A record of Neogene seawater δ11B reconstructed from paired δ11B analyses on benthic and planktic foraminifera

Greenop, Rosanna; Hain, Mathis P.; Sosdian, Sindia M.; Oliver, Kevin I. C.; Goodwin, Philip; Chalk, Thomas B.; Lear, Caroline H.; Wilson, Paul A.; Foster, Gavin L.

doi:https://doi.org/10.5194/cp-13-149-2017

Articles | Volume 13, issue 2

Clim. Past, 13, 149–170, 2017

https://doi.org/10.5194/cp-13-149-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Clim. Past, 13, 149–170, 2017

https://doi.org/10.5194/cp-13-149-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 13, issue 2

Research article 24 Feb 2017

Research article | 24 Feb 2017

A record of Neogene seawater δ¹¹B reconstructed from paired δ¹¹B analyses on benthic and planktic foraminifera

Rosanna Greenop et al.

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Final revised paper (published on 24 Feb 2017)
Supplement to the final revised paper
Preprint (discussion started on 26 Jan 2016)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Review', Anonymous Referee #1, 17 Mar 2016
- AC1: 'RESPONSE TO REFEREE #1', Rosanna Greenop, 18 May 2016
RC2: 'review of Greenop et al.', Anonymous Referee #2, 02 Apr 2016
- AC2: 'RESPONSE TO REFEREE #2', Rosanna Greenop, 18 May 2016

Peer-review completion

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

ED: Reconsider after major revisions (24 May 2016) by Appy Sluijs

AR by Rosanna Greenop on behalf of the Authors (06 Sep 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (21 Sep 2016) by Appy Sluijs

RR by Anonymous Referee #1 (20 Oct 2016)

Suggestions for revision or reasons for rejection

Greenop et al. have revised their manuscript substantially, improved the model tests and tailored it to show the consensus with previously published studies, rather than the focus on minor discrepancies of the original manuscript. I find this manuscript very much improved, but it still needs a few minor but important revisions. For the sake of simplicity, I will list these in chronological order, which is not necessarily the order of importance.

Line 175: spelling of name: Martínez-Botí

Lines 192/238/248…: wuellerstorfi

Lines 205/208: Müller

Lines 274 ff: what makes these “minor effects”? In particular the Miocene is a period of intense organic matter deposition (Monterey Formation!), that indicates systematic and substantial removal of isotopically light carbon from the ocean. Systematic changes in boundary conditions such as d13C-DIC and pCO2 are not captured in the assumed constant uncertainty, which accounts only for random changes in boundary conditions. As such, these new d11Bsw estimates are still not the ultimate answer, which is generally nicely acknowledged in this manuscript, but should be repeated in the conclusions (see comment below).

Line 460: its changes

Lines 474/475: what is the “plausible” range and why? Please define “plausible”.

Lines 584-587: the pH decrease may be at odds with the pCO2 estimates shown in Beerling & Royer (2011), but more estimates have since been published that support the greater pCO2 reconstructed from stomata (as already shown in Beerling & Royer). For instance, Zhang et al. (2013) show Miocene pCO2 from alkenones >400 µatm, Bolton et al. (2016) went a step further and considering calcification changes in coccolithophores, they suggest Miocene pCO2 may have been at least 50% higher compared to the Plio-Pleistocene. Foster et al. (2012) and Greenop et al. (2014) also suggest Miocene pCO2 ~ 400 µatm, although they used an exceedingly low alkalinity of 1292 µmol/kg to yield these estimates, a value that is even more extreme than the alkalinity inventories tested in Fig. 7 (~1800-2600, considering that modern alk is somewhere around 2200 µmol/kg), which are considered “extreme and inconsistent with geological evidence” (Line 1138). Using more reasonable alkalinity values of ~2000 µmol/kg (Ridgwell 2005, Tyrrell & Zeebe 2004), the Miocene boron isotope pCO2 estimates range closer to 500-600 µatm, clearly consistent with global evidence for warmer temperatures, and acidification as implied by models. There is uncertainty in all of these pCO2 estimates but growing evidence for higher Miocene pCO2 should be acknowledged and the argument that “proxy CO2 and surface water pH estimates are not well described by the linear change in pH” should be revised in the light of this growing proxy data evidence.

