New research led by scientists from Oxford University and Exeter University has shown that the invasion of the land by plants in the Ordovician Period (488-443 million years ago) cooled the climate and may have triggered a series of ice ages. During this period sea levels are very high and at the end of the period there was a mass extinction event.
At the beginning of the period, around 480 million years ago, the climate was very hot due to high levels of CO2, which gave a strong greenhouse effect. The marine waters are assumed to have been around 45°C, which restricted the diversification of complex multi-cellular organisms. But over time, the climate become cooler, and around 460 million years ago, the ocean temperatures became comparable to those of present day equatorial waters. The dramatic cooling of the planet between 300 and 200 million years ago was also the result of the evolution of large plants with large rooting systems that caused huge changes in both of these processes. In the current results it was shown that the appearance of the first land plants had a similar effect and much earlier in time.
Land plants evolved from chlorophyte algae, perhaps as early as 510 million years ago; some molecular estimates place their origin even earlier, as much as 630 million years ago. Land plants may have evolved from a branched, filamentous, alga, dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools.
The team set out to identify the effects that the first land plants had on the climate during the Ordovician Period, which ended 444 million years ago. During this period the climate gradually cooled, leading to a series of ice ages. This global cooling was caused by a dramatic reduction in atmospheric carbon, which this research now suggests was triggered by the arrival of plants.
Among the first plants to grow on land were the ancestors of mosses that grow today. This study shows that they extracted minerals such as calcium, magnesium, phosphorus and iron from rocks in order to grow. In so doing, they caused chemical weathering of the Earth’s surface. This had a dramatic impact on the global carbon cycle and subsequently on the climate. It could also have led to a mass extinction of marine life.
The research suggests that the first plants caused the weathering of calcium and magnesium ions from silicate rocks, such as granite, in a process that removed carbon dioxide from the atmosphere and forming new carbonate rocks in the ocean. This cooled global temperatures by around five degrees Celsius.
In addition, by weathering the nutrients phosphorus and iron from rocks, the first plants increased the quantities of both these nutrients going into the oceans, fueling productivity there and causing organic carbon burial. This removed yet more carbon from the atmosphere, further cooling the climate by another two to three degrees Celsius. It could also have had a devastating impact on marine life, leading to a mass extinction whose cause had puzzled scientists.
The team used the modern moss, Physcomitrella patens for their study. They placed a number of rocks, with or without moss growing on them, into incubators. Over three months they were able to measure the effects the moss had on the chemical weathering of the rocks.
They then used an Earth system model to establish what difference plants could have made to climate change during the Ordovician Period.
One of the lead researchers, Professor Tim Lenton of Geography at the University of Exeter said: “This study demonstrates the powerful effects that plants have on our climate. 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.”
Source : exeter.ac.uk
