Oceans Gasping for Breath

The Toarcian occurred at the end of the early Jurassic Period, between 183 million years ago to 175 million years ago.
Credit: UC Berkeley

The debate over climate change and its effects is often headline news. In the near future, scientists are concerned that the Earth will experience a continuing increase in global temperature. New research is showing that even if this increase is only a few degrees, it could have important repercussions for life on our planet.

In a study published in the March issue of the journal Geology, a research team led by Christopher Pearce of the Open University in the UK examined layers of sedimentary rocks from the ocean floor. Their goal was to unravel the story of climate change during the early Jurassic period.

During the Jurassic, abrupt global warming of between 5 and 10 degrees Celsius was associated with severe environmental change. Many organisms went extinct and the global carbon cycle was thrown off balance. One of the most intriguing effects was that the oxygen content of the oceans became drastically reduced. This caused many marine species to die off.

These intervals of reduced oxygen content in the oceans are now known as oceanic anoxic events, or OAEs. OAEs are associated with periods of global warming and have occurred a few times in Earth’s history. In the recent study, researchers focused specifically on the Toarcian OAE, a well-documented OAE from the early Jurassic.

During OAEs, the remains of dead organisms and other organic matter accumulate on the ocean floor and became layers of organic-rich sediments. Today, scientists are examining the chemical and isotopic compositions of these sedimentary deposits in order to determine the actual extent to which the oceans became anoxic. By doing so, they have been able to draw connections between oxygen-depleted oceans and the disruption of Earth’s carbon cycle.

The carbon cycle is one of the most important biogeochemical cycles on Earth. In any given year, tens of billions of tons of carbon move between the atmosphere, hydrosphere, and geosphere. The illustration above shows total amounts of stored carbon in black, and annual carbon fluxes in purple. Click image for larger view.
Credit: NASA/NASA Earth Science Enterprise

The carbon cycle on Earth is one of the most important cycles for life as we know it. Carbon is a primary building block of life and is present in every living organism. In order for life to survive on our planet, carbon must cycle between the atmosphere, geosphere (land), hydrosphere (water) and biosphere (life). If the carbon cycle were to suddenly become disrupted, many forms of life on Earth would not survive. Even minor disruptions in the carbon cycle can have profound consequences for living organisms.

By studying organic-rich marine deposits from the Toarcian OAE, the Open University researchers were able to compare the oxygen levels of ancient seawater to the oceans of today. The sedimentary rocks contain molybdenum, whose isotopic composition is altered depending on how oxygenated the seawater was when the sediments formed. By studying how the isotopic composition of molybdenum changed during the Toarcian OAE, scientists have developed a unique way to trace fluctuations in the oxygen content of Earth’s oceans.

The Open University team determined that major disruptions in the global carbon cycle during the Jurassic period were intimately linked with the development of anoxic oceans and with global warming. Ultimately, this ties global warming to the demise of numerous life forms on Earth millions of years ago. Additionally, the research is providing insight into how the Earth’s oceans and atmosphere evolved over time.

Image of the Earth and Moon taken by Galileo spacecraft. The Earth is the only planet known to harbor life, and life’s future on Earth may depend on our understanding of global climate change.
Credit: NASA

Modern studies of global climate change on Earth usually rely on computer modeling techniques. However, studying the history of our planet through geology can provide information on actual occurrences of climate change in the past. Dr. Anthony Cohen, a member of the research team, commented: “The use of current computer models to try to predict the course of climate and environmental conditions in the longer term is uncertain because of our relatively poor understanding of the great complexity of the Earth’s behaviour. In contrast, marine sedimentary records can provide quantifiable information about precisely how the Earth has responded to severe environmental change in the past. Therefore, these records may also provide valuable constraints for testing the reliability of predictions about environmental change that will continue to occur in the future as a result of man’s activities.”

Although the Toarcian OAE occurred roughly 183 million years ago, the findings of the recent study have important implications for our understanding of climate change today. The rates and magnitude of environmental change during ancient OAEs appear to have been similar to what we see occurring in modern times.

By studying OAEs, scientists are able to gain important clues about how climate change might impact life on Earth in the in the coming centuries. Hopefully, their work will lead to scientific solutions that could prevent the same devastating affects on the Earth’s carbon cycle — and life itself — that were caused by global warming during the Jurassic period.


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