| 08 Dec 2025
Biomarkers and diatoms as tracers of past sea ice conditions and phytoplankton communities in the southwestern Ross Sea, Antarctica: drivers and variability over the last 200 years
Abstract. The Ross Sea, Antarctica, is among the most seasonally productive areas globally, where different classes of phytoplankton, such as diatoms and haptophytes (Phaeocystis antarctica), play key roles in marine ecosystems and the carbon cycle. Sea ice dynamics strongly influence Ross Sea phytoplankton blooms, yet the effects of recent sea ice changes on bloom composition and productivity remain poorly constrained. Seasonally resolved observational records of past Ross Sea phytoplankton and sea ice variability are too short to understand potential future changes in sea ice extent, phytoplankton productivity and community composition, and the resulting consequences on climate. In this study, we investigated phytoplankton-derived lipid biomarkers (fatty acids, highly branched isoprenoids; HBIs, sterols) and diatom assemblages in six marine sediment core tops and three short sediment cores collected along a north-south transect spanning two seasonally recurring polynyas in McMurdo Sound and Terra Nova Bay, to assess how sea ice dynamics and phytoplankton communities drive biomarker signatures and diatom assemblages archived in sediments. For the core-tops, we find that the proportion of open-ocean diatom species and bacterial fatty acid concentrations in core top samples increases towards the southern end of the transect near McMurdo Sound, which is driven by lower summer sea ice extent, a phytoplankton community dominated by diatoms, and higher summer biomass in McMurdo Sound. In contrast, diatom assemblages shift towards sea ice-associated diatoms in the northern end of the transect, characterised by increased concentrations of sea ice diatom-derived fatty acids, sterols, and HBIs (PIPSO25), driven by greater sea ice concentrations. Similarly, Phaeocystis antarctica-derived fatty acid biomarkers increased towards the northern end of the transect, likely driven by differences in the phytoplankton community. For the short cores, we show that an exponential decrease in the fatty acid biomarker signal in the top ~20 cm of sediment is driven by processes such as bacterial biogenesis. Although phytoplankton-derived fatty acids show little change in community composition over the last 200 years, Fragilariopsis curta and PIPSO25 indicate increasing sea ice extent, accompanied by declining Chaetoceros resting spores and open ocean diatom species in the southwestern Ross Sea. Overall, our records reveal a 200-year increasing sea ice trend, consistent with existing regional sea ice extent reconstructions from ice cores. Overall, biomarkers in the southwestern Ross Sea sediment independently distinguish between pelagic diatoms, P. antarctica, and sea ice-associated diatoms, offering a valuable tool for developing decadal resolution records of sea ice and phytoplankton community changes.
Competing interests: Some authors are members of the editorial board of journal Biogeosciences.
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This paper represents a valuable exercise in investigating biomarkers and sea ice proxies in Ross Sea environment, important for continuing the focus on these topics first established by Thomas et al., 2019 and Ashley et al., 20021. Of particular interest is the effort to identify and organically characterize specific biomarkers of different phytoplankton groups and to use them for comparative analysis of bacterial activity in sediments.
Supplemental Pangea dataset (.xlsx) is nice while the in the Supplement Biomarker sediment (.pdf) maybe some basic description, focus on the Appleby, 2001 method, could be useful.
I’d like to point out that the general section on cores, the sedimentological and descriptive reports (cores logs, photos of laminations and a general descriptions) are underdeveloped while some important environmental evidences inserted in a paleoclimatic context are missing. In particular, the important section on polynyas, their importance and their evolution is sparse. Furthermore, some specific expertise are taken for granted, making the article highly specific and lacking in flow and descriptiveness.
While reading the final part of the article, there is some confusion in defining diatom assemblages such as 'sea ice', 'pelagic diatoms', ‘dwelling diatoms’ or 'centric open water diatoms. A quick diagram (like Table 2.), providing the Groups/assemblages and the species that define each of them, could help. Selective dissolution before and after deposition is a little underestimated.
Perhaps a more structured mathematical/statistical analyses as a Cluster Analyses and a Principal Component Analyses (PCA), could be produced on the complete datasets (at least for surface samples), aiming a more quickly highlight of the main variance groups and their correlation (most significant elements)
The general section on cores is underdeveloped, and the sedimentological and descriptive report on the environment and paleoclimatic context is sparse. In particular, the important section on polynyas and their evolution is underdeveloped. Furthermore, some skills are taken for granted, making the article highly specific and lacking in flow and description.
List of corrections/suggestions:
L14: Please, revise the Abstract after the revision, enriching it with a better paleoclimatic framework
L23: Why the Ross Sea polynyas aren’t mentioned? As they are so important for the productivity and water masses circulation in the Ross Embjment, many highlighting publications exist on this topic; some fleeting mentions are present later in the texe but it seems important mention here too. See some examples even in the .pdf: Truax et al., 2024 https://doi.org/10.1016/j.quascirev.2024.108635; Falco et la., 2024 Deep–SeaResearch, II, 218,105429
L107: Please, add the cores RX or, at least, some photos of these crude sediment laminations
L109: As GC72, GC78 and GC80 present a good core sediment record (944 cm, 821 cm and 721 cm respectively) why do you present only the surface (top) sediments? Was it not possible to submit records relating to the same box corers time gap? If not, why?
