the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Mar 2024
Surface processes and drivers of the snow water stable isotopic composition at Dome C, East Antarctica – a multi-datasets and modelling analysis
Abstract. Water stable isotope records in polar ice cores have been largely used to reconstruct past local temperatures and other climatic information such as evaporative source region conditions of the precipitation reaching the ice core sites. However, recent studies have identified post-depositional processes taking place at the ice sheet's surface modifying the original precipitation signal and challenging the traditional interpretation of ice core isotopic records. In this study, we use a combination of existing and new datasets of the precipitation, snow surface and subsurface isotopic compositions (δ18O and d-excess), meteorological parameters, ERA5 reanalyses, outputs from the isotope-enabled climate model ECHAM6-wiso, and a simple modelling approach to investigate the transfer function of water stable isotopes from precipitation to the snow surface and subsurface at Dome C, in East Antarctica. We first show that water vapor fluxes at the surface of the ice sheet result in a net annual sublimation of snow, from 3.1 to 3.7 mm water equivalent per year between 2018 and 2020, corresponding to 12 to 15 % of the annual surface mass balance. We find that the precipitation isotopic signal cannot fully explain the mean, nor the variability of the isotopic composition observed in the snow, from annual to intra-monthly timescales. We observe that the mean effect of post-depositional processes over the study period enriches the snow surface in δ18O by 3.3 ‰ to 6.6 ‰ and lowers the snow surface d-excess by 3.5 ‰ to 7.6 ‰ compared to the incoming precipitation isotopic signal. We also show that the mean isotopic composition of the subsurface snow is not statistically different from that of the surface snow, indicating the preservation of the mean isotopic composition of the surface snow in the top centimetres of the snowpack. This study confirms previous findings about the complex interpretation of the water stable isotopic signal in the snow and provides the first quantitative estimation of the impact of post-depositional processes on the snow isotopic composition at Dome C, a crucial step for the accurate interpretation of isotopic records from ice cores.
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RC1: 'Comment on egusphere-2024-685', Anonymous Referee #1, 20 Apr 2024
Comments on “Surface processes and drivers of the snow water stable isotopic composition at Dome C, East Antarctica – a multi-datasets and modelling analysis” by Ollivier et al.
General comments
The effects of post-depositional processes (water vapor exchanges between surface snow and atmosphere, diffusion of water vapor in the firn, and wind redistribution) on water stable isotopic composition in surface snow in the inland of polar ice sheets strongly limit the quantitative paleo-temperature reconstructions by ice-core stable isotopic records from these regions. In this study, the authors attempted to evaluate the mean effect of post-depositional processes at Dome C using a combination of existing and new dataset of precipitation, surface snow and subsurface isotopic compositions over 2017-2021. It is noted that the sampling schemes are elaborate and the sampling temporal resolution is high, which is not easy to perform at very harsh meteorological conditions such as at Dome C but is very necessary to address the problem of post-depositional processes. I appreciate the authors for their hard work at Dome C and their persistent efforts on this complex issue on the post-depositional processes. Although I agree with the authors for their most interpretations of the observations, I think some explanations should be added and the different post-depositional processes should be clarified.
Specific comments
I note that precipitation, surface snow and subsurface snow show similar δ18O seasonal cycle, but the maximum (minimum) value of δ18O occurs in different month, with highest δ18O value in January and lowest value in May for precipitation, a maximum of δ18O in February and a minimum in October for surface snow, and a maximum of δ18O in March and a minimum in November for subsurface snow. I think it must be due to the post-depositional processes, but why the post-depositional processes can lead to the δ18O shift in time?
In section 3.2.2, the authors find that the surface snow is relatively enriched in δ18O during warm season (from November to April) due to the sublimation of surface snow. Indeed, the calculated surface moisture fluxes also indicate sublimation occurs mainly in summer (Fig. 2). Theoretically, stronger sublimation should cause more enrichment of δ18O in surface snow. As a result, I would like to know if there exists a correlation between the surface snow δ18O and the water vapor flux during the summer. Additionally, the slope between δD and δ18O in surface snow should be lower than the δD- δ18O slope of precipitation. I suggest the authors to add above relevant information to further validate the sublimation process because it is a key post-depositional process in inland of Antarctica.
In section 3.3.2, the authors compared precipitation isotopic composition (δ18O and d-excess) with the simulations of ECHAM6-wiso. Although the daily δ18O modelled by ECHAM6-wiso shows a good agreement with the observations, the ECHAM6 overestimates (underestimates) the δ18O (d-excess) likely due to the warm bias of the model. In addition, the daily d-excess is poorly simulated by the ECHAM6-wiso (Fig. 7b). The possible deficiency of the ECHAM6-wiso for the mismatch between the simulations and observations should be brief explained for guiding scientists to better utilize GCMs-wiso.
