CONCLUSIONS

Carbonate precipitates that were recovered from several depth intervals from sediments overlying the Blake Ridge Diapir are dominated by authigenic aragonite cements. Oversaturation of the pore water with respect to aragonite is provided by anaerobic methane oxidation that causes CaCO₃ precipitation. The methane flux, which is required to sustain these reactions, is most likely driven by the decomposition of methane hydrates above the diapir and seems to be concentrated along fluid conduits that were observed in the form of vertical, gas hydrate-cemented veins in the sediment.

A comparison of the ¹³C values of carbonate cements to the isotopic composition of the carbon pool indicates that the carbonate nodules derived their carbon from a ¹³C-depleted CO₂ pool at shallow depth. The oxygen isotopic composition of the carbonate nodules indicates precipitation in, or near, isotopic equilibrium with the regional bottom water. Variations of the ¹⁸O values with depth might be related to changes in bottom-water conditions over time. The ⁸⁷Sr/⁸⁶Sr ratios of carbonate precipitates and pore-water samples from Site 996 are indistinguishable from contemporaneous seawater, indicating a shallow Sr source.

Based on these data, the authigenic carbonates at Site 996 seem to have formed at or near the sediment/water interface from a ¹³C-depleted carbon pool that is fueled by anaerobic methane oxidation (with methane released by the decomposition of gas hydrates below). Furthermore, there is no indication for ongoing carbonate precipitation with depth. This suggests that these near-surface carbonate deposits are the only preservable expression of fluid venting. Thus, on the basis of the depth distribution of the precipitates, fluid venting has been occurring above the Blake Ridge Diapir for at least 600,000 yr.