Footprint of sustained poleward warm water flow within East Antarctic submarine canyons

Domain: earth_science
Published: 2024-07-17
Editorial outcome: major_revision

Abstract
The intrusion of relatively warm water onto the continental shelf is widely recognized as a threat to Antarctic ice shelves and glaciers grounded below sea level, as enhanced ocean heat increases their basal melt. While the circulation of warm water has been documented on the East Antarctic continental shelf, the modes of warm water transport from the deep ocean onto the shelf are still uncertain. This makes predicting the future responses of major East Antarctic marine-grounded glaciers, such as Totten and Ninnis glaciers, particularly challenging. Here, we outline the key role of submarine canyons to convey southward flowing currents that transport warm Circumpolar Deep Water toward the East Antarctic shelf break, thus facilitating warm water intrusion on the continental shelf. Sediment drifts on the eastern flank of the canyons provide evidence for sustained southward-directed flows. These morpho-sedimentary features thus highlight areas potentially prone to enhanced ocean heat transport toward the continental shelf, with repercussions for past, present, and future glacial melting and consequent sea level rise.

Introduction
The East Antarctic Ice Sheet (EAIS) is receiving increasing attention as it represents the largest potential contributor to both past and future sea level rise, containing at present ca. 52 m of Sea Level Equivalent (SLE1,2). In some sectors, where the EAIS ice is grounded well below the sea level, rapid ice shelf basal melting is occurring due to intrusions of relatively warm Circumpolar Deep Water (CDW3–5), the largest source of heat to the ice sheet6. Defining both the extent and the long-term persistence of intrusion of warm CDW on the continental shelf (e.g. refs. 7–9) is critical because it can substantially increase the vulnerability of marine-based sectors of the EAIS, with major consequences for predicting their future responses to climate warming. Here we focus on the Totten and Ninnis glaciers, which represent key outlets of the two main marine-based sectors of East Antarctica: the Aurora-Sabrina and Wilkes subglacial basins, respectively (Fig. 1).

The Totten Glacier (Sabrina Coast; Fig. 1) has a large marine-based catchment and exhibits among the highest basal melt rates (10.5 m/yr) in the whole East Antarctica10. These rates are comparable with those observed in rapidly retreating and thinning sectors of the West Antarctic Ice Sheet (WAIS11–13). Totten Glacier alone drains a volume of ice above flotation equivalent to >3.5 m of sea-level rise1,14, similar to the entire WAIS10. The rapid basal melting, thinning, and acceleration of the Totten Glacier, particularly evident since 196915, are likely associated with ocean heat transported by CDW intrusions onto the continental shelf16–21, with the morpho-bathymetry playing a key role in steering warm waters from the shelf break toward the glacier14,19–22. On the Sabrina continental shelf, CDW accesses the ice-shelf cavity at the glacier front3. The thick layer of CDW (~1000 m) is transported efficiently toward the Sabrina Coast shelf break by southward currents on the rim of a quasi-stationary cyclonic eddy with a spatial scale of 100–200 km, guided by the underlying bathymetry20. Over the last three decades of observations, there is no evidence for the production of Dense Shelf Water on polynyas found on the Sabrina Coast continental shelf (Fig. 1)3,4,16,17,21, contrary to polynyas from other Antarctic regions. Thus, CDW intrusions, allowing warm water to reach the ice shelf cavities, can cause rapid basal ice melting4, potentially leading to the acceleration of the glacier and reduced buttressing18. Totten Glacier grounding line retreat is predicted to increase 3.5x by 2100 in response to changes in the inflow of warm water onto the Sabrina continental shelf23,24.

Ninnis Glacier (George V Land) holds 95 cm Sea Level Equivalent10, and buttresses the ice sheet draining part of the Wilkes Subglacial Basin (WSB25), the largest marine-based drainage basin in East Antarctica. During the last century, from 1913 to 1962, the Ninnis Glacier tongue retreated about 90 km26, possibly as a result of periodic calving events27. Ninnis Glacier occupies a landward sloping basin1, is grounded below sea level28, and holds potential for rapid retreat should the CDW access the ice sheet margin10,29. Relatively warm water has been found near the shelf break30, and the presence of basins up to 1400 m deep on the continental shelf could convey warm water toward the ice shelves28,31,32. Similar to the Aurora-Sabrina subglacial basins, the Wilkes Subglacial Basin is thus vulnerable to the intrusion of warm CDW across the continental shelf, possibly causing irreversible retreat33. The Ninnis Glacier is currently in contact with cold dense shelf water34,35, forming Adélie Land Bottom Water (25% of the global volume of Antarctic Bottom Water36, and references therein). However, changes in atmospheric and ocean circulation may have driven retreat in the past, as indicated by numerical ice sheet modeling (37 and references therein) and by ice and marine sediment cores (refs. 38–42).

Here we document submarine canyons on the East Antarctic continental rise that host sediment drifts formed by predominantly southwards currents. This evidence argues for the existence of a persistent quasi-barotropic (i.e. from surface to bottom) southward flow, which also favors the transport of the CDW layer toward the continental shelf. This, in turn, may have influenced Totten and Ninnis glaciers in the past, and increases their vulnerability to future ocean warming.

Results
Sub-bottom CHIRP (Compressed High Intensity Radar Pulse) data collected offshore the Sabrina Coast reveal the occurrence of sedimentary deposits on the eastern flanks of the Maadjit and Manang canyons, from ca. 119° E to ca. 121° E (Fig. 1), in water depths ranging from 3250 to 3550 m (Fig. 2). These mounded deposits, ca. 2000–3500 m wide and ca. 50-80 m thick, are asymmetric, with a steeper flank facing the thalweg of the canyon and a gentler eastern flank, and show a slight eastward crest migration. They appear to grow above a higher amplitude mounded surface, possibly representing the harder substratum of a terrace similar to those described by O’Brien et al.43. Internally, their thinly stratified acoustic facies includes low to medium amplitude, laterally continuous seismic reflections, sub-parallel on the gentler side, while converging before terminating on the steeper side (Fig. 2). The configuration of the seismic reflections and the overall morphology of these sediment bodies are diagnostic of bottom current-related deposits44–46 and are similar to those of small scale sediment drifts observed in high- and low latitude continental margins47. Similarly to the Sabrina Coast, a sediment drift has been found on the continental rise in front of the Ninnis Glacier at around 148° E. Morpho-bathymetry data reveal that this area is cut by a canyon system resembling the Maadjit and Manang canyons, with dendritic-like branches in the slope that converge into a single canyon. The thinly stratified, asymmetric sediment drift lies on the canyon’s eastern flank at a depth of 3050 m, is ca. 2750 m wide and ca. 40 m thick, and shows a slight eastward crest migration (Fig. 3).

