Astronomical Time Scale of the Late Pleistocene in the Northern South China Sea Based on Carbonate Deposition Record

Round 1

Reviewer 1 Report

Line 48 I think it is important to state that this is an ocean current and to provide a brief description of its physical properties for the benefit of the reading audience.

Line 61 What materials were analyzed to obtain OSL dates? Quartz and feldspar grains? or something else? Please specify.

Line 65 to the best of my knowledge, OSL dating techniques are applied to siliciclastic sediments. Are they also appropriate for dating marine carbonates? Please clarify what materials were submitted for OSL dating.

Line 65-78 I appreciate the attention devoted to describing the constraints for acquiring radiocarbon dates from these deep sea cores.  

The comprehensive Introduction is one of the several strengths of this research manuscript in my opinion. 

Line 157 I do not see the connection between carbonate deposition and preservation and Milankovitch cycles? Please elaborate on this subject.

Figures 1 and 3. Please check the spelling of place names on these maps. HaiNan should appear as Hainan, TaiWan should appear as Taiwan, LuZon should appear as Luzon.

Line 221 The sedimentary record is...

Line 232 The authors are examining the carbonate content of marine sediments, not soils. Please correct this error.

Line 236 Please move this sub-heading to next page.

Lines 239-240 This appears to be an extraordinarily strong data transformation! Does this filter introduce bias into the transformed data set? Please comment. 

Line 258 38% by weight appears to be a substantial quantity of carbonate in my opinion. 

Lines 259-272 and Figure 2 It is not clear to me how the authors divided the carbonate record into four stages given the large variations in carbonate contents within the individual stages. Please clarify how these stratigraphic boundaries were  established.

Lines 309-310 ...in the carbonate strata, the ...

Lines 319-320 please see my comments in the text

Line 323 please see my comment in the text

Line 332 please see my comment in the text

Line 355 Figure 5 Which control points - SB2.0, SB3.0, SB5.0? Label these points on this graph. Why was the AMS 14C date not used to calibrate this age model? This date does not appear in the figure? Please link the changes in sedimentation rates with the four stages identified in the carbonate stratigraphy. 

Line 363 please see my comment in the text

Lines 397-401 Please link these observations to marine production of carbonates. 

Line 415 Of course, you forced the model to yield these results in my opinion!

Lines 419-420 You should expect this outcome given how you tuned the model.

Line 469 This statement does not make sense?

Lines 476, 477, 479 please see my comments in the text.

I appreciate that these authors presented a plausible explanation for the early Holocene low carbonate event recorded in the core.

I regret that I do not understand the connection between the precession cycle and the relative strength of the Asian monsoon. To the best of my knowledge, the precession cycle is linked to the timing (date) of perihelion and aphelion in Earth's revolution about the Sun. How the timing of these events is linked to insolation in low latitudes and the relative strength of the Asian monsoon is not clear to me.  

References - please see my comments in the text. I note the inconsistent use of capital letters in the literature citations and these minor errors should be corrected in revision. Thank you. 

I learned a lot about the paleoceanography of the South China Sea during my review of the manuscript. As someone whose research has been conducted exclusively north of the Arctic Circle, I appreciated learning about the influence of the Asian monsoon on marine sedimentation in this low latitude marine environment. 

Comments for author File: Comments.pdf

I was impressed by the quality of the English expression demonstrated by the authors. Recommendations for minor revisions are noted in the manuscript. 

Author Response

Dear reviewer,

Thank you very much for your comments and suggestions. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We have studied the comments carefully and have made corrections, which we hope meet with approval. Meanwhile, we tried our best to improve the language of the manuscript and made some changes to the manuscript. These changes will not influence the content and framework of the paper. It is a reason to delay the return of the revised manuscript. Please see if the revised version met the English presentation standard. We attached the revised manuscript in PDF format for your approval. The main corrections in the paper and the responses to the reviewer's comments are as follows.

Point 1: Line 48 I think it is important to state that this is an ocean current and to provide a brief description of its physical properties for the benefit of the reading audience.

Response 1: Thanks for this suggestion. We have made corrections based on the reviewer’s comment and the results are as follows. Please check it.

