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:

• The discussion focuses heavily on using leaf wax carbon isotope values to reconstruct changes in carbon isotope discrimination (Δ) across the O/M boundary – but this was not introduced prior to the discussion and I was left wondering why this was important? Was there a specific hypothesis that you wanted to test?  To resolve this, I suggest some restructuring is required (e.g., talk about Δ during the introduction so that the reader knows the importance…)
• The manuscript explores the many potential controls on carbon isotope discrimination values and states that “…low seasonality and decreased light intensity during the OMT would contribute to a negative δ13C excursion and associated higher Δ” – however, it was unclear what evidence you had to support either low seasonality and/or decreased light intensity during the OMT. Thus, this section currently feels speculative – this is a big part of your manuscript, thus key to resolve.
• The authors suggest a “marked negative carbon isotope excursion … for all chain lengths across the OMT” but the data in Figure 3 and 4 does not appear to show a “marked” decrease at the OMT (i.e. 23 Ma). There is clearly a long-term shift towards more 13C-depleted values from ~25-23 Ma but the OMT data looks pretty similar to values at 24 Ma and 20.5 Ma.
• There was no discussion on how the age model was constructed and the methods would benefit from discussion this further.

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?