Synchronous mid-Holocene marine and terrestrial deglaciation in the Ross Sea, Antarctica

Domain: earth_science
Published: 2025-12-19
Editorial outcome: minor_revision

Abstract
The Ross Ice Shelf buttresses ice draining from both East and West Antarctica and its collapse could accelerate the loss of inland ice sheets, rapidly raising sea level. Documenting the location, timing and rate of past glacial retreat can help reveal processes driving rapid mass loss, informing projections of ice sheet responses to a warming climate. Here, we present a record of mid-Holocene ice retreat from the southwestern Ross Sea using facies succession and paired ramped pyrolysis oxidation 14C/210Pb chronology. This record shows rapid ice shelf retreat from 6.9-5.4 cal kyr BP, coeval with thinning of adjacent outlet glaciers. Our findings reconcile earlier discrepancies in terrestrial and marine reconstructions, and indicate that synchronous grounding line retreat from west of Ross Island to the Siple Coast at ~7-6.2 cal kyr BP was likely driven by warm-water incursions, a process active in parts of Antarctica today.

Introduction
The Ross Ice Shelf buttresses ice draining from the East Antarctic Ice Sheet through the Transantarctic Mountains and from the West Antarctic Ice Sheet via ice streams; these two large catchments represent approximately 12 m of potential global sea level 1. Understanding historical ice shelf stability is crucial for future sea level projections of ice sheet change, which are influenced by local bathymetry and ice-ocean interactions 2. Although currently stable 3, model projections suggest that the Ross Ice Shelf will thin in the coming decades, causing grounding line retreat into the deep basins of West Antarctica and contributing meltwater to the global ocean 4–8. However, the modern observational period is too short to confidently assess long-term responses to climate warming over centuries to millennia.

The nature of ice sheet palaeo-drainage and retreat in the Ross Embayment following the termination of the last glacial maximum (LGM) has significant implications for broader understanding of the dynamics of Antarctica’s ice sheet margins and their sensitivity to climate forcing and local bed topography. Geological reconstructions provide insights into mechanisms of ice retreat and serve as benchmarks for ice sheet models (e.g. 5, 9, 10). However, the timing and pattern of grounding line retreat remain debated due to the inconsistencies between marine and terrestrial records 10–15. In the western Ross Sea, retreat was complex 9, 13, 14, with bathymetric highs stabilising the grounding line 16 and reconstructions of megascale glacial lineations recording shifts in ice flow direction 14. Recent studies indicate that the grounding line retreated inland of its modern position along the Siple Coast during the mid-Holocene and subsequently readvanced 17–20, though this pattern has not been empirically constrained elsewhere in the Ross Sea. Model simulations reproducing this grounding line reversal depend on robust geological reconstructions to refine ocean heat forcings and solid Earth feedback parameterisations 17, 18, 21.

The largest discrepancies between marine and terrestrial reconstructions of Holocene ice sheet retreat based on geological deposits and between geological reconstructions and model simulations, occur in the southwestern Ross Sea. Reconstructions from marine sediment cores dated with foraminiferal radiocarbon (14C) show that the grounding line retreated east of Ross Island by at least 8.6 cal kyr BP 10, reaching McMurdo Sound around ~7.5 ka 22. Rapid ice surface lowering of Mackay and Mawson glaciers in the southwestern Ross Sea occurred between ~7.5 and 5 kyr BP, while raised beaches in McMurdo Sound (6.6 kyr BP) provide key indicators of the final timing of coastal ice retreat 23, 24. However, glacial lowering and modelled grounding line retreat lag empirical marine reconstructions in this region by ~2–5 ka 5, 16, 24, 25. These discrepancies may arise from age model uncertainty in marine records, complex bed topography, local subglacial conditions 5, 16, or negative cold-cavity ice shelf feedbacks 21, 26. Resolving these intra-proxy and proxy-model differences requires further observational constraints on post-LGM grounding in this region.

Accurate 14C dating of marine sediments remains a major challenge for constraining the timing of ice retreat. Paired benthic and planktic foraminiferal assemblages from laminated strata are presumed to most accurately record the timing of sediment deposition, but are sparse or poorly preserved in Antarctica 10, 27. Consequently, bulk sediment acid-insoluble organic matter has commonly been dated to determine the age of sediment deposition. However, glacial processes are known to rework older carbon, either biasing bulk 14C towards older ages (e.g. 28), or towards younger ages if sedimentary sequences are condensed and heavily bioturbated, as is the case in many Ross Sea cores 29. Ramped pyrolysis oxidation (RPO) 14C dating, which thermochemically separates different carbon pools in the sediment, addresses many of these limitations 19, 20, 27, 30, 31. Through ramped pyrolysis, syn-depositional sedimentary marine organic matter from the water column is dated in isolation from pre-aged, transported material 32. This approach is further strengthened by pairing with pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) analysis, which identifies source organic compounds to enable preferential dating of splits containing primary photosynthetic products 31. An additional challenge of dating marine sediments in the Antarctic is the large (~1100 years 23, 33) and variable 34 reservoir age, which must be accounted for when calibrating 14C ages to calendar years. However, application of additional radiometric techniques (U-Th disequilibrium; 210Pb) can provide independent age constraints on both depositional age and the magnitude of the radiocarbon reservoir correction.

Here, we present RPO 14C and 210Pb-constrained retreat histories that reveal synchronous mid-Holocene grounding line retreat and glacial lowering in the Ross Embayment, demonstrating the importance of bed topography and ocean forcing during retreat. Two sites, RS15-GC78 and GC80 are ideally located to assess the relationship between ice sheet grounding line retreat and glacier surface lowering because they are situated within the respective paleo-drainage paths of Mawson and Mackay glaciers, two glaciers with well-constrained ice lowering histories 24, 35. Two additional sites northeast of Ross Island, GC71 and GC72, provide a link to central Ross Embayment retreat 10 (Fig. 1). This network of sites allows us to document the timing of marine-based calving line retreat across this sector of the southwestern Ross Sea and compare it to adjacent terrestrial glacier thinning records with the support of regional ice-sheet modelling. Widespread mid-Holocene grounding line retreat is supported by the pattern of ice retreat in the Ross Sea Embayment; retreat in the region west of Ross Island was contemporaneous with grounding line retreat towards the Siple Coast.

Results
Ross Sea sediment cores reveal a stratigraphic succession from subglacial to open marine environments 10, 22, 36–38 (Figs. 1 and 2; Supplementary Figs. 1–4; Supplementary Table 1). Basal muddy or sandy diamicton facies (Dm and Ds, respectively) are overlain by bioturbated silty clay facies with common clasts (Mc) indicative of a proximal ice shelf grounding line environment NE of Ross Island at GC71, transitioning into a diatom ooze facies (Do) up to ~3.5 m thick, which represents open marine conditions. The transition from diamicton to Do is abrupt in GC78 and GC80, offshore of Mawson Glacier and Granite Harbour, respectively: both cores lack a sub-ice shelf facies. GC72 is located south of GC71 and displays a slightly different stratigraphy; no diamicton was recovered and a basal silty clay facies with intermittent clasts (Mc and Mo) transitions into Do. Dating the lowermost Do deposition, immediately above the transition between underlying diamicton or silty clay, reveals the onset of open marine conditions following calving line retreat at the core site.

We establish robust chronologies for cores GC71, 72, 78 and 80 by combining 210Pb data with RPO-derived 14C ages calibrated using the Marine20 curve and a regional marine reservoir correction (ΔR) of 609 ± 137 14C years 33, 39. Modern, or near modern core tops are confirmed by 210Pb activity profiles for each core (Supplementary Text 1). RPO 14C dates generally increase in age with pyrolysis temperature 32, but in our cores, the second temperature fraction ('split 2') systematically yields younger ages than the first ('split 1'), despite the higher temperature. This reversal, also reported elsewhere in the Ross Sea 30, likely reflects contamination of split 1 with volatile, older material that biases 14C ages older (Supplementary Text 2; Source Data 1 and 2). Analysis of the organic compounds supports this interpretation: pyrolysis gas chromatography mass spectrometry (Py-GC-MS) analysis on a subset of samples shows that split 2 typically displays a more diverse array of organic compounds (Supplementary Fig. 6), indicating that the lower-temperature fraction incorporates low-molecular degradation products. Based on these results, we selected split 2 ages as the most accurate maximum age constraints available for cores GC71, 72, 78 and 80. For GC71 and 72, the uppermost 14C dates calibrated with the regional ΔR alone are significantly older than their 210Pb counterparts, suggesting a greater influence from residual detrital carbon contamination. To correct for this, we apply an additional local contamination offset (LCO) derived from the difference between the regional Ross Sea ΔR and a reverse-calibrated pre-bomb 210Pb-14C pair in GC72 at 10 cm depth 40, 41. The resulting LCO of 870 ± 120 years was also applied to GC71 for which no equivalent pair exists (Supplementary Text 1 and 3). For GC78 and 80, 210Pb chronology indicates that the uppermost 14C dates are post-bomb and therefore cannot be reverse-calibrated to the Marine20 curve (Source data 1). We therefore use the regional ΔR for GC78 and 80 and exclude the uppermost 14C dates in favour of the more precise 210Pb chronology for the surface interval. Full methodological details and calibration rationale are provided in the Supplementary Information (Supplementary Text 1–3).