“The Kuroshio is an important part of the western boundary current of the northern Pacific Ocean, with high temperature and salinity characteristics [9,10].”

Point 2: Line 61 What materials were analyzed to obtain OSL dates? Quartz and feldspar grains? or something else? Please specify.

Response 2: Thanks for this suggestion. We have made corrections based on the reviewer’s comment and the results are as follows. Please check it.

“Zhong et al. and Lin et al. used optically stimulated luminescence of quartz to establish the chronological framework of the ChaoShan Delta and Pearl River Delta, respectively [15,16].”

Point 3: Line 65 to the best of my knowledge, OSL dating techniques are applied to siliciclastic sediments. Are they also appropriate for dating marine carbonates? Please clarify what materials were submitted for OSL dating.

Response 3: Thanks for this suggestion. OSL dating techniques are not suitable for dating marine carbonates. We have made corrections based on the reviewer’s comment and the results are as follows. Please check it.

“The transportation distance, particle size, depositional environment, and other factors can make the degree of solar bleaching of quartz or feldspar difficult to evaluate [14,17].”

Point 4: Line 65-78 I appreciate the attention devoted to describing the constraints for acquiring radiocarbon dates from these deep sea cores. The comprehensive Introduction is one of the several strengths of this research manuscript in my opinion.

Response 4: We are very grateful for your kind appraisal. Thank you for your time and dedication in reviewing our manuscript. We are honored to have had the opportunity to receive feedback from someone with your level of expertise.

Point 5: Line 157 I do not see the connection between carbonate deposition and preservation and Milankovitch cycles? Please elaborate on this subject.

Response 5: Thanks for this suggestion. We have made corrections based on the reviewer’s comment and the results are as follows. Please check it.

“The suitability of carbonate stratigraphy and astrochronology for core SCS1 in the SCS is supported by several factors: a) the widespread availability of carbonate content as a classical paleoenvironmental proxy [58-62]; b) the established link between carbonate content variations and glacial-interglacial cycles [60,63]; c) the influence of monsoon variability on carbonate deposition and preservation [55], and d) the significant effects of Milankovitch cycle-induced climatic oscillations on glacial-interglacial cycles, monsoon variability and ocean productivity [64-68]. The change in carbonate content is affected by dilution and productivity [55]. Both dilution and productivity are related to monsoon variability [55]. Precession forcing has long been considered to be the main force of monsoon variation on orbital time scales [66,68]. It redistributes or splits seasonal in-coming solar radiation, especially at low latitudes where monsoon systems prevail [64-67]. The Summer and winter monsoons both have an impact on carbonate content through productivity and dilution [55]. Thus, the cycle of low-latitude solar radiation is also transmitted to the oceanic carbonate deposition record through the monsoon as a link. Furthermore, the periodic changes in solar insolation with latitude and season caused by changes in the Earth's orbital parameters are the fundamental driving force for climate change and glacial cycles on a millennium to ten thousand years scale [69]. The periodicity controlled by the precession of monsoon variability is considered to be influenced by the Pliocene/Pleistocene glacial-interglacial cycle [55]. At the same time, the carbonate content during the late Pleistocene is also related to the glacial-interglacial cycle [55,60]. These factors suggest that Earth's orbital parameter changes directly impact the deposition and preservation of carbonates in sediments, reflected in the carbonate and other biogeochemical records of ocean sediments.”

Point 6: Figures 1 and 3. Please check the spelling of place names on these maps. HaiNan should appear as Hainan, TaiWan should appear as Taiwan, LuZon should appear as Luzon.

Response 6: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 7: Line 221 The sedimentary record is...

Response 7: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 8: Line 232 The authors are examining the carbonate content of marine sediments, not soils. Please correct this error.

Response 8: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 9: Line 236 Please move this sub-heading to next page.

Response 9: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 10: Lines 239-240 This appears to be an extraordinarily strong data transformation! Does this filter introduce bias into the transformed data set? Please comment.

