Cold spells over Greenland during the mid-Pliocene Warm Period

Domain: earth_science
Published: 2025-02-22
Editorial outcome: minor_revision

Abstract
The mid-Pliocene Warm Period (mPWP; 3.26–3.02 Ma) is an interval often suggested as a potential analogue of the near future climate and fate of the Greenland Ice Sheet (GIS). Here, neodymium and lead isotopes from marine sediment cores collected off the southern Greenland margin suggest pulses of intense glacial erosion of Precambrian terranes during this interval, while grain size data indicate a reduction in the strength of contour currents, both following an obliquity cycle (~41 ka) pacing. These cold spells were thus sufficiently intense to trigger recurrent ice growth over Greenland, even under the high atmospheric CO2 concentration (~400 ppmV) of the interval, before the intensification of the Northern Hemisphere Glaciation (~2.7 Ma). However, the mPWP was marked by a low amplitude in the axial tilt oscillation of the Earth, thus lesser variations in summer insolation at high latitudes than in the present era. Therefore, although it may offer some similarities with the future of the Earth’s climate, the mid-Pliocene cannot be seen as a genuine analogue for predicting the fate of the GIS.

Introduction
The current atmospheric pCO2 is within estimates for the mid-Pliocene Warm Period (mPWP; 3.264–3.025 Ma)1–4. With atmospheric pCO2 ranging from 360 ± 80 ppmv5 to 530 ± 110 ppmv6 and the boreal forest reaching the northernmost Arctic Canada7,8, the mPWP is often seen as a possible analogue of the near future of our planet2, worth investigating as it depicted a land-ocean distribution and an ocean bathymetry similar to those of the present-day1,9. Modelling studies indicate that during the mPWP, the Greenland Ice Sheet (GIS) extent may have been considerably reduced10, thus possibly contributing to ~15 m higher than present sea-level11. Data and model simulations suggest sea surface temperatures (SST) of at least +3 °C in Arctic regions during this interval1, and terrestrial Arctic temperatures 11 to 16 °C warmer than present8. Diverse scenarios have been put forth regarding the extent of the GIS during the mPWP. These scenarios range from a substantial 50% reduction in the size of the GIS12 to a situation where ice caps were confined to the elevated areas of south/southeastern and eastern/northeastern Greenland13,14 (Fig. 1A, B). Some models suggest that the maximum ice extent occurred in northeast Greenland14,15. Still, proxies documenting the mPWP Greenland ice cover are few, and mostly indirect as they are restricted to information from marine records off Greenland margins16.

In this study, we use a sedimentary record from the Integrated Ocean Drilling Program (IODP) on the Eirik Drift off southern Greenland (site U1307; Fig. 1) to indirectly assess the dynamics of the GIS during the mPWP. The Eirik Drift is a contourite drift moulded by the Western Boundary Undercurrent (WBUC)17, and its formation can be traced back to the Miocene18. Previous paleoceanographic studies suggest that the strength of the WBUC was significantly reduced during glacial intervals but exhibited high flow rates notably during Quaternary interglacials19–23.

Ice erosion of Precambrian terranes in Southern Greenland has been shown to deliver sediments of singular radiogenic isotope compositions to the Eirik Drift during the Pleistocene and Holocene19,24. Here, we use the isotope signature of the Precambrian source rocks to document changes in glacial activity over southern Greenland during the mPWP, as recorded in sediment from IODP site U1307. This site was investigated by several authors who set up the first age models20–22. At this site and along the trajectory of the WBUC, grain-size data analyses of surface sediments show that silt outweighs the clay size fraction that may originate from remote sources, north of the Denmark Strait (Fig. 1). Notwithstanding the dominance of silt, the effective grain size of the sediment is sensitive to changes in the strength of the WBUC23–25. Disregarding the content of ice rafted debris (IRD)22, this behaviour results in the deposition of coarser sediments during interglacials due to high shear stress and winnowing26. Whereas sediments originating from the Canadian margin are transported southward through the Labrador Current27, the Eirik Ridge sediments mostly relate to proximal terrestrial source supplies from southeastern Greenland, as documented in Quaternary records19,24.

The terrestrial sediment sources from southeastern Greenland terranes include the Archean Block (AB), the Paleoproterozoic Ketilidian Mobile Belt (KMB), the Paleoproterozoic Nagssugtoqidian Mobile Belt (NMB), and the Paleogene volcanism (PV) that also formed Iceland28,29 (Fig. 1). Each of these terranes holds distinctive neodymium (Nd) and lead (Pb) isotope compositions19,24, as detailed in the supplementary material, thus supplying sediments with distinctive chemical signatures of their source to IODP site U1307 (Fig. 2).

For the documenting of the mPWP interval, we used cores from the A and B holes of IODP site U1307 (58°30’N, 46°24’W, 2575 m water depth; Fig. 1)30 to set a continuous Nd and Pb isotope record throughout the mPWP interval. Earlier studies of U1307, notably by Channell et al.30, Sarnthein et al.20, Mazaud et al.21, Aubry et al.31 and Blake-Mizen et al.22 provided detailed features of paleoceanographic changes at this site, with a chronostratigraphy based on oxygen isotope stratigraphy and paleomagnetic reversal age constraints. Here, we used the most recent age model of Blake-Mizen et al.22 for investigating the 3.22–2.95 Ma interval, i.e., broadly the mPWP episode, with a temporal sampling resolution of ~5 ka. In addition to Nd and Pb isotope measurements (here expressed as εNd(0) and 207Pb/206Pb; methods in supplementary material) performed on the <63 µm sediment fraction, we used calcite contents derived from quantitative X-ray diffraction analysis of bulk sediments and laser measurements of mean grain size, sand contents and sortable silt mean size (SSmean). X-ray-ray-based calcite abundances are used as tracers of biogenic production32, while the coarseness in SSmean and increased sand content are used as indicators of bottom current strength33,34. Details about analytical methods are to be found in the supplementary material. Our main results are illustrated in Fig. 3, together with the δ18O stack of Lisiecki and Raymo35, and the U1307 δ18O measurements of the planktic foraminifera Neogloboquadrina atlantica, by Sarnthein et al.20 for potential comparison with other site records.

Results
The mPWP sedimentary record at Eirik Drift exhibits recurring εNd(0) excursions of −2 to −7 units from background values, with the most radiogenic values ranging from −15 to −11. These excursions are in phase with 207Pb/206Pb upsurges from a background value of ~0.84 toward values exceeding 0.88. As documented for Quaternary glacial intervals in several studies19,24, such unradiogenic Nd/radiogenic Pb excursions record enhanced terrigenous supplies from the old Precambrian craton of southern Greenland (Fig. 3). We will thus now refer to these excursions as “glacials”, and to the interval between them as “interglacial” stages within the mPWP time window.

The overall εNd(0) and 207Pb/206Pb signatures depict trends across the mPWP interval (Fig. 3). The εNd(0) data from both the glacial and interglacial sediments follow a trend towards more radiogenic values through time. The εNd(0) values of interglacial sediments (−15 to −11) suggest an almost even mixing of three end-members (Fig. 2): PV, KMB, and NMB/AB. These values remain significantly less radiogenic than those recorded by Fagel et al.36 in late Holocene and surface sediment clays from the area (εNd(0) = −9.3)36 (Fig. 3). The more radiogenic recent values indicate higher supplies from the Paleogene volcanic sediments36, outcropping on both sides of Denmark Strait (Fig. 1). The 207Pb/206Pb ratios of interglacials are within the range of late Holocene sediments36. The glacial 207Pb/206Pb excursions depict a trend indicating increasing contributions from less radiogenic sources towards the younger part of the record37,38 (Fig. 3).

