West Antarctic ice retreat and paleoceanography in the Amundsen Sea in the warm early Pliocene

Domain: earth_science
Published: 2025-07-01
Editorial outcome: minor_revision

Abstract
Mass loss from polar ice sheets is poorly constrained in estimates of future global sea-level rise. Today, the marine-based West Antarctic Ice Sheet is losing mass at an accelerating rate, most notably in the Thwaites and Pine Island glacier drainage basins. Early Pliocene surface temperatures were about 4 °C warmer than preindustrial and maximum sea level stood ~20 m above present. Using data from a sediment archive on the Amundsen Sea continental rise, we investigate the impact of prolonged Pliocene ocean warmth on the ice-sheet-ocean system. We show that, in contrast to today, during peak ocean warming ~4.6−4.5 Ma, terrigenous muds accumulated rapidly under a weak bottom current regime after spill-over of dense shelf water with high suspended load down to the rise. From sediment provenance data we infer major retreat of the Thwaites Glacier system at ~4.4 Ma several hundreds of km inland from its present grounding line position, highlighting the potential for major Earth System changes under prolonged future warming.

Introduction
Over the past three decades, ice loss from the West Antarctic Ice Sheet (WAIS) has been a major contributor to sea-level rise. Over a 25-year period, the rate of mass loss increased five-fold as a result of dynamic thinning and grounding line retreat of glaciers draining into the Amundsen Sea Embayment, such as Pine Island and Thwaites glaciers1–3. During modern interglacial conditions, glaciers draining into the Amundsen Sea Embayment lose most of their mass through high rates of ocean-driven sub-ice-shelf melt and dynamic thinning upstream, in contrast to other parts of the Antarctic ice sheet, especially in East Antarctica, where warm deep water is upwelling further offshore from the slope and where iceberg calving dominates as a mechanism of mass loss4–7. Interglacial melt rates are primarily driven by the temperature and flux of Circumpolar Deep Water (CDW) as it interacts with ice shelves and grounding zones. Upwelling of CDW along the continental slope and inflow into cross-shelf troughs occurs as a result of a complex interplay of wind-driven and density-driven circulation, which, in turn, is impacted by meltwater release from the ice shelves8–11.

The Amundsen Sea drainage sector of the WAIS holds a 125 cm sea level equivalent12. Given that the drainage basins in this sector, especially that of the Thwaites Glacier, is largely based below sea level on a reverse bed slope, current melting and mass loss there may lead to increasing rates of retreat13 and attendant increase in sea-level rise. Recent meltwater deposits found on the Amundsen Sea continental shelf are interpreted as a signature of enhanced subglacial melt potentially contributing to fast retreat14,15. Ice-sheet dynamics are influenced by subglacial meltwater production and its routing at the ice-sheet bed16 and warming may enhance surface melt and its drainage17. Periodic loss of ice shelves on the warmer Antarctic Peninsula has shown a strong dynamic response of ice flow, faster flow and increased sediment entrainment by the ice, with a possible role for meltwater percolation to the bed18,19.

How these processes involving ocean−ice interactions play out over longer timescales than the observational record and in higher-than-today temperatures is poorly known and contributes to increased uncertainties on how the Antarctic system will evolve under the current climate changes. Whereas high-latitude feedbacks can result in non-linear responses to warming in the ice-sheet-atmosphere-ocean system, the effects may be different depending on local geological boundary conditions20. Ice-sheet modeling based on post-Last Glacial Maximum (ca. 19–23 ka) data from subglacial sediments in the Ross Sea sector of the WAIS suggested that uplift due to glacio-isostatic adjustment (GIA) created a prograde slope at the WAIS bed and, thus, allowed the grounding line to re-advance after initial deglacial retreat, whereas in the Amundsen Sea sector modeled retreat was steady without re-advance21. The variability in ice-sheet response points to the need to assess the regional context of ice-sheet and climate tipping points under warmer climate states than the dominant “icehouse” of the past 3.3 Ma22, including different mechanisms of ice mass loss and how it was partitioned between East and West Antarctica23,24.

During the early Pliocene, ~5.2 to 3.6 Ma, global surface temperatures were ~4 °C higher than pre-industrial under similar atmospheric greenhouse gas concentrations as today25. Sea surface temperatures and pCO2 reached the highest levels of the past 5 Myrs during the early Pliocene26,27, making it an excellent analogue for future evolution of the climate system. Reconstructed sea level highstands from the early Pliocene exceed 20 m above present28 and require contributions from all major ice sheets, including the WAIS. A record of sea surface temperature (SST) variations based on Mg/Ca ratios of planktonic foraminifera at Deep-Sea Drilling Project (DSDP) Site 590 in the SW Pacific Ocean, can be used as a proxy for Southern Ocean subsurface ocean temperature variations25,27, and shows that peak subsurface warming occurred in the early Pliocene. DSDP Site 590 lies at intermediate water depth and is influenced by low salinity and high nutrient Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) that originated directly north of and within the Southern Ocean’s Polar Frontal Zone27. Early Pliocene warming of the Southern Ocean was also noted in temperature reconstructions from the Ross Sea and Prydz Bay sectors that are based on silicoflagellates29,30 and TEX86L biomarker data from the Ross Sea shelf31. Geological archives show that the Antarctic ice sheet retreated from the Ross Sea and Wilkes Subglacial Basin with enhanced iceberg discharge in response to early Pliocene warming32,33. On the Antarctic Peninsula continental margin, prograding sequences on the outer shelf and upper slope and lithofacies in drill cores off Marguerite Trough show evidence for repeated late Miocene−early Pliocene advances and retreats of grounded ice streams across the shelf34,35. Coeval terrestrial evidence from the Antarctic Peninsula is interpreted in the context of a thinner wet-based ice sheet with several periods of ice retreat in the early Pliocene36.

In the light of modern major ice loss in the Amundsen Sea drainage sector of WAIS, we targeted a sedimentary paleoarchive offshore from this sector to reconstruct ice-sheet variability through the warm early Pliocene. Here we use the sedimentological, geochemical and clay mineralogical data collected on the drill cores from the Amundsen Sea continental rise to determine the extent of WAIS retreat during the early Pliocene warm period, and to characterize the mechanisms and impacts of this ice loss. The seismic stratigraphy of the eastern Amundsen Sea outer continental shelf shows evidence of extensive progradation of ~80 km in the early Pliocene. However, shelf progradation became more variable and generally less extensive through time with evidence of buried grounding zone wedges (bGZWs) in the overlying aggradational intervals37. GZWs form at the grounding line under an ice shelf either when the ice-sheet terminus has reached its maximum extent or during short-term pauses that interrupt a time of general retreat. The bGZW in the aggradational Pliocene strata on the Amundsen Sea shelf document grounded ice advances to mid-to-outer shelf positions. Notably, these GZWs are buried under continuous, thick sediment drapes resulting from substantial periods (~300 kyr), during which grounded ice did not re-advance across the shelf and resided in a retreated position37.

Although Pliocene-age sediments were not recovered in the few currently available short drill cores from the Amundsen Sea shelf38, International Ocean Discovery Program (IODP) Expedition 379 Sites U1532 and U1533 recovered a hundreds of meters thick sequence of Pliocene sediments from a drift on the Amundsen Sea continental rise39,40 (Fig. 1). Sites U1532 and U1533 were drilled in ~3900−4200 m water depth, ~250 km from the shelf break, into the Resolution Drift, which is located adjacent to a submarine channel (Fig. 1). Site U1533 was drilled on the western flank of the Resolution Drift directly to the east of the deep-sea channel37,40. The channel is connected to the eastern Amundsen Sea continental slope, offshore from the mouth of the Pine Island-Thwaites paleo-ice stream trough. Site U1532 was drilled near the crest of the same drift39 (Fig. 1). The sites yielded Pliocene sediment archives of unprecedented resolution due to up to 90% core recovery and sedimentation rates up to dm/kyr, with inferred orbital-scale cyclical changes in sedimentation. The Pliocene sediments recovered at sites U1532 and U1533 consist of dark grey laminated silty clays interbedded with massive and bioturbated greenish grey silty clays with variable bioturbation and gravel content (Fig. 2). In preliminary shipboard analysis, four to five lithofacies were distinguished in the Pliocene drift sediments at both sites. These shipboard lithofacies classifications were based on descriptions and interpretations of representative examples of dm-scale intervals of sedimentary structures or clast-rich beds39,40.

