Review of egusphere-2024-1003 Brashear et al., Clim Past Discuss. 2024

Summary

An ultra-high resolution time series for central/eastern Greenland deuterium variability is presented. After correction for diffusion, 7-15-year variability is considered as represented and and interpreted in terms of temperature variability that changes between Greenland stadials and interstadials. Temperature change at the ice core site on decadal scales is compared to sea-ice and sea surface-temperature change in the North Atlantic, and interpreted as primarily arising from sea-ice variability.

Crucially, the authors suggest that a reduction of decadal temperature variability at the ice core site occurs centuries prior to the interstadial warming, and corroborate this with the analysis of model simulations and one high-resolution marine core.

The study is well-written, and the topic is relevant to Climate of the Past. However, there are a few points were more detail, or more stringent acknowledgement of uncertainties, is important.  

Major Points

• Please acknowledge the assumption of the validity of the temperature interpretation of dD at sub-decadal timescales -- as you show, sea-ice variability is highly correlated with temperature, but it is not the only driver.
• Consider, in the Discussion, the robustness of the model-based process interpretation consideringa usefulness of model intercomparison (e.g., [1]), in particular given the complexity of sea-ice models. Similarly, the marine record is a (hand-picked) example, there would be potential for targeted synthesis work here.
• The figures need to be reworked. Standard red/green looks grey to quite a few people (Listen, e.g. to these people describing their experience https://www.youtube.com/watch?v=FKSOe5NK_qQ and imagine how distinguishable colors are in most of your figures...)

Minor Comments

• p1l26/27 This sentence is ambiguously phrased. (Why) should there be a phase offset, and should it be distinguishable? I read this as: "Across stadial/interstadial transitions proxy evidence showed in-phase changes in mean temperature/dust/sea-salt concentration/accumulation rate".
• p1l29/30 You write that high-frequency interannual variability surrounding "mean temperature change" has not been investigated -- please clarify that by mean change you mean centennial-to-millennial scales, and by high-frequency interannual variability. From a lower-resolution marine point of view both timescales are "just" variability.
• p2l71 "tipping-point sea-ice displacement" - The concept of a threshold, below which the ice edge becomes unstable, and fast/complete retreat of perennial sea-ice cover occurs is debated [2,3]. Compared to the ice sheet, sea-ice itself has little memory, but small changes in the ice edge may lead to large impact warming. Please rephrase.
• p3l85 and following: Investigating leads and lags, as well as interannual to multicentennial variability across the LGP and for different timescales ("mean" vs. "variability") and attributing it primarily to local temperature change assumes that dD is faithfully representing local EGRIP site temperature. This is not explicitly mentioned, but is permeating the study, and should be acknowledged explicitly. Sea-ice variability, independently of temperature change at the EGRIP site, can induce d18O variability [4], and, as the authors themselves show with the model-based correlations these variables are colinear. Isotope-enabled simulations could allow, to some extent, to disentangle these relationships.
• p4l141 timestep of 50-200 years -- presumably these are the shifts for the moving windows? Unclear.
• p5l154 correct: preserved
• p6l204: instead of "spectrum of change" suggest rephrasing
• p7l217 increased depletion 
• p8l251 clarify "mean" timescale (see above)
• p8l255 and following: How long are the simulations? How are the degrees of freedom and a significance for the correlations calculated? Are these step-wise simulations, and is the mean change then subtracted prior to correlation? The magnitude of the correlation is surprisingly high.  To what extent are these correlations representative for other models (given the fairly simple sea-ice model in HadCM3)?
• p10l350 Arguably, this is a single core site for which a reduction of sea-ice occurs prior to Greenland isotopic/temperature change, and a single climate model. The correlation patterns of sea-ice variability with EGRIP temperature in other models would be interesting. What is the age model of MD95-2010 based on, and what is the corresponding age uncertainty? Hopefully (or perhaps, evidently, from the results) not tie-points to GICC05. Perhaps this is an age model issue?
• Fig. 3, 4, 6, 7 please avoid red/orange and green as dominant colors in figures (not colorblind friendly)
• Fig. 5, perhaps add the mean position of the sea-ice edge in these figures for the LGM to aid interpretation.
• Data availability: The DOI points to a lower-resolution (5cm) version of the dataset. As such the study is, therefore, not (yet) reproducible.

References

[1] Malmierca-Vallet, I., Sime, L. C., and the D–O community members: Dansgaard–Oeschger events in climate models: review and baseline Marine Isotope Stage 3 (MIS3) protocol, Clim. Past, 19, 915–942, https://doi.org/10.5194/cp-19-915-2023, 2023.

