Ocean heat forced West Antarctic Ice Sheet retreat after the Last Glacial Maximum

Domain: neuroscience
Published: 2026-02-06
Editorial outcome: major_revision

Abstract
The West Antarctic Ice Sheet (WAIS) is thinning at an accelerating rate, driven by melting at its margins by warm Circumpolar Deep Water (CDW). However, this understanding is largely based on observations from recent decades, leaving the long-term influence of ocean temperature on WAIS stability uncertain. Here we reconstruct bottom water temperatures and water mass properties over the past 18 kyr using benthic foraminiferal Mg/Ca and δ¹³C records from sediment cores in the Amundsen Sea. Our data indicate that warm CDW occupied the continental shelf between ~18.0 and 10.1 kyr BP, coincident with major WAIS retreat from the shelf break to near its present-day grounding-line position along the Marie Byrd Land coast. Bottom waters cooled after ~10.1 kyr BP and remained relatively stable thereafter, with no evidence for substantial grounding-line migration. Continued atmospheric warming across West Antarctica until a mid-Holocene thermal maximum (~6–3 kyr BP) without further retreat indicates that ocean heat was the primary driver of WAIS variability since the Last Glacial Maximum.

Introduction
The net loss of glaciers flowing into the Amundsen Sea Embayment (ASE), which accounts for ~35% of the total discharge from the WAIS, has increased by 77% since 1973. Ice sheet thinning has been accompanied by rapid inland retreat of the grounding lines of Pine Island Glacier and Thwaites Glacier potentially initiating an unstable and irreversible retreat. Complete collapse of the glaciers in this region would raise global sea level by ~1.5 m, substantially above the ‘likely’ modelled range of 0.25–0.95 m predicted for the 21st century by the IPCC. The coherent thinning of glaciers across the ASE has been attributed to basal melting of the floating ice shelves by warm CDW upwelling onto the continental shelf. Originating from the Southern Ocean, modified CDW (CDW hereafter) is channelled along landward deepening bathymetric troughs, eroded by palaeo-ice streams. At the ice sheet margin, CDW is ~3.8 °C above the in-situ freezing point and causes vigorous basal melting of the fringing ice shelves that buttress the inland ice.

Oceanographic measurements show the volume and temperature of CDW in Pine Island Bay increased between 1994 and 2009, and this increase coincided with progressive thinning of the ice shelf, glacier speed-up and enhanced ice discharge into the ocean. Ocean models attribute these incursions of CDW to changes in the frequency and strength of the westerly winds near the continental shelf break. Typically these winds are driven by convective anomalies in the equatorial Pacific troposphere (El Niño) that radiate an atmospheric wavetrain toward the region, strengthening winds at the continental shelf edge and leading to greater upwelling of CDW onto the shelf. Wind-induced Ekman pumping may also influence the depth of the on-shelf thermocline and thus drive higher melt rates. One model was able to relate two recent periods of accelerated thinning of Pine Island Glacier in 1974–87 and after 1994 to simulated increases in CDW influx. Conversely, another model was able to show—in combination with ocean measurements—that from 2010 to 2012 oceanic melting of the Pine Island Glacier ice shelf decreased by 50% in response to a major La Niña event. The weak convection in the tropical Pacific resulted in decreased wind stress at the shelf edge, reduced upwelling of CDW and reduced glacier melting in Pine Island Bay, thereby providing a mechanism by which current mass loss could be reduced. Together these studies demonstrated that the mean-ocean state in the Amundsen Sea can vary—at least on annual to decadal time-scales—and provides a mechanism that might increase the rate of ice sheet melting and retreat, or in the case of 2012, reduce localised ice sheet thinning.

However, with only a few decades of ocean temperature data available for the ASE, it remains difficult to assess whether CDW upwelling onto the shelf has been relatively constant, driven on annual to decadal time scales by tropical forcing, or has varied substantially in the past, potentially even promoting thickening and advance of grounded ice during times of reduced upwelling. Atmospheric and ocean modelling studies attempted to address this. Simulations revealed that the mean easterlies were progressively weakened by anthropogenic radiative forcing throughout the twentieth century, causing tropically forced decadal warm ocean anomalies to become more prevalent at the ice-sheet margin. Episodes of particularly strong wind anomalies in the mid-1940, drove strong melting that caused ice streams to unground from seabed ridges and progressively retreat due to ice-ocean feedbacks and increased anthropogenic forcing. However, these studies only examined the last century of change and did not consider long term changes in warm CDW incursions onto the Antarctic shelf and their link with periods of increased glacier discharge. Hillenbrand et al. used a multi-proxy approach to reconstruct water mass variations in the ASE throughout the Holocene (11.7 kyr BP to present). Using the stable carbon isotope (δ¹³C) composition of benthic foraminifera Trifarina angulosa and preliminary Mg/Ca data (n=10), they argued that enhanced CDW upwelling could have forced deglaciation of the innermost part of Pine Island Bay from at least ~10.4 kyr BP until ~7.5 kyr BP. But at the time of publication, no suitable Mg/Ca temperature calibration existed for T. angulosa, meaning that the long-term relationship between ice sheet retreat and ocean heat content remained uncertain.

Given the modelled sensitivity of the WAIS to CDW-induced melting on decadal and millennial timescales and the potential uncertainty in the magnitude of future global sea-level rise from the ASE drainage sector, there is a fundamental need to constrain past oceanographic changes—and particularly ocean temperature—in order to understand whether the ocean-driven melting we see today is unique, has operated in the past (and if it has driven ice sheet retreat), or, if retreat is reversible.

Here we present Mg/Ca and δ¹³C data of benthic foraminiferal calcite (T. angulosa) for 6 sites in the ASE, which together provide a unique record of water mass variability for the past ~18 kyr. No single sedimentary record from the ASE contains sufficient T. angulosa shells throughout this entire period because sites were either covered by grounded ice or affected by iceberg turbation at some points since the Last Glacial Maximum (LGM), so we used a multi-site approach. Furthermore, calcareous microfossils, required not only for analysing the geochemical composition of their shells but also for establishing reliable radiocarbon-based age models, are absent in the vast majority of previously collected cores. Indeed, the cores used here offer the best carbonate preservation and are the only ones suitable and available for this type of work. An estimate of BWT is calculated using an Antarctic-wide calibration for T. angulosa, which demonstrated the sensitivity of this species to BWT based on core-top samples from the Amundsen Sea, Antarctic Peninsula, Weddell Sea and East Antarctic margin (Methods). T. angulosa was used for three reasons: (1) it is common in modern and post-LGM sediments on the Antarctic continental shelf, and particularly in the Amundsen Sea; (2) it is genetically related to Uvigerina spp., which has previously been used for BWT reconstructions; and (3) it is a shallow infaunal species, which is considered less likely to be influenced by the effects of carbonate ion concentration on the Mg/Ca ratios of their tests than epifaunal species (see Methods). To aid interpretation of the Mg/Ca record and provide independent constraints on water mass variability, we also measured the δ¹³C composition of T. angulosa as a proxy for the carbon isotope composition of the dissolved inorganic carbon (δ¹³C_DIC) of the ambient seawater. Because the dissolved inorganic carbon of the two dominant water masses in the Amundsen Sea (CDW and Antarctic Surface Water (AASW)) are distinct, δ¹³C measurements of foraminiferal calcite can be used as a reliable tracer for its source water mass variations. Age constraints for the six cores are based on previously published age data that are supplemented by additional AMS ¹⁴C dates on calcareous microfossils (VC436) and ²¹⁰Pb data (BC431).

Results
Mg/Ca and δ¹³C records of ocean temperature and water mass variability

Resulting BWT, calculated from measured Mg/Ca ratios, are -4.4 to +4.2 °C and are unrealistic given the freezing point of regional surface waters (-1.87 °C) and maximum CDW temperature (+2.0 °C) in the Southern Ocean. Temperatures outside of a realistic range likely reflect limitations of our calibration (see Methods). Because of this, we do not interpret the Mg/Ca-ratios in terms of absolute temperature change(s) and instead focus on trends between ‘warmer’ and ‘cooler’ BWT. While this does not allow direct comparisons between palaeo and measured (modern) temperatures, it does enable us to investigate the response of the ice sheet, which is well established, to changes in ocean forcing. Furthermore, regardless of which calibration or method for calculating the temperature data is used, all curves display consistent trends, suggesting that although estimates of BWT may improve as calibrations evolve, the observed trends are robust. This assumption is supported by the δ¹³C data whereby high BWT correspond to low δ¹³C values that have previously been shown to be indicative of CDW. Thus, and consistent with previous work, we attribute high Mg/Ca and low δ¹³C ratios to enhanced CDW delivery/warm conditions whereas low Mg/Ca and high δ¹³C values indicate a dominance of AASW-like/cooler conditions.

The Mg/Ca and δ¹³C data are presented in Fig. 2, split between outer and inner shelf sites. Outer shelf variations are largely constrained by data from site VC436, which was the first to deglaciate following the LGM and spans the post-LGM to Early Holocene period (~18.0 to 9.8 kyr BP). The Holocene part of VC436, after 9.8 kyr BP, is heavily disturbed due to iceberg turbation, making this interval unsuitable for Mg/Ca and isotope analyses. Relative to inner shelf sites, VC436 is characterised by high Mg/Ca and low δ¹³C values implying that warm CDW-like conditions persisted throughout this interval. Highest Mg/Ca values occur around ~11 kyr BP while lowest values are recorded 14.5–13.9 kyr BP. Data from PS75/190 and PS75/192 in the northeastern ASE, are consistent with warmer BWT during the Late Glacial but are restricted to a single data point in the post-LGM to Early Holocene period (~18.0 to 9.8 kyr BP). Mg/Ca and δ¹³C data from PS75/190 and PS75/192 additionally suggest a transition to cooler BWT and overall reduction in CDW during the Holocene, although the precise timing of this change is poorly constrained due to the limited age constraints. BC431, which only recovered sediments spanning the past 150 years, indicates that Late Holocene BWTs were cooler than from ~18.0 to 9.8 kyr BP.

Inner shelf sites PS75/160-1 and PS75/167 span the time interval after ~10.46 kyr BP, when ice had retreated to a position close to present. They are characterised by a shift to lower Mg/Ca and higher δ¹³C ratios that are indicative of cooler BWTs and an overall reduction of CDW after ~10.0 kyr BP. In detail, Mg/Ca is characterised by periods of lower values 8.9–7.1 kyr BP, 5.2–4.8 kyr BP and 1.8–1.1 kyr BP that are coeval with higher δ¹³C ratios and periods of slightly higher Mg/Ca and lower δ¹³C ratios at 6.3–5.6 and ~3.6 kyr BP, respectively. Generally, both outer and inner shelf records display centennial to millennial-scale water mass variability during the past 18 kyr but note that the resolution of our dataset is insufficient to resolve seasonal variability.

While our composite dataset has limited temporal overlap (~660-year) between outer and inner shelf sites we note two important observations: (1) there is a gradient from high Mg/Ca and low δ¹³C ratios to low Mg/Ca and high δ¹³C ratios during the Early Holocene (~10–8.0 kyr BP). This gradient is better resolved in the δ¹³C data because fewer specimens are required for isotopic analyses, thus allowing measurement of more sediment core intervals and increasing temporal resolution. Furthermore, sites PS75/190 and PS75/192, while also limited in their temporal resolution, provide Mg/Ca and δ¹³C data spanning the past ~16 kyr BP. The Mg/Ca and δ¹³C data at these sites are characterised by cooler BWT after ~10 kyr BP compared to the period 18–10 kyr BP, thereby supporting our interpretation of an embayment-wide change in water mass properties during the earliest Holocene. (2) There is a strong argument that outer shelf oceanographic conditions are representative of the entire shelf because present-day oceanographic observations around Antarctica indicate consistency between the inner and the outer shelf. Presently, and irrespective of the proximity to the ice sheet grounding line, BWTs on the Antarctic shelf are either warm or cold, with little variation between the ice front and the shelf break. Furthermore, numerical modelling demonstrates that the residence time of shelf waters, i.e., the time required to fully replace water properties through advection following a regime shift, is just a few years for small shelf seas such as the Amundsen Sea and up to a few decades for large ones such as the Weddell Sea. Thus, a signal observed in one deep location that is coherent over centuries, is likely to be representative of an entire shelf sea even if the ice sheet configuration changes. Accordingly, our composite dataset indicating warmer BWT on the outer shelf ~18.0 to 10 kyr BP and cooler BWT on the inner shelf after ~10-8.0 kyr BP applies equally to all shelf areas free of grounded ice. Change-point analysis of the composite Mg/Ca and δ¹³C datasets confirm that the transition from warm to cooler BWT occurred between 10.1 and 7.9 kyr BP, with peak BWT identified between 11.0 and 10.1 kyr BP.

Timing of ice sheet and water mass variations since the LGM

Extensive marine geological data from the Amundsen Sea shelf indicate that during the LGM, the WAIS advanced to a position at, or close to the continental shelf break. Grounding line retreat was underway by ~22-19 kyr BP, reaching the mid-shelf at ~13.8–13.5 kyr BP and a position close to modern along the Walgreen, Bakutis and Hobbs Coast (hereafter referred to as Marie Byrd Land coast) before ~11.2 kyr BP, without significant change since then. This time-slice reconstruction is supported by ice sheet modelling, which simulates the majority of Amundsen Sea mass loss as having occurred between 17 and 9 kyr BP. Surface exposure dating in the hinterland around Pine Island Bay documents a period of rapid, but localised, ice sheet thinning between 9 and 6 kyr BP, which has been linked to the collapse of a more extensive ice shelf covering the inner shelf at 7.5 kyr BP, resulting in debuttressing and drawdown of inland ice. Several other studies also point to localised glacier and ice shelf changes during the past 10 kyr, such as the Early-to-Mid-Holocene retreat of the Cosgrove Ice Shelf and Late Holocene ice-sheet thinning in the vicinity of Pope and Thwaites glaciers but without significant increases in mass loss. Sea-level data support the scenario of Early Holocene deglaciation followed by relatively stable ice positions until recent times and suggest that Thwaites and Pine Island glaciers—which are currently the main contributors of WAIS mass loss—have not been substantially smaller than present during the Holocene. Our reconstruction of BWT and δ¹³C data indicates that warmer, CDW-like conditions were persistent on the outer shelf throughout the deglacial phase (~18–10.1 kyr BP). Conversely, cooler BWT and a shift to higher δ¹³C values indicative of more AASW-like deep water properties are recorded after ~10.1–7.9 kyr BP, when WAIS grounding lines along the Marie Byrd Land coast are considered to have been largely stable. This provides compelling evidence that post-LGM WAIS deglaciation was primarily driven by incursion of warm CDW. We now consider the role, and possible interplay between other climatological drivers, focusing first on the deglaciation.