In this study we embed these dm-scale interpretations into a lithofacies assemblage scheme applied to the deposition, over time, of the hundreds of meters thick lower-to-mid Pliocene sequence over multiple glacial-interglacial cycles41. We investigate the offshore supply of glaciogenic detritus through the early Pliocene within a chronological framework of orbital-scale cyclical changes in sedimentation. Using facies models in conjunction with grain-size measurements and detrital provenance, we aim to reconstruct the connections between source-to-sink supply of glaciogenic detritus and WAIS dynamics. Terrigenous sedimentation on the Antarctic continental rise is governed by glacial processes on land as well as the continental shelf that produce terrigenous detritus, downslope transport of this material to the continental slope and rise, and bottom current reworking41–43. Over glacial-interglacial cycles, the depositional signatures of ice-sheet and ice-shelf processes in deep-marine sediment drifts are expressed in muddy turbidites, hyperpycnal plumites, contourites and sand and gravel-rich ice-rafted debris (IRD) beds41–44. Besides IRD, glacial rock flour, which is silt-sized and supplied to the glacio-marine environment, can be distributed in hyperpycnal flows, where suspended particles are redeposited by bottom currents of variable strength.

We deploy a multiproxy approach with bulk trace and Rare Earth Element (REE) geochemistry, including X-ray fluorescence (XRF) scanner data, clay mineralogy, and 40Ar/39Ar geochronology of medium-coarse sand fractions to track the provenance of the fine-grained drift sediments and the coarse-grained ice-rafted component. Grain-size data collected for this study is used to calculate the sortable silt mean size (SS), which can be interpreted as a proxy for bottom-current strength45. The data collected and analyzed post-cruise allows us to build an early to mid-Pliocene time series of changes in the sources of suspended and ice-rafted detritus from the Thwaites and Pine Island Glacier drainage systems that inform the extent in changes in grounding line position within the context of the changing local paleoceanography.

The geology of West Antarctica is the product of multiple tectonic collisions with accompanying magmatic events20,46–48. The Eastern Palmer Land Shear Zone separates the Paleozoic Gondwana terranes of the Eastern Domain from terranes amalgamated during Triassic−Jurassic events (Central Domain)49; the Western Domain, adjacent to the Bellingshausen and Amundsen seas, consists of terranes affected by Cretaceous accretionary processes, rifting, and magmatic events (Fig. 1). Paleozoic metasediments and granitoids related to the active margin of Gondwana dated to ~375 to ~250 Ma are found in Marie Byrd Land, whose ice-sheet cover is partly drained through Thwaites Glacier50. In contrast, granitoids from the Thurston Island block (~240−220 Ma)51, near Pine Island Glacier (~180−170 Ma)52 and from the Ellsworth-Whitmore Mountains (250−170 Ma)53 have Triassic−Jurassic ages. These plutons were emplaced during a younger phase of subduction along the margin of Zealandia51. The boundary of the Central Domain with Triassic−Jurassic arc-volcanic intrusives and the Marie Byrd Land province, which lacks evidence of magmatic activity of this age, runs under Thwaites Glacier52,54.

In the mid-Cretaceous ~110−100 Ma, flare-up arc volcanism affected Thurston Island and the Antarctic Peninsula47,51. In a different tectonic setting, magmatic phases related to the early stages of Cretaceous rifting of Zealandia and West Antarctica are observed in Marie Byrd Land ~100−95 Ma. Isolated highlands of felsic rocks with low magnetic susceptibility detected ~200 km landward of the grounding line under Thwaites Glacier may represent similar anorogenic granites48 (Fig. 1). Mafic igneous rocks and sedimentary basins created during the later stages of Cretaceous-Cenozoic rifting are interpreted to dominate the subglacial geology of the Thwaites Glacier system54,55. Late Cretaceous and Cenozoic rifting created sedimentary basins, which are currently partially covered by the WAIS56. The relatively high kaolinite content of the clay mineral fraction of seafloor sediments in the Amundsen Sea is indicative of a sedimentary rock source57–59. Kaolinite concentrations of >30% are found in modern sediments near Thwaites Glacier, with lower kaolinite contents characterizing sediments with a Pine Island Glacier source55. Basal debris from the Byrd ice-core in the WAIS interior contains ~40% kaolinite and is inferred to derive from the infill of subglacial sedimentary basins60. Kaolinite is also found in pre-Oligocene sediments or sedimentary rocks cropping out on the Amundsen Sea inner shelf57–59.

Here we show that early Pliocene sedimentation on the Amundsen Sea continental rise was sourced within the Amundsen Sea ice drainage sector and strongly coupled to advance and retreat of Thwaites Glacier. Following a period of rapid accumulation of meltwater plume deposits a temporary minimum in kaolinite content and Marie Byrd Land-type geochemical and 40Ar/39Ar IRD provenance signatures in the glaciogenic supply is indicative of major retreat of Thwaites Glacier ~4.4 Ma several 100s of km upstream of its present grounding zone. Biogenic and authigenic geochemical signatures and the sortable silt current proxy indicate that vigorous circulation gave way to slow-moving currents ~4.6−4.1 Ma, whereas meltwater plumes extended into deep water more than 250 km from the shelf break. These results are significant as they highlight the potential impacts of major WAIS retreat after its ice-sheet inception during a prolonged warm period with surface temperatures only a few degrees above those of the present day.

Results
Sedimentary facies
Based on shipboard visual core and smear-slide descriptions, augmented with post-cruise image analysis and new grain-size data acquired post-cruise for this study, three main lithofacies are distinguished in the early to mid-Pliocene section at sites U1532 and U1533: 1) terrigenous dark grey laminated silty clay with or without sand interbeds; 2) terrigenous interlaminated silty clay with reddish color banding, and 3) greenish grey silty clay with variable intensity of bioturbation, contents of gravel and biogenic silica components. Using XRF-scanner data and detailed analysis of a* track data carried out for this study post-cruise, these facies are here placed in a cyclostratigraphic context. These three facies typically occur in an interbedded sequence with the color-banded facies transitioning upward into the greenish grey gravel bearing facies, which is typically overlain with a sharp contact or transition by the dark grey laminated facies (Fig. 2; Supplementary Fig. 1). The dark grey, terrigenous silty clays contain rare dispersed sand or gravel and show normally graded, fanning, deformed, or wavy silt laminations, ripple cross-lamination, and silt lenses, primarily in the lower Pliocene section at both sites39,40. Color-banded silty clays are thinly laminated and locally moderately bioturbated with occasional fragments of siliceous microfossils and rare dispersed sand and gravel. Laminations occur by color and do not signify visible grain-sized differences. A reddish hue is common. Sets of thin sharp-based sand beds occur throughout this facies. The greenish grey facies consists of massive or bioturbated diatomaceous silty clay and generally exhibits an upward transition from a bioturbated mud with dispersed gravel in the lower part of the unit to more gravel-rich mud at the top, which locally exhibits stratification and a higher biogenic content (Fig. 2).

Grain-size distributions for the greenish grey facies are most variable with modes ranging from clay to coarse silt (Supplementary Fig. 2). Dark grey laminated silty clays also show variable silt modes. In contrast, muds of the color banded facies typically consist of more uniform fine silt (Supplementary Fig. 2). A strong correlation between sortable silt and sortable silt mean size is found for the silty clays, indicating a variable bottom current influence. Five samples of gravel-bearing greenish grey lithology and one from the dark grey laminated facies exhibit a uniform fine sand mode with a fine tail. These samples were excluded from the sortable silt calculations.

