The ferruginous, sulfate-rich hypolimnion of a post-mining lake as an analogue to disentangle redox cycling in Paleoproterozoic coastal zones

The shallow marine depositional and early diagenetic conditions in the predominantly anoxic oceans that followed the Great Oxygenation Event (GOE) remain to be fully understood. In post-GOE coastlines, ferruginous seawater was locally admixed with oxidised freshwater carrying products from the enhanced weathering of sulfides on land, to form coastal aquifers likely exhibiting sulfate concentrations significantly higher than those generally estimated for Proterozoic open oceans; e.g., < 400 μM¹. Also, there is mounting petrographic evidence for pseudomorphs after gypsum (or anhydrite) in Paleoproterozoic shallow marine facies, indicating that the penecontemporaneous oxidised sulfur levels in peritidal to intertidal settings were high enough to allow for the formation of primary sulfate minerals. The study of such ancient coastal depositional/early diagenetic conditions throughout modern systems is not straightforward since most of the purposed analogues to Precambrian ferruginous oceans lack environmentally relevant sulfate levels. A combination of spectroscopic and physicochemical measurements of the bottom waters of a meromictic, post-mining lake featuring a dysoxic hypolimnion and an anoxic monimolimnion reveals relatively high concentrations of sulfate ([SO₄^2-]= 19 ± 2 mM) and dissolved iron ([Fe(II)]= 127 ± 17 μM), with redox gradients marked by changes in Fe and N speciation². The oligotrophic artificial lake—known as Lake Medard (Czech Republic)—also features a depth-dependent co-variation in the abundance of volatile fatty acids, pH and alkalinity, together with a lack of dissolved sulfide, which can only be detected (at near quantification limits) in the 60 m depth sediment-water interface (SWI). Within the hypolimnion, changes in the relative abundance of bacterioplankton taxa point to prokaryotes (mostly Proteobacteria) being important for the co-recycling of dissolved C, N, and Fe stocks, but exerting limited sulfate reduction. In the clayey anoxic sediments there is no accumulation of authigenic sulfides but gypsum, and early diagenetic siderite acts as a significant Fe(II) sink. Preservation of P-bearing FeOOH polymorphs were also observed by using a combination of high-resolution synchrotron-based in situ XRF and XRD analyses. In the sediment pile accessory amounts of pyrite (≤ 0.5 wt. %) can be detected as depth increase, suggesting that a high turnover rate of reduced sulfur occurs towards the SWI. Such effect could be tied to sulfur disproportionation. The meromictic, oligotrophic, ferruginous and sulfate-rich study site exhibits chemical conditions that, via extrapolation, could provide insight int the microbial and abiotic pathways that controlled the coupled iron and sulfur geochemistry of shallow marine Paleoproterozoic coastal zones. A study of dissolved sulfate-bound oxygen and sulfur, and iron isotope ratios of the bottom water column is currently underway to constrain iron- vs. sulfate-reducing activity and ongoing re-oxidation processes.

¹Fakhraee, M., Hancisse, O., Canfield, D.E. et al. Proterozoic seawater sulfate scarcity and the evolution of ocean–atmosphere chemistry. Nat. Geosci. 12, 375–380 (2019).

²Petrash, D.A., Jan, J., Sirová, D., et al. Iron and nitrogen cycling, bacterioplankton community composition and mineral transformations involving phosphorus stabilisation in the ferruginous hypolimnion of a post-mining lake. Environ. Sci. Process. Impacts 20, 1414–1426 (2018).