erosion of deccan traps determined by river geochemistry

Erosion of Deccan Traps determined by river geochemistry:impact on the global climate and the 87Sr/86Sr ratio of

seawater

Celine Dessert a;*, Bernard Dupre a, Louis M. Franc°ois a;b, Jacques Schott a,Jeroªme Gaillardet c, Govind Chakrapani a;d, Sujit Bajpai e

a LMTG, Universite Paul Sabatier, 38 rue des 36 Ponts, 31400 Toulouse, Franceb LPAP, Universite de Lie©ge, 5 avenue de Cointe, 4000 Lie©ge, Belgium

c Laboratoire de Geochimie et Cosmochimie, IPGP, 4 place Jussieu, 75005 Paris, Franced Department of Earth Sciences, University of Roorkee, Roorkee 247667, India

e School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India

Received 21 November 2000; accepted 19 March 2001

Abstract

The impact of the Deccan Traps on chemical weathering and atmospheric CO2 consumption on Earth is evaluatedbased on the study of major elements, strontium and 87Sr/86Sr isotopic ratios of the main rivers flowing through thetraps, using a numerical model which describes the coupled evolution of the chemical cycles of carbon, alkalinity andstrontium and allows one to compute the variations in atmospheric pCO2, mean global temperature and the 87Sr/86Srisotopic ratio of seawater, in response to Deccan trap emplacement. The results suggest that the rate of chemicalweathering of Deccan Traps (21^63 t/km2/yr) and associated atmospheric CO2 consumption (0.58^2.54U106 molC/km2/yr) are relatively high compared to those linked to other basaltic regions. Our results on the Deccan andavailable data from other basaltic regions show that runoff and temperature are the two main parameters which controlthe rate of CO2 consumption during weathering of basalts, according to the relationship:

f � RfUC0exp3Ea

R1T3

1298

� �� where f is the specific CO2 consumption rate (mol/km2/yr), Rf is runoff (mm/yr), C0 is a constant ( = 1764 Wmol/l), Earepresents an apparent activation energy for basalt weathering (with a value of 42.3 kJ/mol determined in the presentstudy), R is the gas constant and T is the absolute temperature (³K). Modelling results show that emplacement andweathering of Deccan Traps basalts played an important role in the geochemical cycles of carbon and strontium. Inparticular, the traps led to a change in weathering rate of both carbonates and silicates, in carbonate deposition onseafloor, in Sr isotopic composition of the riverine flux and hence a change in marine Sr isotopic composition. As aresult, Deccan Traps emplacement was responsible for a strong increase of atmospheric pCO2 by 1050 ppmv followedby a new steady-state pCO2 lower than that in pre-Deccan times by 57 ppmv, implying that pre-industrial atmosphericpCO2 would have been 20% higher in the absence of Deccan basalts. pCO2 evolution was accompanied by a rapid

0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 3 1 7 - X

* Corresponding author. Fax: +33-5-61-55-81-38; E-mail: [email protected]

EPSL 5829 18-5-01

Earth and Planetary Science Letters 188 (2001) 459^474

www.elsevier.com/locate/epsl

warming of 4³C, followed after 1 Myr by a global cooling of 0.55³C. During the warming phase, continental silicateweathering is increased globally. Since weathering of continental silicate rocks provides radiogenic Sr to the ocean, themodel predicts a peak in the 87Sr/86Sr ratio of seawater following the Deccan Traps emplacement. The amplitude andduration of this spike in the Sr isotopic signal are comparable to those observed at the Cretaceous^Tertiary boundary.The results of this study demonstrate the important control exerted by the emplacement and weathering of large basalticprovinces on the geochemical and climatic changes on Earth. ß 2001 Elsevier Science B.V. All rights reserved.

Keywords: erosion; rivers; geochemistry; Sr-87/Sr-86; Deccan Traps; Global change; weathering; paleoclimatology; K-T boundary

1. Introduction

Continental erosion is a major geological pro-cess which is responsible for landscape evolutionand exerts a major control on the transport oferosion matters from continents to the oceansand on the cycles of many chemical elements, in-cluding carbon, at the Earth's surface. Moreover,as chemical weathering involves consumption ofatmospheric CO2, a greenhouse gas, it probablystrongly in£uences the secular evolution of theEarth's climate.

Over geologic time scales, three major processesconsume atmospheric CO2 : (1) the chemicalweathering of continental silicate rocks [1^3], (2)the organic carbon burial during sediment depo-sition (e.g. [4^6]) and (3) the chemical weatheringof the oceanic crust [7,8].

Because climatic conditions are dependent onthe atmospheric CO2 levels, several authors havetried to model the evolution of the carbon cycleand other elements over geologic time [2,8^10]. Inorder to quantify the role of silicate weathering inthe carbon cycle, it is necessary to study the geo-chemistry of rivers and to determine the laws gov-erning chemical weathering. Many studies havefocused on river geochemistry on a global [1,11^18] and smaller scale [19^23]. These studies sug-gest lithology to be a dominant parameter whichcontrols intensity of erosion and atmospheric CO2

consumption, followed by climatic parameters(runo¡ and temperature). The importance oflithology is emphasised in the studies of Meybeck[19] and Amiotte-Suchet and Probst [24] whereinit is pointed out that granite and gneiss are lesseasily erodible and hence consume less CO2 com-pared to the volcanic rocks. In recent years,weathering of basaltic rocks has been addressed

in several studies, in particular those of Louvat[21,25] on four volcanic islands: Iceland, Java,la Reunion and Sao Miguel, and that of Gislasonet al. [20] on Iceland. These authors have shownthat basaltic weathering is a major CO2 sink ontime scales of several million years because basal-tic rocks weather easily. Furthermore, Taylor andLasaga [26] have shown, from the modelling ofthe Columbia River basalt, that weathering oflarge igneous provinces can play a signi¢cantrole in the evolution of the marine Sr isotope rec-ord.

