GLOBALBIOGEOCHEMICALCYCLES,VOL. 13,NO 4, PAGES1167-1172,DECEMBER 1999
An ecologicexplanation for the Perlno-Triassic carbon
and sulfur isotopeshifts
Wallace S. Broeckerand SyntePeacock
Lamont-Doherty
EarthObservatoryof ColumbiaUniversity,Palisades,
New York
Abstract. The boundarybetweenLate PermianandEarly Triassicstratigraphicsequences
is
characterized
by the onsetof a markedshiftin the isotopiccompositionof bothmarinecarbonates
and sulfates.The abrupt3%odecreasein the carbonisotoperatio of carbonatesandthe more gradual 5%oincreasein the sulfurisotoperatio of sulfatescan be explainedby changesin the natureof
the Earth'secosystems
inducedby the massivePermo-Triassicextinctions.We proposethat the
continentalecosystem
responsiblefor the preservationandburial of organicmatterduringPermian
time was disruptedandthatthe relativelyefficientmarinefoodweb of Permiantime was replaced
by a lessefficientsystem,whichallowedfor a greaterfractionof the marineorganicmatterto
reachthe seafloor.Thesechangeswerereflectedby a major shift in the depositionalenvironment
for the reducedcomponentof matedhal
accumulatingin sediments.During the Permian,organic
carbonburialon the continentsdominated,whereasduringthe Triassic,depositionof organiccarbonon the seafloordominated,allowingsulfidesto becomean importantcomponentof the reducedphase.
sequentreleaseof phosphatefrom soilsmay have played a key
role in the Permo-Triassicisotopeshifts.Grotzingerand Knoll
It is now well established
that the carbonisotopecomposition [ 1995] andKnoll et al. [ 1996] call on the overturnof a highly anof marine CaCO3 and thereforealso that of oceanX;CO2under- aerobicdeepoceanas an explanationfor this change.Bowring et
1. Introduction
wenta 3%oshiftfromtheunusually
heavyvalues
(•9•3C
= +3%o) al. [1998]reviewa number
of thesehypotheses
in connection
characterizing
LatePermian
timetomorenormal
values
(•9•3C
= with theseprecisedatingsof the P-Tr boundaryandprovideevi0%o)duringthe Early Triassic [Baud et al., 1989; Holser et al.,
1989;Li et al., 1986;Magaritz et al., 1988;Zhouand Kyte, 1988;
Nicora et al., 1984]. This shift has been documentedin sedimentary sequencesfrom Greenland,Spitzbergen,the Alps, Turkey,
Russia,Iran, China, andAustralia(seeFigure 1a). It is alsodocumentedin organicmatter [Wang et al., 1994]. The consensus
is
dencethatthecarbonisotopeexcursionat thisboundaryhad a durationof 165,000yearsor less.We presentyet anotherhypothesis
which buildson that publishedby Berner and Raiswell [1983],
who pointedoutthe importanceof Permianlandvegetationin the
carboncycle.Our hypothesisfocusesexclusivelyon the transition
associatedwith the Permo-Triassicboundary.It dependsheavily
thatthisshiftwasabrupt
(<106years)
andthatitstimingwasco-
on the followingassumptions.
1. The shiftwas triggeredby a vastecologictransitionresulting
from the major extinctionswhich occurredat the close of the
Permian.While we picturetheseextinctionsas the productof a
cometaryimpact,this explanationis by no meansnecessaryto our
hypothesis.
2. No significantchangein the broad paletteof inputsto the
ocean-atmosphere
systemoccurredfrom Permianto Triassictime.
incident with that of the massive extinctions
which characterize
the Permo-Triassic(P-Tr) boundary.Precisegeochronologyby
Bowring et al. [ 1998] suggeststhat this carbonshift occurredin
lessthan 165,000 years. All the recordsshow that this shift constituteda transitionfrom one steadystatemode of operationto
another(seeFigure 1a).
A shift in the isotopiccompositionof marine sulfates(see Figure lb) beganin Early Triassictime [Holser, 1977]. Over a period
Specifically,neitherthe net oxidationstateof the input material
of several
millionyears,
the•34Sforthese
deposits
became
about nor the isotopiccompositionof the carbonand sulfurenteringthe
5%0heavierthanduringthe Late Permian.
