the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
An annually resolved chronology for the Mount Brown South ice cores, East Antarctica
Nerilie J. Abram
Alison S. Criscitiello
Camilla K. Crockart
Aylin DeCampo
Vincent Favier
Vasileios Gkinis
Margaret Harlan
Sarah L. Jackson
Helle A. Kjær
Chelsea A. Long
Meredith K. Nation
Chris T. Plummer
Delia Segato
Andrea Spolaor
Paul T. Vallelonga
Abstract. Climate reconstructions of the last millennium rely on networks of high resolution and well-dated proxy records. This study presents age-at-depth data and preliminary results from the new Mount Brown South ice cores, collected at an elevation of 2,084 metres on the boundary of Princess Elizabeth and Kaiser Wilhelm II Land in East Antarctica. We show an initial analysis of the site meteorology, mean annual chemical species concentrations, and seasonal cycles including analysis of a seasonal cycle in fluoride concentrations with a potential link to sea ice formation. The annually resolved chronologies were developed from this data using a site-specific layer-counting methodology which employed seasonally varying trace chemical species and water isotope ratios, combined with a volcanic horizon alignment approach. The chronologies developed include the ‘Main’ 295 m record spanning 1,137 years (873–2009 CE), and three surface cores spanning the most recent 39–52 years up to the surface age at the time of drilling (austral summer 2017/2018). Mean annual trace chemical concentrations are compared to the Law Dome ice core further to the east and discussed in terms of atmospheric transport, and the uncertainty in the determination of annual horizons via layer counting is quantified. The MBS chronologies presented here – named MBS2023 – will underpin the development of new palaeoclimate records spanning the past millennium from this under-represented region of East Antarctica.
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Tessa R. Vance et al.
Status: final response (author comments only)
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RC1: 'Comment on cp-2023-52', Jacob Chalif, 06 Sep 2023
In the paper “An annually resolved chronology of the Mount Brown South ice cores, East Antarctica,” T.R. Vance and coauthors present four new chronologies for ice cores (three surface cores and one deeper core) from Mount Brown South (MBS) in East Antarctica. These chronologies were developed through a combined multi-researcher annual chemical layer counting and volcanic alignment approach, which is common in dating annually resolved ice core records. Preliminary analyses of ice core chemistry, in particular comparisons with Law Dome and an analysis of the seasonal cycle of certain chemical species used in the annual layer counting effort, are presented as well.
I applaud the authors for their comprehensive discussion of the methods used in analyzing the MBS ice cores. Not all ice core studies are so transparent in detailing their methods of analysis, but the authors do a commendable job of laying out their complete analytical regime, which involved multiple cores, institutions, and types of chemical and physical analyses.
Given the quality of the annual layers in the MBS ice cores, I completely agree with the method the authors chose to use to establish the MBS chronologies.
In Section 4.1, the seasonal cycles of trace chemical species are discussed. This discussion is foundationally important for their dating methodology, but the authors spend the majority of this section proposing a new mechanism to explain the seasonality of the fluoride signal. I do not think that the authors need to explain the origin of the fluoride signal to defend its use in their dating methodology, as the presence of its seasonal cycle is plainly evident regardless of its cause. That said, it is suggested that the fluoride seasonality is linked to sea ice seasonality and the behavior of East Antarctic polynyas. While I do find their hypothesis compelling, the authors do not provide sufficient observational or modeling results to support this hypothesis. I believe that it warrants a deeper investigation to be included in this study, especially given that the authors point out the many uncertainties in their interpretation, including (1) the difficulty in measuring fluoride due to its low concentrations, (2) the inconsistency of its reliability at different periods of the MBS record, (3) the volatility of fluoride, and (4) the existence of alternative explanations for the seasonal signal. Perhaps, as the authors suggest in Lines 394-5, a separate study examining sea ice proxies in the MBS cores would be a better place to introduce and test this hypothesis.
Besides this one issue in Section 4.1, I found that the paper is well-written. This paper will be very useful for future analyses of the MBS ice cores, which will be a valuable archive of East Antarctic climate proxy records. Additionally, given the authors’ thorough discussion of their methods, this paper will be a useful community document detailing the chemical analysis of and chronology development of annually resolved ice cores more broadly.
I recommend its publication after resolving the above point and the following minor points:
- Section 1: I appreciated the extensive discussion of site characteristics, but I wondered why the authors spent so much space discussing the wind characteristics? I don’t believe that they ever returned to this later on in the discussion of the chronology development.
- Figure 1, caption: I suggest the authors replace “fuschia” with “red” or “pink” and “cyan” with “blue” as these would be more universal color labels.
