Loess deposits in the low latitudes of East Asia reveal the ~20-kyr precipitation cycle

Domain: earth_science
Published: 2024-02-03
Editorial outcome: accept

Abstract
The cycle of precipitation change is key to understanding the driving mechanism of the East Asian summer monsoon (EASM). However, the dominant cycles of EASM precipitation revealed by different proxy indicators are inconsistent, leading to the “Chinese 100 kyr problem”. In this study, we examine a high-resolution, approximately 350,000-year record from a low-latitude loess profile in China. Our analyses show that variations in the ratio of dithionite−citrate−bicarbonate extractable iron to total iron are dominated by the ~20-kyr cycle, reflecting changes in precipitation. In contrast, magnetic susceptibility varies with the ~100-kyr cycle and may be mainly controlled by temperature-induced redox processes or precipitation-induced signal smoothing. Our results suggest that changes in the EASM, as indicated by precipitation in this region, are mainly forced by precession-dominated insolation variations, and that precipitation and temperature may have varied with different cycles over the past ~350,000 years.

Introduction
As an important part of the global monsoon system, the Asian summer monsoon is divided into the Indian summer monsoon (ISM), the western North Pacific summer monsoon (WNPSM), and the East Asian summer monsoon (EASM)1. The formation of the modern Asian summer monsoon is linked to seasonal surface temperature gradients. As insolation increases in the Northern Hemisphere, the surface temperature gradient between the Asian continent and the Indo–Pacific Ocean increases, causing winds to blow towards the continent. Simultaneously, water vapor is transported to the continental interior, leading to the formation of Asian summer monsoon precipitation2. The Asian summer monsoon, which can generate abundant precipitation, plays a crucial role in maintaining the ecosystem of East Asia.

Studies of Chinese mid-latitude loess have produced many geological records of the EASM, of which those from the Chinese Loess Plateau (CLP) are the most classic3–6. Magnetic susceptibility (MS) in most cases reflects the dominant 100-kyr cycle of the EASM since the Middle Pleistocene3,7–11. Both δ13C of inorganic carbonate12 and δ13C of land snail shells13 exhibit the same cycle. Because the strong EASM recorded on the CLP occurred during the interglacial period, which was characterized by low global ice volume, the change in the EASM is thought to be driven by global ice volume variability7,13. Unlike the proxies mentioned above, a spliced EASM record since 260 ka from the western CLP reveals a dominant 23-kyr cycle caused by Northern Hemisphere summer insolation forcing14; and a composite microcodium δ18O record since the last interglacial period from the central CLP shows precessional cycles15.

Stalagmites in East Asia have also recorded the summer monsoon changes since the Middle Pleistocene16–19. The stalagmite δ18O records exhibit remarkable spatial and temporal consistency at the orbital scale, and the oxygen isotope indicator in stalagmites is currently considered a proxy for summer monsoon intensity rather than precipitation2,19. In contrast to most loess records, the stalagmite δ18O records display a dominant ~20-kyr period for the summer monsoon, with almost no ~100-kyr periodic component. The stalagmite records suggest that a strong EASM corresponds to a period of high insolation in the Northern Hemisphere. Therefore, the EASM is thought to be driven by precessional processes2,19.

The idea that the EASM is mainly driven by changes in insolation controlled by precession is supported by simulations20,21. Similar results have been obtained with different models, i.e., the southwesterlies and southeasterlies are strengthened and reach farther inland when insolation increases in the Northern Hemisphere, leading to a northwards shift in the rainfall belt, an overall increase in continental precipitation22 and negative δ18O values for continental precipitation23,24. In this scenario, loess and stalagmite records should indicate the same precessional cycle; however, the dominant cycles of the EASM differ in these records, resulting in the so-called “Chinese 100-kyr problem”2,19,25. It has been suggested that the global monsoon should be reconciled with the existing framework of orbital theory19.

It has been speculated that several factors may contribute to the occurrence of the “Chinese 100-kyr problem”: (1) the unique geographical location of the CLP may play a role, as simulations have shown that the effect of precession on precipitation in this region is not significant22–24; (2) the MS does not include changes in deposition rates, and the 20-kyr cycle is nearly as important as the 100-kyr cycle in terms of the MS flux26; (3) loess MS is not a pure monsoon precipitation signal, but a measure of more processes than the intensity of the EASM alone27; (4) there may be some mechanisms that suppress the variability in the magnitude of stalagmite δ18O at the glacial–interglacial scale28 (for example, the moisture transport pathway effect may counteract the forcing of glacial boundary conditions29, resulting in the lack of glacial−interglacial variability in the stalagmite δ18O records); (5) post-depositional processes have smoothed the MS signal, increasing the influence of the ~100-kyr cycles and decreasing the influence of the ~20- and ~40-kyr cycles30; and (6) reduced precipitation during glacials on the CLP may have fallen below the threshold for magnetic enhancement, preventing MS from recording monsoon precipitation during glacials, although it remains recordable during interglacials14.

