Smooth horsetail plants have segmented stems
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A strange plant that has existed since animals first walked on land can distil water to an extreme degree, making it look more like water from meteorites than regular water from Earth. As well as being key to understanding ecosystems today, the plant’s fossilised remains could shed light on Earth’s climate and water systems when dinosaurs were alive.
Almost all the oxygen atoms in water have eight neutrons, but some are rare, heavier isotopes with nine or 10 neutrons. When water evaporates, lighter isotopes evaporate more than the heavier ones, making the ratio change in predictable ways. Scientists can use this to trace the history of a particular water sample, such as whether the water came from the ground or from fog, how quickly the water passed through the plant, or the humidity that that plant experienced in the past.
However, because the heavier isotopes occur in such small quantities, it is difficult to gather good data on how the isotope ratio changes, making some observations difficult for scientists to explain.
When sampling water from desert plants and animals, Zachary Sharp at the University of New Mexico and his colleagues found that their data didn’t fit with what was expected from models based on laboratory readings.
Sharp and his colleagues think they have solved the problem thanks to unusual plants called horsetails, which have grown on Earth since the Devonian Period, around 400 million years ago, and have hollow, segmented stems. “It’s a metre-high cylinder with a million holes in it, equally spaced. It’s an engineering marvel,” says Sharp. “You couldn’t create anything like this in a laboratory.”
When water moves up each segment of the horsetail stem, it evaporates, distilling it many times as it passes through the plant. Sharp and his colleagues sampled the water at many points along the stem in smooth horsetails (Equisetum laevigatum) growing near the Rio Grande in New Mexico.
By the time the water reaches the very top of the stem, the isotope ratio is unlike any other water found on Earth. “If I found this sample, I would say this is from a meteorite, because it’s not from Earth. But in fact, [the oxygen isotope ratios] do go down to these crazy low values,” Sharp told the Goldschmidt geochemistry conference in Prague, Czech Republic, on 7 July.
With these horsetail measurements, Sharp and his team could calculate how the water isotope ratio changes under near-perfect conditions and put these values into their models to make them more accurate.
When they revisited their desert plant data with these updated models, their observations were suddenly explainable. Sharp thinks these values could account for other hard-to-explain observations, too, especially in desert environments.
Ancient horsetails – which grew up to 30 metres high, much taller than today’s descendants – may have provided even more extreme isotope ratios and could be used to understand ancient water systems and climates, says Sharp. Small, sand-like grains called phytoliths in the horsetail stem, which can survive until the present day, have different isotope signatures according to the humidity of the air, because this will affect the amount of evaporation. “We can use this as a palaeo-hygrometer [humidity measurer], which is pretty cool,” says Sharp.
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