The Day the Sea Stood Still
Special to The Washington Post
Wednesday, September 9, 1998; Page H01
It may have been the blast that changed the world.
One day in the Caribbean Sea, at the end of the Paleocene epoch 55 million years ago, a volcano blew, spewing a climate-altering parasol of tiny particles high into the atmosphere. Such events are hardly rare. But this was no ordinary volcano: It was huge. And this was no ordinary time in the planet's history: The environment already was on the threshold of profound change.
According to a new theory proposed by marine geologist Timothy Bralower of the University of North Carolina at Chapel Hill, climatic fallout from the Caribbean eruption pushed Earth beyond that threshold, triggering what has been identified in the last 10 years as one of the most remarkable worldwide transformations known.
In the dry argot of earth scientists, this event is called the Late Paleocene Thermal Maximum, or LPTM. In the language the rest of us speak, it is simply unbelievable.
Its prelude, innocuously enough, was a long-term global warming trend that began about 60 million years ago and weakened the circulatory system of the world's oceans. Five million years of that warming seems to have left Earth's environment vulnerable to catastrophic change.
Then, at the very end of the Paleocene, the circulation system experienced the oceanic equivalent of a heart attack. The global network of ocean currents stopped doing its vital job of delivering cold, oxygen-rich water to the deep ocean. As a result, the abyss warmed and stagnated.
This shock caused a mass extinction of marine organisms, including as many as half of all species of deep-sea foraminifera. This family of ubiquitous one-celled sea animals forms one of the primary links in the oceanic food chain. The mass killings of forams and other species, Bralower says, represent "the biggest extinction event in the deep sea in the last 90 million years. Nothing else even comes close."
When the oceanic heart attack struck, Earth was considerably warmer than today, thanks to the 5-million-year global warming trend. Global average air temperature was several degrees higher than now, chiefly because the poles were far warmer.
Antarctica was glacier-free, possibly draped by forests, and surrounded by sea water with a surface temperatures about 35 degrees Fahrenheit higher than now. Immediately after the heart attack, it became still warmer as a spike of very high global temperatures was superimposed on the already toasty Earth.
According to Gerald Dickens, a geochemist at James Cook University in Australia, a gargantuan gasp of methane, loosed from the sea floor as the deep ocean warmed, may be the best explanation for why Earth seems to have spiked the high fever. Oceanic and climate changes of the LPTM may have peaked in 10,000 years, a mere heartbeat on the geologic time scale.
Methane (CH4), like carbon dioxide, is a rather potent greenhouse gas. Per molecule, it can absorb 10 to 20 times more heat radiation than CO2, trapping that warmth in the air.
Mammals Suddenly Appear
Little was untouched by the higher temperatures. When the heat peaked on land, for example, the group of modern mammals that now dominate Earth -- including primates that later would give rise to our own species -- suddenly made their first appearances on at least two continents.
"I and others in the earth science community are beginning to feel that this is one of the most -- if not the most -- fascinating time intervals in the history of the Earth," Dickens says.
Research into events of this period is part of a larger effort to reconstruct climate changes that occurred millions of years ago. This initiative is driven partly by the need to know how we may be altering the climate through emissions of greenhouse gases. Conducting controlled experiments on the global environment obviously is out of the question. But nature has conducted plenty of her own, and the LPTM is one of the most remarkable.
"We really need geologic records of climate to understand the long-term causes and effects of climate change," Bralower says. "The Late Paleocene Thermal Maximum is relevant because it is the most abrupt warming event ever documented."
That record, preserved in seafloor sediments, provides a warning that Earth's environmental system may not be as stable as we might like to believe, says James Kennett, an oceanographer at the University of California at Santa Barbara. "It's clear that Earth at times develops an environmental system that's extremely sensitive to change and can flip from one state to another, creating bedlam."
Hints that something strange had happened began turning up a decade ago. But convincing evidence came in 1991, in the form of hardened mud from the Antarctic sea floor. Kennett and geochemist Lowell Stott of the University of Southern California found chemical fingerprints in those sediments indicating that the ocean had warmed and changed its circulation pattern dramatically.
Kennett and Stott analyzed the difference in abundance of two different forms of oxygen called isotopes that are preserved within fossil foraminifera skeletons.
As the tiny marine animals drift on currents, they absorb from their surroundings more of the lightest isotope (16O, with eight protons and eight neutrons for an atomic weight of 16) and less of the heavier 18O (which has two extra neutrons) when the water warms.
Evidence of this change is preserved within the foram skeletons. Forams' preference for 16O in warm water occurs because atoms of this isotope vibrate faster than those of 18O. Physics dictates that, as water warms, a foram more easily absorbs the faster-vibrating atom.
