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New study shows value of century-old herbarium specimens for studying global change

A dried plant with yellow flowers pressed flat on a sheet of paper
Deaver Herbarium/Northern Arizona University
Example of a Deaver Herbarium annual plant specimen (Xanthisma gracile) that was harvested in 1964 at the peak of bomb spike in Flagstaff, Arizona, USA.

Anew study from Northern Arizona University traces the carbon in the atmosphere over the last 100 years through a unique scientific instrument: wildflowers. Scientists examined century-old specimens of local plants to get insights into how the climate is changing. KNAU’s Melissa Sevigny spoke with lead author Mariah Carbone about how bombs detonated in the 50s and 60s left a signature behind.

What gave you the idea to look at herbarium specimens? What drew you to doing this research?  

It was kind of accidental… We were looking at getting archived samples to look at how the radiocarbon concentrations in the atmosphere around Flagstaff have changed in the last few years. As we started digging around, we could find samples that were older and older, and then realized there was an herbarium on campus and we could go to the herbarium and collect samples that are more than 100 years old.

So these are plants that someone collected and dried and pressed, and they’re archived?

Exactly…. There was a lot of foresight in collecting samples back in 1900 because they can be used for a lot of purposes, and particularly now, they can be used to study global change.

You mentioned that you wanted to look at radiocarbon concentrations, what exactly does that mean?

Radiocarbon is an isotope of carbon. It is produced naturally in Earth’s atmosphere but also in the 1950s and 60s humans produced radiocarbon in the atmosphere, and they did this with atmospheric thermonuclear weapons, and these high energy explosions created radiocarbon.

So you can actually see that in in the tissues of the plant, that these weapons were put into the atmosphere.

Plants are wonderful samplers of Earth’s atmosphere…. Annual plants do it for a very short period of time, because they complete a life cycle in less than one year, usually just a couple of months.

What did you find out? Did anything surprise you about your discoveries?  

It was reassuring that we found the bomb spike in the herbarium samples, which is what we expected, but we were happy to see that. We found some regional difference in Flagstaff compared to other records that have been made of the bomb spike. I guess that’s not surprising in the other records for the bomb spike are nowhere close to Flagstaff, so there are some localized effects…. We also found the atmosphere in Flagstaff in the last five years has more radiocarbon in it than the background atmospheric records we were comparing it to. We believe this could be due to the burning of biomass. That could be wildfires in the western U.S. or localized forest management practices, and that would be elevated in radiocarbon because this biomass, or these trees, grew in a time when there was more bomb radiocarbon in the atmosphere. So when we burn those tissues that radiocarbon goes into the atmosphere.

What do you think using this technique could show us about global change?

Previously radiocarbon records have been made with tree rings or atmospheric air samples from very few locations on earth. Herbaria samples have been collected all over Earth…. And they’re sitting there ready for us to look at them. So there’s this vast quantity of samples that we could analyze to look at better temporal scales or spatial scales to better quantify what our atmosphere has looked like over time and study the modern carbon cycle.

Mariah Carbone, thank you so much for speaking with me.

Thank you very much.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.