Chemical Geology 285 (2011) 184–193

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Chemical Geology

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Sr, C and O isotopes in carbonate nodules from the Ganga Plain: Evidence for recentabrupt rise in dissolved

87Sr/

86Sr ratios of the Ganga

Waliur Rahaman a,⁎, Sunil K. Singh a, Rajiv Sinha b, S.K. Tandon c

a Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380009, Indiab Engineering Geosciences Group, Indian Institute of Technology, Kanpur 208016, Indiac Department of Geology, University of Delhi, Delhi 110007, India

⁎ Corresponding author.E-mail address: waliur@prl.res.in (W. Rahaman).

0009-2541/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.chemgeo.2011.04.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 June 2010Received in revised form 8 February 2011Accepted 5 April 2011Available online 12 April 2011

Editor: J.D. Blum

Keywords:GangaHimalaya87Sr/

86Sr

Carbonatesδ13C and δ

18O

The evolution of87Sr/

86Sr ratio of the Ganga water during the past ~100 ka has been reconstructed in this study. The

87Sr/

86Sr, δ

13C and δ

18O values have beenmeasured in carbonate nodules from different depths of two sediment cores

raised from the Indian Institute of Technology Kanpur (IITK) campus (50 m long) and Jagdishpur (JP, 25 m long)respectively. The

87Sr/

86Sr ratios of the carbonate nodules range from 0.7142 to 0.7189 in the IITK and 0.7142 to

0.7367 in the JP cores. The nodules in general, display significantly lower87Sr/

86Sr ratio compared to contemporary

Ganga river water at Kanpur, however, values of87Sr/

86Sr ratio are observed in the nodules of the JP core near the

surface are consistent with that of the present day Ganga water at Kanpur as well as groundwater samples fromadjacent areas indicate recent abrupt increase in

87Sr/

86Sr ratio of the Ganga. These findings are also consistent with

the concomitant increase in87Sr/

86Sr ratio of silicates in contemporary sediments of theGangaatKanpur compared to

that of past ~100 ka. The sudden rise of87Sr/

86Sr ratio of the Ganga is probably due to increase in the relative

proportionof Sr fromtheLesserHimalayacontaining silicates andcarbonateswithhigher87Sr/

86Sr ratio. The cause for

the recent increase in Sr contribution from the Lesser Himalaya is not clear; this could be due to enhancedagricultural activities and deforestation and or climatic variability during recent times resulting in more erosion inthe Lesser Himalaya. The δ

13C and δ

18O, measured in the nodules from the IITK core, vary from−6.8 to +1.6‰ and

−8.3 to−5.4‰ respectively. The co-variation of δ13C and δ

18O suggest the impact of paleoclimate/paleovegetation.

Petrography and chemical composition of carbonate nodules indicate little or no diagenetic alteration.

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The interplayof tectonics, erosion and climate continues to be a topicof debate among geochemists (Edmond, 1992; Molnar and England,1992; Raymo and Ruddiman, 1992; Burbank, et al., 2003; Wobus et al.,2005). Long-term erosional patterns over million year time scales arecontrolled by tectonics (Burbank et al., 2003) whereas climate modifiessome of these processes on shorter-time scales (Goodbred et al., 2003;Bookhagen et al., 2005; Clift et al., 2008; Rahaman et al., 2009). Recentstudies of the Ganga–Brahmaputra drainage suggest that the contem-porary chemical and physical erosion of their catchments in theHimalaya are quite high; they together supply one to two billion tonsof particulate matter and ~100 million tons of dissolved material to theBay of Bengal annually (Sarin et al., 1989; Hay, 1998; Galy and France-Lanord, 2001; Singh et al., 2005). In addition, there are indications ofsupply of dissolved material including Sr to the Bay of Bengal bysubmarine groundwater discharge in magnitude comparable to theriverine flux carried by the G–B rivers (Basu et al., 2001).

The role of these rivers and groundwaters in contributing to theisotopic and geochemical budgets of the global ocean since theHimalayan orogeny has been assessed mainly through studies of theircontemporary isotopic and chemical composition. For example, thepresent day Ganga–Brahmaputra waters are characterized by high

87Sr/

86Sr ratio and moderately high Sr concentration which has led to the

suggestion that the G–B rivers play a key role in the Sr isotope evolutionof seawater during the Cenozoic (Veizer, 1989; Edmond, 1992;Krishnaswami et al., 1992; Raymo and Ruddiman, 1992; Richter et al.,1992). The steady increase inmarine

87Sr/

86Sr ratio since the Cenozoic led

to further hypothesize that seawater Sr evolution during this periodwasa result of enhanced silicateweathering resulting from rapid uplift of theHimalaya (Raymo and Ruddiman, 1992; Richter et al., 1992). Thishypothesis assumes that the Sr isotope composition of the Himalayanrivers andhence the silicates undergoingerosion has remained invariantsince the Cenozoic, an assumption that remains to be validated. Studieson clays of the Bay of Bengal (Derry and France-Lanord, 1996) andpaleosols from the Siwaliks (Quade et al., 1997), however, seem toindicate that the Sr isotope composition of the G–B rivers may havevaried overmillion-year time scale, challenging the validity of the aboveassumption. These results prompted us to undertake a study toreconstruct

87Sr/

86Sr ratio of the Ganga during the past by analysing

185W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

secondary carbonates, locally known as “kankar” contained in thealluvial sediments of the Ganga plain. These are soil carbonates andformed alongwith sediments in the plain. They typically contain severaltens to hundreds of ppmSr and therefore archive Sr isotope compositionof ground and soil water fromwhich they form. There have been a largenumber of studies in literature on

87Sr/

86Sr ratio in carbonates. It has been

demonstrated that strontium isotope in soil carbonates has substantialpotential to provide information on geochemical cycles of soils andchemical weathering (Quade et al., 1995a, 1995b; Stewart et al., 1998;Capo and Chadwick, 1999; Chiquet et al., 1999; Van der Hoven andQuade, 2002). Among the limited studies available from the Himalaya,Quade et al. (1997) tracked the Sr isotope composition of theGanga overa million year time scale using paleosol carbonates in the Siwaliks.The

87Sr/

86Sr composition of paleosol carbonate nodules from the Siwalik

Grouphas beenused to reconstruct erosion andweatheringof the LesserHimalayan rocks in the central and theeasternNepalHimalaya (DeCelleset al., 1998). Yang et al. (2000) brought out coupling between thechemical weathering and East Asian monsoon based on calcretes ofChinese loess.

87Sr/

86Sr ratio of soil carbonates has been used as a tracer to

infer about the origin andevolutionof soil carbonates in the inlandSouthAustralia and Victoria (Quade and Lois, 1999) andweathering history ofKohala peninsular, Hawaii (Rosemary et al., 2000).

The oxygen and carbon isotope composition of these carbonates alsohold clues to the paleoenvironmental conditions, particularly temper-ature, rainfall and type of vegetation prevailing at the time of theirformation (Cerling, 1984; Quade et al., 1989, 1995a, 1995b; Quade andCerling, 1995; Cerling et al., 1997; Quade and Lois, 1999; Behrensmeyeret al., 2007). Freshwater carbonates including the “kankars’ from theGanga plain have been studied to infer the paleoenvironmentalconditions during their formation (Srivastava, 2001; Sharma et al.,2004; Sinha et al., 2006). The aim of the present work is to track thetemporal evolution of

87Sr/

86Sr ratio of Ganga water by analysing

carbonate nodules in sediment cores raised from the Ganga plain. Inaddition to

87Sr/

86Sr, their δ

13C and δ

18O values are used to reconstruct

paleoclimate and paleovegetation in the Ganga basin.

Fig. 1. Locationmap of the study area near Kanpur in the southern Ganga plain. Themajor lith

2. Study area: climate and geohydrology

The studyarea is a part of thewesternGangaplainsnearKanpurwithsurface elevation of 120–130 m above the mean sea level. The Gangariver valley is ~20 km wide in this region and the interfluves arecomposed offlood plain deposits of Quaternary alluviumbrought by theGanga river and its headwaters tributaries, the Alaknanda and theBhagirathi rivers. Two cores used for this study are from the IndianInstitute of Technology of Kanpur (IITK) campus and Jagdishpur (JP) inthe Ganga valley fill (Fig. 1) separated by ~25 km from each other. Thecore from the IITK campus represents the Ganga–Yamuna interfluvesregion in thewestern Ganga plain far upstream of the confluence of theGanga and the Yamuna rivers. The core from Jagdishpur is from thevalley fills northeast of the present course of the Ganga.

