Role of dense shelf water in the development of Antarctic submarine canyon morphology
Graphical abstract
Introduction
Increased ocean heat supply to Antarctic continental shelves is projected to cause accelerated ice sheet loss and contribute significantly to global sea-level rise over coming decades to centuries (DeConto and Pollard, 2016; IPCC, 2019). Currently, numerical modelling studies lack necessary resolution and spatial coverage of seafloor morphology data to sufficiently constrain past and future sub-ice shelf melting, ice-sheet collapse, and sea level change estimates (Petrini et al., 2018; Colleoni et al., 2018). This is because one of the major causes of current Antarctic ice sheet retreat stems from increased ocean heat supply to the continental shelves surrounding Antarctica, with atmospheric temperature rise contributing to a lesser extent (Rignot et al., 2019; IPCC, 2019). Recent studies show that heat and volume transport around Antarctica are substantially enhanced where seafloor irregularities, such as submarine canyons, allow dense shelf waters to descend down-slope (Morrison et al., 2020). Warm water incursions onto the shelf can be intermittent and highly localised and can vary depending on the geometry of the ice shelf and seafloor bathymetry (Padman et al., 2018). Lack of necessary resolution and data availability to image these irregularities therefore makes predictions and future estimates of ice sheet and oceanic changes difficult. Thinning of ice shelves, due to increasing ocean temperatures and warm water incursions can lead to rapid ice retreat. This is especially true where marine based ice-sheets occur in conjunction with a landward sloping seabed, seen around the West and East Antarctic Ice Sheet (WAIS and EAIS, respectively) (Joughin and Alley, 2011). This fast ice-shelf retreat can lead to an overall disintegration of the marine-based section of ice-sheets.
Dense shelf water around Antarctica is produced predominantly in the Weddell and Ross Sea polynyas where seabed irregularities, such as canyons and basins, can drive these flows from the shelf to the deep ocean where mixing with ambient water drives the global Meridional Overturning Circulation (Jacobs et al., 2002; Purkey and Johnson, 2010). Changes in temperature or salinity of these waters, such as due to increased meltwater input from ice sheets and seasonal sea ice melt, lead to significant changes to the Meridional Overturning Circulation (Jacobs, 2004; Seidov et al., 2001; Weaver et al., 2003; Seidov et al., 2005). Recent studies show that freshening of bottom waters is already occurring in the Ross and Weddell seas due to factors such as increased ice mass loss in the Antarctic Peninsula and Amundsen Sea (Silvano et al., 2018). This has significant global implications for large-scale effects to ocean and atmospheric circulation, with changes to the strength of the Meridional Overturning Circulation potentially leading to abrupt and global climate changes (Rahmstorf, 1994; Manabe and Strouffer, 1995). Understanding the influence that dense shelf water has on seafloor morphology and vice versa has important implications for determining how these processes changed in the past, in response to different climatic conditions and ice sheet configurations on the continental shelf, and how ice sheet configuration may change in the future. Submarine canyons influence the location and dynamics of intruding warm waters that enhance melting and ice shelf retreat with dense shelf water cascading driving net onshore heat and volume transport of warm currents around Antarctica (Dinniman et al., 2003; Morrison et al., 2020). Thus, understanding how canyon morphology and isobath curvature evolved through time is crucial in understanding factors, processes and feedbacks contributing to past and future ice sheet retreat.
Ice-ocean interactions are arguably the most poorly constrained aspect of ice sheet, ocean and climate models. Extensive Antarctic paleo-climate records are recovered from the flanks of canyons or drifts associated with sediment delivery down-canyon (e.g. Rebesco et al., 2007; Barker and Camerlenghi, 2002), yet very little is known about canyon process across modern, as well as glacial-interglacial timescales. Understanding large-scale oceanic feedbacks as well as the differences in amplitude and frequency of Antarctic continental slope processes under different climatic conditions, noted in far-field paleo-oceanographic records, is essential for constraining how ice sheet and oceanic interactions may change in the future (e.g. Zachos et al., 2001).
