Research Communications

Until recently, most scientists have believed that the extinction of the dinosaurs was caused by a single event: a 10-kilometer-diameter asteroid crashed on the Yucatan peninsula in Mexico, creating the Chicxulub crater, causing global forest fires and tsunamis, collapsing the ecosystems, leading one of the largest mass extinctions 65 million years ago. Some researchers, like Sankar Chatterjee from Texas Tech University, have found another, even larger crater on the western shelf of India and proposed that multiple impacts were responsible for the mass extinction event that marked the time between the Cretaceous (K) and the Tertiary (T) periods, or the KT boundary.

The KT impact theory emerged in the 1980s, when Luis and Walter Alvarez proposed that an asteroid struck the Earth, clogging the atmosphere with pulverized rock and blocking the sunlight. Evidence of an impact includes a thin layer of iridium at the KT boundary that covers several places on Earth. Iridium is rarely found on Earth, but it is abundant in extraterrestrial material, making the impact of an asteroid a likely cause. Along with the iridium spike, another crucial evidence in favor of an impact event is the presence of shocked quartz grains at the KT boundary that show multiple sets of intersecting fracture lines under microscope. These signal disruption of crystal lattice at an extremely high pressure generated by an impact.

Paul Whitfield Horn Professor of Geosciences and Curator of the Paleontology Museum of Texas Tech, Chatterjee has studied many aspects of the history of life and has led several expeditions to China, India, Antarctica, and the American Southwest in search of the dinosaurs and early birds. But the question capturing his attention lately is a basic one that had intrigued paleontologists for more than a century: what killed the dinosaurs? Chatterjee and a growing number of colleagues around the world are turning up evidence that, rather than a single Chicxulub event, a series of asteroid impacts and intensive volcanic eruptions are likely to have stressed the world ecosystem that led to the demise of dinosaurs.

Chatterjee has identified a second crater at the KT boundary, submerged on the western shelf of India around the Bombay High, the largest oil field in India. Named after the Hindu god of destruction and renewal, the Shiva crater is the largest known impact crater on Earth, measuring 500 kilometers in diameter, and it is the buried remnant of the fateful dinosaur-killing event. “The asteroid that created the Shiva crater would have been 40 kilometers in diameter, and the energy created by the impact would have been the largest holocaust in geologic history, equivalent to the explosion of 100 trillion tons of TNT, or about 10,000 times greater than the explosive energy of the world’s entire nuclear arsenal,” says Chatterjee. The impact may have injected a large volume of dust into the atmosphere that blocked sunlight for months and prevented photosynthesis; as a result, plants and animals died, the food chain collapsed, and the dinosaurs perished. The tremendous impact would have ignited global fires, initiated tsunamis, destroyed coastal habitats, produced acid rains, turned seawater acidic, dissolved carbonate-shelled animals and devastated the biosphere. Millions of organisms would have died instantly from the tremendous tremor and the global fire generated from the impact.

Chatterjee thinks that the Shiva impact might have enhanced a spectacular volcanic outburst known as the Deccan Trap when a million cubic kilometers of lava flooded the western part of India in just a few great pulses. The greatest pulse seems to have coincided with the Shiva impact. “The Shiva impact shattered the lithosphere, made the western coast of India seismically active, and caused the plate movement separating India from the Seychelles Island,” Chatterjee claims. The impact probably led to the sudden northward acceleration of the Indian plate, which was then an island continent located south of the equator, to collide with Asia, forming the Himalayas.

His compelling claims follow years of arduous collection of physical and chemical evidence in support of the Shiva crater’s connection to the KT extinction event. Chatterjee and his team have collected a rich treasure trove of the last dinosaurs of India below the KT boundary during extensive field works, funded by the National Geographic Society and Smithsonian Institution. The different KT boundary sections of India show the telltale signatures of the killer asteroid, such as an iridium anomaly, shocked quartz, nickel-rich spinels, high-pressure fullerenes, and glass-altered smectites—all indicative of a very powerful impact event. Chatterjee and his research team report their findings in Shiva Structure: A Possible KT Boundary Impact Crater On The Western Shelf of India, the October 2006 monograph published as part of the Museum of Texas Tech’s Special Publication series.

Chatterjee has just been selected for the L. Rama Rao Birth Centenary Award in recognition of his outstanding contributions to Indian Stratigraphy and Paleontology. The Council of the Geological Society of India agreed that Chatterjee’s work has made significant strides toward understanding the cause and consequences of the death of dinosaurs 65 million years ago, the discovery of Maastrichition dinosaurs and their eggs, and the Cretaceous-Tertiary boundary in and around the Deccan Traps in India. The award will be presented at the Annual Convention of the Geological Society of India to be held at the Wadia Institute of Himalayan Geology, Dehra Dun on Friday, November 24, 2006. Named after the founder editor of the Journal of the Geological Society of India, L. Rama Rao, it is the most prestigious award presented by the Geological Society of India.