To determine the cause of the “Chinese 100-kyr problem”, it is useful to obtain geological records from locations distant from the CLP. Two records from the monsoon marginal zone show a wet‒dry variation similar to that of the East Asian speleothem over the past ~300 kyr14,31. However, records from the monsoon core zone are still lacking. Therefore, the loess in the lower reaches of the Yangtze River was chosen as the subject of this study (Fig. 1). The loess distributed in this region, known as the Xiashu loess, began to accumulate ~0.9 Myr ago32. The loess in Madang, Pengze County, Jiangxi Province, has special characteristics: Madang is located at a low latitude (Fig. 1 and Supplementary Fig. 1), corresponding to the monsoon core zone, while the CLP is located at mid-latitudes, roughly corresponding to the monsoon marginal zone.

Results
Madang profile and loess stratigraphy
The Madang profile (29°59′29″N, 116°39′44″E), with a thickness of 15.9 m, is roughly divided into seven loess–paleosol stratigraphic units, which roughly correspond to the stratigraphic units L1, S1, L2, S2, L3, S3 and L4 of the CLP (Supplementary Fig. 3). A total of 291 samples were collected and analyzed for grain size, MS, frequency-dependent magnetic susceptibility (FDS), dithionite–citrate–bicarbonate (DCB) extractable iron (FeD), total iron (FeT), hematite content and redness. Additionally, six optically stimulated luminescence (OSL) samples were taken from units L1 and S1. The grain size analysis showed that the samples have typical frequency distribution curves for eolian sediments and do not vary much in the profile (Supplementary Fig. 4). The MS and FDS variations show a strong positive correlation (Supplementary Figs. 5 and 6), and their curves are similar to those of the CLP loess. The obtained OSL ages (Supplementary Table 4) are consistent with the stratigraphic sequence, ranging from 30.1 ± 2.2 ka (at 40 cm depth) to 85.5 ± 6.0 ka (at 350 cm depth). The time series of MS defined by OSL ages can be well correlated with that of the Luochuan profile, a representative loess–paleosol sequence from the central CLP. In particular, the MS variation curves of the Madang and Luochuan profiles show striking similarities (Fig. 2). Combined with the OSL dating results, the Madang MS curve can be directly compared with that of Luochuan, with a corresponding time scale established based on the orbital tuning method8. Based on the nine depth–age tie points (Fig. 2 and Supplementary Table 5), the time scale of the Madang profile can be established by linear interpolation and extrapolation. This time scale shows that the bottom of the profile is 346.0 ka, the top is 42.9 ka (Fig. 2 and Supplementary Table 6), and the sampling resolution of the profile is approximately 1 kyr. Using a computational program for unevenly spaced time series33, we estimated the cross-correlation coefficient (rxy) between the Madang and Luochuan MS time series to be 0.88 (Supplementary Fig. 7), indicating a strong positive correlation.

Variation cycles of MS and FeD/FeT
Wavelet analysis and power spectrum analysis show that there is a dominant 101 kyr cycle in the MS curve of the Madang profile (95% robust confidence intervals) (Fig. 3a, b). The ~100-kyr cycle is considered to be an eccentricity-dominated cycle34, which also appears in the integrated MS curve of the CLP (0–350 ka)35. In contrast, there is a distinct 18 kyr cycle in the FeD/FeT curve (95% robust confidence intervals) (Fig. 3c, d). The 18 kyr cycle closely resembles the ~20-kyr precession cycle, suggesting that the FeD/FeT curve is generally dominated by a precession cycle. It is evident that the dominant cycle of the MS curve is significantly different from that of the FeD/FeT curve in the Madang profile.

Methods
Measurement of MS and FDS
A Bartington MS2 System was used for the MS and FDS measurements. Dried and weighed samples were stored in a nonmagnetic sample box. Low-frequency MS at 470 Hz and high-frequency MS at 4700 Hz were measured. FDS was defined as the difference between the measured values at low and high frequencies57.

