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Molybdenum isotope study of three key black shale units of the Paleoproterozoic age indicates a dramatic rise of free oxygen level from ca. 2.32 to 2.15 Ga, followed by a crush at ca. 2.06 Ga. We found evidence for a dynamic oceanic Mo cycle at 2.32 Ga, which gradually stabilized as the seawater Mo reservoir increased in response to oxidative continental weathering. We argue that the seawater Mo reservoir was still small during the Lomagundi Event as a result of widespread weakly euxinic conditions in the oceans. See article by Asael and others, p 275–299.