Reduced North Pacific Deep Water formation across the Northern Hemisphere Glaciation

Domain: earth_science
Published: 2025-03-19
Editorial outcome: major_revision

Abstract
Today, deep waters do not form in the northern high latitudes of the Pacific Ocean, but this may not have been the case during the Pliocene. Evidence suggests there was a Pacific Meridional Overturning Circulation during the warm Late Pliocene, similar to the modern Atlantic Ocean with a weak halocline in the subpolar North Pacific resulting in North Pacific Deep Water (NPDW) formation. However, much of this evidence comes from proxies that can be biased by changes in local productivity. We present a coupled Mg/Ca-δ18O record from the North Pacific which shows two distinct water masses in the Pliocene North Pacific Ocean, with NPDW colder and fresher than the underlying deeper water. Here, we show a decline in NPDW formation during glacials from 2.7 million years ago, which we suggest demonstrates the strong sensitivity of ocean gateways to sea level and ice volume change in shaping deep water circulation, and thus the climate system.

Introduction
Deep water circulation is an important component of the Earth’s climate, moving heat, salt, and nutrients around the globe. Today, the formation of deep waters occurs in the high latitude North Atlantic and Southern Oceans1; there is no deep water formation in the North Pacific due to a strong upper ocean salinity gradient, termed a halocline2,3. This results in a homogenous deep Pacific Ocean with deep waters supplied from the Southern Ocean which travel northwards, lose density, upwell, and return to the Southern Ocean as surface and intermediate waters1.

Though there is no North Pacific Deep Water (NPDW) formation at present, there is evidence for active deep water formation in the geologic past. Studies suggest there may have been NPDW formation in the Paleogene4, the recent deglacial5 and the mid-Pliocene Warm Period6,7 (mPWP, 3.26–3.03 million years ago, Ma). Here we examine the history of NPDW formation over the intensification of Northern Hemisphere Glaciation (iNHG, c. 2.7 Ma) to investigate the sensitivity of deep water circulation to climate change as the Earth’s climate transitioned from a warm climate to a cold one dominated by ice sheet growth and sea level change. Whilst there is evidence for NPDW formation in the Late Pliocene, this is absent in the Early Pleistocene7, suggesting that the iNHG played a role in suppressing this deep water formation. Deep water formation in the North Pacific would have markedly changed the global carbon cycle, increasing global export productivity by 20%8, as well as altering patterns of ocean circulation in the Pacific9 and the Atlantic10, so it is important to understand how past deep water formation has changed in past warm climates. Active deep water formation in the North Pacific would require the removal of the halocline in the subpolar North Pacific, and studies have suggested a weak meridional (north-south) sea surface temperature (SST) gradient7,11, changing monsoon patterns12, and sea level induced changes in ocean gateways13,14 as potential causes.

The evidence for NPDW formation in the Late Pliocene is contested and relies heavily on proxy records that can be biased by changes in local productivity. Many modelling studies of the Late Pliocene fail to generate an active overturning circulation in the North Pacific15; though this may be due to limited model run time6, poor reconstructions of Late Pliocene temperature gradients7, or an inability of the models to accurately simulate changing Pliocene palaeogeography14. Modelling studies that alter cloud physics to simulate a weak meridional Pacific SST gradient, as proxy data suggest was the case during the Pliocene11, generate active NPDW formation7. This water mass structure is supported by carbon isotopes across the Late Pliocene Pacific Ocean which point to NPDW reaching depths of around 3000m in the North Pacific6, though recent work from the subpolar North Pacific questions this16. Critically, these records come from opposite sides of the Pacific Ocean and NPDW formation is expected to occur in the Northwest Pacific Ocean during the Pliocene and is unlikely to show a similar water mass structure in the Northeast Pacific9,17. Carbon isotopes are also readily influenced by changes in local productivity18 and so it is hard to determine which, if any, of these signals are driven by water mass changes. The presence of NPDW in the Pliocene is also supported by opal and carbonate mass accumulation rates19,20 and redox-sensitive trace metal records7 in the subpolar North Pacific, which both suggest an unstratified and well-ventilated water column indicative of active deep water formation as well as upwelling of deep waters in the Late Pliocene7.

This study uses oxygen isotope ratios and minor element concentrations in benthic foraminifera to determine intermediate and deep water mass properties in the North Pacific during the Late Pliocene and Early Pleistocene. Foraminiferal δ18O values are a function of temperature and the δ18O of seawater (δ18Osw), itself a function of salinity, with colder and/or more saline water masses having more positive (heavier) δ18O values18. Therefore, δ18O in benthic foraminifera (δ18Obenthic) can be used as a proxy for water mass density, with denser waters having more positive δ18O values. However, it is possible for δ18Osw to be decoupled from salinity, often due to sea ice formation, such as is observed in the modern Southern Ocean21. Here we use the magnesium to calcium (Mg/Ca) ratios in benthic foraminifera as an independent proxy for bottom-water temperature (BWT) which is coupled with δ18O measurements to deconvolve the temperature and salinity (δ18Osw) contributions22. Additionally, δ18O, δ18Osw and Mg/Ca-derived temperatures are conservative tracers of water masses and so can be used to determine past water mass distributions; this contrasts with seawater stable carbon isotope ratios, which behave non-conservatively due to organic carbon remineralisation processes23.

Results
Oxygen isotopes across a depth transect in the Northwest Pacific indicate there were two distinct water masses in the Late Pliocene North Pacific. We generated a δ18O record at Ocean Drilling Program (ODP) Site 1209 (32°39.10’ N, 158°30.36’ E, 2387m depth), from the Shatsky Rise in the Northwest Pacific and compared it to the nearby, but deeper, ODP site 1208 (3346m depth24) (Fig. 1). During the Late Pliocene (3.3–2.7 Ma), the δ18O values at the two sites show a mean difference of 0.25‰, but this is regularly in excess of 0.4‰ (Fig. 2). This is a significant difference in the δ18O values (p<1031, N=601) far in excess of the measurement uncertainty of ±0.05‰ and does not vary significantly between glacials and interglacials over the Late Pliocene (p=0.8110, N=174). Counter-intuitively, we report heavier δ18O values at the shallower Site 1209 compared to the deeper Site 1208, suggesting denser water at Site 1209; as this is impossible, there must have been two distinct water masses, where one (or both) had a decoupled δ18Osw-salinity relationship bathing the two different sites. We suggest these two water masses were a NPDW, which bathed Site 1209, and a southern-sourced deep water, which likely had a decoupled δ18Osw-salinity relationship, similar to today, and flowed over the deeper site 1208, akin to the modern North Atlantic where AABW underlies NADW despite lighter δ18O25.

This arrangement of water masses seen in the Late Pliocene North Pacific is unlike that of the present Pacific Ocean. Across the modern day North Pacific, core-top benthic foraminiferal δ18O values are relatively homogenous compared to other ocean basins26 and show a positive depth gradient with δ18O values increasing by 0.10±0.03‰ per km (r=0.30, p<0.001)27, as opposed to the decreasing δ18O values with depth seen at the Shatsky Rise in the Late Pliocene. Modern δ18Osw values in the deep North Pacific (between 1000 and 4500m depth) lie within −0.18‰ to −0.04‰ (2σ), a range of 0.14‰, and also show a positive depth gradient with δ18Osw values increasing by 0.021±0.005‰ per km (r=0.46, p=0.0001)28. On the Shatsky Rise, core-top foraminiferal δ18O values from Site 1209 (this study) and 120829 show no significant differences during the last 25 ka (p=0.678, N=22, Supplementary Fig. 3). Modern measurements of salinity and temperature on the Shatsky Rise show similar values at the two sites30 (Fig. 3), with fresher (by 0.004 psu) and warmer (by 0.28°C) waters at Site 1209. From this, we conclude the Shatsky Rise was not bathed by a persistent NPDW in the recent past.

