DRASTIC CHANGES IN DEEP-SEA SEDIMENT POREWATER COMPOSITION INDUCED BYEPISODIC INPUT OF ORGANIC-MATTER

We report the first seasonal time-series observations of porewater com position obtained for deep-sea sediments. We observed considerable tem poral variability of O-2, NO3-, and Mn2+ profiles at the French JGOFS site DYFAMED (western Mediterranean) in response to a pulsed input of organic matter. A delivery of reactive organic matter representing onl y 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 comp osition. The perturbation resulted in dramatic changes in integrated r eaction rates and sediment-water fluxes. O-2 uptake rates increased fr om 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 decr ease 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). Porewater profiles responded rapi dly 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 downslop e transport of resuspended upper-shelf sediments.