Snowball Earth Article

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68 Scientific American January 2000 Snowball Earth

Our human ancestors had it rough. Saber-toothedcats and woolly mammoths may have been day-to-day concerns, but harsh climate was a consum-ing long-term challenge. During the past million years, theyfaced one ice age after another. At the height of the last icy

episode, 20,000 years ago, glaciers more than two kilometersthick gripped much of North America and Europe. The chilldelivered ice as far south as New York City.

Dramatic as it may seem, this extreme climate change palesin comparison to the catastrophic events that some of our ear-liest microscopic ancestors endured around 600 million yearsago. Just before the appearance of recognizable animal life, ina time period known as the Neoproterozoic, an ice age pre-vailed with such intensity that even the tropics froze over.

Imagine the earth hurtling through space like a cosmic snow-ball for 10 million years or more. Heat escaping from themolten core prevents the oceans from freezing to the bottom,but ice grows a kilometer thick in the 50 degree Celsius cold.

All but a tiny fraction of the planets primitive organisms die.

Aside from grinding glaciers and groaning sea ice, the only stircomes from a smattering of volcanoes forcing their hot headsabove the frigid surface. Although it seems the planet mightnever wake from its cryogenic slumber, the volcanoes slowlymanufacture an escape from the chill: carbon dioxide.

With the chemical cycles that normally consume carbondioxide halted by the frost, the gas accumulates to record lev-els. The heat-trapping capacity of carbon dioxidea green-house gaswarms the planet and begins to melt the ice. Thethaw takes only a few hundred years, but a new problemarises in the meantime: a brutal greenhouse effect. Any crea-tures that survived the icehouse must now endure a hothouse.

As improbable as it may sound, we see clear evidence thatthis striking climate reversalthe most extreme imaginableon this planethappened as many as four times between 750million and 580 million years ago. Scientists long presumedthat the earths climate was never so severe; such intense cli-mate change has been more widely accepted for other planets

such as Venus [see Global Climate Change on Venus, by

Snowball EarthSnowball Earth

Ice entombed our planet hundreds of millionsof years ago, and complex animals evolved in

the greenhouse heat wave that followed

by Paul F. Hoffman and Daniel P. Schragby Paul F. Hoffman and Daniel P. Schrag

Ice entombed our planet hundreds of millionsof years ago, and complex animals evolved in

the greenhouse heat wave that followed

Copyright 1999 Scientific American, Inc.


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Mark A. Bullock and David H. Grinspoon; ScientificAmerican, March 1999]. Hints of a harsh past on the earthbegan cropping up in the early 1960s, but we and our col-leagues have found new evidence in the past eight years thathas helped us weave a more explicit tale that is capturing the

attention of geologists, biologists and climatologists alike.Thick layers of ancient rock hold the only clues to the cli-

mate of the Neoproterozoic. For decades, many of thoseclues appeared rife with contradiction. The first paradox wasthe occurrence of glacial debris near sea level in the tropics.Glaciers near the equator today survive only at 5,000 metersabove sea level or higher, and at the worst of the last ice agethey reached no lower than 4,000 meters. Mixed in with theglacial debris are unusual deposits of iron-rich rock. Thesedeposits should have been able to form only if the Neopro-terozoic oceans and atmosphere contained little or no oxy-gen, but by that time the atmosphere had already evolved tonearly the same mixture of gases as it has today. To confound

matters, rocks known to form in warm water seem to have

accumulated just after the glaciers receded. If the earth wereever cold enough to ice over completely, how did it warm upagain? In addition, the carbon isotopic signature in the rockshinted at a prolonged drop in biological productivity. Whatcould have caused this dramatic loss of life?

Each of these long-standing enigmas suddenly makes sensewhen we look at them as key plot developments in the tale ofa snowball earth. The theory has garnered cautious sup-

port in the scientific community since we first introduced the

Snowball Earth Scientific American January 2000 69

GLENALLISONDigitalImagery1999PhotoDisc,Inc.

TOWERS OF ICE like Argentinas Moreno Glacier (above)

once buried the earths continents. Clues about this frozen pasthave surfaced in layers of barren rock such as these hills near thecoast of northwest Namibia (inset).



