PALEOCEANOGRAPHY, VOL. 16, NO. 2, PAGES 155-163, APRIL 2001
An osmium isotopeexcursionassociatedwith the late Paleocene
thermal maximum: Evidenceof intensifiedchemicalweathering
G. Ravizza,R. N. Norris,andJ. Blusztajn
Department
of GeologyandGeophysics,
WoodsHoleOceanographic
Institute,WoodsHole,Massachusetts
M.-P. Aubry
InstitutdesSciencesde l'Evolution,Universit6MontpellierII, Montpellier,France
Abstract.In thelatestPaleocene
anabruptshifttomorenegative
•5•3C
valueshasbeendocumented
atnumerous
marineand
terrestrialsites[Braloweret al., 1997; Crameret al., 1999;Kaiho et al., 1996;Kennettand Stott, 1991; Koch et al., 1992;
Stottetal., 1996;Thomas
andShackleton,
1996;Zachos
etal., 1993].Thiscarbon
isotope
event(CIE) iscoincident
withoxygenisotopedatathatindicatewarmingof surfacewatersat highlatitudesof nearly4ø-6øC [Kennettand Stott,1991]and
moremoderate
warmingin thesubtropics
[Thomas
etal., 1999].Herewereport•87Os/•88Os
isotope
records
fromtheNorth
Atlantic and Indian Oceans which demonstratea >10% increase in the •87Os/•88Osratio of seawater coincident with the late
Paleocene
CIE.Thisexcursion
tohigher•87Os/•88Os
ratiosisconsistent
witha globalincrease
in weathering
rates.Theinferenceof increased
chemical
weathering
duringthisintervalof unusual
warmthis significant
because
it provides
empirical
evidencesupporting
theoperationof a feedbackbetweenchemicalweathering
ratesandwarmglobalclimate,whichactsto
stabilizeEarth's climate [Walker et al., 1981]. Estimatesof the durationof late PaleoceneCIE [Bainset al., 1999; Bralower
et al., 1997;NorrisandROhl,1999;R•hl et al., 2000]in conjunction
withtheOsisotopedataimplythatintensified
chemical
weathering
inresponse
towarm,humidclimates
canoccurontimescales
of 104-105
years.Thisinterpretation
requires
thatthe
latePaleocene
thermalmaximumOs isotopeexcursion
be producedmainlyby increased
Os flux to the oceanratherthana
transient
excursion
tohigher•87Os/•88Os
ratiosinriverrunoff.Although
weargue
thattheformerismorelikelythanthelatter,we cannotruleoutsignificant
changes
in the•87Os/•88Os
ratioof rivers.
thetemperature
teedback
on silicateweathering
rateis largely
unimportant
in regulating
globalclimate[Edmond
andHuh,1997].
The interplaybetweenglobaltemperature
andchemicalweather- Inresponse,
others
havenoted
thatintheabsence
ofaWalker-style
ing ratesis a criticalpartof theongoingdebateovertheunderlying weathering
feedback,
thereis no mechanism
to preventextreme
1. Introduction
mechanisms
thatbothperturbandstabilizeEarth'sclimatesystem.
The hypothesis
that a negativefeedbackbetweenglobaltemperatureandsilicateweatheringratesactsto stabilizethe long-termclimateof the Earth [Walkeret al., 1981] is appealingbecauseof its
simplicitybut is extremelydifficultto test.The feedbackoperates
suchthat rising temperatureacceleratessilicate-weathering
rates
(schematically
represented
by the Urey reaction,CaSiO3+ CO2-•
SiOn_
+ CaCO3),causinga drawdownof atmospheric
CO2.This in
turn leadsto coolertemperatures
via diminishedgreenhouse
warming. The idea that chemicalweatheringplaysan importantrole in
regulatingEarth's climatehistoryledto thedevelopment
of a series
of modelswhichhindcastlong-term(i.e., Phanerozoic)
changesin
atmosphericCO2 content based on geochemicalmass balance
variationsin the CO2 contentof the ocean-atmosphere
system
[Bickleet al., 1998;BroeckerandSanyal,1998].
ThelatePaleocene
thermalmaximum
[Zachos
etal., 1993]representsa uniqueopportunity
to examinetheresponse
of the Earth
systemto unusualwarmth.This eventoccurred-55millionyears
ago [Norris and ROhl, 1999]. It is identified in the marine record
by an abruptnegativecarbonisotopeexcursioncoincidentwiththe
extinction
of 35-50%of deep-sea
foraminifera
[Aubryetal., 1998;
Thomasand Shackleton,
1996].Associated
oxygenisotopedata
indicateseasurfacetemperatures
at highlatitudes
increased
by as
muchas4ø-6øC [Kennettand Stott,1991]duringthecarbonisotope event (CIE) with more modestwarmingat lower latitudes
[Thomas
et al., 1999].Terrestrial
records
indicateunusually
warm
calculations[Berner, 1994, and referencestherein]. Interest in the
and humidconditionsas well [Schroeder,1992].The durationof
the LPTM is difficult to constrain.Interpolationbetweenstratipothesisthat orogenesiscould acceleratethe above reaction and graphicdatums[Braloweret al., 1997],modelsof thecarboncycle
overwhelmthesupposed
negativetemperature
feedback,givingrise whichattributetheCIE to methanehydratedecomposition
[Bains
to glaciation[Raymoand Ruddiman,1992].Recentstudiesof mod- et al., 1999;Dickenset at., 1997],andastronomically
tunedestiern riversshowthatCO2consumption
duringchemicalweathering matesbasedon cyclostratigraphy[Norris and R6hl, 1999; ROhl
is stronglydependent
uponlithologyandtectonicsettingswith lit- eta!., 2000] suggests
the durationof theCIE is roughly2 x 10s
tle or nodependence
on temperature,
leadingto theconclusion
that years.The high-resolution
carbonisotopedataandcyclostratigraphy resultsalsoindicatethat the initial excursionto light carbon
historyof silicateweatheringwas furtherheightenedby the hy-
Copyright
2001by theA•nerican
Geophysical
Union.
isotoperatiosoccurred
in <104years.If theweathering
feedback
proposed
by Walkeret al. [1981]playsan importantrolein regu-
Papernumber 2000PA000541.
