Sr and 87Sr/86Sr in rivers draining the Deccan Traps (India): Implications to weathering, Sr fluxes,...

Sr and 87Sr/86Sr in rivers draining the Deccan Traps (India):Implications to weathering, Sr fluxes, and the marine87Sr/86Sr record around K/T

Anirban DasPlanetary and Geosciences Division, Physical Research Laboratory, Ahmedabad-380 009, India

Temporarily at Department of Earth Sciences, University of Ottawa, Ottawa, Ontario, Canada

S. KrishnaswamiPlanetary and Geosciences Division, Physical Research Laboratory, Ahmedabad-380 009, India ([email protected])

Anil KumarNational Geophysical Research Laboratory, Uppal Road, Hyderabad-500 007, India

[1] The concentration of dissolved Sr and its 87Sr/86Sr has been measured in the headwaters of the KrishnaRiver System and west flowing Western Ghat rivers, all of which have their drainage almost entirely in theDeccan Traps. The Sr concentration follows that of Ca and Mg with Sr/Ca and Sr/Mg ratios similar to thatof Deccan basalts, suggesting that all these alkaline earths are released to waters nearly congruently fromthe Deccan basalts during chemical weathering. The 87Sr/86Sr range from 0.70614 to 0.70986, within thatreported for the Deccan basalts. The dissolved Sr flux from the Deccan calculated from the measured datais �1.35 � 108 moles yr�1, �0.4% of the global riverine flux, which is nearly the same as the proportionof area covered by Deccan basalts relative to the global drainage. The flux, however, is a factor of �3lower than that reported for the Narmada-Tapti-Wainganga (NTW) rivers draining the northern Deccan.This difference in Sr flux among rivers draining the various regions of Deccan could be natural spatial/temporal variations and/or due to supply of Sr to NTW rivers from nonbasaltic sources. Model calculationson the role of emplacement and weathering of Deccan on marine Sr isotope evolution around KTB and lateTertiary show that Deccan basalts can be an important contributor to the decline in 87Sr/86Sr during theseperiods. It is shown that to account for the pre-KTB dip in 87Sr/86Sr, the flux requirement from Deccan atthat time would have to be several times the contemporary flux with 87Sr/86Sr of 0.705–0.708.Considering that the area of Deccan at KTB was about thrice the present area and that the weathering rateof Deccan basalts may have been much higher in early stages following eruption, such high fluxes at aboutKTB seem feasible. The disproportionately higher flux requirement for Sr is similar to that invoked toexplain the pre-KTB decrease in 187Os/188Os. The calculations also show that the supply of unradiogenicSr from basalts can have significant control on the long-term, �66 to �55 Ma, decline of marine 87Sr/86Sr.

Components: 9056 words, 5 figures, 4 tables.

Keywords: Deccan basalts; geochemistry; strontium.

Index Terms: 1039 Geochemistry: Alteration and weathering processes (3617).

Received 19 July 2005; Revised 25 January 2006; Accepted 20 March 2006; Published 15 June 2006.

Das, A., S. Krishnaswami, and A. Kumar (2006), Sr and 87Sr/86Sr in rivers draining the Deccan Traps (India): Implications to

weathering, Sr fluxes, and the marine 87Sr/86Sr record around K/T, Geochem. Geophys. Geosyst., 7, Q06014, doi:10.1029/

2005GC001081.

G3G3GeochemistryGeophysics

Geosystems

Published by AGU and the Geochemical Society

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

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Volume 7, Number 6

15 June 2006

Q06014, doi:10.1029/2005GC001081

ISSN: 1525-2027

Copyright 2006 by the American Geophysical Union 1 of 14

1. Introduction

[2] The dissolved Sr concentration in rivers isderived primarily from weathering of silicates andcarbonates of their drainage basin. The 87Sr/86Sr ofrivers depend on the relative contributions of Srfrom these sources and their 87Sr/86Sr ratios. Marinecarbonate rocks formed several hundreds of millionyears ago have 87Sr/86Sr of �0.707 to �0.709[Palmer and Edmond, 1989; Veizer, 1989]. Excep-tions to this general trend are some of the metamor-phosed and highly radiogenic carbonates reportedfrom the Himalaya [Blum et al., 1998; Singh et al.,1998; Bickle et al., 2001]. The average 87Sr/86Sr ofcrustal silicate rocks is more radiogenic than typicalcarbonates, their isotopic composition, however,has large variability depending on their age andRb content. For example, granites, gneisses andshales of the Himalaya have 87Sr/86Sr ratios rangingfrom �0.71–1.0 [Krishnaswami et al., 1992, 1999;Jacobson et al., 2002]. In contrast, uncontaminatedbasalts, e.g., from the Deccan Traps, have 87Sr/86Srof�0.704 [Mahoney, 1988]. Rivers draining basaltstherefore will generally have 87Sr/86Sr poor inradiogenic 87Sr. Thus, analogous to carbonates, theeffect of mixing Sr from basalts with that from othercrustal silicates would be to bring down the average87Sr/86Sr of global rivers. This coupled with therapid weatherability of basalts and their significantaerial coverage [Dessert et al., 2001, 2003; Das etal., 2005a] makes it important to study their role ininfluencing the Sr flux from global rivers and itsisotopic composition.

[3] Considering this, a systematic study to measurethe Sr concentration and its 87Sr/86Sr in dissolvedphase of selected rivers draining the Deccan Traps,a major global basaltic province, was undertaken.The primary goals of this work are (1) to determinethe dissolved concentration of Sr and its 87Sr/86Srratio in several rivers of the Deccan Traps. Theriver studied are the headwaters of the Krishnasystem and selected rivers of the Western Ghats(WFWG rivers) flowing into the Arabian Sea, (2)to assess the mobility of Sr from basalts to riversrelative to Ca, Mg and

PCations, and (3) to

evaluate the role of Deccan basalts on the globalriverine flux of Sr. The impact of the Deccaneruptions and their weathering in contributing tomarine Sr isotope excursions at �66 Ma, coincid-ing with the major pulse of its eruption and theKTB, is also a topic of interest [Martin andMacdougall, 1991; Vonhof and Smit, 1997; Dessertet al., 2001]. This study also makes efforts toconstrain the role of Deccan basalts in defining

the marine 87Sr/86Sr record at that time and itssubsequent evolution over the next �10 Myr.

