Abrupt weakening of deep Atlantic circulation at the last glacial inception

Domain: earth_science
Published: 2025-08-14
Editorial outcome: major_revision

Abstract
Deglaciations and glacial inceptions are the two equally important transitional periods that bridge the glacial and interglacial climate states, yet our understanding of deglaciations far exceeds that of glacial inceptions. Substantial variations in deep ocean circulation accompanied the last deglaciation, and model simulations recently suggested that a weakening of the Atlantic Meridional Overturning Circulation (AMOC) also occurred at the last glacial inception (LGI; 113-119 thousand years ago), yet evidence of such a change remains inconclusive. Here, we report three Pa/Th records from the western and central North Atlantic that display an abrupt weakening of the AMOC at the LGI. The magnitude of the reconstructed AMOC weakening approaches but never reaches the level of disruptions associated with the Heinrich ice discharge events. Our results may highlight a unique period of orbitally forced abrupt circulation changes and the importance of ocean processes in setting atmospheric CO2 changes in motion.

Introduction
During the last 800 thousand years (kyr), the Earth’s climate oscillated between 100-kyr glaciations and relatively brief interglacial intervals of warmth1. If not for anthropogenic influence, the next climate event after the current interglacial would be the transition to a glacial period2. Any changes observed during the past glacial inception offer potential insights into how the transitions between two very distinct climate states, interglacial and glacial periods, took place. Before the LGI, the cryosphere configuration was analogous to that of today, while the sea level was several meters higher than today3,4 and the global mean temperature was likely 1–1.5 °C higher than pre-industrial levels5. After the LGI, ice sheets started to regrow in the Northern Hemisphere, and would eventually lower the sea level by ~120 m6. Our understanding of glacial inceptions is far inferior to that of deglaciations (Supplementary Fig. 1), and many gaps remain. Recently, a seminal Earth system model study indicated that the AMOC abruptly weakened during the LGI7. However, past studies of the production of North Atlantic Deep Water (NADW) during the LGI have not achieved a consensus, with some indicating active overturning8,9 and others suggesting diminished NADW10–13. Evidence of polar and subpolar cooling during the LGI, of which the AMOC weakening is a sufficient but not necessary cause, is observed in a Greenland ice core record14 and in some reconstructed North Atlantic sea surface temperature records13,15 but not others16–18.

In the North Atlantic, the ratio of radioisotopes 231Pa and 230Th in bulk sediment is a dynamic tracer sensitive to the AMOC strength changes19–22. Unlike their radioactive decay parents, 238U and 235U, which are homogeneously distributed in seawater, 231Pa and 230Th are readily scavenged by particles raining down through the water column. Because of a difference in their timescale of removal, a strong AMOC preferentially exports 231Pa out of the North Atlantic23,24 and leaves behind a low 231Pa/230Th (hereafter Pa/Th) in underlying sediments19,20,22,23. In turn, a weakened AMOC leads to a high Pa/Th approaching the production ratio (0.093) in sediments deposited in the deep North Atlantic.

Here, we report Pa/Th in three sediment cores collected from sites in the western and central North Atlantic (Fig. 1). IODP Site U1313 (41° 0.081’N, 32° 57.421’W, water depth 3413 m) and V30-099 (43° 08.9’N, 32° 26.9’W, water depth 3594 m) sit on the western flank of the Mid-Atlantic Ridge. Core KNR191-CDH19 was retrieved from the Bermuda Rise (33° 41.443’N; 57° 34.559’W, water depth 4541 m). These cores were chosen because they are from locations shown to record circulation dynamics and water mass mixing19,22,25.

Results
The three benthic δ18O records show distinctive features in common that facilitated the target alignment to establish a shared temporal framework (Fig. 2B). The variability of the benthic δ18O record from V30-99 suggests the sediment from this core may have experienced post-depositional mixing, possibly as a result of the core’s relatively low sedimentation rate (~1.5 cm/kyr) and hence susceptibility to bioturbation (Fig. 2B). However, the V30-99 benthic δ18O–based age model is supported by an independent age model based on the constant deposition of 230Th excess and a fixed focusing factor during the study interval26 (Supplementary Fig. 2). The age models created by the two methods differ by at most 847 years. Additionally, we generated the benthic δ18O record for the entire last glacial cycle in V30-99, further bolstering the identification of Marine Isotope Stages (MIS) 5d and 5e in this core (Supplementary Fig. 3).

The benthic δ18O-based age models allow us to compare the three Pa/Th records on a common age scale (Fig. 2C–E). A consistent pattern emerges, indicating that Pa/Th increased at the LGI at all three sites, rising close to, but never quite reaching, the production ratio of 0.093. The elevated Pa/Th contrasts with the relatively low Pa/Th at each location during MIS 5e and after the LGI. In V30-99, Pa/Th increased to the production ratio at ~135 thousand years ago (ka), probably a signal of Heinrich event 11 during the penultimate deglaciation13. During the span of MIS 5e, all three Pa/Th records show a decreasing trend starting from an already low Pa/Th, likely indicating a continued AMOC strengthening after the recovery from Heinrich event 11. The 2σ uncertainty of Pa/Th is relatively high in CDH19 because the sediments have a lower percentage of scavenged 230Th relative to the total 230Th measured. The scatter plots between the preserved opal content and the Pa/Th data show a weakly negative relationship in V30-99 and a weakly positive relationship (R2=0.03019 and R2=0.1892, respectively) in U1313 and CDH19 (Supplementary Fig. 4). A comparison between the Pa/Th and preserved opal time series in V30-99 shows that the opal content stays around 1.5% during 100-135 ka despite Pa/Th increases during the LGI and H11 (~135 ka) (Supplementary Fig. 5). In U1313, the opal content shows relatively more variability but generally is around 2%, although the opal content measurements missed the Pa/Th increases (Supplementary Fig. 5). In CDH19, the opal content is 2–6% and does not seem to covary with Pa/Th (Supplementary Fig. 5). Notably, the increase in opal content after 110 ka is not associated with a concurrent increase in Pa/Th. Our comparison of the opal content and Pa/Th data thus indicates that the opal contents in the three cores are generally low, and the small variations in opal content are unlikely to explain the observed changes in Pa/Th, indicating that the observed Pa/Th increases primarily reflect changes in circulation rather than the preferential scavenging of 231Pa by biogenic opal27.

In addition to opal flux, the basin-wide total particle flux is indicated as another factor to potentially bias the Pa/Th proxy28. We calculated the 230Th-normalized particle flux at our three sites (see “Methods”). During the LGI, the particle flux increased at CDH19, was in an increasing trend but did not reach its maximum at V30-99, and did not increase at U1313 (Supplementary Fig. 6). The scatter plots between particle flux and Pa/Th (Supplementary Fig. 7) show a weak relationship between particle flux and Pa/Th in every case. At CDH19, the only core where the relationship is positive, the R2 value is 0.07. Since these are correlations, they do not require any causality, but they provide a maximum estimate of the influence of one variable on the other. That means that particle flux has at most a 7% influence on the variance of sedimentary Pa/Th in those cases, and possibly less, from the perspective of linear modeling with least squares estimation. We infer that 93% or more of the influence on Pa/Th derives from something other than particle flux, which we interpret to be changing ocean circulation. Although we can’t absolutely rule out the possibility that there are other influences, the sedimentary data are inconsistent with a dominant influence of particle flux.

