Reviewers comment #1

The presence of non-marine diatoms needs some further discussion throughout the paper.

Firstly, in the introduction. Here there needs to be more of an acknowledgement that diatoms are also abundant and diverse in lakes, streams and wet habitats (e.g. soils) in terrestrial environments of Antarctica as well as growing on ice e.g. in cryoconite holes (Verleyen et al 2021, Noga et al 2020, Cavacini 2001, Van de Vijver & Beyens 1998). Thus there are additional sources of diatoms which either can grow in situ or be blown onto the ice which need to be taken into consideration. The authors mention some of these sources later in the paper, but they need to be referred to in the introduction.

Response:

Reviewers comment #2

Diatoms in cryoconites aren’t mentioned at all, are these environments present in their sampled locations? could they contribute an additional source?

Response:

Cryoconite holes are common in Alpine and Artic glaciers. However, in Antarctica, cryoconite holes have been comparatively rare, with most of them located in the McMurdo Dry Valleys (Fountain et al., 2004), some in the Dronning Maud Land Coast, East Antarctica (Weisleitner et al., 2020 and references therein) and just one report of cryoconite holes from the northern AP, in the South Sandwich Islands (Buda et al., 2020). Unlike cryoconite holes in Alpine and Artic Glaciers, Antarctic cryoconite holes are mostly perennially ice-lidded due to low air temperatures year-round, so it is unlikely that they are a prevalent source of diatoms to the ice core records presented in this study. We have addressed this comment by acknowledging cryoconite holes as potential source of diatoms (Introduction – 1^st paragraph) and by describing the scarcity of cryoconite holes (and other non-marine water bodies) in this region and their limitations to become a common source of diatoms to our ice core sites (Discussion – Section 5.1 – 1^st paragraph).

References:

Reviewers comment #3

Secondly, the ‘broad ecological affinities’ of the Cyclotella group (L300) needs a bit more unpacking. Many of these species are associated with freshwaters; the Cyclotella group as defined here may be considered to have a broad affinity but within it specific taxa have much narrower niches. By grouping together in this way information has been lost. Given the authors’ comments about good preservation (e.g L179) it would have been useful to know (perhaps in the methods) the constraints on greater taxonomic resolution. Why was it not possible to identify diatoms to species level? If species had been determined then these could be more confidently assigned to aerial, freshwater, brackish or marine categories – diatom species are pretty faithful, especially in terms of salinity. Cyclotella is an important component at ice cores JUR and SKBL so there needs to be a bit more discussion of the possible genera and species found in the ice cores and where these are found in present day communities. The same point applies to the Navicula and Achnanthes groups (Table 3) as species from these groups/genera can be found in aerial or freshwater habitats as well as marine ones.

Response:

Reviewers comment #4

The fact that marine diatoms consist of just over half (58%) of the assemblages (L307, Table 3) at the 4 sites means you do have to spend a bit of time thinking about diatoms from non-marine environments too. It is misleading to say in terms of marine components that (L330) they are – ‘almost exclusively dominated’ or (L341) there are ‘minor contributions’ from exposed sediments and freshwater and brackish. Stating (L380) that ice cores from this region are uniquely situated to record marine transported diatoms over emphasises this importance of the marine diatoms.

Perhaps this is a missed opportunity? it’s not binary i.e., an ice core could provide useful environmental interpretations from both marine and terrestrial sources if those sources can be well separated which they should be if high enough taxonomic resolution is employed. For example, questions re. extent of terrestrial sources could also be explored.

Response:

We absolutely concur that the non-marine taxa offer the potential for greater environmental understanding and intend to explore this in subsequent work. However, a previous study has already pointed out the numerous limitations to identify the source(s) for non-marine diatoms in Antarctic snow air and snow samples (McKay et al., 2008). Since considerable time investment would be needed (manually locating and re-imaging individual specimens, undertaking several taxonomic investigations, etc.) with no certainty that definitive source region(s) could be identified, we consider this to be beyond the scope of this work.