[2] Serreze, M. Rethinking the sea-ice tipping point. Nature 471, 47–48 (2011). https://doi.org/10.1038/471047a

[3]  Livina, V. N. and Lenton, T. M.: A recent tipping point in the Arctic sea-ice cover: abrupt and persistent increase in the seasonal cycle since 2007, The Cryosphere, 7, 275–286, https://doi.org/10.5194/tc-7-275-2013, 2013.

[4] Rhines, Andrew, and Peter J. Huybers. "Sea ice and dynamical controls on preindustrial and last glacial maximum accumulation in central Greenland." Journal of Climate 27.23 (2014): 8902-8917. 

Review of Shifts in Greenland interannual climate variability lead Dansgaard-Oeschger abrupt warming by hundreds of years by Chloe A. Brashear, Tyler R. Jones, Valerie Morris, Bruce H. Vaughn, William H. G. Roberts, William B. Skorski, Abigail G. Hughes, Richard Nunn, Sune Olander Rasmussen, Kurt M. Cuffey, Bo M. Vinther, Todd Sowers, Christo Buizert, Vasileios Gkinis, Christian Holme, Mari F. Jensen, Sofia E. Kjellman, Petra M. Langebroek, Florian Mekhaldi, Kevin S. Rozmiarek, Jonathan W. Rheinlænder, Margit Simon, Giulia Sinnl, Silje Smith-Johnsen, and James W. C. White; to Climate of the Past (EGUsphere), Juli 2024.

The study by Brashear et al. shows how stable water isotope interannual variability on the Greenland ice sheet changes throughout the Last Glacial, being stronger during stadials than interstadials, with peaks preceding D-O events by hundreds of years. They used CFA to measure high-resolution isotope data, spectral estimates for the correction of isotopic diffusion, and for estimating isotopic variance at interannual frequencies. They hypothesize that sea ice variability in the North Atlantic area and the mean temperature on the Greenland plateau are closely related to isotopic variability at the ice core location, underpinning this hypothesis by using HadCM3 models, and comparing ice sheet temperatures to sea ice dynamics. The study is important for advancing our understanding of the climate system, specifically Greenland variability and the North Atlantic Ocean and AMOC in relation to the global mean climate state, as well as the characteristics of abrupt climate changes by assessing sudden shifts of D-O events. They use adequate methods and contextualize their results within previous studies and hypotheses. We believe this paper includes interesting and relevant results suitable for publication in “Climate of the Past.” However, we have some major concerns that should be addressed before publication, as well as some minor suggestions.

Major concerns: 

Contribution of non-climatic noise on the changes of isotope variability:

The authors cautiously interpret isotopes and do not directly translate them to temperature, which aligns well with current knowledge of the uncertainties regarding isotope interpretation and isotope-temperature translations. They discuss altered source-sink pathways and evaporation sources upstream as other possible influences on isotope variability. Based on their thorough analysis of different frequencies (Figures A2, A5), their results should not be sensitive to time uncertainties within this (not layer counted) record.. As the analyzed core is highly influenced by ice flow, the authors could additionally state why they think upstream effects do not influence their results.

Despite considering all these effects, the fast variability interpreted in this manuscript will still be influenced by non-climate noise. Even nearby ice core isotope records are found to be quite distinct from each other, especially at high frequencies (Münch & Laepple 2018). Estimates of the Signal to Noise Ratio (SNR) in the Greenland NGT stack (Hörhold et al. Extended Data Fig. 1b) suggest an SNR of 3-5 in this frequency band for a stack of 12 records; resulting in an SNR of around 0.3-0.4 for a single record such as EastGRIP. This in turn shows that the majority of the interpreted variance will likely be due to local depositional effects.  Such noise components likely differ across climate states (e.g., GI vs. GS) and introduce isotope variability changes unrelated to climate. One characteristic that could hint towards such systematic influence could be a change in accumulation rates, which is strongly reduced in the cold phases compared to the Holocene. Lower accumulation rates in the last glacial coincide with more precipitation intermittency and stratigraphic noise. We, therefore, suggest that the authors show the accumulation history of the record, how it coevolves with the variability changes, and discuss the possibility of state-dependent noise influencing the discovered changes in variability.

