The ocean is losing its oxygen. Last week, in a sweeping analysis in the journal Science, scientists put it starkly: Over the past 50 years, the volume of the ocean with no oxygen at all has quadrupled, while oxygen-deprived swaths of the open seas have expanded by the size of the European Union. The culprits are familiar: global warming and pollution. Warmer seawater both holds less oxygen and turbocharges the worldwide consumption of oxygen by microorganisms. Meanwhile, agricultural runoff and sewage drive suffocating algae blooms.
The analysis builds on a growing body of research pointing to increasingly sick seas pummeled by the effluent of civilization. In one landmark paper published last year, a research team led by the German oceanographer Sunke Schmidtko quantified for the first time just how much oxygen human civilization has already drained from the oceans. Compiling more than 50 years of disparate data, gathered on research cruises, from floating palaces of ice in the arctic to twilit coral reefs in the South Pacific, Schmidtko’s team calculated that the Earth’s oceans had lost 2 percent of their oxygen since 1960.
Two percent might not sound that dramatic, but small changes in the oxygen content of the Earth’s oceans and atmosphere in the ancient past are thought to be responsible for some of the most profound events in the history of life. Some paleontologists have pointed to rising oxygen as the fuse for the supernova of biology at the Cambrian explosion 543 million years ago. Similarly, the fever-dream world of the later Carboniferous period is thought to be the product of an oxygen spike, which subsidized the lifestyles of preposterous animals, like dragonflies the size of seagulls. On the other hand, dramatically declining oxygen in the oceans like we see today is a feature of many of the worst mass extinctions in Earth’s history.
“[Two percent] is pretty significant,” says Sune Nielsen, a geochemist at the Woods Hole Oceanographic Institution in Massachusetts. “That’s actually pretty scary.”
Nielsen is one of a group of scientists probing a series of strange ancient catastrophes when the ocean lost much of its oxygen for insight into our possible future in a suffocating world. He has studied one such biotic crisis in particular that might yet prove drearily relevant. Though little known outside the halls of university labs, it was one of the most severe crises of the past 100 million years. It’s known as Oceanic Anoxic Event 2.
The Mesozoic era, stretching from 252 to 66 million years ago, is sometimes mistakenly thought of as sort of long and uneventful Pax Dinosauria—a stable, if alien world. But the period was occasionally punctuated by severe climate and ocean changes, and even disaster. Ninety-four million years ago, while the supersonic asteroid that would eventually incinerate dinosaurs was still silently boomeranging around the solar system, a gigantic pulse of carbon dioxide rose from the bottom of the ocean. The Earth warmed, the seas rose, and oxygen-deprived waters spread. The smothering seas mercilessly culled through plankton, bizarre bivalves, and squid-like creatures whose tentacles long dangled from stately whorled shells. For the dolphin-like ichthyosaurs, Oceanic Anoxic Event 2, or OAE 2, might have been the coup de grâce. The ocean reptiles had been patrolling the ancient seas for more than 150 million years before seemingly taking their last gasps suspiciously close to the event.
“Basically the entire continental shelf went anoxic,” says Nielsen. “There was no oxygen at the bottom of the shelf anywhere in the world.”
Today, as much as 90 percent of commercial fish and shellfish are caught on these shallow shelves—the broad flanks of our continents that slip coyly under the sea, sometimes for hundreds of miles, before remembering to drop off into the abyss. And already, spreading anoxia is beginning to advertise its deadly promise on these fishing grounds: In 2006 a seafloor survey off of Oregon revealed that rockfish, familiar fixtures of the rocky bottom, had completely abandoned their haunts, as anoxic water—water with no dissolved oxygen—spread onto the shallow shelf. But 94 million years ago in the Cretaceous, this problem was not just a seasonal nuisance. It was a global catastrophe. If dromaeosaurs had learned to pilot industrial bottom trawlers on the continental shelf, they would have gone bankrupt pulling up empty nets.
The source of the great smothering in the Cretaceous seems to have been a molten font burbling deep beneath an ancient sea that separated North from South America. The lava from these eruptions makes up much of what today is known as the Caribbean large igneous province, a vast expanse of frozen lava that stretches from Ecuador in the Pacific to the Antilles bracing against the open Atlantic. Like many scientific sobriquets, “large igneous province” fails utterly to capture the phenomenon it describes—though no description could ever really succeed in evoking its terrible grandeur.
In the United States, large igneous provinces might be more familiar to Manhattanites gazing across the Hudson at the towering basalt cliffs of the New Jersey Palisades (which, along with volcanic rocks of the same age from Nova Scotia to Brazil, are tied to a catastrophic mass extinction 201 million years ago), or to windsurfers in the black canyons of the Columbia River Gorge (which was formed by a later, smaller eruptive event). The worst mass extinction of all time, the end-Permian mass extinction 252 million years ago, left behind a large igneous province so sweeping that today it blankets much of Siberia. In fact, eruptions on this scale, though geologically brief and thankfully rare, are associated with at least four of Earth’s five major mass extinctions (and most of the dozen or so less severe, though still transformative, prehistoric crises like OAE 2).
