We report the first seasonal time-series observations of porewater composition obtained for deep-sea sediments. We observed considerable temporal variability of O2, NO3-, and Mn2+ profiles at the French JGOFS site DYFA-IVIED (western Mediterranean) in response to a pulsed input of organic matter. A delivery of reactive organic matter representing only 1% of the average organic C content of surface sediments (0.6% wt/wt) was required in order to fuel the observed changes in porewater composition. The perturbation resulted in dramatic changes in integrated reaction rates and sediment-water fluxes. O2 uptake rates increased from 1.26 to 1.82 mmol m-2 d-1, while nitrification rates rose from 0.13 to 0.18 mmol m-2 d-1. These changes were paralleled by a decrease in NO3- effluxes across the sediment-water interface from 0.07 to 0.01 mmol m-2 d-1 and a concomitant rise of denitrification rates from 0.06 to 0.17 mmol m-2 d-1. Pore-water profiles responded rapidly to the sediment pulse deposition, returning to their steady-state values within a few months. The main driving force appeared not to be the seasonal variability of pelagic production but rather the downslope transport of resuspended upper-shelf sediments.