GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 19, PAGES 3145-3148, OCTOBER 1, 2000
Accretionof InterplanetaryDustin Polar Ice
Edward J. Brook
Geology
Department,
Washington
State
University,
Vancouver,
Washington,
USA
Mark D. Kurz and Joshua Curtice
Department
ofMarine
Chemistry
andGeochemistry,
Woods
HoleOceanographic
Institution,
Woods
Hole,
Massachusetts,
USA
Stuart Cowbum
Geology
Department,
Washington
State
University,
Vancouver,
Washington,
USA
Abstract. Measurements
of helium isotopesin particles using
3Heasa proxy[Farley
andPatterson,
1995;Patterson
and
separatedfrom polar ice demonstratethat extraterrestrial3He
Farley, 1998;Marcantonioet al., 1996]. Resultsof thesestudies
dominates
the3HefluxattheGISP2(Greenland)
andVostok areequivocal.Marcantonioet al. [1996]arguedfor minimalIDP
(Antarctica)ice core sites. Replicatemeasurements
of late fluxvariations,
whilePatterson
andFarley[1998]interpreted
Holocene
icesamples
yield3Hefluxesof 0.62+ 0.27x 10-•2cm3 theirdataasevidence
for 100kaperiodicity
in IDP accretion.
STPcm'2ka'• (GISP2)
and0.77ñ 0.25x 10'12cm3STPcm-2ka'l
The noblegascontentof particlesin polarice coresmay
(Vostok),similarto resultsfrom marinesediments.Theseare the
provide another record of IDP flux variations that would shed
firstdetailed
measurements
of 3Hein particles
fromice core lightonsomeof theseissues,
butpriorto thisstudythispotential
samples,
andtheydemonstrate
the utilityof the ice corerecord has not been investigatedin detail. There are a number of
for evaluatingthe temporalhistoryof the extraterrestrial
dust possibleadvantages
of an ice corerecordof IDP flux. First,
flux.Results
fromVostok
samples
from1096-1403
m depth
(75- accumulation
ratesandtime scalesof deeppolarice coresare
97 ka B.P.) are similarto the late Holocenedata,with the wellknown,allowingaccurate
fluxcalculations
andcomparison
exceptionof two highly anomalous
resultsfrom 1307 m. The to otherrecords.Second,
ice corerecords
containa varietyof
latterprobably
indicate
thepresence
of rare,largeor gas-rich climateproxies,includingstableisotopeandtracegasrecords,
extraterrestrial
particles.
allowing
examination
of postulated
linksbetween
IDP variability
andclimatechange.Finally,thedatapresented
heredemonstrate
that3Hein ice coreparticulate
matteris dominated
by
Introduction
extraterrestrial helium.
Accretion
of extraterrestrial
material
to theearthis important
to studies
of theoriginof theatmosphere
andoceans,
dynamics Methods
of dustin planetary
systems,
geochemical
budgets,
astronomical Helium isotopeswere measuredin particles from GISP2
observations,
andpossibly
terrestrial
climatechange
[Loveand (centralGreenland)andVostok(centraleastAntarctica)ice core
Brownlee,
1993;Muller andMacDonald,1995,1997]. Marine samples.GISP2sampleswerefrom 132-138m depthin the G2
sedimentsand polar ice providean archiveof the influx of
extraterrestrial
materialin the form of interplanetary
dust
particles
(IDPs)[Parkinand Tilles,1968]. Highlevelsof 3He
andothernoblegasisotopes
provide
a uniquesignature
of IDP in
pelagic
sediments
[e.g.,ArnariandOzirna,1988]. Thissignal
core and are approximately400-430 years old [Meeseet al.,
1997]. Vostoksampleswere from the 110-111m sectionof the
BH-5 core and from selecteddepthsbetween1096 and 1404 m
fromthe 5G deepcore. The BH-5 samples
weredeposited
about
3.8 ka B.P andthe 5G samplesweredeposited
between75 and97
can,in principle,be usedto constructa recordof extraterrestrial
ka B.P.,usingthetime scaleof Sowerset al. [ 1993].