Line 704: please augment this sentence to “Despite some disagreements, and different uncertainties associated with each approach, the fact that…”. This is just to reiterate that Greenop’s approach, like all others, has large uncertainties as well, albeit due to different factors.

Line 712: controls

Line 1109: please specify where the “modern ocean” data are coming from, i.e. they are globally distributed and exceed the data shown in the Atlantic profiles. The reference to Foster et al. (2012) is not sufficient.

Line 1117: Please clarify that the error bars with 95% confidence due to external reproducibility only apply to the new data by Greenop et al. (this study), but not to Raitzch & Hönisch (2013), where they represent propagated uncertainties of external reproducibilities of time equivalent benthic foraminifer samples from different core sites in different ocean basins.

Line 1122: the color choice for the planktic foraminifera symbols is unfortunate and should be changed - the orange and red colors are nearly indistinguishable.

Figure 8 caption: Please explain the 0.201 line

Figures 2 and 11: The d11Bsw data of Raitzsch & Hönisch (2013) are not plotted correctly, the data are all lower than originally presented, and so are the polynomial fits. The polynomial fits should probably be replaced by something like a 5-point running mean anyway, but I am puzzled why the data are lower than presented in the original study? This is particularly striking in Fig. 11, where the data are shifted even lower than in Fig. 2, below the 39.61‰ modern seawater d11Bsw estimate that the caption claims the data have been adjusted to (Lines 1175/1176). This must be rectified. The original publication presented the data originating at a modern d11Bsw average of 39.6‰, so no adjustment of the original is necessary or justified.

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RR by Philip A.E. Pogge von Strandmann (18 Nov 2016)

Suggestions for revision or reasons for rejection

This study presents a new boron isotope curve of the Neogene based on foraminifera. The authors make the point that the d11B of forams is not an unfractionated archive for seawater, but is instead pH dependent, and therefore must be corrected using modelled ocean pH. The study goes into some depth on this modelling, and the assumptions made, and the results broadly agree with several other studies, although this study has considerably more data. Interestingly, the d11B seawater data also co-vary with records from other isotopic systems like Li, Mg and ca.
Overall this is a well-written manuscript, with a detailed discussion on modelling, and some interesting outcomes in terms of seawater records, and I recommend it be published after some minor amendments listed below.

Line 61: isn’t carbonate weathering also a B source? Presumably they must be if carbonates are a major sink.

Line 63: is this low or high temperature ocean crust alteration?

Line 116/Fig 3c: I would draw this as a line, rather than a series of data points, to make reading the graph easier.

Lines 11-125: for the non-B person, explain how this is not affected by CCD changes and associated pH changes?

Line 145: referencing a manuscript in preparation is a little odd.

Line 147: “nano” rather than “nanno”

Lines 154-161: what secondary standards were measured to ensure precision/accuracy? What is the precision of the d11B measurements?

Section 2.4 and 2.5: you’ve listed all the parameters you used, but it would also be useful if you explained why you selected those parameters for the models.

Line 513: explain this 0.1‰/Myr a bit more, especially given rapid Neogene weathering (and therefore input) changes.

Line 634: you say there are two controls on Mg, but then list three…

Line 674-680: if you want to be inclusive about all the Li hypotheses, there are also modelling papers on the Cenozoic: Li and West 2014, Wanner et al., 2014, Vigier and Godderis, 2015.

