Iodide in the ocean:distribution and impact on iodine flux and ozone loss

Over the last five years, research led by this team has made it apparent that the reaction of iodide with ozone at the sea surface plays an important role in controlling the chemical composition of the troposphere. This process directly controls the deposition of O3 to the oceans and is the dominant source of reactive iodine to the atmosphere, which leads to significant loss of tropospheric O3. Ozone concentrations are directly impacted but through changes to the atmospheric oxidants, indirect changes also occur to methane and aerosols leading to potential ramifications on climate, air quality and food security. This is likely a biogeochemical negative feedback for tropospheric O3 and oxidants, which, since it is dependent on both atmospheric O3 and ocean iodide concentrations, will have changed over time. Iodine is also an essential human nutrient. The transport of iodine from the oceans to the atmosphere and subsequent deposition over land is a pathway by which marine iodine may enter the terrestrial food chain, and iodine radioisotopes released to the sea may be dispersed. These iodine ocean-atmosphere processes are now being incorporated into chemical transport models but critical uncertainties remain. The marine iodide distribution is poorly understood, yet it is likely that it will be subject to change as a result of changes in ocean circulation, biological productivity and ocean deoxygenation.

This proposal brings together marine and atmospheric scientists in order to address uncertainties in the marine iodine flux and associated ozone sink. Specifically, it aims to quantify the dominant controls on the sea surface iodide distribution and improve parameterisation of the sea-to-air iodine flux and of ozone deposition. This will be achieved through a combination of laboratory experiments, field measurements and ocean and atmospheric modelling.

We aim to impact upon five distinct areas:

A. Air Quality and Climate
Beneficiaries: Governmental research agencies e.g. the US EPA; international coordinating agencies e.g. WMO.
Iodine impacts air quality via both the direct contribution of iodide to ozone loss at the sea surface, and through atmospheric iodine chemistry destroying tropospheric ozone. In the UK, air pollution reduces lifespan by an average of 8-14 months. Ozone is also responsible for reductions in crop yields and tree growth. We will facilitate the inclusion of iodine in air quality and Earth system models by providing improved iodine flux parameterisations and iodide fields, including making available a database of global seawater iodide measurements. Understanding these fundamental processes is required to provide appropriate policy advice regarding both air quality and climate.

B. Radionuclide dispersion
Beneficiaries: Environmental consultants and applied research centres e.g. Centre for Environment, Fisheries and Aquaculture Science; government bodies and regulators e.g. Food Standards Agency; industry e.g. Sellafield Ltd.
Iodine radionuclides (129-I, 131-I) are released to the marine environment by the nuclear industry both intentionally and accidentally. Their transport and fate is dependent on their speciation, and they will undergo volatilisation in the same manner as stable iodine species. Improved understanding of these processes is thus relevant to pathway assessment of radioiodine exposure. We will discuss the inclusion of radioiodine volatilisation and speciation changes in exposure assessment tools with relevant research bodies and environmental consultants. Through inclusion in such tools, our results will impact on regulatory bodies and industry.

C. Nutritional iodine deficiency
Beneficiaries: Public Health professionals
Volatilisation followed by atmospheric transport and deposition is a major pathway by which marine iodine may enter the terrestrial food chain. Iodine is an essential human nutrient, deficiency of which is the leading preventable cause of brain damage, and so better understanding of the geochemical controls on iodine levels in food is of interest to nutritionists and epidemiologists. We will host a workshop, leading to a publication on the global iodine cycle, as a contribution to evaluating the role of environmental determinants on iodine deficiency. Deposition flux maps (directly informed by the project deliverables), will be made available for uses such as interpreting historical causes of regional iodine deficiency, developing sampling strategies for epidemiological studies and assessing the impact of changing agricultural practices on iodine levels in food.

D. Ocean deoxygenation
Beneficiaries: Marine Management professionals
The improved constraints on marine iodine cycling achieved in our work will inform the development and evaluation of the carbonate I/Ca ratio as a proxy for past ocean deoxygenation, via direct collaboration with our collaborator Prof. Andy Ridgwell. Better understanding of past ocean deoxygenation will in turn improve our ability to predict the effects of current and future ocean dexoygenation on marine life, including economically important fisheries, ecosystem services, and the biogeochemical production of greenhouse gases (N2O).

E. Public engagement and education
Beneficiaries: The general public; school children in UK and India.
Our subject matter provides an interesting, accessible example of the interactions between biological, chemical, physical and human aspects of the Earth system. We will incorporate it in a set of visualization tools and demonstrations for use in public engagement activities (e.g. Royal Society Summer Exhibition, British Science Festival, school visits). Selected material will be used for engagement with the public and schools in India through collaboration with our project partner Dr Anoop Mahajan.