The aim of this study was to quantify the in£u-ence of the weathering of the Deccan Traps onthe chemistry of seawater and the atmosphericCO2 mass balance. With this purpose, we studiedriver geochemistry in the Deccan Traps, one ofthe largest continental £ood basalts on Earth,erupted around 65.5 Myr ago close to the timeof the Cretaceous^Tertiary boundary (KT bound-ary [27,28]). The present-day rates of erosion andatmospheric CO2 consumption are approximatedfrom the chemistry and the Sr isotope composi-tion of the main rivers draining the Deccan Traps.These results are used in a model presented in thisstudy to calculate the evolution of atmosphericCO2, global surface temperature, carbonate depo-sition on sea£oor and Sr isotopic composition ofseawater following Deccan Traps emplacement.To validate the model, the simulation of oceanic87Sr/86Sr evolution is compared with the actualmarine Sr isotope record around the KT bound-ary [29^31].

2. General setting of Deccan Traps

The Deccan volcanic province is located in the

EPSL 5829 18-5-01

C. Dessert et al. / Earth and Planetary Science Letters 188 (2001) 459^474460

western and central parts of India (Fig. 1). Thesetraps form a high plateau with an average eleva-tion of 750 m. Presently, western India is exposedto a monsoonal climate. The most humid period,between June and September, witnesses the majorpart of the annual rainfall, with a maximum inJuly (1150 mm). The temperature is relatively highwith an annual mean of 25³C and a maximum inMay (35³C).

K^Ar, 40Ar^39Ar and recent Re^Os geochro-nology coupled with palaeomagnetic and palaeon-tological studies suggest that the Deccan Trapswere erupted around the KT boundary 65.5Myr ago [27,28,32], when the Indian plate wasmoving northwards [33]. This volcanic event wasapparently quite short on a geological time scale,spanning less than 1 Myr.

The Deccan Traps are one of the largest basal-tic provinces on the Earth's surface. They have apresent-day volume of V106 km3 and cover anarea of V5U105 km2. Courtillot et al. [27] sug-gested that the total initial volume of lava mayhave reached V3U106 km3. Therefore, some twothirds of the initial basalt must have disappearedin the last 65 Myr. The thickness of the lava pilevaries from 200 m or less in the east to 1500^2000m in the west [27,34]. Javoy and Michard [35]estimated that the total amount of CO2 outgassed

during eruption was 1.6U1018 mol, which wouldrepresent half of the total ocean content.

The western Deccan Traps have been exten-sively studied in terms of geochemistry and strat-igraphy [34,36^38]. Basalts overlie crystallinebasement consisting largely of granites andgneisses, and sediments. The basalts are mainlyof tholeiitic composition and contain phenocrystsof plagioclase, clinopyroxene, altered olivine, opa-que minerals and altered glass. Stratigraphy isbased on the chemical and/or isotopic composi-tion of the £ows. The basaltic sequence is dividedinto ¢ve sub-horizontal distinct formations: frombase to top, the Bushe, Poladpur, Ambenali, Ma-habaleshwar and Panhala. The lowermost forma-tions have high Sr isotopic ratios, with all Busheformations having 87Sr/86Sr values higher than0.713 and all Poladpur formations having 87Sr/86Sr values in the range 0.705^0.713. The otherthree formations have 87Sr/86Sr values around0.705 [34,36]. Older sequences outcrop in thenorth, with a progressive overstepping, andyounger towards the south [34,38]. Cox and Haw-kesworth [36] have shown that observed chemicalvariations are due to heterogeneity of mantlesources, variations in the degree of crustal con-tamination and e¡ects of fractional crystallisation.These formations are separated in some instances

Fig. 1. Location of sampling points in the Deccan Traps volcanic province.

EPSL 5829 18-5-01

C. Dessert et al. / Earth and Planetary Science Letters 188 (2001) 459^474 461

by lateritic soil horizon and regional lateritic car-apaces are preserved in the coastal region [39].

3. Field and laboratory techniques

3.1. Sampling

Water and suspended and bottom sedimentswere collected during July 1998 in three signi¢cantwatersheds, namely the Godavari, Narmada andTapti (Fig. 1). The Godavari drainage area isdominated by basaltic rocks associated with crys-talline rocks of the Indian Shield, `Gondwana'sediments and Proterozoic sediments. The maintributaries are the Wardha (referring to no. 19in Fig. 1) and the Pench (no. 25). The Wainganga(no. 20) and the Kanhan (no. 21) do not £owthrough any basaltic lithology and the Ramakona(no. 23) partly drains basalts. The Narmada wassampled twice within an interval of 10 days (no. 2and no. 28). All Narmada sampled tributariesdrain basaltic rocks except the Silverfall (no. 26)and the Denwa (no. 27). The Tapti (no. 8 and no.12) drains only Deccan Traps and the Purna (no.9 and no. 18) is its main tributary.

3.2. Analyses

pH and alkalinity were measured in the ¢eldduring sampling. The alkalinity was determinedby acid^base titration (Gran method). Water sam-ples were ¢ltered on site through 0.2 Wm celluloseacetate ¢lters (142 mm diameter) using a Sartorius¢ltration unit made of Te£on. Filtered sampleswere acidi¢ed to pH 2 with HNO3 and stored inacid-washed polypropylene bottles. The samplesfor anion determination were not acidi¢ed.

Anions and cations were measured by ion chro-matography with a precision better than 5%.The accuracy of the analysis was assessed by run-ning the SLRS-3 and the SLRS-4 riverine stan-dards. Dissolved silica concentrations were deter-mined by spectrophotometric measurement. Traceelements were measured by ICP-MS after addi-tion of an indium standard solution. 87Sr/86Sr iso-topic ratios were determined using mass spec-trometry.

4. Results

Concentrations of major elements and Srisotopic compositions are listed in Table 1. Theriver waters are mostly alkaline with pH rangingfrom 7.54 to 8.15. Waters sampled in rivers £ow-ing through the basaltic terrains have high ioniccharge (4� V1839^6312 Weq/l) and those drainingpartly through sediments and crystalline rockshave lower ionic charge (153^2722 Weq/l). Watersare characterised by their speci¢c charge balance,as indicated by the normalised inorganic chargebalance (NICB = (4�343)/4�). Values are gener-ally close to 5%, except for the Ramakona(no. 23) and Kulbakera (no. 24), where the imbal-ance can be attributed to less precise alkalinitymeasurements in the ¢eld. In these cases, werecalculated the alkalinity from the charge bal-ance.

