Thermodynamic calculations indicate that most of the pore waters of Leg 168 sediment are supersaturated with calcite and aragonite, with an affinity of reaction around 2 kJ/mol. At colder sites (i.e., the HT sites), there is a return to equilibrium with calcium carbonate at the sediment/basement interface. Calcium carbonate geochemistry at the top of the sediment column at all but Sites 1030 and 1031 is dominated by the diagenetic production of alkalinity and subsequent calcite precipitation.
The decrease in pore-water Sr concentration in the first few tens of meters of the sedimentary cover can be explained by an uptake of Sr by calcite precipitation, but is not consistent with the commonly observed increase in pore-water Sr concentration resulting from the recrystallization of biogenic calcium carbonates. At greater depth in the sediment column, the variation in pore-water Sr concentration is very complex and cannot be explained solely by calcium carbonate precipitation.
At Sites 1030 and 1031, where a diffuse fluid discharge takes place, pore waters are at equilibrium with calcite or aragonite in the lower half of the sediments at Site 1030 and throughout the sediment column except near the seafloor at Site 1031. It appears that equilibrium is achieved because the upwelling of a low-alkalinity fluid counterbalances the increase in alkalinity caused by diagenesis.
The scatter in the analytical data for pore-water compositions does not allow distinguishing between calcite and aragonite precipitation being favored, which would be possible for open-ocean water column compositions. This brings some justification to the use of aqueous calcium carbonate models that have been designed for oceanic conditions. For sediment pore waters that are not too different from standard seawater, our calculations provide some insight into calcium carbonate geochemistry in a low-temperature environment of an oceanic ridge flank.