Asian summer monsoon variability across Termination II and implications for ice age terminations

Domain: earth_science
Published: 2025-05-30
Editorial outcome: major_revision

Abstract
The detailed anatomy of Termination I (TI) is well depicted, but whether changes across Termination II (TII) resemble TI remains controversial. Here we present high-resolution Asian monsoon records covering TII using Shima Cave stalagmites from China. Correlating marine and ice-core records to our U/Th-dated records via millennial-scale variabilities, we find an initial CO2 rise from 139±1 ka BP concordant with boreal summer insolation increase, which was followed by a major rise phase of CO2 between 135.7±1 and 129±1 ka BP. The major rise phases of CO2 were comparable during TI and TII, but the initial CO2 rise before TII was distinct from CO2 behavior before TI, likely forced by the Earth’s internal variabilities, in particular an ice-sheet collapse event and a 50% reduction in southern hemisphere dust flux. Here, we show that ~4000–5000-year-long gradual changes in CO2, along with insolation rise, preconditioned glacial terminations, supporting the “tipping point” theory.

Introduction
Quaternary (≈2.6 Ma to present) climate was characterized by a series of alternations between warm interglacial and cold glacial. Records of glacial-interglacial change typically show a “sawtooth” pattern, where gradual ice sheet accumulation and sea level reduction during glacial periods were followed by ice age terminations with rapid ice sheet melting and sea level rise1–3. In an early study by Broecker and Denton4, glacial terminations were ubiquitously associated with millennial-scale oscillations in ocean circulation and ultimately inferred as the cause of CO2 release into the atmosphere. Several subsequent studies support the views of Broecker and Denton4, in particular identifying the critical role of the Atlantic Meridional Overturning Circulation (AMOC)5–8. During ice age terminations, one or two weak monsoon intervals (WMIs) occurred, coinciding with Heinrich (H) events characterized by massive iceberg discharges, where CO2 rise could be caused via a set of mechanisms ultimately linked to increasing boreal summer insolation9. Hence, Asian summer monsoon records provide insights into the feedback and interactions between millennial- and orbital-scale processes.

Current understanding of ocean-atmosphere-cryosphere interactions during ice age terminations mainly relies on the assessment of Termination I (TI, ~18–11.7 thousand years before the present, hereafter ka BP) due to the abundance of archives with reliable, independent age constraints5. Denton et al.5 suggested that the collapse of northern hemisphere ice sheets, due to rising insolation, disrupted the global oceanic and atmospheric circulations, leading to CO2 release from the Southern Ocean that further augmented global warming during the last termination. However, TI may not be representative of previous glacial terminations. An increasing body of evidence from high-resolution ocean and terrestrial records suggests climatic similarities and dissimilarities between TI and TII, where TII is the penultimate ice age termination (TII, ∼136–129 ka BP), containing the transition from the Penultimate Glacial Maximum (PGM, ~140 ka BP) to the Last Interglacial (∼129–116 ka BP)6,10–18. While the massive continental ice sheets disintegrated as the Heinrich (H) event 11 during TII11,19, no clear evidence of a Younger Dryas-like event in the North Atlantic sea surface temperature (SST)14 nor a Bølling-Allerød-like event in the AMOC intensity occurred during TII6. Importantly, Cheng et al.9 observed that monsoon structure during TII was different from TI, lacking the Bølling-Allerød period. Another discrepancy was the relatively higher atmospheric CO2 concentration by >15 ppm at the end of TII than that of TI20. Accordingly, Barker and Knorr8 suggested that Denton et al.’s model5 could not explain the anatomy of all ice age terminations. To better understand why the two most recent ice age terminations exhibited different characteristics, especially the rise in atmospheric CO2 which plays a critical role in regulating the Earth’s temperature, we must investigate the cascade of climatic elements and dynamics before and during the ice age terminations.

In this work, we present a chronological benchmark for the linkage between major ice-rafted debris (IRD) peaks, variations of atmospheric CO2 and weak monsoon shifts across TII based on four high-resolution and U/Th-dated speleothem records from Shima Cave in China. Particularly, we focus on defining the preconditions during the glacial maximum, which might be critical in initiating CO2 release across TII as a way to understand the roles of external solar forcing and internal variabilities in the ocean and atmosphere system during ice age terminations.

Results
Stalagmite samples
Four broken stalagmite samples SM9, SM12, SM16 and SM17 were collected from Shima Cave, Hunan Province, central China (29°35′N, 109°31′E, 650 meters above sea level) (Supplementary Fig. 1). Shima Cave was developed in the Permian limestone and is strongly influenced by the Asian monsoon system. Modern temperature, precipitation, and monthly simulated precipitation δ18O data all show seasonal variations (Supplementary Fig. 1). The mean annual temperature is ~17 °C and the mean annual precipitation is ~1430 mm, with nearly 57% of the precipitation occurring from May to August when abundant low-δ18O-valued moistures are sourced from remote tropical oceans21.

A total of 26 230Th dates for four samples are presented in Supplementary Table 1. Most subsamples have low 232Th content and high 230Th/232Th activity ratios, leading to small, <100 years, initial detrital 230Th corrections and dating uncertainties of 300–500 years. The age models for Shima samples were obtained using the MOD_AGE model22 (Supplementary Fig. 2) because some dates were reversed within dating errors. Only one age outlier exists in SM9 sample, younger than the modeled age by approximately 600 years. According to age models, SM9, SM12, SM16 and SM17 deposited from 126.7±0.8 to 122.6±0.9 ka BP, 131.7±0.7 to 130.8±0.6 ka BP, 142±1 to 133.6±0.6 ka BP and 129±0.9 to 126.2±0.8 ka BP (BP represents 1950 AD), respectively (Fig. 1a).

A total of 1949 δ18O subsamples were analysed and used for proxy reconstruction. In addition, five subsamples of stalagmite SM7 from our previous study of TI at Shima Cave21, were analysed for triple oxygen isotope composition (Δʹ17O) because SM7’s total δ18O range of 5‰ and the record’s completeness over the glacial termination was conducive for defining trends in Δʹ17O and δ18O space (as per meg/‰) that are linked to the driving hydrologic processes23–25.

Interpretive basis and speleothem oxygen isotope records
The Shima speleothem δ18O records are shown in Fig. 1a, and, based on prior work, we interpret them in the context of the intensity of the Asian summer monsoon (ASM), reflecting the integrated rainfall from the Pacific and Indian Ocean moisture sources to the cave21,26,27. Temporal resolution for the record is better than 20 years, with chronologies constrained by most 230Th dates with uncertainties <400 years for the section younger than 130 ka BP, and 400–500 years for the older part. To alleviate the temporal gap of the abrupt Asian monsoon TII in the Shima record, we combined our records with that of Sanbao Cave (270 km distant, Supplementary Fig. 1), which is well-dated and has dating errors of less than 100 years9 (Fig. 2c), by adding a systematic bias of 0.2‰ to SB25 δ18O data. We can accordingly precisely determine the timing of millennial-scale events evident in ice cores and marine sediments by correlating them to the radiometrically dated cave δ18O record (Fig. 2c).

Calcite precipitation in isotopic equilibrium is a prerequisite for using its δ18O as a climate proxy. Three lines of evidence suggest that Shima δ18O records reflect formation in isotopic equilibrium. First, the Hendy test28 was performed on four individual growth layers of each sample. Most δ18O and δ13C variations along the same layer range from 0.1‰ to 0.3‰ (Fig. 1b), and the absence of positive relationships between δ18O and δ13C are consistent with the equilibrium formation of calcite (Supplementary Fig. 3). Second, replication of different stalagmite δ18O records is reliable to test for equilibrium fractionation in carbonates29. Our Shima records display good consistency with other cave records from southern China on the orbital to millennial scales, including abrupt positive shifts of ~2‰ at approximately 136 ka BP and negative shifts of >4‰ from the TII to the Last Interglacial (Supplementary Fig. 4). Third, the Δʹ17O vs. δʹ18O trend of Shima stalagmite SM7 has a slope of ≈1 per meg/‰ (Fig. 1c), consistent with the predominance of rainout processes in determining δ18O. Similar trends could mathematically come from combinations of multiple processes24. However, even combining trends from, for example, rainout (Rayleigh, 0 per meg/‰) and within-cave kinetic (7 per meg/‰) processes would still require an overwhelming 86% of the signal to be coming from climate-related drivers (i.e., 0.86×0 per meg/‰ + 0.14×7 per meg/‰ = 1 per meg/‰). Changes in Δʹ17O can also come from varying the relative humidity during evaporation at the moisture source (−9.5 per meg/‰) and require only a 37.5% contribution from the climate signal (i.e., 0.375×(−9 per meg/‰) + 0.625×7 per meg/‰ = 1 per meg/‰). These calculations, in combination with the fact that modern precipitation in the Asian monsoon region shows weak, slightly negative to slightly positive trends (Supplementary Fig. 5), also support stalagmite formation in (near-)isotopic equilibrium. Ultimately, while these lines of evidence are useful in establishing that the Shima samples formed in (near-)isotope equilibrium, our fundamental interpretations below are about the timing of change, not the exact interpretation of speleothem δ18O, and thus independent of the driving processes.

