Spatial and temporal variations in surface snow chemistry along a traverse from Dome C toward South Pole in the framework of East Antarctic International Ice Sheet Traverse (EAIIST) project
Abstract. As part of the “East Antarctic International Ice Sheet Traverse” (EAIIST) project, surface snow and snow pit samples were collected along a traverse from Dome C toward the geographic South Pole during the 2019–2020 Antarctic campaign. Results on spatial distribution of major ions are here reported to understand deposition and post deposition processes in sites with very low snow accumulation rate in the East Antarctic Plateau where megadune and wind crust areas are present.
The volcanic signature of Pinatubo eruption (occurred in 1991) was clearly visible in the non-sea salt SO42− stratigraphy from two snow pits (AGO-5 and PALEO) allowing the determination of annual accumulation rates that revealed to be 25.7 and 22.6 mm of water equivalent/year, respectively at the two sites. Moreover, a decreasing trend in accumulation rate as the distance from the Indian Ocean increases was detected. Mineral dust concentration and size show presence of a criptotephra layer in AGO5 and PALEO stratigraphies which is stratigraphically compatible with the deposition of volcanic ash related to the Puyehue-Cordón Caulle explosive eruption occurred in June 2011. The ssNa+ fraction, accounting for the 92.5 % of the total Na+, is preserved stably in the snow layers and was chosen as marker of sea spray deposition. Despite the very low accumulation rate in this area, the main deposition process of sea spray aerosol is the wet deposition. Conversely, both biogenic and crustal nssSO42− are dry deposited, the total flux of nssSO42− resulted to be constant in the Antarctic plateau, but the biogenic to crustal ratio increases as distance from Dome C increases. The presence and quantification (by nssCa2+) of a dry deposited crustal source, as the biogenic one, sheds light on the interpretation of nssSO42− biogenic stratigraphy during glacial and interglacial time in Antarctic ice cores. NssCl− represent the fraction of Cl− deposited as HCl and arises from the exchange reactions between chloride in the sea salt aerosol and acidic species such as H2SO4 and HNO3 that occurs both into the atmosphere (in this case HCl is deposited by wet deposition) and into the snow (at the expenses of NaCl or MgCl2 deposited as sea salt aerosol). The latter process could be particularly efficient in sites affected by wind crust formation, probably because of a longer exposure time of the snow layers to the atmosphere favouring the HCl volatilization . Another important marker in ice core is HNO3, that in the considered sites is found at very high concentration in the most superficial 3 cm of snow due to the uptake by superficial snow and possibly concentration effects from the layers beneath, but it is reversibly deposited. The depth of the active layer for HNO3 reemission was calculated and it spans from 22 cm to 12 cm; in addition, the concentration preserved in the snow decreases as the accumulation rate decreases, but wind scouring increases the efficiency of re-emission processes in the active layer. The knowledge and quantification of all the above reported processes will allow the interpretation of the ice core stratigraphies in low accumulation site likely hopefully recording, at selected sites, the climate history of more than one million years ago.
Simone Ventisette et al.
Status: open (until 21 Jun 2023)
RC1: 'Comment on egusphere-2023-393', Anonymous Referee #1, 26 May 2023
- AC1: 'Reply on RC1', Silvia Becagli, 30 May 2023 reply
Simone Ventisette et al.
Simone Ventisette et al.
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This paper reports major ion chemistry in a series of samples taken on a traverse from Dome C southwards. This is an area of low snow accumulation rates, and understanding processes of chemical deposition and postdepositional loss across the region could be valuable in interpreting deep cores from the same region. The authors make the case that the altitude does not vary much, and therefore argue that the different accumulation rate across the traverse could be critical in determining the observed concentrations.
I agree that this gives the potential for a very useful study of how accumulation rate affects chemical deposition. However unfortunately the samples the authors have collected are completely unsuitable for the study they carry out. Most of the samples are simply surface scrapes of the top 6 cm. This may have been all that could be achieved on a fast-moving traverse. However it has two effects: firstly the concentrations measured have little meaning as 6 cm is of order 1 year, and the well-known variability of accumulation rate in a single year in such a region means that each sample could represent a single season up to 2 years. But secondly it means that the authors simply don’t know the accumulation rate at each site.