The timing of the Pa/Th increase is roughly in the middle of the transition from MIS 5e to 5d. In U1313 and CDH19, the Pa/Th increase occurs after the respective increasing trends in benthic δ18O from MIS 5e to 5d are underway. In V30-99, the Pa/Th increase is nominally spread over a relatively long time (~8 kyr, compared to the <1-kyr duration in U1313 and CDH18), again a likely sign of the potential influence of post-depositional sediment mixing.

Methods
The sediment samples from V30-99 and U1313 were freeze-dried, soaked in deionized water, and disaggregated on the Cambridge washing wheel disaggregator for an hour. The wet samples were washed through 63 μm sieves with the help of the Lamont automated sample sieving bench, and the coarse fraction retained in the sieves was dried and transferred to glass vials. The dried >63 μm fraction was again dry sieved at >150 μm and examined under a microscope. In core V30-99, the benthic foraminifera Cibicidoides wuellerstorfi tests were picked. In core U1313, the tests of both Cibicidoides wuellerstorfi and Uvigerina spp. were picked. The ẟ18O measurements on the benthic foraminifera tests were conducted with a Thermo Delta V Plus gas-source isotope-ratio mass spectrometer equipped with a Kiel IV individual acid-bath sample preparation device at the Lamont-Doherty Earth Observatory of Columbia University stable isotope laboratory. The ẟ18O records were corrected to Uvigerina using an offset of 0.64‰ for Cibicidoides59. The long-term standard deviation of ẟ18O measurements made on carbonate standard NBS19 is 0.06‰. At depths with abundant benthic foraminifera tests, up to four separate stable isotope analyses were carried out. In core CDH19, a benthic ẟ18O record measured on Cibicidoides wuellerstorfi and Nuttallides umbonifera was previously made public21.

The chronostratigraphies of the three cores were established by aligning the benthic ẟ18O records to the LR04 global stack1 using the open-source BIGMACS software (https://github.com/eilion/BIGMACS)60. While the automated BIGMACS alignment procedure performs generally well, because of how short the alignment period is, we also added two alignment data points using the built-in additional age control function. First, a 172 cm sediment depth in V30-99 is given an age of 118 ka. Second, 3322 cm sediment depth in CDH19 is given an age of 115 ka.

Bulk sediment samples of ~100 mg were spiked with 229Th, 236U, and 233Pa, digested, purified61, and analyzed for uranium, thorium, and protactinium isotopic activities. Isotopes were measured on an Element 2 inductively coupled plasma mass spectrometer (ICP-MS) using either Nickle Jet sample cone and X skimmer cone (for Pa) or Standard sample cone and X skimmer cone (for U and Th), and coupled to either a CETAC TM Aridus desolvating nebulizer (For Pa) or an ESI-PC3 Peltier cooled cyclonic spray chamber (for U and Th) at the Lamont-Doherty Earth Observatory of Columbia University. 238U and 232Th were measured in analog mode, and the rest of the isotopes were made in ion counting mode. Tail corrections and mass bias corrections were made, and the analog/counting gain was calculated61. Every batch of 18 samples was accompanied by a procedural blank, an internal standard called the North Atlantic Internal Mega Standard (NAIMS), and a 233Pa/231Pa mixture solution to track the decay of the 233Pa spike since its creation. The procedural blanks from the 11 batches contribute, on average, 3% of the 238U measured from samples, 0.4% of 230Th and 232Th, and 0.9% of 231Pa. The repeated measurements of internal standard NAIMS from the 11 batches determined the 1σ precision to be 8.5% for 238U, 3.4% for 230Th, 8.2% for 232Th, and 9.6% for 231Pa.

The radioactive isotopes 235U (half-life: 704 million years) and 238U (half-life: 4.5 billion years) are highly soluble in seawater and have long residence times (~400 kyr). In contrast, their decay products, 231Pa (half-life: 32.7 kyr) and 230Th (half-life: 75.584 kyr), are highly insoluble and readily scavenged by sinking particles62. The residence time of 231Pa (100–200 years) is shorter than that of 230Th (20–40 years)63 and approaches the Atlantic deep water transit time. As a result, a vigorous AMOC, such as the condition today, exports approximately half of the 231Pa produced in the Atlantic basin towards the Southern Ocean23,24. A weaker AMOC leads to more 231Pa being deposited in the Atlantic sediments, pushing Pa/Th higher to approach its production ratio (0.093).

The measured bulk sediment concentrations of 230Th and 231Pa include contributions of the detrital (produced from the radioactive decay of U in mineral lattices) and authigenic (from the radioactive decay of U that precipitated from the soluble form U(VI) to its insoluble form U(IV) in anoxic, reducing sediments) fractions. In calculating the scavenged portions of 230Th and 231Pa, these other two sources need to be accounted for and corrected. Detrital 230Th and 231Pa can be estimated from the measured concentration of 232Th, which is entirely of detrital origin64. We apply site-specific lithogenic 238U/232Th activity ratios, using 234U/238U to gauge the presence of authigenic U65. After excluding samples with 234U/238U more than 0.96 to account for the loss of 4% of 234U from the detrital sediments by alpha-recoil65, we also excluded one 238U/232Th outlier data point in V30-99 (encircled in Supplementary Fig. 11B). We additionally found samples with abnormally low 234U/238U in U1313 compared to the adjacent samples, all from a single batch of measurements, which we have excluded as well (encircled in Supplementary Fig. 11C). The average of the remaining 238U/232Th data points in each core is used as the local detrital 238U/232Th. The resulting local detrital 238U/232Th estimates are 0.48 at V30-99, 0.57 at U1313, and 0.52 at CDH19. We additionally apply a lithogenic 230Th/238U activity ratio of 0.8166 and a natural 235U/238U activity ratio of 0.04667. We note that Pa/Th is not as affected by the specific choice of lithogenic 238U/232Th activity ratio as other uranium series proxies68,69. Authigenic 230Th and 231Pa are estimated from the non-detrital portion of 238U and 235U by assuming a seawater 234U/238U activity ratio of 1.146870 and correcting for time passed since uranium precipitation.

