Researchers at the University of New Mexico have found that horsetails, ancient plants over 400 million years old, create extreme oxygen isotope signatures in their water that resemble those from meteorites. This discovery, published in the Proceedings of the National Academy of Sciences, offers a new tool for reconstructing ancient climates. By analyzing modern and fossil samples, scientists can now decode humidity levels from the dinosaur era.
A team led by Zachary Sharp, a professor in Earth and Planetary Sciences at the University of New Mexico, investigated horsetails (Equisetum), hollow-stemmed plants that have persisted on Earth for more than 400 million years. Their study, titled "Extreme triple oxygen isotope fractionation in Equisetum," reveals how water moving through these plants undergoes intense natural filtration, resulting in oxygen isotope ratios more extreme than any previously recorded on Earth—similar to those in meteorites or extraterrestrial materials.
The researchers collected smooth horsetails (Equisetum laevigatum) along the Rio Grande in New Mexico and tracked isotope changes from the plant's base to its tip. The upper portions showed values that fell outside known Earth-based ranges. "It's a meter-high cylinder with a million holes in it, equally spaced. It's an engineering marvel," Sharp said. "You couldn't create anything like this in a laboratory."
Sharp presented the findings at the Goldschmidt Geochemistry Conference in Prague in July. "If I found this sample, I would say this is from a meteorite," he noted during the conference. "But in fact, these values do go down to these crazy low levels."
This work clarifies puzzles in oxygen isotope measurements from desert plants and provides a method to reconstruct climates in arid regions. Fossil horsetails, which once reached 30 meters tall, contain phytoliths—tiny silica particles that preserve isotope signatures for millions of years. These act as a "paleo-hygrometer" to measure ancient humidity. "We can now begin to reconstruct the humidity and climate conditions of environments going back to when dinosaurs roamed the Earth," Sharp said.
The study updates models for isotope behavior, aiding understanding of ancient climates through preserved phytoliths.