Our results show that open marine conditions developed in the southwestern Ross Sea following ice retreat between 6.9–5.4 (7.3–5.1, 95% confidence) cal kyr BP (Figs. 2 and 3; Supplementary Figs. 1–4, Source Data 2). At GC71, NE of Ross Island, a grounding-line-proximal to sub-ice-shelf setting developed as grounded ice retreated southward over the core site and open marine conditions were established by 6.4 (7.2–6.2, 95% confidence) cal kyr BP. Further south, at GC72, retreat was likely restricted by pinning around Ross Island, allowing an ice shelf setting to develop and persist before the establishment of open marine conditions by 5.4 (6.0–5.1, 95% confidence) cal kyr BP. Along the western margin of the Ross Sea, cores GC78 and GC80 record a sharp transition from diamicton to overlying Do with no intermediate sub-ice shelf facies, indicating that open marine conditions were established quickly following local grounding line retreat. Ice had retreated beyond GC78 by 6.9 (7.1–6.7, 95% confidence) cal kyr BP and beyond GC80 by 6.5 (7.3–6.3, 95% confidence) cal kyr BP, into the over deepened troughs of Mawson Glacier and Mackay Glacier, respectively.

Methods
Sample collection
Gravity cores RS15-GC71, 72, 78 and 80 were collected from the southwestern Ross Sea by the R/V Araon during the ANA05B Cruise in 2014/2015. The cores were split, imaged and described at the Korea Polar Research Institute (KOPRI) in August 2015.

Geochemistry
Biogenic silica (wt% BSi) was measured at 5 cm intervals on all cores at the University of Otago using an alkaline extraction spectrophotometric method modified from refs. 65, 66 using a Shimadzu UV-1800 spectrophotometer. Samples were frozen and freeze-dried before being homogenised using an agate mortar and pestle. Replicates were measured after every 10th sample and each analytical run contained one blank and one internal standard derived from the sample set. The average standard deviation of the nine internal replicates is 0.55%.

Grain size
Grain size data were analysed at 5 cm intervals at KOPRI on samples treated with H2O2 and HCl to remove organic matter and CaCO3, respectively. The coarse fraction (>63 μm) was analysed with sieves to determine the distribution of gravel to very fine sand sizes. The fine fractions were analysed for grain size distribution using a Micrometric Sedigraph III 5120. Grain size statistics follow the methods of ref. 67 and the lithological classification scheme follows ref. 10.

Core scanning
Unsplit cores were logged after collection onboard the R/V Araon using the Alfred Wegener Institute Geotek Ltd Multi-Sensor core logger. This device measures wet bulk density by gamma ray attenuation, compressional (p-) wave velocity and magnetic susceptibility (e.g. 68). Values of Cs-137 gamma ray attenuation were empirically calibrated to wet bulk density using variable Al-water mixtures to define a density gradient, with pure water having a density of 1.0 g cm−3 and Al 2.7 g cm−3. P-wave velocity was calculated from p-wave travel time through a water standard of known temperature, which allowed for calculation of the p-wave travel time offset (PTO) representing the transit of the acoustic wave through core liner and electronic delay. The PTO is then subtracted from subsequent measurements of p-wave travel time through sediment with core liner to calculate the transit time through sediment of known thickness. Magnetic susceptibility was measured with a loop sensor using a Bartington MS-2 meter set in 'SI' measurement mode.

During the processing of the data, it became clear that the core thickness sensor was experiencing drift over time. Because both density and velocity data are dependent on accurately knowing the core (and thus sediment thickness once liner thickness is subtracted), we institute a correction on the basis that the drift was at a constant rate and as measurements are made systematically down core applying a linear thickness correction over the length of the core was appropriate. We ran the same water standard before and after each core was logged and the deviation from known thickness values for this standard were used to determine the magnitude of correction.

Age model
Lead-210 dating. 210Pb dating of sediment was conducted in the Environment Radioactivity Measurement Centre at ANSTO, Sydney. The alpha-particle spectrometry technique was used and is preferable over gamma-ray spectrometry as it requires small samples and gives lower detection limits. The samples were processed to prepare Po and Ra alpha sources. The activities of 210Po, the proxy for total 210Pb and 226Ra, the proxy for supported 210Pb, from these sources were determined by alpha-particle spectrometry to calculate unsupported 210Pb activities. Approximately 2 g of homogenised sediment were analysed for each sample from 4 to 8 depths in the upper 20–40 cm of sediment for each core (see Supplementary Text 1, Supplementary Fig. 5 and Source Data 1). The chemical separation of Po is achieved through acid digestion to extract metals in the sediment into solution, complexation, pH adjustment of the solution and auto-deposition onto a silver disc. This disc is analysed by alpha-particle spectrometry. Following the removal of Po from solution, Ra is concentrated by co-precipitation, collected by colloidal precipitation of barium sulphate on a membrane filter and analysed by alpha-particle spectrometry. Unsupported 210Pb activities, calculated from the difference between 210Po and 226Ra activities, in the sediment were used to determine sediment ages (see Source Data 1 for 210Pb data) using the constant flux constant sedimentation 210Pb dating model 63, 64.

Ramped pyrolysis oxidation (RPO) radiocarbon dating
Sediment samples for RPO radiocarbon dating were taken from the Do section of each of the four cores (16 samples in total), selecting for 1 cm horizons containing well-defined laminae to reduce the effects of bioturbation (Source Data 2). Samples were dried, homogenised and acid-treated with dilute HCl (1N) to remove carbonate minerals for at least 1 h until effervescence ceased. Acid was removed by centrifuging and rinsing with deionised water until the supernatant reached pH of 7 32.

These samples were prepared using RPO at the College of Marine Science, University of South Florida (USF). For each sample analysed at USF, we followed established protocol from ref. 32. The sample size was determined by using the %TOC required to yield ~100 μmol CO2. Using a standard pyrolysis temperature ramp of 5 °C min−1, complex mixtures of organic carbon in sediment samples are separated by leveraging the thermochemical stability of the acid insoluble organic matter. These pyrolysates are then combusted into CO2 27, 32, 45. Sample CO2 gas aliquots were collected sequentially at intervals of approximately 10, 12, 25, 25, 25 μmol and sealed in pre-combusted and evacuated borosilicate ampoules with 150 mg of copper oxide and 1 cm of silver wire. The lower temperature aliquots are small to minimise the amount of detrital CO2 incorporated. Ampoules with the sample gas, copper oxide and silver wire were heated in a furnace at 525 °C for 2.5 h to remove sulphur, if present, prior to graphitisation.

Radiocarbon measurements of the CO2 gas samples were performed at the Centre for Accelerator Science, ANSTO, Sydney 69. The first two ampoules (containing 10 and 12 µmol of CO2) for each sample and a series of blanks comprising three each of radiocarbon-dead material (graphite) and modern standard material (SRM 4990C oxalic acid, OxI) were analysed at the ANSTO for radiocarbon dating using the ANTARES accelerator 70. This accelerator was used for its ability to process low carbon mass samples with high precision. The CO2 from these samples, blanks and standards was converted to graphite by reduction with excess hydrogen over iron catalyst to produce targets for AMS measurements 71. The graphite was pressed into cathodes and measured using the ANTARES accelerator 72, followed by δ13C measurements on the graphite using IRMS to account for natural isotopic fractionation. Initial measurements were normalised to a δ13C of −25‰ relative to the PDB standard.

RPO-AMS measurements were also performed on three additional samples at the Rafter Radiocarbon Lab (RRL) AMS facility at GNS Science, New Zealand following procedures established by ref. 31 in collaboration with USF. One distinction, however, is that RPO samples are split according to inflection points in the RPO thermographs and quantified after separation at RRL. The CO2 is then recombusted (500 °C, 4 h with silver wire and copper oxide), graphitised and measured for radiocarbon content 73, 74.

Ramped pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS)
Ramped Py-GC-MS was used to determine the composition and quantify the sources of organic carbon in the RPO splits of the three samples analysed at RRL (e.g. 31 Supplementary Text 2; and Supplementary Fig. 6). Samples were from a single horizon from cores GC72, GC80 and GC71 (40, 76 and 301 cm, respectively) and analysed at the GNS/VUW Organic Geochemistry Laboratory at GNS Science, New Zealand using a micro furnace-based Frontier Laboratories double-shot pyrolyser coupled to an Agilent GC-MS system, as reported in ref. 31. In brief, for full, rapid-ramp pyrolysis, samples were ramped from 100 to 650 °C at 50 °C min−1. Then, incremental, partitioned ramped pyrolysis was used to replicate the ramped pyrolysis split temperature ranges for those samples. The resulting pyrolysis products were analysed by GC-MS analysis and relative proportions of compound classes determined (pyrroles, furans, alkanes and alkenes, toluene and other aromatics, thiophenes, polycyclic aromatic hydrocarbons and alkylbenzenes).

