Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier

Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed "corrugations" (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high-resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long-term change in Pine Island Glacier. Corrugations and ploughmarks on a sub-ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed-shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a "stable" grounding-line configuration throughout the Holocene

Responses of the Pine Island and Thwaites Glaciers to Melt and Sliding Parameterizations

The Pine Island and Thwaites glaciers are the two largest contributors to sea level rise from Antarctica. Here we examine the influence of basal friction and ice shelf basal melt in determining projected losses. We examine both Weertman and Coulomb friction laws with explicit weakening as the ice thins to flotation, which many friction laws include implicitly via the effective pressure. We find relatively small differences with the choice of friction law (Weertman or Coulomb) but find losses to be highly sensitive to the rate at which the basal traction is reduced as the area upstream of the grounding line thins. Consistent with earlier work on Pine Island Glacier, we find sea level contributions from both glaciers to vary linearly with the melt volume averaged over time and space, with little influence from the spatial or temporal distribution of melt. Based on recent estimates of melt from other studies, our simulations suggest that the combined melt-driven and sea level rise contribution from both glaciers may not exceed 10 cm by 2200, although the uncertainty in model parameters allows for larger increases. We do not include other factors, such as ice shelf breakup, that might increase loss, or factors such as increased accumulation and isostatic uplift that may mitigate loss

Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica

Ice shelves around Antarctica are vulnerable to an increase in ocean-driven melting, with the melt rate depending on ocean temperature and the strength of flow inside the ice-shelf cavities. We present measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate, and thermal variance dissipation rate beneath Pine Island Glacier ice shelf, West Antarctica. These measurements were obtained by CTD, ADCP, and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV). The highest turbulent kinetic energy dissipation rate is found near the grounding line. The thermal variance dissipation rate increases closer to the ice-shelf base, with a maximum value found ∼0.5 m away from the ice. The measurements of turbulent kinetic energy dissipation rate near the ice are used to estimate basal melting of the ice shelf. The dissipation-rate-based melt rate estimates is sensitive to the stability correction parameter in the linear approximation of universal function of the Monin-Obukhov similarity theory for stratified boundary layers. We argue that our estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting

Characteristics and rarity of the strong 1940s westerly wind event over the Amundsen Sea, West Antarctica

Glaciers in the Amundsen Sea Embayment of West Antarctica are rapidly retreating and contributing to sea level rise. Ice loss is occurring primarily via exposure to warm ocean water, which varies in response to local wind variability. There is evidence that retreat was initiated in the mid-20th century, but the perturbation that may have triggered retreat remains unknown. A leading hypothesis is that large pressure and wind anomalies in the 1940s drove exceptionally strong oceanic ice-shelf melting. However, the characteristics, drivers, and rarity of the atmospheric event remain poorly constrained. We investigate the 1940s atmospheric event using paleoclimate reconstructions and climate model simulations. The reconstructions show that large westerly wind anomalies occurred from ∼1938–1942, a combined response to the very large El Niño event from 1940–1942 and other variability beginning years earlier. Climate model simulations provide evidence that events of similar magnitude and duration may occur tens to hundreds of times per 10 kyr of internal climate variability (∼0.2 to 2.5 occurrences per century). Our results suggest that the 1940s westerly event is unlikely to have been exceptional enough to be the sole explanation for the initiation of Amundsen Sea glacier retreat. Additional factors are likely needed to explain the onset of retreat in West Antarctica, such as naturally arising variability in ocean conditions prior to the 1940s or anthropogenically driven trends since the 1940s.</p

Decadal ocean forcing and Antarctic ice sheet response: Lessons from the Amundsen Sea

Mass loss from the Antarctic Ice Sheet is driven by changes at the marine margins. In the Amundsen Sea, thinning of the ice shelves has allowed the outlet glaciers to accelerate and thin, resulting in inland migration of their grounding lines. The ultimate driver is often assumed to be ocean warming, but the recent record of ocean temperature is dominated by decadal variability rather than a trend. The distribution of water masses on the Amundsen Sea continental shelf is particularly sensitive to atmospheric forcing, while the regional atmospheric circulation is highly variable, at least in part because of the impact of tropical variability. Changes in atmospheric circulation force changes in ice shelf melting, which drive step-wise movement of the grounding line between localized high points on the bed. When the grounding line is located on a high point, outlet glacier flow is sensitive to atmosphere-ocean variability, but once retreat or advance to the next high point has been triggered, ocean circulation and melt rate changes associated with the evolution in geometry of the sub-ice-shelf cavity dominate, and the sensitivity to atmospheric forcing is greatly reduced

