GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 9, PAGES 1321-1324,MAY 1, 1998
Temporal variation in the interhemispheric
offset
F.G. McCormac
1,A.G. Hogg:,T.F.G.Higham:,J. Lynch-Stieglitz
•, W.S.Broexker
•,
M.G.L. Baillie1,J.Palmer
•, L. Xiong•, J.R.Pilcher
1,D. Brown• and$.T. Hoper•
Abstract. Contemporaneoustree-ring dated wood, from trees
carbon dates of wood of the same age are older by about 40
in thenorthern
andsouthern
hemispheres,
givesdifferent14C yrs)[Lermanet a1.,1970]. This is becausethe larger expanseof
dates.Previousstudies[Vogel et al., 1986; 1993] using wood ocean
andslightly
higherwindspeeds
cause
more14Cdepleted
from SouthAfrica and The Netherlandshave shown depletion's
of-4.56 + 0.85%0 and -5.12 + 0.62%0respectively. This translates to age differencesof 36 + 7 and 41 _+5 years(yrs) with the
southern hemisphere giving the older dates. More recently,
Stuiver and Braziunas [1998] have shown that an offset of 23 +
4 yrs exists between combined 19th century wood measurements from Tasmania and Chile in the southern hemisphere
and the west coast of the U.S. (Washington) in the northern
hemisphere. In this studymeasurementson contemporaneous
decadalsamplesof oak from the British Isles and cedarfrom
New Zealandover the period 1725 to 1885 AD show a depletion of-3.4 +_0.589'00(27.2 + 4.7 yrs). However, data after
1895 AD has a mean offset of 0.66 + 1.06%o (-5.3 + 8.5 yrs)
with increasedvariance comparedto 19th century data. This,
we believe, is attributable to anthropogenicfossil fuel, which,
due to its long residencetime in the earth, has long since lost
CO: from the oceanto enter the southern atmospherethan the
northern [Cain and Suess, 1976]. This natural offset was
thought to be constant through time, however, here we show
that, small changeswith time do occur and the burning of fossil fuels in the industrial northernhemispherehas resulted in a
slow changein the natural isotopic balance of the atmosphere
between the hemispheres.
Measurements
In this studywe have usedoak (Quercuspetraea)from Sherwood Forest,England(53ø 12'N 01ø 04'W altitude80m) (1725
- 1745 AD)and Shane'sCastle, Co. Antrim, N. Ireland (54ø
44'N 06ø 16%V altitude 20m) (1755 - 1935 AD) and cedar
(Libocedrusbidwillii) from Hihitahi State Forest Sanctuaryin
the North Island of New Zealand (39ø 32'S 175ø 44'E altitude
any 14Ccomponent
andwhenburnedpreferentially
depletes 976m). Two trees,one cedarone oak, were tree-ring datedand
blocksof 125g were separated.Sampleswere pre-treatedto
thenorthern
hemisphere
atmosphere
of 14C.
cellulose [Hoper et al., 1997] thereby removing all mobile
fractionsandthe 14Cactivitieswere determined
by liquid
scintillation counting of benzene[McCormac, 1992; McCor-
Introduction
The growth rings of trees provide an annually datedrecord
of atmosphericconditions at the location of growth. However, contemporaneous
wood from different locations within a
macet al., 1993]. Smalldifferences
in the 14Cactivitiesof
wood from different locationsare extremely difficult to detect.
If woodfrom different regions is measuredin different laboratories then small systematicdifferencesbetweenlaboratories
givenhemisphere
doesnot alwaysgiveidenticalA14Cmeas- can easily mask any atmospheric signal that may exist
urements [Damon, 1995; McCormac et al., 1995; Stuiver and [Damon et al., 1989; McCormac et al., 1995]. We therefore
Braziunas, 1998]. Speciesto speciesfractionation effects can measuredboth the oak and cedar in the same laboratory and
result in contemporaneous
wood having a different specific ac- then replicatedthe measurements
betweenthe laboratoriesat
tivity but this is corrected
using$13Cmeasurements.