The dissolved silica concentration varies be-tween 187 and 836 Wmol/l for the rivers drainingbasalts, and between 100 and 314 for other rivers.Concentrations in rivers £owing through the Dec-can Traps are similar to those for Reunion Islandrivers (200^800 Wmol/l [21]). HCO3

3 (1148^5494Wmol/l) is always the dominant anion species, fol-lowed by Cl3 (53^1568 Wmol/l), SO23

4 (10^268Wmol/l), NO3

3 (4^141 Wmol/l), and F3 (1.8^21Wmol/l). The lower anionic concentration charac-terises rivers draining sediments and crystallinerocks. This observation is also true for cationiccontent. Among major cations, Na� is dominant(47^2794 Wmol/l). Ca2� (31^919 Wmol/l) and Mg2�

(17^1271 Wmol/l) have the same range of concen-trations whereas concentrations of K� range be-tween 7 and 163 Wmol/l. The sum of Na� and K�

represents around 28% of the total cation charge.The cation concentrations are lower than thosereported in a previous study on the Narmadaand Tapti rivers (e.g. Na� V450^7600 Wmol/l[40]). This di¡erence can be explained by thefact that sampling for the previous study was per-formed in May, when river discharge is lowercompared to that during the south west monsoonperiod.

The concentrations of Sr vary between 1.13 and3.51 Wmol/l for rivers draining basaltic lithologyand between 0.07 and 1.47 for the other rivers.

EPSL 5829 18-5-01


Tab

le1

Che

mic

alco

mpo

siti

onof

diss

olve

dlo

ad

Sam

ple

and

loca

tion

pHN

aN

H4

KM

gC

aF

Cl

NO

3SO

4H

CO

3Si

O2

4�

43

NIC

BSr

87Sr

/86Sr

Na*

K*

Mg*

Ca*

SO4*

Wm

ol/lW

mol

/lW

mol

/lW

mol

/lW

mol

/lW

mol

/lW

mol

/lW

mol

/lW

mol

/lW

mol

/lW

mol

/lW

eq/l

Weq

/l%

Wm

ol/l

Wm

ol/l

Wm

ol/lW

mol

/lW

mol

/lW

mol

/l

Kol

ar(1

)8.

371

1nd

6247

272

44.

415

518

4029

1236

631

6631

693

0.1

1.51

578

6045

772

132

Nar

mad

a(2

)8.

136

9nd

3736

566

94.

293

7726

2338

314

2474

2565

33.

71.

300.

7113

49þ

829

035

356

668

21G

anja

l(3

)7.

976

8nd

5044

085

93.

113

58

4432

4939

534

1434

833

2.0

1.68

0.70

9770

þ21

652

4742

685

637

Mac

hak

(4)

8.7

2077

nd52

1271

588

18.2

372

nd98

5494

552

5846

6079

34.

02.

120.

7094

35þ

1817

5845

1235

581

79K

alim

acha

k(5

)8.

810

92nd

9898

370

29.

211

6467

138

3039

836

4560

4556

0.1

3.11

9577

870

681

79K

hagn

i(6

)8.

129

31.

355

377

749

4.0

117

528

2394

332

2601

2576

1.0

1.21

193

5336

674

622

Cho

tata

wa

(7)

8.7

906

nd99

704

638

6.9

413

nd67

3182

465

3689

3737

31.

32.

130.

7081

98þ

2255

292

664

630

46T

apti

(8)

7.9

659

262

475

740

3.6

256

5463

2847

394

3152

3286

34.

31.

270.

7082

00þ

2143

958

450

735

50P

urna

(9)

813

2312

.611

331

651

56.

965

428

127

2167

247

3110

3109

0.0

2.01

762

101

253

503

93B

ori

(10)

8.3

1271

nd11

457

782

78.

851

413

109

3546

414

4192

4301

32.

62.

340.

7095

65þ

2283

010

452

781

883

Pan

jkra

(11)

7.8

2794

8412

673

591

97.

715

6911

426

842

0344

563

1264

523

2.2

3.51

0.70

9710

þ19

1449

9758

389

018

8T

apti

(12)

8.1

1517

nd93

483

649

5.8

623

4510

430

5536

038

7339

403

1.7

2.20

984

8242

263

872

Ara

naw

ati

(13)

8.1

871

nd50

401

710

4.7

263

2852

2773

396

3143

3173

31.

01.

1364

645

375

706

39A

ner

(14)

8.3

873

1.6

3870

381

54.

627

028

4036

8656

939

4940

683

3.0

1.50

0.70

7584

þ22

642

3367

781

026

Wag

urr

(15)

7.8

382

5.0

113

229

484

3.4

173

nd42

1555

242

1926

1815

5.8

1.20

234

110

212

481

33N

alga

nga

(16)

7.9

1139

37.6

163

190

457

4.9

457

132

8818

5818

726

3426

443

0.4

1.40

747

155

146

449

64M

un(1

7)7.

874

310

.098

249

476

3.5

351

141

7715

2520

123

0121

755.

51.

600.

7088

46þ

1844

292

215

470

59P

urna

(18)

7.9

785

10.3

8316

531

55.

030

018

7312

5616

518

3917

276.

11.

200.

7091

14þ

2352

878

136

309

58W

ardh

a(1

9)8.

251

2nd

7153

478

15.

818

64

3829

7144

132

1432

423

0.9

1.80

0.70

8387

þ17

353

6751

777

828

Wai

ngan

ga(2

0)7.

617

20.

943

160

364

3.0

64nd

1911

4817

112

6412

540.

80.

6011

742

154

363

16K

anha

n(2

1)7.

955

0nd

7430

970

111

.919

923

8721

4926

126

4425

573.

31.

4038

070

289

698

77Ja

m(2

2)8.

238

3nd

4439

085

215

.813

015

258

2197

348

2910

2874

1.2

1.50

0.71

1422

þ23

272

4237

784

925

1R

amak

ona

(23)

7.9

580

nd64

322

695

13.3

167

3414

015

6226

926

7820

5623

.21.

5043

761

306

692

131

Kul

bake

ra(2

4)8.

153

2nd

4935

187

421

.016

636

5321

5127

930

3124

8118

.11.

700.

7149

30þ

1939

046

335

871

45P

ench

(25)

838

3nd

4438

984

914

.213

027

268

2057

348

2904

2765

4.8

1.40

0.71

1439

þ17

272

4137

784

726

1Si

lver

fall

(26)

nm47

1.5

717

31nd

5311

10nm

100

153

830.

