The biogeochemical cycling of Zn, Cu, Fe, Mn, and dissolved organic C in a seasonally anoxic lake

Open Access
Publisher Website
Google Scholar
Abstract
Detailed time‐depth distributions of dissolved Zn, Cu, Fe, Mn, and dissolved organic C (DOC), together with pH, were obtained for the water column and near‐surface sediment pore waters of a seasonally anoxic lake (Esthwaite Water, U.K.) over a 1‐yr period. Dissolved Fe and Mn foll owed well‐known patterns linked to redox recycling. The variation in DOC resulted mostly from its coupling with Fe, DOC being coprecipitated with Fe oxide and released into solution on the reductive remobilization of the oxide. This process produced a > 7‐fold seasonal variation in DOC concentrations in the pore waters, with a maximum of ∼7 mg liter−1. Dissolved Zn and Cu exhibited coherent and similar distribution patterns unrelated in any simple way to the distributions of dissolved Fe and Mn, pH, and DOC. The most ubiquitous feature of the distribution of dissolved Zn, and to a lesser degree that of dissolved Cu, was a concentration maximum at the sediment‐water interface, which disappeared completely only when anoxia was maximal in mid‐ and late summer. The interfacial maxima were attributed to rapid release from freshly deposited particles allied to sulfide precipitation in the sediments. The timing of events, associated with an increase in interfacial dissolved Cu and Zn concentrations before and during a period of ice cover, indicated that the Cu was derived from Fe and possibly Mn oxides and Zn from planktonic algae. The cumulative downward diffusive fluxes of Zn and Cu from the interfacial maxima were unimportant 1% of total) compared to total metal sedimentation rates. Chemical equilibrium calculations indicated that Zn‐ and Cu‐sulfide complexes dominated in the pore waters under reducing conditions, with humic binding being insignificant, and that precipitation as pure sulfides or coprecipitation with FeS were likely major removal processes for Zn and Cu under anoxic conditions.

This publication has 0 references indexed in Scilit: