Marine iodine emissions in a changing world

Iodine is a critical trace element involved in many diverse and important processes in the Earth system. The importance of iodine for human health has been known for over a century, with low iodine in the diet being linked to goitre, cretinism and neonatal death. Research over the last few decades has shown that iodine has significant impacts on tropospheric photochemistry, ultimately impacting climate by reducing the radiative forcing of ozone (O3) and air quality by reducing extreme O3 concentrations in polluted regions. Iodine is naturally present in the ocean, predominantly as aqueous iodide and iodate. The rapid reaction of sea-surface iodide with O3 is believed to be the largest single source of gaseous iodine to the atmosphere. Due to increased anthropogenic O3, this release of iodine is believed to have increased dramatically over the twentieth century, by as much as a factor of 3. Uncertainties in the marine iodine distribution and global cycle are, however, major constraints in the effective prediction of how the emissions of iodine and its biogeochemical cycle may change in the future or have changed in the past. Here, we present a synthesis of recent results by our team and others which bring a fresh perspective to understanding the global iodine biogeochemical cycle. In particular, we suggest that future climate-induced oceanographic changes could result in a significant change in aqueous iodide concentrations in the surface ocean, with implications for atmospheric air quality and climate.

1. Iodine cycling at the ocean-atmosphere interface. I was confused by the description of how iodine and ozone interact to produce IO radicals (Section 1). On lines 63-64 and in Fig. 1, the authors state that the reaction with ozone is a three-step process; iodine compounds are emitted from seawater, are then photolyzed to iodine atoms, and only then react with tropospheric ozone. However, in a number of other places in the manuscript (e.g., lines 137ff., Figure 3), the authors note that tropospheric ozone can invade surface seawater, where it reacts with iodide in situ. (This process is alluded to in Fig. 1, but the reactions are not shown.) So I am left wondering, is iodine 'getting out' of the ocean, or is ozone 'getting in?' Is it both? If so, is there any literature describing which pathway is more important? And , he two pathways have different sensitivities that might be important to consider under the future global change scenarios described in Section 5?
2. Ocean iodine cycling model. The authors describe the recent iodine cycling model of Wadley et al. (2020, ref. [51]) as being the first of its kind. However, this statement is completely accurate as there is an already a global implementation of the iodine cycle in the cGENIE Earth system model described by Lu et al. (2018;doi:10.1126/science.aar5372). Interestingly, Lu's model indicates that the iodate [O<sub>2</sub>] reduction threshold is 30 μmol/kg and that iodide has a lifetime of ~50 years. It would be worthwhile to compare Lu's model results against the other approaches listed in Table 1, and to discuss the key differences between the parameterizations used by Wadley et al. (2020) and Lu et al. (2018).

Deep ocean iodide.
In general I felt there was not much discussion of the marine iodine cycle outside of those transformations occurring in the upper ocean. Indeed, the model described by Wadley et al. (2020) seems to prescribe a deep ocean iodide concentration of 0 nmol/kg. However, deep-ocean iodine sources, such as sediments, can release considerable quantities of iodide into the upper mesopelagic (e.g., Kennedy & Elderfield, 1987;doi:10.1016/0016-7037(87)90301-2;Cutter et al., 2018, ref. [66]). Iodide emanating from sedimentary sources may even outcrop at the ocean surface (e.g., in the Bay of Bengal; Chance et al.,ref. [46]). Given the relatively long residence time of iodide in the mixed layer, I am curious as to whether this iodide source to the surface is significant-does it rival in situ reduction by phytoplankton or is it a mere curiosity?

