doi:10.1016/j.epsl.2020.116400engBlattmann, Thomas M.Lesniak, BarbaraGarcía-Rubio, InésCharilaou, MichalisWessels, MartinEglinton, Timothy I.Gehring, Andreas U.Ferromagnetic resonance of magnetite biominerals traces redox changesART-2020-118617Redox variations govern a multitude of key geochemical and microbiological processes within lacustrine and marine systems, yet the interpretation of these geological archives can be limited because redox-sensitive microorganisms leave behind sparse fossil evidence. Here, we assess a biologically controlled magnetic proxy through investigation of a well-constrained sedimentary record covering a perturbation of redox-conditions driven by a complete trophic cycle in Lake Constance. Ferromagnetic resonance spectroscopy of sediments reveals strong uniaxial anisotropy, indicative of single-domain magnetite particles in intact or fragmentary chain arrangements, which are an unambiguous trait of magnetotactic bacteria (MTB) and their magnetofossil remains. We show that biogenic magnetite formed intra-cellularly in MTB faithfully records changing redox-conditions at or close to the sediment water-interface. Biogenic magnetite within sedimentary records points to the proliferation of MTB parallel to a decline in water column dissolved oxygen and the formation of sulfidic surface sediments in Lake Constance associated with an episode of eutrophication (1955–1991). We conclude that magnetofossils may serve as a sensitive geological proxy to reconstruct dynamic redox-changes along the sediment-water interface and bottom waters.2020http://zaguan.unizar.es/record/9455410.1016/j.epsl.2020.116400http://zaguan.unizar.es/record/94554oai:zaguan.unizar.es:94554info:eu-repo/grantAgreement/ES/DGA/E35-17Rinfo:eu-repo/grantAgreement/ES/MINECO/CTQ2015-64486-REarth and Planetary Science Letters 545 (2020), 116400 1-7by-nc-ndhttp://creativecommons.org/licenses/by-nc-nd/3.0/es/info:eu-repo/semantics/openAccess