The resilience of a marine-based ice sheet is strongly governed
by the stability of its grounding lines, which are in turn sensitive to
ocean-induced melting, calving, and flotation of the ice margin. Since the
grounding line is also a sedimentary environment, the constructional
landforms that are built here may reflect elements of the processes governing
this dynamic and potentially vulnerable environment. Here we analyse a large
dataset (
Marine-based ice sheet stability is strongly influenced by perturbations near the grounding line; the most downstream location ice is in contact with the underlying bed (e.g. Schoof, 2011; Robel et al., 2014). The grounding line position is fundamentally determined by ice thickness relative to water depth, where ice is sufficiently thick to overcome buoyancy (Fig. 1a), and where ice thickness in turn is determined by mass balance at the grounding line. A broad suite of processes and conditions that locally dictate both buoyancy and mass balance make it difficult to reliably distinguish and define grounding line positions as “stable” versus “unstable”. Yet predicting how ice sheet sectors will respond to their grounding lines being dislodged by enhanced melt or rising sea level under future warming scenarios or, conversely, how grounding lines will respond to changes in interior ice flow behaviour is an urgent endeavour.
The flux of ice to grounding lines is highly spatially variable, determined
by the overall flow structure of the ice sheet (Bamber et al., 2000; Rignot
et al., 2011), its basal thermal regime (Kleman and Glasser, 2007), basal
slipperiness due to the distribution and style of meltwater drainage (Stearns
et al., 2008), cyclic responses of subglacial till rheology to tides (Doake
et al., 2002; Anandakrishnan et al., 2003; Gudmundsson, 2007), and the
effects of ice shelf buttressing (Rignot et al., 2008; Hulbe et al., 2008).
Mass loss occurs by calving and by submarine melting of the ice front and
ice shelf, the balance between which can vary enormously, with orders-of-magnitude variability in melt rates (Depoorter et al., 2013; Rignot et al.,
2013). Ocean-driven basal melting of ice shelves is thought to be
concentrated near grounding lines (e.g. Jenkins and Doake, 1991; Rignot and
Jacobs, 2002), and channelised subglacial freshwater emanating at grounding
lines can lead to locally enhanced ice shelf melting (Le Brocq et al., 2013;
Marsh et al., 2016). While the magnitude of these processes and changes
therein may predispose an ice sheet grounding line to advance or retreat, the
In the last decade, observations and measurements from direct access as well as from remote sensing and geophysical data have helped characterise contemporary grounding line environments and the processes acting at the time of observation. At the Whillans Ice Stream grounding line, one of the best studied contemporary grounding lines, a grounding zone wedge is actively forming (Anandakrishnan et al., 2007) and processes including channelised meltwater delivery (Horgan et al., 2013), tidally induced compaction of till (Christianson et al., 2013), and basal melt-out of englacial debris (Christianson et al., 2016) are thought to contribute to grounding line dynamics. Observations and modelling results demonstrate coupling between ice shelf change and grounding line movement, indicating that grounding lines are sensitive to ice shelf buttressing (e.g. Shepherd et al., 2004; Goldberg et al., 2009). Longer-term and larger-scale modelling has shown that grounding lines are sensitive to bed geometry and the presence or absence of topographic pinning points (Jamieson et al., 2012). Despite these advances, most observations of grounding line processes and, importantly, the response of the grounding line to those processes, are limited in spatial coverage and relate to timescales of years to decades at best. A comprehensive understanding of grounding line stability and the rates, magnitudes, and timescales of change is therefore precluded.
Grounding line landforms (grounding zone wedges and moraines, Fig. 1b–d) directly mark present and former grounding line positions and represent the history of sedimentation during periods of grounding line position stability. Sediment is transported by glacial and glaciofluvial processes to the grounding line, where it is either deposited and a landform builds or is further transported into the marine environment by sediment plumes. Terminal moraines, here referring to any moraines that form at a grounding line position, are thought to form by a variety of sedimentation processes, including lodgement and deformation of subglacial till; pushing and squeezing of ice-marginal sediments; rockfall, dumping, and melt-out of englacial debris; and glaciofluvial sediment delivery and suspension settling (Powell and Alley, 1997; Batchelor and Dowdeswell, 2015). Grounding zone wedges are rather distinct landforms with an asymmetric morphology (e.g. Anderson, 1999; Anderson and Jakobsson, 2016; Batchelor and Dowdeswell, 2015). Previously described as till tongues (King et al., 1991), till deltas (Alley et al., 1987, 1989), and diamict aprons (Hambrey et al., 1991; Eittreim et al., 1995), grounding zone wedges are composed of prograding strata of dilatant deforming till (King, 1993; Powell and Alley, 1997; Anderson, 1999; Dowdeswell and Fugelli, 2012; Batchelor and Dowdeswell, 2015; Simkins et al., 2017a). Both types of grounding line landforms have been observed to contain features described as grounding line fans: lobate or bulbous deposits building from a point source and linked to both glaciofluvial deposition at the mouth of a subglacial channel and re-mobilisation of grounding line sediments by gravity flows (Powell and Alley, 1997; Bjarnadóttir et al., 2013).