A numberof explanationshave beenput forth to explain such
shifts.Garrelsand Perry [1974] were the first to point out that in
order to maintain oxidation reduction balance, there should be an
inverse
relationship
between
theshiftsin thei•3C and•34Sfor
marinecarbonatesand sulfates.This relationshipwas modeledby
Garrels and Lerman [1984] and by Berner and Raiswell[1983].
Kump [1988, 1989, 1993] suggested
that forestfires and the conCopyright
1999by theAmericanGeophysical
Union.
Papernumber1999GB900066.
0886-6236/99/1999GB
systemchanged.
3. The extinctionsled to a major revision in the operationof
the global biogeochemicalcycles.In particular,we envisionthat
the extinctionof manyvascularlandplantsled to a drasticdropin
the contributionof the terrestrialbiosphereto carbonstorageand
the disruptionof an efficientPermianmarinefoodweb alloweda
muchlargerfractionof the organicmatterproducedat the seasurfaceto reachthe seafloorbeforebeingeaten.
4. While there appearsto have been a large negativepulse in
carbonisotopecompositionduringthe first 165,000yearsof the
Triassic[Bowringet al., 1998], this pulseis not what interestsus.
Rather,
it is theshiftin thesteady
state•9•3C
formarine
carbon-
900066512.00
1167
1168
BROECKERAND PEACOCK:PERMO-TRIASSICC AND S ISOTOPESHIFTS
S.
ALPS
Idrljca
ANTALYA
R.
•'•r•k
TRANSCACAUS.
Da•
ELBURZ
I•AB-I
MTS
Emarat-2
,
3
SALT
RANGE
CHINA
Nammal Gorge
Meeahan-D
u-
'-
I
u •
O
•
+3
•
O
+3
--
0
+3
•
O
+3
•
=
O
•
0
I
+3
25--
.
U.
I
0
I
•,•
I
I
I
5%,
•
0
+3
Figure la. Summaryby Baud et al. [1989] of carbonisotopeshiftsin marinecarbonatesspanningthe PcrmoTriassic(P-Tr) boundary.
ates,the rather heavy value which characterizedthe Permianto
the 3%0lighter value which characterizedthe Triassic.As the
alsothe split betweenthe burial of carbonas CaCO3and as organicresidues.
residencetime of carbonin the ocean-atmosphere
systemwas un-
likely to haveexceededseveralhundredthousandyears,boththe
PermianandTriassiccompositions
representsteadystates.
3. Carbon Isotope Shift
2. Hypotheses
tioningrequired
to yielda 3%oreduction
in the/)•3Cof marine
Let us first considerthe changein CaCO3:organic
matterparti-
Two end-memberexplanationsfor the shift in the carbonisotope record exist: either the isotopic compositionof the mix of
carbon entering the ocean-atmospheresystemunderwent a 3%0
shift or the split betweenCaCO3 and organicresiduesaccumulating in the sediments
underwenta dramaticchange.We rejectthe
former explanation,for we seeno way to make an abruptchange
in the broadpallet of sourcessupplyingcarbonto oceanand atmosphere.Admittedly,the extrusionof the SiberianTrapswhich
occurredat the very end of Permiantime [Renneand Basu, 1991;
Campbellet al., 1992; Renne et al., 1995] could have releaseda
large amountof CO2into the atmosphere.However,the isotopic
fingerprintof suchan additionwould have persistedfor no more
carbonates.
In Figure2 is showna hypotheticalscenarioby which
thismightbe accomplished.