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idea in the journal Science a year and ahalf ago. If we turn out to be right, thetale does more than explain the myster-ies of Neoproterozoic climate and chal-lenge long-held assumptions about thelimits of global change. These extremeglaciations occurred just before a rapiddiversification of multicellular life, cul-minating in the so-called Cambrian ex-plosion between 575 and 525 millionyears ago. Ironically, the long periods ofisolation and extreme environments ona snowball earth would most likely havespurred on genetic change and couldhelp account for this evolutionary burst.

The search for the surprisingly strongevidence for these climatic events hastaken us around the world. Althoughwe are now examining Neoproterozoicrocks in Australia, China, the westernU.S. and the Arctic islands of Svalbard,we began our investigations in 1992along the rocky cliffs of NamibiasSkeleton Coast. In Neoproterozoictimes, this region of southwestern Africawas part of a vast, gently subsidingcontinental shelf located in low south-ern latitudes.

There we see evidence of glaciers inrocks formed from deposits of dirt anddebris left behind when the ice melted.Rocks dominated by calcium- and mag-nesium-carbonate minerals lie justabove the glacial debris and harbor thechemical evidence of the hothouse thatfollowed. After hundreds of millions ofyears of burial, these now exposedrocks tell the story that scientists firstbegan to piece together 35 years ago.

In 1964 W. Brian Harland of the Uni-versity of Cambridge pointed out that

glacial deposits dot Neoproterozoic rock

outcrops across virtually every continent.By the early 1960s scientists had begunto accept the idea of plate tectonics,which describes how the planets thin,rocky skin is broken into giant piecesthat move atop a churning mass of hotterrock below. Harland suspected that thecontinents had clustered together nearthe equator in the Neoproterozoic, basedon the magnetic orientation of tiny min-eral grains in the glacial rocks. Beforethe rocks hardened, these grains alignedthemselves with the magnetic field anddipped only slightly relative to horizon-tal because of their position near the

equator. (If they had formed near thepoles, their magnetic orientation wouldbe nearly vertical.)

Realizing that the glaciers must havecovered the tropics, Harland became thefirst geologist to suggest that the earthhad experienced a great Neoproterozoicice age [see The Great Infra-CambrianGlaciation, by W. B. Harland andM.J.S. Rudwick; Scientific American,August 1964]. Although some of Har-lands contemporaries were skeptical

about the reliability of the magneticdata, other scientists have since shownthat Harlands hunch was correct. Butno one was able to find an explanationfor how glaciers could have survived thetropical heat.

At the time Harland was announcinghis ideas about Neoproterozoic glaciers,physicists were developing the firstmathematical models of the earths cli-mate. Mikhail Budyko of the LeningradGeophysical Observatory found a wayto explain tropical glaciers using equa-tions that describe the way solar radia-tion interacts with the earths surfaceand atmosphere to control climate.Some geographic surfaces reflect moreof the suns incoming energy than oth-ers, a quantifiable characteristic knownas albedo. White snow reflects the mostsolar energy and has a high albedo,darker-colored seawater has a low albe-do, and land surfaces have intermediatevalues that depend on the types and dis-tribution of vegetation.

The more radiation the planet reflects,the cooler the temperature. With their

high albedo, snow and ice cool the at-mosphere and thus stabilize their ownexistence. Budyko knew that this phe-


EARTHS LANDMASSES were most likely clustered near the equator during the globalglaciations that took place around 600 million years ago. Although the continents havesince shifted position, relics of the debris left behind when the ice melted are exposed atdozens of points on the present land surface, including what is now Namibia (red dot).

HEIDINOLAND

AUSTRALIASOUTH CHINA

ANTARCTICA

AFRICA

INDIA

KAZAKHSTANNORTH AMERICA

NORTHERNEUROPE

SIBERIA

SOUTH AMERICAEASTERN

SOUTH AMERICA

COURTESYOFDANIELP.