[4] Prior to this work, the Narmada, the Tapti andthe Godavari rivers, draining the northern DeccanTraps have been studied for their Sr concentrationand isotopic composition [Trivedi et al., 1995;Dessert et al., 2001]. Major ion abundances indissolved phase of these rivers are distinctly dif-ferent from those in the Krishna system and theWFWG rivers [Das et al., 2005a], suggesting thepossibility of spatial variations in Sr concentrationamong rivers draining different regions of theDeccan Traps. The measurements of Sr and its87Sr/86Sr in rivers from the south-central andwestern Deccan carried out in this study allowthe assessment of such variability and their poten-tial causes. Further, data on Sr concentration and its87Sr/86Sr from rivers draining different regions ofDeccan Traps would provide a more representativeestimate of its flux from the Deccan and its globalsignificance.

2. Geohydrology

[5] The rivers sampled for this study are the head-waters of the Krishna river system and a fewsmaller rivers draining the Western Ghats of theDeccan into the Arabian Sea (Figure 1). The head-waters of the Krishna system include the main-stream Krishna and many of its tributaries andsubtributaries all of which have their basins almostentirely in the Deccan Traps. Among the riversstudied, the Ambika is the northern most, close tothe Tapti studied by Trivedi et al. [1995] andDessert et al. [2001]. The geology, mineralogyand petrology of the Deccan Traps are discussedin detail by Mahoney [1988] and Sen [2001]. Thegeo-hydrology of the Krishna headwaters and thewest flowing Western Ghat rivers is discussed byDas et al. [2005a]. Briefly, the Deccan Traps arelocated in the west-central part of India, presentlyoccupying an area of �0.5 � 106 km2 (Figure 1)with an average thickness of �750 m. The lithol-ogy of the drainage basins for all the riverssampled is tholeiitic basalts, which consist mainlyof plagioclase, augite and olivines [Sen, 2001]. Inaddition, minor abundances of calcites in basalts[Sukeshwala et al., 1972], calcareous tufas in theupland Bhima and the Krishna basins and calciumcarbonate in river sediments are also reported[Pawar et al., 1988; Das et al., 2005a, 2005b].Black colored vertisols dominate the soils of theregion with abundant laterites in the western coastaldistricts and in the Koyna river basin [Widdowson

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and Cox, 1996; Widdowson and Gunnell, 2001].Further, occurrences of saline/alkaline soils in thebasins of the Bhima and its tributaries have alsobeen reported [Bhargava and Bhattacharjee, 1982;National Remote Sensing Agency, 1998]. Monsoon

rainfall is the source of water for these rivers; as aresult about 80% of the annual river dischargeoccurs in the monsoon season (June–September).The runoff is much higher for the rivers flowingwest of the Western Ghats (�1685 mm yr�1),compared to those draining the rain shadow regionsof the eastern side, �463 mm yr�1 [Das et al.,2005a]. The runoff compares with values of�2500 mm yr�1 and �700 mm yr�1 for averagerainfall in these two regions.

3. Sampling and Analyses

[6] The details of samples and the various proper-ties measured in the waters are given by Das et al.[2005a]; some of these (sampling locations, date ofcollection) are listed in Table 1. As mentionedearlier, rivers sampled have their drainage basinswithin the Deccan Traps (Figure 1) and hence themajor source of solutes is chemical weathering ofbasalts. Sampling was carried out during the mon-soon of 2001 and 2002; however, Sr concentrationand isotope measurements were made only insamples of August 2002. Sampling during mon-soon not only allows collection of water fromseasonal rivers, but also minimizes the impact, ifany, of groundwater and anthropogenic inputs torivers. The water samples were collected from themid-stream of the rivers to avoid local heterogene-ity and possible ‘‘near-bank’’ human influences.For Sr isotopes and trace element analysis, �1 Lsamples were collected in precleaned high-densitypolyethylene bottles. The bottles were cleaned inacid, distilled water and were thoroughly rinsedwith the ambient river water. The water sampleswere filtered through 0.4 mm Nucleopore filters atsite and an aliquot (�500 ml) was acidified withhigh purity HNO3 to pH �2 for Sr concentrationand isotope measurements.

[7] Sr concentrations in rivers were measuredusing ICP-AES coupled to an ultrasonic nebulizer[Dalai and Sarin, 2002]. Calibration of the instru-ment was done with standard solutions madefrom analytical grade Sr(NO3)2. The accuracy ofmeasurements was checked by analyzing commer-cial standards (23 element Merck1) and also withsolutions of USGS reference rock standard (G-2).On the basis of replicate analyses, the precision forSr measurements is calculated to be ±4%.

[8] For Sr isotope measurements, about 50 to150 ml of filtered–acidified water was taken inthoroughly cleaned FEP beakers and evaporated tonear dryness. The volume of water used for Sr

Figure 1. (top) Present-day aerial coverage of Deccanbasalts and major rivers draining it. (bottom) Thesampling locations (stars) of this study. The sampleswere collected in 2002. The Western Ghat is shown by aband parallel to the west coast of India. The Krishnaoriginates near Wai town. (Modified from Das et al.[2005a, Figure 1]; copyright 2005 Elsevier Ltd., withpermission from Elsevier.)



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isotope analysis was decided on the basis of theknown Sr concentration in water such that at least afew mg of Sr was available for mass spectrometry.Sr was separated from the dried residue using ionexchange chromatography. The purified Sr fractionwas dried and loaded with H3PO4 on degassed highpurity Ta filaments. Sr isotopes were measuredusing a VG 354 mass spectrometer in a singlecollection mode, at the National GeophysicalResearch Institute, Hyderabad. During the periodof measurements, Sr standard (SRM 987) was runrepeatedly to check for accuracy and reproducibil-ity. The results yielded a value of 0.710226 ±

0.000043 (2s, n = 6). The average proceduralblank for Sr was 2.4 ± 1.8 ng (n = 5).