The Nd-Pb isotopic excursions are preceded by peaking calcite abundance, of variable amplitude, as well as by high mean grain size, sortable-silt size (SSmean) and sand contents (Fig. 3). These features broadly highlight two distinct sedimentary regimes linked to the strength of the WBUC. One is related to strong currents during warm intervals, in particular during their middle to late part32. The other is related to low flow rates during cold intervals. The interglacial layers illustrated in Fig. 3 record an early stage with relatively fine material, followed by a late stage marked by sediment coarsening, which is a feature observed during the present interglacial in the core site vicinity32. The quasi-absence of dolomite leads to the inference that calcite peaking values record enhanced biogenic carbonate fluxes during the late part of the interglacial intervals. A similar feature characterises the present interglacial32.

Estimating changes in sediment accumulation rates between the two contrasted sedimentary regimes would require estimating the duration of the glacials vs interglacials. However, linear interpolations between age anchors cannot provide robust duration estimates due to highly variable sediment accumulation rates between and during both intervals18,39. Nevertheless, the recurrence of six, possibly seven (Fig. 3) isotopic excursions over the 2280–3220 ka interval studied (grey stripes in Fig. 3) would be compatible with a mean duration of ~40 ka for each cold-warm cycle. The ~40 ka-periodicity is not clearly expressed when using the Blake-Mizen et al.22 age model (supplementary material, Fig. S7). This can be due to both uncertainty of anchorages, in particular those of paleomagnetic reversals, and biases linked to linear interpolation between these ages. Assigning the glacial pulses to specific glacial stages defined by the LR04 stack35 throughout the interval constrained by the paleomagnetic anchor ages (supplementary material, Fig. S5) could be a way to define an age model, which would fit better with a ~40 ka cycle forcing (supplementary material, Fig. S5).

Methods
Grain size measurements
Sediment grain size analyses (<2 mm fraction) were performed using a Malvern-Panalytical Mastersizer 3000™ particle-size analyzer equipped with a Hydro LV module following the instrumental conditions outlined in Belzile and Montero-Serrano (2022)61, at the Institut des sciences de la mer (ISMER) of University of Québec in Rimouski (UQAR). Before analysis, sediment samples were pretreated with 10 mL of hydrogen peroxide (H2O2; 30% v/v) to remove organic matter. Carbonates were not removed from sediment samples, as they provide information on primary production and sediment provenance62. Sediment samples were diluted with ~20 mL of sodium hexametaphosphate, sieved at <2 mm, and disaggregated using an in-house rotator for 12 h prior to particle size measurements. The grain size data obtained were processed using the GRADISTAT™ software version 9.163. For the documenting of deep current velocities, the sortable silt mean size (SSmean), which corresponds to the mean grain size of the non-cohesive silt fraction (10–63 μm) and the percentage of sortable silt (SS%) in the <63 μm fraction33 were calculated using the approach developed by McCave and Andrews (2019)34. Results are presented in Supplementary Datasets 1 and 2.

Determination of calcite and dolomite by quantitative X-ray diffraction (qXRD)
Calcite and dolomite were determined by quantitative X-ray diffraction (qXRD) at ISMER-UQAR following the method developed by Eberl (2003)64. For these analyses, ~1 g of each sample (<2 mm fraction) was spiked with 0.25 g of corundum and then ground in a McCrone™ micronizing mill using 5 mL of ethanol to obtain a homogenous powder. The slurry was oven dried in a hood and then slightly homogenized with an agate mortar. Next, 0.5 mL of Vertrel™ solvent was added to the mixture to prevent the possible agglomeration of finer particles. The powder sample was then sieved (<500 µm), side-loaded into the holders and analyzed on a PANalytical X’Pert™ powder diffractometer. Samples were scanned from 5° to 65° 2θ in steps of 0.02° 2θ, with a counting time of 2 s per step. For the quantification of the calcite and dolomite, sediment XRD scans obtained were converted into mineral weight percent (wt.%) using the R package “powdR”65. powdR uses a full pattern summation approach that permits the quantification of whole-sediment mineralogy with an average absolute bias of 0.6% for non-clay minerals65. The quality of the fitting procedure was evaluated for the R-weighted profile (rwp; best least-squares fit between observed and calculated model). The rwp for the samples analyzed ranged from 0.17 to 0.22 (median 0.18). An rwp value between 0.2 and 0.3 indicates a good fit for geological samples66. Results are presented in Supplementary Dataset 3.

Nd and Pb isotope measurements
Chemical separation and Nd-Pb isotopic analyses were conducted at Geotop-UQAM. Prior to analysis, 1 g of each sample was wet sieved through a 64-µm Nitex™ mesh and then dried at the hot plate (~80 °C). Silt was not separated from the clay because grain size analyses showed that clay constitutes only 10% of the samples. In a very conservative estimate, in which clays from the cold spells would have been entirely derived from Precambrian sources (143Nd/144Nd = 0.510978)19, their presence would account for a maximum increment of ~-1 εNd(0) units, meaning that excursions would still be distinguishable from background values. Samples were digested in Teflon™ closed beakers, with a combination of HF, HCl and HNO3, at 110 °C. Pb separation was performed using resin AG1X8™: matrix elution with HBr 0.8 N and Pb recovery with HCl 6 N, following the procedures described in Maccali et al. (2012) and Manhes et al. (1980)67,68. Nd was then separated following analytical procedure described in Maccali et al. (2013) and Song et al. (2022)69,70: digested samples were subjected to iron removal using AG1X8™ resin columns with 6 N HCl twice, replacing the resin each time. The samples were dried, treated with 14 N HNO3, and dried again. Next, light rare earth elements (LREE) separation was performed using TRUSPEC™ resin. The matrix was eluted with 1 N HNO3, and LREE were eluted and collected with 0.05 N HNO3. The concentrated LREE samples underwent Nd separation using LN Spec™ resin. The first elution with 4 mL of 0.25 N HCl removes Ce, allowing Nd collection afterwards, with 2 mL of 0.25 N HCl. Nd and Pb isotope ratios (143Nd/144Nd, 207Pb/206Pb) were analyzed on a Nu Plasma II™, a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) with an Aridus™ II desolvating membrane. Replicate analyses of the standard JNdi-1 yielded mean values of 143Nd/144Nd = 0.512071 ± 0.000029 (2σ; n=15), 0.512084 ± 0.000051 (2σ; n=11) and 0.512079 ± 0.000042 (2σ; n=5), in three batches of analyses. Results on JNdi-1 are slightly below the reference value (0.512115 ± 0.000007)71, as it is typical of Nd measurements on MC-ICP-MS due to mass bias after oxide formation in the plasma72. Therefore, results were normalised to 0.51211571. The external reproducibility (2σ) of εNd(0) values deduced from repeated measurements of the JNdi-1 standard was 0.8 ε units. Nd isotope ratios are expressed as εNd(0) = [(143Nd/144Nd)sample / (143Nd/144Nd)CHUR - 1] × 10000, with the present-day (143Nd/144Nd)CHUR of 0.51263873,74. For Pb measurements, instrumental mass bias was corrected using thallium (Tl) doping and internal normalisation to NBS 981 following Woodhead (2002)75. Reproducibility of Pb measurements was assessed using standard CGPB-1, which yielded 207Pb/206Pb of 0.844396 ± 0.000039 (2σ; n=6). Procedural blanks were negligible compared with the sample size. Results are presented in Supplementary Dataset 4.