It is assumedthat the carbonaddedto
theocean-atmosphere
system
hada/)•3Ccomposition
averaging
5%oduringboth the Permianand the Triassic.The differencein
carbonisotopiccompositionbetweenthe carbonburied as CaCO3
andthatburiedasorganicmatteris assumedto havebeen,asnow,
about25%o.Thus,in orderto achievecarbonisotopicbalance,the
CaCO3:organicmatter split would have to have shiftedfrom 69:31
duringthe Permianto 81:19 duringthe Triassic.Regardless
of
whetheror not the assumedcarbonisotopiccompositions
are correct, it is clearto us that a substantialdrop in the fractionof carbonburiedas organicresiduesmusthaveoccurredfollowingthe
P-Tr boundaryevent.The 12% magnitudeof this shiftis setby
thana fewtimes105years.Rather,
weleantoward
thepossibility theratioof the P-Tr carbonisotopeshift (3%o)to thedifferencein
that a major changeoccurredin the mechanisms
which regulated
the throughputof carbon. Our caserestson the observationthat
about90% of marinespeciesand 70% of continentalspeciessufferedextinctionat the closeof the Permianperiod[Raupand Sepkoski,1982; Erwin, 1994]. As the flow of carbonis governedby
interactionsamongphysical,chemical,and biologicalprocesses,
the suddenreplacement
of the long-standing
Permianecosystems
with new ecosystemsmust have altered this flow and therefore
isotopiccompositionbetweenorganiccarbonand marineCaCO3
(25%0).
4. Oxidation State of SedimentaryC and S
If it is also assumed that the mean oxidation state of carbon and
sulfurenteringtheocean-atmosphere
systemremainedunchanged
betweenPermianandTriassictime,thenthe factthattheisotope
BROECKERAND PEACOCK:PERMO-TRIASSIC C AND S ISOTOPE SHIFTS
could have been causedby a changein the amountof sulfidereleasedfrom the ocean'sspreadingcentersin associationwith the
great continentalbreakupassociatedwith the Siberianflood ba-
lOO
salts.
2oo
P.T.BOUND
_
300
•
_
25500(.9
<
6. Time
Constant
for the Transition
On the basisof the analogyto the modem ocean,the time requiredfor the carbonisotopetransitionto be accomplished
would
beontheorderof a fewtimes105yearsandthatrequired
forthe
sulfur isotopetransitionwould be many millions of years [see
Holser et al., 1988]. The reasonfor a different responsein the
isotopicrecordsof carbonand sulfurto a perturbationis that the
4OO-
>•
1169
meanresidence
time of SO• in the seais 2 ordersof magnitude
greaterthan that of •CO2. Therelbrea transitionin the carbon
-
600-
isotoperecordwould be expectedto be muchmore abruptthan a
transitionin the sulfurisotoperecord.The isotopicrecordfor the
Triassicsuggests
that this was indeedthe case.The constraintof
the timing of the sulfur isotopeshift is somewhatfuzzy because
700 -
/)34S
is generally
measured
in evaporites,
forwhichstratigraphic
800 -
900 I
I
I
I
I
10
15
20
25
30
35
334S(0/00)
Figure lb. Summaryby Holser [1977] of the isotopiccomposition of marinesulfates.Note the 5%oshift duringthe Early Triassic (termedthe Rot event).
recorddemandsthat the carbonbeing buried duringthe Triassic
was more oxidized than that depositedduring the Permian re-
quiresthat this shift was compensated
by a countershift in the
oxidationstateof the sulfurbeing buried.The fractionof sulfur
accumulating
in the reducedstate(i.e., as sulfides)wouldhaveto
have been greaterduringthe Triassicthan duringthe Permian.
Becausethe electronshift for sulfur(8 electrons)is twice as large
asthat for carbon(4 electrons),for everytwo molesof carbonnot
buriedas organicmatter,one extramole of sulfurwouldhaveto
be buriedas sulfide.Of course,this argumentappliesonly to the
new Triassicsteadystate.During the transient,any slack in the
electronbalancewould have beentakenup by a changein atmospheric02 content(seebelow).
5. Sulfur Isotope Shift
A Permian to Triassic shift in the mean oxidation state of the
sulfur buried in sedimentswould lead to a shift in the isotopic
composition
of the sulfurdissolvedin the sea.As is the casefor
carbon,the heavysulfurisotopeis discriminatedagainstduring
the formationof the reducedphase.If duringthe Triassica greater
controlis lessprecise.
A length of time similar to that for the sulfur isotope shift
would be requiredto bring the atmosphere's
02 contentto a new
steadystate.This can be seenas follows. The switchfrom 31% to
19% of carbonburial as organicmatterwould have createda 12%
changein the way carbonwas partitioned.Puttingthe role of sulfur aside for the moment, this would have taken a toll on the at-
mosphere's
02 inventory.Assumingthatthe amountsof •CO2 and
02 presentin the ocean-atmosphere
systemwere roughlythe same
thenasnow(about
0.5molcm'2and7 molcm'2,respectively),
the
12% imbalance would have resulted in an O2 drawdown of about
0.5x 10'6 molcm-2yr-•. At thisrate,it wouldrequire
2 million
yearsto change
the02 content
of theairby 1 molm'2(i.e.,by
1/7th today'samount).While this calculationis useful in setting
the time constantfor the adjustmentof the atmosphere's02 content, when the responseof the sulfur cycle is included,even the
directionof the actual02 shiftis opento question(seebelow).