3Heflux,andthereby
derive
a record
ofIDPaccretion
[Farley Ice sampleswere cut on a bandsaw and weighed.Particles
andPatterson,
1995;Farley,1995;Patterson
andFarley,1998]. were extracted by melting at room temperaturein a glass
MullerandMacDonald
[1995,1997]highlighted
theimportancefiltration funnel in a class 100 clean hood. Meltwater was
of theserecords
by suggesting
thattheearth's100ka glacial- vacuumfilteredthrough0.45 or 0.2 micronpure silver filters
interglacial
cyclemightbe drivenby variationsin the tilt of the
(Osmonics-Poretics). The filters were rinsed with ethanol,
orbital
plane.Mostotherexplanations
ofthe100kacyclerelyon folded,andplacedin aluminumfoil boats. Helium was extracted
astronomically
forcedsolarinsolationchanges[Imbrieet al.,
by meltingthe particles,filter, and foil boat in an ultra-high
1993],andhavedifficulty
explaining
powerin the100ka cycle. vacuumfurnacewith a tantalumcrucible. Heliumwaspurified
Muller and MacDonald[1995]suggested
thatvariabilityin and the isotopes measured using static noble gas mass
interplanetarydust abundancenear the earth could link orbital
spectrometry;
the massspectrometerand proceduresused are
inclinationand climate. Severalgroupshavesearched
marine describedby Kurz et al. [1996]. Blankswere between4 and 6 x
sediment
records
forvariations
in IDP fluxon 100katimescales, 10-• cm3 STP4Hewithapproximately
atmospheric
3He/4He
Copyright2000 by the AmericanGeophysicalUnion.
ratios,andrepresent
smallcorrections
to 4He and3He. The
ultimate
detection
limitof thesystem
is approximately
2 x 10'16
cm3 STP3He(5000atoms);
thesmallest
samples
discussed
here
Papernumber2000GL011813.
are approximately20 times largerthan this detectionlimit. All
0094-8276/00/2000GL011813505.00
heliumconcentrations
arereported
in unitsof cm3 STPg'• ice.
3145
3146
BROOK ET AL.' INTERPLANETARY
DUST IN POLAR ICE
Table 1. Helium IsotopeData for > 0.45 Micron ParticlesFilteredFromIce Samples.
Sample
Depth Age•
(m)
(ka)
Wgt.Ice
4He
3He/4He
(g)
(10'•2cm3g'•ice)
(R/R0
3He
IceAccum.
2
(10'•8cm3g'•ice) (gcm'2a
'•)
3HeFlux
(10'•2cm
3cm'2
ka'•)
GISP 2 (Greenland)
132.7-133.1
0.40
1092.70
0.77 _+0.02
42.83 +_0.28
45.47 +_0.96
22.4
1.02 +_0.02
133.6-133.7
0.41
338.50
0.99 +_0.02
8.75 +_0.25
12.01 _+0.42
22.4
0.27 _+0.01
134.2-134.3
0.41
800.60
4.26 _+0.09
7.31 _+0.06
43.09 _+0.93
22.4
0.97 _+0.02
137.2-137.7a 5
0.43
751.75
0.59 _+0.01
15.70 +_0.25
12.73 _+0.32
22.4
0.29 +_0.01
137.2-137.7b 5
0.43
938.88
