1 East Antarctic Ice Sheet bed erosion indicates repeated large-scale retreat and 2 advance events.

3 Authors

4 Aitken, A.R.A.1, Roberts, J.L.2,3, Van Ommen, T2,3, Young, D.A.4, Golledge, N.R.5,6, Greenbaum, J.S.4, 5 Blankenship, D.D.4, Siegert, M.J.7 6 7 1 - School of Earth and Environment, The University of Western Australia, Perth, Western Australia, Australia 8 2 -Australian Antarctic Division, Kingston, Tasmania, Australia 9 3 -Antarctic Climate & Ecosystems Cooperative Research Centre, The University of Tasmania, Hobart, 10 Tasmania, Australia. 11 4- University of Texas Institute of Geophysics, The University of Texas at Austin, Austin, Texas, USA 12 5 - Antarctic Research Centre, Victoria University of Wellington, Wellington 6140, New Zealand. 13 6 - GNS Science, Avalon, Lower Hutt 5011, New Zealand 14 7 - The Grantham Institute; Imperial College London, London, United Kingdom 15 16 With changing climate, ice sheet retreat and advance is a major source of sea level change, but our 17 understanding of this is limited by high uncertainties. In particular, the contribution of the East Antarctic 18 Ice Sheet (EAIS) to past sea level change is not well defined. Several lines of evidence suggest possible 19 collapse of the Totten Glacier catchment of the EAIS during past warm periods, in particular, during the 20 Pliocene1-4, causing a multi-meter contribution to global sea level. However, the large-scale structure and 21 long-term evolution of the ice sheet have been insufficiently well known to constrain retreat-advance 22 extents in this region. Using aerogeophysical data5,6 we define the erosion of the ice sheet bed as an 23 indicator of past ice sheet activity. We show that deep erosion exposing basement rocks is restricted to 24 two regions: the head of the Totten Glacier, within 150 km of the present grounding line; and deep within 25 the Sabrina Subglacial Basin, 350-550 km from the present grounding line. These regions demarcate the 26 marginal zones of two distinct quasi-stable EAIS configurations corresponding to modern-scale and 27 retreated ice sheets. Despite experiencing both regimes, the 200 to 250 km broad transitional region 28 between the two is less eroded, suggesting a shorter-lived exposure to eroding conditions. Limited 29 transition-zone erosion is well explained by repeated retreat-advance events driven between the quasi- 30 stable states by ocean-forced instabilities. Representative ice sheet models indicate that the global sea- 31 level increase from retreat in this sector can be up to 0.9 m in the modern-scale configuration, and is in 32 excess of 2 m in the retreated configuration.

33 Satellite-based observations indicate that the margin of Totten Glacier may be experiencing greater ice loss 34 than anywhere else in East Antarctica7,8. This, coupled with the presence of the 1.5 km deep Aurora 35 Subglacial Basin upstream, means Totten Glacier and its drainage basin may be at risk of substantial ice loss 36 under ocean warming conditions. Vulnerability of this region to change may be driven by access of warm 37 modified circumpolar deep water to the ice-shelf cavity. At decadal timescales this access is controlled by a 38 combination of polynya activity and bathymetric constraints9,10. Recent studies indicate that potential access 39 pathways exist to access the cavity beneath the Totten Ice Shelf11.

40 Totten Glacier also possesses an inland ice stream that extends far into the continental interior. This ice 41 stream mostly overlies the Sabrina Subglacial Basin (SSB), which is a bowl-shaped depression bounded to the 42 east by the Terre Adelie Highlands, to the south by Highland C and to the west by Highland B (Figure 1). The 43 SSB is underlain by an extensive sedimentary basin (the SSSB) of moderate but quite variable thickness 44 (Figure 1c). This basin probably dates back to at least 40 Ma6, and therefore predates glaciation. Perhaps 45 because of this sedimentary basin, this region has widely distributed subglacial hydrology12 and little large- 46 scale topographic relief5,12,13, although the surface is quite rough at shorter wavelengths12.

47 Here we use geophysical data from the ICECAP program (extended data Figure 1) to define the erosion of 48 the SSB due to the activity of the East Antarctic Ice Sheet (EAIS). We use 2D gravity modelling along flight 49 lines, also including depth to magnetic basement estimates, to understand the thickness of sedimentary 50 rocks in the SSB region. Representative ice sheet models support the interpreted erosion regimes and 51 provide complementary estimates of global sea level change.

