Scientific significance

The manuscript by Acharya et al. presents a compilation of 7 brGDGT-based temperature records from west, southwest and south Greenland (5 new, 2 published) and interprets these data as changes in lake water temperature during the ice-free season (IFS LWT). These trends are used to assess spatiotemporal patterns of Holocene temperature change in this sector of Greenland, with particular attention on the regional timing of Holocene Thermal Maximum. There is obvious heterogeneity across the records and the authors attribute the disparities to differences in lake location and proximity to the coast and/or Greenland Ice Sheet. It is a thoughtful study design and an interesting network of sites that, unlike many lakes from Greenland generally, also capture some or most of the early Holocene (10-8 ka). The sites presented have the potential to fill in important spatial context for the extent and pattern of insolation-driven Holocene warming and subsequent cooling across Greenland.

Scientific quality

The dataset presented here is interesting and an important contribution to the body of evidence documenting the past environment from Greenlandic lake sediments. However, the manuscript presents with several major issues that result in a poorly supported interpretation of the data.

The use of brGDGT distributions to estimate temperature is extensively calibrated and commonly used, but the proxy is also widely documented to respond to other environmental gradients beyond temperature. In particular, there is a growing body of work showing that local lake conditions including redox status can overwhelm or skew temperature (Raberg et al., 2025; Yao et al., 2020; Zander et al., 2024) and evidence that Holocene brGDGT temperature reconstructions at Arctic sites are impacted (de Wet et al., 2019; Kusch et al., 2019; Lattaud et al., 2021). The authors acknowledge this as a potential confounding factor and their data show periods of elevated concentrations of isoGDGT0 along with extremely high isoGDGT0/Cren (“Cald/Cren”) ratios (>200) that indicate methanogen production and thus anoxia is indeed heightened at some sites through parts of the Holocene (e.g., Figs. S6-S10). However, they quickly rule out the likelihood of brGDGT distributions being impacted by site-specific redox conditions, reflected in isoGDGTs, based on 1) A lake modeling exercise that indicates all sites are invulnerable to summer stratification, and thus 2) that isoGDGTs have a different production seasonality (winter, with anoxia driven by prolonged ice cover) than brGDGTs (summer). However, there are obvious problems with the lake model, and the second point lacks any additional data to support it independently, as described below.

Lake Model

The presented lake model output for the suite of lakes demonstrates every lake in the dataset is invulnerable to changes in summer mixing regime, even at summer air temperature perturbations as high as +10 degrees. Supplemental Fig. S16 indicates that +10 deg of JJA air temperature will yield just +2 degrees of IFS surface LWT change at every lake despite morphological and catchment type differences. This same model also shows invariant ice phenology regimes (Fig. S15) between the lakes, despite large differences in both elevation and latitude. These model outputs are contradicted by observational data at sites across Greenland and the Arctic broadly, which demonstrate A) lake water and air temperature are much more closely matched at most sites (Carrea et al., 2025; Kettle et al., 2004; Piccolroaz et al., 2020; Tong et al., 2023), B) Arctic lakes can be vulnerable to summer stratification at even modern warming levels (Antoniades et al., 2024) and with a temperature difference between the epi and hypolimnion as little as 0.5 deg C (Klanten et al., 2024), and there are comparable lakes in this sector of Greenland that are summer-stratified today (Saros et al., 2016), and C) ice phenology (i.e., timing and length of the ice free season) is almost certainly variant at these sites based on the elevation range (Posch et al., 2024).

Additionally, this model indicates that Holocene-scale air temperature changes across Greenland (~ +0-4 deg C) should result in changes that are barely detectable in brGDGT distributions (IFS LWT <1 deg C, as demonstrated in Fig S16). However, the reconstructed brGDGT IFS LWT presented here show anomalies on the order of +5-15 degrees, which would lead to highly unreasonable air temperature estimates even at the low end of those estimates given the relationship between air and water temperatures expressed by the model.

It is therefore very unlikely this lake model is accurately capturing thermal dynamics in these lakes and the subsequent logic that these lakes are invulnerable to summer stratification, and thus anoxia is restricted to the winter and brGDGT production is not impacted, is not supported. At the very least, the model needs to be validated by observational data on temperature and mixing status from these or several similar Arctic lakes that have documented summer temperatures and mixing regimes (e.g., Carrea et al., 2025 and references therein) to see if it captures known conditions.