When forams die, their skeletons rain into the abyss and accumulate as sea floor mud. Over time, these sediments become deeply buried and harden under pressure, locking away an oxygen-isotope record of past water temperature. Scientists can access that record by drilling into the sea floor for a core of sediment.
Kennett's and Stott's analysis of 55 million-year-old forams from Antarctic waters showed that, just before the Paleocene closed, the bottom waters were at 50 degrees, considerably warmer than today's near-freezing temperatures but still quite chilly. Then something forced the temperature of those waters to rise nearly 20 degrees, possibly in less than 10,000 years. Meanwhile, surface waters also warmed, although somewhat less, from 57 to 70 degrees.
At this point, bottom and surface waters were almost uniformly warm. This was shocking. With only a few exceptions, ocean waters almost always are layered, with warm water atop cold bottom water.
If you've ever gone swimming in a lake in the summer, you may have noticed that the surface water was tolerably warm while deeper water was numbingly cold. In fall, lake waters "turn over" as warm and cold layers mix. But this isn't supposed to happen in the ocean.
For example, in the Caribbean today, surfaces average about 81 degrees whereas water half a mile below may be only 40-45 degrees. That difference keeps the layers stable.
Kennett's and Stott's evidence showed that the two layers did mix as they came to the same temperature at the end of the Paleocene. The ocean, Kennett says, "turned over just like a lake."
The warming of deep waters that made the turnover possible occurred more than 6,000 feet below the surface. Kennett and Stott proposed that this deep heating must have been caused by a profound change in the global system of ocean circulation or perhaps a virtual halt.
The Circulatory System
In the years that followed, scientists confirmed that the pattern of change discovered by Kennett and Stott off Antarctica was repeated throughout the world's oceans and that the land had warmed abruptly, too.
For example, oxygen isotope evidence confirmed that the warming of the deep ocean, for reasons unclear at first, seemed to beget more warming, quickly causing global ocean-air temperatures to spike.
But what might have caused ocean circulation to change in the first place?
Ordinarily, a robust difference in temperature between tropics and poles powers large-scale ocean circulation. Nature tries to erase such differences. Because water can carry far more heat than air does, ocean currents are a highly effective way to redistribute heat.
The currents in question are organized into a system that is called thermohaline ("heat-salt") circulation and is analogous to the circulatory system that distributes oxygen-rich blood throughout the body. In the ocean, the circulatory system moves heat toward the poles and cold, oxygen-rich water to the deep reaches.
The system does not move water quickly. It can take 1,500 years to move deep water from the north Atlantic to central Pacific, for example. But it moves as much as 20 times more each second than all of the world's rivers.
Along one portion of the circulatory system, warm surface water from the tropics flows toward the northern high latitudes where it yields much of its heat to the atmosphere by evaporation, which increases its salinity. Now cooler and saltier, it is denser. So it sinks and begins flowing south, sucking in more warm water and maintaining the circulation.
Through many loops and turns, the now deep-flowing current feeds cold, dense water into the ocean bottoms. Finally, deep water percolates slowly to the surface in the tropics, where it is heated by the sun and starts flowing north to begin the cycle anew.
Today, the circulatory system is comparatively strong. But the 5 million years of global warming that began before the end of the Paleocene, perhaps caused by effusive volcanism in the north Atlantic region, would have made it sluggish and weak, according to Bralower.
Computer models suggest that, when Earth's climate warms, temperatures rise more in high latitudes than in tropics and subtropics. And the foram evidence shows that this is what happened between 60 million and 55 million years ago. As a result, the temperature difference between poles and tropics would have narrowed. This, in turn, should have sapped the vigor of ocean circulation, in effect hardening its arteries.
Then, at the end of the Paleocene, came the heart attack. Scientists' best guess is that warm surface water in the tropics stopped flowing toward the northern high latitudes. According to this theory, it began sinking directly into the deep and fanning throughout the world's oceans.
This would have fed the abyss with warm water instead of cold, explaining why bottom waters around the world appear to have warmed. Since warm water holds much less oxygen than cold water, deep-water organisms began suffocating, accounting for mass extinction of foraminifera.
Carbon Ratios Increase
Other evidence in core samples suggested a possible answer to the latter question -- changes in the relative abundance of carbon isotopes that forams incorporated into their bodies during metabolism. The signal in sea floor sediments was clear. As the world's bottom waters began to warm, the ratio of 12C to 13C in forams suddenly increased. Clearly, something rich in 12C and deficient in 13C had flooded the ocean.
A few years ago, Jerry Dickens of Cook University and James O'Neil, a colleague at the University of Michigan, proposed the best explanation so far. They hadn't deliberately set out to do it.
In his laboratory in summer 1994, Dickens was making methane hydrate, a solid substance that consists of methane imprisoned within tiny molecular cages of water. Methane is the carbon-hydrogen compound present in swamp and natural gases and in the digestive activity of many animals.
