Massive volcanic eruptions set the stage for dinosaurs’ demise
Deccan Traps spewed over a million cubic kilometers of rock just before impact.

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It's now widely accepted that the impact of an asteroid at Chicxulub in Mexico's Yucatan region finished off any dinosaurs that we don't currently refer to as birds, while triggering a mass extinction that wiped out a lot of other species. But that hasn't ended the debate regarding the dynamics of the extinction event, with other ecological influences getting consideration as contributing to the dinosaurs' vulnerability.

One potential contributor that's hard to overlook is situated in western India: the Deccan Traps. These enormous deposits are built of layer upon layer of volcanic rock, suggesting a series of flood eruptions took place over thousands of years. These eruptions happened suspiciously close to the start of the mass extinction—close enough that some researchers argued that it was the eruptions that killed off the dinosaurs. There was, after all, precedent; the eruptions that formed the Siberian Traps have been blamed for a mass extinction that was so severe, it's known as the The Great Dying.

To help settle the issue, an international team of researchers has gone back and obtained the most precise dates for the eruptions yet. The dates show that the eruptions started nearly a quarter-million years before the onset of the mass extinction but continued for roughly 750,000 years, meaning they spanned the extinction event. This supports the idea that the eruptions helped set the stage for the end of the dinosaurs.

The dating techniques used in the work—uranium-lead dating of zircons—is pretty standard. But it's really the technical challenges of getting it right in multiple samples from a massive eruption that stand out. It was easy to find zircons and get dates from them, but a number of reasons to expect those dates wouldn't be very precise.

First, the researchers had to ensure that the zircons had formed in the magma involved in the eruptions rather than forming earlier and being included in them. This was handled by visually examining the crystals, which could eliminate non-volcanic zircons. But even then, some of the crystals seem to have taken an extended time to form, as the authors obtained a range of dates from their samples. So the authors looked at the presence of other elements in the sample, finding a set of zircons that had the same chemical signature and thus likely formed in the same magma.

To add an additional layer of control, the authors used the fact that each layer of rock must have erupted after the layers it was sitting on top of. So once dates were obtained, the researchers ordered them by rock layer and ran the whole stack through an analysis that discarded any dates that were out of order.

The authors conclude that it took 750,000 years to put about 80-90 percent of the rock that was ultimately erupted in place. That's a long time, but the Deccan Traps consist of at least 1.1 million cubic kilometers of rock, so that number still represents a pretty staggering series of eruptions. The dates also place the start of the eruptions at 250,000 years prior to the mass extinction event at the Cretaceous-Paleogene boundary.

The start of the eruptions clearly didn't trigger the mass extinction. But a variety of ecological disruptions are apparent in the fossil record leading up to the mass extinction, and it's clear that the Deccan Traps eruptions could have contributed to these disruptions. The duration of the events also means that the eruptions could have been ongoing even as the Chicxulub impact occurred, which really provides just about any species out there a great excuse for dying.