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2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; February 2004; v. 32; no. 2; p. 157160; DOI 10.1130/G20158.1; 3 figure s; Da ta Repository ite m 2004019. 157
Osmium isotope evidence for the regulation of atmospheric CO2
by continental weathering
Anthony S. Cohen*Angela L. Coe
Stephen M. Harding
Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK
Lorenz Schwark Geologisches Institut, Universitat zu Koln, Zulpicher Strasse 49a, D-50674 Koln, Germany
ABSTRACT
The long-term stability of Earths climate throughout the Phanerozoic stands in
marked contrast to the dramatic fluctuations that have taken place on time scales as
short as a few years, reflecting the high efficiency of longer-term climate regulation
through negative feedbacks. A fundamental mechanism is thought to involve control of
CO2 in the ocean-atmosphere system through continental weathering, although un-
ambiguous, high-resolution data supporting this hypothesis have hitherto not been avail-
able. Organic-rich mudrocks from Yorkshire, England, which were deposited during the
Toarcian oceanic anoxic event (ca. 181 Ma, Early Jurassic), contain evidence of an excep-
tionally large excursion in the 187Os/188Os ratio of contemporaneous seawater, from 0.4
to 1.0. The most likely explanation for this excursion is that it resulted from a transient
increase in global continental weathering rates of400%800%. The Os isotope excursion
coincided with a well-documented global 13C excursion of 6 that affected all the
major biospheric reservoirs of the time. Higher mean global temperatures caused global
chemical weathering rates to increase substantially, while, in turn, chemical weathering
was very effective in reducing the elevated levels of atmospheric CO2 and the high tem-
peratures to preexcursion levels.
Keywords: osmium, weathering, Toarcian, oceanic anoxic event, strontium, methane hydrate.
INTRODUCTION
Although rapid oscillations in Earths cli-
mate have occurred frequently in the past (Al-
ley et al., 2003), climate-control processes
have operated during the Phanerozoic to keep
Earths climate within the relatively narrow
bounds required for the continuing success of
life on the planet. Interaction between CO2 inthe ocean-atmosphere system and the silicates
in the continental crust is thought to be a fun-
damental mechanism in the regulation of
Earths climate, involving the release of Ca
and Mg by chemical weathering and, ulti-
mately, the deposition of (Ca,Mg)CO3 in the
oceans (Berner et al., 1983; Broecker and San-
yal, 1998; Kump et al., 2000; Walker et al.,
1981). However, one of the major obstacles in
demonstrating the precise role of chemical
weathering as a regulator of climate has been
the difficulty in finding a distinctive chemical
proxy that responds with sufficient rapidity
and magnitude to changes in global weather-ing. The relatively short seawater resi-
dence time for Os of1040 k.y. (Peucker-
Ehrenbrink and Ravizza, 2000) makes it an
ideal isotopic tracer for the purpose, as long
as the recovered signal is primary and global
in extent. Our approach is based on the ob-
servation that the Os isotope composition of
seawater has varied over geologic time in re-
*E-mail: [email protected].
sponse to changes in the balance between the
major inputs to the oceansprimarily from
continental weathering, the hydrothermal alter-
ation of juvenile oceanic crust, and meteoritic
sources (Cohen et al., 1999; Pegram et al.,
1992; Peucker-Ehrenbrink and Ravizza, 2000).
Because the hydrothermal input of unradiogen-
ic Os to the oceans can be assumed to remainconstant over relatively short intervals, rapid
and transient increases in the seawater 187Os/188Os ratio can be caused only by a sudden
increase in the flux or isotopic composition of
radiogenic Os from continental weathering.
