Supplementary Material - Enhanced reverse weathering in the Late Permian and the Early Triassic Ocean

Chemical weathering and reverse weathering are two critical processes controlling biogeochemical cycles. Geochemical and stratigraphic records suggest that these processes might have varied considerably during the Permian to Early Triassic period influencing biogeochemical cycles during this critical interval of Earth’s history. Here we present new seawater lithium and strontium isotope records derived from bulk carbonates and fossil brachiopods spanning the Permian to Early Triassic period (300 Ma to 247 Ma). The lithium isotope composition of seawater remained constant for most of the Permian until a sharp decrease in the late Wuchiapingian which reached a minimum of around 5‰ in the Changhsingian, before the Permian-Triassic boundary (PTB, ~ 251.92 Ma). This decline occurred several million years prior to the PTB but low seawater Li isotope values (~10‰) persisted throughout the Early Triassic. Based on steady-state box modeling of the marine Li isotope budget, changes in the fluxes and isotopic composition of Li inputs alone (i.e., chemical weathering and hydrothermal) are unable to explain the abrupt decline in seawater Li isotopes. Rather, increased reverse weathering in the ocean is required to initiate the quick drawdown and maintain low Li isotope values in the Late Permian and Early Triassic (253-247 Ma).