She makes dripstones talk

Paleoclimatologist Franziska Lechleitner climbs into caves and examines stalagmites to unravel climatic changes of the past. Those, for example, that may have played a role in the collapse of the Mayan civilisation 1100 years ago.

Franziska, a researcher from Bern, ventured into the jungle of Belize to study the Yok Balum cave. The cave is named Jaguar Paw in English and can be dangerous, not so much for the sake of the predators as for the poisonous snakes. To ensure her safety, a guide with a machete accompanied her on her way to the cave in the southern part of Belize. Her fieldwork regularly takes her to caves all over the world. The reason: stalagmites or dripstones are an excellent environmental archive that can be used to reconstruct the climate of the past.

Yok Balum was not as technically challenging as other caves Franziska has explored. Other expeditions required much more courage. "When I abseil," she admits, "I still have palpitations.” She does not see herself primarily as a speleologist. "Caves are beautiful places and their formation is fascinating, but my main reason for exploring them is to better understand climate and environmental processes."

This time, in Yok Balum cave, Franziska was not looking for stalagmites - the samples she is examining in the laboratory in Bern come from a specimen that was extracted from the cave about 20 years ago - but conducting research on infiltration. Because if you want to understand how environmental information gets into stalagmites, you must know where and how much water seeps into a cave and forms the dripstones with its deposits. The Bernese researcher is primarily interested in the carbon that is brought in. Her topic is the carbon cycle in caves - and thus, indirectly the changes in the ecosystem on the earth's surface. This is because the carbon comes mainly from the soil above the cave and is washed in with rainwater.

A career built on stalagmites

With the help of the stalagmite analysis, it is possible to understand, among other things, how precipitation in Belize has changed over time. "For local agriculture, the timing of rainfall is very important," Franziska emphasises. "With climate change in mind, a better understanding of the past rainfall regime plays a big role for Belize's future."

In her office at the University’s Muesmatt building, the palaeoclimatologist shows what her objects of investigation look like with a stalagmite sample from India: a longitudinal section only a few millimetres thick, polished, and embedded in epoxy resin. The individual layers of the stalagmite are visible to the naked eye. No more than 150 micrograms of material are needed to date them and determine stable carbon isotopes.

Franziska’s scientific focus on dripstones is partly due to chance. During her time as a Master’s student in "Atmosphere and Climate" at ETH, she heard a lecture on stalagmites as natural climate archives, which fascinated her so much that she decided to write her Master's thesis in this field. After that, one thing led to another. The paleoclimatologist completed her doctorate - also at ETH - with a study on the carbon cycle in karst systems and on the carbon isotopes incorporated in stalagmites. These can be used as indirect indicators of past climate. She then spent three years as a postdoctoral researcher at the University of Oxford and, back in Switzerland, received a prestigious Ambizione Fellowship from the Swiss National Science Foundation.

Oeschger Centre attracts young scientists

The ambitious young researcher chose the University of Bern as the location to carry out her research project with this grant. Here she found the measurement technology she needed for her project at the Laboratory for the Analysis of Radiocarbon with AMS (LARA) and attractive opportunities to network across disciplines at the Oeschger Centre for Climate Change Research. "There is a great potential for partners to collaborate with in this climate community."

Franziska’s Ambizione project focuses on stalagmites and the organic material stored in them. The goal is to reconstruct both changes in the past climate and local ecosystems for the first time. Stalagmites as climate archives not only open up such new conceptual possibilities but also allow reconstructions with high temporal resolution. The stalagmites from Belize, for example, provided data in seasonal sequence for the past 2,000 years.

In addition to their practical value, dripstones, compared to ice cores, are more cost effective. "There's no need for expeditions costing millions," says Franziska, "the projects needs less equipment, and you have more freedom to try things out." And another thing: the field of stalagmite research is relatively young and the community of researchers in this field is still small. "People know and help each other," says Franziska. For example, she says, it is no problem to obtain samples of stalagmites from colleagues for one's research.

A new explanation for the decline of Maya culture

And finally, stalagmites are sometimes good for sensational news. "Decline in seasonal predictability possibly destabilised Classic Maya societies" was the title of a recently published study in which Franziska was involved in. She contributed results from the Yok Balum cave, located in one of the former core areas of the Maya culture. The study argues that one of the reasons that led to the abandonment of the highly developed Maya cities about 1100 years ago was a specific climate change.

A key factor for the survival of the cities, the study argues, was the timely arrival of sufficient rainfall. This was the only way to produce enough food for the up to 11 million people who lived in the Mayan population centres. However, a reconstruction of rainfall over the past 2000 years showed that regional climate changes had influenced seasonal rainfall. Their onset became very erratic, and Maya farmers were hardly able to predict them from one year to the next. This had a negative impact on crop yields.

"It can be problematic to bring up climatic changes as the sole cause of social upheavals," says Franziska. That is why studies like the one on the decline of the Maya culture must be carried out in interdisciplinary teams. The archaeologists and anthropologists involved had relativised the paleoclimatologists' statements, which is why the publication now talks about the climate as one of several causes for the decline of the Maya cities. "What is clear, however, is that both happened at the same time," Franziska stresses, "there was definitely a connection."