Environment - First land plants plunged Earth into ice age

Moss age (Image: Philip Silverman/Rex Features)

Never underestimate moss. When the simple plants first arrived on land, almost half a billion years ago, they triggered both an ice age and a mass extinction of ocean life.

The first land plants appeared around 470 million years ago, during the Ordovician period, when life was diversifying rapidly. They were non-vascular plants, like mosses and liverworts, that didn't have deep roots.

About 35 million years later, ice sheets briefly covered much of the planet and a mass extinction ensued. Carbon dioxide levels probably fell sharply just before the ice arrived – but nobody knew why.

Tim Lenton of the University of Exeter, UK, and colleagues think the mosses and liverworts are to blame.

Moss versus rock

It's not the first time that plants have been fingered as a cause of glaciation. Researchers already suspect that the rise of vascular plants in the Devonian period, some 100 million years later, triggered another ice age. The plants' roots extracted nutrients from bedrock, leaving behind vast quantities of chemically altered rock that could react with CO₂ and so suck it out of the atmosphere.

Non-vascular plants like mosses don't have deep roots, so it was thought that they didn't behave in the same way. Lenton suspected they might have played a role nevertheless. To find out, he set up an experiment to see what damage a common moss (Physcomitrella patens) could inflict on granite. After 130 days, rocks with moss living on them had weathered significantly more than bare ones – and about as much as they would have if vascular plants were living on them. "The secret seems to be that the moss secrete a wide range of organic acids that can dissolve rock," Lenton says.

When Lenton added this effect of non-vascular plants to a climate model of the Ordovician, the CO₂ dropped from about 22 times modern levels to just eight times modern levels. That was enough to trigger an ice age in the model of Ordovician Earth.

In his experiments, the non-vascular plants also released lots of phosphorus from rocks. Much of this would have wound up in the ocean, where we know it can trigger vast algal blooms. As other bugs feasted on the algae, they would have used up the oxygen in the water – suffocating oxygen-breathing animals and accounting for the mass extinction of marine life known to have occurred at the end of the Ordovician.

Although the first land plants were responsible for these mass deaths in their ocean-dwelling neighbours, Lenton says they themselves probably came out of the Ordovician ice age largely unscathed. That's because the ice was concentrated around the South Pole, while the plants lived in the tropics.

Life may also have caused an even harsher cold snap much earlier in Earth's history. The first complex animals appeared some time around 800 million years ago, and may have sucked so much CO₂ from the atmosphere that the entire planet froze over in a "snowball Earth".

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1390

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