CDW as the primary driver of ice-sheet retreat following the LGM

The long-held view is that deglaciation of the WAIS following the LGM was driven by a combination of rising sea levels—primarily due Northern Hemisphere deglaciation—warming ocean waters, and changes in atmospheric circulation and/or ice sheet dynamics. Our combined Mg/Ca and δ¹³C dataset provides direct evidence that WAIS retreat in the Amundsen Sea was linked to warm water forcing, whereby high BWTs are recorded on the outer shelf as early as ~18 kyr BP, concomitant with initial retreat of the WAIS from its expanded LGM position. Today, incursions of CDW onto the ASE shelf are tightly coupled to enhanced wind anomalies at the shelf edge, driven by variability in the Amundsen Sea Low, and modulated by both El Niño events and the phase of the Southern Annular Mode (SAM). During La Niña, a cooler ocean state is characterized by a deepened thermocline that separates cold surface water from warmer CDW, reducing the contact area between CDW and the ice-shelf base. Ultimately, a deeper thermocline reduces the volume of warm water in contact with the ice-shelf base, resulting in reduced basal melting, slower ice shelf thinning and a slower decline of glacier buttressing and consequently, less ice discharge. Furthermore, Hillenbrand et al. linked the Early Holocene (~10.4 to 7.5 kyr BP) transition from warm CDW-like to cooler Antarctic Surface Water (AASW)-like conditions in the Amundsen Sea to a northward (equatorward) shift of the SHWW. This interpretation was based on δ¹³C gradients (low to high) between cores PS75/167 and PS75/160, and supported by multi-proxy data from Campbell Island and the southwest Pacific Ocean, which together suggest a pole-to-equator migration of the westerlies after 10 kyr BP. A weakening of the SHWW is also inferred from decreasing sea surface temperature (SST) data on the western Antarctic Peninsula shelf after ~9 kyr BP. Our findings support this hypothesis and additionally reveal a pronounced decrease in BWT after ~10.1 kyr BP, coinciding with the hydrographic transition from CDW- to AASW-like conditions.

These results suggest that the mean latitudinal position of the SHWW played a dominant role in modulating CDW inflow onto the shelf during the Early Holocene but its role in on-shelf incursions of CDW immediately after the LGM remains uncertain. Southern Ocean planktic foraminiferal δ¹⁸O records indicate a ~4° equatorward shift and about a 25% weakening of the SHWW during the LGM compared to the Mid-Holocene (about 6,000 years ago) while alkenone-derived SST reconstructions from Chatham Rise (east of New Zealand) and the east coast of Chile record major warming between ~19.2 and ~12.1 kyr BP. The warming trend is linked to the poleward displacement of the SHWW and the Antarctic Circumpolar Current (ACC). A ~6° poleward shift of the ACC following the LGM is further supported by biomarker-based SST reconstructions from the southern Indian Ocean. The latitudinal position of the SHWW thus emerges as a key control on the exchange of warm water onto the Amundsen Sea shelf. A poleward-shifted SHWW would bring the ACC closer to the continental margin, promoting upwelling and enhancing basal melting. A similar mechanism has been proposed for deglaciation in the western Ross Sea (JOIDES Trough) where retreat after ~19.6 kyr BP coincides with higher subsurface ocean temperatures linked to a southward SHWW shift. However, a notable feature of our CDW record is that it is largely binary through the deglaciation rather than displaying a gradual increase in BWT as the SHWW and ACC migrated poleward. This possibly suggests the existence of a latitudinal threshold beyond which CDW incursions are triggered, or that other mechanisms were also influential. In this context, ocean-driven ice sheet retreat following the LGM may have been amplified by rising sea levels, with additional positive feedback between sea-level rise and the landward deepening Amundsen Sea shelf. Indeed, Smith et al. previously linked a speed-up in grounding line retreat rates in the Amundsen Sea around 14 kyr BP to a positive feedback between sea-level rise (and particularly meltwater pulse 1A) and the retrograde seabed, which deepens significantly inboard of the mid-shelf. In contrast, surface air temperatures likely played a subordinate role to CDW and sea-level during deglaciation, given their limited impact on the current mass balance in the Amundsen Sea sector. Past atmospheric warming would have needed to exceed modern levels by several degrees to drive substantial retreat via surface melting, which is not supported by ice core data. The ultimate cause of latitudinal shifts of the SHWW is debated but likely involved large-scale climate teleconnections. Menviel et al. proposed that a cessation of North Atlantic Deep Water (NADW) formation during Heinrich Stadial 1 (HS1: 18.2–14.6 kyr BP) cooled the North Atlantic and warmed the South Atlantic, shifting the Inter-Tropical Convergence Zone southward which strengthened and shifted the SHWW poleward. In the Amundsen Sea, our data indicate that this reorganisation led to enhanced upwelling of warm CDW onto the shelf, which in turn drove WAIS retreat. However, a fully coupled ocean-atmosphere-ice sheet modelling is needed to fully elucidate the links between global climate, the SHWW position and the interplay between CDW, rising sea level and ice-sheet retreat in the Amundsen Sea.

Divergent drivers of ice sheet change during the Holocene?

Consistent with the earlier study of Hillenbrand et al. we link the well-documented grounding line stability along the Marie Byrd Land coast during the Early Holocene to the observed reduction in CDW upwelling after ~10.1–7.9 kyr BP. An additional observation is that surface air temperatures continued to increase for several millennia, reaching a Mid-Holocene optimum, yet had no significant impact on the grounding lines of Pope, Smith, Thwaites and Pine Island glaciers. This is consistent with the idea that CDW was the dominant driver of past ice-sheet variations and supports modelling results, which indicate that warm water accessing ice shelf cavities and the ice-sheet margin leads to faster retreat. Conversely, once the ocean driver is removed or muted, the grounding line can stabilise on high points in the bed. However, while the grounding lines of Pope, Smith, Thwaites, and Pine Island glaciers remained relatively stable during the Holocene, ice shelves and glaciers on the eastern side of the embayment continued to evolve. The Early-to-Mid-Holocene retreat of the Cosgrove Ice Shelf has been primarily attributed to atmospheric warming, and specifically the strengthening of the Amundsen Sea Low linked to warming in the tropical Pacific. This may imply divergent drivers during the Holocene, whereby variations in WAIS retreat along the Marie Byrd Land coast were muted in response to a reduction in CDW, while other parts of the glacier system remained sensitive to enhanced or peak atmospheric warmth from ~6 to 2 kyr BP. In this context, the depth of the thermocline, modulated by shelf-edge wind stress or other thermodynamic atmospheric processes, might be the critical factor governing past ice-sheet variability. Modelling indicates two ‘threshold’ thermocline depths (700 and 1000 m). When the thermocline shoals above 700 m, grounding lines becomes highly sensitive to melt forcing, potentially triggering rapid and irreversible ice-sheet retreat. Conversely, when the thermocline is lowered below 1000 m, melt rates may be sufficiently reduced to allow ice-shelf thickening and theoretically, ice-sheet re-grounding. A similar threshold may have operated in the past, whereby oceanic conditions in the Amundsen Sea oscillated between warm and cool states on centennial to millennial timescales depending on thermocline depth. During the Holocene, a more northerly position of the SHWW/ACC reduced the influx of warm CDW onto the shelf, deepening the thermocline—similar to conditions observed during contemporary La Niña phases. Under these conditions, incursions of warm CDW onto the shelf still occurred, but at greater depth, below our core records from above 800 m water depth, and therefore did not induce significant ice-shelf or ice-sheet melting along the Marie Byrd Land coast. A deeper thermocline may also help reconcile microfossil data suggesting that incursions of CDW persisted in deep basins, such as Ferrero Bay, during the Mid-to-Late Holocene. However, the benthic foraminifer Bulimina aculeata, used in Minzoni et al. as an indicator of CDW, may not be uniquely diagnostic, because it is only present in some modern surface sediments presently bathed in CDW. Finally, the role of the SHWW in modulating mid-to-late Holocene oceanographic conditions appears complex. While multiple lines of evidence point to an equatorward SHWW shift during the Early Holocene coincident with reduced BWT and muted ice-mass variations in the Amundsen Sea sector, other data suggest that the latitudinal position of the SHWW varied throughout the Holocene without consistently affecting BWT in the Amundsen Sea. Again, this could indicate additional drivers are important i.e., growing influence of ENSO during the Holocene, or simply reflect the variable temporal resolution of our Holocene palaeoceanographic dataset, which is unlikely to capture higher-frequency, decadal-scale variability. What is clear however, is that the amplitude of ocean thermal forcing was greatest during the post-LGM deglaciation (18-10.1 kyr BP) and was reduced during the Holocene, perhaps intensifying again in the twentieth century in response to a combination of consistent ENSO during the 1940s followed by enhanced anthropogenic warming after the 1950s.

Implications for future WAIS retreat

Glaciers in the Amundsen Sea drainage sector are retreating rapidly, with no indication that these changes will be reversed in the coming century. Contemporary mass loss is linked to the poleward transport of warm Circumpolar Deep Water (CDW), which has intensified in recent decades, with the magnitude of ocean-driven melting being considered the key variable determining future sea-level rise from Antarctica. Our reconstruction of BWT and water-mass variability (δ¹³C) suggests that similar processes operated in the past, whereby embayment-wide ice-sheet retreat coincided with the presence of warmer CDW—likely tied to the poleward displacement of the SHWW/ACC following the LGM. Conversely, when the SHWW/ACC were displaced equatorward, reduced CDW inflow limited the delivery of warm water onto the shelf, allowing glacier advance (as during the LGM) or, in the Holocene context, to a period of WAIS grounding-line stability along the Marie Byrd Land coastline. Given that climate models predict a consistent strengthening and poleward displacement of the SHWW in the future, due to enhancement of the SAM as well as a trend toward stronger and more frequent El Niño, the WAIS will likely continue to retreat through mechanisms similar to those reconstructed for the past. Furthermore, since the subglacial beds of Pine Island and Thwaites glaciers deepen inland, pulses of rapid grounding line retreat that have been documented in the past may become more frequent in the future, amplified by positive feedbacks associated with widening ice-shelf cavity geometries and marine ice sheet instability.

Methods
Core material and chronology

Our coring strategy targeted shallower sites because prior experience indicated that this increases the likelihood of recovering records that contain calcareous microfossils but are situated deep enough to capture a CDW signal. Gravity cores PS75/192 (71.74’S, 103.32’W) and PS75/190 (71.87’S, 103.38’W) were collected from the outer shelf (Abbot Trough) during expedition ANT-XXVI/3 of RV Polarstern in 2010, with core recoveries of 2.14 and 2.96 m, respectively. Water depth at site PS75/192 was 793 m and 775 m at site PS75/190. The age models for these sites are from Klages et al. and were generated by AMS ¹⁴C dating on calcareous microfossils in combination with the measurement of relative paleomagnetic intensity (RPI). All previously published ages are presented as calibrated years before present (where present is 1950 CE), unless otherwise stated. New ¹⁴C dates from core VC436 generated for this study were converted to calendar ages using the Marine20 radiocarbon calibration applying a delta ΔR of 813±30 years (#522 in the marine radiocarbon database, http://calib.org/marine/). Calibration was performed using Calib v. 8.1.0. Abundance of calcareous foraminifera is variable downcore but yielded sufficient foraminifera to measure stable isotopes (n=7) and trace elements (n=5) in a handful of horizons. Gravity cores PS75/160 (74.56’S, 102.62’W) and PS75/167 (74.62’S, 105.80’W) were collected on the inner shelf of Pine Island Bay during expedition ANT-XXVI/3. Core recovery at PS75/160 (337 m) and PS75/167 (526 m) were 6.7 m and 9.3 m, respectively. The chronology of these cores has been constrained using AMS ¹⁴C dating on calcareous microfossils. Core PS75/160 spans an age range from 9.2 kyr BP to 1.2 kyr BP. Absence of sediments younger than ~1.2 kyr BP implies the loss of seafloor surface sediment during coring or its recent erosion by either iceberg scouring or intense bottom currents. Core PS75/167 spans an age range from 10.4 kyr BP to 8.2 kyr BP. Lack of calcareous microfossils in the uppermost sediments likely reflects site-specific factors, such as local sea-ice cover or calcite dissolution.

Core BC431 (72.30’S, 118.16’W) was collected using a box corer (BC) on expedition JR141 with RRS James Clark Ross in 2006 from a water depth of 512 m. The chronology of BC431 is based on a constant rate of supply (CRS) ²¹⁰Pb age model (Supplementary Fig. 4, 5). ²¹⁰Pb and ¹³⁷Cs activities were measured on 1 cm thick sediment slices by gamma-spectrometry using Canberra ultra low-background Ge-detectors in the Department of Geography, Durham University. The core showed surface contents of unsupported ²¹⁰Pb of about 81 mBq/g with a tendency for exponential decline with depth in the upper 6.5 cm (Supplementary Fig. 4). Below about 6 cm core depth the ²¹⁰Pb activity was at the detection limit or lower. ¹³⁷Cs concentration was below the detection limit throughout. The 5.9 m long vibrocore VC436 (71.81’S, 117.43’W) was also recovered on expedition JR141 from a water depth of 479 m. The chronology of VC436 is from Smith et al. and newly generated AMS ¹⁴C ages on calcareous microfossils. Chronological data indicate an age reversal at ~60 cm depth in this core, which has been attributed to nearby iceberg scouring. However, here we focus only on the interval from 580 to 160 cm, which yielded ¹⁴C dates in stratigraphic order with an age range of 17.97–9.79 kyr BP.

