BG-2021-87 Review

The manuscript entitled “Subsurface iron accumulation and rapid aluminium removal in the Mediterranean following African dust deposition” by Bressac et al. is an interesting and well-written paper teasing apart the cycling of Fe and Al in the Mediterranean Sea and challenging the use of Al as a tracer of dust inputs to this region. Using a quick response dissolved and particulate water sampling regime, the authors were able to take high resolution time-series (hours to days) observations of two wet-deposition events while at sea and thus capture previously unobserved mechanisms controlling the cycling and removal of Fe and Al. This high-resolution sampling revealed that dissolved Fe increased in the surface layer during and at around 6h after deposition but was quickly scavenged to background concentrations, which is attributable to the saturation of Fe binding ligands in the region. Using Fe/Al concentration ratios in suspended and sinking particles, the authors were further able to show that phytoplankton, especially diatoms, actively accumulate Al. However, comparing Al/Si ratio of particles with published values for diatoms, the authors demonstrate that adsorption of Al onto biogenic particles, rather than active uptake by diatoms is the main sink for dissolved Al in the region.


General comments
The manuscript entitled "Subsurface iron accumulation and rapid aluminium removal in the Mediterranean following African dust deposition" by Bressac et al. is an interesting and well-written paper teasing apart the cycling of Fe and Al in the Mediterranean Sea and challenging the use of Al as a tracer of dust inputs to this region. Using a quick response dissolved and particulate water sampling regime, the authors were able to take high resolution time-series (hours to days) observations of two wet-deposition events while at sea and thus capture previously unobserved mechanisms controlling the cycling and removal of Fe and Al. This high-resolution sampling revealed that dissolved Fe increased in the surface layer during and at around 6h after deposition but was quickly scavenged to background concentrations, which is attributable to the saturation of Fe binding ligands in the region. Using Fe/Al concentration ratios in suspended and sinking particles, the authors were further able to show that phytoplankton, especially diatoms, actively accumulate Al. However, comparing Al/Si ratio of particles with published values for diatoms, the authors demonstrate that adsorption of Al onto biogenic particles, rather than active uptake by diatoms is the main sink for dissolved Al in the region.
As a climate change hotspot, understanding current biogeochemical cycling mechanisms in the Mediterranean is important for ensuring the accuracy of modelled climate change impacts in this region. The mechanisms discussed in this study will be important for both optimisation of regional climate models and for future biogeochemical studies focussing on nutrient cycling in the Mediterranean. In my opinion, the scientific method utilised in this study is robust and the paper seems quite polished. I have few suggestions on the scientific discussion itself and have mainly suggested minor technical corrections. As such, this manuscript will be well suited for publication in Biogeosciences after minor revisions.

Specific comments
Section 3: You refer to Supp. Fig. 3 quite regularly in the results section, and it is quite an interesting figure. It might be worth moving this figure into the text.
Can you comment at all on what impact future changing climate conditions might have on Mediterranean Fe/Al cycling in the context of your results? It seems that there is uncertainty as to whether dust deposition events are predicted to increase or decrease in the region based on observed trends over the last few decades (e.g. https://doi.org/10.1016/j.atmosenv.2020.117736), but though it may seem speculative I think it could still be valuable to mention the impacts that future changes may have on Fe/Al cycling based on your results. Lines 203 -208: this sentence is too long and needs to be broken up to make it easier to read. Suggest adding full stop after '10 th of May' on line 205, removing the next 'with' and then adding 'was observed ' after '…Dulac et al., 1992)' on line 207.
Line 210: change 'was' to 'were' Line 221: replace 'that day' with '11 th of May' and remove from end of sentence.
Line 222: does '…1.5 g m-2 over 6 h, or more in the area of…' mean that there was more than 1.5 g m-2 forecasted at just stations 5, TYR and 6, or more than 1.5 g m-2 forecasted over the whole area? If it's the latter, I would replace 'with up to' with 'with at least', and remove 'or more'.
Line 238: 'R/V on the 5 th …' Line 239: 'clear dust signature' isn't technically wrong, and readers of this paper wouldn't think you meant that the dust was clear, but it's a little ambiguous, so I would recommend changing to '… by a dust signature clearly revealed by the chemical composition of the rain,…' Line 246: 'as do all' Line 279: 'This is an order-of-magnitude…' Line 280: replace 'with' with 'compared to' and 'indicates' with 'indicating' Line 472: This sentence needs a little restructuring. I suggest: '…central Mediterranean, we observed two atmospheric wet deposition events while measuring Al and Fe watercolumn distributions, providing…' Line 474: This sentence could also do with a slight tweak. I suggest: 'The water-column Al inventories were successfully utilised to assess deposition fluxes, complementing atmospheric…' Line 481: change 'and extended until' to 'extending to' and 'excess to dFe' to 'excess of dFe'