Speciation of iodine (¹²⁷I and ¹²⁹I) in lake sediments

Fallout of anthropogenic ¹²⁹I at northern Europe has been occurring since the early 1950. Nevertheless, it is still unclear where and how this radioactive iodine is incorporated in the surface environment. In order to elucidate part of this problem, we here present an investigation of the occurrence modes of ¹²⁹I in varved lake sediments, collected in central Sweden and cover the period 1941-2007. Using a sequential extraction method, the results of ¹²⁹I and ¹²⁷I indicate that the partitioning of the two isotopes were more or less similar, despite the large gradient of ¹²⁹I within the sediment. Organic bound iodine was the dominant form over other fractions, while iodine bound to metal oxides was negligible. The leachable part constituted 5-6% of the iodine. Diagenetic influence seems to exert a limited effect on distribution of iodine in the examined sediment section.