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. In the low-nutrient, redox-stratified Lake Medard(Czechia), reductive Fe(III) dissolution outpaces sulfide generation frommicrobial sulfate reduction (MSR) and ferruginous conditions occur withoutquantitative sulfate depletion. The lake currently has marked overlapping C,N, S, Mn and Fe cycles occurring in the anoxic portion of the water column.This feature is unusual in stable, natural, redox-stratified lacustrinesystems where at least one of these biogeochemical cycles is functionallydiminished or undergoes minimal transformations because of the dominance ofanother component or other components. Therefore, this post-mining lake has scientificvalue for (i) testing emerging hypotheses on how such interlinkedbiogeochemical cycles operate during transitional redox states and (ii) acquiring insight into redox proxy signals of ferruginous sediments underlying asulfatic and ferruginous water column. An isotopically constrained estimateof the rates of sulfate reduction (SRRs) suggests that despite high geneticpotential, this respiration pathway may be limited by the rather low amountsof metabolizable organic carbon. This points to substrate competitionexerted by iron- and nitrogen-respiring prokaryotes. Yet, the planktonicmicrobial succession across the nitrogenous and ferruginous zones alsoindicates genetic potential for chemolithotrophic sulfur oxidation.Therefore, our SRR estimates could rather be portraying high rates of anoxicsulfide oxidation to sulfate, probably accompanied by microbially induceddisproportionation of S intermediates. Near and at the anoxicsediment–water interface, vigorous sulfur cycling can be fuelled by ferricand manganic particulate matter and redeposited siderite stocks. Sulfuroxidation and disproportionation then appear to prevent substantialstabilization of iron monosulfides as pyrite but enable the interstitialprecipitation of microcrystalline equant gypsum. This latter mineralisotopically recorded sulfur oxidation proceeding at near equilibriumwith the ambient anoxic waters, whilst authigenic pyrite sulfur displays a38 ‰ to 27 ‰ isotopic offset from ambient sulfate,suggestive of incomplete MSR and open sulfur cycling. Pyrite-sulfurfractionation decreases with increased reducible reactive iron in thesediment. In the absence of ferruginous coastal zones today affected bypost-depositional sulfate fluxes, the current water column redox stratification in the post-mining Lake Medard is thought relevant forrefining interpretations pertaining to the onset of widespread redox-stratifiedstates across ancient nearshore depositional systems.

Aqueous system-level processes and prokaryote assemblages in the ferruginous and sulfate-rich bottom waters of a post-mining lake