Review of Preunkert et al.” Impact of subsurface crevassing on the depth-age relationship of high alpine ice cores extracted at Col du Dome between 1994 and 2012

Preunkert et al. compare the records of three ice cores drilled at Col du Dome, near Mont Blanc in 1994, 2004, and 2012. The age scale appears intact in the 1994 core (C10) while the age scales are disturbed in the 2004(CDK) and 2012 (CDM) cores in the time period of the 1950s and 1960s. The dating is primarily established by annual layer interpretation of ammonia, but the disturbances are primarily identified by the complexity of the H3 and C137 records. They ascribe the disturbances to the presence of a crevasse upstream. The crevasse, which is sealed near the surface by a snow/ice bridge, allows the accumulation of Pb210 due to the granitic bedrock. I believe the primary argument is that the dated ice in the 1994 core originated when the crevasse was smaller and did not yet intersect the flow path reaching the ice core site. The 2004 and 2012 were disturbed, however, because the crevasse had enlarged and intersected the flow path.

Preunkert et al. present high quality measurements of a large variety parameters and provide a plausible explanation for the disturbed stratigraphy in the two later cores. The use of the bomb horizons to evaluate disturbances is an interesting application. The primary conclusion that care must be taken in interpreting alpine ice core timescales is well supported. The mechanisms of layer skipping and layer doubling is well established. I have a few suggestions to improve the manuscript and make the argument more convincing.

• The extension of the crevasse through time should be presented in more detail. A plan view of the extension would be very helpful. The photos in Figure 1, particularly 1b, is quite poor. Given the popularity of Mt. Blanc, it seems like a long record of photographs exists to validate the hypothesis of crevasse extension. Mapping of the crevasse through time would significantly improve the plausibility of the proposed mechanism.
• I found the discussion of Pb210 and Rn222 to be rather confusing. I didn’t see any data on Rn222 presented and am unclear how this fits into the Pb210 and crevasse story. The authors also reference Pb210 record from 30m away, but this is not shown. It would be helpful to see how this compares to the C10 record and strengthen the arguments. But mainly I remain unclear on why C10 is more enriched in Pb210 if the ice did not intersect the crevasse. This is a complex system which necessitates temporal variations in the crevasse as well as coverage of the crevasse with a snowbridge and the firn/ice transition. A schematic showing different crevasse and firn configurations and the resulting Pb210 anomalies would be very helpful.

A few additional minor comments and/or questions:

• L266 – “reach”
• Have cores been drilled on Dome de Gouter? The ice thickness may be less and the accumulation lower, but couldn’t these cores provide good benchmarks to compare the records collected at Col du Dome?
• Figure 2 – is there an apriori expectation for the H3 and C137 profiles that could be plotted behind the measurements?
• Figure 2 – it would be helpful to have the annual layers marked, at least on the CDM profile
• Figure 4 – please make the y-axes the same on all plots so that the differences in magnitude – which I believe is the primary point – stands out more clearly. And please include the results from the core 30m away
• Figure 5 – make the bedrock a thicker line and different color

The manuscript presents nitrate records obtained from three different ice cores collected at nearly the same location on Col du Dome, Mont Blanc in the years in 1994, 2004 and 2012. Using the records of nitrate and the radionuclides 3H and 210Pb together with a 3D ice flow model the authors argue that there are discontinuities in the depth-age relation of the ice cores drilled in 2004 and 2012, which were caused by the presence of an upstream crevasse.

This is an interesting hypothesis. Although it is common knowledge that areas with upstream crevasses should be avoided as ice core drilling sites, it could be valuable to demonstrate what the effects of such a crevasse are. However, I find the argumentation rather speculative and not well supported by the data, which are often inconclusive. Further, I miss in some part scientific rigorousness as outlined below.  Considering all my concerns as outlined below, this manuscript requires major revisions.

Major comments

210Pb data presented in Fig. 4 have very different time resolution. It is scientifically not sound to compare such data. For instance, the peak at 1970 in CDK would disappear if the same averaging period as in the upper part of the record would be applied. This peak is most likely due to the strong 210Pb seasonality (Eichler et al., 2000). When using the same temporal averaging the postulated anomaly in the 1960s and 1970s in the CDK and CDM cores will be much smaller and may be due to an increased input of 210Pb in the 1970s. Such an increase has been observed already at other glaciers in the Alps, e.g. at Silvretta and Adamello (Festi et al., 2021), at Colle Gnifetti (Gäggeler et al., 1983) and at Grenzgletscher (Eichler et al., 2000) and was attributed to enhanced vertical transport related to the maximum in sulphate aerosol particles acting as transport vehicles (Eichler et al., 2000).  