Timing of the initial rise of CO2 during the Penultimate Glacial Maximum
Based on a mechanical link of abrupt shifts in ASM (under whose influence the wetland is one of the major sources for atmospheric CH4) and CH4 in Antarctic ice cores during ice age termination9,30, we compare the Shima-Sanbao record with CH4 and CO2 records from the EDC ice core on the AICC2012 chronology31,32. Apart from the nearly-consistent abrupt intensification/rise in ASM and CH433 at around 129 ka BP (blue dashed line in Fig. 2b, c), a small peak of CH4 at ~134 ka BP is possibly tied to a short-lived strengthening of the ASM, which has been confirmed in a previous study34 and further constrains the chronology of CO2 and CH4 records for older sections. All three atmospheric CO2 records17,20,31,35 began to rise at ~140 ka BP until 129 ka BP, lasting for ~11,000 years with a total rise of ~100 ppm (Fig. 2a). The onset timing of CO2 rise cannot be determined more precisely due to the low-resolution data of CO2 at ~140 ka BP. However, considering that: (i) the linkage of North Atlantic IRD events or H stadials with rises of CO2 through a mechanism of the AMOC weakening that promotes the release of CO2 from the Southern Ocean to the atmosphere5 and (ii) good correlations of WMIs and IRD events9,21,26, here we can provide constraints on the timing of initial CO2 rises by bridging the ice core, marine and Shima-Sanbao cave records.

There is a broad similarity between our cave record and the North Atlantic SST record at the site of MD01-244436. The intensive monsoon weakening and strengthening, indicated by >2‰ changes in δ18O values, were associated with a cooling of ~4 °C at around 136 ka BP and large-amplitude warming at the end of TII in the North Atlantic (black dashed lines in Fig. 2c, d). The WMI at around 139 ka BP, as evidenced by a positive shift of 1.3‰ in Shima δ18O record and also observed in the Hulu, Sanbao and Dongge records (Supplementary Fig. 4), was coherent with a ~1 °C cooling event in the North Atlantic (Fig. 2d). Since the chronologies for ODP 984 and MD01-2444 cores were well aligned by tie points36, major IRD peaks of H11 and H12 events in core ODP 98436,37 (Fig. 2e) could also be well linked to the sequence of WMIs in the cave record. Obvious peaks for H12 event in other IRD records on independent age models varied between 140 and 138.5 ka BP, and H11 event showed a more complex structure with its onset at approximately 136±1.5 ka BP (Supplementary Fig. 6). These two major IRD peaks were consistent with AMOC weakening, and tightly correlated with WMIs (Supplementary Fig. 6). Based on the evidence presented, we propose that H12, aligning to a WMI event dated at 139±0.6 ka BP, possibly contributed to the initial rise of CO2 through the weakening or shutdown of the AMOC5,9 (Supplementary Fig. 6e, f). The timing of WMI in response to H12 during the PGM in the Shima record is consistent with other independently dated Chinese cave records within their dating errors (Supplementary Fig. 4). By evaluating 230Th age control for cave records (Supplementary Fig. 4) and the original age model for ice cores31,32, we suggest that the initial rise of CO2 for TII occurred at 139±1 ka BP.

Establishing the analogy of the last two ice age terminations
The existing analogy of ice age terminations has been proposed6,8,9, and these hypotheses involve millennial-scale internal variabilities in the Earth’s climate system that setup deglaciations. Here we invoke similar processes in CO2 rise related to monsoon intensity changes to establish an analogy between TI and TII, since CO2 functions as an essential amplifier of global temperature38 and an important tipping forcing for ice age terminations39.

We applied the Change Point detector in software Acycle 2.8 to the EDC CO2 record31 and obtained an abrupt rise of CO2 at 17.4±0.2 ka BP during the early last deglacial period (Supplementary Fig. 7). This result is supported by the determination of the onset of CO2 rise in ref.38. Following the same procedure, we determined the start of the major rise phase of CO2 during TII to be 135.7±2 ka BP (Supplementary Fig. 7). Considering that the tie points of CH4 jump and fast monsoon recovery in the cave record can constrain the timing of CO2 record as discussed above, we narrowed the uncertainty from ±2 to ±1 ka.

After obtaining the onsets of rapid CO2 rises for the last two ice age terminations, we established their analogy (Fig. 3). Changes in monsoon intensity inferred from cave records and Antarctic temperature inferred from ice δD records are comparable. Although the internal sequences of monsoonal events are distinguished between TI and TII9, analogous changes in cave records exist, including: (i) similarly abrupt weakening and fluctuations in the ASM occurring at the transitions from glacial maximums to TI/TII (green bar in Fig. 3), and (ii) a rapid monsoon intensification after the long-term WMIs which marks an end of ice age termination (yellow bar in Fig. 3). The onset and end of TI/TII bracketed the major rise phase of CO2, which lasted ~6000–7000 years. The TI and TII shared a similarly changing amplitude of ~80 ppm during the major rise phases of CO2, regardless of different substages within them (Fig. 3a). Besides, Antarctic δD records40 fluctuated around −440‰ during both glacial maximums and then took similar amounts of time through deglacial processes to reach their interglacial plateaus, despite the interruption of Antarctic Cold Reversal in TI (Fig. 3b). Landais et al.41 reported that atmospheric CO2 concentrations and Antarctic temperature started increasing in phase around 136 ka BP, supporting the result of 135.7±1 ka BP here. The beginning of the rapid CO2 rise at around 135.7±1 ka BP was also widely consistent with abrupt changes in a number of oceanic and terrestrial records like the H11 event, cooling in the North Atlantic SST and European surface temperature, as well as the AMOC weakening (Supplementary Fig. 6). Therefore, abrupt shifts in different archives at 135.7±1 ka BP could be critical changes in the global climate system, which shared similar features with the onset of TI10,14,42,43.

Different routes to ice age terminations
While TI and TII have similarities in their initiations and overall CO2 increment, distinct patterns of CO2 change are observed before ice age terminations. During the PGM, CO2 already increased gradually from 139±1 to 135.7±1 ka BP, leading to a total rise of 15–20 ppm before the abrupt increase (Fig. 4c). However, atmospheric CO2 concentration fluctuated steadily around 190 ppm for about 5000 years before the TI onset (Fig. 4C). Both isolation and ice volume provide critical backgrounds for CO2 variations during glacial-interglacial cycles44, but they cannot explain different CO2 behaviors during glacial maximums. Because northern hemisphere summer insolation45 and global ice volume2 during the LGM (Fig. 4A, B) were broadly similar to those during the PGM (Fig. 4a, b), with insolation on the rising limb from minimums and lasting 4000–5000 years, while LR04 δ18O values varying around 5‰. Instead, the internal elements of the Earth’s climate system might account for differences in CO2 variations.

We notice three significantly different conditions during the PGM and LGM: (i) a more unstable monsoon status during the PGM than LGM, (ii) the meltwater pulse (MWP) event 2A11 associated with the H12 occurring at ~139 ka BP46, and (iii) a significant drop in dust flux recorded in the Antarctic ice cores47. During the LGM, the high-resolution speleothem record26 displays a monsoon intensification trend with centennial-scale variations (Fig. 4G). However, during the PGM, a significant 139±0.6-ka WMI is detected in addition to centennial oscillations (Fig. 4g). This WMI was likely tele-connected with the H12 event and the MWP-2A (Fig. 4d, e), as well as the North Atlantic SST cooling (Supplementary Fig. 6). The H12 event occurred around the boreal summer insolation minimum (Fig. 4a), and created small, partially deglaciated ice sheets prior to TII46. Alike the H11 event which caused an intensive reduction of the northern ice sheets and the MWP-2B11, the H12 event could have caused the MWP-2A. Although not as intensive as MWP-2B which contributed ~70% of deglacial sea level rise11, MWP-2A was the early phase of ice-sheet retreat before TII and contributed about 30 meters of sea level rise48,49. The concurrence of the H12 event, MWP-2A, the WMI and the North Atlantic cooling episode (Figs. 2, 4) supports a clear millennial-scale climatic oscillation at around 139 ka BP.

More importantly, the 50% off in dust flux observed at ~139 ka BP (Fig. 4f) could have also contributed to CO2 rise and ultimately arose from the less exposure of the South American continent due to sea level rise and a relatively humid South America. On millennial timescales, the weakening of the ASM is generally coincident with the strengthening of the South American summer monsoon (SASM)50, and South America, especially Patagonia, is the primary dust source for Antarctica51. At around 139 ka BP, the decrease in dust input to Antarctica and the Southern Ocean might be due to relatively humid conditions50 and the well-developed vegetation52 in South America, both associated with a relatively strong SASM status which was antiphase with the ASM intensity. Increased input of Fe-bearing atmospheric dust to the Fe-deficient Southern Ocean may have stimulated the primary productivity of phytoplankton and enhanced oceanic sequestration of CO2 during glacial periods, in contrast to changes observed during interglacial periods53. It was estimated that ~40 ppm of the change in CO2 concentration during the glacial-interglacial transitions could be caused by changing dust export to the Southern Ocean54. Therefore, a combination of dust flux decreases and the H12 event could possibly cause the initial CO2 rise, leading to different CO2 change patterns during the LGM and PGM.

