Manganese oxides, Earth surface oxygenation, and the rise of oxygenic photosynthesis

Oxygenic photosynthesis is arguably the most important biological innovation in Earth's history, facilitating the transition to a habitable planet for complex life. Dating the emergence of oxygenic photosynthesis, however, has proven difficult with estimates spanning a billion years. Sedimentary manganese (Mn) enrichments represent a potentially important line of evidence given the high redox potentials necessary to oxidize Mn in natural environments. However, this view has been challenged by abiotic and anaerobic Mn oxidation pathways that decouple Mn enrichments from oxygenation. With these in mind, we review Mn oxidation pathways and Mn enrichments and evaluate their relation to Earth's oxygenation. We argue that despite possible alternative pathways, shallow oxygenated seawater is a prerequisite for generating and, importantly, preserving significant sedimentary Mn enrichments (and associated geochemical signals). This implies that Mn enrichments indeed track Earth's oxygenation and oxygenic photosynthesis emerged 100 s of millions of years prior to irreversible atmospheric oxygenation.