Simulation of long-term feedbacks from authigenic carbonate crust formation at cold vent sites

Fluid flow at cold vent sites is usually driven by the ascent of overpressured fluids from subsurface reservoirs. Porosity changes in surface sediments due to precipitation and dissolution of authigenic carbonates affects fluid flow and biogeochemical turnover. From observations, it is known that carbonate precipitates often occur in distinct layers in high concentrations, surrounded by layers with low carbonate content. Using a non-steady state model, we simulate aragonite and calcite precipitation and dissolution in a 2-m long sediment column, located under a bacterial mat at Hydrate Ridge, Cascadia accretionary margin. Assuming a constant pressure gradient over 7000 years, fluid flow, anaerobic oxidation of methane (AOM) rates, and carbonate precipitation and dissolution rates show strong oscillations evoked by changes in permeability and fluid flow over time. The porosity reaches values below 0.35 in the carbonate layers that reduce the fluid flow velocity from an initial value of 30 cm a−1 to a minimum value of about 2 cm a−1. These significant changes in the fluid flow system displace the depth of sulfate penetration. The simulation predicts cycles of carbonate crust formation and dissolution with a duration of 2000–2700 years resulting in several distinct carbonate layers. During periods of high fluid flow, AOM reaches rates over 1000 μmol cm−1 a−1 and methane fluxes out of the sediment reach 200 μmol cm−1 a−1. During periods of low fluid flow, AOM is about 450 μmol cm−1 a−1 and the methane flux into the bottom water vanishes completely. The oscillations are dampened so that fluid flow and biogeochemical turnover slowly approach steady state after about 7000 years towards the end of the simulation period, showing a 1-m-thick area at the surface with carbonate concentrations of about 25 wt.%. Flow oscillations may also impact the colonization of chemoautotrophic larva and bacteria in and on the sediment. The frequency of precipitation–dissolution cycles of 2000–2700 years is long enough for vent biota to react to changes in the sediments, for example explaining the occurrence of buried Calyptogena and Acharax mussel shells at former vent sites that are not presently active. aluation of side scan sonar data reveals large areas of cemented sediments and carbonate pavements north of the investigated site. The situation found in the investigated core may be characterized as intermediate stage of carbonate cementation. This ongoing process will also form a solid carbonate pavement at this site in the future.