The 15–20 ppm CO2 increase before 135.7±1 ka BP might explain why atmospheric CO2 concentration at the end of TII was >15 ppm higher than that at the end of TI. This initial, gradual rise of CO2 can be regarded, according to the “tipping point” theory39, as an early warning signal that leads to the inevitable CO2 rise at the onset of TII. From the viewpoint of physical mechanisms, the abrupt rise of CO2 since 135.7±1 ka BP was likely due to the increased carbon storage in the stratified Southern Ocean during the preceding PGM55. The early warning signal in CO2 implies that a slow, initial process is important for the shift from glacial to interglacial conditions39,56, which might operate through a way similar to that proposed by Zhang et al.57. By defining the onset of the last two ice age terminations, we find that the major deglacial processes did not occur until ~4000–5000 years after the insolation minimums (Fig. 4), and therefore we suggest that a gradual increase in CO2 following boreal summer insolation rise could precondition the ice age terminations, thus supporting the “tipping point” theory.

Methods
230Th dating method
Stalagmites were halved and polished along the growth axis. All samples have a “candle” shape and are composed of pure and transparent calcite. We did not identify deposition hiatuses in these samples, thus indicating continuous deposition (Supplementary Fig. 2). Dating work was performed at the University of Minnesota and Nanjing Normal University, following standard procedures for U and Th separations58. Twenty-six powdered samples for 230Th dating were obtained by drilling along the stalagmite growth axis with a 0.9 mm carbide dental drill. They were then weighed and dissolved in HNO3 in Teflon beakers containing a known quantity of a 229Th-233U-236U triple spike. U and Th were preconcentrated by coprecipitation with iron hydroxide and then separated by passing the solution through a resin column. The U and Th fractions were then dried and diluted in a mixture of 1% HNO3 and 0.05% HF for analysis on a Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). The U and Th isotopic data were acquired by the secondary electron multiplier protocol and then processed in the Excel spreadsheets or an interactive program written with the MATLAB programming language59,60. Corrections for instrument memory, instrumental mass fractionation, tailing effects, spike contributions, abundance sensitivity, dark noise, and blanks were performed during data processing offline. Decay constants of 234U61, 230Th61 and 238U62 were used. Corrected 230Th ages assume the initial 230Th/232Th atomic ratio of (4.4±2.2)×10−6, and those are the values for a material at secular equilibrium, with the bulk earth 232Th/238U value of 3.8. Most speleothem ages are in stratigraphic order with 2σ analytical errors of roughly 0.1–0.6% (Supplementary Table 1).

Stable isotope analyses
A total of 1885 subsamples from Shima stalagmites were drilled along the growth axis with 0.5 mm carbide dental burs for stable isotope analysis. And 64 subsamples for the Hendy test were drilled on four individual growth layers in each sample. Powder samples were measured using a Finnigan-MAT 253 mass spectrometer coupled with a Kiel Carbonate Device at Nanjing Normal University, China. Results are reported as “delta” values, where δ18O = (R sample / R standard − 1) relative to standard Vienna Pee Dee Belemnite (VPDB), and given in permil (‰) notation (where R is the ratio of the heavy isotope to light isotope for a material, e.g., R = 18O/16O). Replicate analyses of an international standard (NBS 19) indicated long-term reproducibility, with precisions better than 0.06‰ for δ18O.

