Reconciling Coupled Thermal-Water Evolution Models of Earth With Observations Through Variable Regassing Efficiency

Abstract

Reconciling Earth's thermal evolution with geochemical observations remains a fundamental challenge, as parameterized convection models typically predict high present-day Urey ratios inconsistent with geochemical estimates. Previous studies propose that if mantle water concentration varies inversely to mantle temperature, it would weaken the feedback between temperature, viscosity, and heat loss, promoting primordial heat retention allowing models to match Urey ratio estimates. However, many previous coupled thermal-water models fail to produce such a water history, leading to Urey ratios that are still too high. We propose that a time-variable regassing efficiency provides a physical mechanism to resolve these discrepancies. We incorporate time-dependent regassing efficiency into parameterized coupled thermal-water models and produce realizations matching present-day constraints on critical mantle variables like mantle temperature, water concentration, and the Urey ratio while aligning with petrological estimates of Archean-Proterozoic mantle temperatures and Phanerozoic freeboard constancy. Variable regassing efficiency allows mantle water concentration to vary inversely with temperature by 1-2 orders of magnitude, drying as the mantle warms in the Archean then rehydrating as the mantle cools; such a scenario keeps viscosity approximately constant and weakens the feedback between mantle temperature and heat loss. Across >95% of successful realizations Earth's total water budget is constrained to 1.25–1.7 ocean masses, with a maximum past surface water mass of ∼1.4 ocean masses. Low total water budgets permit sufficient Archean upper mantle dehydration while still allowing subsequent rehydration to remove excess surface water and match the present day ocean mass.