Measurement of DCB-extractable iron and total iron
A popular technique used to extract secondary iron oxides is the dithionite–citrate–bicarbonate (DCB) method58. Sodium dithionite (Na2S2O4) acts as a strong reducing agent, dissolving secondary iron oxides and reducing Fe3+ to Fe2+, which in turn forms a stable chelate with sodium citrate (Na3C6H5O7·2H2O); additionally, sodium bicarbonate (NaHCO3) acts as a buffer for the above reaction, keeping the pH of the solution at ~7.3. The original sample was ground in an agate mortar to create a powder sample. Approximately 0.1 g of the sample was weighed (this weight was recorded), and 35 ml of 0.3 N sodium citrate solution and 3 ml of 1 N sodium bicarbonate solution were added first, followed by ~2 g of sodium dithionite in excess for the reaction. After sealing, the sample was placed in a shaker at 80 °C and shaken for 90 min until the liquid was gray in color, which indicated that the reaction was complete. Next, the sample was centrifuged at 2795 × g for 15 min, and 5 ml of the supernatant was placed in a small test tube and set aside.

One milliliter of the above liquid was pipetted into a 25 ml colorimetric tube, and 2 drops of 1 N hydrochloric acid and 1.25 ml of 2% (mass%) ascorbic acid solution were added in turn; then, the samples were shaken well. After waiting for 10 min, 2.5 ml of 0.2% O-phenanthroline solution and 4 ml of 25% sodium acetate solution were added and the samples were shaken well before fixing the volume. Then, Fe2O3 solution (100 mg/ml) was used to prepare standard solutions with concentrations of 0, 0.5, 1, 1.5, 2, 2.5 and 3 mg/l. The measured absorbance of the sample was compared with the standard curve obtained from the standard solution to calculate the DCB-extractable iron content. The instrument used was a Shimadzu UV-1800 UV‒VIS spectrophotometer with a colorimetric analysis wavelength of 535 nm. Sample measurements were performed at the Ministry of Education Key Laboratory of Surficial Geochemistry, Nanjing University.

Total iron analyses were performed at Nanjing FocuM Technology Co. Ltd. Approximately 40 mg of powder was mixed with 0.5 ml of 60 wt% HNO3 and 1.0 ml of 40% HF in high-pressure PTFE bombs. These steel-jacketed bombs were then placed in an oven at 195 °C for 3 days to ensure complete digestion. After cooling, the bombs were opened, dried on a hot plate, re-dissolved with 5 ml of 15 wt% HNO3 and 1.0 ml of Rh internal standard, sealed and placed in an oven at 150 °C overnight.

Aliquot of the digestions (dilution factor 500) were introduced into an Agilent Technologies 5110 ICP–OES (Penang, Malaysia) to determine total iron.

OSL age determination
All OSL samples were pretreated and measured at the Luminescence Dating Laboratory of Nanjing University. Under subdued red light, the bulk materials were initially treated with 10% hydrochloric acid (HCl) and 30% hydrogen peroxide (H2O2) to remove carbonates and organic material, respectively. The fine grains (4–11 µm) were separated by settling according to Stokes’ Law and treated with 30% hydrofluorosilicic acid (H2SiF6) for 5–7 days to extract the quartz component. The purity of the quartz fractions was verified using the OSL-IR depletion ratio method59. Samples with OSL-IR depletion ratios deviating from unity by 10% or more were etched again.

The quartz equivalent dose (De) was determined based on a standard SAR protocol60,61, using Risø TL/OSL-DA-20 luminescence readers equipped with blue LEDs (470 nm, ~80 mW cm−2) and infrared LEDs (870 nm, ~135 mW cm−2) and calibrated 90Sr/90Y beta source62. A preheat of 260 °C for 10 s and a cut heat of 220 °C were used. The OSL signal was stimulated by blue LEDs at 125 °C for 40 s and detected using a 7.5 mm thick Schott U-340 glass filter.

If two or more aliquots yielded De values larger than two times the D0 value of the growth function63, then the OSL signal of this sample was considered saturated, and therefore, any calculated age was considered the minimum age. The De value was given as the weighted arithmetic mean of each set of aliquots and presented alongside the standard error.

