The peculiar plants that existed since the dawn of terrestrial animals can process water to remarkable extremes, resembling water from metstones more than typical groundwater. Not only do they play a crucial role in today’s ecosystems, but their fossilized remnants also provide insights into Earth’s ancient climate and hydrological systems during the age of dinosaurs.

Almost every oxygen atom in water contains eight neutrons, though some rare heavy isotopes possess nine or ten neutrons. When water evaporates, lighter isotopes do so more readily than their heavier counterparts, leading to predictable shifts in their ratios. Researchers can utilize this information to trace the origin of a specific water sample, determining whether it originated from groundwater, fog, or the rate at which it traversed through plants and the humidity levels experienced by those plants in the past.

Nevertheless, due to the minimal presence of heavier isotopes, acquiring reliable data on how these ratios fluctuate can be quite challenging, making it hard for scientists to draw definitive conclusions.

During examinations of water samples from desert flora and fauna, Zachary Sharp from the University of New Mexico and his colleagues discovered discrepancies between the observed data and the anticipated outcomes based on laboratory models.

Sharp and his team believe they have addressed the issue through a remarkable plant known as horsetail, which has been on Earth since the Devonian period approximately 400 million years ago and features segmented, hollow stems. “It’s a tall cylinder with countless holes, evenly spaced, a marvel of engineering,” states Sharp. “We couldn’t replicate this design in our lab.”

As water flows through each segment of the horsetail stem, it undergoes a process of repeated distillation. Sharp and his colleagues collected water samples at various points along the smooth idiot stem (Equisetum) cultivated near the Rio Grande in New Mexico.

By the time the water reaches the top of the stem, its isotopic composition markedly differs from other terrestrial waters. “If you encounter this sample, I suspect it originates from metstone, as it doesn’t come from Earth. [The oxygen isotope ratios],” Sharp remarked during a presentation at the Goldschmidt Geochemical Conference in Prague, Czech Republic, on July 7.

These horsetail analyses enable Sharp and his team to ascertain the variations in the water’s isotopic ratios under near-ideal conditions, allowing them to enhance model accuracy with these values.

By reassessing desert plant data with these refined models, previously inexplicable observations suddenly made sense. Sharp posits that these findings could illuminate other challenging observations, especially in arid regions.

Reaching heights of 30 meters, far surpassing today’s descendants, ancient horsetails provide even more extreme isotopic ratios and could serve as a key to understanding ancient water systems and climates, according to Sharp. Small, sand-like grains known as plant stone threads within horsetail stems can endure to the present day and may feature unique isotopic signatures influenced by atmospheric humidity. This factor affects the evaporation rate. “This could serve as a paleofat meter [humidity indicator]—how fascinating,” Sharp concludes.