7. Nature of the Ecological Change
Our hypothesisis that the ecologicalchangeassociated
with
the massextinctionsbroughtto an end a situationwhere the accumulationof organicmatteron land dominatedandinitiatedone
whereaccumulation
of organicmatterat seadominated.The massive extinctionsdestroyedmost of the ecosystemresponsiblefor
the massivecontinentalcarbonstorage.Theseextinctionsalsodestroyedan efficientmarineecosystemin which duringthe Permian, as today,the vastmajorityof the plantmatterproducedin
surfacewaterswas eatenin the upperwater column,leavingonly
a very small fractionto reachthe seafloor.Becausecontinental
watersarelow in SO• content,
organiccarbonwasthedominant
reducedmaterialdepositedduringthe Permian.During the Triassic the storageof organicmaterial shiftednot only to beneath
SO•-richmarinewaterbutto mainlyin anoxicsediments
under
conditions where active sulfate reduction occurred.
In the marinerealm, the ratio of reducedcarbonto reducedsulproportion
of thesulfurwasburiedin thereducedform,massbalfur
buriedin sedimentdependson the ratio of oxygendemandto
ancerequires
thattheisotopiccomposition
of thesulfatedissolved
in the ocean would shift toward heavier values. As summarized in
oxygensupplyjust belowthe sediment-water
interface.If, for exFigure2, this is whatis observed[Holser, 1977].Duringthe Tri- ample,duringTriassictime the amountof organicmattersurviv-
assic,
the•)34S
formarinesulfates
increased
by 5%o.
Wemustad- ing the fall throughthe water column were to have been much
mit, followingWalker[1986],it is possible
thattheobserved
shift
greaterthan today's,then a larger fractionof the oxidationof or-
1170
BROECKER
AND PEACOCK:
PERMO-TRIASSIC
C AND S ISOTOPESHIFTS
tion which must have marked the close of the Permian
I
COMBINED
INPUT
100%
I
I
OUTPUT
AS
OUTPUT
AS
PERMIAN
ORGANICS
CaiO3
31%
69%
I
I
OUTPUT
AS
ORGANICS
OUTPUT
AS
TRIASSIC Ca•O
3
81%
19%
I
I
-25
-20
I
I
I
I
I
-15
-10
-5
0
+5
• 13C(%0)
I
COMBINED
INPUT
100%
fur (continental
watersaretoopoorin the SO• ion to hostlarge-
I
I
OUTPUT
AS SULFIDE
scale sulfur reduction).By contrast,Early Triassic continental
sedimentsare largelydevoidof organic-richsediments[Retallack
et al., 1996]. While this two-reservoirmodel is qualitativelyappealing, as shown in Figure 3, the requiredshift in burial environmentwould haveto havebeenvery large.During the Permian,
OUTPUT
AS SULFATE
PERMIAN
9O%
lO%
OUTPUT
OUTPUT
ASSULFIDE
TRIASSIC
some 90% of the reduced carbon would have to have accumulated
ASSULFATE
25%
led to a
largerfractionof the marinephotosynthetic
productsurvivinguntil it reachedthe seafloor.This alloweda very large fractionof the
photosynthetic
productto be consumedby bacterialiving in the
uppersediment.This wouldhavecausedthe inputrate of organic
matter to bottom sedimentsto exceedthe rate of supplyof 02,
making availableto sulfur-reducinganaerobesa greaterfraction
of the photosynthetic
productand leadingto widespreadanoxiain
marinesediments.Indeed,evidenceexistsfor widespreadanaerobic sedimentaryconditionsduring Early Triassictime [Twitchett
and Wignall,1996; Wignalland Twitchett,1996].
However,a problemarisesin connectionwith this explanation.