Based on their position, progressive crest migration and shape, we argue that these sediment drifts formed because of southward flowing bottom currents focused on the eastern side of the canyon, potentially under the influence of the Coriolis force (e.g. ref. 48). The Coriolis effect is known to strongly influence the internal flow structure of oceanic currents, especially at high latitudes, deflecting them towards the left in the southern hemisphere. As a consequence, sediment-laden currents flowing from the shelf break toward the rise would cause formation of sedimentary deposits on the left, i.e., the western flank of the canyons49. This is inferred to have occurred off the Sabrina Coast during glacial maxima and especially during the build-up of canyon-levees in the so-called “Phase 3” of ref. 50 and references therein (Fig. 2c), tentatively dated early-to-late Miocene in age, when a highly dynamic, polythermal ice sheet delivered large amounts of sediments on the continental slope and rise via slumps and turbidity flows (51 and references therein). The highly asymmetric canyon-levees formed in the continental rise since the mid-Cenozoic reflects non-uniform deposition from downslope gravity flow deviated to the west by the combined effect of the Antarctic slope currents and the Coriolis effect43,51. These processes and related canyon-levee configuration constitute the so-called “paleoconfiguration”. Downslope processes still contributed to shaping the distal margin architecture even during and after the transition to polar ice sheet regimes (i.e., “Phase 4” of ref. 50 and references therein; Fig. 2c), by transferring sediments from the shelf to the rise area through the long-lived canyon systems (51; Fig. 2). Conversely, the sediment drifts that we discovered and discuss here, imaged on the high-resolution CHIRP and TOPAS profiles, are explained with a different mechanism. In the “present day configuration”, the main canyon is smaller compared to the “paleoconfiguration” and is entrenched to the west. The weaker turbidity currents possibly generated throughout the Phase 4 were not able to develop on the steep, up to 700 m-high, “paleo” western levee, being thus only capable to build a small levee to the east (Fig. 2a and Supplementary Fig. 1). On the Sabrina Coast continental rise, the sediment drifts build-up further to the east of this small eastern levee, and their internal reflection configurations indicate that they are coeval or even younger than the levee deposits (Fig. 2a and Supplementary Fig. 1).

Off the Sabrina Coast, ocean currents have been estimated using in situ and satellite measurements20,52. These observations show a southward flow in the Maadjit canyon from the surface to the seafloor (with velocities up to 10 cm/s in the bottom 100 m of the water column). Such southward flows are associated with a deep-reaching semi-permanent cyclonic eddy20,50,53. It is recognized as playing a key role in southward transport of offshore CDW, which is then able to access the continental shelf at around 120° E (18–21). Vertical profiles obtained from conductivity-temperature-depth (CTD) casts carried out during the IN2017-V01 cruise (see Fig. 2d for their location) highlight the presence of CDW, with potential temperature >0 °C, salinity >34.5 (potential density >27.7 kg m−3), and dissolved oxygen <240 uM, between 300 m and 1200 m depth over the continental slope and rise. At the head of the Maadjit and Manang canyons, CDW is found at stations 28 and 29 and at stations 7 and 8. Similarly, a signal of warm water is found in the 100 m-thick bottom layer on the continental shelf at stations 25 and 26 directly to the south of the canyons head (Fig. 4). A signature of Antarctic Bottom Water (AABW), with potential temperature <0 °C, salinity >34.63, dissolved oxygen >240 uM, neutral density >28.27 kg m−3, is instead clear over the slope at stations 28 and 29 below 1300 m, and at stations 7 and 8, below 1800 m depth. Approaching the shelf break (i.e. the head of the canyons), the CTD profiles coherently show that the water column between 1000 and 2500 m is progressively more influenced by colder, fresher and more oxygenated waters of shelf-slope origin, modifying the properties of CDW accessing the continental shelf (Fig. 4b–d).

Methods
SABRINA COAST
Sub-bottom profiler CHIRP, morpho-bathymetry, and CTD data were collected during January–March 2017 on board the research vessel RV Investigator. Details of the survey are available at the Australian Marine National Facility web site under the survey number IN2017-V01 (https://www.cmar.csiro.au/data/trawler/survey_details.cfm?survey=IN2017_V0169).

Sub-bottom profiler CHIRP data were acquired with a Kongsberg SPB120; a linear chirp with a sweep of 2.5–6.5 kHz was used. The pulse length was 6 (in the study area) to 12 milliseconds (during transits in deep water). A gain of 6 dB was applied to the data, followed by a gain correction, matched filter, instant amplitude processing, and a time variable gain to enhance sub-bottom reflections. The dataset was recorded and presented as two-way time sections, saved as an envelope in segy format (Society of Exploration Geophysics Y format) and Kongsberg proprietary .raw files (O’Brien et al. 43). Segy files were then uploaded into a HIS Markit Kingdom software project and viewed in Seisee. The envelope data has a low frequency and contains only positive values, without phase information70.

Key seismic profiles were then re-processed, to enhance the internal configurations of the identified sedimentary features. The envelope raw data were processed using Seismic Unix (https://wiki.seismic-unix.org/start). Due to the great depth (most of the sediment drifts lie at ca. 3500–3650 m depth), the original profiles have a large intertrace (20–22 m). Therefore, the signal was spatially reduced by creating an additional track as a mixture of the neighboring traces. In areas with steep slopes to avoid aliasing effects, the traces were rectified in time before mixing and then shifted.

The gain (compensation of transmission loss, absorption and spherical divergence) was calculated and applied by inverting the amplitude decay curve. A water column muting was performed before creating the envelope data for interpretation.

