Our planet was a warmer place millions of years ago. In fact, the existence of permanent ice at the poles is something recent if we speak in geological terms. And largely to blame for the planet’s cooling was a tiny plant that covered an area as large as half of Europe . How was this possible?
Not all ferns are large-leaved plants that occupy a cool, dark place in the woods. There are aquatic ferns, and among them, the genus Azolla has short roots and leaves smaller than lentils .
These floating plants can live in fresh water and have an amazing growth capacity . Depending on the conditions you are in, they can double in mass in a week. Spread over one hectare, Azolla can absorb two and a half tons of nitrogen and fifteen tons of carbon each year.
They have been used in the rice paddies of Southeast Asia for centuries . When preparing the terraces, they cover the water with this fern to prevent weeds and mosquitoes from growing, since it hinders the development of the larvae due to the lack of oxygen in the water isolated from the outside by the Azolla layer. As soon as the water is withdrawn, the fern dies, leaving behind a nitrogen that serves as fertilizer for the rice. Something similar happened 49 million years ago, but, like any geological process, on a gigantic scale.
Plants mainly need three elements to grow without stopping: nitrogen, carbon and phosphorus . This water fern has a symbiotic relationship with the cyanobacterium Anabaena azollae , capable of fixing nitrogen in amounts beneficial to Azolla . Being a plant, it takes carbon from carbon dioxide that exists in the atmosphere of our planet. We would only lack phosphorus to have a plant with excessive growth. And these perfect conditions for the Azolla fern were 49 million years ago.
At the beginning of the Eocene the Earth had higher average temperatures . Global warming reached such a point that forests grew at the poles . At that time, the arrangement of the continents left the Arctic Ocean virtually isolated , surrounded by continents still in the process of breaking apart. The heat caused the waters in the area to evaporate in large quantities, resulting in heavy rains that formed freshwater rivers that flowed into the Arctic Ocean. The arrival of fresh water was forming a layer that displaced the salt water downwards, with a higher density than the fresh water. The formation of this layer of fresh water on the surface of the Arctic allowed the fern to colonize the area. As if that were not enough, the rivers were rich in minerals and, among them, phosphorus . And, as we know, during the summer the North Pole area receives up to twenty hours of sunlight .
It was the perfect recipe for an overgrowth potion . Azolla was capable of doubling the area it occupied in just three days. It ended up covering the entire surface of the Arctic. It spread across some 4,000,000 km 2 of ocean for 800,000 years . Obviously, such a fact could not go unnoticed by the environment of planet Earth.
The so-called Azolla Event had drastic consequences. While the Azolla layer completely covered the Arctic water mass, it prevented the exchange of gases between the water and the atmosphere, causing the ocean to lose more and more oxygen. When winter came, due to the lack of sun, the water ferns died and fell to the seabed. With the lack of oxygen, they hardly decomposed, so the sediments were covering layers and capable of Azolla that was being fossilized. With the arrival of summer and the hours of sunshine, Azolla covered the entire surface of the Arctic again. This cycle repeated over almost a million years caused a radical decrease in the amount of carbon dioxide in the atmosphere. With the lack of carbon dioxide, temperatures began to drop around the globe, and that is how the 13 °C that could have been in the Arctic, dropped to the current -9 °C. It is estimated that it was this event that would lead to the creation of permanent ice at both poles , as we recognize at the time that human beings live on Earth.
The end of these ferns had several factors, but the main one is related to the separation of the continents. Continental drift allowed masses of salty water to mix with the Arctic Ocean , ending Azolla ‘s life chances.
In 2004, through drilling in the arctic ice, sheets of a few millimeters thick with fossilized Azolla were found. It is fascinating that something so tiny spread over such a large area could change the environmental fate of an entire planet.
Today, the Azolla fern grows in many warm and temperate regions of the world. But in certain countries, such as Spain, it is considered a very dangerous invasive species that has already caused more than one problem in lakes and ponds that are suffocated by this uncontrollable plant.
Brinkhuis, H. et al. 2006. Episodic fresh surface waters in the Eocene Arctic Ocean. Nature 441, 606-609 . DOI: 10.1038/nature04692.Speelman, EN et all. 2009. The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown. Geobiology 7 (2): 155-70 . DOI: 10.1111/j.1472-4669.2009.00195.x.