Carbonate fragments provide remarkable information

Professor Cees W. Passchier and Dr. Gül Sürmelihindi from Johannes Gutenberg University Mainz (JGU), in collaboration with colleagues from France and Austria, have now unraveled the history of the mill complex using calcium carbonate deposits that are now stored in the Archaeological Museum of Arles. These deposits had formed towards the end of the roughly 100-year operational life of the Barbegal water mills on the sides and base of the wooden supply system that conveyed the water to the wheels. “We show that it is possible to reconstruct to a large extent the history of a water mill on the basis of such carbonate deposits,” stated Passchier, head of the JGU team. First, the researchers had to fit some of the total of 140 stored pieces together like a jigsaw puzzle, then they analyzed the layers using various techniques, including mass spectrometry.

Wooden water wheels and gutters were replaced

The researchers have now published their results in Geoarchaeology. “We were able to show, for example, that wooden water wheels and water channels had to be replaced after three to eight years. In at least one case, an old water wheel was replaced by a larger one,” said Passchier. The researchers drew this conclusion from the unusual shape of the carbonate deposits that had formed in the water channel. While the lower and earlier layers indicate that water levels must have originally been relatively low, upper and later carbonate layers indicate a higher water level. The possibility that there was originally less water flowing through the water channel which was subsequently increased was rejected by the researchers. They established that — for a gently sloping water channel and low water level — the amount of water provided would not have been sufficient to drive a mill wheel. Therefore, the inclination of the water channel must have been altered, from what was at first a steeper angle with a low water level to a shallower slope transporting water at a correspondingly higher level. “The entire structure of this water mill must have been modified,” said Passchier. “If you uplift the water channel alone, the water tends to splatter, losing the power to drive the wheel efficiently. Thus, when you uplift the water channel, you also need a larger water wheel.” In fact, a section of carbonate deposit formed on the water wheel corroborates this conclusion as it does not contain all the carbonate layers but only those of the latter years of operation.

Results of isotope analysis provide evidence of the mill’s service life

Using isotope analysis of the carbonate layers, the researchers were even able to ascertain the operating periods before which parts of the mill required renewal. Carbonate contains oxygen and the relative ratios of oxygen isotopes differ depending on water temperature. Based on the isotope composition in the carbonate layers, the researchers were able to infer water temperatures and thus identify the seasons in which the layers were deposited. They concluded that the carbonate from the samples in the Archaeological Museum in Arles had been deposited in the water channels over a period of seven to eight years. “The uppermost and thus youngest carbonate layer contains mollusk shells and fragments of wood, showing that the mill must have been abandoned by then and was disintegrating. The water continued to flow for a while so that carbonate deposits also continued to form, but maintenance of the water channels ceased,” said Passchier.

The researchers were able to answer yet another question. It was not previously known whether the mills had been run in combination by a single operator or whether the 16 water wheels had been used independently of each other. Judging from the layers of three investigated water channels, which are clearly different from each other, the mills were in operation separately — at least towards the end of their lifetime. Moreover, the western side of the complex was abandoned earlier than the eastern side. Finally, long pieces of carbonate from the water channels were later used as partition screens in a water basin for other industrial purposes after the mills had already been abandoned.

“Our study shows that the environmental resilience of S. caninervis is superior to that of some of highly stress-tolerant microorganisms and tardigrades,” write the researchers, who include ecologists Daoyuan Zhang and Yuanming Zhang and botanist Tingyun Kuang of the Chinese Academy of Sciences. “S. caninervis is a promising candidate pioneer plant for colonizing extraterrestrial environments, laying the foundation for building biologically sustainable human habitats beyond Earth.”

A small number of previous studies have tested the ability of microorganisms, algae, lichens, and plant spores to withstand the extreme environments of outer space or Mars, but this is the first study to test whole plants.

Syntrichia caninervis is a common moss species with a widespread global distribution. It grows in remarkably extreme desert environments including Tibet, Antarctica, and the circumpolar regions as part of the biological soil crust — a widespread and resilient type of ground cover often found in arid lands. Given the moss’s ability to survive extreme environmental conditions, the researchers decided to test its limits in the lab.

To test the moss’s cold tolerance, the researchers stored plants at −80°C (in an ultra-cold freezer) for 3 and 5 years and at −196°C (in a liquid nitrogen tank) for 15 and 30 days. In all cases, the plants regenerated when they were defrosted, though their rebound was less rapid compared to control specimens that had been dehydrated but not frozen, and plants that were not dehydrated prior to freezing rebounded more slowly than plants that were dried, then frozen.

The moss also demonstrated the ability to survive gamma radiation exposure that would kill most plants, and doses of 500 Gy even seemed to promote the plants’ growth. For comparison, humans experience severe convulsions and death when exposed to around 50 Gy. “Our results indicate that S. caninervis is among the most radiation-tolerant organisms known,” the researchers write.

Finally, the researchers tested the moss’s ability to endure Mars-like conditions using the Chinese Academy of Sciences’ Planetary Atmospheres Simulation Facility. The simulator’s Martian conditions included air composed of 95% CO₂, temperatures that fluctuated from −60°C to 20°C, high levels of UV radiation, and low atmospheric pressure. Dried moss plants achieved a 100% regeneration rate within 30 days after being subjected to the Martian conditions for 1, 2, 3, and 7 days. Hydrated plants, which were only subjected to the simulator for one day, also survived, though they regenerated more slowly than their desiccated counterparts.

“Although there is still a long way to go to create self-sufficient habitats on other planets, we demonstrated the great potential of S. caninervis as a pioneer plant for growth on Mars,” the researchers write. “Looking to the future, we expect that this promising moss could be brought to Mars or the Moon to further test the possibility of plant colonization and growth in outer space.”

This research was supported by the Chinese Academy of Sciences, the Leading Talents in Technological Innovation Program, and The Third Xinjiang Scientific Expedition Program.