The 1σ uncertainty of the measured isotopes is estimated with the standard deviation of the 200 scans of the isotopes by the ICP-MS. We detect and remove outliers of the 200 scans using the modified z-score71: Mi = xi - x / MAD, where Mi is the modified z-score, xi is the value to be analyzed, x is the median, and MAD is the median absolute deviation. Outliers are defined as values with a modified z-score greater than 2. The uncertainty propagation considers the ICP-MS intensity drift during the 200 scans of each sample and standard (National Institute of Standards and Technology Standard Reference Materials Uranium Standard or SRM). The accepted ratio of the SRM 238U/235U is 137.7145, and we apply a relative standard deviation of 0.5% to the ratio in error propagation. We estimate the relative 1σ uncertainty of the lithogenic 238U/232Th and 230Th/238U activity ratios to be 5%, also propagated while calculating the uncertainty. The conversion from raw protactinium counting data to activities and associated error propagation has been packaged into a Python script named PaxsPy, accessible at https://github.com/yz3062/PaxsPy. The counterpart Python script for thorium has previously been published66 at https://github.com/yz3062/ThxsPy.

To test whether the preferential scavenging of 231Pa by biogenic opal influenced our Pa/Th results27, we measured biogenic opal in V30-99, U1313, and CDH19 following two established methods, one utilizing spectrometry72 and the other with inductively coupled plasma optical emission spectroscopy (ICP-OES)73. At the Bermuda Rise site of CDH19, Pa/Th has previously been shown to be little affected by biogenic opal20. For the spectrometry method, bulk sediment samples of ~100 mg were mixed with sodium carbonate and heated at 85 °C for 5 h to extract opal. Silica concentration was measured with a molybdate-blue spectrophotometry method. For the ICP-OES method, a sub-sample of 0.3 ml from the leachate was added to 10 ml of Milli-Q water in 15 mL centrifuge tubes and neutralized with HNO3 to pH = 6. The final volume was then adjusted to 12 mL with additional Milli-Q water. Silicon concentrations were quantified using an Agilent 720 (axial) ICP-OES housed on the LDEO campus of Columbia University. The standard curve used was brought up in a matching Na2CO3 matrix to minimize any matrix effects between the samples and standards. The standard curve encompassed the expected range of the samples. A drift solution, made of a mixture of samples, was run after every 5th sample to monitor and correct for any changes in sensitivity. Silicon was measured on four different wavelengths (185.005 nm, 250.690 nm, 251.611 nm, and 288.158 nm), all of which yielded good signal intensity. The concentration of Silica was calculated based on each individual wavelength, and they were then averaged together for a final value. Percent opal was estimated from Si concentrations using a formula weight conversion factor of 2.4.