Five ~50 mg powdered samples were drilled for triple oxygen measurements at the IsoPaleoLab, University of Michigan. Samples were acidified and converted to O2 for analysis on a Nu Perspective mass spectrometer following methods63). Isotope data were corrected with a session specific-normalization and normalized to the VSMOW-SLAP scale64. The Δʹ17O values were secondarily normalized to the Wostbrock et al.65 IAEA-603 value of -100 per meg VSMOW-SLAP via analyses of the equivalent IAEA-603 and IAEA-C1 standard to account for fractionations occurring in acid digestion and reduction steps of sample processing. The Δʹ17O value is defined using “delta-prime” notation as: Δʹ17O = δʹ17O – 0.528×δʹ18O, given in per meg, where δxO = ln(R sample / R standard) (e.g., ref. 66).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript titled \"East Asian summer monsoon variabilities across the penultimate deglaciation and implications for ice age terminations\" by Liang et al. focuses on Termination II (TII) and its differences from Termination I (TI) within the Late Quaternary glacial cycles. The authors argue that a new East Asian summer monsoon (EASM) record from Shima Cave in southern China shows a weak monsoon event at approximately 139,000 years ago, preceding TII. They suggest that this precondition may have influenced various paleoclimatic processes in the Southern Hemisphere and eventually triggered TII. However, I remain unconvinced that the 139,000-year event has global paleoclimatic implications. Furthermore, I find the correlations the authors draw among North Atlantic sea surface temperatures (SST), dust flux, freshwater flux, ice-rafted debris (IRD), Antarctic methane (CH4), carbon dioxide (CO2) levels, deltaD, and dust flux to be overstated, given the resolution and values involved. Therefore, I cannot recommend acceptance of this manuscript for publication in Nature Communications. Detailed suggestions for improvement are provided below:\nLine 11 to 22: Please review the English grammar and ensure uniform decimal points for ages. It appears to me that the current version of the abstract lacks emphasis on the importance and novelty of this study.\nLine 56 to 59: This statement may suggest bias, despite ongoing debates regarding the similarities and differences between TI and TII, as outlined by the authors above.\nLine 81 to 82: Authors should correctly refer to \"Mod Age\" as \"MOD-AGE,\" aligning with its original reference..\nLine 90 to 96: Speleothem δ18O records should be assessed for isotopically equilibrium precipitation, despite the absence of absolute methods for verification. However, the millennial-scale oscillations in the SM record examined here cannot undergo such testing due to the discontinuous dataset of the SM record itself and substantial discrepancies between various Chinese cave records presented in this manuscript (Figure 2). This limitation could significantly undermine the authors' arguments.\nFigure 1: The age models of these stalagmites reveal several age outliers and reversed dates based on their stratigraphic order (Figure S2). Additionally, while Figure 1 and Table S1 present 9 ages for SM16, the MOD-AGE model in Figure S2 shows only 8 ages. To accurately estimate petrography and identify possible hiatuses, section images of the SM stalagmites are essential.\nLine 105 to 106: Reference 26 is not suitable for this sentence, as the SM record in this study has not been replicated on a millennial timescale as suggested by Dorale and Liu (2009).\nLine 133 to 135: Authors should provide more detailed information on the statistical analysis in the Methods section, particularly regarding the merging of data from different datasets\nLine 135 to 138: I am not convinced by this interpretation because XBL and JJ caves are located slightly upstream compared to other cave sites, suggesting that these sites may reflect signals of the Indian Summer Monsoon (ISM) or a combination of ISM and East Asian Summer Monsoon (EASM).\nLine 174 to 177: I am not convinced by such a detailed correlation presented by the authors because individual cave records exhibit significantly different changing patterns over time.\nLine 179 to 183: These assertions seem unreasonable due to the millennial-scale differences among Chinese records and the potential data noise levels (also, sampling resolution) of SST and CH4 records.\nLine 206 to 207: Please revise the figure citation to 'Figure 3, left panel' to conform to the manuscript format of Nature Communications.\nLine 205 to 261: The last section titled 'Different routes to ice age terminations' is overly speculative, as the authors' main arguments lack direct support from the SM records.\nLine 206 to 209: I remain unconvinced by the author's argument, as mentioned previously.\nLine 245 to 247: This explanation does not align with the results presented by the authors.\nLine 250 to 252: I find the authors' estimation of CO2 rise unconvincing, particularly due to the slightly higher value at ~142 ka BP compared to 140 ka BP.\nLine 269 to 270: Instead of \"parts mil,\" you should use \"per mil\" or \"‰\" to correctly denote the unit.", "reviewer_2": "Liang et al use 4 speleothem records from a cave in China to examine H11 and Termination II in the EASM region. They examine how the EASM record fits into the sequence of global events to explore drivers and feedback mechanisms. I think the records themselves are extremely valuable and supported with elaborate discussion.\nI have a few comments that will hopefully improve the manuscript:\n- Since the premise of the paper really seems to be examining the sequence of events, it would be great to see the speleothem age-model uncertainties clearly in the plots and in the text. For example, Figure 1 could include age-model uncertainties as shading, and could show expanded plots of significant time periods e.g. H11. Similarly, the text, should include the full ages and uncertainties in brackets e.g. Lines 93 to 95. Similarly for Figures 2 and 3, it would be really useful to see some consideration of temporal uncertainties on the remaining records of ice melt, CO2, temperature etc as well. You could try something like Fig. 7 in this publication: https://cp.copernicus.org/preprints/cp-2024-37/cp-2024-37.pdf\nLine 52: What does 'ocean-forcing sea level rise' mean\nLines 121 to 125: Does TI show this N-S pattern as well?\nLine 177: Positive biases near 133 and 130 ka BP in the list of caves are not immediately apparent in the figure. Perhaps show these with lines / arrows.\nLines 189 and 190: Is the Figure labelling (Figure 3C) correct? I would also reconsider this, I find it hard to se these positive anomalies in the record.\nLine 223: Should be 4000 years rather than 4 ka.\nLine 236: What does a bias in d13C in the Antarctic ice core mean? Is this positive / negative? HWhat does it indicate?\nLine 257: Should be 7000 years rather than 7 ka and 3500 years rather than 3.5 ka."}, "author_response": "Response: Thank you so much for your nice comments and suggestions which help to improve the manuscript. After reading through relevant research, we found that the 139-ka event should be related to Heinrich event 12 as pointed out by Lisiecki and Stern (2016). They also mentioned that the H12 event has a global impact. We then re-evaluated this event in the context of many other different ocean and terrestrial archives, as provided in the figures in the supplementary materials and main manuscript, and reconsidered the correlations between these records. Besides, we removed the discussion of SST and monsoon relationship during the Last Interglacial, which might be overstated.\nReference: Lisiecki, L. E. & Stern, J. V. Regional and global benthic δ18O stacks for the last glacial cycle. Paleoceanography 31, 1368–1394 (2016).\nThe following part is the point-by-point response.\nLine 11 to 22: Please review the English grammar and ensure uniform decimal points for ages. It appears to me that the current version of the abstract lacks emphasis on the importance and novelty of this study.\nResponse: Thank you so much for your nice suggestion. We reviewed the grammar thoroughly and uniformed the decimal points for ages. The abstract was also revised to highlight the importance and novelty. Please see in lines 20–29.\nThe detailed anatomy of Termination I (TI) is well depicted, but to which extent TI represents previous terminations remains controversial. Based on an analogous two-step structure between TI and TII observed in stalagmite-based Asian monsoon records from Shima and nearby caves in China, we found that a major CO rise phase (135.7±1–129 2 ka BP) during TII was similar to that during TI, including rapid shifts and amplitudes. Correlating marine and ice- core records to our U/Th-dated record, we determined an initial 15–20 ppm rise of CO occurring at 139±1 ka BP 2 before major rise phase. This initial rise of CO , distinct from CO behavior before TI, likely resulted from a 50% 2 2 reduction in southern hemisphere dust flux and millennial-scale processes in the ocean and Asian monsoon. We show that the initial rise of CO was one of the key preconditions that ended up glacial, supporting the “tipping 2 point” theory.\nLine 56 to 59: This statement may suggest bias, despite ongoing debates regarding the similarities and differences between TI and TII, as outlined by the authors above.\nResponse: Thank you so much for your nice suggestion. We modified the introduction after considering your suggestions. Please see in lines 45–62.\nCurrent understanding of ocean-atmosphere-cryosphere interactions during terminations mainly relies on the assessment of Termination I (TI, ~18–11.7 thousand years before the present, hereafter ka BP) due to the abundance of archives with reliable, independent age constraints5. Denton et al.5 suggested that the collapse of northern hemisphere ice sheets, due to rising insolation, disrupted the global oceanic and atmospheric circulations, leading to CO release from the Southern Ocean that further augmented global warming during the last termination. However, 2 TI may not be representative of previous terminations. An increasing body of evidence from high-resolution ocean and terrestrial records suggests climatic dissimilarities between TI and TII, where TII is the penultimate ice age termination (TII, ∼136–129 ka BP), containing the transition from the Penultimate Glacial Maximum (PGM, ~140 ka BP) to the Last Interglacial (LIG, ∼129–116 ka BP)6,10–18. No clear evidence of a Younger Dryas-like event in the North Atlantic sea surface temperature (SST)14 nor a Bølling/Allerød-like event in the AMOC intensity occurred during TII6. Importantly, Cheng et al.9 observed that monsoon structure during TII was different from TI, lacking the Bølling-Allerød period. Another notable discrepancy between TI and TII was that atmospheric CO at the end of TII 2 was higher than at the end of TI by >15 ppm19. Accordingly, Barker and Knorr8 suggested that Denton et al.’s model5 could not explain the anatomy of all ice age terminations. To better understand why the two most recent terminations exhibited different characteristics, especially the rise in atmospheric CO which plays a critical role in 2 regulating the Earth’s temperature, we must investigate the cascade of climatic elements and dynamics before and during the ice age terminations.