0883-8305/01/2000PA000541512.00
the late Paleoceneshouldbe accompanied
by a net increasein
lating Earth's climate on this timescale,then the unusualwarmth of
globalsilicateweathering.
155
156
RAVIZZA ET AL.' LPTM Os ISOTOPE EXCURSION
The marineosmium(Os) isotoperecordis theonlypaleoceanographictool currentlyavailablethat hasthe potentialto constrain
changesin chemicalweatheringassociated
with brief climaticexcursionssuchas the LPTM. Both pelagicclaysand metalliferous
carbonates
preservea recordof pastvariationsin the Os isotopic
compositionof seawater[Oxburgh, 1998; Pegram eta!., 1992;
Pegram and Turekian, 1999; Peucker-Ehrenbrinket al., 1995;
Ravizza, 1993; Reuschet al., 1998]. At any point in time, the
•a7Os/•aaOs
ratio of seawaterreflectsthe balancebetweensources
of Os with low ratios,suchas hydrothermaland extraterrestrial
sources,andcontinentalsourceswith muchhigher•a7Os/•aaOs
ratios.In thisrespect,themarineOsisotoperecordis analogous
to the
marineSr isotoperecord,a widelyexploitedrecordof thegeologic
historyof chemicalweathering,
withbothsystems
providingglobally integrated
recordsof dissolved
inputto theoceanthroughtime.
The marineresidence
timeof Sr is approximately
-2 x 106 years
[PalmerandEdmond,1992],toolongto recordshort-duration
perturbationsin oceanchemistryrelatedto intensifiedsilicateweathering. The marineresidencetime of Os is 2-3 ordersof magnitude
0.5
1.0
1.5
2.0
2.5
143
i .... I
DSDP
213
144--..
145
shorterthanthat of Sr [Levasseur
et al., 1999;Oxburgh,1998;
Peucker-Ehrenbrink
and Rav&za, 1996;Sharmaet al., 1999].Consequently,it is possiblethat the marineOs isotoperecordcould
preserveevidence of transientperturbationsin global chemical
weathering
rates.Givenourcurrentunderstanding
of themarineOs
cycle,a pulseof intensified
chemicalweathering
is expected
to pro-
duceanexcursion
to higher•aVOs/•SaOs
ratiosfollowedbya recovery to lower ratios.This studywas designedto seekevidenceof
suchan Os isotopeexcursionin the late Paleocene.
147
2. Samples
In order to construct a record of 187Os/•a8Os
variations acrossthe
LPTM eventwe haveanalyzed
sediments
recovered
fromtwoDeep
Sea Drilling Project (DSDP) sites. One is locatedin the Indian
Ocean (Site 213), and the other is locatedin the North Atlantic
I
148 l........
-- ....., , I , , , •__l , , , ,
-2
-1.5
-1
-0.5
(Site 549). Site 213 wasselectedfor initial analysisbecause
it is
lithologicallysimilarto basalmetalliferouscarbonates
whichaccumulate on recently formed oceaniccrust and have been demon-
strated
torecorddetailedlaVOs/•gaOs
variations
of Neogene
seawater
[Oxburgh, 1998; Peucker-Ehrenbrinket al., 1995; Ravi:•a, 1993;
Reuschet al., 1998].Nannofossil
[AttbryandSanfilippo,1999]and
foraminifera[BerggrenandNorris, 1997]biostratigraphy
at DSDP
213 are consistent
with a late Paleoceneage.Bulk carbonate
8•-•C
datafromSite213 showthatthesesediments
capturethelatterpart
of theCIE butnottheonsetof theevent(Figure1).• Sediments
from
DSDP Site549 wereselected
for furtherinvestigation
because
previouswork demonstrates
this sequence
recordsthe CIE associated
withtheLPTM [Attbry,1998;Stottet al., 1996).Lithologically,
the
sediments
fromDSDP Site549 areredto buffcoloredclaybearing
nannofossil
oozes,suggesting
a significant
component
of hydrogenousFe oxides.Thesesedimentsare not, however,analogousto
basalmetalliferoussedimentswhichhavebeenusedin previousOs
isotopestudies
because
theyarelocatedfar abovebasement
andpresumablyaccumulated
far removedfrommid-oceanridgehydrothermal activity.
•Supporting
datafor FigureI is availableelectronically
at World Data
Center-A for Paleoclimatology,NOAA/NGDC, 325 Broadway, Boulder,
CO 80303 (e-mail: [email protected];
URL: http://www.ngdc.
noaa.gov/paleo).
•
(•180
%0
Figure1. Carbonandoxygenisotope
datafromanalyses
of bulkcarbonate
fi'omDeepSeaDrilling Project(DSDP) Site213. The low 15•Cratiosat the
bottomof thisrecordcorrespond
to thelatterhalfof thecarbon
isotope
event(CIE), whichdefinesthe latePaleocene
thermalmaximum(LPTM).
Depthis reportedin metersbelowseafloor(mbsf).
3. Methodology
Osisotope
analyses
of bulksediments
wereperformed
bymagneticsector
inductively
coupled
plasma
source-mass
spectrometry
(ICP-MS).In thisstudy,cellulose
filterpapers
containing
theOs
preconcentrated
by NiS fire assayweredigestedin closedTeflon
vesselswith 1 mL of nitric acid and then diluted fivefold with
water.
Oswasseparated
fromthissolution
andintroduced
directly
intotheplasma
asOsO4
bybubbling
theAr sample
gasthrough
the
solution
[Hassleretal., 2000].Themainadvantage
of thismethod
relative
to morecommon
thermal
ionization
methods
is thespeed
of analysis.
An individual
Osisotope
analysis
ontheICPMSrequires-15 minof instrument
time.Thepotential
of thismethodto
enhance
sample
throughput
is important
forhigh-resolution
paleoceanographic
studies.
Analyses
wereperformed
overa periodof 6
months
onfourseparate
days.Onthesedaysa totalof 11analyses
RAVIZZA
ET AL.' LPTM Os ISOTOPE EXCURSION
157
Table la. Re-Osand Bulk CarbonateC andO IsotopeData for Site213•
Sample
16R-I, 102-106
16R-I, 141-144
16R-2
36-40
16R-2
100-104
16R-2
140-143
16R-2
140-143
16R-3
41-44
16R-3.
