Metal mobilization in the Gironde Estuary (France):: the role of the soft mud layer in the maximum turbidity zone

Vertical profiles of heavy metals (Cd, Cu, Cr, Fe, Mn, Ni and Ph) in the particulate and dissolved phases and redox sensitive parameters (dissolved oxygen, nitrate, nitrite and ammonium) were analyzed for the first time in the continuum Maximum Turbidity Zone (MTZ)-Fluid Mud (FM; 50 g l(-1) < suspended particulate matter (SPM) < 500 g l(-1))-consolidated sediment of a macrotidal, highly turbid estuary: the Gironde. The results show that the fluid mud is a layer of intense metal mobilization due to redox induced dissolution, releasing trace metals into the water column. This newly identified phenomenon creates a transient situation characterized by the onset of diagenetic sequences in the FM, i.e. in the lower part of the water column. These sequences overlie permanent diagenetic sequences in the consolidated sediment, similar to those typically observed in marine and estuarine sediments. The discontinuity of dissolved Cd, Ni and Pb concentrations at the sediment surface indicates that Mn reduction is faster in the FM than in the upper sediment, isolated from the oxic water column by the suboxic FM. Two separate diagenetic signals are preserved, as the installation of the diagenetic sequence in the FM is faster than the molecular diffusion of dissolved compounds through the FM-sediment interface. The diagenetic signal of trace metals (e.g. dissolved Cr maximum) in the upper sediment layer near the FM-sediment interface is interpreted as a transient record of past hydrologic situations, during which the absence of the FM layer permitted the installation of an oxic/suboxic front in the upper sediment. The resuspension of the FM during the spring tide probably results in a new distribution of these elements in the estuary, where they may be stabilized in the dissolved phase (e.g. by chloride complexes or dissolved organic compounds) or be adsorbed onto reactive particles (e.g. freshly precipitated Mn oxyhydroxides). (C) 2004 Elsevier B.V. All rights reserved.