102

612

307

Den

wa

(27)

nm88

1.1

3156

138

1.8

6526

24nm

112

508

141

0.30

3129

4913

720

Nar

mad

a(2

8)7.

520

40.

938

197

474

3.0

67nd

2115

4721

715

8716

593

4.6

0.80

0.71

0973

þ19

146

3719

147

317

*In

dica

tes

conc

entr

atio

nco

rrec

ted

from

the

rain

inpu

t;nd

indi

cate

sco

ncen

trat

ion

belo

wde

tect

ion

limit

;nm

indi

cate

sco

ncen

trat

ions

not

mea

sure

d.

EPSL 5829 18-5-01


The rivers of the Deccan Traps have higher Srconcentrations than those of the basaltic Reunion,Sao Miguel and Iceland islands (0.026^0.5 Wmol/l,0.18^0.79 Wmol/l and 0.015^0.85 Wmol/l respec-tively [25]). However, the Sr concentrationsmeasured in this study are similar to those re-ported for Java island rivers (0.63^4.08 Wmol/l[25]). The 87Sr/86Sr isotopic ratios of the riversdraining Deccan Traps vary between 0.70758and 0.71493.

5. Chemical weathering rates and isotopicchemistry of Sr

5.1. Chemical weathering rates

To quantify the chemical weathering rates, thechemical compositions of river are corrected foratmospheric inputs. This correction was carriedout using Cl concentrations and oceanic (X)/(Cl)ratio (where X = Na, Mg, SO4, Ca or K) and as-suming that all the Cl content of the dissolvedload has an atmospheric origin. This hypothesis

is validated by the very low Cl concentrations ofbasaltic rocks [34,36]. Cl concentrations in theKalimachak and Panjkra rivers are very high(s 1100 Wmol/l, Table 1), suggesting an anthro-pogenic origin for chloride. Therefore, these riverswere not considered in this study. The correctedconcentrations of Na, Mg, SO4, Ca and K arelisted in Table 1.

The total dissolved solids (TDSw) have beencalculated from the concentrations of the majordissolved elements (Na, K, Ca, Mg, SiO2 andSO4) originating from the weathering of Deccanbasalts. TDSw values range from 46 to 136 mg/l,with a mean value of 81 mg/l (Table 2). From thedata of Dekov et al. [41], we have estimated amean annual runo¡ value (net £ow of water outof watershed) of 463 mm/yr for the Deccan Trapsregion. This value is similar to that for Narmada(452 mm/yr [40]). From mean runo¡ and TDSw

values, we have calculated the speci¢c chemicalweathering rates (rate per unit surface area) listedin Table 2. These rates range between 21 and63 t/km2/yr, with a mean value of 37 t/km2/yr.The annual average of chemical erosion £ux that

Table 2TDSw values (total dissolved solid after rain input corrections), chemical erosion rates and associated atmospheric CO2 consump-tion rates deduced from the dissolved load for the monolithologic rivers draining Deccan basalts

Sample and location TDSw Chemical weathering rate CO2 consumption ratemg/l t/km2/yr 106 mol/km2/yr

Kolar (1) 80 37 1.35Ganjal (3) 89 41 1.50Machak (4) 136 63 2.54Khagni (6) 67 31 1.11Chotatawa (7) 90 42 1.47Tapti (8) 81 37 1.32Purna (9) 65 30 1.00Bori (10) 96 44 1.64Tapti (12) 85 40 1.41Aranawati (13) 81 38 1.28Aner (14) 101 47 1.71Wagurr (15) 52 24 0.77Nalganga (16) 58 27 0.86Mun (17) 55 26 0.76Purna (18) 46 21 0.63Wardha (19) 83 39 1.38Jam (22) 96 44 1.02Kulbakera (24) 75 35 1.25Pench (25) 97 45 0.95

Mean 81 37 1.26

EPSL 5829 18-5-01


results from the weathering of the Deccan area isclose to 18.5U106 t/yr (by considering an area of5U105 km2 for these basalts). The speci¢c chem-ical erosion rates for Deccan Traps are higherthan those for most of the world's largest rivers(25 t/km2/yr for the Amazon and 6 t/km2/yr forthe Zaire [3]) or similar to those for several moun-tainous rivers (42 and 46 t/km2/yr respectively forthe Ganges and the Brahmaputra [3]). If Deccanrivers are compared with other small rivers drain-ing basalts, it appears that chemical weatheringrates for Deccan are similar to those for Sao Mi-guel (35 t/km2/yr [25]) and Iceland rivers (41 and55 t/km2/yr [20,25]). Rivers of Reunion and Javaislands exhibit higher chemical weathering rates(102 and 326 t/km2/yr [21,25]) because of theirhigher runo¡, temperature and relief.

Thus, based on the present study and on studiesof other basaltic rivers, we conclude that theweathering of the Deccan Traps and, more gen-erally, of basalts is of great importance to thetotal £ux of chemical elements washed to theocean.

5.2. Flux of strontium and 87Sr/86Sr isotopic ratios

The speci¢c £ux of strontium derived from theweathering of Deccan Traps ranges between 361and 1080 mol/km2/yr, with an average of 751 mol/km2/yr. The total £ux of strontium is equal to3.75U108 mol/yr and corresponds to 3.7% ofstrontium derived from silicate weathering [3]. Itis interesting to note that the Deccan area repre-sents only 0.5% of the total surface of continents.Thus, the £ux of strontium originated from theweathering of the Deccan basalts should not beneglected in the global balance of strontium to theocean.

The 87Sr/86Sr isotopic ratios measured in Dec-can rivers vary between 0.70758 and 0.71493. 87Sr/86Sr in the Kanhan (samples 21^25) is signi¢cantlyhigher (s 0.710) than that in the samples fromthe Tapti and the Narmada basins, where it variesslightly with many tributaries having 87Sr/86Srclose to 0.709. All these values re£ect the chemicalerosion of the basaltic rocks which have unusuallyhigh 87Sr/86Sr isotopic ratios ranging between0.705 and 0.715 [34,36]. Therefore, the 87Sr/86Sr

ratio of rivers draining Deccan Traps is substan-tially higher than those of volcanic islands studiedby Louvat (0.7035^0.7053 [25]).