100-101
16R-3, 140-143
16R-4. 36-39
16R-4. 73-75
16R-4. 93-95
16R-4, 101-103
16R-4, 122-125
Depth,
Re,
Os,
mbsf
ppt
ppt
143.02
42
143.4l
143.86
144.50
144.90
144.90
145.41
146.00
146.40
146.86
147.23
147.43
147.51
147.72
33
47
46
34
25
31
136
52
38
83
192
233
354
126
42
132
295
167
••?Re/•
aaOs
13.6
2.5
3.4
6.3
2.4
••?Os/•
•Os
b•aC,
b•80,
%
%
0.4089 _+0.0063
2.13
- 1.101
0.3494
0.3416
0.3530
0.3465
0.3443
0.3578
0.3628
0.3417
0.3494
0.3748
0.4017
0.4124
0.4015
2.156
2.23
2.201
2.17
-1.014
-0.94
-0.776
-0.769
2.032
2.038
-0.905
-0.666
1.917
1.724
1.419
1.096
0.92
-0.781
_+0.0023
_+0.0028
_+0.0057
_+0.0039
_+0.012
_+0.004
_+0.0044
_+0.0026
_+0.0022
_+0.0036
_+0.0025
_+0.0119
_ 0.0033
-0.784
- 1.109
-1.539
- 1.887
"Repol'ted
uncertainties
in •87Os/•8•Os
ratiosarebasedon2{5in-runcounting
statistics.
Notethatthecomplete
bulkcarbonate
C andO isotope
datasetis
not tabulatedbut is plottedin Figure 1. Here mbsfis metersbelowseafloorandppt is partsper thousand.
of between400 and 1250 pg of Os standardwereanalyzed.After
discardingoneoutlier,the mean•87Os/•8•Os
ratioaftercorrection
for instrumentalmassbias was 0.1740 +_0.0006 (1 standarddeviation). This valuecompareswell with the acceptedvalueestablished
by repeated thermal ionization analysis of the same solution
(0.1741 +_0.0003;1 standarddeviation)[Hassleret al., 2000]. Mea-
sured•9øOs/•Osratiosfor thestandard
analyses
(1.9864+_0.0035;
I standarddeviation)werealsoindistinguishable
from theaccepted
value (1.9840), indicatingthat thereis no detectablecarryoverof
Os betweensampleand standardanalyses.If carryoverdid occur,
elevated •9øOs/•88Osratios should be measured in standards be-
causeall samplesare spikedwith •9øOsin orderto determineOs
concentrations
by isotopedilution.
Between2 and5 g of sedimentwerefusedfor eachanalysis.The
reagentsused in the NiS fire assaydetermineproceduralblanks.
The magnitudeof the fusion blank correctionis variable because
the massratio of sedimentto reagentsvaried amongthe sample
suite. In all samplesfrom Site 549 and in those samplesfrom
Site 213 representative
of the LPTM, proceduralblank corrections
were 2.2% of the total Os concentration
or less. In the low Os con-
centrationsamplesfrom Site 213, blank correctionwere aslargeas
12%. Uncertainties
4. Results
Measured
187Os/188Os
ratiosat thetwo sites(Tablesla andlb)
rangefrom0.34to0.44.Thesevaluesaresimilartothosepreviously
reportedfor Pacific metalliferoussediments[Peucker-Ehrenbrink
et al., 1995]andPacificpelagicclays[PegramandTurekian,1999]
of Paleoceneto Eoceneage. Os concentrations
of bulk sediment
fromSites213 and549 aresimilarto thosepreviously
measured
in
metalliferous
carbonates
whichhavebeenshownto recordpast
changes
in theOs isotopiccomposition
of seawater[Oxburgh,1998;
Pegrain and Turekian, 1999; Peucker-Ehrenbrinketa!., 1995;
Ravizza, 1993;Reuschet al., 1998]. MeasuredRe/Os ratiosare low
enoughthatcorrections
for in situdecayof •8?Re
areunimportant.
The 187Os/188Os
variationsare well correlatedwith variationsin
stablecarbonisotopes.At boththe North Atlantic andIndianOcean
sitesthe highest187Os/•88Os
ratiosarecloselyassociated
withthe
lowest813Cvalues(Figure2). As81qCshiftstowardhighervalues
nearthe topof thecarbonisotopeexcursion,
•87Os/•88Os
ratiosdecline to lower values.The amplitudeof the excursioncapturedat
bothsitesis -0.06 187Os/188Os
units,an increaseof >10% relativeto
post-LPTM ratios. The North Atlantic record (Site 549), which
associated with fusion blank corrections are the
spans
thecarbonisotope
anomaly,
showsthat187Os/•88Os
isotope
ra-
singlemostimportantfactor limiting the precisionand accuracyof
theselow-concentrationsamples.
tiosrecoverto valuessimilarto pre-LPTM (Figure2b) values,indicating a transientperturbationof the marine Os cycle. There is a
Table lb. Re-Os and Bulk CarbonateC and O IsotopeData for Site 549•
Sample
Depth,
Re,
Os,
mbsf
ppt
ppt
•513C,
•8?Re/•
88Os
•7Os/•Os
%
•180,
%
16R-4, 68-72
16R-4, 98-102
336.68
336.98
35
41
145
129
1.2
1.6
0.3764
_+0.0031
1.457
-0.53
0.3773
_+0.0016
1.388
-0.527
16R-5,
16R-5,
16R-5,
16R-5,
16R-5,
16R-6,
16R-6,
16R-6,
16R-6,
16R-6,
337.29
337.47
337.85
338.17
338.47
338.73
338.96
339.33
339.66
339.97
34
138
119
132
141
159
199
288
243
172
259
1.2
0.3839
0.3779
0.3856
0.4212
0.4350
_+0.0013
_+0.0013
_+0.0017
_+0.0021
_+0.0012
1.192
-0.846
1.147
-0.881
O.637
-1.025
0.4387
_+0.0018
-2.816
0.4000
_+0.0023
-0.473
0.3883
_+0.0021
1.879
-1.003
0.3666
_+0.0073
2.293
-0.651
0.3633
_+0.0014
2.191
-0.425
11-15
29-32
67-71
99-103
129-133
5-8
28-33
65-69
98-102
129-131
69
44
77
50
49
72
65
2.6
1.6
2.4
1.3
0.8
1.5
1.9
"Reported
uncertainties
in 187Os/188Os
ratiosarebasedon2c5in-runcounting
statistics.