Our coupled Mg/Ca and δ18Obenthic values show that the NPDW was colder than the underlying southern-sourced waters during the Late Pliocene (Fig. 4). Our Mg/Ca-derived bottom water temperature (BWT) values at Site 1209 are colder than BWT values at Site 120829 over the period 3.0–2.7 Ma, with a difference in the mean BWT of 0.45°C. When the BWT is coupled with the δ18Obenthic values the colder Pliocene waters at Site 1209 must, therefore, have been fresher than those at the deeper Site 1208 to ensure density balance—in agreement with modelling evidence showing that the NPDW was fresher than underlying southern-sourced waters7 (Supplementary Fig. 4). The derived δ18Osw values at Site 1209 are significantly (p<1010, N=107) heavier than those at Site 1208 by 0.24‰ in the Late Pliocene (Fig. 4). Given the heavier δ18Osw at Site 1209 we therefore infer that the NPDW and southern-sourced deep water formation regions had different salinity-δ18O relationships, with one or both sites showing a decoupling of δ18Osw from salinity, likely due to sea-ice formation31 (Fig. 3). Our results are constrained by the uncertainty in the BWT estimates, calculated by replicate analysis to be ±0.8°C. This uncertainty is greater than the difference in Mg/Ca-derived BWT between 1209 and 1208, with only a few instances (e.g., MIS G9 and G8) during the Late Pliocene where the difference in the inferred temperatures is greater than the uncertainty (Fig. 4).

It has been suggested that NADW may have been advected into the North Pacific during the Late Pliocene32, which could thus be an explanation for the two distinct water masses revealed by the δ18O records. Reconstructed NADW temperature was consistently warmer during the Late Pliocene than deep Pacific waters sourced from the Southern Ocean29. However, our BWT record shows that the temperatures measured at Site 1209 were consistently colder, not warmer, than the temperatures at Site 1208 (Fig. 5). We therefore interpret the more positive δ18O values and colder BWT at Site 1209 compared to Site 1208 as a signal of NPDW formation and export to c. 2500m depth North Pacific in the Late Pliocene, not incursion of NADW into the Pliocene Pacific Ocean.

After iNHG (c. 2.7 Ma), the δ18O records on the Shatsky Rise continue to be offset during interglacials (Fig. 2) and converge during glacials (Fig. 4) indicating that NPDW formation may have been suppressed during post-iNHG glacials. From MIS G6 onwards, the mean δ18O difference decreased to 0.15‰ during Early Pleistocene glacials from 0.25‰ in the Late Pliocene (Figs. 2 and 6). During interglacials, however, the mean difference in δ18Obenthic values actually increases to 0.32‰ during the Early Pleistocene, an increase of 0.07‰ from the Late Pliocene (Figs. 2 and 6). The difference in mean δ18Obenthic shows no significant difference (<0.01‰, p=0.811, N=174) between glacials and interglacials in the Late Pliocene, but a significant difference (0.17‰, p=0.002, N=107) between glacials and interglacials during the Early Pleistocene. This suggests that there is some change over the iNHG which results in glacial-interglacial cycles modulating the formation and export of NPDW to intermediate depths of the North Pacific in the early Pleistocene. The difference in δ18O values after the iNHG is strongly correlated with sea level estimates (Supplementary Fig. 5). Falling sea levels during glacials would close, or at least markedly reduce water mass transport through a number of oceanic gateways, which could have been a control on NPDW formation. Our study shows an arrangement of δ18Obenthic and BWT reconstructions in the Late Pliocene that is explained by two distinct water masses in the intermediate and deep Northwest Pacific Ocean. This deep water mass is not present in the modern day or the Late Pleistocene34 which means the strong subpolar North Pacific halocline seen in the modern Pacific likely formed during the Early Pleistocene.

The glacial-interglacial variability in the difference in δ18Obenthic and BWT after the iNHG (Fig. 6), and the correlation between the difference in δ18Obenthic and Late Pleistocene sea levels (Supplementary Fig. 5) suggests this process is modulated by iNHG ice sheet expansion. We suggest falling sea levels in Early Pleistocene glacials would restrict water mass transport through the Indonesian Gateway and act as a control on the formation of NPDW. Closing the Indonesian Gateway would halt NPDW formation by increasing the precipitation minus evaporation in the North Pacific14. During the Pliocene, warm, saline waters flowed through the Indonesian Gateway originating from the Equatorial and Southern Pacific Ocean35,36. The closure of the Indonesian Gateway would redirect this warm water back to the Equatorial Pacific, warming it relative to the high latitudes37, which would increase the meridional (north-south) SST gradient in the Pacific Ocean. This would increase the precipitation minus evaporation in the North Pacific7,38, decrease surface salinity in the North Pacific, strengthen the subpolar halocline, and thus halt NPDW formation (Fig. 7). Alkenone SST records show a c. 1°C stronger Pacific meridional SST gradient after the iNHG39,40 (Supplementary Fig. 6). Stronger SST gradients during MIS G2, 100, and the first half of MIS 104 correspond to reduced δ18O gradients, while interglacials with weaker SST gradients, such as MIS G3 and 99, see greater δ18O gradients (Supplementary Fig. 6). Similarly, planktic oxygen isotope records from the subpolar North Pacific imply local salinity changes, proportional to the strength of the subpolar halocline, in phase with global sea level changes after 2.7 Ma41. The Indonesian Gateway thus links changes in sea level to the North Pacific meridional SST gradient and subpolar surface salinity, and the 1208–1209 δ18O gradient after the iNHG.

The water mass transport through the Indonesian Gateway, modulated by global sea level changes, is unlikely to be the cause of other instances of NPDW formation in the geologic past. We suggest that although the halocline is eroded during the Pliocene, the resultant NPDW is still fresher than underlying southern-sourced waters. The minimal impact of BWT changes on the density of water masses at Sites 1208 and 1209 highlight the importance of salinity in determining the formation of NPDW (Fig. 3). In the Late Pleistocene, there are suggestions of deep water formation during the LGM5, but in contrast to the Pliocene this was triggered by regional cooling and increased surface salinity in the North Pacific, in part aided by a closed Bering Strait due to lower sea levels5,13,42. That falling sea levels can halt NPDW formation over the iNHG, yet low sea level induces NPDW formation in the Late Pleistocene is likely due to different heat and freshwater balance over these timescales.

Our results show the presence of two distinct water masses in the Late Pliocene North Pacific which we interpret to be NDPW and a Southern Ocean-sourced water mass. During the Pleistocene the signal of NPDW formation weakens during glacials but becomes stronger during interglacials, suggesting some modulation of deep water formation in the North Pacific by glacial-interglacial factors that is not present prior to the iNHG. We suggest that this could be due to falling sea levels reducing the water mass transport through the Indonesian Gateway after the iNHG which would have wide-reaching effects on the Pacific climate. After iNHG, there is a convergence in deep Pacific and Atlantic Ocean characteristics29, suggesting greater homogeneity and connectivity in global deep water characteristics in the Pleistocene compared to the Pliocene. This may be driven by the cessation of NPDW formation allowing for a greater Antarctic Bottom Water extent, as well as stronger Antarctic Bottom Water export from the Southern Ocean following the growth of the Antarctic ice sheet43. This reorganisation in deep ocean circulation had cascading impacts on the carbon cycle and likely resulted in greater inorganic carbon storage influencing atmospheric temperatures and carbon cycling in the Pleistocene12.

Methods
Sample preparation
Ocean Drilling Program (ODP) Leg 192 Site 1209 is located on the Shatsky Rise in the NW Pacific Ocean (32°39.11′N, 158°30.36′E) at 2387m depth44. Samples were washed in deionised water and benthic foraminifera were picked from the >150μm size fraction for geochemical analysis.

Stable Isotopes
Four hundred twenty-five samples of the benthic foraminifera Cibicidoides wuellerstorfi were used for stable isotope analysis. Approximately 5–7 specimens were analysed at c. 4cm resolution along the core for both oxygen and carbon isotopes using a VG ISOGAS SIRE dual inlet isotope ratio mass spectrometer at the Godwin Laboratory for Palaeoclimate Research at the University of Cambridge. The long term analytical reproducibility for NBS-19 is ±0.06‰ and 0.05‰ (1 s.d.) for carbon and oxygen isotopes. All stable isotope data are reported relative to Vienna Pee Dee Belemnite (V-PDB) using the conventional notation. Calibration to reference standards allows for comparison of these records to stable isotope records from Site 120824. Previous stable isotope analysis was also undertaken on the species Uvigerina peregrina for the two sites6,29 at lower resolution and shows the same pattern of heavier oxygen isotopes at the shallower Site 1209 (Supplementary Fig. 8).