RPO data processing
All 14C data collected by RPO must be blank-corrected for contamination introduced into the sample during processing 75. The correction process is consistent for samples prepared at USF and RRL, but the magnitude of the correction differs due to small differences in the reagents and systems used in the two laboratories. At the time of analysis of these samples, 109 geological graphite (devoid of radiocarbon) and Oxalic Acid I (OxI), nearly modern levels of radiocarbon 76 had been processed to assess the USF system’s blank contamination. We calculate the mass of blank contamination which contains no 14C (dead blank mass; 1.61 ± 0.9 μg) and the mass of the blank contamination which contains modern levels of 14C (modern blank mass; 2.46 ± 1.6 μg) using a subset of this data set (19 data points) most appropriate for samples prepared in 2015 (see discussion in ref. 30). An additional (but smaller) blank correction was applied based on the contamination introduced during graphitisation and AMS measurement at ANSTO and the magnitude of these corrections was derived from materials introduced directly into the graphitisation system (14C-free Kapuni CO2 gas and OxI prepared in a large CO2 batch). For RPO performed at RRL, the contribution of modern carbon contamination is monitored with a 14C-free kauri wood and the contribution of dead carbon blank is measured with the OxI modern standard. Blanks encompass the complete process, including contamination from pre-treatment, RPO preparation, graphitisation and AMS measurement. Unlike USF, RRL observes a time-dependency in the blank contamination as part of the RPO process, which is also accounted for in the blank correction 31.

Age model development
The age models for cores GC71, 72, 78 and 80 are based on a combination of RPO-14C radiocarbon and, where available, core-top 210Pb dates. Calibration of 14C dates to calendar years before present (cal yr BP) was performed using the BChron R package 77 and the Marine20 calibration curve, which includes a time-varying global reservoir age of ~603 years 42. The current Ross Sea-wide ΔR of 609 ± 137 14C years is used for all dates 33, 39. We further apply a LCO for GC71 and GC72. Further detail on the radiocarbon reservoir age methods is in Supplementary Text 3.

Glacier thinning from cosmogenic exposure dating
The timing of glacier thinning at Mackay and Mawson glaciers is determined from cosmogenic exposure dating of glacial cobbles at sites adjacent to the glacier 24, 35. The exposure ages were calculated for all sites using iceTEA 78, with the global 10Be production rate calibration dataset 49 and nuclide-specific scaling model 'LSDn' 48. While using a Southern Hemisphere production calibration site would make the exposure ages ~1 kyr older 24, we consider the global dataset and nuclide-specific scaling to produce the most appropriate exposure age estimates based on available evidence in Antarctica. The rate, onset and cessation of the rapid thinning recorded at Mackay Glacier were taken from published estimates 51, which also used the global production calibration dataset and 'LSDn' scaling.

Modelling
We use regional-scale ice sheet modelling to assess the coupling relationship between the recorded grounding-line retreat and inland glacier thinning 16. Modelling was carried out for the southwestern Ross Sea using the finite element model Úa, which solves for ice flow in the grounded and floating portions of the ice sheet simultaneously. In ref. 16, experiments were designed to capture grounding-line migration and corresponding upstream ice thickness change for 9–0 kyr ago, using two different climate forcings and five different combinations of the basal slipperiness and ice rate factor. To determine the likely relative timings between retreat and thinning in the southwestern Ross Sea, we use the five simulations from the experiment that had enhanced early-to-mid Holocene ocean thermal forcing (E2 in ref. 16), as these simulations best fitted the timing and rate of recorded thinning at Mackay and Mawson Glaciers.

