The Deccan Traps, India. Image: Science and M. Widdowson
What do dinosaurs, enormous lava flows, and asteroid impacts have in common? Aside from being fascinating to most 10 year olds, they were all involved in the events that took place about 65 million years ago. The exact timing and relationship between these three topics has long been of interest, and an article published today in Science sheds some new light on the potential impact of the lava flows.
The dinosaurs, and many other species, disappeared from the earth 65 million years ago in one of the largest extinction events the world has known. This extinction event occurred at the end of the Cretaceous period at what is known as the K-T (Cretaceous-Tertiary) boundary. Probably because dinosaurs are such an icon of paleontology, the K-T boundary has been studied intensively.
Presently, the two most common theories for the cause of the K-T extinction event are extensive volcanism and a large asteroid impact. A large impact would have put enough dust in the atmosphere to decrease the sunlight available for photosynthesis for several years and created global firestorms (far more intense heat than a wildfire). Indeed, soot is present at the K-T boundary, and the only animals that survived the event were those that could hide underground (e.g. small burrowing mammals) or underwater.
While there is substantial evidence for a major impact at the K-T boundary, it is also clear that there was extensive volcanism at the time. The lava flows of the Deccan Traps in India date to the end of the Cretaceous period, and their volume is estimated to be 1.3 million cubic kilometers. For reference, India is only 3 million square kilometers. Extensive volcanism can have large climatic consequences because of the aerosols and greenhouse gases it introduces to the atmosphere.
One problem with claiming that volcanism was responsible for the K-T extinction is the uncertainty about the volume of gas put into the atmosphere. Previous estimates had been based on the assumption that these lava flows emitted as much gas as current mid-ocean ridge basalt flows. Today's Science article presents measurements of the gas concentrations preserved in bubbles within the lava flows. The authors searched through 150 rock samples under a microscope to find four samples that contained gas bubbles they could analyze.
The authors confirmed that gas concentrations were similar to those of modern mid-ocean ridge lava flows. This is important because it means that a single eruption event could have put 1000 teragrams of Sulfur Dioxide into the atmosphere. The best comparison the authors can make is to an eruption in 1783 that put a mere 120 Tg into the atmosphere and had significant atmospheric effects. More recent volcanism, such as the eruption of Mt. Pinatubo in 1991, may not be a good analog, because the length of the eruption differs substantially.
While the authors present a significant data point for evaluating the impact of volcanism on the dinosaurs, the puzzle is by no means complete. Improvements to models of paleoclimate, as well as an evaluation of the other major periods of volcanism that have occurred over the last 350 million years, are necessary to further our understanding of the relationship between volcanic eruptions and extensive extinction events.
Science, 2008. DOI: 10.1126/science.1152830