Trace metal analysis
The oxygen isotope measurements were paired with Mg/Ca measurements from the same depth in the core. 135 samples of between 8 and 12 foraminifera of the species Uvigerina peregrina were picked for trace metal analysis. In some samples of greater abundance, replicate analysis was performed (n=9). The samples were crushed between two glass slides. They then underwent a modified reductive and oxidative cleaning process described in ref. 45, 46 respectively. Elemental analyses were performed on a Thermo-Fisher Scientific Element 2 single collector sector field ICPMS at University of Southampton following established methods47. For this study, analytical reproducibility for Mg/Ca was ±3% (2σ) based on repeat measurements of in house carbonate standards.

The samples were measured for Mg/Ca as well as Mn/Ca, Fe/Ca and Al/Ca to monitor for any contamination of the sample from remnant diagenetic coatings or detrital material. Any samples with values of Mn/Ca, Fe/Ca, or Al/Ca over 120 μmol/mol were rejected (Supplementary Fig. 1). There is no correlation or temporal relationship between Mn/Ca, Fe/Ca or Al/Ca and Mg/Ca (Supplementary Fig. 2). Replicate analysis (n=9, separate populations of cleaned foraminifera) have an uncertainty in the Mg/Ca measurements of ±0.085 mmol/mol (1 s.d.).

PSU Solver48 was used to calculate temperature and seawater δ18O. Using the Mg/Ca and δ18O values, PSU Solver uses a Monte Carlo model to simultaneously solve paleotemperature equations and propagate uncertainty. The Mg/Ca-temperature equation used in the model was from ref. 22 for Uvigerina with a 10% adjustment to account for the reductive cleaning step. The temperature-δ18O equation was from ref. 49 for Cibicidoides wuellerstorfi. The uncertainties used were ±0.085 for the Mg/Ca (the uncertainty for replicate analysis) and ±0.05 for δ18O.

Age model
An orbitally tuned age model for Site 1209 was made with HMM-Stack50. We used the HMM-Stack age estimator to tune our record using a probabilistic Bayesian model to align the benthic oxygen isotope record to ProbStack, a probabilistic global benthic δ18O stack50. This was run on Queen Mary’s Apocrita HPC facility, supported by QMUL Research-IT51. Our sampling resolution is 1–4 kyr. The age model for the 1209 core-top samples was adapted from ref. 52.

Changepoint analysis
The benthic δ18O record was analysed using a Bayesian changepoint algorithm53. The algorithm was set up to look for the largest change point in the record, with a minimum spacing between points of 10 ka to avoid a single large glacial or interglacial peak being indicated as a changepoint.

Salinity calculations
Salinity of past water masses is calculated using the δ18Osw estimates from PSU Solver (see above). The δ18Osw is first corrected for the effects of changing ice volume, δ18Oivc, for the specific interval taken from nref. 54. The corrected δ18Osw is compared to the modern δ18Osw from the water depth and latitude in the North Pacific as in ref. 25, to get the difference in δ18Osw. The difference in δ18Osw is converted to a difference in salinity using the North Pacific δ18Osw-salinity relationship of Δδ18Osw = (0.44 × ΔSalinity) from ref. 25. The difference in salinity is then added to the modern salinity for the site to give an estimate of the past salinity of the site. This estimate of salinity is corrected for the change in salinity relationship from changing sea levels using the sea level curve from ref. 54. The difference in sea level, ΔRSL, is divided by the modern average depth of the oceans, 3680 m, and then multiplied by the modern average salinity of the oceans, 34.7 psu, to give the correction added to the salinity estimate to achieve the final salinity estimate.

