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THE DAY THE SEA STOOD STILL Part II

Editor’s note:Analyzing 55-million-year-old fossil foraminifera extracted from deep-sea sediments, research-ers are documenting a period of climatic “bedlam” known as the Late Paleocene Thermal Max-imum (LPTM). The oceans’ circulatory system, weakened by millions of years of globalwarming, inexplicably ground almost to a halt. Starved for cold, oxygen-rich waters, much ofEarth’s deep-sea marine life perished.

The first evidence of this devastating event surfaced nearly a decade ago when researchersbrought up hardened mud filled with foraminifera from the Antarctic sea floor. Oxygen isotopesin the forams revealed that temperatures of the Antarctic’s bottom waters had risen as high as 70degrees Fahrenheit during the late Paleocene, almost equal to its surface-water temperatures. theocean, in effect, had “turned over like a lake,” reported James Kennett, an oceanographer at theUniversity of California at Santa Barbara, and Lowell Stott, a geochemist at the University ofSouthern California. Such deep heating, they proposed, must have been caused by a profoundchange in oceanic circulation patterns. But what had triggered the change?

Tom Yulsman

The Circulatory SystemIn the years that followed, scientists confirmed that thepattern of change discovered by Kennett and Stott offAntarctica was repeated throughout the world’s oceansand that the land had warmed abruptly, too. For example,oxygen isotope evidence confirmed that the warming ofthe deep ocean, for reasons unclear at first, seemed to be-get more warming, quickly causing global ocean-air tem-peratures to spike.

But what might have caused ocean circulation tochange in the first place? Ordinarily, a robust differencein temperature between tropics and poles powers large-scale ocean circulation. Nature tries to erase such differ-ences. Because water can carry far more heat than airdoes, ocean currents are a highly effective way to redis-tribute heat.

The currents in question are organized into a systemthat is called thermohaline (“heat-salt”) circulation and isanalogous to the circulatory system that distributes oxy-gen-rich blood throughout the body. In the ocean, the cir-culatory system moves heat toward the poles and cold,oxygen-rich water to the deep reaches. The system doesnot move water quickly. It can take 1,500 years to movedeep water from the north Atlantic to central Pacific, for

example. But it moves as much as 20 times more each sec-ond than all of the world’s rivers. Along one portion ofthe circulatory system, warm surface water from the trop-ics flows toward the northern high latitudes where ityields much of its heat to the atmosphere by evaporation,which increases its salinity. Now cooler and saltier, it isdenser. So it sinks and begins flowing south, sucking inmore warm water and maintaining the circulation.Through many loops and turns, the now deep-flowingcurrent feeds cold, dense water into the ocean bottoms.Finally, deep water percolates slowly to the surface in thetropics, where it is heated by the sun and starts flowingnorth to begin the cycle anew.

Today, the circulatory system is comparatively strong.But the 5 million years of global warming that began be-fore the end of the Paleocene, perhaps caused by effusivevolcanism in the north Atlantic region, would have madeit sluggish and weak, according to marine geologist Tim-othy Bralower of the University of North Carolina atChapel Hill.

Computer models suggest that, when Earth’s climatewarms, temperatures rise more in high latitudes than intropics and subtropics. And the foram evidence showsthat this is what happened between 60 million and 55 mil-

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Article 24. THE DAY THE SEA STOOD STILL Part II

lion years ago. As a result, the temperature difference be-tween poles and tropics would have narrowed. This, inturn, should have sapped the vigor of ocean circulation,in effect hardening its arteries.

Then, at the end of the Paleocene, came the heart at-tack. Scientists’ best guess is that warm surface water inthe tropics stopped flowing toward the northern high lat-itudes. According to this theory, it began sinking directlyinto 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 worldappear to have warmed. Since warm water holds muchless oxygen than cold water, deep-water organismsbegan suffocating, accounting for mass extinction offoraminifera.

Carbon Ratios IncreaseThis was a tidy explanation. But why would warm sur-face waters start sinking into the depths? And why didwarming of bottom waters seem to beget yet more warm-

BRALOWER ET AL. (1997)

Environmental changes in LPTM at Site 999 (Columbia Basin).Shown from left are biostratigraphy and magnetostratigraphy; lithology, showing position of claystone; depth(meters below sea floor); bulk carbonate §13C values; positions of tephra/ash layers; and age. Note supererup-tion between uppermost pre-excursion and lowermost §13C excursion value. Level of benthic foraminiferal ex-tinction has been obscured by dissolution.

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ing? Other evidence in core samples suggested a possibleanswer to the latter question—changes in the relativeabundance of carbon isotopes that forams incorporatedinto their bodies during metabolism. The signal in sea-floor sediments was clear. As the world’s bottom watersbegan to warm, the ratio of 12C to 13C in forams suddenlyincreased. Clearly, something rich in 12C and deficient in13C had flooded the ocean.

A few years ago, Jerry Dickens of Cook University andJames O’Neil, a colleague at the University of Michigan,proposed the best explanation so far. They hadn’t deliber-ately set out to do it. In his laboratory in summer 1994,Dickens was making methane hydrate, a solid substancethat consists of methane imprisoned within tiny molecularcages of water. Methane is the carbon-hydrogen com-pound present in swamp and natural gases and in the di-gestive activity of many animals. It is “blatantly obvious inthe laboratory that an increase in temperature causes solidhydrate to melt and release methane,” Dickens says. But hedid not immediately see the connection between his labo-ratory 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 strangethings in geochemistry,” including the carbon-isotopeshift during the LPTM. O’Neil wondered what couldhave produced all that 12C. “It was one of those rare mo-ments when everything clicks,” Dickens said, “and I justsaid ‘hydrates.’ ”

Perhaps it was the beer. Dickens remembered thatmany sea floor sediments are rich in methane hydrates.

