The First Plants May Have Been Responsible For The Carboniferous Ice Age

A fossil of Cooksonia, the oldest land plant

The oldest known terrestrial ecosystem was 410 million years old and consisted of a volcanic peat bog in North West Scotland, known as the Rhynie Chert. The rocks contain some of the most detailed fossils on Earth. Every creature is small but can be examined as far down as the cellular level due to the incredibly fine grained nature of the silica minerals in which they were preserved. All the organisms which once inhabited the area are vital to our understanding of evolution, including the oldest insects and land-based predators.

Perhaps the most important are the plants. Organisms such as Cooksonia are the common ancestors of all the vegetation we see around us today. They are responsible for the creation of an ozone layer dense enough to support complex terrestrial organisms, fertile soil and whole ecosystems; for taming the first rivers, transforming the land from a barren wasteland of fast, eroding, rock into a lush haven where life has flourished for hundreds of millions of years.

New studies into their effect upon geology and the carbon cycle suggests that they also may have been responsible for a devastating ice age which destroyed the Carboniferous forests, facilitating the arrival of the harsh Permian scrub land. A team of geologists and palaeoclimatologists, led by Professor Tim Lenton from the University of Exeter, conducted a study to examine the effect that these first terrestrial plants had on the Earth and its climate.

The team set up an experiment using modern plants. They placed rocks, some covered in the moss species Physcomitrella patens and some without in incubators. Over a period of three months, they measured the effects of the chemical weathering caused by the moss.

Then they used what is known as an Earth System Model to simulate and plot what effect plants may have had around 440 million years ago, during the late Ordovician, when plants first appeared on land. They concluded that the plants stripped minerals containing magnesium, phosphorus and calcium from the stones, combining them with carbon dioxide in the atmosphere, excreting the end products to form carbonate rocks.

The model showed that this would have cooled the Earth's global temperature by as much as five degrees Celsius. The unused minerals would have ended up in the oceans, stimulating carbon burial by marine plants and micro-organisms, lowering the temperature again by a further two to three degrees. 'This study demonstrates the powerful effects that plants have on our climate,' said Dr Lenton. 'Although plants are still cooling the Earth's climate by reducing atmospheric carbon levels, they cannot keep up with the speed of today's human-induced climate change. In fact, it would take millions of years for plants to remove current carbon emissions from the atmosphere.