It should follow that if different sets of processes build contrasting landforms, then the landforms themselves can be used to infer conditions and processes operating at palaeo-grounding lines. However, a consistent view of what fundamentally controls why one landform type is produced rather than another is lacking. Whereas terminal moraines are observed in both marine and terrestrial settings, grounding zone wedges are only documented in marine settings largely associated with fast-flowing ice in cross-shelf troughs and fjords (e.g. Batchelor and Dowdeswell, 2015). The presence of an ice shelf is argued by some to be critical to the production of grounding zone wedges (e.g. King, 1993; Dowdeswell and Fugelli, 2012; Batchelor and Dowdeswell, 2015). The restricted vertical accommodation space underneath an ice shelf accounts for the asymmetric wedge morphology and promotes growth by progradation; conversely, a moraine ridge can build at an ice cliff terminus where its vertical growth is unrestricted. Powell and Alley (1997) argue that an ice shelf is not critical, but rather the subglacial thermal and hydrological regimes and their effects on the mode of sediment delivery control terminal landform development are. Dilatant deforming sediment, the production of which is encouraged by meltwater drainage through sediment pore space (Darcian processes), has a low angle of repose and will build a low-relief wedge irrespective of accommodation space. Where meltwater is instead in high abundance and drains through a channelised system, subglacial sediments are less easily deformed and sediment delivery to the grounding line via till deformation may decrease (Simkins et al., 2017b). Where grounding line sedimentation is not dominated by the transport of dilatant till, terminal moraines and fans may build with a higher angle of repose. Bjarnadóttir et al. (2013) challenge this meltwater–sediment delivery model for grounding line landforms, reporting observations of meltwater fans (channelised meltwater) within grounding zone wedges (distributed meltwater). However, in all these cases net addition of sediment to the grounding line implies that the size of an eventual landform will reflect a combination of sediment flux/accumulation and time and should therefore provide some measure of grounding line “stability”.
Enhanced coverage and resolution of bathymetric data (e.g. multibeam sonar) acquired over the last 10–15 years from numerous continental shelf and ice sheet settings reveal vast swathes of grounding line landforms. These provide a wealth of data on grounding line retreat following the last glacial maximum and offer an opportunity to extract information about grounding line processes and sensitivity across a range of glaciological, topographic, and oceanographic settings. Here we characterise morphological traits and the spatial distribution of 6275 grounding line landforms from the western Ross Sea continental shelf, formerly occupied by a marine-based sector of the East Antarctic Ice Sheet, to characterise landform morphology, examine those factors that control landform morphology and distribution, and explore drivers of grounding line stability and instability.
Multibeam bathymetry was collected on cruise NBP1502A aboard the RV
Mapped distribution of grounding line landforms in the western Ross Sea, Antarctica, imposed on the IBCSO bathymetric grid (Arndt et al., 2013). Landforms include recessional moraines, grounding zone wedges, and an isolated field of crevasse-squeeze ridges. The landforms predominantly occur within palaeo-glacial troughs and basins: northern Drygalski Trough (NDT), southern Drygalski Trough (SDT), McMurdo Sound (MS), JOIDES Trough (JT), Pennell Trough (PT), and Central Basin (CB). Transect profiles used in morphometric analyses (Fig. 4) are shown by red lines.
Recessional moraines
Grounding line landforms were mapped based on visual identification and interpretation (Fig. 2). Morphometric parameters of individual landforms were calculated using standard line geometry tools in ArcGIS and a peak picking function in MATLAB from transects across grounding line landforms (Fig. 2). We explore correspondence among morphometry, landform distribution, topography, and sediment distribution. Analyses are detailed in the Supplement (Sect. S1).
