Sources And Composition Of Organic Carbon In Tropical River Systems:Case Studies From India And China

River transport of organic carbon (OC) is an important component of the global carbon cycle; in contrast its compositions extremely sensitive to the local and global environmental perturbation. It plays a crucial role in the phenomenon of continental control of atmospheric CC>2; during re-mineralization and preservation processes in the terrestrial as well as marine reservoirs. The characterization of terrestrial OM across the river systems will provide important information on the carbon sources as well as their controlling processes, which will be helpful to predict their further potential role in the global carbon cycle.Both natural and anthropogenic factors impinge on the biogeochemical cycle of carbon in the terrestrial as well as aquatic environments. The natural factor consists of Variability in the precipitation (e.g. excess (flood/typhoon) and/or less rainfall (drought)) due to the global climate change, whereas the anthropogenic activities include dam construction, deforestation and intensive agriculture due to the global population rise in the last century. Studying the impact of these processes in the tropical region has become major thrust area of global scientific community, which is due to the observed intense impact of global environmental change (changes in precipitation and land use change) on the aquatic ecosystems of the tropical region. The effects of natural and anthropogenic processes on terrestrial OM in tropical region as well as their migration and circulation can be easily identified by understanding the carbon fluxes and their compositional alterations across the tropical river systems.India and China, two developing countries generally experienced a large-scale climate change (e.g. reduced monsoon rainfall and extreme typhoon events) and the transformation of human activities (e.g. land use patterns changes in the watershed, dam construction and population rise). Yet, to date our understanding of the biogeochemical processes across the continental margins of India and China is limited. To our knowledge, only a limited number of studies have reported on the river transport of OM, specifically focusing into their seasonal changes and transportation across the Indian and Chinese rivers. The research focus of this thesis is to identify the alterations in the river transport of OM due to reduced monsoon rainfall, dam construction and human induced activities in the sustainable land use practice. Furthermore, the study not only restricts to the size of the rivers and seasonal variations but also covers the ecosystems in a large geographical range. The sophisticated multiple organic geochemical techniques (elemental and isotopic ratios and biomarkers) used in this study will be helpful to access an in-depth understanding of OM composition and their controlling mechanisms across the river systems of India and China. Collectively, this study focused on 35 small and large river estuaries from the western continental margin of India, a large river in the eastern continental margin of India, as well as the largest river system of China’s tropical Hainan Island. This effort will importantly contribute to the world scientific community in terms of providing baseline data on carbon isotope record (813C) and lignin phenols compositions in particulate and sedimentary environment of different aquatic regime. This study also includes analyses of terrestrial plants and soil samples, which serve as an important carbon end-member to the riverine OM composition and delivered to the ocean. However, the purpose of this study is not only limited to the areas of fulfilling paucity in dataset, but also aims to identify the response of small and large river systems towards the impact of precipitation change, large-scale land-use changes and other important river modulation process on the OM composition.Briefly, the objective of this thesis is as follows:(i) to provide the OM composition status in particulate as well as sediments from the rivers and estuaries of Indian peninsula and tropical Hainan Island; (ii) to identify the factors controlling the OM in space and time scales and (iii) analysis of precipitation changes (typhoons and monsoon rainfall reduction) and human activities (land use change and dam construction), as well as their different effects on OM composition.The elemental analysis (ratio of organic carbon to total nitrogen; C/N),δ13C and lignin (A8) records of sediments from the estuaries of thirty five small and large rivers along the west coast of India (WCI) provides strong evidence of changing land use pattern, precipitation and topography of the Western Ghats, which collectively influence the OC composition in the estuarine region as well as adjacent marine systems (Arabian Sea). Excessive agriculture and bio mass burning dramatically changed the nature of OC as evidenced in the estuarine sediment. Multivariate statistics, such as cluster analysis, identified similar sedimentary chemical properties existing among different estuaries due to the geographical provenance control