A satellite image of the Northern Gulf of Mexico, where 'dead zones' can reach the size of New Jersey. Photo Credit: NASA
Oxygen-starved ocean 鈥渄ead zones,鈥� where fish and animals cannot survive, have been expanding in the open ocean and coastal waters for several decades as a result of human agricultural and industrial activity. Trying to predict the scale and location of future dead zones, scientists have looked to the past for historical clues.
Today鈥檚 largest open ocean dead zone, located in the eastern Pacific Ocean, emerged eight million years ago as a result of increasing nutrient content in听the ocean, an international team of scientists reported in the听Proceedings of the National Academy of Sciences.
While the sources of nutrient enrichment today may be different, the mechanics that created what scientists call 鈥渙xygen-deficient zones鈥� remain the same, according to Boston College Assistant Professor of Earth and Environmental Sciences Xingchen 鈥淭ony鈥� Wang, a lead author of the report. Better understanding ocean dead zones in the past, may help future ocean conservation efforts.
Xingchen 鈥淭ony鈥� Wang (Lee Pellegrini)
鈥淚n order to better protect marine ecosystems and manage fisheries, it is critical to predict how an ocean 鈥榙ead zone鈥� will evolve in the future,鈥� Wang said.
A coastal ocean dead zone is mainly caused by the flow of excess nutrients humans use on land, such as fertilizer application. In the Northern Gulf of Mexico, anthropogenic nutrients delivered by the Mississippi River annually produce a dead zone as big as the state of New Jersey.
These zones also occur naturally in the open ocean, with the largest found in the eastern Pacific Ocean. 鈥淚t remains unclear how these dead zones will change as the planet warms. So, we studied the history of the eastern Pacific dead zone in order to better predict its future behavior,鈥� Wang said.
The researchers from universities in the U.S., Canada, Taiwan, Germany, and Australia set out to determine the evolution of open ocean dead zones before human activity began to impact the ocean, Wang said. In addition, did these dead zones always exist? If so, why?
The team examined the chemical composition of ocean sediments near today鈥檚 largest ocean dead zone, located in the eastern Pacific Ocean. The team obtained sediment samples鈥攖he 鈥渉istory books鈥� of ocean activity鈥攇oing back 12 million years and analyzed nitrogen contained in microfossils, known as foraminifera.
鈥淔ossil foraminifera in ocean sediments are mainly made of calcium carbonate and they have been used to study past climate change for decades,鈥� said Tianshu Kong, a doctoral student in Wang鈥檚 lab who studies foraminifera in her research. 鈥淢ost foraminifera found in ocean sediments actually came from the surface water, so they can tell us what happened in the upper ocean.鈥�
The team looked in the dead zones for signs of denitrification, which occurs when oxygen content is so low that microbes instead use nitrate to power their biological activity. Nitrogen has two stable isotopes, nitrogen 14 and nitrogen-15, and microbes prefer to consume the lighter isotope of nitrogen-14 during denitrification.
When oxygen deficient zones expand, denitrification zones also expand, raising the nitrogen-15 to nitrogen-14 ratio of the remaining nitrate, which is then recorded in ocean organisms such as foraminifera through the cycling of nitrogen in the marine ecosystems, according to the report.
鈥淏y analyzing the nitrogen-15 to nitrogen-14 ratio of foraminifera in ocean sediments, we can reconstruct the history of the extent of oxygen deficient zones,鈥� said Wang, whose work on the project was funded in part by the Simons Foundation.
In addition, the researchers analyzed the phosphorus and iron content of the same sediments, which reveal the ancient nutrient content in the deep Pacific Ocean, according to the report.
鈥淒eep-ocean nutrient content is hard to reconstruct and our record is the first of its kind over the past 12 million years; its trends have important implications for the global carbon cycle and climate change,鈥� said Woodward W. Fischer, a co-author of the study and professor at the California Institute of Technology.
Today鈥檚 largest 鈥渄ead zones鈥� are located in the eastern Pacific Ocean. Symbols and colors indicate the location and thickness of oxygen-deficient water, with darker color indicating thicker 'dead zones.' Photo Credit: Tianshu Kong.
The sedimentary records showed the team that the largest open ocean dead zones gradually expanded over the past eight million years, said Wang, who was joined on the project by colleagues from the Max Planck Institute for Chemistry, Princeton University, National Taiwan University, University of Toronto, Texas A&M University, University of Western Australia, and Caltech.
鈥淔urther, the expansion of these dead zones was mainly caused by nutrient enrichment,鈥� he said. 鈥淭his mechanism is similar to the formation of dead zones in today鈥檚 coastal waters, except that humans are responsible for the current nutrient enrichment.鈥�
These findings may help better predict the future behavior of open ocean dead zones, according to the report. For example, human activities have been adding more and more nitrogen to the ocean. They can support the need to improve climate and ocean models to better gauge the impact of anthropogenic nitrogen on the deoxygenation processes in the open ocean.
The researchers were struck by data that showed much lower nutrient content in the ocean prior to eight million years ago.
鈥淭he nutrient increase since eight million years ago was likely caused by increased weathering and erosion on land which would increase the delivery of phosphorus to the ocean,鈥� said Fischer.
鈥淚n addition, terrestrial ecosystems underwent a major transition between eight and six million years ago,鈥� Wang said. 鈥淢any forests were replaced by less dense grassland, known as the expansion of C4 ecosystems. With more grassland, soil erosion might have increased during this period, and it would have triggered a greater transfer of organic nutrients to the ocean.鈥�
Wang said a likely next step in this research would be to determine how the flow of nitrogen into the ocean from human activity can impact the ocean鈥檚 nutrient cycle.
鈥淭he key questions lie in our coastal zones, where the most anthropogenic nitrogen enters the ocean,鈥� Wang said. 鈥淚f most of anthropogenic nitrogen is removed in coastal regions鈥攅ssentially by denitrification taking place in the sediments鈥攖hen that could lessen the impact on the whole ocean. Our research group at AV研究所 is currently doing some work in the Northern Gulf of Mexico to better understand the fate of anthropogenic nitrogen in the ocean.鈥�
Ed Hayward | University Communications | January 2023