Overview of oceanic iodine cycle
Topic outline
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The bio-geochemical cycle of carbon is one of the most important research theme. This is because carbon dioxide in the atmosphere has a greenhouse effect. The history of the terrestrial carbon cycle is also the history of climate change. It shows where and how much carbon is stored, as well as where it is moving and how fast it is doing so. The carbon in fossils lying dormant in the crust is released into the atmosphere as carbon dioxide, and part of it is stored in the ocean, used by living things, and returned to the atmosphere again. This kind of material circulation influenced by biological activity is called the bio-geochemical cycle.
A bio-geochemical cycle can be conceived for all elements other than carbon. The nitrogen cycle, phosphorus cycle, sulfur cycle…among these is the iodine cycle. In bio-geochemical cycle research, if the carbon cycle occupies the top rung, followed by nitrogen, phosphorus, and sulfur. The iodine cycle would follow (this is my personal opinion) .
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Most terrestrial iodine is in the crust. Iodine in igneous rocks is thought to have been fixed in the crust since ancient times. It is highly concentrated in the saltpeter mined in the Atacama Desert in Chile, South America. It is also abundant in marine sediments and in sedimentary rocks that were isolated in the crust. These comprise over 99% of the iodine on earth. However, the iodine in the crust is almost entirely fixed and immobile (it only dissolves through weathering when coming into contact with river water or groundwater, and it is only slightly released as volcanic gas).
About 99% of the mobile iodine on earth exists as inorganic iodine in seawater. The largest movement of iodine is due to the power of living organisms (marine plants). Around 12×1012 g of iodine is consumed by marine plants annually, and about the same amount returns to seawater. Around 5×1011 g of iodine moves annually between the ocean and the atmosphere. This will be described later, but the release of iodine from the ocean to the atmosphere is also thought to involve marine plants. Marine plants are said to drive the iodine cycle of the earth in this way.
Fuge and Jhonson (1980) The geochemistry of iodine -a review, The environmental geochemistry and health, 8(2), 31-54.
The inflows and outflows should normally be balanced when describing the material cycle. However, the above figure is not balanced. This is probably due to the uncertainty in the estimation of the iodine cycle.
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The illustration below shows an image of the iodine cycle centered on the ocean and atmosphere. Most of the mobile iodine on the surface of the earth exists as inorganic iodine in seawater. Oxygen is abundant in seawater (i.e., it is oxidative), so inorganic iodine exists stably as an iodate (IO3-) ion. Iodate ions are reduced to iodide ions (I-) due to the activity of marine plants. They are useful for enzymatic reactions and bactericidal actions in living organisms as an organic iodine compound. Only a small portion of the organic iodine used in living organisms has a low molecular weight and is volatile (volatile = gas). In other words, the ions are converted into organic iodine gas. If the seawater containing the organic iodine gas is in contact with the atmosphere, the organic iodine gas will be released into the atmosphere. In addition to being released as organic iodine gas, some amount is thought to be released as iodine molecules (I2) from the ocean surface. When organic iodine and iodine molecules are released into the atmosphere, they rapidly photolyze and release iodine atoms into the atmosphere. The iodine atom plays a role in catalytically destroying ozone in the troposphere. Iodine atoms destroy the ozone and are eventually deposited on the earth's surface with rain and return to the sea. Some amounts of iodide ion do not return to the sea and are fixed in the crust. Some marine plants containing iodine are additionally deposited on the seabed and buried in the ground.
This is an overview of the iodine cycle on the earth's surface (over a relatively short period). The illustration below shows a picture of organic iodine (org-I) moving at the boundary between marine sediments and seawater and between the ocean and atmosphere. Organic iodine and organic iodine gas are thought to be the important iodine carriers. Marine plants produce organic iodine. In other words, marine plants are thought to drive the terrestrial iodine cycle. The details will be provided in the next course.
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