The concentrations of uranium, thorium, and potassium were measured using inductively coupled plasma‒mass spectrometry (ICP–MS) and atomic emission spectrometry (AES). The water content was obtained by weighing the sample before and after drying, with an uncertainty of ±10%. For the dose rates of quartz, an alpha efficiency value of 0.04 ± 0.0264 was adopted. The cosmic ray contribution to the dose rate was estimated as a function of sample elevation and burial depth65. Then, all the measured radioactive element concentrations were used to determine the external dose rates of quartz using the conversion factors66.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript entitled “Loess deposit reveals variations of 100-kyr cycle of temperature and 20-kyr cycle of precipitation at the low latitude of East Asia” by Li et al. presents a new set of proxy records reconstructed from the loess deposited over the past ~350 ka at Madang, East Asia. In comparison with loess records from the Chinese Loess Plateau, they suggested that the magnetic susceptibility (MS) record of loess varied at 100-kyr and 40-kyr cycles is essentially a proxy of temperature, while the DCB-extractable iron (FeD) varied at 50-kyr, 20-kyr and 10-kyr cycles is actually a proxy of precipitation. On the basis of these interpretations, the authors suggested that the Asian summer monsoon indicated by the precipitation is mainly forced by the summer insolation. Overall, the new data are valuable and the results/interpretations appear to be sound and interesting. I only have a few suggestions/comments for the improvement pending on which I recommend acceptance of this paper.\n(1) It might be oversimple to attribute the proxies, FeD, MS, hematite content, and redness, to effects from just temperature and precipitation. In theory, the pedogenesis duration also needs to be accounted for as suggested by Cheng et al. (2022).\n(2) The authors suggested that the Madang MS record, varied at 100-kyr and 40-kyr cycles, is basically a proxy of global temperature. This is certainly possible, but the evidence seems tentative, or insufficient.\n(3) All the discussions in the manuscript were presented more or less in a qualitative way, for example, lacking assessment of data uncertainties, and missing correlation coefficients for all comparisons, etc.\n(4) I am not sure to what degree the circular reasoning could occur. This is largely because all the chronologies involved are essentially from the orbital tuning strategy. I suggest having a short discussion.\n(5) Further editing is necessary. For example, the abbreviation DCB needs an elucidation when appears for the first time in the text; and Figs. S8 and S9 seems not be cited anywhere.", "reviewer_2": "Large attempts had been made to solve the important “Chinese 100-kyr Problem”, for example, Cheng et al. made systematic reviews and new attempts to explain and reconcile the related disputes (Cheng et al., 2020 Science China, 2022 The Innovation). The authors aimed at this important issue and provided a loess record from the lower reaches of the Yangtze River spanning past ~350 ka. The magnetic susceptibility (MS) and DCB extractable iron were measured and considered as temperature and precipitation proxies. Spectrum analysis on these records displayed a dominating of 100-kyr and 40-kyr cycles for the MS while 50-kyr, 20-kyr and 10-kyr cycles for the DCB records. They finally draw a conclusion that the temperature and precipitation in the past ~350 ka at the low latitude of East Asia have varied at different cycles, and the change of East Asian summer monsoon indicated by the precipitation in this region is mainly forced by the insolation change due to a variation in the precession. This study will be important if they do provide representative temperature and precipitation proxy and can provide their distinct periodicity evolution pattern. However, the present paper bears some flaws that harm the reliability of the results and the related conclusions. Therefore, the paper needs a major modification to resolve the following problems and concerns.\n1. The very basis of this study is the age model. Especially when the authors talk about the 20 -kyr, 10-kyr cycles using spectrum analysis, an accurate age model is indeed needed to convince us of the reliability of the results and relating conclusions. However, the age model of this 15.9 m thick Madang profile is only based on 2 OSL ages on the top part of the section at 2.4 and 3.5 m, in combination with 9 tie points yielding from a visual comparison between the studied MS curve to the MS of Luochuan profile from the Chinese Loess Plateau (Lu et al., 1999). Not to mention whether some hiatus existed in this section, 2 OSL age only on the top part cannot convince me of the subsequent visual comparison and the accuracy of the 9 tie points. For example: the correlation of the first tie point and the following linear interpolation will cause some errors due to different sedimentation rates because the top age of the Madang MS is unknown. The correlation of the fifth tie point is also inappropriate from the shape of the two curves.