52

53 Figure 1: Interpretation of erosion in the Sabrina Subglacial Basin region: a) Base-ice elevations. Data from 54 ICECAP (bold) and Bedmap214 (muted). The Sabrina Subglacial Basin is the low-lying region bounded by 55 Highland B, Highland C and the Terre-Adelie Highlands. VF – Vanderford Glacier; CG – Cape Goodenough. 56 The sedimentary basin base (b) and sedimentary basin thickness (c) derived from gravity modelling. The SSSB 57 is the centrally located basin of moderate thickness. Much thicker sedimentary basins occur in the Aurora 58 Subglacial Basin and Vincennes Subglacial Basin (VSB). d) From these datasets we interpret regions of 59 differing erosion characteristics for the SSB. See text for further details of these regions. Inset shows the 60 study area location.

61 The gravity model results show the SSSB to extend southeast from the Totten Glacier for over 500 km. Its 62 base is as deep as -4000 m elevation (Figure 1b), but is typically much shallower. The basin base is tilted 63 towards the south or southeast, although several east-northeast oriented faults define smaller-scale 64 perturbations (Figure 2). Allowing for the subglacial topography (Figure 1a), the thickness of the SSSB is 65 defined (Figure 1c). The results suggest an initially broad and generally flat-based geometry, with a probable 66 pre-erosion thickness of approximately 3 km. Error analyses (Figure 2, extended data Figures 2, 4-7) indicate 67 the modelled SSSB thickness error is approximately ±500m. The ASB and VSB have larger uncertainties, up 68 to 2 km, and erosion patterns are not interpreted within these basins.

69 The thickness of the SSSB (Figure 1c) shows high variability within tectonic blocks, and also transgressions of 70 major tectonic structures (see extended data Figure 3, extended data Figure 8) and does not parallel the 71 tectonic structure of the region 6. Rather, the SSSB thickness defines distinct patterns that relate to the 72 erosion of the SSB by the East Antarctic Ice Sheet.

73 Glacial erosion occurs due to basal-sliding, which requires warm-based ice and a sufficiently high basal shear 74 stress to cause motion, and erosion-rate is primarily dependent on the basal velocity of the ice sheet 15. 75 Regions with high erosion potential are typically found near the margins of an ice sheet, and may be 76 selective or distributed. Under selective erosion, deep troughs occur in regions with high lateral 77 convergence of ice flux, as well as high basal-velocity . These convergent zones are often topographically 78 focused within pre-existing valley systems16. Thinner ice promotes stronger selective erosion because 79 highlands are often cold-based and are protected from erosion. The same highlands may be warm-based 80 under thicker ice cover. As a consequence, larger ice sheets are more likely to exhibit distributed erosion, 81 whereas smaller ice sheets are more likely to exhibit selective erosion.

82 For the SSB we interpret several regions of distinctive erosion (Figure 1d) that define two distinct quasi- 83 stable EAIS configurations. We identify a modern-scale configuration, with a marginal zone near the present- 84 day margin, and a retreated configuration, with a marginal zone located far inland. These configurations are 85 not time-specific, although global climate and sea level data suggest a retreated ice sheet was dominant in 86 the Oligocene to mid-Miocene, with a modern-scale ice sheet dominant since the mid-Miocene17.

87 Cumulative glacial erosion is generally low in elevated coastal regions, including Law Dome, the Knox Coast 88 and the Cape Goodenough region. Erosion is also low on the highlands surrounding the SSB and ASB. Ice 89 sheet models suggest that the coastal highlands are either covered with slow-moving ice, or are ice free 90 (Figure 3; Extended data figure 9). Highlands A and C, the upper part of Highland B, Ridge B and Dome C 91 remain ice covered in all models, and it is likely that these highlands were also covered by the early ice sheet 92 5 93