Using high-resolution multibeam bathymetry data, we mapped 6275 grounding
line landforms that visually present two distinct populations (Fig. 2):
quasilinear, closely spaced, symmetric ridges interpreted as moraines
(Fig. 3a–d;
Morphological analyses show that as a population, landforms interpreted as
recessional moraines are low amplitude (
Normalised frequency distribution of morphometric parameters for the
population of western Ross Sea recessional moraines (
We additionally find that the larger the two-dimensional form of the landform, the greater asymmetry it has developed (Fig. 5d), while Fig. 5e, f illustrate that these properties are also correlated with landform sinuosity. Grounding zone wedges in general are found to be more variable in size, sinuosity, and asymmetry compared to the tight distributions and consistent form of recessional moraines. Individual morphometric parameters show overlapping distributions and imply a continuity of form between recessional moraines and grounding zone wedges (Fig. 4). However, a more holistic description that considers two or more landform properties (Fig. 5) tends to separate the landform population into two groups, consistent with visual interpretation of two distinct landform types. Grounding zone wedges and recessional moraines occur within clusters of numerous landforms of the same morphotype, which abruptly transition from one morphological endmember to another both laterally across a single time-synchronous grounding line (Fig. 6a–c) and within a grounding line retreat sequence (Fig. 6d).
Our observations point to variability in grounding line processes and environments that can lead to a spatial (lateral) and/or temporal switch between two distinctly different landform products. We now ask what grounding line settings or processes may control the production of contrasting landforms and what, consequently, can we learn from the style and distribution of grounding line landforms about the (in)stability of a retreating ice sheet?
A state of grounding in marine settings is fundamentally a function of ice thickness and water depth. A range of glaciological and oceanic processes and topographic settings can affect this relationship (Fig. 1a) and, one may hypothesise, also affect the landform product of grounding. Bed topography has a direct control on the relationship between ice thickness and water depth, and thus the grounding line position. Topography also exerts an indirect control on grounding line processes by creating variability in ice flow velocity and basal sediment flux, influencing tides and near-grounding line ocean circulation, and thereby affecting both the tendency towards buoyancy and grounding line mass balance. Grounding line sedimentation, importantly, serves to build relief at the grounding line, which has been identified as a potential feedback on grounding line position stability (e.g. Alley et al., 2007; Christianson et al., 2016). Processes at the ice–bed interface determine subglacial sediment transport mechanisms and fluxes, and basal and near-grounding line sedimentary processes have therefore been considered fundamental to the production of different grounding line landforms (e.g. Powell and Alley, 1997; Bjarnadóttir et al., 2013). Finally, the presence or absence of an ice shelf exerts a major control on ice flow by providing back stress to grounded ice (Fig. 1a; e.g. Scambos et al., 2004; Fürst et al., 2016), affects mass balance at the grounding line via effects on submarine melting and on calving rate, and places a limit on sediment accommodation at the grounding line.
We use our dataset to evaluate three groups of potential controls on grounding line landform morphology: (i) topographic setting, (ii) grounding line sedimentation, and (iii) presence or absence of an ice shelf.
Paired variable plots of landform morphometry typically distinguish
grounding zone wedges from recessional moraines.
Recessional moraines and grounding zone wedges transition from
clusters of one type to the other spatially (laterally:
We consider here that topographic factors including water depth, the bed
slope, and regional topographic configuration could affect landform
development. First, grounding line landforms of both types are widely
distributed across a range of water depths and bed slopes (Figs. 2, 7).
Collectively, they occupy a window of available depths in the western Ross
Sea (Fig. 7a–c), typically within or on the flanks of well-defined glacial
troughs (Fig. 2). Neither landform type occurs in limited areas of
particularly shallow (
Recessional moraines and grounding zone wedges occur in a similar range of
water depths, suggesting water depth alone – and consequent properties such
as buoyancy – does not dictate the formation of one particular landform type
rather than the other (Fig. 7a–c). Furthermore, we find both landforms at
similar water depths (Fig. 6a), recessional moraines shallower than grounding
zone wedges (Fig. 6b) and grounding zone wedges shallower than recessional
moraines (Fig. 6c). There are very weak preferences of landform type for
particular bed slopes or regional topographic configuration. Collectively,
grounding line landforms span the full range of bed slopes that exist in the
western Ross Sea (Fig. 7d–f), but moraines appear to favour particularly low
slope beds (Fig. 7e). The lowest (i.e. flat,
Water depth distribution across the western Ross Sea
The style and magnitude of subglacial to grounding line sedimentation should influence landform growth, and the resulting form has often been linked to basal sediment fluxes and the duration of grounding (Powell and Alley, 1997; Batchelor and Dowdeswell, 2015; Bart et al., 2017). Implicit in this interpretation is that grounding line landforms are depositional and that they grow with sediment input and with time. Here we first examine evidence in our dataset for the mechanisms of sedimentation. We then assess the importance of grounding line sediment accumulation in accounting for differences between landform types.