\n2. This age model cannot convince the robustness of spectrum analysis results. Furthermore, for the spectrum result of the MS record (Fig. 3b), the most significant cycles are ~66-kyr and ~33-kyr instead of 39-kyr and 97-kyr discussed by the authors. The dominating 10-kyr cycle in the DCB Fe record is not significant enough to pass the 95% significant level. For the wavelet results, to keep consistent with the power spectrum result and the fact that the main focus of this study is 100 and 97-kyr cycles, the 100-kyr period should at least be displayed in the wavelet figure (Y-axis), but the current y limit is only ~64 kyr (Fig. 3).\n3. Another key conclusion of this study is that the authors proposed the MS and DCB extractable iron as separated temperature and precipitation proxies. For the MS, this conclusion is derived from a visual comparison between the Madang MS curve with the temperature curve of the Antarctic ice core (Fig. 4a). This kind of visual comparison is usually subjective, for example, the reviewer considers that the correlation since 220 ka is ok, but that between 220-320 ka cannot support a real high correlation. In addition, a visual comparison between MS, redness and hematite content are served as supporting evidence. While, hematite generally develops best under warm and dry conditions, forming from ferrihydrite by a dehydrationrearrangement process (K¨ampf and Schwertmann, 1983; Maher, 1986) and controlled by multiple factors, such as temperature variations (e.g., Schwertmann, 1971; Torrent et al., 2006; Gao et al., 2018), pedogenesis processes in the drainage area, post-depositional dissolution of detrital magnetic minerals and changes in provenance (Yang & Ding, 2003; Fang et al., 2015). Redness of sediments is considered controlled by the content of fine-grained hematite (Schwertmann et al., 1982; Torrent et al., 1983). Therefore, for such an important conclusion, simple visual comparison with Antarctic ice core records as well as hematite and redness records controlled by multi-factors, should be cautious. More evidence, data or comparison methods are suggested to improve this key conclusion's robustness.\n4. In south China, the strong monsoon and potential fluvial process may alter the eolian deposition process, thus leading to some potential hiatus. Further, the potential iron mineral downward migration due to redox reactions may lead to the mismatch or lag between the iron mineral-based proxy and other records in such a humid and wetting region. For example, the highest Fed content is at ~250 ka, but this peak is at top of the L3, which cannot match any proxy (e..g, MS, Sanbao oxygen isotope), but lags behind a strong MS peak at ~240 ka (corresponding to S2 bottom).\n5. It is strange that the authors use hematite and redness to reflect temperature but FeD to reflect East Asian Summer Monsoon. Because most FeD in loess is composed of hematite and goethite. Further, redness can mostly reflect the hematite content. Another point is that during the loess stage, the FeD values are slightly higher than those in the paleosols, so can we expect that the overall weathering intensity is strong at loess? If not, how can we believe the short-term variation reflects the weathering intensity or summer monsoon rainfall? In this regard, such inconsistencies indeed question the validity of the FeD as a proxy of iron mineral weathering in this study. Rather, the FeD may also reflect the overall provenance effect on iron mineral content at loess and paleosols horizons. Because the authors used FeD rather than the FeD/FeT ratio to reflect summer monsoon precipitation, which means that the potential provenance change in iron minerals cannot be excluded. Such provenance effect may exhibit a profound control on changes in the orbital variations."