Article 51

THE DAY THE SEA STOOD STILLAncient Eruptions, Global Warming, and Mass Deathby Tom Yulsman

It may have been the blast that changed the world.

One day in the Caribbean Sea, at the end of the Pale-ocene Epoch 55 million years ago, a volcano blew, spew-ing a climate-altering parasol of tiny particles high intothe atmosphere. Such events are hardly rare. But this wasno ordinary volcano: It was huge. And this was no ordi-

nary time in the planet’s history: The environment al-ready was on the threshold of profound change.

According to a new theory proposed by marine geolo-gist Timothy Bralower of the University of North Caro-lina at Chapel Hill, climatic fallout from the Caribbeaneruption pushed Earth beyond that threshold, triggeringwhat has been identified in the last 10 years as one of themost remarkable worldwide transformations known. Inthe dry argot of earth scientists, this event is called the

T. BRALOWER, UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL

Large blue-grey ash (57–61 centimeters) directly beneath LPTM claystone (47–49 centimeters) at Site 1001 (lower NicaraguanRise). Note red-brown ash level above claystone at 47–49 centimeters.

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Late Paleocene Thermal Maximum, or LPTM. In the lan-guage the rest of us speak, it is simply unbelievable. Itsprelude, innocuously enough, was a long-term globalwarming trend that began about 60 million years ago andweakened the circulatory system of the world’s oceans.Five million years of that warming seems to have leftEarth’s environment vulnerable to catastrophic change.Then, at the very end of the Paleocene, the circulation sys-tem experienced the oceanic equivalent of a heart attack.The global network of ocean currents stopped doing itsvital job of delivering cold, oxygen-rich water to the deepocean. As a result, the abyss warmed and stagnated. Thisshock caused a mass extinction of marine organisms, in-cluding as many as half of all species of deep-sea foramin-ifera. This family of ubiquitous one-celled sea animalsforms one of the primary links in the oceanic food chain.The mass killings of forams and other species, Bralowersays, represent “the biggest extinction event in the deepsea in the last 90 million years. Nothing else even comesclose.”

When the oceanic heart attack struck, Earth was con-siderably warmer than today, thanks to the five-mil-lion-year global warming trend. Global average airtemperature was several degrees higher than now,chiefly because the poles were far warmer. Antarcticawas glacier-free, possibly draped by forests, and sur-rounded by sea water with surface temperatures about 35

degrees Fahrenheit higher than now. Immediately afterthe heart attack, it became still warmer as a spike of veryhigh global temperatures was superimposed on the al-ready toasty Earth.

According to Gerald Dickens, a geochemist at JamesCook University in Australia, a gargantuan gasp of meth-ane, loosed from the sea floor as the deep ocean warmed,may be the best explanation for why Earth seems to havespiked the high fever. Oceanic and climate changes of theLPTM may have peaked in 10,000 years, a mere heartbeaton the geologic time scale. Methane (CH4), like carbon di-oxide, is a rather potent greenhouse gas. Per molecule, itcan 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 ofmodern mammals that now dominate Earth, includingprimates that later would give rise to our own species,suddenly made their first appearances on at least twocontinents. “I and others in the earth-science communityare beginning to feel that this is one of the most—if not themost—fascinating time intervals in the history of theEarth,” Dickens says.

Research into events of this period is part of a larger ef-fort to reconstruct climate changes that occurred millionsof years ago. This initiative is driven partly by the need toknow how we may be altering the climate through emis-sions of greenhouse gases. Conducting controlled experi-ments on the global environment obviously is out of thequestion. 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 under-stand the long-term causes and effects of climate change,”Bralower says. “The Late Paleocene Thermal Maximum isrelevant because it is the most abrupt warming event everdocumented.” That record, preserved in sea-floor sedi-ments, provides a warning that Earth’s environmentalsystem may not be as stable as we might like to believe,says James Kennett, an oceanographer at the Universityof California at Santa Barbara. “It’s clear that Earth attimes develops an environmental system that’s extremelysensitive to change and can flip from one state to another,creating bedlam.”

Hints that something strange had happened beganturning up a decade ago. But convincing evidence camein 1991, in the form of hardened mud from the Antarcticsea floor. Kennett and geochemist Lowell Stott of the Uni-versity of Southern California found chemical finger-prints in those sediments indicating that the ocean hadwarmed and changed its circulation pattern dramatically.

COURTESY OF CLAY KELLY,UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL

Planktonic foraminifera from LPTM at Site 865.

