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: Recent siliceous sediments of the Guaymas Basin, a rapid spreading center in the Gulf of California, are intruded by basaltic sills, which induced significant diagenetic and metamorphic changes in the sediments. The transformation from opal-A to opal-CT, opal-CT to quartz, and opal-A directly to quartz in these sediments, cored on DSDP Leg 64, can be used to infer the temperature history and order of emplacement of intrusives. At or near contacts with sills of 30 m or greater average thickness, opal-A inverts directly to authigenic quartz, but there is less quartz than would be expected from the amount of opal-A dissolution. Opal-CT forms in sediments sandwiched between adjacent sills. Based on high rates of quartz nucleation and growth at high temperatures (〉 150°C), and on considerations of convective solution transfer, opal-CT is thought to form only where temperatures were lower or at positions between sills, an environment which prevents rapid convective dissipation of silica in solution. Where temperatures were higher or convection uninhibited, solutions remained at silica concentrations too low to permit opal-CT crystallization, and opalA inverted directly to quartz by dissolution and reprecipitation. These arguments allow inference of the order of sill emplacement and estimates of thermal history in a high heat flow regime perturbed by local sill intrusions. These are the youngest oceanic siliceous sediments known to have been converted to opal-CT and quartz; in the Guaymas Basin this evolution took only thousands of years, whereas in deep sea oceanic environments far from igneous activity it would have taken millions to tens of millions of years.

Description: As laboratory experiments predict, a Mg-hydroxide-sulfate-hydrate mineral, here named caminite, precipitates in nature from seawater heated in an active submarine hydrothermal system. Caminite is found intergrown with anhydrite in the wall of a black-smoker chimney precipitated around hydrothermal fluids discharging on the East Pacific Rise axis at 2l'N latitude. Caminite is tetragonal (space group I4,/amd) with a = 5.239Å and c = 12.988Å. The five strongest lines appearing in X-ray powder difraction patterns (CuKa radiation) are 3.345 (I/Io = 100; hkl = 103); 3.220 (80; 112); 1.871 (50; I l6); 1.620 (25; 303); 1.609 (20; 224). Bond-strength calculations and experimental results predict that the caminite structure accommodates a range of compositions described by a general formula: MgSO4 . xMg(OH)2 . (1- 2x)H2O, where 0 〈 x 〈 0.5. The caminite in our sample has a composition corresponding to a stoichiometry of MgSO4 . 0.4Mg(OH)2 . 0.2H2O. It is soft (H = 2.5) and apparently colorless. Caminite is uniaxial negative and has low birefringence (0.002). Its indices of refraction are omega = 1.534 and epsilon = 1.532. In the recharge zones of submarine hydrothermal systems, large volumes of convecting seawater heated above approximately 240°C rnay precipitate abundant caminite and anhydrite. Formation of abundant caminite can drastically lower the pH of downwelling seawater in such systems, and rapid removal of sulfate into caminite and anhydrite may prevent the reduction of much seawater sulfate to sulfide within the hydrothermal system. Incorporation of seawater sulfate into caminite and anhydrite at elevated temperatures and subsequent recycling of this sulfate into the oceans by dissolution at low temperatures should affect the oxygen-isotope compositionof seawater sulfate and may play a part in maintaining the oxygen-isotope values of oceanic sulfate in disequilibrium with δO18 of seawater.

Description: Hydrocarbon-rich fluids expelled at mud volcanoes (MVs) may contribute significantly to the carbon budget of the oceans, but little is known about the long-term variation in fluid fluxes at MVs. The Darwin MV is one of more than 40 MVs located in the Gulf of Cadiz, but it is unique in that its summit is covered by a thick carbonate crust that has the potential to provide a temporal record of seepage activity. In order to test this idea, we have conducted petrographic, chemical and isotopic analyses of the carbonate crust. In addition a 1-D transport-reaction model was applied to pore fluid data to assess fluid flow and carbonate precipitation at present. The carbonate crusts mainly comprise of aragonite, with a chaotic fabric exhibiting different generations of cementation and brecciation. The crusts consist of bioclasts and lithoclasts (peloids, intraclasts and extraclasts) immersed in a micrite matrix and in a variety of cement types (microsparite, botryoidal, isopachous acicular, radial and splayed fibrous). The carbonates are moderately depleted in 13C (δ13C = − 8.1 to − 27.9‰) as are the pore fluids (δ13C = − 19.1 to − 28.7‰), which suggests that their carbon originated from the oxidation of methane and higher hydrocarbons, like the gases that seep from the MV today. The carbonate δ18O values are as high as 5.1‰, and it is most likely that the crusts formed from 18O-rich fluids derived from dehydration of clay minerals at depth. Pore fluid modelling results indicate that the Darwin MV is currently in a nearly dormant phase (seepage velocities are 〈 0.09 cm yr− 1). Thus, the thick carbonate crust must have formed during past episodes of high fluid flow, alternating with phases of mud extrusion and uplift. Highlights ► Results of pore fluid modelling indicate low seepage activity at localised sites. ► Pore fluids are supersaturated with respect to hydrocarbons of thermogenic origin. ► AOM supports vent fauna and results in the formation of authigenic carbonates. ► The carbonate crust has a brecciated appearance and mainly consists of aragonite. ► The crust formation seems to be regulated by changes in fluid and mudflow activity.

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.

Description: Analytical challenges in obtaining high quality measurements of rare earth elements (REEs) from small pore fluid volumes have limited the application of REEs as deep fluid geochemical tracers. Using a recently developed analytical technique, we analyzed REEs from pore fluids collected from Sites U1325 and U1329, drilled on the northern Cascadia margin during the Integrated Ocean Drilling Program (IODP) Expedition 311, to investigate the REE behavior during diagenesis and their utility as tracers of deep fluid migration. These sites were selected because they represent contrasting settings on an accretionary margin: a ponded basin at the toe of the margin, and the landward Tofino Basin near the shelf's edge. REE concentrations of pore fluid in the methanogenic zone at Sites U1325 and U1329 correlate positively with concentrations of dissolved organic carbon (DOC) and alkalinity. Fractionations across the REE series are driven by preferential complexation of the heavy REEs. Simultaneous enrichment of diagenetic indicators (DOC and alkalinity) and of REEs (in particular the heavy elements Ho to Lu), suggests that the heavy REEs are released during particulate organic carbon (POC) degradation and are subsequently chelated by DOC. REE concentrations are greater at Site U1325, a site where shorter residence times of POC in sulfate-bearing redox zones may enhance REE burial efficiency within sulfidic and methanogenic sediment zones where REE release ensues. Cross-plots of La concentrations versus Cl, Li and Sr delineate a distinct field for the deep fluids (z 〉 75 mbsf) at Site U1329, and indicate the presence of a fluid not observed at the other sites drilled on the Cascadia margin. Changes in REE patterns, the presence of a positive Eu anomaly, and other available geochemical data for this site suggest a complex hydrology and possible interaction with the igneous Crescent Terrane, located east of the drilled transect.