94 Figure 2: Gravity model along flight line R06Ea (location, Figure 1b). a) Observed and calculated gravity 95 data, including model components from ice and topography, the deep crust and Moho, and the sedimentary 96 basins. b) The model, also showing prior depth to magnetic basement estimates, the tensioned spline fit, 97 and cross-ties with other models. The gravity disturbance was modelled at true observation elevations as 98 indicated by the uppermost grey line c) SSSB thickness and current surface ice velocity (vertically flipped). 99 Average error in ice sheet velocity on this line is 10.5 m/yr18. A|B, B|C and C|HC (HC-Highland C) indicate 100 the locations of the boundaries demarcated in Figures 1 and 3. Note the thin to absent basin downstream of 101 A|B, the sloped base from A|B to B|C, and the anti-correlation between basin thickness and ice-sheet 102 velocity. Dashed lines indicate the inferred prior 3 km thickness of the SSSB, which has been almost entirely 103 removed in Region A, and eroded in line with present-day ice velocity in Region B. See extended data figures 104 4 through 7 for additional representative models throughout the SSB region.

105 Region A surrounds the coastal glaciers, and is typified by a broad region with moderate elevation, cut by a 106 number of smaller channels that are aligned with modern ice sheet flow (Figure 1a). The SSSB thickness is nil 107 for much of Region A and rarely exceeds 1 km (Figure 1c). The A|B boundary is located near a topographic 108 ridge that extends WSW from Cape Goodenough. The ridge is at ca. -200 m elevation, but is cut by channels 109 as deep as -600 m elevation (Figure 1a). The ice-sheet bed within region A is ocean-sloping, with an average 110 measured gradient along flight lines of +2 to +4 m/km.

111 Region B is defined by an almost linear inland-thickening trend to the base of the SSSB, with an upstream 112 limit defined where this thickening trend ceases to be evident. Over ca. 200-250 km, the thickness of the 113 SSSB increases in co-variance with the reduction in ice sheet velocity (Figure 2 and extended data Figures 4- 114 7).

115 Non-selective and inland-reducing erosion within regions A and B points to the activity of an ice sheet with a 116 modern-scale configuration15. Under current climate conditions our ice sheet model indicates high basal

117 velocities within Region A (extended data Figure 9a). With an air temperature, dTa=+4°C and an ocean

118 temperature, dTo=+1°C, the model indicates a retreat of the ice sheet margin to the A|B boundary, with high 119 basal velocities mostly within Region B (Figure 3a). These results indicate a history of repeated smaller-scale 120 retreat and advance cycles within Region A. These are likely orbitally forced2, with a typical length-scale of 121 less than 200 km, and do not invoke collapse.

122 Region B comprises two subregions. Subregion B2 preserves only the modern-scale ice sheet erosion 123 signature, and the SSSB is typically over 2 km thick. In Subregion B1, the SSSB is typically 1-2 km thick and, in 124 addition to the modern-scale ice sheet signature, a muted dendritic pattern is evident (Fig 1). The ice sheet 125 bed in Subregion B1 is beneath sea level and typically inland-sloping, with an average gradient of -1 to -3 126 m/km. Therefore, once the topographic ridge at the A|B boundary is reached, retreat into the inland SSB is 127 subject to ocean-driven instabilities.

128 Ocean-driven instabilities include the marine ice sheet instability (MISI), moderated by ice-shelf 129 buttressing19, and ice-cliff failure augmented by hydrofracturing20,21. From a modern-scale configuration, 130 highly unstable ice sheet reconstructions for the Pliocene, especially those including ice-cliff failure20,21, can 131 breach the A|B boundary. Under similar climate conditions, more stable reconstructions22,23 typically fail to 132 breach this boundary. This suggests that a significant degree of ice-sheet instability is necessary to cause 133 retreat into the SSB interior under Pliocene conditions.

134 Following retreat into the SSB interior, a new ice sheet margin is established in front of Highlands B and C, in 135 a more advanced location than a previous interpretation of the early ice sheet5. Region C (Figure 1) is 136 characterised by a well-developed dendritic erosion pattern, with sufficient erosion to expose basement. 137 This region is consistent with the occupation of a fluvial valley network by a smaller-scale ice sheet 5. 138 Highland C and the Terre Adelie Highlands are significant topographic highs extending well above sea level. 139 Average measured gradient on their frontal slopes is ~+2.5 m/km. Consequently, these highlands are 140 persistent barriers to further ice sheet retreat, even under highly unstable conditions15,21.

