Algae bounced back after a knock
A number of mass extinctions punctuate the fossil record, dealing a sharp blow to life on Earth. The best known (although not the biggest) is the one that did for the dinosaurs, some 65 million years ago. Unlike some mass extinctions, there’s at least one smoking gun: a damn great rock crashed into the planet, somewhere near Mexico. It’s not a closed case, though; there are various theories, including the possibility of an even bigger rock landing near India.
If a massive rock did indeed hit the Earth, it would have made the world’s nuclear weapon stockpile look like a few party poppers. There would then have been earthquakes, volcanic eruptions, fires, colossal tsunamis, acid rain, and darkened skies from the dust thrown up. In short, all the things you get in Horizon, or the more exciting bits of the Bible.
This study looked at the effect of the disaster on algae, by measuring various chemicals, produced by the algae and stored in rocks. Different groups produce different complex chemicals, and the more algae there were, the higher the concentration of the relevant chemical. When the mass extinction hit, the rock laid down records a sharp drop in algal populations, as you’d expect. The dust blocking out the sunlight was probably the key cause—like plants, algae get their energy from sunlight by photosynthesis.
The key question, though, is how quickly they recovered afterwards. From the chemicals, it looks like they bounced back almost at once—within a century, the study’s authors claim, algae were back to more or less the previous density. Algae are at the bottom of the marine foodweb, so that would have set the stage for other species to recover, and for new ones to evolve into the niches left by extinctions.
Distinguishing a mere century after 65 million years is no mean feat. That’s on the same scale as ten seconds seen from a distance of three months. Are we sure that the chemicals are well enough fixed in place in the rock to record that difference? At the layer representing the mass extinction, there is a sharp change in the chemical signal. If the chemicals had ‘run’, we’d expect it to be more smeared out. At that level, however, there is a change in the type of rock (from a marl to a clay) which might affect how well the chemicals were fixed. Finally, the scientists note that this record is only from one place (in Denmark). If the same thing was going on over the whole ocean, we should be able to confirm it by studying sediments elsewhere.
Sepulveda, J., Wendler, J., Summons, R., & Hinrichs, K. (2009). Rapid Resurgence of Marine Productivity After the Cretaceous-Paleogene Mass Extinction Science, 326 (5949), 129-132 DOI: 10.1126/science.1176233