Trace metal analyses and BWT calibration

Sediment samples were washed through a 63 µm sieve and dried for 24 h at 40 °C at the Godwin Laboratory for Palaeoclimate Research at the Department of Earth Sciences, University of Cambridge (UK). The >63 µm fraction was then dry-sieved and specimens of T. angulosa were picked from the 250–355 µm fraction. All picked specimens of T. angulosa belonged to the costate (ribbed) morphotype. T. angulosa is a shallow infaunal species living 0–5 cm below the seafloor surface which is considered less likely to be influenced by the effects of carbonate ion, [CO₃²⁻], concentration on the Mg/Ca ratios of their tests than epifaunal species. In this environment, porewaters tend to equilibrate rapidly with respect to carbonate chemistry, and the saturation state remains near zero. This minimizes the influence of ambient [CO₃²⁻] on calcification. Elderfield et al. demonstrated this effect for Uvigerina spp. and given the close genetic relationship between Uvigerina and Trifarina spp., it is reasonable to assume that Trifarina responds similarly—i.e., that any [CO₃²⁻] effect on Mg/Ca is negligible.

Samples were visually inspected for preservation state, and no obvious signs of degradation/alteration were observed (Supplementary Fig. 9). Approximately 30 specimens were gently crushed between two glass plates and visible contaminants were removed using a fine paintbrush. The fragments then underwent a cleaning procedure adapted from Boyle and Keigwin. This includes: (i) the removal of clay by ultrasonication of test fragments 3x in 18.2 MΩ.cm deionised water and 3x in methanol; (ii) a reductive step to remove any metal oxides using a solution of hydrous hydrazine and citric acid in ammonia; (iii) an oxidative step using a solution of hydrogen peroxide in sodium hydroxide to remove organic matter; and (iv) a dilute nitric acid leach using Optima pure nitric acid. Between the mechanical clay removal step and the reductive step, samples were examined under a binocular microscope and any obvious non-carbonate particles removed. The cleaned samples were dissolved in ultra-pure nitric acid (0.1 M HNO₃), centrifuged, and a ‘concentrate’ aliquot was transferred to a new sample vial to prevent leaching from any remaining contaminants. Trace metal data was analysed using ICP-OES following the method of de Villiers et al. Long-term instrumental precision of element ratio data determined by replicate analyses of a consistency standard prepared at University of Cambridge with a magnesium/calcium ratio = 1.3 mmol/mol is ±0.55%. Accuracy of Mg/Ca determinations was confirmed by replicate analyses of standard JCt-1, where mean Mg/Ca of 1.264±0.005 mmol/mol (r.s.d. 0.43%), measured on 13 occasions during this study, was obtained, in agreement with published results. The cleaning efficiency was monitored by measuring Mn/Ca and Fe/Ca. Maximum Mn/Ca and Fe/Ca are 1.06 and 0.19 mmol/mol (ave. 0.25 and 0.05 mmol/mol) respectively. A weak positive correlation is observed between Mn/Ca and Mg/Ca for the different records (Supplementary Fig. 8). To assess the impact of any diagenetic Mg contribution on the Mg/Ca data we applied a correction to the Mg/Ca data, assuming a Mg/Mn diagenetic coating of 0.15 ± 0.05 mmol/mol following Hillenbrand et al. This value reflects the average composition of Mn nodules and ferromanganese crusts from the South Pacific and the Pacific sector of the Southern Ocean. The corrected Mg/Ca values exhibit the same overall down-core trend as the uncorrected data, though with reduced amplitude, with elevated Mg/Ca ratios prior to ~10 kyr BP (Supplementary Fig. 8a–e). Importantly, the use of alternative Mg/Mn ratios (e.g., 0.20 and 0.25 mmol/mol) for diagenetic coatings does not alter the observed difference between the deglacial and Holocene periods (Supplementary Fig. 8a-e), supporting the robustness of this transition. Propagating both sources of analytical error (σ Mg/Ca =0.005 mmol/mol and σ Mg/Mn =0.05 mmol/mol) yields a mean uncertainty of ±0.0503 mmol/mol for the Mn corrected Mg/Ca dataset. This was further explored using a Monte Carlo approach to probabilistically evaluate the uncertainties (Supplementary Fig. 1a). Monte Carlo simulations (n=10000) produce a mean 2 s.d. uncertainty of ±0.0577 mmol/mol (Supplementary Data 1).

Calculation of BWT followed the Antarctic-wide calibration for T. angulosa from Mawbey et al., which used surface sediment samples from various sectors of the Antarctic shelf. As noted above, applying this calibration (green line Supplementary Fig. 2a) produces an unrealistic temperature range (Supplementary Data 1). Overestimation of ocean temperatures is unlikely to be an analytical error, given our close approximation of Mg/Ca in reference materials/standards. We also rule out diagenetic or reworking issues because of the excellent preservation state of individual tests, which were all inspected under light microscope prior to analysis. Rather, the unrealistic BWT likely reflects the increased uncertainty in the calibration for low BWT due to the limited number of data points in this range. It would be possible to apply a scaling factor of 0.3 and remain within the confines of the 95% confidence intervals of the calibration (blue line Supplementary Fig. 2) to produce BWT closer to the measured modern-day temperature range (-1.89 to +1.79 °C; Supplementary Data 1) but this would require verification by further modern surface sediment calibration. This approximation might offer an alternative input for ice-sheet modelling studies, which aim to investigate the role of ocean forcing on ice-sheet variability. Such studies have previously used outputs from an archived atmospheric-oceanic global climate model simulation that potentially omits regional variability. Propagation of the replicate error (σ Mg/Ca =0.005 mmol/mol), diagenetic coating error (σ Mg/Mn =0.05 mmol/mol) and the calibration error (σ calib =0.098 mmol/mol) gives a 2 s.d. uncertainty of ±1.6 °C for calculated BWT (Supplementary Fig. 2). The calibration uncertainty is estimated following Cléroux et al., based on the standard error of the regression coefficient reported by Mawbey et al.

Stable carbon isotope analyses

In total 3-5 specimens of T. angulosa were hand-picked under a binocular microscope from the >250 μm fraction. δ¹³C analyses were undertaken using a IsoPrime dual inlet mass spectrometer with Multiprep device at the NERC Isotope Geosciences Laboratory in Keyworth (UK). The δ¹³C data are reported as per mil (‰) deviations of isotopic ratios (¹³C/¹²C), calculated to the VPDB scale using a within-run laboratory standard calibrated against NBS-19 standards. Analytical reproducibility of an in-house calcite standard (KCM) is <0.1‰ for the δ¹³C data. The analyses had an average 2 s.d. uncertainty of 0.06‰ on a 2‰ standard for δ¹³C across the analytical period. The δ¹³C of the samples were corrected by +0.85‰ following Hillenbrand et al. to account for species specific vital effects.