0.82 +_0.02
28.58 _+0.28
32.55 _+0.73
22.4
0.73 +_0.02
137.54-138.18
0.42
1186.23
0.86_+0.02
16.17 _+0.18
19.15 _+0.50
22.4
0.43 _+0.01
137.54-138.183
0.42
1186.23
0.30 _+0.01
0.96 _+0.19
0.39 _+0.08
Mean _+2•:
0.62 _+0.27
0.87 _+0.02
Vostok(Antarctica)4
110.0-110.19a 5
3.80
259.69
1.82 _+0.04
173.51 _+0.92
437.02 _+9.04
2.0
110.0-110.19b 5
3.80
280.35
2.22_+0.04
174.11 _+0.78
535.39_+ 10.97
2.0
1.07 _+0.02
110.19-110.38
3.80
265.96
1.33_+0.03
139.07 _+1.14
255.16_+5.51
2.0
0.51 _+0.01
110.38-110.48a 5
3.80
145.15
1.01 _+0.02
159.05 _+1.72
223.08 _+5.07
2.0
0.45 _+0.01
110.38-110.48b 5
3.80
139.50
2.22_+0.04
152.21 _+1.05
466.61 _+9.87
2.0
0.93_+0.02
1096.0
75.26
154.73
5.16 _+0.01
1145.0
77.99
168.92
1220.0
84.04
152.03
1261.0
87.56
158.99
3.60 _+0.07
116.14 _+0.71
1265.0
87.89
215.93
2.39 _+0.05
90.85 _+0.60
1285.0
89.14
254.01
3.00 _+0.06
1303.3
89.94
176.94
2.85 _+0.06
1307.0a 5
90.08
67.44
46.34 _+0.93
1307.0b •
90.08
100.98
1307.0c s
90.08
94.16
1316.6a s
90.87
105.46
1.44 _+0.03
163.28 -+ 1.69
1316.6b s
90.87
106.24
1.44 -+ 0.03
173.75 _+1.77
1321.0
91.28
271.70
2.22 _+0.04
1333.0
92.39
221.45
1341.0
92.99
119.85
1346.0
93.36
1403.6
97.30
Mean _+2•:
0.77_+0.25
1.2
0.97 _+0.02
109.98 _+0.56
785.88 _+16.22
3.15 _+0.06
143.79 _+0.80
626.62 _+13.02
1.5
0.92_+ 0.02
2.37 _+0.05
156.14 _+1.04
510.77 _+10.73
1.5
0.75 _+0.02
578.09 _+12.10
1.4
0.80 _+0.02
300.99 _+6.34
1.3
0.40 _+0.01
159.23 _+0.67
660.49 _+13.50
1.4
0.91 _+0.02
128.74 _+0.88
508.51 _+10.75
1.5
0.76 _+0.02
200.05 _+0.86 12829.29 _+262.45
1.4
17.88 _+0.37
1.60 _+0.03
102.38 _+0.98
1.4
37.28 _+0.75
193.13 _+0.92
227.18 _+5.04
9963.83 _+204.85
0.32 _+ 0.01
1.4
13.89 _+0.29
325.71 -+7.33
1.6
0.52 -+0.01
346.75 _+7.78
1.6
0.55 _ 0.01
147.82 _+0.74
453.21 _ 9.34
1.4
0.65 _ 0.01
2.26 _+0.05
136.33 _+0.76
426.69 _ 8.86
1.4
0.62 +_0.01
8.17 _+0.16
149.61 _+0.72
691.59 _+34.79
1.5
2.47 _ 0.05
207.50
5.60 _+0.11
194.93 _+0.77
1512.07 _+30.83
1.5
2.21 _+0.05
106.56
5.80 _+0.12
101.04 _+0.65
811.55 _+17.04
1.6
1.27 +_0.03
•Ages
forGISP2fromMeese
etal.[1997];ages
fromVostok
fromSPECMAP
timescale
ofSowers
etal [1993].2GISP2:
CuffeyandClow
[1997]'Vostok:
Sowers
etal.(1993).30.2-0.45
m sizefraction.
4Depths
forbelow111.0maretopdepths
of samples
spanning
10-15cm.
5Replicates
forHolocene
GISP2andVostok
samples
wereprepared
bycutting
coresections
longitudinally.
Replicates
of deepVostok
samples
were madeon unorientedvariablesizedpiecesfrom 10-15 cm intervals.
