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.