Bulk geochemical data also reveal clear changes between the facies. XRF-scanner derived Ti/Nb ratios change abruptly near the lower boundary of greenish grey units, and in contrast to Al/Ti ratios, a very gradual decrease is observed near the upper boundary of greenish grey units (Fig. 2). Increases in Ca/Ti and Mn/Ti ratios are found at the tops of greenish grey units. The facies variability is also noted in the down-core distributions of all datasets (Fig. 3). Color-banded intervals have the highest a* values near the tops of the intervals, indicating that they are the most reddish in color, whereas greenish grey units have the lowest a* values. Al/Ti ratios follow a cyclic pattern related to facies with minima in the greenish grey facies only. Greenish grey units have low a* values and higher Ba/Rb ratios and Ce/Ce* values than the laminated silty clays. Greenish grey units also have higher smectite contents, whereas the highest kaolinite percentages coincide with color-banded facies at Site U1533 (Fig. 3). The color-banded terrigenous facies occur primarily within cycles between ~180 and 110 mbsf at Site U1533 and are concentrated between ~170 and 140 mbsf (Fig. 3), whereas color-banding is not as well developed at Site U1532 (Supplementary Fig. 3), which is located near the crest of Resolution Drift, i.e., further away from the deep-sea channel. Similarly, kaolinite contents show large-scale changes over the depth of the drilled interval (Fig. 3). In Hole U1533B the kaolinite content displays a minimum at ~140 mbsf and increases (>12%) between ~130 and 110 mbsf (Fig. 3). GdN/YbN ratios and kaolinite contents generally correlate well, except above ~100 mbsf. A similar pattern is observed in correlative strata at Site U1532 (Supplementary Fig. 3). The sediments of all facies become more diatom-rich in the upper parts of the early-mid Pliocene sections at both sites, albeit without a change in the terrigenous lithological expression of the facies cyclicity (Fig. 3, Supplementary Fig. 3).

Interpretation of dark grey laminated and color-banded facies
The dark grey laminated facies in the Pliocene sections at sites U1532 and U1533 exhibit sedimentary structures consistent with intermittent bottom-current processes, whereas current structures are not ubiquitous in the color-banded facies. Sharp-based sand beds in both the dark grey laminated and color-banded facies are interpreted as turbidites39,40. Both the dark-grey laminated and the color-banded facies are interpreted as a deposit from downslope sediment transport with bottom-current reworking. However, the grain-size distributions for the color-banded sediments (between thin sharp-based sand beds) at Site U1533 exhibit a lower sortable silt mean size than the other facies, confirming deposition of suspended material under more limited bottom-current strength (Supplementary Fig. 2).

Low Eu/Eu* values of the laminated and color-banded facies at Sites U1532 and U1533 (Fig. 3) are typical of a felsic igneous source and point to a supply of detritus from granitoids within the Pine Island Glacier basin, which is characterized with Eu/Eu* values as low as 0.51−0.59 in the mud fraction of modern sediments55 (Supplementary Fig. 4). The multidimensional scaling of trace elements shows that the provenance of the dark grey laminated silty clay and especially the color-banded silty clay facies is most similar to modern sediment in proximity to Pine Island Glacier (Fig. 4). In addition, correlative increases in bulk sediment GdN/YbN ratios and kaolinite contents indicate periodic increases in glaciogenic detritus supply from Thwaites Glacier and ice draining into the western Amundsen Sea embayment54–56. Kaolinite-bearing sediments or sedimentary rocks are present within the Amundsen Sea inner shelf and underneath WAIS in the Byrd Subglacial basin57–60 and higher GdN/Yb values are indicative of the more mafic and younger, less evolved, magmatic signature of the West Antarctic Rift System/Marie Byrd Land province (Supplementary Fig. 4).

While sedimentary structures, such as ripple cross-lamination, silt and fine-sand lenses, and fanning laminations, in the dark-grey laminated facies exhibit evidence of bottom current reworking, the color-banded facies lacks these structures and is primarily composed of fine-grained color-laminated mud with intermittent thin sharp-based sand beds interpreted as distal turbidites. We interpret the color-laminated facies at Site U1533 as a depositional signature of fine-grained suspended sediment plumes originating near the grounding zones in the Amundsen Sea Embayment, either from subglacial channelized flow or ice-shelf melt or both, that were transported to the continental rise as hyperpycnal plumes and flows. Hyperpycnal flows can transport fine-grained detritus to the slope and rise through spill-over from the shelf42,61,62. These deposits are rare on the present-day Antarctic continental margin, but meltwater deposits (plumites) have been described offshore from more temperate glacial margins, where they extend several 100s of kilometers out from a former ice-stream margin, and where suspended sediment is interpreted to have reached the continental slope and rise as hyperpycnal plumes entrained within low-intensity geostrophic flows42,44,61,62.

Interpretation of greenish grey facies
The greenish grey facies with bioturbation, a larger biogenic component, and dispersed sand and gravel, are interpreted as the product of hemipelagic deposition with iceberg rafting at the onsets of glacial retreat. Greenish grey biogenic silica-bearing facies are distinctly different in geochemical and clay mineralogical composition from the terrigenous laminated and color-banded facies (Fig. 2). In contrast to the grey laminated and color-banded facies, the bulk sediment of the greenish grey facies does not exhibit a strong eastern Amundsen Sea Embayment or Pine Island Glacier detrital provenance signal (Fig. 4). Low Al/Ti ratios characterize the greenish grey units at Site U1533 and because Al scavenging by diatoms would elevate Al/Ti ratios and not decrease it, the change in Al/Ti ratios is most likely a detrital signal. High Ti content is found in South Pacific dust. However, Antarctic ice cores have higher dust content in Pleistocene glacials63, while no clear dust signal is detectable in sediments from the Amundsen Sea continental rise during the present interglacial64. Thus, the low Al/Ti ratios more likely indicate reduced terrigenous supply from the WAIS during Pliocene interglacials. High Ti/Nb ratios above the bases of the greenish grey facies (Fig. 2) are a strong indicator of a shift towards mafic igneous source rock contributions at glacial terminations; this elemental ratio is not likely affected by weathering, diagenesis or sediment sorting65. Enrichment in smectite and high Eu/Eu* values in the greenish grey facies (Fig. 3) are consistent with a mafic volcanic source. Detritus with a mafic igneous provenance signature in the Amundsen Sea may be supplied from local West Antarctic volcanic sources54,57, but an offshore marine source is also possible58, which at least for some of the smectite has recently been confirmed by detailed investigations of smectite chemistry66. High Eu/Eu* ratios >0.6 in the greenish grey facies (Fig. 3) are consistent with a detrital mud fraction derived from an offshore South Pacific source, as modern silt and clay-sized detritus in the deep easternmost Pacific sector has Eu/Eu* values ranging from 0.63 to 0.69 (data from Wengler et al.64).

Elevated Mn/Ti ratios at the top of the greenish grey facies and within the transition to the overlying grey laminated facies point to generally well-oxygenated bottom water (Fig. 2). Low terrigenous supply under well-oxygenated bottom water conditions is a favorable setting for the formation and accumulation of authigenic minerals, which is consistent with the high Ce/Ce* values (positive Ce-anomalies) and the elevated Ba/Rb ratios attributed to deposition of water-column derived barium sulphate (barite) under high productivity conditions (Fig. 3). ICP-MS results on a discrete sample at the top of the greenish grey unit (A15) in Core U1533B-17H at ~171.5 mbsf show that the ripple-laminated sands at that level are interbedded with indurated calcium, manganese and phosphorous-rich sediment (Fig. 3; Supplementary Fig. 1). A deformed carbonate-fluorapatite concretion or bed with high Ca/Ti ratio was observed shipboard via XRD analysis in Core U1533B-18H-6 at 185.6 mbsf40. We interpret these carbonate-fluorapatite beds as condensed sections resulting from a decrease in terrigenous supply, biosiliceous bloom activity during interglacials, and bottom-current reworking in ocean frontal zones. Diatoms are efficient fixators of P and are involved in the generation of most marine calcium phosphate deposits, in which Ca can be replaced by Ce under well-oxygenated bottom water conditions67,68.

Correlation of greenish grey units and refined age model
In the lower Pliocene sections of sites U1532 and U1533, greenish grey hemipelagic beds were correlated using the shipboard magnetostratigraphy, the a* record, abundance of biogenic components, and magnetic susceptibility (Fig. 5). Using this approach we were able to correlate ~20 early Pliocene greenish grey beds between the two sites (Table 1). We were also able to tie a distinct bed sequence at U1532C-7F-3, 55 cm (205.37 mbsf) and U1533B-9H-6, 0 cm (99.31 mbsf), dated to 3.92 and 3.91 Ma, respectively, applying the individual linear age models for the two sites. A few greenish grey beds were only present or recovered at one site. The sequence of facies in each cycle drilled at sites U1532 and U1533 is indicative of the transient behavior of the ice-sheet ocean system in the Amundsen Sea sector over Pliocene glacial-interglacial cycles. Following the facies model (Fig. 6), the greenish grey beds are interpreted to represent interglacial stages of deposition. The tops or a* minima of correlative greenish grey bed sequences on the Amundsen Sea continental rise, labeled A1 to A23 (Fig. 5), can be tied to glacial-interglacial transitions derived from the LR04 benthic stack69 (Table 1). This approach was used to attribute ages to the A1−A23 greenish grey units and to refine the age models for both sites through linear interpolation between glacial-interglacial age tie points (Supplementary Fig. 5).