To address this they determine the accumulation rate in 2 snowpits (for some reason not showing us the data from 3 others). They then try to use those two data points, plus one for Dome C, to define a relationship between accumulation rate and distance from the Ocean, which they then apply to all the other sites. This is completely unsafe: the derived accumulation rates, which then translate into fluxes, have no basis. I don’t disagree that accumulation rate likely decreases with distance from the ocean. However trying to derive the relationship from 3 points is impossible. Firstly it is not enough data to give any confidence that the (linear) fit is correct. But also the data points themselves are highly uncertain. The values quoted for Dome C (33 and 35 mm/yr) stand in contrast to the many more data reported by Frezzotti et al (2005) which ranged between 23 and 30 mm/yr within 20 km of Dome C. Using the Frezzotti values would completely change the fit. Secondly the data from the other two sites (AGO5 and PALEO) are clearly not robust: the ratio of depths for Pinatubo (or of derived accumulation rate) is 1.14, while the ratio of depths for the top of the dust layer shown in Fig 4 is 1.6 (55.5/34.5). This shows that far from “increasing the reliability of the accumulation rate” (line 233), the analysis has highlighted its huge uncertainty.
This means unfortunately that no faith can be placed in the derived accumulation rates for the surface sample sites, and (due to the seasonal nature of the surface samples), even less reliance can be placed on the derived fluxes, making most of the rest of the paper unviable. I don’t really see anything the authors can do to solve this problem (unless they do have data for many further pits), and so will be recommending rejection. I will though address a few other points of detail in case the authors can use the data in another way.
Further detailed comments:
The abstract is very long and discursive, perhaps because the conclusions are quite limited.
Fig. 1 would be more useful if lines of latitude and longitude were marked to allow the reader to orient themselves.
The dates of sampling should be shown in Table 1S to allow the reader to see whether the surface represents a similar date in each case.
Table 1. Surely the units for detection limit are ug/L not mg/L.
Section 4.2 dating. I am reasonably happy with the accumulation based on Pinatubo (though noting that the Cole-Dai paper cited actually suggests that the peak at this time includes Pinatubo and Cerro Hudson. The maximum should though be in 1992 as assumed here. It’s unfortunate that the AGO5 data stop in the middle of the peak so we cannot be sure we are at the maximum. I am far less convinced by the identification of the dust peak shown in Fig. 4. It’s impossible to be sure either that it’s the same peak in each core or that it’s related to the Chilean eruption cited (in fact I see no reason to believe the peak is volcanic even). If it is the same peak then it completely undermines the accumulation rate derived from Pinatubo for at least one of the sites. This is very unsatisfying.
Section 4.3 and figure 6. Without a reliable estimate of accumulation rate (except at the two snowpits) the derived fluxes in Figure 6 are not reliable. Indeed this is illustrated in the plot for Ca, where the concentrations in three pit samples show a completely different trend to the surface samples. The use of the flux plots to derive the relative proportion of wet deposition is also based on a misconception. The concept behind such plots is that all the data points are seeing an air mass with the same concentration, so that the accumulation rate alone determines what is deposited. For a given site, but using different years, this might be a reasonable assumption, and a plot of flux against accumulation rate can (at least in theory) be used to derive the proportion of dry and wet deposition; this is similarly true for multiple cores and pits within a small region, where local variability in accumulation rate might be meaningful, and indicate the effect of accumulation under the same air mass. However for this traverse inland, the assumption is surely that air masses are travelling inland from low to high latitude. Taking sodium as an example, it will be deposited along the route, whether by wet or dry deposition, so there will be less Na in the air mass at 81S than at 75S, leading to lower flux at 81S, with no direct implication of whether deposition was wet or dry. (I note in passing that if taken at face value, Fig 6 implies that the Ca flux would be negative by 82 south!). Finally on this issue, the authors should question whether it makes sense that wet deposition would dominate for Na but not for sulfate given that sea salt is known to be on larger aerosol, size ranges than nss-sulfate.
Lines 348-355. I am surprised the authors try to make a statement about marine vs crustal sulfate as they present no evidence that relates to this. There is an active discussion about the significance of sulfate to calcium ratios, ranging from those who imagine that the sulfate is arriving as pure terrestrial gypsum (Goto-Azuma et al 2019) to those who assume that acidic sulfate reacts with terrestrial dust in the atmosphere, and that the sulfate associated with Ca is therefore likely not of crustal origin. Sulfur isotopic data contribute to this discussion but I don’t see what the data here add to this argument.
It's interesting to see the data for nitrate and nss-Cl flux (Fig. 6) and loss from snow pits (Fig 8). However this is quite well-known and results from a complex balance of atmospheric reactions, uptake to snow, photochemical and evaporative losses. The discussion here does not seem to provide new insight compared to earlier work that has also considered atmospheric data.