Weakening of the South Asian summer monsoon linked to interhemispheric ice-sheet growth since 12 Ma

The evolution and driving mechanism of the South Asian summer monsoon (SASM) are still poorly understood. We here present a 12-Myr long SASM record by analyzing the strontium and neodymium isotopic composition of detrital components at IODP Exp. 359 Site U1467 from the northern Indian Ocean. The provenance investigation demonstrates that more dust enriched in εNd from northeastern Africa and the Arabian Peninsula was transported to the study site by monsoonal and Shamal winds during the summer monsoon season. A two-step weakening of the SASM wind since ~12 Ma is proposed based on the εNd record. This observational phenomenon is supported by climate modeling results, demonstrating that the SASM evolution was mainly controlled by variations in the gradient between the Mascarene High and the Indian Low, associated with meridional shifts of the Hadley Cell and the Intertropical Convergence Zone, which were caused by interhemispheric ice-sheet growth since the Middle Miocene.

4. The EICC and WICC stand for east Indian coastal current and west Indian coastal current, respectively, as such, these currents flow along the coast following the bathymetric constriction. In contrast to the EICC and WICC, the authors correctly pointed out that the SMC and WMC stand for summer monsoon current and winter monsoon current, respectively. The WICC or EICC would not deflect away from the coast while carrying buoyant freshwater due to the Coriolis force and rotation of the Earth. Is there a possibility that the authors may have confused these two sets of currents in explaining the Ganges-Brahmaputra transport to the IODP Site U1467? Given the deep bathymetric separation (see the figure below) between India and Chagos-Laccadive Plateau, it would be an unrealistic proposition to receive any detrital or other contribution from the Ganges-Brahmaputra discharge via EICC or WICC at the Site U1467.
5. I wonder whether the authors explored the possibility of any contribution by the Antarctic Intermediate Water (AAIW) borne Nd to the IODP Site 1467, given its location at a shallow water depth of 487 m? 6. I find the paragraphs between lines 224 and 297 are a mix between facts and fiction. The authors suggested that "the changes in the MH and IL pressure system, meridional temperature gradients, and the polar ice-sheet growth were linked coherently, which generated the two-step decline of the SASM since ~12 Ma (lines 237-240)". I had to jog my memory about Zachos contributions to polar ice-sheets growth during the Cenozoic. The Northern Hemisphere's ice sheets began to appear ~8 Ma (Science 292, 2001; Fig.  2), and the Antarctic Ice Sheets (i.e., East ad West) were in their place well before ~8 Ma. Therefore, it would have been better if the authors toned down their claim a bit and used the existing literature to further their hypothesis.
Validity: It is unclear whether the authors validated eNd used in the manuscript as the issues of boundary exchange, fossil fish teeth/debris, and detrital fraction to validate authigenic versus detrital Nd data at the IODP Site U1467. The authors appear to either ignore or unaware of these factors that may influence eNd data. For a quick review of these issues, I would like to suggest that the author consult publications such as Du  Significance: These are the first set of data in the detrital fraction from a "relatively" older stratigraphy (i.e., Miocene) compared to the abundant data for the late Pleistocene. However, Yao et al. appear to set their sight to explain the eNd data as a function of the strength of the SASM without providing any discussion or discounting the impact of the Indian Ocean intermediate and deep circulation and boundary exchange. I would extend my neck and say that there has been a revolution in interpreting eNd data over the last three years compared to traditional views using the eNd data. Therefore, it is expected that the authors would provide multiple hypotheses and then settle on one that supports their data. That scientific thinking or rigor appears to be absent in the manuscript.
Data and Methodology: The authors used an established and time-tested methodology to acquire Nd data from the IODP Site U1467. I am not a modeler; however, I am familiar with and have been using model results obtained by the earlier version of the Community Earth System Model.
Analytical approach: The authors applied acceptable standard international analytical protocol used to acquire Nd data in the manuscript currently used by the Nd community.
Clarity and context: The manuscript text must be contextualized (see above) and improved the text.
References: I would like to suggest that the authors consider improving the list of references by following: (i) first cite references available from the northern Indian Ocean which has been inadequately referenced; (2) reduce the superfluous references that have no bearing to the topic of discussion, and (3) add or cite a few references that have divergent views (it does not mean that the authors need to accept those views).
Your expertise: I am comfortable assessing the content of the manuscript with ease as I am familiar with the Nd data through my publications from the northern Indian and the North Atlantic oceans. I have also provided reviews of manuscripts from numerous journals and research proposals dealing with the Nd geochemistry from the North Atlantic and Indian oceans.