Pre- The Queen'sUniversityof Belfast, N. Ireland andThe Univertreatment
of the wood to {z-cellulose
ensures that the molecular
compositionof the fraction of wood datedis the same for the
differentspeciesused[Stuiveret al., 1984; Hoper et al., 1997]
and so variable lignin and/or other cellulosecompoundscan-
not contribute
to A14C differences.Remainingoffsetsmust
therefore
reflectrealatmospheric
differences
in AI4c evenif
theseare merely the result of early or late onset of tree growth
in a given region with the tree sampling an annually varying
AlnClevel[StuiverandBraziunas,
1998].
sity of Waikato,New Zealand. This approachis vindicatedbecausethe data showsan offset of -10 yrs between laboratories
for identical samples. This does not in any way affect the
measuredrelative differencesbetween the two speciesof tree.
300
]
250
'
+ Wk-Cedar
T•
+ QUB
Cedar
Rings from trees, that grew in the same year, in opposite
hemispheres,give different carbondates(southernhemisphere
Palaeoecology
Centre,TheQueen's
University
of Belfast,Belfast,
Northern Ireland
Radiocarbon
DatingLaboratory,
Universityof Waikato,Hamilton,
501
New Zealand
o
Lamont-Doherty
EarthObservatory
andDepartment
of Earthand
o
o
o
o
o
o
o
,
o
Cal Year AD
Environmental
Sciences,ColumbiaUniversity,Palisades,NY
PlantSciences
Department,
LincolnUniversity,
Lincoln,NewZealand Figure 1. Independent
'nCmeasurements
(yrs BP) on decM•
s•ples of •
•om New Ze•d
(39•S) mMe in Q• and
Copyright1998by theAmericanGeophysical
Union.
W•ato.
E•or b•s for aH figures •e • lo which Mcludesthe
Poisson counting e•or •d an e•r multiplier to account for
Papernumber98GL01065.
0094-8534/98/98GL-01065505.00
overall laboratory reproducibility.
1321
1322
McCORMAC ET AL.: INTERHEMISPHERIC •sCOFFSET
250-
T•
•
0.51%o(19.7 + 4.1 yrs). However Figure 3 showsthat the offset changesconsiderablyafter 1895 and if data before and after
this period are comparedthe offset is -3.40 + 0.589'00(27.2 +
4.7 yrs) for the period up to 1885 and 0.66 + 1.06%o(-5.3 +
8.5 yrs) for the post 1885 interval.
An error weightedrunningmean on the combinedoffset data
is shown in Figure 4. Here sets of 3 decadaloffset measurements from each laboratoryhave been averagedand the age difference plotted againstthe central date. This showssome variability with time of the hemispheric offset notably between
the intervals 1725- 1795 AD (offset 23.1 +_7.0 yrs), 1805-
• vv•-Oak
--o---
200.
QUB Oak
TT/•
T
•-150. i•-
o
100.
1
50
o
Cal Year AD
Figure 2. Independent
anCmeasurements
(yrsBP) on decadal
samplesof oak from the British Isles (53øN, SherwoodForest,
England and 54øN, Shane'sCastle,N. Ireland) made in QUB and
Waikato.
The replicatemeasurements
are shownin Figures1 and2. The
time structure of both the cedar and oak measurements is simi-
lar to previouslypublisheddatafrom the sameperiods[Stuiver
and Becker, 1993; Pearson et al., 1986].
Weighted differencesof the cedar and oak measurements
showsa consistentdepletion in the southernhemisphereof
-3.4 + 0.58%0 until the decade centred on 1885.
results from both laboratories
After that the
show a reversal such that the
1865 AD (offset34.6 + 7.1 yrs) and again the large change
around1895 AD thoughtto be relatedto the anthropogenicinput of fossilfuel CO2 to the northernhemisphereatmosphere.
The pre-anthropogenicchange in offset value for the two earlier intervals given above is 11.5 + 10.0 yrs. Although not
highly significant this result is suggestiveof small temporal
changes in the pre-anthropogenichemispheric offset value.
The changes
in AtnCasa functionof timeareevidentin Figure
5 which shows the combined oak measurements(circles) and
combined cedar measurements(squares)from both laboratories.
