Southern Ocean evidence for recurring West Antarctic Ice Sheet destabilization during Marine Isotope Stage 11

Domain: earth_science
Published: 2025-10-15
Editorial outcome: major_revision

Abstract
Millennial-scale deoxygenation of Antarctic Bottom Water (AABW) in the Atlantic Southern Ocean during past interglacials was linked to West Antarctic Ice Sheet (WAIS) melt-driven suppression of dense water formation along the Antarctic margin. However, the circum-Antarctic extent of these ‘AABW stagnation events’ and drivers of WAIS retreat remain unclear. Here, we identify recurring bottom water O2 minima in the central Pacific Southern Ocean during Marine Isotope Stage (MIS) 11 (424–374 ka ago) that are synchronous with their Atlantic Southern Ocean counterparts. As they (partially) align with Circumpolar Deep Water (CDW) warming above present-day levels and/or a reorganization of deep-ocean circulation, we postulate recurring and synchronized Pacific-Atlantic AABW perturbation events during MIS 11 through WAIS retreat and enhanced exposure to ocean heat (i.e., CDW) from below. This indicates a significant contribution of WAIS meltwater to sea-level high-stands during MIS 11 and, by analogy, to sea-level rise due to ocean warming in the future.

Introduction
Observations show that the Antarctic Ice Sheet (AIS) has lost mass over the past decades1,2. The West Antarctic Ice Sheet (WAIS) is particularly affected as it is prone to instabilities due to below sea-level grounding3, a landward-sloping geometry of the bedrock4, over-deepened continental shelf areas5, and large ice shelf areas rimming the ice sheet2,6. When fully melted, WAIS has the potential to raise global sea-levels by up to 4.3 m eustatic sea-level equivalent7. Antarctic Bottom Water (AABW) that forms by sinking of dense oxygenated shelf waters along the Antarctic margin supplies oxygen to the ocean interior8, and is therefore intricately linked to the geometry and/or advance of the WAIS on the Antarctic continental shelf9. Instrumental records bear witness to hydrographic and geochemical changes in the Southern Ocean that manifest the impact of WAIS retreat and associated meltwater supply on the Southern Ocean9: Antarctic shelf waters became fresher10, and AABW contracted11, lost oxygen12 and became less dense due to combined effects of warming and freshening13. Continued Antarctic ice mass loss, and hence meltwater supply to the Southern Ocean, is expected for the coming decades and centuries14, which may contribute to global sea-level rise15 and alter ocean circulation dynamics16–18.

One of the primary causes of WAIS destabilization from below via ocean warming is the upwelling of warm Circumpolar Deep Water (CDW) underneath vast floating ice shelves19,20 such as the Ross Ice Shelf and/or the Filchner-Ronne Ice Shelf (Fig. 1)6,15. Antarctic ice shelves act as a stabilization band, buttressing hinterland ice sheet flow across the grounding line2. However, they also represent a band of vulnerability making AIS prone to melting from below6. Upwelling of warm CDW near the Antarctic periphery favors WAIS melt and leads to meltwater discharge into the Southern Ocean in the Ross Sea, the Weddell Sea and the Amundsen-Bellingshausen Sea as highlighted by observations10,21 and numerical simulations22. Several processes can control the heat supply from the Southern Ocean to the WAIS margin, such as changes in the formation of Dense Shelf Water (DSW) and the associated density gradients at the continental shelf break keeping warm CDW off Antarctic continental shelves20, adjustments in wind- and gyre circulation propelling CDW across the Antarctic shelf break23, variations in the heat content and/or upwelling rate of CDW offshore24, and/or combinations thereof. Observations10 and numerical models12,16,18 attest that freshwater fluxes from WAIS melt can significantly perturb AABW formation along the Antarctic margin, particularly in the Ross- and Weddell Sea, where the majority of deep water mass formation in the Southern Ocean occurs today8,25. Yet, observational evidence on the interplay between WAIS- and CDW dynamics for different climate boundary conditions in the past remains scarce.

Marine sediment core evidence from Ocean Drilling Program (ODP) Site 1094 in the deep Atlantic Southern Ocean reveals transient decreases in bottom water O2 (BWO) levels during late Pleistocene interglacials26,27. Although some of these phases of low BWO conditions were explained by increases in surface ocean productivity and concomitant respiratory BWO consumption16, organic carbon fluxes can be excluded as the main driver of observed BWO minimum events during Marine Isotope Stage (MIS) 5e at ~127 kiloyears before present (ka BP)27 and during MIS 11 at ~397 ka BP (Fig. 2)26. Instead, these transient lows in BWO levels were found to be linked to short-term disruptions (i.e., a slowdown) in AABW formation in the Weddell Sea Embayment and an associated decline in the dispersal of well-oxygenated AABW northward26,27. These events were thus dubbed ‘AABW stagnation events’27 and were suggested to be driven by meltwater supply to the Weddell Sea26,27. However, transient AABW deoxygenation events during warmer-than-interglacial conditions have so far not been identified outside of the Atlantic Southern Ocean, specifically at a sediment core location other than ODP Site 1094. While WAIS retreat and associated freshwater input into the Southern Ocean were linked to the observed ‘AABW stagnation events’ in the Atlantic Southern Ocean, the spatial extent, and drivers of these events as well as possible associated ocean temperature thresholds of WAIS melt remain largely unconstrained.

Here, we reconstruct BWO concentrations in the central South Pacific Ocean during MIS 11 based on high-resolution reconstructions of authigenic U (aU) enrichments in foraminiferal coatings in sediment cores from International Ocean Discovery Program (IODP) Site U1540 (55°08.467ʹS, 114°50.515ʹW; 3580 m water depth; Fig. 1). Our study site is presently bathed in Lower CDW, which forms through the entrainment of well-oxygenated Ross Sea-derived AABW (Fig. 1a, b)8,25. This makes IODP Site U1540 particularly sensitive to changes in the oxygenation and/or formation of AABW in the Pacific sector of the Southern Ocean and their possible mechanistic link to WAIS retreat. We specifically focus on the climatic optimum of MIS 11, i.e., MIS 11c (~426–396 ka BP28), because it is often considered one of the best analogs for the current interglacial due to several reasons. First, both MIS 11c and the Holocene are characterized by low eccentricity-driven insolation forcing28. Second, the protracted duration of MIS 11c (>20 ka)28 matches the predicted length of the Holocene under unabated anthropogenic greenhouse gas emissions29, and might therefore allow for an assessment of the WAIS response to an extended exposure to ocean heat. Third, global mean temperatures during MIS 11c were up to 2°C higher than pre-industrial levels28,30, matching predictions for the end of this century31. Finally, sea-level high-stands during MIS 11c reached ~6–13 m above present-day levels and suggest significant contribution from the Antarctic ice sheet32,33. Our multi-proxy approach allows an assessment of the drivers governing reconstructed BWO changes at IODP Site U1540 based on combined reconstructions of surface ocean productivity using high-resolution X-ray fluorescence (XRF) Ba/Fe variations and opal (i.e., biogenic silica) weight percentages, deep-water mass provenance changes based on (biology-independent) Nd isotope variations (εNd) of fossil fish teeth/-debris and planktic foraminifera, and bottom water (i.e., CDW) temperature (BWT) estimates via Uvigerina spp. Mg/Ca ratios–thereby circumventing stratigraphic biases between proxy records. A comparison of our data with ODP Site 1094 in the Atlantic sector of the Southern Ocean26, a site presently bathed in Lower CDW primarily influenced by Weddell Sea-derived AABW (Fig. 1a, b)8,25, provides evidence for recurring and synchronous AABW deoxygenation events in the Pacific and Atlantic sectors of the Southern Ocean during MIS 11c and 11b. This suggests widespread WAIS destabilization both in the Weddell- and Ross Sea Embayment linked to the long excess warm climate conditions of MIS 11, leading to meltwater-driven interruptions in deep water formation in the Ross Sea and Weddell Sea. Given the synchrony of these AABW deoxygenation and/or -contraction events, we postulate a significant contribution of WAIS retreat to global sea-level high stands during MIS 11.

Results
Bottom water oxygen changes
The enrichment of aU in foraminiferal coatings indicated by foraminiferal U/Ca and U/Mn ratios have been suggested to reflect redox-driven changes in aU enrichment in marine sediments, and hence BWO levels34. Globigerina bulloides and Uvigerina spp. U/Ca- and U/Mn ratios at IODP Site U1540 during MIS 10 and 12 are higher than during MIS 11, which is consistent with glacial-interglacial variations in sedimentary aU levels in the Atlantic Southern Ocean, including ODP Site 1094 (Fig. 2b–d)26,34. During MIS 11, foraminiferal U/Ca and U/Mn ratios are punctuated by recurring, millennial-scale increases in U/Ca by ~20 nmol/mol (G. bulloides) and ~10 nmol/mol (Uvigerina spp.) as well as rises in U/Mn by ~60 µmol/mol (G. bulloides) and ~50 µmol/mol (Uvigerina spp.), specifically at ~416–407 ka BP, ~402–398 ka BP and ~395–390 ka BP (albeit with a poor expression of the youngest event in G. bulloides U/Ca levels; Fig. 2b, c). The magnitude of these foraminiferal aU enrichments at IODP Site U1540 is roughly a quarter of the MIS 12-to-MIS 11 transition (Fig. 2b, c). Although changes in seawater carbonate ion ([CO32-]) variations alter U/Ca ratios in foraminiferal tests during calcification35, G. bulloides U/Ca shifts of ~20 nmol/mol suggest a surface ocean [CO32-] change of more than 400 µmol/kg, which is unrealistic. Similar U/Ca-[CO32-]-calibrations for Uvigerina spp. do not exist. However, it is unlikely that both the planktic foraminifer G. bulloides and the benthic foraminifer Uvigerina spp. record simultaneous changes in surface and bottom water [CO32-], respectively, especially with the extreme magnitude as implied by existing calibrations35. In contrast, the high U/Ca values recorded at IODP Site U1540 suggest a U enrichment in authigenic foraminiferal coatings to be the main driver of our foraminiferal U/Ca- and U/Mn data. The fact that both planktic and benthic foraminiferal U/Mn records (and U/Ca with potential biases from different test morphometrics34) closely agree with each other reinforces our notion of a primary authigenic influence of BWO variations and sedimentary redox-conditions on our aU-based proxies. We thus refer to the millennial-scale increases in U/Ca and U/Mn levels at IODP Site U1540 at ~416–407, ~402–398 and ~395–390 ka BP as “deoxygenation events” (hereafter referred to as EI, EII and EIII, respectively, Fig. 2), in analogy to the definition of “stagnation event” of ref. 27; yet, we deliberately choose to refer to EI-EIII without a mechanistic implication.