To assess whether particle flux could affect our Pa/Th results, we calculated particle flux F = β * Z / 230Thxs,0, where F is the vertical particle flux, β is the 230Th production rate in the water column, Z is the water depth, and 230Thxs,0 is the excess 230Th corrected for radioactive decay (i.e., the denominator of Pa/Th).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review report\nZhou et al. reconstructed the strength of AMOC during Last Glacial Inception (LGI) from Pa/Th records in 3 sediment cores\nretrieved from the North Atlantic. They interpreted the Pa/Th ratios being close to, but not reaching, their production ratio of\n0.093 during LGI as weakening of AMOC, but not to the extent of the weakening observed elsewhere during Heinrich\nevents. Based on these interpretations, the authors discussed three existing proposals for the causes of\nthe AMOC\ndisruption during the LGI, and implications to the importance of ocean processes in setting atmospheric CO2 changes. The\nauthors carried out reconstruction of AMOC for a period (LGI) that is important but understudied compared with other periods.\nGiven that the results are fully justified, this work could fill in the knowledge gap about AMOC and its implication to the\nclimate relevant to the current and future climate changes. The study produced relevant datasets to support the discussion,\nwith good amount of Pa/Th data, supported by benthic d18O and biogenic opal. The manuscript was well presented with\nclear and accessible texts and figures. Previous studies were well considered, and cited appropriately to support the\ndiscussion of this study. Before accepting the manuscript for publication, however, there are a few major points that need to\nbe addressed.\n1. Interpretation of Pa/Th records as variation in AMOC intensity: The results and discussion rely strongly on AMOC as the\nprimary control of the variations in the Pa/Th ratios observed in the sedimentary records. However sedimentary Pa/Th\nrecords are also influenced by particle flux and composition. The authors ruled out the influence of opal flux based on little\nrelationship between opal flux and Pa/Th ratios (Line 82-84). There are two points that require further clarification here:\n1.1. Biogenic opal fluxes were calculated from opal weight percentage normalized to 230Th concentrations. However,\ncorrection of the presence of detrital 230Th in the measured 230Th assumes the same 238U/232Th ratio of 0.48 for all three\ncores (line 360). This assumption might not be valid due to the different sedimentary settings the three cores were located,\nwith one of the cores (CDH19) in a different basin from the other two cores. The choice of this ratio as the authors pointed out\nmight not affect Pa/Th ratios significantly, but might affect 230Th concentrations alone, and hence calculation of opal flux\nusing 230Th concentrations. It therefore requires either justification of using the same 238U/232Th ratio to correct for 230Th\nconcentrations for all three cores, or adoption of local 238U/232Th instead (Bourne et al., 2012).\n1.2. Apart from biogenic opal flux, particle flux (total of CaCO3, detrital, organic matter, and opal), i.e. scavenging intensity\nnot differentiating composition, could also affect sedimentary Pa/Th ratios (e.g. Missiaen et al., 2020). There lacks\ndiscussion of particle flux and justification that variation in particle flux could be ruled out for interpretation of Pa/Th in the\nmanuscript.\n2. Pa, Th, and U data: The description of the methods to obtain Pa, Th and U data in Methods does not allow evaluation of\nthe quality of the data. I suggest that the details below be included.\n2.1. Measurement on ICP-MS: there lacks information on instrument set-up, ICP-MS method adopted, corrections if any,\nprocedural blanks, etc. If this is a routine measurement published in previous studies, such studies should be provided as\nreference.\n2.2. Uncertainty and error propagation: details of how the uncertainties were assessed are lacking. Brief summary of\nuncertainties for Pa/Th data in all three cores should be provided. Although the authors provided the link to the Python\npackage used for error propagation, brief description of the types of errors that were accounted for and the propagation\nmethodology should be included in Methods.\n3. Discussion: I have a major concern here about the writing of this section. The authors described mechanisms proposed in\nprevious studies in great length (three long paragraphs), but there is limited discussion (relative to the length of the\ndescription of the mechanisms) about the results of this work for or against the mechanisms. Similar to this is the discussion\nof how the results of this study explain moisture initiators, and linkage to CO2 drawdown. For instance, the authors pointed\nout the coincidence between AMOC weakening and delayed CO2 drawdown, but continued with explaining the delay of\nCO2 drawdown by mechanisms proposed in other studies. The results of this study could have been given more focus and\ndiscussion, for instance, to establish the linkage between AMOC weakening and CO2 decrease.\nMinor comments:\n1. Figure 1: check orange symbol for CDH19 on the map is the same size as the other two (it seems smaller).\n2. Line 46: suggest changing “radioisotopic decay” to “radioactive decay”.\n3. Line 46-47: suggest changing the sentence “unlike the radioactive decay parents 238U and 235U, … , their decay\nproducts 231Pa and 230Th are readily… “.\nto, “unlike their radioactive decay parents 238U and 235U, … , 231Pa and 230Th are readily…. “.\n4. Figure S4: error bars should be included.\nReferences:\nBourne, M.D. et al. (2012) ‘Improved determination of marine sedimentation rates using 230Thxs’, Geochemistry,\nGeophysics, Geosystems, 13(9). Available at: https://doi.org/https://doi.org/10.1029/2012GC004295.\nMissiaen, L. et al. (2020) ‘Modelling the impact of biogenic particle flux intensity and composition on sedimentary Pa/Th’,\nQuaternary Science Reviews, 240, p. 106394. Available at: https://doi.org/https://doi.org/10.1016/j.quascirev.2020.106394.", "reviewer_2": "This study investigates the signature of sedimentary 231Pa/230Th extracted from cores in the deep North Atlantic during the\nLast Glacial Inception (119-113 ka) in order to understand, how the AMOC responded to this climatic event that has, in\ncontrast to glacial terminations, not received much attention despite its potential importance and implications for the future\nevolution of climate, given that the next glacial inception will occur in the not so distant future. The exact\nmechanisms are important to understand given that it is not clear what the consequences of man-made climate change during the upcoming\ninception will be.\nThe manuscript presents three new 231Pa/230Th data sets showing positive excursions of different duration during the LGI\nperiod, which are, however, not very pronounced in at least two of these three records. In many cases the data presented\nduring the LGI are not significantly different outside analytical uncertainties from the data before or afterwards. This is\nparticularly the case for Core CDH19, in which the data during MIS5e within error were as high as during the LGI. In the\n231Pa/230Th data set presented for CDH19 and Site U1313, the LGI excursion is defined by only two data points within the\nLGI. What duration of a phase of weakened AMOC would these data indicate (are we really talking about 1 kyr)? In this\nrespect, it is therefore important to present any supporting evidence available for simultaneous climatic events\naccompanying a weaker AMOC during the LGI from other data set as part of the manuscript, which the authors have tried to\ndo in Fig. 2. However, the d13C data and IRD data sets presented show a high variability and to me it is unclear if there are\nany significant minima that could have occurred contemporaneously with a significant short term AMOC weakening during\nthe LGI. The timing of the changes of the measured proxies and the duration of potentially LGI-related changes differs\nstrongly and it is therefore not clear if they reflect the same events and forcings.