| 05 May 2026
Holocene-like summer climate during Marine Isotope Stage 11 in northwestern Greenland
Abstract. Determining the climatic conditions under which the Greenland Ice Sheet (GrIS) was smaller than present is important to quantify GrIS sensitivity to climate change. We use biomarkers in sediment collected beneath the GrIS at Camp Century, northwestern Greenland to reconstruct summer temperature and atmospheric circulation 416,000 ± 38,000 years ago (Marine Isotope Stage 11; MIS 11). We find that northwestern Greenland summer climate during MIS 11 was similar to the middle Holocene but different than the 20th century: air temperature was 4.7 ± 3.2 °C warmer and atmospheric water vapor isotope values were 22 ± 18 ‰ 2H-enriched, indicating a greater contribution of locally evapotranspired moisture. These conditions are similar to or slightly warmer than during peak Holocene warmth on Greenland, when the Camp Century site remained ice-covered, and cooler than the Last Interglacial (LIG). Biomarkers from lower in the section likely represent an earlier ice-free interval, potentially during the Pliocene or early Pleistocene, and record climatic conditions similar to MIS 11. These data add to the sparse available climate data for the early and middle Pleistocene on Greenland, and suggest interglacial periods had similar temperature throughout the Pleistocene. We interpret the reduced GrIS extent in northwestern Greenland during MIS11 compared to the Holocene, despite similar temperature, to indicate ice-sheet response to prolonged warmth, as reconstructed in southern Greenland. Therefore, efforts to reduce both the magnitude and duration of summer warmth in the coming centuries will be important to curbing ice-sheet retreat.
Competing interests: Some authors are members of the editorial board of Climate of the Past for the special issue “The Camp Century ice and sediment core: new science from a 1966 core that touched the base of the Greenland ice sheet (CP/TC inter-journal SI)”
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
Aguilar and co-authors present new lipid biomarker and stable isotope data from the basal sediments of Camp Century in NW Greenland. Along with a meta-analysis of published Greenland paleoclimate records spanning the current and prior interglacials, the authors use these data to frame climate constraints for a smaller-than-present Greenland ice sheet. The rare basal ice sheet sediments and implications for ice sheet climate sensitivity make this a particularly relevant and tantalizing study. Broadly, I found the methods to be appropriate, however, I have comments that I hope the authors can address to expand upon their assumptions and rationale for proxy interpretations (see detailed comments below). Of course, sediments like these come with substantial uncertainties and necessary assumptions, and I think the authors have generally done a good job covering the gamut. Overall, the manuscript is generally well written with nice figures. There is a commendable amount of information and context provided in the main text and supplement, which I think is important for the reader.
Comments
L37: Since we don’t really know when during MIS 11, perhaps better to say at some point during MIS 11?
L52: The Plain Language Summary can be removed…I’m assuming this is legacy from a prior manuscript version
L80-81: …and constrains climate sensitivity, which seems like a major goal of this study? And links to first paragraph
L90: Can you specify the two regions of deepwater formation?
L94: Please clarify how tropical and high-lat temperature are in step…phasing? Magnitude?
L96: temperatures were not the same at the high-lat and tropics during interglacials, which I’m thinking may not be what you intended, but the phrasing of the sentence is currently unclear
L127: May be important to also mention McFarlin et al. (2023) here and the alternative interpretations of depleted lake water stable isotopes being changes in methane cycle
L130: clarify that this is “summer” warmth
L148: Would be helpful for the reader to clarify what methods were used to infer this…cosmogenic exposure dating?
L198: For clarity, reference the later section where you justify that these are lake sediments here
L199-204: I would suggest rephrasing the fractionation factors as “estimated” or “based on compilations”. This is one major source of uncertainty and it will help the reader to not over interpret these as super well constrained
L206: Instead of account, I may suggest using estimate, as similar to my last point there is lots of uncertainty here (for this the temperature estimate too) so important for the language to help reflect this uncertainty
L223: Does ice free here refer to Greenland being ENTIRELY ice free? Please clarify.
L231-232: Based on my understanding of the elevation adjustments, ice being present just outside the Camp Century drill site would have a similar elevation to today (530 m). And then the other end of the potential elevation spectrum is 810 m for no ice sheet on Greenland. Therefore, I don’t necessarily agree that 810 includes uncertainties for unknowns in ice sheet size, particularly if the ice sheet was not that much smaller. Would it make sense to calculate lapse rate adjustments based on the two extremes (530 and 810 m) instead of just 810? This may more accurately capture the possible ranges of uncertainty.
L274: Are you referring to each samples represents an integrated time of centuries to millennia? At first I read this as the resolution between samples so it may help to clarify your intended meaning here.