Response 10: Thanks for this suggestion. This filter introduce does not bias the transformed data set. This step is a key step in time series analysis. Removal of these long-term trends, or detrending, is a critical step for power spectral analysis to ensure that data variability oscillates about a zero mean, and to avoid power leakage from very low-frequency components into higher frequencies of the spectrum.

Point 11: Line 258 38% by weight appears to be a substantial quantity of carbonate in my opinion.

Response 11: Thanks for this suggestion. Our test results are indeed so. The reasons for this high value may be: this period was in the last deglacial period, and the increase in temperature increased the productivity of the calcareous biology. The higher productivity is conducive to the preservation of carbonate and the increase of carbonate content. Moreover, during the last deglacial, the increased overturning circulation disrupts ocean stratification further enhancing surface ocean productivity and simultaneously leading to a rapid rise in atmospheric CO₂. According to carbon cycle models, carbon transfer to the atmosphere will increase deep water [CO^2- ₃] concentration over the whole ocean. Thus, carbon release from the deep ocean would also lead to maximum values of [CO^2- ₃] and carbonate in the deep ocean during the last deglacial.

Point 12: Lines 259-272 and Figure 2 It is not clear to me how the authors divided the carbonate record into four stages given the large variations in carbonate contents within the individual stages. Please clarify how these stratigraphic boundaries were established.

Response 12: We are mainly divided according to the level and change trend of carbonate content. The four stages are divided here to facilitate the description of the change characteristics of carbonate content results in sedimentary record.

“According to the level and change trend of carbonate content, this variability is categorized into four distinct stages (Figure 2): stage I (7.82-5.2 mbsf): carbonate content fluctuates between 10.3% and 15.1%, averaging at 12.7%. This stage is marked by relatively low but gradually increasing carbonate levels from 6.58 to 6.02 mbsf, exhibiting minimal variability; stage II (5.20-3.10 mbsf): here, carbonate content spans 10.4% to 21.1%, with an average of 15.3%. This stage shows moderate variability and an overall increase in carbonate content compared to Stage I; stage III (3.10-0.98 mbsf): carbonate levels oscillate between 9.5% and 38.0%, with an average of 14.5%. This stage features significant variability, with lower content observed from 3.10 to 1.66 mbsf and higher levels noted between 1.66 to 0.98 mbsf; stage IV (0.98-0.02 mbsf): this stage sees carbonate content ranging from 2.8% to 20.1%, averaging at 11.8%. Characterized by large fluctuations, the carbonate content generally decreases towards shallower depths, with the lowest value recorded in Stage IV.”

Point 13: Lines 309-310 ...in the carbonate strata, the ...

Response 13: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 14: Lines 319-320 please see my comments in the text

Response 14: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 15: Line 323 please see my comment in the text

Response 15: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 16: Line 332 please see my comment in the text

Response 16: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 17: Line 355 Figure 5 Which control points - SB2.0, SB3.0, SB5.0? Label these points on this graph. Why was the AMS 14C date not used to calibrate this age model? This date does not appear in the figure? Please link the changes in sedimentation rates with the four stages identified in the carbonate stratigraphy.

Response 17: Thanks for this suggestion. We have labeled these points on this graph. As introduced in the paper, there are many problems in the AMS ¹⁴C dating of marine sediments, such as the surface reservoir age and some other problems have not yet been clarified. Moreover, the dating result of this data is more than 40,000 years old, and the result itself has a large error for the AMS ¹⁴C dating technique. Therefore, this paper does not use it to calibrate this age model, but only refers to this result to identify the carbonate stratigraphic events. Finally, the age of the identified carbonate stratigraphic events is used as the control point.

The changes in sedimentation rate are linked to the four phases identified in the carbonate formation as follows: “According to the variation of sedimentation rate (Figure 5), we found that the average sedimentation rate of Stage I and II in carbonate content record is 6.1 cm/kyr, which belongs to the MIS 3-5 period; the average sedimentation rate of stage III is 17.6 cm/kyr, which belongs to the MIS 2 period; and the average sedimentation rate of stage IV is 8.1 cm/kyr which belongs to the MIS 1 period. The sedimentation rate during the glacial period is higher than that during the interglacial period, with the highest sedimentation rate during the MIS 2 period at the last glacial period.”

