Bottom-water deoxygenation at the Peruvian margin during the last deglaciation recorded by benthic foraminifera

Erdem, Zeynep; Schönfeld, Joachim; Rathburn, Anthony E.; Pérez, Maria-Elena; Cardich, Jorge; Glock, Nicolaas

doi:https://doi.org/10.5194/bg-17-3165-2020

Articles | Volume 17, issue 12

https://doi.org/10.5194/bg-17-3165-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Ocean deoxygenation: drivers and consequences – past, present...

https://doi.org/10.5194/bg-17-3165-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 12

Research article

|

24 Jun 2020

Research article |

| 24 Jun 2020

Bottom-water deoxygenation at the Peruvian margin during the last deglaciation recorded by benthic foraminifera

Zeynep Erdem, Joachim Schönfeld, Anthony E. Rathburn, Maria-Elena Pérez, Jorge Cardich, and Nicolaas Glock

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Final revised paper (published on 24 Jun 2020)
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Preprint (discussion started on 20 May 2019)
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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: 'Review Erdem et al for Biogeosciences', Anonymous Referee #2, 28 May 2019
- AC1: 'Reply to Anonymous Referee #2:', Zeynep Erdem, 23 Jul 2019
RC2: ''Review Erdem et al for Biogeosciences', Anonymous Referee #1, 10 Jun 2019
- AC2: 'Reply to Anonymous Referee #1:', Zeynep Erdem, 23 Jul 2019

Peer-review completion

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

ED: Reconsider after major revisions (15 Aug 2019) by Marilaure Grégoire

AR by Zeynep Erdem on behalf of the Authors (16 Sep 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (21 Sep 2019) by Marilaure Grégoire

RR by Anonymous Referee #3 (15 Nov 2019)

Suggestions for revision or reasons for rejection

This review of Erdem et al. is after the authors have commented to the first round of reviews on the Discussions version. Accordingly, I have especially paid attention to the main issues from these reviews. In general, the authors have answered the questions and issues made by the reviewers in high detail such that when they are implemented the manuscript will be ready for publication. I think this is a very interesting approach to reconstruct paleo-oxygen conditions. And even though it may be not precise enough (yet) to record the smaller changes, i.e. the authors are also honest about current limitations, it does seem to give a reasonable reconstruction. The paper will be an interesting addition to the increasing interest in reconstructing paleo-oxygen.

The reviewers made some valuable comments that were especially pointing towards being precise on the statements that are being made in the paper. I mostly agree with this. The authors have responded in detail to this but I think some parts still require some further explanation. The authors have detailed how to account for different assemblages downcore than the living ones. But I think the reviewers were also concerned on changes in preservation through time. The depositional setting off Peru is pretty harsh towards the preservation of foraminiferal shells and changes a lot through time, i.e. with the changes in upwelling. So the assemblages may be biased by dissolution of more dissolution-susceptible species.

Additionally to this, I have two further points, firstly Bengal Rose was used to identify living specimens, but especially in low-oxygen settings this may largely over-estimate the population of living specimens; and secondly, I would like to see some information on how oxygen changes at these core locations through the year, i.e. seasonal changes. Could this be a reason for the scatter or apparent offset between reconstructed values and values at the time of sampling? I.e. the forams may have been alive during sampling but maybe the conditions were not ideal such that they were “hibernating” and actually have their active life phase during another time of the year with different oxygen concentrations.

The age model is indeed a necessary point to be added. I agree that it appears that there is a prograding trend in changing oxygen values but unless the age models are very precise I would tone this down a bit.

The data used in this manuscript should be reported as a dataset in for example Pangaea. Currently there are only references to the different papers where assemblage, core top and water data have been presented.

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RR by Anonymous Referee #2 (19 Nov 2019)

Suggestions for revision or reasons for rejection

Erdem et al. Biogeosciences review

I am disappointed with the response to reviewers’ comments and only minor revisions that the authors have applied to their paper. If I recall correctly, the editor requested major revisions. The authors appear to agree with various comments that were made by myself and another reviewer, but have not satisfactory revised the manuscript to reflect this. The kind of research the authors pursue is very interesting, and I would really like to see this published, but it needs to properly backed up with robust scientific methods and age models. Furthermore, I would encourage the authors to refrain from making sweeping statements that do not seem to be backed up by the data.