Data availability
The radiogenic isotope data (neodymium and lead)76, along with grain size measurements77 (including sortable silt78) and quantitative X-ray diffraction data79, have been archived in the Pangaea database under license CC-BY-4.0 and are also available in the supplementary datasets. The links to the data in Pangaea repository are: https://doi.pangaea.de/10.1594/PANGAEA.971292. https://doi.pangaea.de/10.1594/PANGAEA.971291. https://doi.pangaea.de/10.1594/PANGAEA.971295. https://doi.pangaea.de/10.1594/PANGAEA.971294.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Dear Editor,\nThe manuscript by Sousa and colleagues investigates sediments collected from offshore southern Greenland to explore the\nglacial-interglacial characteristics of the ice sheet and regional oceanography across a time period in the middle Pliocene\nthat is often thought to be an analogue for future climate change. In summary, using the geochemical composition of the\nsediments the authors use their results to question whether the use of this time period as an analogue is justified, and arrive\nat the conclusion that it is not. Based on their results, I agree with this. The work is of significance to the field of research, the\nmethods are sound and well explained, and overall paper is packaged well. There are, of course, caveats to the timescales\non this work and the ability to go between contemporary timescales and those of the middle Pliocene, but the authors have\npresented these issues clearly. I do not see any major issues in the text, it is largely well written and I believe the authors’\ninterpretations and conclusions are fair and supported by the results. I have a few comments that are attached as tracked\nchanges and some comments where I think there is room for some extra clarity and possibly some minor typographic errors -\nall of which are easily rectifiable. Overall, I do not see any major issues and I recommend this paper for publication.\nIf any of my comments are unclear, then the authors are welcome to contact me.\nBest wishes,\nAndrew Newton - Queen's University Belfast", "reviewer_2": "Manuscript summary\nThis paper presents high-resolution geochemical and grain size analysis of mid-Pliocene (3.2 – 3.0 Ma) sediments from\nIODP Site U1607 offshore southern Greenland. The site is located in Eirik Drift where sediments are delivered from East\nGreenland Current, influenced by the Western Boundary Undercurrent and presently receives IRD from eastern and\nsouthern Greenland iceberg-calving sources. Nd and Pb isotopes of the <63 µm (silt and clay) fraction aimed to distinguish\nbetween bedrock terranes from northern to southern Greenland, while grain size distributions were assessed for bottom\ncurrent strength. Importantly, CaCO3 was not removed for grain size analysis to include dolomite grains (deposited in\nhydrodynamic equilibrium) and biogenic CaCO3, which were differentiated with quantitative XRD. Using a relative\npaleointensity-based magnetostratigraphic age model for U1307 tuned to U1308 by Blake-Mizen et al. (2019), the authors\ndemonstrate clear ~41 kyr paced bi-modal sediment deposition for the mid-Pliocene (3.2 – 3.0 Ma).\nWhile I consider the results of this study to be highly valuable, the manuscript is lacking in clarity and explanation, to the\npoint that the interpretations are not directly supported by the results presented. In fact, I was required to read the Blake-\nMizen et al. (2019) manuscript to gain enough understanding of the geographical and geological context of the site and\nregion.\nDiscussion of concerns\n• The depositional model is very confused and often feels contradicting. This is in part due to i) the paper structure\n(pertinent information to the deposition process and proxy response is scattered throughout the paper), ii) inconsistent use of terms\n(e.g., glacials: used for glacial conditions and glacial cycles; grain size: used stand alone, or with sand, silt, sortable silt,\nSSmean, SS%), iii) references to conditions in Quaternary, Holocene, Miocene and Pliocene with no clear description of\nwhat these conditions are, and iv) a significant lack of explanation linking conditions to processes to proxy response. This\ncould be easily remedied by a table or schematic diagram which should then be adequately described in-text.\n• Throughout the manuscript, references seem to be used in place of explaining key background information that would\nstrengthen the authors conclusions, here are three examples – but there are numerous instances where as a reader I am left\nunsure of the evidence or reasoning.\nLine 88: “…possibly a stronger WBUC, as documented for post-Miocene interglacials 22.” When is post-Miocene? How is\nthis ‘documented’ ?\nLine 89: “Increases in grain size, sand and SSmean are primarily linked to a stronger WBUC flow during interglacials24,33.”\nBy what means is a stronger WBUC flow linked to grain size?\nLine 123: “During these warm intervals, surface conditions were variable38,39.” How were they variable, what proxies, what\nconditions? And importantly, what is the significance of this statement on your results and interpretation?\n• The assumption of CaCO3 (differentiated from dolomite) as biogenic is concerning and reflects a broader lack of reference\nto or use of shipboard visual observations. Further, the use of continuous high-resolution measurements (e.g. color\nreflectance and physical properties) would greatly enhance this study for lithofacies and orbital assessment. However, if we\ntake the authors assumption that CaCO3 is biogenic, then Fig. S1, which shows a near to 1:1 (0.9x) relationship between\nCaCO3 (%) and sortable silt mean size (µm), suggests the coarsening is not a response of increasing shear stress (bottom\ncurrent strength) but purely the result of increased biogenic deposition. And to note the Fig. S1 formula for y is incorrect – it\ndoes not contain x.\n• Regarding the grain size analysis, while the reader is buoyed by the consistency between the various measures\n(sand%, mean grain size, sortable silt and mean sortable silt), it i) appears that the use of different y-axis enables\nbetter comparable range (amplitude) of results are presented and ii) makes all four redundant – it would be better to\nhave this in supplementary and one measure presented with (the background research) providing reasoning of why the\nspecific proxy is used and how it is interpreted. Again, other available data e.g., IRD (Blake-Mizen et al., 2019)\nand/or physical property logs would provide more information.\n• D18O data is poorly introduced, the mention of planktic is separated from specifying N. Atlantica which is solely used to\nrefer to the data in the manuscript otherwise. This is all first presented in the discussion, when as\npreviously published work, it should have been presented in the introduction.\n• Of significant concern is the weak explanation for the Nd and Pb isotope. The results are not unique to the conclusions\ndrawn – there is significant mixing of end members. The poor [science community] knowledge of Greenland geology is not\nadequately addressed. The modern values are the strongest line of evidence to support the interpretation of interglacial\nconditions, but this is barely discussed (again taking by the place of a reference) and does not address the Iceland source\nwhich appears to be a significant influence of sedimentation today (Blake-Mizen et al., 2019). Fundamentally, explanations\nfor why erosion and sediment delivery to the site during glacial conditions would be sourced by central Greenland zones is\nnot apparent in the manuscript – which is central to the study. This is inhibited by a lack of introduction of the ice sheet\nmodels presented in Fig. 1. At no point are the climate conditions for the models presented – I would presume a glacial state\nand interglacial state to be represented but it is unclear what the differences between the models are. I cannot agree with the\nauthors that the results exclude ice sheet scenarios.\n• Figure captions need more detail.