Signals of Holocene climate transition amplified by anthropogenic land-use changes in the westerly–Indian monsoon realm

Burdanowitz, Nicole; Rixen, Tim; Gaye, Birgit; Emeis, Kay-Christian

doi:https://doi.org/10.5194/cp-17-1735-2021

Articles | Volume 17, issue 4

https://doi.org/10.5194/cp-17-1735-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/cp-17-1735-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 4

Research article

|

24 Aug 2021

Research article |

| 24 Aug 2021

Signals of Holocene climate transition amplified by anthropogenic land-use changes in the westerly–Indian monsoon realm

Nicole Burdanowitz, Tim Rixen, Birgit Gaye, and Kay-Christian Emeis

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Final revised paper (published on 24 Aug 2021)
Supplement to the final revised paper
Preprint (discussion started on 17 Nov 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review', Anonymous Referee #1, 01 Feb 2021
- AC1: 'Reply on RC1', Nicole Burdanowitz, 17 Mar 2021
RC2: 'NE Indian Monsoon Holocene evolution', Anonymous Referee #2, 15 Feb 2021
- AC2: 'Reply on RC2', Nicole Burdanowitz, 17 Mar 2021

Peer-review completion

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

ED: Reconsider after major revisions (17 Mar 2021) by Luc Beaufort

AR by Nicole Burdanowitz on behalf of the Authors (15 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (15 Apr 2021) by Luc Beaufort

RR by Anonymous Referee #1 (17 May 2021)

Suggestions for revision or reasons for rejection

Review of revised manuscript “Signals of Holocene climate transition amplified by anthropogenic land-use changes in the Westerly-Indian Monsoon realm” by Burdanowitz et al.. The main thrust of this manuscript is to use high-resolution proxy records from the NE-Arabian Sea reflecting various aspects of Holocene monsoonal changes in the region. The main new data series include Uk37 based SST estimates, alongside a number of proxy time series (Ti/Al, endmember modelled aeolian input and lithogenics mass accumulation rates) reflecting lithogenic input in the region with the latter being of central importance. Based on these data the authors conclude that the Arabian Sea region around 4.6-3kaBP be- came more sensitive to changes in the tropical westerly jet controlling climate in the region. In addition, early agricultural activities in the region, may be reflected in the data. Compared to the initial version of the manuscript, there are areas of improvement in the revised manuscript. There are, however, still a number of issues which prevent recommending publication at this stage. Below there is a list of comments related to specific sections of the manuscript (line referencing based on version with track changes being highlighted). More generally, there are still too many occasions where the aspects focussed on the discussion are not properly described in the results sections and there seems to be some randomness with regard which events to discuss, despite the undiscussed events being of the same size. The results section should be expanded (including all the purely descriptive elements from the discussion). Also, the entire ENSO angle is rather unconvincing. It may help, in a revised version to remove this link and focus more on the change in vegetation.
The list below is in random order or importance/gravity:
Lines 40-42 Can any of these things be proven. WD disturbances on which time scale do they occur? Can this be resolved in the existing data. How exactly are, probably short term changes in the STWJ affecting the ISM? This should be better explained.
Lines 130-131: It is not clear how this is supposed to work. First, what is the average and best resolution of the record. Second, there needs to be a much better explanation why it is justified (if it is) to "invent" data in a part of the record that does not have any measured data?
Line 143: 93 grain size classes between 10 and 3 mm??? - are there no small size classes in the sediments as would need to be expected in this region of the world ocean?
Line151-152: “High SSTs were registered between 8.1 and 5.2 ka BP during the Mid-Holocene.” This is not correct. There is a short term cool period centred at ~7.8 Ka BP. Also, how is the averaging done. Is this just, as stated, a five point moving average (without any time control) or a time controlled filtering (it should be the latter).
Lines 217-220 The link between the 8.2ka event and the G. ruber isotope data is unclear. Also, the Ti/Al ratio change is small and not quite in phase (it seems). This sections would benefit from a rewording.
line 245 “indicated by...” The end of this sentence does not make sense.
lines 248-250 The entire discussion surrounding SST/d18O-data is based on marginally significant data (i.e. most of the signal in either the Uk37 and the d18O ruber data is within the uncertainties for either method). There is some change in those records, but the discussion should also reflect that this is based on a small signal.
Line 253 It is unclear how a climate change in one direction leads to a two (opposite) changes in the early civilizations. Better to remove the link from the study or explain better (interpret jointly with other data in a section on anthropogenic change?).
Line 257 The statement is not correct. The LitMar data over the last 3ka are generally low with some superimposed spikes.
Lines 278 and following: The entire argument surrounding cross-equatorial insolation gradient as a driver for short term change in the record is weak. As the authors noted with regard to summer insolation, smooth changes in the latter are very unlikely to be the main cause for short term change. The gradient records are equally smooth and it is therefore not clear how these help to explain short term change in core SO90-63KA.
Lines 313-317 It is unclear which period is being referred to. In the first sentence a change between 4.2 and 3.5 ka is introduced. Thereafter, suddenly there is a reference to the period 7.5-3.5 kaBP -which is it?
Lines 338-354 This, again, is an unconvincing section. My understanding is that the authors try to discuss a non-finding, i.e. that there is not signal in the data that matches the ENSO signal. It would be better to remove the ENSO link entirely. First, in modern climate the link between ENSO and the ISM strength is complicated and not linear. Second, the change happens on times scales beyond the scope of the study and thirdly, the data of core SO90-63KA do not support a link. It therefore does not seem to make much sense to include a rather long discussion surrounding ENSO in the manuscript.
As indicated above, the manuscript has improved but it is still not in a state supporting publication at this stage. A further revision is recommended.