\nIn addition to the abovementioned limitations, there is obviously no evidence for a major weakening of the AMOC during LGI\nin any other North Atlantic Pa/Th data sets (see figure S5). Why is this or, if turned around, why is the LGI signal visible in\nonly two cores of this study (the CDH19 Pa/Th increase is not statistically significant)? The North Atlantic basin-wide\nintegrated Pa/Th data should show a Pa/Th increase during the LGI or a part of it to support AMOC weakening, which I\ncannot see. In addition, the modelled significant AMOC reduction (Yin et al., 2021) comprises the entire LGI period, which is\ndefinitely not indicated by the Pa/Th data. And in some records, such as MD95-2037, the MIS5e Pa/Th data are even higher\nthan those during the LGI. In that respect, instead of only showing the correlations in S4, I would have liked to see a plot,\nin which the opal time series of the cores are compared to the Pa/Th time series in order to check if there were no simultaneous\nchanges in both data sets that may explain at least a part of the observed Pa/Th variability.\nIn summary, this is overall a well written manuscript but, as outlined above, I am unfortunately not convinced of the\nsignificance of the variations in the new and old Pa/Th time series as indicators of a pronounced AMOC change during the\nLGI. I therefore must recommend rejection of the manuscript. In order to further pursue the investigations of glacial inceptions\nand their impact on AMOC changes in the frame of future work, which I clearly deem worthwhile, it would be worth looking at\nother and better resolved sediment cores and/or other glacial inception periods such as the transitions from MIS 5a to MIS 4\n(the error bars of the Pa/Th data would be smaller because there is still more initially adsorbed 231Pa present in the\nsediments of that age) and combining Pa/Th data with other proxies for AMOC changes such as Nd isotopes.\nAdditional smaller comments:\nLine 156:…could have been enhanced…\nLine 159-160: I think the word ”believe” should not be used here."}, "author_response": "Response #1 – We agree with the reviewer that adopting local detrital 238U/232Th ratios makes\nmore sense, and we have done so and updated the relevant figures and Methods (Line 438)\naccordingly. The detrital 238U/232Th, which is derived from the minimal 238U/232Th at each site, is\n0.22 at V30-99, 0.36 at U1313, and 0.42 at CDH19. As noted by the reviewer, the resulting\nPa/Th records do not deviate much from the original (see figure below for a comparison of\nFigure 2 using a constant 0.48 ratio for all cores (left) and using local detrital 238U/232Th ratios\n(right)).\nThe reviewer also noted that the local detrital 238U/232Th ratio changes could potentially change\nthe opal flux via changes to the 230Th-normalized mass flux. We show that the opal flux vs.\nPa/Th scatter plot (Fig. S4) mostly retains the same negative trend after the detrital 238U/232Th\nratio changes. In the figure below, the left panels were calculated with a constant 0.48 ratio for\nall cores, and the right panels were calculated using local detrital 238U/232Th ratios. Note that we\nmade more opal measurements in response to Reviewer 2’s comments, shown in the right panels.\n1.2. Apart from biogenic opal flux, particle flux (total of CaCO3, detrital, organic matter, and\nopal), i.e. scavenging intensity not differentiating composition, could also affect sedimentary\nPa/Th ratios (e.g. Missiaen et al., 2020). There lacks discussion of particle flux and justification\nthat variation in particle flux could be ruled out for interpretation of Pa/Th in the manuscript.\nResponse #2 – We thank the reviewers for thinking more deeply and\ncomprehensively about the impact particle flux may have on Pa/Th. We have dedicated a\nparagraph to discuss the potential complication from particle flux (Line 98) and added a citation\nto Missiaen et al. (2020):\n“In addition to opal flux, the basin-wide total particle flux is indicated as another factor to\npotentially bias the Pa/Th proxy (Missiaen et al., 2020). We calculated the 230Th-normalized\nparticle flux at our three sites (see Methods). During the LGI, the particle flux increased at\nCDH19, was in an increasing trend but did not reach its maximum at V30-99, and did not\nincrease at U1313 (Fig. S6). Although the particle flux changes at CDH19 possibly has an\nimpact on the Pa/Th record, previous Pa/Th records at Bermuda Rise (Böhm et al., 2015; Henry\net al., 2016; McManus et al., 2004), where CDH19 was retrieved, have been corroborated by\nother proxies (Lynch-Stieglitz et al., 2014; Thiagarajan et al., 2014). A previous basin-wide\ncompilation of particle flux (Zhou and McManus, 2024) also shows no dramatic changes during\nthe LGI (Fig. S7).”\n2. Pa, Th, and U data: The description of the methods to obtain Pa, Th and U data in Methods\ndoes not allow evaluation of the quality of the data. I suggest that the details below be included.\n2.1. Measurement on ICP-MS: there lacks information on instrument set-up, ICP-MS method\nadopted, corrections if any, procedural blanks, etc. If this is a routine measurement published in\nprevious studies, such studies should be provided as reference.\nResponse #3 – Agreed. We have added information on the ICP-MS procedure (Line 388):\n“Isotopes were measured on an Element 2 inductively coupled plasma mass spectrometer (ICP-\nMS) using either Nickle Jet sample cone and X skimmer cone (for Pa) or Standard sample\ncone and X skimmer cone (for U and Th), and coupled to either a CETACTM Aridus desolvating\nnebulizer (For Pa) or an ESI-PC3 Peltier cooled cyclonic spray chamber (for U and Th) at the\nLamont-Doherty Earth Observatory of Columbia University. 238U and 232Th were measured in\nanalog mode, and the rest of the isotopes were made in ion counting mode. Tail corrections and\nmass bias corrections were made, and the analog/counting gain was calculated(Fleisher and\nAnderson, 2003). Every batch of 18 samples was accompanied by a procedural blank, an internal\nstandard called the North Atlantic Internal Mega Standard (NAIMS), and a 233Pa/231Pa mixture\nsolution to track the decay of the 233Pa spike since its creation. The procedural blanks from the\n11 batches contribute, on average, 3% of the 238U measured from samples, 0.4% of 230Th and\n232Th, and 0.9% of 231Pa. The repeated measurements of internal standard NAIMS from the 11\nbatches determined the 1σ precision to be 8.5% for 238U, 3.4% for 230Th, 8.2% for 232Th, and\n9.6% for 231Pa.”\n2.2. Uncertainty and error propagation: details of how the uncertainties were assessed are\nlacking. Brief summary of uncertainties for Pa/Th data in all three cores should be provided.\nAlthough the authors provided the link to the Python package used for error propagation, brief\ndescription of the types of errors that were accounted for and the propagation methodology\nshould be included in Methods.\nResponse #4 – We thank the reviewer for reminding us to be transparent with the error\npropagation procedure and have added text discussing how the uncertainty is constrained by this\nstudy (L425):\n“The 1σ uncertainty of the measured isotopes is estimated with the standard deviation of the 200\nscans of the isotopes by the ICP-MS. We detect and remove outliers of the 200 scans using the\nmodified z-score (Iglewicz and Hoaglin, 1993): 𝑀 = (𝑥𝑖−𝑥̃)/𝑀𝐴𝐷, where 𝑀 is the modified z-score, 𝑥𝑖\nis the value to be analyzed, 𝑥̃ is the median, and MAD is the median absolute deviation. Outliers\nare defined as values with a modified z-score greater than 2. The uncertainty propagation\nconsiders the ICP-MS intensity drift during the 200 scans of each sample and standard (National\nInstitute of Standards and Technology Standard Reference Materials Uranium Standard or\nSRM). The accepted ratio of the SRM 238U/235U is 137.7145, and we apply a relative standard\ndeviation of 0.5% to the ratio in error propagation. We estimate the relative 1σ uncertainty of the\nlithogenic 238U/232Th and 230Th/238U activity ratios to be 5%, also propagated while calculating\nthe uncertainty. The conversion from raw protactinium counting data to activities and associated\nerror propagation has been packaged into a Python script named PaxsPy, accessible at\nhttps://github.com/yz3062/PaxsPy. The counterpart Python script for thorium has previously\nbeen published (Zhou et al., 2021) at https://github.com/yz3062/ThxsPy.”\n3. Discussion: I have a major concern here about the writing of this section. The authors\ndescribed mechanisms proposed in previous studies in great length (three long paragraphs), but\nthere is limited discussion (relative to the length of the description of the mechanisms) about the\nresults of this work for or against the mechanisms. Similar to this is the discussion of how the\nresults of this study explain moisture initiators, and linkage to CO2 drawdown. For instance, the\nauthors pointed out the coincidence between AMOC weakening and delayed CO2 drawdown,\nbut continued with explaining the delay of CO2 drawdown by mechanisms proposed in other\nstudies. The results of this study could have been given more focus and discussion, for instance,\nto establish the linkage between AMOC weakening and CO2 decrease.