Tracing organic and inorganic sources to deep-seated marine interstitial fluids: evidence from iodine and 129I/I systematics
Abstract
Submarine mud volcanoes and cold seeps represent shortcuts in geochemical cycles connecting deeply buried sediments or basement rocks with the global ocean reservoir. Iodine isotope systematics are a powerful tracer for the age and origin of pore waters in such fluid escape structures because of their close association with organic material in deeply buried sediments. We report here iodine concentrations and 129I/I ratios for fluids from five mud volcanoes sampled along an E-W transect through the Gulf of Cadiz in the NE Atlantic Ocean. Iodine concentrations increase consistently seaward from 44 to 180 µM, accompanied by a decrease in 129I/I ratios from 6490 x 10-15 to 660 x 10-15. The exceptionally high 129I/I ratios reflect the addition of fissiogenic 129I, produced by spontaneous fission of 238U in terrigenous sediments from the surrounding continental margins. Common trends of 129I/I and 87Sr/86Sr ratios across the margin indicate a progressive seaward transition from organic-poor/continental to organic-rich/marine fluid sources. Age estimates based on 129I/I ratios and mass balance calculations suggest that most organic iodine in pore fluids is derived from source formations with minimum ages in the early Miocene. 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 iodine isotopes in oceanic settings or volcanogenic sediments reflect release from organic matter diagenesis, 129I systematics in continental rock-hosted aquifers and terrigenous sedimentary systems are dominated by fissiogenic 129I. The Gulf of Cadiz represents the full transition between these continental and marine 129I/I and 87Sr/86Sr end members.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMDI41A1771S
- Keywords:
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- 1030 GEOCHEMISTRY / Geochemical cycles;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 1050 GEOCHEMISTRY / Marine geochemistry;
- 3004 MARINE GEOLOGY AND GEOPHYSICS / Gas and hydrate systems