Layered limestone deposits give unique insight to Roman aqueducts
Mineral-rich waters from Italy’s Apennines flowed through ancient Rome’s Anio Novus Aqueduct, leaving a detailed rock record of past hydraulic conditions, researchers said. Two studies characterizing layered limestone deposits called travertine in the Anio Novus are the first to document the occurrence of antigravity growth ripples and find that these features provide clues to the history of ancient water transport and storage systems.
These multidisciplinary studies, led by geology professor Bruce Fouke of the University of Illinois, Urbana-Champaign and published in the journals Scientific Reports and GSA special papersapply advanced engineering principles and high-resolution microscopy to establish a controversial new theory about how the corrugated travertine formed, Fouke said.
As the water — from the Anio River and an underground lake near Subiaco, Italy — flowed, it left undulating layers of calcium carbonate travertine that accumulated along the interior floors, walls and ceilings of the Anio Novus Aqueduct.
In the field, the researchers collected upstream-downstream oriented travertine samples that exhibit two salient features: light and dark millimeter-scale stratification patterns, and centimeter-scale wavy ripple shapes persist in those layers.
Previous studies have suggested without evidence that the layers in the Anio Novus travertine are the result of changes in flow rate initiated by seasonal changes or engineering methods introduced by the Romans, the researchers said. Travertine with similar layered forms in ancient aqueduct systems, however, occurs worldwide regardless of regional climate or operation.
Fouke’s specialty is interpreting how microbes that thrive in mineral-rich water affect the crystalline architecture of travertine and other similar mineral deposits in nature. His group has worked extensively to reveal the geologic history of stratified mineral formations — and has drawn conclusions about life on Mars via Yellowstone to coral reefs in Australia — and even inside the human body.
“Subiaco’s waters are chemically similar to the waters of Yellowstone National Park, where waterborne microbes form mats and biofilms that play a critical role in the shape and structure of Mammoth Hot Springs’ famous stepped travertine features,” Fouke said. “We also identified fossil microbes and plant remains in the dark layers of the Anio Novus travertine deposits. Once we realized the similarity between Subiaco and Yellowstone waters, we knew we had the knowledge and experience needed to understand the history and mystery of the last stream of the Anio Novus, the longest and most important of the ancient Roman aqueducts.”
Fouke and Marcelo Garcia — a professor of civil and environmental engineering at the U. of I. and co-author of the study — worked with their teams to meticulously measure the geometry of the corrugated layers of the Anio Novus travertines to create a unusual interpretation.
“A geologist will tell you that the only way ripples form is through fluid shear and gravity-dependent sediment transport,” Fouke said. “The theory is that water or wind can move loose sediment into wave-like shapes that progress slowly and are influenced by gravity to form the familiar asymmetric ripple shapes we see along riverbanks, dunes and in the ancient sedimentary rocks deposited in these environments.” .”
However, Fouke’s team argues that the Anio Novus travertine crystals precipitated, grew and accumulated in the flowing water of the aqueduct — independent of gravity and aided by the shape and biochemical composition of microbial colonies — to form what they call “travertine crystal.” to call. growth ripples.”
Although the complex processes that control the ripples of travertine crystals are distinctly different from those that control the ripples of sediment transport, the researchers said they are visually similar. The geometries of ripples along the vertical walls of the aqueduct are identical to those along the floors – proof that the mechanisms that form crystal growth ripples are not dependent on gravity.
Convinced that the structures are ripples reflecting the flow, Garcia and his team measured the geometry of the ripple to reconstruct the volume and velocity of the water that flowed through the aqueduct during ancient Roman times.
“Because few researchers had ever recognized these structures as ripples before, no one had used the power of the shape of a ripple, along with fluid mechanics principles, to produce this kind of reconstruction,” Garcia said.
Using the travertine deposited in direct contact with the original aqueduct mortar, researchers conclude that when the aqueducts were first turned on, water was flowing through them at a rate of about one meter per second — fast enough to cover a football field within an hour. flooding – much faster than previously believed.
The fact that corrugated travertine exists along the ceilings of the aqueduct channels indicates that they were operating at full capacity, the researchers said. This observation suggests that previous studies were incorrect in stating that the layers were formed by seasonal changes in the flow or when Romans used technical means to control the flow rate.
“These aqueducts were much more robust than was ever realized,” Fouke said. “The flow was greater than anticipated, and that flow rate was maintained at a constant rate.”
The researchers are now extracting the ancient fossilized microbes and their biomolecules trapped in the travertine to learn more about what kind of microbes — and possible pathogens — the Romans drank.
“Historians and archaeologists are very interested in what led to the fall of the Roman Empire,” Fouke said. “Since the aqueducts played an important role in the success of the Romans, any information gained from the demise of the aqueducts may be helpful in this endeavour.”
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Duncan Keenan-Jones et al, Travertine crystal growth ripples capture the hydraulic history of ancient Rome’s Anio Novus Aqueduct, Scientific Reports (2022). DOI: 10.1038/s41598-022-05158-2
Mayandi Sivaguru et al, Depositional and Diagenetic History of Travertine Deposited in the Anio Novus Aqueduct of Ancient Rome, GSA special papers (2022). DOI: 10.1130/2022.2557 (26) pubs.geoscienceworld.org/gsa/b … history-of-travertine
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