Table 1: in the Age column is CE Common Era? Equivalent to d. C.?
L118: Hard to under stand for non-experts: the transition from 210Pb to 210Posupported (to be explain) is not immediate. Please, a brief explanation from Appleby, 2001 (or in Sup Mat.) coul help in understanding the geochronological chain and methodology.
Fig. 1: ). Please highlight the presence of different polynyas (biological hotspots and important high salinity shelf areas - HSSW production areas). Is there any known morphological, atmospheric or oceanic dynamic explanation for the presence of >0.8% sea ice cover in the BC04, BC03 and GC78 cores collection area?
L153: instrument available from which Institution?
L187: Absolute Diatom Abundances (ADA) were calculated for each sample while diatoms were ….
L192: 2.7 Proxy rationale
L219: please, which species or Group, have you found them in your samples?
L222: a refuse … than what? dinoflagellates?
L230: refuse: Fragilariopsis obliquecostata
L231: do you intend the ‘Sea ice dwelling diatoms’ reported at L365? If yes, please pay attention in defining the Groups and in keeping the same definition throughout the article
L235: … and iceberg drift
L251: refuse: section 2.7
Fig. 2: maybe to specify in Sup. Mat. how chromatogram works and the meaning of ‘Retention time’ is better
Table 3: absolute diatom abundance (ADA)
Figs 4: for an easier reading, please set the name of the cores and the polynya location
L301: Please, offer the complete list of the fifty-eight species
L303: absolute diatom abundances or ADA values
L309: please, explain better in discussion, why in your opinion, ‘centric cold-water diatoms’ decrease northward
L310: is biological hotspot equivalent to the McMurdo polynya?
L311: Why not also analyze the cores, specifically the same time frame? Are there any sedimentological peculiarities in the core record?
L316: Have you considered the impact of selective dissolution (which is already active along the water column and in the 20 cm down the sediment)? CRS, spores in general and E. antartica are selectively favore dover the others (x es. F. cylindrus and ctyophilic forms) … have you found T. antartica spores or vegetative cells?
Fig. 6: For an easier reading, please set the name of the cores and the polynya location
L330: Perhapes a Cluster Analyses and a Principal Component Analyses (PCA) could be produced by putting together the biomarkers and diatom assemblages datasets (at least for surface samples). This to more quickly highlight the main variance groups and their correlation (most significant elements).
L345: In the Conclusions, you consider Phaeocystis antartica as ‘pelagic‘ but is it ‘Haptophytes’? here is not specified … better explain
L358: the reference is missing
L365: Sea ice dwelling diatoms are usually pennate diatoms, frequently characterized by the presence of raphe (Navicula sp., Pleurosigma sp., Nitzscia sp., even Fragilariopsis sp.. …) (Zhang et al., 2025; https://doi.org/10.1101/2024.11.18.624199) Do you intend F. cryophilic Group?
More controversial is to consider F. curta as ‘sea ice dwelling diatom’ since it can also be a ‘open water/seasonal ice’ transitional form but not specific dwelling into the ice, except as a seed carried by strong polar winds or as stucked into the new forming ice …. (Tetzner et al., The Cryosphere, 16, 779–798, 2022; https://doi.org/10.5194/tc-16-779-2022; Allen et al., Geosciences 2022, 12, 282. https://doi.org/10.3390/geosciences12080282)
L408: Spreen et al., 2008)
L464: Before these results, please, add some general notes on cores’ highlights and differences among the cores (Fig. 7) as ADA values, diatom and biomarker variations, photos of the laminations in BC03 and BC04 with some sedimentological/environmental comment or note (even basic)
L496: Assuming readers are familiar with Antarctic paleoenvironmental and paleoclimatic issues related to the time period examined is, in my opinion, counterproductive. This, in fact, excludes readers who are not specifically and thoroughly informed from easily understanding the study conducted. Therefore, it would be better contextualize:
As in 1950 the atmospheric CO2 concentration was approximately 311 parts per million (ppm), this data can be considered the moment in which the critical threshold of 300 ppm has been exceeded for anthropogenic factors. I think it’s worth highlighting (from climate.nasa.gov)
L503: It’s a guess: if it’s degraded, the result is not certain. Maybe better: ‘suggesting little changes in phytoplankton ….., despite increasing sea-ice influence in this coastal part of the Basin (References indicating this trend…)
Fig. 8: in c) … F. curta (%)
L515: area characterized by the variable presence of coastal polynyas (Falco et al., 2024)
L516: Suggestion:
… (with a minor error of 2 decades max). This centennial time gap (1794-2007 years) represents the time during which the atmospheric CO2 has increate since the first human Industrial Revolution and includes the final part of the colder Little Ice Age interval, during which controversial atmospheric and Ross Sea oceanic dynamics promoted polynyas evolution (Lagorio et al., 2025; Truax et al., 2024; Tesi etal., 2020; Stenni et al., 2017; Mezgec et al., 2017; Rhodes et al., 2012). BCO2, BC03 and BC04 cores offer the opportunity to analyze the biosiliceous and organic records during this period, suggesting a relatively low variability, and allow to test long-core behavior of biomarkers, bacterial activity and preservation processes.
It seems nice to highlight this for a better paleoclimatic contextualization.
L530: little repetition: Trends ….. reveal an increase in sea ice extent in the southwestern Ross Sea …