If the current deoxygenation of the ocean mirrors past events, the area of oxygen-deprived waters might double over the next 100 to 350 years, according to a new study.
But it could also happen much faster than that, the researchers say.
The ocean is losing oxygen due to nutrient pollution and the climate change effects of rising water temperatures and decreased mixing of marine layers. Deoxygenation expands hypoxic “dead zones,” killing off fish, including commercial species such as shrimp, and disrupting the ocean ecosystem.
If deoxygenation occurs at rates similar to those leading up to a period of low-oxygen seas 94 million years ago, there might be a doubling of oxygen-deficient ocean areas in as little as a century, according to the study published last week in the journal Science Advances.
But researchers say it’s possible that deoxygenation, driven by human activities, may occur more quickly. The ocean has already lost about 2 percent of its oxygen in just the last 50 years.
“Human-induced climate change is likely much more rapid than past climate events,” said Sune Nielsen, a geochemist at the Woods Hole Oceanographic Institution and a co-author of the new study.
Nielsen added that it’s “not impossible” that today’s ocean could lose half its oxygen in just 1,000 years. Even the 2 percent already lost has had a significant impact.
“We’re not going to lose half the oxygen in the ocean anytime soon – on average. But we are going to lose it in some places, and some places already have,” said Lisa Levin, a biological oceanographer at Scripps Institution of Oceanography at the University of California, San Diego, who was not involved in the new study.
She said that while some parts of the ocean have seen oxygen concentrations increase, the northern Pacific, Southern Ocean, some parts of the northern Atlantic and most tropical oceans have lost more than that 2 percent net decrease of oxygen.
Some of those waters are among the most biodiverse and economically important ecosystems. Nielsen notes that the vast majority of fisheries are in well-oxygenated waters. “Things would get really bad for fisheries and many marine ecosystems way before half the oxygen is lost from the ocean,” he said.
Past research Levin has conducted found that oxygen levels are the primary driver of biodiversity. “When oxygen goes down, biodiversity goes down,” she said.
The impact of deoxygenation also depends on the existing concentration of oxygen. “If you have an area that is already low-oxygen, [it] could tip you over a threshold from having a lot of animal life to having none,” Levin said.
Ocean oxygen levels have fluctuated throughout time. Two billion years ago, oxygen first spread through the oceans, giving rise to new forms of life. That was followed by periods of widespread deoxygenation, killing off much of that marine life.
One of those eras, what scientists call “Ocean Anoxic Event 2,” happened around 94 million years ago. Knowing how quickly oxygen was depleted during such events may allow researchers to forecast how fast today’s ocean might continue to lose oxygen.
Unlike ocean acidification or rising sea levels, Nielsen says scientists still know little about the mechanisms of ocean deoxygenation – how quickly it’s happened in the past, to what extent and why.
Rocks might be able to change that.
Manganese oxide dissolves in anoxic seawater and is deposited into the sediment below oxygenated waters. The production of thallium isotopes is affected by the presence of manganese oxide in sediments. So by measuring thallium isotopes in samples of rock drilled from the seafloor, the amount of oxygen that was in the waters above it can be quantified.
That’s what Chadlin Ostrander, an Arizona State geology graduate student and lead author of the new study, was counting on. Through this process, the researchers found that the ocean lost about half its oxygen over the course of 20,000 to 50,000 years during Ocean Anoxic Event 2.
Studying elements like thallium could be a key to answering questions about how quickly deoxygenation has occurred in the past and how fast it might occur in coming decades and centuries. “We are just now starting to use thallium isotopes,” said Ostrander. “As we continue to apply the tool, our estimates will be more refined.”
Other elements such as molybdenum and uranium can help researchers quantify past deoxygenation, notes Ostrander. But unlike thallium, non-oxygen-related factors also affect those element’s isotopes in seafloor sediments. Still, studies of other elements could be useful when combined with studies of thallium isotopes, he says, helping refine models of past – and thus future – deoxygenation.
Ostrander hopes the new study will lay the groundwork for research comparing ancient and current rates of deoxygenation with current observations. While other scientists have tried to quantify how much oxygen was lost during anoxic events, the researchers say the new study is the first to quantify the rate at which oxygen was lost leading up to the event, which may be more useful to understanding what currently is happening.
“There is no better time than right now to harmonize our understanding of ancient-Earth catastrophes with the analogous symptoms being observed today,” said Ostrander.