40Ar/39Ar dating of IRD in greenish grey facies
The greenish grey facies contains variable concentrations of dispersed sand and gravel grains interpreted as IRD. 40Ar/39Ar dating of 228 out of 230 individual ice-rafted hornblende, biotite and other mica grains (>250 μm) in the greenish grey units at both sites U1532 and U1533 revealed prevalence of Triassic to Cretaceous ages (Fig. 7; Supplementary Fig. 6), with two grains exhibiting negative ages removed from the dataset. The main modes are concentrated at ~93 and ~170 Ma. The ~170 Ma mode overlaps with the ages of Triassic−Jurassic intrusives known from the Ellsworth-Whitmore and Thurston Island blocks, including ~180−170 Ma granitoids near the downstream end of the Pine Island Glacier catchment51–53. In contrast, the ~93 Ma age is not characteristic for recent IRD sourced from the Amundsen Sea sector and the nearest known outcrops and offshore sediments recording these ages are found along the west side of the Antarctic Peninsula54. Therefore, the ~93 Ma old grain population could represent IRD delivery from icebergs drifting with the westward flowing Antarctic coastal current from sources on the Antarctic Peninsula and adjacent terranes (Fig. 1). Ages of IRD are not uniform and vary between intervals (Fig. 8). Notably, an IRD grain population with dominant Triassic−Jurassic ages is observed in the greenish grey units between 140 and 145 mbsf in Hole U1533B and between 290 and 320 mbsf in Hole U1532C (IRD-bearing beds A12−A13).

Methods
Shipboard, lithological descriptions were carried out on the archive halves of the cores based mainly on visual characteristics, including color, grain-size and sedimentary structures82. Cores were imaged using a Section Half Image Logger (SHIL). Biogenic components were identified and estimated using smear slides. Archive halves were also analyzed with a Section Half Multisensor Logger (SHMSL) at 2 cm intervals. The sampling strategy was designed to avoid aliasing and shipboard track data, in particular a* color reflectance and magnetic susceptibility records, were used to identify different phases of interpreted interglacial−glacial cycles and to sample all representative facies. Subbottom depths were originally reported as mCSF-A82, and these depths are re-designated in this study as meters below seafloor (mbsf) for clarity. For this study, sediments from Hole U1533B between ~84 and 205 mbsf and Holes U1532C and U1532G between ~197 and 531 mbsf, dated between ~3.6 and 5.0 Ma, were analyzed. The shipboard age model was refined through correlation of interglacial beds of greenish grey facies between the two drill sites U1532 and U1533 and tied to orbitally tuned excursions in the LR04 benthic stack69.

Grain-size analysis
Particle size data was collected on 94 samples from Site U1533 to characterize the grain-size variability both within each facies and between the facies. The size fraction 0.02 to 2000 μm of Site U1533 sediments was measured with a Malvern Mastersizer 2000 laser particle sizer at Montclair State University (U.S.A.). This is a dual light source instrument with a large array of detectors and software capabilities including Mie theory for the accurate and precise characterization of fine-grained sediments. Sample pretreatment consisted of boiling the sediments with 30% H2O2 and 10% HCl to remove organic matter and carbonate. Where diatoms were present, samples were heated with 0.2 NaOH in a hot bath at 90 °C for one hour to remove biogenic silica. Sodium pyrophosphate was used as a dispersant. Samples were measured using a Hydro MU sample dispersion unit with a stirred speed of 2000 rotations per minute. Grain-size distributions were generated using the Malvern software using a refractive index of 1.6 and an absorption coefficient of 0.9. Sortable silt mean size was calculated from the bulk particle size distributions. Sortable silt refers to the non-cohesive fine-sediment fraction with a size range from 10 to 63 μm. Sortable silt percent (SS%) and mean sortable silt grain size (SS) were calculated using the equations45,83:

SS% = vol:% [10−63 μm] = vol:% [<63 μm] * 100 (1)

SS = e^θ m with θ m = (Σ i=1,n) θ i fss,i (2)

(θi) is the natural log of the bin’s geometric mid-point diameters and fss,i is the fraction of material associated with the SS range in bin i, with bin 1 representing the bin with its lower bound coinciding with 10 μm, and bin n representing the bin with its upper bound coinciding with 63 μm.

Positive correlations between sortable silt percent and mean sortable silt grain size are interpreted to represent sorting by bottom currents, and this holds for most sediments even when influenced by particle deposition from icebergs, sea ice or meltwater plumes. High IRD input can mask the bottom-current signal, and this is revealed by a poor correlation between the percentage and mean grain size of the sortable silt45.

Clay mineralogy
Clay mineral assemblages were analyzed at the University of Leipzig (Germany) following standard procedures58. The clay fraction (<2 μm) was separated from the bulk sediment in settling tubes. 40 mg clay were dispersed in an ultrasonic bath and sucked through a membrane filter of 0.20 μm pore width. The dried filter cakes were mounted on aluminium tiles and glycolated immediately before the X-ray analyses. Samples were X-rayed on a Rigaku MiniFlex automated powder diffractometer system with CoKα radiation (30 kV, 15 mA) in the range of 3°–40° 2θ with a step size of 0.02° 2θ and a measuring time of 2 s/step, and in the range of 27.5°–30.6° 2θ with a step size of 0.01° 2θ and a measuring time of 4 s/step to better resolve the (002) kaolinite and (004) chlorite peaks. The X-ray diffractograms were evaluated using the interactive “MacDiff” program (freeware available from https://www.uni-frankfurt.de/69528130/Petschick___MacOS_Software). The semiquantitative evaluation of the clay mineral assemblages followed Biscaye84,85.

Bulk geochemical composition
Post-cruise, archive halves of Hole U1533B were scanned at 2 cm resolution with an energy of 10 kV, 30 kV, and 50 kV on an Avaatech XRF core scanner at the IODP Gulf Coast Repository of Texas A&M University in College Station (U.S.A.). in order to provide a record of elemental values for the whole sequence (84−205 mbsf) Specifically, XRF results for Ca, K, Ti, Mn, Rb, Zr, Nb, Ba were used to investigate elemental ratios (Ca/Ti, K/Ti, Mn/Ti, Zr/Rb, Ba/Rb, and Ti/Nb) indicative of biogenic and detrital sediment supply and composition. Bulk geochemical data were collected on 108 samples from sites U1532 and U1533 using a ThermoFisher Quadrupole ICP-MS at Montclair State University (U.S.A.). Approximately 100 mg of powdered sample and 400 mg of lithium metaborate flux were quenched at 1050 °C and digested in 7% nitric acid. The solution was diluted to 0.1% and sample unknowns were analyzed in triplicates with 10 USGS calibration standards. Measurements of standard solutions of Ba, Ce, Pr, Nd and Sm were applied for interference corrections. Ratios of trace and Rare Earth Elements (REE) were interpreted to assess mineral content and bulk provenance for the mudstones. The cerium anomaly (Ce/Ce*) was utilized as a proxy for authigenic mineral content in which the deviation of Ce concentrations is calculated with respect to a smooth REE pattern, with ‘SN’ designating shale-normalized values86:

Ce/Ce* = CeSN / ((LaSN + PrSN) / 2) (3)

Provenance from elemental ratios
Ratios of compatible to incompatible elements were used to infer geochemical provenance. During magmatic processes, compatible elements are incorporated into minerals whereas incompatible elements remain in the melt. We use non-metric multidimensional scaling with Bray-Curtis similarity of a multivariate dataset of elemental ratios to identify similarities in provenance between source rocks and Expedition 379 sediments87. The statistical analyses were carried out in the software package PAST v. 4.0988. The elemental ratios GdN/YbN, La/Th, Sm/Zr, Th/Sc, Zr/Hf and Eu/Eu* differentiate between types of crustal rocks89,90 and show clear spatial distributions in existing source rock and sediment data in West Antarctica54,55,73,91 (Fig. 1; Supplementary Fig. 4). ‘N’ stands for chondrite-normalized values according to Bhatia86. GdN/YbN ratios track the relative abundance of heavy rare earth elements (HREE) and can be used to distinguish different felsic igneous source rocks, ranging from K-rich granites with stable HREE patterns and low GdN/YbN to more Na-rich granodiorite with higher GdN/YbN ratios, which represent different processes of crustal formation92. Th/Sc ratios are low and La/Th ratios are high in mafic to intermediate igneous rocks89,90 and La/Th ratios differentiate arc-volcanic sources due to inheritance of these ratios from the subducting crust72. Magmatic source rocks can further be distinguished using Sm/Zr and Zr/Hf ratios, which are hosted in the chemically and mechanically resistant accessory minerals of monazite, zircon and baddeleyite. Monazite is enriched in Sm and more easily released upon partial melt93, whereas Zr is more easily taken up than Hf upon crystallization of zircon, allowing for compositional differentiation of volcanic and plutonic igneous source rocks94. Europium is a compatible element and partitions into plagioclase upon crystallization in magma chambers, so residual magmas display a negative europium anomaly (high value of Eu/Eu*). The europium anomaly is expressed as Eu/Eu*, where Eu* is the anticipated europium value for a smooth chondrite normalized REE pattern versus atomic number86.