Production Seasonality of archaeal vs. bacterial GDGTs

It is unclear what data exists to support that the production seasonality of archaeal vs. bacterial GDGT production would be so substantially disconnected. As the authors identified, there is data that supports higher production of brGDGTs in the warm season (Raberg et al., 2021; Shanahan et al., 2013), although the reasoning for this is not always clear (Cao et al., 2020), and this observation isn’t necessarily different from the production seasonality of isoGDGTs (Blaga et al., 2011; Li et al., 2025; Zander et al., 2024). There is also relevant data that shows strong increases in production of brGDGTs occurs in anoxic conditions (Raberg et al., 2025; Weber et al., 2018), suggesting brGDGTs could also shift towards dominant production in the winter given development of appropriate conditions like water column anoxia, which may not be widely captured in modern observations if most lakes in the datasets are oxygenated in the winter today (Klanten et al., 2023; Raberg et al., 2025). In absence of other independent data on lake mixing regime, the hypothesis that one signal is winter and the other is summer is interesting and may motivate future work that tests this further but is currently weakly supported by external observations in the existing literature. Furthermore, concentration trends presented by the authors of both isoGDGTs and brGDGTs at the lake sites appear strongly correlated, suggesting production of both groups is responding to similar environmental forcings (Figs. S6-10). Trends in the concentration of isoGDGT0 also appear related to reconstructed IFS temperature. What mechanisms exist to drive this relationship, and can the same production window be excluded? If they are produced during the same season, then within-lake changes can’t be ruled out from interpretation of brGDGT distributions. Nevertheless, the authors could test this hypothesis further by presenting data on the structures specifically within the brGDGTs that are recognized to also respond to redox status: fractional percent of IIIa (e.g., %IIIa, HP5 index) and IIIa’’ (Weber et al., 2015; Yao et al., 2020). The former indices are readily calculated from the data already presented and the latter should be recorded in their HPLC-MS data, given that they used the Hopmans et al. (2016) method.

Summary

A more thorough consideration that lake-specific parameters, including mixing status and oxygen, contribute to brGDGT trends is warranted. Given that the key assumptions that lead to the conclusion that brGDGT distributions are entirely driven by IFS LWT are poorly supported, the discussion of climatic forcings that can explain the heterogeneous pattern of warming is somewhat moot and therefore not extensively evaluated at this stage of review. From this data, can we really be sure that the HTM occurs from ~7-5ka in this sector of Greenland with leads/lags around this timing related to continental position (with even this inconsistent across their dataset), or is it equally or more plausible that higher seasonality and warmer summers in the early Holocene led to mixing regime changes at some of these sites (reflected currently in isoGDGT0 concentration and 0/cren ratios) and consequently increased production of e.g., brGDGT IIIa, creating a cold-biased temperature reconstruction in the early-middle Holocene and catchment-scale heterogeneity across the Holocene.

Other points of consideration:

• The cut-off of the IR_6ME ratio at 0.3 to exclude data is arbitrary and substantially lower than the datasets that provide the reasoning for cut-off (i.e., 0.5, 0.4; Bauersachs et al., 2024; Novak et al., 2025). All data, including those flagged by IR ratio, should be included in the main figure(s), even if distinct symbols/colors are used to flag those datapoints, and a thorough discussion related to the IR ratio needs to be included. How does the data interpretation change if a cut-off of 0.4 or 0.5 is applied? Is this cut-off even still applicable when not using the modified MBT5Me calibration that excludes data above the cut-off?
• The authors compare IFS LWT directly to estimates of air temperature (e.g., from ice cores, chironomid, pollen), which based on their own model, aren’t directly comparable. The GISP2 record that is presented is not corrected for elevation change or seasonal bias (see Axford et al., 2021). The magnitude of warming in these data is much higher compared to existing estimates of temperature change, and this discrepancy warrants discussion and reconciliation with both existing temperature reconstructions and the model that suggests lake water response should be dampened compared to air temperature if these data are interpreted as temperature.
• More justification is needed for the selection of a site-specific calibration dataset from one lake in south Greenland (Zhao et al., 2021) vs. calibration datasets that incorporate more extensive data and many additional Arctic sites (e.g., Raberg et al., 2021) that have greater potential to more adequately cover the range of environmental conditions that occur in these 7 lakes through time.
• Is the same screening for soil-derived brGDGTs (<30% hexa-methylated) used in Cluett et al. for the Lake Gus temperature reconstruction also applied to all other lake brGDGT data presented here?
• Why does the lake model for Lake Gus in Cluett et al. have such different sensitivity between air and lake water temperature compared to the model for the other lakes here? It could be valuable to remodel Lake Gus and model Lake 578 here as well, using the same modeling decisions applied to the other lakes.
• An internal C46 standard is mentioned as added but it is not elaborated on if or how this standard was used in data processing

Presentation Quality

The structure of this manuscript overall flows well. At times the presentation is a little odd though, for example, the supplemental figures are referenced well-before and more often than most of the main figures. It seems like at least some of these data should be moved into the main text (e.g., Figs. S6-10). Some of the background/discussion is not internally consistent. There are minor issues with typos (e.g., surface areas in Table 1 are different from surface areas given in the in-line text; Comarum Sø is spelled incorrectly in parts of the text, Caldarchaeol is spelled incorrectly in some of the figure captions and I did wonder why it’s presented as “Cald” as opposed to the more common presentation of isoGDGT0, etc.). The scaling of deg C on the y-axes on Figures 2, 3 are confusingly unique by each row and make it hard to compare temperature change site to site and record to record; I noted a similar issue with the scaling of cald/cren ratios across Figs. S6-10.