141 Highland B, however, is pierced by three deep fjords providing access to the Aurora Subglacial Basin (ASB)5. 142 This additional vulnerability limits the residence-time of an ice margin in front of Highland B and permits ice 143 sheet retreat into the ASB5,15,21. The B|C boundary links to the central fjord, suggesting that this fjord is the 144 most significant for facilitating further collapse5.

145 The erosion characteristics of regions B1 and C point to the activity of a retreated ice sheet. With dTa=+8°C

146 and dTo=+2°C (Figure 3b) the ice sheet margin retreats to Subregion B1, with high basal velocities mostly

147 within Subregion B1 and Region C. With dTa=+12°C and dTo=+5°C further retreat to the B|C boundary is 148 indicated, with only Region C of the SSB being ice covered (Figure 3c). This retreat scenario is similar in 149 extent to a previous interpretation of the early ice sheet, prior to advance into the SSB5, although the

150 locations of the ice margins differ in location. With dTa=+15°C and dTo=+5°C full retreat into the ASB is 151 indicated (extended data Figure 9b), with ice coverage only on inland highlands. This retreat scenario is 152 somewhat more extensive than a previous interpretation of the early ice sheet, prior to advance into the 153 ASB5.

154

155

156 Figure 3: Ice sheet models with differing climate forcing. Ice sheet models showing ice sheet surface and

157 bed elevations and basal velocity contours. Models were run with air (dTa) and ocean (dTo) temperatures 158 above today’s. Sea level contributions including ice mass loss and isostatic rebound are estimated for all

159 Antarctica (dVA) and for the SSB/ASB sector (dVl), indicated by the green area (inset). With dTa=+4°C and

160 dTo=+1°C (a) the ice sheet margin is located near the A|B boundary and high basal velocities are focused in

161 region B. With dTa=+8°C and dTo=+2°C (b) the ice sheet margin is located in region B and high basal velocities

162 are focused in regions B1 and C. With dTa=+12°C and dTo=+5°C (c) the ice sheet margin is located near the 163 B|C boundary and high basal velocities are focused in region C and the ASB. Additional models are shown in 164 extended data figure 9.

165 Overall, SSB erosion points to large periods of time with a modern-scale ice sheet, with similar structure to 166 todays, and also large periods of time with a retreated ice sheet. Despite experiencing both regimes, 167 Subregion B1 is less eroded than both regions A and C, suggesting that less time has been spent transitional 168 between the two states. Thus, although Highland B is a significant barrier, retreat of the EAIS into the ASB is 169 necessary to explain our observed erosion, and is a characteristic of a fully retreated ice sheet.

170 There is no direct evidence in the erosion record for Pliocene ice-sheet retreat events at Totten Glacier. Ice- 171 rafted detritus from ODP site 1165 at Prydz Bay1 includes a detrital signature that matches closely both the 172 detrital record in Totten-proximal sediment cores24 and the predicted basement geology of Region A6,25. This 173 suggests basement rocks were being eroded in Region A at 7 Ma and at 3.5 Ma1. Therefore, the modern- 174 scale ice sheet has been a regular feature of the EAIS at Totten Glacier since well before 7 Ma.

175 Nonetheless, a significant retreat here may be necessary to generate a sufficient EAIS contribution to 176 Pliocene sea-level highstands of up to 20m above present26. Our representative ice sheet models provide 177 guidance as to the contribution of this sector to global sea level, although it is important to note that 178 different ice sheet models will resolve these pinning points with differing amounts of sea level contribution 179 27428. In addition, transient effects, including the lag-time between ice removal and isostatic response, may 180 generate short-term sea level fluctuations that we do not consider here27.

181 With the bi-stable EAIS configurations we have defined, any Pliocene retreat event was likely restricted to 182 within Region A (< 150 km retreat from present) or involved over 350 km of retreat. A stable long-term ice 183 sheet margin in the intervening zone is less likely, excepting transient pinning at Highland B. Numerical 184 models suggest this transient period in front of Highland B may persist for several thousand years21.

185 The largest retreat under the modern-scale ice sheet configuration (Figure 3a) is associated with a total 186 Antarctic contribution to global sea level of 8.39 m, of which the SSB/ASB sector provides 0.89 m. Allowing 187 for 7.3 m of sea level rise obtained from complete collapse of the Greenland ice sheet29, this is insufficient to 188 explain a 20m highstand.