Methane is incredibly rich in carbon-12. It takes less en-ergy for bacteria that help to transform organic materialinto methane to absorb the lighter isotope, 12C, than theheavier 13C. Calculations on the back of napkins con-firmed that heating of sea-floor sediments from the influxof deep, warm water could have released about 1,000 to2,000 gigatons (billion tons) of the potent greenhouse gas.That would have been enough to account for carbon iso-tope changes seen in foram skeletons worldwide. As themethane bubbled up and into the atmosphere, it couldhave caused more warming, causing or at least contribut-ing to the temperature spike.

A Parfait of AshMost 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, pushingthe ocean-climate system over the threshold?

In early 1996, Bralower was bobbing on Caribbeanswells aboard JOIDES Resolution, a 470-foot drill shipchartered by the international Ocean Drilling Program.For weeks, the crew had been hauling 30-foot-long cylin-ders of sea-floor sediment to the surface for analysis. Forthe most part, these cores were unremarkable. One day, acolorful surprise greeted the scientists. Sandwiched be-tween gray sediment above and below were red, greenand blue layers—a parfait of volcanic ash. According toBralower, analysis of this and other cores revealed that aCaribbean volcano blew its top 55 million years ago. Sed-iments immediately above the ash contained the chemical

BRALOWER ET AL. (1998)

Simplified cartoon showing the proposed origin of the Late Paleocene Thermal Maximum. North AtlanticIgneous Province (NAIP) activity (1) leads to global warming concentrated at high latitudes (2). The Caribbeansupereruption(s) (3) decrease(s) the rate of warming at low latitudes (4). Downwelling of relatively dense, sub-tropical surface waters ensues (5). These warm waters cause methane-hydrate dissociation in slope sediments(6) that fuels LPTM warmth and causes the carbon-isotope excursion (7)

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Article 24. THE DAY THE SEA STOOD STILL Part II

signature of the oceanic heart attack and abrupt climaticfever and evidence of the mass extinction. This position-ing meant that the volcano blew first, convincingBralower that it triggered the LPTM.

But volcanic eruptions usually cause climate cooling. Sohow could the eruption have been related to globalwarming? A volcano cools climate by lofting aerosols ofsulfate high into the atmosphere, where they block sun-

light. When Mount Pinatubo in the Philippines blew in1991, its aerosols circled Earth in three weeks, covering 42percent of the planet’s surface in just two months. Theumbrella effect of the aerosols lowered average globaltemperatures by about 1° F, with the greatest cooling inlow latitudes over the oceans.

Bralower estimates that the Caribbean volcano emittedseveral thousand times as much aerosol mass as Pi-natubo, “far larger than anything we’ve seen in historictimes.” Based on thickness of the ash layers, he estimatesthat the volcano lofted more than 10 billion tons of sulfateaerosols into the atmosphere. Like Pinatubo, the Carib-bean volcano was at a low latitude, so the cooling itcaused probably was greatest over low-latitude oceans.This cooling, Bralower says, pushed Earth over thethreshold.

With ocean circulation already made sluggish by long-term warming, warm surface water in the tropics was beingdrawn only weakly toward the poles. After the eruption,slight cooling of tropical surface waters narrowed the dif-ference in temperature between the tropics and high lati-tudes even more. This further weakened the force drivingocean circulation—enough, Bralower hypothesizes, to al-low the now somewhat cooler, and therefore denser, sur-face 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 themethane hydrate and allowing those gigatons of thegreenhouse gas to bubble to the surface and cause the al-ready-warm climate to spike a higher fever.

T. BRALOWER, UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL

Shiptrack of ODP Leg 165, showing the location of Sites 999 and 1001.

BRALOWER ET AL. (1998)

Frequency of tephra layers recovered at Site 1001 in 0.25 m.y. incre-ments through upper Paleocene and lower Ecocene. Gaps indicate in-tervals of time for which no section was recovered. Timing of the NorthAtlantic Igneous Province is also indicated.

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The most compelling evidence that this scenario is cor-rect, Bralower says, is that the volcano was “right at thescene of the crime” in the tropics and blew immediatelybefore the global warming began. “This can’t be a coinci-dence,” he says.

Others are not sure. “Volcanoes erupt all the time,”Kennett notes. Dickens also is skeptical, saying, “I don’tbelieve there has to be a trigger. The ocean-climate systemsimply passed a threshold because of the long-termwarming.”

Skepticism is how science weeds out mistaken ideas,so it’s possible that Bralower’s hypothesis eventually willbe 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 humanitymight be causing is leading to a reprise of the astounding

events of 55 million years ago. A greater difference intemperature exists today between poles and tropics, pro-tecting us from that particular brand of mayhem. But thatstill may not let us off the hook.

“The interesting thing about the LPTM is that it’s theextreme,” Bralower says. “We may not get that far, but wemay go part of the way.”

Tom Yulsman School of Journalism, University of Colorado, Boulder, Colo. 80309 Tom Yulsman, an associate professor of journalism at the University of Colo-rado, is former editor-in-chief of Earth magazine.

This article first appeared in somewhat different form in TheWashington Post’s “Horizon” section, Sept. 9, 1998.

From Geotimes, March 1999. © 1999 by the American Geological Institute.

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