For hundreds of years, water that rushed through Roman aqueducts left behind thick layers of sediment caked to the channels’ walls. These sediments—mostly calcium carbonate—may contain chemical records of the region’s climate, similar to the stalactites and stalagmites that scientists study in caves.
“What we hope to do is to obtain information about paleoenvironmental conditions during Roman times and see if it’s different from nowadays,” Cees Passchier, a structural geologist at the Johannes Gutenberg University in Mainz, Germany, and leader of a research project investigating the layers of sediment, told Eos.
Studying these ancient sediments “is a brilliant idea,” said Amy Frappier, an assistant professor in paleoclimatology at Skidmore College. Because aqueducts, one of the greatest engineering feats of the Roman Empire, were so widespread—stitching across Europe and northern Africa—they could offer a unique look at how climate changed on all sides of the Mediterranean from approximately 31 BCE to 476 CE.
“Those aqueducts have been around for thousands of years,” Frappier said. They “should have been recording all sorts of environmental changes that were happening.”
Many large and reliable Roman aqueducts were sourced from limestone cave springs. Waters deep within these springs are rich in dissolved calcium carbonate and carbon dioxide, thanks to material dissolving from cave walls and the closed environment of the cave system, which prevents carbon dioxide from escaping.
When the water emptied into an aqueduct from a spring high in the hills, it equilibrated with the open air, releasing carbon dioxide, which lowered the water’s acidity. The lower acidity caused crystals of calcium carbonate to rain out along the aqueduct. These deposits of calcium carbonate are called sinter.
In the same way that hard water residue cakes the insides of today’s pipes, groundwater rushing through Roman aqueducts over hundreds of years deposited layers upon layers of sinter. In fact, nearly 40% of the 1400 known major aqueducts from Roman times are lined with sinter, according to Passchier and colleagues.
Extracting Climate Signatures
Similar to rings of growth found within cave stalactites and stalagmites, layers of calcium carbonate in aqueducts roughly follow annual seasonal cycles. These tend to manifest as alternating dark and light stripes that each keep a chemical record of the environment in which it was formed, said Gül Sürmelihindi, a postdoctoral researcher at Johannes Gutenberg University.
To study these season cycles, Sürmelihindi turned to geochemical analysis. She looked at the abundance of an isotope of oxygen—oxygen-18 (18O )—within the layers. Because 18O has two more neutrons than the more common 16O, it is slightly heavier and will be more likely to precipitate out of the rushing water to form sinter.
Because calcium carbonate can more readily stay dissolved in colder water, any sinter that forms during the chill of winter will contain higher concentrations of 18O because heavy oxygen will be the first oxygen isotope to precipitate out of solution, Sürmelihindi said.
In many of the darker-colored stripes, Sürmelihindi found a higher abundance of the heavy oxygen isotope, which meant that these dark stripes were formed during the colder parts of the year. In contrast, the lighter-colored stripes tended to have less of the heavy oxygen isotope, which told Sürmelihindi that these layers were deposited in a warmer environment.
However, other factors also may create the banding. “These distinct layers can mean many things, such as a drought period, very wet period, or even human activity like a cleaning process of the subject aqueduct,” Sürmelihindi said. Another factor adding to the dark color of these stripes could be the presence of biological material, she noted.
Unlike stalactites and stalagmites, however, the layers of carbonate mineral formed in aqueducts provide a more highly resolved look into the past because of their thickness, Passchier said. Whereas mineral layers in cave formations can be less than a millimeter thick, the layers on aqueducts can be a centimeter thick.
In fact, some sequences of sinter coating the aqueducts can be about a meter thick, representing hundreds of years of deposition, Passchier noted. The more material there is for scientists to analyze, the better picture they can get of the depositional environment.
Climate and Society
From these chemical signatures, it should be possible to study how climate changes affected ancient societies, said Frappier, who has used cave deposits to study climate changes faced by the ancient Mayan empire.
Scientists have found that human activity has been responsible for air pollution as far back as the Roman Empire, so “these aqueducts could possibly be picking up signatures from regional air pollution, which would be interesting to look at,” Frappier said.
Dating the Past
However, before they can piece together a climatic history of the region, Passchier and Sürmelihindi must first be able to accurately date each layer. Although the relative ages of the layers are apparent, the exact age can be gleaned only from radioactive dating and comparing those relative ages to known, finely detailed climate records sourced from tree rings. This comparison has the potential to resolve the age of each layer to within 5 years, said Passchier.
The better scientists can understand the environmental changes that occurred during historical periods such as the rise and fall of the Roman Empire, “the better we can learn about the cultural context and how people responded to changes,” Frappier said.
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2015), Ancient Roman aqueducts could spill climate secrets, Eos, 96, doi:10.1029/2015EO026629. Published on 19 March 2015.