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Planktic foraminifera test iodine to calcium ratios represent an emerging proxy method to assess subsurface seawater oxygenation states. Several core-top studies show lower planktic foraminifera I/Ca in locations with oxygen depleted subsurface waters compared to well oxygenated environments. The reasoning behind this trend is that only the oxidized species of iodine, iodate, is incorporated in foraminiferal calcite. The I/Ca of foraminiferal calcite is thought to reflect iodate contents in seawater. To test this hypothesis, we compare planktic foraminifera I/Ca ratios, obtained from plankton tows, with published and new seawater iodate concentrations from 1) the Eastern North Pacific with extensive oxygen depletion, 2) the Benguela Current System with moderately depleted oxygen concentrations, and 3) the well oxygenated North and South Atlantic. We find the lowest I/Ca ratios (0.07 µmol/mol) in planktic foraminifera retrieved from the Eastern North Pacific, and higher values for samples (up to 0.72 µmol/mol) obtained from the Benguela Current System and North and South Atlantic. The I/Ca ratios of plankton tow foraminifera from environments with well oxygenated subsurface waters, however, are an order of magnitude lower compared to core-tops from similarly well-oxygenated regions. This would suggest that planktic foraminifera gain iodine post-mortem, either when sinking through the water column, or during burial. 

Abstract. A fast-response (10 Hz) chemiluminescence detector forozone (O3) was used to determine O3 fluxes using the eddy covariancetechnique at the Penlee Point Atmospheric Observatory (PPAO) on the southcoast of the UK during April and May 2018. The median O3 flux was −0.132 mg m−2 h−1 (0.018 ppbv m s−1),corresponding to a deposition velocity of 0.037 cm s−1(interquartile range 0.017–0.065 cm s−1) – similar to thehigher values previously reported for open-ocean flux measurements but notas high as some other coastal results. We demonstrate that a typical singleflux observation was above the 2σ limit of detection but hadconsiderable uncertainty. The median 2σ uncertainty of depositionvelocity was 0.031 cm s−1 for each 20 min period, whichreduces with the square root of the sample size. Eddy covariance footprintanalysis of the site indicates that the flux footprint was predominantlyover water (> 96 %), varying with atmospheric stability and, toa lesser extent, with the tide. At very low wind speeds when the atmospherewas typically unstable, the observed ozone deposition velocity was elevated,most likely because the footprint contracted to include a greater landcontribution in these conditions. At moderate to high wind speeds whenatmospheric stability was near-neutral, the ozone deposition velocityincreased with wind speed and showed a linear dependence with frictionvelocity. This observed dependence on friction velocity (and therefore alsowind speed) is consistent with the predictions from the one-layer model ofFairall et al. (2007), which parameterisesthe oceanic deposition of ozone from the fundamental conservation equation,accounting for both ocean turbulence and near-surface chemical destruction,while assuming that chemical O3 destruction by iodide is distributed overdepth. In contrast to our observations, the deposition velocity predicted bythe recently developed two-layer model of Luhar et al. (2018) (whichconsiders iodide reactivity in both layers but with molecular diffusivitydominating over turbulent diffusivity in the first layer) shows no majordependence of deposition velocity on wind speed and underestimates themeasured deposition velocities. These results call for further investigationinto the mechanisms and control of oceanic O3 deposition. 

We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O₃) concentrations, except in polar regions where the model now underestimates O₃ concentrations. Halogen chemistry reduces the global tropospheric O₃ burden by 18.6 %, with the O₃ lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28  ×  10⁶ molecules cm⁻³ are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH₄ lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH₄ by Cl is small (∼  2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼  15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde.