One of the best-known mass extinction events in Earth’s history was the Cretaceous-Paleogene (K-Pg) extinction event. Dinosaurs were among the species that perished. It is widely believed that a large comet or asteroid impacted Earth and caused the extinction. Scientists are still refining our understanding, however, of the intricacies of the impact’s effects. We spoke to Michael Henehan, a scientist at the GeoForschungsZentrum Potsdam – the German National Research Centre for Geosciences. He specializes in Earth surface geochemistry and has studied the ecological devastation caused by the impact. He shared his experience learning about the impact’s consequences. He furthermore spoke about what the impact teaches us about Earth’s interactions with space objects, and why this is relevant today.
What have you learned about the asteroid that killed the dinosaurs?
By measuring the chemistry of fossil organisms that lived during the first hundreds of thousands of years after the asteroid’s impact, we have worked out that the impact led to a sharp reduction in the pH of the ocean. In other words, it made the oceans more acidic. Previously, people had suggested this could have happened because of the chemistry of the rocks that the asteroid hit. The rocks were rich in sulfur, so as a result of the impact, sulfuric acid would have rained down in the years after impact. Our data confirmed that this did indeed happen.
Our findings help explain some of the subsequent patterns of extinction that have long been noted. After the impact, organisms that made their shells out of calcium carbonate were the worst affected. Organisms that did not make such sorts of shells weren’t so badly affected… and this makes sense because organisms that make their shells out of calcium carbonate struggle to do so in more acidic waters. Also, ecosystems in the middle of ocean gyres were worse hit than those near the coasts, where sediments could dissolve to buffer the water. So, in terms of extinction in the ocean, our findings really helped us nail down the mechanisms behind ecological collapse.
Furthermore, and perhaps this is also critical for life on land, our findings indicate how Earth’s carbon cycle responded to the impact. Normally, the ocean is critical for cycling carbon (i.e. CO2). Organisms in the surface ocean take up CO2during photosynthesis, they die, and they bring this organic carbon to the seafloor. This is what’s known as the “biological pump” or “soft tissue pump”. At the same time, there’s also the “alkalinity pump”, which is driven by organisms making shells out of CaCO3in the surface ocean and then sinking to the bottom.
What we learned is that both of these processes were hugely weakened by the asteroid impact. At first their being weakened would have caused a big spike in atmospheric CO2because photosynthesis wasn’t taking up CO2from the surface ocean and atmosphere anymore. But then over time, because Earth’s rivers were delivering alkalinity to the ocean – from the dissolution of rocks on land – and because the “alkalinity pump” wasn’t working either, the surface ocean got more alkaline. More alkalinity caused the ocean to essentially suck CO2out of the air, driving down atmospheric CO2. There were thus 100 to 200 thousand years of extreme volatility in atmospheric CO2levels after the impact.
This volatility means that even if a species on land were lucky enough to survive both the searing heat and the cold winter after the asteroid hit Earth, they would still have had to cope with a prolonged period of environmental instability. Ultimately, this could have contributed to the disappearance of many species on land.
What are some areas of confusion that remain about the asteroid, and do you plan on studying them?
For us, one of the big questions that remains is the timescales involved. We know that the acidification due to the impact should have been very rapid, on the timescales of years. Earth would have gradually begun returning to some kind of “normal” from that point on as the acid was buffered by dissolution of rocks and sediments.
Knowing that, consider this: if a sample is showing us that there was acidification of .23 pH units, it is very important to know how close in time this sample is to the impact. If the sample came from 1,000 years after the impact, then it is likely that Earth already had had some time to recover, and the acidification was much worse right after the asteroid hit. But if the sample captured the first few tens of years after the impact, then that would imply that .23 pH units is about as bad as acidification got. And if .23 pH units was all it took to prompt a mass extinction, this is important because, if our CO2 emissions don’t slow, we’re on course for the oceans to drop by more than that amount by the year 2100. So, to get some more clarity on this issue, we’re always on the lookout for new sample sites to get a more resolved interval of time around the K-Pg boundary.
What does the impact teach us about the interaction between Earth and space objects?
To me, what this shows is that because of the complexity of our Earth system, even a sudden and instantaneous event like an asteroid impact can have effects that last for hundreds of thousands of years. There are so many cycles interacting on different timescales. Once you wipe out a huge section of an ecosystem, like what happened in the ocean after the K-Pg impact, it takes a long, long time to rebuild all of those geochemically important processes that regulate Earth’s climate. And for us, who as a species are acidifying the oceans at a pace that is very rapid from a geological perspective, I think that should be a cautionary tale!
For people who would like to study how the Earth system is affected by space objects, what are some interesting areas of research you think are worthy of focus?
I think one area of research which is still really interesting is figuring out why some impacts have such profound effects on life while others do not. For instance, there was an impact in the Chesapeake Bay area in the Eocene that had a similar-looking geochemical signature in the rock record according to some metrics, but we don’t see indications that this impact led to a big extinction event. How many detrimental effects depend on the rocks where an asteroid hits? How much does it matter if an asteroid hits a marginal marine area in the tropics as opposed to on land somewhere near the poles? These are the sorts of questions that are still open and are waiting to be answered.