Reconciling Seawater δ18O Records with Solid Earth System Evolution

Abstract:

A quantitative understanding of seawater compositional evolution can help to unravel a variety of solid Earth phenomena, including the history of crustal alteration, the extent of volatile degassing and regassing, and the style of mantle convection. In this study, we present a new model of coupled crust-mantle-ocean evolution to investigate the geological processes that are responsible for the evolution of seawater ¹⁸O. This work builds from several recently generated novel records—from iron oxides and ancient altered oceanic crust—of the oxygen isotope composition of seawater (Galili et al., 2019; Johnson and Wing, 2020). In our model, the changing rates of low- and high-temperature crustal alteration are constrained by the history of mantle cooling and crustal evolution. We can reconcile the seemingly contrasting evolutionary trends of seawater ¹⁸O provided by recent studies. Further, our results suggest that the evolution of seawater ¹⁸O favors rapid crustal growth during the early Earth. The declining influence of low-temperature crustal erosion and alteration since 3 Ga, which is consistent with an early emergence of continental crust, dominates the long-term evolution of seawater ¹⁸O. The rapid growth of continental crust and its early emergence provide important constraints on surface environment, tectonics, and the evolution of life in the early Earth.