4. Results and Discussion

4.1. Geochemistry of Sr in the Basin

[9] Sr concentrations and the 87Sr/86Sr isotoperatios of river waters analyzed are given in Table 1.Also presented in this table are Si, HCO3, Ca* andMg* concentrations in the dissolved phase (wherethe asterisk indicates concentration after correctionfor atmospheric inputs [Das et al., 2005a]). The

Table 1. Ca, Mg, Na, and Si (mM), Sr (nM), and 87Sr/86Sr in Rivers Along With the Sampling Locations and Date ofSamplinga

Code River Lat, �N Long., �E Date (D/M/Y) Ca* Mg* HCO3 Si 87Sr/86Sr Sr

Krishna MainstreamKRS-1 Krishna 16.79 74.63 28/08/02 419 255 1468 334 0.70686 592KRS-2 do 16.65 74.64 28/08/02 294 191 862 291 0.70706 567KRS-3 do 17.3 74.19 29/08/02 455 280 1653 350 0.7069 705KRS-4b do 17.4 74.11 29/08/02 610 360 2226 419 0.70682 798KRS-5 do 16.8 74.57 28/08/02 355 232 1326 327 0.70683 679KRS-6b do 17.94 73.9 3/9/02 868 563 3252 520 0.7069 1394

Bhima SystemBHM-1 Bhima 19.03 73.63 24/08/02 147 95 585 309 0.70822 378BHM-3b do 18.57 74.38 25/08/02 733 464 2464 396 0.70977 1029BHM-4b do 18.4 74.57 26/08/02 787 532 2499 390 0.70969 1240

Tributaries of BhimaGHOD-1b Ghod 19.08 73.77 24/08/02 575 325 1999 418 0.70858 907MTH-1b Mutha 18.57 74.34 25/08/02 736 449 2501 398 0.70986 930

Tributaries of KrishnaKYN-1 Koyna 17.27 74.18 29/08/02 274 186 1111 353 0.70692 499KYN-2 Koyna 17.93 73.61 3/9/02 46 44 300 179 0.70698 164VRN-1 Varna 16.87 74.36 29/08/02 262 170 990 292 0.70695 352PGN-1 Panchganga 16.69 74.6 28/08/02 173 110 611 238 0.70718 368PGN-2 Panchganga 16.76 74.26 31/08/02 177 123 680 257 0.70713 346GTP-1 Ghataprabha 16.01 74.27 1/9/02 39 37 212 126 0.70842 149HRN-1 Hiranyakeshi 16.36 74.15 31/08/02 36 39 216 128 0.70819 134TPN-1 Tambrapani 15.92 74.29 1/9/02 72 59 319 166 0.70842 194DDG-1 Doodhganga 16.43 74.14 31/08/02 254 153 835 268 0.70733 322VDG-1 Vedganga 16.36 74.15 31/08/02 137 95 562 247 0.70751 301BGW-1 Bhogwati 16.5 74.05 31/08/02 158 113 603 271 0.70717 360

West Flowing RiversARJ-1 Arjuna 16.65 74.02 2/9/02 144 116 633 366 0.70683 206GAD-1 Gad 16.26 73.72 1/9/02 130 99 604 323 0.70984 210SUKH-1 Sukh 16.56 73.63 1/9/02 140 115 624 332 0.70687 204KJL-1 Kajli 16.93 73.51 2/9/02 139 113 633 353 0.70614 251SHT-1 Shastri 17.18 73.55 2/9/02 158 112 678 386 0.70652 281VAT-1 Vashisthi 17.53 73.54 2/9/02 174 127 725 391 0.70711 225

OthersAMB-1 Ambika nm nm 4/9/02 504 335 1876 685 0.70786 493

aCa, Mg, Na, and Si values are in mM; Sr values are in nM. Typical errors (±2s) in 87Sr/86Sr is 0.00004. The asterisk in Ca* and Mg* indicates

concentrations corrected for rain inputs. For Sr a rain input of 30 nM is assumed. nm, not measured.bThe data were not used for statistical analyses, as they do not meet selection criteria (see text for details).



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geochemistry of major elements and their implica-tions to chemical and silicate weathering in theKrishna basin was presented by Das et al. [2005a].

[10] Dissolved Sr in the samples varies from�130 nM to �1390 nM (average �490 nM) with87Sr/86Sr ratio from 0.70614 to 0.70986, about twothirds of the samples have ratios in the narrowrange of 0.706–0.708 (Table 1). Samples with thehigher 87Sr/86Sr, 0.708–0.710, mainly belong tothe Bhima, Ghod and the Mutha rivers. The rangein Sr concentration and 87Sr/86Sr are lower than thevalues of the Narmada, the Tapti and the Godavaririvers draining the northern Deccan Traps [Trivediet al., 1995; Dessert et al., 2001], but are similar tothose in rivers from other global basaltic provinces,such as those reported for the Reunion [Louvat andAllegre, 1997].

[11] The Krishna river was sampled at six locations.The Sr concentration in the sample close to its origin(KRS-6) is the highest with a value of �1390 nMwhich decreases to a near-constant value of �650 ±100 nM in downstream samples (Table 1). The causefor such a high concentration in KRS-6 is unclear;however, all the major ions also show high concen-tration in this sample (Table 1) [Das et al., 2005a].This sample is collected close to Wai town(Figure 1), which raises concerns about anthropo-genic influence on the water chemistry. The87Sr/86Sr ratios of the samples are nearly invariantwith values in the range of 0.7068–0.7070 (Table 1).This near constancy in 87Sr/86Sr is not unexpectedconsidering that the drainage is made almost entirelyof Deccan basalts and the tributaries Koyna, Varnaand Panchganga merging with the mainstream allhave ratios in the range of 0.7069–0.7072, nearlythe same as that of the Krishna.