As hasbeen shownby Berner [1987], in modemreducingsediments,the contentof authigenicsulfidesis correlatedwith the organiccarboncontent.This suggests
that a dropin the foodweb efficiencywould lead to an increasein the rate of burial of both reducedsulfurand reducedcarbon.In contrast,our explanationrequiresthe rate of burial of reducedsulfur went up while that of
reducedcarbonwent down. One way aroundthis problemis to
think of organiccarbonburial in terms of two reservoirs:one on
the continents
with relativelylittle reducedsulfurandthe otheron
the seaflooralongwith appreciablereducedsulfur.Duringthe late
Paleozoic,prolific vascularland plantsled to the creationof continentalsedimentsrich in organiccarbonbut poor in reducedsul-
in sedimentsfree of reducedsulfur.By contrast,duringthe early
Triassic,this fractionwouldhavedroppedto about15%. In order
75%
I
I
I
I
I
I
I
-30
-20
- 10
0
+ 10
+20
+30
o
• 34S(%0)
Figure 2. (top) The carbonisotopiccompositionof Permianmarine carbonatesaveragedabout 3%0heavier than that of Triassic
marinecarbonates.
Assumingthat the isotopiccompositionof the
combinedcarboninputsremainedunchanged,
this requiresa shift
in the ratio of carbonateto organicmatterbeingburiedfrom 69:31
duringthe Permianto 81:19 duringthe Triassic. If then,as now,
theturnover
timeof ocean
carbon
wasontheorderof 105years,
this new split must have been maintainedfor many, many ocean
residencetimes;that is, it representeda steadystateflow of carbon throughthe ocean-atmosphere
system.(bottom) The sulfur
isotopecompositionof Permianmarine sulfatedepositsaverage
about5%olighter than that for their Triassicequivalents.Assuming that the isotopiccompositionof the combinedsulfur inputs
remainedunchanged,this requiresa shift in the ratio of sulfateto
sulfide being removedfrom 90:10 during the Permianto 75:25
duringthe Triassic.
to compensatefor this switch, the fraction of sulfur buried as sul-
fide wouldhaveto have shiftedfrom 10% duringthe Permianto
25% duringthe Triassic.Further,in orderto achievemassbalance
for boththe isotopesand for the oxidationstateof the output,a
seeminglyunreasonablyhigh ratio of reducedsulfur to reduced
carbonis required.For modemanoxicsediments,Berner [1982]
and Berner and Raiswell [1983] have shownthat the molar ratio
of reducedsulfurto reducedcarbonaverages0.15. As canbe seen
in Figure 3, in order to achieveboth oxidationand isotopebalance,we require a ratio greaterthan 0.75. Were the ratio lessthan
this, continentalorganiccarbonburial duringthe Triassicwould
have to have been negative.While erosionof Permiancontinental
organicmattermight createsucha negativebalance,we consider
it unlikely.
8. Shiftin Atmospheric
With this scenarioin mind, let us considerthe situation for atmospheric02. We assumethat the partial pressureof 02 in the
atmosphere
servesas the policemanwhich at steadystateassures
ganicmatterwould havetakenplacein the upperfew centimeters that the net oxidation state of the material entering sediments
of the sedimentcolumn,raisingthe likelihoodthat anoxiccondi- matches that of the material being supplied to the oceantionsexisted.This doesnot requirethat the deepseawas anoxic. atmospheresystem.The switch from conditionsexistingin the
Rather, it requiresthat organicmatter be deliveredto the sedi- Permian world where organic matter buried on the continents
mentsmorerapidlythanO2is ableto diffuseintothem.If organic dominated the removal of reduced matter to conditions in the Trimatter falls into an anoxic environment,then the survival prob- assicworld where organicmatter and iron sulfideburied on the
ability is greater.We hypothesizethat the great ecologicaltransi- seafloor dominated the removal of reduced matter must have re-
BROECKER
ANDPEACOCK:
PERMO-TRIASSIC
C AND SISOTOPESHIFTS
CARBON
CONT.
