Iodine as a tracer of organic material: I results from gas hydrate systems and fore arc fluids

The strong association of iodine with organic material and the presence of the cosmogenic radioisotope I make the iodine isotopic system useful in tracing and dating studies of organic materials and their derivatives. We present here results from two new applications of this system, investigations of gas hydrates from the Peru Margin (ODP 201, Site 1230) and of fluids collected from the fore arc region of the North Island, New Zealand. Pore fluids from Site 1230 are strongly enriched in iodine and show a distinct decrease in I/I ratios from 920x10 close to the surface to 140x10 at a depth of 200 mbsf, suggesting the presence of a shallow, young source and deep, old source of fluids. The fore arc fluids from New Zealand are also enriched in iodine and show a similar range in I/I ratios. In both cases minimum ages are calculated to be between 40 and 60 Ma for these fluids. The results are in good agreement with earlier investigations of gas hydrate systems at Blake Ridge and Nankai Trough and of fore arc fluids from Central America and Japan, but are not compatible with derivation from subducting sediments in active margins. The results indicate that continental margins contain large amounts of old iodine, reflecting the presence of large quantities of organic material stored in these regions. Results for gas hydrate systems and fore arc fluids show similar characteristics, but differ strongly from those obtained for fluids collected from the main zones of volcanic activity associated with active margins.