By Alastair G.C. Graham, Anna Wåhlin, Lauren Simkins, Claus-Dieter Hillenbrand, James A. Smith, and the THOR and TARSAN projects of the International Thwaites Glacier Collaboration (ITGC)
As our understanding of the Antarctic continent has improved, we have come to realise that certain parts of the ice sheet are more susceptible to rapid change than others. Large and fast-flowing glaciers, whose bases are bathed by relatively warm ocean waters, and which rest on beds that deepen towards the ice sheet interior, are the cause for most concern. These conditions dispose glaciers to a potential runaway retreat aided by ocean melting1. Thwaites Glacier in West Antarctica ticks all these boxes2.
In January 2019, our research team set sail for the Amundsen Sea embayment, West Antarctica, aboard the US Antarctic Program ice-breaker R.V. Nathaniel B. Palmer. The voyage was the first in a 5-year project dubbed Thwaites Offshore Research (THOR). Our overall aim: to use the geological record of the sea floor in front of Thwaites to reconstruct the history of the glacier, and to investigate how the glacier has responded to forcing by the atmosphere and ocean through past centuries and millennia.
Covering an area ~70,000 km2, Thwaites Glacier is the size of Florida and, along with its neighbours, holds back enough continental ice to raise sea level by more than a metre. From 30 years of observations from satellites, we know that the glacier is thinning and retreating3,4, as well as speeding up, resulting in a measurable mass loss large enough to impact sea level globally (it currently accounts for c. 4% of global sea level rise5). Computer models warn that over the next few decades, the glacier may lose ice more rapidly, giving a significant increase in sea levels within just a couple of lifetimes6. Despite these warnings, the models of today are suffering from a lack of observations against which to compare, or calibrate, their predictions. Since the models are trying to reconstruct a process that has not yet happened on Earth (or at least not while humans were observing it), there is a deep uncertainty that we try hard to overcome by historical detective work. The goal is to be able to accurately predict if, when, and through what mechanisms Thwaites Glacier will change in coming decades to centuries.
The International Thwaites Glacier Collaboration (ITGC) between the US and UK, of which THOR is one component, aims to resolve this shortcoming. Although the glacier has been recognised as a key player in Antarctica’s future for nearly half a century7, the remoteness of the field site combined with its challenging environment means that the Thwaites region has been largely unexplored. Prior to 2019, only ~100 people had ever stepped foot onto the glacier itself and no ship had sailed within c. 45 km of the glacier grounding line (the point at which the kilometres-thick ice sheet comes afloat). Many of its boundary conditions – simple things such as the shape of the seabed near to and under the ice, or even the thickness of the ice itself – remained elusive. The paths for and patterns of warm ocean water that circulate under and melt the floating fringes of Thwaites had not been ascertained before our expeditions, and the key processes that trigger or accompany phases of glacier retreat were unknown. Importantly, the window of time over which we have been able to witness change in Thwaites Glacier is too short to say, with any certainty, whether the glacier is responding to short-term forcing or is being driven by changes instigated many decades or centuries ago. We know neither how the glacier has evolved in the face of changing climate in the past, nor the processes that have been involved in the retreat of Thwaites from previously largely extents.
Clues at the seabed
At most glacier forelands around the world, evidence can be found for the past extent and glaciological processes in operation from the geomorphology and composition of sediments leading away from the ice front. At Thwaites, we anticipated that such a record of the ice scraping and making tracks in the seabed might also exist near to the ice margin from which a more detailed history of glacier change could be derived8.
Accessing this landscape, however, poses significant challenges. The sought-after archive of past change at Thwaites is submerged many hundreds of metres beneath the polar ocean. In any given year, the coastal vicinity of the glacier is usually encased by a wide band of fast ice on the ocean surface. Historical ice limits digitised from satellite images record the repeated advance and calving of the Thwaites Ice Tongue9 that periodically produced enormous icebergs – for example, the 40-mile wide B-22 iceberg that calved in 2002 – that have consistently restricted access for vessels and have served to retain and solidify an impenetrable and persistent floating ice melange.
The 2019 sea ice year was a fortuitous one for our voyage. Inner shelf polynyas opened up early in the austral season (part of an overall decadal trend of declining summer sea ice extent and season length in the Amundsen Sea10 and, assisted by strong off-ice winds, the sea ice at Thwaites cleared. Previous expeditions had mapped the sea bed farther offshore11–13. Now, we finally had access to an area of open and previously uncharted water roughly the size of Houston in front of the glacier.
Rán: The Orange Goddess of the Sea
A multi-national research team from another ITGC project, TARSAN (which stands for Thwaites-Amundsen Regional Survey and Network Integrating Atmosphere-Ice-Ocean Processes), worked alongside us on the expedition. Aligned closely to the THOR objectives, TARSAN aims to understand how of the ocean is currently influencing Thwaites. TARSAN has been making use of a variety of instruments (gliders, seals and ship-based tools) to trace water masses, understand their spatial and temporal variability, and monitor mixing and melting in the ocean near Thwaites as well as underneath in its vast ice cavity.