Statistical analysis
The significance of any similarities in datasets (in either the δ18O and Mg/Ca record for both Sites 1209 and 1208) was measured using a Welch’s T-test. A standard T-test was not used as the δ18O or Mg/Ca records for both Sites 1209 and 1208 were not normally distributed. This was tested by a Kolmogorov–Smirnov test (comparing the distribution to a normal distribution) and a Shapiro–Wilk test, the results of these tests are provided Supplementary Table 1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The paper “North Pacific Deep Water Formation controlled by Sea Level Changes” investigates ocean circulation during the\nLate Pliocene, focusing on the mechanisms and the controlling factors.\nThe paper provides new data on stable isotopes and Mg/Ca ratio on foraminiferal tests from Site 1209 (North Pacific) and\ncompared it to the nearby site 1208, also from the North Pacific.\nThe comparison between the records of the two cores suggests that, in the comparison to the deeper Site 1208, colder\nand/or more saline water was present at the shallower Site 1209. Since the deepest site should be characterised by denser\nwater, the authors determine that the only possible reason to cause this difference is the presence of two distinct water\nmasses in the area, a North Pacific Deep Water, and a southern-sourced deep water, which must have had different\ntemperatures and δ18O values. Consequently, they analyze the record of the two cores using the differences between them\nas a proxy for the export of North Pacific Deep Water. They also discuss how this process might have happened during\ndifferent time periods.\nMy research mainly focuses on shallow-water successions (although I have worked on stable isotopes and Mg/Ca ratio on\nforaminifera and therefore I know how variable these data can be). Therefore, I may not be accustomed to many\npaleoceanographic details. However, I find two major flaws in the paper that require a moderate revision of data presentation\nand a major revision of the text, namely insufficient presentation and analysis of the data and improper organization of the\ndiscussion.\nThe main focus of the paper is the difference between the δ18O of the two cores intended as a proxy for the differences\nbetween the water masses. These differences are very subtle, especially in the way in which they are depicted into the\ngraphs, and they are only qualitatively discussed.\nIn figure 2 the differences are associated with a shading that indicates the uncertainty. Although these shading are too faint\nto be properly appreciated, there is a clear superposition, especially in the estimated temperature of bottom waters. This type\nof superposition between the data clearly highlights that the differences are not so large. They are there, but they are not so\nclear. In my opinion this requires a little bit of statistical analyses to prove that the differences are statistically relevant\n(especially taking into account the variability of replicate analysis of each sample). On the other hand, from line 103 to 107\nwhere the differences in δ18O are discussed, the manuscript is quite generic:\n“During MIS G4, G2, and the second half of MIS 104, there was a <0.5°C difference in BWT between the two sites, and a\n<0.1 ‰ difference in δ 18 O, compared to a >0.4 ‰ difference in δ 18 O values in interglacials MIS G3, G1, and 103 (Figure\n2)”\nAs the whole papers is based on this evidence, I believe that a more extensive discussion and a more detailed statistical\ntests of the relevance of these differences is required. In this regard I think that Extended Data Figure 4 is much more\neffective in conveying the differences between the water masses. Extended Data Figure 4 includes only some data (maybe\naverages values? In the caption an explanation is not provided), however, a similar figure with all the data and their scatter,\ncoupled with a statistical analysis would be much more effective in conveying the results of the paper.\nIndeed, following the brief discussion of the data from line 103 to line 107, the paper essentially becomes a review of\nliterature data on various proxies related to ocean circulation during the Pliocene – Pleistocene interval. This\ndiscussion/review is much more extensive than the analysis of the data provided by the authors and relatively disconnected\nfrom the latter. Furthermore, this part is not properly organized and the narrative wonders between various subjects and\nelements (too many for a short paper) without a clear direction.\nIn order to improve its impact the paper needs to be better organized: 1) Better and more clear presentation of the data 2)\nMore detailed analysis of the data and their significance, using statistical methods to test the significance of the observed\ndifferences taking into account all the uncertainties. 3) Shorter and more impactful discussion of the existing literature to\nsupport the evidence obtained from the analyzed dataset.\nThere are other minor flaws that should also be addressed (how many samples were analysed? In certain figures, like\nextended figure 4, it is unclear if average values or selected values are being displayed). I have provided some other small\nnotes in the annotated version of the manuscript.\nI hope that these suggestions might help the authors and the future impact of their paper to the general scientific community.\nBest regards", "reviewer_2": "Review of the manuscript “North Pacific Deep Water Formation controlled by Sea Level Changes”, by de Graaf et al.\nThe manuscript presents a very interesting benthic δ18O record from the North Pacific (ODP 1209, at 2387 m depth) that,\ncompared to the δ18O record of a deeper site (ODP 1208, at 3345 m depth) from the same region, has been interpreted as a\nresult of the presence of different water masses along the water column. The authors interpreted that ODP 1209 record\nindicates North Pacific deep water formation was active before the intensification of the Northern Hemisphere glaciation.\nThey also linked the cessation of North Pacific deep water formation to the restriction in the water mass transport through the\nIndonesian Gateway, as suggested by ocean modelling studies.\nThe hypothesis proposed is interesting, however, I do not clearly see in the presented graphics a dataset that supports a\nclear cessation of North Pacific deep water formation after the intensification of the Northern Hemisphere glaciation. Figure 1\nshows the difference benthic δ18O difference between the two sites as a proxy for the strength of North Pacific deep water\nformation but there is no marked reduction. The discussion about BWT is not easy to follow in figure 2, a gradient would be\nbetter to show if there is a difference between both sites that changed at the intensification of the Northern Hemisphere\nglaciation. Same thing for δ18Ow. Figure 2d is intriguing, it seems that instead of a cessation in North Pacific deep water\nformation, this process was enhanced during interglacials after 2.7 Ma. The support for the presence/absence of North\nPacific deep water formation must be clearly explained indicating the differences between the Southern Ocean deep water\nand the NPDW.\nIn addition, the text tries to justify the hypothesis from the very beginning. I think it would be better to present first the modern\nconditions and water masses characteristics; then indicate how the interval before the intensification of the Northern\nHemisphere glaciation shows different conditions. The discussion about possible drivers for this change is also repeated in\ntwo different sections. I suggest reading again all the manuscript and try to arrange the story and the data in a more clear\nway.\nMinor comments\nLine 57: I believe the authors wanted to report that “A positive difference indicates more positive δ18O values at Site 1209”\n(not in 1208)\nLine 63: specify benthic foraminifers"}, "author_response": "Note: Reviewer comments are in green text, author replies are in black text, and\nmanuscript edits are in black text with track changes. The annotated line numbers refer\nto those lines in the final document.\nWe would like to thank the two reviewers for their helpful feedback, and we have\nchanged the paper accordingly. The two reviewers commented that the central\nhypothesis of the paper, the formation of North Pacific Deep Water and its cessation\nover the iNHG, was not presented clearly enough either graphically or statistically, we\nhave addressed these points in this new draft and believe that the paper is better for it.\nIt was also noted that the structure of the paper could be made clearer, with a greater\nfocus on the results, which we have done as well as adjusting the paper to fit the Nature\nCommunications structure. In light of these changes, we have also changed the title of\nthe paper, to “Reduced North Pacific Deep Water Formation across the Northern\nHemisphere Glaciation”, to put the results at the centre of this paper rather than\nhypotheses for what may have caused them. We believe that this paper is now a more\nfocused, data-led manuscript and would like to thank the reviewers for their helpful\ncomments in getting the paper to this stage.\nIndividual points raised by the reviewers are addressed below.\nReviewer #1 (Remarks to the Author):\nThe paper “North Pacific Deep Water Formation controlled by Sea Level Changes”\ninvestigates ocean circulation during the Late Pliocene, focusing on the mechanisms\nand the controlling factors.\nThe paper provides new data on stable isotopes and Mg/Ca ratio on foraminiferal tests\nfrom Site 1209 (North Pacific) and compared it to the nearby site 1208, also from the\nNorth Pacific.\nThe comparison between the records of the two cores suggests that, in the comparison\nto the deeper Site 1208, colder and/or more saline water was present at the shallower\nSite 1209. Since the deepest site should be characterised by denser water, the authors\ndetermine that the only possible reason to cause this difference is the presence of two\ndistinct water masses in the area, a North Pacific Deep Water, and a southern-sourced\ndeep water, which must have had different temperatures and δ18O values.\nConsequently, they analyze the record of the two cores using the differences between\nthem as a proxy for the export of North Pacific Deep Water. They also discuss how this\nprocess might have happened during different time periods.