SAMPLES AND RESULTS
The samples that we selected for study are
Toarcian organic-rich mudrocks from expo-
sures at Saltwick Bay, Port Mulgrave, and
Hawsker Bottoms, Yorkshire, England (Hes-
selbo and Jenkyns, 1995; Howarth, 1962,
1992). The widespread deposition of marine
organic-rich mudrocks across the exaratumBiosubzone of the lower falciferum Biozone
(Toarcian, Early Jurassic, ca. 183178 Ma [Palfy
et al., 2000]), with as much as 15% organic
carbon, is a primary feature of the Toarcian
oceanic anoxic event (Jenkyns, 1988; Jenkyns
et al., 2002). Although the duration of the ex-
aratum Subzone is not accurately constrained,
it is estimated from annual sediment-layer
couplets to have been 150 k.y. (Cope, 1998;
Hesselbo et al., 2000). The rich marine fauna,
sedimentary facies, stratigraphic context, and
paleogeographic position of these exposures
all provide evidence that the mudrocks were
deposited under open-marine conditions (Hes-
selbo and Jenkyns, 1995; Howarth, 1962,
1992).
Our new Re and Os abundance data, and C
isotope results, are shown in Figure 1 and re-
ported in Tables DR1 and DR21. Suites of
mudrock samples from the exaratum and fal-
ciferum Subzones define Re-Os isochron ages
of 181 13 Ma (Cohen et al., 1999) and 178
5 Ma (this study, Fig. 2), respectively, thus
confirming that the Re-Os isotope system in
these successions has not been disturbed since
deposition and that the initial Os isotope com-
position is primary (Cohen et al., 1999). There
is a major, transient increase in the calculated187Os/188Os ratio of seawater from 0.4 to
1.0 (Fig. 1) that occurred mostly during de-
position of the exaratum Subzone. Despite its
relative brevity, with an apparent duration of
100 k.y., the magnitude of the Os isotope
excursion exceeded the total variation in the187Os/188Os ratio of seawater over the past
35 m.y. (Pegram et al., 1992).
DISCUSSIONThe magnitude of the Os isotope excursion
reported here would have demanded an in-
crease of800% in the continental weather-
ing flux, if present-day isotopic ratios for the
end-member components are used (Peucker-
Ehrenbrink and Ravizza, 2000). This excur-
sion could, in theory, have involved a very
abrupt and transient increase in the 187Os/188Os ratio of the global continental weather-
ing component, rather than in its flux. How-
ever, we consider such an eventuality to be
very remote because large-scale changes in
the worldwide balance of rocks that are avail-
able for weathering at Earths surface requirethe action of long-term (millions of years) tec-
tonic processes. Such processes could not
have operated on the much shorter (thousands
1GSA Data Repository item 2004019, TablesDR1 and DR2 (sample locations, stratigraphic po-sitions, Re and Os abundances, Os isotope and 13Cdata), is available online at www.geosociety.org/pubs/ft2004.htm, or on request from [email protected] or Documents Secretary, GSA, P.O.Box 9140, Boulder, CO 80301-9140, USA.
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158 GEOLOGY, February 2004
Figure 1. Os, C, and Sr isotope variations in seawater for part of Toarcian (Lower Jurassic); plots display large, concurrentchanges in seawater 187Os/188Os ratios (Table DR1; see footnote 1), 13C values for bulk organic matter (this study, TableDR2; see footnote 1; VPDB is Vienna Peedee belemnite), and seawater 87Sr/86Sr ratios (McArthur et al., 2000) that took placeduring deposition of exaratum Subzone; gray band across isotope curves indicates exaratumSubzone. Sample positionsrelate to composite stratigraphic section from three localities in Yorkshire, England, shown here as summary graphic log(for details see Table DR1; see footnote 1). Lithostratigraphy and biostratigraphy are from Hesselbo and Jenkyns (1995) andHowarth (1962, 1992). P. Protogrammoceras, D. Dactylioceras, cl. clevelandicum, ten. tenuicostatum. Lithologiesinclude dark gray mudrocks (dark gray tone), medium gray mudrocks (gray tone), and carbonate bands and nodules (white).Early Jurassic seawater Os isotope compositions were calculated from present-day Re and Os abundances and Os isotopecompositions of mudrock samples by assuming closed-system behavior (as indicated by isochron relationships, this study,and Cohen et al. [1999]) and using depositional ages indicated by isochrons (Table DR1; see footnote 1).