Blank measurementson unusedfilters and proceduralblanks GISP2 siteare - 10 timeshigherthan at Vostok(Table 1), the
of the GISP2samples
is roughlyhalf of
using deionized,filtered water were indistinguishable
from mean4Heconcentration
furnaceline blanks. As a test of filtration efficiencyand yield the mean for the HoloceneVostok samples. This probably
of terrestrial
dust
onesamplewasfilteredfirstthrougha 0.45micronfilterandthen reflectsthe relativeproximityandabundance
through
a 0.2micron
filter.Thequantity
of3Hetrapped
onthe sourcesat the GISP2 site, althoughdifferencesin the age,
0.2 micron filter was lessthan 2% of the total trappedby the two
composition,
or particlesizeof the dustmayalsocontribute.
filters(Table1), indicating
thatthe0.45 micronfilterseffectively 3He/4Heratiosfor GISP2 samplesare between7.3 and42.8 Ra
trapmostof the particulate
3He. Thisis consistent
with (Raistheatmospheric
3He/4He
ratio,
1.384
x 10'6).Vostok
ratios
predictionsof the size distributionof He-retainingIDPs [Farley
arehigher,between
91 and200 Ra(Table1). Estimates
of the
etal., 1997],whichsuggest
thatthemajority
ofIDP-borne
3Heis 3He/4Heratiofor IDPs rangefrom 173 Ra [Nier andSchlutter,
carriedin particleswith diametersgreaterthan 1 micron. A 1992]for stratospheric
particles
to 310 Raforthesolarwind
greater
quantity
of 4Hepassed
through
thelargerfilter(-26%of [Geiss
etal.,1971].Thehigh3He/4He
ratios
foricecoresamples
thetotal;Table1)suggesting
thatsome
4Heiscarried
in smaller indicatethat virtuallyall of the 3He they containis
particles,probablyterrestrialdust.
Results
extraterrestrial.Using a two componentmixing model [e.g.,
Patterson
andFarley,1998]thefraction
3Heet/3Hetotal
isgreater
than 99.8% for Greenland samplesand >99.99% for Antarctic
samples. This calculationassumesthat 3Heet/3Hetotal
=(4He/3Heobs-4He/3Hecrust)/(4He/3Hct-4He/3Hecrust),
3He/4Heet=
290
4.26x 10-•2cm3STPg'•ice(Table1). 4Heconcentrations
ofthe
The 4Hecontentof the GISP2 samplesrangesfrom 0.30 to
Ra. Thecrustal
ratiomaybeashigh
Vostoksamples
arehigher,reflecting
thelowericeaccumulationRa,and3He/4Hec•ust=0.015
ratesat Vostok. AlthoughHoloceneaccumulation
ratesat the as 0.33 Ra [Marcantonioet al., 1998]. However,evenusingthis
BROOK ET AL.; INTERPLANETARY
ratio3HeinVostok
samples
is> 99.7% extraterrestrial
and3He
in Greenlandsamplesis > 98.0% extraterrestrial.
DUST IN POLAR ICE
3147
The expectedstatisticaldistributionof resultsfor the Holocene
Vostoksamples
(meanAT=0.01m2a)wasmodeled
usingthe
Treatingthe 6 GISP2samples
as replicates,
the mean3He methodof Farley et al. [1997]. (We focuson thesesamples
concentration
is 28 + 12x 10'•s cm3 STPg'• ice(all means becausethey are most likely to yield statisticallymeaningful
reportedas+ 2 x standarderror). For the five VostokHolocene results). We employedthe predictedsize distributionand flux of
samples
themean
3Hecontent
is384+ 123x 10'Iscm
IDPsthatwereheatedto lessthan600øCuponatmospheric
entry
TheGISP2samples
spananagerangeof about40 years.If there (andtherefore
retained
3He)fromthatstudy.A MonteCarlo
were variability in IDP flux to the ice surface on decadal or methodwasusedto calculatethe expecteddistribution
of relative
shortertime scales[suggested
by Kayset et al., 1998] this 3Hecontent
withrespect
totheglobal
mean
value,
assuming
that
uncertaintyestimatewould be a maximum. There also could be
the3Heconcentrations
arecorrelated
withparticle
surface
area
scatter in the results due to seasonalvariations in snowfall. The
average annual snow accumulation rate at the site is
[Farley et al., 1997], generatinga syntheticdistributionof 2000
approximately50% or more of annualsnowfall. Given the small
randomlyfrom the 2000 points.
