above the sea floor.  (a) shows the non-alignment of ribs to underlying lineations (b), rib formation on terraces (c) and the ‘beading’ (red circle) and overprinting (red arrow) of existing subglacial features (d) and (e) are multibeam hillshades showing fine-scale landforms, less than 20 centimeters high, crossing lineation ridges and grooves. (f) Shows the profile for the line XX′, demonstrating the subtle geometries of some of the “ribs” (5–20 centimeters) and their surprising depth (more than 740 meters). 

What mechanism could make such remarkable similar, parallel structures at nearly 700 meters under water? Alastair’s group puzzled it out, and it’s pretty slick.

Parallel rib construction, Thwaites Ice Shelf front, Alastair G.C. Graham et al., “Rapid Retreat of Thwaites Glacier in the pre-satellite era,” Nature, Sep. 5, 2022

In (a), at low tide levels, the glacier plows ahead, generating a small underwater terminal moraine in front of it, 10 to 40 centimeters high. Remember the glacier is flowing at a very fast pace. In (b), at high tides the snout of the glacier is lifted up enough to allow the slightly warmer waters of the Southern Ocean to circulate through the newly created channel, melting the underbelly of the ice sheet. In (c), low tide collapses the ice sheet back onto the sea floor, just a meter or so further back because of the melt. The glacier continues to push forward and creates a new small moraine, a tidal day’s distance behind the crest of the last small moraine.

It doesn’t happen everywhere under Thwaites. Obviously, it won’t work on a rock sea bottom, or where strong currents would erode away the parallel moraines. But where conditions were right, Thwaites has left an exquisite set of footprints that allow the glaciologists to determine just how much the glacier has receded each tidal day. To the millimeter. How cool is that?

The data so far suggest that it’s not a steady process. The rate of retreat varies, depending what is exposed as the tide lifts up ice sheet every day. Presently, the rate of retreat has slowed as Thwaites’ snout has snagged on rocky ridges. But the historic record is there to be read. Written by the glacier in the sea floor.

Alastair’s group called the formations ribs, which they sorta kinda resemble, Which is why WC titled the post as he did.

See why WC loves geology?

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