Fundamental Science: The oceans run out of steam

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We all instinctively know the importance of oxygen for life on Earth. Not only do humans and land animals need this gas to live, but as any aquarium aficionado knows, it is important to keep the water well oxygenated so that marine animals can thrive. We do have evidence, however, of several moments in the planet’s previous history when the oceans suffered from a lack of oxygen – and that something similar is happening today.

These episodes are called Oceanic Anoxic Events, and are usually marked by the mass extinction of several marine species. Its causes are varied, but in general they involve the increase in the average temperatures of the globe and the amount of nutrients thrown into the sea water. Oxygen is more soluble in colder waters, therefore, a warming of the seas causes the concentration of this element to decrease.

As for nutrients, if excessive they cause a phenomenon known as “red tide”, which occurs when a huge mass of simple organisms such as algae is attracted by the abundance of food in surface waters. The abundance of algae produces a kind of blanket on the surface of the seawater that ends up preventing sunlight from reaching the deeper waters, where various organisms depend on it to survive.

Increases in temperature and nutrients have occurred in the geological past due to various natural factors. One of the main ones is the intensification of volcanic activity at specific periods in the Earth’s history. When tectonic plates are separating from each other, they undergo a thinning in the region of separation that ends up causing the breakup of supercontinents like Pangea. This thinning allows a considerable amount of heat from the mantle to be expelled in these zones, generating enormous volcanic activity.

The more active volcanoes, the greater the volume of gases such as CO2 expelled into the atmosphere, causing a greenhouse effect and raising global temperatures. At the same time, volcanic rocks in plate thinning zones are particularly rich in nutrients such as phosphorus. If many of these rocks are exposed to the earth’s surface, they undergo accelerated weathering and erosion, and rivers and winds carry excess phosphorus and other nutrients to the seas.

Back to the beginning of this text: in recent years, a similar phenomenon has been happening in different areas of the oceans around the world. The so-called Minimum Oxygen Zones have been expanding at high rates, producing the death of countless organisms in the coastal areas of several continents, and directly affecting not only the marine ecosystem, but also the fishing communities whose survival depends on these ecosystems.

Why are Low Oxygen Zones getting bigger and bigger? The reasons do not differ from the previous ones: climatic changes concomitant with an increase in the level of nutrients in the oceans. But, as you, the reader, may already imagine, now we are responsible for this phenomenon. The gigantic amount of CO2 thrown into the atmosphere since the Industrial Revolution has given rise to very important climate changes that directly affect the solubility of oxygen in seawater. And the waste and garbage generated by human beings, rich in nutrients, lead to anthropogenic red tides.

Investigating the geological past, we can identify several moments in which the increase in temperature and in the number of nutrients, due to natural factors, led to mass extinctions, since the absence of oxygen and sunlight shook marine ecological chains. It only remains for us to know how to read these warnings from other eras and prevent human actions from resulting in similar catastrophes in the near future.

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Fabrício Caxito is a professor of geology, principal researcher in the GeoLife MOBILE project and philosopher at UFMG.

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