Feature

Can nuclear weapons falloutmark the beginning of theAnthropocene Epoch?

Colin N. Waters, James P. M. Syvitski,Agnieszka Gałuszka, Gary J. Hancock,Jan Zalasiewicz, Alejandro Cearreta,Jacques Grinevald, Catherine Jeandel,J. R. McNeill, Colin Summerhayes, andAnthony Barnosky

AbstractMany scientists are making the case that humanity is living in a new geological epoch, the Anthropocene, butthere is no agreement yet as to when this epoch began. The start might be defined by a historical event, such asthe beginning of the fossil-fueled Industrial Revolution or the first nuclear explosion in 1945. Standard strati-graphic practice, however, requires a more significant, globally widespread, and abrupt signature, and thefallout from nuclear weapons testing appears most suitable. The appearance of plutonium 239 (used in post-1945 above-ground nuclear weapons tests) makes a good marker: This isotope is rare in nature but a significantcomponent of fallout. It has other features to recommend it as a stable marker in layers of sedimentary rock andsoil, including: long half-life, low solubility, and high particle reactivity. It may be used in conjunction withother radioactive isotopes, such as americium 241 and carbon 14, to categorize distinct fallout signatures insediments and ice caps. On a global scale, the first appearance of plutonium 239 in sedimentary sequencescorresponds to the early 1950s. While plutonium is easily detectable over the entire Earth using modern meas-urement techniques, a site to define the Anthropocene (known as a Ògolden spikeÓ) would ideally be locatedbetween 30 and 60 degrees north of the equator, where fallout is maximal, within undisturbed marine or lakeenvironments.

KeywordsAnthropocene, golden spike, nuclear weapons fallout, radioactive isotope, radiogenic signature, Trinity test

Seventy years agoÑat 5:30 a.m. onJuly 16, 1945Ñthe worldÕs firstnuclear device exploded at the

Trinity Test Site in what was then theAlamogordo Bombing and GunneryRange in New Mexico. After an intense

flash of light and heat, and a roaring shockwave that took 40 seconds to reach theclosest observers, a fireball rose into thesky, forming a mushroom cloud 7.5 mileshigh. J. Robert Oppenheimer later wrotethat he and other Manhattan Project

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scientists who had gathered to watch thetest Òknew the world would not be thesame.Ó The Ònuclear ageÓ had begun(Ackland and McGuire, 1986; Eby et al.,2010; Groves, 1962).

Arguably, Trinity was also the begin-ning of something even bigger: a newgeological epoch (Zalasiewicz et al.,2015). Human activities have had such agreat impact upon the Earth that manyresearchers suggest we are no longerliving in the Holocene Epoch (a termdescribing the most recent slice of geo-logical time that literally means ÒentirelynewÓ), but instead within a brand-newtime unit: the Anthropocene, from theGreek words for ÒhumanÓ and Ònew.Ó

Since 2009, a small group of us, com-posed of geoscientists and other expertsfrom across the globe, have assembled todevelop a proposal for this new termin-ology and to make recommendations tothe official bodyÑknown as the Interna-tional Commission on StratigraphyÑthatdetermines geological time units. Toaccomplish this, our panel, the Anthropo-cene Working Group, has not only beenexamining the evidence for the Anthro-poceneÕs existence but attempting todetermine the duration of this potentialnew unit (Zalasiewicz et al., 2012). Thegroup will also make recommendationsabout where the Anthropocene, if itdoes exist, fits into the hierarchy of geo-logical time: period, epoch, or age (per-haps even within the Holocene Epoch).