Though the link between these eruptions and the choking seas that accompany them isn’t immediately obvious—that is, how exactly it is that one drives the other—the answer lies in life itself. And strangely, the same mechanisms that pushed the Cretaceous oceans to the edge are also driving the worrying modern expansion of anoxia in today’s oceans.
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Last summer, scientists in the Gulf of Mexico watched with growing alarm as the largest dead zone in recorded history spread across the sea, from Texas to the mouth of the Mississippi. This almost-9,000-square-mile swath of oxygen-poor ocean rendered one of the country’s most productive fishing grounds almost completely lifeless. Similar low-oxygen seas are spreading around the world.
Though not as exciting as Jurassic Park, summertime boating in the lifeless Gulf is just about as close as you can get to experiencing the Late Cretaceous planet of OAE 2. “The Gulf of Mexico today is a good analogy,” says Nielsen. “The best way to think about OAE 2 is just gigantic dead zones all over the world.”
Today’s expanding dead zones are driven, perhaps counterintuitively, by plant food. When farmers in the country’s breadbasket spread phosphorus- and nitrogen-based fertilizers on their crops, much of that Miracle-Gro eventually washes into streams and rivers, and then on into the mighty Mississippi. Where the Mississippi meets the Gulf of Mexico south of Louisiana, this plant food from the heartland proves to be as good as advertised, fertilizing huge blooms of algae that, when they die, decompose and rob the seas of oxygen.
“It may seem counterintuitive at first—you think, ‘I’m putting lots of nutrients into the ocean, that’s great,’” says Nielsen. But “it actually strips the oxygen out of the ocean.”
In 2014, fertilizer from soy and corn farms in Ohio fueled an algae bloom on Lake Erie so large and noxious that it shut down drinking water for the city of Toledo. Erie vacationers have grown accustomed to the annual appearance of toxic-slime season.
In dinosaur society, agriculture presumably played a limited role, and if tyrannosaurs had vast sewer systems, paleontologists haven’t found them yet. So what was driving the global dead zones of the Late Cretaceous? That leads back to the molten forge burbling insidiously under the Caribbean. “The magmatism definitely drove an increase in marine productivity [like we see today],” says Chris Lowery, a paleontologist at the University of Texas at Austin. “How you connect those things though—there’s still some debate.”
One of two things seems to have been happening. On the one hand, this strange volcanism could have been seeding the metastasizing algae blooms directly, by injecting a blast of trace metals, like iron, into the seawater. This would have fertilized the ancient oceans (much like some brash geoengineers have proposed doing today to sequester carbon in the ocean).
On the other hand, the volcanism might have fueled these runaway plankton blooms more obliquely. By injecting huge amounts of carbon dioxide into the oceans and atmosphere, they drove global warming and more intense weather, as inevitably happens when you inject too much CO2 into the atmosphere. Indeed, carbon-dioxide-driven global warming is a feature of many of the worst mass extinctions in Earth history. In the Late Cretaceous, this hot, stormy world would have worn down continental rock more quickly, releasing more nutrients like phosphorus from the land, which would have then washed into the rivers. Just like today’s fertilizers, this nutrient-rich brew would have been carried into the open sea, where it would have fueled explosions of algae that would die and take the ocean’s oxygen with it.
On top of all that, warmer water is just able to hold less oxygen, a phenomenon documented in the modern oceans as well. Perhaps, most likely, all of these mechanisms were working in concert, as they will be in our near future.
Who knows what legacy humans will eventually leave in the geological record, but the residue of Oceanic Anoxic Event 2 is painted in rocks around the world, most strikingly in the precipitous Furlo Gorge in central Italy. The gorge is carved out of the chalky submarine snowdrift of Cretaceous sea life—a seafloor that was shoved into the air during later tectonic collisions and which is part of a vast pile of ocean rock that makes up much of the Apennine Mountains. It’s a predictably beautiful limestone canyon, long traversed by Roman and Etruscan traders. But between stacks of this healthy white Cretaceous seafloor, a line of sickly black shale cuts through the walls.
This shale marks OAE 2. Organic sea life that died during the episode was allowed to fall and gather on the stifling sea bottom, where it couldn’t decay. Eventually it became this carbon-rich black shale, and carbon isotopes in rocks all over the world indicate a massive global burial of life in these deadly seas. (Unsurprisingly, the black rocks of OAE 2, rich with the carbon of ancient marine life, have proven attractive to oil prospectors.)
The dark dash in the Italian limestone isn’t far from a more famous rock outcrop where Walter and Luis Alvarez described a younger line in the rocks marking the dinosaurs’ eventual extraterrestrial doomsday. Like that later boundary, the dreadful delineation of OAE 2 shows up in similar blemishes of the same age around the world, from rock outcrops in Germany and Morocco, to drill cores in the Atlantic, Indian, and Pacific oceans, testifying to Late Cretaceous seas everywhere briefly seized by suffocation.
