Geochemical focusing of manganese and iron in lakes and its impact on vivianite occurrence
Scholtysik, Grzegorz; Littke, Ralf (Thesis advisor); Arz, Helge Wolfgang (Thesis advisor)
Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022
Lakes are highly susceptible to human impact and environmental forcing. Especially since the last two centuries, the environment has changed drastically due to climate change and human activities related to the Industrial Revolution. Because of the high susceptibility to environmental changes, lake sediments are excellent archives recording those changes. To enable a reconstruction of those records, paleolimnology uses sedimentary proxies like element contents or fossil remains of various organisms, especially plants. In this study, sediment profiles, sedimenting material (seston) water samples and porewater profiles were sampled and compared with lake monitoring records as well as historic notes. The goal was to develop new or improve existing redox proxies. In particular, we studied the still insufficiently understood process called geochemical focusing that describes a resedimentation process of redox-sensitive elements. Two lakes that have undergone eutrophication in their recent history are described in this study: Lake Stechlin and Lake Arendsee. In both lakes, pronounced enrichments of reactive and redox-sensitive manganese (Mn), iron (Fe) and phosphorus (P) were observed. In Lake Stechlin, enrichment of Fe was found in sediments deposited under oligotrophic conditions. Since the lake has begun to get mesotrophic, Mn contents of even up to 18 wt.% were deposited. In Lake Arendsee, Mn and Fe were enriched in sediments deposited during mesotrophic conditions. We attributed the enrichment to geochemical focusing, which was favored by enhanced organic matter sedimentation and decomposition resulting in reducing conditions. Based on a sampling of sedimenting material, we have shown that shallow-water sediments are most probably the source of dissolved Mn and Fe, which, after diffusion out of the sediment, are released to the water column and get oxidized allowing them to be transported through the water column close to the sediment-water-interface towards the deepest sites. Mineralogical methods (wet chemical fractionation, x-ray diffraction, microscope), supported by porewater profiles, have shown that diagenetic transformations have led to authigenesis to vivianite (Fe2[PO4]2×8H2O), calcium-rich rhodochrosite (Mn(Ca)CO3), and possibly other Mn-Fe carbonates similar to ankerite (CaFe[CO3]2). Approximately since the onset of mesotrophic conditions in Lake Stechlin and since the onset of eutrophic conditions in Lake Arendsee, geochemical focusing and the enrichment of Fe (up to 8 wt.%) ceased. This fact is explained by fixation of Fe in shallow sediments as starting pyrite (FeS2) formation that has reduced Fe mobility. We conclude that geochemical focusing is favored in deep and well-mixed oxygenated lakes, like those in this study. We have shown that elemental composition strongly depends on the water depth of the sampling site. This finding may influence the understanding of sedimentary redox proxies. We have shown that whether an element may be redistributed depends not only on redox conditions but also on diagenetic transformations, which may either fix it at shallow sites or cause its dissolution and further transportation towards deepest sites. It was also shown that because of the large Fe enrichments at the deepest sites, vivianite presence depends on the Fe enrichments and water depth. Furthermore, first estimates showed that vivianite may be responsible for a significant P-sequestration in lacustrine ecosystems.
- Division of Earth Sciences and Geography 
- Institute for Geology and Geochemistry of Petroleum and Coal