Lithium isotope and mercury evidence for enhanced continental weathering and intense volcanism during the Ordovician-Silurian transition

Abstract

The Ordovician-Silurian transition (OST) was characterised by climatic fluctuations (warming in the Katian and glaciation in the Hirnantian) and mass extinctions. However, the mechanisms driving the climatic and biological variability remain under debate. In order to reveal the relationships between volcanism, climate, and continental weathering, we measured lithium (Li) isotopes and mercury (Hg) concentrations in a carbonate-dominated marine section from South China. The reconstructed δ⁷Li_seawater values were generally ∼ 21 ‰ during the Ordovician-Silurian transition, with negative excursions towards ∼ 16 ‰ in the latest Katian and the latest Hirnantian intervals. We infer that changes in continental weathering affected dissolved riverine Li fluxes and δ⁷Li values, and thereby exerted a major control on the seawater δ⁷Li variations, while changes in temperature that influenced isotope fractionation during weathering and reverse weathering exerted a secondary control. In the Late Katian, intense volcanic activity (high Hg/TOC ratios and low δ¹³C values) likely initiated the climatic warming (late Boda warming), which was sustained by enhanced clay formation (δ⁷Li_seawater values of ∼ 21 ‰). The intense volcanism also contributed to the high primary productivity and expansion of ocean anoxia, accounting for the Katian extinction. In the latest Katian and latest Hirnantian, enhanced and more congruent weathering (δ⁷Li_seawater values of 16 ‰) likely contributed to the initiation of global cooling and further glaciation. Meanwhile, the weathering-induced expansion of euxinic seawater could have driven the Late Ordovician Mass Extinction (LOME) events. During the Hirnantian glacial intervals, decreased and incongruent weathering could have contributed to reduced CO₂ drawdown, ultimately allowing warming and climatic recovery. Overall, the climatic fluctuations during the OST were related to changes in continental weathering, while the multi-phase biotic extinctions could be attributed to volcanism and/or weathering-induced oceanic anoxia.