[12] The Bhima river was sampled at three loca-tions, the 87Sr/86Sr of these samples, 0.70822 to0.70977, is marginally more radiogenic than thoseof the Krishna. The concentration of Sr in two ofthese samples is also high, 1029 and 1240 nMrespectively. Two of Bhima’s tributaries, the Muthaand the Ghod also have Sr concentration and87Sr/86Sr ratios (0.70986 and 0.70858), similar tothat of the Bhima. Some of these samples also havehigh Cl, SO4 and NO3, which led Das et al.[2005a] to suggest that these rivers may be receiv-ing solutes from saline/alkaline soils and anthro-pogenic sources. The role of these sources incontributing to Sr and 87Sr/86Sr budget of theserivers needs to be assessed. The other east flowingtributaries and subtributaries of the Krishna have Srranging from �134 nM to �500 nM (average of

�290 nM) with 87Sr/86Sr of 0.70692 to 0.70842(Table 1). In contrast, the small rivers flowing westinto the Arabian Sea have Sr within a narrow rangeof about 240 ± 40 nM and 87Sr/86Sr from 0.70614to 0.70984. The low concentrations of Sr in thesesamples parallel that of major ion abundances inthem (Table 1).

[13] The Sr abundance in the rivers analyzed in thepresent study are lower than that reported [Trivediet al., 1995; Dessert et al., 2001] for the Narmada-Tapti-Wainganga (NTW) systems (Figure 2). Asimilar trend was also noticed for major ion abun-dances [Das et al., 2005a]. This can be due eitherto spatial variability in chemistry of rivers drainingthe Deccan basalts, or supply of Sr (and majorions) to NTW rivers from resources other thanbasalts [Das et al., 2005a]. The average Sr in theKrishna system is 490 nM, a factor of �2 lowerthan that of global average river value of �890 nM[Palmer and Edmond, 1989]. This indicates dilutenature of rivers sampled in this study.

[14] The Sr isotope composition of the riversreflects the 87Sr/86Sr of Deccan basalts throughwhich they flow. The headwaters of the Krishnasystem and the WFWG rivers primarily flowthrough the Ambenali, Mahabaleshwar and Polad-pur Formations [Mitchell and Cox, 1988] of theDeccan basalts. The Sr concentration and its iso-topic composition in many of the Deccan TrapFormations are well documented [Mahoney, 1988;Peng et al., 1994; Subbarao et al., 2000] (Table 2).These show significant variations, with 87Sr/86Srvalues in the range of 0.7035 to 0.719, and averageSr concentration in the range of 171 ± 55 to 254 ±3 mg g�1. The 87Sr/86Sr of the Ambenali and theMahabaleshwar Formations are �0.7055, close topristine mantle values. The Podalpur Formation ismore radiogenic with values ranging from 0.705 to0.713, the higher values resulting from crustalcontamination of these basalts [Mahoney, 1988].

[15] In contrast to the Krishna headwaters and theWFWG rivers analyzed in this work, the Narmada-Tapti rivers flowing through the northern DeccanTraps and the Krishna and the Godavari in theirlower reaches [Trivedi et al., 1995; Dessert et al.,2001] carry more radiogenic Sr with 87Sr/86Srvalues in the range of 0.7076 to 0.7152(Figure 2). Majority of these samples have ratios�0.709 (Figure 2). Dessert et al. [2001] attributedthe more radiogenic Sr isotope composition of theNarmada-Tapti waters to chemical erosion ofbasalts with unusually high 87Sr/86Sr. As men-tioned earlier, the unusually high 87Sr/86Sr of some



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of the Formations of the Deccan basalts has beenattributed to widespread crustal contamination[Mahoney, 1988]. Further, some of these rivershave multilithological basins and hence could bereceiving Sr from lithologies in addition to Dec-can basalts. For example, the Krishna in its lowerreaches flows through Archean cyrstallines, whichcan contribute to its higher 87Sr/86Sr at Vijaywada[Trivedi et al., 1995]. The Godavari, particularly

its lower reaches, has part of its drainage throughsedimentary sequences and high-grade metamor-phic terrains [Trivedi et al., 1995; Sarin et al.,2002], potential sources of more radiogenic87Sr/86Sr to this river. Similarly, the Narmadaand Tapti have Vindhayan sediments/alluvialdeposits as a part of their basins. These are unlikethe rivers sampled in this study, which almostentirely drain the Deccan basalts.

Figure 2. Frequency distribution of Sr and 87Sr/86Sr in rivers analyzed in the present study and those reported forthe Narmada-Tapti-Wainganga (NTW) rivers [Trivedi et al., 1995; Dessert et al., 2001]. A distinct shift toward higherSr concentration and 87Sr/86Sr is observed for the NTW rivers. The dotted lines are the global average values,�890 nM for Sr abundance and 0.7119 for the 87Sr/86Sr [Palmer and Edmond, 1989].

Table 2. Mg, Ca, and Sr Abundances and 87Sr/86Sr in Different Formations of Deccan Trapsa

Formation n Mg, wt.% Ca, wt.% Sr, mg g�1 87Sr/86Srb

Poladpur 4 3.71 ± 0.49 7.65 ± 0.36 224 ± 13 0.705–0.713Ambenali 3 3.67 ± 0.43 7.93 ± 0.43 227 ± 3 <0.705Mahabaleshwar 3 3.35 ± 0.13 7.65 ± 0.79 227 ± 14 �0.705Bushe 3 3.82 ± 0.95 7.15 ± 0.43 171 ± 55 0.713–0.719Thakurwadi 6c 3.91 ± 0.86 6.93 ± 1.14 249 ± 10 -Bhimashanker 3 3.59 ± 0.05 7.00 ± 0.29 254 ± 3 -Overall 22 3.74 ± 0.66 7.29 ± 0.86 228 ± 32

aMg, Ca, and Sr abundances from Subbarao et al. [2000].

bDevey and Lightfoot [1986]; Lightfoot et al. [1990].

cFor Thakurwadi, n = 9 for Ca and Mg.



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[16] The 87Sr/86Sr in all samples analyzed in thisstudy is also lower than the value of 0.7119,reported for global average river water [Palmerand Edmond, 1989]. This is expected consideringthat samples of the present study drain Deccan

basalts, which are far less radiogenic than othercrustal silicates.

