A new approach for modeling Cenozoic oceanic lithium isotope paleo-variations: the key role of climate

Vigier, N.; Goddéris, Y.

doi:https://doi.org/10.5194/cp-11-635-2015

Articles | Volume 11, issue 4

https://doi.org/10.5194/cp-11-635-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/cp-11-635-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 11, issue 4

Research article

|

01 Apr 2015

Research article |

| 01 Apr 2015

A new approach for modeling Cenozoic oceanic lithium isotope paleo-variations: the key role of climate

N. Vigier and Y. Goddéris

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Final revised paper (published on 01 Apr 2015)
Preprint (discussion started on 28 Jul 2014)

Interactive discussion

Status: closed

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

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- Supplement

RC C1429: 'Comments for cp-2014-77', Christoph Wanner, 03 Sep 2014
RC C1451: 'Vigier & Godderis manuscript', Paul Tomascak, 06 Sep 2014
RC C1535: 'Review of Vigier and Godderis 2014', Anonymous Referee #3, 22 Sep 2014
- RC C1575: 'Corrected Comment', Anonymous Referee #3, 25 Sep 2014
AC C1738: 'Answer to Reviewers', Nathalie Vigier, 23 Oct 2014

Peer-review completion

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

AR by Nathalie Vigier on behalf of the Authors (02 Dec 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Jan 2015) by Appy Sluijs

RR by Christoph Wanner (10 Feb 2015)

RR by Anonymous Referee #3 (04 Mar 2015)

Suggestions for revision or reasons for rejection

I am pleased to have had the opportunity to look at the revised version of this manuscript from Vigier and Godderis (V&G), and to also consider their reply to my previous comments. In my view, the revised submission of this paper is improved, to the extent that the authors are now more upfront in the text about some of the basic assumptions, unresolved questions, and uncertain parameterizations in their approach, which were highlighted during the review process. Perhaps some of the simplicity and nice flow of the original text has now been interrupted, but I think this is a small price to pay for the more complete discussion provided in this revision. There are a few very minor mistakes in grammar that have crept into this version, but these can surely be fixed during the editing process.

I want to preface my more substantive comments by saying that my initial review was perhaps too lengthy and maybe overly focused on comparison of the V&G results with the Li and West (2014) analysis of the Cenozoic Li record, although I do think that effort was useful and (I hope) constructive. I disagree with some of the details of the comments made by V&G in their reply to my previous review, but I am not sure how useful it will be to get bogged down in debating these at this stage. I certainly agree with V&G that the setup of the two models is different in a couple of respects, and given the major uncertainties in the parameterization of both models, there is little point to argue whether one is “better.” I apologize if my previous comments came across as suggesting that V&G’s approach is not a useful and novel addition (that is not what I intended, because I do indeed think it is both useful and novel).

I want to keep the focus of my comments here on how I see this current submission, standing on its own, and focusing on the big picture. I continue to see this paper as providing an important contribution, because it points out that there are two mathematically viable solutions to the mass balance model for the Cenozoic seawater Li isotope record. This is an important fact overlooked in prior studies. One of the two solutions requires a very large change in the isotopic composition of dissolved Li from the continents; the other requires a very large change in the flux. Previous analyses have focused on the first solution and suggested that this solution might reflect an “erosional” control on global chemical fluxes over the Cenozoic. V&G suggest that the second solution could be consistent with a “climatic” control on global chemical fluxes (like V&G, I am treating this climate vs. erosion question as a useful duality, understanding that the reality is more complicated). Although I think there are many, many uncertainties and assumptions in both interpretations, it is vital to know (which we now do, thanks to this paper) that both are theoretically viable. So I think V&G are absolutely right (as they state in their reply to my review) to argue that their paper is important because the solution implying a “climatic” control has not been previously developed and discussed. I totally agree and again am sorry if this was not clear previously in my comments.

Still in this version, the authors continue to frame their interpretations and discussion around the “new” solution requiring changing flux, in their view implying a climate control on soil formation. For example, they conclude their abstract by saying that “climate exerted a dominant control on soil production rates during the last 70 Ma” (this is just one example of the concept at the center of their interpretation). This may or may not be the case; it is certainly an interesting working hypothesis, and it would be great to find evidence to test it. But the Li isotope record has two physically plausible solutions, and we do not currently have enough a priori understanding of the Li cycle to rule out one or the other, so the model does not make it possible to distinguish between them.

So what then is the justification for choosing one of the two solutions? I am sure that V&G would agree that just because the erosional interpretation has been discussed in prior work does not really justify presenting the alternative as the (more) likely explanation. Ultimately I think we want independent evidence that can help tease apart the roles of erosion and climate over Cenozoic timescales (or at least to shed more light on the complicated interplay of erosion, weatherability, and climate). V&G nicely show that, based on what we know now about the Li system, the Li isotope record does not provide persuasive, unique evidence for an erosional forcing of Cenozoic change, because it could alternatively be consistent with a climate control on soil production rates. My own view is that their paper would be strongest if they highlighted this key point and did not over-interpret the evidence and model solutions as support for a climate-dependence, which I see as being just as non-unique a solution as the erosion-dependent solution. Nonetheless, however the authors decide to present their interpretations, I think this paper is interesting and will doubtlessly stimulate continued discussion about the meaning of the Li isotope record of seawater through the Cenozoic.

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ED: Publish subject to minor revisions (review by Editor) (04 Mar 2015) by Appy Sluijs

AR by Nathalie Vigier on behalf of the Authors (14 Mar 2015) Author's response Manuscript

ED: Publish as is (16 Mar 2015) by Appy Sluijs

AR by Nathalie Vigier on behalf of the Authors (16 Mar 2015)

Short summary

We develop here a new approach that couples the carbon and lithium cycles for reconstructing the Cenozoic Li isotope record. We show that this record does not provide persuasive, unique evidence for Cenozoic change in erosional forcing as it could, alternatively, be consistent with climatic control on soil production rates. The Li storage in continental secondary phases plays a key role, in particular, during the Early Cenozoic.