A planet in crisis
At this unassuming outcrop 42 km north of Yichang city, a bronze plaque marks a line in the rock. This is a Global Boundary Stratotype Section and Point (GSSP), a location that is the official, internationally agreed-upon reference for a major geological time boundary. The line at Wangjiawan separates the Ordovician Period from the Silurian Period, pinpointing the climax of the Late Ordovician Mass Extinction around 443 million years ago. This event was the first of the 'Big Five' mass extinctions and eliminated about 85% of all marine species.
The extinction unfolded in two distinct pulses, driven by a severe and rapid ice age. During the Late Ordovician, the supercontinent Gondwana drifted over the South Pole, triggering the formation of massive ice sheets. This Hirnantian glaciation locked up immense volumes of water, causing a sea-level drop of up to 100 meters. The draining of shallow epicontinental seas destroyed the habitats for a huge number of species adapted to warm, tropical waters. This first pulse of extinction devastated communities of trilobites, brachiopods, and corals. The second pulse occurred as the ice age abruptly ended. Glaciers melted, sea levels rose rapidly, and a flood of freshwater disrupted ocean circulation. This led to widespread anoxia, a severe depletion of oxygen in the oceans, which proved fatal for many of the disaster's initial survivors.