Modelling
Therelativedepletionof 14Cin the southern
hemisphere
northernhemisphereis slightly depleted andthereafterthe depletion appearsto alternatebetweenhemispheres. A similar
change in hemispheric offset has been recently reported
[Stuiver and Braziunas, 1998]. Table ! gives our individual
measurementson decadalsamples of oak and cedarfrom both
ernOcean,
surface
ocean14C islow,reflecting
theexposure
of
laboratories.
Atlantic surfacewatersremainshigh reflecting their origins in
Errors on the measurements
include
both
the
Poissoncountingerror and an error multiplier to accountfor
overall laboratoryreproducibility. Also given are the per mil.
differencesbetweencedarandoak determinedindependentlyin
both
laboratories.
Table 2 showsthe errorweightedmeanof the differencesbetween oak and cedarfor selectedperiods.The offset betweencedar and oak over the entire
set of measurements
is -2.46
+
atmospherebefore 1885 can be attributedto the differencesin
oceancirculationbetweenthe two hemispheres. In the Southold, deep waters at the surface. While there is a similar but
smallerdepletion
of 14Cin theNorthPacific,the 14Cin North
low latitudes.Theoceaneffecton atmospheric
•4C gradients
has previously been shown in both a simple one-dimensional
atmosphericmodel [Levin et al., 1987] as well as an atmospheric transport model from the GISS GCM [Braziunaset al.,
1995].
Here, we estimate the magnitude of this preanthropogenic interhemispheric gradient by using a simple
two box (northern and southern hemispheres) atmospheric
Table 1. tnCMeasurements
(yrsBP)onDecadal
Samples
of CedarandOakandHemispheric
Differences
in 'nC(%0)Determined
Independently
in Belfast(QUB)andWaikato(Wk)
Year AD
1725
1735
1745
1755
1765
1775
1785
1795
1805
1815
1825
1835
1845
1855
1865
1875
1885
1895
1905
1915
1925
1935
Cedar-Wk
Cedar-QUB
162+20
213 + 19
232+19
224 + 18
248+20
218+19
246+19
242+20
176-+ 19
129+20
135+ 19
194 + 20
224+19
249 + 19
203+18
192+18
234+18
230+ 18
164+ 19
154+ 19
140+20
141+18
165 + 20
180+ 19
155_+19
167+17
158+18
120+ 17
147 + 19
136+20
178 + 19
173_+20
151+19
159+20
146 -+ 20
152+20
165+14
142+13
152+20
138+ 17
108 + 17
101+19
165 + 19
146+21
Oak-Wk
130+20
184 + 20
208_+19
208 -+ 18
208_+20
197_+18
226-+20
212_+20
124-+20
121_+20
104_+19
112-+18
144 _+20
147-+20
125_+18
148-+17
123_+20
126-+20
127 _+20
154-+21
164 -+ 20
187_+20
Oak-QUB
135+20
167 + 21
201+20
Diff.-Wk%o
184+18
170+19
209+18
208+20
121+ 19
134+ 19
110+20
91_+21
130_+21
104-+20
127+14
-4.0 + 3.5
-3.6 + 3.5
-3.0-+3.4
-2.0 _+3.1
-5.0_+3.5
-2.6_+3.2
-2.5-+3.5
-3.8+3.5
-6.5-+ 3.4
-1.0_+3.5
-4.5-+3.2
-3.6_+3.1
-2.6 _+3.5
-4.1-+3.5
-3.8-+3.2
143-+22
135+20
164+ 17
96 + 17
139+20
128 + 19
186+20
-2.4-+3.0
-4.4-+3.1
0.8-+3.0
-2.5 _+3.5
2.2+3.5
-1.7 _+3.4
1.7_+3.1
Diff.-QUB%o
0.0+3.5
-3.4 -+ 3.6
-2.9-+3.5
-2.4+3.1
-2.8+3.1
-3.1+3.1
-2.8-+3.4
-5.4-+3.4
-2.5_+3.4
-5.1-+3.5
-8.5_+3.5
-2.0 _+3.6
-6.0-+3.5
-4.8-+2.5
0.1-+1.9
-2.1-+3.5
3.2-+3.0
-1.5 _+3.0
4.8+3.5
-4.6 _+3.4
5.0_+3.6
McCORMACET AL.: INTERHEMISPHERIC•nCOFFSET
5O
Table 2. Cedar Minus Oak DeterminedIndependently
in Both Laboratories(offsets given in yrs)
TTTTT-I-T
• 251725-1935
1725- 1885
21.5 + 5.8
27.7 + 6.6
17.9+ 5.8
26.8+ 6.6
1895- 1935
QUB
Combined
19.7 + 4.1
-0.4 +
27.2 + 4.7
8.50
x = Z (x/o,2)/Z (1/oi
2) and1/•= Z (1/oi
2)
model in which
0
12.4
-9.6+ 11.7
-5.3 +
TTTTT•T?o??øa,
oTT
o
•
Waikato
1323
,_ -:25
ø-500
we assume that the concentration
666
of atmosCal Year AD
pheric14Cin eachhemisphere
reflectsa balance
between
pro-
duction, exchange with the ocean, and exchange between Figure 4. Three decaderunning mean of hemisphericdifferin 14C.Modelresults
aresuperimposed
(solidline).