Many scientists agree that the Earthhas left the Holocene behind and is nowin the Anthropocene, but there is lessagreement about when the Anthropo-cene began. Some researchers makegood arguments for dating the beginningof this new epoch to the advent ofagriculture, or to the increase in fossilfuel consumption that ushered in the

Industrial Revolution, or some othermajor shift that left its mark on the geo-logical record. One recent paper arguesfor either 1610 (when atmosphericcarbon dioxide levels dipped after thearrival of Europeans brought death toabout 50 million native people in theAmericas) or 1964 (based on peakcarbon 14 fallout signatures) as potentialkickoff dates (Lewis and Maslin, 2015).But if geoscientists want to establish astarting point for the Anthropocene, Tri-nity and the nuclear bombings and teststhat followed it from 1945 to the early1960s created an extremely distinctiveradiogenic signatureÑa unique patternof radioactive isotopes captured in thelayers of the planetÕs marine and lakesediments, rock, and glacial ice that canserve as a clear, easily detected book-mark for the start of a new chapter inour planetÕs history.

Does it really matter what epochwe are living in? ItÕs obviously importantto geologists and other Earth scientists,who use the geological timescale to meas-ure, describe, and compare events andchanges that happened in our planetÕspast. For many people outside thesefields, though, the potential designationof a new epoch has political overtones.As an editorial in a leading scientific jour-nal observed a few years ago, the Anthro-pocene Òreflects a grim reality on theground, and it provides a powerful frame-work for considering global change andhow to manage itÓ (Nature, 2011).

Although the Anthropocene has, inthe public sphere, become closelyassociated with climate change and par-ticularly the burning of fossil fuels, it ismuch bigger than that. We and other sci-entists who are considering whether anew epoch has begunÑand if so, howbest to mark its onsetÑare examining a

Waters et al. 47

host of environmental changes wroughtby humans, from the domestication ofplants and animals to the nuclear armsrace. Public discussion of these changescan only lead to a growing awarenessthat humans have left an enormousfootprint on the EarthÑand not just acarbon oneÑand may help increasepublic understanding of how a warmingclimate relates to other momentousglobal changes.

Origins of the Anthropocene

In the geological timescale used by Earthscientists, the Holocene Epoch beganabout 11,700 years ago, after the planetÕslast glacial phase came to an end. Whenthe Anthropocene concept (Crutzen,2002; Crutzen and Stoermer, 2000) wasinitially proposed, the Industrial Revolu-tion was suggested as its starting point.The reasoning was that industrializa-tionÕs accelerated population expansion,technological changes, and economicgrowth caused increased urbanization,mineral exploitation, and crop cultiva-tion; these factors in turn elevated atmos-pheric carbon dioxide and methaneconcentrations enormously (Waterset al., 2014; Williams et al., 2011).

Alternatively, the proponents of anÒearly AnthropoceneÓ or ÒPalaeoan-thropoceneÓ interval that preceded theIndustrial Revolution (Foley et al., 2013)emphasize that this interval had a diffusebeginning, with signatures associatedwith the onset of deforestation, agricul-ture, and animal domestication; somescientists propose that these changesbroadly coincide with the beginningof the existing Holocene Epoch (Smithand Zeder, 2013).

But there is growing evidence foranother, later starting point for the

Anthropocene: the range of globallyextensive and abrupt signatures duringthe mid-20th century (Waters et al.,2014) that coincide with the ÒGreatAccelerationÓ of population growth, eco-nomic development, industrialization,mineral and hydrocarbon exploitation,the manufacturing of novel materialssuch as plastics, the emergence of mega-cities, and increased species extinctionsand invasions (Steffen et al., 2007, 2015).Some researchers even suggest that theonset of the Anthropocene is marked bya ÒdiachronousÓ boundary in sedi-mentsÑone in which a boundary betweenhuman-modified and ÒnaturalÓ groundcan be found that is of different ages atdifferent locationsÑand thus is not a geo-logical time unit (Edgeworth et al., 2015).

The standard accepted practice fordefining geological time units duringthe current eon (which began about 541million years ago) is to identify a singlereference point (or Ògolden spikeÓ), at aspecific location, that marks the lowerboundary of a succession of rock layersas the beginning of the time unit. Thisinternationally agreed-upon physicalreference point is representative of thesum of environmental changes that jus-tify recognition of the time unitÑtheappearance or extinction of a fossil spe-cies, say, or a geochemical signature leftby a massive volcanic eruption (Smith,2014). For example, the boundarybetween the Cretaceous and PaleogenePeriods has as its golden spike the baseof an iridium-enriched layer of rock in ElKef, TunisiaÑa marker for the debrisspewed into the atmosphere when ahuge meteorite struck the Earth and forthe mass extinctions of dinosaurs andother creatures that followed that event.