6. Atmospheric CO2 consumption by chemicalweathering

The consumption rate of atmospheric CO2 as-sociated with the chemical weathering of basaltswas calculated from riverine HCO3

3 concentra-tions. During the weathering of silicate rocks, alldissolved bicarbonates originate from atmospher-ic/soil CO2. Values of CO2 consumption rateslisted in Table 2 are very high, ranging from0.58 to 2.54U106 mol/km2/yr with a mean valueof 1.26U106 mol/km2/yr. The annual averageCO2 consumption that results from the weather-ing of the Deccan Traps basalt province is closeto 0.63U1012 mol/yr. The amount of CO2 con-sumed by the weathering of Deccan basalts rep-resents 5% of total CO2 consumption £ux derivedfrom silicate weathering [3]. It must be highlightedthat this £ux is higher than the silicate alkalinity£ux determined for the Amazon (2.75% [3]), theGanges^Brahmaputra system (2.3% [17]) and theCongo (1.7% [3]).

We have plotted in Fig. 2 atmospheric CO2

consumption rates as a function of runo¡ forthe Deccan region and small rivers draining onlybasaltic or granitic lithology [19^21,23,25,42]. Itcan be seen that lithology, runo¡ and temperatureexert a major control on atmospheric CO2 con-sumption. It ¢rst appears that rivers draining ba-saltic rocks have higher HCO3

3 concentrationsthan those draining granitic rocks; HCO3

3 con-centrations of `basaltic rivers' vary between 400and 2700 Wmol/l, whereas those of `granitic rivers'vary between 40 and 500 Wmol/l. Note also that,for a given runo¡ value, CO2 consumption ratesduring basalt weathering are signi¢cantly higherthan those for granites. This ¢rst observation con-¢rms the important in£uence of the basaltic lith-ology on CO2 consumption rates as noted previ-ously by several authors [19^21,24].

Fig. 2 also highlights the in£uence of runo¡and temperature on CO2 consumption rates bybasalts. For a given temperature, bicarbonate

EPSL 5829 18-5-01


concentrations are constant and independent ofruno¡. Thus, CO2 consumption rates appear tobe approximately proportional to runo¡. Unlikeruno¡, HCO3

3 concentrations are directly depen-dent on temperature. Indeed, if we consider aconstant value of runo¡, the observed increaseof CO2 consumption rate re£ects an increase oftemperature. The temperature e¡ect can be moreeasily observed in Fig. 3 where, by analogy withexperimental studies on the kinetics of mineralweathering, the logarithm of HCO3

3 concentra-tions has been plotted versus the reciprocal ofabsolute temperature (1/T in ³K) for rivers drain-ing basaltic rocks. A good linear relationship isobserved between the two parameters. From theslope of this relation, a value of Ea = 42.3 kJ/molcan be deduced for the apparent activation energyof basalt weathering. The relationship betweenHCO3

3 concentrations and temperature (T) canbe de¢ned as:

logCHCO33� 3

EaUln10RT

� b �1�

where R represents the gas constant and b theintercept of the linear regression ( = 10.66). Thespeci¢c rate of CO2 consumption of basalts f(mol/km2/yr) can be expressed as:

f � RfUCHCO33

�2�

where Rf (mm/yr) represents the value of runo¡.Combining Eqs. 1 and 2 gives:

f � RfUC0exp3Ea

R1T3

1298

� �� 3�

where C0 = 10�b3Ea=298Rln10� = 1764 Wmol/l.This equation is not felt to be valid for the

weathering rate on granites. In fact, parametersother than runo¡ and temperature in£uence thechemical weathering of granites such as soil thick-ness and physical erosion intensity [13,15,22] ;hence, it is di¤cult to characterise weathering ofgranite with a simple relation as obtained for ba-salt.

Fig. 2. Plots of mean atmospheric CO2 consumption rates versus runo¡ for rivers draining basaltic or granitic lithologies. a:[25]; b: [42]; c: [19]; d: [20]; e: [23]. The values in parentheses correspond to the main surface temperature of each region.Dashed straight lines reported in this ¢gure represent constant HCO3

3 concentrations from 50 to 5000 Wmol/l.

EPSL 5829 18-5-01


7. E¡ect of Deccan Traps emplacement on climateand chemistry of oceans

A simple model is next generated to calculatethe evolution of atmospheric CO2 level, surfacetemperature and strontium isotopic compositionof seawater following Deccan Traps emplacement.Using the results inferred from the dissolved load(Section 5) of Deccan Traps rivers, this modellingwork allows us to quantify the impact of trapemplacement on global evolution.

7.1. Model description

The model used in this work is a modi¢ed andadapted version of those developed by Franc°oisand Walker [8] and Franc°ois et al. [43]. In thiswork, we model the geochemical cycles of inor-ganic carbon, alkalinity and strontium.

In the case of carbon, the weathering of carbo-nates (Fcw) is the largest source of inorganic car-bon. However, this £ux is not the primary sourcedriving the evolution of the system, since depar-tures from equilibrium in the weathering^deposi-tion cycle of carbonates remain limited even in thetransient experiments performed here. The secondsource of inorganic carbon is volcanic CO2 emis-sion. Let FD

vol be the CO2 emission from DeccanTraps and FR

vol the CO2 emission from the rest of

the world, including mid-ocean ridges. In themodel reference simulation, we assume that thetime span of Deccan Traps emplacement is 105

years [32]. The major sink of inorganic carbon iscarbonate deposition on the ocean £oor (Fcd).This £ux is calculated in the model, based onthe lysocline depth for calcite and aragonite, ina simple ocean sub-model which is outlined be-low. In the long term, the carbon budget in agiven reservoir is described by a di¡erential equa-tion of the form:

dCT

dt� FD

vol � FRvol � F cw3F cd �4�

where CT is the amount of carbon in the ocean^atmosphere system.