Eu/Eu* = EuSN / ((SmSN + GdSN) / 2) (4)

Different calculations of the europium anomaly are used by individual authors and where sample Eu/Eu* values are compared to existing published data, the Eu/Eu* ratios for the published data are recalculated using the same equation from Bathia86.

40Ar/39Ar geochronology of coarse sand fraction
40Ar/39Ar geochronology was applied to 230 hornblende, biotite and other mica grains picked from the sieved >250 μm fractions of samples from the greenish grey units, interpreted as coarse-grained IRD. 40Ar/39Ar determinations were performed at the Argon Geochronology for the Earth Sciences (AGES) Laboratory at Lamont Doherty Earth Observatory of Columbia University (U.S.A.), using a fully automated Micromass VG5400 mass spectrometer with a CO2 laser extraction system and operated by MassSpec software from Al Deino (Berkeley Geochronology Center). Minerals were co-irradiated with monitor standards offsite to produce 39Ar from 39K and to define the “J-value”, used for the age calculations.

Age model
We used the shipboard age models for Sites U1532 and U153339,40, which in the Pliocene are based on magnetostratigraphic age tie points constrained with diatom and radiolarian biostratigraphy (Supplementary Fig. 5a). We refined the shipboard age models by correlating marker beds between the two drill sites and then further refined the age model through correlation of the sequence stratigraphy to the LR04 benthic stack69,95 (Supplementary Figs. 5b and 5c).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript from Passchier et al. is well written and adresses an important topic that has interest for the larger Earth science community and beyond.\nPasschier et al. applies a multi proxy approach to infer sediment provenance and processes from a really nice sediment stratigraphy from the Amundsen Sea continental rise.\nBased on the data they present and their discussion, I find their conclusions that grounded ice in the Twaites sector of the West Antarctic Ice Sheet retreated during the early Pliocene both robust and compelling.\nWhile the figures and text are generally clearly presented, I feel that the manuscript would have benefitted from illustrations of the processes behind the different sedimentary facies encountered in the record, including the modern processes that is referenced several times in the manuscript but not thoroughly described. This would especially benefit non-specialists reading this paper.\nSome comments on figures: The figures is generally clearly presented and readable, however I miss some things regarding captions and labeling etc:\nFig. 4: Units of the a* and mSS panels are missing (I suppose the mSS should be µm)\nFig. 5: the a* records lacks scale (where are the low values..)\nFig. 8: lowermost panel is not mentioned in the caption, it would also be useful to label the panels a)-C)\nFig. 9: labels a)-h) are missing in the figure", "reviewer_2": "The manuscript investigates cored materials from the Amundsen Sea in the Pliocene to explore the coeval evolution of the West Antarctic Ice Sheet and oceanographic conditions. The methods presented are robust (though some of the geochemiical methods I confess to only having a working understanding of rather than a detailed knowledge) and the inferences from the results are fair and well argued.\nIn reality, with the caveat of my gap in knowledge of the geochemical methods, this was a very easy paper to review and I had very little to negatively comment upon in the manuscript. It appears to be in a very mature condition, it is well written, the figures help to make the case, and throughout the context is well referenced. Given the potential analogy of this warm period the authors investigate, I would state that I think the paper provides an important contribution - though the authors could perhaps consider elaborating on this further. I have some minor suggestions in the pdf.", "reviewer_3": "The authors present an interesting manuscript aimed at understanding the glacial history of the West Antarctic Ice Sheet (WAIS) by examining sediment archives from the deep-sea Antarctic Continental Rise, located downstream from the \"doomsday\" glaciers of Pine Island and Thwaites. Their goal is to use various proxies (color, grain size, XRF, 40Ar/39Ar) to interpret the extent and dynamics of different WAIS glaciers in relation to Pliocene climate and oceanography.\nThe study design is solid, the methods align well with the objectives, and I did not find major flaws in the data interpretation.\nHowever, after multiple readings, I believe several significant issues need to be addressed before the manuscript can be considered for publication.\nBelow, I provide major comments to help the authors enhance their manuscript, followed by specific feedback in the marked-up version.\nMajor comment 1:\nThe manuscript lacks a clearly articulated research question or problem, which is reflected in the overall structure (see also Major Comments 2 and 3). The general aim appears to be an overview of WAIS evolution, primarily focusing on Pine Island and Thwaites glaciers during the Pliocene, to provide analogs of past warm periods and help constrain sea-level rise projections. However, sea-level only appears in the introduction and conclusion, and it’s unclear whether the observed retreat from 4.6 to 4.4 Ma was more or less significant than today’s retreat. The manuscript would benefit from a stronger comparison with present-day conditions to help readers grasp the research implications. A clearer focus on the broader problem, followed by the knowledge gap this study aims to address, would enhance the paper's impact. Additionally, the introduction could be better structured: start with the broader problem, state the knowledge gap, and present the methodology, followed by the regional setting and previous work. Much of the material currently in the methods and results sections, such as data from the IODP report and bedrock geology, belongs in this section. Also, geochemical provenance is barely mentioned in the conclusions, despite its focus in the methods\nMajor comment 2:\nThe manuscript would greatly benefit from a more detailed results section. It is crucial to differentiate between what this study accomplished and what was published during IODP Expedition 379. The authors rely on previously interpreted lithofacies and use them as templates for sedimentation variations but present minimal new results. There are very few numbers or statistics provided, which makes it difficult to understand the dataset or assess the authors’ interpretations. I recommend presenting the results from each analysis, such as mean element content by lithofacies. Currently, it feels like the results were skipped, and the manuscript jumps straight into a lengthy discussion.\nMajor comment 3:\nThe discussion would benefit first from an overview of the chronology/the sequence. As of now it is hard to follow track of what happened in chronological order.\nYou can start by a sub-section of discussion that states what that based on your results, the lihtofacies 1 (terrigenous) exhibit characteristics a, b, c. Characteristic a can be associated with , characteristic b, can be associated with…etc. The authors kind of do a very brief version of that in the section \"Interpretation of sedimentary facies\", but I think it is too short – from the text it mostly/solely is: terrigenous facies is interpetation as glacial conditions based on the fact that it is terrigenous; color-banded is associatd with the deglacial phase and ice-stream activation based on the high rates of sedimentation; and, the greenish facies are interpreted as interglacial conditions based on low Al/Ti ratios.\nAfter you have interpreted the facies in term of what they mean in general conditions, present the sequence conditions : based on these facies you have 19 changes (A series) in background conditions that can be tied to LRD04 benthic stack. After correlating tie-points you have fewer A series than glacial-interglacial cycles as suggested by benthic stacks. Discuss why, and if it means something about the WAIS sensibility to orbital forcing + here you can introduce the variability of in the different A series in chronological order (IRD provenance, sedimentation rates, terrigenous sand content, kaolinite content, etc.) This will set the framework for the subsequent interpretations of changes in the cores.\nMinor comments:\nI have provided a marked-up Word version of the manuscript with detailed comments to make the revision process easier for the authors. This includes direct suggestions and queries.\nComments on figures:\nFigure 1:\nThis figure is difficult to read due to the numerous black lines. Consider dividing it into two panels: one showing topography/bathymetry (with color), glacier extent, and catchments; and another showing geology and Zr/Hf values. Ensure that the colors in the legend match the map for clarity Something like this with catchements on them and the age of each geological unit:\nFigure 2:\nThe figure would be more effective if it showed a transition from glacial to interglacial (dark gray to color-banded to green), rather than starting with color-banded. Also, arrange the cores and history from oldest to youngest, left to right, to match the natural reading flow. Aligning all cores horizontally would make it easier to add curves and improve readability.\nFigure 3:\nAdd transparency and outlines to the facies circles to ensure no data points are hidden by others.\nFigure 4:\nInclude both cores and all curves referenced in the text. Consider removing the core number and recovery information, as these can be found in the IODP report. Align the lithology box with the analyzed interval and ensure the lithology reflects the facies discussed in the text. An idea would be to add the lithology color scheme to the whole figure/under the curves. This would really help with linking proxies and facies.