Abstract
Replying to C. T. Reinhard & N. J. Planavsky , doi:10.1038/nature09959 (2011) ; N. Dauphas & J. F. Kasting doi:10.1038/nature09960 (2011) ; C. Goldblatt & K. Zahnle doi:10.1038/nature09961 (2011) Reinhard and Planavsky 1 and Dauphas and Kasting 2 question whether the formation of Fe2+/Fe3+-rich sediments and the preservation of magnetite in banded iron formations (BIFs) constrains atmospheric . Goldblatt and Zahnle 3 question whether reduced albedo in combination with a slightly increased greenhouse effect could compensate for the reduced luminosity of the faint young Sun. Reinhard and Planavsky1 and Dauphas and Kasting2 suggest that BIF mineralogy is solely controlled by diagenetic and metamorphic processes decoupled from ocean–atmosphere chemistry. We focus on the persistent preservation of magnetite in all Archean BIFs, and of magnetite and siderite in many BIFs4, and estimate the partial pressure of CO2 during the Archean ( ) using the least number of assumptions concerning unconstrained parameters. Our conclusions5 are not undermined by any geochemical data that support high .
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