Point 18: Line 363 please see my comment in the text

Response 18: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 19: Lines 397-401 Please link these observations to marine production of carbonates.

Response 19: Thanks for this suggestion. We have made corrections based on the reviewer’s comment and the results are as follows. Please check it.

“Thus, the phase relationship between carbonate cycles and precession parameters is characterized by a correspondence where the lowest (highest) carbonate levels coincide with the precession index's peak (trough) values. Precession dominates productivity and Asian winter monsoon changes in the SCS [66-68,112-114]. Meanwhile, the productivity change in the northern SCS is affected by the Asian winter monsoon [113,115,116]. The minimum value of the precession causes the enhancement of the Asian winter monsoon, which leads to the enhancement of carbonate productivity, and the opposite is true when the precession is at the maximum value. A detailed discussion is presented in Section 5.2.”

Point 20: Line 415 Of course, you forced the model to yield these results in my opinion!

Response 20: Thanks for this suggestion. We obtained tuned carbonate time series date after tuning based on the phase relationship between carbonate record and precession; this data was later used to perform a power spectral analysis in Acycle software. Power spectral analysis evaluates the distribution of time series variance (power) as a function of frequency. The primary use of power spectral analysis is for the recognition of periodic or quasi-periodic components in a data series. Here the method we have chosen is Multi-taper method (MTM) and robust AR (1) red noise model. Thereby this result is obtained.

Point 21: Lines 419-420 You should expect this outcome given how you tuned the model.

Response 21: Thanks for this suggestion. The tuned carbonate time series date was obtained by using QAnalySeries software, and then we used Acycle software to perform Evolutionary Spectral Analysis on the tuned carbonate time series date. The method used is Fast Fourier transform, and the Evolutionary Spectral Analysis results are obtained to visualize the periodical changes of the data series. The Evolutionary Spectral Analysis results also show the accuracy of the tuning results.

Point 22: Line 469 This statement does not make sense?

Response 22: Thanks for this suggestion. We have made corrections based on the reviewer’s comment and the results are as follows. Please check it.

“The carbonate content increased from the Last Glacial Period to the Holocene [62,120]. However, an abrupt decrease in carbonate content during the early Holocene is noted, with the content dropping from 20.1% to 7.9%, a 61% reduction (Figure 7A). Following this decrease, the sediment's carbonate content gradually recovers (Figure 7).”

 Point 23: Lines 476, 477, 479 please see my comments in the text.

Response 23: Thanks for this suggestion. We have made corrections based on the reviewer’s comment. Please check it.

Point 24: I regret that I do not understand the connection between the precession cycle and the relative strength of the Asian monsoon. To the best of my knowledge, the precession cycle is linked to the timing (date) of perihelion and aphelion in Earth's revolution about the Sun. How the timing of these events is linked to insolation in low latitudes and the relative strength of the Asian monsoon is not clear to me.

Response 24: Precession forcing has long been considered to be the main force of monsoon variation on orbital timescales. The presence of the precession significantly affects the seasonal differences in insolation, especially at low latitudes. It redistributes or splits seasonal incoming solar insolation. Because the global monsoon is more distributed in the lower latitudes, and more and more monsoon climate record there are precession cycles. Therefore, precession plays an important role in the evolution of monsoon.

In this study, the precession curve of the tuned carbonate time series is consistent with that of the northern hemisphere summer solar insolation (Figures 6E and F). It shows that the change in carbonate content is influenced by solar insolation. The greater the solar in-solation, the higher the temperature, and thus the more carbonate is formed. Moreover, spectral analysis underscores the dominant role of precession forcing in modulating productivity changes within the SCS region [112]. The evident strong precession cycle in core SCS1's carbonate record suggests that productivity changes significantly affect carbonate preservation at this site [113]. The variability in surface ocean productivity in the SCS, particularly in its northern parts, is closely linked to the intensity of the Asian winter monsoon systems [113,115,116]. Sedimentary records from the SCS further reveal that these monsoon systems are largely influenced by the precession cycle [66-68,114]. Analysis of insolation changes induced by orbital parameters over the last 300,000 years [65,132] shows that minimum precession coincides with maximal northern hemisphere summer insolation and minimal winter insolation, and vice versa for maximum preces-sion [69]. When the northern hemisphere winter insolation caused by precession is weakened (enhanced), the temperature decrease (increase) in the southern East Asia continent is significantly greater than the temperature change in the neighboring west-ern Pacific region, thus strengthening (weakening) the latitudinal land-sea thermal contrast, and then strengthening (weakening) the winter monsoon in the southern East Asia region [67,133]. Thus, when the precession is at a minimum, the Northern Hemi-sphere winter insolation is low, the East Asian Winter Monsoon is stronger [66,67,114]. The strong winter monsoon enhances surface seawater productivity during minimum precession phases, favoring carbonate deposition and preservation; the opposite occurs during maximum precession phases.

