Late Ordovician and early Silurian warming sustained by enhanced clay formation

The Ordovician–Silurian transition (O-S) was a period of dramatic climatic, environmental, and biological changes marked by severe mass extinction, glaciation, intense volcanism, marine anoxia, and widespread deposition of organic-rich shale. Silicate weathering has been proposed as a potential driver for the extreme climate change and invoked as a driver for marine anoxia during this time. However, the changes in chemical weathering across O-S transition are poorly constrained. Here, we present high-resolution Li isotope (δ⁷Li) records of marine shales from South China, spanning the Upper Ordovician to Lower Silurian to track changes in continental weathering across the O-S transition. We find significant positive δ⁷Li excursions in the Late Ordovician (Katian stage) and early Silurian (Rhuddanian stage), reflecting a shift to incongruent weathering, associated with secondary clay formation. Clay formation can retain cations on the continents, resulting in inefficient atmospheric CO₂ consumption through silicate weathering. We therefore propose that enhanced clay formation may have sustained the long-term greenhouse conditions during Early Silurian, although volcanic degassing may have acted as a trigger. The greenhouse conditions would have reduced the thermohaline circulation and oxygen solubility, facilitating the development of prolonged anoxia throughout the Early Silurian and delayed the biotic recovery of marine ecosystems during this period. Marina anoxia could enhance the burial of huge amounts of organic matter in the sedimentary record as globally distributed organic-rich black shales, which ultimately caused the drawdown of atmospheric CO₂ and allowed the climate recovery.