Transient early diagenetic processes in Rhône prodelta sediments revealed in contrasting flood events - Université d'Angers Accéder directement au contenu
Article Dans Une Revue Continental Shelf Research Année : 2018

Transient early diagenetic processes in Rhône prodelta sediments revealed in contrasting flood events

Résumé

Floods carry sediments to river deltas and the coastal zone, but little is known about the geochemical evolution of this particulate material deposited over a short period of time. Here, we studied two recent contrasting flood deposits in the Rhône River prodelta area (northwestern Mediterranean Sea). We monitored the porewater and solid-phase chemistry over periods ranging from a few days to 6 months after deposition. Non-steady state diagenetic processes associated with episodic deposition promote a wide spectrum of transient redox conditions in the shallow prodelta region of the Rhône. Specific attributes of diagenetic responses depend on the sources of flood material and scale (thickness) of deposition. The first flood unit of 20-30 cm was composed of light gray mud, poor in organic carbon and rich in reactive manganese oxides. The short-term responses of early diagenetic processes contrasted with a rapid consumption of O2 and NO3-over a few hours just after the deposition event, accompanied by a slower build-up of Mn2+ concentration, and a lagged response in Fe2+ concentration over a few days or weeks. This difference was due to the redox capacity of the sediment, evolving from oxidized, during the flood layer deposition, to more reducing conditions, after a few days or weeks, allowing Fe2+ to build up and remain in solution. Sulfate reduction may have started within a few days within the flood deposit and was greatly enhanced just below the former redox front due to a fresh input of organic matter (OM). This large production of H2S probably led to the precipitation of sulfide minerals in close vicinity to the former redox front, limiting the accumulation of Fe2+ and H2S. The unit was sampled repeatedly three times during the six months following the flood event, and showed that manganese oxides were reduced at a rate of 1.8 mmol m−2 d−1, whereas the iron oxide concentration did not vary substantially.
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Dates et versions

hal-02421439 , version 1 (29-06-2020)

Identifiants

Citer

L. Pastor, C. Rabouille, E. Metzger, A. Thibault de Chanvalon, E. Viollier, et al.. Transient early diagenetic processes in Rhône prodelta sediments revealed in contrasting flood events. Continental Shelf Research, 2018, 166, pp.65-76. ⟨10.1016/j.csr.2018.07.005⟩. ⟨hal-02421439⟩
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