Carbon clues to dino extinction
doi:10.1038/nindia.2008.200 Published online 12 May 2008
Detection of a new form of carbon in volcanic rock samples from Anjar town in Gujarat in western India has revived the debate on what killed the dinosaurs.
Dinosaurs and almost 80 per cent of Earth's other organisms were wiped out 65 million ears ago at the so-called K-T boundary (KTB) that marks the end of Cretaceous (K), and beginning of Tertiary (T) periods in the geological calendar.
But what caused the 'K-T extinction' is an issue that has divided scientists into two camps.
Some say it was the result of extraterrestrial objects hitting the earth, a theory originally proposed by the Nobel physicist Luis Alvarez. Others blame it on vast clouds of climate-altering gases released by eruptions that buried western India under layer upon layer of basaltic lava flows nearly 3,500 meters thick.
Now, researchers from India's three national laboratories have joined the fray. They report that their discovery of a new phase of fullerene (or Carbon-60) in the Anjar sedimentary rocks bolsters the impact theory.
According to this theory, a meteorite the size of a small city that landed in Chicxulub in Mexico 65 million years ago, coinciding with KTB, kicked up so much dust that it caused a global blackout triggering mass extinction.
The key evidence of impact, besides the crater, is the abnormally high concentration of element Iridium found at K–T boundaries across the world. Iridium is more abundant in meteorites than in Earth's crust and so it was hypothesized that the meteorite vaporized after crashing into Mexico and spread the Iridium into the atmosphere.
Since Alvarez and his team proposed the impact hypothesis in 1980, over one hundred KTB sections have been identified worldwide based on the presence of anomalously high Iridium.
The Anjar volcano-sedimentary sequence located in the Kutch region of Gujarat consists of nine lava flows covering a time span from 68.7 to 61.0 million years ago. The sediments trapped between the third and fourth flow have been dated to be close to 65 million years old, the age of KTB.
The Indian scientists who analyzed samples from this sedimentary layer report they have found, along with Iridium, a type of fullerene that can be formed only under conditions of high pressure and temperature (HPT) that can exist during an impact.
"Our findings of HPT phase of fullerenes along with Iridium in the K-T boundary layer at Anjar lends weight to the impact theory," Gopalakrishnarao Parthasarathy of the National Geophysical Research Institute (NGRI) in Hyderabad and the lead author of the paper told Nature India.
"This is further supported by the absence of either Iridium or fullerenes in adjacent lava flows (which took place before or after the KTB) in the same section," he said. Scientists from the Indian Institute of Chemical Technology, also in Hyderabad and the Physical Research laboratory in Ahmedabad collaborated in the work.
According to this group, the HPT fullerenes formed in the hot atmospheric plume of the Chicxulub crater may have been carried by the soot to distant places of the globe and got deposited at the KTB sites.
"People have found normal fullerene at some other K-T-boundary sites. But they may not have been aware of this HPT phase, which is reported by us for the first time in natural samples," Parthasarathy said. The researchers were able to distinguish HPT fullerene from normal fullerene by spectroscopic techniques.
But Princeton University paleontologist Gerta Keller, who has long challenged the impact theory and her collaborator Thierry Adatte at the University of Neuchatel, Switzerland, are not convinced.
They point out that the Anjar section has three small Iridium anomalies, not one, suggesting multiple Iridium producing events. Furthermore, the Iridium concentrations are too low to be of impact origin, and they are "clearly of volcanic origin".
Keller also questions the conclusion that presence of fullerene is evidence of Chicxulub impact saying the section examined by the Indian group is older than 65 million years (the KT boundary).
But Parthasarathy says the age of Anjar section has been independently confirmed by a French group to be 64.8 million years through Argon dating and palaeo-magnetic data also supported the K-T boundary at Anjar.
The fullerene in Anjar rocks is far from solving the mystery of what turned off life on Earth 65 million years ago. Keller and Adatte however admit that the Indian authors have raised 'an interesting dilemma' by detecting fullerenes (that are not known to be of volcanic origins) along with Iridium anomalies of volcanic origins. "These require further investigations," they say.
- Parthasarathy, G. et al. High-pressure phase of natural fullerene C60 in iridium-rich Cretaceous–Tertiary boundary layers of Deccan intertrappean deposits, Anjar, Kutch, India. Geochim. Cosmochim. Ac. 72, 978-987 (2008)
- Courtillot, V. et al. Cosmic markers, 40Ar/39Ar dating and paleomagnetism of the KT sections in the Anjar Area of the Deccan large igneous province. Earth Planet. Sci. Lett. 182, 137-156 (2000)