The major sources of alkalinity are the weath-ering of continental carbonates and silicates (Fcw

and Fsw). FDsw is the alkalinity £ux from Deccan

basalt weathering. As for carbon, the carbonatedeposition on the sea£oor (Fcd) is the major sinkof alkalinity. The long-term budget can be writtenas:

dAT

dt� 2�FD

sw � FRsw � F cw3F cd� �5�

where AT is the amount of alkalinity in the ocean.The source of ocean strontium is the weather-

ing of silicate and carbonate rocks and its majorsink is the carbonate precipitation. The strontiumbudget is hence determined by:

dSrT

dt� kD

swFDsw � kR

swFRsw � kcwF cw3kcdF cd �6�

where kDsw, kR

sw, kcw and kcd are proportionalityfactors used to transform carbon or alkalinity£uxes into strontium £uxes [44]. They are consid-ered constant and are obtained from the ratiosbetween strontium and carbon £uxes of today(Table 3).

The evolution of the 87Sr/86Sr isotopic ratio ofocean water over geologic time can be writtenas:

SrTdroc

dt� �rD

sw3roc�kDswFD

sw � �rRsw3roc�kR

swFRsw�

�rcw3roc�kcwF cw � �rsfw3roc�ksfwFRvol �7�

2.0

2.5

3.0

3.5

4.0

3.30

30 25 20 15 10 5 0

3.35 3.40 3.45 3.50 3.55 3.60 3.65

log

HC

O(µ

mol

/l)3

1/T 1000 (K )x -1

T (°C)

Ea = 42.3 kJ/mol

Fig. 3. Logarithm of HCO33 concentrations (in Wmol/l) versus

reciprocal of absolute temperature for rivers draining basalticrocks [19,20,23,25,42]. From the slope of the linear regres-sion, we estimate an activation energy of 42.3 kJ/mol.

EPSL 5829 18-5-01


ksfwFRvol is the exchange £ux of Sr during sea£oor

basalt alteration which is considered to be propor-tional to the non-Deccan volcanic CO2 £ux FR

vol.roc is the strontium isotopic ratio of the ocean [45]and rD

sw, rRsw, rcw and rsfw are the mean strontium

isotopic ratios originating from Deccan basalts,silicate weathering, carbonate weathering and sea-£oor basalt alteration [46]. Note that carbonatedeposition is not taken into account in the iso-topic budget (Eq. 7) because it is assumed thatno fractionation occurs during precipitation. Thediagenetic £ux is small enough that it can beignored with no signi¢cant lack of accuracy inEqs. 6 and 7. The values of some present-daymodel parameters are given in Table 3 with rele-vant references. The riverine 87Sr/86Sr ratio com-ing from Deccan Traps weathering (rD

sw) used inthe model is the present-day ratio. Because of thelow ratio Rb/Sr of Deccan basalts [34,36], thechronological correction is insigni¢cant and wecan suppose that the riverine 87Sr/86Sr ratio com-ing from Deccan Traps weathering has notsigni¢cantly changed over the last 65 Myr. Theoceanic 87Sr/86Sr ratio before Deccan Trapsemplacement was determined from the actual ma-rine Sr isotope record 65 Myr ago [45] and isequal to 0.7078. From this value and the

present-day £uxes of strontium, we have calcu-lated the pre-Deccan 87Sr/86Sr ratio of globalriverine £ux (0.7103) and riverine £ux comingfrom silicate weathering excluding Deccan(rR

sw = 0.7115).Over geologic time, weathering £uxes are af-

fected by various factors such as continental sur-face (A), global average runo¡ per unit area (Rf ),surface temperature (T) and atmospheric CO2

concentration (pCO2). In this model, we use theBLAG relations [2] that link Rf to T and pCO2 toT :

Rf

Rf0� 1� 0:038U�T3T0� �8�

T � T0 � 2:88UlnpCO2

pCO2;0

� ��9�

where the 0 subscript refers to present-day values(280 ppmv and 288 K respectively for pre-indus-trial pCO2 and T).

In the case of silicate rocks, weathering £uxesare proportional to runo¡, temperature and con-tinental area. The global £ux (Fsw) is the sum ofthe Deccan Traps contribution and that of therest of the world. Each contribution is described

Table 3Main parameters of the geochemical model

Parameter Value Reference

CO2 consumption £ux by carbonate weathering 12.3U1012 mol/yr [3]CO2 consumption by silicate weathering 11.7U1012 mol/yr [3]CO2 release from Deccan Traps pulses 1.6U1018 mol [35]CO2 consumption by weathering of Deccan basalts 0.63U1012 mol/yr This studyRiverine £ux of Sr 28.4U109 mol/yr Calibrated from CO2 £ux and

k valueskcw 1/1250 [44]kR

sw 1/305 [44]kD

sw 1/840 This studyksfw 1/140 [44]Oceanic 87Sr/86Sr ratio before Deccan Traps emplacement, roc 0.7078 [45]Global riverine 87Sr/86Sr ratio before Deccan Traps emplacement 0.7103 Calibrated to obtain an oceanic

87Sr/86Sr of 0.7078Riverine 87Sr/86Sr ratio coming from carbonate weathering, rcw 0.7080 [46]Riverine 87Sr/86Sr ratio coming from Deccan Traps weathering, rD

sw 0.7097 This studyRiverine 87Sr/86Sr ratio coming from silicate weathering, rR

sw(excluding Deccan)

0.7115 Calibrated to obtain a riverine87Sr/86Sr of 0.7103

87Sr/86Sr ratio coming from sea£oor basalt weathering, rsfw 0.7030 [45]

EPSL 5829 18-5-01


by the following equation:

F isw

F isw0

� AA0

URf

Rf0Uexp 3

Eai

R1T3

1T0

� �� 10�

with i = D or R.A/A0 is the area (Deccan Traps or rest of the

world) normalised to today's value. R is the gasconstant and Ea is the activation energy of themineral dissolution reaction. An apparent energyof 42.5 kJ/mol and 40 kJ/mol is used for Deccanbasalt dissolution (determined in Section 5) anddissolution of silicates from the rest of the world(unpublished data of Oliva), respectively. Beforethe emplacement of Deccan basalts, we supposethat this area was constituted by silicates as in therest of the world (with Ea = 40 kJ/mol) and thatthe system was at steady state. To calculate car-bonate weathering, it is assumed that streamwater draining carbonate lithologies reaches equi-librium with calcite and an e¡ective pCO2 value(pCOeff

2 ), which is a harmonic mean between soiland atmospheric pCO2. Soil pCO2 is calculatedaccording to Volk's [47] study with a present val-ue 100 times higher than the atmospheric value.