As reported in our results section, at least 58% of the classified diatoms from each site were exclusively marine. These correspond to all the unambiguous classifications. The remaining fraction, the ambiguous group, comprised specimens identified as benthic, brackish and/or freshwater (some could also be marine e.g. Navicula) and diatoms that were, at best classified to genus level (excluding exclusively marine genus).

The broad ecological affinities of these groups prevented us from establishing a clear origin for the “non-exclusively marine” fraction of the assemblage. We acknowledge there is a possibility that the whole “non-exclusively marine” fraction of the assemblage could be entirely comprised by non-marine diatoms. To address this comment, we have modified the discussion section, accounting for the potential inputs from non-marine sources (Discussion – Section 5.1 – 1^st paragraph).

Reviewers comment #5

In sections 2.1 2.2 and 2.3 current conditions in terms of oceanographic, climate and sea ice are detailed but there is no mention of what is happening on the land...if we accept that there is, or at least could be, a terrestrial signal in the ice cores then a brief discussion of recent terrestrial changes would be justified.

Response:

Reviewers comment #6

In Table 2 high proportions of diatoms are ‘unclassified’ e.g. in JUR 544 diatoms out of 1149 (i.e. c. 50%) but also high at other sites (e.g. SHIC c. 25%), I assume these are the (L157) “unidentified (obscured, undiagnostic or indistinct)” diatoms. With so many in this category (I acknowledge when doing diatom analysis allocating some diatoms to this category is inevitable, but in my experience, these would be unusually high proportions) there needs to be a bit more detail as to why and whether this has any consequences. For example, if they are obscured then by what? What is meant by undiagnostic? And indistinct? Most diatoms can be identified to species level (or if not identified with confidence to a described species at least put into separate taxonomic units) so is this an issue with the technique, for example the use of SEM rather than light microscope? Some comments would be useful. What I am worried about here is some sort of systematic bias, for example if some genera are more undiagnostic than others due to being say harder to identify.

Response:

The percentage diatom assemblage data presented in Figures (2 -4) are “normalised” to the main species (i.e. recalculated to 100%) but when you have varying amounts of unidentified species it makes it difficult to compare between sites. Ideally, you need to look at the whole assemblage if not then there is an assumption that all the unidentified diatoms are a random subsample (not biased) of the assemblage? Is that a reasonable assumption?

I assume that as with light microscopy fragments are counted e.g. a fragment of a raphid diatom central area is usually distinct enough to assign to a genus if not a species. But if fragments form part of the ‘indistinct’ category then there is a need to square this with the pristine nature (ornamentation and chains are mentioned L409) of the diatoms. Again, this could be covered with more detail on what constitutes the ‘unclassified’ portion.

Response:

Reviewers comment #8

Fig 1 the locations of the ice cores aren’t particularly clear – an inset would help here (like that of Figure 6..or just refer to that) and the caption needs to include the names of the ice cores. Locations such as Amundsen-Bellinghausen seas, Ellsworth Land need to be added.

Response:

Reviewers comment #9

What is the justification of the decadal subsets? if these are ”to assess the consistency of the assemblage” (L160) there might be better ways to do that e.g. DCA axis scores to see if there are changes in ‘turnover’. Subsequently there is more emphasis on using the decadal data sets to examine changes in diatom concentration (number per L) between the 2 time periods (L404) and these are compared directly but the time intervals aren’t the same e.g. JUR is 1992-2001 and 2002-2012 whilst SHBL and SHIC are 1999-2008 and 2009-2019. So I am unclear as to the justification for these decadal means when all 3 cores show positive trends over the whole sampled period and section 2 doesn’t discuss climate/sea ice changes at these time intervals.

Response:

Our reason to analyse temporal changes in the assemblages and on the diatom concentration was to assess the consistency of the diatom record in response to recent environmental changes in the region. In particular, to determine if there were shorter-term shifts in the diatom concentrations caused by changes in certain diatom species or if there was a general shift in all diatom species that shape the main assemblage. We decided to assess this at a decadal timeframe to reduce the potential imprints of interannual variability. We did not discuss specific changes in climate/sea ice during the assessed decades because trends in both, wind strengthening and sea ice retreat, have been sustained over the last decades.