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Article 51. THE DAY THE SEA STOOD STILL

Kennett and Stott analyzed the difference in abundanceof two different forms of oxygen, called isotopes, that arepreserved within fossil foraminifera skeletons. As the tinymarine animals drift on currents, they absorb from theirsurroundings more of the lightest isotope (16O, with eightprotons 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 pre-served within the foram skeletons. Forams’ preference for16O in warm water occurs because atoms of this isotopevibrate faster than those of 18O. Physics dictates that, aswater warms, a foram more easily absorbs the faster-vibrating atom. When forams die, their skeletons rain intothe abyss and accumulate as sea-floor mud. Over time,these sediments become deeply buried and harden underpressure, locking away an oxygen-isotope record of pastwater temperature. Scientists can access that record bydrilling into the sea floor for a core of sediment.

Kennett’s and Stott’s analysis of 55-million-year-oldforams from Antarctic waters showed that, just before thePaleocene closed, the bottom waters were at 50 degrees,considerably warmer than today’s near-freezing temper-atures but still quite chilly. Then something forced thetemperatures of those waters to rise nearly 20 degrees,possibly in less than 10,000 years. Meanwhile, surface wa-ters also warmed, although somewhat less, from 57 to 70degrees. At this point, bottom and surface waters were al-most uniformly warm. This was shocking. With only afew 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 tolera-bly warm while deeper water was numbingly cold. In fall,lake waters “turn over” as warm and cold layers mix. Butthis isn’t supposed to happen in the ocean. For example,in the Caribbean today, surfaces average about 81 degrees

T. BRALOWER, UNC/CHAPEL HILL

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

T. BRALOWER, UNC/CHAPEL HILL

Above: Reconstruction of the Caribbean region at 55 Ma, showing thelocation of the two ODP sites.

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whereas water half a mile below may be only 40–45 de-grees. That difference keeps the layers stable.

Kennett’s and Stott’s evidence showed that the twolayers did mix as they came to the same temperature atthe end of the Paleocene. The ocean, Kennett says,“turned over just like a lake.” The warming of deep wa-ters that made the turnover possible occurred more than6,000 feet below the surface. Kennett and Stott proposedthat this deep heating must have been caused by a pro-

found change in the global system of ocean circulation orperhaps a virtual halt.

Tom Yulsman School of Journalism, University of Colorado, Boulder, Colo. 80309

Tom Yulsman, an associate professor of journalism at the University of Col-orado, is former editor-in-chief of Earth magazine.

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

Warmth and Mammals

Striped rock formations that rise above the badland canyons of Wy-oming’s Bighorn Basin contain the most complete fossil record ever discovered of the late Paleocene and early Eocene epochs—the pe-riod between about 60 million and 50 million years ago. According to that record, before the Paleocene drew to a close, no modern mam-mals were in the area. Then sud-denly, 55 million years ago, at precisely the same time as the Late Paleocene Thermal Maximum (LPTM), they just appear, “seem-ingly out of nowhere,” says Wil-liam Clyde, a paleontologist at the University of New Hampshire.

Of course, those animals didn’t evolve in a geologic instant. Pale-ontologists believe that they evolved at a more leisurely pace

during the Paleocene, leaving behind their bones in an as yet undiscovered modern-mammal motherland. Some experts believe that Asia is a good possibility, Clyde says. Then, the sudden warming allowed them to fan out quickly into North America and elsewhere.

How? During the Paleocene, Clyde says, North America, Eu-rope, and Asia were connected in the far north, providing a theoreti-cal corridor for migration. Despite this connection, it would have been too cold for newly evolved modern mammals to survive a trek through the far north until the sudden warming of the LPTM. It may have enabled modern mammals to pass from their motherland to other continents, including North

America, where their arrival is re-corded in the Wyoming rock. “You can think of it like a door that opened, temporarily allowing a bunch of new taxa [animal types] to rush in,” Clyde says.

The advent of modern mammals seems to have increased biodiver-sity significantly in the Bighorn Ba-sin, at least for a short while. “The number of species went way, way up for a short period of time and then tapered off to a stable level that was still higher than it was be-fore,” Clyde says. “Almost imme-diately, the new [species] became very important.” Eventually, of course, the old groups just faded away, leaving modern mammals to inherit the Earth.

—Tom Yulsman

Reprinted with permission from Geotimes, February 1999, pp. 14-17. © 1999 by the American Geological Institute.

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