}, "author_response": "RESPONSE TO REVIEWERS\nWe thank the reviewers for providing valuable comments and insightful suggestions on\nour manuscript. These comments and suggestions have been extremely helpful in\nimproving the quality of our work. After careful consideration of the comments, we\nhave fully revised our manuscript to address all of the reviewers’ concerns. Our detailed\nresponses are set out below (highlighted in blue). The main changes have been made in\nthe following areas:\n(1) Four additional OSL samples were measured, and new OSL ages were used to prove\nthe validity of the first tie point (56 ka) of the timescale;\n(2) Total iron (FeT) values were measured for 291 samples, and the FeD proxy indicator\nis replaced by the FeD/FeT ratio in order to eliminate the provenance effect;\n(3) The fifth tie point of the age controls was adjusted from the depth of 820 cm to 860\ncm, as suggested. Similarly, the third tie point was moved from the depth of 460 cm\nto 445 cm. As a result, the timescale of the Madang profile was revised accordingly;\n(4) When comparing time series of different proxy indicators, the cross-correlation\ncoefficient (r ) was calculated using a program specifically designed for unevenly\nxy\nspaced time series;\n(5) Uncertainty for each proxy indicator was estimated by replication analysis or\ncomparison with standard (known) samples;\n(6) The Vostok temperature curve was replaced by the EDC curve because the timescale\nof the EDC core was well constrained.\nReviewer #1 (Remarks to the Author):\nThe manuscript entitled “Loess deposit reveals variations of 100-kyr cycle of\ntemperature and 20-kyr cycle of precipitation at the low latitude of East Asia” by Li et\nal. presents a new set of proxy records reconstructed from the loess deposited over\nthe past ~350 ka at Madang, East Asia. In comparison with loess records from the\nChinese Loess Plateau, they suggested that the magnetic susceptibility (MS) record of\nloess varied at 100-kyr and 40-kyr cycles is essentially a proxy of temperature, while\nthe DCB-extractable iron (FeD) varied at 50-kyr, 20-kyr and 10-kyr cycles is actually a\nproxy of precipitation. On the basis of these interpretations, the authors suggested\nthat the Asian summer monsoon indicated by the precipitation is mainly forced by the\nsummer insolation. Overall, the new data are valuable and the results/interpretations\n1\nappear to be sound and interesting. I only have a few suggestions/comments for the\nimprovement pending on which I recommend acceptance of this paper.\n(1) It might be oversimple to attribute the proxies, FeD, MS, hematite content, and\nredness, to effects from just temperature and precipitation. In theory, the pedogenesis\nduration also needs to be accounted for as suggested by Cheng et al. (2022).\nResponse: We appreciate the reviewer’s helpful comments. When we state that an\nindicator can be interpreted as a proxy for temperature (precipitation), we do not mean\nthat the indicator is a pure proxy for temperature (precipitation). A more precise\nexpression is that the indicator can be interpreted as a temperature-dominated\n(precipitation-dominated) proxy. Regardless of whether they are biological or\ngeochemical indicators, it is important to acknowledge that their changes are influenced\nby a variety of factors from different perspectives. Taking climate as an example, the\nchange in biological or geochemical indicators is mainly driven by the seasonal\ncombination of water and heat. It is actually impossible to extract an indicator as a pure\nproxy for temperature (precipitation). Ideally, an indicator can only be interpreted as a\ntemperature-dominated (precipitation-dominated) proxy. To avoid misunderstandings,\nwe have revised our expression in the text by adding \"-dominated\". The pedogenesis\nduration suggested by Cheng et al. (2022) was added as an effect; and much analyses\nwere added to the revised manuscript.\n(2) The authors suggested that the Madang MS record, varied at 100-kyr and 40-kyr\ncycles, is basically a proxy of global temperature. This is certainly possible, but the\nevidence seems tentative, or insufficient.\nResponse: In principle, pedogenesis is generally considered to be the mechanism that\nenhances magnetic susceptibility (MS). However, pedogenesis is largely associated\nwith biological activity which is controlled by temperature, precipitation, their seasonal\ncombination, and parent materials, etc. Due to the spatially synchronous change of\nprecipitation and temperature in East Asia, it is difficult to determine which factor is\nthe main driving force by comparing the MS of modern surface samples only. Therefore,\nwe cannot directly prove that the Madang MS is a proxy for temperature in terms of\nmechanism. Nevertheless, since the change in loess MS at low latitude (Madang) is\nsimilar to that at mid-latitude (Chinese Loess Plateau), this implies that loess MS is a\nproxy for a climatic factor that can show a consistent variation over a large area (from\nmid-latitude to low latitude). In this sense, the Madang MS is more likely to be a proxy\nfor temperature than for precipitation, because temperature presents a large space\n2\nconsistent during the glacial-interglacial alternations in Northern Hemisphere, whereas\nprecipitation often shows a spatial discrepancy. Assuming that the Madang MS is a\ntemperature-dominated proxy, its change must be generally similar to the known\ntemperature record. Since the Madang MS shows a very similar trend of change to\nglobal temperature, it is reasonable to deduce that the Madang MS is a temperature-\ndominated proxy.