Figure 2. Re-Os isochron diagram for all sixmudrock samples from falciferumSubzone.Regression age is 178.2 5.6 Ma (meansquare of weighted deviates 3.0). Initial187Os/188Os ratio (0.4 0.15) reflects that ofcontemporaneous seawater (Cohen et al.,1999; Peucker-Ehrenbrink and Ravizza,2000).
of years) time scale over which the global sea-
water Os isotope excursion in the exaratum
Subzone was established. Nevertheless, if we
consider an extreme event in which the 187Os/188Os ratio of the global weathering flux sud-
denly increased by 50% to 2 (compared
with its present-day value of 1.4), then the ob-
served Os isotope shift would have still re-
quired an additional increase in weathering
flux of 400%. Thus, the magnitude of the tran-sient excursion in the Os isotope composition
of seawater during deposition of the exaratum
Subzone was so large that it would have in-
volved a relatively brief interval of greatly en-
hanced continental weathering, on a global
scale, for any realistic Os isotope composition
of the weathering flux.
The Os isotope excursion coincided with a
global 13C shift of 6 (Fig. 1) that was
one of the largest of the Phanerozoic, affecting
all the major biospheric carbon reservoirs at
that time, i.e., bulk marine organic matter
(Hesselbo et al., 2000; Kuspert, 1982; Schou-
ten et al., 2000), primary organic production
biomarkers (Schouten et al., 2000), marine
carbonate (Kuspert, 1982; Schouten et al.,
2000), and terrestrial carbon (Hesselbo et al.,
2000). Hesselbo et al. (2000) demonstrated
that the only geologic process consistent with
a global C isotope excursion of this magnitudewould have been the dissociation of very large
amounts of methane hydrate, an immediate
consequence of which would have been its
rapid oxidation to CO2 and a threefold in-
crease in atmospheric CO2 levels (Beerling et
al., 2002). A crucial point is that the precise
interval over which the levels of atmospheric
CO2 were high is clearly defined by the highly
distinctive light C isotope composition im-
parted by the dissociated and oxidized meth-
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GEOLOGY, February 2004 159
Figure 3. Variations in Jurassic seawater87Sr/86Sr ratio. A: High-resolution profile forupper Pliensbachianlower Toarcian (Mc-Arthur et al., 2000), showing variations withstratigraphic height. B: Variations in 87Sr/86Sr ratio of Jurassic seawater, based on in-tegrated data set (Jones et al., 1994a,1994b). Bold arrow indicates approximateduration of eruption of Karoo-Ferrar igneousprovince (Duncan et al., 1997).
ane hydrate (Fig. 1). The concurrent Os andC isotope excursions in the Toarcian also co-
incide with an exceptionally sharp increase in
the seawater Sr isotope composition (Figs. 1
and 3) (Jones et al., 1994b; McArthur et al.,
2000). Because these synchronous C and Sr
isotope shifts were demonstrably global in ex-
tent (Hesselbo et al., 2000; Jenkyns et al.,
2002; Jones et al., 1994b), they provide ad-
ditional compelling evidence that the seawater
Os isotope excursion, which occurred over the
same short interval, also represented a global
signal.