points. The distributionhas a meanvalue closeto the global
approximately
22 cm/yr,butsummer/fall
accumulation
is slightly mean,but a standarddeviationof over 100%, reflectingthe
greater(-10-20%) thanwinter/spring
accumulation
[Shuntan
et statisticallysmall numbersof IDPs in sampleswith low AT
al., 1995]. Althoughthe seasonof deposition
is unknown,the values. We then created 1000 model samplesof n=5 for
samples
areat least10cmthick(Table1) andtherefore
represent comparisonwith the Vostok resultsby repeatedlyselecting
seasonalvariationsat this site this issue is probablynot
Althoughdataare limitedat thispoint,the impliedagreement
significant.Theuncertainty
for theVostokreplicates
is 32 % of of ourreplicatesappearsto be betterthanpredictedby the model
themean(Table1), andthesesamples
represent
deposition
over treatment.The probabilityof obtainingan uncertaintyof 32% in
- 25years.
a sample
of n=5forAT=0.01m2aisslightly
lessthan15%based
Duplicate
3Hemeasurements
at1316
matVostok
(Table
1)' onthemodel.
Although
more
data
arerequired
tocharacterize
yieldbetteragreement,
within6.4 % of themean(Table1). thestatistics
of extraterrestrial
3Hein ice coresamples
one
Given
theconsistency
ofthisduplicate
measurement,
results
for' explanation
forthedifference
between
thereplicate
results
and
triplicatemeasurements
of the 1307 m sample(Table 1) are
the model predictionis that the IDP size distributionusedin the
difficultto interpret.One replicatehad a concentration
of 227 +
model is inaccurate. For example,the numberflux of small
5 x 10-]acm3 STPg-•,similar,
though
slightly
lowerthan, particles(10 micronand less)couldbe underestimated;
this is a
relativelylow valuesfoundin thetwo samples
belowthisdepth plausiblehypothesisbecausethe IDP size distributionis not well
(Table1). The3Hecontent
of theothertworeplicates
of this knownfor smallparticles.Fragmentation
of particlesduring
sampleareextremelyhigh,12,829+ 263 and9,964+ 205 x 10'•a atmospheric
entryor in the ice is anotherpossible
explanation
cm3STPg'],- 10times
higher
thanthemean
forallother
Vostok [Peucker-Ehrenbrink
and Ravizza,2000]. Futureanalysisof
samples.
Thesesamples
alsohad5-20 timesmore4Hethanthe size-fractionated
samples
mayshedlightonthisissue.
others(Table 1). The origin of thesehigh concentrations
is
Theaccumulation
rateattheGISP2sitewas22.4g cm'2yr'• in
unclear
butmayresultfromsampling
relatively
raregas-rich
or thedepthrangeof thesamples
analyzed
[CuffeyandClow,1997].
largeIDPs[Patterson
andFarley,1998]. 3He/4He
ratiosfor 3Hefluxeswerecalculated
assuming
that•He•ux=•Heconc
* b,
thesetwo replicates
are200 and193Ra,thehighestin thedata where b is accumulationrate. At Vostok accumulationrates were
set,supporting
thisconclusion.
