The global signature of catastrophe
Around 66 million years ago, Earth experienced a deep crisis. The Cretaceous-Paleogene (K-Pg) extinction event eliminated about 75% of all species, including the non-avian dinosaurs. Scientists first identified the global record of this event in the 1980s: a thin, worldwide layer of sediment containing unusually high concentrations of the element iridium. Iridium is rare in Earth's crust but much more common in asteroids and other extraterrestrial materials. This discovery led to the widely accepted hypothesis that the impact of a large asteroid near Chicxulub, Mexico, caused the mass extinction.
At the very same time, geologically speaking, a different kind of cataclysm was unfolding on the other side of the planet. In what is now the Maharashtra region of India, one of the largest volcanic features on Earth was forming. The Deccan Traps are a massive province of flood basalt, a series of colossal eruptions that buried a vast area under lava. These eruptions began before the Chicxulub impact and continued for some time after, spanning the K-Pg boundary. The puzzle remains in sedimentary layers, known as intertrappean beds, found between the ancient lava flows. Here, too, scientists have found an iridium anomaly, forcing a question: did this iridium rain down from the asteroid, or did it well up from the Earth's mantle?
A mantle plume or asteroid dust?
The Deccan Traps eruptions were of an almost unimaginable scale. Over a period of roughly a million years, they spewed out more than a million cubic kilometers of lava, covering an area that may have originally been as large as 1.5 million square kilometers. In some places, the stacked layers of basalt are over two kilometers thick. This volcanic activity was likely caused by a deep mantle plume, a column of superheated rock rising from deep within the Earth.
This mantle origin is one hypothesis for the iridium layer. Because iridium is a siderophile, or "iron-loving" element, it is more concentrated in Earth's mantle and core than in the crust. A powerful mantle plume could have transported iridium to the surface and ejected it during eruptions.
The alternative hypothesis is that the iridium is purely extraterrestrial. During lulls in the volcanic eruptions, which could last for thousands of years, dust and sediment accumulated on the hardened lava fields. According to this view, iridium-rich dust from the vaporized Chicxulub asteroid settled out of the atmosphere across the globe, including on the Deccan Traps. This dust was then buried and preserved by the next lava flow. Detailed studies, particularly in the Anjar region, have identified iridium-rich layers associated with dinosaur eggshells, suggesting the element was present before the final extinction. The debate continues, with some evidence showing multiple impacts and others to complex interactions between the impact event and the ongoing volcanism.