[17] Figures 3a–3c are scatterplots of Sr* with Ca*,Mg* and

PCat*, respectively. Corrections for rain-

water contributions have been made following Daset al. [2005a]. For Ca and Mg, rainwater concen-trations from a few locations in the region areavailable [Parashar et al., 1996], for Sr, a value of30 nM has been assumed. Figures 3a–3c includeonly those samples which are undersaturated incalcite (calcite saturation index, CSI < 0) and whichare impacted minimally from anthropogenic andsaline/alkaline salt contributions to major ions.The latter requirement is assumed to be met byselecting samples with Cl < 300 mM and SO4 <100 mM [Das et al., 2005a]. It is seen that in all theseplots Sr shows strong positive correlation with themajor ions (Table 3). For example, Sr is stronglycorrelated with Ca* (r2 = 0.78), similar to thatobserved for various peninsular rivers of India[Trivedi et al., 1995]. The slope of the Ca*-Sr* [Sr(nM)/Ca (mM)] best fit line for the Deccan Traprivers is 1.14 ± 0.13 (Figure 3a; if the data of theAmbika river, which flows far north of the otherrivers, is excluded from the regression analysis theslope increases to 1.36 ± 0.12, r2 = 0.86). This slopecompares with the Sr/Ca abundance ratio of 1.4 ±0.2 in basalts (Table 3). The Sr*/Ca* ratios ofindividual rivers, though show wide range from0.92 to 3.05, majority of the samples have ratios inthe band of 1.6 ± 0.4. The outliers are samples fromthe Ambika river, which has the lowest ratio of 0.92,and the Koyna and Ghataprabha rivers, which havethe highest ratio of �3. In the latter rivers, themeasured Ca and Sr are the lowest; therefore thecorrections for atmospheric contribution in thesesamples are quite significant, which may contributeto larger uncertainties in their Ca* estimates(Table 1). The overall range in Sr*/Ca* of majorityof rivers, however, is similar to the range of 0.74 to1.8 for Sr/Ca in basalts (mean 1.4 ± 0.2). Theobservations that the mean Sr/Ca ratio in basalt

Figure 3. Scatterplots of Sr* with (a) Ca*, (b) Mg*,and (c) SCat *(=Ca + Mg + Na + K)*, where theasterisk indicates measured concentrations are correctedfor atmospheric contribution. The data show a goodpositive correlation. The slope of the lines overlap withthe respective ratios in basalts, suggesting near con-gruent release of Sr and other alkaline earths duringchemical weathering of Deccan basalts.

Table 3. Statistical Analyses of Sr-Major Ion Data

Pair n r2 Slope InterceptRatio inBasalts

Sr*-Mg* 23 0.73 1.86 ± 0.25 59 ± 39 1.69 ± 0.3822a 0.84 2.34 ± 0.22 5.5 ± 33 -

Sr*-Ca* 23 0.78 1.14 ± 0.13 84 ± 32 1.4 ± 0.222a 0.86 1.37 ± 0.12 50 ± 27

Sr*-P

Cat* 23 0.78 0.63 ± 0.07 145 ± 65 0.69 ± 0.11(in Eq units) 22a 0.86 0.75 ± 0.07 80 ± 56

aExcludes Ambika.



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overlaps with the slope of the line (Figure 3a) andaverage Sr*/Ca* in waters within errors, suggeststhat both Sr and Ca are mobilized to rivers fromDeccan basalts roughly in the same proportion astheir abundances in basalts.

[18] Sr also exhibits strong correlation with Mg*(Figure 3b) and

PCat* (Figure 3c) with slopes of

1.86 ± 0.25 and 0.63 ± 0.07 respectively (P

Cat* =Na* + K* + Mg* + Ca*, expressed as mE; Table 3).The slope of the Sr*–Mg* line compares with theSr/Mg abundance ratio of 1.69 ± 0.38 (nM/mM) inDeccan basalts again indicating that Sr and Mg arereleased to rivers in roughly the same proportion astheir abundances. Considering that the dominanthost mineral for Sr in Deccan basalts is plagioclaseand that for Mg it is pyroxenes and olivines, theirnear congruent release to rivers leads to infer thatthese minerals are chemically weathering roughlyin proportion to their abundances. These inferencesrequire that Mg, Ca and Sr in rivers are derivedonly from basalts. This requirement may not befully met for Ca, as it can also be supplied to riversfrom minor amounts of carbonates present inbasalts and sediments. Budget calculations [Daset al., 2005a], however, show that the contributionof Ca from carbonates is generally low, only in therange of �1% to �29% of measured Ca. Similarityin the slope of Sr*–

PCat* line to their abundance

ratio in basalts also attests to the near congruentrelease of (Na + K + Mg + Ca) and Sr from basaltsto rivers. This is also borne out from the significantSr*–HCO3 covariation (r2 = 0.71) as carbonic acidis the primary weathering agent for the Deccanbasalts [Das et al., 2005a]. In contrast, the plot ofSr with Si (Figure 4) does not seem to follow asingle trend, the data of the Ambika and the

WFWG rivers fall distinctly below the trend setby the headwaters of the Krishna and the Bhima.Variation in Sr/Si among the different drainagebasins can contribute to this; the basins of theWFWG are more abundant in lateritic soils, whichare generally deficient in alkali and alkaline earthelements.

[19] The results of Dessert et al. [2001] for theNarmada-Tapti systems also show positive corre-lation of Sr* with Ca* (r2 = 0.4, slope = 1.93 ±0.47) and Mg* (r2 = 0.6, slope = 2.66 ± 0.43). Theslopes of these best fit lines, are however, factors of�1.5 to 2 higher than those deduced in this workfor the Krishna and WFWG system. Many of theNarmada-Tapti river waters are supersaturatedw.r.t. calcite, which can contribute to the removalof Ca and other alkaline earths to varying degreesfrom solution. If such a process is operative itwould modify the Sr/Ca and Sr/Mg ratios withwhich they are released to river waters frombasalts. The effect of precipitation would be toenhance the Sr/Ca in solution as the partitioncoefficients of Sr � Ca [Jacobson et al., 2002].

