Changes in productivity and intermediate circulation in the northern Indian Ocean since the last deglaciation: new insights from benthic foraminiferal Cd&thinsp;∕&thinsp;Ca records and benthic assemblage analyses

Ma, Ruifang; Sépulcre, Sophie; Licari, Laetitia; Haurine, Frédéric; Bassinot, Franck; Yu, Zhaojie; Colin, Christophe

doi:https://doi.org/10.5194/cp-18-1757-2022

Articles | Volume 18, issue 8

https://doi.org/10.5194/cp-18-1757-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/cp-18-1757-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 8

Research article

|

03 Aug 2022

Research article |

| 03 Aug 2022

Changes in productivity and intermediate circulation in the northern Indian Ocean since the last deglaciation: new insights from benthic foraminiferal Cd ∕ Ca records and benthic assemblage analyses

Ruifang Ma, Sophie Sépulcre, Laetitia Licari, Frédéric Haurine, Franck Bassinot, Zhaojie Yu, and Christophe Colin

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Final revised paper (published on 03 Aug 2022)
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Preprint (discussion started on 18 Nov 2020)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'referee comment', Anonymous Referee #1, 01 Apr 2021
- AC1: 'Reply on RC1', Ruifang Ma, 26 Jul 2021
RC2: 'Review of Ma et al.', André Bahr, 04 Apr 2021
- AC2: 'Reply on RC2', Ruifang Ma, 26 Jul 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (23 Oct 2021) by Arne Winguth

AR by Ruifang Ma on behalf of the Authors (17 Nov 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (19 Dec 2021) by Arne Winguth

RR by André Bahr (07 Jan 2022)

RR by Anonymous Referee #1 (17 Feb 2022)