\nLine 81 to 82: Authors should correctly refer to \"Mod Age\" as \"MOD-AGE,\" aligning with its original reference..\nResponse: Thank you so much for your nice suggestion. Changed accordingly. Please see in line 83.\nThe age models for Shima samples were obtained using the MOD_AGE model21 ...\nLine 90 to 96: Speleothem δ18O records should be assessed for isotopically equilibrium precipitation, despite the absence of absolute methods for verification. However, the millennial-scale oscillations in the SM record examined here cannot undergo such testing due to the discontinuous dataset of the SM record itself and substantial discrepancies between various Chinese cave records presented in this manuscript (Figure 2). This limitation could significantly undermine the authors' arguments.\nResponse: Thank you so much for your nice suggestion. Previously, while we found a general changing pattern of the monsoonal deglaciation in different cave records (that is, changing from more positive values to more negative values in δ18O data), we also tried to say something about the regional discrepancies, which might be confusing. In the revised version, regional discrepancies are not discussed anymore and therefore we removed several calcite records. Considering your suggestion, we did three tests in order to test the isotopic equilibrium precipitation of speleothem δ18O, including Hendy test, Replication test and triple oxygen isotope analyses. Please see details in lines106–128.\nStalagmite precipitation in isotopic equilibrium is a prerequisite for using its δ18O as a climate proxy. Three lines of evidence suggest that Shima δ18O records reflect formation in isotopic equilibrium. First, the Hendy test27 was performed on four individual growth layers of each sample. Most δ18O and δ13C variations along the same layer range from 0.1‰ to 0.3‰ (Fig. 1b), and the absence of positive relationships between δ18O and δ13C are consistent with the equilibrium formation of calcite (Supplementary Fig. 3). Second, replication of different stalagmite δ18O records is a reliable method to test for equilibrium fractionation in carbonates28. Our Shima records display good consistency with other cave records from southern China on the orbital to millennial timescales, including abrupt positive shifts of ~2‰ at approximately 136 ka BP and negative shifts of >4‰ from the TII to LIG (Supplementary Fig. 4). Third, the Δʹ17O vs. δʹ18O trend of Shima stalagmite SM7 has a slope of ≈ 1 per meg/‰ (Fig. 1c), consistent with the predominance of rainout processes in determining δ18O23. Similar trends could mathematically come from combinations of multiple processes. However, even combining trends from, for example, rainout (Rayleigh, 0 per meg/‰) and within-cave kinetic (7 per meg/‰) processes would still require an overwhelming 86% of the signal to be coming from climate- related drivers (i.e., 0.86×0 per meg/‰ + 0.14×7 per meg/‰ = 1 per meg/‰). Changes in Δʹ17O can also come from varying the relative humidity during evaporation at the moisture source (-9.5 per meg/‰) and require only a 37.5 % contribution from the climate signal (i.e., 0.375×(-9 per meg/‰) + 0.625×7 per meg/‰ = 1 per meg/‰). These calculations, in combination with the fact that modern precipitation in the Asian monsoon region shows weak, slightly negative to slightly positive trends (Supplementary Fig. 5), also support stalagmite formation in (near-)isotopic equilibrium. Ultimately, while these lines of evidence are useful in establishing that the Shima samples formed in (near-)isotope equilibrium, our fundamental interpretations below are about the timing of change, not the exact interpretation of speleothem δ18O, and thus independent of the driving processes.\nFigure 1: The age models of these stalagmites reveal several age outliers and reversed dates based on their stratigraphic order (Figure S2). Additionally, while Figure 1 and Table S1 present 9 ages for SM16, the MOD-AGE model in Figure S2 shows only 8 ages. To accurately estimate petrography and identify possible hiatuses, section images of the SM stalagmites are essential.\nResponse: Thank you so much for your nice suggestion. Yes, there are some reversed dates in samples and therefore we chose to use modeling for the chronology reconstruction. We checked the dates and age models for these samples again. There are 8 dates for SM16. Besides, we put the section images for these samples in Supplementary Fig. 2 as you suggested. Short-lived hiatuses might exist, but considering dating errors for several centuries, the exact timing for hiatuses is hard to detect. Nonetheless, modeled chronologies run through most 230Th dates.\nLine 105 to 106: Reference 26 is not suitable for this sentence, as the SM record in this study has not been replicated on a millennial timescale as suggested by Dorale and Liu (2009).\nResponse: Thank you so much for your suggestion. The replication test is important for stalagmite studies, and selecting proper records is necessary before comparison. We removed records from northern China because moisture sources and atmospheric circulation systems for northern China are different from those for southern China (Chu et al., 2019; Zhang et al., 2021), which could cause different isotopic signals in rainfall and speleothem. We also removed Xiaobailong and Jiangjun records which have very low resolutions and fewer dating constraints. All the records from southern China presented show an overall similarity, including the 139-ka and 136-ka weak monsoon intervals and a rapid monsoon intensification around 129 ka BP (Supplementary Fig. 4 Supplementary Fig. 4). Besides, to alleviate the temporal gap of the abrupt Asian monsoon TII in Shima record, we combined our records with that of Sanbao Cave (270 km distant), which is well-dated and has dating errors of less than 100 years by adding a systematic bias of -0.2‰ to SB25 δ18O data.\nReferences:\nChu Q et al. Roles of moisture sources and transport in precipitation variabilities during boreal summer over East China. Clim Dyn 53, 5437–5457 (2019).\nZhang R et al. Summertime Moisture Sources and Transportation Pathways for China and Associated Atmospheric Circulation Patterns. Front. Earth Sci. 9:756943 (2021).\nLine 133 to 135: Authors should provide more detailed information on the statistical analysis in the Methods section, particularly regarding the merging of data from different datasets\nResponse: Thank you so much for your nice suggestion. We deleted the merging data from different datasets which is not important in the present version.\nLine 135 to 138: I am not convinced by this interpretation because XBL and JJ caves are located slightly upstream compared to other cave sites, suggesting that these sites may reflect signals of the Indian Summer Monsoon (ISM) or a combination of ISM and East Asian Summer Monsoon (EASM).\nResponse: Thank you so much for your nice suggestion. According to published papers, the XBL and JJ caves should reflect more of the signals of the Indian Summer Monsoon (ISM) or a combination of ISM and East Asian Summer Monsoon (EASM) (Wassenburg et al., 2020; Cai et al., 2015). In the revised version, we use the term “Asian summer monsoon”, which includes both systems of the ISM and EASM for discussion. This interpretation is generally accepted in previous studies (e.g. Cheng et al., 2016; Kelly et al., 2006; Wang et al., 2008).\nReference:\nWassenburg, J. A. et al. Penultimate deglaciation Asian monsoon response to North Atlantic circulation collapse. Nat. Geosci. 14, 937–941 (2021).\nCai, Y. et al. Variability of stalagmite-inferred Indian monsoon precipitation over the past 252,000 y. Proc. Natl. Acad. Sci. U.S.A. 112, 2954–2959 (2015).\nWang, Y. et al. Millennial- and orbital-scale changes in the East Asian monsoon over the past 224,000 years. Nature 451, 1090–1093 (2008).\nCheng, H. et al. The Asian monsoon over the past 640,000 years and ice age terminations. Nature 534, 640–646 (2016).\nKelly, M. J. et al. High resolution characterization of the Asian Monsoon between 146,000 and 99,000 years B.P. from Dongge Cave, China and global correlation of events surrounding Termination II. Palaeogeogr. Palaeoclimatol. Palaeoecol. 236, 20–38 (2006).\nLine 174 to 177: I am not convinced by such a detailed correlation presented by the authors because individual cave records exhibit significantly different changing patterns over time.\nResponse: Thank you so much for your nice suggestion. We deleted this expression.\nLine 179 to 183: These assertions seem unreasonable due to the millennial-scale differences among Chinese records and the potential data noise levels (also, sampling resolution) of SST and CH4 records.\nResponse: Thank you so much for your nice suggestion. We modified this part and deleted some overstated expression. The link between 134-ka CH peak and a short-lived strengthening of the Asian summer monsoon might 4 be correct because it has been discussed by Schmidely et al. (2021).\nReference:\nSchmidely, L. et al. CH and N O fluctuations during the penultimate deglaciation. Clim. Past 17, 1627–1643 4 2 (2021).\nLine 206 to 207: Please revise the figure citation to 'Figure 3, left panel' to conform to the manuscript format of Nature Communications.\nResponse: Thank you so much for your nice suggestion. We modified the figure citation in the revised version thoroughly.\nLine 205 to 261: The last section titled 'Different routes to ice age terminations' is overly speculative, as the authors' main arguments lack direct support from the SM records.\nResponse: Thank you so much for your nice suggestion. In the revised version, we highlighted the older section of Shima records and discussed more comprehensively the preconditions before termination II. Compared with other Chinese cave records, Shima record has smaller 230Th age errors for this part and higher data resolutions (Supplementary Fig. 4). Based on a combined record of Sanbao and Shima, we used cave-marine and cave-ice core correlations to establish the linkages between ocean-atmosphere processes, which allowed us to discuss the main points regarding the 139 ka event. Although we did not change the title, we modified this section to be more readable. Please see in lines 206-263.\nDifferent routes to Ice Age terminations\nWhile TI and TII have similarities in their overall rate of CO change and the presence of millennial-scale 2 events, they have distinct patterns before the onset of the terminations. During the \"preparing stage\" of the PGM, CO had already increased gradually from 139±1 ka BP to 135.7±1 ka BP, leading to a total rise of 15–20 ppm 2 before the abrupt increase (Fig. 3a). However, CO concentration was stable at 190 ppm for about 4000 years prior 2 to the onset of TI. Orbital forcing and global ice volume during the LGM (Fig. 4A, B) were similar during the PGM (Fig. 4a, b), both of which provide critical backgrounds for CO variations during glacial-interglacial cycles42, and 2 therefore cannot explain differences in CO variations between terminations.