\nWe would like to thank the reviewer for their comments. We have edited the paper to\npresent the data more clearly, better highlight the significance of the results, and have\nrestructured the discussion to better reflect this.\nMy research mainly focuses on shallow-water successions (although I have worked on\nstable isotopes and Mg/Ca ratio on foraminifera and therefore I know how variable these\ndata can be). Therefore, I may not be accustomed to many\npaleoceanographic details. However, I find two major flaws in the paper that require a moderate revision of data\npresentation and a major revision of the text, namely insufficient presentation and\nanalysis of the data and improper organization of the discussion.\nThe main focus of the paper is the difference between the δ18O of the two cores\nintended as a proxy for the differences between the water masses. These differences\nare very subtle, especially in the way in which they are depicted into the graphs, and\nthey are only qualitatively discussed.\nThis is a good point, and we have addressed this in three principal ways:\n1. We have performed two-tailed Welch’s T-tests on the δ18O from 1208 and 1209,\nfor the period before and after the iNHG to show quantitatively that the mean\nvalues are different before and after the iNHG. Furthermore, the differences\nbetween the two records before and after the iNHG are discussed more\nqualitatively, and the qualitative way in which the oxygen isotope records differed\nwas removed.\nDuring the Late Pliocene (3.3 – 2.7 Ma), the δ18O values at the two sites show a mean\ndifference of 0.25‰, but this is regularly in excess of 0.4 ‰ (Figure 1). This is a significant\ndifference in the δ18O values (p < 1031, N = 601) far in excess of the measurement uncertainty\nof ±0.05 ‰ and does not vary significantly between glacials and interglacials over the Late\nPliocene (p = 0.8110, N = 174). (Lines 91 – 95)\nAfter iNHG (c. 2.7 Ma), the δ18O records on the Shatsky Rise continue to be offset\nbenthic\nduring interglacials (Figure 1) and converge during glacials (Figure 3) indicating that NPDW\nformation may be suppressed during post-iNHG glacials. From MIS G6 onwards, the mean\nδ18O difference decreased to 0.15 ‰ during Early Pleistocene glacials from 0.25 ‰ in\nbenthic\nthe Late Pliocene (Figures 1, 5). During interglacials, however, the mean difference in\nδ18O values actually increases to 0.32 ‰ during the Early Pleistocene, an increase in\nbenthic\n0.07 ‰ from the Late Pliocene (Figures 1, 5). The difference in mean δ18O shows no\nbenthic\nsignificant difference (< 0.01 ‰, p = 0.811) between glacials and interglacials in the Late\nPliocene, but a significant difference (0.17 ‰, p = 0.002) between glacials and interglacials\nin the Early Pleistocene. (Lines 147 – 155)\n2. We have included a histogram of the distribution of δ18O and BWT from 1208 and\n1209 during glacials and interglacials before and after the iNHG in Figure 5 which\nshows the above results in a more intuitive form for the reader. As well as this, we\nhave removed the lines joining the binned mean difference in δ18O values in\nFigure 1 (in grey) so that the 10-ka filtered difference is clearer which better\nhighlights the glacial-interglacial disparity in Δδ18O post-iNHG.\n3. We have included a greater qualitative discussion of the modern arrangements in\nthe deep North Pacific Ocean to make clear that the difference in oxygen\nisotopes in the Late Pliocene is not akin to modern day ocean dynamics\nThis arrangement of water masses seen in the Late Pliocene North Pacific is unlike that of the\npresent Pacific Ocean. Across the modern day North Pacific, core-top benthic foraminiferal\nδ18O values are relatively homogenous compared to other ocean basins27 and show a positive\ndepth gradient with δ18O values increasing by 0.10 ± 0.03 ‰ per km (r = 0.30, p < 0.001)28,\nbenthic\nas opposed to the decreasing δ18O values with depth seen at the Shatsky Rise in the Late\nbenthic\nPliocene. Modern δ18O values in the deep North Pacific (between 1000 – 4500 m depth) lie\nsw\nwithin -0.18 ‰ to -0.04 ‰ (2σ), a range of 0.14 ‰, and also show a positive depth gradient\nwith δ18O values increasing by 0.021 ± 0.005 ‰ per km (r = 0.46, p = 0.0001)29. On the\nsw\nShatsky Rise, core-top foraminiferal δ18O values from Site 1209 (this study) and 120830 show\nno significant differences in the last 25 ka (p = 0.678, Supplementary Fig. 3). Modern\nmeasurements of salinity and temperature on the Shatsky Rise show similar values at the two\nsites31 (Figure 2), with fresher (by 0.004 psu) and warmer (by 0.28°C) waters at Site 1209.\nFrom this, we conclude the Shatsky Rise was not bathed by a persistent NPDW in the recent\npast. (Lines 103 – 115)\nIn figure 2 the differences are associated with a shading that indicates the uncertainty.\nAlthough these shading are too faint to be properly appreciated, there is a clear\nsuperposition, especially in the estimated temperature of bottom waters. This type\nof superposition between the data clearly highlights that the differences are not so large.\nThey are there, but they are not so clear. In my opinion this requires a little bit of\nstatistical analyses to prove that the differences are statistically relevant (especially\nconsidering the variability of replicate analysis of each sample). On the other hand,\nfrom line 103 to 107 where the differences in δ18O are discussed, the manuscript is\nquite generic:\n“During MIS G4, G2, and the second half of MIS 104, there was a <0.5°C difference in\nBWT between the two sites, and a <0.1 ‰ difference in δ 18 O, compared to a >0.4 ‰\ndifference in δ 18 O values in interglacials MIS G3, G1, and 103 (Figure 2)”\nWe would like to thank the reviewer for this comment and have addressed this in the\nmanuscript, which we believe has made the paper better. As reflected in the changed\ntitle of the paper, we have changed the emphasis of the paper towards what the data\ncan show and reduced the weight given to speculation that can not be supported\ncompletely by the present data.\nWe have made the uncertainty envelopes on Figures 3 (previously Figure 2) and Figure 4\nclearer so the reader can more easily see where the errors overlap.\nWe have also discussed, with use of statistics, where there are differences in BWT\nrecord as shown below, which replaced the previously more qualitative wording.\nOur coupled Mg/Ca and δ18O values show that the NPDW was colder than the\nbenthic\nunderlying southern-sourced waters during the Late Pliocene (Figure 3). Our Mg/Ca derived\nbottom water temperature (BWT) values at Site 1209 are colder than BWT values at Site\n120830 over the period 3.0 – 2.7 Ma, with a difference in the mean BWT of 0.45°C. (Lines\n117 - 120)\nWhen the BWT is coupled with the δ18O values the colder Pliocene waters at Site 1209\nbenthic\nmust, therefore, have been fresher than those at the deeper Site 1208 to ensure density\nbalance – in agreement with modelling evidence showing NPDW was fresher than underlying\nsouthern-sourced waters7 (Supplementary Fig. 4). However, the derived δ18O values at Site\nsw\n1209 are significantly (p < 1010, N = 107) heavier than those at Site 1208 by 0.24 ‰ in the\nLate Pliocene (Figure 3). (Lines 120 - 125)\nThe Mg/Ca record for the Early Pleistocene also suggests that there is a convergence in water\nmass properties in the North Pacific during glacials but not interglacials. Over the iNHG, the\nmean derived BWT at Sites 1209 and 1208 increased by 1.0 ± 0.8°C and 1.1 ± 0.8°C\nrespectively (Figure 3). This is likely due to a greater influx of warm NADW into the\nSouthern Ocean and a greater stratification of Southern Ocean surface waters following the\niNHG which would warm the southern sourced waters flowing into the Pacific30. Due to the\nreduction in NPDW formation after the iNHG, both sites would have experienced a greater\nshare of Southern Ocean sourced waters which would have increased the BWT at both sites.\nWhile at Site 1209, there was effectively no difference in mean BWT between Early\nPleistocene glacials and interglacials (0.7°C and 0.6°C respectively), Site 1208 saw a 0.5°C\ndifference develop between mean glacial and interglacial BWT (1.0 ± 0.8°C and 1.4 ± 0.8°C\nrespectively). There was a slight decrease in the mean BWT gradient between Site 1209 and\n1208 during Early Pleistocene glacials (0.3°C) and an increase in the mean BWT gradient\nduring interglacials (0.8°C). This supports the idea of a convergence of water mass properties\nduring glacials and a divergence during interglacials as seen in the δ18O record, and lends\nbenthic\ncredence to the idea that glacial-interglacial changes in the Early Pleistocene exerted a control\non water mass formation in the Pacific that they did not during the Late Pliocene. (Lines 164\n- 180)\nFinally, we have discussed where in the record there is a quantitative difference in BWT\nrecord and where the current data can not completely support the current hypothesis.\nOur results are constrained by the uncertainty in the BWT estimates, calculated by replicate\nanalysis to be ± 0.8°C. This uncertainty is greater than the difference in Mg/Ca-derived BWT\nbetween 1209 and 1208, with only a few instances (e.g. MIS G9 and G8) during the Late\nPliocene where the difference in the inferred temperatures is greater than the uncertainty\n(Figure 3). (Lines 128 - 132)\nThese results are caveated by the high uncertainties in BWT estimates (±0.8°C), however,\nthere are stretches of MIS 101, 103, G1 and G3 when the BWT estimates at 1208 is warmer\nthan 1209 outside of uncertainties (Figure 3) which is not seen during glacials, supporting the\nidea that there is some glacial-interglacial control on water mass properties in the deep North\nPacific during the Pleistocene. (Lines 180 - 184)\nAs the whole papers is based on this evidence, I believe that a more extensive\ndiscussion and a more detailed statistical tests of the relevance of these differences is\nrequired. In this regard I think that Extended Data Figure 4 is much more effective in\nconveying the differences between the water masses. Extended Data Figure 4 includes\nonly some data (maybe averages values? In the caption an explanation is not provided),\nhowever, a similar figure with all the data and their scatter, coupled with a statistical\nanalysis would be much more effective in conveying the results of the paper.