Variable silicate weathering and enhanced reverse weathering during the Ordovician-Silurian transition

The causes of climate change during the Late Ordovician and Early Silurian remain debated. In order to reveal the relationship between climate, continental weathering, and reverse weathering during this interval, this study presents new records of Li isotopes in marine shales (δ⁷Li_shale) from the Wangjiawan section (Global Boundary Stratotype Section and Point for the base of the Hirnantian Stage) and boreholes BD-1 (shallow shelf setting) and YD-1 (deep shelf setting) in South China. The BD-1 borehole shows consistently low δ⁷Li_shale values in the early Rhuddanian (∼ 0 to 5‰). The YD-1 borehole exhibits similar trends to the existing Dob’s Linn section from Scotland, with higher δ⁷Li_shale values in the early Hirnantian (∼ 11‰) than in the late Katian and late Hirnantian-Rhuddanian (∼ 0‰). The high δ⁷Li_shale values during the Hirnantian Glaciation are likely attributed to more incongruent weathering. In contrast, the Wangjiawan (WJW) section is characterised by extremely variable, and often high, δ⁷Li_shale values (−4.2‰ to 18.9‰) throughout the Ordovician-Silurian transition (OST). Given that the maximum values of δ⁷Li_shale fall into the range of reconstructed δ⁷Li_seawater for this interval (16‰ to 23‰), muted isotopic fractionation between marine authigenic clay and seawater (Δ_mac-seawater =  − 8‰) is tentatively inferred, consistent with previous studies. In this case, the high δ⁷Li_shale values were possibly related to enhanced marine authigenic clay formation and/or reverse weathering, caused by the widespread deposition of volcanic ash and the enrichment of biogenic silica on deep-water shelves. From the variability in shale Li isotope compositions across the OST, possible intervals of δ⁷Li_river evolution are inferred based on Rayleigh model calculations (Pre-glaciation: δ⁷Li_river ∼ 10‰; Hirnantian Glaciation: δ⁷Li_river ∼ 26‰, Post-glaciation: δ⁷Li_river ∼ 10‰). Hence, the variations of global δ⁷Li_seawater values across the OST could be explained predominantly by changes in the riverine dissolved Li fluxes and δ⁷Li_river values caused by changes in continental weathering intensity on top of a potentially enhanced reverse weathering baseline. High reverse weathering could have contributed to the warm climate in the early Paleozoic, with variable silicate weathering contributing to climate stabilisation. However, more global sections should be explored to better quantify the magnitude of reverse weathering during the OST.