Chemical Compositions And Sr-Nd Isotopic Fingerprints Of Sediments In The Selected River Systems Of India And China

Use of radiogenic isotopes of Sr and Nd are widely embraced in geochemical studies to understand the signals of chemical and physical erosion processes present in environmental samples(suspended particulate matter(SPM)and sediments).During the process of chemical weathering in minerals and rocks,Sr isotopes follow non-stochiometric release,so vary temporally in the riverine fluxes due to rainfall pattern variability e.g.tropical vs.non-tropical watersheds.In contrast,the Nd isotopes unambiguously fractionate during surficial processes such as erosion,transportation and diagenesis,therefore strongly preserve the provenance information.Collectively,the studies for the provenance determination and physical erosion processes employ both Sr and Nd isotopes under the assumption that their source signatures remain preserved in sedimentsPresent thesis documented extensive study of Sr-Nd isotopic ratios in acid insoluble(silicate)and acid soluble fractions present in sediments and bulk SPM collected from Indian summer monsoon(or south west monsoon)dominated watersheds e.g.Narmada(large river)and Netravati(small river)of western continental margin of India along with the large Changjiang(CJ)of China Geochemical concentrations of trace and major elements measured alongside the Sr-Nd isotopic ratios,to tag chemical weathering intensities and potential anthropogenic perturbations prevalent across these watersheds.The enormous sampling includes seasonal sample collection during three seasons(pre-monsoon,during southwest monsoon and post-monsoon)from Narmada and Netravati,followed by a strong spatially resolved sampling in Changjiang and a time series observation for a one-year period in the lower reaches at Xuliujing(XLJ).Chemically and ion exchange chromatographically purified samples analyzed with methodology modified from the conventional ones and described in Chapter II.High Resolution Inductively Coupled Plasma Mass Spectrometry(HR-ICP-MS)and a Multi-Collector Inductively Coupled Plasma Mass Spectrometry(MC-ICP-MS)used to determine the elemental concentrations and isotopic compositions respectively.Results indicated that sedimentary Sr isotopic ratios in acid soluble fractions from Narmada and Netravati Rivers fall within the proximity of global average modern seawater(0.7092)and river water(0.7119)therefore could be representative of authigenic origin in the same rivers.A simple end-member mixing result revealed provenance of sediments in Narmada as mid-late Proterozoic Vindhyan(-30%)and Quaternary Deccan deposits(-70%)with Mesoproterozoic to Mesoarchean TDMNd ages.Contrastingly,sediments in Netravati holds signatures of peninsular gneisses and gneisses schists(?30%)and dominant granadiorite(?70%)with Paleoarchean TDMNd ages.Moreover,both the watersheds showed strong variation in Sr-Nd isotopic compositions in SPM during different seasons.During southwest monsoon season,SPM from Narmada characterized by less radiogenic Sr isotope and more radiogenic?Nd(0)values,whereas SPM in Netravati exhibit more radiogenic Sr and less radiogenic eNd(0)together evidenced as control of basin lithology in seasonal variation of Sr-Nd in these watersheds.The weighted average Sr and Nd fluxes showed roughly similar signatures during southwest monsoon(Sr:0.7155;Nd:0.5120)and non-monsoon(Sr:0.7143;Nd:0.5122)in Narmada.In contrast,the fluxes of Sr and Nd showed contrast signatures during southwest monsoon(Sr:0.7269;Nd:0.51136)and non-monsoon(Sr:0.7135;Nd:0.5112)in Netravati.Such contrasts reflects strong control of difference in the amount and frequency of rainfall as well as sediment load between these two watersheds among different seasons.Elemental geochemistry of sediment indicated moderate chemical weathering in Narmada,whereas it suggest intense chemical weathering in Netravati.Overall REE patterns in SPM and sediments of Narmada and Netravati Rivers highlighted towards enrichment of LREE in Netravati than Narmada.In addition,REEs in sediments from Netravati constitutes signature of more fractionation than that in Narmada,evidenced as control of source rocks in the drainage basin in both the watersheds.Collectively,the elemental stoichiometry indicated towards a moderate state of alteration in Narmada as well as Netravati,exceptional is that the state of alteration in Netravati is due to accumulation of few elements,which indicate that Narmada to be influence by more anthropogenic forces than NetravatiThe isotopic compositions in Changjiang(CJ)revealed differences in Sr isotope in SPM as well as sediments relates to the difference in the mineral phase involved in the weathering reactions and suggests difference in the calcium(CaO)content in the upper to lower CJ basin.The large differences in ?Nd(0)values among acid soluble and acid insoluble fraction indicates that the bed load sediments in CJ are isotopically very in-homogeneous and that the leachable reservoir contains a mineral phase with high 143Nd/144Nd and 147Sm/144Nd ratios.Based on the Sr-Nd isotopic ratios,sediments in CJ basin are notably from(1)weathering products of adjacent bedrock,(2)distal bedrock that extend across major geological boundaries and(3)dissolution products of bedload,which corresponds to mixture of local bedrock as well as components from rock units exposed in upstreamThe times series observation in SPM sample for one-year period in the lower stream of CJ at XLJ showed similar REE concentrations and fractionation patterns to the upper CJ sediments suggesting influence of source rocks.The isotopic results showed lower Sr ratios and higher ?Nd(0)values during the high discharge period and reverse during for low discharge period.The seasonal variations primarily reflect the controls of provenance rocks,therefore showed strong contribution from upper CJ endmember such as Tibetan Plateau(TP)during the period of high discharge(70.5±1.1%),whereas contribution from lower CJ endmember such as South China Granites(SCG)during the period of low discharge(64.3±4.4%)in the SPM of XLJ.In addition,transport of sediment from the upper CJ during the period of high water discharge highlighted the condition of lower sediment flux from lower CJ.Higher contribution of SCG during the period of low water discharge is indicative towards channel erosion process of the lower reaches owing to Three Gorges Dam construction and reflected through radiogenic Sr and non-radiogenic Nd signatures in the SPM of XLJ.Based on the changes in the Sr-Nd isotopic signals the sediment flux transported from the upper basin to lower basin of the CJ accounts to 59%from the upper basin and 41%from the lower basinDominant control of the lithologies in the upper reaches incorporated mainly in mafic fragments,and granitoids in the middle lower reaches of the CJ evidenced from elemental geochemistry of SPM and sediments in this study.Signals of apparently weak weathering found in upper reaches sediment,whereas strong degree of weathering found in middle-lower reaches.Contributions from ultramafic rocks to the upper reaches sediment,whereas lithic fragments and detrital feldspars,of lighter minerals in the fine-grained fraction to the Middle-lower reached evidenced in this study.When compared with the historical values,the selected elemental concentrations in SPM and sediments from CJ showed higher limits due to changing global conditions,which affected most of the river systems in the world.Collectively,overall pattern of weathering and temporal variations of Sr-Nd isotopes along with elemental geochemistry in a suite of small to mid-sized and extremely large watersheds revealed important information of comparative analysis of controls of geology,geomorphology and climate on erosion regime in this study.Extending this state of knowledge,present thesis also synthesize a global pattern based on the Sr-Nd isotopic compositions found in different rivers of world,which showed an additional control of physiographic properties acted along the watersheds become a maj or outcome of the idea for further research.