Point 25: References - please see my comments in the text. I note the inconsistent use of capital letters in the literature citations and these minor errors should be corrected in revision. Thank you.

Response 25: Thanks for this suggestion. We have made corrections based on the reviewer's comment and JMSE guidelines. Please check it.

We look forward to hearing from you in due time regarding our submission and to respond to any further questions and comments you may have. Thanks for your time.

Best wishes

Chunhui Zhang

Author Response File: Author Response.pdf

Reviewer 2 Report

Please see the attached file.

Comments for author File: Comments.pdf

Minor editing of English language required

Author Response

Dear reviewer,

Thank you very much for your comments and suggestions. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We have studied the comments carefully and have made corrections, which we hope meet with approval. Meanwhile, we tried our best to improve the language of the manuscript and made some changes to the manuscript. These changes will not influence the content and framework of the paper. It is a reason to delay the return of the revised manuscript. Please see if the revised version met the English presentation standard. We attached the revised manuscript in PDF format for your approval. The main corrections in the paper and the responses to the reviewer's comments are as follows.

 Point 1: [Abstract] Citations in the Abstract (i.e., Laskar et al., 2004 and Wang et al., 1995) should be avoided: the Abstract should focus on the original research by the Authors and not on the work of others; moreover, the Abstract should be self-contained and fully understandable without reference to other sources.

 Response 1: Thanks for this suggestion. We have made corrections based on the reviewer's comment and the results are as follows. Please check it.

“Variations in solar insolation caused by changes in the Earth's orbit—specifically its eccentricity, obliquity, and precession—can leave discernible marks on the geologic record. Astrochronology leverages these markers to establish a direct connection between chronological measurements and different facets of climate change as recorded in marine sediments. This approach offers a unique window into the Earth's climate system and the construction of high-resolution, continuous time scales. Our study involves comprehensive bulk carbonate analyses of 390 discrete samples from core SCS1, which was retrieved from the deep-sea floor of the northern South China Sea. By utilizing carbonate stratigraphic data, we have developed a carbonate stratigraphic age model. This was achieved by aligning the carbonate sequence from core SCS1 with the established carbonate standard stratigraphic time scale of the South China Sea. Subsequently, we construct an astronomically tuned time scale based on this age model. Our findings indicate that sediment records in this core have been predominantly influenced by a 20,000-year cycle (precession cycle) throughout the Late Pleistocene. We have developed an astronomical time scale extending back approximately 110,000 years from the present, with a resolution of 280 years, by tuning the carbonate record to the precession curve. Time-domain spectral analysis of the tuned carbonate time series, alongside the consistent comparability of the early Holocene low carbonate event (11-8 kyr), underscores the reliability of our astronomical time scale. Our age model exposes intricate variations in carbonate deposition, epitomizing a typical "Pacific-type" carbonate cycle. Previous research has illustrated that precession forcing predominantly influences productivity changes in the South China Sea. The pronounced precession-related cycle observed in our record suggests that changes in productivity significantly impact carbonate content in the area under study. Furthermore, the clear precession period identified in the carbonate record of core SCS1 reflects the response of low-latitude process to orbital parameters, implying that carbonate deposition and preservation in core SCS1 are chiefly influenced by the interplay between the Intertropical Convergence Zone (ITCZ) and the monsoon system within the precession band. Our astronomical time scale is poised to enhance paleoceanographic, paleoclimatic, and correlation studies further. Additionally, the independent evidence we provide for using proxy records for astronomical age calibration of marine sediments lends additional support to similar methods of astronomical tuning.”

