Status: this preprint is open for discussion and under review for Climate of the Past (CP).
Extinction, Turnover, and the Reorganization of Diatom Communities Across the Eocene/Oligocene Boundary: Equatorial Atlantic Perspective
Volkan Özen,Johan Renaudie,and David Lazarus
Abstract.Marine diatoms couple the global carbon and silicon cycles, and their fossil record tracks oceanographic and climatic changes in deep-time. The Eocene/Oligocene Transition (EOT) marks the onset of Antarctic glaciation and major ocean reorganization and is a key interval in diatom evolutionary history. Although high-latitude plankton responses to polar cooling are extensively studied, it remains challenging to determine how cooling-driven changes in circulation, stratification and nutrient supply propagated and shaped low-latitude assemblages. Here we reconstruct species-level diatom diversity from exhaustive full-assemblage counts and integrate these data with diatom and radiolarian productivity from Deep Sea Drilling Project (DSDP) Site 366 (Sierra Leone Rise, equatorial Atlantic) spanning 38–32 Ma. Diatom diversity at DSDP 366 varies in step with Southern Ocean diversity records across the same interval. Extinction rates and community-structure metrics indicate a major reorganization of tropical diatom communities that is consistent with changes in upper-ocean stratification. We identify a sharp shift in community structure at ~33.5 Ma, pointing to a rapid ecological response in the earliest Oligocene.
Received: 21 Feb 2026 – Discussion started: 05 Mar 2026
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The manuscript is carefully structured, written in excellent English, in a straightforward and convincing manner. The aims, methods, and conclusions are clearly formulated, and the authors use a wide array of numerical methods that strongly support their arguments. Also, a vast array of published data is used to provide a broader context for interpreting the data. The manuscript is also beautifully illustrated. My rating of criterion 2 (Scientific quality) is "good" only because there is no intermediate between "excellent" and "good", and suggest a minor revision, requesting the authors to relate to the points made below.
There are several issues that, in my opinion, require addressing by the authors:
1. First, the East/West Atlantic comparisons rely heavily on the paper by Nilsen and colleagues. As discussed in the 2024 paper by Bryłka and colleagues, this paper is controversial with respect to biogenic silica, as the visual core descriptions for the relevant interval from Site 925 do not mention the presence of biogenic siliceous particles. Have the authors verified whether biogenic silica is actually preserved at Site 925? If Site 925 data were to be verified negatively, does the interpretation presented in the manuscrit still hold?
2. I am surprised to see a highly relevant publication missing from the reference list. Were the identifications made by the authors consistent with those of Suto & Tanaka (2017, https://doi.org/10.2517/2016PR019)? Is there a specific reason for excluding the paper by Suto & Tanaka?
3. No magnification is given for the light microscope observations. A related issue is that the observations were made on residues that were sieved through a 10 micron mesh, which is likely to have eliminated part of the diatom assemblage. The authors openly discuss various sources of potential bias in the diatom diversity records - I think this one should be included as well.
4. One other potential source of bias in diversity records could result from nomenclatural overlap arising from multiple scientific names given to seasonal/functional stages of the same diatom (such as vegetative form and the resting spore it produced, or valves representing different parts of a chain). While it is obvious that there is often no way of knowing these, it should be made clear to the readers that such biases may arise. Or have these been already corrected for in the statistical treatment that the authors applied?
5. A recent paper by Figus and colleagues (https://doi.org/10.5194/bg-22-3029-2025), albeit not based on diatom assemblage data, posits that "that through most of the Palaeogene, low to mid-latitude areas, especially in the Atlantic Ocean, were the locus of widespread biogenic silica deposition and burial". Do the findings from Site 366 challenge or support this view? Or are these two studies entirely incompatible? Please relate.
6. A minor nomenclatural issue: the current accepted genus name for Stephanopyxis is Eupyxidicula.
Özen et al Extinction, Turnover, and the Reorganization of Diatom Communities Across the Eocene-Oligocene Boundary: Equatorial Atlantic PerspectiveVolkan Özen et al. https://doi.org/10.5281/zenodo.18723713
Volkan Özen,Johan Renaudie,and David Lazarus
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We studied how diatoms (photosynthetic algae with silica shells) responded to major climatic and ocean changes between 38 and 32 million years ago, when the climate shifted into a colder state and Antarctic ice sheets expanded. Equatorial Atlantic sediment records show a sharp reorganization of diatom communities and an extinction pulse around 33.5 million years ago, coinciding with the largest cooling step of this transition.
We studied how diatoms (photosynthetic algae with silica shells) responded to major climatic and...
The manuscript is carefully structured, written in excellent English, in a straightforward and convincing manner. The aims, methods, and conclusions are clearly formulated, and the authors use a wide array of numerical methods that strongly support their arguments. Also, a vast array of published data is used to provide a broader context for interpreting the data. The manuscript is also beautifully illustrated. My rating of criterion 2 (Scientific quality) is "good" only because there is no intermediate between "excellent" and "good", and suggest a minor revision, requesting the authors to relate to the points made below.
There are several issues that, in my opinion, require addressing by the authors:
1. First, the East/West Atlantic comparisons rely heavily on the paper by Nilsen and colleagues. As discussed in the 2024 paper by Bryłka and colleagues, this paper is controversial with respect to biogenic silica, as the visual core descriptions for the relevant interval from Site 925 do not mention the presence of biogenic siliceous particles. Have the authors verified whether biogenic silica is actually preserved at Site 925? If Site 925 data were to be verified negatively, does the interpretation presented in the manuscrit still hold?
2. I am surprised to see a highly relevant publication missing from the reference list. Were the identifications made by the authors consistent with those of Suto & Tanaka (2017, https://doi.org/10.2517/2016PR019)? Is there a specific reason for excluding the paper by Suto & Tanaka?
3. No magnification is given for the light microscope observations. A related issue is that the observations were made on residues that were sieved through a 10 micron mesh, which is likely to have eliminated part of the diatom assemblage. The authors openly discuss various sources of potential bias in the diatom diversity records - I think this one should be included as well.
4. One other potential source of bias in diversity records could result from nomenclatural overlap arising from multiple scientific names given to seasonal/functional stages of the same diatom (such as vegetative form and the resting spore it produced, or valves representing different parts of a chain). While it is obvious that there is often no way of knowing these, it should be made clear to the readers that such biases may arise. Or have these been already corrected for in the statistical treatment that the authors applied?
5. A recent paper by Figus and colleagues (https://doi.org/10.5194/bg-22-3029-2025), albeit not based on diatom assemblage data, posits that "that through most of the Palaeogene, low to mid-latitude areas, especially in the Atlantic Ocean, were the locus of widespread biogenic silica deposition and burial". Do the findings from Site 366 challenge or support this view? Or are these two studies entirely incompatible? Please relate.
6. A minor nomenclatural issue: the current accepted genus name for Stephanopyxis is Eupyxidicula.