\nFigure 5:\nAlign the start and end points of the three curves and orient the figure horizontally.\nFigure 6:\nAdd all argon-argon histograms from Figure 7. Remove the outline from the bins and use color fill instead to reduce visual clutter.\nFigure 7:\nIf the curves are added to Figure 4, this figure may no longer be necessary.\nFigure 8:\nLabel each chart with a, b, c, and explain each in the main text.\nIt’s unclear where the data points originate. To improve clarity, I recommend differentiating between each core and deriving separate curves for each. This would help determine whether distinct trends or narratives emerge from the individual cores, which isn’t evident in the current presentation.\nFigure 9:\nExcellent figure overall. However, I suggest flipping it to read from left to right, with the oldest data on the left. The gray area of core 32 is a bit lost and could just be in another color. You could present all cureves that relates to core 33 in black and to core 32 in blue, in this figure and in all others. I would also add benthic stack at the top or bottom, it is the reference of the study."}, "author_response": "Reviewer #1 (Remarks to the Author):\nThe manuscript from Passchier et al. is well written and adresses an important topic that has\ninterest for the larger Earth science community and beyond.\nPasschier et al. applies a multi proxy approach to infer sediment provenance and processes from a\nreally nice sediment stratigraphy from the Amundsen Sea continental rise.\nBased on the data they present and their discussion, I find their conclusions that grounded ice in\nthe Twaites sector of the West Antarctic Ice Sheet retreated during the early Pliocene both robust\nand compelling.\nRe: We thank Reviewer #1 for this positive evaluation of our work.\nWhile the figures and text are generally clearly presented, I feel that the manuscript would have\nbenefitted from illustrations of the processes behind the different sedimentary facies encountered\nin the record, including the modern processes that is referenced several times in the manuscript\nbut not thoroughly described. This would especially benefit non-specialists reading this paper.\nRe: We have added a diagram (Figure 6) providing a generalized interpretation of the facies in the\nsequences.\nSome comments on figures: The figures is generally clearly presented and readable, however I miss\nsome things regarding captions and labeling etc:\nFig. 2: The labels referred to in the caption are not on the figure. (from PDF).\nRe: Labels (a) through (f) were added, and description for (f) was added to the caption.\nFig. 4: Units of the a* and mSS panels are missing (I suppose the mSS should be μm).\nRe: (Now Figure 3). The unit μm was added and a* is a green-red spectral ratio and does not carry a\nunit.\nFig. 5: the a* records lacks scale (where are the low values..).\nRe: included green and red labels and modified the text.\nFig. 8: lowermost panel is not mentioned in the caption, it would also be useful to label the panels\na)-C)\nRe: (Now Figure 9). The Zr/Hf panel was removed as it was not referred to specifically in the text\neither. The linear sedimentation rate was added; the caption was modified to reflect these changes.\nFig. 9: labels a)-h) are missing in the figure\nRe: labels a) - j) were added to the figure (now Figure 10).\nReviewer #2 (Remarks to the Author):\nThe manuscript investigates cored materials from the Amundsen Sea in the Pliocene to explore the\ncoeval evolution of the West Antarctic Ice Sheet and oceanographic conditions. The methods\npresented are robust (though some of the geochemiical methods I confess to only having a working\nunderstanding of rather than a detailed knowledge) and the inferences from the results are fair and\nwell argued.\nIn reality, with the caveat of my gap in knowledge of the geochemical methods, this was a very easy\npaper to review and I had very little to negatively comment upon in the manuscript. It appears to be\nin a very mature condition, it is well written, the figures help to make the case, and throughout the\ncontext is well referenced.\nRe: We appreciate these encouraging comments by Reviewer #2.\nGiven the potential analogy of this warm period the authors investigate, I would state that I think the\npaper provides an important contribution - though the authors could perhaps consider elaborating\non this further. I have some minor suggestions in the pdf.\nRe: We now elaborated more on the major findings by framing the results from the Pliocene within a\npresent-day context. We also took note of the minor editorial suggestions in the PDF and modified\nthe text accordingly.\nReviewer #3 (Remarks to the Author):\nThe authors present an interesting manuscript aimed at understanding the glacial history of the\nWest Antarctic Ice Sheet(WAIS) by examining sediment archives from the deep-sea Antarctic\nContinental Rise, located downstream from the \"doomsday\" glaciers of Pine Island and Thwaites.\nTheir goal is to use various proxies (color, grain size, XRF, 40Ar/39Ar) to interpret the extent and\ndynamics of different WAIS glaciers in relation to Pliocene climate and oceanography.\nThe study design is solid, the methods align well with the objectives, and I did not find major flaws\nin the data interpretation.\nRe: We are grateful to Reviewer #3 for these positive comments.\nHowever, after multiple readings, I believe several significant issues need to be addressed before\nthe manuscript can be considered for publication.\nBelow, I provide major comments to help the authors enhance their manuscript, followed by\nspecific feedback in the marked-up version.\nMajor comment 1:\nThe manuscript lacks a clearly articulated research question or problem, which is reflected in the\noverall structure (see also Major Comments 2 and 3). The general aim appears to be an overview of\nWAIS evolution, primarily focusing on Pine Island and Thwaites glaciers during the Pliocene, to\nprovide analogs of past warm periods and help constrain sea-level rise projections. However, sea-\nlevel only appears in the introduction and conclusion, and it’s unclear whether the observed\nretreat from 4.6 to 4.4 Ma was more or less significant than today’s retreat. The manuscript would benefit\nfrom a stronger comparison with present-day conditions to help readers grasp the research\nimplications. A clearer focus on the broader problem, followed by the knowledge gap this study\naims to address, would enhance the paper's impact. Additionally, the introduction could be better\nstructured: start with the broader problem, state the knowledge gap, and present the methodology,\nfollowed by the regional setting and previous work. Much of the material currently in the methods\nand results sections, such as data from the IODP report and bedrock geology, belongs in this\nsection. Also, geochemical provenance is barely mentioned in the conclusions, despite its focus in\nthe methods\nRe: We restructured the manuscript to better reflect how our findings build on existing knowledge\nand provided the background on the regional setting and geology in the introduction.\nMajor comment 2:\nThe manuscript would greatly benefit from a more detailed results section. It is crucial to\ndifferentiate between what this study accomplished and what was published during IODP\nExpedition 379. The authors rely on previously interpreted lithofacies and use them as templates for\nsedimentation variations but present minimal new results. There are very few numbers or statistics\nprovided, which makes it difficult to understand the dataset or assess the authors’ interpretations. I\nrecommend presenting the results from each analysis, such as mean element content by\nlithofacies. Currently, it feels like the results were skipped, and the manuscript jumps straight into a\nlengthy discussion.\nRe: The notion that “minimal new results” are presented is based on a misunderstanding of\nshipboard data collection. For example, all particle size data for the facies were collected post-\ncruise at Montclair State University, as we state in the methods section, and were not generated\nduring the expedition, so they were not previously published in the IODP Expedition 379\nProceedings volume. The same applies to the clay mineral data, which were all analyzed post-\ncruise at the University of Leipzig (only preliminary shipboard clay-mineral data had been presented\nin the IODP 379 Proceedings). Additionally, the lower Pliocene lithofacies sequence presented in\noriginal Figure 4 (now Figure 3) and Supplemental Figure 2a of our study was also not previously\ndefined and interpreted; instead, in the IODP 379 Proceedings volume only individual lithofacies\nand two short upper Pleistocene sections with facies associations were described and interpreted\nwithout stratigraphic context. Lastly, we present the results for each analysis in various graphic\nways, which also display the variations in geochemical ratios by lithofacies. To further clarify the\npresentation of the data, we added 95% confidence loops to the MDS plot, as suggested by the\nreviewer in the PDF. Following the referee’s recommendation, we have now separated the results\nsection entirely from the discussion section.\nMajor comment 3:\nThe discussion would benefit first from an overview of the chronology/the sequence. As of now it is\nhard to follow track of what happened in chronological order.\nYou can start by a sub-section of discussion that states what that based on your results, the\nlihtofacies 1 (terrigenous)exhibit characteristics a, b, c. Characteristic a can be associated with ,\ncharacteristic b, can be associated with…etc. The authors kind of do a very brief version of that in\nthe section \"Interpretation of sedimentary facies\", but I think it is too short– from the text it\nmostly/solely is: terrigenous facies is interpetation as glacial conditions based on the fact that it is\nterrigenous; color-banded is associatd with the deglacial phase and ice-stream activation based on\nthe high rates of sedimentation; and, the greenish facies are interpreted as interglacial conditions\nbased on low Al/Ti ratios.