To obtain an estimation of the thickness of the sediment drifts, we applied a time-depth conversion, by using the seismic velocities derived from each high-resolution multichannel seismic profile collected in the study area during the IN2017_V01 cruise, as described in 53. Seismic velocities from the seafloor to 100 ms depth range from 1571 to 1603 m/s.

The morpho-bathymetry was acquired using two multibeam echosounders, a Kongsberg EM122 and an EM710. The former is a 12 kHz full ocean depth multibeam echosounder and was operated at all depth ranges on the Sabrina Seafloor Survey. The EM710 is a high resolution 70–100 KHz sounder used for mapping on the continental shelf and upper slope to depths of up to 2000 m. Sound velocity corrections were undertaken using data from CTDs and eXpendable BathyThermographs (XBTs). Data have been processed within Caris HIPS and SIPS version 9.1.9. Further details about both data collection and processing are provided in 54.

The oceanographic data presented in this study are part of the 31 conductivity–temperature–depth (CTD) vertical profiles (labeled 1–11, 13–23, 25–33) acquired in the study region (Fig. 2) during the IN2017_V01 cruise using a Seabird SBE9 plus (pump-controlled) probe and SBE11 plus V2 Deck Unit. The probe was integrated with an additional sensor for measuring dissolved oxygen concentration (DO, SBE43), and an altimeter (PA500). A SBE32 Carousel Water Sampler was equipped with 36 Niskin bottles (Ocean Test Equipment Inc. Florida; 12 L capacity each one, mounted on the rosette frame). Further details about both data collection and processing are provided in 21.

GEORGE V LAND
About 4000 km of sub-bottom Topas Kongsberg PS18 0.5-6 kHz, ca. 10.000 km2 of multibeam Kongsberg MBES EM304 Frequency range (26–34 KHz), were collected during the XXXVIII Italian Expedition on board of R/V Laura Bassi in the framework of PNRA Collapse Project (P.I. L. DeSantis), in water depth ranging from 700–3500 meters. The investigated area lies between ca. 65° S and 64° S and 145° E and 152° E (https://www.pnra.aq/index.php/it/project/528/cook-glacier-ocean-system-sea-level-and-antarctic-past-stability).

The main acquisition parameters of the sub-bottom TOPAS profiles are briefly reported here. Transmitter: Normal transmit mode; external trigger mode; CHIRP pulse form; start frequency 2 kHZ; stop frequency 6 kHz; CHIRP length 20 ms; output decibel −9°; output level 12.6%; auto beam control; transducer speed 1500 m/s. Receiver: manual delay control; sample rate 64 kHz; trace length 500; HP-filter 2.0 kHz.

Sub-bottom profiler data were acquired using a keel mounted, parametric Kongsberg-Geoacoustic TOPAS echosounder, connected to the Motion Reference Unit of the ship to automatically compensate for the pitch, roll, and heave components; in order to maximize the penetration, a 20 ms long chirp signal, with an upsweep ranging from 2 kHz to 6 kHz, was used. The data, recorded in two way time and double polarity, were saved in the proprietary Kongsberg raw format; they were then converted to SEGY through the TOPAS PS18 software.

The data were processed with the SCHLUMBERGER Vista 19 software. The first step was the application of the static corrections (stored in an appropriate header) for a proper time delay compensation; a gain function and a spiking deconvolution (with an operator length of 2 ms) were successively applied to compensate for the amplitude decay and to increase the temporal resolution, respectively. Finally, a top (sea bottom) mute followed by the application of a Hilbert envelope function was adopted.

To obtain an estimation of the thickness of the sediment drifts, we applied a time-depth conversion, by using the seismic velocities derived from each high-resolution multichannel seismic profile collected off Sabrina Coast.