The first two increases in foraminiferal U/Ca and U/Mn levels during MIS 11c during EI and EII match within age uncertainties with millennial-scale peaks in bulk sedimentary aU levels at ODP Site 1094 in the Southern Atlantic Ocean26, while the agreement is less clear for EIII (Fig. 2b–d). The age model of ODP Site 1094, which is based on benthic foraminiferal δ18O tuning36 to the LR04 benthic δ18O stack37 (similar to IODP Site U1540; Methods), is linked with high age uncertainties given sparse benthic δ18O constraints in the study interval (Fig. 2f). While the chronology at IODP Site U1540 is confirmed by an excellent agreement of Neogloboquadrina pachyderma abundance-based summer sea surface temperature (SST) estimates at the study site with water isotope variations (δD) in the Antarctic EPICA Dome C ice core30 based on the AICC2023 ice age scale38 (Fig. 2e), a similar independent test is lacking for ODP Site 1094. An adjustment of the existing age model at ODP Site 1094 within its uncertainties and adhering to the original age model premise36 emphasizes ambiguities regarding the relative timing of EIII at IODP Site U1540 and ODP Site 1094 (Fig. 2f). Specifically, considering a slight shift of the tie point at 395.2 ka BP at ODP Site 1094 (by ~2 ka) maintaining a good match between the benthic foraminiferal δ18O record to the LR04 δ18O stack, shifts the aU enrichment event at ODP Site 1094 at 397 ka BP by ~2 ka towards younger ages (Fig. 2d, f). This sensitivity test highlights that the foraminiferal U/Ca and U/Mn increase during MIS 11b at IODP Site U1540 (i.e., EIII) might have coincided with a similar aU enrichment event at ODP Site 1094 (Fig. 2d). Without high-resolution δ18O data and/or additional age control for ODP Site 1094 it can, however, not be excluded that EIII represents a local event at IODP Site U1540.

Surface ocean productivity changes
To deconvolve possible biological, dynamical and/or physical influences on BWO variations at the study site, we use for comparability the same marine sediment core proxy as applied at ODP Site 1094, i.e., high-resolution XRF log(Ba/Fe) ratios (Fig. 3d), to approximate the flux of organic matter to the sediment39, in addition to temporal changes in opal weight percentages40. At IODP Site U1540, XRF log(Ba/Fe) ratios are higher during MIS 11 than during the adjacent glacial intervals MIS 10 and 12, showing similar trends39 as seen at ODP Site 1094 (Fig. 3d). These trends are consistent with an increase in coccolith accumulation rates and temperature-corrected coccolith Sr/Ca ratios during MIS 11 in nearby South Pacific core PS75/059-2 (Fig. 3e), which reflect enhanced coccolithophore productivity in the central South Pacific during that time41. However, opal weight percentages at IODP Site U1540 are lower during MIS 11 than during MIS 12 and MIS 10, which is consistent with opal content data from core PS75/059-2 (Fig. 3f)40,42. While this may indicate major shifts in ecosystem structures (i.e., siliceous versus calcareous organisms) over glacial-interglacial timescales, systematic, marked increases in surface ocean productivity, and by inference the export of organic matter to the seafloor, are not observed during periods of elevated G. bulloides and Uvigerina spp. U/Ca and U/Mn ratios at IODP Site U1540 (i.e., EI-EIII; Fig. 3b).

Variations in deep water provenance
Given the uncertainties in unraveling the drivers governing BWO variations from reconstructions of export production changes alone, we combine our analyses with Nd isotope (i.e., 143Nd/144Nd) estimates (expressed as εNd) from fossilized bio-phosphates (fossil fish teeth and/or -debris) and Fe-Mn-encrusted planktic foraminifera43 that serve as indicators of deep water provenance/-mixing changes, and hence deep-ocean circulation dynamics at IODP Site U1540. Paired εNd measurements of fossil fish teeth/-debris and planktic foraminifera (n=3) match within 2σ-uncertainties of εNd = 0.3. The combined εNd record at IODP Site U1540 indicates higher (i.e., more radiogenic) values during MIS 10 and 12 (εNd = −6.3 ± 0.3 (n=1) and εNd = −5.9 ± 0.5 (n=19), respectively) than during MIS 11 (εNd = −7.2 ± 0.3, n=60; Fig. 3g). In addition, overall low εNd values during MIS 11 are interrupted by shifts toward higher εNd values by 0.4–1.1 εNd units, which exceeds the long-term 2σ-external reproducibility of 0.3 εNd units (Fig. 3g). These εNd shifts occur at the same time as the observed increases in G. bulloides and Uvigerina spp. U/Ca and U/Mn levels during EII and EIII at our study site, while the εNd shift during EI is less pronounced (Fig. 3b, g).

Ice-rafted detritus supply
The abundance of ice-rafted detritus (IRD) indicates the presence of icebergs at IODP Site U1540, which is a function of their production rate at the Antarctic margin and their transport efficiency to the study site. IRD abundances are highest during MIS 12, and low or (near-) zero during MIS 11 and 10 (Fig. 3a). During MIS 11c and MIS 11b, however, short-lived IRD peaks at respectively ~410–408 ka BP and ~394–390 ka BP coincide with elevated G. bulloides and Uvigerina spp. U/Ca and U/Mn ratios during EI and EIII, respectively (Fig. 3a, b).

Bottom water temperature changes
Uvigerina spp. Mg/Ca ratios were shown to reflect BWT variations44, and serve here to estimate CDW temperature variations at our study site. To evaluate the bias from contamination on Uvigerina spp. Mg/Ca ratios, we measured Al/Ca, Fe/Ca and Mn/Ca ratios on the same samples. Uvigerina spp. Al/Ca is below the limit of detection (<4.7 μmol/mol) in all except 4 out of 80 samples and does not show a statistically significant correlation to Uvigerina spp. Mg/Ca ratios within 95% confidence levels (r²=0.15, p=0.61). Uvigerina spp. Fe/Ca is below the limit of detection (<2.1 μmol/mol) in all except 14 samples and does not correlate with Uvigerina spp. Mg/Ca values (r²=0.04, p=0.47). Uvigerina spp. Mn/Ca values vary between 50 and 220 µmol/mol and show a statistically significant correlation with Mg/Ca values (r²=0.56, p=0.0001), likely indicating climatically driven changes in the precipitation of Mn-oxyhydroxide in marine subsurface sediments rather than a significant bias affecting the Mg/Ca-based BWT estimates at our study site45. Indeed, assuming a Mg/Mn ratio in authigenic coatings45 of 0.1 mol/mol found in oceanic ferromanganese crusts, contaminant Mg/Ca would contribute a maximum of 0.02 mmol/mol to overall Mg/Ca values and would affect the BWT results insignificantly (<0.2°C). The combined evidence suggests that the impact of contamination on reconstructed BWT based on Uvigerina spp. Mg/Ca ratios at IODP Site U1540 is negligible.

While Uvigerina spp. Mg/Ca-derived BWT approach the freezing point during MIS 10 and 12, they reach present-day annual-mean levels (~1.1°C)46 during the climatic optimum of MIS 11 (MIS 11c: 1.1 ± 0.8°C, n=50; Fig. 3h). In addition, during phases of MIS 11c, i.e., ~426–416 ka BP and ~410–406 ka BP, BWT at IODP Site U1540 exceed present-day levels by ~1°C, before they progressively decrease to MIS 10 levels, with a strong BWT minimum at ~−2°C during MIS 11b, i.e., ~396–390 ka BP (Fig. 3h). Periods with elevated foraminiferal U/Ca and U/Mn ratios at our study site are therefore characterized by BWT exceeding present-day levels (EI), declining BWT (EII) and BWT levels near freezing (EIII; Fig. 3h). This is confirmed by similar BWT estimates at deep Southeast Pacific45 ODP Site 1123 and Atlantic Southern Ocean36 ODP Site 1094 (Fig. 3h).

Methods
Study area
Sediment cores from IODP Site U1540 (55°08.467ʹS, 114°50.515ʹW; 3580 m water depth) were drilled on the eastern flank of the East Pacific Rise north of the modern sub-Antarctic Front in the central South Pacific (Fig. 1)69. They contain well-preserved calcareous ooze69 during MIS 11 and siliceous ooze in MIS 10 and 12. The high carbonate preservation and high sedimentation rates averaging ~7.5 cm/ka during MIS 11 make the core site ideally suited to study the nature, drivers and impacts of bottom water O2 variations on (sub-) millennial timescales and their links to AABW- and Lower CDW dynamics in the central South Pacific Ocean.

