Investigation of Near-Surface Production of Iodocarbons - Rates and Exchange (INSPIRE)
Lead Research Organisation:
Plymouth Marine Laboratory
Department Name: Plymouth Marine Lab
Abstract
Iodine is a vital element. For example, human growth hormones contain iodine so it is an essential part of our diet and a deficiency in this will severely affect how we grow and our brains develop. Also, in the atmosphere iodine is involved in chemical processes that contribute to climate change such as ozone depletion and cloud formation. The persistence of life on earth depends upon the constant recycling of essential elements that occurs via the major biogeochemical cycles that are maintained by the activities of microorganisms. It is crucial that we know how iodine is cycled between the oceans, air and land. The oceans contain a large proportion of the total iodine on the planet and the transfer of this element from seawater to the atmosphere is known to be an important part of its global cycle. One group of compounds that play a major role in transferring iodine across the sea surface are the volatile iodocarbons, this includes CH3I, CH2I2 and CH2ClI. Although we can measure the iodocarbons in seawater we do not fully understand what controls their concentrations in seawater and crucial parts of the jigsaw are still to be discovered. INSPIRE will compare iodocarbon concentration distributions with measurements that indicate biological, chemical and photochemical processes, and carry out experiments in the laboratory and during two research cruises to work out what the main controls on the concentrations of these compounds are. For example, the project will examine how the exposure of water samples to sunlight, zooplankton grazing of phytoplankton, plankton death and decay and bacterial growth influences the concentrations of the iodocarbons we measure in seawater. Once we have identified this we will then produce a mathematical model to simulate iodocarbon production to allow us to predict how much iodine is transferred from the oceans to the atmosphere. This will help us to understand how the iodine biogeochemical cycle operates much better and how it might alter with future climatic change.
Organisations
Publications
Dixon J
(2011)
Rapid biological oxidation of methanol in the tropical Atlantic: significance as a microbial carbon source
in Biogeosciences
Dixon J
(2012)
Fine-scale variability in methanol uptake and oxidation: from the microlayer to 1000 m
in Biogeosciences
Smirnov A.
(2009)
Maritime Aerosol Network as a component of Aerosol Robotic Network
in JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES
Smyth T
(2011)
Penetration of UV irradiance into the global ocean
in Journal of Geophysical Research: Oceans
Turk KA
(2011)
Nitrogen fixation and nitrogenase (nifH) expression in tropical waters of the eastern North Atlantic.
in The ISME journal
| Description | Iodine is a vital element. For example, in humans iodine deficiency can severely affect how we grow and our brains develop. Iodine is also important in the atmosphere where it is involved in processes that contribute to climate change e.g. ozone depletion and cloud formation. Life on earth depends upon the constant recycling of essential elements through the major biogeochemical cycles that are maintained by the microbial activity. The oceans contain a large proportion of Earth's iodine and it is crucial that we know how iodine is cycled from the sea to air and land surfaces. The volatile iodocarbons, including CH3I, CH2I2 and CH2ClI, play a major role in transferring iodine across the sea surface. It has been possible to quantify iodocarbons in seawater for some time but we do not fully understand what controls their concentrations in seawater - crucial parts of the jigsaw are still to be discovered. During the INSPIRE project we did laboratory experiments, a field campaign, and modelling with the aim of better understanding the sources and sinks of volatile iodocarbons in seawater. Whilst our preliminary cruise was cancelled, the main INSPIRE D325 research cruise went ahead in late 2007. We worked in the Cape Verde region in the tropical North Atlantic Ocean, a windy region downwind of Mauritanian upwelling, where high dust input from Africa delivers nutrients and enhances marine primary production. We chose this area because the tropics are highly significant for ozone production and loss processes that are affected by volatile iodine compounds. Our earlier work there showed high seawater CH3I concentrations and suggested this area as a strong iodocarbon source region despite relatively low nutrient and primary production levels. We worked at 6 sites chosen by studying pictures depicting seawater chlorophyll concentrations observed by satellites in space. These daily 'snapshots' were important because they allowed us to sample waters with contrasting levels of primary production. Seawater collected at each site was analysed immediately to determine iodocarbon concentration distributions and used for a range of incubation experiments to assess how iodocarbon concentrations vary with indicators of biological, chemical and photochemical processes. Key results from the cruise, and associated INSPIRE laboratory and modelling studies include the following: 1) In contrast with 2 studies published in the 1990's we found little photochemical production of CH3I; 2) Cultures of Prochlorococcus marinus, a key primary producer in nutrient poor seawaters, produced more CH3I under stress conditions and calculations show this species could contribute significantly to CH3I production in the tropical Atlantic Ocean; 3) ERSEM modelling supported the idea that phytoplankton stress affects CH3I production and highlight that bacteria may play a significant role in total CH3I production; 4) Laboratory and shipboard studies revealed a novel, and potentially ubiquitous, sea-surface source of compounds CH2I2, CHClI2 and CHI3,, due to production via reaction of dissolved organics with hypoiodous acid/molecular iodine, formed when ozone reacts with iodide at the sea surface; 5) a novel radiocarbon technique revealed that bacterial CH3I oxidation rates but these were very much lower than observed for other biogenic gases (eg DMS) suggesting low biological removal relative to removal by air-sea flux and nucleophillic substitution; 6) novel isotopic enrichment experiments showed us that photochemical loss of CH2I2 and subsequent production of CH2ClI were in line with rates seen in laboratory experiments; 7) rates of iodocarbon formation were measured in plankton detritus samples but they were not high enough to sustain the seawater concentrations observed. |
| Sectors | Communities and Social Services/Policy,Environment |