Anoxic and iron-rich (ferruginous) conditions prevailed in the ocean under the low-oxygen atmosphere that occurred through most of the Archean Eon. While euxinic conditions (i.e. anoxic and hydrogen sulfide-rich waters) became more common in the Proterozoic, ferruginous conditions persisted in deep waters. Ferruginous ocean regions would have been a major biosphere and Earth surface reservoir through which elements passed through as part of their global biogeochemical cycles. Understanding key biological events, such as the rise of oxygen in the atmosphere, or even the transitions from ferruginous to euxinic or oxic conditions, requires understanding the biogeochemical processes occurring within ferruginous oceans, and their indicators in the rock record. Important analogs for transitions between ferruginous and oxic or euxinic conditions are paleoferruginous lakes; their sediments commonly host siderite and Ca carbonates, which are important Precambrian records of the carbon cycling. Lakes that were ferruginous in the past, or euxinic lakes with cryptic iron cycling may also help understand transitions between ferruginous and euxinic conditions in shallow and mid-depth oceanic waters during the Proterozoic. Modern ferruginous meromictic lakes, which host diverse anaerobic microbial communities, are increasingly utilized as biogeochemical analogues for ancient ferruginous oceans. Such lakes are believed to be rare, but regional and geological factors indicate they may be more common than previously thought. While physical mixing processes in lakes and oceans are notably different, many chemical and biological processes are similar. The diversity of sizes, stratifications, and water chemistries in ferruginous lakes thus can be leveraged to explore biogeochemical controls in a range of marine systems: near-shore, off shore, silled basins, or those dominated by terrestrial or hydrothermal element sources. Ferruginous systems, both extant and extinct, lacustrine and marine, host a continuum of biogeochemical processes that highlight the important role of iron in the evolution of Earth’s surface environment.