SCHRAG

WEST

AFRICA



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nomenon, called the ice-albedo feed-back, helps modern polar ice sheets togrow. But his climate simulations alsorevealed that this feedback can run outof control. When ice formed at latitudeslower than around 30 degrees north orsouth of the equator, the planets albedobegan to rise at a faster rate because di-rect sunlight was striking a larger surface

area of ice per degree of latitude. Thefeedback became so strong in his simula-tion that surface temperatures plummet-ed and the entire planet froze over.

Frozen and Fried

Budykos simulation ignited interestin the fledgling science of climatemodeling, but even he did not believethe earth could have actually experi-enced a runaway freeze. Almost every-one assumed that such a catastrophewould have extinguished all life, andyet signs of microscopic algae in rocksup to one billion years old closely re-semble modern forms and imply a con-

tinuity of life. Also, once the earth hadentered a deep freeze, the high albedoof its icy veneer would have driven sur-face temperatures so low that it seemedthere would have been no means of es-cape. Had such a glaciation occurred,Budyko and others reasoned, it wouldhave been permanent.

The first of these objections began to

fade in the late 1970s with the discoveryof remarkable communities of organ-isms living in places once thought tooharsh to harbor life. Seafloor hot springssupport microbes that thrive on chemi-cals rather than sunlight. The kind ofvolcanic activity that feeds the hotsprings would have continued unabatedin a snowball earth. Survival prospectsseem even rosier for psychrophilic, orcold-loving, organisms of the kind livingtoday in the intensely cold and drymountain valleys of East Antarctica.Cyanobacteria and certain kinds of algaeoccupy habitats such as snow, porousrock and the surfaces of dust particles en-cased in floating ice.

The key to the second problemre-versing the runaway freezeis carbondioxide. In a span as short as a humanlifetime, the amount of carbon dioxidein the atmosphere can change as plantsconsume the gas for photosynthesis andas animals breathe it out during respi-ration. Moreover, human activities suchas burning fossil fuels have rapidly

loaded the air with carbon dioxidesince the beginning of the IndustrialRevolution in the late 1700s. In theearths lifetime, however, these carbonsources and sinks become irrelevantcompared with geologic processes.

Carbon dioxide is one of several gas-es emitted from volcanoes. Normallythis endless supply of carbon is offsetby the erosion of silicate rocks: Thechemical breakdown of the rocks con-verts carbon dioxide to bicarbonate,which is washed to the oceans. Therebicarbonate combines with calciumand magnesium ions to produce car-bonate sediments, which store a greatdeal of carbon [see Modeling the Geo-

Snowball Earth Scientific American January 2000 71

ROCKY CLIFFS along Namibias Skele-ton Coast (left) have provided some of

the best evidence for the snowball earthhypothesis. Authors Schrag (far left) andHoffman point to a rock layer that repre-sents the abrupt end of a 700-million-year-old snowball event. The light-coloredboulder in the rock between them proba-bly once traveled within an iceberg andfell to the muddy seafloor when the icemelted. Pure carbonate layers stackedabove the glacial deposits precipitated inthe warm, shallow seas of the hothouseaftermath. These cap carbonates are theonly Neoproterozoic rocks that exhibitlarge crystal fans, which accompany rapidcarbonate accumulation (above).

CAPCARBONATES

GLACIALDEPOSITS

COURTESYOFDANIELP.

SCHRAG

COURTESYOFGALENPIPPAHALVERSON

CRYSTALFANS



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hama Banks in what is now the AtlanticOcean. If the ice and warm water hadoccurred millions of years apart, no onewould have been surprised. But thetransition from glacial deposits to thesecap carbonates is abrupt and lacksevidence that significant time passed be-tween when the glaciers dropped theirlast loads and when the carbonates

formed. Geologists were stumped to ex-plain so sudden a change from glacial totropical climates.

