Recent acceleration of Denman Glacier (1972-2017), East Antarctica, driven by 1 grounding line retreat and changes in ice tongue configuration. 2

: After Totten, Denman Glacier is the largest contributor to sea level rise in East 11 Antarctica. Denman’s catchment contains an ice volume equivalent to 1.5 m of global sea-level 12 and sits in the Aurora Subglacial Basin (ASB). Geological evidence of this basin’s sensitivity 13 to past warm periods, combined with recent observations showing that Denman’s ice speed is 14 accelerating, and its grounding line is retreating along a retrograde slope, have raised the 15 prospect that its contributions to sea-level rise could accelerate. In this study, we produce the 16 first long-term (~ 50 years) record of past glacier behaviour (ice flow speed, ice tongue 17 structure, and calving) and combine these observations with numerical modelling to explore 18 the likely drivers of its recent change. We find a spatially widespread acceleration of the 19 Denman system since the 1970s across both its grounded (17 ±4% acceleration; 1972-2017) 20 and floating portions (36 ±5% acceleration; 1972-2017). Our numerical modelling experiments 21 show that a combination of grounding line retreat, ice tongue thinning and the unpinning of 22 Denman’s ice tongue from a pinning point following its last major calving event are required 23 to simulate an acceleration comparable with observations. Given its bed topography and the 24 geological evidence that Denman Glacier has retreated substantially in the past, its recent 25 grounding line retreat and ice flow acceleration suggest that it could be poised to make a 26 significant contribution to sea level in the near future.

Denman's ice tongue from a pinning point following its last major calving event are required 23 to simulate an acceleration comparable with observations. Given its bed topography and the 24 geological evidence that Denman Glacier has retreated substantially in the past, its recent 25 grounding line retreat and ice flow acceleration suggest that it could be poised to make a 26 significant contribution to sea level in the near future. Over the past two decades, outlet glaciers along the coastline of Wilkes Land, East Antarctica, 30 have been thinning (Pritchard et al., 2009;Flament and Remy, 2012;Helm et al., 2014;31 large uncertainties exist over the magnitude and rates of any future sea level contributions. accessing the cavity below Totten Ice Shelf (Greenbaum et al., 2015;Rintoul et al., 2016; numerical modelling to explore the possible drivers of Denman's long-term behaviour. The 65 following section outlines the methods (section 2) used to generate the remote sensing 66 observations (section 3) and we then outline the numerical modelling experiments (section 4) 67 that were motivated by these observations, followed by the discussion (section 5) and 68 conclusion (section 6).  72 We use a combination of imagery from the ARGON (1962), Landsat-1 (1972-74), Landsat 4-73 5 (1989)(1990)(1991), RADARSAT (1997) and Landsat 7-8 (2000Landsat 7-8 ( -2018 satellites to create a time Antarctic Data Centre, and whilst we could not access the full resolution image, the preview 81 image was sufficient to determine the approximate location of the ice-front and confirm that a 82 major calving event took place shortly before the image was acquired (Fig. S1).  Scherler et al., 2008). This requires pairs of cloud-free images where surface 88 features can be identified in both images. We found three suitable image pairs from the older 89 satellite data: Nov 1972-Feb 1974, Feb 1989-Nov 1989, and Nov 2001-Dec 2002. We 90 used a window size of 128 x 128 pixels, before projecting velocities onto a WGS 84 grid at a 91 pixel spacing of 1 km.

92
To reduce noise, we removed all pixels where ice speed was greater than ±50% the MEaSUREs

Ice tongue calving cycles and structure
122 Throughout our observational record (1962 -2018) Denman Glacier underwent only one major 123 calving event, in 1984, which resulted in the formation of a large 54 km long (1,800 km 2 ) 124 tabular iceberg (Fig. 2). Since this calving event in 1984 the ice-front has re-advanced 60 km 125 and there have been no further major calving events (Fig. 2b, c), as indicated by minimal changes to the geometry of its 35 km wide ice front. As of November 2018, Denman Glacier's 127 ice-front was approximately 6 km further advanced than its estimated calving front position 128 immediately prior to the major calving event in 1984 (Fig. 2b, c). However, given the absence 129 of any significant rifting or structural damage, a calving event in the next few years is unlikely.

130
This suggests the next calving event at Denman will take place from a substantially more 131 advanced position (>10 km) than its last observed event in 1984.