The authors conclude that the mean effect of post-depositional processes enriches surface snow δ18O by 3.3‰ to 6.6‰ and lowers the snow surface d-excess by 3.5‰ to 7.6‰. As the authors analyzed, summertime sublimation should contribute significantly to the mean effect. After comparing Fig. 3b and Fig. 6c, I found the δ18O values of surface snow during wintertime are also higher than the values of precipitation. This suggests that the post-depositional processes rather than sublimation are also significant in winter. So, what kind of post-depositional processes lead to the enrichment of surface snow δ18O in winter? Is it the diffusion of water vapor? The authors should discuss the post-depositional processes in wintertime.
Citation: https://doi.org/10.5194/egusphere-2024-685-RC1 - AC1: 'Reply on RC1', Inès Ollivier, 19 Jul 2024
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RC2: 'Comment on egusphere-2024-685', Anonymous Referee #2, 01 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-685/egusphere-2024-685-RC2-supplement.pdf
- AC2: 'Reply on RC2', Inès Ollivier, 19 Jul 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-685', Anonymous Referee #1, 20 Apr 2024
Comments on “Surface processes and drivers of the snow water stable isotopic composition at Dome C, East Antarctica – a multi-datasets and modelling analysis” by Ollivier et al.
General comments
The effects of post-depositional processes (water vapor exchanges between surface snow and atmosphere, diffusion of water vapor in the firn, and wind redistribution) on water stable isotopic composition in surface snow in the inland of polar ice sheets strongly limit the quantitative paleo-temperature reconstructions by ice-core stable isotopic records from these regions. In this study, the authors attempted to evaluate the mean effect of post-depositional processes at Dome C using a combination of existing and new dataset of precipitation, surface snow and subsurface isotopic compositions over 2017-2021. It is noted that the sampling schemes are elaborate and the sampling temporal resolution is high, which is not easy to perform at very harsh meteorological conditions such as at Dome C but is very necessary to address the problem of post-depositional processes. I appreciate the authors for their hard work at Dome C and their persistent efforts on this complex issue on the post-depositional processes. Although I agree with the authors for their most interpretations of the observations, I think some explanations should be added and the different post-depositional processes should be clarified.
Specific comments
I note that precipitation, surface snow and subsurface snow show similar δ18O seasonal cycle, but the maximum (minimum) value of δ18O occurs in different month, with highest δ18O value in January and lowest value in May for precipitation, a maximum of δ18O in February and a minimum in October for surface snow, and a maximum of δ18O in March and a minimum in November for subsurface snow. I think it must be due to the post-depositional processes, but why the post-depositional processes can lead to the δ18O shift in time?
In section 3.2.2, the authors find that the surface snow is relatively enriched in δ18O during warm season (from November to April) due to the sublimation of surface snow. Indeed, the calculated surface moisture fluxes also indicate sublimation occurs mainly in summer (Fig. 2). Theoretically, stronger sublimation should cause more enrichment of δ18O in surface snow. As a result, I would like to know if there exists a correlation between the surface snow δ18O and the water vapor flux during the summer. Additionally, the slope between δD and δ18O in surface snow should be lower than the δD- δ18O slope of precipitation. I suggest the authors to add above relevant information to further validate the sublimation process because it is a key post-depositional process in inland of Antarctica.
In section 3.3.2, the authors compared precipitation isotopic composition (δ18O and d-excess) with the simulations of ECHAM6-wiso. Although the daily δ18O modelled by ECHAM6-wiso shows a good agreement with the observations, the ECHAM6 overestimates (underestimates) the δ18O (d-excess) likely due to the warm bias of the model. In addition, the daily d-excess is poorly simulated by the ECHAM6-wiso (Fig. 7b). The possible deficiency of the ECHAM6-wiso for the mismatch between the simulations and observations should be brief explained for guiding scientists to better utilize GCMs-wiso.
The authors conclude that the mean effect of post-depositional processes enriches surface snow δ18O by 3.3‰ to 6.6‰ and lowers the snow surface d-excess by 3.5‰ to 7.6‰. As the authors analyzed, summertime sublimation should contribute significantly to the mean effect. After comparing Fig. 3b and Fig. 6c, I found the δ18O values of surface snow during wintertime are also higher than the values of precipitation. This suggests that the post-depositional processes rather than sublimation are also significant in winter. So, what kind of post-depositional processes lead to the enrichment of surface snow δ18O in winter? Is it the diffusion of water vapor? The authors should discuss the post-depositional processes in wintertime.
Citation: https://doi.org/10.5194/egusphere-2024-685-RC1 - AC1: 'Reply on RC1', Inès Ollivier, 19 Jul 2024
-
RC2: 'Comment on egusphere-2024-685', Anonymous Referee #2, 01 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-685/egusphere-2024-685-RC2-supplement.pdf
- AC2: 'Reply on RC2', Inès Ollivier, 19 Jul 2024
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