L323: I very much appreciated this section dedicated to proxy systematics and interpretation, and inclusion of the Iso2k framework
L341: Is there a reason why the authors choose to refer to GDGT-0 as caldarchaeol? While this is fine and a matter of choice as it is a formal name, it is far more often referred to as GDGT-0 in the literature and may be clearer for the reader to use this terminology
L345: For soil comparisons, have the authors considered including the relatively recent dataset from Raberg et al. (2024)? These include a bunch of additional high-latitude sites as well as a compilation of others that don’t appear to be included in this paper’s compilation
L349: There is no reference provided and I couldn’t find results provided in the supplement for the BIGMaC source classification? This would be important to include somewhere if the authors wish to retain this statement.
L365: I would suggest changing to “likely” absence. Blaga et al. (2009) define values over 2 as indicating anoxic conditions although I don’t necessarily think there is strong justification for this. In any case, reducing the certainty in the statement would be best since we don’t really know what the threshold is or how that varies across space and time. I agree that values less than 10 probably indicate a lack of reducing conditions.
L406: Note that Dion-Kirschner et al. (2020) find that terrestrial plants produce up to 30x more plant waxes than aquatic plants
L415: Modern soil water isotopes are rare, so it may be helpful to reference Harning et al. (2024) here as they report that soil water isotopes likely have a summer bias in Iceland. Since I’m not aware of similar studies on Greenland, these data may be transferrable
L423: Stating that lake water evaporation is limited, assumes the lake basin is opened. If the lake is hydrologically closed, summer evaporation can significantly alter lake water stable isotopes in high latitude lakes (e.g., Kjellman et al., 2022; Akers et al., 2024; Harning et al., 2024).
L424: Akers et al. (2024) and Cluett and Thomas (2001) discuss the impact of summer evaporation on lakes, particularly in the Camp Century region of Greenland, so they don’t really support the statement. I would suggest revising to acknowledge the potential impact of summer lake water evaporation, and including references from prior comment on L423. Since we don’t know whether the sediments studied were originally deposited in open or closed lakes, this therefore likely reflects a “minimum” difference between precip and lake water-inferred isotopes
L430: Suggesting changing account to estimate or something similar
L494: You previously conclude that the biomarkers were deposited in a lacustrine environment…if fluvial, this would require you to also consider soil brGDGT calibrations and substantially revise your interpretative framework for biomarkers. This seems like a major point to address.
L520-521: I believe at least some of these lake records are suggested to have been impacted by Early Holocene anoxia…how was this taken into account when deriving comparative temperature anomalies? Were temperatures estimated as maximum or minimums? How reliable are these?
L602-603: Could this reflect a cold bias if the Holocene sediments are being impacted by anoxia (e.g., Weber et al., 2018)?
L606-607: What about Kusch et al. (2019)? Although this record is impacted by anoxia and not inferred to accurately record Holocene temperature
L658-660: This sentence needs a reference
L737: CC doesn’t need to be abbreviated here
L738-739: This is a very important point that occurred to me reading the paper, but this is only brought up at the very end. I would encourage the authors to incorporate this line of reasoning from the outset. How does this change your choice to use warmest 2 thousand year comparison? How do those latter averages compare with the full Holocene for instance?
Text S1: At end of second paragraph, MAF precip units should be cm I believe, and not m
Text S3.1: Clarify if fractional abundances for branched and isos are based on total total or total branched and isos, respectively. I also liked the approach of combining Arctic brGDGT lake temperature calibrations. In addition to the Full set from Raberg et al. (2021), did you also try the Meth set calibration that separate structural differences?
Text S3.6 (and main text): Were datasets used for comparison recalculated in terms of brGDGT-inferred temp using the combined lake calibration approach used for Camp Century sediments? If not, this would be good to make datasets more comparable.
References
Harning, D.J., et al. (2024). Spatiotemporal variation of modern lake, stream, and soil water isotopes in Iceland. Hydrol. Earth Sys. Sci. 28, 4275-4293.
Kjellman, S.E., et al. (2022). Arctic and sub-Arctic lake water δ2H and δ18O along a coastal-inland transect: Implications for interpreting water isotope proxy records. J. Hydrol. 607, 127556.
Kusch, S., et al. (2019). Holocene environmental history in high-Arctic North Greenland revealed by a combined biomarker and macrofossil approach. Boreas 48, 273-286.
Raberg, J.H., et al. (2024). BrGDGT lipids in cold regions reflect summer soil temperature and seasonal soil water chemistry. Geochim. Cosmochim. Acta 369, 111-125.
Weber, Y., et al. (2018). Redox-dependent niche differentiation provides evidence for multiple bacterial sources of glycerol tetraether lipids in lakes. Proc. Natl. Acad. Sci. USA 115, 10926–10931.