There are several distinct sites of AABW formation in the Antarctic periphery with two major sites in the Ross Sea and Weddell Sea located along the WAIS margin8,25. The formation of AABW is generally driven by the entrainment of CDW into DSW (i.e., the AABW precursor water mass) that forms on the Antarctic continental shelf and spills over the Antarctic continental margin into the abyssal Southern Ocean due to its high density8. Although DSW formation is mostly controlled by brine formation during sea ice formation in polynyas and cooling beneath vast Antarctic ice shelves52,70,71, spatial differences exist in the formation mode, the hydrographic characteristics and the pathway of AABW (and its precursor water masses) in the abyssal ocean8,25. AABW dispersal in the global deep ocean is strongly bathymetry-controlled, with AABW sourced from the Weddell Sea and the Ross Sea occupying mostly the Atlantic sector and Indo-Pacific sector of the Southern Ocean, respectively8,25 (Fig. 1b). While Lower CDW at our study site, i.e., IODP Site U1540, is primarily influenced by Ross Sea-sourced AABW, ODP Site 1094 is mostly bathed in Lower CDW that is impacted by Weddell Sea-derived AABW (Fig. 1b).

Dense bottom water formation in the Ross Sea accounts for ~25% of AABW formation at present-day8. In this region, dense water formation primarily occurs during brine rejection during sea ice formation with only a minor contribution from supercooling of seawater under the Ross Sea Ice Shelf62. The western Ross Sea produces the saltiest variety of bottom waters (S > 34.72) with relatively high temperatures of −0.6–-0.3°C, while bottom waters that originate in the eastern Ross Sea are characterized by low salinities (S < 34.70) and low temperatures (< −0.8°C)8. AABW emerging from the Ross Sea is O2-enriched ([O2] = 210–225 μmol/kg)48 and spreads into the deep Southeast Pacific along the East Pacific Rise, into the deep Southwest Pacific east of Campbell Plateau or westward into the Australian-Antarctic Basin (Fig. 1b)8,25.

The dominant production site of AABW in the global ocean today is the Weddell Sea (~60%)8. Unlike in the Ross Sea, AABW precursor water masses in the Weddell Sea are formed by both brine rejection during sea ice formation in polynyas and supercooling underneath the floating Filchner-Ronne Ice Shelf8. Bottom waters derived from the Weddell Sea comprise the coldest (< −1°C) and freshest (S < 34.64) AABW variety found in the global ocean8. Steered by South Atlantic bathymetry, well-oxygenated Weddell Sea-sourced AABW ([O2] > 250 μmol/kg)8,48 is exported northward into the Argentine Basin and Cape Basin, with only weak dispersal further eastward into the Indian Southern Ocean and no contributions to the central South Pacific (Fig. 1b)8,25.

Sampling and sample processing
In total, 169 sediment samples (10 cm3) were taken from the shipboard splice69 of IODP Site U1540 at 5 cm-increments for MIS 10 and 12, and at 2 to 3 cm-increments for MIS 11. All samples were freeze-dried and then washed with deionized water over a sieve with a mesh size of 150 µm. The coarse fraction of the sediment was then dried overnight in an oven at a temperature of 40–50°C. Dry bulk sediments and the coarse fraction of the sediment samples (>150 µm) were weighed with a Sartorius PT310 toploading balance with a precision better than 0.01 g.

Sedimentary census counts
Planktic foraminiferal assemblages as well as the abundance of rock and mineral fragments (i.e., IRD) were determined based on the coarse fractions (>150 µm) through sedimentary census counts72. Specifically, the samples were split with a micro-splitter, were equally distributed on the counting tray and all intact sedimentary grains were counted under a stereomicroscope until a minimum of 300 intact planktic foraminiferal specimens was reached. The percentage of the planktic foraminifer N. pachyderma was then calculated relative to the total number of planktic foraminifera. Neogloboquadrina pachyderma abundances were then converted73 to summer SST. The presence of IRD was quantified as the number of IRD per dry weight of the coarse fraction (#IRD/g > 150 µm). The 1σ-uncertainty of the census counts was estimated based on 13 replicate counts of the same sample aliquot and is 0.9 % for the abundance of N. pachyderma and 3.4 #IRD/g > 150 µm.

Stable oxygen isotope analyses of benthic foraminifera
For stable oxygen isotope analyses, two to five specimens of the epibenthic foraminifera Cibicidoides wuellerstorfi and Cibicides kullenbergi larger than 250 µm were handpicked with a brush under the microscope to obtain at least 20–80 μg CaCO3. Foraminiferal stable oxygen isotopes were measured with a Thermo Fisher Scientific MAT 253 isotope ratio mass spectrometer coupled to a Kiel IV carbonate preparation device at the Leibniz Laboratory for Radiometric Dating and Stable Isotope Research at Kiel University, Germany. The data are expressed in delta-notation (δ18O) with respect to the Vienna Pee Dee Belemnite (VPDB) standard. The analytical precision of the δ18O analyses is better than 0.08‰ (1σ), as derived from repeated measurements of different internal and international standards (i.e., NBS19, IAEA-603). The 1σ-uncertainty of full replicate C. wuellerstorfi and C. kullenbergi δ18O analyses amounts to 0.06‰ VPDB for C. kullenbergi (n=14) and 0.04‰ VPDB for C. wuellerstorfi (n=3), which is smaller than the analytical uncertainty. The C. wuellerstorfi and C. kullenbergi δ18O records were adjusted by +0.64‰ VPDB to account for disequilibrium effects74.

Trace element analyses of foraminifera
To reconstruct BWT variations at IODP Site U1540 via Uvigerina spp. Mg/Ca ratios and BWO variations via the U and Mn enrichment in foraminiferal coatings (i.e., U/Ca- and U/Mn ratios, respectively)34, the planktic foraminifer G. bulloides (20–25 specimen; 250–300 µm size fraction) and the benthic foraminifer Uvigerina spp. (15–20 specimen; >250 µm size fraction) were handpicked under the microscope to obtain a total weight of 300–500 µg CaCO3. The selected foraminifera were crushed and were oxidatively cleaned following established protocols75. In brief, the first oxidative cleaning step includes several milli-Q and ultrapure methanol rinses to remove adhering clay from the sample. The organic matter was oxidatively removed by adding alkali-buffered 1% H2O2 solution to the samples that were subsequently placed into a hot water bath at ~90–100°C for 10 min. Then, the samples were rinsed with milli-Q twice and the remaining silicate particles were individually removed from the samples with a fine brush when present. Before the measurement, the samples were weakly acid-leached with ultrapure 0.001 M HNO3 and dissolved for analysis in ultrapure 0.1 M HNO3 to obtain the sample concentrate.

To quantify major element/Ca ratios (i.e., Mg/Ca, U/Ca, Mn/Ca, Fe/Ca and Al/Ca), the samples were initially diluted by adding 25 µl sample concentrate to 275 µl 2% ultrapure HNO3 – the amounts were adjusted if needed to obtain a Ca concentration of 20 µg/g. The samples were then measured on an inductively coupled plasma-optical emission spectrometer (ICP-OES, Spectro ARCOS II) at the Institute of Geosciences at Kiel University, Germany. The results were calibrated with single element standards and drift-corrected based on multiple measurements of ECRM 752-1 (i.e., limestone powder with Mg/Ca = 3.824 mmol/mol)76. The external measurement accuracy of the Mg/Ca ratios was determined based on the reference material JCt-1 (giant clam Tridacna gigas)77. Our measured Mg/Ca ratio of JCt-1 is 1.245 ± 0.011 mmol/mol (1σ, n=18), which matches within uncertainty the reported value (1.289 ± 0.045 mmol/mol)77. The analytical uncertainty of Uvigerina spp. Mg/Ca ratio is based on four repeated measurements of the same sample solution and is on average 0.005 mmol/mol. Uvigerina spp. Mg/Ca ratios were converted to BWT using the calibration of ref. 44 to ensure comparability to the BWT reconstructions at ODP Site 1123 in the Southwest Pacific45.

To reconstruct BWO variations at IODP Site U1540, we measured U/Ca- and U/Mn ratios of both the planktic foraminifer G. bulloides and the benthic foraminifer Uvigerina spp. The foraminiferal U/Ca- and U/Mn ratios reflect the post-depositional, redox-driven authigenic precipitation of U in the sediment, which also occurs around deposited foraminiferal tests, under depleted O2 conditions in bottom waters and marine sub-surface sediments34. Because the U concentration in the authigenic coatings of foraminifera (up to 800 nmol/mol) is much higher than the lattice-bound U concentrations of G. bulloides and benthic foraminifera (0–20 nmol/mol)35, the measured U/Ca ratio of weakly or oxidatively cleaned foraminifera primarily indicates the redox-driven U concentration in the authigenic coatings34. A normalization of G. bulloides and Uvigerina spp. U/Ca ratios with Mn/Ca reduces test-morphological biases of measured U/Ca levels34.

To estimate G. bulloides and Uvigerina spp. U/Ca and U/Mn levels, the sample concentrates were diluted based on ICP-OES-measured Ca levels to a Ca concentration of 20 µg/g. G. bulloides and Uvigerina spp. U/Ca and Mn/Ca levels were then measured on an Agilent 7900 inductively coupled plasma-mass spectrometer (ICP-MS) at the Institute of Geosciences at Kiel University, Germany. The drift correction of the samples is based on repeated measurements (every tenth sample) of the JCt-1 standard77. The analytical uncertainty of the measurements is based on the relative standard deviation of the signal ratios and is on average ~3.6% for U/Ca and ~1.2% for Mn/Ca. The precision of the U/Ca analyses is based on repeat measurements of the reference material KCp-1 (U/Ca = 1031 ± 139 nmol/mol, 1σ, n=13, fossil coral)78 and HST 210 (U/Ca = 228 ± 27 nmol/mol, 1σ, n=11, Globigerina mud from Sulu Sea, Institute of Geosciences, Kiel).