132
Following the production of the large tabular iceberg from Denman Glacier in 1984, it drifted 133 ~60 km northwards before grounding on the sea floor (Fig. 2f), and remained near stationary  Mawson, 1915), but not in 1931(Mawson, 1932, suggest that these low-frequency, 137 high-magnitude calving events are typical of the long-term behaviour of Denman Glacier. In 138 1962, our observations indicate a similar large tabular iceberg was present at the same location 139 (Fig. 2d) and, through extrapolation of the ice-front advance rate between 1962 and1974 (Fig. 140 2b), we estimate that this iceberg was produced at some point in the mid-1940s. However, the 141 iceberg observed in 1962 (~2,700 km 2 ) was approximately 50% larger in area than the iceberg 142 produced in 1984 (~1,700 km 2 ), and 35% longer (73 km versus 54 km). Thus, whilst Denman's 143 next calving event will take place from a substantially more advanced position than it did in 144 1984, it may not be unusual in the context of the longer-term behaviour of Denman Glacier 145 (Fig. 2b).

146
There are clear differences in the structure of Denman Glacier between successive calving 147 cycles. In all available satellite imagery between the 1940s and the calving event in 1984 (e.g. 148 1962, 1972 and 1974) an increasing number of rifts (labelled R1 to R7) were observed on its 149 ice tongue throughout this time (Fig. 2e, f). The rifts periodically form ~10 km inland of 150 Chugunov Island (Fig. 2e), on the western section of the ice tongue, before being advected    159 We observed widespread increases in ice speed across the entire Denman system between  Where is the rate factor with its corresponding stress factor n, h is the vertical ice thickness,   ii.

Ice Speed
In the Úa ice model, the grounding line position is not explicitly defined by the user but 247 is instead a direct result of ice thickness, bedrock depth and the relative densities of ice and sea 248 water. As such, the two ways to perturb a given grounding line are to either modify the ice 249 thickness or the bedrock depth. Modifying the bedrock depth is the less disruptive approach 250 because the resulting effect upon velocity is not biased by an imposed change in ice thickness 251 at the grounding line effecting the regional ice velocity field due to flux conservation, in addition to that caused by shifting the grounding line. Note that raising the bedrock to meet the 253 underside of the ice shelf in this way is not a representation of any real earth processes, it is 254 merely forcing the model to have the grounding line in a particular location, that than enables   288 We show observed 2009 ice speed relative to each of the seven simulations which represent can be explained by internal processes following interactions with local pinning points during 339 the ice shelf's calving cycle (Gudmundsson et al., 2017). In contrast, the widespread 340 acceleration of outlet glaciers in the Amundsen Sea sector (Mouginot et al., 2014) is linked to 341 enhanced intrusions of warm ocean water increasing basal melt rates (e.g. (Thoma et al., 2008;342 Jenkins et al., 2018), leading to ice shelf thinning (Paolo et al., 2015) and grounding line retreat 343 (Rignot et al., 2011a). Thus, in the following section we discuss whether the observed speed-344 up of Denman since the 1970s (Fig. 3) is more closely linked to variations in its calving cycle 345 349 We observe a spatially widespread acceleration of both Denman's floating and grounded ice. period has not been constant (Fig. 3b, c). Between 1972 to 1990, observations indicate that ice 359 accelerated 26 ±5% on the ice tongue (Fig. 3b) and 11 ±5% at the grounding line (Fig. 3c) in 360 comparison to more limited accelerations of 9 ±1% and 3 ±2%, respectively, between 1990- Denman's ice tongue during its re-advance following its 1984 calving event, the ice tongue 413 now makes limited contact with Chugunov Island (e.g. Fig. 4e) and has a very limited effect 414 on ice flow speeds (e.g. E4; Fig. 5e ).

415
The acceleration of Denman's ice tongue following its last major calving event in 1984 may Ice Shelf and Scott Glacier (Fig. 3a). We also observe the lateral migration of the shear margins

Future evolution of Denman Glacier
In the short-term, an important factor in the evolution of the wider Denman/Shackleton system 443 is Denman's next calving event. Whilst our observations do not suggest that a calving event is 444 imminent (next 1-2 years), our calving cycle reconstruction indicates that a calving event at 445 some point in the 2020s is highly likely. Because the calving cycle of Denman Glacier has 446 demonstrated some variability in the past (e.g. Fig. 2), the precise geometry of its ice tongue 447 after this calving event cannot be accurately predicted. stability, but there is a possibility that projected increases in surface melt (Trusel et al., 2015) 463 could increase the ice shelves vulnerability to meltwater-induced hydrofracturing. The recent changes in the Denman system are important because Denman's grounding line 477 currently rests on a retrograde slope which extends 50 km into its basin (Morlighem et al., 478 2020;Brancato et al., 2020), suggesting clear potential for marine ice sheet instability. Given 479 the large catchment size, it has potential to make globally significant contributions to mean sea 480 level rise in the coming decades (1.49 m; Morlighem et al., 2020). Crucial to assessing the 481 magnitude of any future sea level contributions is improving our understanding of regional 482 oceanography, and determining whether the observed changes at Denman are the consequence 483 of a longer-term ocean warming. This is in addition to monitoring and understanding the 484 potential impact of any future changes in the complex Shackleton/Denman ice shelf system.    Table 1). Note that red indicates areas where ice is