Data availability
All data generated or analysed during this study are included in this published article (and its Supplementary information files). Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present here new marine radiocarbon data based on ramped pyrolysis (RP) from the western Ross Sea that\nbear on the timing of ice recession following the last glacial maximum. They integrate these data with others from the region,\nas well as cosmogenic nuclide ages of ice thinning of nearby outlet glaciers, to present a synthesis of the timing and pattern\nof Holocene ice retreat.\nThe authors are to be commended in their attempt to bring together a wide range of different data types in their synthesis.\nHowever, this paper suffers from issues that should be addressed prior to publication.\nIn my opinion, the main issues can be summarized as:\n1) The new delta R is not justifiable both in its calculation and application. It also leads to inconsistencies in the dataset that\nwould not exist if they used a standard delta R for this region (which is well-known).\n2) The paper needs more specific information on how calculations were done and on how ages presented in the text were\ndetermined. There are also numerous inconsistencies and/or typos in the text and figures that need resolving prior to\npublication.\n1. Delta R\nMarine dates in this study are based on ramped pyrolysis (RP), which aims to separate out different carbon fractions that\nhave different origins, with the thought being that the more labile fractions reflect more recent marine carbon deposition and\nthe more refractory components issuing from reworked ancient material. Ideally, one would be able to isolate only the\nrecent fraction in order to obtain accurate ages for Ross Sea sediments. Dating has come a long way since the early studies\nin this region using simple TOC – which produced grossly old (and erroneous) ages for deglaciation. However, RP is not\nperfect and 100%, confident separation of the fractions cannot at present be achieved. This has been noted in prior studies\n(e.g., Venturelli et al., 2023) and appears to be the case in this present study. In the Supplemental Information (SI) the authors do a\ngood job of explaining which fractions are thought to be autochthonous and which are reworked. There is also a substantial\nnumber of compounds that could be from both sources. Table S2 is especially helpful. However, from a combination of\nTable S2 and Fig. S6, one can see that even in the preferred Split 2, only roughly half of the compounds can be confidently\nattributed to autochthonous sources. Another ~40% could be either and about 7-10% are from refractory sources (ancient).\nThus, the resulting radiocarbon dates from this split have a likely component of old material of at least 7-10% and maybe as\nmuch as 50%. This must affect the age, which means that these RP ages should be considered maximum ages only. Lines\n225-226 of the SI refer to the resulting radiocarbon ages correctly as “maximum ages,” a point that seems lost when\ncalculating and applying the delta R in the main paper.\nIf these RP ages are only maximum limiting ages, then they cannot be used in the manner proposed here to derive a\nmarine delta R, because they are not the true radiocarbon age of the core top. At best, it is a maximum possible delta R for\nthat place and time. As the proposed delta R (1480 yrs) calculated by this dubious manner is more than twice that of other\nmeasured values in the area (~610 yr; recalculated in Gao et al., 2022 and Hall et al., 2023) – in an area that already has\none of the largest delta Rs in the world – it seems likely that the delta R proposed here is too large and should not be used.\nThe authors suggest that the commonly used delta R value of ~610 yr for McMurdo Sound (site of the original Hall et al.\ndifferences. However, similar delta R values also exist for Terra Nova Bay to the north (Hall et al., 2010), near the Siple\nCoast grounding line (Venturelli et al., 2023) nearly 1000 km away, and in the entire circum-Antarctic region based on\nmarine shells collected prior to 1940 (when recalculated; Berkman and Forman, 1996), suggesting that this ~610 yr delta R\nvalue has robust regional significance. A quick look at the Marine Chrono database reveals that the average delta R for all\nRoss Sea samples is ~620 yrs. Thus, this delta R of 1480 yr calculated here is more than 3 sd from the mean and needs to be\nreevaluated critically. The authors do at one point mention that their delta R might be only applicable to their local site,\nbut they then nevertheless apply it widely in the region, including to their two core sites on the western coast, one of which\n(McKay) is at the north end of McMurdo Sound. Applying their large delta R to these two core sites (GC78 and 80) results in\nthe core top ages dating well into the future. All of this suggests that the proposed delta R is simply too large, most likely\nbecause no matter how good the RP and how much of an improvement over prior methods (which it is), there is some\ncomponent of old material still left, which is affecting the age, making the delta R calculation an overestimate.\nThe authors suggest that the exact ages (which differ by nearly 1000 yrs) don’t matter, just the pattern. I disagree. The\npattern has been emerging for some time, with early models of a “swinging gate” giving way over the last decade to “saloon\ndoors” with the center of the embayment opening up first. The approximate timing of retreat – mid-Holocene event - also has\nbeen known for some time. It is at the point where we need precise chronology and reliable onshore-offshore correlations –\nwhich is why exact ages do matter.\n2. Need for specific information and correction of inconsistencies.\nThis paper needs more detailed methodological information. Readers are left at critical points wondering exactly how certain\nages were calculated, whether the number referred to is actually a delta R (as opposed to a reservoir effect), what\nproduction rates and scaling were used for cosmogenic ages, were internal or external errors used in comparisons, etc… There are\nalso several instances of figures not matching captions and text not matching tables. Line examples for all of these are\nbelow.\nLine comments:\n35-36 – I am not sure that the results as interpreted here reconcile long-standing differences. In fact, by introducing such a\nlarge reservoir effect, the marine ages are now younger, in my opinion, than the cosmogenic ages at McKay Glacier or the\ncoastal marine deposits of McMurdo Sound.\n69 – Is Ross Island really the southwestern Ross Sea? There is a lot more Ross Sea south of Ross Island.\n98 – this reference is out of date, with more recent papers and the Marine Chrono database suggesting 1100 yrs (e.g., Hall et\nal., 2010).\nFig. 1 – needs lat./long.\nFig. 2 – larger text would make this more readable\n159 – I assume from the extended data table that the authors compared the samples at 10 cm depth and not at the core top.\nThere needs to be specific information about which two dates were compared, if not here then definitely in the SI. This gets\nback to Point 2 above.\n160 – how did you reverse calibrate? Neither of the cited references give you how. Perhaps cite Reimer and Reimer 2017 if\nyou followed their method.\n161-163 As mentioned, this delta R should be treated with a large dose of skepticism. It is not “slightly older” than that\nreported from the corals of 609 yr. It’s 900 yrs larger, which in the world of deltas R is a massive difference.\n165 – how can you be sure of minimal contamination when the fraction that is confidently autochthonous is relatively small\nand there is a non-zero refractory component, which is almost certainly old?\n168 – this argument would be more convincing if similar reservoir effects to McMurdo Sound were not seen elsewhere\n(Point 1 above) = This implies to me that the reservoir age from site GC72 is either too old because of contamination by old\nmaterial or that it is very site and time specific and should not be applied elsewhere.\n171 – Because I am not confident in the applied reservoir correction, I am not confident in the ages in this paragraph for ice\nretreat.\nI think it is also important to stress that your dates are for the onset of open marine conditions, which is not necessarily\nthat of grounding-line retreat. It’s a minimum age for grounding-line retreat. This point gets lost at various stages of the\npaper.\nPerhaps the use of “before” rather than “by” (e.g., line 181, 182) would help make this point. Hall et al. (2010) showed that\ncoral was already growing in southern McMurdo Sound by ~ 6 ka from U-series dating. Thus, McMurdo Sound had opened\nup before that time, which is older than the dates you propose for sites 78 and 80. Moreover, if one applies\nthe McMurdo delta R of Hall et al. (2010) to McMurdo Sound carbonates, which seems more appropriate given their\nproximity to the calibration site, then the sound must have opened before 7.5 ka (Licht et al., 1996 date from sound) to 6.5\nka (oldest shells in beach deposits along McMurdo Sound – Hall et al., 2004). This leaves interpretation of the GC80 core\nsite with two options – 1) the 1480 yr reservoir correction is too large, or 2) the minimum-limiting date for open marine\nconditions does not closely constrain the timing of grounding line retreat (the authors make a case against this from the lack of\nsub ice-shelf facies).\nFig. 3 – this figure is difficult to follow. In panel A, one cannot distinguish between grounded and floating ice. In addition, the\ntime slices given do not match those in the caption. For example, (iii) says <5.2 ka on the figure but 5.8-5.2 ka in the caption.\nWhat is the blue shading along the coast in (iii)?\nPanel B – If the authors actually applied the smaller reservoir correction to their McKay and Mawson cores, I think they\nwould find their data agree better with the cosmogenic ages of ice thinning.\nThe text needs more information on the exposure age data beyond a simple reference to the Jones papers. What production\nrate was used? Scaling? Were external errors used or just internal ones? Were ages recalculated to be consistent between\nthe two datasets? Looking up the original papers, it appears that Borchers and LSDn were used in the Mawson paper. The\nBorchers et al. rate is almost certainly too high, based on rates from both New Zealand and South America. LSDn generally\ngives younger ages than St or Lm. Using Borchers and LSDn then gives a young combination for the exposure ages\nmeaning that if one chose a different production rate or scaling scheme, ages would get older. The point here is that these\nexposure ages are about as young as they can be within existing production rate and scaling frameworks and thus any\nchanges will likely bump them older, increasing rather than decreasing the gap between the proposed marine ages and the\nthinning history from the glaciers.\nThe Mawson data are very messy, and could give a wide range of thinning scenarios. McKay, on the other hand, is a\nbeautiful dataset and shows the ice surface dropped like a rock at 7 ka, reaching the present glacier surface level at about\nthat same time. This is quite a bit older than the 5.3 ka proposed age for the core near its mouth. A more realistic delta R\nwould improve the data fit to the cosmogenic data, as well as to those two orange bands that represent modelled grounding\nline retreat.\nFor Mawson, I would mention that Mt. Murray is well inland of Bruce Pt and thus the ages are not surprisingly younger.\nWould it be more useful to just plot the Bruce Pt samples as those are ones with the closest relationship to the dates in the\noffshore core?\nFig. 3 Caption – please specify throughout whether you mean grounded ice or an ice shelf.\nline 205 in caption – “results in good agreement between simulated and recorded …” I’m not sure I would characterize it as\ngood agreement – the modelled thinning history seems nowhere near as fast as the documented.