-0.056
-1.764
-0.721
-2.022
0.627
-1.169
158
RAVIZZA
ET AL.' LPTM
54.3
54.4
Os ISOTOPE
EXCURSION
AGE (Ma)
(a)
54.5
2'41'[[
54.6
[
54.7
]
54.8
]
]t
__•_,•0..0• •;)•!180½lC]•t.t•BuIk
Carbonate
0.44
0.42 (•
1.6!'
0.40
'-
0.38
o
r,,.
0.36
0.8
: .....
• , , ,O
143
144
145
146
0.34
148
147
Depth (mbsf)
AGE (Ma)
54.6
54.7
54.8
54.9
,
,
,
,
(b)
55.0
55.1
•
M.
subbotinae
'l•Et•'•I;• / CO-/tEl'
©• - 0.44
0.42
4-.m
B.4a
--E•. •
o
2
---BulkCarbonate
/"
O
••
•,
\
O
_
\
0
- 187
Os/188
Os
-• /
-2-'
,,
336
,,
• •,
337
•
\/
, , • , , ,•,
338
oo
-
o
- 0.38
r,,.
_
(20
- 0.36
'-
_
• ....
339
• ....
340
0.34
341
Depth (mbsf)
Figure2. Comparison
of bulksediment
[•?Os/•Osvariations
to carbonisotopevariations
in (a) Site213 and(b) Site549.Carbon
isotopedatafor theplanktonicforaminiferM. subbotinae
at Site549 arefi'omStottet al. [1996].Notethatthelargest[87Os788Os
ratiosare associated
with the lightestcarbonisotoperatiosat bothsitesandthat the amplitudeof Os isotopeexcursionis similarat
bothsites.Depthis reportedin metersbelowseafloor(mbsf).The lightcarbonisotoperatiosthatcorrespond
to the CIE providethe
basisfor correlatingthe two Os isotoperecords.Age assignments
are baseduponthe carbonisotopestratigraphy
andan ageof 55
Ma for the initiationof theCIE [NorrisandRShl, 1999]andan estimateddurationfor theCIE of 220 kyr. [Rfihlet al., 2000]. These
ageassignments
are intendedto facilitatecorrelationof the two recordsbasedon our interpretation
of the carbonisotopedata.Note
thatthe 55 Ma agefor the LPTM conflictswith currentlyassignedagesfor nearbybiostratigraphic
andmagnetostratigraphic
datums
[Berggrenet al., 1995].This inconsistency
reflectsthe needfor an improvedand internallyconsistent
geochronology
for the upper
Paleoceneto the lowermostEocene[Attb•3, et al., 1998].
systematic
offsetin [•?Os/•SOs
ratiobetweenthetwositesof-0.03
•?Os/•Os units,with the North Atlantic beingmoreradiogenic
than the Indian Ocean. The highestOs concentrationsat both sites
occurcloseto butnotexactlycoincident
withthehighest
•7Os/•8Os
ratios. Bulk sediment Os concentrations and •?Os/•S•Os ratios do not
show the hyperbolicrelation requiredby two-componentphysical
mixing models.This indicatestwo-componentphysicalmixing of
isotopicallydistinctend-membersis not controllingthe observed
Os isotopevariations.
5. Discussion
5.1. A Seawater Os Isotope Record of the LPTM
from DSDP Sites 213 and 549
We interpretthe •7Os/•Os variationsat Sites213 and549 asa
recordof the Os isotopiccompositionof seawateracrosstheLPTM.
Severallinesof argumentsupportth•s•nterpretation.
First,by analogy to other marine sedimentsused to reconstructthe seawater
Os isotoperecord [Ravizza, 1993], the Os concentrations
at both
sitesare sufficientlyhighthathydrogenous
(i.e., seawaterderived)
Os shoulddominatethe sedimentOs budget.Second,the absolute
•?Os/•S•Osratios are similar to one anotherand to existingdata
fromPacificsedimentsof similarage(seeTable2 anddiscussion
of
LL44-GPC3 in section).It is unlikely that sucha similarity would
resultas a consequence
of physicalmixing of isotopicallydistinct
components.
Third, the Os isotopeexcursionat bothsitesmustbe
nearly synchronousbecauseit coincidesso with the CIE. Fourth,
theamplitudeof the •?Os/•SOsexcursion,
asdefinedby themaximum 187Os/188Os
and the post-CIEplateau,at thesewidely separatedsitesis essentiallythe same.
As notedabove,there is a systematicoffset betweenthe North
AtlanticandIndianOceansitesof-0.03 187Os/•8Os
units.It is pos-
RAVIZZA ET AL.: LPTM Os ISOTOPE EXCURSION
Table 2. Summaryof •S7Os/•SSOs
Variationsin Cores
From the Pacific, Indian, and Atlantic Oceans
Location/Site
North Pacific/LL44-GPC3
Indian Ocean/DSDP 213
North Atlantic/DSDP 549
Approximate
Duration"
-- 3 million years
-- 220,00 years
= 220,00 years
159
5.2.1. Changes in total Os flux to the ocean associatedwith
enhanced continental input during the LPTM. The simplest
interpretationof the LPTM Os isotope shift is that it reflects an
187Os?SSOsepisode of increased chemical weathering on the continents,
which resultsin an increasein the total flux of radiogenicOs to
0.32-0.41
0.34-0.41
0.36-0.44
the oceans. A crude estimate of the relative increase in Os flux can
beobtainedby considering
the •87Os/•8SOs
ratioof seawater
to be
the result of mixing betweenunradiogenichydrothermalOs and
"Duration estimates are based on sedimentation rate estimates for GPC3
radiogeniccontinentalOs with fixed •7Os/•Os for both end[Janecekand Rea, 1983] and the estimatedduration of LPTM carbon
members.
