Using the titanium isotopic make up of shales, Greber et al. develop a proxy for reconstructing the SiO2 content of shale source rocks and thus the chemical composition of emerged crust. From this they construct a mixing model to translate titanium isotope values of shales into an estimate of the crustal composition, and an emergence of predominantly felsic crust since at least 3.5 Ga.
The character of this crust influenced both atmospheric and oceanic chemistry on Earth. In addition to changes in Ni chemistry, the authors observe a change in quantity of the biologically important nutrients P and N to the oceans across the Archean-Proterozoic boundary, likely derived from crustal weathering. With more nutrients to stimulate biological productivity, this may have paved the way for an oxygenated atmosphere.
Fig 1: Correlation between Ti isotopic composition and SiO concentration in igneous rocks. Data used in the paper are either tabulated in the supplementary material, published in the cited references, or archived in the PetDB Database
Data used in the paper are either tabulated in the supplementary material, published in the cited references, or archived in the PetDB Database
Greber, et al.,Titanium isotopic evidence for felsic crust and plate tectonics 3.5 billion years ago, Science Sep 2017:Vol. 357, Issue 6357, pp. 1271-1274 DOI: 10.1126/science.aan8086