15-Dec-2020
Dear Professor Carpenter, On behalf of the Editor, I am pleased to inform you that your Manuscript RSPA-2020-0824 entitled "Marine iodine emissions in a changing world" has been accepted for publication subject to minor revisions in Proceedings A. Please find the referees' comments below.
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We welcome submissions of images for possible use on the cover of Proceedings A. Images should be square in dimension and please ensure that you obtain all relevant copyright permissions before submitting the image to us. If you would like to submit an image for consideration please send your image to proceedingsa@royalsociety.org Once again, thank you for submitting your manuscript to Proceedings A and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. Comments to the Author(s) Carpenter and co-authors present a review of recent findings in surficial iodine chemistry. Such a review is timely and of will be of broad interest to readers in atmospheric sciences, chemical and biological oceanography, and Earth system modeling. The manuscript is well-written, has clear figures, and was a fascinating read. I recommend publication in Proceedings A, though would ask the authors to first consider a number of points of clarification, detailed below.

Best wishes
1. Iodine cycling at the ocean-atmosphere interface. I was confused by the description of how iodine and ozone interact to produce IO radicals (Section 1). On lines 63-64 and in Fig. 1, the authors state that the reaction with ozone is a threestep process; iodine compounds are emitted from seawater, are then photolyzed to iodine atoms, and only then react with tropospheric ozone. However, in a number of other places in the manuscript (e.g., lines 137ff., Figure 3), the authors note that tropospheric ozone can invade surface seawater, where it reacts with iodide <em>in situ</em>. (This process is alluded to in Fig. 1, but the reactions are not shown.) So I am left wondering, is iodine 'getting out' of the ocean, or is ozone 'getting in?' Is it both? If so, is there any literature describing which pathway is more important? And , he two pathways have different sensitivities that might be important to consider under the future global change scenarios described in Section 5? 2. Ocean iodine cycling model. The authors describe the recent iodine cycling model of Wadley et al. (2020, ref. [51]) as being the first of its kind. However, this statement is completely accurate as there is an already a global implementation of the iodine cycle in the cGENIE Earth system model described by Lu et al. (2018; doi:10.1126/science.aar5372). Interestingly, Lu's model indicates that the iodate [O<sub>2</sub>] reduction threshold is 30 μmol/kg and that iodide has a lifetime of ~50 years. It would be worthwhile to compare Lu's model results against the other approaches listed in Table 1, and to discuss the key differences between the parameterizations used by Wadley et al. (2020) and Lu et al. (2018).

Deep ocean iodide.
In general I felt there was not much discussion of the marine iodine cycle outside of those transformations occurring in the upper ocean. Indeed, the model described by Wadley et al. (2020) seems to prescribe a deep ocean iodide concentration of 0 nmol/kg. However, deep-ocean iodine sources, such as sediments, can release considerable quantities of iodide into the upper mesopelagic (e.g., Kennedy &amp; Elderfield, 1987;doi:10.1016/0016-7037(87)90301-2;Cutter et al., 2018, ref. [66]). Iodide emanating from sedimentary sources may even outcrop at the ocean surface (e.g., in the Bay of Bengal; Chance et al.,ref. [46]). Given the relatively long residence time of iodide in the mixed layer, I am curious as to whether this iodide source to the surface is significant-does it rival <em>in situ</em> reduction by phytoplankton or is it a mere curiosity?
• Lines 77-80: Authors state that iodinated organic compounds are not major sources of atmospheric iodine. However, in Fig. 1, these sources are shown to comprise >40 % of the oceanic outward fluxes. This mismatch requires clarification.
• Lines 215-217: This statement needs revising in light of the recent work by Cutter et al. (2018). There are significant accumulations of iodide present below the mixed layer, even relatively far from the continental margin. • Line 263-266: Our team recently measured iodate reduction in the Eastern Tropical North Pacific OMZ using the radiotracer-incubation method and confirmed that the rates are indeed slow (see Hardisty et al., in press;doi:10.1016/j.epsl.2020).
• Lines 311-313: For comparison with the residence times of iodide in the sea surface, could the authors describe iodine residence times in the troposphere and stratosphere? Decision letter (RSPA-2020-0824.R1)

28-Jan-2021
Dear Professor Carpenter I am pleased to inform you that your manuscript entitled "Marine iodine emissions in a changing world" has been accepted in its final form for publication in Proceedings A.
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