Reviewers comment #10

Line 267 S. gracilis (not gracilics)

Response:

 Additional modifications:

Based on the reviewer’s comments, we have decided to modify the title of the manuscript, acknowledging the records we present are not exclusively marine and to emphasise the spatial component.

Reviewers comment #1

Figure 5 and MSA: It is mentioned in the methods that the ROIC ice core was dated using the annual cycles of major ion concentrations, and MSA is used in Figure 5c to identify the austral summer based on the maxima of MSA. As this study crosses multiple disciplines, not every reader will know what MSA is and understand its use here. MSA should be introduced earlier on in the manuscript (only mention is line 129) as to what it is, how it is interpreted at this site, and what the maximum represents.

Response #1:

Revised as suggested. Details about MSA and how it can be interpreted are now included in the methods section.

“The MSA is the oxidized product of DMS, an organic sulphur compound from marine biogenic emissions. Measurements of MSA were used to estimate the temporality of the ROIC ice core. The MSA record was used as it has demonstrated to present a clear seasonal cycle in Antarctic ice cores, with a sharp austral summer maximum and a broad winter trough (Abram et al., 2013).”

Reviewers comment #2

This may be a missed opportunity in the discussion for ROIC site, as MSA can be an indicator of winter sea ice extent or summer primary productivity (i.e., Thomas et al., 2019 as cited; Abram et al., 2013), and the relative timing of the MSA peak to the diatom peaks could add an interesting aspect to the discussion.

Response #2:

The link between MSA and diatoms in ice cores is not straightforward. Despite MSA being an indicator of primary productivity, this does not necessarily imply a direct relationship between the concentration of MSA and the diatom abundance in ice cores. To date, there is uncertainty on the amount of DMS (turned into MSA after oxidized) produced by each diatom species. We acknowledge that exploring the relationship between MSA and diatom abundance could provide interesting insights but it is beyond the scope of this research article. We hope to explore the relationship between MSA and ice core diatom assemblages in subsequent work.

Reviewers comment #3

Line 331/376: The text emphasizes the ‘dominance’ of marine diatoms in the record, when they are just over 58% (line 307) in some instances. The authors also state they cannot “rule out minor contributions from exposed sediments in fresh/brackish-water bodies.” in line 341. Given that in some instances marine diatoms only make a little over half the assemblages I suggest the authors rephrase these sentences as ‘dominance’ could overemphasize the marine contribution. The same could be said about ‘minor’ minimizing contributions from fresh/brackish water bodies.

Response #3:

Marine diatoms account for at least 58% of the main diatom assemblage preserved in ice cores presented in this work. We have modified the introduction and discussion sections, accounting for the potential inputs from non-marine sources. We have also revised the text to better acknowledge the contribution of “non- marine” taxa. Finally, we amended our description of the Antarctic marine taxa as ‘prevalent’ within the diatom assemblage rather than ‘dominant’ throughout the whole manuscript.

Example: “Whilst the SO is the principal source of diatom to ice cores in this region, we cannot rule-out contributions from exposed diatom-bearing sediments (most reported from sites within the distally located Transantarctic Mountains (Barrett, 2013)) and fresh/brackish-water bodies. Antarctic non-marine water bodies could potentially contribute with diatoms to the ice core record.”

Reviewers comment #4

Line 344: SSIZ and POOZ are used to describe ‘seasonal sea ice zone’ and ‘permanently open ocean zone’ with a reference to section 2. However, neither of these acronyms are used or defined in section 2. SSIZ is defined in the caption of Figure 1. POOZ isn’t mentioned until the caption in Table 3. As these acronyms are used extensively in the discussion, they should be defined and described in main text in section 2 and not just in the captions.

Response #4:

We have modified section 2.1 to define and describe the SSIZ and POOZ as suggested.