At steadystatethemolefractionF of 188Os
in seawater
isotopeevent [RiJhleta!., 2000].
derived fi'om continentalweatheringis
sible that this difference is the result of a subtle offset between the
F = (R,-
R•,)/(R,- R•,),
(1)
•87Os/1S8Os
of thebulksediment
andthatof contemporaneous
seawhereR denotesthe iS7Os/188Os
ratioandthesubscripts
s, h, andr
denoteseawater,hydrothermal,and rivers, respectively.To comOceans.Analysesof the surfacesof manganesecrustsindicatea paretherelativeincreasein continentalOs duringtheLPTM peak
with thepost-LPTMplateau,we assumeeachcanbe approximated
similar contrast may exist in the modern ocean [Burton et al.,
steadystatemixtureandcalculatetheratioof themix1999]. In spiteof the uncertaintyas to the causeof this systematic asa separate
offset betweenSites 213 and 549, synchronousOs isotopeexcur- ing fractionsas
sionsof similar amplitudeat thesetwo widely separatedsitesproFl.l,r•/Fr•,,t_l.l,l•
= [R,(LPTM)- R•,]/[R•(post-LPTM)
- R•,].
(2)
vide strongevidencefor a transientwhole-oceanshift to more
water. It is equally likely that this differencereflectsa true contrast
in the 187Os/188Osratio between the North Atlantic and Indian
continent-like
•87Os/•8SOs
ratiosduringtheLPTM.
Using•S?Os/•SSOs
ratioof thehydrothermal
end-member
(0.12)and
Theyoungest
samplefromtheSite213 recordhasa •87Os/188Os
rationearlyas high assamplesfrom theLPTM intervalof thiscore.
It is difficult to judge the significanceof this samplebecauseit is
an isolatedoccurrence.If abruptexcursionsof thissortarecommon
thelS7Os/•SSOs
ratiosfortheLPTM andpost-LPTM
fromSite549
yieldsa fractionratioof 1.23.Thisratioindicates
a 23% largercontributionof continental
Os to seawaterat theheightof theLPTM
comparedto the period immediatelythereafter.Using data from
in the marine Os record, the likelihood that the correlatedOs and
Site213 to makethe samecalculation
yieldsa 26% largerconticarbonisotopevariationsdescribedaboveis fortuitousincreases. nentalOs contributionduringtheLPTM.
It is alsopossiblethatthissamplerecordspartof a minorwarming 5.2.2. Changesin the •S?Os/•SSOs
of rivefineinput duringthe
eventfollowingthe LPTM because
it coincides
with a localmini- LPTM. The magnitudeof thechangein riverine•S?Os/•SSOs
ratio
mumin bulkcarbonate
8•sO(Figure1) andis notdiscordant
in the
that would have been requiredto producethe observedseawater
•S?Os/•SSOs-(5
•sOcorrelation(Figure3a). This studyof theLPTM
Os recordcanbe estimated
by assuming
otherrelevantparameters
anda studyof recentglacialepisodes
[Oxburgh,1998]aretheonly (F andRn) remainedconstant.As above,we assumethat the LPTM
high-resolution
studiesof seawaterOs isotopevariationsmadeto andtheperiodof timeimmediatelythereaftercanberepresented
as
date.Consequently,
thereis little basisfor evaluatinghow com- separatesteady state mixtures of Os derived from continentaland
monlyabruptexcursions
in themarineOsisotoperecordoccur.The hydrothermalsources.In thiscase,the mixingequationis recastin
relativelygoodagreement
betweenNeogenerecordsbasedon dif- the form
ferentsedimentcoressampledat low temporalresolution[Pegram
R,,•.= R,F + Rh(I-F),
(3)
etal., 1992;PegrainandTurekian,1999;Peucker-Ehrenbrink
et al.,
1995; Ravizza, 1993; Reuschet al., 1998] suggestthat suchlarge-
amplitude,
abruptexcursions
arenotubiquitous.
Nevertheless,
additionalhightemporalresolution
Os isotopedatasetsareneededto
better address this concern.
wherethe notationusedis identicalto (1). Writing separateequations
for LPTM andpost-LPTM conditionsandsubtracting
themyields
R,w(LPTM)- R,,(post-LPTM) = F [R,(LPTM) - R,(post-LPTM)]. (4)
For a widerangeof assumed
•8?Os/•a8Os
for post-LPTMinput,
5.2. Cause and Significanceof the LPTM
•S7Os/•SSOsExcursion
Our preferredinterpretationof the seawaterOs isotoperecordis
thattheexcursion
to higherseawater
•87Os/•SSOs
ratiosresultsfrom
rangingfrom0.5 to 1.5,theincreasein •a?Os/•aaOs
in LPTM riverine input requiredto producethe Os isotopeexcursionobserved
in seawater
corresponds
to a •8?Os/•SSOs
ratioroughly20% larger
thanpost-LPTMvalues.Giventhelargerangeof •a?Os/•a8Os
ratio
measured in modern rivers (0.4-
2.5) [Levasseur et al., 1999;
an increasein the total flux of Os suppliedto the oceanby continental weathering. The LPTM Os isotope excursion could also
Sharmaet al., 1999],we cannotruleoutthepossibilitythatchanges
sive. Throughoutthis discussionwe make the implicit assumption
that fluxes of unradiogenicOs to the ocean, related to seafloor
hydrothermalactivity and/orcosmicdustdeposition,remainedconstantduring the LPTM becauseat present,there is no evidenceto
suggesta transientreductionof theseinputs.
sibleto statewith any degreeof certaintythatincreasedcontinental
Os flux to the oceanduringthe LPTM to the oceancausedthe ex-
in the •a?Os/•aaOs
of riverineinputto the oceansis an important
resultfroman increasein theaverage•87Os/•88Os
ratioof soluble contributorto the LPTM Os isotopeexcursion.
Os from the continents.Althoughwe considerthesetwo scenarios 5.2.3. Discerningbetweenchangesin the flux and isotopiccomseparatelybelow, these two possibilitiesare not mutually exclu- positionof rivefine Os during the LPTM. Althoughit is notpos-
cursion
to higher•8?Os/•88Os
ratios,thereareseverallinesof argument that are supportiveof this interpretation.These include (1)
independent
evidenceof anaccelerated
hydrologiccycleduringthe
160
(a)
RAVIZZA
ET AL.' LPTM
EXCURSION
Gibsonet al., 1993;Kaiho et al., 1996;RobertandKennett,1994].