The Ganga plain experiences subtropical monsoonal climate withaverage precipitation of ~800–1000 mm/year (Singh, 1994). In thecentral Ganga plain averageminimum andmaximum temperatures are21 °C for November to February and 27 °C to 33 °C for March–Junerespectively (Sharma et al., 2004). The onset of south-west monsoon isgenerally in mid-late June and its retreat begins during October. Thesouth-west monsoon accounts for more than 85% of rainfall in thisregion. The average annual discharge of the Ganga river at Kanpur is~900 m3/s and the bank-full discharge is 8500 m3/s (Sinha et al., 2007).

The IITK core site is about 80 km north of the present location of theYamuna. The sediments of theYamuna arederived fromtheboth cratonic(through Chambal) and the Himalayan sources (Singh et al., 2008).However, the IITKcore is located far fromnorthern limitof craton-sourcedsediments in the subsurface and litho-stratigraphic study has clearlyshown that this core is composed entirely of sediments derived from theHimalayan source at least for the past ~100 ka (Sinha et al., 2009).

2.1. Carbonates in the Ganga plain

Carbonates occur abundantly in the Ganga plains and have precipi-tated at different time scales as the sediment deposition proceeded in the

ologies supplying dissolved and particulate materials to the Ganga River are also shown.

Table 1Elemental compositions,

87Sr/

86Sr, δ

13C and δ

18O of carbonate nodules from IITK core.

Depth (m) Cinorg Ca Mg Mn Sr 87Sr/86Sr δ13C δ18O

wt.% μg g−1 (‰ PDB)

1.25 0.04 – – – – 0.71888 – –

1.85 6.6 10.8 0.3 238 215 0.71559 −3.1 −7.72.50 6.7 10.3 0.3 207 222 0.71606 −0.2 −6.32.85 – – – – – 0.71666 – –

2.85-R – – – – – 0.71676 – –

3.15 6.4 9.8 0.3 292 232 0.71666 −3.6 −7.14.03 6.6 10.3 0.3 184 156 0.71683 −3.1 −6.84.03-R1 – – – – – 0.71687 – –

4.03-R2 – – – – – 0.71676 – –

6.80 – 9.7 0.3 157 173 0.71700 −6.8 −8.36.80R – – – – – – −6.8 −8.110.30 6.6 10.9 0.4 541 245 0.71418 0.9 −5.411.70 7.3 12.1 0.3 261 187 0.71714 −3.0 −6.711.70-R – – – – – 0.71717 – –

13.12 6.0 8.8 0.4 734 551 0.71590 −2.4 −5.913.12-R – – – – – 0.71590 – –

13.54 6.1 9.9 0.3 764 172 0.71534 1.1 −5.814.16 6.5 24.7 0.6 1426 488 0.71530 1.6 −5.814.46 8.1 11.5 0.8 1003 479 0.71588 1.5 −5.516.23 6.9 12.0 0.3 257 217 0.71721 −3.4 −6.816.23-R – – – – – 0.71721 – –

23.85 6.6 11.2 0.3 314 218 0.71722 −5.5 −7.723.85-R – – – – – 0.71726 – –

28.40 0.1 – – – – 0.71751 – –

39.02 7.6 10.5 0.2 135 189 0.71745 −0.4 −6.839.02-R – – – – – 0.71748 – –

39.60 8.0 11.9 0.2 82 250 – – –

40.00 6.2 8.7 0.2 106 265 0.71715 −0.2 −6.740.00-R – – – – – 0.71718 – –

40.40 7.4 8.6 0.2 106 201 0.71674 −0.8 −6.843.18 6.0 10.9 0.3 250 233 0.71725 – –

43.72 4.9 7.5 0.2 61 168 0.71690 −1.5 −7.244.42 3.4 6.3 0.2 58 167 0.71744 −2.9 −7.044.82 4.5 6.6 0.1 247 94 0.71761 −3.8 −7.544.82-R – – – – – 0.71758 – –

48.31 8.6 12.3 0.3 120 269 0.71835 −0.8 −7.648.31-R – – – – – – −0.8 −7.748.40 8.1 10.9 0.3 120 240 0.71833 −0.8 −7.848.40-R – – – – – 0.71831 – –

R: replicate analysis of the same aliquot.

186 W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

plain and continues even today (Agarwal, 1992; Singh, 1992; Srivastava,2001; Pal et al., 2003). These carbonates are generally formed in semi-humid to semi-arid climate. Part of the overlandflowof theGanga plain isendoreic which coupled with semi-arid climatic conditions leads torepeated cycles of drying and wetting of the plain which promotesprecipitation of calcite and other saline/alkaline minerals (Srivastava,2001; Pal et al., 2003; Rai et al., 2010). These carbonates known as“kankar” occur at different depth horizons of the Ganga plain with moreextensive occurrences in the western, central and the southern parts andhave been characterized by earlier workers on the basis of theirpetrography, genesis and carbon and oxygen isotopic composition(Salomons et al., 1978; Alam et al., 1997; Srivastava, 2001; Sinha et al.,2006).

Sinha et al. (2006) classified the carbonates of the Ganga plain aspedogenic and non-pedogenic or groundwater carbonates based ontheir field and petrographic characteristics. Srivastava et al. (2010)has provided micromorphological description of pedosedimentaryfeatures of the IITK core indicating several weakly developedpaleosols at different depths and one well-developed paleosol layerat ~45 m depth. Both weakly and well-developed paleosol horizonshave formed both pedogenic and non-pedogenic carbonates whichshow distinct micromorphological characterstics (Srivastava et al.,2010). Majority of the secondary carbonates analysed in this study istaken from the paleosol layers identified by Srivastava et al. (2010),however some of them are taken from sand/silt layers and could begroundwater carbonate. These are early carbonate cements and theirisotope signatures can be considered similar to the paleosolcarbonates (Quade and Lois, 1999).

3. Materials and methods

3.1. Samples

Lithostratigraphy and chronology of the two cores sampled aredescribed by Sinha et al. (2007). The entire length of the IITK core isdominated by silty sediments with a few layers of sand and paleosolswith intermittent calcretes. The Jagdishpur core consists of two majorsand bodies separated by two intra-valley floodplain depositsdispersed with carbonates (Sinha et al., 2007). Carbonate nodulesare not distributed throughout the core but are present only at certaindepths, and therefore, they provide only discontinuous records. Inaddition, samples of shells from the bank of the Ganga at Kanpur, theGanga water and groundwater samples were also collected at Kanpurand from adjoining areas.

Pedogenic carbonate nodules of about 0.5 to 5 cm size werehandpicked from various depths of the cores. They are white to off-white in colour and irregular in shape, typical of “kankars” found in theGanga plain. These carbonates were generally associated with siltymaterial. Carbonate nodules were powdered using agate mortar andpestle and sieved to b100 μm size using nylon mesh and homogenized.

3.2. Experimental procedure

3.2.1. Sr isotopesIt is important to ensure that procedures used for extracting Sr from

the carbonatephasedonot leachSr fromsilicateminerals cemented intonodules. In viewof this, the sampleswere leachedwith dilute acetic acidat room temperature, which is known to dissolve only the carbonatephase without attacking the silicates (Asahara et al., 1995; Hart et al.,2004). Further, the leaching was carried out for short time and at roomtemperature. Approximately 0.5 g of dried powdered samples wastreatedwith 5% acetic acid (Suprapure,Merck) at room temperature for~15 min. The slurry was centrifuged and Sr was separated from theleachates by ion exchange procedures (Rai and Singh, 2007; Singh et al.,2008). The Sr isotope composition of the pure Sr fractionwasmeasuredusing ISOPROBE-T Thermal Ionization Mass Spectrometer (TIMS) in

static multi-collection mode. Measured data were corrected for massfractionation using

86Sr/

88Sr of 0.1194. The mean

87Sr/

86Sr ratio of the

standard NBS 987 based on repeat measurements during the course ofthis study is 0.710229±0.000028 (2σ, n=85). The reproducibility of87Sr/

86Sr data was checked by measuring several samples in duplicates

(Tables 1 and2). In addition to these carbonates, a few shells, riverwatersamples of the Ganga and groundwater samples were also analysed fortheir Sr isotope composition. Several procedural blanks weremeasuredduring the courseof these analyses. The total procedural blank for Srwas~500 pgwhich is about three orders of magnitude lower than that of Srprocessed from the samples, and therefore data given in Tables 1–3a,3bare uncorrected for blank.