The entire depth records of 210Pb should be shown and not only the interval between 40 and 130 m in Fig. 4. Without the upper part, it is impossible to see if there is a decrease of 210Pb with depth at all and if the surface activity is in the range expected for glaciers in the Alps.

In the C10 core, 210Pb was determined by gamma-spectrometry (Vincent et al., 1997), whereas for the CDK and CDM cores 210Pb was analyzed by alpha-spectrometry of its decay product 210Po after chemical enrichment, which is the much more sensitive method. Gamma-spectrometry is  rather insensitive due to the high conversion of the low energy gamma-line at 46 keV (96% in the form of electron and only 4% in the form of gamma-emission) and the rather low efficiency of gamma-detectors. This method is normally used for samples with high activity concentrations of 210Pb, e.g. from lake sediments. For low-activity ice samples, the uncertainty is high (more than 50%, Vimeux et al., 2008). Especially in the region, where the anomaly was observed in the C10 core, also 137Cs activity concentrations are high due to the fallout from nuclear tests. This must have resulted in a high background in the gamma-spectrum. These uncertainties need to be discussed.

It is unclear, which 210Pb decay correction was made. In Fig. 4 it is stated that for the C10 core the 1994 activity is shown. However, 210Pb activity concentration are much higher than in the original publication (Vincent et al., 1997). For a comparison between the cores, the activity should be corrected to the same reference date.

I cannot follow the argument how the presence of the crevasse caused such a large 210Pb anomaly in the C10 core, but did not affect the stratigraphy, while in the other two cores the stratigraphy was disturbed, but the 210Pb anomaly was much smaller if present at all. This is a contradiction to me.

The agreement between the nitrate records obtained at a nearly identical location (please add coordinates to support this statement) is not as good as I would expect. Maybe plotting them against a m water equivalent scale would make it easier to identify common features. Generally, I find Fig. 2 difficult and confusing. What are the 250 ppb and 400 ppb levels?

What is also puzzling is that 14% of the nitrate values (and even 30% of the ammonium data) were discarded. What is the basis for that? Which criteria did you use to identify contaminated values?

Why are the tritium records not continuous? With a discontinuous record it is difficult to identify the 1963 maximum. In the case of the CDK core the maximum might be at 86 m.

Minor comments

Bachelor thesis’s cited (Waldner, Zipf) are not publicly available. Include information in supplement.  

103: Despite the undersized core section available at CEP, the nitrate profile obtained at DRI and IUP are in very good agreement (Fig. 2). Do you mean DRI and CEP?

177: How was the winter to summer layer thickness ratio obtained?

216:  Result of annual layer counting, what do you mean with that?

239: For 210Pb seasonality include reference Eichler et al., 2000.

245-250: A zero 210Pb level can only be seen if the values are blank corrected and if the ice does not contain any supported 210Pb from mineral dust (see e.g. Gäggeler et al., 2020). Did you do a blank correction and what was the blank?

Figure 3: C10 was drilled in 1994. Why do the records of annual layer thickness and nitrate concentration continue to the year 2000?

References

Eichler, A., Schwikowski, M., Gäggeler, H.W., Furrer, V., Synal, H.A., Beer, J., Saurer, M., Funk, M., 2000. Glaciochemical dating of an ice core from upper Grenzgletscher (4200 m a.s.l.). Journal of Glaciology 46, 507-515.

Festi, D., Schwikowski, M., Maggi, V., Oeggl, K., Jenk, T.M., 2021. Significant mass loss in the accumulation area of the Adamello glacier indicated by the chronology of a 46 m ice core. The Cryosphere 15, 4135-4143.

Gäggeler, H.W., Tobler, L., Schwikowski, M., Jenk, T.M., 2020. Application of the radionuclide 210Pb in glaciology – an overview. Journal of Glaciology 66, 447-456.

Vimeux, F., de Angelis, M., Ginot, P., Magand, O., Casassa, G., Pouyaud, B., Falourd, S., Johnsen, S., 2008. A promising location in Patagonia for paleoclimate and paleoenvironmental reconstructions revealed by a shallow firn core from Monte San Valentín (Northern Patagonia Icefield, Chile). Journal of Geophysical Research: Atmospheres 113, D16118.