However,the timescaleof kaolinite formationis long compared
to the durationof the LPTM [Thiry, 2000]; thereforeclay mineral-
0.44
0.42
ogy cannot provide time-resolved information about continental
weathering intensity. Climate simulations [Sloan and Thomas,
ß
1998] indicatethat increasedprecipitationover land surfaceswas
likely duringthe LPTM. Changesin nannofossilmorphologyassociatedwith LPTM suggestan adaptationtowardmore buoyant
•(••
0.40[] i
forms, consistent with the inference of decreased surface water
• 0.38
....
ß[]
0.36
0.34
Os ISOTOPE
-2.5
-2
[][]•l
ß
-1.5
-1
-0.5
density[Aubryeta!., 2000]. In the contextof theseindependent
lines of evidence,it is reasonableto interpretthe shift in the
•?Os/•88Osratio of seawater at the LPTM as the result of increased
ß
[]
0
DSDP
DSDP
0.5
549
213
1
(per
(b)150
-t
Silicate Weathering
I Activation
Energy
for
Os flux from the continents.The Os isotopedata make a significant contributionto this line of investigationbecausethey provide
the only compellingevidencethat enhancedweatheringandrunoff
was globally rather then locally significant.
Laboratory measurementsof the activationenergy of silicate
weathering[Kumpet al., 2000; Whiteet al., 1999]predicta specific
relationshipbetweensilicateweatheringratesandtemperature.At
both Sites 213 and 549 the 45•O of bulk carbonate and •a?Os/laaOs
are correlatedwith one another(Figure 3a). Consideringoxygen
isotopesasa proxyfor localtemperatureandOs isotopesasa proxy
for weatheringflux, thisplot qualitativelyindicatesweatheringflux
=
70 kj/mol
increases
with increasingtemperature.
To examinethisrelationship
c
100
---• - 30 kj/mol
in a more rigorousway would requireknowledgeof an average
temperatureincreaserepresentative
of weatheringcontinentalenvironments.We contendthat a reasonableguessat sucha temperature
.•_
increaseis in the rangeof 4ø-7øC. This is basedon seasurfacetemperatureestimates[Kennettand Stott, 1991; Thomasand Shackleton, 1996; Thomas et al., 1999] showing increasesof 40-6 ø and
r..)
1%4ø at high and low latitudes,respectively.This temperature
rangeand the 20-30% increasein weatheringflux estimatedabove
are consistentwith an effective activationenergyfor weatheringof
-30 kJ/mol (Figure 3b), a value at the low end of the rangesmea0
1
2
3
4
5
6
7
suredin silicate weatheringrate experiments[White et al., 1999].
Delta T (øC)
Althoughthiscomparisonis a grossextrapolation,it showsthatinterpretingthe Os isotopeexcursionas a proxyfor weatheringflux
Figure3. (a) The plotof ]•7Os/•aOsversus$•O of bulkcarbonate,
which yieldsan estimatedweatheringrate increasethat is consistentwith
suggests
thatseawater
•87Os/188Os
ratiosincrease
withincreasing
temperaavailable data constrainingboth the LPTM temperatureincrease
tures.Note that bulk carbonateoxygenisotopedata are only a proxy for
and the temperaturedependenceof silicateweatheringrates.
local temperaturevariations.The samplefi'omSite 549 that plotsaway from
the correlationis from the $•*C minimumandhasexperienced
the most
The abruptness
of the onsetof the LPTM alsosuggests
that the
extensivecarbonatedissolution;percentcarbonatein this sampleis only
associated
Os isotopeexcursionis morelikely relatedto changing
i
x
50
-'t-
3% versus40-50% aboveand below the CIE. Thus we suspectthe larger
ratherthanchanging•7Os/•Os
$•aOof thissamplereflectsthe influenceof dissolution-resistant
benthic totalOs flux fi'omthe continents
of river input. The short and transientnatureof the Os isotope
excursionassociated
with theLPTM makesit unlikelythattectonic
ciated with the LPTM, which are used to define the dark field labeled
processescould producethe observedOs isotopicexcursionby
"LPTM." The curvesrepresentthe calculatedtemperaturedependenceof
varying the natureof the exposedlithologiesduring the course
silicateweatheringratesbasedon activationenergiesmeasuredin the laboof
the LPTM. This is a fundamentallyimportantcontrastbetween
ratory.The overlapof the LPTM field with the 30 kJ/toolactivationenergy
curveindicatesthat Os isotope-based
weatheringflux estimatesare consis- interpretingshort-termvariationsin the Os isotopiccomposition
tent with available data constrainingboth the LPTM temperatureincrease of seawater,as we do here, and interpretinglong-termCenozoic
andthe temperaturedependenceof silicateweatheringrates.
trends. There is one particular scenario of variable rivefine
foraminifer.(b) The percentincreasein weatheringflux, inferredfi'om Os
isotopedata, and the estimatedaveragesurfacetemperatureincreaseasso-
•a?Os/•88Os
throughtheLPTM thatwarrants
specificconsideration:
LPTM, (2) the magnitudeof theincreasein temperature
andweatheringflux estimatedfor theLPTM, and(3) the abruptonsetof the
LPTM.
Each of these is discussed in turn below.
the possibilitythat the Os isotopeexcursionmight reflect a brief
episode of erosion of sedimentary organic matter with large
•8?Os/•aaOs
ratios.The clearcorrelationof high •a?Os/•aaOs
with
lightiS•*Cvalues(Figure2) motivates
usto address
thisscenario
ex-
Severalindependent
linesof evidencesuggestan accelerated plicitly becausea similarcorrelationis apparentin theNeogeneOs
hydrologiccycleoperatedduringtheLPTM. Many workershave isotoperecord [Ravizza, 1993], albeit on a much longertimescale.
arguedthat increasedabundanceof kaolinite and/or increased In addition,exactlythis mechanismis invokedto explainthe longkaolinite/illite ratiosassociatedwith someLPTM sectionsprovide
direct evidence of warm, humid climates [Cramer et al., 1999;
term shift in bulk carbonisotopesto lighter valuesas a resultof the
initial stagesof the collision of India and Asia from the Paleocene
RAVIZZA
ET AL.' LPTM Os ISOTOPE EXCURSION
161
Median Grain Size ( phi - 50)
(Janecek and Rea 1 983)
to the Eocene and to suggestthat the unusualwarmth associated
with thisperiodmay have resultedfrom a relatedincreasein atmospheric CO2 levels [Beck et al., 1995, 1998]. If the CIE is attributed
to a net decreasein the sizeof the sedimentaryorganiccarbonreservoir rather than gas hydrate release,then the excursionto higher
8.2
8.4
8.6
8.8
9.0
9.2
9.4
LL4 4- GPC3
•*7Os/•'8Os
caneasilybeattributed
toerosionof radiogenic
ancient
sedimentaryorganic matter. Other workers have arguedthat the
CIE is too large and abruptto be explainedby burial and erosionof
sedimentaryorganicmatter,and favor attributingthe CIE to gashydratedecomposition
[Dickenset al., 1997;Thomasand Shackleton,
1996].Recentworkdemonstrating
a veryabrupt(<104years)onset
for the CIE [Bains et al., 1999; Norris and RShl, 1999; ROhl et al.,
2000] stronglysupportsthislatterview andargueagainstinterpreting the Os isotopeexcursionas the resultof the erosionof ancient
sedimentaryorganic matter.
