Controls on Lithium Incorporation and Isotopic Fractionation in Large Benthic Foraminifera

Round 1

Reviewer 1 Report

Manuscript ID:  minerals-2089087

Title: Controls on lithium incorporation and isotopic fractionation in large benthic foraminifera

Laurie M. Charrieau, Claire Rollion-Bard, Anja Terbrueggen, David J. Wilson. Philip A. E. Pogge von Strandmann, Sambuddha Misra and Jelle Bijma

This manuscript addresses the issue of the dependency of the Li isotopic composition of large benthic foraminifera on either pH or DIC. To resolve this the authors conducted culture experiments using the large benthic foraminifera Amphistegina lessonii under decoupled pH and DIC conditions, using different light treatments and in both normal and Li-enriched seawater. Analyses of δ7Li and Li/Ca ratios in the tests did not reveal significant relationships between these values and either pH or DIC.

The authors used two light treatments at natural seawater Li concentration to investigate the potential role of symbionts and, in a second experiment used a single light treatment but varied the concentration of Li, x1, x5 and x10 relative to modern seawater.

I found the results for the normal and x5 Li-enriched seawater easy to understand but it was much more difficult to understand those for the x10 Li-enriched seawater. These results should be tabulated along with those for the x5 Li-enriched seawater in order to make the authors’ arguments easier to follow.

It appears that there are some discrepancies in the x10 Li-enrichment data which need to be addressed. The ∂7Li of x 10 Li-enriched seawater was the same as x5 Li-enriched seawater and the Li/Ca of foraminifera were also the same.  Why is ∂7Li of x10 Li-enriched seawater not lower?  I would expect to see a mixing line between normal seawater & the added LiCl.  Please can the authors add more explanation as to how the x10 Li-enrichment experiment was conducted and how the results were measured or calculated.

The authors arguments with respect to the experiments in normal and x5 Li-enriched seawater are well presented and the manuscript is clearly written and presented. I believe this paper should be acceptable for publication in Minerals after explanation of how the x10 Li enriched data were measured and inclusion of the x10 Li enrichment data either in Table 2 or a separate table.  Other than this only minor editing of the manuscript is required.

Specific comments, listed below. 

Fig 1 – I assume the errors for pH & DIC are negligible compared to symbol size but please can the authors confirm this or, if not they should be included in the figure.

Fig 2 – How robust is the single high Li/Ca data point at 27.6 +/- 5.9 µmol/mol? 

Fig 3 Interpretation of this figure depends considerably on the x10 Li-enrichment data and hence on how the x10 Li-enrichment results were measured or calculated. 

Line;

111-114 ~50 specimens grown in x10 Li enriched water.  

Were the conditions the same as for the control with 5x Li-enriched seawater shown in Table 1?  Assuming this is so then please can the authors state this explicitly in the text, or in a footnote to Table 1, as it was difficult to find the conditions for x10 Li-enriched seawater.

124 x10 Li-enriched seawater

Results are not shown in Table 2 and need to be included, either by adding to Table 2 or in a separate table.

131 pH was measured with a multimeter

How was the pH meter calibrated? Please state what calibration buffer solutions were used.

145 dilute HCl

Dilute is too vague, please state the actual concentration used.

154      why is ∂7Li of x10 Li-enriched seawater not lower?  I would expect to see a mixing line between normal seawater & the added LiCl

174 The beam diameter was set at 1.5 mm 

I presume there is a typo here.

175 the specimens were ablated in the knob,

Please can the authors explain what the phrase “in the knob” means? 

180-184 x10 Li enriched seawater experiments

I find this paragraph confusing. Was Li/Ca actually measured for the x10 Li-enriched experiments?

198 ∂7Li result for x10 enriched seawater needs to be included in Table 2

202 How robust is the single high Li/Ca data point at 27.6 +/-5.9 µmol/mol?

Much off the interpretation of Figure 2 is dependent on this high data point.

210 significantly?  This depends very much on the single high Li/Ca data point, so using the term significantly may be overstating things.

310 I am confused as to how the Li/Ca ratios for the x10 Li-enriched medium were measured. See lines 180-184 and previous comment above.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Editor, please find below my revision of the manuscript I received on December 2nd 2022:

“Controls on lithium incorporation and isotopic fractionation in large benthic foraminifera"

 by Laurie M. Charrieau *, Claire Rollion-Bard, Anja Terbrueggen, David J. Wilson, Philip A. E. Pogge von Strandmann, Sambuddha Misra, Jelle Bijma

 [Paper  Ref: minerals-2089087] 

submitted for the publication in Minerals, Special Issue: Biomineralization of Organisms Used as Environmental Proxy Archives.

Recommendation: Publish after minor changes made by authors

 The manuscript shows the results of culture experiments on the larger benthic foraminifer Amphistegina lessonii. The main goal is to test the dependence of d⁷Li on either pH or DIC, in view of the use of these large benthic foraminifera to reconstruct the past seawater chemistry and infer variations in chemical weathering, provided d⁷Li of carbonates be a reliable archive of past seawater composition.

The topic, relatively recent, is very interesting considering that the larger foraminifera, such as A. lessonii, are good bio-indicators of the tropical carbonate platforms/reefs and consequently potential tools for paleoenvironmental reconstructions. The manuscript is clearly written, and the methods are well explained. Moreover, culture experiments are not easy to carry out.

 Although I have not major points/changes to suggest, I report below two comments/suggestions the authors may take into account.

 Lines 72-74. “They are major carbonate producers, as well as key bio-indicators in tropical carbonate platforms (review in [36]), and are ubiquitous through the geological record,....”

The author should explain better what they mean with “ubiquitous”. Do they mean that large benthic foraminifera can be found wherever through the geological record?

 Line 82: “two light treatments” and Lines 100-102: ”two light treatments were applied: all the beakers went through the same day/night cycles, but half of them were wrapped in black mesh”.

Could the authors explain/specify if and to what natural condition/environment the two light treatments would correspond? The dark treatment would represent a condition of increased weathering, possible increased turbidity of the water and consequently less light for photosynthesis?

 In summary, I recommend to publish this very interesting manuscript. I thank you very much for offering the opportunity to review this interesting paper.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I have read the revised manuscript and the authors' response to my review and the authors have answered all of the points that I raised.

I believe that this manuscript is suitable for publication without further modification.