One of the main factors governing rates of
chemical weathering is temperature (Kump etal., 2000). Beerling et al. (2002) calculated
that the global 13C isotope excursion of
6 in the Toarcian would have required the
dissociation of5000 Gt of methane hydrate
and that average global temperatures would
have increased by at least 3 C as a conse-
quence of the threefold increase in atmospher-
ic CO2. Direct evidence for a sudden, large
increase in seawater temperatures at that time
also comes from O isotope analyses (Jenkyns
et al., 2002; McArthur et al., 2000) and Mg/
Ca ratios (McArthur et al., 2000) from bel-
emnites, some of which were used to define
the contemporaneous seawater Sr isotope
curve. The 18O values suddenly decreased by
3 at the same point that marked the start
of the Os and C isotope excursions, while Mg/
Ca ratios approximately doubled, from 0.2 to
0.4. Both data sets independently suggest that
average seawater surface temperatures during
deposition of the exaratum Subzone were asmuch as 10 C higher than those before and
after (Jenkyns et al., 2002; McArthur et al.,
2000).
Although the precise relationship between
continental weathering rates and mean global
temperature in the Toarcian would have de-
pended on the highly complex interplay be-
tween vegetation and hydrology as well as on
atmospheric CO2, the 400%800% increase in
weathering rates in the exaratum Subzone of
the Toarcian is nevertheless fully consistent
with estimates of the temperature dependence
of current chemical weathering rates (Gaillar-
det et al., 1999; Kump et al., 2000). An anal-ysis of the dissolved loads carried by the
worlds rivers shows that a 500% increase in
weathering rate occurs for every 5 C rise in
temperature where weathering rate is not con-
trolled by water supply (Gaillardet et al.,
1999). The sudden and large increase in con-
tinental weathering rates in the greenhouse
world of the Toarcian is commensurate with
highly efficient hydrologic and weathering
systems where groundwater flow predominat-
ed, as was the case in the Late Cretaceous
when groundwater flow exceeded runoff by a
factor of six (Floegel et al., 2003). For com-
parison, during the present-day icehouse con-
ditions, global runoff exceeds groundwater
flow by a factor of three.
The transient increase in continental weath-
ering rates in the exaratum Subzone resulted
in an increased flux of Ca and Mg to the
oceans, which would have served to balance
the higher levels of CO2 in the oceans and
atmosphere. The approximate time scale of the
C (and Os) isotope excursions may be esti-
mated from global sedimentation and CO2consumption rates; this calculation also pro-
vides an independent check of the feasibility
of enhanced continental weathering havingacted as the negative feedback that regulated
atmospheric CO2. Sediment supply to the
oceans was relatively low in the Early Jurassic
and, outside the exaratum Subzone, may have
been as little as 12%20% of its current level
(Floegel et al., 2000). Present-day chemical
weathering of Ca- and Mg-bearing silicates
consumes 0.085 Gt of C annually (Gaillardet
et al., 1999); thus, depending on the exact
choice of values, the excess CO2 derived from
the oxidation of methane hydrate during the
exaratum Subzone would have been con-
sumed in 37123 k.y. This result is fully
consistent with the other estimates for the du-
ration of the Os and C isotope excursions.
The dramatic rise in the 87Sr/86Sr ratio of
seawater during deposition of the exaratum
Subzone represented one of the fastest rates of
increase of the Phanerozoic. It was superim-
posed on a longer-term increase (Fig. 3) that
extended from the late Pliensbachian to theearly Bajocian. The singular change in slope
of the seawater Sr isotope curve in the late
Pliensbachian coincided with the main erup-
tive phase of the Karoo-Ferrar igneous prov-
ince at 183 1 Ma, according to the dates of
Duncan et al. (1997). Under the assumption
that the contribution of unradiogenic Sr from
seafloor spreading remained essentially con-
stant over this interval (Rowley, 2002), we
suggest that the eruption of the Karoo-Ferrar
igneous province caused both the initial rise
in seawater 87Sr/86Sr ratio in the late Pliens-
bachian and its longer-term increase. This oc-
curred through volcanic CO2 outgassing,which led in turn to higher mean global tem-
peratures and enhanced continental weather-
ing. We further suggest that the transient and
exceptionally high rate of increase in seawater87Sr/86Sr ratio during deposition of the exar-
atum Subzone was caused by the acceleration
of weathering rates resulting from higher at-
mospheric CO2 and mean global temperatures
following methane hydrate destabilization
(Beerling et al., 2002; Hesselbo et al., 2000).