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Editorial Note: Parts of this Peer Review File have been redacted as indicated to remove third-party material where no permission to publish could be obtained.
This paper presents a geological record and a modelling approach to provide an insight about the mechanisms and causes behind the evolution off the South Asian Summer Monsoon over the last 12 million years. The rationale for the study is that the SASM has been understudied compared to the East Asian Monsoon due to a scarcity of suitable records. However, it is, today, an important climatic system that affects the wellbeing of billions of people. We are here presented with a multi proxy record, using strontium and neodymium isotopic compositions of the sediments collected at IODP Exp. 359 Site U1467 in the Maldives Inner Sea. The paradigm here is that the source of terrigenous components in the ocean can be traced using Sr and Nd isotope ratios. In addition, variation in the 87Sr/86Sr ratio can be used to reflect the degree of weathering on the continent, therefore a climatic signal. Based on previous studies, the authors have defined different sources of the terrigenous component based on the !Nd versus 87Sr/86Sr. Data from site U1467 are compared to other records which contain proxies as indicators of terrigenous sources (chlorite/smectite). The second component of the paper is a modelling approach of climatic conditions under different scenarios in relation to ice-sheet development in the high latitudes of both hemispheres. The strength of the study is a methodological approach that is robust and a dataset showing changes concomitant with a global cooling signal, highlighting the weakening of the SASM over the last 12 million years, including an abrupt decline around 7 to 5 Ma. The mechanisms to explain the overall decline and abrupt decrease are also effectively discussed using the reconstructed wind speed, precipitation and sea level pressure scenarios. However, there are some weaknesses in this paper that need to be addressed. Firstly, why not analyse the chlorite/smectite composition in site U1467 so comparison between proxies would be stronger; in this paper, cores ODp658 and U1447 are in a different basin compared to U1467. Secondly, the structure of the paper is not adequate in place. The location of the core is rather important to understand why the sediments there can be used to study eolian components. I would suggest moving the text from lines 155 to 175 to before line 103. Figure 3 should be presented before figure 2a, that means to move lines 79-86 after line 133. Below are some suggestions corrections: Line 54: add "and wellbeing" after "economic" Line 95: correct "was" with "were" Line106: replace B-G with G-B Line 126: replace river with River Line 156 replace enclose with encloses Line 156 replace is with are Line 188: Add "a" after as Figure 1 caption is incomplete, add d after the c in line 601 Figure 2: redraw your !Nd using a smaller X-axis scale (from -8 to -13.5). Line 649: correct deonotes with denotes Figure 6: the red square should also be in maps c and d.  Reply: Many thanks for the valuable suggestions. Nearly all the papers suggested relate to water column/dissolved Nd inferred from selectively extracted authigenic fraction.
We have, however, consulted these papers and also several other papers related to Nd isotopic study in the marine sediments in the Indian Ocean, and have cited some of the most relevant publications in the text. By citing these papers, we have discussed the potential influence of boundary exchange, ocean circulation (e.g. AAIW) borne Nd on our sediment Nd isotope data, and concluded that these processes have no pronounced BF?DL>F<> GF KA> T1= G? KA> =>KIBK:D ?I:<KBGF :K GLI JKL=B>= JBK> !5BK> 7'(*+"% Accordingly, the following text has been added in the revised manuscript (P5, Line

