All animals, whether they live on land or in the water, need oxygen to breathe. But today the oceans of the world are losing oxygen, due to a combination of rising temperatures and changing sea currents. Both factors are driven by man-made climate change.
This process has the potential to disrupt marine food chains. We already know that large hypoxic or oxygen-depleted zones can be fatal. If the hypoxia expands in both size and duration, it is possible to eradicate marine life on a large scale, which happened earlier in the history of the earth.
We investigate natural, old changes in ocean oxygenation and biological effects as a way to understand the natural response to possible future climate scenarios. In a recent study we investigated the links between a large volcanic event that took place millions of years ago and changes in the oxygen level of the sea. Just like human activities today, this event brought huge amounts of carbon dioxide and other greenhouse gases into the atmosphere.
We discovered that this episode caused significant oxygen losses in the ocean of the world that lasted more than a million years. Our research contributes to growing evidence that the oxygen content in the seas is dramatically influenced by warming temperatures and other climate-related feedback caused by the emission of greenhouse gases.
Are our oceans suffocating?
Scientists widely agree that human activities – mainly burning of fossil fuels, deforestation and farming practices – release carbon dioxide and methane into the atmosphere in unprecedented quantities. Over the past decades, research into the impact of climate change has concentrated on the greenhouse effect, sea level rise and acidification of the ocean. The loss of oceanic oxygen is now starting to get attention.
The world's oceans have lost more than 2 percent of their dissolved oxygen reservoir in the last five decades. In many places, local factors such as nutrient pollution make the problem worse. In the American waters, large hypoxic zones regularly occur in the Gulf of Mexico, the Great Lakes and along the Pacific coast. Other coastal waters are affected in the same way all over the world.
Hypoxia can destroy fish. For example, a major fish death in the Philippines in 2002 was directly associated with falling oxygen levels in the water. A similar event occurred in Redondo Beach, California, in 2011, when the local fish population was decimated by hypoxic conditions for several days. Ultimately, these events have significant consequences for humans, as 40 percent of the world's population lives within about 60 kilometers of the ocean. Millions of people depend on fish for food, income or both.
Linking old oxygen loss to extinction of the sea
Early volcanic eruptions are probably our only old analogues for modern greenhouse gas emissions from human activities. To understand how these events affected the oceans, we have turned to ancient sea-rocks that can establish the relationship between carbon dioxide from volcanoes, marine oxygen levels and extinction events.
One such event, which took place 183 million years ago during the early Jurassic, is called the Toarcian Oceanic Anoxic Event. It is known for the great volcanism and the seventh largest mass extinction in the history of the earth, which took place mainly in the oceans. The volcanism that took place was much larger in size than all modern volcanoes and would have released enormous amounts of greenhouse gases into the atmosphere, causing the planet to warm up dramatically.
We have used a new and new tool – thallium isotopes – to determine the timing and amount of oxygen loss of the oceans during this event. Thallium is a soft, silvery metal that is found in various ores, including manganese balls on the ocean floor. Isotopes are atoms of the same element with small mass differences, because they contain different numbers of neutrons.
Numerous minerals form in the ocean, often through reactions involving oxygen. But the amount of free oxygen in seawater is not constant in the modern ocean and also varies over time. When oxygen is abundant in the ocean, manganese oxides precipitate at the bottom of the ocean and thallium – and especially the heavier isotopes – continue to stick to them. By analyzing old marine sediments and looking for shifts in the isotopic value of thallium, we hypothesized that we could follow the progressive loss of oceanic oxygen.
To do this, we collected specific dark sediments from this period at locations in Canada and Germany, representing two different oceans from antiquity. We then dissolved each layer of rock to form a liquid and isolated and purified the thallium in each sample.
We found that thallium isotopes shifted in two phases during this event. First, the oceans became less oxygen-rich during the onset of mass volcanism, some 183.8 million years ago to 183.1 million years ago. Then the oceans lost even more oxygen, coinciding with the most intense phase of volcanism, which took place from 183.1 million years ago to 182.6 million years ago.
This work shows for the first time that the ocean lost oxygen worldwide, coincidentally with the onset of volcanism. It is important that this happened at the beginning of a known eradication, the Pliensbachian-Toarcian mass extinction event. In other words, the first signs of extinction in the fossil record coincide with oxygen loss in the oceans.
We now think that this state of oxygen-depleted marine conditions lasted more than a million years and about two extinction pulses. The second phase of deoxygenation was more expansive, causing a greater extinction. It happened even though the atmosphere contained enough oxygen to sustain life, just like today. Moreover, the duration of oxygen-depleted conditions was comparable to another event that occurred 94 million years ago with biological consequences.
A threshold for global warming?
The Intergovernmental Panel on Climate Change has recently issued a special report on global warming of 1.5 ° C, which called for immediate action to limit climate change to levels that minimize the stress of the environment and the ecosystem. Scientists generally agree that this means preventing the global average temperature from rising more than 1.5 degrees Celsius above pre-industrial levels.
The report notes that if the temperature rises by 2 ° C instead of 1.5 ° C, significantly more oxygen loss will occur in the oceans. This makes it important to study the old effects of oxygen loss on the extinction record, so that scientists can better predict future climate scenarios. It is also important to identify areas that will be most affected by ocean loss and to reduce the environmental impact that will occur if our planet continues to warm up.