It is "blatantly obvious in the laboratory that an increase in temperature causes solid hydrate to melt and release methane," Dickens says. But he did not immediately see the connection between his laboratory epiphany and the carbon isotope changes.
Then, in late 1994, "I was at a bar with Jim O'Neil," Dickens says. "He was talking about his favorite strange things in geochemistry," including the carbon-isotope shift during the LPTM. O'Neil wondered what could have produced all that 12C. "It was one of those rare moments when everything clicks," Dickens said, "and I just said 'hydrates.' "
Perhaps it was the beer. Dickens remembered that many sea floor sediments are rich in methane hydrates. Methane is incredibly rich in carbon-12. It takes less energy for bacteria that help to transform organic material into methane to absorb the lighter isotope, 12C, than the heavier 13C.
Calculations on the back of napkins confirmed that heating of sea floor sediments from the influx of deep, warm water could have released about 1,000 to 2,000 gigatons (billion tons) of the potent greenhouse gas. That would have been enough to account for carbon isotope changes seen in foram skeletons worldwide.
As the methane bubbled up and into the atmosphere, it could have caused more warming, causing or at least contributing to the temperature spike.
A Parfait of Ash
Most loose ends seemed tied up. Except for the big one: What had triggered the heart attack in the first place, causing warm tropical waters to start sinking, pushing the ocean-climate system over the threshold?
In early 1996, Bralower was bobbing on Caribbean swells aboard JOIDES Resolution, a 470-foot drill ship chartered by the international Ocean Drilling Program. For weeks, the crew had been hauling 30-foot-long cylinders of sea floor sediment to the surface for analysis. For the most part, these cores were unremarkable.
One day, a colorful surprise greeted the scientists. Sandwiched between gray sediment above and below were red, green and blue layers, a parfait of volcanic ash.
According to Bralower, analysis of this and other cores revealed that a Caribbean volcano blew its top 55 million years ago. Sediments immediately above the ash contained the chemical signature of the oceanic heart attack and abrupt climatic fever and evidence of the mass extinction. This positioning meant that the volcano blew first, convincing Bralower that it triggered the LPTM.
But volcanic eruptions usually cause climate cooling. So how could the eruption have been related to global warming?
A volcano cools climate by lofting aerosols of sulfate high into the atmosphere, where they block sunlight.
When Mount Pinatubo in the Philippines blew in 1991, its aerosols circled Earth in three weeks, covering 42 percent of the planet's surface in just two months. The umbrella effect of the aerosols lowered average global temperatures by about 1 degree F, with the greatest cooling in low latitudes over the oceans.
Bralower estimates that the Caribbean volcano emitted several thousand times as much aerosol mass as Pinatubo, "far larger than anything we've seen in historic times." Based on thickness of the ash layers, he estimates that the volcano lofted more than 10 billion tons of sulfate aerosols into the atmosphere.
Like Pinatubo, the Caribbean volcano was at a low latitude, so the cooling it caused probably was greatest over low-latitude oceans. This cooling, Bralower says, pushed Earth over the threshold.
With ocean circulation already made sluggish by long-term warming, warm surface water in the tropics was being drawn only weakly toward the poles. After the eruption, slight cooling of tropical surface waters narrowed the difference in temperature between the tropics and high latitudes even more.
This further weakened the force driving ocean circulation -- enough, Bralower hypothesizes, to allow the now somewhat cooler, and therefore denser, surface waters simply to sink in place rather than flow north.
That water, however, still carried a huge amount of heat. So it would have warmed the abyss, thereby melting the methane hydrate and allowing those gigatons of the greenhouse gas to bubble to the surface and cause the already-warm climate to spike a higher fever.
The most compelling evidence that this scenario is correct, Bralower says, is that the volcano was "right at the scene of the crime" in the tropics and blew immediately before the global warming began. "This can't be a coincidence," he says.
Others are not sure. "Volcanoes erupt all the time," Kennett notes. Dickens also is skeptical, saying, "I don't believe there has to be a trigger. The ocean-climate system simply passed a threshold because of the long-term warming."
Skepticism is how science weeds out mistaken ideas, so it's possible that Bralower's hypothesis eventually will be relegated to the compost pile. But he, Dickens, Kennett and others do agree that the LPTM may hold vital lessons.
That's not to say that whatever global warming humanity might be causing is leading to a reprise of the astounding events of 55 million years ago. A greater difference in temperature exists today between poles and tropics, protecting us from that particular brand of mayhem. But that still may not let us off the hook.
"The interesting thing about the LPTM is that it's the extreme," Bralower says. "We may not get that far, but we may go part of the way."
Tom Yulsman, an associate professor of journalism at the University of Colorado, is former editor-in-chief of Earth magazine.
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