Morpho-bathymetry data (MBES) were acquired between 06/03/2022 and 18/03/2022 by means of the 30 kHz Hull mounted Kongsberg EM304 1°(tx) x 2°(rx) in mainly uncharted areas and processed by means the SW ‘QPS Qimera’. Kongsberg EM304 MBES system uses separate transmit and receive transducers in a Mills Cross configuration. The transmit fan generated is divided into individual sectors for unique control, enabling active stabilization in real time to correct for any yaw and pitch movement of the vessel, while roll stabilization is applied on the receiving beams. Up to 800 individual beams are normally available, or up to 1600 if the dual swath mode is enabled. In dual-swath mode two individual transmitting fans are generated with a small difference in tilt to provide a constant sounding separation along track, resulting in a dense sounding pattern on the seafloor. R/V Laura Bassi acquired MBES data using two different software packages at the same time, i.e. both with ‘QPS Qinsy’, the main navigation program used during the campaign and with ‘SIS5’, supplied by Kongsberg, the same brand that supplied the 2 MBESs equipped with the ship, the Kongsberg EM304 and EM2040c. As well as Qinsy data format, the KMall KONGSBERG logging format supplied by ‘SIS5’ contains already calculated latitude, longitude, depth, time, and ellipsoidal height, which allowed us to combine the two above mentioned data formats within the used ‘QPS Qimera’ processing program. Velocity profiles used during the acquisition worked quit well, as did the calibration performed before the start of the acquisition. The dataset was initially examined line by line and cleaned up by manual despiking using the SWATH EDITOR, SLICE EDITOR and 3D EDITOR to remove most of the residual outliers not filtered out by the software during the acquisition phase, and then, in a second phase, the same lines were re-examined and further cleaned up using some statistical data validation functions provided by “QPS Qimera” (CUBE, SPLINE). Following the analysis of the preliminary DTMs obtained, lines that could potentially cause some residual noise were again inspected and surgically treated. Finally, 2 different Digital Terrain Models were created with grids of different sizes (20 m and 100 m) depending on the depth investigated. Part of the morpho-bathymetry data shown here were re-gridded by using a grid cell 50 m wide.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript submitted by Donda et al. documents the presence of small drift-like features on the\neast side of canyons in the Sabrina Coast region of East Antarctica. The authors infer that these have\nbeen formed by ‘sustained’ southward (up-slope) flowing warm water masses associated with warm\nCircumpolar Deep Water, and that these show long-term heat transport towards the East Antarctic ice\nshelf.\n\nThe paper addresses a very important topic. Clearly, understanding the history of warm-water\nincursion towards the ice shelf around Antarctica is important to understand potential current and\nfuture changes that could impact melt rates. The authors propose an interesting hypothesis, with\nrecent oceanographic data showing the presence of semi-permanent eddies in the region (Hirano et\nal., 2021), and with analogues elsewhere in Antarctica, such as the Ross Shelf (Morisson et al., 2020),\nshowing focused poleward warm-water transport along canyon systems.\n\nHowever, I have several concerns about the manuscript as it is presented. The major issues are as\nfollows:\n\nInterpretation of sediment drifts in the canyons: “The configuration of the seismic reflectors and the\noverall morphology of these sediment bodies are diagnostic of bottom current-related deposits” (lines\n94-95) – This is an overstatement, based on the information presented. First, the CHIRP profile is\nrelatively low quality, and the interpretation in Fig 2.b is ambiguous. Could this be a slump feature\nwithin the canyon for example? Or some aggradation on a terrace within the canyon? The text infers\nthat there are more than one of these, but only one is shown in each canyon. This feature is also not\nclear on the MBES data, and I would expect that the authors show a much higher-resolution,\nannotated version of this data to justify their explanation. The single colour image does not show\nthese ‘drifts’ at all clearly. This is crucial to the argument of the manuscript, and the authors should\nprovide a much more convincing case that these are indeed sediment drifts, and document their\nlateral extent and locations more clearly. As a more general point, the only original data presented by\nthe authors are two CHIRP profiles, and MBES data at a very small scale in the Maadjit canyon region\n– not sufficient to justify the interpretation presented.\n\nInference of southward flowing bottom currents: “The sediment drifts that we identify on the eastern\nlevee of the present-day canyon-levee systems can only have been shaped by a southward flowing\nbottom current.” (Lines 112-113) This again seems to me to be an oversimplification and an\noversimplification. If these are indeed drifts, they form within the canyons. As the canyons are several\nhundred metres deep, any downslope currents (mainly turbidity currents) would have to have flow\nthicknesses higher than the canyon depth to experience flow stripping and sedimentation on the outer\nlevees, which in this case should be on the west of the channels due to Coriolis. However, if the flows\nare confined to the channel, this would not occur. In this case, the hydrodynamics of the flow would be\nmore complicated, and could result in net deposition on the terrace to the east, without the need for\nCoriolis-driven transport to the left, up-channel. The authors do refer to the analogue data from the\nRoss Sea (Morisson et al., 2020), which is intriguing, but they don’t show any morphological evidence\nof bedform migration, for example, that would justify that these drifts are forming directly from\nupslope-flowing currents.\n\nIn addition to this, the argument would benefit from more observational data of up-canyon flow from\nADCP data (for example; data from Hirano et al., 2011 are for surface flow). I note that the ‘Yamazaky et al., 2023’\npaper that is cited for ADCP data does not appear in the reference list so this data is not\navailable for context. The reference to Hirado et al., for intermediate water depth velocities are\nactually for surface flow, and an assumption that near-barotropic flow would result in broadly similar\nvelocities at greater depth, but they don’t have any actual velocity measurement for the deepwater\nwater masses.\n\nDriver for southward flowing bottom currents: the main analogue that the authors use to justify their\ninterpretation for strong southward flowing currents is from Morisson et al., 2020, where the up-\ncanyon currents form specifically in response to cascading dense shelf water. But the authors state\nthat “In contrast with other Antarctic regions with polynyas, there is no evidence over the last three\ndecades of Dense Shelf Water (DSW) production in the Dalton Polynya (lines 59-61). This appears to\nbe a problem for their process interpretation, as the currents associated with the eddies documented\nby Hirado et al. do not appear to be strong enough to entrain sediment and form large contourite\ndrifts. However, numerical modelling results in the Morisson et al., paper do indicate strong DSW\nproduction in the study area. This needs to be addressed in any future revisions.