\nThe models in Fig 1b-c need a specific time or condition they are modelled for – are these peak mPWP interglacial\nconditions, glacial periods, early Pliocene, Late Pliocene? More geological/ geographical context should be added in the\nsite map (e.g. other sites and currents).\nFig. 3 δ18O of N. atlantica needs to be more specific – Planktic (N. atlantica) δ18O stratigraphy from IODP Site U1307 27…\nagain using references in place of explanation.\nFig 4 does not particularly add anything – there is no interpretation with this and is simply explained in text. Would prefer this\nwas a sediment model.\nRecommendations\nAll of the above concerns can be addresses with more detailed introduction including site location, modern deposition\nincluding all oceanography – only WBUC mentioned, published data, Greenland ice sheet history – particularly from ODP\n646, ODP 907 and previous work from U1307. Simplification of grain size analysis for a very clear depositional model that\nexplains the proxy, the process and environmental interpretation. The concerns about the CaCO3 need to be addresses –\nbut this should be as simple as using visual observation from shipboard descriptions. References should not be used in\nplace of explanation, or require the reader to have read all of these references – the relevance to this study needs to be\nsummarised.\nHowever, the conclusions are not uniquely supported by the data presented. The results do not preclude ice sheet\nconfigurations as has been proposed here. The results have not provided adequate reasoning for changes in\nstrength of the WBUC, with further interpretation of long-term trends for a 200 kyr record. Particularly, the focus on global orbital conditions\nduring the mid-Pliocene as an inappropriate analogue is out of place - as stated in the abstract “… low overall summer\ninsolation conditions at high latitudes, a feature not anticipated in the near future… the mPWP interval cannot be seen as a\ngenuine analogue specifically for predicting the fate of the GIS.” A 1.2 Myr node in obliquity has been noted in numerous\nstudies and is in fact a primary reason for studying the mPWP – to test the influence of precession – this is not new but is\nseemingly presented as such.\nThe weighting of various conclusions in the title, abstract and discussion are not consistent and leave the reader confused\nas to the point of the paper.\nThe very important result of obliquity pacing of sediments off Greenland is of significant value (can’t overstate this enough!!!!)\nto address the question of climate pacing in Greenland when distal IRD records that have multiple sources are unable to\ndistinguish this. Specifically, this highlights variability in polar ice sheet response and global records (EAIS –\neccentricity; Patterson et al., 2014)(WAIS – obliquity; Naish et al., 2009)(Iceberg Alley – precession; Reilly et al., 2021)(sea-level –\nprecession; Grant et al., 2019)(LR04 – eccentricity; Lisiecki and Raymo, 2005). While refining the deposition model is\nneeded to convince the reader of the processes it reflects (bottom water currents or ice sheet position and erosion),\neither process paced by obliquity would be of interest and contribution to the community.\nI hope the authors find this feedback useful. I am eager to see these results published. With some simplification and\nclarification a much better paper will emerge.\nKind regards,\nDr. Georgia Grant, GNS Science\nNew Zealand", "reviewer_3": "The authors have investigated geochemical and sedimentological changes in a core collected from the Eirik Drift near\nGreenland during the Pliocene, a time interval that is of interest to paleoclimate science both for its similarities and\ndifferences to the Earth’s present climate state. New data include neodymium and lead isotope ratios of fine-fraction\nsediments (<63 µm) as well as % carbonate and grain size (including sortable silt) of bulk sediment. The observed\nexcursions in Nd and Pb isotopes are interesting, as is the long-term drift observed in maximum Pb isotope values and both\nmax and min of Nd isotope values.\nHowever, the limitations of the age model and the uncertainty regarding whether specific isotope excursions occur during\nglacial or interglacial intervals limit the insight that can be gained from these data alone. Specifically, large gaps in the stable\noxygen isotope record make it difficult to A) align with the global curve and/or B) assign glacial/interglacial intervals to the\ngeochemical record. The authors do discuss these issues, but the addition of more d18O observations to fill gaps in the\noriginal record from Sarnthein et al. (2009) could have placed the other geochemical records on a firmer foundation for\ndiscussion of glacial-interglacial climate variability and trends.\nIn addition, it is puzzling that in this study, the Nd and Pb isotope analyses were done on the entire <63 micron size fraction,\nwhen it has been previously documented that the clay fraction (<3 micron) and silt fraction (3 – 63 micron) from samples in\nthis region are chemically distinct and reflect different processes and sources. It is not clear why the authors cited prior\nstudies that used a similar isotope provenance approach (Colville et al., 2011; Reyes et al., 2014) as proof of concept, but\nthen did not follow those methods. In the Colville et al. and Reyes et al. studies, workers physically separated the 3-63\nmicron fraction (silt) via Stokes-Law settling, and then performed geochemical analysis on the silt-only fraction. In contrast,\nthe present study did not separate clay from silt prior to analysis, and the reason for this was not discussed in either the main\ntext or the supplement. Colville et al. (2011) suggested that the coarser silt fraction is primarily derived from glacial erosion\nand supplied from Greenland, while the finer clay fraction can be supplied from much more distant sources. The reason for\nlumping/analyzing them together in the present study is not clear, and this difference in sample processing makes direct\nanalogy to previously published results in this region problematic.\nThe authors make a good point regarding the unsuitability of the mPWP as an analogue for near-future Earth climate – the\norbital oscillations for these two intervals are different, and their results suggest that obliquity in particular is important for the\nbehavior of high-latitude ice. Overall, this study does present some very intriguing patterns and trends for an interesting time\nperiod, from a site proximal to a globally important ice sheet. The interpretations are not as strongly supported by the data as\nthey might be given stronger age control, or at least enhanced ability to assign glacial vs. interglacial conditions to\nexcursions in Nd and Pb isotopes and other sediment properties. In addition, these records are inherently indirect measures\nof Greenland Ice Sheet behavior, being located offshore and downstream of the ice itself, making their link to the object of\nstudy somewhat tenuous. There was not much discussion of how the marine records compared with terrestrial evidence,\nwhich might strengthen the discussion."}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer #1\nLine Comment/Suggestion (in red) Action\nSheet (GIS) extentmay have been considerably\n34 mostly indirect as they are restricted to information from marine records Corrected\noff Greenland margin\n- contourite drift molded by the Western Boundary Undercurrent (WBUC) The paragraph was\nand its formation can be tracked back to the Miocene. Radiogenic isotope modified during the\ncomposition of sediments from this drift has been proven as a suitable review and does not\ntracer for glacial landscape erosion during the Pleistocene and the feature the passages that\nHolocene. We thus use a similar approach here to document the needed correction\noccurrence of glacial activity in southern Greenland during the mPWP. anymore.\n- As Eirik Ridge sediments are supplied by the WBUC (Colville et al. 2011; The sentence was\nInnocent et al. 1997; McCartney 1992), their grain size modified during the\nreview and does not\nfeature the passages that\nneeded correction\nanymore.