\nResponse #5 – Agreed. We have added a paragraph (L158) describing how our results could\npotentially support Proposal 1:\n“Our study cannot definitively determine which of the three proposals is the most likely scenario,\nbut we find tentative clues in the particle flux supporting Proposal 1. Specifically, 230Th-\nnormalized particle flux measurements from site CDH19 show an increase during the LGI\nconcurrent with the Pa/Th rise. Particle flux increase could indicate increased export\nproductivity, ice rafting of detrital materials, dust flux, or underwater density flow. These local\nor regional changes are more likely to be caused by processes that originated in the Northern\nHemisphere, which exists in Proposal 1 only. On the other hand, increases in particle flux are\nmuch more muted at sites U1313 and V30-99 during the LGI, and the particle flux increases also\nexist during other periods with little changes in Pa/Th. Therefore, the interpretation that our\nfindings support Proposal 1 is only speculative, and future research on the bi-polar and -subpolar\nregions during the LGI will shed light on the causes of the AMOC weakening.”\nWe have also rephrased the text linking AMOC weakening and CO2 decrease to put the results of\nthis study at the center of the discussion (L205).\nMinor comments:\nFigure 1: check orange symbol for CDH19 on the map is the same size as the other two (it seems\nsmaller).\nResponse #6 – We checked, and at least programmatically, the symbols are supposed to be the\nsame sizes. We do agree with the reviewer that the orange symbol looks smaller and we are\npuzzled by this discrepancy. It is possible that the darker background makes the orange circle\nseem smaller.\nLine 46: suggest changing “radioisotopic decay” to “radioactive decay”.\nResponse #7 – Done.\nLine 46-47: suggest changing the sentence “unlike the radioactive decay parents 238U and\n235U, … , their decay products 231Pa and 230Th are readily… “.\nto, “unlike their radioactive decay parents 238U and 235U, … , 231Pa and 230Th are readily…. “.\nResponse #8 – Done.\nFigure S4: error bars should be included.\nResponse #9 – Done.\nReviewer #2 (Remarks to the Author):\nThis study investigates the signature of sedimentary 231Pa/230Th extracted from cores in the\ndeep North Atlantic during the Last Glacial Inception (119-113 ka) in order to understand, how\nthe AMOC responded to this climatic event that has, in contrast to glacial terminations, not\nreceived much attention despite its potential importance and implications for the future evolution\nof climate, given that the next glacial inception will occur in the not so distant future. The exact\nmechanisms are important to understand given that it is not clear what the consequences of man-\nmade climate change during the upcoming inception will be.\nThe manuscript presents three new 231Pa/230Th data sets showing positive excursions of\ndifferent duration during the LGI period, which are, however, not very pronounced in at least\ntwo of these three records. In many cases the data presented during the LGI are not significantly\ndifferent outside analytical uncertainties from the data before or afterwards. This is particularly\nthe case for Core CDH19, in which the data during MIS5e within error were as high as during\nthe LGI. In the 231Pa/230Th data set presented for CDH19 and Site U1313, the LGI excursion is\ndefined by only two data points within the LGI. What duration of a phase of weakened AMOC\nwould these data indicate (are we really talking about 1 kyr)? In this respect, it is therefore\nimportant to present any supporting evidence available for simultaneous climatic events\naccompanying a weaker AMOC during the LGI from other data set as part of the manuscript,\nwhich the authors have tried to do in Fig. 2. However, the d13C data and IRD data sets presented\nshow a high variability and to me it is unclear if there are any significant minima that could have\noccurred contemporaneously with a significant short term AMOC weakening during the LGI.\nThe timing of the changes of the measured proxies and the duration of potentially LGI-related\nchanges differs strongly and it is therefore not clear if they reflect the same events and forcings.\nResponse #10 – We thank the reviewers for reminding us to interrogate the veracity of the data,\nespecially given the observed changes are relatively small and short-lived. To answer the\nreviewer’s question about the duration of the observed AMOC changes, in V30-99, the two\nPa/Th data points during the LGI represent 1780 years. In U1313, they represent 543 years. In\nCDH19, they represent 623 years. The short duration of the AMOC weakening is similarly\ncaptured by one or two Pa/Th data points in previous studies, such as during H3-10 in Böhm et\nal. (2015) and non-HE stadials in Henry et al. (2016), demonstrating the sensitivity of Pa/Th as a\nproxy to short-term, abrupt AMOC changes.\nWe added two North Atlantic records, a sortable silt record from Hall et al. (1998) and a Cd/Ca\nrecord from Adkins et al. (1997), to our Fig. 2 (see below). We used the accompanying benthic\nδ18O of these two studies to update the age models of the two records for consistency. The\nbenthic δ18O records have been aligned to LR04 using BIGMACS via the same procedure as the\nthree cores of this study (Fig S9, 10).\nSortable silt is a proxy for bottom flow speed, while Cd/Ca is conventionally used as a water\nmass tracer because of its empirical covariation with the deepwater nutrient level. Because these\nproxies target different paleoceanographic processes than Pa/Th, the circulation changes implied\nby these two records differ from our Pa/Th data in timing and magnitude. Nevertheless, they\nprovide more corroborative evidence of a period of circulation instability during the LGI.\nThe reviewer rightly pointed out that the duration of AMOC weakening simulated by Yin et al.\n(2021) is much longer than our results suggest (~7 kyr). However, the speed of AMOC recovery\ncan be highly dependent on the model setup. In the simulations run by Yin et al. (2021), only\ninsolation and CO2 were allowed to vary with time. Using the same version of the model\n(LOVECLIM1.3) as Yin et al. (2021) but including a realistic NH ice sheet evolution, Loutre et\nal. (Loutre et al., 2014) found an LGI AMOC weakening of a similar magnitude as Yin et al.\n(2021) but much shorter in duration (~1.5 kyr) that better resembles our Pa/Th reconstructions\n(their topoGR run as seen in Fig. 4b red curve within). Loutre et al. (2014) suggest that the NH\nice sheets, as they grow in size during the LGI, have an increasingly stabilizing effect on the\nAMOC due to the lower sea surface temperature in the vicinity of the ice sheets that is conducive\nto deepwater formation. Therefore, the longer duration of the LGI AMOC weakening simulated\nby Yin et al. (2021) could be due to the experimental design. Indeed, following the Yin et al.\n(2021) proposal that an insolation threshold exists below which the AMOC destabilizes, the\nduration of the AMOC weakening could vary depending on the insolation threshold, which is\ncontrolled by the model parameters. The simulation of Yin et al. (2021) could easily reproduce\nthe timing and magnitude of the AMOC weakening we observe. This is because a minimum in\nsummer insolation at 65°N occurs at 114 ka, concurrent with or slightly before the Pa/Th\nincreases seen in our records. A lower insolation threshold for AMOC weakening, perhaps when\nthe model includes NH ice sheet evolution, would then be consistent with our observation.\nThe IRD flux record (Starr et al., 2021) is also longer in duration than the Pa/Th increases in our\nrecords. The longer duration of the IRD flux peak could be due to the way IRD flux was\ncalculated. The IRD flux in that study was derived from the benthic δ18O-based age model and\nthe resultant linear sedimentation rate. This method of calculating flux leads to over-smoothing\nthe flux because of the limited age model resolution achievable and the interpolation required –\nunderestimating the flux peaks and overestimating the flux troughs. This is especially true during\nIRD-rich zones in the sediment, where benthic foraminifera can be rare, and the resolution of the\nbenthic δ18O record within the zone becomes poor. As a result, the IRD flux record (Starr et al.,\n2021) could potentially be consistent with the duration of the Pa/Th peaks in our records if a\nbetter flux calculation method is used.\nIn summary, our observed AMOC weakening during the LGI is supported by paleo-\nreconstructions and models in the literature. While the literature data differ slightly in timing and\nvariability, they offer corroborative evidence of circulation instability that our new Pa/Th data\nbring into sharper relief.