Geochemical focusing and sequestration of manganese during eutrophication of Lake Stechlin (NE Germany)

Grzegorz Scholtysik,Patricia Roeser,Helge Wolfgang Arz,Olaf Dellwig,Michael Hupfer,Tobias Goldhammer,Peter Casper,Christiane Herzog

doi:10.1007/s10533-020-00729-9

Abstract

Significant sedimentation of manganese (Mn) in form of manganese oxides (MnOx) and the subsequent formation of authigenic calcium-rich rhodochrosite (Mn(Ca)CO3) were observed in the seasonally stratified hard water Lake Stechlin in north-eastern Germany. This manganese enrichment was assumed to be associated with recent eutrophication of the formerly oligotrophic lake. The mechanisms and processes involved were examined by analysing: (i) short sediment cores obtained from seven locations along a depth transect ranging from 69.5 m (the deepest point) to 38 m; (ii) sediment traps located at 20 m and 60 m water depths; (iii) water column profiles; and (iv) porewater profiles at 69.5 m and 58 m depths. Sedimentary Mn enrichment was observed at water depths below 56 m and increased to more than 25 wt% at the deepest site. Between 2010 and 2017, Mn accumulation at the deepest site was 815 g Mn m−2. Transfer of Mn from the shallower towards the deepest parts of the lake was initiated by reductive dissolution of MnOx and diffusion of dissolved Mn from the sediment to the overlying water column. Manganese was then dissipated via turbulent mixing and subsequently oxidised to MnOx before being transported towards the deepest zone. Transformation of the redeposited MnOx to Mn(Ca)CO3 favoured the final burial of Mn. We show that eutrophication and the areal spreading of anoxic conditions may intensify diagenetic processes and cause the spatial redistribution of Mn as well as its effective burial. Contrary to many previous findings, we show that increases of Mn and Mn/Fe can also be used as indicators for increasing anoxic conditions in previously oligotrophic lakes.

Highlights

Manganese (Mn) is a redox-sensitive element and an important terminal electron acceptor in the anaerobic mineralisation of organic matter in aquatic ecosystems
We have shown that Mn enrichment of Lake Stechlin is linked to a recent period of eutrophication in this once oligotrophic lake
Dissolved Mn is oxidised to manganese oxides (MnOx) particles, transported through the water column close to the sediment surface, and is deposited in the deepest areas of the lake as a part of a varved section

Summary

Introduction

Manganese (Mn) is a redox-sensitive element and an important terminal electron acceptor in the anaerobic mineralisation of organic matter in aquatic ecosystems. Mn forms oxide (oxidation state (IV), carbonate (II), or sulphide (II) mineral phases (Suess 1979; Davison 1993; Dellwig et al 2010; Hausler et al 2018). Depending on the morphological and hydrophysical characteristics of lakes and marine basins, redox changes may cause a spatial redistribution of particulate Mn in lakes (Delfino et al 1969; Ostendorp and Frevert 1979; Schaller and Wehrli 1996; Dellwig et al 2018) and marine basins (Rajendran et al 1992; Lenz et al 2015a, b; Hausler et al 2018; Schaller and Wehrli 1996) introduced the term ’’geochemical focusing’’ to address the mobilisation of Mn from shallow sediments, its transportation within the water column, and sedimentation in the deep zones of lakes. In marine geology and sedimentology, the term ‘‘shelf-to-basin shuttle’’ is more common for this transport process (Lyons and Severmann 2006; Scholz et al 2013)

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