Science1 publisherNot yet confirmed elsewhere2 min readPublished
Seafloor wedge in Vincennes Bay records East Antarctic ice advancing 65 km mid-retreat
Kiel and AWI researchers imaged a 580-cubic-kilometre sediment wedge off East Antarctica that records ice halting its retreat and advancing about 65 km. The authors link the stall partly to the seafloor itself, so ice-sheet models now have a site where simulated retreat has to pause and push back.
The Scientist · Science desk
What happened
- At roughly 260 metres high and 65 kilometres long, it is the largest known isolated grounding-zone wedge on any glaciated continental shelf.
- The team mapped it with 340 kilometres of high-resolution seismic reflection lines from Polarstern Expedition PS141 in early 2024, plus bathymetry and sediment echo-sounder data.
- After that readvance the grounding zone retreated permanently, as the ice sheet continued withdrawing after the last glacial period.
- The results appear in Geophysical Research Letters, with partners from the University of Bremen, the University of Tasmania and an Australian Antarctic science centre.
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Why it matters
- constraint Simulations of East Antarctic retreat since the last glacial period get a site-specific check: in Vincennes Bay the grounding line has to stall, push about 65 km seaward, then retreat for good.
- decision Models that set grounding-line retreat by climate alone leave out a control the authors call significant, the shape and composition of the seafloor beneath each bay.
- capability Imaging a wedge's internal layers gives a step-by-step history of how a grounding line moved, a sequence models can be tested against beyond a single end position.
The wedge's height is about one 250th of its length [17]. On the floor of Vincennes Bay it is a long, low ramp of sediment, laid down where grounded ice met the floating ice shelf [4].
Two figures in the account are the same. The grounding zone advanced seaward by about 65 kilometres [5], and the wedge is 65 kilometres long [2][18]. The match fits the process the authors describe. While the grounding zone holds position, sediment carried beneath the ice is deposited there. The growing wedge lets the grounded ice move slightly seaward, new sediment lands on its seaward side, and the grounding line works its way out to sea [10]. In the seismic profiles the layers step seaward, recording material from the hinterland carried under the ice and dropped at the grounding zone [14].
"We can clearly see that an ice sheet does not simply retreat uniformly across all bays," said Chiara Tobisch, the study's first author and a doctoral researcher at Kiel University [8][16]. "Depending on the characteristics of the substrate, the grounding zone can remain locally stable over long periods of time and may even shift seaward again due to sediment deposition." [8]
Relatively little had been known about the inside of such wedges [12]. "What's new is that, for the first time, we were able to image these dynamics very precisely using high-resolution methods and reconstruct the formation process based on the internal structure of the grounding-zone wedge," Tobisch said [9]. The bay lies offshore from the Aurora Subglacial Basin [13].
The authors conclude that grounding-zone stability depends on climate and also, to a significant extent, on the shape and composition of the seafloor [6]. At this site the seafloor's slope changes at the wedge's landward end, where retreat halted and the stall began [15]. The account lists the wedge's age among the reference points it offers modellers [11] but does not report it, or how long the stall lasted.
I'd treat Vincennes Bay as a strong test at one site and weak evidence about the rest of the coast. The wedge stands out because it is the largest of its kind known [3]. A record that is notable for its size shows what the ice sheet did in one bay. Knowing how often East Antarctic retreat paused and pushed back elsewhere would take a count of wedges across many bays.
What to watch
- Publication of the wedge's age and the length of the stall, both needed to use Vincennes Bay as a timed check on models.
- Whether East Antarctic deglaciation simulations reproduce a stall and a roughly 65 km readvance at this site.
- Seismic surveys of other East Antarctic bays that find wedges at seafloor slope breaks, to show whether this readvance was typical or exceptional.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
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- [1]
Researchers led by Kiel University (CAU) and the Alfred Wegener Institute (AWI) investigated an exceptionally large sedimentary body in the seafloor of Vincennes Bay off East Antarctica, a grounding-zone wedge.
- [2]
The grounding-zone wedge is symmetrical, wedge-shaped, approximately 260 meters high and 65 kilometers long.
- [3]
With a volume of more than 580 cubic kilometers, it is the largest known isolated grounding-zone wedge on a glaciated continental shelf to date.
- [4]
The sediment wedge formed within the grounding zone, the transition area between ice resting on the seafloor and a floating ice shelf.
- [5]
During the retreat of the East Antarctic Ice Sheet following the end of the last glacial period, the grounding zone temporarily stabilized, then advanced seaward by about 65 kilometers and finally retreated permanently.
- [6]
The researchers demonstrated that the stability of the grounding zone depends not only on climatic changes but also, to a significant extent, on the shape and composition of the seafloor.
- [7]
The study was published in Geophysical Research Letters and provides reference points for modeling the behavior of the East Antarctic Ice Sheet.
- [8]
"We can clearly see that an ice sheet does not simply retreat uniformly across all bays. Depending on the characteristics of the substrate, the grounding zone can remain locally stable over long periods of time and may even shift seaward again due to sediment deposition," says Chiara Tobisch.
- [9]
"What's new is that, for the first time, we were able to image these dynamics very precisely using high-resolution methods and reconstruct the formation process based on the internal structure of the grounding-zone wedge."
- [10]
If the grounding zone remains stationary for a long time, subglacial sediment is deposited there; the wedge allows grounded ice to advance slightly seaward, resulting in new sediment deposition on the seaward side; the combination leads to a gradual advance of the grounding line toward the sea.
- [11]
The location, age and formation mechanism of grounding-zone wedges serve as important reference points for verifying simulations of past and present ice-sheet dynamics.
- [12]
Until now relatively little was known about the internal structure and formation processes of these structures; the international team included scientists from Kiel University, the AWI, the University of Bremen, the University of Tasmania and the Australian Center for Excellence in Antarctic Science.
- [13]
The study is based on 340 kilometers of high-resolution seismic reflection data collected during Expedition PS141 aboard the research vessel Polarstern in early 2024 in Vincennes Bay, offshore from the Aurora Subglacial Basin, combined with bathymetric and sediment echo-sounder data.
- [14]
The data reveal distinct sediment layers progressing seaward, documenting how material from the hinterland was transported beneath the ice and deposited at the grounding zone.
- [15]
The landward end of the grounding-zone wedge marks the point where retreat halted and grounding-zone stabilization began; there, the slope of the seafloor changes.
- [16]
Chiara Tobisch is first author of the study and a doctoral researcher in the Marine Geophysics and Hydroacoustics research group at the Institute of Geosciences at Kiel University.
- [17]
The wedge's length is about 250 times its height.
- [18]
The roughly 65 km seaward advance of the grounding zone equals the 65 km length of the wedge.
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Topics
- Antarctic ice-sheet dynamicsFollow
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Entities
- Kiel UniversityFollow
- Alfred Wegener InstituteFollow
- Chiara TobischFollow
- East Antarctic Ice SheetFollow
- PolarsternFollow
- Geophysical Research LettersFollow
- Vanderford GlacierFollow