\nAfter you have interpreted the facies in term of what they mean in general conditions, present the\nsequence conditions: based on these facies you have 19 changes (A series) in background\nconditions that can be tied to LRD04 benthic stack. After correlating tie-points you have fewer A\nseries than glacial-interglacial cycles as suggested by benthic stacks. Discuss why, and if it means\nsomething about the WAIS sensibility to orbital forcing + here you can introduce the variability of in\nthe different A series in chronological order (IRD provenance, sedimentation rates, terrigenous sand\ncontent, kaolinite content, etc.) This will set the framework for the subsequent interpretations of\nchanges in the cores.\nRe: The detailed facies description and interpretation was moved to the results section. Many\ndetails of the facies description and interpretation were previously woven into the discussion. The\ndiscussion now includes a chronological order of events, as suggested by the reviewer.\nMinor comments:\nI have provided a marked-up Word version of the manuscript with detailed comments to make the\nrevision process easier for the authors. This includes direct suggestions and queries.\nComments on figures:\nFigure 1:\nThis figure is difficult to read due to the numerous black lines. Consider dividing it into two panels:\none showing topography/bathymetry (with color), glacier extent, and catchments; and another\nshowing geology and Zr/Hf values. Ensure that the colors in the legend match the map for clarity\nSomething like this with catchements on them and the age of each geological unit:\nRe: We changed the inset figure to one with catchments. We provided clarity on the black lines in\nthe legend. We did not expand to a full geological map in addition to the outcrop and rock and\nsediment sample data our map is based on, because it would include controversial interpretations\nof geophysical data beyond the scope of this paper. We did add information about the assumed\nThwaites Glacier sub-ice geology as inferred from aerogeophysical data (Jordan et al., 2023).\nFigure 2:\nThe figure would be more effective if it showed a transition from glacial to interglacial (dark gray to\ncolor-banded to green), rather than starting with color-banded. Also, arrange the cores and history\nfrom oldest to youngest, left to right, to match the natural reading flow. Aligning all cores\nhorizontally would make it easier to add curves and improve readability.\nRe: Stratigraphically, the color-banded facies is the transition from glacial to interglacial. Showing\nthe continuous XRF data for all three facies in the detailed view of one core section is only possible,\nif the thinnest greenish grey facies is the center unit in the view. Therefore, we selected the section\nshown in Figure 2. We furthermore point out that the natural flow of reading is not universal, and for\nmany paleoclimatologists it is right to left, old to young, which is our consistent choice throughout\nthe manuscript (on an x-axis of a mathematical visualization, zero is the origin on the left and values\nincrease to the right). To accommodate those reading time series from left to right, we have now\nadded “old” and “young” labels in the figures to guide the observations of the core sections and\ndata. Just to give one example: The frequently cited paleoclimate paper about the LR04 benthic\nstack also displays oldest data on the right and youngest data on the left (Lisiecki and Raymo, 2005;\nhttps://doi.org/10.1029/2004PA001071).\nFigure 3:\nAdd transparency and outlines to the facies circles to ensure no data points are hidden by others.\nRe: Many data points plot on top of each other in this figure (now Figure 4) because they have similar\nvalues. However, we carefully chose the symbols and hope that all individual data points are\nrecognizable now. Furthermore, we have added 95% confidence ellipses to the MDS as suggested\nby Reviewer #3 in their comments on the PDF.\nFigure 4:\nInclude both cores and all curves referenced in the text. Consider removing the core number and\nrecovery information, as these can be found in the IODP report. Align the lithology box with the\nanalyzed interval and ensure the lithology reflects the facies discussed in the text. An idea would be\nto add the lithology color scheme to the whole figure/under the curves. This would really help with\nlinking proxies and facies.\nRe: In the revised paper we present the downcore data plot for Site U1533 in Figure 3 and that for\nSite U1532 in Supplemental Figure 5. We prefer not to add the Site U1532 down-core data plot to\nthe main manuscript as it would either overload Figure 3 or add an additional figure, which seems\nunnecessary. We now highlight the greenish grey interglacial beds with green shading under the\ngraphs in the revised Figure 3. The lithological log is based on visual information and smear slides,\nand provides an overview of the sedimentology, which can be used to characterize the depositional\nsetting and processes of the sediments. The analytical datasets are strongly governed by the\nprovenance of the supplied terrigenous detritus, which is not necessarily the same for identical\nfacies in the stratigraphic column because of changes in maximum and minimum ice-sheet extent\nthrough different glacial-interglacial cycles.\nFigure 5:\nAlign the start and end points of the three curves and orient the figure horizontally.\nRe: In this paper, drill-site data on a depth scale are consistently displayed vertically, illustrating the\norientation of the layers from which the samples were collected (The only exception is revised\nFigure 2, where just one core section is displayed). Figure 5 displays the stratigraphy on a depth\nscale, a vertical cross-section of the geological stacking of layers on top of each other. The depth\nscale is a spatial scale indicating a position beneath the seafloor, which in the reference frame of a\nperson on the surface of the Earth across a drill site is vertically down, so it is natural to read this\ndata vertically. Core data on an age scale are displayed horizontally as a time series, because the\nspatial data is converted to a time series using an age model and no longer a true spatial\nrepresentation.\nFigure 6:\nAdd all argon-argon histograms from Figure 7. Remove the outline from the bins and use color fill\ninstead to reduce visual clutter.\nRe: We now present the histograms of the individual stratigraphic intervals in a separate figure, i.e.,\nFigure 8 in the revised manuscript.\nFigure 7:\nIf the curves are added to Figure 4, this figure may no longer be necessary.\nRe: The curves were added to Figure 4 (now Figure 3) and the original Figure 7 was removed, as\nrequested.\nFigure 8:\nLabel each chart with a, b, c, and explain each in the main text.\nIt’s unclear where the data points originate. To improve clarity, I recommend differentiating between\neach core and deriving separate curves for each. This would help determine whether distinct trends\nor narratives emerge from the individual cores, which isn’t evident in the current presentation.\nRe: Labels a-c were added to the charts as requested and the interpretation for each panel is given\nin the figure caption (now Figure 9). Data from the different sites are now marked by different\nsymbols to show that there is overlap between the data from the two sites, and that no trends\nemerge from either site.\nFigure 9:\nExcellent figure overall. However, I suggest flipping it to read from left to right, with the oldest data\non the left. The gray area of core 32 is a bit lost and could just be in another color. You could present\nall cureves that relates to core 33 in black and to core 32 in blue, in this figure and in all others. I would also add\nbenthic stack at the top or bottom, it is the reference of the study.\nRe: We added the benthic stack at the top of the figure (now Figure 10). As mentioned above, the\noriginal reference for the benthic stack (Lisiecki and Raymo, 2005) ‒ and also newer versions (see\nAhn et al. (2017) ‒ consistently displays the oldest data on the right, as in the submitted version of\nour manuscript. Therefore, we decided to keep the orientation as is. We did not change the grey\nareas as we think this visualization of the U1532 data works well."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Dear Passchier et al.,\nI am satisfied with the reply to the reviewers comments and the changes made to the manuscript.\nI recommend publication.", "reviewer_3": "I appreciate the efforts the authors have made in revising the manuscript. Overall, I find that the adjustments to the structure and the incorporation of suggestions have improved the readability and clarity of the paper. The manuscript is now more cohesive and better organized.\nThat said, I believe the introduction remains too long (currently spanning five pages) and contains some redundancies, particularly in the statement of the study’s objectives, which are repeated multiple times (see marked-up copy). Additionally, I suggest that the last paragraph of the introduction be moved to the discussion, conclusions, or abstract, as it reveals key findings that should not be preemptively disclosed.