Two styles of sediment accumulation at grounding lines can conceptually be distinguished: (1) deformation of sediments at the grounding line by push and squeeze and (2) deposition (i.e. net input of sediment) at the grounding line supplied by mobilised subglacial sediments and release of debris from overlying ice by melt-out at or immediately in front of the grounding line. However, do these contrasting sedimentation styles produce two distinct landform morphotypes?
Landform aspect with respect to bed slope for
Sub-bottom acoustic data across
In our dataset, there are morphological signs of scour and push at the
lateral transition from grounding zone wedges to recessional moraines along
single grounding line positions (Fig. 6a, c). In Fig. 6c, recessional
moraines appear to “peel off” from grounding zone wedges, where wedge
sediment appears to be pushed forward to form a narrower ridge. In these
examples, there is some element of push of grounding line sediment over a
distance that is comparable to grounding zone wedge widths (
Grounding zone wedges have been widely shown to be depositional products that accumulate by progradation of sediments that are delivered to the grounding line from a conveyor belt of deforming till at the base of the ice sheet (e.g. Anderson, 1999; Batchelor and Dowdeswell, 2015) and subsequently transported down the foreset slope of the wedge by sediment mass movement (Simkins et al., 2017a). Here, the asymmetric morphology and distinct stoss–lee slope transitions of grounding zone wedges (Fig. 3e, f) are consistent with landform topset aggradation and foreset progradation, although individual topset and internal foreset beds are not resolved in our high-frequency acoustic data from these relatively small landforms, contrary to lower-frequency seismic records of larger documented grounding zone wedges (Fig. 1d; e.g. Batchelor and Dowdeswell, 2015). Undisturbed buried horizons beneath a variety of grounding zone wedges in our dataset (Figs. 9b, c, 10b) clearly indicate that wedge relief is due to the addition of material at the grounding line. Landform arrangements further reveal that wedges have prograded over older recessional moraines and grounding zone wedges (Fig. 3e, g). Side-scan sonar data document small-scale slumping on the foreset of a grounding zone wedge in the southern JOIDES Trough (Fig. 11), perhaps indicating the delivery of relatively cohesive sediment to the grounding line conducive to viscous sediment gravity flows. In our dataset, we do not observe any lobate or bulbous fan deposits along wedge fronts (e.g. McMullen et al., 2006; Bjarnadóttir et al., 2013; Fig. 1c) that would indicate focal points for deposition from glaciofluvial transport.
Side-scan sonar image from cruise NBP95-01 across a grounding zone wedge in the southern JOIDES Trough, showing slumps on the grounding zone wedge foreset surface.
Acoustic profiles of some smaller grounding zone wedges show signs of active
deformation through
Landform width and amplitude are positively correlated (Fig. 5a–c) in the case of both recessional moraines and grounding zone wedges. At the smallest end of the global population, the tight morphological clustering of moraines in the western Ross Sea may suggest that there is a limit imposed on their eventual size. Such a limit must be either inherent to their process of relief creation or due to a limited net input of sediment due to low delivery flux and/or occupation time of a grounding position. Among grounding zone wedges, increases in width and amplitude are accompanied by development of landform asymmetry and sinuosity (Fig. 5d–f). These relationships suggest that grounding zone wedges grow as a function of sediment supply over time and that variability in accumulation in both space and time will yield variable morphologies (Howat and Domack, 2003), expressed here by heightened sinuosity and asymmetry. Since growth is inherently a function of both sediment availability and time, these properties can be difficult to disentangle. Does a larger grounding line landform represent more time or a greater basal sediment flux?
A paired group of grounding zone wedges and recessional moraines within a
trough, in which grounding zone wedges laterally transition to recessional
moraines (Fig. 6a), allows us to isolate the time factor of sediment
accumulation. We select a sequence of these landforms that is bounded by a
laterally continuous grounding line at both a distal and retreated position,
each representing a time-synchronous grounding line position (Fig. 10a). In
this group, an individual grounding zone wedge has an average cross section
(i.e. sediment content) 8.3 times larger than that of an individual moraine.