on sedimentary OC composition from south towards north of WCI. Results from an end-member mixing model indicated that terrestrial OC sources (C3 plants, C4 plants, and soil) contribute -80% to the sedimentary OC, whereas remaining 20% contributed by marine OC sources (marine plankton and estuarine macrophytes). C4 plants and soil OM are major contributors of OM in large river estuaries, such as the Narmada and Sabarmati River, which is likely to be as a result of widely, distributed C4 vegetation and agriculture in their watershed. Most of the estuaries were observed with low nitrogen content (C/N> 15-20), and highly degraded plant OM i.e. lignin ((Ad/Al)v= 0.4 to 0.6 and DHBA/V= 0.16-0.34), are indicative of biodegradation of terrestrial OM and demineralization soil OM owing to humification of OM across their river catchments. The OM in plants and soil of WCI discovered to be undergone into degradation, due to the impact of biomass burning, intensive farming and multiple human activities. The A8 composition and ratios were affected by biomass burning which was revealed by negative relationship of A8 and degradation index (Ad/Al)v in the burning residues as compared to that in the un-burnt plants and soil. The A8 was found to be a function of lignin phenol vegetation index (LPVI) in river bank and agriculture soils due to variability in mixing of different plant tissues and differences in their diagenetic behaviors.The impact of dam controlled water discharge in a small river system (Zuari River) along the WCI brings froth changes into its seasonal characteristics of hydrographic properties as well as geochemical compositions of OM. The short transit time of riverine water due to hydrologic intervention and elongated estuarine turbidity maxima (ETM) largely corresponds to the OC composition along the Zuari River system (ZRS). Multivariate statistics identified two major factors:mixing and biological production, which predominantly controls the OC compositions in the SPM of ZRS.The terrestrial OC compositions were largely influenced by the variability in south west monsoon (SWM) rainfall and dam construction along the main channel of Godavari River system. The results highlighted on short term phasing of terrestrial OC embedded in the riverine SPM and sediments thereby discharged from Godavari River system into the Bay of Bengal (BOB). Changes in the δ13C signatures indicated that upper tributaries of the Godavari River drained heavier carbon (δ13C= 20.4±2.2%o) than the lower tributaries (δ13C=-25.4±1.5%o) due to change in the vegetation cover in the catchment. The OC content was higher in SPM and in the sediments of the region after the middle reach dam (Sriram Sagar) than before (2.2±1.6 vs.1.0 ±0.1% OC and 2.±2.3 vs.0.6 ±0.2% OC, respectively). The A8 was lower in SPM and in the sediments after the dam impoundment than before (0.37 vs.1.94 mg/100 mg OC and 2.9±1.1 vs.5.4±2.3 mg/100 mg OC, respectively) due to an increased contribution of lignin free OC from algae and degraded soil. The lignin flux from largest tropical river (Godavari) to BOB was calculated as 7.26 ×103t yr-1, which is much lower than those of most world rivers except the rivers from polar arctic.The temporal investigation on OC bio geochemistry in Nandujiang (Nandu River system) of Hainan Island draining into the South China Sea (SCS) indicated large variability of OM composition between typhoons affected wet period (August, 2011) and a normal wet period (October,2012). Amongst, the former brings forth large disparity in fresh water discharge so as to compositions particulate organic matter (POM) in the Nandu River system (NDR). In the riverine region, contributions from soil (53±13%) and river plankton (73±1%) into the SPM were dominated during August,2011 and October,2012 respectively. This is in contrast to the estuarine region, where the contribution from marine plankton predominated and contributes 45±6% and 57±3% during August,2011 and October,2012 respectively. The nature of POM changed in the vicinity of copious precipitation during typhoon and prolonged anthropogenic processes such as land-use change across the Hainan Island are the key factors affecting the carbon cycling of NDR and the adjacent SCS.Overall, this thesis provides wealth of first-hand information and new insights into the impact of coupled natural (precipitation change) and anthropogenic (land use change, dams, etc) activities on the biogeochemical cycling of terrestrial OM in tropical river basins of India and China. The case studies displayed cohesive response of land-use changing to the precipitation variability in tropical India and China. Results from this study have importance for our understanding of the variability in amount and frequency of precipitation, its impact on vegetation cover and anthropogenic activities on the river sedimentation. Both these processes affect the terrestrial carbon cycle and human civilizations in the tropical region. The results also have implications for the use of lignin phenols in temporal phasing of terrestrial OC and to understand the degree of anthropogenic influence to carbon cycling across the tropical region.