\nIn addition to the abovementioned limitations, there is obviously no evidence for a major\nweakening of the AMOC during LGI in any other North Atlantic Pa/Th data sets (see figure S5).\nWhy is this or, if turned around, why is the LGI signal visible in only two cores of this study (the\nCDH19 Pa/Th increase is not statistically significant)? The North Atlantic basin-wide integrated\nPa/Th data should show a Pa/Th increase during the LGI or a part of it to support AMOC\nweakening, which I cannot see. In addition, the modelled significant AMOC reduction (Yin et\nal., 2021) comprises the entire LGI period, which is definitely not indicated by the Pa/Th data.\nAnd in some records, such as MD95-2037, the MIS5e Pa/Th data are even higher than those\nduring the LGI. In that respect, instead of only showing the correlations in S4, I would have liked\nto see a plot, in which the opal time series of the cores are compared to the Pa/Th time series in\norder to check if there were no simultaneous changes in both data sets that may explain at least a\npart of the observed Pa/Th variability\nResponse #11 – We agree with the reviewer that our Pa/Th data should be compared with the\npreserved opal content to check its influence. We made more opal measurements to double the\namount of data points compared to our last submission. Opal measurements are now also\navailable for core CDH19. A comparison between the Pa/Th and preserved opal time series in\nV30-99 shows that the opal content stays around 1.5% during 100-135 ka despite Pa/Th\nincreases during the LGI and H11 (~135 ka). In U1313, the opal content shows relatively more\nvariability but generally is around 2%, although the opal content measurements missed the Pa/Th\nincreases. In CDH19, the opal content is 2-6 % and does not seem to covary with Pa/Th.\nNotably, the increase in opal content after 110 ka is not associated with a concurrent increase in\nPa/Th. We present opal content to better illustrate the relatively low preserved opal content\nfound in our cores, as the concentration accounts for the fact that the relative opal content in the\ntotal particle flux can potentially influence Pa/Th without changes in the opal flux (Lippold et al.,\n2019, 2016).\nThe scatter plots between the preserved opal content and the Pa/Th data show a weakly negative\nrelationship in V30-99 and a weakly positive relationship (R2=0.0191 and R2=0.1208,\nrespectively) in U1313 and CDH19. Our comparison of the opal content and Pa/Th data thus\nindicates that the opal contents in the three cores are generally low, and the variations in the opal\ncontents are unlikely to explain the observed Pa/Th changes.\nOur finding of low correlation between the preserved opal content and Pa/Th is consistent with\nprevious studies that looked into the same correlation in western and central North Atlantic sites\n(Guihou et al., 2010; Lippold et al., 2019, 2012; Ng et al., 2018), including studies that\ninvestigated the same sites as our study or sites very close to ours (Guihou et al., 2011; Henry et\nal., 2016; Lippold et al., 2016; McManus et al., 2004). Also consistent with previous studies, we\nfind the preserved opal content in the western and central North Atlantic is generally very low.\nAlthough variable opal preservation could complicate the interpretation of the preserved opal\ncontent, the more corrosive Antarctic Bottom Water tends to be found in the eastern North\nAtlantic during periods of North Atlantic Deep Water shoaling (Zhou and McManus, 2023).\nAdditionally, our measured opal contents do not show distinct trends between the last interglacial\nand the LGI. Only minimal opal dissolution probably occurred during the last interglacial when a\nvigorous AMOC bathed our sites with newly formed deep water. Opal dissolution would have to\nmatch the increase in opal production to produce a mostly constant opal content from the last\ninterglacial to the LGI. Therefore, although we cannot rule out that opal dissolution occurred at\nthe three sites of our study and impacted our interpretation of the relationship between opal\ncontent and Pa/Th, we deem the scenario unlikely.\nThe reviewer is right to point out that there was no indication of an AMOC slowdown in the\nexisting literature. The lower resolution of the existing Pa/Th records might have prevented the\nidentification of this event. Since the Pa/Th increase during the LGI does not reach the\nproduction ratio and is relatively short-lived, it is best captured by high-resolution Pa/Th records.\nIn summary, this is overall a well written manuscript but, as outlined above, I am unfortunately\nnot convinced of the significance of the variations in the new and old Pa/Th time series as\nindicators of a pronounced AMOC change during the LGI. I therefore must recommend rejection\nof the manuscript. In order to further pursue the investigations of glacial inceptions and their\nimpact on AMOC changes in the frame of future work, which I clearly deem worthwhile, it\nwould be worth looking at other and better resolved sediment cores and/or other glacial inception\nperiods such as the transitions from MIS 5a to MIS 4 (the error bars of the Pa/Th data would be\nsmaller because there is still more initially adsorbed 231Pa present in the sediments of that age)\nand combining Pa/Th data with other proxies for AMOC changes such as Nd isotopes.\nAdditional smaller comments:\nLine 156:…could have been enhanced…\nResponse #12 – Done.\nLine 159-160: I think the word ”believe” should not be used here.\nResponse #13 – Agreed. We have rephrased this sentence to be:\n“An increase in subpolar North Atlantic sea-ice formation, as laid out in Proposal 1, could have\nfurther suppressed the oceanic moisture supply.”\nLastly, we identified a typo in the caption for Fig. 2. The colored lines in the benthic δ18O panels\nare the averages of multiple data points at the same depth if available, not the three-point\naverages."}, {"round": 2, "reviewer_comments": {"reviewer_1": "As a reviewer, I appreciate all the effort Zhou et al. have made to improve the manuscript based on the reviewer’s comments. However, there is one major concern based on the new figure/data presented in the revised version (Authors’ Response #2). The authors presented in Figure S6 particle fluxes for three cores (minor comment here: x axis label and label of the colour band for LGI and MIS5e were missing). After seeing this new figure, I am not convinced that the authors could rule out the influence of the particle flux in their interpretation of the Pa/Th. For core CDH19, the authors stated that high particle flux was observed and might contribute to high Pa/Th. However the authors then argued that they could be ruled out by referring to other work. McManus et al. (2014) did measure the same core as used in this study. They however presented results without high enough time resolution to provide support for this study. That is why this study providing Pa/Th records that were not presented before could fill the knowledge gap in the first place. On top of that, McManus et al. ruled out the influence of particle flux on Pa/Th by showing higher Pa/Th observed coincide with when lower particle flux was observed. The new result/figure shown in this manuscript could not provide evidence that the similar is true for when high Pa/Th was observed during LGI, with evidence however showing the opposite, i.e. high Pa/Th coinciding with high particle flux. For the same reason, Bohm et al. (2015), quoted in the manuscript, does not also support the authors’ argument. Another quoted article, Henry et al. (2016) presented records spanning 20-60 kyr BP, which is beyond the period of the core in this study. I could not see how these records could be used to rule out the influence of particle flux in this study. The basin-wide particle flux, which show little change during the LGI does not really provide strong support having shown the down core records of variation in particle flux in Fig.S6. For core V30-99, there is increasing trend that coincide with the increase in Pa/Th. There lacks the discussion for this core in the revised manuscript. In summary, having seen the particle flux data shown in Figure S6. and not been convinced by the authors’ approach to rule out the influence of the particle flux on interpreting Pa/Th as solely indicating AMOC strength, I could not recommend this manuscript for publication.