A major study led by the University of Otago in collaboration with University of Oxford researchers Hugh Jenkyns, Alex Dickson and Don Porcelli, throws new light on the role of oxygen depletion in global warming.
The study, published in the Proceedings of the National Academy of Sciences (PNAS), examined a well-known period of global warming around 94 million years ago, when oceans became widely de-oxygenated. This period of Earth’s geological history, known as an Oceanic Anoxic Event (OAE), has given scientists an extreme case-study to help understand how oceans are effected by high atmospheric CO2 emissions. Hugh Jenkyns worked on the initial research into OAEs in the 1970s when data from the Deep Sea Drilling Project, which was to become the International Ocean Drilling Program (IODP), provided conclusive evidence for the global nature of a phenomenon previously thought of as merely local.
Research Fellow Dr Matthew Clarkson and Professor Claudine Stirling, of The University of Otago’s Chemistry Department, applied a novel technique to examine how the oceans responded to climate change in the past. The scientists measured naturally occurring uranium isotopes from ancient sediments to create a geochemical record of the oxygen levels in the ocean millions of years ago. They applied this technique to sediments that are today preserved on land as the white chalk cliffs on the south coast of England, and also in the mountains of southern Italy.
They found that the likely driving mechanism of this anoxic, or deoxygenation, event was nutrient run-off, itself driven by high CO2 emissions and warmer temperatures; and that when CO2 emissions reduced, along with nutrient levels, global oceans recovered for a period.
Areas of ocean deoxygenation, known as “dead zones”, can be found currently in a number of oceans around the world including in the eastern parts of the tropical Pacific, Atlantic and Indian Oceans. The “dead zones” occur because it is harder to dissolve oxygen in water when the oceans are warm, and also more oxygen is used up during the breakdown of marine planktonic whose productivity increased when nutrient levels in the oceans became higher. Some of these nutrients came from run-off in rivers, and some from upwelling of deep ocean water.
Dr Clarkson explains the importance of the study:
“From studies like this, scientists can describe the link between increased global temperatures and increased global weathering rates, which drive a high input of nutrients into the ocean. This leads to high primary productivity in the oceans and eventually the loss of oxygen as the organic matter degrades by aerobic respiration. This process is similar to eutrophication, which happens in many lakes and rivers due to the input of fertilisers, but in this case it occurred on a global oceanic scale.
“Through comparison to other geochemical data, and simulating the event with a new biogeochemical model, we present strong evidence for the nutrient input hypothesis as a driving mechanism for anoxia (deoxygenation).”
The event was most likely caused by increased CO2 emissions from volcanic activity, over hundreds of thousands of years. Marine fauna suffered heavily during this event, although it is not considered as one of the major mass extinctions of Earth’s history. This particular Oceanic Anoxic Event was also previously thought to have lasted continyously for around 1 million years, but the new data show for the first time that the global oceans briefly recovered in the middle of the event, before returning to widespread anoxia again.
This research was undertaken primarily at the University of Otago and involved collaborators from the Universities of Oxford, Exeter and London. The researchers were mainly supported by the Marsden Fund (managed by The Royal Society of New Zealand), and also The Natural Environment Research Council (UK), The Royal Society of London and The European Research Council.
Paper: Uranium isotope evidence for two episodes of deoxygenation during Oceanic Anoxic Event 2 by Matthew O. Clarkson, Claudine H. Stirling, Hugh C. Jenkyns, Alexander J. Dickson, Don Porcelli, Christopher M. Moy, Philip A. E. Pogge von Strandmann, Ilsa R. Cooke and Timothy M. Lenton was published in Geology PNAS
Image: Chalk Cliffs of Beachy Head, Sussex where the most important samples used in this study were obtained. Image by Ian Stannard from Southsea, England (Beachy Head Uploaded by snowmanradio) [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons