Earth's early O-2 cycle suppressed by primitive continents

摘要

Free oxygen began to accumulate in Earth's surface environments between 3.0 and 2.4 billion years ago. Links between oxygenation and changes in the composition of continental crust during this time are suspected, but have been difficult to demonstrate. Here we constrain the average composition of the exposed continental crust since 3.7 billion years ago by compiling records of the Cr/U ratio of terrigenous sediments. The resulting record is consistent with a predominantly mafic crust prior to 3.0 billion years ago, followed by a 500-to 700-million-year transition to a crust of modern andesitic composition. Olivine and other Mg-rich minerals in the mafic Archaean crust formed serpentine minerals upon hydration, continuously releasing O-2-scavenging agents such as dihydrogen, hydrogen sulfide and methane to the environment. Temporally, the decline in mafic crust capable of such process coincides with the first accumulation of O-2 in the oceans, and subsequently the atmosphere. We therefore suggest that Earth's early O-2 cycle was ultimately limited by the composition of the exposed upper crust, and remained underdeveloped until modern andesitic continents emerged.