Chief amongst the arsenal of research tools on the Palmer was the University of Gothenburg’s Kongsberg HUGIN AUV ‘Rán’ – a 6.5-m long, 1-m diameter orange submarine (https://www.gu.se/en/skagerak/auv-autonomous-underwater-vehicle). Rán is named after the Norse goddess of the sea and tooled to the hilt with geophysical instrumentation for sea-floor mapping and sensors for underway oceanographic observation.
AUVs provide a unique and valuable platform for work in icy environments14. Aside from their manoeuvrability and endurance, they can operate and remain in areas where ships cannot, even under extensive floating ice shelves. They can capture a multi-frequency view (both in space and time) of the sea bed and water column; and, compared to surface vessels they offer an unrivalled picture of sea floor landforms and processes that simply cannot be achieved by ships, owing to their ability to track and survey the sea bed at close distance with considerable stability.
We deployed and recovered the AUV from the stern of the vessel, using small zodiac boats and grapples to reign in the vehicle. In choppy waters, high winds, and icy seas, and without a dedicated launch and recovery system, this was a challenging task made to look easy by the Palmer’s expert crew, marine technicians and the AUV’s Swedish support team. Over two extended survey missions, we targeted submarine troughs previously hypothesised as conduits for warm water access to the glacier grounding line. At both sites, the sea-floor also comprised of sills that likely served as former stabilising points for the grounding zone of Thwaites. Ran collected stunning imagery of the sea floor, including geomorphological features formed by Thwaites Glacier in the past that are the focus of the present paper, published in Nature Geoscience. The AUV also obtained invaluable data on the whereabouts and circulation of warm ocean currents in the Thwaites vicinity (published recently in Science Advances15). Both missions included forays under the floating ice tongue, making this expedition pioneering as the first scientific deployment of any kind into the Thwaites ice shelf cavity.
Ribs at tidally-modulated grounding lines
Our new paper presents the highest resolution geophysical images of the ocean bed anywhere offshore of the West Antarctic Ice Sheet. We found ribbed ridges across the top of a large sea-bed high -- a bump that mimics the modern-day grounding zone setting for several parts of the Thwaites system -- that were formed in series, one ridge per day, as the Thwaites grounding line pulled back in the past. The tides modulate the retreat and the ribs preserve the tidal signal in their shapes, size, and spacing.
We knew we had found something extraordinary straight away. Having sighed with relief at recovering the AUV from an intensive 20-hour mission away from the ship, we were astonished by the images downloaded from the sub. Little did we know that at the same time as we were making our discoveries, a team led by Prof. Julian Dowdeswell, at the University of Cambridge, was making very similar maps from the tops of ancient grounding zone features off of the Larsen Inlet, Antarctic Peninsula. Interestingly, both teams mapped very similar sets of landforms and arrived independently at very similar mechanisms for forming the ridges16. Critically we were able to show the tidal signature of the grounding line rising and falling, superimposed on the overall retreat, that allowed us to confidently interpret the former retreat rate and recent grounding zone behaviour.
For Thwaites, the importance of our results lie in two take-home messages. The first is that Thwaites has undergone significantly faster rates of retreat in the past than it is experiencing right now (>2 kilometres per year compared to the present somewhere between 0.3 and 1 kilometre per year). This finding raises the potential upper ‘’speed limit’ on our expectations for Thwaites behaviour in the near future. Second, our results show that very short (several months duration) pulses of non-linear grounding zone retreat occur, especially where the ice is retreating from flattish sea floors or flat-topped ridges. These phases of rapid retreat are corroborated by modern observations17, and demonstrate, collectively, that drastic changes in grounding line position might be expected, even from one field season to the next (often the observational sampling rate of Antarctic glaciers). Furthermore, the Dowdeswell et al. data suggest that some phases of retreat could take place at even faster speeds; 5 or 10 times what we are witnessing from year to year at Thwaites right now, albeit under different forcing conditions.
What comes next? Direct sampling of the ribs could tell us a lot about the grounding line processes that form the ribbed ridges. Unfortunately, an ice shelf collapse in February 2019 stopped our efforts to obtain sediment records at the time; and, with heavy sea ice preventing a return to the site in two further THOR/TARSAN field expeditions in 2020 and 2022, our exploration of the processes that form ribs will turn next to mathematical simulations, specifically focused on replicating rib evolution in a model linking the ice, tides, and sediment bed. Meanwhile, the ITGC continues to combine better observations, past and present, to pursue an answer to how much, and how quickly Thwaites will commit to ongoing sea level rise over the next 75 years.
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