New study reveals a 'nuclear winter' may not have killed the dinosaurs

A new study published in the journal Geology, seems to indicate that a long "nuclear winter" did not occur after the asteroid impact 66 mya.
Christopher McFadden
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The plot thickens...


66 million years ago, something terrible happened to Earth that wiped out 75 percent of species, including all nonavian (bird-like) dinosaurs. Most scientists think a massive asteroid impact at Chixculub, Mexico, caused this and plunged the entire planet into nuclear winter.

But a new study published in the journal Geology may have found proof that this didn't happen to the extent believed, if at all.

The current theory is that a 60-mile-wide (10-kilometer) asteroid hit what was the Yucatán Peninsula one spring. This event was so violent that it kicked up so much dust and debris into the atmosphere that it blocked out the sun for months, perhaps decades. Likened to a "nuclear winter," this would have killed off many plants, leading to the partial or total collapse of the food chain on Earth. The dust shroud would also have caused large-scale cooling of the planet as the heat from sunlight would have been blocked from reaching the Earth's surface.

But, as the new study's researchers found, this does not appear to correlate with some key fossils that tend to be a good proxy for the Earth's climate over time. "We found that there was no evidence for the 'nuclear winter,'" Lauren O'Connor, a geoscientist at Utrecht University in the Netherlands and first author of the study, told Live Science. "At least, not in the resolution of our study," which, Live Science reports, "would have detected temperature declines spanning 1,000 years or more." 

O'Connor and her team analyzed bacteria fossilized in coal samples from before, during, and after the Chicxulub impact. In response to temperature changes, these bacteria thicken or thin their cell walls "like putting a blanket on or taking one-off," she said. 

The scientists discovered that the bacteria didn't appear to be "gaining weight" for several millennia after the impact. Instead, scientists discovered a warming trend that lasted around 5,000 years and stabilized rapidly. These warm years may have resulted from supervolcanoes spewing CO2 into the atmosphere in the millennia before the abrupt end of the Cretaceous epoch. This is interesting in and of itself, as volcanism is another competing theory of the extinction of the dinosaurs.

If this is correct, the asteroid would have been the coup de grâce that finally killed them off rather than the cause. But it is essential to note that this new study does not necessarily support this view.

The O'Connor team concurred that the beginning of the end-Cretaceous extinction was probably preceded by a brief period of cold and darkness. However, it doesn't appear to have started a long cooling trend. According to O'Connor, their findings suggest that Earth might be able to recover from a climate change catastrophe faster than previously thought, but only if it doesn't result in mass extinction.

You can read the study for yourself in the journal Geology.

Study abstract:

"The Cretaceous-Paleogene (K-Pg) boundary marks one of the five major mass extinctions of the Phanerozoic. The ways in which the climate system responded to a bolide impact and extensive volcanism at this time over different time scales are highly debated. We used the distribution of branched tetraether lipids (brGDGT) from fossil peats at two sites in Saskatchewan, Canada (paleolatitude ~55°N), to generate a high-resolution (millennial) record of mean annual air temperature (MAAT) spanning the last ~4 k.y. of the Cretaceous and the first ~30 k.y. of the Paleogene. Our study shows that MAATs ranged from 16 to 29 °C, with the highest value in the first millennia of the Paleogene. The earliest Paleogene averaged ~25 °C—maintaining or enhancing warmth from the latest Cretaceous—followed by a general cooling to ~20 °C over the following ~30 k.y. No abrupt post boundary cooling (e.g., an “impact winter”) or abrupt warming is evident in our data, implying that if such phenomena occurred, their duration was relatively short-lived (i.e., sub-millennial-scale). Further, no long-term impact- or volcanism-driven warming is evident. The range of temperature change observed is considerably greater than that derived from marine proxy records over the same time interval. Our findings, therefore more properly place bounds on the magnitude and duration of temperature change on land during this critical interval—the main setting for the demise of nonavian dinosaurs and the rise of mammals."

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