Description: Barium concentrations were measured in pore fluids and sediments from the shallow forearc of the Costa Rica subduction zone to investigate the impact of progressive barite dissolution coupled to [SO4]2- depletion on the residual sediment Ba flux to the volcanic arc and mantle. At the Costa Rica subduction zone, the entire sediment section entering the trench is underthrust beneath the prism sediments of the overriding plate. Dissolved [SO4]2- concentrations measured in the reference sediment section in the incoming plate at ODP Site 1039/1253 vary between 13 and 29 mM. At ODP Site 1040/1254, 1.6 km arcward from the trench, [SO4]2- is depleted in the ~370 m of prism sediments as well as in the upper 30 m of the underthrust sediments. This suggests that, upon subduction, [SO4]2- diffusion from seawater into the underthrust sediment section ceases and the available pore fluid [SO4]2- at the top of the section is consumed by active microbial [SO4]2- reduction. Because the only remaining source of [SO4]2- is in the underthrust sediments, the depth of [SO4]2- depletion in the underthrust sediments must increase with distance from the trench. Dissolved Ba2+ concentrations in the uppermost underthrust sediments at Site 1040/1254 are several orders of magnitude greater than in the reference sediment section at Site 1039/1253, indicating intense barite dissolution coupled to [SO4]2- depletion. This is corroborated by a 50% decrease in the barite content within this unit. As a result of tectonic compaction, the dissolved Ba2+ released from barite dissolution is transported seaward and reprecipitated as barite when reaching [SO4]2- -rich fluids. As [SO4]2- depletion continues arcward, greater losses of sedimentary barite must occur in the subducting sediments. If all the barite is dissolved from the subducting sediment section, 60% of the incoming bulk sediment Ba will be distilled from the sediments in the shallow forearc. Balancing the Ba output flux with this lower input flux requires a much larger sediment component recycled to the volcanic arc than previously suggested. These results indicate that diagenetic mobilization of Ba from barite can have a profound impact on the chemical composition of sediments recycled to the arc and mantle, and should be considered in the global budget of subducted sediments.

Description: In this paper we present an in-depth analysis and synthesis of published and newly acquired data on the chemical and isotopic composition of forearc fluids, fluid fluxes, and the associated thermal regimes in well-studied, representative erosional and accretionary subduction zone (SZ) forearcs. Evidence of large-scale fluid flow, primarily focused along faults, is manifested by widespread seafloor venting, associated biological communities, extensive authigenic carbonate formation, chemical and isotopic anomalies in pore-fluid depth-profiles, and thermal anomalies. The nature of fluid venting seems to differ at the two types of SZs. At both, fluid and gas venting sites are primarily associated with faults. The décollement and coarser-grained stratigraphic horizons are the main fluid conduits at accretionary SZs, whereas at non-accreting and erosive margins, the fluids from compaction and dehydration reactions are to a great extent partitioned between the décollement and focused conduits through the prism, respectively. The measured fluid output fluxes at seeps are high, ∼15–40 times the amount that can be produced through local steady-state compaction, suggesting that in addition, other fluid sources or non-steady-state fluid flow must be involved. Recirculation of seawater must be an important component of the overall forearc output fluid flux in SZs. The most significant chemical and isotopic characteristics of the expelled fluids relative to seawater are: Cl dilution; sulfate, Ca, and Mg depletions; and enrichments in Li, B, Si, Sr, alkalinity, and hydrocarbon concentrations, often distinctive δ18O, δD, δ7Li, δ11B, and δ37Cl values, and variable Sr isotope ratios. These characteristics provide key insights on the source of the fluid and the temperature at the source. Based on the fluid chemistry, the most often reported source temperatures reported are 120–150 °C. We estimate a residence time of the global ocean in SZs of ∼100 Myr, about five times faster than the previous estimate of ∼500 Myr by Moore and Vrolijk, similar to the residence time of ∼90 Myr for fluids in the global ridge crest estimated by Elderfield and Schultz, and ∼3 times longer than the 20–36 Myr estimate by German and von Damm and Mottl. Based on this extrapolated fluid reflux to the global ocean, subduction zones are an important source and sink for several elements and isotopic ratios, in particular an important sink for seawater sulfate, Ca and Mg, and an important source of Li and B.

Description: Kessler et al. (Reports, 21 January 2011, p. 312) reported that methane released from the 2010 Deepwater Horizon blowout, approximately 40% of the total hydrocarbon discharge, was consumed quantitatively by methanotrophic bacteria in Gulf of Mexico deep waters over a 4-month period. We find the evidence explicitly linking observed oxygen anomalies to methane consumption ambiguous and extension of these observations to hydrate-derived methane climate forcing premature.