Suggestions for revision or reasons for rejection

Review of manuscript “Changes in productivity and intermediate circulation in the northern Indian Ocean since the last deglaciation: new insights from benthic foraminiferal Cd/Ca records and benthic assemblage analyses” by Ma et al.. This is a revised version of a manuscript that presents data from a sediment cores from the northern Indian Ocean (Arabian Sea and Bay of Bengal), comprising geochemical time series generated on benthic foraminifera as well as census data for planktic and benthic foraminifera. Based on these data, the authors deduce monsoon driven changes in productivity mainly dominating the various records during the Holocene and changes in intermediate water chemistry during the deglaciation. Whilst the revised version of the manuscript has improved by addressing some of the issues raised, there are still a number of problems hampering wholehearted support at this stage
These are some of the issues (line references are based on the "...ACT1" version of the manuscript that highlights changes in the document):
a) One issue still relates to the consistency of interpreting the Cdw records (a part of this section is unchanged text from the previous review). In chapter 5.4 the authors claim that the Cdw values during the deglaciation are lower than during the Holocene. First, this statement is only correct if longer term averages are considered. On short time scales (which need to be considered, given that this is a chapter on millennial scale change), the youngest Cdw data in core MD77-191 (2-1.5 KaBP) are comparable to the low YD and HS1 values. Up to this point a big effort has gone into establishing Cdw as reflecting productivity variations at the sea surface and the related flux of organic carbon. Now the focus shifts to bottom water ventilation changes being recorded. If general water ventilation would play are role in setting the recorded Cdw values, this has to apply to the Holocene too and would therefore need to be considered there. The revised version of the text does contain some extra lines, but the arguments made are rather unconvincing. First, the discrepancies between the Cdw and the G. bulloides/Corg data in figure 5 during the Holocene are not “little” as stated in line400. There seems to be some covariation at times, but there are also significant offsets during other periods, in particular in the youngest part of the record. Both, G. bulloides records in figure 5 as well as the Corg record suggest that the there is high productivity in the latest part of the Holocene. Yet, the Cdw values in core MD77-191 drop back to very low values. Taking this observation as face value, the long-terms trends between the records do show differences too. The authors then argue that the Cdw values in the late Holocene are too high (quoting a value of 1.59 nmol/kg which is difficult to reconcile with figures 4 and 5) to be explained by water mass related changes (here discussed as NADW influence). This refers back to the Cdw record of core MD77-191, however, which does show very low values in the top section, which would be in line with a low Cdw water mass incursion and are not consistent with the high productivity values indicated in the other data in figure 5 for the latest Holocene. This part of the discussion should contain a more in-depth evaluation of all possible factors explaining the Cdw record. In addition, ignoring (too a large extend) the millennial scale change in the Holocene in Cdw records may turn out to be a missed chance.
b) Usage of term NADW: In response to my previous comment on the referral to NADW, the authors did change the manuscript. It has improved to some degree but contains a misleading use of NADW. Initially NADW is introduced as a contributing water mass to IDW. The first problem is that usage thereafter only mentions NADW, whilst, at most, I guess, it would reflect an added contribution to IDW (?). Also, it is claimed that IDW would occur in the northern Indian Ocean between 1500 and 3800m (according to Talley 2011). Based on the usage of NADW in latter parts of the manuscript the reader is led to believe that NADW would also be occurring at water depths as shallow as 1500m. Yet, using the salinity maximum, referred to in Talley (2011, chapter 11, figure 11.16), as an indicator, NADW occurs below roughly 2000m, in line with the text in that chapter. In addition, the only region original (largely unmixed) NADW occurs is off the southeast coast of Africa, with the northward propagation blocked by the Davie Ridge (although there is some discussion in relation to a potential northward spillover occurring). In order to substantiate their argument, a) the hydrography section still needs further improvement and b) there needs to be a more in-depth explanation how (even contributions) of a deepwater mass, currently occurring below ~2km, affect sediment cores at true intermediate water depth. The changes in the revised version are not entirely convincing. This affects large parts of the discussion.
c) The usage of PCA’s, assemblages and/or individual species concentrations is rather confusing, and pieces of information are missing. In lines 238-239, e.g. the authors state that the PCA analysis yielded 3 factors, referring to table 1. Yet table 1 only shows 2. In lines 246-248, referral is made to assemblage 3 (PC3) without figures 3 and S2 actually showing PC3 scores. This should be clarified. In addition, the usage of assemblage and individual species concentrations is rather confusing. Given that the factor analysis did produce groupings of species, it would help to add plots showing change in the grouped assemblages. It would help the flow of the text and may add robustness to the results. This requires changes across the manuscript.
d) (somewhat minor point) Lines 543/4 Figure 7 needs a much better embedding in manuscript. This is first and only time it is mentioned. There is no real stipulation of the main findings. The reader to left to deduce this alone.

Detailed comments:
Lines 35-36 the ages in Monnin are not in line with this statement
line 93 this statement is not true. The figures reach back 17-18kaBP in most cases, which includes the deglaciation
chapter 4.1:This chapter is not placed correctly. It would be better placed in the methods section.
Line 248-250 This statement is inconsistent with figure 3. The figure clearly shows that the statement only applies to parts of the Holocene and is not valid for the entire period. Also, the description of the counts is focussed on the Holocene whilst earlier description cover the entire 18ka. This leaves the reader very confused as to the main thrust of the manuscript.
Lines 337-338 this does not make sense. There is confusion in the use of "since". This includes the title.
Figure 4: The application of a 5 point running average has led to offsets between the timing of the actual peak and those shown in the smoothed record. A time-controlled box car filter should be applied.

Overall, the manuscript has improved a little. There are, however, issues remaining that require a further rewrite.

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ED: Reconsider after major revisions (09 Mar 2022) by Arne Winguth

AR by Ruifang Ma on behalf of the Authors (05 Apr 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (06 Jul 2022) by Arne Winguth

AR by Ruifang Ma on behalf of the Authors (14 Jul 2022) Author's response Manuscript

Short summary

We provide high-resolution Cd / Ca records of benthic foraminifera on two cores from the northern Indian Ocean since the last deglaciation. We reconstructed intermediate Cd_w records based on Cd / Ca. Combined with benthic foraminiferal assemblages, we show that intermediate Cd_w during the last deglaciation was mainly influenced by the ventilation of intermediate–bottom water masses. Thereafter during the Holocene surface productivity is the main forcing factor related to monsoon precipitation.