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.

中文翻译：

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Stechlin 湖（德国东北部）富营养化过程中锰的地球化学聚焦和封存

在德国东北部季节性分层的硬水湖 Stechlin 中观察到以氧化锰 (MnOx) 形式存在的锰 (Mn) 的显着沉积和随后形成的自生富钙菱锰矿 (Mn(Ca)CO3)。这种锰富集被认为与以前贫营养湖泊最近的富营养化有关。所涉及的机制和过程通过以下分析来检验：(i) 从 69.5 m（最深点）到 38 m 的深度横断面上的七个位置获得的短沉积岩心；(ii) 位于 20 m 和 60 m 水深的沉积物陷阱；(iii) 水柱剖面；(iv) 69.5 m 和 58 m 深度的孔隙水剖面。在低于 56 m 的水深处观察到沉积 Mn 富集，并在最深处增加到 25 wt% 以上。2010 年至 2017 年间，最深处的 Mn 积累量为 815 g Mn m-2。Mn 从湖的较浅部分向最深部分的转移是通过 MnOx 的还原溶解和溶解的 Mn 从沉积物扩散到上覆水柱而开始的。然后锰通过湍流混合消散，随后被氧化成 MnOx，然后被输送到最深的区域。再沉积的 MnOx 转化为 Mn(Ca)CO3 有利于 Mn 的最终埋藏。我们表明，富营养化和缺氧条件的区域扩散可能会加剧成岩过程并导致锰的空间重新分布及其有效埋藏。与之前的许多发现相反，我们表明 Mn 和 Mn/Fe 的增加也可以用作增加先前贫营养湖泊缺氧条件的指标。