[20] In summary, the 87Sr/86Sr ratio in rivers of theKrishna headwaters and the WFWG rivers areconsistent with the range of 87Sr/86Sr valuesreported for the Deccan basalts, but lower thanthose reported for the rivers draining the northernDeccan Traps [Dessert et al., 2001] and globalaverage river. Dissolved Sr shows good correlationwith dissolved Ca*, Mg* and

PCat*. The dis-

solved Sr to major ion ratios overlap within errorswith their abundance ratios in Deccan basalts,indicating that Sr and the major ions are releasedto rivers in roughly the same proportion as theirabundances. Scatterplot of Sr with Si shows thatthe data of WFWG rivers plot significantly awayfrom the trend set by other rivers, possibly reflect-ing the Sr/Si ratios of their drainage basins.

[21] The dissolved Sr flux out of the Deccan Trapsbased on the Sr concentration of KRS-2 andaverage for the WFWG rivers (�230 nM) is1.35 � 108 moles yr�1. The calculation uses avalue of 463 mm yr�1 and 1690 mm yr�1 forrunoff in these two regions (eastern and the westernsides of the Western Ghats) and aerial coverage of94% and 6% respectively. The flux correspondsto �0.4% of the global river dissolved Sr flux[Palmer and Edmond, 1989], roughly the sameproportion as the fraction of Deccan area to theglobal drainage. The Sr flux from the northernDeccan Traps, based on the data of Narmada-Tapti[Dessert et al., 2001], is a factor of �3 higher than

Figure 4. Variation of Sr (nM) with Si (mM). The datashow two distinct trends. The lower slope (Sr/Si) of thedashed line is for the WFWG and AMB-1 rivers.



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that determined in this study for the south-centralDeccan. This difference is also seen in the majorion data which Das et al. [2005a] attributed tocauses such as spatial variations in the river watercomposition among different regions of the DeccanTraps and contributions of solutes to the NTWrivers from lithologies in addition to basalts.

[22] The above conclusions based on dissolved Srand major ions are also consistent with thosederived from their abundances in sediments ofthese rivers (A. Das et al., Elemental geochemistryof sediments from the Deccan Traps: Implicationsto source of elements and their mobility duringbasalt-water interaction, submitted to ChemicalGeology, 2006). The sediment data show that Sris quite mobile from basalts to waters, and isstrongly interrelated with Ca and Mg. The Sr/Caand Sr/Mg ratios in sediments are similar to thosein basalts and in dissolved phase attesting to theirnear-congruent release during chemical weatheringof basalts.

4.2. Role of Deccan in Sr IsotopeExcursions of Oceans Around KTBand Late Tertiary

[23] There have been a number of investigations onthe nature of 87Sr/86Sr evolution at about KTB, theresults reported to date, however, have been con-troversial. The controversy pertains to the existenceor otherwise of a positive anomaly in 87Sr/86Sr atKTB. Hess et al. [1986] first reported such a spike.Subsequent studies from other locations, however,yielded results both supporting and contradictingthese findings [Martin and Macdougall, 1991;McArthur, 1994; Meisel et al., 1995; Sugarmanet al., 1995; Vonhof and Smit, 1997; McArthur etal., 1998]. Studies that support the presence of thespike have attributed it to enhanced continentalweathering and associated supply of radiogenic Srto the oceans caused by the effects of either abolide impact [Martin and Macdougall, 1991;Vonhof and Smit, 1997] or Deccan volcanism[Courtillot et al., 1986, 1988; Officer et al.,1987; Dessert et al., 2001]. Recently, Dessert etal. [2001] using a coupled CO2-Sr model showedthat the observed 87Sr/86Sr excursions at KTB areconsistent with the after effects of Deccan trapemplacement. Some of the key parameters deter-mining the impact of Deccan volcanism in con-tributing to the 87Sr/86Sr spike are the timing of themajor Deccan eruptions, their duration and theamount of acidic gases released during the erup-tion, issues which are also in dispute. For example,

some of the recent Ar-Ar geochronological resultssuggest that the major pulse of Deccan volcanismpredate KTB [Pande, 2002]. In contrast to theabove studies that have reported and/or supportedthe presence of a spike, there are also investiga-tions which did not observe any excursion in87Sr/86Sr at KTB [McArthur, 1994; McArthuret al., 1998]. These studies have attributed thereported observations of positive anomalyin 87Sr/86Sr to sample diagensis and analyticaltechniques.

[24] The 87Sr/86Sr record of late Cretaceous showsa steady increase toward KTB. This trend reversesclose to KTB and the 87Sr/86Sr continues to de-crease for several millions of years [McArthur etal., 2001]. The details of 87Sr/86Sr evolution duringthe transition period at about KTB, however, aredifficult to judge as the available data are incon-sistent. Vonhof and Smit [1997] in their highresolution Sr isotope study of KTB sections fromBidart (France) and ElKef (Tunisia) reported twomaxima, one predating KTB by �300 Kyr and theother at KTB. The authors suggested the onset ofemplacement of Deccan Traps as the cause for themaximum prior to KTB and estimated that87Sr/86Sr of global river runoff would have de-clined by 0.0004 units because of Deccan volca-nism. Similar to Sr isotopes, marine 187Os/188Osrecord also shows a significant decrease predatingKTB [Ravizza and Peucker-Ehrenbrink, 2003].The decline in 187Os/188Os also has been attributedto supply of nonradiogenic Os from weathering ofDeccan basalts. Ravizza and Peucker-Ehrenbrink[2003] calculated that about 9% of total Os flux toocean would have to be from Deccan basalts tobalance the measured pre-KTB decline in187Os/188Os. These observations that Sr and Osisotope excursions are predating KTB, is anotherindication that Deccan eruption preceded KTB[Pande, 2002].

[25] The origin of the second maxima in the87Sr/86Sr curve, coinciding with KTB as mentionedearlier, is controversial, whether it is a result ofchanges in ocean chemistry at that time [Vonhofand Smit, 1997] or an artifact of sample diagenesis[McArthur et al., 1998] needs to be ascertained.