\nThe 15–20 ppm CO increase before 135.7±1 ka BP might explain why atmospheric CO concentration at the 2 2 end of TII is >15 ppm higher than the Holocene. This initial, gradual rise of CO can be regarded, according to the 2 “tipping point” theory36, as an early warning signal that leads to the inevitable CO rise at the onset of TII. From the 2 viewpoint of physical mechanisms, the abrupt rise of CO since 135.7±1 ka BP was likely due to the increased 2 carbon storage in the stratified Southern Ocean during the preceding PGM53, and thus the early warning signal for TII arose from the southern hemisphere. In contrast, the early warning signal for TI was particularly expressed in the northern hemisphere36,54,55. Brovkin et al.36 suggested a tipping point at 18 ka BP when AMOC collapsed, before which a repeated build-up of a subsurface heat reservoir in the subpolar Atlantic was robust54. That the early warning signal may occur in either hemisphere suggests that a slow, initial process, regardless of location, is a precondition for shifting from glacial to interglacial conditions.\nIn Denton et al.’s model5, higher CO driven by the reorganization of ocean and atmospheric circulation is a key 2 factor in completing the termination. Here our hypothesis further points to its importance in initiating the termination (Fig. 5). Under similar orbital configurations of large ice sheets and low northern hemisphere solar insolation (NHSI), the PGM experienced more dramatic climate oscillations. The H12 and the MWP-2A events caused a reduction in ASM and ~30m sea level rise46,47, leading to an abrupt 50% decrease in southern hemisphere dust flux. All these processes led to the initial rise in CO that ultimately gave feedback on the extensive ice sheets ready for the onset of 2 TII. When gradual changes in ocean temperature or CO crossed the threshold for “tipping”, forced by rising NHSI, 2 the deglaciation process started, following the sequence of massive ice sheet melting, reduced AMOC, weakened ASM and southward Intertropical convergence zone (ITCZ)/westerlies as summarized by Denton et al.5. All these processes caused continuous CO release from the Southern Ocean to sustain ice sheet melting, which then ended up 2 with a last hit of the 1000-year-long “pause”.\nLine 206 to 209: I remain unconvinced by the author's argument, as mentioned previously.\nResponse: Thank you so much for your suggestion. We re-evaluated global geological archives and found that the 139 ka event should have occurred and had a global impact because it is a Heinrich-type event (Lisiecki and Stern, 2016). Please see the modifications in lines 206-263.\nLine 245 to 247: This explanation does not align with the results presented by the authors.\nResponse: Thank you so much for your nice suggestion. Original writing might be confusing and so we rewrote this part in the section “Different routes to Ice Age Terminations”. Please see in lines 206-263.\nLine 250 to 252: I find the authors' estimation of CO2 rise unconvincing, particularly due to the slightly higher value at ~142 ka BP compared to 140 ka BP.\nResponse: Thank you so much for your nice suggestion. We provide constraints on the timing of initial CO rises by 2 bridging the ice core, marine and cave records, as discussed in the section “Timing of the initial rise of CO during 2 the Penultimate Glacial Maximum”. From the long-term development of the termination, we recognize that the initial CO rise might start from ~140 ka BP, and this timing point is supported by three CO records. Please see in 2 2 lines 130–159.\nTiming of the initial rise of CO during the Penultimate Glacial Maximum 2\nBased on a mechanical link of abrupt shifts in ASM (under whose influence the wetland is one of the major sources for atmospheric CH ) and CH records in Antarctic ice cores during ice age termination9,29, we compare the 4 4 Shima-Sanbao record with CH and CO records from the EDC ice core on the AICC2012 chronology30,31. Apart 4 2 from the nearly-consistent abrupt intensification/rises in ASM and CH at around 129 ka BP (blue dashed line Fig. 4 2b, c), a small peak of CH at ~134 ka BP is possibly tied to a short-lived strengthening of the ASM, which has been 4 confirmed in a previous study32 and further constrains the chronology of CO and CH records for older sections. All 2 4 three atmospheric CO records began to rise at ~140 ka BP until 129 ka BP, lasting for approximately 11000 years 2 with a total rise of >100 ppm (Fig. 2a). The onset timing of CO rise cannot be determined more precisely due to the 2 low-resolution data of CO at ~140 ka BP. However, considering that: (i) the linkage of North Atlantic ice-rafted 2 debris (IRD) events or H stadials with rises of CO through a mechanism of the AMOC weakening that promotes the 2 release of CO from the Southern Ocean to the atmosphere5 and (ii) good correlations of WMIs and IRD events20,25, 2 here we can provide constraints on the timing of initial CO rises by bridging the ice core, marine and composite 2 Shima-Sanbao cave records.\nThere is a broad similarity between our cave record and a North Atlantic SST record33, including dramatic monsoon weakening and strengthening associated with a cooling of ~4°C at the beginning of TII and a large amplitude warming at the end of TII in the North Atlantic (black dashed lines in Fig. 2). Two moderate excursions in the SST record could also correlate to, monsoon weakening and strengthening at approximately 137.5±0.4 and 134±0.6 ka BP. Since the chronologies for ODP984 and MD01-2444 cores were well aligned by tie points33, major IRD peaks in core ODP98434 can also be well linked to the sequence of WMIs in the cave record, including H12 and H11. Other IRD records on independent age models also show clear peaks and support the tight correlations with WMIs (Supplementary Fig. 6). Based on the evidence presented, we propose that H12, aligning to a WMI event dated at 139±0.6 ka BP, possibly drove the initial rise of CO through the weakening or shutdown of the AMOC5,9 2 (Supplementary Fig. 6). The timing of WMI during the PGM in the Shima record is consistent with other independently dated Chinese cave records within their dating errors (blue arrows in Supplementary Fig. 4). By evaluating 230Th age control for cave records (Supplementary Fig. 4) and the original age model for ice cores30,31, we suggest that the initial rise of CO for TII occurred at 139±1 ka BP. 2\nLine 269 to 270: Instead of \"parts mil,\" you should use \"per mil\" or \"‰\" to correctly denote the unit.\nResponse: Thank you so much for your nice suggestion. Changed accordingly. Please see in line 273.\n.... and given in per mil (‰)....\nReviewer #2 (Remarks to the Author):\nLiang et al use 4 speleothem records from a cave in China to examine H11 and Termination II in the EASM region. They examine how the EASM record fits into the sequence of global events to explore drivers and feedback mechanisms. I think the records themselves are extremely valuable and supported with elaborate discussion.\nI have a few comments that will hopefully improve the manuscript:\nResponse: Thank you so much for your comments which help to improve the manuscript. We modified the content point-by-point as follows.\n- Since the premise of the paper really seems to be examining the sequence of events, it would be great to see the speleothem age-model uncertainties clearly in the plots and in the text. For example, Figure 1 could include age- model uncertainties as shading, and could show expanded plots of significant time periods e.g. H11. Similarly, the text, should include the full ages and uncertainties in brackets e.g. Lines 93 to 95. Similarly for Figures 2 and 3, it would be really useful to see some consideration of temporal uncertainties on the remaining records of ice melt, CO2, temperature etc as well. You could try something like Fig. 7 in this publication: https://cp.copernicus.org/preprints/cp-2024-37/cp-2024-37.pdf\nResponse: Thank you so much for your nice comments. According to your suggestion, we plot Fig. 1a with line shading of age-model uncertainties. Because our record does not cover a complete H11, we do not show an expanded plot. Besides, we add descriptions of age uncertainties in the revised version throughout the paper in records of stalagmite and CO . Age uncertainties in other cave records are shown in Supplementary Fig. 4, which 2 was part of the previous Fig. 2. Uncertainties for ice core and marine records are as large as ~1500 to 3000 years across penultimate termination according to references (Mokeddem and McManus, 2016; Irvalı et al., 2016; Böhm et al., 2015; Lisiecki and Stern, 2016; Bazin et al., 2013; Tzedakis et al., 2018; Martrat et al., 2014; Marino et al., 2015; Deaney et al., 2017), and some marine records were tuned to AICC2012 chronology (Deaney et al., 2017). Most of these age models have been well discussed, verified and tested (some of them have been compared with stalagmite-based chronologies), and thus we think it is proper to use records on their independent chronologies. We also presented a temporal uncertainty plot and added it to Supplementary Fig. 6. Because different records for SST, AMOC and IRD inputs have different uncertainties, we here apply the smallest age errors with the best age constraints provided by references in Supplementary Fig. 6.\nBesides, the age errors for CO are constrained by U/Th-dated cave records and narrowed to 1000 years because (i) 2 Asian monsoon (under whose influence the wetland is one of the major sources for atmospheric CH ) can be 4 mechanically linked to atmospheric CH as shown in Fig. 1, and (ii) the chronology for EDC CH and CO records 4 4 2 are all on the AICC2012 chronology (Bazin et al., 2013). An estimation of 1000 years approximate to cave dates Supplementary Fig. 4) is used for temporal uncertainties on CO .\nMokeddem, Z. & McManus, J. F. Persistent climatic and oceanographic oscillations in the subpolar North Atlantic during the MIS 6 glaciation and MIS 5 interglacial. Paleoceanography 31, 758–778 (2016).\nIrvalı, N. et al. Evidence for regional cooling, frontal advances, and East Greenland Ice Sheet changes during the demise of the last interglacial. Quat. Sci. Rev. 150, 184–199 (2016).\nLisiecki, L. E. & Stern, J. V. Regional and global benthic δ18O stacks for the last glacial cycle. Paleoceanography 31, 1368–1394 (2016).\nDeaney, E. L., Barker, S. & Van De Flierdt, T. Timing and nature of AMOC recovery across Termination 2 and magnitude of deglacial CO change. Nat. Commun. 8, 14595 (2017).\nMartrat, B., Jimenez-Amat, P., Zahn, R. & Grimalt, J. O. Similarities and dissimilarities between the last two deglaciations and interglaciations in the North Atlantic region. Quat. Sci. Rev. 99, 122–134 (2014).\nMarino, G. et al. Bipolar seesaw control on last interglacial sea level. Nature 522, 197–201 (2015).\nTzedakis, P. C. et al. Enhanced climate instability in the North Atlantic and southern Europe during the Last Interglacial. Nat. Commun. 9, 4235 (2018).\nBazin, L., et al. An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology (AICC2012): 120– 800 ka, Clim. Past, 9, 1715–1731 (2013).\nLine 52: What does 'ocean-forcing sea level rise' mean\nResponse: Thank you so much for your question. It is wrongly spelled and should be “Sea level rise”. But this sentence was removed in adjustment to the Introduction.