Controls on the ¹²⁹I/I ratio of deep-seated marine interstitial fluids: 'Old' organic versus fissiogenic 129-iodine

Iodine and its radioisotope ¹²⁹I have been successfully used to trace the origin of pore waters in submarine fluid escape structures because of their close association with organic material in deeply buried sediments. We report here halogen concentrations and ¹²⁹I/I ratios for fluids of five mud volcanoes sampled along an E-W transect across the Gulf of Cadiz in the NE Atlantic Ocean. Concentrations of iodide and bromide increase consistently seaward accompanied by a decrease in ¹²⁹I/I ratios from 6490×10^-15 to 663×10^-15. The exceptionally high ¹²⁹I/I ratios at the near-shore locations reflect the presence of fissiogenic ¹²⁹I, produced in situ by spontaneous fission of ²³⁸U within terrigenous sediments and released into pore water during clay mineral diagenetic processes. The observed ¹²⁹I/I and halogen trends, together with similar changes in ⁸⁷Sr/⁸⁶Sr ratios, indicate a progressive seaward transition from inorganic-terrigenous to organic-marine fluid sources.Comparison of our results with literature data for varying geological settings reveals a general relationship between fissiogenic ¹²⁹I, radiogenic ⁸⁷Sr and the lithology or provenance of rocks and sediments, respectively. While ¹²⁹I/I ratios in continental rock-hosted aquifers and terrigenous sedimentary systems are dominated by in situ production of fissiogenic ¹²⁹I, iodine isotopes in oceanic settings or volcanogenic marine sediments reflect the release of 'old' iodine from deeply buried organic matter. The Gulf of Cadiz represents the full transition between these continental and oceanic ¹²⁹I/I and ⁸⁷Sr/⁸⁶Sr end members. This is the first systematic investigation of fissiogenic ¹²⁹I production in marine sedimentary environments.