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To elucidate the role of (bio)geochemical processes that fueled iron and carbon cycling in early Earth oceans, modern environments with similar geochemical conditions are needed. As the range of chemical, physical, and biological attributes of the Precambrian oceans must have varied in time and space, lakes of different compositions are useful to ask and answer different questions. Tropical Lake Matano (Indonesia), the largest known ferruginous lake, and Lake Pavin (France), a meromictic crater lake, are the two best studied Precambrian ocean analogs. Here we present seasonal geochemical data from two glacially formed temperate ferruginous lakes: Brownie Lake (MN) and Canyon Lake (MI) in the Upper Midwest, USA. The results of seasonal monitoring over multiple years indicate that (1) each lake is meromictic with a dense, anoxic monimolimnion, which is separated from the less dense, oxic mixolimnion by a sharp chemocline; (2) below this chemocline are ferruginous waters, with maximum dissolved iron concentrations >1 mM; (3) meromixis in Brownie Lake is largely anthropogenic, whereas in Canyon Lake it is natural; (4) the shallow chemocline of Brownie Lake and high phosphorus reservoir make it an ideal analog to study anoxygenic photosynthesis, elemental ratios, and mineralogy; and (5) a deep penetrating suboxic zone in Canyon Lake may support future studies of suboxic microbial activity or mineral transformation.Plain Language Summary Earth's atmosphere acquired oxygen around 2.46-2.33 billion years ago from oxygen-producing bacteria. Researchers are interested in how elements, for instance, iron and carbon, cycled between dissolved and solid phases in the ocean before oxygen was produced. In past oceans with little oxygen, all life would have been microbial. In order to study elemental cycling on early Earth, modern water bodies where oxygen is absent in deep water are needed. Water samples were analyzed from different depths within Brownie Lake in Minneapolis, MN, and Canyon Lake in the Upper Peninsula of MI. We found that these lakes have two distinct chemical layers like past oceans: a top section that has oxygen and a bottom section that has no oxygen. The bottom sections never mix with the water on top and so remain without oxygen year-round. Brownie and Canyon Lakes have different, but applicable water chemistries as compared to well-studied field sites in Europe, Africa, and Asia. With the addition of Brownie Lake and Canyon Lake as early Earth analogs, researchers will have the potential to answer many questions about early Earth's oceans.

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Uranium isotopes (238U/235U) have been used widely over the last decade as a global proxy for marine redox conditions. The largest isotopic fractionations in the system occur during U reduction, removal, and burial. Applying this basic framework, global U isotope mass balance models have been used to predict the extent of ocean floor anoxia during key intervals throughout Earth's history. However, there are currently minimal constraints on the isotopic fractionation that occurs during reduction and burial in anoxic and iron‐rich (ferruginous) aquatic systems, despite the consensus that ferruginous conditions are thought to have been widespread through the majority of our planet's history. Here we provide the first exploration of δ238U values in natural ferruginous settings. We measured δ238U in sediments from two modern ferruginous lakes (Brownie Lake and Lake Pavin), the water column of Brownie Lake, and sedimentary rocks from the Silurian‐Devonian boundary that were deposited under ferruginous conditions. Additionally, we provide new δ238U data from core top sediments from anoxic but nonsulfidic settings in the Peru Margin oxygen minimum zone. We find that δ238U values from sediments deposited in all of these localities are highly variable but on average are indistinguishable from adjacent oxic sediments. This forces a reevaluation of the global U isotope mass balance and how U isotope values are used to reconstruct the evolution of the marine redox landscape.

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