hemispheres(Table 3). Our "best guess"valuesfor exchange ences
terms and surfaceoceanvaluesgive a depletionin the southern
hemisphereof-3.0 %0. This value is sensitive to the interhemisphericmixing time, the accuracyof our reconstruction
of proportionto their areas and gas exchangerates. The magnipre-anthropogenic
surface
oceanA14Candtheproduction
rate tude of the calculatednorthernhemispheredepletion is consisof 14C. Whilewepresent
results
for our"bestguess"
values, tent with the observationsfrom the tree rings (Figure 4). The
gradient
in A14Cinduced
by the inputof fossil
reasonableadjustmentsin any of thesepoorly constrainedpa- atmospheric
rameterscan bring the calculatedaverageinterhemisphericgra- fuel carbon into the atmosphereis, like the preindustrial gradidient into agreementwith the tree ring measurementsgiven ent, sensitiveto the interhemisphericmixing time, but is also
above (-3.4%0) as well as the more complexmodels of Levin et sensitiveto the location of the uptake of fossil fuel carbon. If
al. [1987] (-4 %0)andBraziunaset al. [1995] (-3.1%o). Also, we put 80% of the fossil fuel that doesnot remain in the atwhile we presentthe differencebetweenaveragevaluesin each mosphereinto a northernhemispheresink [Tans et al., 1990]
hemisphere,
the moredetailedstudyof Braziunaset al. [ 1995] the magnitudeof the interhemispheric gradient drops only to
showsthat there is considerablestructurein A14C within each 0.9 %o in 1950 vs. 0.2%0 when 41% of the carbon is absorbed
hemisphere.
WhileweshowtheA14Cgradient
hemisphere,with the differencefor the latitudesof the trees in in thenorthern
induced
by
the
input
of
fossil
fuel carbon into the atmosphere
this studyapproximately30% higher than the averageinterafter 1952, in reality the initiation of atmospherictesting of
hemisphericgradient
Theswitchto relativedepletion
of 14Cin the northern atomicweaponsat this time wouldonce again producehigher
hemisphere
is a consequence
of the additionof fossil fuel car-
atmospheric
InC in thenorthern
hemisphere.
bon (no 14C)primarilyto thenorthernhemisphere
atmosphere. We can calculate the expectedchange in the atmos-
phericAI4c gradient
by calculating
theincrease
in carbon
in
each hemisphereof our two box model dueto the addition of
fossil fuel carboninto the northernhemisphereand the uptake
of part of this carbonby oceanand/or land biota. We addfossil fuel carbon to the northern hemisphere atmosphereusing
the fluxes from Keeling [1973] (before 1950) and Marland and
Rotty [1984] (after 1950). It is assumedthat 60% of the fossil
fuel emissions remain in the atmosphere, consistent with
ocean carbon cycle models [Siegenthaler and Joos, 1992].
While there is much debate [Tans et al., 1990; Heimann and
Keeling, 1989; Broecker and Peng, 1982] as to where the excess carbon is actually absorbedinto the ocean and atmosphere,we simplyassumea null biosphereand addthe carbonto
the northern (41%)and southern(59%)hemisphere ocean in
Conclusions
14C measurements
madeoncontemporaneous
decadal
blocksof
tree rings from the British Isles and New Zealand show a depletion of-3.4 _+0.58 %0for the period 1725 to 1885 AD and a
mean offset of 0.66 + 1.06 %0for the period post 1895 AD.