Pondering our field observations fromNamibia, we realized that this change isno paradox. Thick sequences of carbon-ate rocks are the expected consequenceof the extreme greenhouse conditionsunique to the transient aftermath of asnowball earth. If the earth froze over,an ultrahigh carbon dioxide atmospherewould be needed to raise temperaturesto the melting point at the equator. Once

melting begins, low-albedo seawater re-places high-albedo ice and the runawayfreeze is reversed [see illustration below].The greenhouse atmosphere helps todrive surface temperatures upward to al-most 50 degrees C, according to calcula-tions made last summer by climate mod-eler Raymond T. Pierrehumbert of theUniversity of Chicago.

Resumed evaporation also helps towarm the atmosphere because watervapor is a powerful greenhouse gas,and a swollen reservoir of moisture inthe atmosphere would drive an en-hanced water cycle. Torrential rainwould scrub some of the carbon diox-ide out of the air in the form of carbon-ic acid, which would rapidly erode therock debris left bare as the glaciers sub-sided. Chemical erosion productswouldquickly build up in the ocean water,leading to the precipitation of carbon-

ate sediment that would rapidly accu-mulate on the seafloor and later be-come rock. Structures preserved in theNamibian cap carbonates indicate thatthey accumulated extremely rapidly,perhaps in only a few thousand years.For example, crystals of the mineralaragonite, clusters of which are as tallas a person, could precipitate only from

seawater highly saturated in calciumcarbonate.Cap carbonates harbor a second line

of evidence that supports Kirschvinkssnowball escape scenario. They containan unusual pattern in the ratio of twoisotopes of carbon: common carbon 12and rare carbon 13, which has an extraneutron in its nucleus. The same pat-terns are observed in cap carbonatesworldwide, but no one thought to in-terpret them in terms of a snowballearth. Along with Alan Jay Kaufman,

Scientific American January 2000 73

D

AVIDFIERSTEIN

Concentrations of carbon dioxide in the atmosphere increase1,000-fold as a result of some 10 million years of normal vol-canic activity. The ongoing greenhouse warming effectpushes temperatures to the melting point at the equator.Asthe planet heats up,moisture from sea ice sublimating nearthe equator refreezes at higher elevations and feeds thegrowth of land glaciers. The open water that eventuallyforms in the tropics absorbs more solar energy and initiatesa faster rise in global temperatures.In a matter of centuries,abrutally hot,wet world will supplant the deep freeze.

As tropical oceans thaw, seawater evaporates and worksalong with carbon dioxide to produce even more intensegreenhouse conditions. Surface temperatures soar to morethan 50 degrees Celsius,driving an intense cycle of evapora-tion and rainfall.Torrents of carbonic acid rain erode the rockdebris left in the wake of the retreating glaciers. Swollenrivers wash bicarbonate and other ions into the oceans,where they form carbonate sediment. New life-formsen-gendered by prolonged genetic isolation and selective pres-surepopulate the world as global climate returns to normal.

Stage 3

Snowball Earthas It Thaws

Stage 4

Hothouse Aftermath

GLACIERS CARBONATE

SEDIMENT

Snowball Earth

. . . AND ITS HOTHOUSE AFTERMATH



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an isotope geochemist now at the Uni-versity of Maryland, and Harvard Uni-versity graduate student Galen PippaHalverson, we have discovered that theisotopic variation is consistent overmany hundreds of kilometers of ex-posed rock in northern Namibia.

Carbon dioxide moving into theoceans from volcanoes is about 1 per-cent carbon 13; the rest is carbon 12. Ifthe formation of carbonate rocks werethe only process removing carbon fromthe oceans, then the rock would have thesame fraction of carbon 13 as thatwhich comes out of volcanoes. But thesoft tissues of algae and bacteria growingin seawater also use carbon from the wa-ter around them, and their photosynthet-ic machinery prefers carbon 12 to carbon13. Consequently, the carbon that is leftto build carbonate rocks in a life-filledocean such as we have today has a high-er ratio of carbon 13 to carbon 12 thandoes the carbon fresh out of a volcano.