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This is a well-written and well-illustrated manuscript. Holocene temperature changes in ice-free Greenland were first reconstructed from marine bivalves from raised marine deposits and later by pollen analyses of lake and mire deposits. The former method is problematical because it is difficult to find continuous records and the latter is problematical because it is time-consuming, because of delayed immigration of many plant species and because of long-distance transport of pollen grains from the south. Later studies of lake sediments have used stable isotopes, chironomids, biomarkers and other proxies, but each method has its own problems. However, we still do not have a good picture of the timing of the Holocene Thermal Maximum. Did it vary from area to area? Did it end gradually or more abrupt? And how warm was the HTM in SW Greenland?

I am impressed to see that the authors have generated five new brGDGT-inferred ice-free season lake water temperature series from Holocene lake deposits in southwestern Greenland, yielding a total of seven Holocene brGDGT timeseries in this region. Most proxy studies of lake sediments from Greenland have only dealt with a single or a few lakes.

I am not an expert on brGDGT but I note that Kusch et al. (2019) tried to use brGDGT on lake sediments from North Greenland and concluded that the method did not work in the far north. From the discussion about calibrations, I get the feeling that more work is needed to better infer past temperatures from data on brGDGT in SW Greenland. I am surprised to learn that the authors reconstructed temperatures up to 21.8 °C – I think this are high temperatures for a lake in Greenland. Even more surprisingly, this high temperature was found for Marshall at an elevation of 862 m.

The authors conclude that five of the seven timeseries from South and Southwest Greenland indicate that the Holocene Thermal Maximum occurred between approx. 7000 and 5000 years BP. I note that Fredskild 1983 suggested that the warmest period in SW Greenland occurred from 7500 BP and the following millennia (The Holocene vegetational development of the Godthåbsfjord area, West Greenland. Meddelelser om Grønland, Geoscience 10, https://doi.org/10.7146/moggeosci.v10i.140322). I also note that Bennike et. al. 2010 suggested peak Holocene temperatures from c. 7000 to 6500 cal. years BP based on the occurrence of a warmth-demanding ostracode in lake sediments at the head of Kangerlussuaq (Holocene palaeoecology of southwest Greenland inferred from macrofossils in sediments of an oligosaline lake. Journal of Paleolimnology 43, 787–798

Specific comments:

Line 11, 41. early and middle-Holocene – these are formal units, I suggest to write Early and Middle Holocene

Line 11. Delete likely – it has been demonstrated in numerous studies.

Line 51. defined herein the region – change to: defined herein as the region

Line 51. I would change Nugaatsiaq (spelled Nuugaatsiaq on Fig. 1) to Nuussuaq. Nugaatsiaq is a small abandoned settlement, whereas Nuussuaq is a large, well-known peninsula.

Line 63-64: “reveals a Holocene thermal maximum (HTM) at ~7.9 ka which is temperature ~2.9 ± 1.4 °C warmer than the recent”. Rewrite

Also, strictly speaking, temperatures cannot be warm or cold – they can be high or low.

The high temperatures were recorded at 7960 ± 30 years B.P. – during a short-lived period. The period from 8000 to 6000 years BP shows declining temperatures – you may argue that the HTM is dated to 8000–7000 at Summit.

Line 65. change between ~6 to 4 ka to: between ~6 and 4 ka

Line 98 etc. I would expect some more notes on the setting in this chapter. Including some notes on ocean currents, air temperatures in the region and notes on the strong temperature gradient from the cold outer coast to the warm inland near the margin of the ice sheet. It would also be great if you can provide some data about measured lake water temperatures.

Line 363. katabatic winds are usually warm

Line 364 etc. Here you discuss the distance to the Greenland Ice Sheet. It should be distance to the margin of the ice sheet.

Line 433. The similarity between the GISP2 and Lake Marshall records is not great.

Line 444. both Comarum Sø and Kløft Sø are mis-spelled.

Line 448. The pollen-based temperature reconstruction from Qipisarqo lake is mainly based on Alnus pollen from Labrador.

Line 463. Not sure what you mean by this: due to low temporal resolution of the time series between 2 to 4 ka, anomaly period.

Fig. 1. You can argue that the East Greenland Current continues around Cape Farewell and a little up the coast of West Greenland. The region of locality 13 to 18 is strongly influenced in late summer by sea ice from the Arctic Ocean.

Fig. 3. The temperature record from GISP2 is “upside-down”, so that the 8.2 event appears visually as a warm period (temperatures  ~ –34°C) and the HTM appears as a cold period.