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doi:10.2204/iodp.proc.314315316.220.2012

Iodine geochronology in marine environments

Iodine concentrations in marine interstitial fluid from continental margins have been reported to rapidly increase with depth (Martin et al., 1993; Fehn et al., 1992; Tomaru et al., 2007b, 2009). This increase is ascribed to the strongest association of iodine with organic matter; sedimentation and subsequent degradation of iodine-rich organic matter at depth liberates iodine into the interstitial fluid, resulting in iodine concentrations considerably higher than typical seawater levels of 0.4 µM (Broecker and Peng, 1982; Burton, 1996; Muramatsu and Wedepohl, 1998).

The presence of the long-lived cosmogenic radioisotope ¹²⁹I (T_1/2 = 15.7 m.y.) has been used to date the time of separation of organic matter from the marine system (Fabryka-Martin et al., 1987; Fehn et al., 2000). The initial ratio for dating of iodine in the marine environment has been determined to be (1500 ± 150) × 10^–15, which results in a dating range of ~80 m.y. for the ¹²⁹I isotopic system (Fehn et al., 2007). The ¹²⁹I/I ratio of interstitial fluid is sometimes modified by the input of fissiogenic ¹²⁹I, which is produced by the spontaneous fission of ²³⁸U and released into interstitial fluid during migration through the sediment column. In the case here, however, fissiogenic input is negligible because of relatively low U concentration of 0.99 ppm in the subducting sediments in the Nankai Trough and high interstitial fluid iodine concentration (Fabryka-Martin et al., 1989; Plank and Langmuir, 1998; Tomaru et al., 2007a). Standard decay of ¹²⁹I thus provides the elapsed time since the organic matter enriched in iodine has been isolated from seawater as (Fehn et al., 2007)

where

• R_obs = observed ¹²⁹I/I ratio,

• R_i = initial seawater ¹²⁹I/I ratio of 1500 × 10^–15,

• λ₁₂₉ = decay constant of ¹²⁹I (4.41 × 10^–8 y^–1), and

• t = years since iodine deposition.

Although fissiogenic ¹²⁹I or anthropogenic ¹²⁹I are unlikely to have contributed to interstitial fluid in locations like those studied here (e.g., Fehn et al., 2000; Tomaru et al., 2007a), contribution from either source would have raised the ratio. The values calculated for the samples here are therefore minimum ages.

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