- Line 111: It is stated that there is a mean sample resolution of 10 samples y-1. As resolution decreases with depth, it might be more useful to give a range of sample resolutions along the core (i.e., perhaps a mean sample resolution for a section of core near the top and a mean sample resolution for a section of core near the bottom).
- Section 2.4: It would be useful to include a schematic of the CFA melter system used, either in the Appendix or as a main figure. If space limitations are a concern, I would suggest that Figure 3 could be combined with a CFA schematic into one new, slightly larger figure. See example CFA schematics in Figure 2 of Hoffman et al. (2022) or Figure 2 of Osterberg et al. (2006), among other papers.
- Line 228: The proper name for the “National Ice Core Facility” is the “National Science Foundation Ice Core Facility” (previously, it was named the “National Ice Core Laboratory,” hence the confusion).
- Section 2.8, and Line 480, and Line 505: A simple schematic or table, even in the Appendix, illustrating the authors’ 4-step dating method would be useful. I reread the dating section a few times and still struggle to understand when various chemical species were used in different dating schemes.
- Line 365: I believe the authors mean “DMS” (dimethyl sulfide) when they write “MSA”. Algae produce DMS, not MSA, and DMS is oxidized in the atmosphere into MSA and/or non-sea-salt sulfate, among other products and intermediates (see Figure 1 of Fung et al. (2022) for a nice overview of DMS oxidation chemistry).
- Figure 5: It would be useful to include row labels (“Main core non-satellite era”, “Main core satellite era”, “Charlie surface core”) at the side similar to the column labels at the top.
- Figure 5: The y-axis limits on the left 3 columns are such that the seasonal cycle is very clear, but for fluoride the axis limits are much wider than the fluoride seasonal signal, making its seasonality stand out less. I am wondering why the authors chose to minimize the apparent magnitude of its seasonal signal?
- Lines 455-8: The authors suggest that a key difference between the MBS and Law Dome ice cores is that MBS exhibits clear seasonality of fluoride, but then note that there is no fluoride dataset from Law Dome. I may be misunderstanding this, but the authors should not call this a difference between the sites if there is no evidence for the lack of fluoride seasonality at Law Dome. If they mean that this is not a difference between the site characteristics, but only a difference in how ice cores from the two sites were dated, this should be specified.
- Line 472: couldn’t the hypothesis that there was a “training” period for the researchers near the top of the core be eliminated by repeating the layer-counting, at least for the top section of core?
- Figure B1: Y-axis labels for each series would be very useful.
Typographical errors:
- Lines 90: I believe the word “was” is missing between “weight” and “recorded”
- Line 132: “non sea salt component” should be changed to “non-sea-salt component” in order to maintain consistency with the rest of the usages of “non-sea-salt” throughout the paper
- Line 179: there is a closing parenthesis, “)”, where there should be none after the word “capillary”
- Table 3, caption: there is a missing closing parenthesis, “)” after “(see Plummer et al. (2012)”
- Lines 349-51: The usage of “e.g.”’s are not consistent. In Line 349, e.g. begins a parenthetical, whereas in line 350, e.g. comes after a comma. I believe they both should begin parentheticals
- Line 361: “MBS-main” should have a capitalized “M” in “main”
- Line 442: There is an extra opening parenthesis, “(“, before “van Ommen”
References
Fung, Ka Ming, Colette L. Heald, Jesse H. Kroll, Siyuan Wang, Duseong S. Jo, Andrew Gettelman, Zheng Lu, et al. “Exploring Dimethyl Sulfide (DMS) Oxidation and Implications for Global Aerosol Radiative Forcing.” Atmospheric Chemistry and Physics 22, no. 2 (February 1, 2022): 1549–73. https://doi.org/10.5194/acp-22-1549-2022.
Hoffmann, Helene M., Mackenzie M. Grieman, Amy C. F. King, Jenna A. Epifanio, Kaden Martin, Diana Vladimirova, Helena V. Pryer, et al. “The ST22 Chronology for the Skytrain Ice Rise Ice Core – Part 1: A Stratigraphic Chronology of the Last 2000 Years.” Climate of the Past 18, no. 8 (August 10, 2022): 1831–47. https://doi.org/10.5194/cp-18-1831-2022.
Osterberg, Erich C., Michael J. Handley, Sharon B. Sneed, Paul A. Mayewski, and Karl J. Kreutz. “Continuous Ice Core Melter System with Discrete Sampling for Major Ion, Trace Element, and Stable Isotope Analyses.” Environmental Science & Technology 40, no. 10 (May 1, 2006): 3355–61. https://doi.org/10.1021/es052536w.
