All animals, whether they live on land or in the water, require oxygen to breathe. But today, the oceans of the world lose oxygen, due to a combination of rising temperatures and changed ocean currents. Both factors are driven by human induced climate change.
This process has the potential to interfere with marine food chains. We already know that large hypoxic or low acidic zones can be fatal. If hypoxia expands in both size and duration, it is possible to cause widespread extinction of marine life that has previously occurred in the earth's history.
We investigate natural, ancient changes in ocean oxidation and the biological effects as a way to understand the natural response to potential future climate scenarios. In a recent study, we investigated links between a major volcanic event that occurred millions of years ago and changes in the ocean's oxygen content. Just like human activities today, this event released massive amounts of carbon dioxide and other greenhouse gases into the atmosphere.
We found that this section seemed to trigger significant acid losses in the world's ocean which lasted over a million years. Our research contributes to increasing evidence that the oxygen content is affected drastically by heating temperatures and other climate-related feedbacks caused by greenhouse gas emissions.
Is our sea suffocating?
Scientists agree that human activities – primarily the combustion of fossil fuels, deforestation and agricultural methods – release carbon dioxide and methane into the atmosphere at unprecedented prices. Over the last decades, climate change research has focused on global warming, sea level rise and ocean acidification. Now the loss of oxygen starts to get attention.
The oceans of the world have lost more than 2 percent of their loose oxygen reservoirs over the past five decades. In many places, local issues like nutrient contamination make the problem worse. In American waters, major hypoxic zones are formed regularly in the Gulf of Mexico, the Great Lakes and along the Pacific Ocean. Other coastal waters are affected the same way around the world.
Hypoxia can destroy fish catches. For example, a large fish death in the Philippines in 2002 was directly associated with decreasing oxygen levels in the water. A similar event occurred in Redondo Beach, California in 2011 when hypoxic conditions over several days decimated the local fish population. In the end, these events have significant effects on humans, as 40 percent of the world's population lives within about 60 miles of the ocean. Millions of people are addicted to fish for food, income or both.
Coupling of old acid loss into a marine mass destruction
Earlier volcanic eruptions are probably our only ancient analogues of modern greenhouse gas emissions from human activities. To understand how these events affected the oceans, we turned to old marine rocks that can record the relationship between carbon dioxide emissions from volcanoes, marine acid levels and extinction events.
Such an event, which occurred 183 million years ago during the early Jurassic, is called Toarcian Oceanic Anoxic Event. It is known for greater volcanicism and the seventh largest mass eradication in the history of the earth, which occurred mainly in the oceans. The volcano that occurred was much larger in scale than all modern volcanoes, and would have released massive amounts of greenhouse gases into the atmosphere and dramatically warm the planet.
We applied a new and new tool – thallium isotopes – to determine the amount of time and amount of oxygen loss from the oceans during this event. Thallium is a soft silver melted metal found in different ores, including manganese balls on the seabed. Isotopes are atoms of the same elements that have light mass differences because they contain varying numbers of neutrons.
Many minerals form in the ocean, often through reactions involving oxygen. But the amount of free acid in sea water is not constant in the modern sea and has also varied in time. When oxygen is abundant in the ocean, manganese oxides on the seabed and tallium – especially its heavier isotopes – adhere to them. By analyzing ancient marine sediments and looking for shifts in the isotopic value of thallium, we suggested that we could track the progressive loss of hydrochloric acid.
To do this we collected specific dark colored sedimentary rocks from this time in places in Canada and Germany, representing two different ancient oceans. We then dissolved each layer of stone to form a liquid and insulated and purified tallium in each sample.
We found that thallium isotopes are changed in two steps during this event. First, the oceans became less oxygenated during massive volcanism, about 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 occurred 183.1 million years ago to 182.6 million years ago.
This work shows for the first time that the global sea lost oxygen temporarily with the beginning of volcanism. It is important that this happened at the start of a known extinction called the Pliensbach-Toarcian mass extermination event. In other words, the first signs of extinction coincide with the fossil record with oxygen loss in the oceans.
We now believe that this state of marine low oxygen conditions lasted over one million years and over two eradication pulses. The second phase of deoxigenation was more expansive, leading to greater eradication. It also happened if the atmosphere contained enough oxygen to support life, much like today. Furthermore, the duration of low oxygen conditions was similar to another event that occurred 94 million years ago with biological consequences.
A global warming threshold?
The Intergovernmental Panel on Climate Change recently released a special report on global warming of 1.5 ° C, which required immediate action to limit climate change to levels that will minimize environmental and ecosystem tensions. Researchers agree that this means that global average temperatures rise from above 1.5 degrees Celsius over pre-industrial levels.
The report notes that if the temperature increases by 2 ° C instead of 1.5 ° C, significantly more oxygen loss will occur in the oceans. This makes it important to continue studying the old effects of oxygen loss on the extinction record, so researchers can better predict future climate scenarios. It is also important to identify areas that are most affected by oxygen loss in the ocean and limit the environmental impact that will occur when our planet continues to warm.