\n\nAge of drifts: the authors state that “The seismostratigraphy reveals that sediment drifts are coeval or\neven younger than the levee deposits on the western flanks” (line 97-98), referring to previously\npublished interpreted seismic profiles. However, the terraces/canyon drifts are not easily resolved on\nthe multichannel seismic profiles. The CHIRP profiles in Figs 2 and 3 are of higher resolution, but\nrelatively low quality, and the drifts are separated from the levees by canyon walls dominated by\nerosion. It is very difficult to correlate the stratigraphy between the drifts and levees, or to tell which\nis older, and this statement is unjustified based on the evidence presented. Thus the “fossil\nconfiguration” and present-day configuration model in Fig. 5 seems to be speculative. Furthermore,\nthe age model presented is difficult to justify. The authors refer to a piston core a long distance to the\nwest of the study area, but there are other cores much closer to the study location – for example, the\nPC-05 core published in O’Brien et al., 2020, which is dated. Other cores in or near the canyons should\nalso be considered – sediment accumulation rates in the canyon and on a drift system are likely to be\nsubstantially different to those on the open slope. The authors should take care to justify the age\nmodel, and acknowledge uncertainties.\n\nOther minor issues include:\n- References and values quoted not always appropriate or correct. For example, they state that\n“Totten Glacier alone drains a 49 volume of ice above flotation equivalent to more than 3.5 m of sea-\nlevel rise (Rignot et al., 2019), similar to the 50 entire WAIS (Greenbaum et al., 2015).” However, the\nvalue of 3.5 m for Totten is from Greenbaum et al (Rignot et al., quote a value of 3.85 m). The\nGreenbaum paper focuses specifically on the Totten Glacier, so is inappropriate as a citation for the\nWAIS.\n- Figure 1, the bathymetry is not very clear – consider using colour spectrum, as well as shaded relief.\nConsider a cross section showing shelf and subglacial topography, grounding lines, etc.\n- Figure 2. Line location symbology are different and hard to see.\n- There are a number of spelling and grammatical issues that should be addressed.", "reviewer_2": "Footprint of sustained poleward warm water flow within East Antarctic submarine canyons\nThe data this manuscript have a relative high quality taking into account the remote area and the\ndifficult to obtained such type of data. These data were obtained with the appropriate techniques as\nultra-high resolution seismic (Sub-bottom CHIRP). The main fortress of the manuscript is the potential\nsignificance of the results. In this way, authors point out to periodic intrusion of relatively warm deep\nwater onto the continental shelf as a potential cause of melting of ice-caps in Antarctica. This\nintroduce that fact the deep oceanic circulation of warm currents around the Antarctica as a threat to\nAntarctic ice shelves ice-caps and glaciers grounded below sea level.\n\nThe major ice-sheets around Antarctica (e.g. Weddell & Ross Sea Ice-Caps) acts as the main source of\ncold deep waters distributed around the world´s oceans, playing an important role in “cooling” the\nEarth during the last geological history, Miocene to present. After my own experience of having carried\nout eight oceanographic expeditions to Antarctic, I support that role of deep currents in ice-cap\nmelting as observed in the Weddell Sea and, even more, into their role in the distribution of Antarctic\ndeep waters as forming the great system of oceanic´s world circulation.\n\nThe intrusion of warm water onto the continental, as proposed in this manuscript by accelerating the\nmelt of ice-caps might increase the cold deep water as the formation of Antarctic Bottom Waters\n(AABW) and their influence in the global ocean circulation. At geological scale, during deglacial\nPleistocene times, the rapid retreat of the Antarctic ice-caps have been explained by increasing of\ntemperatures in the southern oceans, and finally resulting rapid sea-level rise as consequence of the\nrapid retreat of massive ice-caps.\n\nThe only weakness that I observe in this manuscript is how to explain the water depth as much of\n3,050 meters (4.2 s Two-way Travel Time) of the formation of the plastered drifts into deep incised S\nto N canyons as consequence of along-slope currents as the Circumpolar Deep Water (CDW). The\nplastered drift seems clearly associated to “upwelling currents” rising along a S to N submarine canyon\nand deflected to left by Coriolis effect. Authors explains these upward currents as results of semi-\npermanent cyclonic gyres formed by the interaction of cross-slope CDW current with the incision of the\ndeep submarine canyons.\n\nIn my opinion, it seems much more difficult to explain the plastered drifts formed along the S-N\ncanyons as results of orthogonal currents flowing in along-slope direction than from “upwelling” of N-S\nsourced deep waters.\n\nIt has been also proposed other mechanism to explain the spatial and temporal connection between\nupwelling of CDW and cascade of DSW (Dense Shelf Water) transport formed beneath ice-sheet. In\nthis way, as a pulse of DSW descends the continental slope, the dense water displaces less dense\nwater within the canyon allowing upwelling of CDW warm waters (see Morrison et al. 2020).\n\nProbably, authors should improve the explanation(s) and discussion on the mechanism proposed\ndealing with the formation of cyclonic gyres enough or not to transport “upwelling” CDW warm waters\nonto the shelf.\n\nNevertheless, given the potential significance of the results, I strongly recommend the publication of\nthis manuscript after making some further explanation and discussion on the proposed mechanism of\nintrusion/upwelling of warm waters onto the shelf.", "reviewer_3": "Review for Nature Communications 463358, \"Footprint of sustained poleward warm water flow within\nEast Antarctic submarine canyons\". Overall, I see no Major Weakness in the manuscript or research\npresented therein. First I'll summarized some overall thoughts before several minor weakness and\nline-by-line comments/edits.\n\nOverall, this work represents an incremental but important step in Antarctic ice-ocean interactions, i.e.\npresenting new evidence for sustained meridional flow in two East Antarctic systems. The material is\nmostly presented in a clear fashion, and the research methods are sound - I do not have any\ncriticisms of either their stratigraphic or hydrographical interpretations. In fact, I think they are\nexceptionally clear examples of the hypothesis processes at play, and illustrates how to tie\noceanography and solid-earth disciplines. However I have a few concerns about the overall impact of\nthe work. As the authors themselves state, their work confirms that southward flow can be inferred\nusing multichannel seismic/CHIRP, something that has been postulated by theory and confirmed\nelsewhere in Antarctica. While this is certainly a finding worthy to share with the community, for me it\ndoes not transform our thinking of the processes at play or the fate of glaciers in East Antarctica. The\nreader is told that the poleward flow of CDW to the ice sheet is critical for projecting future ice mass\nloss and sea level rise, but in its present form, the only result from this paper related to these topics\nare that the identified features \"represent at least a ca. 440 to 1300 kyr long record of southward\nflowing currents\". How does this help improve/understand projections of future glacial melt and SLR?