the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Mar 2022
Late Holocene glacier variations in the central Tibetan Plateau indicated by the δ18O of ice core enclosed gaseous oxygen
Abstract. The δ18O of gaseous oxygen in ice core air bubbles (δ18Obub) has been widely used for reconstruction of climate changes in polar glaciers. Yet, less is known about its climatic implication in alpine glaciers as the lack of continuous record. Here, we present a long-term δ18Obub record from the Tanggula glacier in the central Tibetan Plateau (TP). It shows that there is a good correlation between the variation of the δ18Obub in this alpine ice core and the accumulation or melting of the glacier. The more developed the firn layer on glacier surface, the more positive the δ18Obub is. The more intense the glacier melting, the more negative the δ18Obub is. Combined with the chronology of the ice core air bubbles, we reconstructed the glacier variations since the late Holocene in the central TP. It showed that there were four accumulation and three deficit periods of glaciers in this region. The strongest glacier accumulation period was from 1610–300 B.C., which corresponding to the Neoglaciation. The most significant melting period was the last 100 years, which corresponding to the recent global warming. During the Little Ice Age, glacier accumulation in the central TP was not significant, and even short deficit events occurred. Comparisons of the late Holocene glacier variation in the central TP with hemispheric glacier and climate variations showed that it was closely related to the North Atlantic Oscillation.
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RC1: 'Comment on tc-2022-43', Anonymous Referee #1, 22 Mar 2022
This manuscript suggests that the oxygen isotope composition of air bubbles in alpine glaciers can be used as a proxy for aridity versus intervals of snowfall sufficient to maintain a firn layer. The authors describe results from an ice core in the Tibetan Plateau, in which a correlation is seen between the d18O of air bubbles in the ice, and the aridity, as inferred from the presence or absence of a firn layer.
Unfortunately, the analysis is based on simple correlation, and is not rooted in a knowledge of the relevant physical processes such as isotope fractionation by dissolution of oxygen gas in meltwater, or gravitational settling, or kinetic fractionation during disequilibrium dissolution of oxygen gas in liquid water. Thus the proposed proxy is not likely to be reliable. In other words, a mechanistic understanding of the causes of the d18O variations is needed if the proposed aridity proxy is to be reliable and useful.
Indeed, it is quite possible that the observed correlation will not hold up, on other glaciers, because isotope fractionation can easily happen via processes that are not related to aridity.
So this manuscript must unfortunately be rejected. The scientific foundation is simply lacking.
Citation: https://doi.org/10.5194/tc-2022-43-RC1 -
AC1: 'Reply on RC1', J.-L. Li, 24 Mar 2022
Many thanks for your comments on our manuscript.
In our study we found a close relationship between the oxygen isotope composition of ice core air bubbles and the variation (accumulation or melting) of glaciers in the central Tibetan Plateau. This was based on the analysis about what caused the unusual fluctuation of the d18O of ice core air bubbles compared to the d18O of atmospheric air. As we mentioned in our study, it was not only related to the thickness of the firn layer which could lead to the gravitational settling of oxygen isotope, but also to the complex isotope fractionation processes happened during the closure and storage of air in the ice through a series of physical and chemical processes via strong ultraviolet radiation at high altitude. These processes would happen when the regional temperature was high and there was meltwater in the firn layer. These conclusion was consistent with what you mentioned about the knowledge of the relevant physical processes such as isotope fractionation by dissolution of oxygen gas in meltwater, or gravitational settling, or kinetic fractionation during disequilibrium dissolution of oxygen gas in liquid water. So, it was not just as a proxy for aridity versus intervals of snowfall sufficient to maintain a firn layer, but a proxy for increase or melting intensity of the firn which is closely related to the advance or melting of the glacier.
This conclusion and correlation are strongly supported by the comparison between the variations of d18O of ice core air bubbles and the regional glaciers. So, it could be used to reflect the regional glacier variation in the central Tibetan Plateau.
Thanks again for your comments.
Citation: https://doi.org/10.5194/tc-2022-43-AC1
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AC1: 'Reply on RC1', J.-L. Li, 24 Mar 2022
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RC2: 'Comment on tc-2022-43', Anonymous Referee #2, 07 Apr 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-43/tc-2022-43-RC2-supplement.pdf
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AC2: 'Reply on RC2', J.-L. Li, 10 Apr 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-43/tc-2022-43-AC2-supplement.pdf
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AC2: 'Reply on RC2', J.-L. Li, 10 Apr 2022
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RC3: 'Comment on tc-2022-43', Anonymous Referee #3, 10 Apr 2022
This paper contains some clever ideas for interpreting climate records from ice cores in (relatively) warm regions. However, it has 3 basic problems. First, the analytical method is not fully described, and some of the results are very hard to understand if all the analyses are accurate. Second, the paper deduces correlations between various climate records, but does not validate these correlations statistically. Third, some basic physics is invoked but not described. For example, there is no explanation for how water and O2 can exchange isotopes fast enough to influence the isotopic composition of O2 in trapped gases.
Specific concerns include:
Total air content was determined by an indirect qualitative method and was apparently not checked against robust observations. (Section 2.2)
In Figure 2, the data does not constrain annual layer thicknesses well. Bomb radioisotopes are invoked but the data are not included.
The analytical method for measuring and standardizing d18O of O2 was not fully described.
Figure 3: The relation between TSI and air content is not validated by a simple x-y plot showing the relationship between the 2 properties, or other approaches. The high value for air content comes around 1640, but there is no TSI maximum at this time.
Table 1: the authors do not explain how they measured d15N, which is needed to calculate d18Oatm. I could not find information about the reference gas.
Figure 4: there is no statistical documentation for a relationship between climate and d18Obub. Also in Fig. 4: d18Obub reaches +2 per mil, which would require a firn column thickness of about 200 meters thick at certain times. This seems unlikely to say the least.
