A chemical fingerprint at world's end
Around 66 million years ago, as the Cretaceous period drew to a close, western India experienced volcanism on an almost unimaginable scale. Over hundreds of thousands of years, a series of eruptions buried an area of at least 500,000 square kilometers under layers of basalt lava, in some places more than two kilometers thick. This geological feature is the Deccan Traps. Between these immense, ancient lava flows are thin sedimentary layers, known as intertrappean beds, that preserve a record of the environment between eruptions.
Scientists analyzing these layers—along with marine sediments from the same period found globally—found an important clue to the dinosaurs' demise: a dramatic spike in mercury. Volcanoes are the planet's largest natural source of atmospheric mercury. Geochemical analysis shows that mercury concentrations in sediments laid down during the most intense phases of the Deccan eruptions are orders of magnitude higher than normal background levels. This mercury signature is a direct fingerprint of massive, sustained volcanic outgassing.
A planet on the brink
The elevated mercury levels are a proxy for a much larger environmental crisis. The same volcanic vents that released mercury also spewed enormous quantities of sulfur dioxide and carbon dioxide into the atmosphere. This triggered a cascade of destructive effects across the globe. The release of CO2 caused a period of intense global warming, raising global temperatures by 3–4°C. Sulfur gases likely led to devastating acid rain, altering soil and ocean chemistry.
Evidence from fossil shells shows that ocean acidification was widespread, making it difficult for marine organisms like planktic foraminifera to build their shells. The ecosystem was under severe stress for at least 250,000 years before the Chicxulub asteroid impact. The mercury anomaly, correlated with pulses of eruptive activity, shows that the Deccan Traps created a toxic, unstable world. Life on Earth was already struggling for survival when the asteroid delivered the final blow. Some studies of ancient fossil shells have found mercury levels comparable to modern industrial pollution sites.