\n- As illustrated onFig.1 -\n68 performed on the<63 µm sediment fraction Corrected\n- reversals and ∂18O stratigraphy to set an age model due to highly variable Corrected\n- As the duration of glacials remains unascertained, thecorollary is that The sentence was\ninterglacials are also difficult to estimate. modified during the\nreview and does not\nfeature the passages that\nneeded correction\nanymore.\nmodified during the\nreview and does not\nfeature the passages that\nneeded correction\nanymore.\n- fewer documentationof potential linkages with the obliquity cycle, a The paragraph was\nresolution not necessarily reached using current approaches such as the modified during the\noxygen isotope stratigraphy and paleomagnetic reversal ages. These review and does not\napproaches were indeed used for the setting of variable age models for feature the passages that\ncore U1307(Sarnthein et al. 2009; Channell, Hodell, and Curtis 2016), needed correction\nincluding the Blake-Mizen et al. age model(Blake-Mizen et al. 2019) anymore.\nreferred to on Fig. 3. The oxygen isotope data of planktic foraminifera\nwere initially reported by Sarnthein et al. (Sarnthein et al. 2009) and show\nd18O\n- age model resolution is insufficient for the higher-order timescale The sentence was\ninvestigated here modified during the\nreview and does not\nfeature the passages that\nneeded correction\nanymore.\n185 at high latitudes (Lee and Poulsen 2008)that wouldallow Corrected\n199 the interval are currently based on hypothetical correlation using the Corrected\navailable data .\n211 marked by aprogressively Corrected\n1) I think I understand what this means, that through time the dominant source of the sediments progressively shifts\nnorth? I think this more needs to be written more clearly exactly what it means as it is not clear - “erosion\nprogressing” is somewhat ambiguous. Basically this needs more clearly explained for a multi-disciplinary journal\nlike this.\nYou understood it correctly; we modified the sentence to make it clearer. The initial sentence “Thus, throughout\nthe mPWP, the glacial advances resulted in erosion progressing from the Archean Block and Nagssuqtoqidian\nMobile Belt to the Paleogene volcanic terranes in eastern Greenland (Fig. 1)” has been replaced by:\n“Thus, throughout the mPWP, the southeastern Greenland ice mass occupying KMB/AB terranes expanded\nprogressively northward from one glacial pulse to the next, possibly merging with ice from the northeastern\nmountainous PV/NMB area (Fig. 1B). Consequently, there is a clear trend of increasing overall contribution of\nthe PV terrane towards the upper part of the record. During the cold spells, enhanced ice erosion mainly released\ndetrital supplies from the Archean Block (AB) and Nagssuqtoqidian Mobile Belt (NMB) (Figs. 1B and 2).\nConversely, during interglacials, ice erosion was likely restricted to the mountainous KMB-AB area of\nsoutheastern Greenland (Figs. 1B; 2). This would explain the narrow cluster of Pb and Nd isotope compositions\nof interglacial detrital supplies between three endmembers (KMB, AB/NMB and PV): the KMB and AB/NMB\nendmembers, linked to ice erosion, and the PV endmember linked to increased WBUC supplies from the Denmark\nStrait (Fig. 2).”\n2) “Nevertheless, the recurrence of six, possibly seven (Fig. 3) climatic oscillations over the 2.80 to 3.22 Ma interval\nwould be compatible with a mean duration of ~ 40 ka for each glacial-interglacial cycle, taking into account\nuncertainties about the age model (see below).” Room for clarity here as the timeline wording and younger date\ndoes not match the Fig. Are the climatic oscillations the pulses of sediment? Needs clearer wording.\nWe tried to clarify as follows:\n“Nevertheless, the recurrence of six, possibly seven (Fig. 3) isotopic excursions over the 2280-3220 ka interval\nstudied (grey stripes in Fig. 3) would be compatible with a mean duration of ~40 ka for each cold-warm cycle.\nHowever, the ~40 ka-periodicity is not clearly expressed when using the Blake-Mizen et al.28 age model\n(supplementary material, Fig. S7). This can be due to both uncertainty of anchor ages, in particular those of\npaleomagnetic reversals, and biases linked to linear interpolation between these ages. Assigning the glacial pulses\nto specific glacial stages defined by the LR04 stack37 throughout the interval constrained by the paleomagnetic\nanchor ages (supplementary material, Fig. S5) could be a way to define an age model, which would fit better with\na ~40 ka cycle forcing (supplementary material, Fig. S5).”\n3) Not sure on what the journal requirements are, but ka is usually seen as a date, not a duration (ka).\nAs the journal accepts both ka and kyr, we maintained ka to avoid changing some figures.\n4) Myr? Depends on journal preferences for dates and durations.\nIbid. Ma is used.\nReviewer #2\n1) The depositional model is very confused and often feels contradicting. This is in part due to i) the paper structure\n(pertinent information to the deposition process and proxy response is scattered throughout the paper), ii)\ninconsistent use of terms (e.g., glacials: used for glacial conditions and glacial cycles; grain size: used stand alone,\nor with sand, silt, sortable silt, SS , SS%), iii) references to conditions in Quaternary, Holocene, Miocene and\nmean\nPliocene with no clear description of what these conditions are, and iv) a significant lack of explanation linking\nconditions to processes to proxy response. This could be easily remedied by a table or schematic diagram which\nshould then be adequately described in-text.\ni) We tried to improve the description of the sediment accumulation on Eirik drift, by adding to the\ncorresponding paragraph in the introduction, which now reads as follows:\n“Previous paleoceanographic studies suggest that the strength of the WBUC was strongly reduced during glacial\nintervals but exhibited high flow rates notably during Quaternary interglacials 20-24. At this site and along the\ntrajectory of the WBUC, grain-size data analyses of surface sediments show that silt overweighs the clay size\nfraction that may originate from remote sources, north of the Denmark Strait (Fig. 1). Notwithstanding the\ndominance of silt, the effective grain size of the sediment is sensitive to changes in the strength of the\nWBUC20,24,29. Disregarding the IRD content28, this behavior results in the deposition of coarser sediments during\ninterglacials due to high shear stress and winnowing30.”\ni) We added a new reference which further documents the relationship between the glacial/interglacial\nchange in the WBUC strength and the sediment grain size on Eirik Drift (Davies Stow, and Nicholson\n2021).\nii) With reference to grain size, the fractions of concern are more precisely mentioned in the revised version.\nWe also made it clear that we mention glacials/interglacial intervals with reference to the corresponding\nsedimentary units, but to \"cold spells\" for climate conditions leading to ice expansion as documented in\nthe present record, i.e., spanning the 2980-3220 ka time window.\niii) All references cited in the text now include explicit information about the paleoceanographic or\npaleoclimate conditions of concern.\niv) In Fig. 3, we added an arrow indicating “coarser sediments = stronger WBUC”. In the same figure, the\ngrey bars indicate the cold spells.\n2) Throughout the manuscript, references seem to be used in place of explaining key background information that\nwould strengthen the authors conclusions, here are three examples – but there are numerous instances where as a\nreader I am left unsure of the evidence or reasoning. i) Line 88: “…possibly a stronger WBUC, as documented\nfor post-Miocene interglacials 22.” When is post-Miocene? How is this ‘documented’ ? ii) Line 89: “Increases in\ngrain size, sand and SSmean are primarily linked to a stronger WBUC flow during interglacials24,33.” By what\nmeans is a stronger WBUC flow linked to grain size? iii) Line 123: “During these warm intervals, surface\nconditions were variable38,39.” How were they variable, what proxies, what conditions? And importantly, what\nis the significance of this statement on your results and interpretation? iv) The assumption of CaCO3\n(differentiated from dolomite) as biogenic is concerning and reflects a broader lack of reference to or use of\nshipboard visual observations. v) Further, the use of continuous high-resolution measurements (e.g. color\nreflectance and physical properties) would greatly enhance this study for lithofacies and orbital assessment.