\nIndeed, following the brief discussion of the data from line 103 to line 107, the paper\nessentially becomes a review of literature data on various proxies related to ocean\ncirculation during the Pliocene – Pleistocene interval. This discussion/review is much\nmore extensive than the analysis of the data provided by the authors and relatively\ndisconnected from the latter. Furthermore, this part is not properly organized and the\nnarrative wonders between various subjects and elements (too many for a short paper)\nwithout a clear direction.\nThanks for this comment, we agree with the points here and have made adjustments\naccordingly. The paper has been updated to give a more quantitative basis to the claims\nmade in the paper, as discussed above. We have also included a version of Extended\nData Figure 4 in the paper as Figure 2, though with only some of the data from the study\nso as to maintain legibility. The figure caption on this figure has been updated to better\nconvey this information to the reader.\nFigure 2. Derived densities of past and modern water masses.\nTemperature and salinity of past water masses from ODP site 1208 (green, 3346 m depth) and\n1209 (orange, 2387 m depth) for Late Pliocene (stars), Early Pleistocene glacials (circles) and Early\nPleistocene interglacials (triangles). The temperature is the mean of the BWT estimates for each site\nover all intervals for the Late Pliocene (2.7 – 3.0 Ma) or the Early Pleistocene (2.4 – 2.7 Ma). The\nmean of all the δ18O estimates from the sites are used to determine the salinity estimates (see\nsw\nMethods). Isopycnals of σ are calculated for 2300 m depth in the North Pacific using the Gibbs-\nare greater than those at (deeper) site 1208 for the Late Pliocene and Early Pleistocene (hollow\nsymbols) which suggests that the derived δ18O values for 1208 and/or 1209 cannot be linearly\nsw\nconverted to past salinities. Modern values of temperature and salinity (diamonds) from Argo float\ndata31, and calculated temperatures and salinities from core-top mean δ18O and Mg/Ca values\nbenthic\nfrom 1209 and 120830 for the Holocene (0 – 12 ka, squares) are shown highlighting that modern and\nHolocene δ18O measurements do appear correlated to salinity (filled symbols).\nsw\nThe literature review on possible causes for water mass changes in the North Pacific\nhas been generally cut down to a few lines in the introduction (see below), allowing for\nmore of a focus on the data presented in the paper.\nActive deep water formation in the North Pacific would require the removal of the halocline\nin the subpolar North Pacific, and studies have suggested a weak meridional (north-south) sea\nsurface temperature (SST) gradient7,11, changing monsoon patterns12, and sea level induced\nchanges in ocean gateways13, 14 as potential causes. (Lines 45 - 48)\nIn order to improve its impact the paper needs to be better organized: 1) Better and\nmore clear presentation of the data 2) More detailed analysis of the data and their\nsignificance, using statistical methods to test the significance of the observed\ndifferences taking into account all the uncertainties. 3) Shorter and more impactful\ndiscussion of the existing literature to support the evidence obtained from the analysed\ndataset.\nWe would like to thank the reviewer again for their comments, which have been very\nhelpful. We have changed the structure of the paper, firstly to reflect the Nature\nCommunications style guide, but also in response to this review to better highlight the\nclaims made by the evidence and to put less of an emphasis on the review of the\nprevious literature.\nWe have included statistical tests on all the claims of difference between records for\nvarious intervals in this paper – some of which have highlighted that the changes are not\nas large as suggested in the original paper and the language of the paper has been\nadjusted to reflect this. In particular, the differences between the bottom water\ntemperature records of the Sites 1209 and 1208 during the Early Pleistocene are shown\nto not be outside the analytical uncertainty and therefore cannot be said to show\nunequivocally that there is a difference in water masses present at Site 1209 between\nglacials and interglacials in the Early Pleistocene. The presentation of this, and the\nconclusions from this about the causes of this water mass change over the iNHG, have\nbeen made more equivocal.\nThe literature review section of the paper has been moved to introduction and now\nfocuses on those issues which are specifically pertinent to the data here presented,\nhighlighting the challenges to the PMOC hypothesis as well as its adherents, to make\nclear how our data fits into the wider discussion around NPDW in the Pliocene.\nThere are other minor flaws that should also be addressed (how many samples were\nanalysed? In certain figures, like extended figure 4, it is unclear if average values or\nselected values are being displayed). I have provided some other small notes in the\nannotated version of the manuscript.\nThe notes on other issues were welcomely received and the paper has been adjusted\naccordingly.\nThe small corrections on lines 14 – 18, line 20, and line 44 are no longer relevant as the\nrelevant lines have been changed.\nThe keys for Figures 1, 3, and 4 are now also included in the figure as well as the in the\nfigure caption.\nThe shading for Figure 2 (now Figure 3) has been made darker so as to be more visible.\nThe suggestion on Line 163 is no longer relevant as this section has been removed as it\nis no longer supported by the data.\nThe suggestions on Lines 178, 182, 198 and Lines 212 – 214 are no longer relevant as\nthis section has been removed and is now included (in much shorter form) in the\nintroduction.\nThe suggestions on Lines 226 – 227 are no longer relevant as this section of the\nparagraph has been removed to make the paragraph more focused.\nOur coupled δ18O and Mg/Ca values show that the NPDW was colder, and fresher than\nbenthic\nthe underlying southern-sourced waters during the Late Pliocene (Figure 3). (Lines 117 –\n118)\nSchematic diagram showing ocean circulation in the Pacific Ocean during the Late Pliocene with mid\nPliocene sea levels highlighting how an open Indonesian Gateway could promote NPDW formation.\n(Figure 6 Caption)\nStable Isotopes. 425 samples of the benthic foraminifera Cibicidoides wuellerstorfi was used\nfor stable isotope analysis. (Lines 244 – 245)\nThe long term analytical reproducibility for NBS-19 is ±0.06 ‰ and 0.05 ‰ (1 s.d.) for\ncarbon and oxygen isotopes. (Lines 248 – 249)\n135 samples of between 8 – 12 foraminifera of the species Uvigerina peregrina were picked\nfor trace metal analysis. (Lines 256 - 257)\nI hope that these suggestions might help the authors and the future impact of their\npaper to the general scientific community.\nReviewer #2 (Remarks to the Author):\nReview of the manuscript “North Pacific Deep Water Formation controlled by Sea Level\nChanges”, by de Graaf et al.\nThe manuscript presents a very interesting benthic δ18O record from the North Pacific\n(ODP 1209, at 2387 m depth) that, compared to the δ18O record of a deeper site (ODP\n1208, at 3345 m depth) from the same region, has been interpreted as a result of the\npresence of different water masses along the water column. The authors interpreted\nthat ODP 1209 record indicates North Pacific deep water formation was active before\nthe intensification of the Northern Hemisphere glaciation. They also linked the\ncessation of North Pacific deep water formation to the restriction in the water mass\ntransport through the Indonesian Gateway, as suggested by ocean modelling studies.\nThe hypothesis proposed is interesting, however, I do not clearly see in the presented\ngraphics a dataset that supports a clear cessation of North Pacific deep water formation\nafter the intensification of the Northern Hemisphere glaciation. Figure 1 shows the\ndifference benthic δ18O difference between the two sites as a proxy for the strength of\nNorth Pacific deep water formation but there is no marked reduction.\nWe would like to thank the reviewer for their helpful comments. As the new proposed\ntitle of the paper shows, we have changed the paper to focus on the evidence for NPDW\nformation in the Late Pliocene as well as the cessation in the Early Pleistocene, and\nreduced the focus on mechanisms for the changes in NPDW formation which are less\nsupported by the data of the paper, and we believe that the paper is now more data-\ndriven than previously.\nWe have changed Figure 1, to better show both the deep water formation prior to the\nINHG and the reduction in NPDW strength during Early Pleistocene glacials. We have\nsimplified Figure 1 to better highlight how the glacial Δδ18O is smaller than the\ninterglacial Δδ18O , seen in the blue 10-kyr smoothed average of Δδ18O, and have\nremoved the lines connecting the binned average Δδ18O values as this complicated\nrather than clarified the figure.\nWe have softened the language around the change in the δ18O before and after the\nbenthic\niNHG. Our new statistics analyses do show there is a decrease in the gradient, but not a\ncomplete collapse in the gradient between the two sites during the glacials.\nDuring the Late Pliocene (3.3 – 2.7 Ma), the δ18O values at the two sites show a mean\ndifference of 0.25‰, but this is regularly in excess of 0.4 ‰ (Figure 1). This is a significant\ndifference in the δ18O values (p < 1031, N = 601) far in excess of the measurement uncertainty\nof ±0.05 ‰ and does not vary significantly between glacials and interglacials over the Late\nPliocene (p = 0.8110, N = 174). (Lines 91 – 95)\nAfter iNHG (c. 2.7 Ma), the δ18O records on the Shatsky Rise continue to be offset\nbenthic\nduring interglacials (Figure 1) and converge during glacials (Figure 3) indicating that NPDW\nformation may be suppressed during post-iNHG glacials. From MIS G6 onwards, the mean\nδ18O difference decreased to 0.15 ‰ during Early Pleistocene glacials from 0.25 ‰ in\nbenthic\nthe Late Pliocene (Figures 1, 5). During interglacials, however, the mean difference in\nδ18O values actually increases to 0.32 ‰ during the Early Pleistocene, an increase in\nbenthic\n0.07 ‰ from the Late Pliocene (Figures 1, 5). The difference in mean δ18O shows no\nbenthic\nsignificant difference (< 0.01 ‰, p = 0.811) between glacials and interglacials in the Late\nPliocene, but a significant difference (0.17 ‰, p = 0.002) between glacials and interglacials\nin the Early Pleistocene. (Lines 147 – 155)\nWe have also included Figure 5, which shows the distribution of Δδ18O values in both\nglacials and interglacials and highlights how the glacial Δδ18O is considerably reduced\ncompared to the interglacial difference.