the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Dust deposition fluxes at the gateway to the Southern Ocean: investigating the use of lithogenic tracer measurements in aerosols collected in Tasmania, Australia
Abstract. Australia contributes a significant amount of dust-borne nutrients (including iron) to the Southern Ocean, which can stimulate marine primary productivity. A quantitative assessment of the variability of dust fluxes from Australia to the surrounding ocean is therefore important for investigating the impact of atmospheric deposition on the Southern Ocean’s carbon cycle. In this study, lithogenic trace metals (aluminium, iron, thorium and titanium) contained in aerosols collected between 2016 and 2021 from kunanyi/Mount Wellington in lutruwita/Tasmania (Australia) were used to estimate dust deposition fluxes. Lithogenic fluxes were calculated using each tracer individually, as well as an average using all four tracers. This latter approach enabled an assessment of the uncertainty associated with flux calculations using only individual tracers. Elemental ratios confirmed the lithogenic nature of each tracer in aerosols when compared with both Australian soil samples and the average Earth’s upper continental crust. Determined lithogenic flux estimates were consistent with a regular dust deposition peak during the austral summer, in line with the dust storm season in the southeast of Australian, and a low atmospheric deposition in winter. This study provides an insight into the seasonal and interannual variability of dust deposition fluxes from the southeast of Australia based on aerosol sample measurements. This information will enhance our understanding of nutrient-bearing dust deposition to the Australian sector of the Southern Ocean and may prove useful in refining modelling estimates of southern hemisphere atmospheric deposition fluxes and their subsequent impact on global biogeochemical cycles.
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Status: open (until 05 Oct 2024)
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RC1: 'Comment on ar-2024-21', Zongbo Shi, 08 Sep 2024
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This is a solid paper. It proposed a multi-tracer method to estimate dust fluxes. It then estimated the dust flux at a strategic location in the Southern Ocean. The methodology is robust. The results are well presented and the conclusion is well justified.
I only have a few minor comments for consideration.
- Title – be more concise. Words like “investigating” are a waste of space
- Line 12-12: I suspected that you mean the flux estimated is between a peak dust deposition event and a low event. If so, the writing as it is does not represent this. Please clarify this and revise accordingly.
- Line 16-19: This is a bit wordy. The first part of sentence appears to repeat the previous sentence. The main point appears to be something like: the data provided here will help to constrain model estimates of ….
- Line 136: explain why 125 samples only for 6 years? E.g., give information on the sample duration and frequency.
- Line 196-197: how an aliquot be DRY sieved?
- I am sorry if I have missed but how the total mass of aerosols was estimated? This is important to mention as it determines the accuracy of the Fe/total aerosol mass ratio. It should be noted that there may well be sea salt and other natural/anthropogenic aerosols. This could reduce the total Fe content in the total aerosol. Similar applies to Al. This may partially explain the low mean Al/Fe contents in aerosols. It would be great if a mass closure (e.g., sulfate, nitrate, sea salt, OC, EC, dust etc.) is given if such data are available. This comment is also relevant for points raised in the paragraph starting line 286.
- Table S2 – total Fe content in soil appears to be very low. Yes, there may be spatial variabilities. But could there also be a possibility of the size dependence? The size cut here is about 63 um. And in reality, you are unlikely going to see many particles of that size at the sampling location due to long range transport (not to say that it is impossible). I suggest that the authors look at literature and see how other studies have estimated the total Fe content, both in terms of the size cut of the particles, and methodology. Secondly, can you show all other elements you measure for all soil samples. They are very useful reference data for future research.
- Line 266: this is an interesting point. Later sentences supported this argument. Are there representative back trajectories that you can show to support this point? It would be good to have the back trajectories from high and low dust flux seasons.
- Line 316 – spelling error
- Figure 3 – please mention briefly what ratios are being used? UCC or Australian soil results?
- In Figure 1, would it be appropriate to consider add the locations by Strzelec et al. (and any other studies) where the dust flux was estimated?
- There are mentions of fire and related dust. This is an interesting point but I do wonder whether you can provide any supporting evidence, such as higher K+ concentrations. I presume you haven’t analysed levoglocosan?
- Paragraph starting 397: I wonder whether you can compare the estimated fluxes with more modelling studies. I think there are several global modelling studies of dust deposition fluxes. For example, Mahowald et al. 2005. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2004GB002402
- Uncertainties: I agree that the multi-tracer method is more reasonable and better than a single tracer estimate. However there are still uncertainties and I suggest that you add a paragraph or section to discuss specifically about all the possible uncertainties, e.g., ratios, deposition velocities. It does not affect the conclusion of this paper but it will help readers to understand the manuscript better, and to use it more appropriately. If you can give an estimate of the uncertainty range, e.g., 2 times, that would be helpful. But I suspect it is not going to be an easy job. It may worth mentioning that models still have large uncertainties so the uncertainties from observation-based flux estimates are still relatively small.
- Conclusions – this is rather long. Most of the points in the conclusions have already been mentioned in abstract. I wonder whether the final section as “Atmospheric implications” might be more useful to readers. Can you tell us a bit more about the implications of the results reported here? You mentioned nutrient inputs – are the inputs, in different seasons, likely to be important for ocean plankton and biological pump?
- Can you also have a short paragraph, perhaps at the end of the “atmospheric implications” if you decide to have one, about what future research should be done? You did mention somewhere in the text about the research needs – but they could be at one place of the manuscript.
Citation: https://doi.org/10.5194/ar-2024-21-RC1
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