the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Jan 2025
Survival strategies of Antarctic vegetation during extensive glacial expansion across the Oligocene/Miocene Transition
Abstract. Antarctica’s terrestrial ecosystems are at risk from a rapidly changing climate. Investigating how Antarctica’s vascular plants responded to major climatic variations in the geological past, especially under atmospheric CO2 values similar to modern and future projections, may provide insight into how organisms could migrate across the continent as conditions change. Here, we investigate vegetation trends across the Oligocene/Miocene Transition (OMT, ~23 Myr), one of the largest transient glaciations of the Cenozoic. Despite extensive ice sheet expansion, Antarctic vegetation survived throughout this glacial episode. We use compound specific isotope trends (δ13C and δ2H) of plant waxes in an Antarctic proximal sediment core from the Ross Sea (Deep Sea Drilling Project site 270) to investigate the response and survival mechanisms of Antarctic vegetation during this event. We detect the first observation of a marked negative n-alkane δ13C excursion over the OMT, coupled with a shift to more positive n-alkane δ2H. We interpret this as plants sacrificing water use efficiency to maintain photosynthesis and carbon uptake during increasing glacial conditions, as atmospheric CO2 decreased and orbital configurations favoured shorter, colder growing seasons with lower light intensity. We consider further drivers of these isotopic trends to be enhanced aridity, and a shift to a stunted, low elevation vegetation. These findings establish the adaptability of ancient Antarctic vegetation under atmospheric CO2 conditions comparable to modern, and mechanisms that allowed vegetation to keep a foothold on the continent despite prolonged hostile conditions.
- Preprint
(1313 KB) - Metadata XML
-
Supplement
(59 KB) - BibTeX
- EndNote
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-4021', Anonymous Referee #1, 22 May 2025
-
EC1: 'Comment on egusphere-2024-4021', Yves Godderis, 24 Jun 2025
Given that I haven't been able to find a second reviewer, and I've let time slip by, I've personally revised this submission, despite not being a specialist in these matters.
The article is very technical. I don't have any specific comments. It describes a significant dataset.
I think it's missing a section containing a real confrontation with climate change, even if speculative. But that's just a suggestion.
I recommend the authors respond to all of reviewer 1's questions.
Citation: https://doi.org/10.5194/egusphere-2024-4021-EC1 -
RC2: 'Comment on egusphere-2024-4021', Irina Rogozhina, 18 Jun 2026
Survival strategies of Antarctic vegetation during extensive glacial expansion across the Oligocene/Miocene Transition by Duncan et al., in review by CP
Review by Irina Rogozhina
First of all (and on behalf of co-editors in chief of Climate of the Past, CP), I would like to apologize for the long duration of the review and open discussion stages. The handling editor could not find a second reviewer, even though it was a pre-condition for publishing with EGU journals. Although the research methods used in this study are well-established, they don´t seem to attract enthusiastic reviewers. The editor contacted 14 potential referees, and only one of them, in addition to agreeing to reviewing the manuscript, had actually submitted their review. Then I (as a co-editor in chief) agreed to provide a second review, even though it did require quite some homework from my side, since I am on the numerical modeling side of things. However, in the meantime I got sick and ended up in hospital. With this first paragraph I conclude my apology and come on to the review itself.
In general, I have a mixed impression from reading and thinking about this manuscript – I believe that both the writing style and structure of chapters/analyses should improve and even more so the interpretation of the results in terms of scientific findings. Messages should be bold and clear, with the appropriate discussion of the uncertainties.
Writing:
The manuscript is written in such a technical way that anyone who is not directly involved in the analyses in question would grind their teeth trying to extract the most important scientific findings buried under all the equations, formulas and symbols. There are many numbered equations, but these numbers are never mentioned in the text. Also, the manuscript introduces an overwhelming number of abbreviations and notations that may be familiar to experts but for a reader with a more general background, such as mine, it is difficult to follow through more technical sections such as methods and especially results, but also parts of the discussion. While it is rather common for the methods section, it is much less so for the other two above-mentioned sections. Look at for example, section 3.1 - in one paragraph there are 3 acronyms without reminding what they mean. In the texts that are so abundant in special symbols and equations I would avoid introducing even more acronyms.
From my point of view, when submitting a biology-heavy manuscript to a journal like CP, one needs to make sure that it is written in a relatable and easy-to-grasp manner so that climate modelers, paleoglaciologists, geomorphologists, and other interested parties can understand and use these results. The subject and the results of this manuscript are clearly within the scope of CP, but its presentation style is not so much. I encourage the authors to polish their text and also visual materials.
Then I was surprised to find quite a lot of grammatical errors and typos, since the author team includes a lot of native speakers – I even consulted with a native speaker from my team. The manuscript was probably written in a hurry, which unfortunately did not result in fast publication. Also, there are some assumptions that are made in the text about the prior knowledge of literature. For example, in lines 201 – 204 there is a hidden link to some earlier literature discussing an internal parameter from an equation published elsewhere. At least I have not found where it was shown in this manuscript.