Thesehigh3He/4He
ratiosare basedon the accumulation-temperature
relationship[Sowerset
fluxesare0.62+ 0.27x 10'•2cm3
similarto thoseobtained
for individual
IDPs[NierandSchlutter, al., 1993](Table1). Resulting
1992], illustrating that extraterrestrialhelium dominatesthe STPcm'2ka-• for GISP2and0.77+ 0.25x 10-•2cm3 STPcm'2ka'
particulatematterheliumbudget. The occurrence
of thesetwo
• forVostok.These
results
aresimilar
toprevious
estimates
of
highest
concentrations
in ourdatasetat thesamedepthlevelis theextraterrestrial
3Hefluxtomarine
sediments
overthepast200
curious,
andunlikelyif particles
arerandomlydistributed.This ka (Figure 1) [Pattersonand Farley, 1998; Marcantonioet al.,
may indicatethat there were temporalvariationsin the size
1996,1999].3Hefluxes
atVostok
inthe1096.0-1403.6
mdepth
distribution
of3He-bearing
extraterrestrial
particles.
interval(Figure2) arebetween0.32-2.47x 10'•2 cm3 STPcm'2
Discussion
measurements
of the 1307 m sample). The mean flux in this
intervalis 0.94+ 0.32x 10']2 cm3 STPcm'2ka-]. Thelattervalue
ka-• (excluding
theanomalous
datafortwoof threereplicate
Thenumberof IDPs in a sampleis proportional
to samplesize
and inverselyproportionalto accumulationrate. Following
Farley et al. [ 1997] the total accumulationof ice or sedimentin a
2.0
samplecan be describedby the ratio of the samplemassto
accumulation
rate,or "area-timeproduct"(AT). Largervaluesof
=,
AT are preferableto reducestatisticaleffects relatedto the small
numberof IDPs in typical samples(Farley et al., 1997). The
meanAT forGISP2samples
discussed
hereis 0.004m2a,andfor
Vostok
samples
themeanAT is0.01m2a.Forcomparison,
most
marinesediment3He datacomesfrom sediments
with AT values
•-n
1. 0
0.5
-
GISP21
Pacific
2
•
Pacific4
•
Atlantic
3J[
indian•
VostokI
of 0.02 to 0.5 m2a[Farleyand Patterson,1995; 1997;
Marcantonio et al., 1996; Patterson and Farley, 1998].
Uncertaintyin multiplereplicatesof marinesediments
from ODP
Figure 1. Comparison
of ice coredatapresented
herewith recent
estimates
of theextraterrestrial
3Heflux.1 = thispaper.2 =
Site806 withAT of 0.125m2aranges
from15-185%(one Averagefor last 200 ka from Marcantonioet al. [1996]. 3 =
standard deviation) with the more extreme values driven Averagefor last 20 ka from Marcantonio et al. [1998]. 4=
primarily by one or two anomalouslyhigh concentrations averagefor the past114ka fromPattersonand Farley [ 1998]. 5
[Pattersonand Farley, 1998].
= averagefor thelast200 ka fromMarcantonioet al. [ 1999].
3148
BROOK ET AL.' INTERPLANETARY DUST IN POLAR ICE
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1). However,thereis no significantdifferencebetweenthe mean
Cosmochim.Acta, 62, 1535-1543, 1998.
of all of the Holoceneresults(GISP2 and Vostok, n=l 1) and Marcantonio,F., K.K. Turekian,S. Higgins,R.F. Anderson,M. Stute,and
glacial results (Vostok, n=15) at the 95% confidenceinterval
(two-tailedstudents
t-test,t=1.240, outliersat 1307m neglected).
The overallmeanflux for the dataset,(neglectingthe outliersat
1307m)is0.83+ 0.20x 10'12cm3STPcm'2ka'l. Thisvaluemay
P. Schlosser,
The accretionrate of extraterrestrial
3He basedon
oceanic
23øTh
flux andtherelationto Os isotope
variation
overthe
past 200,000 years in an Indian Ocean core, Earth and Planet. Sci.
Lett, 170, 157-168, 1999.
