the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Post-depositional modification on seasonal-to-interannual timescales alters the deuterium excess signals in summer snow layers in Greenland
Abstract. We document the isotopic evolution of near-surface snow at the EastGRIP ice core site in the Northeast Greenland National Park using a time-resolved array of 1-m deep isotope (δ18O, δD) profiles. The snow profiles were taken from May–August during the 2017–2019 summer seasons. An age-depth model was developed and applied to each profile mitigating the impacts of stratigraphic noise on isotope signals. Significant changes in deuterium excess (d) are observed in surface snow and near-surface snow as the snow ages. Decreases in d of up to 5 ‰ occurs during summer seasons after deposition during two of the three summer seasons observed. The d always experiences a 3–5 ‰ increase in d after aging one year in the snow due to a broadening of the autumn d maximum. Models of idealized scenarios coupled with prior work (Wahl et al., 2022) indicate that the summertime post-depostional changes in d (Δd) can be explained with surface sublimation, forced ventilation of the near-surface snow down to 20–30 cm, and isotope-gradient-driven (IGD) diffusion throughout the column. The interannual Δd is also partly explained with IGD diffusion, but other mechanisms are at work that leave a bias in the d record. Thus, d does not just carry information about source region conditions and transport history as is commonly assumed, but also integrates local conditions into summer snow layers as the snow ages. Finally, we observe a dramatic increase in the seasonal isotope-to-temperature sensitivity occurs, which can be explained solely by IGD diffusion. Our results are dependent on the site characteristics (e.g. wind, temperature, accumulation rate), but indicate that more process-based research is necessary to understand water isotopes as climate proxies. Recommendations for monitoring and physical modeling are given, with special attention to the d parameter.
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RC1: 'Comment on egusphere-2023-2462', Anonymous Referee #1, 30 Jan 2024
The manuscript by Dr. Town and others continues the series of publications of the EGRIP team devoted to the study of the post-depositional changes in the isotopic composition of the surface and near-surface snow. In this manuscript the results of the field experiments are shown that are aimed at observing “in real time” the evolution of the oxygen 18 and dxs values in given snow layers during a summer season and between summers.
Such studies are strongly needed to understand how the climatic signal is modified between a precipitation event and a “lock in” of this signal in an ice core.
I have only a few minor comments to the manuscript.
Line 6: “The d always experiences a 3-5 ‰ increase in d” – the last “in d” is redundant.
Lines 12-13: “Finally, we observe a dramatic increase in the seasonal isotope-to-temperature sensitivity occurs, which can be explained solely by IGD diffusion” – “occurs” is unnecessary.
Table 1 – does the limit of 1 cm for the surface snow’s lower boundary is defined somewhere? Or is it just related to the sampling procedure? In the previous papers (e.g., Wahl et al., 2022) I remember the surface sampling meant sampling of the upper 0.5 cm.
Line 47: “The critical distinctions here are if the it precipitation.” – sorry, did not understand this.
Line 48: “we refer to is as” – to it?
Line 147 – is it important that it is a national park in the context of the paper? More important that it’s an ice sheet. But ok, If you like so.
Lines 200-204 – what is the uncertainty of dxs?
Lines 273-274: “The potential influence of forced ventilation on near-surface snow due to tapers off dramatically after about 50 cm” – it looks like something is missed in this sentence.
Figure 7 – panel b is empty.
Line 367 – useful to remember?
Line 429-430 – why do you assume equilibrium? In the Wahl et al., 2022 paper they show that kinetic fractionation much better explained the isotopic modification of the surface snow. Ok, I see the answer lower in the text (471-472).
Line 442 – if you assume equilibrium fractionation, then why does dxs change?
Section 4.1.5 – is it really needed here? Your paper is devoted to another topic, not to the accumulation rate.
Lines 533-534: “7.87 o/oo · o/oo−1” – what is this dot between ‰ and ‰?
Line 543: “It seems logical that the large reservoir of the atmosphere because” – something seems to be missed in this phrase.
Lines 544-545: “causes a negative Δd during summer through surface sublimation but possibly through force ventilation of near-surface snow” – the same.
Line 557: “We suggest is to dividing the contribution” – better “we suggest to divide the contribution”?
Citation: https://doi.org/10.5194/egusphere-2023-2462-RC1 -
RC2: 'Comment on egusphere-2023-2462', Mathieu Casado, 15 Apr 2024
Review of « Post-depositional modification on seasonal-to-interannual timescales alters the deuterium excess signals in summer snow layers in Greenland» by Town and others.
This manuscript describes measurements of short snow pits gathered at EASTGRIP, in Greenland. The authors use the variation in snow isotopic composition after the deposition took place to study the post deposition processes that alter the isotopic composition of near surface snow. Specifically, they consider the impact on the d-excess of these post deposition processes in summer due to latent heat flux near the surface.
This work relies on a new set of short pits that they were obtained across a few field seasons as well as an exhaustive dataset of meteorological measurements. A depth adjustment has been implemented to align the profiles taking into account the elevation offset due to the accumulation during the years, as well as the spatial heterogeneity of snow accumulation in Polar Regions. This provides the authors with comparable isotopic profiles where they can evaluate the change of isotopic composition after the snow has been deposited. The authors quantified whether the changes were statistically significant, but some improvements on this aspect would be beneficial to the manuscript. Modelling of the effects of post-deposition processes is realised to support the qualitatively the observations both at the seasonal and interannual scale.
The manuscript is well written, albeit possibly too long. Some elements in the figure were missing. Apart from a couple of minor comments, I would recommend to accept the manuscript for publication.
Minor Comments:
Line 7: I don’t think citations are supposed to be present in abstracts.
Line 195: “The snow was cut in an open-faced tray using a 0.10-cm thick blade.”
It should be “0.1cm”, or “1mm”.
Line 309: “Significant increases (p < 0.05) in δ18O are seen the summers of 2017 and 2019, down to 20-30 cm.”
It seems to me in these figures that the increases are really near the surface, like the first 10-15cm. Also, this might be explained later, but how are these increase significant while most of the bottom part of the cores are decreasing? This should be mentioned here, as the increase in the first 10 cm is more or less of the same amplitude than the decrease between 30 and 50 cm in Figure 4.
Figure 7: Panel b is empty.
Line 418: “We see an attenuation in peak δ18O of up to 2 o/oo due to IGD diffusion (See Figure 11).”
There is no d18O in Figure 11.
Line 558: “The historical approach has been to consider the atmosphere as the source of the climate signal, and the deep firn as a source of noise to be inverted”
I don’t think noise is the right term, as diffusion, advection and thinning are not adding noise to the signal, but acting respectively as low pass filter, phase modulation, and z-axis compression. I guess more as a “transfer function”.Line 560: “As stated earlier, the atmosphere is often represented by climate-to-isotope sensitivities that reduce to temperature-to-isotope sensitivities,”
This is nonsensical.
Conclusion: Considering the extent of the discussion, I would recommend shortening the conclusion.
Citation: https://doi.org/10.5194/egusphere-2023-2462-RC2
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