Line 671-698: this could be considered a bit oversimplified, given controls by carbonate and dolomite formation on Ca and Mg isotopes, and in turn their link with enhanced cation supply through weathering, and also potential temperature controls on dolomite formation. Also, Li is only affected by silicate weathering, whereas Ca and Mg are not – and Sr isotopes tell us that the Himalayas are dominantly (metamorphosed) carbonates.
However, I agree that the cross-plots are compelling – but I think probably worth backing off a bit from the statements that it’s only rivers, given, for example, that it’s very hard to actually model Li by jut invoking rivers (see Li and West 2014).

It’s also interesting that there is an order of magnitude difference in residence time of the different elements – perhaps something worth mentioning and discussing. Why, for example, does the correlation between Li and B seem best, given the widest residence time difference?

In terms of the Ca record, I would take a look at Fantle and Tipper 2014 – especially their compilation of Neogene Ca isotopes (and their corrigendum to that graph), which suggests that the Griffith data are offset from others.

P. Pogge von Strandmann
UCL, UK

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ED: Publish subject to minor revisions (review by Editor) (18 Nov 2016) by Appy Sluijs

AR by Rosanna Greenop on behalf of the Authors (04 Dec 2016) Author's response Manuscript

ED: Publish subject to technical corrections (09 Jan 2017) by Appy Sluijs

Dear Dr. Greenop,

I am happy to accept your manuscript for publication in Climate of the Past. Both reviewers advised publication and I agree with them. The paper is in a very good state and I have spotted no typo’s in my final editorial check. I do advise to slightly revise the abstract, split a couple very long paragraphs and make sure to consistently use terminology (isotope vs isotopic) and epochs / ages. See below for these suggestions.

Sincerely,

Appy Sluijs
Editor

Line 17 – 21. Use Boron isotope composition or Boron isotopic composition (both fine with me but please use consistently; if a chemist would mind which one you use, please use the most correct one)

Abstract might be phrased more directly. Here a proposal:
The boron isotopic composition (d11B) of foraminiferal calcite reflects the pH and the boron isotopic composition of the seawater the foraminifer grew in. For pH reconstructions, the d11B of seawater must therefore be known but information on this parameter is limited. Here we reconstruct Neogene seawater d11B based on the d11B difference between paired measurements of planktic and benthic foraminifera and an estimate of the coeval water column pH gradient from their d13C values. Carbon cycle model simulations underscore that the DpH/Dd13C relationship is relatively insensitive to ocean and carbon cycle changes, validating our approach.
Our reconstructions suggest that d11Bsw was ~37.5 ‰ during the early and middle Miocene (roughly 23 – 12 Ma) and rapidly increased during the late Miocene (between 12 and 5 Ma) towards the modern value of 39.61 ‰. Strikingly, this pattern is similar to the evolution of the seawater isotopic composition of Mg, Li and Ca, suggesting a common forcing mechanism. Based on the observed direction of change, we hypothesize that an increase in secondary mineral formation during continental weathering affected the isotopic composition of riverine input to the ocean since XXX Ma.

Please make sure you indicate one actual age for the XXX. As it stands now, it seems like you are referring to a time interval rather than one point in time with an uncertainty in the age.

91. new paragraph starting at “Geochemical modelling”
109. new paragraph starting at “The fourth approach”
173. new paragraph starting at “Although”
356. new paragraph starting at “The ensemble spans”
the first paragraph of 3.2 might be split in at least 2 to optimize readability

559-561. sentence unnecessary could be deleted.

In Conclusions, make sure to name both the epochs and absolute ages according to Gradstein et al for both the lower and upper part of the studied interval.

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AR by Rosanna Greenop on behalf of the Authors (17 Jan 2017) Author's response Manuscript

Short summary

Understanding the boron isotopic composition of seawater (δ¹¹B_sw) is key to calculating absolute estimates of CO₂ using the boron isotope pH proxy. Here we use the boron isotope gradient, along with an estimate of pH gradient, between the surface and deep ocean to show that the δ¹¹B_sw varies by ~ 2 ‰ over the past 23 million years. This new record has implications for both δ¹¹B_sw and CO₂ records and understanding changes in the ocean isotope composition of a number of ions through time.