\nLine 212 – see Point 1 above\nline 216 – see Point 1 above and comments for line 171\nline 220 – what is the basis for this range of retreat dates? Give the basis here to remind the reader.\nline 233-234 – elaborate – the McMurdo Sound shell and beach dates when corrected with the regular delta R align pretty\nwell with the McKay Glacier cosmogenic record, so the authors might want to be specific about what did not align (e.g., the\nCoulman High core and other marine 14C records based on TOC).\n261 – these dates (7.7-6.1 ka) don’t appear to come from your results. Based on Fig. 4C it looks as if they come from a\nrecalculation (?) of the CH-2 core (mentioned in passing in the paper in captions up until this point) of McKay et al. and the\nVenturelli et al. Mercer Lake dates, which have not been mentioned at all in the text to this point. If retreat of the whole\nsouthern embayment (the main difference between 4B and 4C) is going to hinge on these two dates then these sites and\ndata need to be adequately described and referenced.\n261 – do you mean an embayment in the grounded ice sheet?\n267 – unsure where these 7-5 ka ages appear from – based on what? Prior paragraph said 7.7-6.1. Overall, this section\nneeds better explanation, referencing, and consistency of dates used.\n269 – Actually, glacier surface lowering in the southern TAMS begins quite a bit earlier than the dates of the cores – e.g.,\nSpector et al., 2017.\n272 – the average reader will not know where these glaciers are so they should be in a map.\nThe 6 ka grounding line north of McMurdo Sound is not in accord with shell dates from the coast or from the floor of\nMcMurdo Sound when a reasonable reservoir correction is used. And, it is not in agreement with the U/Th dates of coral\nfrom the ice shelf region.\nThere is no way to match the ages on fig 4C with the references that produced them and many have not been discussed at\nall in the text (e.g., Anderson, Stutz, Balco, Goehring etc…). These data need to be given somewhere (even if only in the SI).\nWhat is the triangle dated to >1.7 ka under the WAIS? There is no triangle in the legend.\nNot sure what is meant in the legend by “coastal lowering date”. Two of those appear to be relative sea-level curves, which\ngive the timing of unloading and development of open marine conditions.\nDates of >5.8 and >5.4 for the Mawson and McKay core sites on fig. 4C contrast with how these data are presented in the\ntext as the actual (rather than minimum) ages of grounding-line recession. I agree with the > symbol and suggest making\nthe minimum nature of these ages for grounding-line recession more clear in the text. In addition, in line 223, these dates are\nlisted as 5.9 and 5.3 ka. They should be consistent throughout.\nline 283 – as mentioned above, I think recalibrating any of the dates from McMurdo Sound or Terra Nova Bay based on the\n1480 yr delta R is not justifiable. Were the Venturelli et al dates also recalibrated? Table in SI lists the date for that site as 6.3\nka (as does the original publication), not >6.1 ka as given in fig. 4C.\nline 540 – What thickness of core was required to arrive at those sample sizes? 1 cm? 5 cm? 10 cm? Give a sense as to the\ndegree of time averaging captured by these dates.\nline 602 – see Point 1\nSupplemental Information\nPlease be careful throughout not to use the terms reservoir age and delta R interchangeably. They are very different things.\nPlease reference Extended Data 2 for the RP dates in the captions, so that the reader can find the original radiocarbon ages\nand metadata.\nAccording to Ext Data table 2, the raw core top age for Split 2 of GC78 is 1440 yrs, which is less than the applied delta R of\n1480 yrs (and the implied ocean reservoir of about 2080 yrs if mean ocean value is about 600 yrs). This alone demonstrates\nthat the applied reservoir correction is too large by a lot. So, I do not understand why such a large delta R is applied here,\nwhen the authors’ own data so explicitly contradict it. In contrast, use of the conventional 610 yr delta R gives an age of\nabout 290 +/- 270 yrs at about the same depth as the 37 +/- 15 yr Pb sample, which is spot on the Pb data.\nThe core top radiocarbon age for GC80 is similarly too young to apply such a large delta R.\nText S2\nYou may want to reference Venturelli in this section.\nFig. 6 shows that while Split 2 likely contains the highest proportion of autochthonous carbon (line 165), it does not show\nthat all of the sediment is autochthonous. A non-zero percent (looks like 10-12% in GC80) is likely reworked and another\nsubstantial percent, nearly half, could be either autochthonous or reworked, according to Table S2 and the text. To me, this\nreads that for core GC80, only roughly a third of the compounds can confidently be assigned an autochthonous origin likely\nof the correct age (although I suspect even that cannot account for resuspension and resettling of local material) and even\ntaking everything except the PAHs to be autochthonous still gives ~10% refractory and likely old carbon contamination.\nThus, any radiocarbon ages are maximum ages only. This point is stated in lines 225-226. But this concept vanishes in the\nmain paper and the ramifications of this fact for calculating the reservoir age are not appreciated.\nFig S6 caption – are these data for core-top sediments? What is the depth?\nText S3\nLine 236 – the terms R, delta R, and reservoir are used inconsistently. This is the first place R is used and may confuse a\nreader. It is unclear on line 236 what the R is that is referred to – not the corals (R= 1144) and not the R calculated at site\n72 (which would be over 2000 yrs). I suggest just using delta R throughout the paper.\nLines 233 – see Point 1\nLines 240-241 – The timing is highly sensitive to the reservoir age and a correct reservoir age is necessary to make\nmeaningful onshore-offshore comparisons, which is one of the stated goals of this paper. It does have the potential to\nchange the narrative of retreat.\nTable S3 –\nIt is unclear how the ref 14 coral reservoir effect was calculated in this table. It looks as if 603 yrs was simply subtracted\nfrom the published R value and the same error used. This is incorrect. This 1144 yr age offset was calculated with an early\nversion of the Marine radiocarbon database and cannot be used with Marine20. The original calibration data have\nsubsequently been recalculated by at least two studies and new delta Rs specific to Marine20 have been presented in both\nGao et al., 2022 (609 +/- 137 yr) and Hall et al., 2023 (610 +/- 110 yr). The Marine20 compliant delta R also can be\ncalculated from the Marine Chrono database.\n4.3 ka age on fig. 4C is not in the table\nLine 288-289 – units?\n305-307 – I would agree with “provides a more reliable local marine reservoir correction” than some of the previous\nattempts using dates of bulk TOC. However, that does not mean it is reliable enough to produce a reservoir correction that\nis then applied widely. Closer to the true value does not mean correct.\n312 – The production rate and scaling used need to be presented here.", "reviewer_2": "The work is very interesting and presents significant results, making it certainly worthy of publication following minor to\nmoderate revision.\nThe issue of deglaciation after the Last Glacial Maximum (LGM) in the Ross Sea -marked by the retreat of the grounding\nline of the marine-based Ross Ice Shelf, which extended to the edge of the continental shelf- has been widely debated and\nremains a central topic of interest. This is because past changes in the grounding zone are critical for understanding the\nvulnerability and resilience of ice sheets to climate change and to variations in ocean temperature and current circulation.\nSeveral models have been proposed to reconstruct the retreat phases of the grounding line in the Ross Sea following the\nLGM. Among them are the \"swinging gate\" model by Conway et al. (1999), which suggests a general retreat across all\ntroughs, and the “saloon door” model by Ackert (2008), which hypothesises that different troughs experienced distinct\nretreat phases. More recently, Halberstadt et al. (2016) proposed a marine-based model characterized by nine retreat\nstages of the Antarctic Ice Sheet, constrained by numerous radiocarbon dates (see also Prothro et al., 2020).\nRadiocarbon dates obtained from marine organisms provide a wealth of information for dating the deglaciation phases\nthat accompanied relative sea-level rise and the emergence of ice-free areas. These newly exposed coastal regions became\navailable for recolonization by penguins and elephant seals following the LGM retreat of the glacial system along the\nVictoria Land coast (e.g., Baroni and Hall, 2004; Hall et al., 2004, 2006, 2023; Gao et al., 2022).\nRecent studies suggest that the retreat phase was interrupted by one or more re-advances of the grounding line during the\nHolocene (e.g., Greenwood et al., 2018), and likely also during the Antarctic Cold Reversal (Baroni et al., 2022; Giorgetti et\nal., 2024).\nThese studies highlight the strongly regional behaviour of the glacial system involved in the progressive retreat and\ndeglaciation of coastal areas. However, the primary challenge facing all research on Antarctic deglaciation lies in accurately\nconstraining the timing of these phases. In particular, there are difficulties in correlating ages derived from marine\nsediment cores with radiocarbon dates from marine-derived materials collected in deglaciated coastal areas -such as shells\nfrom Holocene raised beaches, and penguin remains from abandoned colonies and nesting sites.\nAmong the strengths of this work, I would like to highlight the following:\na) The authors’ effort to correlate the timing of events recorded in marine sediments with evidence of post-LGM glacial\nretreat on land—documented through thinning of outlet glaciers along the Victoria Land coast and progressive deglaciation\nof coastal areas—is commendable and significantly contributes to the understanding of regional ice dynamics.\nb) The application of ramped pyrolysis oxidation (RPO) radiocarbon (^14C) dating is particularly interesting. This method,\nwhich thermochemically separates different carbon pools in the sediment, addresses several limitations related to\ncontamination by old carbon in marine sediments.\nAdditionally, the combined use of ^210Pb and RPO ^14C dating provides valuable results for estimating reservoir ages in\nsediment cores.\nAmong the weakness of this work, I indicate the following:\na) Authors state that the “reconstructed pattern of deglacion is supported by regional-scale modelling in the SW Ross Sea,\nthat shows glacial retreat is initially slow as the grounding line is pinned to shallow bathymetry to the north of Ross Island,\nbut then retreat accelerates into deeper water”: actually, I did not find where results coming from modelling are presented\nin the paper to really support the reconstruction here proposed.\nb) Reconstruction of deglaciation phases presented in figure 4 is well depicted but would need a check of the calibrated\ndates obtained from different DRs coming from different materials (see suggestion for improvement).\nc) Authors exclude that “marine and terrestrial ice sheets deglaciated independently”, because this is not consistent with\ntheir “understanding of the relationship between grounding line retreat and upstream dynamic thinning of outlet glaciers”.\nActually, grounding line retreat occurred at different velocity and at different time to the west and to the East of Ross\nIsland, as clearly stated by ages from foraminifera collected in core CH-2 (Fig. 1) and dates coming from McMurdo Sound\n(both on land as indicated by Hall et al., 2004, 2006, 2023; and from ages coming from RPO dates here presented).