Numerical modeling of carbonate crust formation at cold vent sites: significance for fluid and methane budgets and chemosynthetic biological communities

摘要

At many cold vent sites authigenic carbonates precipitate due to the release of carbonate alkalinity during the anaerobic oxidation of methane. Carbonate precipitation often induces the formation of massive crusts at the sediment surface or within surface sediments. The range of physical and biogeochemical conditions allowing for the formation of carbonate crusts is largely unknown so that the significance of these widespread manifestations of fluid flow is unclear. Here, we use numerical modeling to investigate the conditions that induce carbonate crust formation in the sediment and the effect of crust formation on sediment porosity and fluid flow rate. Starting with the conditions prevailing at a previously investigated reference site located on Hydrate Ridge, off Oregon, several parameters are systematically varied in a number of numerical experiments. These parameters include coefficients of bioturbation and bioirrigation, sedimentation rate, fluid flow velocity, methane concentration in the ascending vent fluids, and pH and saturation state at the sediment–water interface. The simulations show that carbonate crusts in the sediments only form if the fluids contain sufficient dissolved methane (>50 mM) and if bioturbation coefficients are low (<0.05 cm~2 a~(-1)). Moreover, high sedimentation rates (>50 cm ka~(-1)) inhibit crust formation. Bioirrigation induces a downward displacement of the precipitation zone and accelerates the formation of a solid crust. Crusts only form over a rather narrow range of upward fluid flow velocities (20–60 cm a~(-1_), which is somewhat enlarged (up to 90 cm a~(-1)) if the overlying bottom waters are supersaturated with respect to calcite. At higher flow rates, methane is rapidly exported into the water column so that methane oxidation and carbonate precipitation cannot proceed within the surface sediment. The formation of a several centimeters thick carbonate crust in surface sediments is typically completed after a few hundred years (100–500 a). Crust formation reduces the supply of methane to surface sediments which imposes a strong resistance against diffusive and advective methane transport. Therefore, rates of anaerobic methane oxidation and sulfide production are diminished and thus the density and metabolism of chemosynthetic biological communities is limited by crust formation. Due to the moderate flow rates and the slow diffusive transport, only very little methane escapes into the bottom water overlying carbonate-encrusted vent areas.