Change point analysis

Standard mean change point (CP) analysis was calculated using the ‘changepoint’ package in R to determine the periods when significant trend changes occurred in the Mg/Ca and δ¹³C datasets (Supplementary Fig. 6). Analysis was performed on both the split (inner and outer shelf) and composite record. Segmented mean analysis of the inner shelf data revealed change points (CPs) at 8248 yr BP for Mg/Ca and 7977 yr BP for δ¹³C. For the outer shelf record, the Mg/Ca dataset exhibited two CPs, at 11043 yr BP (indicating a shift to warmer BWT) and 10050 yr BP (shift to cooler BWT). A single CP at 10911 yr BP was identified for the outer shelf δ¹³C data. Analysis of the composite record yielded consistent CPs at 10135 and 9113 yr BP for Mg/Ca, and 10911 and 7938 yr BP for δ¹³C, which is used to define the transition from warmer (10135 yr BP) to cooler (7938 yr BP) BWTs. We attribute the minor offsets between Mg/Ca- and δ¹³C-derived CPs to the higher resolution of the δ¹³C record.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review for NCOMMS-25-17907-T\nMawbey et al. present carbon isotope and Mg/Ca data for the Amundsen Sea\nEmbayment (ASE), a critical region for understanding the trajectory of future sea-level\nrise and relate this to the presence/absence of Circumpolar Deep Water (CDW), a\nprobable driver of ice loss. The study extends records beyond the Holocene and is\ntherefore an important contribution to understanding the long-term nature of the West\nAntarctic Ice Sheet. However, I find several problems that need to be addressed before\npublication:\n1. D13C and Mg/Ca records from all sites are combined in Fig. 2 and 3a to\nrepresent changes in CDW versus AASW for the ASE in the text. However, Site\nPS75/167 and PS75/160 is in the inner ASE, several hundred kilometres from\nPS75/191, PS75/190, VC436 and BC431 on the outer shelf. These records should\nbe separated and discussed separately: one record for the outer shelf discussing\nCDW upwelling; the other for the inner ASE discussing incursion further south\n(although this is already covered in Hillenbrand et al. 2017 but the Mg/Ca can be\nused to confirm these findings).\n2. Following from the previous point, I think the records of outer shelf CDW\npresence/absence should be highlighted as the main finding of this study as\nthese are new records that extend to ~18 ka BP. Any conclusions for the inner\nASE should be toned down as these stem from the previous study of Hillenbrand\net al. (2017) and start at ~10 ka BP.\n3. The trends in the data need to be handled statistically instead of relying on the\nreader to pick out the perceived trends in the data, which are subjective. For\nexample, change-point analysis could be used to determine if the jump at ~10 ka\nis statistically significant. I expect the d13C would see a significant jump, but the\nMg/Ca ratios would not. This should be done after the inner ASE records\n(PS75/167 and PS75/160) are separated from the outer ASE (PS75/191,\nPS75/190, VC436 and BC431).\n4. You mention the carbonate ion e]ect but don’t state how you determined if this\ne]ect was influencing your data. Some evidence to show constant carbonate ion\nconcentrations or some other screening method would be vital here. The ASE is\nunder influence from large amounts of freshwater discharge from nearby glaciers\nsuggesting variable alkalinity across the shelf. Carbonate preservation also\nappears to be highly variable suggesting a dynamic carbonate ion concentration\nacross the shelf.\n5. The wider interpretation is very selective and focuses on a marine geoscience\nperspective, a common problem among marine paleoclimate records, whilst\nneglecting to mention other drivers of ice loss since the LGM such as\natmospheric and sea level forcing. More southernly SHW may drive CDW\nincursions onto the shelf, but it also brings warm, moist air to West Antarctica\nleading to surface melting. Sea-level rise from the LGM to Holocene would have\ndestabilised ice sheets leading to dynamic calving. It is likely the combination of\nwarm ocean and atmospheric currents and sea level rise would have combined\nto drive ice loss post LGM. This paper supports the ocean forcing contribution\nwhich is important but not the whole story. A wider context is therefore needed.\n6. We see a gradual increase in sea level and southward transition of the SHW in\nFig. 3, but Mg/Ca ratios are flat. Why do we not see gradually more CDW on the\nshelf with this change? This suggests the d13C and Mg/Ca ratios are instead\ndisplaying a binary signal, i.e., CDW is either present or not, which needs to be\ndiscussed. Furthermore, other records for SHW migration show a continued\npoleward migration until the mid-Holocene. This may suggest ocean forcing is\nimportant before 10 ka but atmospheric or sea level forcing is important after 10\nka – an interesting finding that has not been discussed.\nAfter these changes are made, I expect you will have an important study of outer shelf\nCDW changes that can inform on the contribution of ocean forcing on ASE ice loss, and\nmaybe more muted CDW influence on continued loss during the Holocene. This finding\nshould be placed within the context of other important forcing mechanisms such as\natmospheric currents and sea-level rise.\nOther comments:\nFig. 1 b and text referring to ASE reconstructions: this reconstruction and the relevant\ntext refers to the pre-Holocene records and might be misleading. Several studies since\nhave shown extensive ice shelf retreat during the Holocene. For example, Minzoni et al.\n(2017) recorded around 50 km retreat of the Cosgrove Ice Shelf during the Holocene.\nFurthermore, ice loss post LGM was of an unbounded ice shelf on the outer ASE. It is ice\nshelf retreat when the ice shelf is bounded by topographic highs in the embayment,\nsuch as during the Holocene, that is important for understanding buttressing related to\ninstabilities mentioned in the introduction (e.g., Pattyn, 2018).\nLine 125: here, I think you need to highlight the distance between these sites (several\nhundred kilometres) and separate the sites into 2 or 3 records. The inner ASE (PS75/167\nand PS75/160) and western outer ASE (VC436 and BC431) and eastern outer ASE\n(PS75/191 and PS75/190). The data in the plots should be separated into di]erent\npanels accordingly and discussed separately.\nLine 139: expand on how you discount carbonate ion concentration influence on you\nMg/Ca ratios. As you mention in line 131, carbonate preservation is highly variable\nacross the shelf suggesting carbonate ion concentration is highly variable.\nLine 152: I am having trouble following this comparison. Maximum CDW temperatures\nof >1.5oC is confusing. Just put the range of CDW values.\nFig. 2: Separate the data into 4 panels i.e., red and blue circles in two panels and open\nsquares, green triangles, and purple triangles in the other two. Change-point analysis\nshould be conducted to see if there is a statistically significant shift in the data. Other\ntests are available such as ANOVA. There appears to be no change in the Mg/Ca ratios\nwhen split between inner and outer ASE, but statistical tests will be able to tell you.\nLine 173: There is quite a lot of scatter in the Mg/Ca ratios. If you focus on the outer ASE\nrecords, I cannot see a clear cooling trend. Do you have any statistics that support this\nassertion? D13c is a bit clearer but there is still a lot of scatter.\nLine 179: I think this quite a narrow view. There are lots of records of Holocene change,\nthey are just not related to ocean forcing (e.g., Minzoni et al. 2017; Sproson et al., 2022;\nJohnson et al., 2014). Sea level was also a likely contributor to ice loss in the post LGM\nperiod that may not be as important now.\nLine 197: This is misleading. I think you can say changes across the outer shelf but not\nembayment-wide as the inner ASE record starts at ~10ka.\nLine 205: This sentence contradicts the next sentence. Lots of evidence for Holocene\nretreat (e.g., Minzoni et al. 2017; Sproson et al., 2022; Johnson et al., 2014; 2017; 2020)\nand it is likely that the final 10s of kilometres of retreat (during the Holocene) are the\nmost important for understanding contemporary ice loss as this is when ice shelves\nbecome pinned by plateaus causing the buttressing e]ect (Pattyn, 2018).\nLine 213: CDW would have contributed to grounding line retreat. What about\natmospheric and sea level forcing?\nLine 218-221: This is a finding from Hillenbrand et al. (2017) and nothing new here\nexcept further support from Mg/Ca ratios.\nLine 232-233: I think this is the main finding of the study and should be the main\nhighlight. You can say a lot about incursion onto the outer shelf from 18 ka. Not a lot can\nbe said about the inner ASE as that record begins at 10 Ka.\nLine 244-255: How does this mechanism interact with sea-level rise? Could rising sealevel\nbring CDW further up the shelf? We see a rise from 20 to 8 ka BP in Fig. 3f.\nFig. 3a: the inner and outer records need to be separated out and treated\nindependently.\nFig. 3c and d: Other records for SHW migrations are available that show continued\npoleward migration during the Holocene (Saunders et al., 2018; Voight et al., 2015; Ai et\nal. 2021). SHW migrations are a contentious issue and hotly debated so you should\nreflect this in the discussion as a possibility and not something that is unequivocally\nknown.\nFig. 3e: More important here is to look at accumulation rates in relation to temperature\nfor the WAIS Divide core (Fudge et al., 2016). Anomalous accumulation rates in the\npost-LGM era suggest atmospheric forcing of the Amundsen Sea sector of the WAIS.\nFig. 3g: sea-level is rising when CDW is present. How does sea-level influence CDW on\nthe shelf? This is not discussed.\nLine 301-305: This is pushing it to say the least. I don’t think you can say this based on\nyour data. Retreat continued into the Holocene; it is just ocean forcing played a\ndiminished role (Sproson et al., 2022). Modelling suggests a greater contribution of\ntropical warming on warm air influencing the WAIS during the Holocene (Skinner et al.,\n2020; Dang et al., 2020; Scott et al., 2019).\nLine 310: and surface melt from atmospheric warming (Ding, 2011; Steig et al., 2013;\nWille et al., 2019).\nLine 314: SHW poleward migration also caused warm, moist air to be delivered to West\nAntarctica leading to surface melting.\nReferences:\nAi, X. E. et al. Southern Ocean upwelling, Earth’s obliquity, and glacial-interglacial\natmospheric CO2 change. Science 370, 1348–1352 (2020).\nDang, H. et al. Pacific warm pool subsurface heat sequestration modulated Walker\ncirculation and ENSO activity during the Holocene. Sci. Adv. 6, eabc0402 (2020).\nDing, Q., Steig, E. J., Battisti, D. S. & Küttel, M. Winter warming in West Antarctica\ncaused by central tropical Pacific warming. Nat. Geosci. 4, 398–403 (2011).\nFudge, T. et al. Variable relationship between accumulation and temperature in West\nAntarctica for the past 31,000 years. Geophys. Res. Lett. 43, 3795–3803 (2016).\nJohnson, J. S. et al. Rapid thinning of Pine Island Glacier in the early Holocene. Science\n343, 999–1001 (2014).\nJohnson, J. S. et al. The last glaciation of Bear Peninsula, central Amundsen Sea\nEmbayment of Antarctica: Constraints on timing and duration revealed by in situ\ncosmogenic 14C and 10Be dating. Quat. Sci. Rev. 178, 77–88 (2017).\nJohnson, J. S. et al. Deglaciation of Pope Glacier implies widespread early Holocene ice\nsheet thinning in the Amundsen Sea sector of Antarctica. Earth Planet. Sci. Lett. 548,\n116501 (2020).\nMinzoni, R. T. et al. Oceanographic influences on the stability of the Cosgrove Ice Shelf,\nAntarctica. Holocene 27, 1645–1658 (2017).Hillenbrand, C.-D. et al. West Antarctic Ice\nSheet retreat driven by Holocene warm water incursions. Nature 547, 43–48 (2017).\nPattyn, F. The paradigm shift in Antarctic ice sheet modelling. Nat Commun 9, 2728\n(2018).\nSaunders, K. M. et al. Holocene dynamics of the Southern Hemisphere westerly winds\nand possible links to CO2 outgassing. Nat. Geosci. 11, 650–655 (2018).\nScott, R. C., Nicolas, J. P., Bromwich, D. H., Norris, J. R. & Lubin, D. Meteorological\ndrivers and large-scale climate forcing of West Antarctic surface melt. J. Clim. 32, 665–\n684 (2019).\nSkinner, C. B., Lora, J. M., Payne, A. E. & Poulsen, C. J. Atmospheric river changes\nshaped mid-latitude hydroclimate since the mid-Holocene. Earth Planet. Sci. Lett. 541,\n116293 (2020).\nSproson, A. D., Yokoyama, Y., Miyairi, Y., Aze, T. & Totten, R. L. Holocene melting of the\nWest Antarctic Ice Sheet driven by tropical Pacific warming. Nat. Commun. 13, 2434\n(2022).\nSteig, E. J. et al. Recent climate and ice-sheet changes in West Antarctica compared\nwith the past 2,000 years. Nat. Geosci. 6, 372–375 (2013).\nVoigt, I. et al. Holocene shifts of the southern westerlies across the South Atlantic.\nPaleoceanography 30, 39–51 (2015).\nWille, J. D. et al. West Antarctic surface melt triggered by atmospheric rivers. Nat.\nGeosci. 12, 911–916 (2019).", "reviewer_2": "Smith et al present new d13C and Mg/Ca data from the Amundsen Sea Embayment (ASE). The dataset advances our\nunderstanding of ocean forcing on ice sheet retreat. It extends the sparce temporal Holocene constraints published by\nHillenbrand et al. (2017) Nature, back through to the last deglaciation for this critical West Antarctic glacier system. The\npower of the new data is the demonstration of two modes/states of the ocean system, and associated ice sheet (in)stability.\nDuring 18-10ka the average temperature of bottom water on the outer shelf was warmer compared to after 10ka when bottom\nwater conditions were cooler. Temperature profiles near respective core sites show how incredibly warm CDW are, and\nproximal to the ice sheet.\nThe nature of continental shelf means that numerous core sites across the ASE are needed to measure both a meaningful\nnumber of rare foraminifera samples, and capture fully the last deglaciation. The novelty in the findings relies on the record\ngenerated in V436 which covers the deglaciation back to 18.2 ka. A step change in ocean conditions is observed which the\nauthor link to ice sheet retreat based on existing multi-proxy reconstructions of the region. The significance of this work is the\nclear shift in baseline bottom water temperature conditions, which puts into context the millennial variability observed since\nthe early Holocene in Hillenbrand et al. (2017). This study suggests the same mechanism proposed by Hillenbrand et al.\ncontrolled the BWT on the shelf, with the additional details provided by ENSO observations on the controls on thermocline\ndepth in setting the two modes of ocean forcing.\nThe data are of the highest quality analytically, the age models are clear, and the authors have dealt conservatively with\nextension of proxy data to temperature given the limits of the calibration/observations at the cooler end.\nThe manuscript is well written, comprehensively building on and advancing the latest understanding of this important sector\nof the ice sheet. I fully recommend publication of the manuscript. Below are considerations for the authors.\nComments:\nCan the authors provide an explanation for the wide range on d13C, Mg/Ca. It is not only the late Holocene where a wide\nrange of values are observed over ~1000-500 years, the deglacial section also show ~0.3-0.4 mmol/mol Mg/Ca range. Do\nthe proxies capture the large seasonal variability in thermocline and water mass properties observed on the shelf? The\nstrength of the new data presented is that it’s time resolution is sufficient to observe the mode of forcing and provides context\nfor centennial scale variability in CDW proxies. Can the author provide insight into controls on sea ice thresholds, or have\nany data to support threshold conditions in sea ice state? Very different sea ice conditions likely contribute to the depth of the\nmixed layer/thermocline.\nMore clarity around the temperature ranges is needed: seasonal variability in deep water temperature on Sup Fig 3, are\nthese from mooring measurements over 1,2 or 3 years? How can the variability be larger than the min and max values at\neach site? It would be helpful to show where these profiles are from if multiple CTD stations were compiled? Or add\nadditional plots to show the raw data from which the averages are compiled. These observations could support explanation\nof variability in proxy data related to previous comment?\nFigure 2: Suggest you match the colour of squares in the d13C plot to the legend colour, as it’s different to the colour of\nsquares in the Mg/Ca plot beneath, and the colour of the square symbol in Figure 1.\nFigure 3: misspelling of alkenones in c.", "reviewer_3": "The manuscript by Mawbey and co-authors deals with the retreat of the west Antarctic Ice Sheet and how it evolved during\nthe last 20 ka. In detail the authors provide new geochemical constraints on the potential role of CDW in driving the retreat or\nthe stabilization of WAIS.\nOverall, this is an interesting and very timely study that provides an interesting dataset and interpretation. The conclusions\ndrawn by the authors are for sure suitable for a journal like Nature Communications.