\nLines 121 to 125: Does TI show this N-S pattern as well?\nResponse: Thank you so much for your question. This is a good question, and we are also interested in finding such a pattern. But up till now we have not seen any published results from Shangxiaofeng Cave during TI, and we cannot say definitely whether N-S pattern existed during TI. Future work is necessary.\nLine 177: Positive biases near 133 and 130 ka BP in the list of caves are not immediately apparent in the figure. Perhaps show these with lines / arrows.\nResponse: Thank you so much for your nice suggestion. We modified this part in the revised version and deleted the comparison of different patterns between northern and southern China. Because we want to focus more on the discussion of termination. Hope you find the revised version interesting.\nLines 189 and 190: Is the Figure labelling (Figure 3C) correct? I would also reconsider this, I find it hard to see these positive anomalies in the record.\nResponse: Thank you so much for your nice suggestion. In the previous version, the label should be Figure 3A. Considering your suggestion, we deleted the discussion of SST and monsoon correlation during the Last Interglacial, which might be overstated.\nLine 223: Should be 4000 years rather than 4 ka.\nResponse: Thank you so much for your nice suggestion. Changed accordingly and we modified similar expressions in the revised version thoroughly.\nLine 236: What does a bias in d13C in the Antarctic ice core mean? Is this positive / negative? What does it indicate?\nResponse: Thank you so much for your question. According to the study by Schneider et al. (2013), it is a ~0.1– 0.4‰ decrease to lighter values of the δ13C level from ∼140 000 yr BP and it can be explained by changes in atm global carbon storage. Generally, a shift to lighter δ13C values indicates a decline in the terrestrial biosphere. In atm the revised version, we removed this expression.\nLine 257: Should be 7000 years rather than 7 ka and 3500 years rather than 3.5 ka.\nResponse: Thank you so much for your nice suggestion. Changed accordingly and we modified similar expressions in the revised version thoroughly."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Liang et al. have extensively revised their original manuscript in response to the reviewers' comments. I am generally satisfied with the improvements in the authors' explanations, which now focus more on their dataset.\nHowever, I would like to request further clarification on the detailed correlations of H12 in Figs. 2 and 4, as well as the pause at the end of TII.\nIn the abstract, you mention that the initial rise in CO2 was one of the key preconditions that led to the end of the glacial period in TII. Could you explain why this is the case?\nWhat specific evidence supports the proposed causes of the different preconditions between TI and TII, as outlined in the revised manuscript?", "reviewer_3": "I was brought in for the second round of reviews, so I'll limit my discussion to what I deem as appropriate considering the edits which were made after the first round. In general this is a well written article, and I support publication of it in Nature Communications. However there are some issues that need to be addressed before I can sign off on publication. I will list them now in order of importance.\n1) Given the errors in the age models, and the distribution of the U-Th dates relative to the d18O features, too much emphasis has been placed on “the pause” in the manuscript. While it *may be a real feature of the TII, the age constraints in these records have too much uncertainty to say for sure that it did indeed occur. There are 4 records that span ‘the pause’, but only one of them was used as the primary chronology: SB25. Given the time constraints among the records of the before ~ 128 ka, I can see why the authors choose this particular record. However two other records from the same cave show a large d18O decrease occurring 1000 years before SB25. While the record from Dongge agrees with the timing as SB25, the uncertainty on the U-Th ages in Dongge are large. Furthermore, there is a considerable slowdown in growth rate and possible signs of a hiatus in SB25 after 400 mm/before 129 ka (Cheng et al., Science, 2009: Figure S2, Table S1) – which is exactly when ‘the pause’ occurs. In fact, there is a clear degradation in the temporal resolution during this timeperiod in the main figures 2-4. Alternate plausible age models using the exact same U-Th ages would have NO pause and either a hiatus during this time - or even a more gentle change in d18O. Given the uncertainty in the age models, much of the discussions surrounding “the pause” is not supported and is conjecture at best. It should be only a small mention with large caveats about the uncertainty.\n2) I'm not sure what the authors mean by ‘preparing’ and ‘quasi-equilibrium’ stages in Figure 4. This entire argument seems very odd, and I'm not sure where it's coming from or why the nomenclature is chosen. If it is based on the CO2 curves, they look almost identical to me. The change point algorithm (Figure 7 in SI) shows that they contemporaneously within error. The older CO2 record just looks like a smoothed moving average of the higher resolution younger record (most likely due to compaction of the ice), and I see no real functional difference between them. It can’t be based on insulation as the authors even state that insulation during those two times is very similar. The dust records are also similar during these ‘stages’. Therefore I'm not sure why these two terms are being used. They are not clearly defined, and I advocate to drop those two terms from the manuscript.\n3) It would be much more illustrative to depict figures 3 and 4 as kilo years since glacial maximum (or time since glacial maximum or years since glacial maximum, e.g TSGM). They would both then start 0 and count upwards, such that one could then easily compare changes between the two records without having to do the math in their head. By subtracting off the time period since the glacial maximum (either last or penultimate) this would help to compare the timing of events in the records between CO2 meltwater pulses Heinrich Asia monsoon etc during deglaciation events. Also, starting TI at 23 ka and not 24 ka cuts of H2 from Figure 4. Then both deglaciations would have two Heinrich events in the figure. Seems arbitrary to start at 23 and have that white space on the right of the plot. The label on top of LGM in the rectangle shows the state of land ice/sea level from LR04.\n4) It would be very informative for the reader to label the arrows/lines in figure 5 with some sort of nomenclature to say exactly which of the time series are being used to come up with those bullets. Currently, Figure 5 reads much more like a story rather than a logical argument. By tying these arrow bulleted points to actual points in the time series in Figures 2-4, the reader could follow along the mechanism with quantitative time series. This was partially done in the right panel of figure 6 of the supplementary information by labeling the different events on the second time scale on the right, such that one can partially follow along with the color-coded event and the color-coded time series. However, it is a little difficult to go back and forth between them just based on the color coding, and some sort of other indicator would be more helpful for the reader to follow along through the mechanism that the authors are proposing in Figure 5 of the main manuscript.\n5) Some of the arrows in the figures are misleading, and I see no reason as to why they're there. For example, the two gray arrows pointing downwards on the right panel of F&G in Figure 4. The Gray arrow in the curve during the LGM at approximately 17.5 ka. This centennial scale oscillation in the record looks much like many other ones and there's no reason to call that particular one out other than the authors subjective selection. I see no quantifiable changes in the record to support it unlike the record from PGM where at 136 ka there a change in the average values during that step down phase showing a clear change in the record. Only use arrows when the math says they should be there - not to try and convince the reader to ‘see’ something."}, "author_response": "Response: Thank you so much for your comments which help a lot in improving this manuscript. According to your suggestion, we added clarification on the detailed correlations of H12 in Figs. 2 and 4. We also deleted the overstated discussion of the pause at the end of TII.\nThe revised text is in lines 150-158: Since the chronologies for ODP984 and MD01-2444 cores were well aligned by tie points34, major IRD peaks of H11 and H12 events in core ODP98435 could also be well linked to the sequence of WMIs in the cave record (Fig. 2e). Obvious peaks for H12 event in other IRD records on independent age models varied between 140 and 138.5 ka BP and H11 event had a more complex structure with its onset at approximately 136±1.5 ka BP (Supplementary Fig. 6). These two major IRD peaks were consistent with AMOC weakening, and tightly correlated with WMIs (Supplementary Fig. 6). Based on the evidence presented, we propose that H12, aligning to a WMI event dated at 139±0.6 ka BP, possibly contributed to the initial rise of CO through the weakening or shutdown of the AMOC5,9 (Supplementary Fig. 2 6e, f).\nAnd in lines 214-223: The H12 event occurred near the boreal summer insolation minimum (Fig. 4a), and may have preconditioned TII by creating smaller, partially deglaciated ice sheets42. Alike the H11 event which caused an intensive reduction of the northern ice sheets and the MWP-2B11, the H12 event could have caused the MWP-2A. Although not as intensive as MWP-2B which contributed ~70% of deglacial sea level rise11, MWP-2A was the early phase of ice-sheet retreat before TII and contributed about 30 meters of sea level rise44,45, which may have disturbed Southern Ocean stratification and lead to deep ocean CO release, as well 2 as impacting the monsoon intensities. The coincidence of the H12 event, MWP-2A, the WMI and the North Atlantic cooling episode (Figs. 2, 4) support a clear millennial-scale climatic oscillation at around 139 ka BP.