\n(3) All the discussions in the manuscript were presented more or less in a qualitative\nway, for example, lacking assessment of data uncertainties, and missing correlation\ncoefficients for all comparisons, etc.\nResponse: We thank the reviewer for bringing this issue to our attention. To address the\nconcern, we have performed uncertainty assessments for all data and compared the\nresults with the variation amplitude of each indicator in the Madang profile (Table S3).\nIt is evident that the data uncertainty is significantly smaller than the variation\namplitude, demonstrating that the observed indicator variation in the profile is true to\nreveal the past climate change. We also performed correlation analyses between the two\ntime series. Using a computational program designed for unevenly spaced time series\n(Polanco-Martinez, et al., 2019), we estimated the cross-correlation coefficient (r )\nxy\nbetween the different time series. The correlation coefficients are presented in Figure 4\nand discussed in the text.\nTab. S3 Estimated uncertainty for each proxy parameter measured in this study\nVariation\nEstimation Relative Standard\nParameter Aliquots Samples Error in Madang\nmethod error deviation\nprofile\nMean grain Replication 10.6-13.5\n24 &$)SE\nsize (Mz) analysis SE\nMagnetic\nReplication 0.5×10- 13-150×10-\nsusceptibility 20\nanalysis 8m3kg-1 8m3kg-1\n(MS)\nDCB-\nReplication\nextractable 21 0.02% 1.16-4.63%\nanalysis\niron (FeD)\nTotal iron Standard\n<2% 4.14-6.38%\n(FeT) sample\nHematite Known\n7 0.03% 0.19-0.88%\n(Hm) sample\nRedness (a*) Replication 60 0.12 6.82-10.77\n3\nanalysis\n(4) I am not sure to what degree the circular reasoning could occur. This is largely\nbecause all the chronologies involved are essentially from the orbital tuning strategy.\nI suggest having a short discussion.\nResponse: Thanks for the useful suggestion. A paragraph has been added to the\nDiscussion section to address this issue, namely “The timescale of the Madang profile\nwas established by magnetic susceptibility stratigraphic correlation with the Luochuan\nloess profile, the chronology of which was constrained by orbital tuning. This approach\nmay lead to circular reasoning for the cycle analysis. However, the orbital tuning did\nnot result in a significant distortion of the actual timescale. The reliability of the orbital-\ntuned chronology of the Luochuan loess profile has been validated by the absolute\ndating of densely sampled OSL samples over the last three glacial-interglacial cycles\n(Zhang et al., 2022)”. (See Fig. R1 below).\nRedacted\nFig. R1 Bacon age-depth model for the Luochuan section of the Chinese Loess\nPlateau (Zhang et al., 2022).\n(5) Further editing is necessary. For example, the abbreviation DCB needs an\n4\nelucidation when appears for the first time in the text; and Figs. S8 and S9 seems not\nbe cited anywhere.\nResponse: Abbreviations were expanded at first use. The citations were inserted into\ntext.\nReviewer #2 (Remarks to the Author):\nLarge attempts had been made to solve the important “Chinese 100-kyr Problem”, for\nexample, Cheng et al. made systematic reviews and new attempts to explain and\nreconcile the related disputes (Cheng et al., 2020 Science China, 2022 The Innovation).\nThe authors aimed at this important issue and provided a loess record from the lower\nreaches of the Yangtze River spanning past ~350 ka. The magnetic susceptibility (MS)\nand DCB extractable iron were measured and considered as temperature and\nprecipitation proxies. Spectrum analysis on these records displayed a dominating of\n100-kyr and 40-kyr cycles for the MS while 50-kyr, 20-kyr and 10-kyr cycles for the DCB\nrecords. They finally draw a conclusion that the temperature and precipitation in the\npast ~350 ka at the low latitude of East Asia have varied at different cycles, and the\nchange of East Asian summer monsoon indicated by the precipitation in this region is\nmainly forced by the insolation change due to a variation in the precession. This study\nwill be important if they do provide representative temperature and precipitation\nproxy and can provide their distinct periodicity evolution pattern. However, the\npresent paper bears some flaws that harm the reliability of the results and the related\n the related\nconclusions. Therefore, the paper needs a major modification to resolve the following\nproblems and concerns.\n1. The very basis of this study is the age model. Especially when the authors talk about\nthe 20 -kyr, 10-kyr cycles using spectrum analysis, an accurate age model is indeed\nneeded to convince us of the reliability of the results and relating conclusions.