On a day some sixty-six million years ago, life on Earth was suddenly and irreversibly altered. An enormous asteroid or comet struck the Earth next to what is now the Yucatan peninsula. After that, life on the Earth would never be the same. Dinosaurs, apart from birds (of course), died out. Even the birds, however, suffered dramatic losses. Extinctions struck plants and insects. Mammals may have lost more than 90% of their species. In the sea, the enormous marine reptiles, as well as the elegant cephalopods know as ammonites, disappeared forever. The tiny floating single celled foraminifera virtually died out. Life survived, but it would never be the same. To a large extent, the diversification of mammals and the eventual spread of humans was contingent (as Steve Gould would say) on that devastating event.
Within the past year, three papers have shone new light on that terrible day and its aftermath. They also demonstrate how modern paleontology and geosciences are multidisciplinary, using cutting edge field and analytical methods, and drawing on the expertise of large numbers of scientists.
In 2016, multiple cores were drilled into the 200 km diameter Chicxulub crater left by the impact. In the wonderfully titled paper “The first day of the Cenozoic,” Gulick and a large team of scientists (https://www.pnas.org/content/116/39/19342) go step-by-step through the minutes and hours following the impact, documented by 130 meters of rocks filling the crater. Most of this section is a complicated mix of shattered and melted rock. At the impact site, the initial stages were dominated by the formation of the 600–1000 m deep crater. The impact produced a huge tsunami, which propagated across the ancestral Gulf of Mexico, inundating shorelines throughout the region and then being reflected back to the crater, filling it.
Being far from the impact would not have helped. At this time, what would become North Dakota was along the shores of an elongate inland sea, the last remnant of the Western Interior seaway that once covered much of North America. The force of the impact and the resultant earthquake almost instantaneously generated a wave surge, a seiche, within the sea. As described in a highly publicized paper by DePalma and co-authors (https://www.pnas.org/content/116/17/8190), fish and other organisms living along the shores were catastrophically buried. Even more amazing, their gills contained tiny spheres of material ejected from the impact. They died that day!
If the tsunami and the seiche wasn’t bad enough, the heat released by the impact set fires within 1500 km of the impact. As the debris from the impact rained back down, it would have also been hot enough to set numerous additional fires. Soot from the fires would reach high in the atmosphere, shutting down photosynthesis across the world. Gulick and his colleagues found evidence for the fires at the top of the crater fill.
And it gets worse. The rocks where the impact hit were formed by evaporation when the Atlantic opened up many millions of years earlier; these rocks were rich in minerals containing sulfur. This sulfur was released into the atmosphere by the impact, producing sulfuric acid, which would have come down as acid rain and dropping the pH of the oceans. New evidence for this comes from a seemingly unlikely source, the element boron.
Boron has two isotopes, 10B and 11B , which are incorporated into the calcium carbonate of the shells of marine organisms such as foraminifera. The ratio of the two isotopes in the shells depends on the pH of the seawater. Using this as a tool, Henehan and numerous colleagues (https://www.pnas.org/content/early/2019/10/15/1905989116#abstract-2) demonstrated a rapid drop in oceanic pH following the impact, consistent with a near doubling of carbon dioxide in the atmosphere. The rapid acidification of the oceans and a resulting huge decline in marine primary productivity were probably the main causes of ecosystem collapse.
Tsunamis, seiches, fires, darkness, acid rain and oceans. And I haven’t mentioned the huge volcanic eruptions occurring at the same time in India. Overall, it was a terrible, horrible, no good, very bad day, even in Australia (apologies to Judith Viorst).