The full assemblage of retreating grounding zone wedges has 4.55 times more
sediment (in cross profile) than the neighbouring assemblage of moraines,
while there are twice as many individual moraines. Therefore, the sediment
flux at each grounding line position is higher in the grounding zone wedge
group
Furthermore, we find spatial variability in sediment thickness within a
single grounding zone wedge in the southern JOIDES Trough. Sub-bottom acoustic
data detect a buried surface beneath the acoustically transparent grounding
zone wedge sediment unit, enabling us to map the spatial distribution of
sediment accumulation on top of the underlying (i.e. older) substrate
(Fig. 10b). The sediment thickness at the grounding zone wedge front is
laterally variable, with peak thickness in the centre west (
In several groups of grounding zone wedges in our dataset, we observe
embayments in the wedge front that contain channels (Fig. 12; Simkins et al.,
2017b). This suggests a link between the position of subglacial meltwater
channels and the development of sinuosity in the grounding line. Contrary to
cases in which subglacial conduits are thought to provide point sources of fan
sedimentation at a grounding line (Powell and Alley, 1997; McMullen et al.,
2006; Bjarnadóttir et al., 2013), here we observe reduced availability of
sediment at the grounding line associated with basal channels. We hypothesise
this may result from non-deposition of sediment at the grounding line due to
enhanced transport by glaciofluvial processes. Alternatively, embayments may
be due to porewater drainage by the channel, sediment stiffening, and reduced
subglacial transport by deformation processes (e.g. Christianson et al.,
2013). In the latter case, we would expect excessive thickening of sediment
behind a grounding zone wedge embayment. We observe along-flow thickening to
the wedge front (Fig. 10b) but to a
Development of sinuosity via spatially reduced or enhanced deposition is a function of variable sediment supply and of time. A longer duration of standstill will permit the variability in transport and deposition rates to enhance the sinuosity of the eventual form, while a longer duration also allows grounding zone wedge asymmetry (in the distal direction) to increase by continued progradation. Since these are progressively developing traits and typical of grounding zone wedges, we might conclude that that low-amplitude recessional moraines are the proto-feature that, given sufficient construction time and supply, would develop into a grounding zone wedge. This idea is perhaps difficult to reconcile with asymmetric moraines (e.g. Larsen et al., 1991; Flink et al., 2015), although these are typically proximal asymmetric, resulting from the magnitude of push, rather than asymmetry being a consequence of time and growth. It is more difficult to reconcile moraines as a proto-feature with the occurrence of much larger terminal moraines globally (e.g. Ottesen et al., 2005; Fig. 5c), which clearly have had plentiful sediment supply and yet a wedge morphology has not developed. An additional factor, other than the incoming grounding line sediment flux and construction time, must explain why larger moraine morphologies build in preference to grounding zone wedges. Notwithstanding this missing element, our dataset shows that time and sediment supply are both important controls on landform type and on the paired development of landform asymmetry and sinuosity.
Ice shelf presence or absence has been postulated as an explanation for contrasting grounding line landforms, where accommodation space at the grounding line is limited under an ice shelf and promotes low-relief, asymmetric grounding zone wedge development, while an ice cliff has unlimited accommodation and a moraine can build upward (Powell, 1990; Dowdeswell and Fugelli, 2012; Batchelor and Dowdeswell, 2015). The existence of two endmember landform types is tempting to explain by a mechanism with two equivalent endmember states. As moraines are observed in both terrestrial and marine environments (e.g. Boulton, 1986), it is even more tempting to invoke moraine formation at grounding lines expressed by an ice cliff – possible above or below sea level – and grounding zone wedge construction only in marine environments where ice shelves can form. If grounding line landform morphology could clearly be associated with ice shelf configuration, then we could use the presence of landform type as a proxy for palaeo-ice shelf presence or absence and identify grounding lines that could have been influenced by ice shelf back stresses.
Our data show that grounding zone wedges in the western Ross Sea have a higher amplitude than recessional moraines (Fig. 5); landforms reported from other deglaciated margins show overlapping moraine and wedge amplitudes (Fig. 5b, c). The condition for the argument given above is therefore not upheld or, at least, an additional process or factor is required to account for inhibited moraine growth. Furthermore, we might expect topographic highs to maintain grounded ice whilst ice over deeper troughs would tend towards flotation and preferentially form an ice shelf, as is argued for late-stage deglaciation in the western Ross Sea (Yokoyama et al., 2016). However, there is not a consistent relationship among recessional moraines and grounding zone wedges and their topographic context (e.g. Fig. 7b, c) that would support this association. Transitions between the two landform types along a single continuous grounding line (Fig. 7a) in a comparable topographic setting are also not straightforward to reconcile with an ice shelf or without an ice shelf.