The mid-20th century saw substantialchanges to living things and their

48 Bulletin of the Atomic Scientists 71(3)

ecological relationsÑalso known asbiotic changes (Barnosky, 2014)Ñbutthose changes have not yet been wellenough documented from the strati-graphic perspective to be the primarymarker for the Anthropocene. The 1945detonation of the Trinity device wouldmake a well-defined, historically import-ant reference point, but a single deton-ation lacks a clear signature in the globalgeological record, even though nucleartesting converted sand into a glass-likesubstance known in the United States asÒtrinititeÓ (Eby et al., 2010) and in Kaz-akhstan as Òkharitonchik.Ó This may wellbe considered a durable signature in thestratigraphic record, but one that is verylocalized in extent.

In contrast, the fallout from thenumerous thermonuclear weapons teststhat began in 1952 deposited largeamounts of radionuclides in the environ-ment and left a well-defined radiogenicsignature. That level would provide aneffective global signal that marks thebeginning of the Anthropocene, in com-parison to using the Trinity Test as amarker. The difference between the twosuggested levels is just seven years, andrepresents only fine-tuning of a generallymid-20th-century boundary; ultimately,the choice between them will dependon analysis and debate of the wholeensemble of stratigraphic evidence cur-rently being assembled.

Sources of human-maderadiation

Admittedly, the fallout from nucleartesting is not the only source ofradiation that would show up in thegeological record. Naturally radioactivesubstances have increased worldwide,due to the mining of ore and the

burning of coal and other fossilfuels, initially beginning during theIndustrial Revolution and then rapidlyincreasing after 1945. Such increases,however, do not provide a clearmarker because the radioactive sub-stances are not uniquely anthropogenicin origin and may be locally abundantfor other, natural reasons.

In addition to these sources, themedical use of radiation representsthe earliest anthropogenic source ofradiation exposure (see Figure 1). Diag-nostic medical examinations currentlycontribute the largest dosage afternatural exposure (UNSCEAR, 2000)but they target individuals, not the envir-onment, so their impact in the geologicrecord would be small. Medical wasteincinerators and uncontrolled disposalof equipment can produce local contam-ination, but they are not useful as wide-spread stratigraphic signatures.

Besides these sources, there arethe contributions of nuclear power.The first commercial nuclear powerplant, at Calder Hall in northern England,opened in 1956. Nuclear power grew rap-idly from 1970 to 1985, but the growthstopped after the 1986 Chernobylaccident. The 2011 Fukushima disasterproduced similar hemispheric fallout,though with less discharge. Radioactivereleases from reprocessing plants, whichrecover uranium and plutonium fromspent fuels, are typically greater thanfor nuclear power plants (Jeandel et al.,1980). Such controlled releases, mainlyuranium series isotopes from sites suchas the Sellafield (United Kingdom) andLa Hague (France) reprocessing plants,peaked in the mid-1970s (Aarkrog, 2003).Radioactive waste dumping causedlocalized contamination mainly in theNortheast Atlantic until 1982 and the

Waters et al. 49

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50 Bulletin of the Atomic Scientists 71(3)

Kara Sea near Novaya Zemlya and theSea of Japan until 1993, despite theLondon Convention of 1972 banningthis practice (Livingston and Povinec,2000). The accidental discharges atChernobyl and Fukushima and con-trolled releases at Sellafield from 1952to the mid-1980s contributed only smallamounts (UNSCEAR, 2000).

