Diagenesis of magnetic minerals in the recent sediments of a eutrophic lake

Abstract

Magnetic mineral assemblages in lake sediments are usually interpreted as reflecting changing catchment erosion. Magnetic and chemical assemblages in a small eutrophic lake (Lough Augher, Northern Ireland) suggest diagenetic dissolution of fine‐grained magnetite, however, probably as a result of changing redox conditions associated with eutrophication. Water‐column results show internal cycling of Mn and Fe from the sediments into the anoxic hypolimnion. Calculations suggest that late‐summer hypolimnetic Fe concentrations are controlled by formation of authigenic vivianite; sedimentary pore‐water concentrations of total soluble Fe and P suggest a similar process in the sediments. The loss of Mn from the lake by diffusion from the sediment and washout at overturn is indicated by Mn budget calculations and by a marked decrease in its sedimentary concentration, reflecting onset of eutrophication as indicated by diatom biostratigraphy. The changes in the sediment Mn profile are coincident with, and in some cases identical to, changes in several magnetic parameters. The changes in magnetic stratigraphy suggest a coarsening of the magnetic grain size and reduction in the total concentration; agreement with chemical profiles indicative of changing redox suggest that this coarsening results from dissolution of fine‐grained magnetite. Catchment stability, together with other stratigraphic profiles of eutrophication and changing redox, supports the conclusion that the magnetic profiles are reflecting in situ diagenetic changes rather than altered catchment processes. With the data available it is not possible to identify the actual dissolution process involved or to determine whether it is chemically or bacterially controlled.