Nd isotope reconstructions of seawater
To reconstruct the Nd isotopic composition of seawater as a water provenance indicator, 22 samples of fossilized bio-phosphate (fossil fish teeth and/or -debris) and 58 samples of Fe-Mn-encrusted mixed planktic/mono-specific foraminifera (primarily Globorotalia inflata) were picked from the coarse fraction (>150 µm) of IODP Site U1540 sediments (at least 2–3 fish teeth or 5–10 pieces of fish debris and ~40 mg foraminiferal CaCO3). The selected sample material was subsequently cleaned and prepared for analysis following existing protocols55. Briefly, planktic foraminiferal samples were gently crushed to open all chambers. Subsequent physical cleaning of detrital particles involved multiple rinses and ultrasonication in Optima grade methanol, followed by multiple rinses in deionized water to ensure any attached clay particles were removed. Cleaned foraminiferal fragments were assessed under the microscope for any remaining clays or extraneous detrital material.

After cleaning, the foraminiferal samples were dissolved in 5% Optima Acetic Acid and fish teeth/-debris samples were dissolved in Optima Aqua regia (1:3 mixture of HNO3 and HCl). Subsequently, the samples were transferred into Teflon beakers and dried down on a hot plate. Nd was isolated by two-step column chemistry to avoid interferences with other elements. For the rare earth element (REE) separation, 100 µl shrink-fit Teflon columns were used that are loaded with Eichrom TRU-spec resin (100–150 µm mesh). A procedural blank was included with each TRU-spec batch. The eluted sample containing only REEs was then dried down and ready for Nd separation. Calibrated Teflon columns pre-loaded with Eichrom LN-spec resin (50–100 µm mesh) were used to isolate Nd from the other REEs. Subsequently, collected Nd fractions were dried down, followed by the removal of any potential organic compounds that could have leaked from the columns using a concentrated 1:3:1 Optima grade mixture of HNO3, HCl and H2O2. Finally, the samples were dried down and redissolved in 2% Optima grade HNO3 for analysis.

All reported Nd isotope data were analyzed at Pennsylvania State University, University Park, PA, USA, using a Thermo Fischer Scientific Neptune-Plus multicollector-ICP-MS (MC-ICP-MS). A desolvation introduction system (an Apex nebulizer) was used for sample introduction into the MC-ICP-MS. Measured 143Nd/144Nd ratios were corrected using a reference value of 146Nd/144Nd = 0.7219 and an exponential mass fractionation law. Mean 143Nd/144Nd ratios of repeated measurements of the international standard JNdi-1 for a given analytical session and the published 143Nd/144Nd of JNdi-1 (0.512115)79 were used to report normalized 143Nd/144Nd ratios of the samples. Nd isotope results are reported in ε-notation (εNd) as parts per 10,000 deviations from Chondritic Uniform Reservoir levels80 of 143Nd/144Nd = 0.512638. The reproducibility of the εNd analyses was estimated based on repeated measurements of the international standard JNdi-1, alongside the samples, and is 0.3 ε-units (2σ).

X-ray fluorescence scanning and sedimentary opal content
Non-destructive XRF scanning was conducted on the shipboard splice of IODP Site U1540 with 2 cm-resolution using an Avaatech XRF core scanner at Texas A&M University, College Station, TX, USA. Split core surfaces of the archive halves were scraped cleaned and covered with a 4 µm-thin SPEX CertiPrep Ultralene foil to avoid cross-contamination and were then subjected to XRF scanning during consecutive 10 kV-, 30 kV- and 50 kV runs. We used the Ba and Fe intensities (area counts) from the 50 kV run to calculate the logarithmic ratio of log(Ba/Fe). The sedimentary opal content was measured on 30 samples through wet-chemical digestion and spectrophotometry following established methods81. Repeated measurements of the same sample yield an analytical uncertainty of ~0.11 weight-% opal (1σ, n=13). The analysis of internal standards shows a long-term reproducibility of 0.47 weight-% opal (1σ). Our discrete opal measurements are plotted on the high-resolution opal record from IODP Site U1540 that was obtained using the polynomial regression from core PS75/56-02 from the same study location, which itself was calibrated with the gamma ray attenuation record69 and discrete opal measurements42.

Chronology
The age model for the interval of interest at IODP Site U1540 was established by a stratigraphical alignment of our high-resolution benthic foraminiferal (C. wuellerstorfi and C. kullenbergi) δ18O record to the benthic foraminiferal LR04 δ18O stack37. The chronology is based on a linear interpolation between the resulting seven tie points. Resulting sedimentation rates vary between 6–7 cm/ka during MIS 10 and 12, and increase up to 9.3 cm/ka during MIS 11 (Fig. 2a). Our age model results in an excellent agreement of changes in N. pachyderma abundance-based summer SST at the study site with EPICA Dome C (EDC) δD variations30 based on the AICC2023 ice age scale38 (Fig. 2a, e).