\nHowever, if we take the authors assumption that CaCO3 is biogenic, vi) then Fig. S1, which shows a near to 1:1\n(0.9x) relationship between CaCO3 (%) and sortable silt mean size (µm), suggests the coarsening is not a response\nof increasing shear stress (bottom current strength) but purely the result of increased biogenic deposition. v)And\nto note the Fig. S1 formula for y is incorrect – it does not contain x.\nAs mentioned in item iii) above, all references are now complemented with specific information about the features\nthey address.\ni) Several studies document the intensification of the WBUC since the Miocene. We refer to one of these\n(Müller-Michaelis and Uenzelmann-Neben 2014) and have added a few words about their findings, from\nseismic profile studies. Post-Miocene refers to the Plio-Quaternary.\nii) A stronger WBUC flow results in coarser grain deposition, which is seen in sortable silt percentage,\nsortable silt mean size and sand percentage. We added appropriate reference. The paragraph was\nrestructured and now reads as follows:\nThe Nd-Pb isotopic excursions are preceded by peaking calcite abundance, of variable amplitude, as well as by\nhigh mean grain size, sortable-silt size (SSmean) and sand contents (Fig. 3). These features broadly highlight two\ndistinct sedimentary regimes linked to the strength of the WBUC. One is related to strong currents during warm\nintervals, in particular during their middle to late part22. The other is related to low flow rates during cold intervals.\nThe interglacial layers illustrated in Fig. 3 record an early stage with relatively fine material, followed by a late\nstage marked by sediment coarsening, which is a feature observed during the present interglacial in the core site\nvicinity22. The quasi-absence of dolomite leads to the inference that calcite peaking values record enhanced\nbiogenic carbonate fluxes during the late part of the interglacial intervals. A similar feature characterizes the\npresent interglacial22.\niii) More information is now provided about the proxies used for reconstructing surface conditions during\nthe warm intervals (planktic foraminifera assemblages and abundances).\niv) Hillaire-Marcel et al. (1994) as well as Dmitrenko, Sivkov, and Rusakov (2009) have shown that\nbiogenic calcite fluxes in the region are linked to an enhanced coccolithophore production and occur in\nthe <10 µm fraction. Additionally, the sand-sized fraction measurements of Blake-Mizen et al. (2019)\nwere performed after removal of carbonates. Their peaking percentages of the ≥ 63µm sediment fraction\nare coeval with our peaks of mean grain size (µm), sand (%), SS (µm) and SS (%) (see Supplementary\nmean\nMaterial, Fig. S3).\nv) We added color reflectance, magnetic susceptibility and NGR data to the Fig. S3 in the supplementary\nmaterial.\nvi) Fig. S1 illustrates the fact that biogenic micrite calcite production increases during the late warm\nintervals marked by an enhanced WBUC.\nvii) The peaking calcite concentrations (red dots in Fig. S1) plot outside the 1:1 trend defined by calcite vs.\nSS , meaning that the anomalous calcite contents are not accompanied by coarsening.\nmean\nFigure S1: This figure was updated in the supplementary material.\n3) Regarding the grain size analysis, while the reader is buoyed by the consistency between the various measures\n(sand%, mean grain size, sortable silt and mean sortable silt), i) it appears that the use of different y-axis enables\nbetter comparable range (amplitude) of results are presented and makes all four redundant – it would be better to\nhave this in supplementary and one measure presented with (the background research) providing reasoning of\nwhy the specific proxy is used and how it is interpreted. ii) Again, other available data e.g., IRD (Blake-Mizen\net al., 2019) and/or physical property logs would provide more information.\ni) We agree that all grain size parameters illustrated in Fig. 3 follow nearly similar patterns and may be\nconsidered redundant. We thus only plot the SS and the mean grain size in the main figure, whereas\nmean\ntheir correlation with SS (%) and sand (%) remains documented in the supplementary material (Fig. S2).\nFigure S2. Correlation between SSmean (µm) and SS(%), and mean grain size (µm) and sand (%).\nii) We added IRD from (Blake-Mizen et al. 2019) and shipboard measurements (natural gamma radiation,\nmagnetic susceptibility and color reflectance) to the supplementary material. Blake-Mizen et al.'s IRD\ndata are split in two fractions: ≥ 63 µm and ≥212 µm. The fraction ≥63 µm mostly includes the sand-size\nfraction; its relative abundance does not necessarily represent the IRD flux vs other sedimentary supplies,\nbut mostly the winnowing impact of an intensified WBUC concentrating coarse fractions and removing\nthe fines. This may be illustrated by the present day WBUC velocities of up to ~ 30 cm.s-1 (Holliday et\nal. 2007).\n4) 18O data is poorly introduced, the mention of planktic is separated from specifying N. Atlantica which is solely\nused to refer to the data in the manuscript otherwise. This is all first presented in the discussion, when as\npreviously published work, it should have been presented in the introduction.\nUnfortunately, there is no continuous δ18O record available for Site U1307. The existing N. atlantica data from\nSarnthein et al. (2009) exhibit significant gaps. We mention this problem in the introduction and further elaborate\non the importance of these data despite the impossibility to use them for the setting of an age model. The δ18O\ndata may still help developing correlations with the LR04 stack (Lisiecki and Raymo 2005). However, as now\ndocumented in the supplementary material of the revised version (Fig. 3 and Fig. S4), we tentatively set a closer\ncorrelation of the sequence analyzed here with the LR04 stack, based on the glacial/interglacial succession within\nthe paleomagnetic intervals investigated.\n5) i) Of significant concern is the weak explanation for the Nd and Pb isotope. ii) The results are not unique to the\nconclusions drawn – there is significant mixing of endmembers. The poor [science community] knowledge of\nGreenland geology is not adequately addressed. iii) The modern values are the strongest line of evidence to\nsupport the interpretation of interglacial conditions, but this is barely discussed (again taking by the place of a\nreference) and iv) does not address the Iceland source which appears to be a significant influence of sedimentation\ntoday (Blake-Mizen et al., 2019). v) Fundamentally, explanations for why erosion and sediment delivery to the\nsite during glacial conditions would be sourced by central Greenland zones is not apparent in the manuscript –\nwhich is central to the study. vi) This is inhibited by a lack of introduction of the ice sheet models presented in\nFig. 1. At no point are the climate conditions for the models presented – I would presume a glacial state and\ninterglacial state to be represented but it is unclear what the differences between the models are. I cannot agree with the\nauthors that the results exclude ice sheet scenarios.\ni) We added a new section in the supplementary material addressing the geology of Greenland terranes and\ndiscussing the meaning of the Nd and Pb signatures.\nii) As illustrated in Fig. 2, three/four endmembers are variably mixed. Previous works have modelled the\nmore recent contribution of different terranes to a nearby site (Reyes et al. 2014; Fagel and Hillaire-\nMarcel 2006) and showed that unradiogenic Nd primarily originates from Archean sources in southern\nGreenland and can thus be used as a tracer of related inland ice erosion in this area.