While ice grounding is fundamentally dictated by water depth, at a regional
scale grounding line landform morphology does not appear to be strongly
governed by properties of the bed topography such as water depth and bed
slope. This furthermore implies that interdependent properties such as ice
velocity and ice shelf/cliff presence also have a limited effect. Locally, the
presence of topographic relief (banks, flanks, and seamounts) encourages the
construction of grounding zone wedges; flat (
Our observations suggest that both local push and squeeze
There are many processes that vary across a continuum that all likely influence grounding line configuration and sedimentation, so why do we not observe morphological products that also vary across a continuum? Rather, we observed a binary product – either grounding zone wedge or recessional moraine. Individual morphological and spatial landform characteristics may overlap but in combination they produce two species that are visually very distinct from each other. A mechanism that itself is binary is an appealing way in which to explain a set of products that is binary. However, our data do not offer such a solution, and instead, several factors offer partial yet inconclusive explanations of landform morphology. Of these, time appears to be important to eventual grounding zone wedge morphology, and it therefore follows that these landforms hold some information about the duration, or the stability, of grounding line positions. In the next section we explore to what degree landform morphology and landform distribution may lead us to interpret aspects of grounding line (in)stability.
Grounding line stability can be conceptualised in numerous ways:
sensitivity to change in position (e.g. likelihood of a grounding line
response to certain forcings); duration that a grounding line position is occupied; magnitude of the retreat event when a grounding line vacates a former
position; consistency (or predictability) in both occupation time and retreat event
magnitude.
Given these different facets of stability, should we define, for example, retreat events of small (large) magnitude punctuated by short (long) periods of grounding line position occupation as stable or unstable retreat? We find from our data that we must consider stability as a multifaceted concept.
Retreat sequences defined by recessional moraines indicate short-distance
retreat steps (mean spacing
As with their spacing, recessional moraines have a tight size distribution
(Figs. 4, 5), indicating not only consistency in retreat event magnitude but
also in the duration that a grounding line position is occupied. Their small
size would suggest that this duration is typically short (Sect. 4.2.2).
Features of comparable scale to our Ross Sea moraine population (e.g.
Fig. 5c) are commonly interpreted as de Geer moraines, considered to form
annually or sub-/multi-annually (Lindén and Möller, 2005; Todd et
al., 2007; Ojala et al., 2015). Grounding zone wedges represent longer-duration grounding line positions, indicated by both their size and, we argue
here, by their greater sinuosity and asymmetry that both develop with growth.
Published estimates of grounding zone wedge formation time suggest timescales
of decades to millennia (Anandakrishnan et al., 2007; Nygård et al.,
2007; Jakobsson et al., 2012; Klages et al., 2014; Bart et al., 2017), though
these typically relate to individuals larger than those found within our
dataset (Fig. 5b). Our paired group of small-scale (
Figure 13a, b conceptually summarise two modes of retreat: (i) grounding zone wedges represent longer-duration standstills, larger retreat events, and inconsistency in both of these; (ii) recessional moraines indicate a regular and consistent retreat mode, with small retreat steps, yet punctuated by short-lived grounding positions. Given that retreat magnitude and standstill duration work against each other, the net rate of grounding line retreat on a regional scale may not differ between these two scenarios.
Much of our landform population in the western Ross Sea comprises individuals arranged in groups of alike morphotypes (Figs. 2, 3, 7). This clustering of distinct endmember landform types indicates that (i) the timescale for a change in process or grounding line setting that would yield a different type of product is extremely abrupt, based on the lack of transitional landform types, and (ii) that once the formational process/environment has changed, it is maintained for a duration significantly longer than the construction time for a single landform.
Retreat of a grounding line must be fundamentally driven by a change to the buoyancy condition that causes ice at the grounding line to lift off from one position, or by grounding line ablation via melt or calving that exceeds the incoming grounding line ice flux (Fig. 1a). The regularity of moraine sequences suggests a cyclic process that would produce short-lived grounding but controlled and small-scale retreat magnitude (Fig. 13b). This retreat style is most likely driven by changes in ablation (mass balance) conditions. Possible mechanisms for cyclic control on grounding line retreat could include annual/multi-annual sea ice variability that has been linked to reduced calving and alters continental shelf ocean circulation (Hellmer et al., 2012), climatic phenomena like El Niño–Southern Oscillation, which can alter ice shelf mass balance (Paolo et al., 2018), tidal cycles causing sufficient calving and/or basal melting to drive grounding line retreat (Jakobsson et al., 2011), or regularly paced subglacial meltwater drainage events that could cause plume-driven melting (Le Brocq et al., 2013; Alley et al., 2016). Sequences of clustered moraines would suggest that these processes, in and of themselves, are not enough to trigger exceptional large-magnitude – one may argue unstable – retreat events, but rather produce steady, controlled retreat. Recessional moraine sequences are rarely terminated by a large-magnitude retreat event that would represent an unstable threshold response to prolonged small-scale ablation forcing.