Meese,D. A. et al., The GreenlandIce SheetProject2 depth-agescale;
be more appropriatefor comparisonto marine sedimentdata
methodsandresults,J. Geophys.Res.,102, 26,411-26,423,1997.
whichare averagedover longtime periodsby stripsampling[e. Muller, R.A. and G.J. MacDonald, Glacial cyclesand orbital inclination,
Nature, 377, 107, 1995.
g., Pattersonand Farley, 1998]. Thereis alsoapparenttemporal
Muller, R.A. and G.J. MacDonald, Glacial cycles and astronomical
variability in the Vostok results for the samplesdeposited
forcing,Science,277, 215-218, 1997.
between 75 and 97 ka (Figure 2). Apart from the highly Nier, A.O. and D.J. Schlutter, Extraction of helium from individual
anomalousresultsfrom the sampleat 1307 m, the Vostok results
interplanetaryparticles by step-heating,Meteoritics, 27, 166-173,
indicatean apparent
variabilityin the3Heflux of a factorof
almost 4 on time scalesof a few thousandyears (Figure 2).
1992.
Parkin, D.W. and D. Tilles, Influx measurementsof extraterrestrial
material, Science,159, 936-946, 1968.
Peucker-Ehrenbrink,
B. andG. Ravizza,The effectsof samplingartifacts
Apparent
fluxesbetween
97 and93 kaare1.27- 2.47x 10'22cm3
on cosmic dust flux estimates: A re-evaluation of non-volatile tracers
STPcm-2ka'•. In thesubsequent
periodbetween
92 and84 ka
(Os,Ir), Geochmim.Cosmochim.Acta, 64, 1965-1970.
fluxesrangedfrom0.32 - 0.91 cm3 STP cm'2ka'•, andthen
3He in seafloor
returned
to highervaluesbetween
0.92and0.97x 10'•2cm3 STP PattersoreD.B. and K.A. Farley, Extraterrestrial
sediments,Evidencefor correlated100 kyr periodicityin the accretion
cm'2ka'• by78 ka.). Suchrapidchanges
notappear
in marine rateof interplanetarydust,orbitalparameters,andQuaternaryclimate,
sedimentdata,but bioturbationand samplingtechniquessmooth
Geochim. Cosmochim.Acta, 62, 3,669-3,682, 1998.
the marine record to varying degrees,while ice cores should Sowers,T. et al., A 135,000 year Vostok-SPECMAP commontemporal
framwork,Paleoceanography,
8, 737-766, 1993.
preservea higherresolutionrecordof flux variations.
There are a number of possiblereasonsfor the apparent Shuman,C.A., R.B. Alley, S. Anandakrishnan,J.C. White, P.M. Grootes,
temporal
3Hefluxvariations
intheVostokcore.First,theymay
reflectthe statisticsof smallIDP populations.Second,theremay
be terrestrialfactorsthat affectthe IDP contentof polarice. For
example, variations in wind velocity might changethe size
distribution
of deposited
IDPs. Finally,the3Hefluxvariability
may reflect changesin the dynamics of interplanetarydust
production,transport, or accretion. This latter possibility is
intriguing,as it implies that ice core samplescould be usedto
examineIDP flux variationsoverthe past-• 400 ka.
Acknowledgments. We thank the National Science Foundation
(OPP-9909384, OPP-9909663, and EAR-9807951) and NASA (NAG59345) for support,Ken Farley andKarl Turekianfor helpfulreviews,and
DempseyLott for importantlaboratoryassistance.
and C.R. Stearns,Temperatureand accumulationat the Greenland
summit:Comparisonof high-resolutionisotopeprofilesand satellite
passivemicrowavebrightnesstemperaturetrends,J. Geophys.Res.,
100, 9,165-9177, 1995.
E. Brook and S. Cowburn, Geology Department,WashingtonState
University, 14204 NE SalmonCreek Ave, Vancouver,WA 98686 (email:
brookor cowburn•vancouver.wsu.edu)
M. Kurz and J. Curtice, Department of Marine Chemistry and
Geochemistry,WoodsHole Oceanographic
Institution,WoodsHole, MA,
02543 (e-mail:mkurzorjcurtice•whoi.edu).
(Received
April2, 2000;revised
June27,2000;
accepted
August8, 2000.)