The emergence of subaerial crust and onset of weathering 3.7 billion years ago

Abstract

Reconstructing the emergence and weathering of continental crust in the Archean is crucial for our understanding of early ocean chemistry, biosphere evolution and the onset of plate tectonics. However, considerable disagreement exists between the elemental and isotopic proxies that have been used to trace crustal input into marine sediments and data are scarce prior to 3 billion years ago. Here we show that chemical weathering modified the Sr isotopic composition of seawater as recorded in 3.52-3.20 Ga stratiform barite deposits from three different cratons. Using a combination of Sr, S and O isotope data, barite petrography and a hydrothermal mixing model, we calculate a novel Sr isotope evolution trend for Paleoarchean seawater that is much more radiogenic than the curve previously determined from carbonate rocks. Our findings require the presence and weathering of subaerial and evolved (high Rb/Sr) crust from 3.7 ± 0.1 Ga onwards. This Eoarchean onset of crustal weathering affected the chemistry of the oceans and supplied nutrients to the marine biosphere 500 million years earlier than previously thought.