\nThe discussion about BWT is not easy to follow in figure 2, a gradient would be better to\nshow if there is a difference between both sites that changed at the intensification of\nthe Northern Hemisphere glaciation. Same thing for δ18Ow.\nThis is a good point, and we have changed the discussion of the BWT and δ18O to\nsw\nmake it clearer what points are being made. We have included some statistical tests to\nhighlight the size and significance of any of the changes. We have focussed the\ndiscussion on the qualitative differences (such as in the mean BWT) that can be seen\nbetween the two records which should make the discussion easier to follow.\nAdditionally, we have included a gradient in the BWT and δ18O in the supplementary\nsw\ninformation as Supplementary Figure 7.\nWhen the BWT is coupled with the δ18O values the colder Pliocene waters at Site 1209\nbenthic\nmust, therefore, have been fresher than those at the deeper Site 1208 to ensure density\nbalance – in agreement with modelling evidence showing NPDW was fresher than underlying\nsouthern-sourced waters7 (Supplementary Fig. 4). However, the derived δ18O values at Site\nsw\n1209 are significantly (p < 1010, N = 107) heavier than those at Site 1208 by 0.24 ‰ in the\nLate Pliocene (Figure 3). (Lines 120 - 125)\nThe Mg/Ca record for the Early Pleistocene also suggests that there is a convergence in water\nmass properties in the North Pacific during glacials but not interglacials. Over the iNHG, the\nmean derived BWT at Sites 1209 and 1208 increased by 1.0 ± 0.8°C and 1.1 ± 0.8°C\nrespectively (Figure 3). This is likely due to a greater influx of warm NADW into the\nSouthern Ocean and a greater stratification of Southern Ocean surface waters following the\niNHG which would warm the southern sourced waters flowing into the Pacific30. Due to the\nreduction in NPDW formation after the iNHG, both sites would have experienced a greater\nshare of Southern Ocean sourced waters which would have increased the BWT at both sites.\nWhile at Site 1209, there was effectively no difference in mean BWT between Early\nPleistocene glacials and interglacials (0.7°C and 0.6°C respectively), Site 1208 saw a 0.5°C\ndifference develop between mean glacial and interglacial BWT (1.0 ± 0.8°C and 1.4 ± 0.8°C\nrespectively). There was a slight decrease in the mean BWT gradient between Site 1209 and\n1208 during Early Pleistocene glacials (0.3°C) and an increase in the mean BWT gradient\nduring interglacials (0.8°C) supporting the idea of a convergence of water mass properties\nduring glacials and a divergence during interglacials as seen in the δ18O record, lending\nbenthic\ncredence to the idea that glacial-interglacial changes in the Early Pleistocene exerted a control\non water mass formation in the Pacific that they did not during the Late Pliocene. (Lines 164\n- 180)\nWe have also included a section discussing the limitations of the differences in the BWT\nestimates and what can, and more importantly, what cannot be inferred from our\nrecord.\nOur results are constrained by the uncertainty in the BWT estimates, calculated by replicate\nanalysis to be ± 0.8°C. This uncertainty is greater than the difference in Mg/Ca-derived BWT\nbetween 1209 and 1208, with only a few instances (e.g. MIS G9 and G8) during the Late\nPliocene where the difference in the inferred temperatures is greater than the uncertainty\n(Figure 3). (Lines 128 - 133)\nThese results are caveated by the high uncertainties in BWT estimates (±0.8°C), however,\nthere are stretches of MIS 101, 103, G1 and G3 when the BWT estimates at 1208 is warmer\nthan 1209 outside of uncertainties (Figure 3) which is not seen during glacials, supporting the\nidea that there is some glacial-interglacial control on water mass properties in the deep North\nPacific during the Pleistocene. (Lines 180 - 184)\nWe have included a gradient in the BWT and δ18O in the supplementary information as\nsw\nSupplementary Figure 7. This is not included in the main paper was deemed to make\nthe Figure 3 too confusing, particularly as, due to poor temporal overlap between the\nrecords, any attempt to determine a gradient had a much lower resolution than the\noriginal records.\nFigure 2d is intriguing, it seems that instead of a cessation in North Pacific deep water\nformation, this process was enhanced during interglacials after 2.7 Ma. The support for\nthe presence/absence of North Pacific deep water formation must be clearly explained\nindicating the differences between the Southern Ocean deep water and the NPDW.\nThis is a really good point. We have used statistical tests to show the magnitude of the\ndifference in BWT between glacials and interglacials. However, as this difference is\nwithin the analytical uncertainty we have also changed the language and wording\naround the cessation of NPDW after the iNHG to be more equivocal.\na greater influx of warm NADW into the Southern Ocean and a greater stratification of\nSouthern Ocean surface waters following the iNHG which would warm the southern sourced\nwaters flowing into the Pacific30, 35. Due to the reduction in NPDW formation after the iNHG,\nboth sites would have experienced a greater share of Southern Ocean sourced waters which\nwould have increased the BWT at both sites. While at Site 1209, there was effectively no\ndifference in mean BWT between Early Pleistocene glacials and interglacials (0.69°C and\n0.63°C respectively), Site 1208 saw a 0.5°C difference develop between mean glacial and\ninterglacial BWT (0.99°C and 1.4°C respectively). There was a slight decrease in the mean\nBWT gradient between Site 1209 and 1208 during Early Pleistocene glacials (0.30°C) and an\nincrease in the mean BWT gradient during interglacials (0.79°C) supporting the idea of a\nconvergence of water mass properties during glacials and a divergence during interglacials as\nseen in the δ18O record, lending credence to the idea that glacial-interglacial changes in\nbenthic\nthe Early Pleistocene exerted a control on water mass formation in the Pacific that they did\nnot during the Late Pliocene. These results are caveated by the high uncertainties in BWT\nestimates (±0.8°C), however, there are stretches of MIS 101, 103, G1 and G3 when the BWT\nestimates at 1208 is warmer than 1209 outside of uncertainties (Figure 3) which is not seen\nduring glacials, supporting the idea that there is some glacial-interglacial control on water\nmass properties in the deep North Pacific during the Pleistocene. (Lines 167 – 184)\nWe highlight the greater interglacial difference in δ18O after the iNHG in Figure 5 as well.\nHowever, we do not expand on this idea too much as we thought that to fully discuss\nthis idea would complicate the paper too much and distract from the main findings of\nthe paper. We agree that it is interesting that there is an increase in the interglacial\nΔδ18O as well as a decrease in the glacial Δδ18O but found that it was too difficult to tie\nin with the rest of the paper without stretching the paper too thin.\nFigure 2d was also removed from Figure 2 (now Figure 3 in the updated manuscript) as it\nwas thought to complicate the figure too much, and the glacial/interglacial mean Δδ18O\nwas calculated from the binned data from Figure 1 which introduced uncertainty into\nthe figure which was not communicated effectively.\nIn addition, the text tries to justify the hypothesis from the very beginning. I think it\nwould be better to present first the modern conditions and water masses\ncharacteristics; then indicate how the interval before the intensification of the Northern\nHemisphere glaciation shows different conditions. The discussion about possible\ndrivers for this change is also repeated in two different sections. I suggest reading again\nall the manuscript and try to arrange the story and the data in a clearer way.\nMany thanks for this comment, we have taken this on board and have restructured the\npaper accordingly and in line with Nature Communications style guide. Firstly, the\nintroduction has been rewritten to be more even-handed on the concept of a Pliocene\nPMOC.\nThe evidence for NPDW formation in the Late Pliocene is contested and relies heavily on proxy\nrecords that are easily biased by changes in local productivity. Many modelling studies of the\nLate Pliocene fail to generate an active overturning circulation in the North Pacific15; though\nthis may be due to limited model run time6, poor reconstructions of Late Pliocene temperature\ngradients7, or an inability of the models to accurately simulate changing Pliocene\npalaeogeography14. Modelling studies that alter atmospheric cloud physics to simulate a weak\nmeridional SST gradient suggested by proxy data during the Pliocene generate active NPDW\nformation in the Pliocene7. This water mass structure is supported by the arrangement of carbon\nisotopes across the Late Pliocene Pacific Ocean which point to NPDW reaching depths of\naround 3000 m in the North Pacific6, though recent work from the subpolar North Pacific\nquestions this16. Critically, these records come from opposite sides of the Pacific Ocean and\nNPDW formation is expected to occur in the Northwest Pacific Ocean during the Pliocene and\nis unlikely to show a similar water mass structure in the Northeast Pacific9,17. Carbon isotopes\nare also readily influenced by changes in local productivity18 and so it is hard to determine\nwhich, if any, of these signals are driven by water mass changes. The presence of NPDW in the\nPliocene is also supported by opal and carbonate mass accumulation rates19,20 and redox-\nsensitive trace metal records7 in the subpolar North Pacific, which both suggest an unstratified\nand well-ventilated water column indicative of active deep water formation as well as\nupwelling of deep waters in the Late Pliocene7. (Lines 50 – 67)\nSecondly, we have reduced the discussion on the possible drivers for NPDW formation to\ninstead focus on the data that is presented here in the paper. The possible theories are\ndiscussed briefly in the introduction:\nActive deep water formation in the North Pacific would require the removal of the halocline in\nthe subpolar North Pacific, and studies have suggested a weak meridional (north-south) sea\nsurface temperature (SST) gradient7,11, changing monsoon patterns12, and sea level induced\nchanges in ocean gateways13,14 as potential causes. (Lines 45 – 48)\nAnd then the discussion at the end of the paper is much reduced (Lines 194 – 211) and\nfocusses on what can be conclusively drawn from the data and removes much of the\ndiscussion of alternatives as seen in the original paper (Lines 169 – 202 in the original\npaper, now removed).