Middle Ordovician climate modulation by enhanced continental and reverse weathering: Evidence from lithium isotopes

Abstract

The Middle Ordovician represents a critical period of climate and oceanic transitions that is notable for a rapid decrease in the marine 87Sr/86Sr ratio, fluctuations in atmospheric CO2 levels, and a brief warming in sea surface temperatures that interrupted a general cooling trend. To investigate the relationship between weathering processes and climate during this period, we analyzed lithium isotopes (δ7Li) in Middle Ordovician carbonate rocks from the Taebaeksan Basin, South Korea. We identified a pronounced ∼6 ‰ negative excursion in δ7Li values during the Middle Darriwilian, followed by a sustained period of low δ7Li values. Using a dynamic box model, we demonstrate that enhanced continental weathering, driven by Taconic arc exhumation, could have initiated the δ7Li decline. Concurrently, reverse weathering may have intensified, as weathering-derived element input to the ocean promoted marine authigenic clay formation. The effects of increased weathering intensity, possibly linked to a brief warming event, drove the subsequent recovery of δ7Li values. During the Middle to Late Darriwilian, enhanced reverse weathering – resulting from the less efficient biological Si uptake – likely reduced the isotopic fractionation between clay minerals and seawater, contributing to a secondary decline in seawater δ7Li values. The active reverse weathering may have played a role in stabilizing atmospheric CO2 levels during the Middle Ordovician by counteracting the CO2 drawdown driven by active continental weathering. Overall, our findings highlight how tectonically-driven weathering modulated global climate during the transition from the warm Early Ordovician to the cooler Late Ordovician environment. © 2025 The Authors