Data availability
All shipboard data from IODP Expedition 383, including for Site U1540, are available from the IODP data archive (https://web.iodp.tamu.edu/LORE/) and the scientific proceedings of IODP Expedition 383 (ref. 69). New data presented here are accessible via the PANGAEA database (https://doi.pangaea.de/10.1594/PANGAEA.974158)82.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of Jebasinski et al.\nThis is an impressive study of deep sea oxygenation changes in the Southern Ocean during a past warm climate in the ice\nage era. The authors have multiple proxies to draw from to conclude that there were millennial-scale low oxygen events in\nthe deep Pacific, which tied together with published results from the deep Atlantic, point to circum-Antarctic ocean response\nto ice sheet melting. Authigenic uranium, neodymium isotopes, bottom water temperatures, barium and opal content provide\na very careful analysis of the suggested events. Honestly, I find the study to be technically impeccable and recommend this\nbe published with a wide audience to incorporate these types of events into climate modeling for the future.\nTo go further, I can offer only one or two reflective comments. The linkage between synchronous changes in both Ross Sea\nand Wedell Sea bottom water formation is very interesting. Could the authors suggest a possible test for future evaluation of\nwhether the “c” or “d” scenarios (from Figure 4) for EI and EII would have been more likely to have occurred?\nI also see the difference in the Events I and II versus III, where the bottom water temperature drop during EIII suggests a\n“cold” stagnation event, versus the “warm” melt-driven events EI and EII. Are there any other interglacial periods where there\nis similar bottom water temperature data available as possible other examples of these “cold” stagnation events?\nFinal thought regarding the interpretation of the epsilon Nd data. Line 222. “These shifts occur mostly at the same time as the\nobserved MIS11 increases in …U/Ca…”. Could this assessment be tightened up more statistically or specifically? From my\nreading of Figure 3, it seems like EIII and EII have clear increases in eNd but E1 could be arguable.", "reviewer_2": "Review: “SouthernOceanEvidenceforRecurringWestAntarcticIceSheetDestabil- \nization During Marine Isotope Stage 11” \nAuthors: L. Jebasinski, D. A. Frick, A. K. I. U. Kapuge, C. Basak, M. Saavedra-Pellitero, G. \nWinckler, F. Lamy, J. Gottschalk \nSummary \nThis study uses a multi-proxy approach to identify recurring millennial-scale deoxygenation \nepisodes in AABW in the central Pacific during MIS 11, concurrent with similar events in the \nAtlantic. These synchronized AABW stagnation events are linked to WAIS retreat driven by \nsubsurface warming from CDW. The results suggest a circumpolar response to ocean heat, with \nWAIS meltwater contributing significantly to MIS 11 sea-level high stands and potentially to \nfuture sea-level rise under continued warming. \nThe authors present interesting and timely results, highlighting the physical and dynamical \nroles of WAIS meltwater and CDW in driving recurring AABW deoxygenation events, with \nrelevance to ongoing and future climate change. Overall, I find this to be a well-written and \nwell-structured paper. However, I have several major concerns, particularly regarding the \ninconsistentproxypatternsacrosstheproposedAABWstagnationeventsandtheinterpretation \nof key physical processes, as detailed below. \nMajor comments \n(1) Recurring events: I appreciate the authors’ use of a multi-proxy approach in Figures \n2 and 3, which reveals generally good agreement among several variables during MIS 11. \nHowever, not all proxies exhibit consistent recurring patterns across the EI–EIII events. \nFor example, summer SST/water isotope (δD) and benthic foraminiferal δ18O (Fig. 2), as \nwell as BWT and relative sea level (Fig. 3), do not show clearly repeated signals. Could the \nauthorsclarifytheReasonsforthesediscrepancies? Inaddition,theterm“AABWstagnation \nevents” is not clearly defined in the manuscript. At times, two events are mentioned; at \nother times, three, depending on the interpretation of the IODP site data. I suggest the \nauthors clearly specify how many such events are explicitly discussed and on what criteria \n1 \nthis identification is based. \n(2) Negative feedbacks between WAIS retreat and AABW contraction: I have a \nconcern regarding the authors’ statement that “meltwater supply into the Southern Ocean \nmight have steepened the isopycnal gradient along the shelf break” (lines 376–377). Whether \nsuch steepening occurs likely depends on how effectively the meltwater is retained on the \ncontinental shelf. Given the large volume of meltwater input (∼50 m sea-level equivalent, \nas shown in Fig. 3i), a substantial portion would likely have escaped to the open ocean. In \nthat case, the West Antarctic shelf would not remain isolated, and the isopycnal gradient \nalong the shelf break would more likely be flattened rather than steepened. \n(3) Development of an open-ocean polynya: I do not agree with the authors’ statement \nthat “expanded Antarctic sea ice and northward shifted westerly winds could also have pro- \nmoted the development of an open-ocean polynya favoring deep-reaching convection” (lines \n381–383). While I understand that this conclusion is based on the assumption of “a north- \nward shift of the westerlies”, it is important to note that this occurred under warming \nconditions (as indicated by SST in Fig. 2e). In general, open-ocean polynyas are expec- \nted to decrease in a warmer climate due to enhanced stratification and reduced sea ice \nformation. \n(4) CDW upwelling in Ross Sea (Fig. 4c): I cannot follow the authors’ statement “warm \nCDW intrusions onto the continental shelf of the Ross Sea Embayment, presumably along \nthe eastern limb of the Ross Gyre” (lines 399–401). Are the authors referring to CDW itself, \nor to the Antarctic Slope Current (ASC) as the transport mechanism? I do not understand \nhow CDW would move along the Ross Gyre as shown in Fig. 4c. \nMinor comments \nIn Sections 2.1 and 2.2, please refer specifically to the subpanels in Figures 2 and 3 (e.g., \nFig. 2a, 2b, Fig. 3a, 3b, etc.) where appropriate. Given the large number of subplots, it is \ndifficult for readers to follow the discussion without clear and direct references to the relevant \npanels. \nFigure 1b: I am not sure it is appropriate to plot tracer concentrations originating from both \n2 \nthe Weddell Sea and the Ross Sea on the same figure. The overlapping region within the \nWeddell Sea makes the results difficult to interpret. \nFigure 2d caption: Change “age model (see Results)” to “age model (see Methods)”. \n3", "reviewer_3": "This manuscript presents a new multi-proxy and high time-resolution assessment of the impact of past West Antarctic Ice\nSheet (WAIS) retreat events via their impact on Antarctic Bottom Water formation as recorded by bottom water oxygen\n(BWO) concentrations. Previously, only one site in the Atlantic sector of the Southern Ocean has revealed intervals of low\nBWO, linked to WAIS retreat and the impact of a warmer ocean. Here, the authors move to the Pacific sector of the Southern\nOcean to test these hypothesized processes and feedbacks in another region, but one which remains influenced by WAIS\nbehavior. The study targets a particularly long interglacial (MIS 11) which has many relevant properties for us to consider in\nprojecting future climate changes (outlined in detail, and clearly, in the manuscript). Of importance is the new finding that\nthese low BWO events are also found in MIS 11 in the Pacific sector, revealing that there was a dynamic WAIS here which\nalso impacted AABW formation and properties. Because this also has implications for our sea-level budget contributions\nfrom WAIS in the past (and future), the results of this work will be of interest to several audiences: paleoceanographers,\ngeochemists, climate and ice sheet modelers, and glaciologists.\nThe manuscript is well-written and the graphics are clear. The authors do an excellent job in the early pages at setting out\nthe modern context and concerns, as well as the paleo-environmental evidence for these BWO events in the recent\ngeological past. The authors have amassed a detailed suite of geochemical indicators, at high temporal resolution, and\ncombined the results in a detailed evaluation which ensures that the key processes can be explored and explained. The\nDiscussion is well-written, and balances evaluation and detail with clear interpretations. My comments here reflect a need\nfor increased clarity in a few places, which should ensure that the nuances as well as the highlights of the results are evident\nto readers; they do not represent any significant flaws to the work being presented.\n1) The authors highlight the synchrony or near-synchrony of events between this site (U1540) and the previous record in the\nAtlantic (ODP 1094). The age model of ODP 1094 is important here, because the new data seems well-constrained with a\nhigh resolution tuning to the EPICA ice core. In line 173 the authors note that “a slight shift of the tiepoint at 395.2 kyr bp …\nmaintains a good match”, but I don’t find the rationale for why this shift was made. It doesn’t seem to have been made with\nthe assumption that the BWO events should be synchronous, especially given the different process driving this event in\nU1540. The text here seems to imply that the authors didn’t try to align the BWO signals, but a sentence or two is needed to\nclarify what was done and why.\n2) The Discussion on Drivers of AABW stagnation (starting line 323) includes mention of atmospheric and ocean\nmechanisms which could lead to WAIS destabilisation which includes mention of ocean warming (line 334). As the ocean\nwarming is discussed in a later section I suggest that the authors flag here that this mechanism will be returned to. I initially\nread this section with some skepticism having inferred that the authors were proposing ocean temperatures as the driver for\nall three events, when MIS 11b is different. A brief note to clarify this would help future readers.\n3) The role of CDW in AABW stagnation events (line 387 onwards) considers the role of ocean temperatures. The first few\nparagraphs detail the warming effect, whereas a later paragraph highlights that the event in MIS 11b occurs during cooling\n(and in fact when bottom water temperature reach their minimum). However, the opening sentences don’t note that only 2 of\nthe 3 stagnation events occur with warmer bottom waters. Rather than saying “events…can be partially linked to warm and\nwarmer-than-present…” (line 388-389) it would be more clear if the authors stated that 2 of the 3 events occurred under\nthese conditions.\n4) Lines 424-426 describe a relatively locally important process to account for the low BWO event EIII. Does this mean that\nwe should not expect a similar event to be expressed in the Atlantic sector (influenced by the Weddell Sea), or could it\naccount for the temporal offset? With this in mind, the unclear reason for shifting a tie-point in ODP 1094 emerges again.\nMinor comments:\nLine 55-6: this is a long and complicated sentence. Rather than trying to combine the stabilization and vulnerability in a\nsingle sentence, this may be more clear if 2 sentences are used here, one for each scenario.\nLine 76-77: unusual expression of dates. If referring to “thousand years ago” then ka can be used (127 ka), rather than 127\nkyr bp. The lowercase bp is confusing because capitalized BP is used to refer to radiocarbon dates.