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All told, such discharges composeonly a small, regional component ofthe total anthropogenic radionuclidebudget, so they are poor candidatesfor defining the beginning of the Anthro-pocene. In contrast, atmospheric nuclearweapons testing, or wartime usage in thecase of Hiroshima and Nagasaki,resulted in regional to global distributionof fallout. Most anthropogenic radio-nuclides in the environment today,locked in soils and sediments, originatedfrom atmospheric testing that tookplace over a 35-year period from 1945 to1980. This fallout started abruptly andshows distinct, globally recognizablephases, such as a rapid decline after thePartial Nuclear Test Ban Treaty of 1963.(Underground tests, on the other hand,had much lower yields and releases tothe atmosphere.) The fallout signatureis locally augmented by accidentaldischarges from power stations, repro-cessing plants, and satellite burn-up onatmospheric reentry.

The signature of nuclear weaponstesting

The case for using the fallout fromnuclear weapons testing as a marker forthe onset of the Anthropocene is strong.There were 2,053 nuclear weapons testsfrom 1945 to 1998 (Figure 1), mainly incentral Asia, the Pacific Ocean, and thewestern United States (see Figure 2); 543were atmospheric tests. Test frequencypeaked in 1951”1958 and especially1961”1962, interrupted by a moratorium(UNSCEAR, 2000). Underground testsoccurred at the rate of 50 or more peryear from 1962 to 1990. From 1945 to1951, the tests involved fission (ÒatomicÓ)weapons producing fallout along testsite latitudes in the lowest layer of theatmosphere (Aarkrog, 2003). Largerfusion (ÒthermonuclearÓ or ÒhydrogenÓ)weapons tests, starting in 1952, producedhigher-altitude fallout dispersed overthe entire Earth surface, with a markedpeak in fallout yields in 1961�1962.Radionuclide fallout rapidly declinedduring the late 1960s, when tests weremainly underground, and effectivelyceased in 1980.

The geographical distribution ofradionuclides associated with fallouthas been measured for the commonercomponents, such as strontium 90(Figure 2) and cesium 137; comparablemeasurements for plutonium 239 and240 are not available. Strontium 90 fall-out is concentrated in the mid-latitudes(30”60 degrees) of each hemisphere(Figure 2), and is smallest at the polesand equator (Aarkrog, 2003; Livingstonand Povinec, 2000). Approximately76 percent of all radionuclide falloutwas in the northern hemisphere, wheremost testing occurred (Livingston andPovinec, 2000).

Waters et al. 51

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52 Bulletin of the Atomic Scientists 71(3)

The best radioactive markers forthe Anthropocene

To define the Anthropocene boundary,radioactive isotopes should ideally beabsent or rare in nature; have long half-lives that provide a long-lasting signa-ture; have low solubilities and highreactivities so that they are less mobileand form a fixed marker in geologicaldeposits; and be present in sufficientquantity to be easily detectable. Theshort half-lives of cesium 137, strontium90, and tritium (a short-lived radio-nuclide of hydrogen associated withfusion bombs) limit their potential toserve as geologically ÒpermanentÓ mar-kers. Americium 241 has a deeper-waterdistribution than plutonium 239 and 240,being more readily transported to thebottom of deep oceans on sinkingorganic particles (Cochran et al., 1987;Lee et al., 2005), and so may be a moresuitable signature in the comparativelyundisturbed deep-water environments.However, the lower abundance of ameri-cium 241 and its 432-year half-life wouldmake it useful for only one or two millen-nia. Radiocarbon (carbon 14) shows aÒbomb peakÓ at 1963�1964 in mostcarbon reservoirs, including peat depos-its, soil, wood, and coral (Hua et al., 2013;Reimer et al., 2004), and this has beenproposed by Lewis and Maslin (2015) asa potential marker for the base of theAnthropocene. This spike will be detect-able for about 50,000 years into thefuture, so it represents a long-lastingand important signature on land.However, the high solubility and lowreactivity of carbon 14 in marine sedi-ments (Jeandel et al., 1981; Livingstonand Povinec, 2000) limit its suitabilityas a marker in the worldÕs oceans.Lewis and Maslin (2015), too, depart

from normal stratigraphic practice inplacing their suggested beginningAnthropocene level at the peak of the sig-nature, rather than at its onset.

