PRIMITIVE MAFIC CONTINENTS IMPEDED OXYGENATION DURING THE ARCHEAN

The development of Earth’s O₂ cycle 3.0-2.4 Gyr ago is one of the most important geochemical processes in the history of system Earth. In spite of its fundamental importance, this process is not well understood; its causes and governing mechanisms are yet to be fully characterized. For decades, suggestions have been made of a link between the global O₂ cycle – most notably oceanic atmospheric oxygenation – and the changing composition of the continental crust. Archean crustal evolution, however, is still highly debated and any such link is obscured. Preservation of Archean crust is a prime issue in this regard; fragments of such crust are typically too strongly overprinted or altered to precisely estimate average crustal composition and may otherwise be subject to preservation bias. To circumvent this issue, we use the composition of terrigenous sediments deposited during the past 3.7 Gyr to estimate average continental composition and changes therein. We use Cr/U as a new compositional tracer in these sediments. This elemental ratio has demonstrably strong resolving power for (source-)lithology. In addition, both elements are expected to show similar behaviour during detrital transport and weathering under variably oxidative conditions. The Cr/U data reveal a striking secular change in the composition of the exposed continental crust during the Archean. Before 3.0 Gyr ago, the crust was mafic to the extent that it contained olivine. Such crust supported serpentinization, which – in present-day analogues – causes strong oxygen scavenging in the local environment. The decline in olivine-bearing continental crust was coeval with the first accumulation of O₂ in the oceans and the disappearance of such crust was followed almost instantly by atmospheric oxygenation. This systematic leads us to conclude that the early O₂ cycle was ultimately limited by the composition of the exposed upper crust and remained underdeveloped until modern andesitic continents emerged.