Understanding the dynamics of processes operating in submarine canyons more widely is of global importance. This can improve understanding of hazards such as turbidity currents which transport the greatest sediment volumes on earth (Talling, 2014). These flows have significant influence on the global carbon flux (Galy et al., 2007) and can influence shelf and deep-sea ecosystems through supplying nutrients (Canals et al., 2006). They contribute to continental margin and fan construction and aid transportation of pollutants to the deep-sea (Nilsen et al., 2008). On glaciated margins, canyons are not ubiquitous as on low latitude margins and are particularly rare on certain margins (Rui et al., 2019). Their study contributes to a better understanding of the dynamics of ice buildup and retreat and associated glacigenic sediment transport which is crucial in understanding the spatial and temporal variability of glacimarine and oceanographic processes operating on high-latitude margins.
We investigate how dense shelf water influences submarine canyon morphology by analysing new geophysical and oceanographic data from a region of significant and prolonged dense shelf water export, the Hillary Canyon in the Eastern Ross Sea, Antarctica (Fig. 1) (Bergamasco et al., 2002; Orsi and Wiederwohl, 2009; Morrison et al., 2020). We present a quantitative analysis of the main morphological features at the Hillary Canyon head, including gullies incising the modern seafloor and buried paleo-gullies and discuss processes influencing their distribution and formation. We discuss the ability for dense shelf water to influence canyon morphology and the modern and past implications of this before discussing the effects of dense shelf water around Antarctica and other high-latitude continental margins more widely.
Section snippets
Study area
The Hillary Canyon is a 180 km long, non-shelf incising canyon on the Eastern Ross Sea continental margin, Antarctica (Fig. 1). The morphology of the canyon is largely unknown due to a paucity of seafloor hydrographic data with much of the seafloor lacking sufficient resolution data. The continental shelf landward of the Hillary Canyon spans ~300 km to the modern front of the Ross Ice Shelf and is dissected by numerous deep, glacially carved troughs. The canyon lies at the mouths of two of
Methodology
Geophysical and oceanographic data were collected in austral summer 2017 by R/V OGS Explora during the EUROFLEETS-funded ANTSSS expedition. The geophysical data covers 1575 km2 at the Hillary Canyon head. Data were collected using a Reason SeaBat 7150 multibeam echosounder with an operating frequency of 12 kHz and swath width of 150°. Data were processed using PDS2000 to 50 m grid size. A Benthos Chirp III collected acoustic sub-bottom profiler data (frequency range 2–7 kHz; maximum ping rate
Shelf-edge and canyon morphology
The Hillary canyon head spans ~85 km along the shelf edge of the Eastern Ross Sea. The outer shelf is predominantly landward-deepening at the mouth of the Glomar Challenger Basin (from ~175°5′W to ~176°45′W). In the Pennell Basin, most of the shelf deepens landward, with the very outer shelf sloping seaward due to the presence of a sill at the mouth of the trough (west of ~176°45′W). The continental slope at the Glomar Challenger Basin mouth is characterised by an average slope gradient of
Discussion
In this section we discuss new geomorphic, sub-surface and oceanographic results from the Hillary canyon in relation to the glacial, oceanographic and sedimentary processes influencing canyon morphology. We discuss the influence of dense shelf water on gully morphology more widely and compare characteristics of submarine gullies observed on other high-latitude continental margins.
Conclusions
Shelf-slope processes and climatic variations can have significant influence on seafloor morphology, especially in Polar regions, where climate, ice sheet and sea level changes play a crucial role (Fig. 9). New geophysical and oceanographic data show that the Hillary Canyon is the main conduit for cascading flows of dense shelf water to the abyss with canyon levees likely formed of overbank deposits indicating a prolonged history of down-slope flows. Incisional gullies occur at the canyon head,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the EUROFLEETS Funding Program (ANTSSS project) and Italian National Antarctic Research Program (PNRA16 00205; ODYSSEA) projects. The authors thank Captain Franco Sedmak and crew of R/V OGS Explora; party chief Riccardo Codiglia; the technicians and scientific party of the expedition; We thank Laura de Steur for supplying the L-ADCP during the ANTSSS expedition and for processing of L-ADCP data. We acknowledge IHS Markit, Paradigm, Schlumberger for Kingdom and Vista
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