I went through the manuscript titled “Controls on Palaeogene deep-sea diatom-bearing sediment deposition and comparison with shallow marine environments”, submitted by Figus and co-authors, to be considered for publication.

The manuscript addresses a very relevant issue, i.e. the relationships between pelagic and shallow marine diatomaceous deposition and the climatic, oceanographic and tectonic reconfigurations occurred during the Palaeogene. A noteworthy task, approached through the review of an impressive amount of data from DSDP, ODP and IODP oceanographic campaigns. The Palaeogene diatomaceous deposition is poorly studied if compared to the Neogene one, and this makes authors’ efforts even more appreciable. Works like this are absolutely needed to fix ideas, highlighting the controversies and raising new research questions. Even if the ‘smoking gun’ is still difficult to be found, this is already an important step beyond. Therefore, on my opinion this manuscript definitely deserves to be published.

Below I provide some suggestions and I raise some questions mostly aimed at improve the discussion. I am very looking forward to know authors’ opinion about.

General comments

Proxies (isotopes etc.) should be introduced in Materials and methods, while they suddenly appear at lines 140-145. I suggest to include a new paragraph where proxies are presented, and their significance for this investigation briefly described.

I believe that in the Discussions the authors should consider, besides the work of Muttoni and Kent (2007), the very recent results presented by Varkouhi et al. (2024) in a paper titled ‘Pervasive accumulations of chert in the Equatorial Pacific during the early Eocene climatic optimum’ (Marine and Petroleum Geology 167, 106940). Another relevant work to be considered is Cermeño (2016) - The geological story of marine diatoms and the last generation of fossil fuels (Perspectives in Phycology 3-2, 53-60).

On my opinion, the role of tectonics is rather vague, and must be better addressed. Among others, I suggest to consider the paper of Rea et al. (1990) - Global change at the Paleocene-Eocene boundary: climatic and evolutionary consequences of tectonic events (Palaeo3 79, 1-2, 117-128).

Did you consider the possible role of sea-level changes occurred during the Palaeogene in governing the shallow marine vs deep-sea diatomaceous deposition?

Although I know that this may go a bit far beyond the aims of the present paper, I believe that the discussion might benefit from a brief evaluation of the ecological meaning of the major diatom taxa identified in deep-sea Palaeogene sediments. Two of the authors provided excellent contributions about (see Renaudie et al., 2010 - Siliceous phytoplankton response to a Middle Eocene warming event recorded in the tropical Atlantic (Demerara Rise, ODP Site 1260A). Palaeo3 286, 3-4, 121-134; Renaudie et al., 2018 - The Paleocene record of marine diatoms in deep-sea sediments. Fossil Record 21, 183-205; Witkowski et al., 2020 - Early Paleogene biosiliceous sedimentation in the Atlantic Ocean: Testing the inorganic origin hypothesis for Paleocene and Eocene chert and porcellanite. Palaeo3 556, 109896, etc.). In these works, the widespread occurrence of representatives of the order Hemiaulales is reported. Authors are surely aware that this order comprises diatoms often involved in diatom-diazotroph associations (DDAs) in stratified, oligotrophic waters. Don’t authors think that would be interesting to deepen this aspect, especially in the light of the variations of nitrogen isotopes (see for example paragraph 3.3. in Knies et al., 2008 - Surface water productivity and paleoceanographic implications in the Cenozoic Arctic Ocean. Palaeocenography and Palaeoclimatology 23, 1)?

What can be inferred from the Palaeogene sedimentary record about the other components of the pelagic trophic chain (not only radiolarians)? Any significant event among other groups of phyto- or zooplankton (or among marine metazoans) that can be relevant for the present study?

Specific comments

Line 14: ‘in the oceans’ – ‘in the modern oceans’

Line 18: ‘supplied’ – ‘mostly supplied’; a reference is needed here, I suggest Tréguer et al. (2021) - Reviews and syntheses: The biogeochemical cycle of silicon in the modern ocean. Biogeosciences 18 (4), 1269-1289.

Line 22: ‘gravitational sinking of diatoms in the water column’ – Some references are needed here; moreover, I would expand a bit this passage. The sinking of diatom aggregates is much more than a passive process controlled by gravity. I recommend the seminal works authored by Smetacek and Grimm about this aspect. For example: Smetacek (1985) – Role of sinking in diatom life-history cycles: ecological, evolutionary and geological significance. Marine Biology 84, 239-251; Grimm et al. (1997) – Self-sedimentation of phytoplankton blooms in the geologic record. Sedimentary Geology 110 (3-4), 151-161.

Line 58: ‘opal-CT’ – ‘opal-CT and quartz’.

Line 61: ‘opal-CT (siliceous microfossils)’ – Unclear, I suggest to rephrase. It seems from this passage that siliceous microfossils are by definition composed of opal-CT.

Line 62: ‘diatoms are usually obliterated’ – An interesting exception, just for authors’ information: Hein et al. (1990) - Eocene diatom chert from Adak Island, Alaska. Journal of Sedimentary Research 60 (2), 250-257. When dealing with the diagenesis of biosiliceous remains, I suggest to cite the processes of reverse weathering (see Michalopoulos & Aller, 1995 – Science 270, 5236, 614-617) and pyritization. This latter can be very interesting for this case study, because it has been described in Palaeogene diatom-bearing sediments of Northern Europe: De Jonghe et al. (2011) - Middle Eocene diatoms from Whitecliff Bay, Isle of Wight, England: stratigraphy and preservation. Proceedings of the Geologists’ Association 122, 472-483.

Line 126: ‘surface/volume ratio’ – References are needed here.

Line 225: ‘(…) shallow marine diatomite(s) deposited in open ocean conditions’ – A bit confusing…

Figures

It would be nice to see some maps in the maintext, similar to those reported in Figures 1, 4 and 5 in Figus et al. (2024).

A very simple, resuming sketch (or a flow chart) could greatly help the reader to fix the major points discussed in the text.