3.2.2. Carbon and oxygen isotopesCarbon and oxygen isotope measurements were carried out in the

powdered carbonate samples. Few milligrams of powdered sampleswere reacted with 100% orthophosphoric acid at 25 °C under vacuumin water bath for 30 min and the CO2 liberated was purifiedcryogenically and analysed in a GEO 20–20 dual inlet massspectrometer along with internal laboratory standards, Z-Carrara(δ

13C=+2.3±0.1‰ and δ

18O=−1.3±0.1‰) and Mac. Marble

(δ13C=+3.8±0.1‰ and δ

18O=−10.6±0.1‰). Reproducibility was

checked by repeat measurements of several samples. Isotopic ratios ofcarbon and oxygen are presented in the usual δ notation with respectto international standard Peedee belemnite (PDB). The precision ofmeasurements is ±0.2‰ (2σ).

Table 2Sr isotope composition of carbonate nodules from Jagdishpur.

Sample ID Depth(m) 87Sr/86Sr Δ (%)

JP-2.25(A) 2.25 0.73651 0.019JP-2.25(B) 2.25 0.73665JP-2.80 2.8 0.73514JP-9.4 9.4 0.71416JP-13.0 13.0 0.71961JP-21.20(A) 21.2 0.72064 0.118JP-21.20(B) 21.2 0.72149JP-23.75 23.75 0.71521JP-24.45 24.45 0.71858JP-24.65(A) 24.65 0.71685 0.004JP-24.65(B) 24.65 0.71688

(A) and (B) of the same samples represent replicate analysis on separately handpickednodules of carbonates from same depths. Δ is the difference in the

87Sr/

86Sr of the

replicates normalized to mean ratio.

Table 3bSr isotope composition of miscellaneous samples from Allahabad after the confluence ofYamuna.

Sample ID Date Location Type 87Sr/86Sr

RW 07.02.06 Allahabad River water 0.71699GW-1 07.02.06 Allahabad Ground Water 0.71761GW-2 07.02.06 Allahabad Ground Water 0.71760KANKAR-1 07.02.06 Allahabad Kankar 0.71761KANKAR-2 07.02.06 Allahabad Kankar 0.71536

This data set is taken from S. K. Rai (personal communication).

187W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

3.2.3. Elemental and photomicrograph studiesThe abundance of Ca,Mg, Sr andMnweremeasured in the acetic acid

leaches of these samples. For elemental measurements approximately0.2 g of dried powdered sample was treated with 5% acetic acid(Suprapure, Merck) for ~15 min at room temperature. The slurry wascentrifuged and Ca, Mg, Sr and Mn were measured in the leaches usingICP-AES (Jobin Yvon, Model 38S), the precision of measurements isbetter than ~5% (Dalai et al., 2002). Inorganic carbon measurements inthe powdered carbonate samples were made using UIC CoulometerModel 5012 (UIC Inc., Illinois, USA) following the same protocol ofBhushan et al. (2001). Measurement precision was ~3% based onreplicate analysis. SEM photomicrographs of the nodules were taken onoptically polished thin sections using a Cameca EPMA Model SX100.

3.3. Chronology of IITK and JP cores

Sinha et al. (2007) reported the chronology of IITK core based onluminescence dating of etched K-feldspars. Four samples of the IITKcore from 11.6, 21.5, 31.9 and 41.9 m depths were dated whichyielded ages of 30.3±3.4, 38.7±3.7, 63.4±4.0 and 86.0±7.4 karespectively (Fig. 4). The age of one carbonate nodules from 2.3 mdepth of this core was determined using

14C dating technique. It

yielded an age of 5.4±0.4 ka (Rahaman et al., 2009). The radiocarbonage may represent the minimum age for the sediment depositionevent as carbonate can precipitate either during or after the sedimentdeposition. Luminescence dates were used to establish an age modelfor this core which indicates that the sedimentation rate between thedepth intervals 2.3 to11.6 m, 21.5 to 31.9 m and 31.9 to 41.9 m is thesame at about 0.41±0.03 m/ka, whereas it is higher, 1.2±0.7 m/ka,between the depths 11.6 and 21.5 m. Luminescence ages of thesediments provide the upper limit of the age of carbonate precipita-tion as carbonate could precipitate during or after the deposition ofthe sediments. In this study luminescence ages of sediment has beenassumed as age of the carbonates. This assumption is validated as theextrapolated age for 2.3 m depth based on the four IRSL ages agree

Table 3aSr isotope composition of miscellaneous samples from Kanpur.

Sample ID Date Type Latitude Longitude

GP-134 3/3/2009 Groundwater 26°37.055′ 80°16.766GP-134R 3/3/2009 Groundwater 26°37.055′ 80°16.766GP-137 3/3/2009 Shells 26°36.648′ 80°16.615GP-135 3/3/2009 River water 26°36.716′ 80°16.598RW7 (3/82) 03/82 River waterRW7 (11/83) 11/83 River water

1-Palmer and Edmond, 1992.

well with the measured14C age of carbonate precipitated at that depth

within the uncertainties associated with the individual ages.The chronology of the JP core was also determined by Sinha et al.

(2007) based on luminescence dating of etched K-feldspars. In the JPprofile, three ages 6.8±0.7, 9.2±1.2 and 26.2±5.1 ka for depths 4.6 m,7.6 m and 18.5 m respectively are available. The top of the lower sandbody has yielded 26.2±5.1 ka. The upper sand body is well constrainedwith an age of 9.2±1.2 ka for the lower part and 6.8±0.7 ka for theupper part.

4. Results

In this work, a total of twenty six carbonate samples from the IITKcore and eight samples from the JP core have been analysed for their Srisotope composition. In addition, the IITK carbonateswere also analysedfor their major element composition, inorganic carbon and δ

13C and δ

18O

values. The inorganic carbon abundance of the IITK samples analysed inthis study ranges from 3.4 to 8.6 wt.% (Table 1) corresponding to CaCO3

of 28 to 72 wt.%;with two samples (at depth 1.25 m and 28.4 m)havingvery low abundance, ≤0.1% of inorganic carbon. The Ca, Mg, Sr and Mnabundances given in Table 1 are based on the analysis of the acetic acidleachates and reported with respect to total sediment weight. Srconcentrations in these samples vary from 94 to 551 μg g−1 (Table 1)and seem to depend on their carbonate contents. These carbonates haveminor amounts ofMg, 0.1 to 0.8% (Table 1) similar to pedogenic low-Mgcalcite. The

87Sr/

86Sr ratio of the IITK carbonate nodules range from

0.7142 to 0.7189,with the lowest value at a depth of 10.3 m (Table 1). Inthe JP core,

87Sr/

86Sr ratio is higher and ranges from 0.7142 to 0.7367

(Table 2). The highest87Sr/

86Sr ratio in the JP core is near its surface,

at 2.25 m depth. The87Sr/

86Sr ratio of this sample is close to that of

present day Ganga river water at Kanpur (Table 3a). Carbonate nodulesfrom three depths of the JP core were analysed for their

87Sr/

86Sr ratio in

duplicate using separate nodules from each depth to assess heteroge-neity in their

87Sr/

86Sr ratio. It is observed from this exercise that the

variations in Sr isotope composition fromthe samedepthhorizondue tosample heterogeneity are of 0.004%, 0.019% and 0.118% (Table 2). Basedon this exercise, a maximum uncertainty of 0.1% on

87Sr/

86Sr ratios of

the carbonates at a given depth is assigned. The δ13C (PDB) and δ

18O

(PDB) values, analysed only in the IITK carbonate samples, vary from−6.8 to+1.6‰ and−8.3 to−5.4‰ respectively. The variability of δ

13C

in the carbonate nodules is higher than that of δ18O (Table 1).

Location 87Sr/86Sr Uncertainty Reference

′ Ganga valley, Kanpur 0.72934 0.00001 This study′ Ganga valley, Kanpur 0.72930 0.00001 This study′ River bank, Kanpur 0.72936 0.00001 This study′ River bank, Kanpur 0.73240 0.00001 This study

Kanpur 0.7312 1Kanpur 0.7373 1

188 W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

5. Discussion

5.1. Petrography and major element composition

Thin sections of these carbonate nodules indicate the dominance ofmicriteswithmicrospar. The groundmass is commonly characterized bya gremulose (mottled) texture, created by complex and highly irregularvariability in crystal sizes. Fine-grained, dark micrites are abundant andcompacted (Fig. 2). All the nodules in the SEM study exhibit α- typefabric (Wright and Tucker, 1991). SEM photomicrographs of thesecarbonate nodules (Fig. 2) do not show any evidence of overgrowth ofcalcite crystal or transition frommicrite to sparwhichsuggestsminimal/or no significant diagenetic effect. Conventional petrographic andtextural criteria of these carbonate nodules are not always conclusiveagainst the diagenesis and alteration effects (Leier et al., 2009). It hasalso been shown that isotope redistribution in carbonates may notalways be associated with the change in their petrographic character-istics (Denison et al., 1994). Therefore, a combined photographic, traceelement and stable isotope study has been carried out to identifypossible diagenesis and alteration effects in the carbonates.

