The lithologic composition of Earth’s emerged lands reconstructed from the chemistry of terrigenous sediments

The lithologic and chemical composition of the continental crust impacts Earth atmosphere and environment through e.g. weathering feedbacks and nutrient supply. However, despite being important for  the biological and atmospheric evolution of our planet, the question of how the lithological composition of Earth’s landmasses evolved from around 3.5 Ga to present is still a matter of considerable debate.

Here I will present a summary of the work that has been conducted by my colleagues and myself over the past five years and that improved our understanding of the chemical and lithological evolution of Earth landmasses since 3.5 Ga. Reconstructing the composition of past continents is difficult because erosion and crustal reworking may have modified the geologic record in deep time, so direct examination of the nature of igneous rocks could provide a biased perspective on the nature of the continents through time. A less biased record is likely provided by terrigenous sediments that average the composition of rocks exposed to weathering on emerged lands and we therefore use major and trace element concentrations and stable isotope compositions of shales as a proxy for the average composition of the emerged continents in the past. Applying a three-component mixing model to the sediment record shows that since 3.5 Ga, the landmasses that were subjected to erosion were dominated by felsic rocks. Furthermore, our reconstructed relative abundance of felsic, mafic and komatiitic rocks in the Archean is close to that currently observed in these ancient terrains. While our model does not suggest a strong change in the lithologic composition of Earth continents, we find a secular change in the average major and trace element concentration, with incompatible elements being more depleted and compatible elements being more enriched in the old landmasses.