Lithium isotope evidence shows Devonian afforestation may have significantly altered the global silicate weathering regime

Abstract

The Devonian Period (∼359–419 Ma) documents significant environmental changes and marine species turnover, but whether these changes were linked to terrestrial weathering remains unknown. Here, we use lithium isotopes in brachiopods and bulk marine carbonates (δ7Licarb) from the Devonian Period to investigate changes in silicate weathering, which represents the primary long-term atmospheric CO2 sink. A rise of ∼ 10 ‰ in δ7Licarb values (from ∼ 8 ‰ to ∼ 18 ‰) is observed across the Mid-Devonian (∼378–385 Ma), suggesting a major change in the seawater Li cycle. We attribute the rise in δ7Licarb values to an increase in the dissolved riverine Li flux and δ7Liriver values, which likely arose from increases in both weathering intensity and regolith thickness, related to the expansion of deep-rooted plants. However, the presence of such terrestrial ecosystems would also have restricted the continuous weathering of silicate rocks. In order to maintain high δ7Liseawater values in the Late Devonian, we propose that repeated cycles of destruction and regeneration of terrestrial forest ecosystems could have occurred, which would have prevented a supply-limited weathering regime from being permanently established. Such a process would potentially have caused oscillations in marine nutrient availability and redox conditions, thereby contributing to prolonged marine biodiversity loss during the Late Devonian.