Theuse of δ13C and δ

18O values to infer about thepaleoenvironmental,

paleovegetation and87Sr/

86Sr ratio to reconstruct the Sr isotopeevolution

of the paleo Ganga waters require that these carbonates have not beenaltered by diagenesis. The relative abundances of Ca, Sr, Fe andMnof the

200μm

c

a

b

500μm

200μm

Fig. 2. SEM images of selected carbonate nodules (a) dense cementation of micritecrystals with quartz matrix; (b and c) abundance of anhedral to subhedral calcitecrystals of micrite and microspar with the ground mass of quartz.

marine carbonates have been used to study their later diagenesis(Veizer, 1983). Repetitive dissolution and precipitation decreases Sr/Caratio of the carbonatesdue to lowpartition coefficientof Sr in carbonates(Banner and Hanson, 1990). In marine carbonates, decrease in Sr/Caratio is associated with increase in Mn concentration (Veizer, 1983). Insamples of this study, Sr/Ca ratio is independent of Mn (Fig. 3a)suggesting very little diagenetic alteration, if any. Similarly,

87Sr/

86Sr

ratios of these carbonates are in general independent of their Mn/Srratios (Fig. 3b) suggesting preservation of original isotope signatures. Itis very unlikely that the meteoric waters can alter the

87Sr/

86Sr ratio of

existing carbonates as they contain very little Sr, (~16 nmol/l, Galy et al.,1999) compared to the Sr abundance in carbonate nodules (Table 1).These lines of evidence suggest very little diagenetic alteration of thesecarbonates. Therefore, the measured

87Sr/

86Sr ratios of these carbonates

are representative of that of the water from which they precipitated.

5.2. How faithfully do these carbonates record riverine Sr isotopesignature?

The use of pedogenic carbonates (kankars) to reconstruct87Sr/

86Sr

ratio of river water requires that they (i) faithfully record87Sr/

86Sr ratio

of river water and (ii) retain these signatures through time. Toconfirm that these carbonates indeed record the

87Sr/

87Sr of rivers,

samples of contemporary river water, groundwater, shells fromsurface sediments collected around Kanpur were analysed for theirSr isotope composition. The results are listed in Table 3a.

87Sr/

86Sr ratio

of river water (0.73240) is slightly higher compared to that ofgroundwater (0.72934) and adjacent shells (0.72936). Measurementsof both groundwater and shells average the

87Sr/

86Sr ratio of several

years whereas the river water data is an instantaneous value. Thus,the difference between these two sets is most likely the result ofaveraging effects. Further, the available data on

87Sr/

86Sr ratios of river

Mn(μμg/g)

1000

*Sr/

Ca(

wt

rati

o)

0

2

4

6

8

Mn/Sr

0 400 800 1200 1600

30 1 2 4 5

87Sr

/86 S

r

0.712

0.714

0.716

0.718

0.720b

a

Fig. 3. Scatter plot of (a) Sr/Ca vs. Mn and (b)87Sr/

86Sr ratios vs. Mn/Sr of the carbonate

nodules.

Fig. 4.87Sr/

86Sr ratios in carbonate phase of the nodules from the IITK profile. The

lithostratigraphy and chronology of the core are also presented (Sinha et al., 2007).

Fig. 5. Variations in87Sr/

86Sr ratios of carbonate nodules with depth in JP profile.

Lithostratigraphy and chronology (Sinha et al., 2007) of the core are also presented. The87Sr/

86Sr ratio of carbonates shows sudden rise near the surface at ~2.5 m depth in JP.

189W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

water from the Himalaya show seasonal variations (Bickle et al., 2003;Rai and Singh, 2007; Tripathy and Singh, 2010) which constrains theuse of a single season measurement as the average value for the year.In addition,

87Sr/

86Sr ratio of river water and kankar of the Ganga at

Allahabad (after its confluence with Yamuna) and adjacent ground-water samples nearly overlap with each others, with values of ~0.715(Table 3b). The data of this study and those from Allahabad thusindicate that the shells and carbonates of the Ganga plain incorporateSr with isotope composition within the range of river and ground-waters (Tables 3a and 3b), satisfying the first requirement. The secondrequirement pertains to the preservation of

87Sr/

86Sr ratio with time. In

this study attempts were made to evaluate this by measuring Srisotope composition in separate aliquots of carbonate nodules takenfrom the same depth with the premise that the extent of alteration ofdifferent nodules may be different, thereby impacting their Sr isotoperatio differently. The result of this exercise shows that

87Sr/

86Sr ratio of

separate nodules from same depth agrees within ~0.1% (Table 2),providing an upper limit on the extent of variability.

In addition to diagenesis, contamination of calcretes/kankars withclays and detrital carbonate derived from the Himalaya can also leadto difference in

87Sr/

86Sr ratio compared to that of the groundwater

from which they form. Efforts were made to minimize the impact ofsuch contamination by selecting larger nodules for analysis whichincreases the carbonate content with respect to detrital carbonate.Most of the calcretes used in this study are of a few cm in size. Further,the presence of ~5% detrital carbonates with

87Sr/

86Sr ratio ~0.72

(typical Himalayan carbonates, Singh et al., 1998) in these calcretes,can alter their

87Sr/

86Sr ratio only at 1–10 ppm level, much lower than

the variability observed in87Sr/

86Sr ratio of the calcretes of various

depths in this study. Hence, it can be concluded that inclusion ofdetrital carbonates from the Himalaya into the calcretes has very littleinfluence on the Sr isotope composition of the calcretes used in thisstudy. Further, the consistency in the

14C ages of the carbonates and

the IRSL/OSL ages of Feldspar/quartz, suggest that the contribution ofdetrital carbonates to calcretes analysed, if any, is only minor. Inaddition, significant co-variation of δ

13C and δ

18O of these carbonates,

generally not evident in detrital carbonates (Singh et al., 1998), rulesout any significant detrital contamination. The δ

18O–δ

13C analysis of

soil carbonates from Dagshai formation of the Himalayan forelandbasin shows that burial diagenesis does not result in completehomogenization of δ

18O of secondary carbonates. In addition, the δ

18O

of dense micrites in that of the carbonates seem to preserve thepristine δ

18O values (Bera et al., 2010).

The evidence discussed earlier suggest that the Sr isotope signaturesin the carbonate nodules are of authigenic origin and that they retainisotopic and chemical signatures of the waters from which they wereprecipitated.

5.3. Sr isotopic composition in carbonate

Depth profiles of Sr isotope composition in carbonates from bothcores show that except for two depths at the top of the JP core whichhave high

87Sr/

86Sr ratios ~0.736, the

87Sr/

86Sr ratios vary within a small

range ~0.71416 to 0.72149 (Figs. 4 and 5). Themagnitude of the abruptincrease in

87Sr/86Sr ratio of the JP corenear surface far exceeds the errors

due to analytical uncertainties and sample heterogeneity. Surfacesamples from the JP core have

87Sr/

86Sr ratio within the range of the

values from contemporary river and groundwater samples at Kanpur(Table 3a, Palmer and Edmond, 1992). Therefore, the increase in

87Sr/

86Sr

ratio of near-surface carbonates can be interpreted in terms of temporalvariation in the riverine Sr isotope composition over thousand year timescales, with significantly higher

87Sr/

86Sr ratios during the recent past

compared toolder samplesup to ~100 ka. It is difficult to assign aprecisetime frame to the surface samples from the JP core and hence to theabrupt rise in their

87Sr/

86Sr ratio. Sinha et al. (2007) demonstrated that

the surface sedimentsof the JP core represents recentdeposition and are

younger (b6.8 ka) compared to that of surface sediments from IITK core.The increase in

87Sr/

86Sr ratios of contemporarywater and the pedogenic

carbonates of the Ganga compared to the past ~100 ka is also supportedby a concomitant increase in

87Sr/

86Sr ratios of silicate component of

contemporary sediments. The87Sr/

86Sr ratio of silicate component of

contemporary sediments of theGanga atKanpur (Singh et al., 2008) is atits highest for the past ~100 ka (Rahaman et al., 2009). During the past~100 ka, the

87Sr/

86Sr ratios of silicates of the sediments of the Ganga

have not exceeded 0.767 and generally have remained between 0.73and 0.76 (Rahamanet al., 2009),whereas the present ratio is the highestever, ~0.78 (Singh et al., 2008).

