Ocean Drilling Program, United States

Thermodynamic calculations were used to investigate the calcite and aragonite saturation states of sediment pore waters collected during Ocean Drilling program (ODP) Leg 168 on the eastern flank of the Juan de Fuca Ridge. An aqueous carbonate model was used, based on apparent stability constants designed for standard seawater at oceanic conditions, which is the base of the computer program CO2SYS. We discuss possible biases that may result from the application of such models to oceanic sediment pore waters that are slightly altered seawater where calcium has replaced magnesium. The geochemistry of calcium carbonates at the top of the sediment column at all sites except Sites 1030 and 1031 is dominated by the diagenetic production of alkalinity and subsequent calcium carbonate precipitation. Our calculations show that no calcium carbonate mineral is at equilibrium with the pore waters at the shipboard conditions (20 degrees C, 1 bar). The scatter in the analytical data (especially pH) for pore-water compositions does not allow us to distinguish between calcite and aragonite. At Sites 1023-1029 and 1032, the saturation indices of calcium carbonate minerals calculated for the in situ temperatures and pressures increase with depth from close to equilibrium values at the seafloor to an almost constant supersaturation at depth as indicated by an affinity of the dissolution reaction around 2 kJ/mol. At colder sites, there is a return to equilibrium near the sediment/basement interface, whereas at all other sites (except Sites 1030 and 1031) supersaturation is maintained down to basement. The decrease in pore-water strontium concentration in the first few tens of meters of sedimentary cover can be explained by an uptake of Sr resulting from calcite precipitation, which is consistent with our calculations, but not with the commonly observed increase in pore-water Sr concentration caused by recrystallization of biogenic calcium carbonates. At greater depth in the sediment column, the variation in pore-water Sr concentration is complex and cannot be explained solely by calcium carbonate precipitation. At all sites, the pore-water Mg/Ca ratio displays variations similar to the Sr/Ca ratio. Sites 1030 and 1031 display a diffuse fluid discharge. Pore waters are at equilibrium in the lower half of the sediment column at Site 1030. Site 1031 shows equilibrium throughout nearly the entire sediment column, except for the topmost section where slight supersaturation is found. The tendency toward chemical equilibrium at these two sites results from competition between the advection of a low-alkalinity, upwelling basement fluid and alkalinity production by organic matter oxidation.