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Drastic changes in deep‐sea sediment porewater composition induced by episodic input of organic matter

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 DYFAMED (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 and a concomitant rise of denitrification rates from 0.06 to 0.17 mmol m−2 d−1. Porewater 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.