144-161).
"It has been highlighted that the Nd cycle in the ocean is complicated and could be influenced by factors such as fluvial input, boundary exchange, and oceancirculation borne Nd [37][38][39][40] . Boundary exchange has been used mainly to explain the effect of sediments on dissolved Nd in seawater and not vice versa 41 , since the Nd concentration in sediments is 2-3 orders of magnitude higher than in dissolved seawater.
Moreover, the lithic grains in the study area were primarily of eolian origin with relatively slow accumulation rates, and thus were less influenced by the boundary exchange which is often significant in the continental margins with high sediments supply 37,41 . Therefore, we could preclude a significant influence of these processes on   (Fig. 2). Further, the miss-match between !Nd data at IODP Site U1447 and chlorite/smectite at the ODP Site 758 in the distal Bengal Fan and the overall decreasing trend in the smectite/(illite+chlorite) should have provided additional insights which the authors did not capitalize. For the record, some published !Nd data for the same time interval from the ODP Site 758 (Gourlan et al., 2008;2010) are available, which the authors may wish to consider when charting out the next course for their manuscript.
Reply: It is indeed that the !Nd between 4.80 and 6.58 Ma are fairly less radiogenic.
We interpret this pattern was caused by addition of materials with significant less I:=BG@>FB< T1= !-20 to -18) from the southern Indian continent (Please see below for more detailed interpretation). We have re-examined the clay mineral data of IODP Site 1447 and ODP Site 578 in the northern Indian Ocean, we found that the ratio of kaolinite/chlorite could reflect the relative strength of the summer monsoon, as the kaolinite and smectite are mainly products of intense chemical weathering in warm humid climate, whereas the chlorite is related to the physical erosion under cold and dry climatic conditions. We thus have used this proxy to compare with our Nd record and these two records show largely consistent pattern (Fig. 3). We have also cited As suggested by the reviewer, the seawater !Nd data from the ODP Site 758 has been cited and compared with our detrital !Nd record to clarify the influence of AAIW on our data. Please see the reply to comment 1.
"The CIA index at the ODP Site 1148 from the northern South China Sea also reveals similar monsoonal evolutional pattern with our Nd records 49 (Fig. 3e) 19 , to the core site (Fig. 2). These materials were possibly transported by the strengthened winter monsoonal winds, as previous studies have argued for a cold-drying climate and intensified winter monsoon during this time interva l1,48,53 . Our interpretation is consistent with a two-fold increase in sediment accumulation rate at Site U1467 during the interval of ~7-5 Ma 7 (Supplementary Fig. 1a). This inference is also supported by an abrupt increase in the ratio of chlorite/smectite from ODP Site 758 in the northeastern Indian Ocean (Fig. 3f), which implies a higher contribution of materials derived from the physical erosion of highlands of the Himalayan river basins 54  Reply: Thanks for the very good suggestions that further validate our interpretation of provenance change. High-quality !Nd dataset from core-top sediments in the Arabian Sea reported by Sirocko (1994) 29 , and from marine sediments and rocks along the margins of the northeastern African and the southern Arabian Peninsula 33 have been cited to plot the provenance diagram (Fig. 2). We also generated a new figure   (Supplementary Fig. 2 Line 252-256, Line 263-269), and these corrections do not alter the main conclusion in the manuscript. "There was a major expansion of the East Antarctic ice sheet from ~13.9 Ma onwards and a further expansion of the West Antarctic ice sheet until ~8 Ma 60,64,65 (Fig.   4d) leading to faster cooling, and consequently to enhanced meridional temperature gradients, in the Southern Hemisphere relative to the Northern Hemisphere 62 (Fig. 4b).
"The development of ephemeral Northern Hemisphere ice sheet between ~6-8 Ma 60,66 , followed by the significant expansion of the Greenland ice sheet commenced at ~4 Ma 60,66,67 (Fig. 4d), causing a cooling in the mid-latitude Northern Hemisphere and enhanced meridional temperature gradients 62 (Fig. 4b). This pushes the Hadley Cell back southward, possibly leading to the subsiding and ascending branches of the Hadley Cell deviated from the MH and IL in the opposite direction."  interpreting !Nd data over the last three years compared to traditional views using the !Nd data. Therefore, it is expected that the authors would provide multiple hypotheses and then settle on one that supports their data. That scientific thinking or rigor appears to be absent in the manuscript.
Reply: Please see the reply to comment 1. Reply: Done.