In contrast, the use of plutonium as amarker offers several advantages.Plutonium isotopes have low solubilityand high particle reactivity, rapidly asso-ciating with clay or organic particles.The long half-life of plutonium 239(24,110 years) makes it the most persist-ent artificial radionuclide, detectable bymodern mass spectrographic techniquesfor about 100,000 years (Hancock et al.,2014). Plutonium 240, also a productof nuclear weapons testing, is lessabundant and has a shorter half-life(6,563 years), and hence will decaybelow easily detectable levels sooner.Few direct plutonium fallout measure-ments were made during the 1950s and1960s, but the historical pattern of pluto-nium isotope distribution is believed tobe similar to that of the widely studiedisotope cesium 137, especially after 1960(Hancock et al., 2014).

WhatÕs more, plutonium in the airis now dominated by atmosphericdischarge from nuclear power plantsand the re-suspension of plutonium-bearing soil particles, whereas during1945”1960 the major source of plutoniumwas nuclear weapons testing. Since 1960,plutonium concentrations in the atmos-phere have been decreasing almostexponentially following internationaltest-ban treaties (Choppin and Morgen-stern, 2001).

Another benefit to using plutonium isthat this element mostly binds withdecayed plant material and iron oxideson the surface of soil particles (Chawlaet al., 2010), thus locking it in place;plants mobilize only a little plutoniumthrough uptake by roots. But a drawback

Waters et al. 53

may be that plutonium can migratein peat profiles and can probably movedownward in organic-rich soils andsediments (Quinto et al., 2013), addinganthropogenic plutonium to layers thatwere deposited before nuclear weaponstesting. This may limit the application ofplutonium as an Anthropocene signaturein acidic, organic-rich environments.

Within the oceans, plutonium sticksto the surface of suspended matter thatfalls through the water column, andconsequently its distribution in theocean is affected by currents and bymovement of sediment (Zheng andYamada, 2006). Plutonium in particularaccumulates in coastal sediments,especially in low-oxygen, organic-richenvironments (Livingston and Povinec,2000) where few bottom-dwelling ani-mals can survive, and so their move-ments do not disrupt the radioactivesediment layers. Plutonium is taken upby organic material in shallow sunlitlevels of the sea and then released backinto solution when reaching a depth ofseveral kilometers (Livingston and Povi-nec, 2000). Coral skeletons thus becomearchives of plutonium contaminationhistory, with plutonium concentrationsin their growth bands reflecting theplutonium levels in the oceans (Lindahlet al., 2012).

Most Ògolden spikesÓ lie withinmarine sedimentary successions be-cause they tend to be more continuousthan terrestrial strata and containtraces of plant and animal life thatcan be easily matched with sedimentsat other sites. These criteria hold truefor the potential use of a radiogenicsignature. However, dynamic transportof radionuclides in the water col-umnÑthrough erosion, suspension, andre-sedimentation and via the biological

food chain (Livingston and Povinec,2000)Ñcan modify the radiogenic signa-ture to the point where it no longerrepresents a time series of discrete fall-out events that can be precisely corre-lated from one location to the next.

Another consideration is the delaybetween detonation and eventual fallout,with radioactive debris residence timesin the troposphere of about 70 days forsmall-yield detonations (Norris andArkin, 1998) and 15 to 18 months inthe stratosphere for large thermonucleartests (Zandler and Araskog, 1973). Suchdelays account for why radionuclidessuch as cesium 137 reach peak abundanceseveral years after the maximum intro-duction of fallout in the atmosphere.This delay is exacerbated in oceanenvironments as fallout is transferredthrough the water column to bottomsediment. From 1973 to 1997, themaximum plutonium signature in theNorthwest Pacific descended from 500meters to 800 meters below the oceanÕssurface through gradual settling of theearly-1960s fallout peak at an averagerate of 12.5 meters per year (Livingstonet al., 2001), and in the mid-latitudesmore than 70 percent of plutonium 239and 240 still remains in the watercolumn (Lee et al., 2005). This suggestsat least decadal residence times inoceanic waters, and a resultant smearingof potential plutonium signatures inmarine sediments, such that annualresolution of sediment layers in theoceans is unlikely.