\nLine 147 needs to define what the “E” is an abbreviation of (event? And were these events defined by the study of ODP\n1094?) They are defined in the figure captions, but they are also never again referred to in the text, which uses the ages\ninstead in the Results and Discussion. The authors should consider whether this labelling of “events” is actually necessary.\nLine 188: clarify that you are referring to opal in U1540 and not another site.\nLine 374: millennia rather than millennials"}, "author_response": "Response to Reviewers\nManuscript Title: Southern Ocean Evidence for Recurring West Antarctic Ice\nSheet Destabilization During Marine Isotope Stage 11\nManuscript number: NCOMMS-25-28167A\nJournal: Nature Communications\nWe sincerely thank all three Reviewers for their constructive feedback and for pointing\nout the importance of our study. Below, we provide a detailed, point-by-point response to\nall raised comments in blue. Line numbers refer to the manuscript file with tracked\nchanges. Italicized text highlights new text implemented in the revised manuscript.\nReviewer #1:\nThis is an impressive study of deep sea oxygenation changes in the Southern Ocean during a\npast warm climate in the ice age era. The authors have multiple proxies to draw from to conclude\nthat there were millennial-scale low oxygen events in the deep Pacific, which tied together with\npublished results from the deep Atlantic, point to circum-Antarctic ocean response to ice sheet\nmelting. Authigenic uranium, neodymium isotopes, bottom water temperatures, barium and\nopal content provide a very careful analysis of the suggested events. Honestly, I find the study\nto be technically impeccable and recommend this be published with a wide audience to\nincorporate these types of events into climate modeling for the future.\nWe thank Reviewer #1 for the constructive feedback and appreciate his/her time and effort\nin reviewing our manuscript. We are grateful for the positive assessment.\nTo go further, I can offer only one or two reflective comments. The linkage between\nsynchronous changes in both Ross Sea and Weddell Sea bottom water formation is very\ninteresting. Could the authors suggest a possible test for future evaluation of whether the “c” or\n“d” scenarios (from Figure 4) for EI and EII would have been more likely to have occurred?\nBased on our data, we unfortunately cannot evaluate which scenario, or whether a\ncombination of both, was more likely to have occurred – this in fact goes beyond the\nscope of our paper, in our view. However, we argue that additional proxy analyses from\nmarine sediment cores on the continental shelf of the Amundsen/Bellingshausen Sea and\nthe Ross Sea, including CDW temperature and -salinity reconstructions might allow an\nassessment of whether scenario “c” or/and “d” would have been more likely. These efforts\nwould need to be combined with numerical model simulations using coupled atmosphere-\nocean-ice sheet models to identify the most vulnerable areas of the West Antarctic Ice\nSheet as a function of degree and/or time of exposure to ocean heat and the geometry of\nthe Ross Gyre. We have accordingly added a sentence in the revised manuscript (lines\n450-454), stating “Whether CDW heat supply to the Ross Sea or the Amundsen- and\nBellingshausen Sea, or in fact a combination of both, was more likely to have occurred\nneeds to be further tested with numerical model simulations ideally using coupled\natmosphere-ocean-ice sheet models that allow an assessment of WAIS stability as a\n1\nfunction of changes in CDW temperature and/or -exposure time as well as the geometry\nof the Ross Gyre.”.\nI also see the difference in the Events I and II versus III, where the bottom water temperature\ndrop during EIII suggests a “cold” stagnation event, versus the “warm” melt-driven events EI\nand EII. Are there any other interglacial periods where there is similar bottom water temperature\ndata available as possible other examples of these “cold” stagnation events?\nWe indeed differentiate EI and EII from EIII on the basis of possibly different\nmechanisms driving the events (“warm” versus “cold”). In our manuscript, we show\nbottom water temperature reconstructions from ODP Sites 1123 (Elderfield et al., 2012)\nand 1094 (Hasenfratz et al., 2019) that are in strong agreement with our BWT record at\nIODP Site U1540, in support of our distinction of the events (Figure 3h). However, we\nare not aware of sufficiently high-resolution data of bottom water temperature and -\noxygenation from other interglacial periods and from other locations bathed in CDW to\nput further constraints on these different events. Frankly, we also believe that comparison\nof our MIS11 data to other interglacial periods goes beyond the scope of the paper, as\nthese likely are characterized by different climate boundary conditions, durations and/or\norbital forcing. Nonetheless, we emphasize in the revised manuscript the consistency of\nour bottom water temperature estimates with above mentioned records for MIS11 (lines\n278-279 and 457-459). Furthermore, we now clarify that additional data from other\ninterglacials would be helpful to further analyze the occurrence of “warm” versus “cold”\nAABW deoxygenation events: “Additional high-resolution proxy data for MIS11 or other\ninterglacial periods for instance26,27 from MIS5e would be helpful to better constrain the\noccurrence, timing and mechanisms governing such “cold” AABW deoxygenation events\nin the Southern Ocean.” (line 479-481).\nFinal thought regarding the interpretation of the epsilon Nd data. Line 222. “These shifts occur\nmostly at the same time as the observed MIS11 increases in …U/Ca…”. Could this assessment\nbe tightened up more statistically or specifically? From my reading of Figure 3, it seems like\nEIII and EII have clear increases in eNd but E1 could be arguable.\nOne strength of our study is the multi-proxy approach, where we apply the reconstruction\nof key parameters on one and the same sediment core sequence. This erases stratigraphic\nbiases between proxies that would occur when comparing the same or different proxies\nin different cores. This is now emphasized in the revised manuscript: “combined\nreconstructions of […] – thereby circumventing stratigraphic biases between proxy\nrecords” (line 108-113). Particularly important to our study is the  record as a\nNd\nbiologically independent indicator for water mass provenance changes at our study site\nin the past. We can thus elegantly address the question whether the deoxygenation signal\n(U/Ca, U/Mn) is influenced by export production and/or changes in water mass\ncirculation. We also now decided to graphically start the vertical bar of EI at 416 ka BP\nwith the onset of elevated authigenic U levels at ODP Site 1094 and of foraminiferal U/Ca\nand U/Mn levels at our study IODP Site U1540 (Figure 2 and 3), which coincides with\nan increase in  , yet a weak one, at the same site. We therefore now specify in the revised\nNd\nmanuscript, that “These ε shifts occur at the same time as the observed increases in G.\nNd\n2\nbulloides and Uvigerina spp. U/Ca and U/Mn levels during EII and EIII at our study site,\nwhile the ε shift during EI is less pronounced (Fig. 3b, g).” (lines 238-241) and “we\nNd\nobserve an increase of ε values by 0.4–1.1 ε units to ε values of -7.0±0.4 coinciding\nNd Nd Nd\nwith the identified AABW deoxygenation events at our study site (Fig. 3g). This is most\npronounced during EII and EIII (Fig. 3g).” (lines 379-382). We hope that this sufficiently\nspecifies the alignment between proxy data in our new high-resolution records.\nReviewer #2:\nThis study uses a multi-proxy approach to identify recurring millennial-scale deoxygenation\nepisodes in AABW in the central Pacific during MIS 11, concurrent with similar events in the\nAtlantic. These synchronized AABW stagnation events are linked to WAIS retreat driven by\nsubsurface warming from CDW. The results suggest a circumpolar response to ocean heat, with\nWAIS meltwater contributing significantly to MIS 11 sea-level high stands and potentially to\nfuture sea-level rise under continued warming.\nThe authors present interesting and timely results, highlighting the physical and dynamical roles\nof WAIS meltwater and CDW in driving recurring AABW deoxygenation events, with\nrelevance to ongoing and future climate change. Overall, I find this to be a well-written and\nwell-structured paper. However, I have several major concerns, particularly regarding the\ninconsistent proxy patterns across the proposed AABW stagnation events and the interpretation\nof key physical processes, as detailed below.\nWe thank Reviewer #2 for evaluating our manuscript and providing such valuable and\nconstructive feedback, which helped to strengthen the quality of our manuscript. We hope\nthat the clarifications in the revised manuscript suffice to address all important points\nraised.\nMajor comments\n(1) Recurring events: I appreciate the authors’ use of a multi-proxy approach in Figures 2 and\n3, which reveals generally good agreement among several variables during MIS 11. However,\nnot all proxies exhibit consistent recurring patterns across the EI–EIII events. For example,\nsummer SST/water isotope (δD) and benthic foraminiferal δ18O (Fig. 2), as well as BWT and\nrelative sea level (Fig. 3), do not show clearly repeated signals. Could the authors clarify the\nreasons for these discrepancies?\nWe appreciate the praise of Reviewer #2 of our multi-proxy approach and the resulting\n“good agreement” of our data. We believe that this makes our study particularly strong.\nIn contrast to what Reviewer #2 might imply, we do indeed compare in detail our BWO\nsignals with BWT (starting line 415) and global sea-level (starting line 495).\nWe believe that the absence of millennial-scale variability in the benthic foraminiferal\nδ¹⁸O at IODP Site U1540 (Fig. 2f) cannot be used to invalidate the findings of consistent\nchanges in reconstructed BWO, IRD input and  -based water mass proportions, because\nNd\nbenthic foraminiferal 18O reflects changes in global ice volume as well as BWT, bottom\n3\nwater salinity and foraminiferal vital effects. Benthic foraminiferal 18O alone may\ntherefore not be ideal to reconstruct transient local Pacific Southern Ocean bottom water\ndeoxygenation events at our study site (for instance through meltwater supply) due to a\nstrong global overprint, although the deconvolution of these processes to extract the\nmagnitude of bottom water salinity changes from benthic foraminiferal 18O at the study\nsite has merit and is planned for a future study.\nFurthermore, summer sea surface temperature at our study site and EDC δD are strongly\ninfluenced by numerous processes including variations in atmospheric temperatures,\nmixed layer depths, seasonality, frontal movements, SWW intensity and/or position as\nwell as greenhouse gas forcing (e.g., Timmermann et al., 2014; Vimeux et al., 2002). They\nare therefore governed by additional and/or different mechanisms than those necessarily\ndriving bottom water deoxygenation events at our study site, which may explain the\napparent inconsistency between records.\nThe link between our reconstructed BWT and our BWO signals is detailed in the\ndiscussion, starting in line 415. Our multi-proxy dataset from one core sequence (i.e.,\nwithout stratigraphic biases) allows us to differentiate between “warm” AABW\ndeoxygenation events (EI and EII) and a “cold” AABW deoxygenation event (EIII). Our\nBWT reconstructions thus highlight a complex southern high-latitude ocean-cryosphere\nsystem that goes beyond a one-to-one correlation, which we address in the study.