\nSuggestions for Improvement\nTo improve the manuscript, I suggest that the authors further clarify the following key issues, in addition to addressing the\nlist of minor comments provided below.\nThe primary concern, in my opinion, relates to the correction of radiocarbon ages from marine sediments and the\ncalculation of ΔR values used for calibration of radiocarbon dates from different organic materials. Specifically, I question\nwhether the reservoir age derived from marine sediments can be reliably used to calculate ΔR values for marine-living\norganisms, which yield significantly different values (e.g., Hall et al., 2010, ref. 419; Gao et al., 2022, ref. 40).\nFurthermore, the authors should discuss whether the reservoir age they obtained has local or regional applicability. In\nother words, can this reservoir age be applied consistently across areas ranging from Coulman Island to Ross Island, and to\nthe entire dataset of radiocarbon dates considered in the study? Please elaborate on this point and, if necessary, consider\napplying different ΔR values for different types of organic materials (specifically dates from TOC sampled in sediment cores\nand dates from marine-living organisms collected on coastal deglaciated areas).\nAbout results from modelling of glacier retreat I suggest revising this part and furnish more information on this point or\nhold down the support furnished by the model to the proposed reconstruction of glacial retreat.\nAs concerns grounding line retreat to the East and to the West of Ross Island, I suggest to revise the discussion taking into\naccount the ages coming from all the cores, including dates from CH-2.\nSpecific issues\nRows 162-163: “Our RPO reservoir age is slightly older than that reported from the western Ross Sea based on carbonate\ndates from corals (∆R of 609 years40,41).”\nActually, ∆R of 609 ± 137 furnished by Gao et al 2022 (ref. 40) was obtained by:\na) conventional 14C dates of known-age marine-life samples from the Ross Sea (penguins and seals of known age\ncollected during the heroic age of Antarctic exploration) (n = 13); and\nb) paired 14C and U/ Th ages of coral samples from Terra Nova Bay and McMurdo Ice Shelf of Ross Sea (Hall et al., 2010,\nref. 41) (n = 39).\nThe total number of samples used for calculating DR to insert in Marine20 CALIB program is 52.\nIn my opinion, DR calculated by Gao et al (2022) should be used for calibrating samples collected on coastal ice-free areas,\nand to calibrate ages of marine-living organism.\nDifferent is the case for RPO ages obtained from marine sediments cores, for which the DR obtained using RPO analysis is\na new relevant contribute to correct and to calibrate the ages obtained from TOC.\nFig. 1: Please, insert geographic coordinates and toponyms of sites cited in the text (e.g. Siple Coast.\nFig. 2: Please, insert geographic coordinates in “A”.\nSupplementary Information\nThe supporting information includes extended data for core sites GC71, 72, 78 and 80 (described in detail in Figure S1-4).\nPlease, insert coordinates and depth of coring site as well as other information on the cruise (for each site in the figure\ncaption or, better, listed in supplementary table).\nReferences\nAckert, 2008. Swinging gate or Saloon doors: Do we need a new model of Ross Sea deglaciation? Fifteenth West Antarctic\nIce Sheet Meeting, Sterling, Virginia.\nAnderson et al., 2014. Ross Sea paleo-ice sheet drainage and deglacial history during and since the LGM. Quaternary\nScience Review, 100, 31-54. Doi: 10.1016/j.quascirev.2013.08.020\nBaroni et al., 2022. Antarctic Ice Sheet re-advance during the Antarctic Cold Reversal identified in the Western Ross Sea.\nGFDQ, 45 (1), 3-18. https://doi.org/10.4461/GFDQ.2022.45.1\nConway et al., 1999. Past and Future Grounding line Retreat of the West Antarctic Ice Sheet. Science, 286 (5438), 280-283.\ndoi: 10.1126/science.286.5438.280\nGiorgetti et al., 2024. Post-LGM glaciomarine processes revealed by inner shelf sedimentary facies analysis (Terra Nova\nBay, Western Ross Sea, Antarctica). Quaternary International, 697, 64-77. https://doi.org/10.1016/j.quaint.2024.06.012\nGreenwood et al., 2018. Holocene reconfiguration and readvance of the East Antarctic Ice Sheet. Nature Communications,\n9:3176. Doi: 10.1038/s41467-018-05625-3\nHalberstadt et al., 2016. Past ice-sheet behaviour: Retreat scenarios and changing controls in the Ross Sea, Antarctica.\nCryosphere 10, 1003-1020. Doi: 10.5194/tc-10-1003-2016\nHall et al., 2006. Holocene elephant seal distribution implies warmer-than-present climate in the Ross Sea. Proceedings of\nthe National Academy of Sciences of the United States of America, 103 (27), 10213-10217. doi:10.1073/pnas.0604002103\nHall et al., 2023. Widespread southern elephant seal occupation of the Victoria Land Coast implies a warmer-than-present\nRoss Sea in the mid-to-late Holocene. Quaternary Science Reviews, 303, 107991. doi: 10.1016/j.quascirev.2023.107991\nProthro et al., 2020. Timing and pathways of East Antarctic Ice Sheet retreat. Quaternary Science Re- views, 230 (106166),\n1-20. Doi: 10.1016/j.quascirev.2020.106166", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": "We thank Reviewer 1 for their thoughtful and constructive feedback. We appreciate the recognition of our effort to\nsynthesise multiple datasets, including new radiocarbon results, previously published marine records, and cosmogenic\nnuclide data, to better constrain Holocene ice retreat in the western Ross Sea.\nWe also acknowledge the concerns raised and are particularly grateful for the time this reviewer has invested to engage\ndeeply with our work. We have thoroughly revised the manuscript to address both general and specific issues identified in\nthe following comments, and we believe these changes have made it a stronger paper. The point-by-point response below\ndetails our response to each recommendation.\nWe thank the reviewer for highlighting the need for greater clarity and consistency in our methodological descriptions and\ndata presentation. We have made substantial revisions throughout the manuscript to address these issues, including a\ncareful review and correction of inconsistencies between figure captions and figures, and between text and tables and text. All\nreferenced data now align across text, tables, and figures, and we have also double-checked for formatting and citation\nerrors.\nWe thank the reviewer for their careful analysis of our approach on our use of RPO dates and the calculation and\napplication of local marine reservoir corrections (ΔR). Based on this feedback, we have revisited our approach to ΔR for\nthese sites. Our revised manuscript now adopts an estimate of ΔR of 609 ±137 years from Hall et al. (2010) and Gao et al.\n(2022), and includes a discussion of how we approach the much older core top ages for GC71 and GC72 (Text S3).\nWe also agree that RPO, while a significant improvement over bulk TOC dating, still cannot perfectly isolate purely\nautochthonous fractions, and that some residual contamination from older, reworked carbon may remain. We have\nupdated the main text and SI to better reflect this limitation and its implications for our chronological interpretations.\nThe revised text emphasises that even though our preferred fraction (split 2) has been demonstrated to concentrate\nprimary photosynthetic products based Py-GC-MS analysis, it still contains a mixture of carbon sources. This includes a\nfraction of material of reworked or ambiguous origin, as shown by the compound classification table (Table S2) and Py-GC-\nMS data (Fig. S6). Accordingly, we treat our calibrated split 2 RPO 14C dates as best maximum ages and have made this\ntreatment more explicit throughout the manuscript. We also clarify that our ΔR estimate is derived from a comparison\nbetween the maximum age from 14C and 210Pb date from GC72 at 10 cm depth, which is not applied across all cores (e.g.\nlines 145-146 in main text; ‘...and a reverse-calibrated pre-bomb 210Pb-14C pair in GC72 at 10 cm depth40,41’ and line 256 in\nSI; ‘We reverse-calibrated a paired pre-bomb 210Pb and 14C date from GC72 at 10 cm depth’).\nIn response to the specific concern about the derived ΔR of 1480 year being too large for broad application, we have\nrevised the manuscript and SI to clearly state that this value includes a local contamination offset, of 870 years, which is\nthe difference between our ΔR value and the regional ΔR of 609 ± 137 years. It is unique to cores GC71 and GC72, rather\nthan a revised regional ΔR. This offset was calculated only to correct those two cores, based on a pre-bomb 210Pb-14C pair,\nof which no pair exists for GC78 and GC80. As described above, we now apply the regional ΔR of 609 ±137 years (compiled\nby Gao et al., 2022) throughout our analysis, including all of our cores. GC78 and GC80 show smaller offsets between 210Pb\nand 14C dates, and we avoid applying any additional correction to them. We exclude the surface 14C dates in these cases\n(post-bomb) and instead rely on the 210Pb for upper age control. This revised approach is explained in the main text (lines\n128-153 and the SI (Text S1 and S3).\nIn response to the final comment above – we agree that exact timing is important and the text in question has been\nremoved.\nThank you for highlighting this here and below. In response, we have made substantial revisions throughout the\nmanuscript to move key methodological details from the SI into the main text to improve transparency and accessibility for\nreaders. We have clarified our use of the ΔR value and reservoir effect throughout (e.g. lines 129-130: ‘...calibrated using\nthe Marine20 curve and a regional marine reservoir correction (ΔR) of 609 ± 137 years33,39’) and included further detail on\nthe cosmogenic ages (e.g. Methods section ‘Glacier thinning from cosmogenic exposure dating’). See detailed responses\nbelow.\nWe have also clarified our terminology regarding marine radiocarbon calibration, specifically the applied ΔR value, and now\nuse this terminology consistently throughout the manuscript (e.g. line 129-130, 142, 145, 150 and in the methods section\n‘Age model development’). In addition, we have added specific detail on our approach to cosmogenic nuclide exposure\ndating, including scaling scheme, production rates and the use of internal vs. external uncertainties in age comparisons.\nSee response below for full details.\nWhere inconsistencies were identified (e.g. figure captions, table references and text), we have corrected these to ensure\nalignment across the manuscript and SI.\nFollowing our adoption of the Gao et al. ΔR value, our results do agree well, which is illustrated in the updated Fig. 3.\nThe retreat age of GC80 (located at the head of the overdeepend trough of Mackay Glacier), aligns nicely with the rapid glacier\nlowering of Mackay Glacier.\n“Southwestern Ross Sea” is a common geographic descriptor for the sector of the Ross Sea around Ross Island, bounded to\nthe south by the front of the Ross Ice Shelf. However, we take the point that, when discussing the retreat timing of ice\nsheets and ice shelves, the entire Ross Sea (including areas presently covered by the ice shelf) should be acknowledged.\nWe have updated the text in line 110 to reflect this but elect to retain “southwestern Ross Sea” elsewhere given its\nfamiliarity to our expected readers.\nUpdated to reference the more recent papers of Hall et al. (2010) and Gao et al. (2022) in lines 93: ‘An additional challenge\nof dating marine sediments in the Antarctic is the large (~1100 years23,33) and variable34 reservoir age’\nGood spot – lat/long now added.\nFont increased from 5 to 7 pt.\nYes we compared the 210Pb and 14C ages from 10 cm depth and have made this clearer throughout the manuscript (e.g.\nlines 145-146: ‘...ΔR and a reverse-calibrated pre-bomb 210Pb-14C pair in GC72 at 10 cm depth40,41’).