By Fabrício Caxito
The signs of a not-so-friendly relationship
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We already know that through their weathering and erosion, mountains may have influenced the emergence of complex life forms, providing the nutrients and oxygen necessary for the metabolism of animals (as I discussed in a previous text, “The mountains that created life” ). But has the connection between them and life on the planet always been so friendly?
In some regions of the globe, such as the southwest coast of Africa, fishermen are already familiar with a phenomenon: the waters become cloudy and acquire a strong greenish, red or yellowish color. Despite the beauty of the spectacle, caused by an uncontrolled proliferation of microscopic algae, these “red tides” or “algal blooms” can be quite harmful to the marine ecosystem. By blocking sunlight and decreasing the amount of oxygen in the water, these microorganisms can expel or kill fish and other complex organisms. Currently, one of the most common reasons for eutrophication, which is the name scientists give to the phenomenon, is human pollution dumped into the oceans, rich in nutrients for algae, which thus reproduce in excess.
And why do algal blooms help us understand the relationship between mountains and complex life on the planet?
In the Ediacaran period –600 million years ago–, some novelties emerged: the first great mountain ranges in a modern style, that is, as high and continuous as the Himalayas; early life forms more complex than simple bacteria and protozoa, such as animals; the shells and other protective mechanisms and ornaments. These innovations and the accelerated development of organisms were possible due to the erosion of mountain ranges that, newly formed, delivered to the adjacent seas a balanced amount of nutrients and oxygen.
For the life forms that inhabited some seas, however, the story did not have a happy ending. In many of them, today located on continents of the northern hemisphere, the first complex organisms gave way to the rich and varied forms of life that characterize the so-called Cambrian explosion, in the geological period that followed. In some ancient seas preserved in the southern hemisphere, such as the Bambuí Sea, in Minas Gerais, Bahia and Goiás, there are no records of this explosion of complex life.
The Bambuí Sea was developed in a peculiar situation in the Ediacarano: it began around 630 to 600 million years ago, beside a large mountain range whose remnants are found today in central Brazil. The erosion of these mountains provided the nutrients and oxygen necessary for the development of Ediacaran life forms such as organisms of the type. Cloudina sp., the first to present calcareous shells to protect themselves from predators on the seabed. However, soon after, some 540 million years ago, Bambuí was surrounded by mountains on all sides, and ended up becoming a closed basin, similar to what is now the Dead Sea. The erosion of these mountains provided an uncontrolled amount of nutrients to the basin, and the lack of connection with the ocean prevented the renewal of the waters. Due to the stagnation of the waters, the basin suffered strong eutrophication – toxic algal blooms depleted oxygen and impeded the development of typical Cambrian fauna.
There is a “Goldilocks” effect in the relationship between mountain ranges and complex life. In the story, the girl can’t eat Papa Bear’s porridge because it’s too hot, nor Mother Bear’s porridge because it’s too cold, but Bear Son’s porridge is at the right temperature for her, as well as the teddy’s bed and so on. against. Effects of this type or optimal level are known and discussed in various areas, such as defining the habitable zones of stellar systems, where a planet must be at an optimal distance from a star. If mountains provide the necessary factors for life, an excess of them around an ancient sea may have had an opposite effect, deleterious to complex life. We talk about that in this article recently published in Scientific Reports.
The history recorded in the ancient seas can teach us. We read in the rocks the devastation and even the extinction of ancient life forms, caused by the excess of nutrients in the waters. Unfortunately, we are now simulating these natural effects by releasing pollutants into the oceans at much faster intervals than Earth’s history has ever known. It’s up to us to understand the warnings the planet gives us, and to avoid deliberately following paths that have already proven catastrophic.
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Fabrício Caxito is a geology professor and philosophy student at UFMG.
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