\nFinally, the link between global sea level reconstructions and our BWO signals is\ndescribed in the manuscript, starting in line 495. We highlight for instance: “Indeed, these\nevents [the observed AABW deoxygenation events during MIS11c] match with peaks in\nreconstructed global sea-levels (Fig. 3i)49,55, reaching consensus levels of up to 6–13 m\nabove present-day32. Further, during MIS11b (i.e., EIII), the global sea-level\nreconstruction49 from ODP Site 1123 suggests a transient sea-level high-stand of several\nm (Fig. 3i).” (line 500-504). We also compare these findings against model simulations\n(Mas e Braga et al., 2021).\nWe strongly believe that these efforts are sufficient to address the association of the vast\nnumber of proxies that we present in our comprehensive study, and hope that Reviewer\n#2 agrees.\nIn addition, the term “AABW stagnation events” is not clearly defined in the manuscript. At\ntimes, two events are mentioned; at other times, three, depending on the interpretation of the\nIODP site data. I suggest the authors clearly specify how many such events are explicitly\ndiscussed and on what criteria this identification is based.\nThe term “AABW stagnation event” was originally defined by Hayes et al. (2014) based\non findings in the Atlantic Southern Ocean, which we clearly outline in the Introduction\n(line 78-82). In our manuscript, we mostly deliberately use “deoxygenation event” rather\nthan “stagnation event” to use a term free of a mechanistic implication (please also see\nour response to a comment of Reviewer #3), and to allow for a discussion of the\nmechanisms driving these events without implying a driver through their designation at\nthe first instance.\nmanuscript, we are thus able to define our three events (EI-EIII) more clearly: “We thus\nrefer to the millennial-scale increases in U/Ca and U/Mn levels at IODP Site U1540 at\n~416–407, ~402–398 and ~395–390 ka BP as “deoxygenation events” (hereafter referred\nto as EI, EII and EIII, respectively; Fig. 2) […]” (line 165-167). We now address the\nevent(s) more specifically throughout the revised manuscript (by using the abbreviations\nEI, EII and/or EIII), which we believe enhances the clarity of our study. Examples are\n“The first two increases in foraminiferal U/Ca and U/Mn levels during MIS11c during EI\nand EII […]” (line 171-172), “ […] abundance peaks of IRD at IODP Site U1540 during\ntwo of the identified AABW deoxygenation events in MIS11c and MIS11b (i.e., EI and\nEIII) […]” (line 365-366) and “Two of our three identified AABW deoxygenation events\nat IODP Site U1540 (i.e., EI and EII during MIS11c) can be linked to […]” (line 415-\n416).\n(2) Negative feedbacks between WAIS retreat and AABW contraction: I have a concern\nregarding the authors’ statement that “meltwater supply into the Southern Ocean might have\nsteepened the isopycnal gradient along the shelf break” (lines 376–377). Whether such\nsteepening occurs likely depends on how effectively the meltwater is retained on the continental\nshelf. Given the large volume of meltwater input (∼50 m sea-level equivalent, as shown in Fig.\n3i), a substantial portion would likely have escaped to the open ocean. In that case, the West\nAntarctic shelf would not remain isolated, and the isopycnal gradient along the shelf break\nwould more likely be flattened rather than steepened.\nWe must admit that our statement on changes of the isopycnal gradient along the shelf\nbreak due to meltwater supply was imprecise, and accordingly thank the reviewer for\npointing that out. Rather than “steepening”, we meant to say that “strong meltwater\nsupply into the Southern Ocean might have strengthened the density gradient along the\nshelf break between the interior open ocean and the shelf environment64, protecting the\nWAIS margin from open ocean heat,” (as now revised in lines 399-401), independent from\nthe amount of meltwater input (i.e., ~6-13 m sea-level equivalent; Dutton et al., 2015) or\nthe steepness (i.e., the geographical tilt) of the isopycnals that may have flattened due to\nthe mechanisms that Reviewer #2 describes. For our argument, however, the magnitude\nof the seawater density gradient across the shelf break matters, i.e., between the shelf and\nthe Southern Ocean interior, which we now better clarify in the revised manuscript.\n(3) Development of an open-ocean polynya: I do not agree with the authors’ statement that\n“expanded Antarctic sea ice and northward shifted westerly winds could also have promoted\nthe development of an open-ocean polynya favoring deep-reaching convection” (lines 381–\n383). While I understand that this conclusion is based on the assumption of “a northward shift\nof the westerlies”, it is important to note that this occurred under warming conditions (as\nindicated by SST in Fig. 2e). In general, open-ocean polynyas are expected to decrease in a\nwarmer climate due to enhanced stratification and reduced sea ice formation.\nOur intention is to highlight the possibility of deep-reaching convection initiated by open-\nocean polynyas as one possible mechanism out of many that may drive recurring cycles\nof de-/re-oxygenation and contraction/expansion of AABW – an important observation\nin our view. Although Reviewer #2 is of course right that open-ocean polynyas decline\n5\nunder warm conditions, we argue that the meltwater supply (that may disperse into the\nopen ocean as Reviewer #2 rightly mentions in his/her comment (2) above) may act to\nenhance sea ice formation (due to a fresher surface). Furthermore, our sea surface\ntemperature reconstructions and the EDC D-based Antarctic temperature record indicate\ndecreasing temperatures directly after EI (Fig. 2e), which would again promote the\nformation of Antarctic sea ice. We therefore find that our assertion is not unlikely, and\nthus decided to keep our argument on the possibility of open-ocean polynyas included in\nthe revised manuscript. We have, however, adjusted our statement to clarify that:\n“Expanded Antarctic sea ice due to SST decline (i.e., following EI; Fig. 2e) and a fresher\nocean surface (i.e., during meltwater supply) and northward shifted westerly winds could\nalso have promoted the development of an open-ocean polynya favoring deep-reaching\nconvection65 (in contrast to coastal polynyas common today), thereby possibly\ncontributing to drive recurring cycles of de-/re-oxygenation and contraction/expansion of\nAABW observed at our study site during MIS11.” (line 405-411). And we further\nemphasize: ”Mechanisms driving these cycles, however, need to be further tested with\nadditional proxy data and/or numerical model simulations.” (line 411-413).\n(4) CDW upwelling in Ross Sea (Fig. 4c): I cannot follow the authors’ statement “warm CDW\nintrusions onto the continental shelf of the Ross Sea Embayment, presumably along the eastern\nlimb of the Ross Gyre” (lines 399–401). Are the authors referring to CDW itself, or to the\nAntarctic Slope Current (ASC) as the transport mechanism? I do not understand how CDW\nwould move along the Ross Gyre as shown in Fig. 4c.\nThe flow path of CDW to the Antarctic margin along the eastern limb of the Ross Gyre\nhas been described in detail by Orsi and Wiederwohl (2009) and Prend et al. (2024),\nand others, see review of Bennetts et al. (2024) and references therein. This is a widely\naccepted process: “The [Ross] gyre acts as a conduit between the open ocean and the\nAntarctic margins, carrying CDW poleward to the continental slope on its eastern edge\nand exporting shelf waters equatorward along its western flank […]” (Prend et al., 2024)\nand “The warm CDW then rises along density surfaces as it crosses the ACC and\ncirculates through the subpolar gyres of the Southern Ocean until it eventually sits at\ndepths comparable to the sea floor of the continental shelf.” (Thompson et al., 2018). This\nis analogous to how CDW was found to enter the West Antarctic continental margin in\nthe Atlantic Southern Ocean, namely through the eastern boundary of the Weddell Gyre\n(Hellmer et al., 2012; Rintoul et al., 2001; Schröder & Fahrbach, 1999). Ross- and\nWeddell Gyre circulation thus allows CDW to impinge and/or cross the ASC to arrive at\nthe shelf break of the West Antarctic continental margins of the Ross Sea and Weddell\nSea, and we argue that these dynamics may have controlled heat supply to WAIS in the\npast. We thus keep our postulations in the revised manuscript (Fig. 4c), and specify in the\nrevised manuscript “[…] reconstructions argue for millennial-scale (excess) warm CDW\nintrusions onto the continental shelf of the Ross Sea Embayment, presumably along the\neastern limb of the Ross Gyre – a major conduit of ocean (i.e., CDW) heat between the\nopen ocean and the West Antarctic margin today23,66 (Fig. 4b, c).” (line 427-430).\nWe believe that this is important because this conduit has likely operated, yet varied in the\n6\npast, which as we argue has implications for the heat supply to the Antarctic margin, and\nultimately for the ventilation of the deep ocean via AABW.\nMinor comments\nIn Sections 2.1 and 2.2, please refer specifically to the subpanels in Figures 2 and 3 (e.g.,\nFig. 2a, 2b, Fig. 3a, 3b, etc.) where appropriate. Given the large number of subplots, it is\ndifficult for readers to follow the discussion without clear and direct references to the relevant\npanels.\nSpecified, where possible.\nFigure 1b: I am not sure it is appropriate to plot tracer concentrations originating from both\nthe Weddell Sea and the Ross Sea on the same figure. The overlapping region within the\nWeddell Sea makes the results difficult to interpret.\nThank you for this important observation regarding the distribution of the tracer\nconcentration, specifically in the Weddell Sea. The main purpose of Figure 1b is to convey\nthe different preponderances of Weddell Sea- and Ross Sea-derived AABW in the Atlantic\n(ODP Site 1094) and Pacific sector of the Southern Ocean (IODP Site U1540),\nrespectively. For clarification, we have included the contours highlighting the areas with\ntracer concentrations larger than 0.02 for both the Weddell Sea- and Ross Sea experiments\nto indicate that there is no strong overlap of these water masses in the Weddell Sea\n(according to the analyses of Solodoch et al., 2022, their Figure 1a, c), please also see\nrevised Figure 1b and associated revised caption. We hope that these changes satisfy the\nvalid comment of Reviewer #2.\nFigure 2d caption: Change “age model (see Results)” to “age model (see Methods)”.\nOur tiepoint adjustment for ODP Site 1094 is described in the Results chapter (line 181-\n191). Therefore, we retain the phrasing “age model (see Results)” in the caption of Figure\n2d to refer the readers to the correct section.\nReviewer #3:\nThis manuscript presents a new multi-proxy and high time-resolution assessment of the impact\nof past West Antarctic Ice Sheet (WAIS) retreat events via their impact on Antarctic Bottom\nWater formation as recorded by bottom water oxygen (BWO) concentrations. Previously, only\none site in the Atlantic sector of the Southern Ocean has revealed intervals of low BWO, linked\nto WAIS retreat and the impact of a warmer ocean. Here, the authors move to the Pacific sector\nof the Southern Ocean to test these hypothesized processes and feedbacks in another region,\nbut one which remains influenced by WAIS behavior. The study targets a particularly long\ninterglacial (MIS 11) which has many relevant properties for us to consider in projecting future\nclimate changes (outlined in detail, and clearly, in the manuscript). Of importance is the new\nfinding that these low BWO events are also found in MIS 11 in the Pacific sector, revealing that\nthere was a dynamic WAIS here which also impacted AABW formation and properties. Because\nthis also has implications for our sea-level budget contributions from WAIS in the past (and\n7\nfuture), the results of this work will be of interest to several audiences: paleoceanographers,\ngeochemists, climate and ice sheet modelers, and glaciologists.