δ18O(‰ PDB)

δ13 C

(‰

PD

B)

-12

-8

-4

0

4b

-8

-4

0

4a

-12 -10 -8 -6 -4 -2 0

-10 -9 -8 -7 -6 -5

Sinha et al., 2006Srivastava, 2001Alam et al., 1997This study

-4

Fig. 6. Scatter diagram of δ13C and δ

18O in carbonate nodules from IITK profile. Data show

significant correlation (r=0.73, n=21, pb0.01, (a)). Covariation plot of δ13C and δ

18O of

carbonates from different areas of the Ganga plain are presented which are available inthe Ganga river literature and compared with our data (b).

190 W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

Dissolved Sr of the Ganga river water is controlled mainly by thethree main tectonic units of the Himalaya; (i) the Lesser Himalayancarbonatesdominatedby lower

87Sr/

86Sr ratiowith scatteredoccurrences

of metamorphosed carbonates with higher87Sr/

86Sr ratio (ii) the Higher

and the LesserHimalayanCrystallines (HHC)withhigh87Sr/

86Sr ratio and

(iii) the Tethyan Sedimentary Sequences (TSS) with low87Sr/

86Sr ratio.

The dissolved Sr of the Ganga river water in study area is a mixture ofcontribution from silicates containing higher

87Sr/

86Sr ratio, and

carbonates in general having lower87Sr/

86Sr ratio (Krishnaswami et al.,

1992; Quade et al., 1997; Singh et al., 1998; Krishnaswami et al., 1999;English et al., 2000; Bickle et al., 2003; Oliver et al., 2003; Tipper et al.,2006). Among these three tectonic units, the Lesser Himalayacontributes 38±8% of contemporary dissolved Sr budget of the Ganga(Bickle et al., 2003). These studies also inferred that the meta-carbonates with higher

87Sr/

86Sr ratio to river waters could be important

in supplying higher87Sr/

86Sr ratio at a local scale; however, their

contribution at basin wide scale is expected to be only minor owing totheir limited exposure in the basin (Singh et al., 1998; Galy et al., 1999;English et al., 2000). Our data on Sr isotope composition of the carbonatenodules does not follow the distribution of Sr in the silicates of the core(Rahaman et al., 2009); however, this comparison is limited in scope ascarbonates are present at selected depths only.

One of the key observations of this study is that all the carbonatenodules of the IITK core and the deeper samples of the JP core (Tables 1and 2) have significantly lower

87Sr/

86Sr ratio than that of the present day

Gangawater at Kanpur (Table 3a of this study and Palmer and Edmond,1992) except for the two near surface samples of the JP core.

5.4. Carbon and oxygen isotope composition

Stable carbon and oxygen isotope compositions of calcretesdeposited in the Ganga plain and the Thar Desert have been previouslyused to reconstruct variations in past climate (Alam et al., 1997;Andrews et al., 1998; Srivastava, 2001; Sinha et al., 2006) with thepremise that the carbonate noduleswere formed in isotopic equilibriumwith soil water. The carbonate nodules collected from IITK core in thisstudy were analysed for their carbon and oxygen isotope compositionswith the aim of reconstructing the past climate and vegetation patternthat prevailed during their formation. There is a large spatial variabilityin climatic pattern and hydrological system across the Ganga plain(Sinha et al., 2005) and from foothills of the Himalaya to Bangladesh(Singh et al., 2008). The study location lies in semi-humid regionwheresummer temperature exceeds 45 °C with scanty precipitation duringsummer (April–May; Singh, 1994). Under this condition evaporation ismore extensive and calcite super-saturation and its precipitation arecommon phenomena (Jacobson et al., 2002; Dhir et al., 2004). Thesource of dissolved inorganic carbon is chemical weathering of silicateand carbonate rocks with soil CO2. It is estimated that 50% of the totalalkalinity budget of the Himalayan rivers is from the dissolution ofcarbonate rocks of the Himalaya (Galy et al., 1999). The δ

13C

and δ18O values of the Precambrian carbonates from India Himalaya

are 0.7±1.2‰ and −9.1±2.9‰ respectively (Singh et al., 1998). Theδ13C of reservoir from which carbonate precipitate is controlled by the

carbon isotope composition of DIC which includes the signature of thelocal soil CO2 derived from the vegetation in the basin (Cerling, 1984).The vegetation assemblage could comprise of various proportions of C3and/or C4 type depending on the climatic as well as paleohydrologicconditions. The δ

13C (PDB) values of C3 plants range from −20‰ to

−35‰with an average of−27‰ (Farquhar et al., 1989) and that of C4type vegetation ranges from−6‰ to−19‰with an average of−13‰(Smith and Epstein, 1971).

Carbonate nodules from the IITK core show a range of variability of~3‰ in their δ

18O (PDB, Table 1). For paleoclimatic interpretation of

the δ18O, it is required that the carbonate nodules are formed in

isotope equilibrium with soil water. Sinha et al. (2006) demonstratedthat the pedogenic carbonates precipitated in areas around Kanpur

are in isotopic equilibriumwith soil water. The calculated δ18O value of

pedogenic carbonates (−7.3‰, PDB), taking the δ18O value of soil

water to be −5.4‰ (SMOW) and mean annual temperature of 25 °C(Singh, 1994) is same as that of shallow groundwater near Kanpur(Krishnamurthy and Bhattacharya, 1991). This value is very close tothe average δ

18O value −7‰ (PDB) obtained for the carbonate

samples from the Ganga valley near Kanpur (Sinha et al., 2006). Theδ18O value in carbonate nodules in this study range between−8.3 and

−5.4‰, similar to the range observed by Sinha et al. (2006); hence,these carbonates can also be considered to have formed in isotopicequilibrium rendering them suitable for paleoclimatic reconstruction.The δ

18O value of carbonates indicates significant positive correlation

with δ13C (r=0.73, n=21, pb0.01; Fig. 6a) which can be interpreted

as a manifestation of evaporation influencing the oxygen isotopecomposition and varying vegetation type controlling the carbonisotope composition. For example, the higher O and C isotope valuesfor calcretes observed in this study could arise due to higherevaporation and increase in proportion of C4 vegetation (Andrewset al., 1998). The positive correlation between δ

18O and δ

13C values of

calcretes could also arise due to kinetic fractionation during theirprecipitation (Hendy, 1971; Mickler et al., 2006). In such cases the Cand O isotope composition of carbonates are not quite suitable toextract information for paleoclimate and paleovegetation (Hendy,1971). However, kinetic fractionation affects the calcite precipitationin caves where rapid degassing takes place and not in case of thepedogenic carbonates.

The δ18O and δ

13C variations are expected to be the direct

consequences of climatic changes whereas the Sr isotope compositionvariation is causedby thevariation in theproportion of its sourceswhichin turn also seems to be controlled by climate. It is noteworthy toobserve that δ

13C of calcretes in the IITK profile varies synchronously

191W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

with δ18O and both correlate negatively with their

87Sr/

86Sr ratio; i.e. the

‘peaks’ of the δ13C and δ

18O coincide with the ‘lows’ of the

87Sr/

86Sr ratio

(Fig. 7). In general,87Sr/

86Sr ratio in calcretes of IITK core shows a uniform

composition of ~0.717 throughout the profile except that decreases at~32 ka and ~27 ka and ~4 ka. As mentioned earlier, the occurrence ofcalcretes in the core is non-uniform and at many depths it was absent.However, the available data from the IITK core suggests a decreasein

87Sr/

86Sr ratio from a uniform value of 0.717 to 0.7142 at ~27 ka and a

further decrease at ~4 ka. The most depleted values of δ18O (−8.3‰;

PDB) and δ13C (−6.8‰; PDB)are observedat6.8 mdepth corresponding

to ~17.5 ka of depositional age. This δ18O value is lower than that

expected fromcontemporary rainwater close to the core location (Sinhaet al., 2006). Possible explanation of the lower δ

18O value at ~17.5 ka

could be the increased supply of snowmelt water from the Himalayaand/or increase in rainfall during the declining phase of LGM. Theincrease in snowmeltwater component could alsoexplain the lower δ

13C

values and increase in87Sr/

86Sr ratio due to enhanced silicate weathering

(Das et al., 2005) resulting from exposure of fresh and fine silicatematerial from the glacial moraines during the decline of the LGM phase(Vance et al., 2009). Silicate weathering derived HCO3

− has moredepleted δ

13C than that of the carbonates. Decrease in

87Sr/

86Sr ratio and

corresponding increase in δ18O and δ

13C at similar depths (Fig. 7) could

result from the enhanced monsoon causing increase in carbonateweathering.