\nWe reverse calibrated the 14C age by inputting the 14C age and 210Pb age (known age) at the same depth into the ΔR\ncalculator (http://calib.org/deltar). This is the correct citation, but we now also cite Reimer and Reimer (2017), which\ndetails this method (main text lines 145-146: ‘...ΔR and a reverse-calibrated pre-bomb 210Pb-14C pair in GC72 at 10 cm\ndepth40,41’ and SI Text S3).\nThe reviewer’s skepticism is valid, and we believe that revising the manuscript to address it has made the paper stronger.\nAs detailed in our response to Point (1), we have overhauled this paragraph to reflect the recalibration and re-evaluation of\nour age models (e.g. lines 128-153). As such, we have removed this and other text associated with the application of the\nreservoir age we used in the original submission.\nAs above, we have revised this paragraph to acknowledge that RPO reduces but does not entirely remove the influence of\nreworked carbon. As highlighted in the reviewer comment, we cannot be sure of minimal contamination, only that there is\nless old carbon contamination than bulk TOC 14C-derived data.\nThough we view it as an interesting possibility for future investigation, we have removed speculative text discussing the\nways that differences in ocean water masses, including waters emerging from the ice shelf cavity, could be influencing the\nreservoir age in the SW Ross Sea region. As per our response to Point (1), the large reservoir age generated at site GC72\nreflects local contamination, calculated as 870 years, which is the difference between a locally calculated ΔR value (based\non the paired 14C and 210Pb date from GC72) and the regional ΔR of 609 ± 137 years (e.g. lines 144-148: ‘To correct for this,\nwe apply an additional local contamination offset derived from the difference between the regional Ross Sea ΔR and a\nreverse-calibrated pre-bomb 210Pb-14C pair in GC72 at 10 cm depth40,41. The resulting local contamination of 870 ± 120\nyears was also applied to GC71 for which no equivalent pair exists (Supplementary Text 1, 3).’\nWe have recalibrated and re-evaluated the ΔR value as suggested, resulting in changes to the ages of retreat at each core\nsite. This paragraph is extensively revised.\nThank you for this comment. We agree that the dates reflect the onset of open water conditions, rather than grounding\nline retreat itself. We have updated the text to add clarity: ‘open marine conditions were established by…’ (e.g. lines 159,\n161-162, 164-165).\nFollowing the recalibrated and updated age models, the minimum calibrated age for grounding line retreat at site GC78 is\n6.9 (6.7-7.1 95% CI) cal yr BP and at site GC80 is 6.5 (6.3-7.3 95% CI) cal yr BP. The opening of McMurdo Sound before 7.5\n(reworked shell; Licht et al., 1996) to ~6ka (based on the U-series dating from corals; Hall et al., 2010) fits nicely with our\nrevised ages of retreat.\nFigure 3 is extensively updated. We acknowledge it is difficult to distinguish between grounded and floating ice – we have\nadded hatched shading to denote floating ice/ice shelf, which is over core site CH-2 and GC72 in both A(i) and (ii). The blue\nshading along the coast was used to denote ice free conditions (and therefore open ocean) along the coast following\nunloading of the ice sheet. We’ve changed this colour to light brown, representing submarine shoreface in areas where\nbathymetric information is not available. This ice/ocean interface has been difficult to represent in our schematics, but we\nhope these changes have made it clearer.\nAs the timing of open marine conditions at each core site has changed, the particular time slices we illustrate in A have also\nchanged. We now show A(i) >7.2 cal kyr BP, when ice had not retreated beyond any of the core sites (older than the\nmaximum 95% CI of all cores); A(ii) ~6.1 cal kyr BP when open marine conditions were established at core sites GC72, 78\nand 80 (younger than 95% CI of these cores), but open marine conditions were not established at GC71 (so, older than the\n95% CI of this core site); A(iii) <5.4 cal kyr BP when open marine conditions were present at all cores sites (younger than all\n95% CI for all cores). We have made sure there is consistency between the time slices and the figure caption and made\nmore explicit that we are referring to the 95% CI intervals, rather than the ‘best’ ages, which is more obvious in panel B(i)\nnow.\nThe reviewer is absolutely right. After recalibrating the 14C ages with the ΔR value of 609 ± 137 years, our marine retreat\nages align nicely with the cosmogenic ages of ice thinning, particularly those of Mackay Glacier.\nThe exposure ages from Mackay and Mawson glaciers were both calculated using the Borchers et al. (2016) production\nrate and LSDn scaling scheme, as reported in the Jones et al. (2020) paper, and ages with external errors (including\nproduction rate uncertainty) are used for comparisons in this study. We now explicitly state this information in the\nMethods ‘Glacier thinning from cosmogenic exposure dating’.\nThe LSDn scaling scheme is the most up-to-date and accurate scaling scheme, and there is no reason to think that the older\nSt or Lm schemes are equally as appropriate to use. But we acknowledge that the exact production rate for Antarctica is\nunknown, and using Southern Hemisphere calibration sites instead of a global calibration dataset scaled to Antarctica\nwould make the exposure ages older. This was demonstrated in Jones et al. (2015) for Mackay Glacier. However, Antarctica\nhas particular conditions (e.g. Stone, 2000), and there is no evidence that New Zealand or South America production rates\nare more suitable for Antarctica than the scaled global calibration dataset. There are also local influences on the\nproduction rate and resulting exposure age (e.g. erosion, glacial isostatic adjustment, snow cover) that, if corrected for,\ncould feasibly make the exposure ages older; we discussed that this is unlikely in Text S4 (now in main text lines 238-256).\nFor 10Be in Antarctica, there is a higher chance that the ages are partially impacted by cosmogenic inheritance. Accounting\nfor any inheritance would make the exposure ages younger, but we also consider this to be minimal. In summary, there are\nvarious factors that could potentially make the exposure ages younger or older, yet we consider these factors to be\nminimal. Nevertheless, we agree that this should be more fully acknowledged and have expanded the text in the main text\n(lines 238-256).\nFollowing recalibration and re-evaluation of the ΔR, the updated timing of retreat at GC80, near Mackay Glacier mouth,\naligns very nicely with the cosmogenic data and orange bands that represent the modelled grounding line retreat.\nWe have made the Mawson data grey with smaller symbols, and similarly changed the modelled ice surface elevation for\nMawson Glacier to grey and in the background. By doing so, we still acknowledge Mawson Glacier and show that it is\nbroadly consistent with Mackay Glacier data, but makes Mackay Glacier data the focus.\nWe have made this clearer throughout the caption for Fig. 3. Thanks.\nWe assume that this comment is referring to the rates of the modelled thinning vs. the rate of thinning implied by the\ncosmogenic data. This fit was made in the original paper by Jones et al. (2020) where the two experiments were very\ndifferent, so it is understandably harder to see without that context. Compared to the rest of the deglaciation, there is a\nsignificantly faster rate of thinning in both the cosmogenic records and the model at the same time; the absolute rate of\nthinning may not be correct, likely due to uncertainty in the true climate forcing and model parameters.\nThanks for flagging – have updated to ‘Combined with marine dates in the southwestern Ross Sea, recalibrated with the\nupdated Marine20 curve42 and the Ross Sea regional ∆R33,39…’ (lines 172-174)\nAs per our response to Point (1) and comment for line 171, we have re-calibrated the Coulman High grounded ice retreat\ndate using the Ross Sea regional ΔR value of 609 ± 137 years, resulting in the minimum age of ungrounding at this site to be\n8.6 (9.2-8.1, 95% confidence) cal kyr BP (lines 175-176 ‘...minimum age of ungrounding at Coulman High10 becomes 8.6\n(9.2-8.1, 95% confidence)...).\nHave updated this sentence to ‘Based on calibrated 14C ages following the transition from (sub)glacial to open marine\nfacies in cores GC71 and 72, we constrain southwest retreat towards Ross Island to between 6.4 and 5.4 cal kyr BP.’ (lines\n181-183).\nWe’ve added a sentence to be more specific about which marine dates have not previously aligned with terrestrial\nchronologies; ‘This discrepancy is largest with the Coulman High core (8.6 cal kyr BP) and sites dated using the acid-\ninsoluble organic fraction of bulk sediment’ (lines 191-193).\nThank you for highlighting this area of confusion. We have carefully assessed the dates we refer to in this paragraph. We\nrefer to the minimum statistically likely ages of retreat given by the 95% CI of each date/site in Fig. 4, and therefore use\nthese minimum ages in this text. The dates (7.7-6.1) indeed come from the recalibration of CH-2 (Mckay et al., 2008) and\nthe Mercer Lake dates (Venturelli et al., 2023), which are now updated to 8.1-6.2 cal kyr BP. We have added text about\nwhat sites we refer to, to make it clear for the reader – lines 284-286 ‘Our results suggest that the embayment in grounded\nice, bounded by Ross Island to the west and the Siple coast, formed rapidly, at the latest, between ~8.1 (site CH-210) and 6.2\n(Mercer Subglacial Lake20) ka BP’.\nThank you for this clarifying question, we have updated the text (line 284).\nWe have overhauled this paragraph as per the comment for line 261 (lines 280-290).\nYes, surface lowering of southern TAMs glaciers (Reedy, Scott, Beardmore) occurs earlier than the dates of retreat\nrecorded in the cores. This is consistent with our reconstruction of retreat in the Ross Embayment. We now clarify in the\n‘Topography controls ice sheet retreat’ section (lines 302-315) that retreat would have occurred earlier in the central Ross\nSea with corresponding surface lowering of glaciers in the southern TAMs, with delayed retreat in the SW Ross Sea due to\nthe topographic control.\nThank you for highlighting. We acknowledge that the location of these glaciers may not be known by the average reader.\nWe have added a table in the supplementary information (Table S4) that includes the latitude/longitude, age and reference\nfor glaciers shown in Figure 4.\nWe have updated the position of the 7 and 6 ka grounding lines in line with the recalibrated ages. We’ve added the\nrecalibrated date from the reworked shell (Licht et al., 1996) – thanks for spotting this missing date.\nAs per the comment above for lines 272, we have added Table S4 to the supplementary information with the\nlatitude/longitude, age and reference for each glacier shown in Figure 4. We refer to the TAM glaciers collectively in lines\n305-306: ‘...with the exact timing varying between glaciers along the Transantarctic Mountains (Supplementary Table 4)...’.\nThe triangle represented the grounding line retreat over the Bindschadler Ice Stream, Siple Coast from Neuhaus et al.\n(2021). However, we’ve now changed that symbol to a square, as this date reflects marine retreat. The date has also been\nupdated to reflect the minimum statistically possible age of grounding line recession (see Table S3), in line with all of the\nmarine retreat dates in this figure.\nThanks for spotting – we have updated the black circles to ‘unloading of grounded ice’ and the black inverted triangles to\n‘glacier surface ice lowering’.\nWe have added this sentence: ‘Note the marine retreat ages are the minimum 95% confidence interval value, and therefore\nthe youngest statistically likely timing of retreat at these sites (Supplementary Table 3)’ to Figure 4 caption to make this\npoint clearer. We hope that this distinction is now clearer.\nTable S3 has been updated to reflect the recalibration of our own dates as described above.