\nMinor:\nAuthors’ response #1: The authors adopted the local detrital 238U/232Th as suggested by the reviewer to correct for the presence of detrital 230Th. Their choice of local detrital 238U/232Th is based on the minimum of 238U/232Th in the core. This needs to be clarified with more details. Bourne et al. (2012) suggested that the 238U/232Th ratios that represent the local detrital 238U/232Th are those that are at the depth without the presence of authigenic U. Not having the information about the presence/absence of authigenic U in the cores, it is not possible to make sense why authors’ choice of using minimum 238U/232Th in the core to represent local detrital 238U/232Th.", "reviewer_2": "This is the revised version of a study that I reviewed previously and that investigates the signature of sedimentary 231Pa/230Th extracted from three cores in the deep North Atlantic during the Last Glacial Inception (119-113 ka) in order to understand, how the AMOC responded to this climatic event that has, in contrast to glacial terminations, not received much attention despite that the next glacial inception will occur in the not so distant future.\nThe authors have added some new data for comparison to their data sets and despite that the number of data points capturing the LGI Pa/Th maximum is small, the possibility is high that there indeed was an AMOC instability during the LGI. I am still not 100% convinced that the data obtained from three cores undoubtedly reflect a short AMOC weakening of up to 1800 years but at the same time I cannot disprove it. I am therefore willing to consider the possibility that it occurred. Given the importance of such a finding I think that the community should now decide itself if this weakening may have occurred based on the presented data and at best produce further well resolved 231Pa/230Th records for this period of time to support or disprove the conclusions presented in this manuscript. In the end the 231Pa/230Th proxy is not supposed to show the same signal in each and every core but it is rather the basin-integrated 231Pa deficit (low 231Pa/230Th) that documents a strong AMOC whereas a lack or a decrease of the basin-integrated deficit will document an AMOC weakening (see original publication on the use of 231Pa/230Th as a paleo circulation tracer in the Atlantic Ocean by Yu et al. 1996). As such, further data will improve or disprove the reliability of the presented conclusions.\nIn summary, based on the above considerations I am now supporting publication of the manuscript.\nMinor suggestion for a change of the text in Line 161: This particle flux increase could…."}, "author_response": "Response #1 – We agree with the reviewer that the influence of particle flux on Pa/Th deserves to be examined more closely. We changed the presentation of the manuscript to make a more effective argument (L101). We continue to believe the influence of particle flux on Pa/Th in our records is minimal.\nThe scatter plots between particle flux and Pa/Th (Fig. R1) show a weak relationship between particle flux and Pa/Th in every case, with the strongest correlation at site U1313, where the association is negative, the opposite of any inferred influence of particle flux.\nAt the other two sites, there is an extremely weak relationship, with R2 values of 0.003 and 0.07. Since those are correlations, they do not require any causality at all, but they provide a maximum estimate of the influence of one variable on the other. That means that particle flux has at most a 0.3-7% influence on the variance of sedimentary Pa/Th in those cases, and possibly less, from the perspective of linear modeling with least squares estimation. We infer that 93% or more of the influence on Pa/Th derives from something other than particle flux, which we interpret to be changing ocean circulation. Although we can't absolutely rule out the possibility that there are other influences, the sedimentary data are inconsistent with a dominant influence of particle flux.\nWe also examined the opal, CaCO3, and lithogenic fluxes of V30-99 and CDH19 to discern which sedimentary type drives the particle flux changes during the LGI (Fig. R2). The CaCO3 record from V30-99 is admittedly very low in resolution (Ruddiman and Farrell, 1996). Nevertheless, the data suggest that the causes of particle flux increases in V30-99 and CDH19 are different; CaCO3 flux drives the particle flux increase in V30-99, while lithogenic flux drives the particle flux increase in CDH19.\nLithogenics and CaCO3 scavenge Th more intensely than Pa (Chase and Anderson, 2004; Hayes et al., 2015; Luo and Ku, 2004; Missiaen et al., 2020a, 2020b; Rempfer et al., 2017; Scheen et al., 2025), and an increase in the fluxes of these two particle types, based on fractionation alone, should theoretically lead to a decrease in sedimentary Pa/Th. The particle types may at least partially explain the relatively weak (<7%) influence of the particle flux on sedimentary Pa/Th in V30-99 and CDH19 (Fig. R1). Previous studies have noted that the influence of particle flux on sedimentary Pa/Th can be counteracted by some particle types, including lithogenics (Siddall et al., 2005) and CaCO3 (Missiaen et al., 2020b).\nLastly, the observed LGI Pa/Th increases coincide with changes in several circulation and water mass tracers, as well as the simulation of the AMOC, which provides independent evidence indicating that the AMOC changes are the likely cause of the observed changes (Fig. 2).\nMinor:\nAuthors’ response #1: The authors adopted the local detrital 238U/232Th as suggested by the reviewer to correct for the presence of detrital 230Th. Their choice of local detrital 238U/232Th is based on the minimum of 238U/232Th in the core. This needs to be clarified with more details. Bourne et al. (2012) suggested that the 238U/232Th ratios that represent the local detrital 238U/232Th are those that are at the depth without the presence of authigenic U. Not having the information about the presence/absence of authigenic U in the cores, it is not possible to make sense why authors’ choice of using minimum 238U/232Th in the core to represent local detrital 238U/232Th.\nResponse #2 – We thank the reviewer for reminding us to engage more thoroughly with the literature. Indeed, Bourne et al. (2012) suggested using 234U/238U to gauge the presence of authigenic U. Samples with lower 234U/238U are more likely to have little to no authigenic U, and the average 238U/232Th of these samples can be used to estimate the local detrital 238U/232Th. We have now adopted this approach from Bourne et al. (Fig. R3). After excluding samples with 234U/238U more than 0.96 to account for the loss of 4% of 234U from the detrital sediments by alpha-recoil (Bourne et al., 2012), we also excluded one 238U/232Th outlier data point in V30-99 (encircled in Fig. R3B). We additionally found samples with abnormally low 234U/238U in U1313 compared to the adjacent samples, all from a single batch of measurements, which we have excluded as well (encircled in Fig. R3C). Despite the exclusion of more samples in U1313, the prominent pattern of 238U/232Th increase during the LGI makes the identification of authigenic U relatively straightforward.\nThe resulting local detrital 238U/232Th estimates are 0.48 at V30-99, 0.57 at U1313, and 0.52 at CDH19. This is in contrast to the previous methodology we employed of using the minimum 238U/232Th as the local detrital 238U/232Th, which was 0.22 at V30-99, 0.36 at U1313, and 0.42 at CDH19. The updated detrital 238U/232Th values do not alter our results in any way that would change the conclusions of this study (Fig. R4).\nResponse #3 – We thank the reviewer for the positive assessment of our revised manuscript.\nMinor suggestion for a change of the text in Line 161: This particle flux increase could….\nResponse #4 – Done."}, {"round": 3, "reviewer_comments": {"reviewer_1": "After the previous review of the revised manuscript aimed at reconstructing the strength of AMO during Last Glacial Maximum from sedimentary Pa/Th records, the authors provided additional assessment of the particle type and rewording to address the influence of the particle flux on the interpretation of Pa/Th records. The authors also conducted reanalysis of the correction to obtain the scavenged fraction of the Pa/Th sediments, as suggested in the previous review, and updated the results in this revised version of the manuscript. Both of the work very well addressed the points raised in the previous review. I am happy to recommend the publication of this manuscript.\nMinor:\nFigure S6, and Figure S7: units for particle fluxes are incorrect, missing subscript (Fig. S6), and having typos (Fig. S7)."}, "author_response": "Response #1 – We thank the reviewer for the positive assessment of our work.\nResponse #2 – We thank the reviewer for catching the typos. We have corrected the particle flux units in both figures (see below)."}]

Decision letter