\nFurthermore, although the CTD analysis is fairly sound, single hydrographic profiles cannot in any way\n'confirm' a sediment record thousands of years long. In summary, this work, as submitted, provides\nvery clear and compelling arguments for extended periods of southerly flow along these canyons, but I\ncame away unconvinced of its importance to the stated motivations (i.e., ice melt and sea level rise)\n\nLn 24: Subject?\nLn 25: What is the justification of looking at these two glaciers?\nLn 26: What about the canyons isy being ‘documented’?\nLn 31: Does it say something about SLR directly, or just indirectly through glacial melt?\nLn 59-61: This sentence is ambiguous. As written it sounds like there are “Dalton Polynyas” in \"other\nAntarctic regions”. Also, are you saying that the Dalton Polynya hasn’t formed? Hasn’t produced DSW?\nLn 69: The bathymetry resolution in some areas seems too low. Can you use Bedmap3? It appears to\nbe available via SCAR as of this summer.\nAlso, the ‘Depth’ scale doesn’t mean much, and topography looks ‘inverted’ in 1b inset, maybe due to\nhillshade angle?\nFinally, perhaps this Figure 1 is not needed? All the reader gets (in its present form) is the location\nand trough/slope shape overview. Readers already get the bathymetry in more detail in Figs 2 and 3.\nSo consider eliminating Fig 1 and add a circum-Antarctic Overview inset to each Figs 2 and 3?\nLn 74: Really ‘one of the largest’?\nLn 93: “are”?\nLn 104: “argue”?\nLn 105: How wide is the canyon/current? Is the Rossby radius of deformation relevant here? I know it\nis difficult to calculate given the dearth of measurements, but can it be estimated?\nLn 113: “Only”? Is this strong of a statement defensible?\nLn 119: Capitalize? And add “.”\nLn 131: In addition to the cartoon in Figure 5, the discussion of these drift and levee features could\nbenefit from a plan view zoom-in of a single feature (like 10 km wide version o)\nLn 143: It would be VER helpful for a zoomed in location plot for the CTDs shown in Figure 4, perhaps\nas an inset?\nLn 144: “Potential” temperature? (theta)\nLn 146: Apostrophe?\nLn 153: As written, the “properties” access the continental shelf. I suggest “modifying the properties\nof CDW accessing the continental shelf.”\nLn 163: Is this specific to “East” Antarctica?\nLn 165: “barotropic”?\nLn 173: Remove comma.\nLn 184: Interesting! I was wondering about ‘timing’ earlier in the paper. Is there a way to elevate this\npoint or move it up?\nThis leads me to ask, can you compare southward flow during the different epochs? Even if relatively?\nWas it stronger/weaker or more/less consistent?\nLn 196: What does “rugged mean here? Have you made the case why it’s relevant?\nLn 199: I don’t get the point of this short paragraph. Either clarify, or eliminate.\nLn 212: This confuses me, as throughout the rest of the paper we are told that this southward flow is\nextremely persistent. If so, what is its role in “destabilization”?\nLn 219: In general I really like this cartoon, however some things are not very clear, especially the\n“sediment drift” features.\nAlso, \"glacials\" to “glacial periods”?\nLn 222: Remove “were” and “giant”?\nLn 224: “flowing flows”? Redundant.\nLn 237: This list is hard to follow and therefore has little meaning.\nLn 255: Add space.\nLn 280: This seems like a less-detailed, redundant paragraph. Suggest removing it.\nLn 295: I think “easy” is a bit too colloquial (and subjective) for a research article.\nLn 296: Too qualitative for me. Can you put numbers on this noise? If not, remove.\nLn 303: Define “DTM”.\nLn 310: “going to be”? This is not good practice."}, "author_response": "Following the Reviewer’s comment, we added a new CHIRP profile and expanded both the “Results” and the\n“Discussion” sections in order to improve the information on the studied features.\n\nFirst, the CHIRP profile is relatively low quality, and the interpretation in Fig 2.b is ambiguous.\nWe have re-processed two CHIRP profiles, i.e., the profile 110 already shown in the previous manuscript\nversion, and a new profile 72_004, collected further to the north. See the next response for clarity on how the\nreprocessed data improve our interpretation.\n\nCould this be a slump feature within the canyon for example? Or some aggradation on a terrace within the\ncanyon?\nWe thank the Reviewer for this comment, which forced us to perform, for the Sabrina coast dataset, a re-\nanalysis of the CHIRP profiles, particularly focused on the re-processed lines, and of the multibeam bathymetry.\nOur re-analyses thus complement the outcomes of the previous geomorphological study of O’Brien et al.,\n(2020), where the first geomorphological map of the Sabrina Coast continental slope and rise was discussed. In\nthe study area, those Authors already evidenced how the western flank, i.e. the steeper flank of the Maadjit\nCanyon is commonly scalloped, and represents areas of sediment mass movement , where arcuate features\nconstitute headwall scarps of slumps; downslope of them, slump deposits are found (O’Brien et al., 2020). The\nnew CHIRP profile we have included in this revised version clearly shows the prominent slide scar on the\nwestern flank of the Maadjit canyon (Fig. 2b), whereas the revised Supplementary Figure 1 outlines the\nhummocky internal configuration characterizing the slided material (the western flank of the Maadjit canyon),\ncompared to the well stratified features, which we interpret as sediment drift. In fact, on the eastern flank of\nthat canyon, CHIRP profiles and multibeam bathymetry do not reveal the occurrence of such prominent\nheadwall scarps and slump features. The feature we interpret as sediment drifts is well stratified and\nprogressively growing with an eastward crest migration, which is more clearly recognizable after the data re-\nprocessing. The elongated feature within the Maadjit canyon, more clearly visible in the new Supplementary\nFigure 2, possibly represents an ancient terrace, above which sediment drifts locally form. In fact, based on Fig.\n6 of O’Brien et al. (2020), terraces have been already identified along the adjacent Jeffrey Canyon (see Figs. 6\nand 10a of O’Brien et al., 2020), and are characterized by “low-relief hummocky surfaces” (O’Brien et al., 2023),\ni.e., they appear very different to the well-stratified features discussed in this study. We have modified the text\naccordingly.\n\nThe text infers that there are more than one of these, but only one is shown in each canyon.\nFor the area located offshore Totten Glacier, we have inserted a new CHIRP profile, i.e. line 72_004, collected\nwithin the same canyon system (i.e., Maadjit Canyon) further to the north. All the three profiles, i.e., those\npresented in Figs. 2a and 2b and that shown in Supplementary Figure 1 have been reprocessed in order to\noutline the internal reflectors configurations.\n\nThis feature is also not clear on the MBES data, and I would expect that the authors show a much higher-\nresolution, annotated version of this data to justify their explanation.