Joint Oceanographic Institutions, United States

Dissolved fluoride was determined for pore waters at eight sites drilled on Hydrate Ridge during Ocean Drilling Program (ODP) Leg 204 and one site drilled in the Peru Trench during ODP Leg 201. All nine sites contain a shallow (<20 m) sulfate-methane transition (SMT) above abundant methane including gas hydrate. For Sites 1248, 1249, and 1250 on the crest of Hydrate Ridge, F (super -) concentrations are significantly lower than that of seawater in the shallowest samples (<50 mu M), rise to a broad maximum, and generally decrease with depth. The low values at the top are consistent with rapid F (super -) removal at or near the seafloor, and the relatively smooth F (super -) profiles are consistent with high upward fluid fluxes. In contrast, Sites 1244, 1245, 1247, 1251, and 1252 from the flanks and slope basins of Hydrate Ridge and Site 1230 from the Peru Trench have F (super -) profiles apparently characterized by two lows with an intervening high. Processes involving sediment components appear to consume F (super -) at shallow depth, release F (super -) at intermediate depth, and consume F (super -) again at deeper depth. The upper low in F (super -) concentrations consistently lies near the SMT where pore water alkalinity and Mg (super 2+) profiles suggest precipitation of Mg-rich carbonate. A similar pattern occurs at other sites drilled into methane-charged sediment. We speculate that Mg-rich carbonates (e.g., high-Mg calcite, protodolomite, and dolomite) remove F (super -) from pore water near the SMT but, with burial and recrystallization, return F (super -) to pore waters at depth. Authigenic Mg-rich carbonates conceivably represent a major sink of F (super -) from the ocean, although additional work is needed to confirm this idea.