The carbon isotopes in the Neopro-terozoic rocks of Namibia record a dif-

ferent situation. Just before the glacialdeposits, the amount of carbon 13plummets to levels equivalent to the vol-canic source, a drop we think recordsdecreasing biological productivity as iceencrusted the oceans at high latitudesand the earth teetered on the edge of arunaway freeze. Once the oceans icedover completely, productivity wouldhave essentially ceased, but no carbonrecord of this time interval exists be-cause calcium carbonate could not haveformed in an ice-covered ocean. This

drop in carbon 13 persists through thecap carbonates atop the glacial depositsand then gradually rebounds to higherlevels of carbon 13 several hundred me-ters above, presumably recording therecovery of life at the end of the hot-house period.

Abrupt variation in this carbon iso-tope record shows up in carbonaterocks that represent other times of massextinction, but none are as large or aslong-lived. Even the meteorite impactthat killed off the dinosaurs 65 million

years ago did not bring about such a

prolonged collapse in biological activity.Overall, the snowball earth hypothe-

sis explains many extraordinary obser-vations in the geologic record of theNeoproterozoic world: the carbon iso-topic variations associated with theglacial deposits, the paradox of cap car-bonates, the evidence for long-livedglaciers at sea level in the tropics, and the

associated iron deposits. The strength ofthe hypothesis is that it simultaneouslyexplains all these salient features, noneof which had satisfactory independentexplanations. What is more, we believethis hypothesis sheds light on the earlyevolution of animal life.

Survival and Redemption of Life

In the 1960s Martin J. S. Rudwick,working with Brian Harland, pro-posed that the climate recovery follow-

ing a huge Neoproterozoic glaciationpaved the way for the explosive radia-tion of multicellular animal life soonthereafter. Eukaryotescells that havea membrane-bound nucleus and fromwhich all plants and animals descend-edhad emerged more than one billionyears earlier, but the most complex or-ganisms that had evolved when the firstNeoproterozoic glaciation hit were fila-mentous algae and unicellular proto-zoa. It has always been a mystery whyit took so long for these primitive or-

ganisms to diversify into the 11 animal

body plans that show up suddenly inthe fossil record during the Cambrianexplosion [see illustration on this page].

A series of global freeze-fry eventswould have imposed an environmentalfilter on the evolution of life. All extanteukaryotes would thus stem from thesurvivors of the Neoproterozoic calam-ity. Some measure of the extent of eu-

karyotic extinctions may be evident inuniversal trees of life. Phylogenetictrees indicate how various groups of or-ganisms evolved from one another,based on their degrees of similarity.These days biologists commonly drawthese trees by looking at the sequencesof nucleic acids in living organisms.

Most such trees depict the eukaryotesphylogeny as a delayed radiation crown-ing a long, unbranched stem. The lack ofearly branching could mean that mosteukaryotic lineages were pruned dur-

ing the snowball earth episodes. Thecreatures that survived the glacial epi-sodes may have taken refuge at hotsprings both on the seafloor and near thesurface of the ice where photosynthesiscould be maintained.

The steep and variable temperatureand chemical gradients endemic to eph-emeral hot springs would preselect forsurvival in the hellish aftermath tocome. In the face of varying environ-mental stress, many organisms respondwith wholesale genetic alterations. Se-

vere stress encourages a great degree of


ALL ANIMALS descended from the first eukaryotes, cells with a membrane-bound nucleus, which appeared about two billion years ago. By the time ofthe first snowball earth episode more than one billion years later, eukary-otes had not developed beyond unicellular protozoa and filamentous algae.But despite the extreme climate, which may have pruned the eukaryotetree (dashed lines), all 11 animal phyla ever to inhabit the earth emergedwithin a narrow window of time in the aftermath of the last snowball event.The prolonged genetic isolation and selective pressure intrinsic to a snow-ball earth could be responsible for this explosion of new life-forms.

BACTER

IA

ARCHAEA

SNOWBALL

EARTH

EVENTS

3,50

0

2,50

070

0

Time (millions of years ago)

600

500

800

900

3,000

2,00

0

1,50

0 0

Mollusks

Platyhelminths

Priapulids

Nematodes

Arthropods

Brachiopods

Chordates

Echinoderms

Cnidarians

Poriferans

Annelids

EUKA

RYOTES

EMERGENCE

OF ANIMALS

x

x

HEID

INOLAND



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