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 (δ18O) 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 δ18O alteration rate of continental crust (Rδ18O-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 Rδ18O-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 Rδ18O-CC shows an overall bell-shape long-term evolution centered at ∼2 Ga superposed with variations that are coupled with supercontinental assembly cycles. The long-term evolution of the reconstructed Rδ18O-CC seems to be correlated with solid-earth CO2 degassing expected from the age distribution of deleted mantle and the supercontinental cycles. Thus, the Rδ18O-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 Rδ18O-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.