The marine iodine cycle has significant impacts on air quality and atmospheric chemistry. Specifically, the reaction of iodide with ozone in the top few micrometres of the surface ocean is an important sink for tropospheric ozone (a pollutant gas) and the dominant source of reactive iodine to the atmosphere. Sea surface iodide parameterisations are now being implemented in air quality models, but these are currently a major source of uncertainty. Relatively little observational data is available to estimate the global surface iodide concentrations, and this data has not hitherto been openly available in a collated, digital form. Here we present all available sea surface (<20 m depth) iodide observations. The dataset includes values digitised from published manuscripts, published and unpublished data supplied directly by the originators, and data obtained from repositories. It contains 1342 data points, and spans latitudes from 70°S to 68°N, representing all major basins. The data may be used to model sea surface iodide concentrations or as a reference for future observations.

Oxygen concentrations [O 2 ] in the oceans have been decreasing and oxygen minimum zones (OMZ) expanding since 1960 (Schmidtko et al., 2017). Climate models also show this decrease and predict that this trend will not only continue but accelerate in the future (Oschlies et al., 2008(Oschlies et al., , 2018. To better understand the longer-term oxygen cycle in the past, on time scales typically exceeding centuries to millennia, we can use proxy reconstructions (Moffitt et al., 2015). Most proxy reconstructions assess in-situ bottom water [O 2 ] and rely on sedimentary samples (e.g.,

&lt;p&gt;The Pacific Ocean hosts one of the most extensive areas of oxygen deficient waters at present with well-defined areas of oxygen minima existing both north and south of the equator along the eastern basin. This deficiency in oceanic O&lt;sub&gt;2&lt;/sub&gt; concentrations is mainly due to a combination of upwelling induced high primary productivity and poorly ventilated intermediate waters. Across the Miocene-Pliocene the Pacific Ocean is thought to have been distinctly different with an elevated water column temperature profile, reduced Walker circulation, active deep-water formation in the north Pacific, high primary productivity, and differences in its fundamental configuration with gateway changes occurring at the eastern and western margins. Collectively, and individually, these different factors will have had implications on Pacific Ocean O&lt;sub&gt;2&lt;/sub&gt; distribution. To better understand the past oxygenation of Pacific waters amidst this backdrop of climatic and geographical changes we reconstruct iodine/calcium ratios from planktic foraminifera across multiple Pacific Ocean sites. Our I/Ca records extending from the mid-late Miocene through to Pleistocene show the progressive reduction in oceanic O&lt;sub&gt;2&lt;/sub&gt; content across the Pacific. We place these records in the context of changes in the Central American Seaway and the resultant changes in oceanic circulation.&lt;/p&gt;

&lt;p&gt;Current climatic trends are expected to lead to expansion of oxygen minimum zones and an overall decrease in oxygen concentration [O&lt;sub&gt;2&lt;/sub&gt;] in the oceans. In order to improve predictions of future trends we need to create a better understanding of the natural oxygen cycle. The iodine to calcium ratio (I/Ca) of planktonic foraminifera is an increasingly popular proxy to assess upper water column oxygenation. Recent studies suggest that this ratio is mainly controlled by subsurface water dissolved oxygen concentrations. A thorough assessment of the proxy has been carried out for the South Atlantic, but is currently lacking for the Indian and Pacific Oceans, which contain the worlds&amp;#8217; most intense and large oxygen minimum zones. Here we present results of recent (Holocene) planktonic foraminifera (mixed layer and deep dwelling species) I/Ca measurements across a range of oceanographic conditions ([O&lt;sub&gt;2&lt;/sub&gt;] varies between &lt; 10 &amp;#181;mol/kg to &gt; 200 &amp;#181;mol/kg) from the Indian and Pacific Ocean to further refine the proxy, using sample material provided by Lamont-Doherty Core Repository.&lt;/p&gt;

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.