the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 May 2026
Extending planktic foraminiferal Mg/Ca palaeothermometry into polar temperature ranges: crust- and lamellae specific calibrations and non-thermal controls
Abstract. The rapidity of climate change in the polar regions underscores the need for improved understanding of its impacts on ocean circulation at both regional and global scales. Reconstructions of past polar ocean-cryosphere interactions can provide this context, but large uncertainties in existing proxies limit the utility of such studies. For instance, there are currently no low-temperature (<9 °C) culture-based Mg/Ca-calibrations for planktic foraminifera, a key tool for reconstructing past changes in ocean temperatures. There is also limited understanding of non-thermal influences on Mg/Ca in Neogloboquadrina pachyderma, the only modern polar planktic foraminifera. Moreover, this species exhibits considerable levels of heterogeneity in composition precipitating a thick lower-Mg/Ca outer crust over higher Mg/Ca inner lamellae calcite; specimens with predominantly, albeit variable crust–lamellae proportions, are thus thought to introduce substantial uncertainty into high-latitude palaeotemperature reconstructions. Here, we used N. pachyderma cultured across a range of temperatures, salinities, and carbonate chemistry conditions including experiments in which pH and [CO32-] either covaried or were decoupled. By using a laser ablation approach to analyse crust and lamellae separately, we present new Mg/Ca-temperature calibrations for each component that extend culture-based calibrations in N. pachyderma down to the lower temperature-range (2–9 °C). The crust-specific calibration is of particular importance in high-latitude downcore records where N. pachyderma are commonly observed to preserve predominantly or only crust. Our results show significant carbonate chemistry influence on Mg/Ca with opposite influences from pH and carbonate ion concentration, when these variables changed in isolation. Additionally, we show that environmental conditions regulate crust-lamellar proportions, where increased salinity and temperature, and lower pH lead to less crust formation with implications for future ocean acidification and Arctic Atlantification, and for downcore reconstructions.
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Status: open (until 30 Jul 2026)
- RC1: 'Comment on egusphere-2026-2415', Lennart de Nooijer, 24 Jun 2026 reply
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RC2: 'Comment on egusphere-2026-2415', Anonymous Referee #2, 06 Jul 2026
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The manuscript of Westgård and coauthors on 'Extending planktic foraminiferal Mg/Ca palaeothermometry into polar Temperature ranges: crust- and lamellae specific calibrations and non-thermal controls' presents a novel and interesting approach and valuable new data for better calibration of the Mg/Ca temperature proxy for paleoceangraphic and paleoclimate reconstruction. The paper merits publication in BG following some revision of the text, which is poorly written in places. In particular, the Discussion chapter would need to be written as a discussion, and not repetition of Results plus some references from the literatur. Having said this, most of the classical references on N. pachyderma a missing so far, and would add value to the topic discussed here when being cited. When rewriting the Discussion chapter in discussion style, while preserving the correct scientific content, the author may think about using AI. In addition to these more general comments, please see some more specific comments in the following.
To my impression, the term lamellae is often used in an unfortunate kind of way. For example, for the title, I would suggest: 'Extending planktic foraminiferal Mg/Ca palaeothermometry into polar temperature ranges for crust and ontogenetic calcite'; I would drop 'and non-thermal controls', because it could be misunderstood, and is not possibly important to be mentioned in the title. For the line 29 (abstract), I would suggest: '... ontogenetic shell calcite. Specimens with predominantly albeit variable crust–shell proportions, ...'. Please have a look at the other places in the text and consider accordingly.
The referencing is rather limited and should be extended. In line 52, at least the first Mg/Ca paper of Nürnberg et al. (1996) should be cited. Other papers on the population dynamics (Volkmann, Carstens, and coauthors), ecology (Spindler and coauthors), and species concept (Darling and coauthors) may be referred to, to put the finding of this paper into a wider context.
Line 66ff: In open waters, it can possibly be assumed that the life-cycle of N. pachyderma is four weeks long and adjusted to synodic lunar cycle similar to other planktic forams (Schiebel et al. 2017). This would be important for the understanding of the formation the ontogenetic shell calcite and the Mg/Ca signature.
Line 177: Please give details (light-dark cycles?, light intensity, etc) on the light programme, to make it reproducible.
Lines 271-273: Comparing pH with salinity gets me no-where. What are you aiming for here?
Lines 319-322, write straight: The weak positive relationship between Mg/Ca and salinity is not significant (p>0.1, Fig. 2). Due to the methods used to modify the seawater salinity, DIC, [HCO3-], and total alkalinity (TA) co-varied linearly with salinity (Table 2). The [CO32−] concentration varied non-linearly across the salinity treatments.
Lione 350: 2-9 C, and not 2-7 C !?
Line 358: give ref. for <0 to >10 C
Lines 381-382: 'A comparable...', modify sentence to make it a perspective for future work, to make it a positive statement.
Line 406: yes, well, CO32- and pH co-vary. Please re-write
Line 415-416: change to: 'As pH and [CO32-] co-vary, the driving parameter would be between these two.'
Line 420-435: Only saying that / if other peoples finding are the same or not does not make a good discussion.
From line 420 onward (at least), this is not a discussion but a rambing collection of own findings and selected references, often without any outcome. The discussion needs to be entirely rewritten to make it a convincing plea for the great findings detailed in the Results.
For example:
Line 442: Citing the Table 2, it may still be said what this should tell us.
Lines 444-447: 'Although...' Why is this mentioned? What can we learn?
Line 447-449: 'This difference...' Guessing doesn't provide any useful information. Better skip.
Lines 457-458: This is not a contradiction.
Line 460: Are all of these parameters not sign. at 95% confidence level?
