Pore fluid advection and reaction in sediments of the eastern flank, Juan de Fuca Ridge, 48°N

Pore water data collected during Ocean Drilling Program (ODP) Leg 168 provide an opportunity to study advection and reaction in hemipelagic sediments in an off-axis hydrothermal setting. We have modelled the pore water SO42− profiles to obtain estimates of vertical advective (Darcy) velocities. Strong downwelling at rates of 0.3–0.9 mm a−1 occurs at a site 4 km from the boundary between sediment-free and sediment-covered ridge. Downwelling velocities weaken away from the boundary. Twenty kilometres to the east, strong upwelling at >2 mm a−1 is indicated at two sites (1030 and 1031) drilled above a basement high. Further eastwards, where sediment thicknesses are in excess of 200 m, vertical advective pore fluid flow through the sediment column is weak, confining flow to the basement aquifer. Extension of the SO42− model to pore fluid alkalinity, Mg2+ and Ca2+ profiles enables the effects of reactions within the sediment column to be examined. Sulfate reduction leads to the production of alkalinity, but the ΔAlk/ΔSO42− ratios are less than the theoretical ratio of 2 at all the sites studied. The discrepancy is largely explained by the precipitation of calcium carbonate and by the diagenetic reactivity of magnesium in the pore fluids. Alkalinity results are consistent with both sources and sinks for Mg2+ within the sediment column, dependent on the direction of pore fluid advection.