\nThe manuscript is well-written and the graphics are clear. The authors do an excellent job in the\nearly pages at setting out the modern context and concerns, as well as the paleo-environmental\nevidence for these BWO events in the recent geological past. The authors have amassed a\ndetailed suite of geochemical indicators, at high temporal resolution, and combined the results\nin a detailed evaluation which ensures that the key processes can be explored and explained.\nThe Discussion is well-written, and balances evaluation and detail with clear interpretations.\nMy comments here reflect a need for increased clarity in a few places, which should ensure that\nthe nuances as well as the highlights of the results are evident to readers; they do not represent\nany significant flaws to the work being presented.\nWe sincerely thank Reviewer #3 for the constructive and valuable feedback on our\nmanuscript.\n(1) The authors highlight the synchrony or near-synchrony of events between this site (U1540)\nand the previous record in the Atlantic (ODP 1094). The age model of ODP 1094 is important\nhere, because the new data seems well-constrained with a high resolution tuning to the EPICA\nice core. In line 173 the authors note that “a slight shift of the tiepoint at 395.2 kyr bp …\nmaintains a good match”, but I don’t find the rationale for why this shift was made. It doesn’t\nseem to have been made with the assumption that the BWO events should be synchronous,\nespecially given the different process driving this event in U1540. The text here seems to imply\nthat the authors didn’t try to align the BWO signals, but a sentence or two is needed to clarify\nwhat was done and why.\nWe intend our age model adjustment of ODP Site 1094 (as described in the “Results”)\nto reflect a transparent sensitivity test to assess the timing of EIII at IODP Site U1540 and\nODP Site 1094 in more detail, within viable age uncertainties. We thereby deliberately\nadhere to the original age model premise of Hasenfratz et al. (2019) of a good match\nbetween the benthic foraminiferal 18O record of ODP Site 1094 and the LR04 18O stack\nby shifting the tiepoint at 395.2 ka BP by 2 ka within uncertainties of the reported age\nmodel (Figure 2d, f). Indeed, without strictly forcing an alignment of the BWO signals,\nwe find that the events occur more closely in time during MIS11b (i.e., near EIII), while\nmaintaining a good match of the benthic 18O records. We thus specify in the revised text:\n“An adjustment of the existing age model at ODP Site 1094 within its uncertainties and\nadhering to the original age model premise38 emphasizes ambiguities regarding the\nrelative timing of EIII at IODP Site U1540 and ODP Site 1094 (Fig. 2f). Specifically,\nconsidering a slight shift of the tiepoint at 395.2 ka BP at ODP Site 1094 (by ~2 ka)\nmaintaining a good match between the benthic foraminiferal 18O record to the LR04\n18O stack shifts the aU enrichment event at ODP Site 1094 at 397 ka BP by ~2 ka towards\nyounger ages (Fig. 2d, f). This sensitivity test highlights that the foraminiferal U/Ca and\nU/Mn increase during MIS11b at IODP Site U1540 (i.e., EIII) might have coincided with\na similar aU enrichment event at ODP Site 1094 (Fig. 2d). Without high-resolution 18O\ndata and/or additional age control for ODP Site 1094, it can, however, not be excluded\n8\nthat EIII represents a local event at IODP Site U1540.” (line 181-191). We hope that these\nchanges and details suffice to adequately address the valid comment of Reviewer #3.\n(2) The Discussion on Drivers of AABW stagnation (starting line 323) includes mention of\natmospheric and ocean mechanisms which could lead to WAIS destabilisation which includes\nmention of ocean warming (line 334). As the ocean warming is discussed in a later section I\nsuggest that the authors flag here that this mechanism will be returned to. I initially read this\nsection with some skepticism having inferred that the authors were proposing ocean\ntemperatures as the driver for all three events, when MIS 11b is different. A brief note to clarify\nthis would help future readers.\nWe agree and now make a distinction between the events and refer to further details later\nin the text: “Our proxy results highlight the synchronous occurrence of AABW\ndeoxygenation- (and stagnation-) events in the Pacific and Atlantic Southern Ocean\nsuggesting WAIS destabilization in the Weddell- and Ross Sea Embayment during\nMIS11c and 11b, likely with a significant role of increased ocean heat exposure of ice\nshelves in these regions during MIS11c (i.e., EI and EII), which we discuss in detail\nbelow.” (line 355-360).\n(3) The role of CDW in AABW stagnation events (line 387 onwards) considers the role of ocean\ntemperatures. The first few paragraphs detail the warming effect, whereas a later paragraph\nhighlights that the event in MIS 11b occurs during cooling (and in fact when bottom water\ntemperature reach their minimum). However, the opening sentences don’t note that only 2 of\nthe 3 stagnation events occur with warmer bottom waters. Rather than saying “events…can be\npartially linked to warm and warmer-than-present…” (line 388-389) it would be more clear if\nthe authors stated that 2 of the 3 events occurred under these conditions.\nWe agree that the differentiation between EI and EII (i.e., the “warm” events) and EIII\n(i.e., “cold” event) needs to be made clearer. We adjusted our statement in lines 415-417\naccordingly: “Two of our three identified AABW deoxygenation events at IODP Site\nU1540 (i.e., EI and EII during MIS11c) can be linked to warm and warmer-than-present\nLower CDW […].”. Please also refer to our response to comment (1) of Reviewer #2.\n(4) Lines 424-426 describe a relatively locally important process to account for the low BWO\nevent EIII. Does this mean that we should not expect a similar event to be expressed in the\nAtlantic sector (influenced by the Weddell Sea), or could it account for the temporal offset?\nWith this in mind, the unclear reason for shifting a tie-point in ODP 1094 emerges again.\nWe agree that processes governing EIII are described with a focus on the Ross Sea\n(given our study site and the robust age constraints we have for IODP Site U1540 based on\nindependent age constraints i.e., SST-EDC D alignment; Fig. 2e, f). Yet, we did not\nintend to exclude the possibility that similar processes operated in the Weddell Sea during\nthe same time. In our view, a near-alignment of the BWO signal during EIII in the Pacific-\n(IODP Site U1540) and Atlantic Southern Ocean (ODP 1094) (lines 187-190) is equally\nlikely as two locally BWO EIII events with a temporal offset (lines 190-191), given the\nsignificant age uncertainties of ODP Site 1094 during EIII (Hasenfratz et al., 2019) and\n9\nthe results of our age model sensitivity test. We believe that highlighting both in the\nrevised manuscript is transparent, and therefore the best way forward.\nIn the revised manuscript, we thus add: “During the identified AABW deoxygenation\nevent during MIS11b (i.e., EIII), CDW temperatures strongly decreased, approaching the\nfreezing point of seawater, similar to the near synchronous stagnation event identified in\nthe Atlantic Southern Ocean26 at ODP Site 1094 that also coincides with low BWT (Fig.\n3h)38.” (line 456-459) and “Similar processes might have operated in the West Antarctic\ncontinental shelf region of the Weddell Sea given the presence of the vast Filchner-Ronne-\nIce Shelf, driving bottom water deoxygenation and -cooling in the Atlantic Southern\nOcean26 during MIS11b; however, the timing of this event at ODP Site 1094 relative to\nEIII at IODP Site U1540 given the large age uncertainties at ODP Site 1094 remains\nunclear (see Results).” (line 467-472). We hope that these additions sufficiently address\nthe important comment of Reviewer #3.\nMinor comments:\nLine 55-6: this is a long and complicated sentence. Rather than trying to combine the\nstabilization and vulnerability in a single sentence, this may be more clear if 2 sentences are\nused here, one for each scenario.\nAmended.\nLine 76-77: unusual expression of dates. If referring to “thousand years ago” then ka can be\nused (127 ka), rather than 127 kyr bp. The lowercase bp is confusing because capitalized BP is\nused to refer to radiocarbon dates.\nAs per international convention (ISO 31 - Quantities and units, International Organization\nfor Standardization, 1993) we decided to use the term ka (kilo-annum) before present\n(BP) for both the age and duration of an event. We accordingly changed all “kyr bp” to\n“ka BP” in the revised manuscript, including text, figures and figure captions.\nLine 147 needs to define what the “E” is an abbreviation of (event? And were these events\ndefined by the study of ODP 1094?) They are defined in the figure captions, but they are also\nnever again referred to in the text, which uses the ages instead in the Results and Discussion.\nThe authors should consider whether this labelling of “events” is actually necessary.\nWe intend the abbreviation “E” to stand for “deoxygenation event” to reduce complexity\n(i.e., this designation is free of a mechanistic implication) and enhances in our view the\nreadability of our manuscript. Therefore, we maintain the use of this abbreviation in the\nrevised manuscript but apply it more consistently throughout. We also define its use at\nthe beginning of the “Results” section more clearly: “We thus refer to the millennial-scale\nincreases in U/Ca and U/Mn levels at IODP Site U1540 at ~416–407, ~402–398 and\n~395–390 ka BP as “deoxygenation events” (hereafter referred to as EI, EII and EIII,\nrespectively; Fig. 2), in analogy to the definition of “stagnation event” of ref. 27; yet, we\ndeliberately choose to refer to EI-EIII without a mechanistic implication.” (line 165-169).\nLine 188: clarify that you are referring to opal in U1540 and not another site.\nLine 374: millennia rather than millennials\nAmended (now line 397).\nPlease note that, based on a minor adjustment of the MIS11c time interval to the margins defined\nin the introduction (line 98), some of the reported mean values in the manuscript have been\nrevised (e.g., line 236 and 271); however, these minor changes do not affect the interpretation\nor conclusions of this study."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my concerns in the revised manuscript.", "reviewer_2": "The authors have adequately addressed my concerns, and I am satisfied with this version of the manuscript. I have no\nfurther comments or suggestions.", "reviewer_3": "The authors have addressed all of the comments I made in my original review, and have made the required changes to the\ntext in a clear way. I have also checked the replies to the other reviewers. Here, there is a range of complexity in the\nresponses, due to the complicated ocean circulation systems in this region and the uncertainties around available\ncomplementary data which could be used to support the interpretations. But I am satisfied that the authors have responded\nappropriately and included the suggested edits and clarifications suggested by the other two reviewers. All three reviewers\nemphasise the importance of the data and the well-written and well-presented manuscript."}, "author_response": ""}]

Decision letter