MARINE
SULFUR
MARINE
ORGANIC ORGANIC CACO
3
TOTAL
C
MARINE
MARINE
TOTAL
SULFIDE SULFATE S
CASE #1 MARINE SEDIMENT SIC = 1.00
PERMIAN
27.0
4.0
69.0
100.0
4.0
36.0
40.0
T.REDUCED SPECIES 27.0 + 4.0 + 2 x 4.0 = 39.0
TRIASSIC
3.0
16.0
81.0
100.0
10.0
30.0
40.0
T.REDUCED SPECIES 3.0 + 16.0 + 2 x 10.0 = 39.0
z•)13C= -3 %0
z•)34S= +5%0
CASE #2 MARINE SEDIMENT SIC = 0.75
5.3
69.0
100.0
4.0
36.0
40.0
T. REDUCED SPECIES 25.7 + 5.3 + 2 x 4.0 = 39.0
TRIASSIC
0.4
18.6
81.0
100.0
10.0
30.0
40.0
T.REDUCED SPECIES 0.4 + 18.6 + 2 x 10.0 = 39.0
A•)13C= -3 %0
experienced
a greatcatastrophe.
Perhapsit was struckby a comet.
As a result, a major fraction of organismson Earth were killed.
The resultingextinctionsdisruptedthe food webs which had developedduringthe Paleozoic.The land plantsresponsible
for the
generationof a major portion of the reduced carbon buried in
Permian sedimentswere largely wiped out. Further, the inefficiency of the new Triassicmarine ecosystemsallowed a substantial amountof the marinephotosynthetic
productto reachthe seafloor, creatingextensiveregionswhereanoxicsedimentary
conditions prevailed.This allowed the formationand burial of sulfide
minerals.It is possiblethat this initially resultedin too great a
burial of reducedcarbonand sulfur.However,as a consequence
of this excessburial,the O2contentof the atmosphere
gradually
rose,causingthe realmof anoxiato shrinkuntil a new steadystate
was achieved. The result was a transition from a world where re-
PERMIAN
25.7
1171
z•)34S= +5 %0
ducedmaterial (mainly carbon) accumulatedon land to a world
where reducedmaterial (both carbonand sulfur) accumulated
mainly at sea.The legacyof thistransitionis carriedby the carbon
andsulfurisotopecomposition
of marinecarbonates
andsulfates.
We arethe firstto admitthat the scenariopresented
hereis not
entirelyoriginal;rather,it buildson that of Berner [1987]. We
also are the first to admit that our scenario,while plausible,is
certainlynot unique.While the truth of the mattermay neverbe
fully known, plausiblescenarioscouldbe better constrainedif the
Figure 3. Hypotheticalscenariofor the causeof the P-Tr shiftsin
time historiesof the 834Stransientandof the durationof anoxic
the•}•3Cfor marinecarbonates
andthe •}34Sfor marinesulfates.
conditions
werebetterdocumented.
While admittedlyimperfect,
This scenario,which is basedon the assumption
that neitherthe
ourhypothesis
leanson something
whichsurelydid happen(i.e.,
isotopiccompositionnor the oxidationstateof the carbonor the
ratherthanon conjectured
events(Kump'sforestfires
sulfuraddedto the oceanchangedacrossthe P-Tr boundary,ac- extinctions)
countsfor the observedisotopeshifts. The ratio of sulfurto carbon flux to sedimentsis setby the followingconstraint:Fs/Fc=
31-19/[2(25-10)] = 12/30 = 0.40, where 31-19 is the shift in the
percentof carbonsedimented
asorganicmaterialand25-10 is the
shift in the percentof sulfursedimentedas sulfide.The factorof 2
andKnoll's oceanturnovers).
Acknowledgments.We thankBob Bemerfor correctingour initially
mistakeninterpretation
of his S/C ratiosfor contemporary
sediments.
Financialsupportwas providedby grantsfrom the National ScienceFoundation.
takes into account the fact that the electron shift for sulfur is twice
that for carbon.The magnitudeof the shiftsare setto yield the -
3%0shiftfor•}•3Cofmarine
CaCO3
andthe+5%0
shiftformarine
CaSO4. The ratio of reduced S to C in marine sediments is arbi-
trarilyset,but, ascanbe seen,a balancecanbe achieved
onlyif it
is greaterthan0.75 (i.e., it mustbe at least5 timeshigherthanthat
whichBerner [1982]findsfor contemporary
sediments).
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(ReceivedJuly 15, 1998;revisedJuly 26, 1999;
acceptedAugust9, 1999.)