5.5. Recent abrupt rise in87Sr/

86Sr ratios of the Ganga and its implications

The inference that87Sr/

86Sr ratio of the contemporary Ganga is

significantly higher than the values measured for the past 100 ka isintriguing. Some of the earlier studies have reported a change in

87Sr/

86Sr

ratio on million year times scale (Quade et al., 1997; Derry and France-Lanord, 1996). Theobservationof sudden rise in

87Sr/

86Sr ratio inpaleosol

carbonates from the Siwalik during Late Miocene was explained interms of exhumation and weathering of higher

87Sr/

86Sr ratio metali-

mestones in the central Himalaya (Quade et al., 1997). A probableexplanation for the recent sudden increase in

87Sr/

86Sr ratio is the

relatively enhanced contribution of higher87Sr/

86Sr ratio from silicates,

calc-silicates and metacarbonates of the Lesser Himalaya. These

87Sr/86Sr

Age

(ka)

0

20

40

60

80

100

δδ13C (

0.712 0.716 0.720 -8 -4

MIS 3

MIS 1

MIS 3MIS 4

MIS 5

L

MIS 2

Last

Fig. 7. Temporal variations in87Sr/

86Sr, δ

13C and δ

18O of the carbonate nodules from IITK profile

corresponding increase in δ18O and δ

13C at certain depths. MIS stages and events are based

lithologies of the Lesser Himalaya are characterized by higher87Sr/

86Sr

ratio, enhanced weathering of which could contribute the excess87Sr to

the contemporary Ganga. The reason for enhanced weathering of theLesser Himalaya lithologies with higher

87Sr/

86Sr ratio during last few

thousands of years is not clear. It could either be climatic, tectonic oranthropogenic. The Holocene intensification of monsoon or theanthropogenic activities such as expansion of cultivated land anddeforestation in the Lesser Himalaya since the onset of humanoccupation could be the plausiblemechanisms to enhancemore erosionover the Lesser Himalaya.Morework is required to fully understand thecausative factors for the recent increase in

87Sr/

86Sr ratios of the Ganga

water. The paucity of data on the exact timing of the increase in87Sr/

86Sr

ratio of the carbonates and hence of the Ganga water further constrainsinferring about its causes.

The results of this study show recent abrupt increase in87Sr/

86Sr

ratios of the Ganga water from ~0.718 to ~0.735 which had remainedwithin a narrow range of ~0.713 to 0.718 prior to that for the past~100 ka. If this observation is representative of the entire Ganga basin,it would suggest that the present day

87Sr/

86Sr ratio of the Ganga water

is not representative of its value since the Cenozoic as has beenassumed to evaluate the impact of the Himalayan orogeny on the Srisotope evolution of seawater. If the significantly higher

87Sr/

86Sr ratio

of contemporary Ganga water is a result of some recent perturbation,then the earlier estimates of the long-term contribution of the Gangato the Sr isotope evolution of the seawater may be overestimated.

6. Conclusions

87Sr/

86Sr ratios, δ

18O and δ

13C of carbonate nodules analysed from

two cores in the Ganga plain show significant temporal variationsduring the past ~100 ka. Chemical composition coupled withpetrography of these carbonates suggest their minimal diagenesisthereby facilitating the preservation of their pristine chemical andisotopic signatures. d18O and d13C show significant co-variation. Thiscould indicate climatic variation as decrease in temperature andhumidity enhances evaporation causing enrichment in δ

18O. These

climatic conditions will also be associated with the increase in theproportion of C4 plants and therefore higher δ

13C. The

87Sr/

86Sr ratios of

‰ PDB) δδ18O (‰ PDB)

0 4 -10 -8 -6 -4

Hypsithermal

ast Glacial Maxima (24-21 ka)

interstade (58 - 24 ka)

. The87Sr/

86Sr ratios of the calcretes of IITK core is generally ~0.717 which decreases with

on Martinson et al. (1987) and Goodbred et al. (2003) and references there in.

192 W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

the calcretes are controlled by the composition of the water fromwhich they precipitate. This study shows that the Sr isotope compo-sition of the Ganga water was low compared to contemporary waterduring the last 100 ka except during the very recent past.

87Sr/

86Sr ratio

of the Ganga water has increased recently which could be a result ofenhanced weathering of the Lesser Himalaya containing lithologieswith higher

87Sr/

86Sr ratio. Recent enhanced weathering in the Lesser

Himalaya could result from human occupation and associated anthro-pogenic activities over the mountains and/or due to climatic vari-ability. This study emphasizes the need of reconstructing temporalvariation in

87Sr/

86Sr ratio of the paleo Ganga at its outflow to assess its

impact on oceanic87Sr/

86Sr ratios evolution.

Acknowledgements

Discussions with Prof. S. Krishnaswami helped to improve thismanuscript significantly.We thankDr. S. K. Rai for providingfive data ofSr isotopes mentioned in Table 3. We thank Dr. Ravi Bhushan, D. Pandaand Prof. S.K. Bhattacharya for their help in inorganic carbon, for takingSEM photographs and stable isotope measurements respectively.Reviews of Drs. Jay Quade, Albert Galy and two anonymous reviewersimproved this manuscript considerably. We thank Dr. Joel D Blum foreditorial handling and useful comments on this manuscript.

References

Agarwal, A.K., 1992. Carbonate deposits in the Ganga plain. In: Singh, I.B. (Ed.), Gangeticplain. Terra Incognita. Geology Department, Lucknow University, pp. 35–43.

Alam, M.S., Keppens, E., Paepe, R., 1997. The use of oxygen and carbon isotopecomposition of pedogenic carbonates from Pleistocene palaeosols in NWBangladesh, as palaeoclimatic indicators. Quaternary Science Reviews 16, 161–168.

Andrews, J.E., Singhvi, A.K., Kailath, A.J., Kuhn, R., Dennis, P.F., Tandon, S.K., Dhir, R.P.,1998. Do stable isotope data from calcrete record Late Pleistocene monsoonalclimate variation in the Thar Desert of India? Quaternary Research 50, 240–251.

Asahara, Y., Tanaka, T., Kamioka, H., Nishimura, A., 1995. Asian continental natureof

87Sr/

86Sr ratios in north central Pacific sediments. Earth and Planetary Science

Letters 133, 105–116.Banner, J.L., Hanson, G.N., 1990. Calculation of simultaneous isotopic and trace element

variations during water–rock interaction with applications to carbonate diagenesis.Geochimica et Cosmochimica Acta 54, 3123–3137.

Basu, A.R., Jacobsen, S.B., Poreda, R.J., Dowling, C.B., Aggarwal, P.K., 2001. Largegroundwater strontium flux to the oceans from the Bengal Basin and the marinestrontium isotope record. Science 293, 1470.

Behrensmeyer, A.K., Quade, J., Cerling, T.E., Kappelman, J., Khan, I.A., Copeland, P., Roe,L., Hicks, J., Stubblefield, P., Willis, B.J., Latorre, C., 2007. The structure and rate oflate Miocene expansion of C4 plants: evidence from lateral variation in stableisotopes in paleosols of the Siwalik Group, northern Pakistan. Geological Society ofAmerica Bulletin 119, 1486–1505.

Bera, M.K., Sarkar, A., Tandon, S.K., Samanta, A., Sanyal, P., 2010. Does burial diagenesisreset pristine isotopic compositions in paleoso carbonates? Earth and PlanetaryScience Letters 300, 85–100.

Bhushan, R., Dutta, K., Somayajulu, B.L.K., 2001. Concentrations and burial fluxes oforganic and inorganic carbon on the eastern margins of the Arabian Sea. MarineGeology 178, 95–113.

Bickle, M.J., Bunbury, J., Chapman, H.J., Harris, N.B.W., Fairchild, I.J., Ahmad, T., 2003.Fluxes of Sr into the headwaters of the Ganges. Geochimica et Cosmochimica Acta67, 2567–2584.