\nSaying this, I have to raise a couple of points and comments regarding the current version of the manuscript. Based on the\nsum of these comments and suggestions and their implications I recommend to revise the submitted manuscript.\nMy points are listed below in the order they appear in the text.\nLines 59-60: by how much did the mentioned temperature increase in this interval? This would be good to mention to\ncompare it with the findings of this study.\nData in figure 2: The authors mention that the Mg/Ca values and therefore ultimately their BWT data have been corrected for\ndiagenetic coatings. They refer to the method section but there it is only stated that the correction was use following another\nstudy. Since the Mg/Ca data look very different between different sites and there is a large potential for trends being\ndiscussed as real shifts in temperate are simply changes between sites and therefore different preservation of the used\nforaminifera, it is necessary to show the Mn/Ca data and the responding corrections n a cross plot against Mg/Ca and do this\nsite specific so the reader can judge how much trend and difference is site specific or how much might be a real signal.\nWithout this information it is nearly impossible to check the reliability of the presented data.\nAnother aspect here would be the information of error bars for the data shown. How much of an error is added when the\nMn/Ca correction is done? For the shown BWT data, this would add to the measurement error and the error of the used\ncalibration. This combined error is nowhere in the manuscript shown or mentioned. Since the changes in Mg/Ca are rather\nsmall within a single core, large parts of the interpreted signal might be in error of the method used. I do not say (and also do\nnot believe) that the signal is not true, but the reader needs to know this information to judge the reliability of the data\npresented.\nLines 189-191: the mentioned trend is only observed in a single datapoint that is a little bit higher than the others of site\nPS75/167 and for Site PS75/190 it is also only one point that is still relatively close to the values of the other (younger) data\nfrom this site. So this comes again back to the discussion about the error of the measurements.\nLines 218-221: What is this statement related to in figure 2? I do not see such a shift in the data (definitely not outside the\nestimated error).\nLine 231: here the authors infer a ‘pronounced decrease in BWT’. This is simply not seen in the data and needs much more\ndiscussion and careful phrasing when errors are involved.\nFigure 3: The authors discuss and interpret the data of the last decades as a real increasing trend in BWT and infer that this\nis supporting the observations of a melting WAIS. However, the data are all very scattered for a single site and the observed\ntrend is well in the range of the scatter observed for the other sites. So how can this be treated as a real trend?\nMethods: It would be great to have some SEM images of the used foraminifera from the single sites to get a feeling for the\npreservation. Just under the light microscope it is impossible to judge recrystallization or dissolution."}, "author_response": "Review for NCOMMS-25-17907-T\nWe thank the reviewers for their thorough and constructive feedback. Below we provide a\npoint-by-point response to these comments (response in red). Line numbers refer to text\nedits in the ‘clean’ (rather than track-change) version.\nReviewer #1 (Remarks to the Author):\nMawbey et al. present carbon isotope and Mg/Ca data for the Amundsen Sea\nEmbayment (ASE), a critical region for understanding the trajectory of future sea-level\nrise and relate this to the presence/absence of Circumpolar Deep Water (CDW), a\nprobable driver of ice loss.\nThe study extends records beyond the Holocene and is therefore an important contribution\nto understanding the long-term nature of the West Antarctic Ice Sheet. However, I find\nseveral problems that need to be addressed before publication:\n1. D13C and Mg/Ca records from all sites are combined in Fig. 2 and 3a to represent\nchanges in CDW versus AASW for the ASE in the text. However, Site PS75/167 and\nPS75/160 is in the inner ASE, several hundred kilometres from PS75/191, PS75/190, VC436\nand BC431 on the outer shelf. These records should be separated and discussed\nseparately: one record for the outer shelf discussing CDW upwelling; the other for the inner\nASE discussing incursion further south\n(although this is already covered in Hillenbrand et al. 2017 but the Mg/Ca can be used to\nconfirm these findings).\nWe thank the reviewer for their constructive comment regarding the structure of the results\nand the spatial distribution of our core sites. In response, we have restructured the Results\nsection to present and describe the outer and inner shelf records separately (lines 181-204).\nWe hope this distinction improves clarity and more explicitly acknowledges spatial variability\nacross the shelf.\nAs noted in our original submission, the spatial and temporal variability in our dataset—split\nbetween outer shelf cores (PS75/192/190, VC436, and BC431) and inner shelf cores\n(PS75160/167) —is an unavoidable consequence of the region’s glacial history and the\npatchy preservation of microfossil-bearing sediments. These factors have constrained site\n(and sample) availability. It is worth reiterating that together with our US and German\npartners, we have recovered over ~400 sediment cores from the Amundsen Sea\nEmbayment (ASE). Of these, the sites included in this study are the only ones suitable for\ndetailed reconstruction of bottom-water temperature (BWT) and water mass properties over\nthe deglacial to Holocene interval. While in an ideal scenario, sites such as VC436 would\nspan the entire 18–0 kyr BP interval, and we would have comparable microfossil-rich cores\nfrom the mid-shelf for direct comparison with inner shelf sites, such records unfortunately do\nnot exist at present.\nRegarding the reviewer’s question about the representativeness of the outer shelf sites for\noceanographic conditions on the inner shelf: we argue that the outer shelf records do\nprovide meaningful constraints on inner shelf ocean properties. Present-day\noceanographic observations from around Antarctica suggest a broad coherence of water\nmass properties between the inner and outer shelf. Specifically, regardless of the distance\nbetween the ice sheet and the shelf break, the Antarctic Slope Front (e.g., see Whitworth III\net al., 1985; Thompson et al., 2018), act as a relatively porous barrier to exchange between\nthe Southern Ocean and the shelf seas (Schmidtko et al., 2014). On the shelf, near-seabed\nocean temperatures are typically either warm (CDW-like waters) or cold (AASW-like waters),\nwith limited horizontal variability from the ice front to the shelf break.\nTo support this, we include a figure (Figure 1 below) from Zhou et al. (in prep) that presents\na climatological synthesis of near-bottom temperatures across Antarctic shelf seas over the\npast 50 years. Two representative transects, extending from ice shelves to the open ocean\nin both warm shelf (most of West Antarctica) and cold shelf (most of East Antarctica)\nscenarios, illustrate the coherent nature of shelf water masses across spatial scales relevant\nto our study (see also Schmitdko et al. 2014).\nIn addition, numerical modelling supports the view that oceanographic signals recorded at a\nsingle deep location on the shelf can be representative of broader shelf conditions.\nSpecifically, model simulations indicate that the residence time of shelf waters—that is, the\ntime required to fully replace water properties through advection following a regime\nchange—is relatively short. For the Amundsen Sea, residence times are on the order of a\nfew years (Nakayama et al., 2014), while for larger Antarctic shelf seas, residence times are\nestimated to be on the order of a few decades (Naughten et al., 2021). This implies that, in\nthe absence of persistent localized forcing, a coherent signal observed over centennial or\nlonger timescales at a single site is likely to reflect conditions integrated across the broader\nshelf. Consequently, we consider the oceanographic signals recorded in our outer shelf\ncores to be representative of larger-scale changes across the ASE shelf, including areas\ncloser to both present and former grounding lines.\nImportantly, we observe remarkably consistent behaviour between the inner and outer shelf\nrecords during the critical transition around ~10 ka BP and extending through to the present\nday. This cross-shelf coherence supports the interpretation that the shelf was responding to\nthe same large-scale oceanographic forcing. Furthermore, the spatial consistency of the\nsignal along the outer shelf—persisting over thousands of years across a relatively small\nshelf width—suggests that the inner shelf remained in equilibrium with the outer shelf. This\nreinforces the validity of using outer shelf records as proxies for conditions influencing ice-\nsheet dynamics near the grounding line.\nIn summary, the reviewer has raised a valuable point, and we acknowledge that our original\nsubmission did not sufficiently explain the relationship (and linkages) between the inner and\nouter shelf sites. We have revised the manuscript to better articulate this connection and to\njustify the representativeness of the outer shelf records (lines 181-204 and 228-230).\nFigure 1. Cold and warm oceanographic regimes on the Antarctic shelf (from Zhou et al.,\nsubmitted) showing water mass consistency between outer and inner shelf settings. We also\nnote that on palaeo timescales, the outer shelf signal is transmitted to the inner shelf\ninstantaneously (years to decades). The data from Zhou et al. is made available under a\nCCBY 4.0 license (https://creativecommons.org/licenses/by/4.0/) following the information in\nthis link; https://www.seanoe.org/data/00886/99787/. No changes were made.\n2. Following from the previous point, I think the records of outer shelf CDW\npresence/absence should be highlighted as the main finding of this study as these are new\nrecords that extend to ~18 ka BP. Any conclusions for the inner ASE should be toned down\nas these stem from the previous study of Hillenbrand et al. (2017) and start at ~10 ka BP.\nWe agree that the outer shelf core data are critical to our overall interpretation and have\naccordingly emphasized their importance in the revised manuscript (lines 181-204).\nHowever, we also wish to highlight that the inner shelf Mg/Ca data presented here are both\nnovel and significant. While they follow the pattern previously inferred from δ¹³C records, it is\nimportant to note that—aside from only few pilot Mg/Ca data published in Hillenbrand et al.\n(2017) —our study is the first to present a reconstruction of post-LGM Mg/Ca-based BWT\nfrom the Antarctic continental shelf.\nOur earlier work (Hillenbrand et al., 2017) established that CDW was present on the inner\nshelf of the eastern ASE as soon as grounded ice had retreated from there, and that the\ntransition from CDW- to AASW-dominated conditions occurred around 8 ka BP. However,\ndue to the absence of direct proxy data for ocean temperature changes during the retreat\nphase before ca. 9 ka BP, we could not establish a robust causal link between ocean thermal\nforcing and grounded ice-sheet retreat from the outer to middle shelf. The additional new\nMg/Ca data now provide that missing evidence, directly linking bottom-water warming and\nsubsequent cooling to changes in ice-sheet dynamics.\nNevertheless, in line with the reviewer’s recommendation, we have revised the manuscript to\nplace greater emphasis on the outer shelf records, and we have added new text (lines 181-\n204) clarifying how oceanographic conditions on the outer and inner shelf relate to one\nanother (lines 216-225).\n3. The trends in the data need to be handled statistically instead of relying on the\nreader to pick out the perceived trends in the data, which are subjective. For example, change-point\nanalysis could be used to determine if the jump at ~10 ka is statistically significant. I expect\nthe d13C would see a significant jump, but the Mg/Ca ratios would not. This should be done\nafter the inner ASE records (PS75/167 and PS75/160) are separated from the outer ASE\n(PS75/191, PS75/190, VC436 and BC431).\nAs requested, we have performed change-point (CP) analysis on both the Mg/Ca and the\nδ13C data with corresponding text in the manuscript (lines 228-230), a new Supplementary\nFigure 6 and details of how this was done in the Methods (lines 573-584). We perform CP\nanalyses on the inner/outer records as well as the composite record (for the reasons outlined\nabove). All datasets yield consistent CPs.\nSimple (segmented mean) analysis of the Mg/Ca data indicates significant CPs at 8248 yr\nBP (inner shelf) and 11043 and 10050 yr BP (outer shelf). The two CPs of the outer shelf\nMg/Ca data CP indicate transitions to warmer (11043 to 10050 yr BP) and then cooler\n(<10050 yr BP) BWTs respectively. Analysis of the δ13C data reveals CP at 7977 yr BP (inner\nshelf) and 10911 yr BP (outer shelf). Analysis of the composite datasets reveals broadly\nconsistent results, yielding CPs at 10135 and 9113 yr BP (Mg/Ca) and 10911 and 7938 yr\nBP (δ13C), respectively. We attribute the difference in CPs between the Mg/Ca and δ13C\ndataset to resolution, with the latter having more datapoints. When discussing the transition\npoint between ‘warm’ deglacial and ‘colder’ Holocene conditions we now quote the time\ninterval 10.1 to 7.9 ka BP. This is taken from the composite dataset 10135 to 7938 yr BP.\nNote that the result of this analysis is broadly consistent with our original submission, which\nstated a change from high/low (low/high) at 10-9 ka BP for the Mg/Ca (δ13C) data.\nFigure 2. Change-point analysis of Mg/Ca and δ13C data using programme ‘changepoint’ in\nsoftware package R. a. Inner-shelf Mg/Ca (CP at 8248 yr BP). b. Outer-shelf Mg/Ca with\nCPs at 11043 yr BP (warming) and 10050 yr BP (cooling). c. Inner shelf δ13C (CP at 7977 yr\nBP). d. Outer-shelf δ13C with a CP at 10911 yr BP. e-f. Composite Mg/Ca records showing\nCPs at 10135 and 9113 yr BP, and δ13C CPs at 10911 and 7938 yr BP. Vertical dashed lines\ndenote the CP, while horizontal black lines denote the segmented means.\n4. You mention the carbonate ion effect but don’t state how you determined if this effect was\ninfluencing your data. Some evidence to show constant carbonate ion concentrations or\nsome other screening method would be vital here. The ASE is under influence from large\namounts of freshwater discharge from nearby glaciers suggesting variable alkalinity across\nthe shelf. Carbonate preservation also appears to be highly variable suggesting a dynamic\ncarbonate ion concentration across the shelf.\nWe apologize if this was not clearly explained in our original submission. In response to the\nreviewer’s comment, we have added clarifying text to the revised manuscript (lines 501-508).\nTo summarize: Previous studies have suggested that seawater carbonate ion concentration\n[𝐶𝑂2−] could exert a secondary influence on foraminiferal Mg/Ca ratios—in addition to the\n3\nprimary temperature control—particularly in epifaunal species, such as Cibicidoides\nwuellerstorfi and C. mundulus, which calcify at or just above the sediment–water interface\n(Elderfield et al., 2010; Elderfield et al., 2006). In contrast, Trifarina angulosa, the benthic\nforaminifera species used in our study, is an infaunal species, meaning it lives and calcifies\nwithin the top 1–5 cm of the sediment column (Mackensen et al., 1990; Schumacher, 2001).\nIn this environment, porewaters tend to equilibrate rapidly with respect to carbonate\nchemistry, and the saturation state remains near zero. This minimizes the influence of\nambient [𝐶𝑂2−] on calcification (Elderfield et al., 2006; 2010). Elderfield et al. (2010)\n3\ndemonstrated this effect for Uvigerina spp., and given both the close genetic relationship\nbetween Uvigerina and Trifarina spp. (Schweizer et al., 2005) and the similar Mg/Ca-BWT\nrelationship between the two species (Mawbey et al., 2020), it is reasonable to assume that\nTrifarina responds similarly—i.e., that any [𝐶𝑂2−] effect on Mg/Ca is negligible.\nestimate a sensitivity of approximately 0.002 ± 0.001 mmol/mol per µmol/kg change\nin ∆[𝐶𝑂2−] for Uvigerina spp., with higher [𝐶𝑂2−] leading to elevated Mg/Ca values. Modern\n3 3\nCDW typically has a [𝐶𝑂2−] concentration of ~75–80 µmol/kg, while Antarctic Surface Water\n3\n(or Winter Water) reaches up to ~120 µmol/kg (Bostock et al., 2011; Gottschalk et al., 2015;\nYu et al., 2014). Assuming a conservative maximum ∆[𝐶𝑂2−] of 40 µmol/kg (a likely\n3\noverestimate, given modification of CDW on the shelf), the resulting Mg/Ca bias would be\napproximately +0.08 mmol/mol (using Uvigerina sensitivity tests performed by Elderfield et\nal. 2010). This is small compared to the magnitude of Mg/Ca changes observed in our\nrecord. Moreover, since the main earliest Holocene transition in our data is characterized by\na decrease in Mg/Ca values (interpreted as cooling and a reduced CDW influence), any\n∆[𝐶𝑂2−] effect would dampen the observed signal. Thus, the true temperature signal may be\n3\neven stronger than inferred (see also discussion under Trace metal analyses in the\n‘Methods’ section of Hillenbrand et al. 2017).