\nHowever, the age model of this 15.9 m thick Madang profile is only based on 2 OSL\nages on the top part of the section at 2.4 and 3.5 m, in combination with 9 tie points\nyielding from a visual comparison between the studied MS curve to the MS of\nLuochuan profile from the Chinese Loess Plateau (Lu et al., 1999). Not to mention\nwhether some hiatus existed in this section, 2 OSL age only on the top part cannot\nconvince me of the subsequent visual comparison and the accuracy of the 9 tie points.\nFor example: the correlation of the first tie point and the following linear interpolation\nwill cause some errors due to different sedimentation rates because the top age of the\nMadang MS is unknown. The correlation of the fifth tie point is also inappropriate from\nthe shape of the two curves.\nResponse: We appreciate this comment and agree with the reviewer's emphasis on\nensuring the accuracy of the chronology. In fact, the age model of the Madang profile\nwas established by linear extrapolation and interpolation of 9 tie points determined by\nMS stratigraphic correlation. The two OSL ages were only used to check the validity of\nthe stratigraphic correlation in the original manuscript. We agree with the reviewer that\nthe first tie point cannot be well verified by the two OSL ages. Therefore, we performed\nage measurements on four additional OSL samples. The results (Tab.S4, in red) show\nthat these four OSL ages are consistent with the stratigraphic sequence. The first tie\npoint (56 ka) falls exactly in the period between 39.6±2.7 ka and 66.8±4.8 ka (as shown\nin the updated Fig. 2). The new OSL ages provide further evidence for the reliability of\nthe first tie point.\nTab. S4 OSL dating results of quartz of Madang profile\nDepth Grain size Water content Dose rate Aliqout\nSample U (ppm) Th (ppm) K (%) De (Gy) Age (ka)\n(cm) !SE\" (%) (Gy/ka) number\nA8-40 40 4-11 17.1 114.0±2.3 114.0±2.3 114.0±2.3 114.0±2.3 4.03±0.28 30.1±2.2 6\nA17-85 85 4-11 17.1 149.6±1.8 149.6±1.8 149.6±1.8 149.6±1.8 4.12±0.28 39.6±2.7 6\nMD-P-0.6 155 4-11 13.2 3.06±0.15 13.42±0.67 1.75±0.03 246.7±4.2 3.69±0.26 66.8±4.8 6\nMD-P-1.2 215 4-11 15.4 3.07±0.15 13.66±0.68 1.71±0.03 262.6±9.3 3.58±0.25 73.3±5.8 6\nMD-P-1.5 245 4-11 17.4 3.21±0.12 13.90±0.38 1.71±0.05 271.5±5.2 3.56±0.25 76.3±5.5 6\nMD-P-4.5 350 4-11 17.6 2.44±0.10 15.40±0.42 1.96±0.06 312.8±6.7 3.65±0.25 85.6±6.0 6\nprofile at low latitude and the Luochuan profile at middle latitude (Lu et al., 1999).\nFour data in the red circle are newly measured OSL ages.\nBased on the characteristics of two MS curves, we made adjustments to the third and\nfifth tie points. As suggested, the fifth tie point was shifted from the depth of 820 cm to\n860 cm. Similarly, the third tie point was shifted from the depth of 460 cm to 445 cm.\nThese modifications resulted in the generation of a new timescale, which was then used\nfor further analysis (Fig. R2). The cross-correlation coefficient (r ) between the MS\nxy\ntime series of the Madang and Luochuan profiles was calculated to be 0.88, indicating\nthe reliability of the Madang timescale (Fig. R3).\n7\n0 50 100 150 200 250 300 350\n0\n2\n4\n6\n8\n10\n12\n14\nFig. R3 Comparison of Madang and Luochuan MS curves. The strong correlation (r\nxy\n= 0.88) demonstrates that the timescale of Madang, generated using nine tie points, is\nreliable.\nResponse: The overall similarity in the stratigraphic and proxy records across numerous\nprofiles suggests that, generally, loess accumulation is likely continuous on orbital\ntimescales throughout the main body of the Chinese Loess Plateau (Zhang et al., 2022;\nLu et al., 2022). This suggestion is further supported by densely-sampled OSL dating,\nwhich reveals that loess accumulation at Luochuan was continuous over the last three\nglacial–interglacial cycles on orbital timescales (Fig. R1). Several factors lead us to\nbelieve that the Madang profile is continuous on orbital timescales. These include the\noverall similarity in MS stratigraphy (updated Fig. 2), the absence of erosion surfaces,\n8\n)m(htpeD\nAge (ka)\nOld\nNew\n300 180\n200 120\n100 60\n0 0\n)1-gk3m8-01(SM\n)1-gk3m8-01(SM\nLuochuan r = 0.88\nxy\nMadang\nAge (ka)\nand the lack of abnormal sedimentation rates as illustrated in the age-depth plot (Fig.\nR2).\nThe records (MS, FeD/FeT, and Hm) analyzed with the new timescale reveal clearer\norbital cycles (refer to updated Fig. 3 and Fig. S8). Specifically, both MS and Hm curves\nexhibit a dominant cycle of 101-kyr, while the FeD/FeT curve displays a distinct cycle\nof 18-kyr. It is evident that the former can be interpreted as a temperature-dominated\ncycle, whereas the latter can be attributed to a precession-dominated cycle.\nUpdated Fig. 3 Wavelet and spectral analysis of MS (a, b) and FeD/FeT (c, d) of the\nMadang profile\n9\n8H=:K>=1B@$6"}]

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