There are alternative environmentsin which a Ògolden spikeÓ section couldbe located: for example, undisturbedlake deposits where fallout material hasaccumulated, or where sediment accu-mulation is too rapid for the layering tobe disrupted by burrowing animals

54 Bulletin of the Atomic Scientists 71(3)

(Hancock et al., 2014). There is someprecedent for this; the base of theHolocene Epoch is defined in a Green-land ice core (Walker et al., 2009), andthis type of deposit might also be usedto define the base of the Anthropocene.Such cores can provide annual recordsthrough layer-counting known Saharandust events, volcanic eruptions, and the1963 tritium horizon when abundances ofthis radionuclide peaked. Plutoniumappears to be immobile in ice (Gabrieliet al., 2011; Koide et al., 1979), and high-resolution records of plutonium fallouthave been measured in polar ice cores(Koide et al., 1977, 1979). With greaterfallout of radionuclides in the mid-lati-tudes, alpine glaciers may be more suit-able. Ice cores from Swiss and Italianalpine glaciers display the earliest riseof plutonium 239 fallout from 1954 to1955, with subsequent peaks in 1958 and1963 and a sharp decrease following thePartial Nuclear Test Ban Treaty in 1963(Gabrieli et al., 2011). The worldÕs icecaps, however, are undergoing increasedwastage through global warming, and sotheir potential to provide a long-termrecord may be limited.

A time of global changes

If we want to use the fallout from nuclearweapons to mark the beginning ofthe Anthropocene Epoch, the 1945 Ala-mogordo nuclear weapons test marksthe start of the nuclear age but lacks aclear radiogenic signature in the globalgeological record. By comparison, themost pronounced rise in plutoniumdispersal commences in 1952 and canprovide a practical radiogenic signaturefor the beginning of the Anthropocene.

Although the Anthropocene may be atime of global warming, climate change

itself is a poor geological indicator for anew epoch, at least when viewed over arecent timescale of decades. There is asignificant time lag between the recentstriking increase in atmospheric carbondioxide levels and significant climateand sea level changes, with the lattereffects not yet clearly expressed in geo-logical deposits.

The advent of the nuclear age in itselfdoes not merit the identification of a newgeological epoch. The signature of weap-ons testing coincides with a range ofhuman-driven changes that have pro-duced stratigraphic signals that indicatea dramatic shift in the Earth systemaround the mid-20th century, which intotal may be considered the distinctivefeature of the Anthropocene. The factthat the plutonium 239 signature is coin-cident with other changes makes it auseful tool for defining the Anthro-poceneÕs base.

A summary of the evidence and rec-ommendations for defining an Anthro-pocene Epoch will be presented at thenext International Geological Congressin 2016. The Anthropocene WorkingGroup is still collecting evidence;nuclear sciences are likely to be criticalto the definition of the Anthropocene,and contributions to this discussionwould be welcomed.

AcknowledgementsThe authors thank Irka Hajdas for her comments onradiocarbon signatures.

Funding

Colin Waters publishes with the permission of theExecutive Director, British Geological Survey(BGS), Natural Environment Research Council,funded with the support of the BGSÕs EngineeringGeology science program. This research received noother specific grant from any funding agency in thepublic, commercial, or not-for-profit sectors.

Waters et al. 55

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Author biographies

Ten of this articleÕs 11 authors are members of the38-person Anthropocene Working Group,established to examine the evidence for theAnthropocene Epoch of geological time. Thegroup includes members from a broad range ofgeosciences and social sciences. In addition,Gary Hancock, a senior research scientist atthe Commonwealth Scientific and IndustrialResearch Organisation (or CSIRO, AustraliaÕsnational science agency) and an expert on pluto-nium radioisotopes in the environment, contrib-uted to the article. The lead author can becontacted at cnw@bgs.ac.uk.

Waters et al. 57

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