\nIn this revised version of the manuscript, we have inserted a revised version of the multibeam bathymetry,\nwith (Fig. 2) and without annotations (Supplementary Figure 2)\n\nThe single colour image does not show these ‘drifts’ at all clearly. This is crucial to the argument of the\nmanuscript, and the authors should provide a much more convincing case that these are indeed sediment\ndrifts, and document their lateral extent and locations more clearly. As a more general point, the only original\ndata presented by the authors are two CHIRP profiles, and MBES data at a very small scale in the Maadjit\ncanyon region – not sufficient to justify the interpretation presented.\nWe recall here the previous comment: For the area located offshore Totten Glacier, we have inserted a new\nCHIRP profile, i.e. line 72_004, collected within the same canyon system (i.e., Maadjit Canyon) further to the\nnorth. All the three profiles, i.e., those presented in Figs. 2a and 2b and that shown in Supplementary Figure 1\nhave been reprocessed in order to outline the internal reflectors configurations. MBES data are now shown in\nFigures 2 and 3 and in the Supplementary Figure S2 with a colour spectrum, which emphasizes the\nmorphobathymetric setting.\n\nInference of southward flowing bottom currents: “The sediment drifts that we identify on the eastern levee of\nthe present-day canyon-levee systems can only have been shaped by a southward flowing bottom current.”\n(Lines 112-113) This again seems to me to be an oversimplification and an overstatement. If these are indeed\ndrifts, they form within the canyons. As the canyons are several hundred metres deep, any downslope currents\n(mainly turbidity currents) would have to have flow thicknesses higher than the canyon depth to experience\nflow stripping and sedimentation on the outer levees, which in this case should be on the west of the channels\ndue to Coriolis.\nWe do agree with the Reviewer comment: our statement at lines 112-113 needed to be deeper discussed. This\nwas not possible in the first version of the manuscript, because of the limited number of words allowed in\nNature Geoscience, where we submitted the paper in a first instance. Here, we improved this part by adding\nseveral statements concerning the relationship between the sediment drifts location and bottom current\nregime, in the two different configurations, i.e. the “Paleo configuration” (former “fossil configuration”) and\nthe present-day one. Moreover, we agree with the Reviewer comment “As the canyons….due to Coriolis”, and,\nbased on that, that’s exactly what happened when the “Paleo configuration” was shaped: turbidity currents\nwere predominant especially when the ice sheet was able to reach the shelf break. These downslope fluxes\nwere then deviated to the west by both the Coriolis effect and, possibly, the westward-flowing Slope Current,\nleading to flow stripping and sedimentation on the western levee more than on the eastern one (please see the\nasymmetry of the levees in the three channel systems shown in Fig. 2 c). The drifts are thus produced by less\npowerful upslope flows deviated to the east and depositing the sediment suspended within the canyon.\n\nHowever, if the flows are confined to the channel, this would not occur. In this case, the hydrodynamics of the\nflow would be more complicated, and could result in net deposition on the terrace to the east, without the\nneed for Coriolis-driven transport to the left, up-channel. The authors do refer to the analogue data from the\nRoss Sea (Morisson et al., 2020), which is intriguing, but they don’t show any morphological evidence of\nbedform migration, for example, that would justify that these drifts are forming directly from upslope-flowing\ncurrents.\nThese Reviewer comments highlight that even here we were not clear enough. We do agree with their\ncomment, indeed: we suggest that in the present day configuration, where the channel is small compared to\nthe “Paleo configuration” and entrenched to the west, the downslope flows were (are?) much more weaker\nthan during the shaping of the “Paleo configuration”, and thus they were not able to exceed the western, huge\n“paleo” levee, being then only capable to build the small levee to the east (please see Fig. 2 a and Fig.\nSupplementary 1). The features we interpret as sediment drifts lie further to the east of this small levee, and\nthanks to the data reprocessing, the migration of the sediment drifts crest toward east is now more\nrecognizable. We added some statements in order to clarify all these aspects. Concerning the reference of\nMorisson et al., 2020, we realized that it does not fit as an analogue because the southward flows in the\nRoss Sea derive from the gravity currents due to dense water formation on the shelf, which does not occur in our\nstudy area. In the Ross Sea, the southward flow on the eastern side of the trough is a response to the\nnorthward flow associated with dense water overflow on the western side. Therefore it is a different system.\n\nIn addition to this, the argument would benefit from more observational data of up-canyon flow from ADCP\ndata (for example; data from Hirano et al., 2011 are for surface flow). I note that the ‘Yamazaky et al., 2023’\npaper that is cited for ADCP data does not appear in the reference list so this data is not available for context.\nThe reference to Hirado et al., for intermediate water depth velocities are actually for surface flow, and an\nassumption that near-barotropic flow would result in broadly similar velocities at greater depth, but they don’t\nhave any actual velocity measurement for the deepwater water masses.\nWe agree with the reviewer that the text was not clear enough in terms of available observations and the\nfeatures of the flow over the slope off the Sabrina Coast. Here a large scale (100-200 km) quasi-stational eddy\ncreates a persistent southward flow in the canyon region. Hirano et al. 2021 in Figure 3 combine surface\nvelocity derived by satellites with sub-subsurface geostrophic velocity (derived by hydrographic data) to obtain\nthe full-depth profile of absolute velocity (not only the surfacecomponent). They show how southward\nvelocities (up to >0.1 m/s) are observed from the surface to the seafloor (barotropic). This is confirmed by\nYamazaki et al. (under review-in Progress in Oceanography) using ADCP data near the seafloor. We have\nmodified the text to better highlight all these points.\n\nDriver for southward flowing bottom currents: the main analogue that the authors use to justify their\ninterpretation for strong southward flowing currents is from Morisson et al., 2020, where the up-canyon\ncurrents form specifically in response to cascading dense shelf water. But the authors state that “In contrast\nwith other Antarctic regions with polynyas, there is no evidence over the last three decades of Dense Shelf\nWater (DSW) production in the Dalton Polynya (lines 59-61). This appears to be a problem for their process\ninterpretation, as the currents associated with the eddies documented by Hirado et al. do not appear to be\nstrong enough to entrain sediment and form large contourite drifts. However, numerical modelling results in\nthe Morisson et al., paper do indicate strong DSW production in the study area. This needs to be addressed in\nany future revisions.