Basal terraces on melting ice shelves

Ocean waters melt the margins of Antarctic and Greenland glaciers, and individual glaciers' responses and the integrity of their ice shelves are expected to depend on the spatial distribution of melt. The bases of the ice shelves associated with Pine Island Glacier (West Antarctica) and Petermann Glacier (Greenland) have similar geometries, including kilometer-wide, hundreds-of-meter high channels oriented along and across the direction of ice flow. The channels are enhanced by, and constrain, oceanic melt. New meter-scale observations of basal topography reveal peculiar glaciated landscapes. Channel flanks are not smooth, but are instead stepped, with hundreds-of-meters-wide flat terraces separated by 5–50 m high walls. Melting is shown to be modulated by the geometry: constant across each terrace, changing from one terrace to the next, and greatly enhanced on the ~45° inclined walls. Melting is therefore fundamentally heterogeneous and likely associated with stratification in the ice-ocean boundary layer, challenging current models of ice shelf-ocean interactions

Ice-shelf retreat drives recent Pine Island Glacier speedup

Speedup of Pine Island Glacier over the past several decades has made it Antarctica’s largest contributor to sea-level rise. The past speedup is largely due to grounding-line retreat in response to ocean-induced thinning that reduced ice-shelf buttressing. While speeds remained fairly steady from 2009 to late 2017, our Copernicus Sentinel 1A/B–derived velocity data show a >12% speedup over the past 3 years, coincident with a 19-km retreat of the ice shelf. We use an ice-flow model to simulate this loss, finding that accelerated calving can explain the recent speedup, independent of the grounding-line, melt-driven processes responsible for past speedups. If the ice shelf’s rapid retreat continues, it could further destabilize the glacier far sooner than would be expected due to surface- or ocean-melting processes

The impact of the Amundsen Sea freshwater balance on ocean melting of the West Antarctic Ice Sheet

The Amundsen Sea has the highest thinning rates of ice shelves in Antarctica. This imbalance is caused by changes in ocean melting induced by warm Circumpolar Deep Water (CDW) intrusions. The resulting changing freshwater balance could affect the on‐shelf currents and mixing. However, a clear understanding of the sources and sinks of freshwater in the region is lacking. Here we use a model of the Amundsen Sea, with passive freshwater tracers, to investigate the relative magnitudes and spatial distributions of the different freshwater components. In the surface layer and as a depth average, all freshwater tracer concentrations are of comparable magnitude, though on a depth average, sea ice and ice shelf are largest. The total freshwater tracer distribution is similar to that of the ice‐shelf tracer field. This implies a potential for ice‐shelf meltwater feedbacks, whereby abundant ice‐shelf meltwater alters the ocean circulation and stratification, affecting melting. Ice‐shelf and sea‐ice freshwater fluxes have the largest interannual variability. The effect of including grounded icebergs and iceberg freshwater flux are studied in detail. The presence of icebergs increases CDW intrusions that reach the base of ice shelves. This suggests another possible feedback mechanism, whereby more icebergs induce greater ice‐shelf melting and hence more icebergs. However, the strength of this potential feedback is dependent on poorly constrained sea‐ice model parameters. These results imply that poorly constrained parameters relating to the ocean freshwater balance, such as those relating to icebergs and sea ice, impact predictions for melting of the West Antarctic Ice Sheet

Pathways of ocean heat towards Pine Island and Thwaites grounding lines

In the Amundsen Sea, modified Circumpolar Deep Water (mCDW) intrudes into ice shelf cavities, causing high ice shelf melting near the ice sheet grounding lines, accelerating ice flow, and controlling the pace of future Antarctic contributions to global sea level. The pathways of mCDW towards grounding lines are crucial as they directly control the heat reaching the ice. A realistic representation of mCDW circulation, however, remains challenging due to the sparsity of in-situ observations and the difficulty of ocean models to reproduce the available observations. In this study, we use an unprecedentedly high-resolution (200 m horizontal and 10 m vertical grid spacing) ocean model that resolves shelf-sea and sub-ice-shelf environments in qualitative agreement with existing observations during austral summer conditions. We demonstrate that the waters reaching the Pine Island and Thwaites grounding lines follow specific, topographically-constrained routes, all passing through a relatively small area located around 104°W and 74.3°S. The temporal and spatial variabilities of ice shelf melt rates are dominantly controlled by the sub-ice shelf ocean current. Our findings highlight the importance of accurate and high-resolution ocean bathymetry and subglacial topography for determining mCDW pathways and ice shelf melt rates