BROMINE AND IODINE IN PERU MARGIN SEDIMENTS AND PORE FLUIDS - IMPLICATIONS FOR FLUID ORIGINS

At the Peruvian convergent margin. two distinct pore fluid regimes are recognized from differences in their Cl- concentrations. The slope po re fluids are characterized by low Cl- concentrations, but elevated Br - and I- concentrations due to biogenic production. The shelf pore flu ids exhibit elevated Cl- and Br- concentrations due to diffusive mixin g with an evaporitic brine. In the slope pore fluids, the Br-, I-, and NH4+ concentrations are elevated following bacterial decomposition of organic matter, but the I- concentrations are in excess of those expe cted based on mass balance calculations using NH4+ and Br- concentrati ons. The slope sediment organic matter, which is enriched in iodine fr om oxidation-reduction processes at the oxygenated sediment-water inte rface, is responsible for this enrichment. The increases in dissolved I- and the I- enrichments relative to NH4+ and Br- correlate well with sedimentation rates because of differential trapping following regene ration. The pore-fluid I-/Br ratios suggest that membrane ion filtrati on is not a major cause of the decreases in Cl- concentrations. Other possible sources for low Cl- water, including meteoric water, clathrat e 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 s ediment 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 mixin g with a brine that formed from seawater concentrated by twelve to nin eteen times and probably was modified by halite dissolution. This dens e brine, located below the sediment sections drilled, appears to have flowed a distance >500 km through the sediment.