\nSeveral reasons explain why the initial rise in CO during the glacial maximum was one of the key 2 preconditions that led to the end of the glacial period in TII. Firstly, CO is one of the primary greenhouse 2 gases, and an essential amplifier of global temperature (Shakun et al., 2012). Simulations show that changes in CO can contribute to 40–65% of the temperature changes during transitions between glacial and interglacial 2 periods (Lorius et al., 1990). The initial CO increase by ~15–20 ppm equals 20–25% of the total CO 2 2 increment during TII (80 ppm), and it might contribute to nearly 10% of the global temperature increase if we multiply by 40–65% (Lorius et al., 1990). Secondly, the amount of initial CO rise is nearly equivalent to that 2 during Heinrich events. Studies show that CO increased during Heinrich events before warming in the 2 northern hemisphere and continued rising after the onset of Dansgaard-Oeschger event (Bauska et al., 2021; Ahn and Brook, 2008). Changes in atmospheric CO can lead to or may be required for Dansgaard-Oeschger 2 variability (Menviel et al., 2020). It is also estimated that the centennial-scale feedback strength of CO to 2 global mean temperature changes is 0.155 ± 0.035 Wm−2 K−1 during Dansgaard-Oeschger events (Liu et al., 2022). These lines of evidence indicate that even with a small amount of increase like during Heinrich events, changes in CO were able to influence global or hemispheric temperature. Thirdly, using the chronological 2 benchmark of speleothem record as described in the manuscript, the initial CO rise started at 139±1 ka BP, 2 nearly consistent with or slightly lagging the minimum of 21 June insolation at 65°N and followed the insolation rising until a rapid increase at ~135.7±1 ka BP. The 135.7±1 ka BP is also a time point when many climatic elements started the deglacial process together, including massive ice sheet collapse, the AMOC stagnation, monsoon weakening and Antarctic warming. The initial CO rise thus led the onset of TII by 2 thousands of years, within the estimated range of the lagged response of ice sheets to CO change by 1000 to 2 4000 years (Ruddiman, 2006). Therefore, considering the feedback of CO on atmospheric warming and ice 2 sheet melting, we suggest that ~4000–5000-year-long gradual changes in CO along with insolation rise could 2 have preconditioned ice-age terminations and contributed to the following deglaciation process.\nIn the manuscript, Figure 5 shows that the orbital forcing (rising from the low insolation values) and global ice volume (with d18O values of around 5 per mil) were almost similar during the LGM and PGM, while internal climatic oscillations were different, especially the millennial-scale IRD events and southern hemisphere dust flux. We pointed out the most two important causes in the revised abstract in lines 24–27:\nThe major rise phases of CO were comparable during TI and TII, but the initial CO rise before TII was 2 2 distinct from CO behavior before TI, likely forced by the Earth’s internal variabilities, in particular an ice- 2 sheet collapse event and a 50% reduction in southern hemisphere dust flux.\nand also, in discussion in lines 237-238:\nTherefore, a combination of dust flux decreases and the H12 event could possibly cause the initial CO rise, 2 leading to different CO change patterns during the LGM and PGM.\nReferences:\nShakun, J. D. et al. Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation. Nature 484, 49–54 (2012).\nLorius, C., et al. The ice-core record: climate sensitivity and future greenhouse warming. Nature 347, 139–145 (1990).\nBauska, T.K., et al. Abrupt changes in the global carbon cycle during the last glacial period. Nat. Geosci. 14, 91–96 (2021).\nAhn, J., & Brook, E. J. Atmospheric CO and climate on millennial time scales during the last glacial period. 2 Science 322,5898, 83–85 (2008).\nMenviel, L.C., et al. An ice–climate oscillatory framework for Dansgaard–Oeschger cycles. Nat Rev Earth Environ 1, 677–693 (2020).\nLiu, M., et al. Past rapid warmings as a constraint on greenhouse-gas climate feedbacks. Commun Earth Environ 3, 196 (2022).\nRuddiman, W. F. Ice-driven CO feedback on ice volume. Clim. Past (2006).\nPatterson, V. L., et al. (2024). Contrasting the Penultimate Glacial Maximum and the Last Glacial Maximum (140 and 21 ka) using coupled climate-ice sheet modelling. Climate of the Past, 20(10), 2191-2218.\nZhang, X., et al. Abrupt glacial climate shifts controlled by ice sheet changes. Nature 512, 290–294 (2014).\nWang, H., et al. Westerly and Laurentide ice sheet fluctuations during the last glacial maximum. npj Clim Atmos Sci 7, 213 (2024).\nResponse: Thank you so much for your comments which help a lot in improving this manuscript. Your concern about \"the pause\" is worthy of consideration. Although Dongge and Hulu records can somewhat support the \"pause\" in Sanbao record (Supplementary Fig. 6), they have much lower resolution and larger dating uncertainties. We agree with your suggestion and deleted the discussion over the \"pause\" and focus more on our new data. Please see the text in lines 177-195:\nAfter obtaining the onsets of rapid CO rises for the last two ice age terminations, we established their analogy 2 (Fig. 3). Changes in monsoon intensity inferred from cave records and Antarctic temperature inferred from ice δD records are comparable. Although the internal sequences of monsoonal events are distinguished between TI and TII9, analogous changes in cave records exist, including: (i) similarly abrupt weakenings and fluctuations in the ASM occurring at the transitions from glacial maximums to TI/TII (green bar in Fig. 3), and (ii) a rapid monsoon intensification after the long-term WMIs which marks an end of ice age termination (yellow bar in Fig. 3). The onset and end of TI/TII bracketed the major rise phase of CO , which lasted approximately 6,000 2 to 7,000 years. The TI and TII shared a similarly changing amplitude of ~80 ppm during the major rise phases of CO , regardless of different substages within them (Fig. 3b). Besides, Antarctic δD records fluctuated 2 around -440‰ during both glacial maximums and then took similar amounts of time through deglacial processes to reach their interglacial plateaus, despite the interruption of Antarctic Cold Reversal in TI. Landais et al.38 reported that atmospheric CO concentrations and Antarctic temperature started increasing in phase 2 around 136 ka BP, supporting the result of 135.7±1 ka BP here. The beginning of the rapid CO rise at around 2 135.7±1 ka BP was also widely consistent with abrupt changes in a number of oceanic and terrestrial records like the H11 event, cooling in the North Atlantic SST and European surface temperature, as well as the AMOC weakening (Supplementary Fig. 6). Therefore, abrupt shifts in different archives at 135.7±1 ka BP could be critical changes in the global climate system, which shares similar features with the onset of TI10,14,39,40.\nResponse: Thank you so much for your comments. Sorry for the confusion in the statements. The \"quasi- equilibrium\" is generally used for describing the LGM because the cold climate persisted for millennia and the climate was broadly in a near-equilibrium state (Schneider von Deimling et al., 2006; Annan et al., 2022). The \"preparing\" stage was aimed to mention that the climate was more variable during the PGM and was preparing for the onset of TII. Considering your comments and the readability of the manuscript, we removed the terms from the text and used glacial maximums instead.\nReferences:\nSchneider von Deimling, T., et al. (2006) Climate sensitivity estimated from ensemble simulations of glacial climate, Clim. Dynam., 27, 149–163.\nAnnan, J. D., et al. (2022) A new global surface temperature reconstruction for the Last Glacial Maximum, Clim. Past, 18, 1883–1896.\nResponse: Thank you so much for your comments. We modified figures 3 and 4 according to your suggestions and they look better. We start the sequences from glacial maximums and start TI at 24 ka. Attached are the revised figures below.\nFigure 3\nFigure 4\nResponse: Thank you so much for your comments. Considering the readability, we deleted Figure 5.\nResponse: Thank you so much for your suggestions. We modified Figure 4 according to your suggestions.\nFigure 4"}, {"round": 3, "reviewer_comments": {"reviewer_1": "I generally agree with the authors' additional responses, but I think there are too much speculations based on the subjective comparisons between the data presented by the authors and others. I think the authors' claims through these comparisons are not appropriate to be treated as important content in this paper, and if this part is greatly reduced and the expression is refined, I believe it can be published in Nature Communications.", "reviewer_3": "In general, they did a good job at responding to the reviews. Nice update on the figures. I appreciate it that they removed all mention of “the pause”, as the uncertainties in the records did not support that argument. Along the same line of logic, two of the dotted lines in Figure 2 are also not supported by uncertainties in the age models. They are cherry picked for sure. One would be hard pressed to get an objective scientist not associated with the work to look at the time series and draw those connected lines. I am referring to lines 146-148 in the revised manuscript: “Two moderate excursions in the SST record could also correlate to millennial-scale moderate monsoon shifts at approximately 137.5±0.4 and 134±0.6 ka BP.” I recommend that the lines be removed, but the authors are free to say this in text form – although it is not supported by the data.\nI see where the authors are going with the whole ‘tipping point’ theory, as a gradual increase in insolation and CO2 cannot explain the abrupt changes during the deglaciation. There are lots of missing dominos in the deglacation not included in this manuscript, but it is not a comprehensive review. One wording to change would be ‘element’ in line 169. Technically, CO2 is referred to as a forcing, not an element, with regards to climate. Please update the text accordingly.\nOther than these small changes, the manuscript is ready for print."}, "author_response": "Response: Thank you so much for your comments and approval. According to your suggestion, we removed sentences including “which could …” from the section “Different routes to ice age terminations”. Please see in lines 215 to 216:\nThe H12 event occurred around the boreal summer insolation minimum (Fig. 4a), and created small, partially deglaciated ice sheets prior to TII46.\nand in lines 217-219:\nAlthough not as intensive as MWP-2B which contributed ~70% of deglacial sea level rise11, MWP-2A was the early phase of ice-sheet retreat before TII and contributed about 30 meters of sea level rise49,50.\nWe removed this sentence below:\nNearly synchronous changes in Antarctic dust flux and atmospheric CO from ~139±1 to 2 135.7±1 ka BP imply that changes in biological pump and net primary productivity in the Southern Ocean were tightly coupled and already began before the onset of TII.\nWe also modified the last paragraph by removing the discussion over the locations of the early warning signals as below in lines 239-243:\nFrom the viewpoint of physical mechanisms, the abrupt rise of CO since 135.7±1 ka BP was 2 likely due to the increased carbon storage in the stratified Southern Ocean during the preceding PGM56. The early warning signal in CO implies that a slow, initial process is 2 important for the shift from glacial to interglacial conditions39,57, which might operate through a way similar to that proposed by Zhang et al.58.\nResponse: Thank you so much for your comments. According to your suggestion, we removed this sentence and also removed two dashed lines in Figure 2.\nResponse: Thank you so much for your comments. Updated accordingly. Please see in line 186:\n…important tipping forcing for ice age terminations39.\nResponse: Thank you so much for your approval."}]

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