After the deposition of the exaratum Subzone,
the seawater Os isotope composition and the
rate of increase of the seawater 87Sr/86Sr ratio
returned rapidly to levels similar to those be-
fore the methane hydrate destabilization, in-
dicating a relaxation in continental weathering
rates.
An alternative explanation for the steep rise
in the seawater 87Sr/86Sr ratio across the ex-
aratum Subzone, proposed by McArthur et al.
(2000), is that it is an artifact of unusually
slow sedimentation rates in the exaratum Sub-
zone alone. These authors make the explicit
assumption that the rate of increase of sea-
water 87Sr/86Sr ratio in the early Toarcian was
constant. They conclude that the exaratum
Subzone is greatly condensed compared withthe strata above and below, and that it was
deposited over an interval of 1 m.y. How-
ever, the assumption upon which this approach
is basedthat the rate of increase in seawater87Sr/86Sr ratio was constantis hard to justify
in view of the dramatic reversal of slope of
the seawater Sr isotope curve in the latest
Pliensbachian, perhaps no more than 1 m.y.
earlier (Fig. 3).
The transient geochemical and climatic
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160 GEOLOGY, February 2004
changes that occurred as a result of methane
hydrate destabilization during the Toarcian are
among the largest that have been reported for
the Phanerozoic. Lesser changes of a similar
nature also occurred at the Paleocene-Eocene
boundary (ca. 55 Ma) (Dickens et al., 1995),
when a smaller gas hydrate release was re-
sponsible for a 13C isotope shift of2.5
and an increase in mean global temperature of
2 C. Then, an 26% increase in continental
weathering flux was inferred from an 10%increase in the seawater 187Os/188Os ratio
(Ravizza et al., 2001). In the more recent past,
seawater 187Os/188Os ratios for warmer inter-
glacial periods were found to be 5% higher
than for colder glacial periods (Oxburgh,
1998). Although the implications of these two
studies are in full accord with the results pre-
sented herethat continental weathering rates
increase with higher mean global tempera-
turesin both these cases the changes in the
seawater Os isotope composition could have
been caused by relatively minor changes in
the Os isotope composition of the continental
component, rather than in its flux.
CONCLUSIONS
The seawater 187Os/188Os ratio suddenly in-
creased from 0.4 to 1.0 for a short interval
during deposition of the Toarcian exaratum
Subzone. The magnitude of the Os isotope ex-
cursion indicates an increase in continental
weathering flux of400%800% and was so
great that it rules out the possibility that the
excursion was caused by a change in Os iso-
tope composition of the weathering flux alone.
High levels of CO2 in the oceans and atmo-
sphere, which were the likely consequence of
massive methane hydrate dissociation, led to
elevated mean global temperatures and higher
rates of chemical weathering. Enhanced chem-
ical weathering resulted in higher fluxes of Ca
and Mg to the oceans, which consumed CO2and thereby caused mean global temperatures
and weathering rates to fall. The large, syn-
chronous changes in the Os, Sr, C, and O iso-
tope ratios of Early Jurassic seawater provide
very strong evidence supporting the hypothe-
sis (Walker et al., 1981) that continental
weathering rates are temperature dependent
and that weathering provides a rapid and ef-
fective negative feedback that can moderatemajor climatic perturbations through CO2consumption.
ACKNOWLEDGMENTS
We thank K.W. Burton, N.B.W. Harris, N.W.Rogers, R.A. Spicer, N. Vigier, and three anony-mous reviewers for comments on an earlier versionof this manuscript, and the Natural Environment Re-search Council and the Open University for finan-cial support. We are grateful for journal reviews
provided by S.P. Hesselbo and an anonymousreviewer.
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Manuscript received 5 September 2003Revised manuscript received 24 October 2003Manuscript accepted 27 October 2003
Printed in USA
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