Fig. 5. There is no statistical evidence showing coherency between Tangguula and other records.
Lines 220-225: There is no evidence that water and O2 exchange isotopes fast enough to impact the isotopic composition of O2 in ice cores. At least the authors do not make a case that extensive exchange is plausible.
Citation: https://doi.org/10.5194/tc-2022-43-RC3 -
AC3: 'Reply on RC3', J.-L. Li, 12 Apr 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-43/tc-2022-43-AC3-supplement.pdf
-
AC3: 'Reply on RC3', J.-L. Li, 12 Apr 2022
Status: closed
-
RC1: 'Comment on tc-2022-43', Anonymous Referee #1, 22 Mar 2022
This manuscript suggests that the oxygen isotope composition of air bubbles in alpine glaciers can be used as a proxy for aridity versus intervals of snowfall sufficient to maintain a firn layer. The authors describe results from an ice core in the Tibetan Plateau, in which a correlation is seen between the d18O of air bubbles in the ice, and the aridity, as inferred from the presence or absence of a firn layer.
Unfortunately, the analysis is based on simple correlation, and is not rooted in a knowledge of the relevant physical processes such as isotope fractionation by dissolution of oxygen gas in meltwater, or gravitational settling, or kinetic fractionation during disequilibrium dissolution of oxygen gas in liquid water. Thus the proposed proxy is not likely to be reliable. In other words, a mechanistic understanding of the causes of the d18O variations is needed if the proposed aridity proxy is to be reliable and useful.
Indeed, it is quite possible that the observed correlation will not hold up, on other glaciers, because isotope fractionation can easily happen via processes that are not related to aridity.
So this manuscript must unfortunately be rejected. The scientific foundation is simply lacking.
Citation: https://doi.org/10.5194/tc-2022-43-RC1 -
AC1: 'Reply on RC1', J.-L. Li, 24 Mar 2022
Many thanks for your comments on our manuscript.
In our study we found a close relationship between the oxygen isotope composition of ice core air bubbles and the variation (accumulation or melting) of glaciers in the central Tibetan Plateau. This was based on the analysis about what caused the unusual fluctuation of the d18O of ice core air bubbles compared to the d18O of atmospheric air. As we mentioned in our study, it was not only related to the thickness of the firn layer which could lead to the gravitational settling of oxygen isotope, but also to the complex isotope fractionation processes happened during the closure and storage of air in the ice through a series of physical and chemical processes via strong ultraviolet radiation at high altitude. These processes would happen when the regional temperature was high and there was meltwater in the firn layer. These conclusion was consistent with what you mentioned about the knowledge of the relevant physical processes such as isotope fractionation by dissolution of oxygen gas in meltwater, or gravitational settling, or kinetic fractionation during disequilibrium dissolution of oxygen gas in liquid water. So, it was not just as a proxy for aridity versus intervals of snowfall sufficient to maintain a firn layer, but a proxy for increase or melting intensity of the firn which is closely related to the advance or melting of the glacier.
This conclusion and correlation are strongly supported by the comparison between the variations of d18O of ice core air bubbles and the regional glaciers. So, it could be used to reflect the regional glacier variation in the central Tibetan Plateau.
Thanks again for your comments.
Citation: https://doi.org/10.5194/tc-2022-43-AC1
-
AC1: 'Reply on RC1', J.-L. Li, 24 Mar 2022
-
RC2: 'Comment on tc-2022-43', Anonymous Referee #2, 07 Apr 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-43/tc-2022-43-RC2-supplement.pdf
-
AC2: 'Reply on RC2', J.-L. Li, 10 Apr 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-43/tc-2022-43-AC2-supplement.pdf
-
AC2: 'Reply on RC2', J.-L. Li, 10 Apr 2022
-
RC3: 'Comment on tc-2022-43', Anonymous Referee #3, 10 Apr 2022
This paper contains some clever ideas for interpreting climate records from ice cores in (relatively) warm regions. However, it has 3 basic problems. First, the analytical method is not fully described, and some of the results are very hard to understand if all the analyses are accurate. Second, the paper deduces correlations between various climate records, but does not validate these correlations statistically. Third, some basic physics is invoked but not described. For example, there is no explanation for how water and O2 can exchange isotopes fast enough to influence the isotopic composition of O2 in trapped gases.
Specific concerns include:
Total air content was determined by an indirect qualitative method and was apparently not checked against robust observations. (Section 2.2)
In Figure 2, the data does not constrain annual layer thicknesses well. Bomb radioisotopes are invoked but the data are not included.
The analytical method for measuring and standardizing d18O of O2 was not fully described.
Figure 3: The relation between TSI and air content is not validated by a simple x-y plot showing the relationship between the 2 properties, or other approaches. The high value for air content comes around 1640, but there is no TSI maximum at this time.
Table 1: the authors do not explain how they measured d15N, which is needed to calculate d18Oatm. I could not find information about the reference gas.
Figure 4: there is no statistical documentation for a relationship between climate and d18Obub. Also in Fig. 4: d18Obub reaches +2 per mil, which would require a firn column thickness of about 200 meters thick at certain times. This seems unlikely to say the least.
Fig. 5. There is no statistical evidence showing coherency between Tangguula and other records.
Lines 220-225: There is no evidence that water and O2 exchange isotopes fast enough to impact the isotopic composition of O2 in ice cores. At least the authors do not make a case that extensive exchange is plausible.
Citation: https://doi.org/10.5194/tc-2022-43-RC3 -
AC3: 'Reply on RC3', J.-L. Li, 12 Apr 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-43/tc-2022-43-AC3-supplement.pdf
-
AC3: 'Reply on RC3', J.-L. Li, 12 Apr 2022
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