[26] In the data of Vonhof and Smit [1997], theminimum between the two maxima occurs at�150 Kyr prior to KTB. The difference in87Sr/86Sr between this minimum and the peak�200 Kyr prior to it is �2 � 10�5 units (basedon the smoothened 87Sr/86Sr curve in Figure 1[Vonhof and Smit, 1997]). The rate of change in



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seawater 87Sr/86Sr during this interval is aboutan order of magnitude more than the long-term(�10 Myr) average rate of decline between �65and �55 Ma [McArthur et al., 2001].

[27] The significance of Deccan eruptions in con-tributing to the pre-KTB dip and the subsequentlong-term decrease in the marine 87Sr/86Sr can beestimated from the commonly used Sr isotopebudget models [Brass, 1976; Richter et al., 1992;Taylor and Lasaga, 1999]. The different sourcescontributing Sr to oceans are rivers (riv), hydro-thermal circulation (h) and sediment diagenesis (d).The balance equations for 87Sr/86Sr of the oceanbefore and after the emplacement of Deccan Trapsare as follows:

Before N d Rswð Þ=dt ¼Jowr Rowr � Rswð Þ þ Jac Rac � Rswð Þþ Jh Rh � Rswð Þ þ Jd Rd � Rswð Þ ð1Þ

After N d Rswð Þ=dt ¼Jowr Rowr � Rswð Þ þ Jdtr Rdtr � Rswð Þþ Jh Rh � Rswð Þ þ Jd Rd � Rswð Þ ð2Þ

where N is the total moles of Sr in oceans, J and Rare the Sr-fluxes from different sources and theircorresponding 87Sr/86Sr ratios, Rsw is the seawater87Sr/86Sr. The river input (Jriv, Rriv), in equations (1)and (2), has been split into two components. For pre-Deccan, one group of rivers are those which weredraining the Archean crust (a lithology exposedprior to the emplacement of Deccan Traps overthem) and the second group made of all other worldrivers. For post Deccan, these are rivers draining the

Deccan basalts and the other world rivers. Thesubscripts owr, ac, dtr, h, d refer to other worldrivers, Archean crust rivers, Deccan Trap rivers,hydrothermal and diagenetic inputs respectively.

[28] The emplacement of unradiogenic Deccanbasalts over the radiogenic Archean crustal silicateswould decrease the global riverine 87Sr/86Sr andhence that of the oceans. The magnitude of thisdecrease would be determined by the difference in87Sr/86Sr between the Archean crust and the Dec-can basalts and the extent of aerial coverage of theArchean crust by Deccan basalts through time.Data on these are scanty and hence for calculationsplausible values for these parameters have beenassumed, the various ratios and fluxes used in themodel are given in Table 4. Rriv, corresponding tothe pre-KTB peak can be calculated to be 0.70913using Rsw of 0.70786 at that time and 3.3 � 1010

moles yr�1 for Jriv. This Rriv and assigned valuesfor Rac and Jac(= 1.5% of global riverine Sr flux)would yield 0.7088 for Rowr. It is assumed that Srflux from Archean crust and Deccan Traps aresame (Jac = Jdtr).

[29] The evolution of 87Sr/86Sr for �10 Myr fromthe pre-KTB maximum at �66 Ma to �55 Ma hasbeen calculated using equations (1) and (2)(Figure 5) and the various parameters given inTable 4. These calculations have been made as-suming that the Deccan emplacement occurredover a time period of either 0.1 Myr or 1.0 Myrand that during this period, the 87Sr/86Sr of riversdraining the Archean crust-Deccan Trap region,linearly declined from a value of 0.7300 [Vonhofand Smit, 1997] to 0.7050 or 0.7080, typical valuesof Deccan rivers. (The 87Sr/86Sr of the Archeancrust underlying the Deccan Traps is not wellestablished; Gopalan et al. [1979] have reporteda value of 0.769 for basement gneisses fromWestern India.) The flux of Sr from the regionwas kept at 5 � 108 moles yr�1, 1.5% of global Srflux (Table 4). This is about thrice the contempo-rary flux out of Deccan as measured in this study.Such a high flux out of Deccan during early stagesfollowing its emplacement is conceivable consid-ering that the present aerial exposure of Deccan isonly about a third of its original [Courtillot andRenne, 2003]. Further, the weathering rate at thattime could have been higher as the basalts wererelatively ‘‘fresh’’ and likely to be richer in moreeasily weatherable volcanic glasses and covered toa lesser extent by soil and vegetation.

[30] The results of the calculations show that Rsw

values after the pre-KTB maximum (1) decline

Table 4. Parameters Used in the Model Calculations

Parameter Value Comments and Reference

Jriv 3.3 � 1010 riverine Sr flux assumedconstant through time

Jh 1.0 � 1010 Palmer and Edmond [1989];Richter et al. [1992];assumed constant

Jd 0.3 � 1010 Palmer and Edmond [1989];assumed constant

N 1.13 � 1017 moles Martin and Macdougall[1991]

Rdtr 0.705–0.708 this workRowr 0.70881 calculated for pre-KTB peak

assuming Rsw = 0.70786,Jac = 0.7300 and Jdtr =5 � 108 moles yr�1

Rh 0.7035 Palmer and Edmond [1989]Rd 0.7084 Palmer and Edmond [1989]Rac 0.7300 assumed value



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more slowly with time for the initial several Ma, asthe period of emplacement is increased from 0.1 to1 Ma, and converging subsequently (2) are mar-ginally higher, particularly in the later stage, as Rdtr

is increased from 0.705 to 0.708, keeping Jdtrconstant, and (3) decrease more rapidly as Jdtr isincreased from 5 � 108 moles yr�1 to 10 � 108

moles yr�1, keeping Rdtr constant. The calculations(Figure 5) show that to reproduce the short-termpre-KTB dip in 87Sr/86Sr (�2 � 10�5 units in over�200, 000 years [Vonhof and Smit, 1997]) it wouldrequire Sr fluxes from Deccan �(6–7) � 108

moles yr�1, for Rdtr of 0.705 and 0.708, respec-tively, several times the contemporary measuredvalues. The required flux corresponds to about�2% present-day global riverine Sr flux, but com-parable to or marginally higher than the fraction ofthe aerial coverage of Deccan relative to globaldrainage at the time of its emplacement. The modelcalculations thus show that the weathering of

Deccan Traps has to be much higher in the pastthan at present to account for the dip in reportedmarine 87Sr/86Sr. Ravizza and Peucker-Ehrenbrink[2003] also brought out the need for disproportion-ately higher weathering of the Deccan basaltscompared to its aerial coverage, to explain thepre-KTB decline in 187Os/188Os observed in themarine Os isotope record.