Even though it is likely that the cyclic processes that take place at grounding lines expressed as moraines are also ongoing at positions marked by grounding zone wedges, sensitivity to processes occurring at more-or-less regular intervals appears to be reduced where grounding zone wedges are present. This suggests that the grounding line is buffered from processes that drive short-term (annual/multi-annual) variability in ablation. Such a buffer could be due to (i) an increase in the ice thickness-to-water depth relation (reduced buoyancy), such that the grounding line ice flux overrides any small-scale variability in ablation rates; (ii) a long-term shift in ocean access to the grounding line (e.g. circulation change, change in ice shelf and/or sub-ice shelf cavity geometry) such that calving and basal melt rates fall below a threshold (relative to ice flux) for enacting grounding line change; or (iii) a feedback with the processes of wedge construction itself. A fundamental change in the ice thickness – water depth relation away from floatation (e.g. pinning points, Fig. 8d–f) might desensitise grounding lines to terminal ablation processes and promote longer occupation of a grounding line position. Where grounding zone wedges are not associated with pinning on antecedent topography but rather occur at the same water depths and bed slopes as recessional moraines, the greater occupation time must therefore be either a function of a change in mass balance (ablation rate or ice flux) or a feedback with construction of sedimentary relief and sediment flux to the grounding line.
Grounding zone wedges in the western Ross Sea are both longer-lived and show signs of local ice advance compared to recessional moraines. Sediment aggradation and progradation at the grounding line is accompanied in several cases by topset development of subglacial lineations. These observations suggest that grounding zone wedges stabilise grounding lines and even allow for ice advance; as sediment is added to the landform the depth of the seabed relative to ice thickness is reduced and allows the position of grounding to advance. Although grounding zone wedge growth initially encourages prolonged occupation of grounding positions, does it also promote greater instability in the context of the magnitude of retreat events? Retreat events from such stabilised positions tend to be large (Fig. 4); the sensitivity to small changes in the buoyancy relation as the grounding line advances over its wedge does not manifest as incremental retreat steps. Larger retreat events associated with grounding zone wedges suggest a threshold of stability is reached that causes inherent instability.
We interpret grounding zone wedge asymmetry and sinuosity as signatures of both stabilising and destabilising feedbacks, respectively, that develop with landform growth (Figs. 5d–f, 13c). Asymmetry is a morphological expression of ice advance due to landform aggradation and progradation and therefore reflects the stabilising aspect of grounding zone wedges. We argue that the development of sinuosity, however, leads to a threshold of maximum stability and grounding line retreat. Several processes could lead to grounding line destabilisation associated with sinuosity, including (i) increased contact of the ice front with ocean water, which could lead to increased melting; (ii) channelised meltwater drainage at grounding lines, which is associated with the development of embayments and the release of meltwater plumes that contribute to melting of the ice front and ice shelf (if present) and/or increased tidal pumping; and (iii) laterally variable stresses that might produce localised shear zones or reduce lateral drag that could promote enhanced calving and crevassing potential. Such processes may promote and reinforce highly spatially variable ablation, creating sinuosity (embayments) in the larger grounding zone wedges that far exceeds the spatial scale of retreat steps associated with smaller landforms, and potentially creating lasting change to the structure of the grounding line such that eventual destabilisation of the grounding position is larger and less predictable than in the case of smaller-scale, more ordered retreat.
Unlike grounding zone wedge growth that can both stabilise and destabilise grounding lines, grounding lines expressed as recessional moraines are not clearly influenced by landform presence and growth. This leads us to conclude that processes driving retreat from moraines should be independent of grounding line sedimentation. Implicit in the above is that grounding lines producing moraines and those producing grounding zone wedges have different sensitivity to processes that trigger grounding line retreat.