Point 2: [Keywords] The keyword "precession" alone does not appear too effective. I would replace (or integrate) it with a more specific keyword.

Response 2: Thanks for this suggestion. We have made corrections based on the reviewer's comment and the results are as follows. Please check it.

“Keywords: carbonate; astrochronology; Milankovitch cycles; low carbonate event; insolation”

 Point 3: [Introduction and all over the manuscript] According to JMSE guidelines, references should be indicated in order of appearance and indicated by a numeral or numerals in square brackets.  

Response 3: Thanks for this suggestion. We have made corrections based on the reviewer's comment and JMSE guidelines. Please check it.

 Point 4: [Core description] It is unclear when and how the 7.82 m long core SCS1 was retrieved from deep sea basin in the northern South China Sea. Could the Authors explain?

Response 4: Thanks for this suggestion. We have made corrections based on the reviewer's comment and the results are as follows. Please check it.

"Core SCS1, measuring 7.82 meters in length, was collected from a deep-sea basin in the northern SCS using a gravity column sampler on June 19, 2017, at coordinates 18°30'22.56"N, 116°16'5.28"E, and a water depth of 3770 meters (Figure 1)."

 Point 5: [Conclusion] Overall, this section is well-structured. However, the Authors should mention the limitations of their study and offer suggestions for relevant future studies.

Response 5: Thanks for this suggestion. We have made corrections based on the reviewer's comment and the results are as follows. Please check it.

“In this study, we have detailed a high-resolution carbonate record from core SCS1, collected from the depths of the SCS. By integrating carbonate stratigraphy with time-series analyses of the carbonate record, we have successfully established an age-calibrated astronomical time scale for core SCS1. The initial analysis of an untuned carbonate stratigraphic age model revealed that precession is the most prominently ex-pressed astronomical parameter. Consequently, an astronomical time scale covering the past ~110 kyr was developed by aligning the time domain carbonate data with the La2004 precession curve. This astronomical time scale reveals that the carbonate record of core SCS1 exhibits a "Pacific-type" carbonate cycle. Moreover, the timing and duration of the low carbonate event identified in core SCS1 align with findings from previous research. This event is primarily attributed to dilution effects coupled with a reduction in productivity. The pronounced precession cycle observed in the carbonate record of core SCS1 underscores that changes in carbonate content are driven by precessional forcing, indicative of low-latitude processes. Within the precession band, the interplay between the Intertropical Convergence Zone (ITCZ) and the monsoon system significantly influences carbonate deposition and preservation by modulating productivity variations in the SCS. The insights gained from this study significantly enhance our understanding of paleoceanographic and paleoclimatic evolution in the northern SCS, highlighting the intricate relationship between astronomical parameters and marine sedimentation processes. However, this study did not reconstruct the paleoclimate evolution of the northern SCS, so in future studies, we can combine a variety of climate proxy indicators to carry out research on the paleoclimate evolution of the northern SCS.”

Point 6: [References]

Ref.#37. This reference is incomplete! Even the book title is missing.

Ref.#41. Does this reference correspond to an available paper?

Ref.#56. At line 727 it reads “Paleoceanography, 20(1).”, but I would specify “Paleoceanography, 20(1), PA1003.”.

Ref.#140. At line 895 it reads "Frontiers in Marine Science, 9.", but I would write "Front. Mar. Sci. 9:1030841".

Response 6: Thanks for this suggestion. We have made corrections based on the reviewer's comment and JMSE guidelines. Please check it. The Reference (Kotov, S.; Paelike, H. QAnalySeries-a cross-platform time series tuning and analysis tool. In Proceedings of the AGU Fall Meeting Abstracts, 2018; pp. PP53D-1230.) is a published Abstract of the conference.

We look forward to hearing from you in due time regarding our submission and to respond to any further questions and comments you may have. Thanks for your time.

Best wishes

Chunhui Zhang

Author Response File: Author Response.pdf