Biogeochemical Cycle of Iodine-from the Earth\u27s Mantle to Human Thyroid

Iodine is a biophilic element and play an important role in thyroid function. Nearly 90% of iodine is concentrated in a small organ, thyroid gland (ca. 20 g), in human body. Iodine is essential not only for mammalian but also for other animals such as fishes and amphibian. However, it is not known when and how iodine was utilized through the evolution of organisms in the earth history. In the early history of the earth, iodine is thought to be released from the mantle into the atmosphere due to degassing and it was dissolved in seawater when the ocean was born. Iodine concentration in the primeval seawater should be much higher than the present level (we estimated about 100 times higher), because of the absence of the accumulation processes into the ocean sediments under anoxic condition. After the oxygen level increased due to the appearance of photosynthetic organisms (e.g. cyanobacteria), iodine accumulation in ocean sediments might be started. In this study we have carried out iodine analysis in different geochemical, biological and nutritional materials and also performed radiotracer experiments to understand its levels and behavior in the environment. We also studied the age and origin of iodine-rich brines (typically containing methane) in Chiba, Japan, where one third of world iodine is producing. Some important pathways and mechanisms to understand biogeochemical cycle of iodine are discussed in this paper.Based on our analytical data for iodine in many different materials, which make up the earth crust and are present in the environment, the inventory of iodine in the crust was calculated. Nearly 70% of the iodine is present in oceanic sediments, followed by continental sedimentary rocks (ca. 27%). These enrichments are expected to be related to the activities of marine organisms. Iodine concentrations in soil samples collected from different places ranged between 0.5 and 60 ppm. These values are much higher than those in their parent materials (mostly