There are other scenariosthat might give rise to largeand rapid
Grain
Size
10
187Os/ 188Os
changesin the •7Os/•88Osof rivefineinput that cannotbe discounted.For example,shiftsin continentalclimate,suchaschanges
in precipitationpatternsor/andmeanannualtemperatures,
couldbe
calleduponto producethis effect eitherby increasingweatheringin
one region at the expenseof anotheror by changingthe extent to
whichan isotopicallyheterogeneous
terrainis weathered.Therefore
our interpretationof the LPTM Os isotopeexcursionas evidence
for increasedcontinentalweatheringratesduringthe LPTM must
be regardedas tentative.Better constrainingthe processes
responsiblefor thisOs isotopeexcursionis importantbecausethe implied
increasein chemical weatheringduring this episodeof unusual
warmthsupportsthe operationof a feedbackin whichatmospheric
CO2 levelsare regulatedby the temperaturedependence
of silicate
weatheringrates.
5.3. Comparison of the LPTM to Recent Glacial-Interglacial
Variations in the Seawater Os Isotope Record
15
Pegionof
P-E Epoch boundary
2O
25
0.2
0.4
0.6
During the last two majorglacialperiods(isotopestages2 and6),
brief excursions to lower •*?Os/•88Osratios have been found in Pa-
cific metalliferoussedimentsand interpretedas an indication of
diminishedchemicalweatheringon the continentsduringcold, arid
glacial periods [Oxburgh, 1998]. Both climatically and in terms
of the directionof the Os isotopeexcursion,the LPTM is the oppositeof the muchmorerecentglacialepisodes.Justasglobalaridity
andcoldertemperatureswereinvokedby Oxburghto explainshifts
to lower•87Os/•Osduringrecentglacialperiods,we attributethe
higher•87Os/•Osat theLPTM to generallywarmer,morehumid
conditionsasdescribedabove.The coherentresponseof the marine
Os isotoperecord to these contrastingclimate regimes supports
the notionthat high-resolutionOs isotopestudiesmay be broadly
applicableto studyingthepossiblefeedbackbetweenglobalweathering ratesandclimatechange.
Two additionalaspectsof the comparisonbetweenrecentglacial
episodesand the LPTM deservemention.First is the significant
difference of the duration of the excursion. While the •'7Os/•Os
minima associatedwith recentglacial episodesspanat mosta few
thousandyears, the LPTM event spannedroughly 200,000 years
[Bainset al., 1999]. Thus the slopeof the seawaterOs curveduring
the LPTM recovery is modest comparedto that associatedwith
glacial transitions.Second,interpretationof the LPTM Os excursion is lesscomplicatedthan thoseassociatedwith recentglacials.
The late Paleoceneand early Eoceneare consideredto be ice-free
periods[Sloanand Thomas,1998;Zachoset al., 1993]. Therefore
other potential explanationsof glacial Os isotopeexcursions,re-
0.8
1
1.2
187Os/188Os
(Pegram and Turekian 1 999)
Figure 4. A reinterpretation
of thetimingof Os isotopevariationsin LL44GPC3 [Pegramand Turekian,1999]basedon grainsizedata[Janecekand
Rea, 1983].The abruptshiftto smallermediangrainsizeof eolianmaterial
(largerphi values)marksthe Paleocene-Eocene
(P-E) epochboundaryin
severalPacificcores[Rea, 1998].This correlation
indicates
thatthelong-
term•S7Os/•aaOs
of seawater
is characterized
by a broadminimumin the
vicinity of the P-E boundary.
latedto weatheringof glaciatedterrains[Peucker-Ehrenbrink
and
Blum, 1998], are not directly relevant.
5.4. Paleocene-Eocene
Transition
as Recorded in LL44-GPC3
and Its Relationship to the LPTM
The mostcompletesingleOs isotoperecordis fromLL44-GPC3,
a North Pacific pelagic clay sequence[Pegram and Turekian,
1999]. A dramatic decreasein eolian grain size occursin several
Pacific corescloseto the Paleocene-Eocene(P-E) epochboundary
[Rea, 1998] includingGPC3. This changein grain size in GPC3
[Janecekand Rea, 1983] providesa stratigraphic
markerwhichallows identificationof the portionof the GPC3 Os isotoperecord
that includesthe LPTM and P-E boundaryintervals(Figure4). Detailed correlationof Os isotopedata betweenPacific GPC3 and
the North Atlantic and Indian Oceanis not possiblebecauseof the
162
RAVIZZA ET AL.: LPTM OsISOTOPEEXCURSION
relativelylow temporalresolutionof theGPC3 record.We discuss thisminimum,
such
asophiolite
weathering
andrecycling
ofextratheGPC3 Os isotoperecordherefor two reasons.
First,thelower- terrestrial
Osderived
fromtheK-Tboundary
impact
[Pegram
and
resolution
GPC3 recordprovidesa broadertemporalcontextin
whichlong-termwarmingfrom the Paleocene
intotheEocenecan
Turekian,
1999],remainreasonable
hypotheses.
beexamined.
Second,
andmoreimportantly,
thesimilarity
in abso- 6. Conclusions
lute •87Os/•8OsratiosbetweenGPC3 and the North Atlanticand
a large(10%) increase
in the•8?Os/•aaOs
IndianOceansites(Table2) provides
strongempiricalevidence We havedocumented
sitesthatis coincident
withthelate
thatOsisotopic
variations
atallthreesitesarecontrolled
mainlyby ratioat two widelyseparated
Paleocene
thermalmaximum.On the basisof the coherence
of
changes
in the•87Os/•a8Os
ratioof seawater
through
time.