\niii) We added a couple of sentences in the revised to clarify this aspect. The modern value provide evidence\nfor some PV (Paleogene volcanism) source material supplies with the modern strong WBUC, either from\ndirect erosion through the Greenland Strait, or from ice calving in ice terminals of NE Greenland, as\nGreenland is presently fully glaciated. This was not the case during the Pliocene, prior to the\n“intensification of the Northern Hemisphere Glaciation” at ~2.7 Ma (Abell et al. 2021; Naafs et al. 2010;\nHaug et al. 1999)\niv) The Iceland source is within the terrane called PV (Paleogene volcanism) as well as the northeastern\nGreenland source, as documented in the Blake-Mizen et al. (2019) paper. PV is composed of basalts\n( up to +4). Sediments from this source thus drive  towards higher values, but do not provide a\nNd(0) Nd(0)\nmeans to distinguish Greenland vs. Iceland supplies.\nv) The current understanding of Greenland terranes (e.g., MacGregor et al., 2023) enabled us to identify,\nsouth to north, the Ketilidian Mobile Belt (KMB), the Archean Block (AB), Nagssugtoqidian Mobile\nBelt (NMB), and the Paleogene volcanism (PV). During the mPWP glacials we see enhanced source\nsupplies not from \"central\" Greenland (NMB and PV), but rather from southeastern Greenland\n(KMB/AB). Worth to mention is the fact that ice streams transport material to the sea mostly only over\na few km (see Paulen et al.,,R.C. and McMartin, I.E. 2009)). Assuming a weak WBUC during glacials,\none would not expect significant long-distance transport of glacially eroded material, at least in coarse\nfractions.\nvi) We tried to clarify the model-based possible distributions of ice during the mPWP in the introduction.\nFigs. 1B,C show two models of ice masses during the Pliocene. In one scenario, ice is restricted to the\nmountains in south and east Greenland (Fig. 1B) and in the other, ice is restricted to northern Greenland\n(Fig. 1C). Our data ( and 207Pb/206Pb) are compatible with scenario 1B, i.e., as the sediment\nNd(0)\ncomposition during warm intervals is mostly controlled by two endmembers: Paleogene volcanics and\nthe KMB terrane. These intervals were the intervals with minimal ice cover.\n6) i) The models in Fig 1b-c need a specific time or condition they are modelled for – are these peak mPWP\ninterglacial conditions, glacial periods, early Pliocene, Late Pliocene? ii) More geological/ geographical context\nshould be added in the site map (e.g. other sites and currents). iii) Fig. 3 δ18O of N. atlantica needs to be more\nspecific – Planktic (N. atlantica) δ18O stratigraphy from IODP Site U1307 27… again using references in place\nof explanation. iv)Fig 4 does not particularly add anything – there is no interpretation with this and is simply\nexplained in text. v)Would prefer this was a sediment model.\ni) Figs. 1B,C are ice sheet surface elevation model outputs with minimal ice cover during the warmer part\nof the Pliocene (~ 3.26-3.02 Ma; Koenig et al. 2015). We added more information about each model in\nthe revised version.\nii) The most important currents (in the context of our research), likely active during the mPWP, are\nindicated in the schematic map of Fig. 1. More detailed information about currents during this interval\nwould be largely speculative.\niii) We improved the information about N. atlantica, with relevant data from U1307.\niv) Although the relationship between obliquity nodes and cooling trends has been mentioned in general\nterms (e.g., O’Neill and Broccoli 2021), we discuss it here with specific reference to the mPWP interval.\nIt constitutes an important element that may help understanding why ice could grow under the high pCO\n2\nof the interval. As such an orbital situation cannot be expected in the near future, we think that the mPWP\ninterval cannot be considered as a real analogue of the near future of the ice cover of Greenland. Thus,\nwe would like to keep the Fig. 4, because this element is discussed in the text. It seems uneasy to infer a\nsediment model for Eirik Drift during the mPWP based on the scarce information existing.\nv) Unfortunatly, we cannot add to the sedimentological studies of site U1307 cores (Sarnthein et al. 2009;\nBlake-Mizen et al. 2019) and present a comprehensive sediment model, because a figure combining the\nIce-Sheet distribution over Greenland and the WBUC strength would be very speculative given the data\nwe have.\n7) i) All of the above concerns can be addresses with more detailed introduction including site location, modern\ndeposition including all oceanography – only WBUC mentioned, published data, Greenland ice sheet history –\nparticularly from ODP 646, ODP 907 and previous work from U1307. ii) Simplification of grain size analysis for\na very clear depositional model that explains the proxy, the process and environmental interpretation. iii) The\nconcerns about the CaCO3 need to be addresses – but this should be as simple as using visual observation from\nshipboard descriptions. iv) References should not be used in place of explanation, or require the reader to have\nread all of these references – the relevance to this study needs to be summarised.\ni) The introduction has been updated to include more details, as suggested, and several passages lacking\nexplanation, are now better developed. About Greenland Ice Sheet history, most studies focus on the\nonset of the Northern Hemisphere Glaciation (long-term cooling of Greenland) or have no comparable\ntemporal resolution. This is a gap in knowledge that our work is filling. Apart from that, the models for\nthe GIS in the Pliocene are the ones cited on Figs. 1B,C, the captions of which were improved.\nii) Fig. 3 was improved to include information about current strength and clarify the link between the results\nand the oceanographic interpretation.\niii) We further discussed the CaCO content and added shipboard measurements to the supplementary\n3\nmaterial. There are no smear slides from the targeted interval, but IDR>63 µm form (Blake-Mizen et al.\n2019) were measured after carbonate removal and depict peaks coeval with our coarse fraction contents.\nThese data were added to the supplementary material.\niv) We reviewed all references to make sure they are not used without explanation about their scientific\npertinence.\n8) However, i) the conclusions are not uniquely supported by the data presented. ii) The results do not preclude ice\nsheet configurations as has been proposed here. The results have not provided adequate reasoning for changes in\nstrength of the WBUC, with further interpretation of long-term trends for a 200 ka record. iii) Particularly, the\nfocus on global orbital conditions during the mid-Pliocene as an inappropriate analogue is out of place - as stated\nin the abstract “… low overall summer insolation conditions at high latitudes, a feature not anticipated in the near\nfuture… the mPWP interval cannot be seen as a genuine analogue specifically for predicting the fate of the GIS.”\nA 1.2 Myr node in obliquity has been noted in numerous studies and is in fact a primary reason for studying the\nmPWP – to test the influence of precession – this is not new but is seemingly presented as such.\ni) In the revised version, we think we highlight better how our conclusions are supported by the data\n(periodic erosion of Precambrian terranes = ice advances, coarser sediments = stronger current).We also\nimproved the discussion about why our data are more compatible with one of the ice-sheet\nconfigurations. The strength of the WBUC is inferred by the SS and SS(%), while erosion of\nmean\nPrecambrian terranes is inferred from radiogenic isotope signatures. Our data do not allow us to clearly\ninfer what could have been the drivers of WBUC changes, but we added a reference that discusses a\nsouthward shift of the deep-water formation regions during “glacials” (implying that that the main North\nAtlantic Deep Water route affected Eirik Drift only during warm intervals) (Müller-Michaelis and\nUenzelmann-Neben 2014).