\nIn summary, while we acknowledge that ∆[𝐶𝑂2−] effects cannot be completely ruled out, the\n3\navailable evidence indicates that any bias is negligible and does not affect the core\ninterpretations of our study. This is particularly true given that our conclusions are based on\nthe relative magnitude of changes rather than on absolute temperature reconstructions. We\ntherefore do not attempt to correct the record for ∆[𝐶𝑂2−] nor do we use Mg/Ca–BWT\n3\ncalibrations that include carbonate ion corrections.\n5. The wider interpretation is very selective and focuses on a marine geoscience\nperspective, a common problem among marine paleoclimate records, whilst\nneglecting to mention other drivers of ice loss since the LGM such as\natmospheric and sea level forcing. More southernly SHW may drive CDW\nincursions onto the shelf, but it also brings warm, moist air to West Antarctica\nleading to surface melting. Sea-level rise from the LGM to Holocene would have\ndestabilised ice sheets leading to dynamic calving. It is likely the combination of\nwarm ocean and atmospheric currents and sea level rise would have combined\nto drive ice loss post LGM. This paper supports the ocean forcing contribution\nwhich is important but not the whole story. A wider context is therefore needed.\nWe appreciate the reviewer’s suggestion and have restructured the discussion to consider\nadditional deglacial drivers beyond oceanic forcing. As noted in both our original submission\nand previous publications on the timing of ice sheet retreat in the ASE (e.g. Smith et al.\n2011; 2014), global sea-level rise is likely to have contributed to grounding-line retreat during\ndeglaciation. Given the relatively early deglaciation of the ASE shelf, however, atmospheric\nand sea-level forcing probably only played a subordinate role in the post-LGM deglaciation\nwhen compared to ocean forcing (as we previously pointed out in Hillenbrand et al., 2017).\nThe exact contribution of sea-level rise cannot be directly assessed using the proxies in our\nstudy. While we acknowledge that rapid sea-level rise could have played a role—particularly\nduring meltwater pulses—we are limited in our ability to quantify or disentangle this effect\nfrom the available data. We discuss this in the revised manuscript (lines 302-312, 322-325)\nand suggest that future fully coupled ocean-atmosphere-ice sheet modelling efforts are\nneeded to better refine this interplay.\nIn contrast, we do not consider that atmospheric drivers, particularly changes in surface air\ntemperature (SAT), have been the primary control, although a poleward shift in the SHW\nwould, as the reviewer correctly states, not only drive (more) CDW onto the ASE shelf but\nalso move warm air masses further south. West Antarctic ice core data indicate indeed that\nSATs increased during the outer to inner shelf deglaciation of the eastern ASE shelf from ca.\n19 ka to 11 ka BP, when the modern temperature was finally reached (Cuffey et al., 2016;\nFudge et al., 2016). In order to drive substantial surface melt and runoff sufficient to\ndestabilize the ice sheet, however, SATs would likely have needed to be several degrees\nwarmer than modern levels—but post-LGM SAT maxima in West Antarctica were less than\n1.5 ºC higher than present, and, importantly, these maxima were only reached after\ngrounded ice had retreated from the ASE shelf (Cuffey et al. 2016; Fudge et al. 2016).\nConsequently, while we now more explicitly acknowledge the role of atmospheric and sea-\nlevel forcing, we continue to interpret ocean-driven melting—linked to the incursion of warm\nCDW—as the dominant driver of grounding-line retreat in the ASE. However, we have also\nadded additional text stating the atmospheric drivers are likely to have played a role in\nHolocene ice shelf and grounding line variations in some near-coastal parts of the\nembayment, e.g., Cosgrove Ice Shelf (lines 360-374).\n6. We see a gradual increase in sea level and southward transition of the SHW in\nFig. 3, but Mg/Ca ratios are flat. Why do we not see gradually more CDW on the\nshelf with this change? This suggests the d13C and Mg/Ca ratios are instead\ndisplaying a binary signal, i.e., CDW is either present or not, which needs to be\ndiscussed. Furthermore, other records for SHW migration show a continued\npoleward migration until the mid-Holocene. This may suggest ocean forcing is\nimportant before 10 ka but atmospheric or sea level forcing is important after 10\nka – an interesting finding that has not been discussed.\nThis is a good point and one we have also considered. We agree that CDW\npresence/absence is likely to be binary (for this proxy). In relation to the SHW, we suggest\nthere might be a latitudinal threshold, beyond which CDW is forced on (or off) the shelf.\nWe discuss this in our revision (lines 302-316, 387-399). However, it is also worth reiterating that\nprevious work, including our own, shows that retreat (along Marie Byrd Land coast) since 10\nka has been very limited.\nAfter these changes are made, I expect you will have an important study of outer shelf\nCDW changes that can inform on the contribution of ocean forcing on ASE ice loss, and\nmaybe more muted CDW influence on continued loss during the Holocene. This finding\nshould be placed within the context of other important forcing mechanisms such as\natmospheric currents and sea-level rise.\nThank you, we appreciate your constructive review and agree that our findings are\nimportant.\nOther comments:\nFig. 1 b and text referring to ASE reconstructions: this reconstruction and the relevant\ntext refers to the pre-Holocene records and might be misleading. Several studies since\nhave shown extensive ice shelf retreat during the Holocene. For example, Minzoni et al.\n(2017) recorded around 50 km retreat of the Cosgrove Ice Shelf during the Holocene.\nFurthermore, ice loss post LGM was of an unbounded ice shelf on the outer ASE. It is ice\nshelf retreat when the ice shelf is bounded by topographic highs in the embayment,\nsuch as during the Holocene, that is important for understanding buttressing related to\ninstabilities mentioned in the introduction (e.g., Pattyn, 2018).\nWe thank the reviewer for highlighting an important oversight in our original submission,\nwhich relates to the distinction between changes to the WAIS grounding lines (which we\ndefine as those along the eastern Marie Byrd Land coastline – including Pine Island,\nThwaites, Smith, Pope glaciers etc.) and changes to other glacier/ice shelves in the\nAmundsen Sea e.g., along the western Ellsworth Land coast. We have added additional text\nto more clearly define what part of WAIS we are referring to (lines 237-239). It was not our\nintention to overlook the important work relating to the Cosgrove Ice Shelf, which is\ndiscussed on lines 244-247 and 362-368.\nLine 125: here, I think you need to highlight the distance between these sites (several\nhundred kilometres) and separate the sites into 2 or 3 records. The inner ASE (PS75/167\nand PS75/160) and western outer ASE (VC436 and BC431) and eastern outer ASE\n(PS75/191 and PS75/190). The data in the plots should be separated into different\npanels accordingly and discussed separately.\nAs previously noted, we have followed the reviewer’s recommendations and separated the\nouter/inner shelf data into different panels and discuss separately. We also refer to our\nearlier response re: outer shelf being indicative to inner shelf changes.\nLine 139: expand on how you discount carbonate ion concentration influence on you\nMg/Ca ratios. As you mention in line 131, carbonate preservation is highly variable\nacross the shelf suggesting carbonate ion concentration is highly variable.\nSee corresponding response above.\nLine 152: I am having trouble following this comparison. Maximum CDW temperatures\nof >1.5oC is confusing. Just put the range of CDW values.\nWe have modified the original text accordingly.\nFig. 2: Separate the data into 4 panels i.e., red and blue circles in two panels and open\nsquares, green triangles, and purple triangles in the other two. Change-point analysis\nshould be conducted to see if there is a statistically significant shift in the data. Other\ntests are available such as ANOVA. There appears to be no change in the Mg/Ca ratios\nwhen split between inner and outer ASE, but statistical tests will be able to tell you.\nDone - see our earlier response.\nLine 173: There is quite a lot of scatter in the Mg/Ca ratios. If you focus on the outer ASE\nrecords, I cannot see a clear cooling trend. Do you have any statistics that support this\nassertion? D13c is a bit clearer but there is still a lot of scatter.\nYes, the Mg/Ca data are inherently noisy (see below) and of lower resolution compared to\nthe δ13C data. The clearest indication for a ‘gradient’ is seen in the δ13C data. CP analysis\nindicates that this transition from low(high) to high(low) δ13C(Mg/Ca) occurs between 10.3-\n7.8 ka BP.\nLine 179: I think this quite a narrow view. There are lots of records of Holocene change,\nthey are just not related to ocean forcing (e.g., Minzoni et al. 2017; Sproson et al., 2022;\nJohnson et al., 2014). Sea level was also a likely contributor to ice loss in the post LGM\nperiod that may not be as important now.\nWe recognise that the issue may stem from a lack of clarity in how we defined the West\nAntarctic Ice Sheet (WAIS) in the context of our study area. Specifically, when we refer to\nWAIS retreat to the inner shelf by ~10 ka BP, we are referring to the sector of the ice sheet\nalong the eastern Marie Byrd Land coast, which includes Pine Island, Thwaites, Smith, and\nKohler glaciers (see lines 237-238). This region likely contributed disproportionately to ice\nmass loss during deglaciation and is the primary focus of our reconstruction. It was not our\nintention to discount or neglect other records from the Amundsen Sea. Rather, our aim was\nto highlight the overall reduction in mass loss and the apparent grounding-line stability along\nthe Marie Byrd Land sector during the Holocene, as constrained by our and other datasets.\nWe agree with the reviewer that sea-level rise may have played a role in driving ice-sheet\nretreat (lines 306-312), and we stated this explicitly in both the original manuscript and\nrelevant earlier work. However, we acknowledge that our current dataset does not allow us\nto isolate or directly assess the contribution of sea-level forcing, and we now clarify this\nfurther in the revised text (line 321-325). Moreover, we point out that along different parts of\nthe western Marie Byrd Land coast (120-158 ºW) grounded ice had retreated to the inner\nshelf already by 15.6 ka BP and 20.9 ka BP (Klages et al., 2014), respectively. This retreat\nwell before Meltwater Pulse (MWP) 1A at 14.6 ka BP and the MWP at 19 ka, respectively,\nsuggests that sea-level forcing played only a subordinate role in driving the deglaciation of\nthe majority of the Amundsen Sea shelf.\nLine 197: This is misleading. I think you can say changes across the outer shelf but not\nembayment-wide as the inner ASE record starts at ~10ka.\nSee our earlier response relating to Amundsen Sea wide inference. However, we have\namended this sentence (lines 255).\nLine 205: This sentence contradicts the next sentence. Lots of evidence for Holocene\nretreat (e.g., Minzoni et al. 2017; Sproson et al., 2022; Johnson et al., 2014; 2017; 2020)\nand it is likely that the final 10s of kilometres of retreat (during the Holocene) are the\nmost important for understanding contemporary ice loss as this is when ice shelves\nbecome pinned by plateaus causing the buttressing effect (Pattyn, 2018).\nWe have re-structured this section, adding a discussion about Holocene glacier variability vs.\nWAIS retreat i.e., glaciers draining the Marie Byrd Land coast (lines 237-247 and 353-364).\nFurthermore, we point out that several of the papers cited in this regard by the reviewer\n(Johnson et al.) actually refer to ice-sheet thinning in the hinterland of the ASE but not\ngrounding-line retreat. Both processes were apparently decoupled in the ASE, with the\nformer lagging the latter by several kyrs. This is explained by the existence of large ice\nshelves that buttressed ice further upstream until the collapse of these ice shelves allowed\nupstream thinning at ca. 10.6 ka BP in the Dotson-Getz Trough area (Smith et al. 2011;\nJohnson et al. 2020) and ca. 7.5 ka BP in the Pine Island Trough area (Hillenbrand et al.\n2017; Johnson et al. 2021).\nLine 213: CDW would have contributed to grounding line retreat. What about\natmospheric and sea level forcing?\nSee earlier responses (and lines 306-316).\nLine 218-221: This is a finding from Hillenbrand et al. (2017) and nothing new here\nexcept further support from Mg/Ca ratios.\nOur previous work (Hillenbrand et al. 2017) indicated presence of warm water on the inner\nASE shelf as soon as grounded ice had retreated from there, suggesting that this retreat was\nforced by CDW incursions. The lack of evidence about ocean properties during the earlier\ngrounded ice retreat from the outer and middle shelf, however, prevented us from\nestablishing a firm causal link between ocean forcing and ice-sheet retreat. We agree with\nthe reviewer that our manuscript fills this gap by presenting observations going back ~20 ka\nBP, and thereby provide a new line of evidence that the retreat was indeed forced by the\nocean.\nLine 232-233: I think this is the main finding of the study and should be the main\nhighlight. You can say a lot about incursion onto the outer shelf from 18 ka. Not a lot can\nbe said about the inner ASE as that record begins at 10 Ka.\nSee our earlier response.\nLine 244-255: How does this mechanism interact with sea-level rise? Could rising sea level\nbring CDW further up the shelf? We see a rise from 20 to 8 ka BP in Fig. 3f.\nIt’s certainly possible but there is limited information on whether rising sea level facilitates\nenhanced delivery of the CDW onto the shelf. However, we do think that the combined\nimpact of rising sea level and warming ocean was important in driving deglaciation.\nFig. 3a: the inner and outer records need to be separated out and treated\nindependently.\nDone.\nFig. 3c and d: Other records for SHW migrations are available that show continued\npoleward migration during the Holocene (Saunders et al., 2018; Voight et al., 2015; Ai et\nal. 2021). SHW migrations are a contentious issue and hotly debated so you should\nreflect this in the discussion as a possibility and not something that is unequivocally\nknown.\nWe now acknowledge this debate (lines 387-395).\nFig. 3e: More important here is to look at accumulation rates in relation to temperature\nfor the WAIS Divide core (Fudge et al., 2016). Anomalous accumulation rates in the\npost-LGM era suggest atmospheric forcing of the Amundsen Sea sector of the WAIS.\nSee our earlier response. We again refer to the temperature record from the WAIS Divide Ice\nCore since the LGM, which does not show SATs more than 1.5 ºC higher than today (Cuffey\net al. 2016; Fudge et al. 2016). Poleward SHW shifts may, as the reviewer highlights, cause\nhigher precipitation which may not be accompanied by an SAT increase (Fudge et al. 2016)\nor outweigh its effect by stabilising the ice sheet.\nFig. 3g: sea-level is rising when CDW is present. How does sea-level influence CDW on\nthe shelf? This is not discussed.\nSee our earlier response.\nLine 301-305: This is pushing it to say the least. I don’t think you can say this based on\nyour data. Retreat continued into the Holocene; it is just ocean forcing played a\ndiminished role (Sproson et al., 2022). Modelling suggests a greater contribution of\ntropical warming on warm air influencing the WAIS during the Holocene (Skinner et al.,\n2020; Dang et al., 2020; Scott et al., 2019).\nAgain, we think this comes down to whether one refers to the main WAIS outlet systems\nalong the southern ASE coast (Pine Island, Thwaites glaciers etc.) or small glacial outlets\ndraining into bays along the eastern ASE coast. Our revised text should address this.\nLine 310: and surface melt from atmospheric warming (Ding, 2011; Steig et al., 2013;\nWille et al., 2019).\nYes, but further to our responses above, we consider our interpretation of incursions of CDW\nas the dominant driver of mass loss in the Amundsen Sea sector of the WAIS and ‘the key\nvariable in determining future sea-level rise from Antarctica’ to be the most plausible. We\ndiscuss the role of atmospheric warming on lines 314-316 and 362-368.\nLine 314: SHW poleward migration also caused warm, moist air to be delivered to West\nAntarctica leading to surface melting.