\nWe have not updated the dates from Venturelli et al. 2023 as the authors derived a local reservoir age from extant\namphipods collected from the marine cavity at their site during borehole operations, which result in an average 14C age of\n1101 ± 38 years (Kingslake et al., 2018). They applied a standard deviation of ± 120 as recommended by Hall et al. (2010) to\naccount for variation in the local reservoir in sub ice shelf and grounding line proximal environmental due to different\nwater masses and DIC pools that are advected under the Ross Ice Shelf. The resulting local reservoir age used is 1101 ± 120\nyears, and using the Marine20 calibration curve, results in a ΔR value of ~500 ± 120 years. This value is comparable to the\nRoss Sea average ΔR value.\nFigure 4C shows the minimum statistically likely ages of retreat (based on 95% CI), which in the case of Venturelli is 6.2 ka,\nwhich we have updated in the figure to correct our mistake.\nSample thickness was 1 cm – have added detail to text (line 393).\nThanks for pointing this out. We have made a conscious effort to make the distinction clear and refer to ΔR throughout the\ntext, which prevents confusion with the reader also.\nDone, thanks.\nThank you for this helpful observation, which we agree with. As noted in our response to point (1), we now apply the\nregional Ross Sea ΔR rather than the previously larger correction. This approach is consistent with the 210Pb constraints and\navoids overestimation of the reservoir effect.\nWe have chosen not to cite Venturelli et al. (2023) in this section as the work is based on RPO, not Py-GC-MS.\nWe thank the reviewer for this careful observation and agree with their assessment. We now explicitly state throughout\nthe main text that split 2 radiocarbon ages are maximum ages due to the presence of reworked and refractory carbon (e.g.\nlines 140-141: ‘...we selected split 2 ages as providing the most accurate maximum age constraint for cores GC71, 72, 78\nand 80’). We also acknowledge that deconvolution modelling could help quantify autochthonous material in the split and\nassess age contribution from various carbon pools, and we look to explore those connections in future work.\nNo, they’re samples from downcore – the depths are in the figure and also now added to the figure caption.\nWe have overhauled this text (‘Reservoir correction and age model development’) to reflect the changes we’ve made\nregarding the ΔR used to calibrate the 14C dates and generate chronology. In doing so, we now only refer to ΔR as\nsuggested throughout the text.\nText S3 has been extensively revised. We refer to reservoir ages in the context of ΔR throughout the text now, not R in this\ncase, as used in the original submission.\nWe agree that the timing of retreat is highly sensitive to the choice of reservoir age. We have removed this sentence and\nclarified our use of the regional Ross Sea ΔR. This makes our age models more robust and allows a more meaningful\ncomparison between onshore and offshore records.\nAll data has been recalibrated using the ΔR of 609 ± 137 years and the resulting calibrated ages are updated in Table S3.\nThanks for pointing this out – an oversight on our part. We have added this date into Table S3. This date is from the\nWhillans Subglacial Lake, Siple Coast from Neuhaus et al. (2021). It is modelled grounding line retreat derived from a\ncomparison of the measured pore water ionic concentrations from a sediment core compared to modelled values using\ntheir ionic diffusion model. It is still a date reflecting marine retreat, and we have therefore changed the symbol to a\nsquare.\nText S4 has been moved to the main text and the unit ‘km’ is added here (line 302).\nWe agree. This text is updated and has moved to the main text on lines 233-236: ‘However, as discussed above, by\ncombining 210Pb data with RPO-derived 14C ages, we reduce contamination by ancient detrital carbon and calibrate using\nthe Marine20 curve and a Ross Sea average ∆R value of 609 ± 137 years33,39 to produce more accurate timings for the\nestablishment of open marine conditions at the core sites.’\nWe now describe how the cosmogenic ages were calculated, including the production rate and scaling used, in the\nMethods. We also discuss uncertainty in these ages in the main text, within the revised section ‘Inland ice drawdown\ncoupled to rapid grounding line retreat’.\nText S4 has been moved to the main text under the section ‘Inland ice drawdown coupled to rapid grounding line retreat’.\nWe thank the reviewer for their thoughtful and positive evaluation of our manuscript. We also appreciate the recognition\nof the study’s contribution to understanding post-LGM deglaciation dynamics in the Ross Sea region and the appreciation\nfor the application of 210Pb and RPO 14C dating.\nThe regional-scale modelling was done in a previous study, and here we use the model results in combination with our new\nretreat ages to provide a physically-based explanation for the timing, pattern and mechanism of retreat. We have now\nmade sure to cite the original study wherever we mention this modelling, have moved relevant discussion from Text S4 to\nthe ‘Inland ice drawdown coupled to rapid grounding line retreat’ section in the main text, and have added a figure to the\nsupplementary information (Fig. S7) to highlight the relationship between modelled and recorded retreat in the SW Ross\nSea.\nWe agree that ΔR values derived from different archives can yield different results. In light of this, and as detailed in our\nresponse to Reviewer 1, we have revised our approach and now apply the most recent regionally constrained compiled\nRoss Sea ΔR value rather than relying solely on values derived from our 210Pb-RPO 14C data. We have clarified this in the\nmethods and results sections and we no longer use our locally derived ΔR value for calibration of radiocarbon ages.\nThe section ‘Inland ice drawdown coupled to rapid grounding line retreat’ has now been revised to more comprehensively\ndiscuss the modelling of glacier retreat in the context of our new retreat ages and supporting thinning chronologies to\nsupport our proposed reconstruction.\nThe discussion has been thoroughly revised (lines 281-290) and CH-2 has also been added to Fig. 3, panel B to facilitate\ncomparison of retreat timing across all sites around Ross Island.\nWe thank the reviewer for clarifying the methods used by Gao et al. (2022) to derive the ΔR of 609 ± 137 years from\nknown-age samples, including marine-life specimens (penguins and seals) and paired 14C–U/Th coral ages. We fully agree\nthat this ΔR is an appropriate regional value for calibrating ages of modern marine organisms and materials from coastal\nice-free areas. In our revised manuscript, we now state explicitly that this Gao et al. (2022) value is applied as the baseline\nΔR for all of our marine sediment core calibrations, except in two cases (GC71 and GC72) where we applied an additional\nlocal contamination offset of 870 years to account for a much older core-top 14C age relative to paired 210Pb data. This\noffset is no longer intended to be used as a revised regional ΔR, but is instead a site-specific correction. For other cores\n(GC78, GC80), we did not observe a similarly large offset and so we rely on 210Pb for surface age control, avoiding over-\ncorrection. We also clarify that our RPO-based ages are treated as best maximum age estimates due to potential residual\nreworked carbon, and we have expanded the text and SI to highlight how this uncertainty is incorporated into our\ninterpretation (e.g. lines 128-153 and Text S3).\nDone, thanks.\nGood spot - have added the coordinates.\nWe have added the cruise details (vessel, expedition and year) to the introduction of the Supplementary information (R/V\nAraon, Expedition ANA05B-RS15, 2015) to avoid duplication in the figure captions. The coordinates, depth of coring site\nand core length have been added to respective figure captions (Figures S1-S4).\nWe thank Rev. 3 for reviewing our manuscript. We appreciate the role of Early Career Researchers in the peer review\nprocess and are grateful for the time and effort dedicated to evaluating our work.\nWe are very grateful to our three reviewers for their highly constructive feedback, which has significantly\nstrengthened our manuscript. Their thoughtful engagement with the data at a granular level exemplifies the\nbest of peer review. We have made the requested corrections and clarifications, and we look forward to seeing\nthe manuscript published in Nature Communications.\nWe thank Rev. 1 for their careful consideration of our revisions. We are very pleased that our efforts to\naddress the initial round of comments were found satisfactory.\nWe greatly appreciate Rev. 2’s thoughtful engagement and detailed assessment of our revisions. We are glad\nthat the clarifications and additional figures (e.g. Fig. S7) were found to strengthen the manuscript, and that\nthe revised discussion and inclusion of core site CH-2 improved the comparison of retreat timing.\nAs noted, we have corrected the citation for Baroni et al. to volume 45 (not 43) and year 2022 (not 2023). We\nthank the reviewer for catching this error."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I reviewed this paper previously and had alot of comments on that intial draft. In my opinion, the authors have made a good-\nfaith effort to thoroughly address those comments, and I have no further changes to suggest.", "reviewer_2": "The authors have addressed all the suggestions and clarified the points I previously indicated as unclear or problematic.\nIn particular, I appreciate the use of a regional ΔR applied within the Calib program, with the exception of two cases (GC71\nand GC72), where the authors applied an additional local contamination offset of 870 years to account for a significantly\nolder core-top ^14C age obtained from paired ^210Pb data (site-specific correction).\nRegarding the regional-scale modeling applied in this study, the authors have better clarified that they used model results\npreviously published in combination with newly provided retreat ages from this paper; they have modified the text to better\nexplain this point. Furthermore, they have added a figure to the supplementary information (Fig. S7) to illustrate the\nrelationship between modeled and recorded retreat in the southwestern Ross Sea.\nThe section titled ‘Inland ice drawdown coupled to rapid grounding line retreat’ has been revised to provide a more\ncomprehensive discussion of glacier retreat modeling in the context of the new retreat ages presented here, along with\nsupporting thinning chronologies, to substantiate the proposed reconstruction.\nThe discussion has been extensively revised (lines 281-290), and core site CH-2 has been incorporated into Fig. 3 to\nenable a comprehensive comparison of retreat timing across all sites surrounding Ross Island.\nSpecific issues have been addressed, and corresponding modifications have been made to the text and figures accordingly.\nRegarding the inserted citations, please note that citation number 55 (Baroni et al.) refers to volume 45 (not 43) of Geografia\nFisica e Dinamica Quaternaria, and the publication year is 2022 (not 2023)."}, "author_response": "We thank Rev. 1 for their careful consideration of our revisions. We are very pleased that our efforts to\naddress the initial round of comments were found satisfactory.\nWe greatly appreciate Rev. 2’s thoughtful engagement and detailed assessment of our revisions. We are glad\nthat the clarifications and additional figures (e.g. Fig. S7) were found to strengthen the manuscript, and that\nthe revised discussion and inclusion of core site CH-2 improved the comparison of retreat timing.\nAs noted, we have corrected the citation for Baroni et al. to volume 45 (not 43) and year 2022 (not 2023). We\nthank the reviewer for catching this error."}]

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