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RR by Anonymous Referee #2 (21 May 2021)

Suggestions for revision or reasons for rejection

The manuscript has been clearly improved, both in terms of its structure (with a “data results” section that was missing in the original version) and content (with a clarified and much improved discussion).

Yet, I still have some questions/commentaries and suggest that the manuscript could be accepted but with additional minor improvements.

1/ LIT MAR: Considering the key importance of the LIT MAR record in the discussion, this proxy deserves an in-depth presentation in the manuscript. Instead of just providing the final, computed LIT MAR record, I would like to see also a figure with the bulk data that have been used to construct this record, namely: (i) the dry bulk density record, (ii) the non-carbonate % and (iii) the sedimentation rate record derived from the numerous 14C dates (with the location of those dates along the core). This could be done in the main manuscript, or in the supplementary material.

Not only should the bulk data for LIT MAR be provided to the readers, but they should also deserve an in-depth discussion too.
It is striking that some of the main LIT MAR events described in the text (eg. LIT MAR peaks at ca 8.4 ka, ca 3.4 ka and ca 2.8 ka) are just defined by 1 or 2 data points only.. I would be interested to know what explains those extremely narrow and high amplitude changes (do they reflect peaks in sedimentation rate and a potential issue with dating? Do they reflect peaks in DBD or peaks in the lithogenic material %? Do they result from a combination of those three elements?). How confident can we be regarding those peculiar peaks? How sensitive is the Wavelet Analysis to the presence of such short, high amplitude peaks?

3/ Line 143: the authors state that the last 3 ka show more variable SST that before. Couldn’t this be - at least partially - attributed to the fact that the temporal resolution is also higher over the top 3 ka than in some older parts of the record ? I counted, for instance, about 20 data points in the SST record over the 4.5-6.5 ka interval, but ~ 40 points over the 0-2 ka time interval, thus twice as much. Change in sampling resolution should have an effect on our capacity to resolve rapid SST variations. The d18O record, which displays a more constant sampling resolution along the core than the SST record, suggests a relatively similar/invariant type of periodicity across the entire Holocene. Was this record run through Wavelet Analysis? How this compares with the LIT MAR spectral evolution?

4/ Wavelet analysis: I wonder why wavelet analyses were not performed/shown for the other proxies as well. Is LIT MAR the only proxy that reveals a shift in periodicity across the Holocene? Shouldn’t we expect to see also a change in periodicity in the EM3 record, for instance? Is it seen when performing wavelet analysis?

5/ Figure 2: Is the “factor 1 – SST” (figure 2b; Giosan et al.) used and discussed in the text ?? I couldn’t find a call to this figure 2b in the text.

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (15 Jun 2021) by Luc Beaufort

AR by Nicole Burdanowitz on behalf of the Authors (09 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (29 Jul 2021) by Luc Beaufort

AR by Nicole Burdanowitz on behalf of the Authors (03 Aug 2021) Author's response Manuscript

Short summary

To study the interaction of the westerlies and Indian summer monsoon (ISM) during the Holocene, we used paleoenvironmental reconstructions using a sediment core from the northeast Arabian Sea. We found a climatic transition period between 4.6 and 3 ka BP during which the ISM shifted southwards and the influence of Westerlies became prominent. Our data indicate a stronger influence of agriculture activities and enhanced soil erosion, adding to Bond event impact after this transition period.