Oxygen isotopic alteration rate of continental crust recorded by detrital zircon and its implication for deep-time weathering

Weathering plays a significant role in the Earth system through the exchange of material among the lithosphere, atmosphere, hydrosphere, and biosphere. Variation of continental weathering in deep-time, however, remains elusive. This work investigates continental weathering recorded by detrital zircon. Zircon can record the oxygen isotopic composition (delta O-18) of its parent crust at the time of crystallization, the value of which principally reflects the time-integrated effect of crustal alteration. The Hf isotopes and U-Pb isotopes of zircon also help to constrain the alteration history between crust generation and zircon crystallization. A new algorithm is introduced to reconstruct the average delta O-18 alteration rate of continental crust (R delta 18O-CC) through time by solving a set of linear equations based on a large population of detrital zircons with varying temporal coverage across the history of crustal alteration. A nearly three-billion-year history of R delta 18O-CC from 3.2 Ga to 0.3 Ga can be reconstructed using more than 5,000 globally distributed detrital zircons with coupled U-Pb-Hf-O isotopic records. The reconstructed R delta 18O-CC shows an overall bell-shape long-term evolution centered at similar to 2 Ga superposed with variations that are coupled with supercontinental assembly cycles. The long-term evolution of the reconstructed R delta 18O-CC seems to be correlated with solid-earth CO2 degassing expected from the age distribution of deleted mantle and the supercontinental cycles. Thus, the R delta 18O-CC is interpreted to reflect weathering considering the control of solid-earth CO2 degassing on the total weathering flux of continental crust. However, independent evidence on the solid-earth CO2 degassing is unavailable, interpreting R delta 18O-CC as a weathering record requires further testing. Nevertheless, this work provides an example of how the time-integrated signal, with large noise-to-signal ratio, preserved in geological archives can be deconvolved using a large dataset. The result also demonstrates the great potential that weathering history may have in reconstructing the operation of the Earth system across deep-time. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Weathering plays a significant role in the Earth system. This study investigates continental weathering using detrital zircon and reconstructs a nearly three-billion-year history of weathering. The results show a correlation between long-term weathering evolution and supercontinent assembly cycles, but further testing is required.