Effects of frequent or massive sediment supplies on sediment redox dynamics: a microcosm simulation

Marine coastal environments are subject to depositional events generated for instance by turbidites, spring tides and estuarine floods. Biogeochemical stability in sediments was already shown to be disrupted following the deposition of a new sediment layer. The present study aims to understand the successive recovery stages of early diagenetic processes following sediment deposition events of varying intensity and frequency, using microcosms simulations. Over a five-week period with weekly sampling, high spatial resolution methods were applied to characterize the behavior of oxygen, nitrogen, manganese and iron in response to i) a single 2.7 cm sediment deposit (“one-time high volume”; OHV) and ii) a series of weekly 0.4 cm thick deposit (“frequent low volume”; FLV), compared to an undisturbed control microcosm. Results indicate that oxygen, ammonium and nitrite concentration in porewaters exhibited similar vertical distributions in FLV and control microcosms. In the OHV microcosm, oxygen penetration depth was returned to control levels after 3 weeks, whereas denitrification fronts within the newly deposited sediments reestablished within one week after disturbance. Regarding manganese and iron, both OHV and FLV treatments showed that the Mn/Fe oxide-enriched surface layer became anoxic after sediment deposition, triggering bacterial reduction of these oxides. Dissolved manganese production occurred one week before dissolved iron production, highlighting preferential organic matter remineralization pathways. These findings suggest that the transient biogeochemical response to sediment deposition can therefore be appreciated through manganese and iron cycling.