Reviewers comment #5

Section 5.2 Inter-annual variability: This section provides some interesting insights, but the assessment of changing environmental conditions influencing diatom concentrations could be strengthened (from line 413 onwards). First off, regarding the decadal variability. Why were these subsets chosen? As they differ between sites, they seem arbitrary. The discussion could instead focus on the overall increasing trend, rather than the differences between the subsets, particularly as the discussion of the changes in atmospheric circulation and sea ice dynamics is only regarding ‘recent decades’ and not these specific periods. However, since these data were subdivided, it would strengthen the discussion to be more specific about the trends in environmental conditions over each decade analysed. For example in Line 423: Regarding the ‘recent decrease in the area of the ABS SSIZ mentioned in Parkinson, 2019– the overall trend is negative, but there has been a slight increasing trend since ~2010. This would then perhaps suggest an increase in the distance of the SSIZ relative to the 1999-2008 period. While data for the specific decades analysed may not be readily available for all environmental conditions mentioned in the text, the authors should make improvements where possible.

Response #5:

Similar points were raised by Prof. V. Jones in her Reviewers comment #9. We have modified the Discussion section of the manuscript to incorporate them.

“Our reason to analyse temporal changes in the assemblages and on the diatom concentration was to assess the consistency of the diatom record in response to recent environmental changes in the region. In particular, to determine if there were shorter-term shifts in the diatom concentrations caused by changes in certain diatom species or if there was a general shift in all diatom species that shape the main assemblage. We decided to assess this at a decadal timeframe to reduce the potential imprints of interannual variability. We did not discuss specific changes in climate/sea ice during the assessed decades because trends in both, wind strengthening and sea ice retreat, have been sustained over the last decades.

We acknowledge we did not specify that wind and sea ice trends have been sustained over time or refer to the different timing of the decadal subsets across the sites. To address this comment, we have modified the manuscript, specifying our reasons to analyse the dataset in decadal subsets (Methods – Section 3.2 – 6th paragraph), we have clarified that recent trends have been sustained over the last decades (Discussion – Section 5.2 – 5th paragraph) and we specified the timeframes considered when comparing recent changes in the diatom concentration (Discussion – Section 5.2 – 4th paragraph).”

Reviewers comment #6

Abstract, line 16: “yield a novel wind paleoenvironmental proxy” – suggest ‘paleoenvironmental proxy’ as authors acknowledge other environmental factors may influence diatom content in ice cores such as sea ice extent.

Response #6:

Modified as suggested

Reviewers comment #7

Line 28: “over long distances” – if possible, be more specific here (ex: over XX kms)

Response #7:

Revised as suggested. Now we specify that diatom transport distances can range from some kilometres to intercontinental scales.

Reviewers comment #8

Line 44: “ocean” is capitalized

Response #8:

Modified as suggested

Reviewers comment #9

Figure 1: SSIE and PSIE are difficult to read/see in the main figure – perhaps outline these in white (but keep them filled in with color) to make them stand out more?

Response #9:

Modified as suggested

Reviewers comment #10

Coastal polynyas: The transect in Figure 1 identifies coastal polynyas as a feature in this area. There are several in this region, yet no mention of coastal polynyas is made in the text or how polynya variability may impact these records.

Response #10:

This paper aims to determine the regional and temporal variability of diatom records preserved in ice cores from the AP and EL regions. Results presented in this paper allow us to broadly outline diatom sources and suggest potential processes driving the variability of the record. We agree polynyas in this region could potentially contribute diatoms to ice core sites and have included this in the discussion section of our revised manuscript.

Reviewers comment #11

Lines 89 and 95: “XXth century”

Response #11:

Modified as suggested

Reviewers comment #12

Table 1 caption: When describing SIE – perhaps refer to section 3.3 which provides the source of the data. The caption does an excellent job of explaining how these distances were calculated but does not identify the source of the data which is presented later.

Response #12:

Modified as suggested

Reviewers comment #13

Figures 2-5, Part C: The color used for SIDI can be hard to differentiate from the total diatom concentration (particularly with a printed version)– an alternate color or line marker may be more suitable. However, this is up to the author and editor’s discretion.

Response #13:

Modified as suggested