Calculations using a simple two box atmospheric model are
consistent
withthis beingcaused
by a depletionof the 14C
contentof the atmosphereas a result of anthropogenicinput of
fossil fuel CO2into the northern hemisphereatmospheresince
the time of the industrial revolution. A small change in the
pre-anthropogenicoffset beweenthe periods 1725 - 1795 AD
and 1805 - 1865 AD is suggestive of a natural time varying
componentto the hemisphericoffset.
100-
,'T
50-
T
/
'•
•
o -50-
--o--1 O0
o
6
QUB diff
6
6
6
•
•
o
o
o
o
o
Cal Year AD
Cal Year AD
Figure5. A•C (%o)values
forthecombined
cedar
(southern
Figure 3. Differences
in •4Cdates(yrs) of southernhemi- hemisphere)
andcombined
oak (northern
hemisphere)
meassphere(cedar)andnorthern hemisphere(oak) wood measured urements.
Thedisappearance
of the hemispheric
offseta•und
by QUB and Waikato (Wk).
1895 AD is evident.
1324
'
McCORMAC
ETAL.:INTERHEMISPHERIC
•nC
OFFSET
Table3. Parameters
Usedto Calculate
Steady
Stateinc Concentration
in Northern
andSouthern
Hemisphere
Atmosphere.
Air-sea
Fluxes
ofCarbon
andincareCalculated
for10øLatitude
Bands
forEach
Ocean
1.2 yr
Timeconstant
forinterhemispheric
exchange
ofCO21
Northern Hemisphere
26 x 10•5molesC
CO2inatmosphere
2
14Cproduction
rate
3
230moles
•4C/yr
2.5x 10•5moles
C/yr
Air-Seaexchange
4
-55 %0
AIncofCO2transfered
fromseato•
SouthernHemisphere
26 x 1015molesC
CO2inatmosphere
2
14Cproduction
rate
3
230moles
•4C/yr
4.2x 10•5moles
C/yr
Air-Seaexchange
4
-65 %0
AI4CofCO2transferred
fromseatoair•
I Tansetal.[1978] 2Broecker
andPeng[1982]3Broecker
andPeng[1974]
4 Air-Sea
fluxisarea
weighted
gross
fluxusing
wind-speed
dependent
exchange
relationship
inTans
etal.[1990],
pCO
2= 280.
Sea-air
exchange
isarea
weighted
gross
fluxusing
wind-speed
dependent
exchange
relationship
andsurface
ocean
ApCO
2inTans
etal.[1990],
mean
ocean
pCO:z
= 280.Modern
ApCO
2areadjusted
everywhere
by6 Ilatm
fornonetfluxofCO2totheocean.
5 Surface
ocean
InC(pre-bomb)
from
Broecker
andPeng
[1982]
corrected
topre-anthropogenic
values
byadding
10%0to
warm
surface
water
values.
Thesurface
ocean
AI4cisweighted
bytheCO2fluxfromtheocean
foreach10ølatitude
band
and
each ocean.
F.G.,Liquidscintillation
counter
characterization,
optimizaAcknowledgements.This work was supportedby a researchgrant McCormac,
from the Natural Environment Research Council (NERC), Grant No
GR9/02597to Dr F.G. McCormacat The Queen's Universityof Belfast
andby a grantto Dr A.G. Hogg at The Universityof Waikato,by the
New ZealandFoundationfor Research,Scienceand TechnologyGrant
number UOW-508.
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puritycorrection,
Radiocarbon,
34,37-45,1992.
McCormac,
F.G., R.M.,Kalin,andA. Long,Radiocarbon
datingbeyond
50,000yearsby liquidscintillation
counting,
in Liquid
Scintillation
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byNoakes,
J.E.,Schonhofer,
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H.,pp.125-133,
Radiocarbon,
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Arizona,
1993.
McCormac,
F.G.,M.G.L., Baillie,J.l•, Pilcher,
andR.M.Kalin,Loca-
tiondependent
differences
intheinccontent
of wood,
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