\nWe agree, but presently the Amundsen Sea sector experiences only very limited surface\nmelting which rarely occurs on more than 30 days per year (including years with extreme\nmelt) and is almost exclusively restricted to the near-coastal areas along the eastern ASE\ncoast (Donat-Magnin et al., 2020; Tedesco and Monaghan, 2009). Mass loss is dominated\nby calving and ice shelf melting and melting at the grounding lines by warm CDW. Peak\nHolocene temperatures less than 1.5°C higher than today are unlikely to have made a\nfundamental difference to this situation.\nReviewer #2 (Remarks to the Author):\nSmith et al present new d13C and Mg/Ca data from the Amundsen Sea Embayment (ASE).\nThe dataset advances our understanding of ocean forcing on ice sheet retreat. It extends the\nsparce temporal Holocene constraints published by Hillenbrand et al. (2017) Nature, back\nthrough to the last deglaciation for this critical West Antarctic glacier system. The power of\nthe new data is the demonstration of two modes/states of the ocean system, and associated\nice sheet (in)stability. During 18-10ka the average temperature of bottom water on the outer\nshelf was warmer compared to after 10ka when bottom water conditions were cooler.\nTemperature profiles near respective core sites show how incredibly warm CDW are, and\nproximal to the ice sheet.\nThe nature of continental shelf means that numerous core sites across the ASE are needed\nto measure both a meaningful number of rare foraminifera samples, and capture fully the last\ndeglaciation. The novelty in the findings relies on the record generated in V436 which covers\nthe deglaciation back to 18.2 ka. A step change in ocean conditions is observed which the\nauthor link to ice sheet retreat based on existing multi-proxy reconstructions of the region.\nThe significance of this work is the clear shift in baseline bottom water temperature\nconditions, which puts into context the millennial variability observed since the early\nHolocene in Hillenbrand et al. (2017). This study suggests the same mechanism proposed\nby Hillenbrand et al. controlled the BWT on the shelf, with the additional details provided by\nENSO observations on the controls on thermocline depth in setting the two modes of ocean\nforcing.\nThe data are of the highest quality analytically, the age models are clear, and the authors\nhave dealt conservatively with extension of proxy data to temperature given the limits of the\ncalibration/observations at the cooler end.\nThe manuscript is well written, comprehensively building on and advancing the latest\nunderstanding of this important sector of the ice sheet. I fully recommend publication of the\nmanuscript. Below are considerations for the authors.\nComments:\nCan the authors provide an explanation for the wide range on d13C, Mg/Ca. It is not only the\nlate Holocene where a wide range of values are observed over ~1000-500 years, the\ndeglacial section also show ~0.3-0.4 mmol/mol Mg/Ca range.\nBenthic foraminiferal Mg/Ca measurements are inherently noisy, even when analytical\nuncertainties are accounted for (de Nooijer et al., 2014; de Nooijer et al., 2009; Rosenthal et\nal., 1997; Sadekov et al., 2005)). This stochastic ‘geological/biological’ noise likely arises\nfrom a combination of factors, including vital effects, ontogenetic variability, and calcification\nprocesses, none of which are yet fully understood (e.g., Skinner and Elderfield, 2007). At\nleast some of these factors are likely to be responsible for the δ13C variability, too. Some of\nthe observed variabilities in the Mg/Ca and δ13C data may reflect genuine interannual or,\nmuch more likely, interdecadal (and in some instances possibly even intercentennial)\nchanges in water mass properties (see response on resolution below), but given current\nlimitations, we cannot confidently disentangle these signals from the underlying stochastic\nvariability.\nWe also note that the magnitude and character of variability in our dataset are consistent\nwith other benthic foraminiferal Mg/Ca records, including those produced in the ‘Elderfield\nLab’ (e.g., Skinner et al. 2003; Skinner and Elderfield, 2007; Sadekov et al. 2014) as well as\nfrom a broad range of paleoceanographic studies from different settings and time periods\n(Lawson et al., 2024; Lear et al., 2015; Martin et al., 2002; Nürnberg and Groeneveld, 2006).\nDo the proxies capture the large seasonal variability in thermocline and water mass\nproperties observed on the shelf? The strength of the new data presented is that its time\nresolution is sufficient to observe the mode of forcing and provides context for centennial\nscale variability in CDW proxies. Can the author provide insight into controls on sea ice\nthresholds, or have any data to support threshold conditions in sea ice state? Very different\nsea ice conditions likely contribute to the depth of the mixed layer/thermocline.\nThis is a good question which we have considered carefully. We agree with the reviewer that\na key strength of our new dataset is its temporal resolution, which is sufficient to investigate\nthe mode of oceanic forcing and ice-sheet response. We also fully acknowledge that\nthermocline depth—and by extension, BWT—is sensitive to a range of processes, including\nsome seasonal ones, but especially interannual variability in sea ice, wind stress/wind curl,\nand surface heat fluxes (Dutrieux et al., 2014; Weber et al., 2014; Yang et al., 2024;\nZheng et al., 2021). Supplementary Fig. 3 was intended to illustrate some of this variability,\nalthough we recognize that the figure caption may not have communicated this clearly\n(please see Figure 2 and our response to the related comment below).\nThat said, we do not believe our data can resolve seasonal-scale variability. The nature of\nbenthic foraminiferal Mg/Ca records—combined with sedimentation rates and sample\nspacing—suggests that our data most likely reflect decadal to centennial-scale averages\n(see next response below). In this context, we interpret the signal as binary or stepwise (i.e.,\npresence/absence or more/less CDW influence) rather than as capturing finer-scale\nseasonal changes. Seasonality has been proposed as a factor influencing foraminiferal\ngrowth windows in some environments (Fontanier et al., 2003; Skirbekk et al., 2016), and\nthis has also been demonstrated based on δ13C data for an epifaunal benthic foraminifera\nspecies (Cibicides wuellerstorfi) in the Southern Ocean (Mackensen et al., 1993). Seasonal\nhabitat preferences of Trifarina spp., however, remain unstudied. For this reason and\nb and\nbecause of the temporal resolution of our datasets (see next response), we refrain from\ninterpreting our data in terms of seasonal-scale variability in water mass properties. We\nassume, however, that the geochemical composition of the analysed foraminifera shells\nreflects mainly the oceanographic conditions during the spring-summer season, i.e., when\nplankton blooms occur and, therefore, Trifarina is most likely to feed and calcify. This is the\nsame season when the CTD data were collected (see Figure 3 below).\nAdditionally, palaeo sea-ice reconstructions from the Amundsen Sea are scarce. We are\naware of only two studies, i.e., Kim et al. (2021), which focuses on the past few centuries,\nand Lamping et al. (2020), who applied biomarker proxies (IPSO25 and modified PD-\nIPSO25) to a core spanning the Holocene. Importantly, however, both records reflect local\npolynya dynamics that are not representative for the wider ASE shelf as both cores were\nrecovered from within the Amundsen Polynya (located on the inner shelf north of the Dotson\nand eastern Getz ice shelves). None of the cores studied by us lies within the Amundsen\nPolynya (see Fig. 1 in the main paper).\nIn the revised manuscript, we now explicitly state that our data are unlikely to resolve\nseasonal variability (line 203-205). We also note that ongoing work seeks to apply\nbiomarker-based approaches (e.g., IPSO25, TEX86L) to the same cores presented here.\nThese efforts may ultimately allow us to better link surface processes—such as sea-ice\ncover and surface heat fluxes (affecting sea-surface temperature [SST])—with bottom water\nproperties in future studies.\nMore clarity around the temperature ranges is needed: seasonal variability in deep water\ntemperature on Sup Fig 3, are these from mooring measurements over 1, 2 or 3 years? How\ncan the variability be larger than the min and max values at each site? It would be helpful to\nshow where these profiles are from if multiple CTD stations were compiled? Or add\nadditional plots to show the raw data from which the averages are compiled. These\nobservations could support explanation of variability in proxy data related to previous\ncomment?\nThank you for highlighting this — we agree that the caption for Supplementary Figure 3 did\nnot adequately describe the data plotted. We have now revised the caption to clarify that, for\neach site, we plot the average of all available CTD data within a 60 km radius around each\ncore site spanning the full instrumental period (1994 to present), plus the maximum and\nminimum temperatures at the water depths of our records (see Figure 3, below). This\napproach is intended to illustrate the range of interannual variability in water column\nproperties at each location because the foraminifera have likely lifespans between ca. 1 and\n13 years (Hayward et al., 2014; Hohenegger, 2018).\nWe acknowledge that temporal oceanographic variability on decadal timescales may\naccount for some of the variability in our Mg/Ca data between samples of similar age from\nthe same core. However, as noted above, the temporal resolution of our sediment core data\nis insufficient to isolate or resolve seasonal or even annual signals, and we assume the\nMg/Ca value of each analysed shell to reflect an integrated multi-annual to decadal average\nin BWT because the lifespan of benthic foraminifera is usually 1-13 years (Hayward et al.\n2014; Hohenegger 2018). As we combined ca. 30 foraminifera shells from each 1 cm thick\nsample horizon for one Mg/Ca measurement, the Mg/Ca data point is likely to give a decadal\nto centennial average in BWT given the wide range of sedimentation rates in our cores. This\nconclusion is based on the following considerations: One of the highest sedimentation rates\nin our cores is the early Holocene section (230-900 cm core depth) in core PS75/167, which\nwas deposited between 8.3 and 10.4 ka BP (Hillenbrand et al. 2017), providing a\nsedimentation rate of 3.2 mm/yr. This implies that even if Trifarina specimens lived only in\nthe top layer of the seabed sediments (0-1 cm) with an (unlikely) lifespan of just 1 year, a\nsample from the early Holocene section of this core is likely to provide a Mg/Ca signal\naveraged over 3 years. However, there is clear evidence that Trifarina lives down to a depth\nof 5 cm below the seafloor surface (Schumacher 2001), i.e., such a sample most likely also\ncontains specimens that lived up to 15 years after the deposition of the sampled horizon, so\nthe Mg/Ca value actually gives a signal integrated over 15 years. Conversely, one of the\nlowest sedimentation rates in our cores is the late Holocene section (133-230 cm core\ndepth) in core PS75/160-1, which was deposited between 1.7 and 4.4 ka BP (Hillenbrand et\nal. 2017), providing a sedimentation rate of just 0.36 mm/yr. This implies a 1 cm thick sample\nfrom this section provides an Mg/Ca signal integrated over 28 years at best (Trifarina living\nonly in the top centimeter) and over 140 years at worst (Trifarina living down to 5 cm within\nthe seabed).\nPotential temperature (oC)\nFigure 3. All available CTD data plotted as potential temperature (top panel) spanning the\nfull instrumental period (1994 to present) within a 60 km radius of the core sites (bottom\npanel).\nFigure 2: Suggest you match the colour of squares in the d13C plot to the legend colour, as\nit’s different to the colour of squares in the Mg/Ca plot beneath, and the colour of the square\nsymbol in Figure 1.\nChanged, thanks.\nFigure 3: misspelling of alkenones in c.\nChanged, thanks.\nReviewer #3 (Remarks to the Author):\nThe manuscript by Mawbey et al is a revised version of a manuscript I already reviewed\nduring the initial submission. The authors have done a good job in answering most of the\nquestions and comments the reviewers (including myself) raised. The revised manuscript is\nmuch better to understand and to follow and I think that the study and the underlying data\nare now better presented than before.\nThank you. We appreciate the efforts of the reviewer and are happy to provide further\ndetails.\nSaying this, however, I still have comments regarding the BWT reconstruction using benthic\nforaminiferal Mg/Ca the authors use to justify and interpret the inflow of colder waters onto\nthe shelf region after approx. 10 ka. The authors have done a much better job to describe\nhow the Mg/Ca data have been analyzed and how they were treated. However, looking at\nthe now included errors of the data, all the trends and therefore the interpretation regarding\nwarmer or colder water masses are well within the error of their analyzes (see new figure in\nSI). Especially in the final conclusion and establishment of the proposed interpretation, the\nauthors repeatedly claim colder temperatures after 10 ka (speaking about ‘pronounced\ndecreases’ in BWT and ‘warmer BWT’ before 10 ka), a fact that simply is not really seen in\nthe data.\nWe thank the reviewer for highlighting this point, as it prompted us to examine our error\ncalculations more carefully.\nAn important first point, is that our primary interpretations do not rely on absolute BWT\nvalues, given the limitations of both global and regional Mg/Ca–temperature calibrations\ndiscussed in our original submission (and again below). For this reason—and as stated\npreviously—we did not propagate analytical or calibration uncertainties in our initial\nsubmission.\nIn our revised submission we reported a two-standard deviation (2 SD) error calculated from\nthe mean of the composite Mn corrected Mg/Ca dataset (± 0.2745 mmol/mol; ± 26.31%).\nThis was a mistake, for which we apologise. In retrospect we should have plotted the\nMg/Ca data with the analytical uncertainty as per the δ¹³C data. Note that the analytical\nuncertainty of replicate measurements for Mg/Ca is very small (± 0.0057 mmol/mol (±\n0.55%)), so is typically not shown in Mg/Ca records e.g. Elderfield et al., 2010; 2012; Kender\net al., 2016; Lawson et al., 2024; Yang et al., 2025). In contrast, one or two standard error or\nstandard deviation is commonly applied when converting to temperature (see below), as this\ncalculation encompasses multiple sources of error, particularly the calibration error.\nBecause the main focus of our manuscript is on the changes of Mg/Ca ratios (and not\nconverted temperature values), and to be consistent with Mg/Ca literature - we quote\nanalytical error in our 2nd revision. Note that in our earlier study (Hillenbrand et al., 2017), we\nquoted an uncertainty of ± 0.05 mmol/mol for Mg/Ca, representing the analytical uncertainty\nin the Mg/Mn composition of the diagenetic coating (e.g., 0.15 ± 0.05 mmol/mol).\nWe follow this approach but additionally incorporate the analytical uncertainty of the Mg/Ca\nmeasurement. Propagating both sources of analytical error yields a mean 2 SD uncertainty\nof ± 0.0503 mmol/mol. To further validate this, we have performed Monte Carlo simulations\n(n = 10000) to probabilistically evaluate uncertainties (e.g., Thornalley et al., 2018; Yu et al.,\n2019; Dai and Yu, 2025). Monte Carlo simulations produce a mean error of ±0.0577\nmmol/mol (Figure 1 below). These results are included in Supplementary Figure 1a and\nTable 1. Note that we also have deleted Supplementary Figure 1b since we do not refer to\nthis in our interpretation.\nFigure 1. 10000 Monte Carlo uncertainty simulations on Mn-corrected Mg/Ca dataset,\nresulting in a mean 2SD error of ± 0.0577 mmol/mol (see Supplementary Table 1).\nWe think that our revised error analysis is robust – and in line with Mg/Ca literature -\nbut if the reviewer favours a different way of representing error, we are happy to\nfollow their recommendations.\nUltimately there is a 0.62 mmol/mol shift in Mg/Ca data between the deglacial phase (18-\n10.1 ka) and the Holocene (7.9-0 ka). Given that our propagated analytical error is ± 0.0577\nmmol/mol (Figure X) we have high confidence that the signal - and our interpretation - is\nrobust. We also emphasize that the inferred transition from “warmer” to “cooler” conditions is\nconsistent with the δ¹³C data, which the reviewer describes below as “convincing”.\nFor completeness, we have also propagated uncertainties associated with the BWT\ncalculations (Supplementary Figure 1) following standard methods and consistent with\nrecent studies in Nature Communications (e.g., Yang et al., 2025\nhttps://www.nature.com/articles/s41467-025-62446-x). This approach incorporates (i) the\nreplicate analytical uncertainty (±0.0057 mmol/mol), (ii) the Mn/Ca correction uncertainty\n(±0.05 mmol/mol), and (iii) the calibration uncertainty (±0.098 mmol/mol). The calibration\nuncertainty is estimated following Cléroux et al. (2008), based on the standard error of the\nregression coefficient reported by Mawbey et al. (2020). Propagation of these uncertainties\nyields a temperature error of approximately 1.6 °C for each sample.