Shedding light on manganese cycling in the early oceans

Agreement is often in short supply within the community of researchers asking questions about Earth’s earliest history. One exception, however, is the near-consensus view that oxygen (O2) was mostly absent from the atmosphere and oceans before about 2.4 to 2.3 Ga—the so-called Great Oxidation Event (GOE). It is also widely accepted that the cycles for iron (Fe) and manganese (Mn) in the early oceans are essential themes in our still nascent understanding of the earliest chapters of life’s coevolution with its host environments at Earth’s surface. Yet, despite this community accord over the importance of Fe and Mn, we still know very little about the principal pathways of oxidation for these metals and the implications for early O2. There are three fundamental possibilities: 1) biologically mediated reactions in the presence of even small amounts of O2, 2) anoxygenic photosynthesis with Fe(II) or Mn(II) as electron donors yielding oxidized forms of those metals as by-products, and 3) sunlight-catalyzed photochemical oxidation by ultraviolet (UV) radiation without contributions from life or O2. The latter is perhaps the least appreciated among these three, even though abiotic reactions likely had elevated importance in early oceans as life was finding its first footholds. Liu et al. (1) refocus attention on the photochemical possibility by exploring the details of photooxidation of Mn(II) in the mineral rhodochrosite (MnCO3; Fig. 1) in a series of elegant experiments and by examining the implications of their results in terms of oceanic and atmospheric chemistry during the Archean Eon (4.0 to 2.5 Ga). Fig. 1. Gem-quality crystalline rhodochrosite (MnCO3)—an eye-catching version of the far more mundane small mineral grains that may have settled through the early oceans and accumulated on the seafloor. As the grains settled and accumulated, UV radiation may have oxidized the … [↵][1]1To whom correspondence may be addressed. Email: timothy.lyons{at}ucr.edu. [1]: #xref-corresp-1-1