Uncertainty of the diffusion correction:

The authors estimate the diffusion length in the spectral domain. As shown by Jones et al., 2017 and by Kahle et al. 2018, in CFA systems, some noise is added to the isotopic signal on the preparation side of the system that, after the smoothing of the CFA system, leads to red noise at the higher frequency end (which would, with discrete measurements, be white), as visible in Appendix Figure A4. This red noise can interfere with the diffusion length estimate as it is difficult to distinguish from a diffused signal. Therefore techniques to account for this have been developed (Kahle et al.,  2018, Improved methodologies for continuous-flow analysis of stable water isotopes in ice cores, most authors from this paper are also on this new manuscript). The red noise might also influence the analyzed frequencies and the diffusion correction possibly amplifies this high frequency noise.

We suggest that the authors use or at least discuss the diffusion length estimation method they introduced in Kahle et al., 2018  for CFA measured data. Further, the authors could show that their results are robust by elaborating on how the variability changes are also detectable on the diffused record (Figure 1c).

Variability leading abrupt change or variability just depending on the mean state? We suggest that the authors interpret their results on the variability ‘leading’ abrupt climate change with more caution. They write, ‘Such a large phase offset between two climate parameters in a Greenland ice core has never been documented for D-O cycles’ (Lines 34ff). To play devil’s advocate, at least visually, the minima in δD also seem to lead the onset of the interstadial periods (their Fig. 3). This may be due to the definition of the onsets, which is set at a certain magnitude of change in the proxy records over time (Rasmussen et al. 2014), combined with the typical shape of the isotope changes. The counter-hypothesis would thus be that the variability depends on the mean isotope value (their Figure 1b), and this dependency (which is interesting in itself) already explains the time-lag. We therefore suggest that the authors either refute this simpler counter-hypothesis, or if this is not possible, one down their interpretation of their results

Minor comments:

42 “Thus, both paleoclimate proxy evidence and model simulations suggest that sea ice plays a substantial role in high-frequency climate variability prior to D-O warming.” - Argument unclear. You mean paleoclimate proxy evidence including the ice core records as well as the open ocean biomarkers? The ice cores alone do not evidence that, so maybe mention the biomarkers as being part of the “paleoclimate proxy evidence” you are referring to, or delete “Thus” as: “Both paleoclimate proxy evidence as well as these model simulations suggest…”

63 References unclear. Which literature explains D-O warming events being related to sea ice and which literature just generally associates sea ice with abrupt warming? Do all of the studies do both? Then maybe add a : between the two sentences?

134 “On average, temporal differences in adjacent data points range from sub-weekly in the Holocene to sub-monthly during the LGP”. Can you clarify what you mean by “adjacent”? Temporally closest together?

151 pore “close-off”

139 Can you state why you choose not to include lower frequencies - e.g., because of prior expectations regarding sea ice variability?

154: persevered or better “preserved”?

185: The method description is too short to be reproducible.  If I understand it right, it needs to assume  / assumes that 1.) P0(f) is not frequency dependent and the fit takes only place on frequencies lower than a manually chosen fc to ensure that the spectrum is dominated by the diffusion signal in this range of frequencies and measurement noise can be ignored. 

1. Event 2.2 is mentioned for the first time, please define what that is. It’s not in the table.

F2: Please define 2.1 and 2.2 events

F2 and F3: Could you please put the names/numbers of the D-O events into the graphic to make it easier to follow? Right now, if you write about a specific event, one has to check the table for the D-O event’s time and then search for it in the graph.

Literature

Hörhold, M., Münch, T., Weißbach, S., Kipfstuhl, S., Freitag, J., Sasgen, I., Lohmann, G., Vinther, B., Laepple, T., 2023. Modern temperatures in central–north Greenland warmest in past millennium. Nature 613, 503–507. https://doi.org/10.1038/s41586-022-05517-z

Jones, T.R., White, J.W.C., Steig, E.J., Vaughn, B.H., Morris, V., Gkinis, V., Markle, B.R., Schoenemann, S.W., 2017. Improved methodologies for continuous-flow analysis of stable water isotopes in ice cores. Atmos. Meas. Tech. 10, 617–632. https://doi.org/10.5194/amt-10-617-2017

Kahle, E. C., Holme, C., Jones, T. R., Gkinis, V., & Steig, E. J. (2018). A generalized approach to estimating diffusion length of stable water isotopes from ice‐core data. Journal of Geophysical Research: Earth Surface, 123(10), 2377-2391.

Münch, T., & Laepple, T. (2018). What climate signal is contained in decadal-to centennial-scale isotope variations from Antarctic ice cores?. Climate of the Past, 14(12), 2053-2070.

Rasmussen, S. O., Bigler, M., Blockley, S. P., Blunier, T., Buchardt, S. L., Clausen, H. B., ... & Winstrup, M. (2014). A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy. Quaternary science reviews, 106, 14-28.