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 organ