General comment:
This paper presents a geological record and a modelling approach to provide an insight about the mechanisms and causes behind the evolution off the South Asian Summer Monsoon over the last 12 million years. The rationale for the study is that the SASM has been understudied compared to the East Asian Monsoon due to a scarcity of suitable records. However, it is, today, an important climatic system that affects the wellbeing of billions of people. We are here presented with a multi proxy record, using Ma. The mechanisms to explain the overall decline and abrupt decrease are also effectively discussed using the reconstructed wind speed, precipitation and sea level pressure scenarios. However, there are some weaknesses in this paper that need to be addressed.

Comment 1:
Firstly, why not analyze the chlorite/smectite composition in site U1467 so comparison between proxies would be stronger; in this paper, cores ODP758 and U1447 are in a different basin compared to U1467.
Reply: This is a very good suggestion. We have tried to extract the clay fraction (< 2 UE" BF J:EHD>J G? 5BK> 7'(*+ KG H>I?GIE KA> 94/ :F:DOJBJ% 7F?GIKLF:K>DO# N> =B= FGK get sufficient clay fraction after removing the carbonates and organic matter to do this analysis, as the carbonates content of samples are generally higher than 80% and mostly above 90%. Although ODP Sites 758 and U1447 located in a different basin compared with our site, the clay minerals in these sites can still be used to reflect monsoon information within a large regional scale. Consequently, these sites have been used for comparison in the revised text. And a good match is seen. Thank you.

Comment 2:
Secondly, the structure of the paper is not adequate in place. The location of the core is rather important to understand why the sediments there can be used to study eolian components. I would suggest moving the text from lines 155 to 175 to before line 103. Figure 3 should be presented before figure 2a, that means to move lines 79-86 after line 133.

Reply:
We totally agree with the reviewer, and have adjusted the order of these two paragraphs, and consequently the Figures in the revised manuscript (changed the order of Fig.2 and Fig. 3).

Other comments:
Below are some suggestions corrections: Line 54: add "and wellbeing" after "economic" Line 95: correct "was" with "were" Line106: replace B-G with G-B Line 126: replace river with River Line 156 replace enclose with encloses Line 156 replace is with are Line 188: Add "a" after as Line 649: correct deonotes with denotes Figure 6: the red square should also be in maps c and d.

Reply:
We are grateful to the reviewer pointing out these errors. We have corrected all of them and checked the grammar throughout the text carefully. Comment 2: Supplementary Fig. 2. I wonder whether the authors would consider plotting the eNd value rather than plotting the core-tops position and marine sediments' location. The lack of these data prevents us from further assessing the nature of provenances and their respective eNd values. It would also be worthwhile to submit these data as an xls file (as a supplementary file) with the latitude, longitude, water depth, eNd, etc.
Reply: As suggested by the reviewer, we also plot the eNd value along a W-E direction within 0-30° N regions in this Figure. As these data were not generated by this study and can be easily accessed from ref 29

Comment 4:
The authors could use "geological editing" as many "unscientific terms" are prevalent in it. For example, "riverine materials," "eolian materials," "By contrast, the Sr and Nd isotopic composition of the Indian Thar deserts "sands" showed, "sands by definition are SiO2 and do NOT have any isotopic composition". I could go on, but Redacted Editorial Note: Parts of this Peer Review File have been redacted as indicated to remove third-party material where no permission to publish could be obtained.