PHYSICAL AND BIOGEOCHEMICAL CONSTRAINTS ON CARBONATE CRUST FORMATION AT COLD VENT SITES: SIGNIFICANCE FOR FLUID AND METHANE BUDGETS

We use numerical modeling to investigate the conditions which induce carbonate crust formation at cold vent sites. The simulations show that carbonate crusts are only formed if the vent fluids contain sufficient dissolved methane (>50 mM) and if bioturbation coefficients are low (<0.2 cm2 a-1). Moreover, high sedimentation rates (>50 cm ka-1) inhibit crust formation. Bioirrigation induces a downward displace of the precipitation zone into deep sediment layers. Crusts are only formed 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 exported into the water column so that methane oxidation and carbonate precipitation can not proceed within the surface sediment. The formation of a several centimeter thick carbonate crust is typically completed after a few hundred years (100 - 500 a). Crust formation reduces the supply of methane to surface sediments due to the chemical clogging of pore space which imposes a strong resistance against diffusive and advective methane transport. Therefore, rates of anaerobic methane oxidation and sulfide production are diminished so that 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.