Lines 457-469: For each of the stements in this section, figures and/or tables need to be referred.
Line 473: change to: 'To the temperature interval from 2–9C in addition to the interval from 9-12C (Davis et al. 2017).'
Lines 517-519: This sentence make no sense. Are both assumed to be driven by physiological processes such as calcification?
Lines 534-535: This would not make much sense, and would possibly invalid other assumption on similar processes in benthic and planktic foraminifers.
Lines 559-560: Please take out. This is another publication.
Wording and syntax:
Always add a comma before 'respectively'.
Check the entire manuscript for the correct use of tenses; e.g., line 271: 'observed'
Line 235: Porites sp.
Line 255: spell out 'percent'
Lines 78, 265, and others: delete 'ratio' as Mg/Ca is a ratio
Lines 277-280: change to: 'Based on tests where both laboratory-grown crust and ontogenetic calcite was analysed, the total wall thickness decreased with decreasing pH and temperature (|R|>0.5, p>0.05), and decreased with increasing [CO32-] (R= -0.8, p>0.05).
Figure 1, line 289: I see grey shading, not green. Line 290: Table in upper case.
Line 292, and following: distribution in lower case.
Figure 2: Salinity has no unit, not even psu. The grey and green color are difficult to tell apart. Make the green line maybe bright blue to also facilitate comprehension for the red green-blind men.
Line 359: change 'curve' to 'relationship'
Line 375: change 'comparable with' to 'similar to'
Line 411: change to '... on Mg/Ca. However, ...'
Line 419: change 'that to' to 'to that'
Line 461: %crust. This is not a term. Do you mean proportion of crust vs. ontogenetic calcite?
Line 493-496: Less impactful than where? This is a relative clause which should be closed.
Citation: https://doi.org/10.5194/egusphere-2026-2415-RC2
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- 1
Review Westgard et al., Biogeosciences (Egusphere-2026-2415)
Dear editor,
with great interest I read the manuscript of Westgård and co-workers. Although many results from the experiments were presented earlier, the elemental data on the cultured N. pachyderma are very interesting and form the basis of an important contribution to our field. I understand that culturing planktonic foraminifera is very challenging and this alone makes that the authors should be applauded for their work. Overall, the manuscript is well-written and the data is sound. However, I do have some issues with the translation from data to results and the use of statistics (or absence thereof). The annotated pdf contains a number of smaller suggestions, and some of them overlap with the major concerns, which are listed below. Another approach to the data may lead to different conclusions, so that some of the discussion may have to be rewritten (which I therefore did not always comment on in detail).
Sincerely,
Lennart de Nooijer
Statistics
To start with, there should be a separate section on the statistics used (lines 260-266). How exactly were the regressions calculated and was there (e.g.) a strategy to identify/ exclude outliers? Why were the uncertainty envelopes not calculated for the plots with the median Mg/Ca and DMg? Related: I am not a big fan of R or R2 as a metric. It says something about the spread of the data, but not about the significance of the correlation (R can be very low, but still p<<< and vice versa). This means that for figure 1, I don’t see the reason for including a trendline if R is >0.5 even when p>0.1 (i.e. no significant trend).
Such a section would also have to explain, for example, why the chosen response model for the regression analysis is the most appropriate. For example, the choice for a linear relationship for the upper panels of figure 3: it seems that an asymptotic function would be just as appropriate (and wouldn’t result in negative Mg/Ca at very high pH). In short, please include a justification for the chosen function.
Maybe more importantly, the authors use the data for isolated analyses of Mg/Ca-environmental pairs. This is a shame, however, as for all treatments, all parameters are measured. I would argue that particularly a dataset like this would be best analysed by some sort of multivariate statistical approach.
Interpretation of trends
Another reason for my suggestion is that (table 2) although the ‘pH stable, varying [CO32-] treatment, the pH is actually not that stable (varies from 7.83 to 8.12). This seems the case for many parameters, which are less stable than suggested in the text: this makes it hard to contribute changes in Mg/Ca to (e.g.) [CO32-] only. In this particular case, see also the significant correlation between pH and [CO32-] in Table S1. Rather than focusing on the trends per treatment (as done now), all of the data combined may provide a much more robust idea on the effect of all (calculated) parameters (and their interaction) on the Mg/Ca. This is important, as it matters a lot for most of the discussion (do pH and carbonate ion concentration have opposite effects on Mg incorporation?). A more rigorous analysis of the data may lead to more reliable statements of what is, and what is not correlated in this dataset.
Adjustment of the discussion: the section on biomineralization (4.5) makes perfect sense given the preceding sections, but given my comments on the relation between pH and [CO32-] in this dataset, may need to be adjusted. Given the content of the discussion in this section, the analysis by Ries (2011), GCA 75: 4053 (http://dx.doi.org/10.1016/j.gca.2011.04.025), may be a good framework to place some of the discussion on this point in.
While looking into the numbers, I noticed that the data in figure 3 and in table 2 don’t match. I tried to do some statistics on the data, but I get different results: for example, the right panels in figure 3 have data for four [CO32-], but table one only has three [CO32-] (64, 195 and 206 µmol/kg). Please check carefully.
At various places, trends are named ‘significantly different’, but an actual statistical test to base this on, is lacking. For example, lines 310-311 and figure 2: Since the LC+CC should be an average of the LC and CC, but clearly isn’t, it is hard to believe that the trendlines for only CC and only LC are significantly different.
Miscellaneous
Lines 311-312 and figure 2: what does it mean that the median Mg/Ca is also correlated to temperature? I don’t see the added value of showing both the median Mg/Ca and the DMg, as they are essentially the same (due to the similar seawater Mg/Ca across this dataset).