\nAs mentioned above, the comment of the Reviewer helped us to realize that Morrison et al. (2020) is not a\ngreat analogue because in the Ross Sea southern flowing currents are associated to downslope, northward\nflowing currents related to dense water formation on the shelf. However, Morrison et al (2020) state the\nfollowing “The DSW is deflected to the western side of the canyons as it descends, due to the Coriolis force,\nwhile the CDW ascends in the center or eastern side of the canyons, depending on the narrowness of the\nparticular canyon”. This statement is important for our discussion. Offshore Totten Glacier, no dense water is\ncurrently produced, so that downslope flows are expected to be strongly reduced or absent. Similarly to\nLützow-Holm Bay, where southward-flowing warm waters are conveyed by the cyclonic Weddell Gyre toward\nthe Shirase Glacier Tongue leading to high basal melt rates (Hirano et al., 2020). Offshore Totten Glacier, warm\nCDW is transported toward the south by the cyclonic eddy system identified by Hirano et al. (2021). Yamazaki\net al. (under review- in Progress in Oceanography) show how southward velocities >0.1 m/s are observed near\nthe seafloor using ADCP data. This flow velocity is similar to that found on the Drift 7 in Antarctic Peninsula\n(0.06 m/s; Camerlenghi et al., 1997), which is longer and (150 km long, 70 km wide and up to 700 m high)\nhigher than the sediment drifts in our study areas.\n\nAge of drifts: the authors state that “The seismostratigraphy reveals that sediment drifts are coeval or even\nyounger than the levee deposits on the western flanks” (line 97-98), referring to previously published\ninterpreted seismic profiles. However, the terraces/canyon drifts are not easily resolved on the multichannel\nseismic profiles.\nDrifts are not recognizable on the multichannel seismic profiles, due to their narrow width and the low\nresolution seismic data, which makes it difficult to discern these features on that dataset. Instead, CHIRP and\nTOPAS profiles collected offshore Totten and Ninnis glaciers respectively, allow identification and\ncharacterisation of these previously undiscovered depositional bodies. We decided to insert the already\npublished seismic profile RAE 5108 and related interpretation to help the Reader understand the difference\nbetween the “paleo configuration”, characterized by the giant channel-levee systems recognizable on that\nmultichannel seismic line, and the “present day configuration”, where channels and related levees are strongly\nreduced in size compared to the former.\n\nThe CHIRP profiles in Figs 2 and 3 are of higher resolution, but relatively low quality, and the drifts are\nseparated from the levees by canyon walls dominated by erosion. It is very difficult to correlate the\nstratigraphy between the drifts and levees, or to tell which is older, and this statement is unjustified based on\nthe evidence presented. Thus the “fossil configuration” and present-day configuration model in Fig. 5 seems to\nbe speculative.\nIn our opinion, the reprocessing of the data allows us to better image the seismic reflector configuration and\nthe relationship between the levee deposits and the sediment drift. Moreover, we have improved the\ndescription and discussion about the two main configurations in the results and in the discussion sections, and\nwe think the sketch shown in Fig.5 is more readable now.\n\nFurthermore, the age model presented is difficult to justify. The authors refer to a piston core a long distance\nto the west of the study area, but there are other cores much closer to the study location – for example, the\nPC-05 core published in O’Brien et al., 2020, which is dated. Other cores in or near the canyons should also be\nconsidered – sediment accumulation rates in the canyon and on a drift system are likely to be substantially\ndifferent to those on the open slope. The authors should take care to justify the age model, and acknowledge\nuncertainties.\nWe agree with the Reviewer comment: we were not clear enough in the mentioning of that sediment core,\nwhich is exactly that the Reviewer suggests to consider, i.e., piston core PC05. We added this information in the\ntext. Moreover, a new piston core collected in the study area in the frame of the same survey, i.e., PC08, has\nbeen recently published (Sadatski et al., 2023). Similarly to PC05, this core has been collected on the crest of a\nsediment ridge located between the Maadjit and the Boongorang canyons. The sedimentation rates at the\nlocation of PC08 core are 3.3 to 5.7 cm/kyr, similar to those found at the PC05 core. We agree with the\nReviewer comment that sediment rates in the canyon and on a drift system are likely to be different. What\nmakes the difference is that the thalweg of a canyon is generally affected by episodic coarse grained bedload\n(highly erosive in the upper course of the canyon and resulting in high accumulation rates only locally and at\nthe mouth of the canyon), whereas on drifts, levees, and other elevated ridges adjacent to canyons the fine\ngrained suspended load can be deposited in a relatively continuous way with moderate sedimentation rates.\nSedimentation rate on the crest of sediment drift 7 (Antarctic Peninsula) is 5 cm/kyr, whereas on the same\ndrift but closer to the Alexander Channel it is of 9-12 cm/kyr (Venuti et al., 2011), thus being (both of them)\ncomparable with those measured in the study area. Also, sedimentation rates estimated for the plastered drift\non the continental shelf of the Amundsen Sea Embayment formed by the southward inflow of CDW are ca. 14\ncm/kyr (Uenzelmann-Neben et al., 2022). In all these cases from different Antarctic sediment drifts and ridges,\nthe sedimentation rates are of few cm/kyr (from 5 to 14 to be precise) and we don’t have any element for\ninferring a significantly different sedimentation rate. At the moment, in our knowledge, no analogues of our\nstudy cases do exist, thus highlighting the novelty of our contribution, and, at the same time, the need of\nfurther investigations, which we are confident to perform in the framework of the new, recently approved\nproject PNRA DIONE (Dynamic behavior of the East Antarctic Ice Sheet in the Sabrina Coast; PI: F. Donda)\n\nOther minor issues include:\n- References and values quoted not always appropriate or correct. For example, they state that “Totten Glacier\nalone drains a 49 volume of ice above flotation equivalent to more than 3.5 m of sea-level rise (Rignot et al.,\n2019), similar to the 50 entire WAIS (Greenbaum et al., 2015).” However, the value of 3.5 m for Totten is from\nGreenbaum et al (Rignot et al., quote a value of 3.85 m). The Greenbaum paper focuses specifically on the\nTotten Glacier, so is inappropriate as a citation for the WAIS.\nCorrected.\n\n- Figure 1, the bathymetry is not very clear – consider using colour spectrum, as well as shaded relief. Consider\na cross section showing shelf and subglacial topography, grounding lines, etc.\nWe redrawn Figure 1 by removing the multibeam maps, which are now shown in Figure 2 and 3 and in the\nSupplementary Figure S2. Multibeam maps are now provided as a colour spectrum.\n\n- Figure 2. Line location symbology are different and hard to see.\nWe modified them according to the Reviewer comment.\n\n- There are a number of spelling and grammatical issues that should be addressed.\nWe apologize for these annoying errors, which we are confident to have solved."}]

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