Below are my detailed comments:

METHODS:
In the manuscript, the authors still do not present arguments why a relationship of living benthic foraminifera abundance with dissolved oxygen concentration can be applied to a dead assemblage. In the response to reviewers comments the authors agree there are substantial differences between the two populations.

ACTION:
The authors need to include statements in their manuscript highlighting that the dead and living populations are intrinsically different at the sites, and discuss how this influences the results. For example, could this explain the huge differences in dissolved oxygen in Holocene versus present day oxygen concentrations? Or do the authors think there has been a huge improvement over the last ca 5 kyr (which would be a big thing?) Either way this needs to be thoroughly explained in the main text.
Would it not make more sense to develop a relationship between recent dead populations with dissolved oxygen concentrations to apply down-core? If the authors believe this is not a valid approach, this should be discussed in the manuscript.
I am not sure what the difference is between making quantitative statements about specific periods compared with making quantitative statements at one location between specific periods?
How were standard deviations calculated?

AGE MODEL:
The information provided is not satisfactory.

ACTION:
The authors need to visually show how benthic d18O records were correlated to stacked records of Antarctic ice cores (this information is not in the referenced publications). I am especially curious to find out how this was done for the Holocene and how the early and mid Holocene were differentiated for core 52-2. In addition, the authors need to describe how radiocarbon ages and oxygen isotope dates were determined within the hiatuses of 47-2 and 50-4.
For the shallow core, there are only two ‘ages’ from the late deglaciation and early Holocene. How can you confidently constrain ages between 13 and 22, including the LGM interval?

The authors need to show the evidence (d18O graph for each site and correlation to which record) in the supplementary information. Stern and Lisiecki (2014) provide stacked records for different ocean basins and water depths, it is more appropriate to use these for correlation purposes.

Other comments and observations that need actioning:

In the abstract it is stated that ‘Each core displayed a similar trend of decreasing oxygen levels since the LGM.’ This is not true, the shallow core is an exception.

The statement about the changes being time transgressive is strange and does not seem corroborated by the data. The shallower cores (at 1013 m and 997 m from 8 degrees South and 4 degrees South) start changing around the same time, whereas the deeper core seems to change later. In addition, for the deeper core the age model is based on two AMS ages around 13 and 22 cal ka, so exact timing of change is not constrained.

Does the last sentence of the abstract refer to data in the article? If not it needs a reference.

Page 13 1st sentence: what does this mean, ‘a slight recovery of the OMZ’? Did it become less or more oxygenated?

Figure 1 and 2 need legends for oxygen concentrations.

What does Figure 4 show? Discuss in detail in the main text.

What is the purpose of Table 1? Generally oxygen threshold are related to redox reactions, not biota abundance.

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ED: Reconsider after major revisions (22 Jan 2020) by Marilaure Grégoire

AR by Zeynep Erdem on behalf of the Authors (18 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (18 Mar 2020) by Marilaure Grégoire

RR by Anonymous Referee #2 (20 Apr 2020)

ED: Publish subject to minor revisions (review by editor) (22 Apr 2020) by Marilaure Grégoire

ED: Publish subject to minor revisions (review by editor) (28 Apr 2020) by Marilaure Grégoire (Co-editor-in-chief)

AR by Zeynep Erdem on behalf of the Authors (29 Apr 2020) Author's response Manuscript

ED: Publish subject to technical corrections (15 May 2020) by Marilaure Grégoire

ED: Publish subject to technical corrections (15 May 2020) by Marilaure Grégoire (Co-editor-in-chief)

AR by Zeynep Erdem on behalf of the Authors (18 May 2020) Manuscript

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Short summary

Recent observations from today’s oceans revealed that oxygen concentrations are decreasing, and oxygen minimum zones are expanding together with current climate change. With the aim of understanding past climatic events and their relationship with oxygen content, we looked at the fossils, called benthic foraminifera, preserved in the sediment archives from the Peruvian margin and quantified the bottom-water oxygen content for the last 22 000 years.