The composition and weathering of the continents over geologic time

Abstract: The composition of continental crust records a history of construction by tectonics and destruction by physical and chemical erosion. Quantitative constraints on how both igneous addition and chemical weathering have modified the continents' bulk composition are essential for understanding the evolution of geodynamics and climate. We have extracted temporal trends in sediments' protolith composition and weathering intensity from the largest available compilation of sedimentary major-element compositions, of ~ 15,000 samples from 4.0 Ga to the present. To do this we used a new analytical method which inverts whole sedimentary compositions for protolith composition and weathering intensity simultaneously. We find that the average Archean upper continental crust was silica rich and had a similar compositional diversity to modern continents. This is consistent with an early-Archean, or earlier, onset of plate tectonics. In the Archean, chemical weathering was ~ 25 % more efficient at sequestering CO2 than in subsequent time periods. Since 2.0 Ga, over long (> 0.5 Ga) timescales, the crustal weathering intensity has remained largely constant. On shorter timescales over the Phanerozoic, the intensity of weathering is correlated to global climate state, consistent with silicate weathering feedback acting in response to changes in CO2 outgassing.

Status: Published at Geochemical Perspectives Letters.