the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Millennial and orbital-scale variability in a 54,000-year record of total air content from the South Pole ice core
Jenna A. Epifanio
Edward J. Brook
Christo Buizert
Erin C. Pettit
Jon S. Edwards
John M. Fegyveresi
Todd A. Sowers
Jeffrey P. Severinghaus
Emma C. Kahle
Abstract. The total air content (TAC) of polar ice cores has long been considered a potential proxy for past ice sheet elevation. Recent work, however, has shown that a variety of other factors also influence this parameter. In this paper we present a high-resolution TAC record from the South Pole (SPC14) ice core covering the last 54,000 years and discuss the implications of the data for interpreting TAC from ice cores. The SPC14 TAC record shows multiple features of interest, including (1) long-term orbital-scale variability, (2) millennial-scale variability in the Holocene and last glacial period, and (3) a period of stability from 35 ka to 25 ka. The longer, orbital-scale variations in TAC are highly correlated with integrated summer insolation (ISI), corroborating the potential of TAC to provide an independent dating tool via orbital tuning. Large millennial-scale variability in TAC during the last glacial period is positively correlated with past accumulation rate reconstructions as well as the δ15N of N2, a firn thickness proxy. These TAC variations are too large to be controlled by direct effects of temperature and too rapid to be tied to elevation changes. We propose that grain size metamorphism near the firn surface is likely to explain these changes. We note, however, that at sites with different climate histories than the South Pole, TAC variations may be dominated by other processes. Our observations of millennial-scale variations in TAC show a different relationship with accumulation rate than observed at sites in Greenland.
Jenna A. Epifanio et al.
Status: open (until 27 Jun 2023)
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RC1: 'Comment on egusphere-2023-578', Anonymous Referee #1, 06 Jun 2023
reply
The total air content of the SPC14 ice core is displayed here at very high resolution over the last 54,000 years. It shows orbital (albeit on a short period) and variability and variability at millenial scale on some periods. By comparing this TAC record to records of other proxies and integrated insolation curves, the authors elaborate on the mechanisms which can explain the observed variations. Accumulation rate seems to be an important control on the variations but it also seems that the mechanism at play is not the same as in Greenland.
In general, the manuscript is well written and well illustrated. I recommend its publication after the following comments are taken into account.
- 105 : Replace « = » by « is »
- 153 : Can you explain how you estimate accurately the line temperature for the portion outside of the GC oven ?
- 154 : remove « is »
- 165 : the use of V1 and V2 are confusing since V1 was used and defined before (eq. 1, l. 105) and I am not sure that it refers to the same volume. Or is it the same volume ? Please clarify.
- Similarly, I am not sure that you refer to the same P1 and P2 than before (in equation 3, P1 and P2 were the pressures of first and second expansion).
- 175 – can you explain clearly what is the ratio of pressures ? Which pressures ?
- The confusions between notations noted above make it very difficult to properly understand the description of the analytical device and the way TAC is calculated. This part should be thoroughly rewritten.
- 195 and after – not enough information is given on the cut-bubble correction for this study. Can you explain in more details how the correction has been derived and how the micro-CT measurements have been used ? Is it possible to show the correction for the top 200 m since it appears that this correction is variable from sample to sample ?
- 206 : Did you try to have the TAC also on a gas age ?
- Also, as you compare it later with the d15N of N2, I imagine that d15N of N2 is on gas age and TAC on ice scale – what is the mechanistic link between the two if they are not on the same age scale ?
- 241 : change the « x » symbol
- 251 : explain what are standardized versions of « TAC" and « Vcr* »
- 290 : I know that it is explained in other places in the manuscript but it is important to document here the speed of the change. In particular, it is important to document the speed of the change because you mention that it is « abrupt ».
- 345 : why do you mention only the resemblance between TAC and accumulation rate and between TAC and d15N of N2 ?
- First, you should explain on which timescale the different records are compared (the sentence « « are also highly correlated with d15N-N2 at all depth » is quite confusing – indeed, if TAC and d15N-N2 are correlated on a depth timescale, then I do not understand why TAC should be on an ice scale since d15N-N2 is on a gas timescale)
- Second, why don’t you also mention the resemblance between TAC and d18O of ice ? I imagine that there is also a good correlation ? What would be the r2 for the correlation between TAC and d18Oice
- Any link between millennial variations of TAC and millennial variations of dust concentration ? What would be the r2 ? Dust load can indeed also influences grain size and this influence has not been discussed in this manuscript. It is important to add a few sentences on this possible influence in a revised manuscript.
- It is really interesting that the TAC signal at SP can not be explained the same way as the TAC signal at NGRIP. However, it would be great to ellaborate a bit more and provide one figure showing the comparison between the two records and their relationship with accumulation rate so that the reader understands clearly the different relationships between TAC and accululation rate in the two sites.
- I am not sure to support the first sentence of section 3.5. Indeed, if the dependence of TAC on accumulation rate (or other influences) is not the same on different sites (+ this study does not provide a clear mechanism), we should be very cautious in using the finding on SP to better interpret « future TAC record » since the controls may be different.
- The multiple regression is a bit difficult to follow. Indeed, while we can assume that ISI and accumulation rate are largely independent, there is strong links between d189ice, d15N-N2, Dage and accumulation rate so that I do not really understand why the multiple regression is not simply done on ISI and accumulation rate (or ISI and d15N-N2) ? The choice of the multiple regression on 4 parameters, 3 of them being strongly linked should be much better explained.
- ISI and accumulation account for 14 and 15% of the multiple regression (l. 422). This is quite weak. Would these proportions be larger if the multiple regression is done only on ISI and accumulation rate ?
- The influence of the dDage/dt is discussed but does not help to identify the mechanism at play (l. 439 : « the reason dDage/dt helps explain TAC changes in the firn is not at first clear ») so why not exploring the influence of dAccu/dt or d(d15N-N2)/dt or … ? The choice of the parameters used in the multiple regression line should be much more discussed.
- 445 : The influence of ISI on TAC is not so obvious because the record is short. Is is possible that the effect of accumulation rate on TAC is inhibited because the ISI is on a minimum and thus inhibits the metamorphism mechanism leading to grain size modification ?
- The conclusion starting on l. 467 is surprising : why isn’t the influence of accumulation rate on Dage and d15N-N2 not mentionned ? How much can the influence of accumulation rate on both TAC and d15N-N2 (Dage) explain the strong link between TAC and d15N-N2 (Dage) ? I feel that some explanations are missing here so as not to give the impression of a circular reasoning.
Citation: https://doi.org/10.5194/egusphere-2023-578-RC1
Jenna A. Epifanio et al.
Jenna A. Epifanio et al.
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