Bromine and iodine in Peru margin sediments and pore fluids: Implications for fluid origins

Abstract

At the Peruvian convergent margin, two distinct pore fluid regimes are recognized from differences in their Cl ≈ concentrations. The slope pore fluids are characterized by low Cl − concentrations, but elevated Br − and I − concentrations due to biogenic production. The shelf pore fluids exhibit elevated Cl − and Br − concentrations due to diffusive mixing with an evaporitic brine. In the slope pore fluids, the Br −, I −, and NH 4 + concentrations are elevated following bacterial decomposition of organic matter, but the I − concentrations are in excess of those expected based on mass balance calculations using NH 4 + and Br − concentrations. The slope sediment organic matter, which is enriched in iodine from oxidationreduction processes at the oxygenated sediment-water interface, is responsible for this enrichment. The increases in dissolved I − and the I − enrichments relative to NH 4 + and Br − correlate well with sedimentation rates because of differential trapping following regeneration. The pore-fluid I − Br − ratios suggest that membrane ion fiitration is not a major cause of the decreases in Cl − concentrations. Other possible sources for low Cl − water, including meteoric water, clathrate dissociation, and/or mineral dehydration reactions, imply that the diluting component of the slope low-Cl − fluids has flowed at least 1 km through the sediment. The low bottom-water oxygenation in the shelf is responsible for the low (if any) enrichment of iodine in the shelf sediments. Fluctuations in bottom-water oxygen concentrations in the past, however, may be responsible for the observed variations in the sediment I Br ratios. Comparison of Na + Cl − and Br − Cl − molar ratios in the pore fluids shows that the shelf high-Cl − fluid formed from mixing with a brine that formed from seawater concentrated by twelve to nineteen times and probably was modified by halite dissolution. This dense brine, located below the sediment sections drilled, appears to have flowed a distance >500 km through the sediment.