Citation: https://doi.org/10.5194/cp-2023-52-RC1 -
RC2: 'Comment on cp-2023-52', Holly Winton, 19 Sep 2023
Vance et al. present the chronology for a new ~300 m ice core recovered from Mount Brown South in East Antarctica filling a void in the spatial array of ice core records in the region. The manuscript describes the drilling, processing and analytical procedures and presents the age-depth model. The core was dated using a range of annually resolved chemical species and a number of volcanic tie points. I enjoyed reading about the group approach to annual layer counting and the use of two independent annual layer counts to derive the final age-depth model. The authors also describe the seasonality of fluoride in the core. Fluoride is rarely detected in Antarctic ice cores and in the atmosphere over the Southern Ocean and thus the sources and atmospheric processes of fluoride in the region are not well understood. Below are suggestions that I hope will improve the manuscript before publication in Climate of the Past.
Main comments
Ice chemistry analysis and figures of merit
The ICP-MS methods section reports figures of merit including LOD and reproducibility. Some LOD data are presented in Table 4. Please report accuracy of the ICP-MS measurements. Did you measure sulphur or aluminium via ICP-MS? As you have done for the ICP-MS, please report figures of merit for IC and CFA measurements including blank concentrations, accuracy, precision and also note the concentration range of calibration standards. How do sodium and calcium concentrations compare between ICP-MS, IC and CFA measurements?
Depth scale
As the focus of this manuscript is the age-depth scale of the MBS core, I encourage the authors to include the description of the scaling and shift factors. This would be useful for the community as discrepancies between field and lab depth scales and core breakage is not unique to the MBS core. Can you estimate a depth uncertainty of the master depth model?
Age uncertainty
An age uncertainty of ±2 years is reported in the conclusions. How was this derived? Please report in the abstract and main text.
Fluoride
The detection of fluoride and its seasonality is an interesting finding. Given fluoride has a different seasonality to the other markers, it is helpful to identify the annual layers and thus useful in this context. Yet, fluoride is largely unexplored in Antarctic ice cores and the modern atmosphere over the Southern Ocean so much so that we know little about the sources and photochemical processes in this unique and pristine environment and without this understanding, interpreting ice core fluoride is largely speculative. A study understanding the air-snow transfer of fluoride and the post-depositional processes along with exploring ice core fluoride relationships with a range of climate variables over the instrumental era would be incredibly valuable to further understand the potential as a sea ice proxy. Given fluoride is volatile, the first step is to understand how photochemistry between the atmosphere and surface snow impacts the archived fluoride signal. For example, over a decade has been dedicated to understanding these processes for ice core nitrate. Snice this information is currently lacking for ice core fluoride, I suggest focussing manuscript on the use of fluoride as an annual marker and moving the discussion on fluoride as a potential sea ice proxy to a separate manuscript dedicated to understanding fluoride deposition at the MBS south.
The detection of fluoride in MBS raises many questions. For example, what were the summer and winter concentrations of fluoride in the MSB core and how do they compare to Severi et al. (2014) and Morganti et al. (2007). Is the seasonality the same between the three studies? How does the seasonality of fluoride compare to nitrate, bromine and iodine which also undergo photochemical/ post-depositional processes? What information is known about fluoride from Southern Ocean aerosol studies? I understand, from personal communication, aerosol fluoride also exhibits a seasonal cycle at the Cape Grim Baseline Air Pollution Station.
Data availability
Note that the age-depth model not supplied and not yet available on the Australian Antarctic Data Centre.
Specific comments
L78 water isotopic ratios (here and throughout)
L105 and L120 how are the 3 cm resolution chemistry samples mentioned here different to the 3.5 cm chemistry samples mentioned in L107?
L105-117 how many samples per year result from this sampling resolution?
L124-125 how did you mitigate this? This organic contamination could be drill fluid contamination which has been observed in some ice core samples where drill fluid has contaminated the core through micro fractures and impacts the shoulder of the MSA peak.
L154 hydrogen peroxide
L167 sodium
L170 number of particles. Add reference.
L172 add resistivity of Milli-Q water.
L173 calibration standards
L177 manuscript uses both “mL” and “ml”
L180 CRDS
L185 delete ‘halogens’?
L185 how were these sub-sampled?
L195 ICP-MS tubing?
L205-206 reported in Table 4
L268-270 references required here to justify assignment of these peaks to 1 January.
L313 in the case of an uncertain counted year, where did you place the annual marker? e.g. on the nss-sulphate or water isotope peak?
L330 e.g., extreme precipitation events (Turner et al. 2019)
L423 which is the “prior study”?
L429 MSA is a proxy of sea ice in some regions of Antarctica.
Figure 1 A scale bar on panel b would be helpful to see the extent of the snow features. Add snow pit label to panel b.
Figure 3 Add dimensions
Figure 4 Y-axis label missing
Citation: https://doi.org/10.5194/cp-2023-52-RC2
Tessa R. Vance et al.
Tessa R. Vance et al.
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