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2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

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: a.s.cohen@open.ac.uk.

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 editing@geosociety.org 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

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