Bookhagen, B., Thiede, R.C., Strecker, M.R., 2005. Late Quaternary intensified monsoonphases control landscape evolution in the northwest Himalaya. Geology 33, 149–152.

Burbank, D.W., Blythe, A.E., Putkonen, J., Pratt-Sitaula, B., Gabet, E., Oskin, M., Barros, A.,Ojha, T.P., 2003. Decoupling of erosion and precipitation in the Himalayas. Nature426, 652–655.

Capo, R.C., Chadwick, O.A., 1999. Sources of strontium and calcium in desert soil andcalcrete. Earth and Planetary Science Letters 170, 61–72.

Cerling, T.E., 1984. The stable isotopic composition of modern soil carbonate and itsrelationship to climate. Earth and Planetary Science Letters 71, 29–240.

Cerling, T.E., Harris, J.M., MacFadden, B.J., Leakey, M.G., Quade, J., Eisenmann, V.,Ehleringer, J.R., 1997. Global vegetation change through the Miocene/Plioceneboundary. Nature 389, 153–158.

Chiquet, A., Michard, A., Nahon, D., Hamelin, B., 1999. Atmospheric input vs in situweathering in the genesis of calcretes: an Sr isotope study at Gálvez (CentralSpain). Geochimica et Cosmochimica Acta 63, 311–323.

Clift, P.D., Giosan, L., Blusztajn, J., Campbell, I.H., Allen, C., Pringle, M., Tabrez, A.R.,Danish, M., Rabbani, M.M., Alizai, A., Carter, A., Luckge, A., 2008. Holocene erosion ofthe Lesser Himalaya triggered by intensified summer monsoon. Geology 36, 79–82.

Dalai, T.K., Singh, S.K., Trivedi, J.R., Krishnaswami, S., 2002. Dissolved rhenium in theYamuna River System and the Ganga in the Himalaya: role of black shale

weathering on the budgets of Re, Os, and U in rivers and CO2 in the atmosphere.Geochimica et Cosmochimica Acta 66, 29–43.

Das, A., Krishnaswami, S., Bhattacharya, S.K., 2005. Carbon isotope ratio of dissolvedinorganic carbon (DIC) in rivers draining the Deccan Traps, India: sources of DICand their magnitudes. Earth and Planetary Science Letters 236, 419–429.

DeCelles, P.G., Gehrels, G.E., Quade, J., Ojha, T.P., Kapp, P.A., Upreti, B.N., 1998. Neogeneforeland basin deposits, erosional unroofing, and the kinematic history of theHimalayan fold-thrust belt, western Nepal. Geological Society of America Bulletin110, 2–21.

Denison, R.E., Koepnick, R.B., Burke,W.H., Hetherington, E.A., Fletcher, A., 1994. Constructionof the Mississippian, Pennsylvanian and Permian seawater curve

87Sr/

86Sr. Chemical

Geology 112, 145–167.Derry, L.A., France-Lanord, C., 1996. NeogeneHimalayanweathering history and river

87Sr/

86Sr: Impact on the marine Sr record. Earth and Planetary Science Letters 142, 59–74.

Dhir, R.P., Tandon, S.K., Sareen, B.K., Rao, T.K.G., Kailath, A., Ramesh, R., 2004. Calcretesin the Thar desert: genesis, chronology and Palaeoenvironment. Proc. Ind. Acad. Sci.(Earth Planetary Sci.) 113, 473–515.

Edmond, J.M., 1992. Himalayan tectonics, weathering processes, and the strontiumisotope record in marine limestones. Science 258, 1594–1597.

English, N.B., Quade, J., DeCelles, P.G., Garzione, C.N., 2000. Geologic control of Sr andmajor element chemistry in Himalayan Rivers, Nepal. Geochimica et CosmochimicaActa 64, 2549–2566.

Farquhar, G.D., Ehleringer, J.R., Hubick, K.T., 1989. Carbon isotope discrimination andphotosynthesis. Annual Review of Plant Physiology and Plant Molecular Biology 40,503–537.

Galy, A., France-Lanord, C., 2001. Higher erosion rates in the Himalaya: geochemicalconstraints on riverine fluxes. Geology 29, 23–26.

Galy, A., France-Lanord, C., Derry, L.A., 1999. The strontium isotopic budget of Himalayanrivers in Nepal and Bangladesh. Geochimica et Cosmochimica Acta 63, 1905–1925.

Goodbred Jr., S.L., Kuehl, S.A., Steckler, M.S., Sarker, M.H., 2003. Controls on faciesdistribution and stratigraphic preservation in the Ganges–Brahmaputra deltasequence. Sedimentary Geology 155, 301–316.

Hart, W.S., Quade, J., Madsen, D.B., Kaufman, D.S., Oviatt, C.G., 2004. The87Sr/

86Sr ratios of

lacustrine carbonates and lake-level history of the Bonneville paleolake system.Geological Society of America Bulletin 116, 1107–1119.

Hay, W.W., 1998. Detrital sediment fluxes from continents to oceans. Chemical Geology145, 287–323.

Hendy, C.H., 1971. The isotopic geochemistry of speleothems-I. The calculation of theeffects of different modes of formation on the isotopic composition of speleothemsand their applicability as palaeoclimatic indicators. Geochimica et CosmochimicaActa 35, 801–824.

Jacobson, A.D., Blum, J.D., Walter, L.M., 2002. Reconciling the elemental and Sr isotopecomposition of Himalayan weathering fluxes: insights from the carbonategeochemistry of stream waters. Geochimca et Cosmochimica Acta 66, 3417–3429.

Krishnamurthy, R.V., Bhattacharya, S.K., 1991. Stable oxygen and hydrogen isotoperatios in shallow ground waters from India and a study of role of evapotranspi-ration in the Indian Monsoon. In: Taylor Jr., H.P., O'Neil, J., Kaplan, I.R. (Eds.), Stableisotope geochemistry: a tribute to Samuel Epstein: Special Publication-Geochem-ical Society, 3, pp. 187–193.

Krishnaswami, S., Trivedi, J.R., Sarin,M.M., Ramesh,R., Sharma,K.K., 1992. Strontiumisotopesand rubidium in the Ganga–Brahmaputra river system: weathering in the Himalaya,fluxes to the Bay of Bengal and contributions to the evolution of oceanic

87Sr/

86Sr. Earth

and Planetary Science Letters 109, 243–253.Krishnaswami, S., Singh, S.K., Dalai, T., 1999. Silicate weathering in the Himalaya: role in

contributing to major ions and radiogenic Sr to the Bay of Bengal. In: Somalyajulu,B.L.K. (Ed.), Ocean Science, Trends and Future Directions. Indian National ScienceAcademy and Akademia International, New Delhi, pp. 23–51.

Leier, A., Quade, J., DeCelles, P., Kapp, P., 2009. Stable isotopic results from paleosolcarbonate in South Asia: paleoenvironmental reconstructions and selectivealteration. Earth and Planetary Science Letters 279, 242–254.

Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J.D., Moore, T.C., Shackleton, N.J., 1987.Age dating and the orbital theory of the ice ages: development of a high-resolution0 to 300,000-year chronostratigraphy. Quaternary Research 27, 1–29.

Mickler, P.J., Stern, L.A., Banner, J.L., 2006. Large kinetic isotope effects in modernspeleothems. Geological Society of America Bulletin 118, 65–81.

Molnar, P., England, P., 1992. Climate and landscape response. Nature 355, 306.Oliver, L., Harris, N., Bickle, M., Chapman, H., Dise, N., Horstwood, M., 2003. Silicate

weathering rates decoupled from the87Sr/

86Sr ratio of the dissolved load during

Himalayan erosion. Chemical Geology 201, 119–139.Pal, D.K., Srivastava, P., Bhattacharyya, T., 2003. Clay illuviation in calcareous soils of the

semiarid part of the Indo-Gangetic Plains, India. Geoderma 115, 177–192.Palmer, M.R., Edmond, J.M., 1992. Controls over the strontium isotope composition of

river water. Geochimica et Cosmochimica Acta 56, 2099–2111.Quade, J., Cerling, T.E., 1995. Expansion of C4 grasses in the late Miocene of northern

Pakistan: evidence from stable isotopes in paleosols: palaeogeography. Palaeocli-matology, Palaeoecology 115, 91–116.

Quade, J., Lois, J.R., 1999. The stable-isotope composition of early ground-watercements from sandstone in paleoecological reconstruction. Journal of SedimentaryResearch 69, 667–674.

