ALBERT

Description: 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- concentrations are significantly lower than that of seawater in the shallowest samples (〈50 µM), rise to a broad maximum, and generally decrease with depth. The low values at the top are consistent with rapid F- removal at or near the seafloor, and the relatively smooth F- 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- profiles apparently characterized by two lows with an intervening high. Processes involving sediment components appear to consume F- at shallow depth, release F- at intermediate depth, and consume F- again at deeper depth. The upper low in F- concentrations consistently lies near the SMT where pore water alkalinity and Mg2+ 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- from pore water near the SMT but, with burial and recrystallization, return F- to pore waters at depth. Authigenic Mg-rich carbonates conceivably represent a major sink of F- from the ocean, although additional work is needed to confirm this idea.

Description: Leg 201 of the Ocean Drilling Program (ODP) focused on understanding subsurface microbial communities and their influence on the chemistry of the surrounding environment (D'Hondt, Jørgensen, Miller, et al., 2003, doi:10.2973/odp.proc.ir.201.2003). During the cruise, sediment cores were collected from four different marine environments of the eastern Pacific Ocean: deep open ocean beneath the moderately productive upwelling regime of the equator (Sites 1225 and 1226), shallow waters of the highly productive Peru shelf (Sites 1227-1229), a deepwater gas-charged zone in the Peru Trench (Site 1230), and a deep open-ocean area under oligotrophic waters (Site 1231). Each of these sites had been drilled previously (during either Deep Sea Drilling Project [DSDP] Leg 34, ODP Leg 112, or ODP Leg 138), although not for detailed microbiological or geochemical investigations. Interstitial waters are routinely analyzed for major cations aboard the JOIDES Resolution drillship. An exception was made during Leg 201 because other dissolved species (e.g. Fe2+, Mn2+, Ba2+, and acetate) were of more immediate interest and because low-resolution profiles of major cations already existed from the earlier cruises (D'Hondt, Jørgensen, Miller, et al., 2003, doi:10.2973/odp.proc.ir.201.2003). Consequently, major cation analyses were withheld for shore-based work. We present here the Ca2+, Mg2+, K+, Na+, and Sr2+ concentrations for pore waters from six of the sites, Sites 1225-1230. Pore waters from Site 1231 are not included in this report because of data inconsistencies.