\nThe following minor comments have been addressed:\nMinor comments\nLine 57: I believe the authors wanted to report that “A positive difference indicates more\npositive δ18O values at Site 1209” (not in 1208)\nA positive difference indicates more positive δ18O values at Site 1209. (Figure 1 Caption)\nLine 63: specify benthic foraminifers\nThis has been fixed and greater clarity added throughout the paper that the stable\nisotope measurements have been taken from benthic foraminifera rather than simply\nforaminifera."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised version of “Reduced North Pacific Deep Water Formation across the Northern Hemisphere Glaciation”\nsignificantly improved the shortcomings of the original version. The data are now better discussed, associated with a\nstatistical analysis, and supported by clear figures, making the message conveyed by the manuscript much clearer and more\nimpactful.\nI believe that the paper is now suitable for publication. I have noted some small typos here and there in the annotated\nversion of the manuscript.\nI hope that this small correction can help the authors, and I hope to see the manuscript published soon.\nBest regards", "reviewer_3": "Response:\nIn regard to the North Pacific Deep Water formation in the Late Pliocene, there is some controversial on the proxy and model\nclimate. In this work, Dr. Graaf et al. present a coupled Mg/Ca-δ18O record from the North Pacific which shows two distinct\nwater masses in the Pliocene North Pacific Ocean, with NPDW colder and fresher than the underlying deeper water. Using\ntwo cores from different water depth, they further prove the NPDW formation decline during glacials from 2.7 million years\nago, which highlight the impact of sea level via ocean gateways in shaping deep water circulation in the climate system.\nAfter the new version and response letter, I think the authors have gave a good reply and made the modifications in the\nmanuscript. I give some comments on this work.\nFirstly, in the Abstract part: I suggest that the author should add some highlights in the last sentence in this part. In this\nversion, I cannot get your key points in your study.\nSecondly, I suggest that you should add a figure 1 about the location in the North Pacific Ocean. In this case, you can\npresent the deep-water circulation in the North Pacific Ocean.\nAfter some improvement in this part, I suggest this manuscript can be accepted after the minor revision."}, "author_response": "Note: Reviewer comments are in green text, author replies are in black text, and\nmanuscript edits are in black text with track changes. The annotated line numbers refer\nto those lines in the final document.\nWe would like to thank the reviewers for their helpful feedback, which we have taken on\nboard, and which have improved the paper. We have changed wording where this is\nambiguous and corrected any typos highlighted by the reviewers. In response to the\nthird reviewer, we have included a new figure (Figure 1) which provides useful context on\nthe location of the sites within the North Pacific and in relation to the modern\ndistribution of water masses in the North Pacific. We have also clarified the wording of\nthe abstract to better highlight the main points of the article.\nA point-by-point response to the reviewer comments is included below.\nReviewer #1 (Remarks to the Author):\nThe revised version of “Reduced North Pacific Deep Water Formation across the\nNorthern Hemisphere Glaciation” significantly improved the shortcomings of the\noriginal version. The data are now better discussed, associated with a statistical\nanalysis, and supported by clear figures, making the message conveyed by the\nmanuscript much clearer and more impactful.\nI believe that the paper is now suitable for publication. I have noted some small typos\nhere and there in the annotated version of the manuscript.\nI hope that this small correction can help the authors, and I hope to see the manuscript\npublished soon.\nBest regards\nWe would like to thank the reviewer for their comments and their help in improving this\nmanuscript. We have corrected the typos as below:\nEvidence suggests there was a Pacific Meridional Overturning Circulation during the warm\nLate Pliocene, similar to the modern Atlantic Ocean with a weak halocline in the subpolar\nNorth Pacific resulting in North Pacific Deep Water (NPDW) formation. (Lines 15 – 18)\nModelling studies that alter cloud physics to simulate a weak meridional Pacific SST\ngradient, as proxy suggest was the case during the Pliocene11, generate active NPDW\nformation. (Lines 56 – 57)\nTherefore, δ18O in benthic foraminifera (δ18O ) can be used as a proxy for water mass\nbenthic\ndensity (Lines 74 – 75)\nforaminiferal δ18O values from Site 1209 (this study) and 120830 show no significant\ndifferences during the last 25 ka (Lines 112 – 113)\nin agreement with modelling evidence showing that the NPDW was fresher (Lines 123 – 124)\nHowever, our BWT record shows that the temperatures measured at Site 1209 were\nconsistently colder (Lines 138 – 139)\nindicating that NPDW formation may have been suppressed during post-iNHG glacials\n(Lines 147 – 148)\nDuring interglacials, however, the mean difference in δ18O values actually increases to\nbenthic\n0.32 ‰ during the Early Pleistocene, an increase of 0.07 ‰ from the Late Pliocene (Lines\n150 – 151)\nThe difference in mean δ18O shows no significant difference (< 0.01 ‰, p = 0.811, N =\nbenthic\n174) between glacials and interglacials in the Late Pliocene, but a significant difference (0.17\n‰, p = 0.002, N = 107) between glacials and interglacials during the Early Pleistocene.\n(Lines 152 – 154)\nhowever, there are stretches of MIS 101, 103, G1 and G3 where the BWT estimates at 1208\nare warmer than 1209 outside of uncertainties (Lines 179 – 180)\nWe suggest falling sea levels in Early Pleistocene glacials would restrict water mass transport\nthrough the Indonesian Gateway and act as a control on the formation of NPDW (Lines 192 –\n193)\nSimilarly, planktic oxygen isotope records from the subpolar North Pacific imply local\nsalinity changes, proportional to the strength of the subpolar halocline, in phase with global\nsea level changes after 2.7 Ma43. The Indonesian Gateway thus links changes in sea level to\nthe North Pacific meridional SST gradient and subpolar surface salinity (Lines 205 – 208)\nto give the correction added to the salinity estimate to achieve the final salinity estimate. (Lines\n293 – 294)\nSchematic diagram showing ocean circulation in the Pacific Ocean during glacials of the Early\nPleistocene with sea levels 60 m lower than modern (Lines 542 – 543)\nElevation above 1000 m is shown in brown. Location of ODP Sites 1209 (orange) and 1208 (blue) are\nshown as circles. (Lines 552 – 553)\nReviewer #3 (Remarks to the Author):\nResponse:\nIn regard to the North Pacific Deep Water formation in the Late Pliocene, there is some\ncontroversial on the proxy and model climate. In this work, Dr. Graaf et al. present a\ncoupled Mg/Ca-δ18O record from the North Pacific which shows two distinct\nwater masses in the Pliocene North Pacific Ocean, with NPDW colder and fresher than the\nunderlying deeper water. Using two cores from different water depth, they further prove\nthe NPDW formation decline during glacials from 2.7 million years ago, which highlight\nthe impact of sea level via ocean gateways in shaping deep water circulation in the\nclimate system. After the new version and response letter, I think the authors have gave\na good reply and made the modifications in the manuscript. I give some comments on\nthis work.\nFirstly, in the Abstract part: I suggest that the author should add some highlights in the\nlast sentence in this part. In this version, I cannot get your key points in your study.\nSecondly, I suggest that you should add a figure 1 about the location in the North Pacific\nOcean. In this case, you can present the deep-water circulation in the North Pacific\nOcean.\nAfter some improvement in this part, I suggest this manuscript can be accepted after\nthe minor revision.\nWe would like to thank the reviewer for their comments. We have reworded the final line\nof the abstract as follows:\nHere, we show a decline in NPDW formation during glacials from 2.7 million years ago,\nwhich we suggest demonstrates the strong sensitivity of ocean gateways to sea level and ice\nvolume in shaping deep water circulation, and thus the wider climate system. (Lines 21 – 24).\nWe have also included a new figure, Figure 1, which shows both the location of Sites\n1208 and 1209 in the North Pacific and the paths of major water masses in the deep\nNorth Pacific. This should make it clearer to readers where the sites are located and\nhow they relate to modern ocean circulation.\nFigure 1. Location of ODP Sites 1208 and 1209 and major water masses in the modern\nPacific Ocean.\nSalinity of the modern Pacific Ocean below 1000 m depth along WOCE Transect P13\nrunning at 165°E from 4°S to 54°N61. Locations of the Ocean Drilling Program (ODP) Sites\n1208 and 1209 are shown as circles. Major water mass trajectories in the North Pacific are\nshown with white arrows. AABW = Antarctic Bottom Water, CDW = Circumpolar Deep\nWater, NPIW = North Pacific Intermediate Water. Bathymetry derived from eWOCE online\narchive62. (Lines 482 – 488)"}]

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