Nielsen’s team, led by Chadlin Ostrander at Arizona State University along with Jeremy Owens at Florida State University, decided to study one such core, this one drilled off the coast of Suriname. They wanted to illuminate, in high-res, the grisly timetable of this global asphyxiation, and doing so required a stroll through the lonelier reaches of the periodic table. The group knew that when there’s oxygen in the ocean, the seafloor becomes littered with manganese oxides.* These minerals precipitate out of oxygen-rich seawater all over the world today, coating sand grains and forming hunks of the stuff on the seabed—and providing an irresistible trove of rare metals for the burgeoning industry of seafloor mining.
The group also knew that when manganese oxides form, they just so happen to suck up the sea’s reserves of heavy thallium as well. So by studying the ratio of heavy to light thallium in the ancient Suriname mud, the group was able to reconstruct—over a fine-scale time span of tens of thousands of years—exactly how fast oxygen dwindled in an ancient ocean shrouded by 94 million years of history.
When Nielsen described this forensic legerdemain to me in his office on Cape Cod, I shook my head in awe. Who ever came up such an ingenious system? He winced and laughed, seeming to conceal years of academic trauma. “That’s basically what I’ve been working on for the last 15 years.”
What his team found (and published in a recent paper in Science) was that OAE 2 itself lasted for almost half a million years. But it took only on the order of thousands of years of diminishing oxygen to reach its choking crescendo. “The rates between now and OAE 2 are actually pretty comparable,” he said. “Dead zones today are expanding at a global scale, pretty much everywhere you see around the world. Around the continental shelves you see larger and more persistent dead zones, and that’s what you’d expect if the ocean is losing its oxygen.”
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OAE 2 marked something of an end for a strange, broader era of stress in Earth’s oceans, a history hinted at by the disaster’s sequel status (Ocean Anoxic Event 2: Just when you thought it was safe to go back in the water ...). Almost 30 million years before, the similarly dramatic Early Aptian Oceanic Anoxic Event throttled ancient ocean life, as did a number of lesser events peppered throughout the Cretaceous. Even earlier, the Jurassic period suffered its own anoxic spasms.
Each summer, Rowan Martindale, from the University of Texas at Austin, ventures to ancient seafloors in Slovenia and Morocco to study the so-called Toarcian Oceanic Anoxic Event of 183 million years ago, a disaster fueled once again by CO2-spewing volcanism as Antarctica tore from Africa—a crisis that wiped out strange reef-building bivalves, corals, and a slew of other ocean critters. It’s a disaster she says has many of the hallmarks of other mass extinctions.
“You have your initial eruption, which puts a massive amount of carbon dioxide into the atmosphere,” she says. “This causes your atmospheric carbon dioxide to rise and temperature to rise, which can result in a whole other host of environmental changes, like the release of terrestrial methane and methane clathrates on the seafloor, ocean acidification, and all of these other knock-on effects. So we see warming and expanded oxygen-minimum zones, which manifest as oceanic anoxic events in the rock record.”
But after the Late Cretaceous, and that black line in the Furlo Gorge of Italy, the age of mass suffocation was largely over. “OAE 2 is really the last big one,” says Lowery, the University of Texas paleontologist.
As the continents carried on their eternal wander, vast new oceans opened up between them. Others closed. It may have been that 94 million years ago, this roaming world accidentally created a planet uniquely primed to go anoxic. Though Pangaea had long since blown to pieces, it took time for the great continental migration to reshape the planet, and the continents still huddled closely around their growing Atlantic toddler. Where New Jersey and Morocco once described the same unbroken expanse, the widening gulf between them had, by now, become a proper North Atlantic Ocean. But the South Atlantic remained little more than a narrow channel—the jigsaw puzzle of South America and Africa only slightly jostled.
“Before the South Atlantic opened up, the North Atlantic and the [proto-Mediterranean and Indian Ocean] were kind of these little, fairly restricted seas,” Lowery says. “And so it kind of lets you build up these low-oxygen areas where you’re not having a lot of circulation and current coming through and aerating the water. But then after the South Atlantic opened, global circulation changed and everything was just kind of freshened up. So you lost the preconditions for having worldwide oceanic anoxic events.”
Today, the preconditions might be back, though in a form unlike anything in Earth’s history. It’s not nearly as warm as it was during the Cretaceous greenhouse, a circumstance that helped lead the oceans closer to the edge—though that may change in the coming centuries. And the continents are arranged more favorably than in the stagnant bathtub of the Late Cretaceous. Only a global technological civilization of billions of people, drenching the world’s shallow seas with phosphorus and nitrogen and blasting the atmosphere with greenhouse gases, could summon OAE 2 back from the fossil record.
The circumstances of the Earth’s ancient anoxic events might have been strange, but not nearly as strange as our modern world. As with global warming, sea-level rise, and ocean acidification, humanity still has time to avoid the grislier scenarios promised by spreading anoxia. But as Nielsen, Ostrander, and Owens write: “Ancient OAE studies are destined to become uncomfortably applicable in the not-so-distant future.” In other words, our project as a species may well ultimately be the same as that of a large igneous province—producing in our eruptions of carbon dioxide and nutrient pollution an increasingly tenantless and sickly ocean beloved by bacteria.
* This article originally stated that the seafloor becomes littered with magnesium oxides. We regret the error.
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