The natural 129I system has received considerable attention recently because it covers an age range of great interest to geologic applications. Examples of recent applications are the dating of pore waters associated with gas hydrates, the tracing of marine sediments in subduction zones and the dating of source material in hydrocarbon reservoirs. Crucial for the application of the 129I system is the pre-anthropogenic input ratio in the marine system. This ratio has been originally determined in marine sediments sampled off the coast of the Carolinas in the Atlantic Ocean (Fehn et al., 1986), and later in a series of samples from other marine locations (Moran et al., 1998). The determinations carried out in these two papers resulted in an input ratio for pre-anthropogenic 129I of Ri = (1500±150)x10-15, which has been used in most of the investigations based on pre-anthropogenic 129I. Because of the importance of this ratio for the application of the 129I system, we carried out a number of measurements on algae samples collected before the onset of the nuclear age and reviewed the literature for existing data on this ratio. Most of the data support the ratio presented in Moran et al. (1998), but there are also some which are considerably lower than 1500x10-15. Because of the close association of iodine with organic matter, these lower ratios might reflect the input from sources with old organic matter. We will present the results of this review together with a discussion of the influence of old organic material on the isotope ratio in the marine system and of the potential for contamination by recent anthropogenic 129I.
Fehn et al., Geophysical Research Letters, 13, 137 139, 1986. Moran et al., Chemical Geology, 152, 193-203, 1998.
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