(Table T1) Pore-water strontium content, and Sr isotope composition and ages of pore waters and sedimentary carbonates of ODP Leg 171B sites

Strontium concentrations and 87Sr/86Sr values were measured on pore-water and sedimentary carbonate samples from sediments recovered at Sites 1049-1053 on the Blake Spur during Ocean Drilling Program Leg 171B. These sites form a 40-km-long depth transect extending along the crest of the Blake Spur from near the upper edge of the Blake Escarpment (a steep cliff composed of Mesozoic carbonates) westward toward the interior of the Blake-Bahama Platform. Although these sites were selected for paleoceanographic purposes, they also form a hydrologic transect across the upper eastern flank of the Blake-Bahama Platform. Here, we use pore-water strontium concentrations and isotopes as a proxy to define patterns of fluid movement through the flanks of this platform. Pore-water strontium concentration increases with depth at all sites implying that strontium has been added during sediment burial and diagenesis. The isotopic values decrease from seawater-like values in the shallow samples (~0.70913) to values as low as 0.707342 in one of the deepest samples (~625 meters below seafloor). The change in pore-water strontium isotopic values is independent of the strontium isotopic compositions predicted from the host sediment age and measured on bulk carbonate in some samples. In most cases the difference between predicted sediment strontium isotopic composition and measured value is less than ±2 about the mean of the measured strontium value. Both the increase in concentration and the decrease in the strontium isotope values with increasing depth indicate that strontium was expelled from older carbonates. The strontium concentration and isotope profiles vary between sites according to their proximity to the Blake-Bahama Platform edge. Profiles from Site 1049 (nearest the platform edge) show the greatest amount of mixing with modern seawater, whereas the site most distal to the platform edge (Site 1052) shows the most significant influence of older, deeper carbonates on the pore-water strontium isotopic composition.

Iodine ages of pore water at Hydrate Ridge, Cascadian continental margin

Hydrate Ridge off the coast of Oregon, USA, is a prime example for gas hydrate occurrences in active margin settings. It is part of the Cascadia Margin and was the focus of Ocean Drilling Program (ODP) Leg 204, which successfully recovered fluids from nine sites from the southern part of the ridge. Iodide concentrations in pore fluids associated with gas hydrates are strongly enhanced, by factors up to 5000 compared to seawater, which allows the use of this biophilic element as tracer for organic source regions. We applied the cosmogenic isotope 129I (T1/2=15.7 Ma) system to determine the age of the organic source formation responsible for the iodide enrichment. In all sites at ODP Leg 204, 129I/I ratios were found to decrease with depth to values around 250x10**-15, corresponding to minimum ages of 40 Ma, but in several sites, maxima in the 129I/I ratios point to the local addition of young iodide. The results indicate that a large amount of iodide was derived from deep accreted sediments of Eocene age, and that additional source regions provide iodide of Late Miocene age. The presence of old iodide in the pore waters suggests that fluid pathways are open to allow transport over large distances into the gas hydrate fields. The strong correlation between iodide and methane in hydrate fields coupled with the similarity in transport parameters in aqueous solutions suggests that a large fraction of methane in gas hydrates also has old sources and is transported into the present locations from source regions of Eocene age.