Structure:
The structure of the discussion section is quite confusing. It feels like the authors are pulling rabbits from a magician´s hat, and each rabbit is from a different continent. There is no cohesion in the narrative of this section, while it is clearly much needed, especially in the discussion. It would be better to avoid mixing actual scientific findings with new equations, introduction of new symbols and concepts, and in some places long discussions of contradictions in earlier studies and current assumptions. Some of these belong to the methods and results (with references to the equation numbers where these symbols are explained). Other issues would be better addressed in the introduction and then referred to in the discussion section. Finally, you should design a separate sub-section in the discussion that describes and analyzes uncertainties in your findings.
In conclusion: Emphasize your scientific findings and innovations. Currently it looks like a methodological publication, scientific results are poorly represented. Explain how your data synthesis was happening.
Interpretation:
I do understand that going into the deep past is not an easy task and many analogues for the past plant types that we can use today are not ideal matches for such interpretations. However, I also spotted some confusing discussions that are going back and forth on whether a plant-state indicator should increase or decrease in response to wetter/drier or colder/warmer climate conditions and also depending on the latitudes and sun light regimes. The discussion has to become more structured and relatable for a reader with a more general background to be useful for the future scientific progress.
Could sea level drop also influence plant migration to lower elevations?
You are presenting a description of many measured factors but with a very limited interpretation of their climate significance. This might be confusing for the reader.
Specific comments:
Lines 137-138: Is there any idea about the origin of this deviation?
Lines 203-204: Has this been demonstrated or only assumed?
Line 206: Why temperate?
Lines 207-2010 and similar: Why not place these numbers in tables?
Line 250: Permafrost could have also developed during Mi-1.
Paragraph of line 246+: There are significant differences between plant types in the Northern and Southern Hemispheres even now. Doesn´t it have to be discussed?
Lines 263-271 (and similar): There are so many different factors that affect delta (and other indicators). How reliable is the presented interpretation?
Citation: https://doi.org/10.5194/egusphere-2024-4021-RC2
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 6,153 | 838 | 188 | 7,179 | 336 | 305 | 424 |
- HTML: 6,153
- PDF: 838
- XML: 188
- Total: 7,179
- Supplement: 336
- BibTeX: 305
- EndNote: 424
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
In this paper, the authors evaluate changes in Antarctic vegetation during the latest Oligocene to early Miocene (~25 -20 Ma). They evaluate this by measuring the distribution and compound specific stable carbon and hydrogen isotopic composition of long-chain n-alkanes in marine sediments from DSDP Site 270. The authors document a gradual long-term negative excursion in n-alkane d13C values and positive shift in d2H values during the late Oligocene. The authors interpret this to represent: 1) a shift in climate to colder, shorter and more arid growing seasons with lower light intensity, 2) a decrease in catchment area and reduction in growing region (both space and elevation), 3) a shift to more shrub-like growth forms.
Whilst the biomarker data appears robust, the interpretation and structure of this manuscript requires improvement. My main concerns are listed below:
Other comments:
The Methods (L56) is referred to as section 4 (and lipid biomarkers as 4.1) – but assume this meant to be section 2?
L62: after 3:1 add (v/v)
L63: should be total lipid extract, not total solvent extract
L69: no need to capitalise Mass
After line 86, equation for stable isotopes states dD not d2H
L113: I would argue that a CPI >1-2 is not a reasonable threshold for modern plant material – most modern plants typically have CPI values between 3 to 30 – anything below 2 is likely affected by diagenesis. I suggest looking at the supplements in Bush and McInernery and re-assessing this in modern plants (e.g, calculate the CPI for 90% of modern plant data) …
L114: very rare to get CPI < 1 except in hypersaline environments
L137: not sure you can report to 2 SFs (i.e. -38.13)
L137: when you say “most values”, define how many
L137: “One sample each for n-C25, n-C27 and n-C29, and three samples for n-C31 were excluded as outliers for those specific chain lengths, as they were different from adjacent samples by more than 2 ‰” – I don’t understand the rationale for excluding this data. Can you explain why you excluded this?
Figure 3: suggest showing individual data points on each black line as this helps show data resolution
Discussion 3.1 – I wasn’t totally sure what the aim of this section was – perhaps better to start by discussing you ACL data and THEN comparing this to pollen data. Or might be better to fold in the palynology/vegetation discussion into d2H interpretation
L169: How does a CPI value of 2.4 argue against reworking?
L298: “plant wax δ2H values trend from more negative in the late Oligocene to more positive over the OMT” – please state the values (i.e. from -xxx to -xxx per mil)
L334-338: “We attribute n-alkane δ13C and δ2H trends to several environmental changes that affected Antarctic vegetation during the OMT (Fig. 5.). These are: 1) a shift in climate to colder, shorter and more arid growing seasons with lower light intensity, 2) a decrease in catchment area and reduction in growing region (both space and elevation), 3) a shift to more shrub-like growth forms” - what is evidence for shorter colder, more arid growing seasons with less light, a decrease in catchment area, and more shrub like growth forms?