189 The retreated ice sheet models (Figures 3b and 3c) are associated with a total Antarctic contribution to 190 global sea level of 16.5 m and 21.5 m respectively, of which the SSB/ASB sector provides 2.18 m and 2.89 m 191 respectively. Full retreat into the ASB (extended data Figure 9b) is associated with a total Antarctic 192 contribution of 29.1 m, with 4.29 m being sourced from the SSB/ASB sector.

193 Therefore, the influence of Totten Glacier instability on global sea level is clearly significant, but for any 194 particular warm period, it is also highly uncertain and subject to progressive instability. Our results suggest 195 that the first discriminant is the development of sufficient retreat to breach the A|B boundary ridge. This 196 causes an instability-driven transition from the modern-scale configuration to the retreated configuration. 197 Under ongoing retreat, the breaching of Highland B causes further collapse into the ASB. Each of these 198 changes in state is associated with a significant increase in both the absolute and proportional contribution 199 of this sector to global sea level.

200 METHODS * To be redone*

201 DATA 202 Collection, processing, imaging, availability

203 GRAVITY MODELLING

204 UNCERTAINTIES ASSOCIATED WITH GRAVITY MODELLING

205 ICE SHEET MODELLING

206 CODE AVAILABILITY

207 The ice sheet surface and base were derived from laser and radar sounding as described in earlier ICECAP 208 publications5, interpolated by kriging. Gravity modelling used the Free-Air gravity anomaly corrected for 209 latitude, instrument drift, elevation, aircraft kinematic accelerations and the Eotvös Effect. Modelling along 210 flight lines used an iterative combination of forward modelling and inversion using a commercial two- 211 dimensional code based upon the principles of Talwani30. This code can successfully model 1D and 2D 212 structures, but not complex 3D structures. Our approach is valid for the ice sheet surface and the SSSB base, 213 but some topographic features are three-dimensional in form and cause localised errors. Only relative 214 gravity differences can be modeled, and results were levelled to a common baseline. Cross-line differences 215 were reduced by applying a constant value to a tensioned spline fit through the basin base on each line so as 216 to minimise the misfit between that line and all cross-ties. Median cross line difference is 200 m, and the 217 arithmetic mean is 300 m, with the highest values associated with rugged topography, certain flight lines, 218 and the deepest basin regions. The central Sabrina Subglacial Basin provides consistently low cross-line 219 differences (Extended Data Fig. 2). We imposed the following density structure: Ice sheet density 920 kg m-3, 220 crystalline basement density 2670 kg m-3, lower crust density 2800 kg m-3, mantle density 3200 kg m-3. 221 Sedimentary rock density was varied between a lower limit of 2200 kg m-3 and an upper limit of 2500 kgm-3 222 to create the ranges of sediment thickness shown in Fig. 2c and extended figures 4b-7b. The former was 223 based on a reasonable maximum porosity of 25% given the thickness of ice and likely age of the rocks; the 224 latter was defined by the limit where basin thickness departs considerably from magnetic depth to basement 225 estimates6. This uncertainty is the error shown in Figure 2. It varies linearly and predictably with density 226 contrast, and has no influence on the pattern of basin thickness, only the magnitude. The Moho and a 13 km 227 thick lower crust were defined by a flexural model that accounts for ice and topographic loads. An elastic 228 thickness of 25 km was used, although due to the long-wavelength of the loads involved little variation in 229 Moho structure is observed between 10 km and 50 km elastic thickness.

230 REFERENCES

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298 ACKNOWLEDGEMENTS

299 ICECAP data collection was supported by National Science Foundation grant PLR-0733025, National 300 Aeronautics and Space Administration grants NNX09AR52G, NNG10HPO6C and NNX11AD33G (Operation Ice 301 Bridge and the American Recovery and Reinvestment Act), Australian Antarctic Division projects 3013 and 302 4077, NERC grant NE/D003733/1, the Jackson School of Geosciences and the Australian Governm.ent's 303 Cooperative Research Centres Programme through the Antarctic Climate & Ecosystems Cooperative 304 Research Centre (ACE CRC). Stewart Jamieson is thanked for his comments and for supplying model images 305 for comparison. This is UTIG contribution #.