\nAt this stage, my primary concerns are editorial in nature, and I have provided detailed suggestions in the marked-up copy. While I appreciate the inclusion of the new figure illustrating processes, I find that the revisions to the other figures were somewhat minimal. Rather than fully engaging with the constructive feedback, the authors have justified retaining their original figures and made only minor modifications. While these choices do not invalidate the study’s conclusions, I believe they represent missed opportunities to enhance clarity and effectively communicate key results. Per example, I suggested that the core proxies be presented from left to right, progressing from older to younger, as this follows the natural reading order in English and aligns with how information is typically processed in an English-language journal. Since the authors are presenting values in years before present (ie., negative values), the use of a standard x-axis convention—where zero is positioned on the right—would be more intuitive. Adjusting these aspects would improve clarity and better align with standard conventions for chronological data presentation.\nFigures should prioritize the most essential information to support the paper’s arguments. Currently, many figures feel overcrowded or could be optimized for readability. Below, I highlight some specific concerns:\nFigure 1: The inclusion of hillshade beneath the ice introduces visual noise, making it difficult to discern key features. Consider whether this layer is necessary. Additionally, the figure contains many overlapping black lines, reducing readability. A possible solution would be to split the figure into two panels: one focusing on topography/bathymetry (with glacier extent and catchments) and another dedicated to geology and Zr/Hf values. Ensuring that legend colors match the map would also improve clarity.\nFigure 3: The resolution of the curves appears low or blurry. Additionally, the alignment of lithology, magnetic susceptibility, and core recovery should match the start of the curves for consistency.\nFigure 5: The start and end points of the three curves should be aligned. It is unclear why the benthic stack appears to \"float\" on the left.\nFigures 7 and 8: The current bin outlines add visual clutter. I recommend either removing the outlines and using color fills instead or add color to the curves o enhance contrast and improve readability.\nWhile these are not fundamental issues, I believe addressing them would substantially improve the manuscript’s presentation and accessibility to readers. I encourage the authors to consider these refinements to enhance the overall clarity of their figures."}, "author_response": "Reviewer #1 (Remarks to the Author):\nDear Passchier et al.,\nI am satisfied with the reply to the reviewers comments and the changes made to the\nmanuscript.\nI recommend publication.\nRe: We thank the reviewer for the positive recommendation.\nReviewer #3 (Remarks to the Author):\nI appreciate the efforts the authors have made in revising the manuscript. Overall, I find\nthat the adjustments to the structure and the incorporation of suggestions have improved\nthe readability and clarity of the paper. The manuscript is now more cohesive and better\norganized.\nThat said, I believe the introduction remains too long (currently spanning five pages) and\ncontains some redundancies, particularly in the statement of the study’s objectives, which\nare repeated multiple times (see marked-up copy).\nRe: We agree that after moving the discussion of all previously published data to the\nintroduction, that the introduction is now too long. We shortened the introduction by\nmoving the detailed description of the experimental approach and proxies to the Methods\nsection. In addition, we moved the interpretation of published bedrock trace element data\nand the review of published Ar/Ar ages of marine sediments back to the discussion section.\nIn the present annotated file the reviewer marked as redundant: “In the light of modern\nmajor ice loss in the Amundsen Sea drainage sector of WAIS, we targeted a sedimentary\npaleoarchive offshore from this sector to reconstruct ice-sheet variability through the\nwarm early Pliocene. Here we use the sedimentological, geochemical and clay\nmineralogical data collected on the drillcores from the Amundsen Sea continental rise to\ndetermine the extent of WAIS retreat during the early Pliocene warm period, and to\ncharacterize the mechanisms and impacts of this ice loss.”The only sections marked as\nrepetition were: “we aim to reconstruct the connections between source-to-sink supply of\nglaciogenic detritus and WAIS dynamics” and “the extent in changes in grounding line\nposition within the context of the changing local paleoceanography”, which to us do not\nseem to be repetitions, rather clarifications.\nAdditionally, I suggest that the last paragraph of the introduction be moved to the\ndiscussion, conclusions, or abstract, as it reveals key findings that should not be\npreemptively disclosed.\nRe: The formatting instructions for Nature Communications ask for the inclusion of this\nsection in the Introduction. The formatting instructions state: The final paragraph must\nbegin with a phrase like “In this work” or “Here, we show”, and contain a brief summary of\nthe major results and conclusions of the current work, written in the present tense. The\nformatting instructions also call for the abstract to be no longer than 150 words, and the\nmain text not to include a Conclusion section.\nAt this stage, my primary concerns are editorial in nature, and I have provided detailed\nsuggestions in the marked-up copy. While I appreciate the inclusion of the new figure\nillustrating processes, I find that the revisions to the other figures were somewhat minimal.\nRather than fully engaging with the constructive feedback, the authors have justified\nretaining their original figures and made only minor modifications. While these choices do\nnot invalidate the study’s conclusions, I believe they represent missed opportunities to\nenhance clarity and effectively communicate key results. Per example, I suggested that the\ncore proxies be presented from left to right, progressing from older to younger, as this\nfollows the natural reading order in English and aligns with how information is typically\nprocessed in an English-language journal. Since the authors are presenting values in years before\npresent (ie., negative values), the use of a standard x-axis convention—where zero is\npositioned on the right—would be more intuitive. Adjusting these aspects would improve\nclarity and better align with standard conventions for chronological data presentation.\nRe: In a survey of geology journals with a broad audience, Reshnik et al. (2024) found that\nnearly 55% of authors used the right-to-left time scale, whereas only 20% used the left-to-\nright time scale. While this may be counter-intuitive for some, most other authors\npublishing on Pliocene paleoclimate data work with a right-to-left timescale: Lisiecki and\nRaymo in Paleoceanography (2005), McKay et al. in PNAS (2012), Federov et al. in Nature\nGeoscience (2013), Tierney et al. in GRL (2019), Dumitru et al. in Nature (2019). We argue\nthat the convention in displaying paleoclimate data time series for the Pliocene is from\nright to left, and that users of Pliocene paleoclimate data would prefer to read all data\nseries in the same direction.\nFigures should prioritize the most essential information to support the paper’s arguments.\nCurrently, many figures feel overcrowded or could be optimized for readability. Below, I\nhighlight some specific concerns:\nFigure 1: The inclusion of hillshade beneath the ice introduces visual noise, making it\ndifficult to discern key features. Consider whether this layer is necessary. Additionally, the\nfigure contains many overlapping black lines, reducing readability. A possible solution\nwould be to split the figure into two panels: one focusing on topography/bathymetry (with\nglacier extent and catchments) and another dedicated to geology and Zr/Hf values.\nEnsuring that legend colors match the map would also improve clarity.\nRe: We have now outlined the catchments in white, which eliminates the crossing of black\nlines. Our study discusses the evacuation of rock materials from the different catchments,\nso the catchment lines are necessary. Both the basement topography or “hillshade” and\nthe Zr/Hf values are data on the bedrock geology, and they should occur together in the\nsame figure panel for clarity and not in separate panels. Our approach is similar to\nMarschalek et al. 2021, published in Nature, who provide a “hillshade” type of base map for\na Miocene provenance study. We improved the consistency of the legend colors.\nFigure 3: The resolution of the curves appears low or blurry. Additionally, the alignment of\nlithology, magnetic susceptibility, and core recovery should match the start of the curves\nfor consistency.\nRe: The lithology and data curves are now aligned. In response to the comment about\nfigures being overcrowded, we have now removed the Sm/Zr panel, because it was only\nmentioned once in the results and was not specifically mentioned in the interpretation. The\nblurriness resulted from the conversion to PDF during production of the manuscript file.\nFigure 5: The start and end points of the three curves should be aligned. It is unclear why\nthe benthic stack appears to \"float\" on the left.\nRe: We have moved the benthic stack to the right of the curves and lined out the top of the\nbenthic stack with the downhole curve for Site U1533. The benthic stack is on an age scale,\nwhereas the data is on a depth scale and because of the variable sedimentation rates,\nthere is no one-to-one correlation possible.\nFigures 7 and 8: The current bin outlines add visual clutter. I recommend either removing\nthe outlines and using color fills instead or add color to the curves o enhance contrast and\nimprove readability.\nRe: We removed the outlines and added a grey color to the bins.\nWhile these are not fundamental issues, I believe addressing them would substantially\nimprove the manuscript’s presentation and accessibility to readers. I encourage the\nauthors to consider these refinements to enhance the overall clarity of their figures."}]

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