Diagenetic mobility of trace elements in sediments covered by a flash flood deposit: Mn,Fe and As

The vertical distribution and concentrations of Fe, Mn, and As were measured in porewaters and sediments recovered with box-cores taken each year since the 1996 flood along the main axis of the Saguenay Fjord and in the Baie des Ha! Ha!. Time-series analyses reveal that Mn remobilization was not significantly affected by the accumulation of the flood deposit. Authigenic Mn oxi-hydroxides present at the original sediment–water interface and in the flood material were progressively reduced, and much of the Mn(II) diffused to the current interface, where it was oxidized and precipitated. First-order rate constants for the reduction and precipitation of the authigenic Mn oxi-hydroxides were estimated by fitting the time-series data to a diagenetic model. On the basis of these rate constants and given that the maximum reactive solid Mn remains concentrated in the thin, oxic layer near the sediment-water interface, an upper limit to the biodiffusion coefficient, D_B, was also evaluated. In contrast to Mn, Fe associated with the authigenic oxi-hydroxides at the original interface was mostly trapped as acid volatile sulfides (i.e., AVS) under the SO₄-reducing conditions established rapidly after the flood. Most of the As associated with the authigenic oxi-hydroxides at the original (buried) interface also appears to be trapped with the authigenic sulfides, thus limiting its diffusion through the flood deposit.