At Hydrate Ridge (HR), Cascadia convergent margin, surface sediments contain massive gas hydrates formed from methane that ascends together with fluids along faults from deeper reservoirs. Anaerobic oxidation of methane (AOM), mediated by a microbial consortium of archaea and sulfate-reducing bacteria, generates high concentrations of hydrogen sulfide in the surface sediments. The production of sulfide supports chemosynthetic communities that gain energy from sulfide oxidation. Depending on fluid flow, the surface communities are dominated either by the filamentous sulfur bacteria Beggiatoa (high advective flow), the clam Calyptogena (low advective flow), or the bivalve Acharax (diffusive flow). We analyzed surface sediments (0 to 10 cm) populated by chemosynthetic communities for AOM, sulfate reduction (SR) and the distribution of the microbial consortium mediating AOM. Highest AOM rates were found at the Beggiatoa field with an average rate of 99 mmol m -2 d -1 integrated over 0 to 10 cm. These rates are among the highest AOM rates ever observed in methane-bearing marine sediments. At the Calyptogena field, AOM rates were lower (56 mmol m -2 d -1 ). At the Acharax field, methane oxidation was extremely low (2.1 mmol m -2 d -1) and was probably due to aerobic oxidation of methane. SR was fueled largely by methane at flowimpacted sites, but exceeded AOM in some cases, most likely due to sediment heterogeneity. At the Acharax field, SR was decoupled from methane oxidation and showed low activity. Aggregates of the AOM consortium were abundant at the fluid-impacted sites (between 5.1 × 10 12 and 7.9 × 10 12 aggregates m

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[1] Fluid distribution in convergent margins is by most accounts closely related to tectonics. This association has been widely studied at accretionary prisms, but at half of the Earth's convergent margins, tectonic erosion grinds down overriding plates, and here fluid distribution and its relation to tectonics remain speculative. Here we present a new conceptual model for the hydrological system of erosional convergent margins. The model is based largely on new data and recently published observations from along the Middle America Trench offshore Nicaragua and Costa Rica, and it is consistent with observations from other erosional margins. The observations indicate that erosional margins possess previously unrecognized distinct hydrogeological systems: Most fluid contained in the sediment pores and liberated by early dehydration reactions drains from the plate boundary through a fractured upper plate to seep at the seafloor across the slope, rather than migrating along the décollement toward the deformation front as described for accretionary prisms. The observations indicate that the relative fluid abundance across the plate-boundary fault zone and fluid migration influence long-term tectonics and the transition from aseismic to seismogenic behavior. The segment of the plate boundary where fluid appears to be more abundant corresponds to the locus of long-term tectonic erosion, where tectonic thinning of the overriding plate causes subsidence and the formation of the continental slope. This correspondence between observations indicates that tectonic erosion is possibly linked to the migration of overpressured fluids into the overriding plate. The presence of overpressured fluids at the plate boundary is compatible with the highest flow rates estimated at slope seeps. The change from aseismic to seismogenic behavior along the plate boundary of the erosional margin begins where the amount of fluid at the fault declines with depth, indicating a control on interplate earthquakes. A previously described similar observation along accreting plate boundaries strongly indicates that fluid abundance exerts a first-order control on interplate seismogenesis at all types of subduction zones. We hypothesize that fluid depletion with depth increases grain-to-grain contact, increasing effective stress on the fault, and modifies fault zone architecture from a thick fault zone to a narrower zone of localized slip.Components: 9574 words, 6 figures.

A large number of mound-shaped structures that originated from mud extrusions is present along the convergent continental margin off Costa Rica and Nicaragua. Active fluid venting is indicated by the existence of CH 4 -and H 2 S-rich pore fluids as well as associated benthic fauna and authigenic carbonates. End-member fluid samples from all mounds are significantly depleted in dissolved Cl and other major elements, suggesting a general process of freshwater addition and thus a common source of the fluids. Our data clearly rule out dilution by gas hydrate dissociation as a dominant source of the freshwater. Enrichments of the fluids in B (up to 2 mmol/L) and inversely correlated ␦ 18 O vs. ␦D values point to clay-mineral dehydration as the cause for these anomalies. Calculations assuming a ␦ 18 O vs. ␦D equilibrium between the pore fluid and clay minerals at depth of formation indicate temperatures of dehydration between 85 and 130 ؇C. This temperature range is in agreement with the B enrichments and the presence of thermogenically formed CH 4 . Because temperatures above 50 ؇C are not reached within the sediment cover of the upper plate, the fluids most likely form within the subducted sediments and flow upward along deep-seated faults from Ն12 km depth. Mound-related fluid expulsion may contribute significantly to the recycling of mineral-bound water.Figure 1. Bathymetric map showing mound locations. Solid line indicates position of seismic transect SO 81-4 shown in Figure 2.

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