\nWe have updated Figures 2 and 3 in the main manuscript, as well as Supplementary Figures\n1 and 7, to reflect these revised uncertainties. Details of the error analysis have been added\nto the Methods section, and all relevant figure captions have been updated accordingly. As\nnoted above, we are happy to adopt an alternative uncertainty treatment if the reviewer has\na preferred approach.\nAlso the now used change-point analysis shows only trends and a change point if the used\ncores are combined. How does it look like when the single cores are used? Is there really a\nsolid and statistically sound change point? I would argue, that only if the data of multiple\ncores are combined, the change point and also the overall trend is visible. Combining cores,\nhowever, is prone to introduce different influences (preservation, geochemically slightly\ndifferent water masses, etc.) a fact that is not convincingly described and discussed in the\nmanuscript.\nWe politely point the reviewer to our previous rebuttal and supplementary information. We\ndid perform CP analysis on both the composite dataset and ‘single’ cores. CPs are\nconsistent for all composite/individual and Mg/Ca and δ¹³C datasets.\nI have to say, that the data for C-isotopes are much more convincing than the Mg/Ca data,\nbut with the former ones alone, the study misses a new and convincing dataset.\nThank you - we agree that our δ¹³C dataset is ‘convincing’ and indicates that warmer CDW-\nlike waters prevailed on the Amundsen Sea shelf between 18–10 ka, while cooler, AASW-\nlike waters dominated on the Amundsen Sea shelf after ~10-7.9 ka, while. As outlined in our\noriginal submission, the dominance of cooler waters after ~7.5 ka was established by our\nearlier work based on δ¹³C and a limited number of Mg/Ca measurements (n=10;\nHillenbrand et al., 2017, Nature, https://doi.org/10.1038/nature22995).\nThe strength of the present study lies in the integration of an extensive new Mg/Ca\ndataset (18–0 ka) with additional δ¹³C data for the deglacial interval (18–10 ka).\nTogether they provide a coherent and robust reconstruction of water-mass variability over\nthe past 18 ka.\nA second point I would like to raise (and would like to ask the authors to deal with in a\nrevised version of the manuscript) is the assumption, that the water masses are evenly\ndistributed across the shelf (i.e., inner to outer shelf are influenced by the same water mass),\nan assumption that is based on modern observations. How does this might change when\nyou have completely different background conditions like an retreating marine ice shelf as it\noccurred during the study interval? Shouldn’t this significantly change the outer vs. inner\nrelationship and factors influencing it?\nYes, we are happy to provide additional details re: “how does this might change when you\nhave completely different background conditions like a retreating marine ice shelf as it\noccurred during the study interval’…”\nAn important first point is that the ice sheet had already retreated to near its modern\nposition before at least ~11 ka in the eastern part of our study area and before at least\n~12‒13 ka in its western part (Larter et al., 2014; see Fig. 1b), i.e. well before the\nobserved Mg/Ca and δ¹³C shifts. Therefore, the ‘background conditions’ have remained\nstable since that time, and there is no evidence to suggest that the pattern of shelf-sea\ncirculation since this retreat and the present differed significantly from today. A second\npoint is that we have rapidly thinning ice shelves and rapidly retreating grounding\nzones in the Amundsen Sea today and there is very little change in the CDW temperature\nas it traverses the shelf.\nThus, together with the modelling results highlighted in our earlier rebuttal (Nakayama et al.,\n2014; Naughten et al., 2021; Zhou et al., in prep.), which indicate residence time of shelf\nwaters — that is, the time required to fully replace water properties through advection\nfollowing a regime change – is relatively short and occurs over years to decades, we are\nconfident that the water masses were evenly distributed across the shelf during both the\ndeglacial phase and during the post-10 ka “modern” background conditions.\nWe have now added text to clarify this in the revised manuscript (line 227)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately answered all of my points and this manuscript now represents a remarkable study of ocean drivers of Antarctic Ice Sheet loss worthy of publication in Nature Communications.\nI have no further comments.", "reviewer_3": "The manuscript by Mawbey et al is a revised version of a manuscript I already reviewed during the initial submission. The\nauthors have done a good job in answering most of the questions and comments the reviewers (including myself) raised.\nThe revised manuscript is much better to understand and to follow and I think that the study and the underlying data are now\nbetter presented than before.\nSaying this, however, I still have comments regarding the BWT reconstruction using benthic foraminiferal Mg/Ca the authors\nuse to justify and interpret the inflow of colder waters onto the shelf region after approx. 10 ka. The authors have done a\nmuch better job to describe how the Mg/Ca data have been analyzed and how they were treated. However, looking at the\nnow included errors of the data, all the trends and therefore the interpretation regarding warmer or colder water masses are\nwell within the error of their analyzes (see new figure in SI). Especially in the final conclusion and establishment of the\nproposed interpretation, the authors repeatedly claim colder temperatures after 10 ka (speaking about ‘pronounced\ndecreases’ in BWT and ‘warmer BWT’ before 10 ka), a fact that simply is not really seen in the data. Also the now used\nchange-point analysis shows only trends and a change point if the used cores are combined. How does it look like when the\nsingle cores are used? Is there really a solid and statistically sound change point? I would argue, that only if the data of multiple\ncores are combined, the change point and also the overall trend is visible. Combining cores, however, is prone to\nintroduce different influences (preservation, geochemically slightly different water masses, etc.) a fact that is not convincingly\ndescribed and discussed in the manuscript.\nI have to say, that the data for C-isotopes are much more convincing than the Mg/Ca data, but with the former ones alone, the\nstudy misses a new and convincing dataset.\nA second point I would like to raise (and would like to ask the authors to deal with in a revised version of the manuscript) is the\nassumption, that the water masses are evenly distributed across the shelf (i.e., inner to outer shelf are influenced by the\nsame water mass), an assumption that is based on modern observations. How does this might change when you have\ncompletely different background conditions like an retreating marine ice shelf as it occurred during the study interval?\nShouldn’t this significantly change the outer vs. inner relationship and factors influencing it?"}, "author_response": "We thank Reviewer #1 for their thorough review and positive comments regarding our\nrevision.\nReviewer #3 (Remarks to the Author):\nThe manuscript by Mawbey et al is a revised version of a manuscript I already reviewed\nduring the initial submission. The authors have done a good job in answering most of the\nquestions and comments the reviewers (including myself) raised. The revised manuscript is\nmuch better to understand and to follow and I think that the study and the underlying data\nare now better presented than before.\nThank you. We appreciate the efforts of the reviewer and are happy to provide further\ndetails.\nSaying this, however, I still have comments regarding the BWT reconstruction using benthic\nforaminiferal Mg/Ca the authors use to justify and interpret the inflow of colder waters onto\nthe shelf region after approx. 10 ka. The authors have done a much better job to describe\nhow the Mg/Ca data have been analyzed and how they were treated. However, looking at\nthe now included errors of the data, all the trends and therefore the interpretation regarding\nwarmer or colder water masses are well within the error of their analyzes (see new figure in\nSI). Especially in the final conclusion and establishment of the proposed interpretation, the\nauthors repeatedly claim colder temperatures after 10 ka (speaking about ‘pronounced\ndecreases’ in BWT and ‘warmer BWT’ before 10 ka), a fact that simply is not really seen in\nthe data.\nWe thank the reviewer for highlighting this point, as it prompted us to examine our error\ncalculations more carefully.\nAn important first point, is that our primary interpretations do not rely on absolute BWT\nvalues, given the limitations of both global and regional Mg/Ca–temperature calibrations\ndiscussed in our original submission (and again below). For this reason—and as stated\npreviously—we did not propagate analytical or calibration uncertainties in our initial\nsubmission.\nIn our revised submission we reported a two-standard deviation (2 SD) error calculated from\nthe mean of the composite Mn corrected Mg/Ca dataset (± 0.2745 mmol/mol; ± 26.31%).\nThis was a mistake, for which we apologise. In retrospect we should have plotted the\nMg/Ca data with the analytical uncertainty as per the δ¹³C data. Note that the analytical\nuncertainty of replicate measurements for Mg/Ca is very small (± 0.0057 mmol/mol (±\n0.55%)), so is typically not shown in Mg/Ca records e.g. Elderfield et al., 2010; 2012; Kender\net al., 2016; Lawson et al., 2024; Yang et al., 2025). In contrast, one or two standard error or\nstandard deviation is commonly applied when converting to temperature (see below), as this\ncalculation encompasses multiple sources of error, particularly the calibration error.\nBecause the main focus of our manuscript is on the changes of Mg/Ca ratios (and not\nconverted temperature values), and to be consistent with Mg/Ca literature - we quote\nanalytical error in our 2nd revision. Note that in our earlier study (Hillenbrand et al., 2017), we\nquoted an uncertainty of ± 0.05 mmol/mol for Mg/Ca, representing the analytical uncertainty\nin the Mg/Mn composition of the diagenetic coating (e.g., 0.15 ± 0.05 mmol/mol).\nWe follow this approach but additionally incorporate the analytical uncertainty of the Mg/Ca\nmeasurement. Propagating both sources of analytical error yields a mean 2 SD uncertainty\nof ± 0.0503 mmol/mol. To further validate this, we have performed Monte Carlo simulations\n(n = 10000) to probabilistically evaluate uncertainties (e.g., Thornalley et al., 2018; Yu et al.,\n2019; Dai and Yu, 2025). Monte Carlo simulations produce a mean error of ±0.0577\nmmol/mol (Figure 1 below). These results are included in Supplementary Figure 1a and\nTable 1. Note that we also have deleted Supplementary Figure 1b since we do not refer to\nthis in our interpretation.\nFigure 1. 10000 Monte Carlo uncertainty simulations on Mn-corrected Mg/Ca dataset,\nresulting in a mean 2SD error of ± 0.0577 mmol/mol (see Supplementary Table 1).\nWe think that our revised error analysis is robust – and in line with Mg/Ca literature -\nbut if the reviewer favours a different way of representing error, we are happy to\nfollow their recommendations.\nUltimately there is a 0.62 mmol/mol shift in Mg/Ca data between the deglacial phase (18-\n10.1 ka) and the Holocene (7.9-0 ka). Given that our propagated analytical error is ± 0.0577\nmmol/mol (Figure X) we have high confidence that the signal - and our interpretation - is\nrobust. We also emphasize that the inferred transition from “warmer” to “cooler” conditions is\nconsistent with the δ¹³C data, which the reviewer describes below as “convincing”.\nFor completeness, we have also propagated uncertainties associated with the BWT\ncalculations (Supplementary Figure 1) following standard methods and consistent with\nrecent studies in Nature Communications (e.g., Yang et al., 2025\nhttps://www.nature.com/articles/s41467-025-62446-x). This approach incorporates (i) the\nreplicate analytical uncertainty (±0.0057 mmol/mol), (ii) the Mn/Ca correction uncertainty\n(±0.05 mmol/mol), and (iii) the calibration uncertainty (±0.098 mmol/mol). The calibration\nuncertainty is estimated following Cléroux et al. (2008), based on the standard error of the\nregression coefficient reported by Mawbey et al. (2020). Propagation of these uncertainties\nyields a temperature error of approximately 1.6 °C for each sample.\nWe have updated Figures 2 and 3 in the main manuscript, as well as Supplementary Figures\n1 and 7, to reflect these revised uncertainties. Details of the error analysis have been added\nto the Methods section, and all relevant figure captions have been updated accordingly. As\nnoted above, we are happy to adopt an alternative uncertainty treatment if the reviewer has\na preferred approach.\nAlso the now used change-point analysis shows only trends and a change point if the used\ncores are combined. How does it look like when the single cores are used? Is there really a\nsolid and statistically sound change point? I would argue, that only if the data of multiple\ncores are combined, the change point and also the overall trend is visible. Combining cores,\nhowever, is prone to introduce different influences (preservation, geochemically slightly\ndifferent water masses, etc.) a fact that is not convincingly described and discussed in the\nmanuscript.\nWe politely point the reviewer to our previous rebuttal and supplementary information. We\ndid perform CP analysis on both the composite dataset and ‘single’ cores. CPs are\nconsistent for all composite/individual and Mg/Ca and δ¹³C datasets.\nI have to say, that the data for C-isotopes are much more convincing than the Mg/Ca data,\nbut with the former ones alone, the study misses a new and convincing dataset.\nThank you - we agree that our δ¹³C dataset is ‘convincing’ and indicates that warmer CDW-\nlike waters prevailed on the Amundsen Sea shelf between 18–10 ka, while cooler, AASW-\nlike waters dominated on the Amundsen Sea shelf after ~10-7.9 ka, while. As outlined in our\noriginal submission, the dominance of cooler waters after ~7.5 ka was established by our\nearlier work based on δ¹³C and a limited number of Mg/Ca measurements (n=10;\nHillenbrand et al., 2017, Nature, https://doi.org/10.1038/nature22995).\nThe strength of the present study lies in the integration of an extensive new Mg/Ca\ndataset (18–0 ka) with additional δ¹³C data for the deglacial interval (18–10 ka).\nTogether they provide a coherent and robust reconstruction of water-mass variability over\nthe past 18 ka.\nA second point I would like to raise (and would like to ask the authors to deal with in a\nrevised version of the manuscript) is the assumption, that the water masses are evenly\ndistributed across the shelf (i.e., inner to outer shelf are influenced by the same water mass),\nan assumption that is based on modern observations. How does this might change when\nyou have completely different background conditions like an retreating marine ice shelf as it\noccurred during the study interval? Shouldn’t this significantly change the outer vs. inner\nrelationship and factors influencing it?\nYes, we are happy to provide additional details re: “how does this might change when you\nhave completely different background conditions like a retreating marine ice shelf as it\noccurred during the study interval’…”\nAn important first point is that the ice sheet had already retreated to near its modern\nposition before at least ~11 ka in the eastern part of our study area and before at least\n~12‒13 ka in its western part (Larter et al., 2014; see Fig. 1b), i.e. well before the\nobserved Mg/Ca and δ¹³C shifts. Therefore, the ‘background conditions’ have remained\nstable since that time, and there is no evidence to suggest that the pattern of shelf-sea\ncirculation since this retreat and the present differed significantly from today. A second\npoint is that we have rapidly thinning ice shelves and rapidly retreating grounding\nzones in the Amundsen Sea today and there is very little change in the CDW temperature\nas it traverses the shelf.\nThus, together with the modelling results highlighted in our earlier rebuttal (Nakayama et al.,\n2014; Naughten et al., 2021; Zhou et al., in prep.), which indicate residence time of shelf\nwaters — that is, the time required to fully replace water properties through advection\nfollowing a regime change – is relatively short and occurs over years to decades, we are\nconfident that the water masses were evenly distributed across the shelf during both the\ndeglacial phase and during the post-10 ka “modern” background conditions.\nWe have now added text to clarify this in the revised manuscript (line 227)."}, {"round": 3, "reviewer_comments": {"reviewer_3": "I would like to thank the authors for doing the requested changes in the manuscript. They have adequately answered all of\nmy points (including the new error analysis).\nI believe that this study will advance our knowledge of the AIS dynamics and recommend publication in Nature\nCommunications."}, "author_response": ""}]

Decision letter