Welcome to Part II of our conversation with Antoine Bercovici, a Peter Buck postdoctoral fellow in the Department of Paleobiology, who is using fossil pollen to solve a paleontological mystery about the K/Pg extinction. We left off with Antoine telling us about a strange site in North Dakota (read Part 1 here), where they found Cretaceous pollen in rocks above where they thought the K/Pg boundary should be.
What happened next?
This is where I started to get into palynology (the study of plant pollen and spores) - to try to solve this problem and better understand what was going on. While still an undergraduate, I started training myself by looking at samples, working with the paleontologist who did the first analysis, the late Doug Nichols from the Denver Museum of Nature & Science. I quickly started to realize something - the usual way that biostratigraphy works is on presence and absence, and that wasn’t going to be good enough here.
For example, usually, if a certain type of pollen from a certain type of plant is found, whether it’s one single grain or a thousand grains, it means that type of plant was present – we don’t typically look any deeper. The key at this strange North Dakota site was to look at this in a different way, from an ecological perspective, using relative abundance (how much is there, rather than just if something is there or not).
Looking at the Cretaceous pollen taxa in our samples, we had about 20-30% abundance and then, at the boundary, it dropped down to almost zero - but not all the way to zero, which was an important point. There was still a very small number of pollen taxa present after what we thought was the boundary. This made us realize that, in this particular case, we couldn’t necessarily count solely on traditional biostratigraphy as the tool that would help us put our finger on the boundary, because Cretaceous pollen does exist after the boundary.
This is because the K/Pg extinction wasn’t just a single event that wiped everything out at once, and these rocks showed how it happened in stages?
Yes. So the asteroid was the trigger for all the catastrophe that caused the extinction approximately 66 million years ago. Obviously the asteroid killed animals and plants locally – having that drop on your head would not be good! But it was not the only extinction mechanism. It triggered all sorts of disasters, like tsunamis, climate change, massive cooling, darkness that stopped photosynthesis, etc. None of these were as instantaneous as the impact was, so some organisms might have survived for a short period of time.
However, geologically, it was still a very fast process that took maybe months to decades. This is a timeline that you can see at the small resolution of pollen in the rock record, millimeter by millimeter. We realize that what was potentially happening with the pollen at our site was that we were seeing this record of plants that survived the trigger event, but then eventually got replaced by competition and all the environmental change.
What you may also have is reworking, which means something excavated older sediments and then they got included in more modern ones. For instance, in France, there is a very famous deposit where you have trilobites in a Tertiary formation, which is strange because trilobites are Paleozoic (meaning they lived much earlier). What probably happened is that these trilobites were part of a Tertiary river system that eroded Paleozoic rock, so they were excavated naturally by the river and then reincorporated into the Tertiary sandstone.
A lycopodium spore from the Hell Creek Formation (66 million years old) viewed under a microscope, adjusting the focus back and forth.
So reworking might have occurred with the pollen at your site?
Yes, this can happen with pollen as well, because it’s very small. Think about it – you have all these Cretaceous rocks that are being eroded by Paleocene river systems and Cretaceous pollen grains being reincorporated into newer rocks in small amounts. This could be an explanation for the persistence of those pollen grains for a short period of time, since the Cretaceous isn’t that far away from the Paleocene - it’s just past the boundary, so you can easily excavate those rocks and reincorporate material. There are also so-called "Tertiary dinosaurs" that have been found the same way, in big river systems where those bones have been re-excavated by erosion and then reincorporated. That’s why there have been claims of Tertiary dinosaurs, but that isn’t the case – they’ve been reworked. We think the same thing happened with pollen on a smaller scale.
Further studies on other sites in North Dakota showed that this anomaly was the rule, rather than exception. My findings of using relative abundance data give us a much more precise positioning of the K/Pg boundary. This is especially important because understanding the mass extinction relies on the capability of pinpointing precisely the position of the K/Pg boundary. This gives us a precise timeline that we can use to compare flora and fauna from below and above, without any mixing.
For more information on fossil pollen and the K/Pg boundary, check out the exhibition The Last American Dinosaurs: Discovering a Lost World.
By Abree Murch, Museum Technician, National Museum of Natural History