U/Th systematics and ages of authigenic carbonates from Hydrate Ridge, Cascadia Margin: Recorders of fluid flow variations

摘要

Uranium (U) concentrations and activity ratios (delta(234)U) of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon-rich fluids ("cold seeps") at Hydrate Ridge, off the coast of Oregon, USA. The low U concentrations (mean: 1.3 +/- 0.4 mug/g) and high delta(234)U values (165-317parts per thousand) of gas hydrate carbonates reflect the influence of sedimentary pore water indicating that these carbonates were formed under reducing conditions below or at the seafloor. Their Th-230/U-234 ages span a time interval from 0.8 to 6.4 ka and cluster around 1.2 and 4.7 ka. In contrast, chemoherm carbonates precipitate from marine bottom water marked by relatively high U concentrations (mean: 5.2 +/- 0.8 mug/g) and a mean delta(234)U ratio of 166 +/- 3parts per thousand. Their U isotopes reflect the delta(234)U ratios of the bottom water being enriched in U-234 relative to normal seawater. Simple mass balance calculations based on U concentrations and their corresponding delta(234)U ratios reveal a contribution of about 11% of sedimentary pore water to the bottom water. From the U pore water flux and the reconstructed U pore water concentration a mean flow rate of about 147 +/- 68 cm/a can be estimated. Th-230/U-234 ages of chemoherm carbonates range from 7.3 to 267.6 ka. Th-230/U-234 ages of two chemoherms (Alvin and SE-Knoll chemoherm) correspond to time intervals of low sealevel stands in marine isotope stages (MIS) 2, 4, 5, 6, 7 and 8. This observation indicates that fluid flow at cold seep sites sensitively reflects pressure changes of the hydraulic head in the sediments. The delta(18)O(PDB) ratios of the chemoherm carbonates support the hypothesis of precipitation during glacial times. Deviations of the chemoherm delta(18)O values from the marine delta(18)O record can be interpreted as to reflect temporally and spatially varying bottom water and/or vent fluid temperatures during carbonate precipitation between 2.6 and 8.6degreesC. Copyright (C) 2003 Elsevier Ltd. [References: 58]