This interesting publication by Suchéras-Marx et al. (2020) highlights peculiar aspects of geochemistry in nannofossils, specifically coccolithophorids. One of the main application of geochemistry on fossil shells is to get hints on the physiology of such extinct taxa. Here, the authors try to get information on the calcification mechanism and processes in Jurassic coccoliths. Coccoliths build a test made of calcium carbonate and one of the most common geochemical proxies used for this fossil group is the Sr/Ca ratio. This isotopic ratio has good chances to be successfully used as a robust proxy for paleoenvironmental reconstruction, but, concerning Jurassic coccoliths things seem to be not straightforward.
The authors managed to compare the isotopic value of Sr/Ca measured on Jurassic coccoliths from different taxonomic groups: the murolith Crepidolithus crassus and the placoliths Watznaueria contracta and Discorhabdus striatus. The results they got clearly show that the Sr/Ca ratio cannot be used as a universal proxy because these species exhibit very different values despite coming from the same stratigraphic level and having undergone minimal diagenetic modification. Data seem to point to a Sr/Ca ratio up to 10 times higher in the murolith species than in the placolith taxa (Suchéras-Marx et al., 2020). One of the explanation given here takes advantage of modern coccolith data and hints to specific polysaccharides that would control the growth of the long R unit in the murolith species. As always, there is plenty of space for additional research, possibly on modern taxa, to sort out the scientific questions that arise from this work.
Suchéras-Marx, B., Giraud, F., Simionovici, A., Tucoulou, R., & Daniel, I. (2020). Evidence of high Sr/Ca in a Middle Jurassic murolith coccolith species. PaleorXiv, dcfuq, version 7, peer-reviewed by PCI Paleo. doi: 10.31233/osf.io/dcfuq
first and foremost, let me send you our deepest apologies for the delay in receiving your revisions. PCI struggled over two months to find a suitable handling editor as none of the listed recommenders felt having enough skills in handling the manuscript. I was asked direclty twice and finally I humbly accepted the task. I sent your manuscript to 13 possible reviewers and most of them either refused the task or took immensely long to get me back their comments. Covid crisis took part in this delay as well. I finally got one excellent revision by one reviewer and one more succint but with nice hints from an early career scientist. Additionally I personally went trough the text highlithing some parts that can be modified.
Based on the revisions I have got I am convinced that your paper can be a very intersting contribution in PCI Paleo but some more extended discussion might increase the readership and might get the paper more to the topic. Please, check directly the pdfs I attach to this letter and in those you will find all specific comments and hints.
As usual, a rebuttal letter is expected, shall you decide to submit again the revised version; make sure you indicate your response for each single comment.
Kind regards and, again, our most sincere Apologies,
Antonino BriguglioDownload recommender's annotations (PDF)