The GPC3recordis characterized
by a minimum(•a?Os/•a•Os
-- these
twonewrecords
withoneanother
andwithexisting
data
we haveconcluded
thattherewasa tran0.32) duringthe periodof mostrapidgrainsizedecrease
and fromPacificsediments
ratios
higher•a?Os/•aOs
ratios(= 0.41)beforeandafterthegrainsize sientwhole-oceanshift to higherseawater•87Os/•88Os
transition
(Figure3). This featureprobablyspansseveralmillion during the LPTM. The transientnatureof the excursionand indeyearson the basisof sedimentation
rate estimates[Janecekand
Rea, 1983;Kyteet al., 1993].The higher187Os/188Os
ratiosafter
theP-E epochboundary
are substantiated
by a spotanalysis
of
Pacificmetalliferous
sediments
of earlyEoceneage(•aVOs/laaOs
-0.48[Peucker-Ehrenbrink
etal., 1995].A singleanalysis
of leachableOsin a pelagic•clay
of latePaleocene
age(187Os/188Os
-• 0.45)
[Peucker-Ehrenbrink
et al., 1995]is distinctlyhigherthenthe
GPC3databut similarto thehighestvaluesmeasured
at Site549.
pendent
supporting
dataindicative
of an accelerated
hydrologic
cyclesuggest
thatthisOsisotope
excursion
isevidence
of a globallysignificant
increase
in chemical
weathering.
Thisinterpretation supportsthe operationof a feedbackbetweenclimate and
chemical
weathering
ratesandsuggests
thatCO2drawdown
associatedwithsucha feedback
mayhaveplayedanimportant
role
in arresting
LPTM warming.
Thevalidityof thisinterpretation
dependscriticallyuponour assumption
that the •87Os/•88Os
of
The rangeof •a?Os/ta•Os
ratiosmeasuredin Sites 213 and 549 riverinefluxdidnotchange
significantly
duringtheLPTM.Comarevery similarto thosereportedfromGPC3 (Table2). However, parisonof theLPTM Os excursion
reportedhereto thoseassocithe shapeof the high- and low-resolutionrecordsis different. atedwithrecentglaciation
reveala coherent
picturein whichbrief
The higher-resolution
datasetsspanning
theLPTM arecharacter- periods
of extreme
warmthandhumidity
arecharacterized
byexizedbya localmaximum
coincident
withtheCIE (Figure2), while cursions
to high18708/18808
ratios,whiletheopposite
is trueof
the GPC 3 recordis characterized
by a broadminimumas de- briefepisodes
of extremecoldandaridity.Thusavailable
Osisothata weathering-based
mechanism
doesindeed
scribed
above.In total,thedatasuggest
thatthelong-term
transi- topedatasuggest
tionfromthelatePaleocene
intotheearlyEoceneis characterized actto ameliorate
Earth'sclimate[Walkeretal., 1981]duringcliby generallylow •?Os/•aOs.ratioswithat leastonebriefexcursion maticextremeson timescales
of 104-10s years.We stressthat
to higher•VOs/•8•Os
withinthebroadminimum
defined
by the thesehigh temporalresolutionstudiesreveal little aboutthe factorscontrolling
thelong-term
evolution
of theOsisotopic
comIt is noteworthy
thattheagemodelfor GPC3[Kyteet al., 1993] position
of seawater.
In thecontextof establishing
thecoherence
datafromLL44GPC3,
usedby PegrainandTurekian[1999]yieldsa mid-Paleocene
age of thesenewLPTM Osdatawithexisting
GPC3 record.
for the localminimum in the seawatert87Os/188Os
recordratherthan
we havewe reinterpreted
age-depth
relationsin thiscoreandinfer
epochboundary
falls withina broad
the Paleocene-Eocene
boundary
agewe suggest
here.In using thatthe Paleocene-Eocene
record,indicativeof a relatheeoliangrainsizedatato reinterpret
thetimingof thisminimum, minimumin the seawater187Os/188Os
we open the possibilitythat the generallylow •a?Os/•a•Os
ratio tiveincrease
in theamount
of seawater
Osderived
fromyoung
of seawaterduringthe Paleocene-Eocene
transitionresultedfrom
increased
supply
of unradiogenic
Osfromrecent
magmatic
activity.
mantle-derived rocks. This is consistentwith evidence of widespread volcanism at this time.
Severalpotentialsourceshavebeenidentified:(1) increased
seafloorhydrothermal
activityrelatedto platereorganization
[Owen
andRea, 1985],(2) extensive
NorthAtlanticvolcanism
[Thomas Acknowledgments.We thankLary Ball and the W.H.O.I. ICP Facility
for operationandsupportof theFinniganMAT sectorfieldinductivelycouand Shackleton,1996], and (3) Caribbeanvolcanism[Bralower pledplasmamassspectrometer
(SF-ICPMS) in makingthesemeasurements.
et al., 1997].The prospectof drawinga causativelink between GR thanksB. Peucker-Ehrenbrink
and J. Howard for helpfulcomments
thelow •VOs/•aaOs
ratioof seawater
andanyof thesemagmatic on earlydraftsof this manuscript.T Bralower,R. Oxburgh,andan anony-
episodes
is important
because
eachof thesehasbeensuggested
as mousreviewel'offeredmanyconstructivecomments.Curatorsof theOcean
DrillingProgramprovidedsamplematerialessential
forthisstudy.Thiswork
a potentialtriggerfor methane
hydratedecomposition.
At present, wassupportedby NSF A wardsOCE-99190500 (RDN), OCE-9819296 (GR)
thislink is onlyspeculative
andotherpossibilities
for explaining and EAR-9804864 (GR).
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M.-P. Aubry, lnstitut des Sciencesde l'Evolution, Universit• Montpellier II, 34095 Montpellier Cedex 05, France.
J. Blusztajn, R. N. Noms, and G. Ravizza,
Departmentof GeologyandGeophysics,
Woods
Hole OceanographicInstitute,WoodsHole, MA
02543. ([email protected])
(Received
May19,2000;revised
December
14,
2000; acceptedDece•nber27, 2000.)