The ocean sub-model allows us to estimate at-mospheric pCO2 and Fcd values for all CT and AT

combinations determined at each time step of thenumeric resolution (Eqs. 4 and 5). This sub-modelcontains three surface pools: the atmosphere, thesurface ocean and the deep ocean, and distributesinstantaneously CT and AT between these threeboxes. As these boxes are assumed to be in equi-librium with each other, the model cannot be usedto analyse the evolution of the system at timescales shorter than 1000 years (oceanic circula-tion). Carbonate speciation in each oceanic reser-voir is taken into account to calculate atmospher-ic pCO2 in equilibrium with the surface ocean andto determine the lysocline depth for aragonite andcalcite from the calculated CO23

3 concentrations.Carbon is transferred between the surface and thedeep ocean reservoir through advective exchange(ocean circulation) and sedimentation of biologi-cal particles. The biological new production isconstant in all of the simulations. The £ux ofcarbonate deposition Fcd is composed of the ara-gonite and carbonate tests deposited above their

respective lysocline (i.e. these £uxes are propor-tional to the fraction of the sea£oor area locatedabove the aragonite or calcite lysocline), as well asof coral reef formation on the equatorial shelf.

7.2. Results

7.2.1. Reference simulationThe reference run combines all the e¡ects pre-

sented previously. The calculated evolutions ofsurface temperature (³C), atmospheric pCO2

(ppmv), carbonate deposition on sea£oor (1013

mol/yr), surface ocean pH and 87Sr/86Sr ratio ofseawater during the emplacement of the traps areshown in Figs. 4^6.

The variation of atmospheric pCO2 is relativelyimportant. In fact, pCO2 increases by 1050 ppmvfrom its pre-Deccan steady-state value. It thenprogressively decreases to reach a new (post-Dec-

Fig. 4. The evolution of atmospheric CO2 partial pressure(ppmv) and surface temperature (³C) calculated from the dif-ferent model runs described in the text.

EPSL 5829 18-5-01


can) steady state after a few million years. Notethat only 1.2 Myr is necessary to return to thepre-Deccan value. After 1.2 Myr, pCO2 continuesto decrease and the new steady-state pCO2 is low-er than the pre-Deccan one by 57 ppmv, as aresult of the very e¤cient weathering of the Dec-can basalts. Thus, the emplacement of DeccanTraps appears to be responsible for a 20% reduc-tion of atmospheric pCO2 (with respect to pre-industrial pCO2). This decrease is accompaniedby a global cooling of 0.55³C.

The variation of carbonate deposition is greaterthan in the case of atmospheric pCO2 and surfacetemperature (Fig. 5). Indeed, this £ux ¢rst de-creases by 0.87U1013 mol/yr (345%) from itspre-Deccan value in only 20 000 years, as a resultof a rapid rise of the lysocline. This is followed bya rapid rise in carbonate deposition of 2.05U1013

mol/yr, resulting in a strong increase of continen-tal erosion. Finally, it progressively decreases to

reach steady state after 1.4 Myr. On the otherhand, it can be seen that the surface ocean pHdecreases rapidly from 8.13 to 7.80 in 0.1 Myr.Note that this decrease is more rapid during the¢rst 20 000 years. After 0.1 Myr, the time corre-sponding to the end of Deccan emplacement, thepH progressively increases to reach a steady-statevalue of 8.17. These two parameters demonstratethe immediate response of ocean and continentsto regulate the strong input of CO2 in the atmos-phere.

After the beginning of Deccan eruption, the87Sr/86Sr ratio of seawater increased from itspre-Deccan value of 0.70782 to 0.70789, beforecoming down to 0.70777 (Fig. 6). This spike ischaracterised by a duration of some 4 Myr anda peak amplitude of about 7U1035. The transientpeak in the Sr signal is related to the strong in-crease of continental weathering during and afterthe eruption. As pCO2 decreases, continentalweathering rates are reduced leading to a decreasein oceanic 87Sr/86Sr ratio. Furthermore, as rD

sw islower than rR

sw, the weathering of Deccan Trapstends to reduce the global isotopic ratio of stron-tium originating from silicate weathering.

Note that the Deccan Traps cover 0.5U106

km2, but their original area was three or fourtimes larger. In this respect, the impact of thesetraps is underestimated.

7.2.2. Test simulationsIn order to explore the contribution of various

parameters, several sensitivity tests were per-formed such as the duration of the emplacement,the pre-Deccan atmospheric pCO2, the contribu-tion of the erosion of continental carbonates andthe dependence of silicate weathering on atmos-pheric pCO2. As most of the results of the simu-lations are relatively close to that of the referenceone, only the two more signi¢cant ones are pre-sented in this study (Figs. 4^6).

The ¢rst important sensitivity test (run 1) wasperformed to analyse the impact of the durationof emplacement of Deccan Traps. In this test, the1.6U1018 mol of CO2 were released to the atmos-phere in 1 Myr instead of 105 years in the refer-ence run. The characteristic peak of pCO2 duringDeccan Traps emplacement is strongly reduced in

Fig. 5. The evolution of the carbonate deposition £ux in theocean (1013 mol/yr) and surface ocean pH calculated fromthe di¡erent model runs.

EPSL 5829 18-5-01


this simulation, pCO2 increasing by only 317ppmv. This smaller variation reduces the impacton global temperature. The return to the post-Deccan steady-state value is longer (1.7 Myr in-stead of 1.2 Myr) and the spike of carbonate dep-osition is weak. Indeed, after a small decrease of5%, carbon deposition £ux progressively increasesby 0.6U1013 mol/yr, before returning to steadystate, about 2 Myr after the beginning of Deccaneruption. Similarly, in comparison with the refer-ence simulation we do not observe any importantvariation of surface ocean pH. A delay is alsoobserved in the Sr signal, where the peak appears1.4 Myr after the beginning of the eruption. Thissimulation illustrates the role of duration of Dec-can Traps emplacement at short time scales.