Grounding line landforms have the potential to inform us of the processes governing the stability and retreat of palaeo-ice sheet grounding lines. From a large dataset of mapped grounding line landforms, individual morphometric properties indicate a continuum of form. However, multi-parameter analyses support a visual classification of a binary landform product that expresses lateral (i.e. along a single grounding line) and temporal (i.e. within a retreat sequence) transitions between clusters of two endmember morphotypes: moraines and grounding zone wedges. It is an appealing idea that a different set of controls and/or processes should dictate the formation of two different landform types. Yet, of the potential controls on landform morphology that we have explored here, we find inconclusive evidence that a distinct set of controls/processes can wholly explain the formation of either morphotype.
Landform morphotype is not fundamentally controlled by water depth or bed slope, although grounding zone wedges are observed on isolated pinning points likely associated with locations of enhanced grounding line position stability. Neither can the presence or absence of an ice shelf be convincingly demonstrated to control the type of landform that results. Inconsistent spatial arrangements of moraines and grounding zone wedges with respect to topography are difficult to reconcile with plausible ice shelf–ice cliff configurations, and the greater amplitude of grounding zone wedges than moraines suggests vertical accommodation space does not dictate landform morphology. This argument does not reject an ice shelf–cliff control, but additional factors are required to limit moraine growth in this setting.
We find that both sediment supply to the grounding line and the duration of grounding line position occupation are important, most notably expressed in cases in which grounding zone wedges laterally transition to recessional moraines: grounding zone wedges represent both a higher basal sediment flux and a longer duration of grounding than do recessional moraines. This is consistent with the development of landform shape (asymmetry and sinuosity) with the size of the landform. A tempting conclusion is that given sufficient time and supply, a moraine would seed and develop into a wedge. This remains, however, difficult to reconcile with larger terminal moraines in other glaciated settings.
With this large dataset of morphological features associated with palaeo-grounding lines that progress tens to hundreds of kilometres in the retreat direction, we are able to explore what landforms reveal about grounding line stability. Recessional moraines are associated with short-lived grounding line positions yet record steady, small-magnitude retreat events. This suggests that a regular process drives grounding line retreat, linked to steady and cyclic net loss of mass. While grounding zone wedges represent longer periods of position stability, the magnitude of retreat events is larger and more variable. Reduced ablation or a grounding line buffered against cyclic ablation processes may prolong grounding line occupation, while sediment aggradation and progradation in wedge growth may independently enhance grounding line stability. This stable phase is reflected as asymmetry in landform morphology and in lineations on the wedge topset. However, some threshold of stability is reached to result in large unstable retreat events. The development of landform sinuosity due to spatial variability in sediment transport to and deposition rates at grounding lines could potentially destabilise otherwise stable grounding lines. In this regard, channelised meltwater delivery to the grounding line, ice sheet–shelf configuration, and the access of ocean water to the grounding line are likely of fundamental importance in governing grounding line shape and therefore ultimate stability.
Grounding line retreat in the western Ross Sea is characterised by either (i) short-lived grounding line positions that backstep with small-magnitude retreat events or (ii) longer-duration grounding line positions followed by major destabilisation in the form of larger-magnitude retreat events (Fig. 13a, b). These contrasting behaviours vary abruptly in space and time, yet neither can be explicitly characterised as slow or fast retreat, nor can a single descriptor as stable or unstable be applied without further qualification. Stability may be conceptualised in numerous and sometimes contradictory ways. Bart et al. (2017) describe prolonged grounding line occupation and large-magnitude retreat as a paradox; here we find this is a common trait of grounding line behaviour. Given non-uniform ablation and non-uniform sediment supply to a prograding landform at the grounding line, an ice margin may, over time, become increasingly prone to unstable (large-magnitude) retreat. This study highlights the importance of understanding thresholds – potentially in the grounding line sedimentation system itself – which may destabilise a system from an apparent state of stability, and of controls on grounding line dynamics on short (annual) to long (centennial to millennial) scales in order to project future changes in ice sheet mass balance.
Multibeam bathymetric data are available through
LS and SG conceived the project and ran analyses. The interpretations and ideas put forward here were developed by LS, SG, and JA. LS and SG wrote the paper with input from JA.
The authors declare that they have no conflict of interest.
The authors thank the crew and science support personnel aboard cruise NBP1502A, as well as students from Rice University, the University of Houston, Louisiana State University, and the University of Silesia for assisting in cruise data collection. Special thanks go to Lindsay Prothro, who provided an early draft of the grounding line retreat style schematic. This project was supported by the National Science Foundation (NSF-PLR 1246353, John B. Anderson) and the Swedish Research Council (D0567301, Sarah L. Greenwood). Edited by: Chris R. Stokes Reviewed by: two anonymous referees