\nii) Indeed, the potential role of low obliquity conditions for ice growth has been evoked in particular by\nO’Neill and Broccoli (2021) and we do not claim any originality in mentioning it. However, as\nmentioned above, the very low obliquity values of the mPWP interval may account for the ice growth\npulses over Greenland, despite the high pCO value of the time. We think important to mention this\n2\naspect in the abstract, especially with respect to the absence of such orbital conditions in the near future\nof the Earth, whereas several papers refer to the mPWP as a potential analogue of the future of the planet.\nWe added a reference to the O’Neill and Broccoli (2021) paper to clarify this aspect.\n9) i)The weighting of various conclusions in the title, abstract and discussion are not consistent and leave the reader\nconfused as to the point of the paper. ii) The very important result of obliquity pacing of sediments off Greenland\nis of significant value (can’t overstate this enough!!!!) to address the question of climate pacing in Greenland\nwhen distal IRD records that have multiple sources are unable to distinguish this. Specifically, this highlights\nvariability in polar ice sheet response and global records (EAIS – eccentricity; Patterson et al., 2014)(WAIS –\nobliquity; Naish et al., 2009)(Iceberg Alley – precession; Reilly et al., 2021)(sea-level – precession; Grant et al.,\n2019)(LR04 – eccentricity; Lisiecki and Raymo, 2005). iii) While refining the deposition model is needed to\nconvince the reader of the processes it reflects (bottom water currents or ice sheet position and erosion),\neither process paced by obliquity would be of interest and contribution to the community.\ni) We restructured several paragraphs to emphasize the periodic enhanced glacial erosion of Archean\nterranes under high pCO conditions of the mPWP, which is the central finding of our work. Nonetheless,\n2\nour data also allow us to discuss the strength of WBUC based on the grain size measurements.\nii) Indeed, the obliquity pacing is of significant value, but due to uncertainties on the age model, we tried\nto discuss it with some caution and even added a possible way to revise the chronostratigraphy through\na direct correlation of our glacial/interglacial sequence with the LR04 stack spanning the same\npaleomagnetic boundaries (see supplementary material, Fig. SX)\niii) We did not include a diagram with the depositional model because the distribution of the ice mass outlets,\nas well as the morphology of Eirik Drift are not well documented for the mPWP interval.\nReviewer #3\n1) Specifically, large gaps in the stable oxygen isotope record make it difficult to A) align with the global curve\nand/or B) assign glacial/interglacial intervals to the geochemical record. The authors do discuss these issues, but\nthe addition of more d18O observations to fill gaps in the original record from Sarnthein et al. (2009) could have\nplaced the other geochemical records on a firmer foundation for discussion of glacial-interglacial climate\nvariability and trends.\nThe gaps in the 18O records are due to the lack of foraminifera in some parts of the cored sequences.\nUnfortunately, to our best knowledge, there are no other available records with the temporal resolution of our\nown study. Nonetheless, we added shipboard measurements to the supplementary material and tried at a direct\ncorrelation of our glacial/interglacial sequence with the LR04 stack spanning the same paleomagnetic boundaries\n(see supplementary material, Fig. S5).\n2) In addition, it is puzzling that in this study, the Nd and Pb isotope analyses were done on the entire <63 micron\nsize fraction, when it has been previously documented that the clay fraction (<3 micron) and silt fraction (3 – 63\nmicron) from samples in this region are chemically distinct and reflect different processes and sources. It is not\nclear why the authors cited prior studies that used a similar isotope provenance approach (Colville et al., 2011;\nReyes et al., 2014) as proof of concept, but then did not follow those methods. In the Colville et al. and Reyes et\nal. studies, workers physically separated the 3-63 micron fraction (silt) via Stokes-Law settling, and then\nperformed geochemical analysis on the silt-only fraction. In contrast, the present study did not separate clay from\nsilt prior to analysis, and the reason for this was not discussed in either the main text or the supplement. Colville\net al. (2011) suggested that the coarser silt fraction is primarily derived from glacial erosion and supplied from\nGreenland, while the finer clay fraction can be supplied from much more distant sources. The reason for\nlumping/analyzing them together in the present study is not clear, and this difference in sample processing makes\ndirect analogy to previously published results in this region problematic.\nIt is unfortunate that there is no consensus in the community about the proper size to use for the documenting of\nsediment sources of hemipelagic sediments from radiogenic isotope measurements. Here, the silt fraction was not\nseparated from the clay as grain size analyses showed that clay constitutes only 10% of the overall sediment. In\na very conservative estimate, in which clays from the cold spells would have been entirely derived from\nPrecambrian sources (143Nd/144Nd=0.510978; Reyes et al. 2014), their presence would account for a maximum\nincrement of ~-1  units, meaning that excursions would still be distinguishable from background values. We\nNd(0)\nadded this information and discussion to the supplementary material. Thus, the Nd and Pb isotopic signatures\nfrom the < 63 m sediment fraction are thought representative of the mixing of sediments from proximal glacial\nerosion and long-distance transport of fine particles, this, in particular during interglacials, when the WBUC was\nstronger.\n3) There was not much discussion of how the marine records compared with terrestrial evidence, which might\nstrengthen the discussion.\nVery few terrestrial data are available for the targeted timeframe in Greenland, especially with high temporal\nresolution. We mentioned Csank et al. 2011), who estimated land surface temperatures and suggested\ntemperatures ~11 to 16 °C warmer than present. We added a new reference (Bennike et al. 2002), who show that\nseawater temperatures were higher than today during the Pliocene (although the work has not the same temporal\nresolution as the one presented in this study). A similar conclusion can be made with respect to the very recent\npublication of Pang et al. (2024) who tries to document paleosea-surface conditions between 3.65–3.37 Ma, i.e.,\nprior to the mPWP interval."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am satisfied that the authors have addressed the concerns I raised during my review. I have also looked through the issues\nraised by the other reviewers and believe that the authors have addressed them as effectively as possible, within the\nlimitations/caveats of the data that are available for this work. I do not see any major edits that the authors could implement\nto improve the paper. This is an interesting piece of work and I recommend publication.", "reviewer_2": "To the authors,\nThe results describing southwestern Greenland Ice Sheet dynamics conclude strong evidence for obliquity pacing of glacial\n-interglacial changes to the sediment delivery forming the Eirik Drift. Where periods of radiogenic Nd isotopes comparable to\npresent day are interpreted as interglacials, while glacial periods are denoted by depleted Nd (in phase with 207Pb/206Pb).\nThese results are linked to erosion of Greenland bedrock where sediments derived from the central interior are consistent\nwith glacial periods and interglacial sediments appear more mixed.\nThis study highlights an absence of Greenland Ice Sheet dynamics for the warmer past and will be a helpful addition to the\ncommunity to further understanding of ice sheet configuration.\nThe previous remarks made have been adequately addressed by the authors.\nKind regards,\nDr. Georgia Grant"}, "author_response": ""}]

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