306 AUTHOR CONTRIBUTIONS

307 ARAA undertook data processing, analysis and modelling and wrote the paper. JLR, TVO, MJS,DAY, DDB, JSG 308 coordinated the field program and/or undertook data collection, quality control and processing. N.R.G. 309 Conducted the ice sheet modelling. All Authors contributed significantly to interpretation, discussion and 310 manuscript preparation.

311 312 Extended Data

313

314 Extended Data Figure 1: Geophysical Data in the study region. Free-air gravity data (a) and magnetic 315 intensity (b); also showing interpreted basement faults6.

316

317 Extended Data Figure 2: Model intersection misfits for the gravity models. The dots show the tensioned 318 spline fit after levelling. Levelling applied a constant value to each line to achieve optimal least-squares fit to 319 the cross-ties. 320

321 Extended Data Figure 3: SSSB base morphology. This map also shows basement faults (thick dashed), and 322 the locations for line R06Ea and additional representative model profiles along Y07b (Extended Data Figure 323 4), R03Ea (Extended Data Figure 5), R08Eb (Extended Data Figure 6) and GL0092a (Extended Data Figure 7).

324

325 Extended Data Figure 4: Gravity model along flight line Y07b (location, Extended Data Figure 3). a) 326 Observed and calculated gravity data, including model components from ice and topography, the deep crust 327 and Moho, and the sedimentary basins. b) The model, also showing depth to magnetic basement estimates, 328 the tensioned spline fit, and cross-ties with other models. c) SSSB thickness and current surface ice velocity 329 (flipped). Average error in ice sheet velocity on this line is 8.4 m/yr18. 330

331 Extended Data Figure 5: Gravity model along flight line R03Ea (location, Extended Data Figure 3). Observed 332 and calculated gravity data, including model components from ice and topography, the deep crust and 333 Moho, and the sedimentary basins. b) The model, also showing depth to magnetic basement estimates, the 334 tensioned spline fit, and cross-ties with other models. c) SSSB thickness and current surface ice velocity 335 (flipped). Average error in ice sheet velocity on this line is 9.7 m/yr18. TAH - Terre-Adelie Highlands. 336

337 Extended Data Figure 6: Gravity model along flight line R08Eb (location, Extended Data Figure 3). 338 Observed and calculated gravity data, including model components from ice and topography, the deep crust 339 and Moho, and the sedimentary basins. b) The model, also showing depth to magnetic basement estimates, 340 the tensioned spline fit, and cross-ties with other models. c) SSSB thickness and current surface ice velocity 341 (flipped). Average error in ice sheet velocity on this line is 8.6 m/yr18. HC - Highland C. 342

343 Extended Data Figure 7: Gravity model along flight line GL0092a (location, Extended Data Figure 3). 344 Observed and calculated gravity data, including model components from ice and topography, the deep crust 345 and Moho, and the sedimentary basins. b) The model, also showing depth to magnetic basement estimates, 346 the tensioned spline fit, and cross-ties with other models. c) SSSB thickness and current surface ice velocity 347 (flipped). Average error in ice sheet velocity on this line is 10.6 m/yr18. ASB - Aurora Subglacial Basin. 348

349 Extended Data Figure 8: Gravity models including basement density variations. a) line R03Ea, b) line 350 R06Ea, c) line R08Eb. The models involve an attempt to generate the flattest basin topography with block- 351 scale density contrasts. Long-dashed dashed lines indicate major faults. The dotted line indicates the basin 352 structure with a homogenous basement density of 2670 kg/m3. Blocks are changed within 25 kg/m3 of this 353 value. Differences are substantial in places but the overall pattern is preserved.

354 355 356 Extended Data Figure 9: Representative ice sheet models for retreat states. Each image represents the ice 357 sheet extent and thickness after a long-term 20 ky run under consistent climate forcing. Each model has 358 different air/ocean temperatures. a) with air/ocean temperatures of today (0/0°C), b) with air/ocean 359 temperatures of 4/1°C above today's, c) with air/ocean temperatures of 8/2°C above today's, d) with 360 air/ocean temperatures of 12/5°C above today's, e) with air/ocean temperatures of 15/5°C above today's. 361 Stated values indicate the total Antarctic contribution and the component from western Wilkes Land.