In the second sensitivity test (run 2), the pre-Deccan atmospheric pCO2 is assumed higher thanin the reference one. To get high CO2 levels, theCO2 release from pre-Deccan volcanism (FR

vol) was

increased by a factor of 1.5. This larger volcanic£ux resulted in a pre-Deccan atmospheric CO2 of1435 ppmv and a global surface temperature of292.7 K. Under such high levels of CO2, the evo-lution of pCO2 following Deccan Traps emplace-ment is perceptibly di¡erent. Atmospheric pCO2

increases by 1792 ppmv after emplacement andreturns to a new steady-state pCO2 lower thanthat in pre-Deccan times by 260 ppmv. On theother hand, the increase of pre-Deccan pCO2

has a smaller e¡ect on the variation of globaltemperature. Indeed, the warming related to theCO2 degassing phase is smaller than the referenceone (2.3³C instead of 4.1³C) and the global cool-ing is unchanged. We note that, as a result ofhigher volcanic pCO2 £uxes, the carbonate depo-sition £ux is higher than the reference one. ThepH is relatively low (as expected under highpCO2) and its variation is smaller. The increaseof pre-Deccan pCO2 also has an e¡ect on the

Fig. 6. The evolution of the seawater 87Sr/86Sr ratio calculated from the di¡erent model runs. By comparison, the data pointsrepresent the marine Sr isotope record taken from Martin and Macdougall [31] around the KT boundary. *: absolute age basedon Martin and Macdougall was recalibrated from 65 Ma (KT boundary).

EPSL 5829 18-5-01


marine Sr record since the spike is characterisedby an amplitude (peak at 0.70788) and a duration(3 Myr) smaller than the reference one. This sim-ulation illustrates that, even if the increase of pre-Deccan pCO2 has a global e¡ect on silicate weath-ering and the Sr cycle, the impact of DeccanTraps emplacement is reduced.

7.2.3. Comparison between results of model andmarine sediment records

We have shown that Deccan Traps emplace-ment had multiple e¡ects on the global environ-ment. We propose that this event caused a sharpdecrease in sedimentation (345% in only 20 kyr),induced by a strong change in lysocline depth.This disturbance and decrease of carbonate de-position has also been observed at the KT bound-ary by several authors including Smit and Romein[48] and Kaiho et al. [49] who inferred a 13 kyrinterval of productivity shutdown. The simula-tions involve a rapid increase of seawater 87Sr/86Sr ratio of 8U1035 with a duration of 4 Myr;therefore the emplacement and weathering ofthese continental £ood basalts may explain thevery abrupt KT boundary spike in 87Sr/87Sr [29^31].

In order to validate the model, we compare oursimulations of oceanic 87Sr/86Sr evolution with theactual marine Sr isotope record around the KTboundary of Martin and Macdougall [31] in Fig.6. To reduce variability due to chronological un-certainty, sediment mixing, analytical uncertaintyand diagenesis, the authors have smoothed andreplotted the data. In this study the KT boundaryis assimilated to the spike in 87Sr/86Sr at 66.4 Ma.We recalibrated the age of this boundary at 65Ma, its well known age. The data of Martin andMacdougall show a spike with a peak value ofabout 0.7079, a maximum amplitude of order7U1035 and a maximum duration of 3 Myrthat is relatively similar to the evolution proposedin our model. This study allows us to suggest anorigin for the Sr isotope anomaly at the KTboundary.

The results of the model show that a number ofgeochemical characteristics of the KT boundarycan be explained by Deccan Traps emplace-ment.

8. Conclusion

This study presents a coupled geochemical in-vestigation of major and trace elements of themain rivers draining the Deccan Traps provinceas well as a simple model making it possible toquantify the impact of Deccan Traps emplace-ment.

b The chemical composition of the dissolved loadin the main rivers draining Deccan Traps ba-salts allows us to calculate chemical weatheringrates (21^63 t/km2/yr) and associated atmos-pheric CO2 consumption rates (0.58^2.54 106

mol C/km2/yr). The results obtained in thisstudy con¢rm that denudation rates of basalticrocks are much higher than those for other sil-icate rocks. Furthermore, it is suggested thatruno¡ and temperature are the most importantparameters which control the chemical weath-ering and CO2 consumption rates, which can berather accurately described by a simple expo-nential law:

f � RfUC0exp3Ea

R1T3

1298

� ��

where f is the speci¢c CO2 consumption rate(mol/km2/yr), Rf is runo¡ (mm/yr), C0 = 1764Wmol/l, Ea represents an apparent activationenergy for basalt weathering (42.3 kJ/mol), Ris the gas constant and T is the absolute tem-perature (³K).

b Our model allows one to quantify the multiplee¡ects of Deccan Traps emplacement on theglobal environment. This emplacement is re-sponsible for a 20% reduction of atmosphericpCO2, accompanied by a global cooling of0.55³C, and has therefore produced a net CO2

sink on geologic time scales. A peak in the 87Sr/86Sr isotopic ratio in the oceans following Dec-can Traps emplacement is also predicted by themodel and indeed allows one to understand theorigin of the Sr isotope anomaly at the KTboundary.

b Overall, this study emphasises the major impactof basalt weathering on geochemical cycles suchas the carbon cycle. It shows that the emplace-

EPSL 5829 18-5-01


ment and the weathering of large basaltic prov-inces cannot be neglected when attempting tobetter understand the geochemical and climaticevolution of the Earth.

Acknowledgements

This work was supported by the French pro-gramme PROSE (Programme Recherche Sol Ero-sion) funded by INSU. We also thank the ForeignO¤ce and the French Ministry of Education, Re-search and Technology for funding and providingpost-doctoral positions for G.J.C. and S.B.L.M.F. was supported by the FNRS (Belgian Na-tional Foundation for Scienti¢c Research) andobtained from a visiting research position fromCNRS. We would like to thank V. Subramanianand the technicians of the School of Environmen-tal Sciences, New Delhi, as well as N. Vigier andT. Allard for assistance in obtaining river sam-ples. We are also grateful to P. Brunet, J. Escalier,P. Oliva, B. Reynier, M. Valladon of the Tou-louse laboratory and to F. Capmas, C. Gorgeand M. Pierre of IPGP for their support in thedi¡erent analyses. L. Turpin, N. Vigier, J. Viersand P. Oliva are thanked for their helpful com-ments. We thank V. Courtillot and C. France-Lanord for their very constructive reviews ofthis manuscript.[AC]

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