Quade, J., Cerling, T.E., Bowman, J.R., 1989. Systematic variations in the carbon and oxygenisotopic composition of pedogenic carbonate along elevation transects in the southernGreat Basin. United States: Geological Society of America Bulletin 101, 464–475.

Quade, J., Cater, J.M.L., Ojha, T.P., Adam, J., Harrison, T.M., 1995a. Dramatic carbon andoxygen isotopic shift in paleosols from Nepal and late Miocene environmentalchange across the northern Indian sub-continent. Geological Society of AmericaBulletin 107, 1381–1397.

193W. Rahaman et al. / Chemical Geology 285 (2011) 184–193

Quade, J., Chivas, A., McCulloch, M.T., 1995b. Strontium and carbon isotope tracers andthe origins of soil carbonate in South Australia and Victoria. PalaeogeographyPalaeoclimatology Palaeoecology 113, 103–117.

Quade, J., Roe, L., DeCelles, P.G., Ojha, T.P., 1997. The late neogene87Sr/

86Sr record of

lowland Himalayan rivers. Science 276, 1828–1831.Rahaman, W., Singh, S.K., Sinha, R., Tandon, S.K., 2009. Climate control on erosion

distribution over the Himalaya during past 100 ka. Geology 37, 559–562.Rai, S.K., Singh, S.K., 2007. Temporal variation in Sr and

87Sr/

86Sr of the Brahmaputra:

Implications for annual for fluxes and tracking flash floods through chemical andisotope composition. Geochemistry Geophysics Geosystems 8, 14.

Rai, S.K., Singh, S.K., Krishnaswami, S., 2010. Chemical weathering in the plain andpeninsular sub-basins of the Ganga: impact on major ion chemistry and elementalfluxes. Geochimica et Cosmochimica Acta 74, 2340–2355.

Raymo, M.E., Ruddiman, W.F., 1992. Tectonic forcing of late Cenozoic climate. Nature359, 117–122.

Richter, F.M., Rowley, D.B., DePaolo, D.J., 1992. Sr isotope evolution of seawaters the roleof tectonics. Earth and Planetary Science Letters 170, 197–204.

Rosemary, C.C., Whipkey, C.E., Blachère, J.R., Chadwick, O.A., 2000. Pedogenic origin ofdolomite in a basaltic weathering profile, Kohala peninsula, Hawaii. GeologicalSociety of America 28, 271–274.

Salomons, W., Gourdie, A., Mook, W.G., 1978. Isotopic composition of calcrete depositsfrom Europe, Africa and India. Earth Surface Processes 3, 43–57.

Sarin, M.M., Krishnaswami, S., Dilli, K., Somayajulu, B.L.K., Moore, W.S., 1989. Major ionchemistry of the Ganga-Brahmaputra river system: weathering processes andfluxes to the Bay of Bengal. Geochimica et Cosmochimica Acta 53, 997–1009.

Sharma, S., Joachimski, M., Sharma, M., Tobschall, H.J., Singh, I.B., Sharma, C., Chauhan,M.S., Morgenroth, G., 2004. Lateglacial and Holocene environmental changes inGanga plain, Northern India. Quaternary Science Reviews 23, 145–159.

Singh, I.B., 1992. Geological evolution of Gangetic plain: present status. In: Singh, I.B. (Ed.),Gangetic plain. Terra Incognita. Geology Department, Lucknow University, pp. 1–14.

Singh, R.L., 1994. India: a Regional Geography. National Geographical Society of India,Varanasi.

Singh, S.K., Trivedi, J.R., Pande, K., Ramesh, R., Krishnaswami, S., 1998. Chemical andstrontium, oxygen, and carbon isotopic compositions of carbonates from the LesserHimalaya: implications to the strontium isotope compositionof the sourcewaters of theGanga, Ghaghara, and the Indus rivers. Geochimica et Cosmochimica Acta 62, 743–755.

Singh, S.K., Sarin, M.M., France-Lanord, C., 2005. Chemical erosion in the easternHimalaya: major ion composition of the Brahmaputra and δ

13C of dissolved

inorganic carbon. Geochimica et Cosmochimica Acta 69, 3573–3588.Singh, S.K., Rai, S.K., Krishnaswami, S., 2008. Sr and Nd isotopes in river sediments from

the Ganga Basin: sediment provenance and hotspots of physical erosion. Journal ofGeophysical Research 113, F03006.

Sinha, R., Jain, V., Prasad, Babu G., Ghosh, S., 2005. Geomorphic characterization anddiversity of the fluvial systems of the Gangetic plains. Geomorphology 70, 207–225.

Sinha, R., Tandon, S.K., Sanyal, P., Gibling, M.R., Stuben, D., Berner, Z., Ghazanfari, P.,2006. Calcretes from a Late Quaternary interfluve in the Ganga Plains, India:carbonate types and isotopic systems in a monsoonal setting. Palaeogeography,Palaeoclimatology, Palaeoecology 242, 214–239.

Sinha, R., Bhattacharjee, P.S., Sangode, S.J., Gibling, M.R., Tandon, S.K., Jain, M., Godfrey-Smith, D., 2007. Valley and interfluve sediments in the Southern Ganga plains, India:exploring facies and magnetic signatures. Sedimentary Geology 201, 386–411.

Sinha R., Kettanah Y., Gibling M.R., Tandon S.K., Jain M., Bhattacharjee P.S., DasguptaA.S., Ghazanfari P., 2009. Craton-derived alluvium as a major sediment source inthe Himalayan Foreland Basin of India. GSA Bulletin, 121, 1596–1610.

Smith, B.N., Epstein, S., 1971. Two categories of13C/

12C ratios for higher plants. Plant

Physiology 47, 380–384.Srivastava, P., 2001. Paleoclimatic implications of pedogenic carbonates in Holocene

soils of the Gangetic Plains, India. Palaeogeography, Palaeoclimatology, Palaeoe-cology 172, 207–222.

Srivastava, P., Rajak, M.K., Sinha, R., Pal, D.K., Bhattacharyya, T., 2010. A high-resolutionmicromorphological record of the Late Quaternary paleosols from Ganga–Yamunainterfluve: stratigraphic and paleoclimatic implications. Quaternary International227, 127–142.

Stewart, B.W., Capo, R.C., Chadwick, O.A., 1998. Quantitative strontium isotope modelsfor weathering, pedogenesis and biogeochemical cycling. Geoderma 82, 173–195.

Tipper, E.T., Bickle, M.J., Galy, A., West, A.J., Pomies, C., Chapman, H.J., 2006. The short termclimatic sensitivity of carbonate and silicate weathering fluxes: insight from seasonalvariations in river chemistry. Geochimica et Cosmochimica Acta 70, 2737–2754.

Tripathy, G.R., Singh, S.K., 2010. Chemical erosion rates of river basins of the Gangasystem in the Himalaya: reanalysis based on inversion of dissolved major ions, Sr,and

87Sr/

86Sr. Geochemistry Geophysics Geosystem 11, Q03013.

Van der Hoven, S.J., Quade, J., 2002. Tracing spatial and temporal variations in thesources of calcium in pedogenic carbonates in a semiarid environment. Geoderma108, 259–276.

Vance, D., Teagle, D.A.H., Foster, G.L., 2009. Variable quaternary chemical weatheringfluxes and imbalances in marine geochemical budgets. Nature 458, 493–496.

Veizer, J., 1983. Trace elements and isotopes in sedimentary carbonates. Carbonates:Mineralogy and Chemistry 11, 265–299.

Veizer, J., 1989. Strontium isotopes in seawater through time. Annual Review of Earthand Planetary Sciences 17, 141–167.

Wobus, C., Helmsath, A., Whipple, K., Hodges, K., 2005. Active out-of-sequence thrustfaulting in the central Nepalese Himalaya. Nature 434, 1008–1011.

Wright, V.P., Tucker, M.E., 1991. Calcretes: an introduction. In: Wright, V.P., Tucker, M.E.(Eds.), Calcretes. Repr. Ser. Intern. Assoc. Sediment, 2, pp. 1–22.

Yang, J., Chen, J., An, Z., Shields, G., Tao, X., Zhu, H., Ji, J., Chen, Y., 2000. Variations in87Sr/

86Sr

ratios of calcites in Chinese loess: a proxy for chemicalweathering associatedwith theEast Asian summer monsoon. Palaeogeography, Palaeoclimatology, Palaeoecology157, 151–159.