[31] The long-term decline in 87Sr/86Sr during�66–55 Ma requires significant change in theproportion of 87Sr supply from rivers and hydro-thermal circulation. The emplacement of DeccanTraps and their weathering can contribute to such achange in 87Sr from rivers. The model calculationscarried out indeed show that for emplacementperiod of 0.1 Ma and Jdtr of 5 � 108 moles yr�1,Rsw decreases from the pre-KTB maximum of0.70786 to 0.707591 and 0.707624 at 10 Ma forRdtr of 0.7050 and 0.7080 respectively. The model

Figure 5. Temporal evolution of marine 87Sr/86Sr resulting from Sr contribution from Deccan Traps emplaced overArchean crust. Model calculations have been made for two time periods: (1) for �66 to �55 Myr ago and (2) forabout a Myr around KTB, with emplacement times of 0.1 and 1 Myr and 87Sr/86Sr of 0.705 and 0.708 for Deccanrivers. The results show that the supply of nonradiogenic Sr from Deccan basalts can be a significant contributor tothe reported pre-KTB dip and the general decrease in 87Sr/86Sr during �66 to �55 Ma (see text for details). KTB isabout 300 Kyr after the pre-KTB peak in 87Sr/86Sr. Sr fluxes (Jdtr) are in moles yr�1.



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values are lower than the measured ratio of�0.70775 at that time [McArthur et al., 2001].The measured and model values can be made tomatch by decreasing the Jdtr values. Thus the short-term pre-KTB dip and Ma timescale long-termdecline in 87Sr/86Sr can be roughly reproduced byhaving the Jdtr change through time, with valuesseveral times more than the contemporary flux atthe time of major Deccan eruption decreasing topresent-day flux. It is, however, recognized thatthere are other parameters in the Sr balance (equa-tions (1) and (2)), variations of which can alsocontribute to the decline in 87Sr/86Sr through time,quantification of all these variations are required tobetter assess the role of Deccan Trap emplacementin the Sr isotope evolution of oceans. For example,if the 87Sr/86Sr of the Archean crust, Rac, is takenas 0.7200 instead of the value of 0.7300 used in thecalculations, then the flux requirement from Dec-can Traps (Jdtr) to account for the pre-KTB dipwould be much higher than the values estimatedabove raising concerns about its feasibility. Incontrast, if the Archean crust is more radiogenic,�0.7700, the flux requirement would be muchlower. The present exercise nonetheless indicatesthat supply of nonradiogenic Sr from weathering ofDeccan basalts can be important in determining thenature of Sr isotope evolution and the magnitude ofits decrease near and post KTB.

[32] The model presented above has not addressedto the role of Deccan emplacement in generatingpositive excursion in 87Sr/86Sr [Dessert et al.,2001]. The observations that the 87Sr/86Sr spikeat about KTB is not global, that there concernsabout adequate preservation of samples which haveyielded the spike and that estimates of CO2 fluxfrom Deccan during its eruption varies over 1–2 orders magnitude [Dessert et al., 2001; Caldeiraand Rampino, 1990] all contribute to much uncer-tainty to this issue.

5. Conclusions

[33] The measurement of Sr concentration and87Sr/86Sr ratios in the Krishna headwaters and theWFWG rivers all of which have their basins withinthe Deccan Traps, lead to the following inferences/conclusions regarding the weathering geochemistryof Sr, its flux out of the Deccan and its role on Srisotope excursions of oceans around KTB and thelate Tertiary.

[34] 1. The Sr concentrations in rivers vary from�130 nM to �1390 nM with an average of

�490 nM which yield present-day dissolved Srflux of �1.35 � 108 moles yr�1 from the Deccan.Sr* shows good correlation with Ca*, Mg* andP

Cat* and suggest near congruent release of thesecations and Sr from basalts to waters duringchemical weathering. The 87Sr/86Sr in these riversvaries from 0.70614–0.70986, with two-thirds ofthe samples having ratios within a narrow range of0.706–0.708. The Sr concentration and 87Sr/86Srof the Krishna headwaters and WFWG rivers arelower compared to those reported for the Narmada-Tapti-Wainganga (NTW) draining the northernDeccan Traps and those reported for global meanriver water. The higher ratios observed in the NTWrivers can be a result of their interaction withbasalts having more crustal contamination and/orimpact of contribution from lithologies in additionto basalts.

[35] 2. Model calculations on the evolution ofmarine 87Sr/86Sr based on results from this studyand various assumptions show that supply ofunradiogenic Sr from Deccan basalts could havea major control on its decline from pre-KTB toseveral Ma subsequently. The rapid decline duringpre-KTB can also result from Deccan basalt weath-ering if the Sr flux from this source is several timesthe present-day value. The impact of Deccan basaltweathering on longer-term decline in 87Sr/86Sr(�66 Ma to �55 Ma), would depend on the fluxesof Sr from various sources to the oceans, theirtemporal variations and that of the Archean crustwhich was covered by the Deccan Traps. If thesefluxes are maintained constant at present values,the model results show that Deccan basalt couldhave a significant role in the 87Sr/86Sr declineduring this period.

Acknowledgments

[36] We thank M. M. Sarin and Sunil K. Singh for their

guidance during Sr measurements and Sr isotope analysis,

respectively, and J. P. Bhavsar for help during the field

campaign. We also acknowledge B. Peucker-Ehrenbrink, H. B.

Vonhof, and Laurent Labeyrie for their comments and reviews,

which helped in improving the quality of the manuscript.

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