Chemical Weathering Processes And Dissolved Carbon Dynamics In The Upper Indus River Basin,Western Himalaya

Rock weathering is therefore a crucial mechanism in the evolution of the earth’s surface and the knowledge of today’s weathering products and control parameters is needed to understand the global carbon cycling.Riverine water chemistry shows the weathering of a river and also sheds light on the exogenic cycles of the elements.There is limited study on chemical weathering by multi-isotopes in mountain rivers in the Himalayas zone.The major emphasis of this thesis is to understand and quantify the chemical weathering processes and dissolved carbon dynamics in rivers,the western edges of the Himalayas.Additionally,dissolved carbon transformation dynamics are profoundly important to ascertain the sources of dissolved organic matter（DOM）and their production from glacier’s photosynthetically-derived primary producers（e.g.phytoplankton）in the glacial surface along with its subsequent degradation during meltwaters transportation from glaciers to streams-tributaries-mainstream Indus River as well as in individual ecosystems（hot springs,saline lake,pond,and groundwater）in the UIRB are unclear to date.Particularly,occurrence of photosynthesis in glacier’s surface by autotrophs（cyanobacteria,algae,diatoms）and their uptake of atmospheric CO₂ are absent on a global scale.Thus,this study was hypothesized to assess three key fundamental research issues choosing the glaciers and its downward margins in the UIRB.To achieve these research issues,this study were designed and planned to set up the following aims and objectives:（1）To understand chemical weathering processes and constrain the sources of solutes,and calculate silicate and carbonate weathering rates as well as associated CO₂ consumption rates.（2）To define the moisture sources of the studied river basin and to reconstruct the modern isotope–elevation relationships for quantitative paleoelevation reconstructions.（3）To ascertain the mechanistic insights into the production of DOM from glaciers’photosynthetically-derived primary producers or organic matter（e.g.phytoplankton）and their subsequent transformation dynamics from glaciers to streams-tributaries-mainstream Indus River for a better understanding of dissolved organic matter cyclings in the UIRB.It is the first time that the multi geochemical studies were conducted in the UIRB in the Western Himalayas.To achieve these aims and objectives,water samples have been sampled during the summer and analysed for major ion chemistry,stable isotope(δD,δ¹⁸O,andδ¹³C-DIC),dissolved organic carbon（DOC）concentration,fluorescent dissolved organic matter（FDOM）,EEM-PARAFAC modelling was applied to quantify and characterize the FDOM components,physicochemical properties,and N-nutrients.The main results of this study were:The first study focused on the chemical weathering processes in which the catchment lithology exerts a strong control on the abundances of major ions in the UIRB.The total cation charge（TZ⁺,1611μEq）and average total dissolved solids(TDS,133 mg L^-1)of the Indus River waters are relatively higher than those of the world’s major rivers.HCO₃^-and Ca²⁺dominate the ionic composition of the river water.Quantitative investigation of hydro-geochemistry shows that carbonate weathering dominates the chemical properties of river water which account for about 80.5%.The total dissolved load of the analyzed rivers is mainly influenced by major reservoirs（such as carbonates,silicates,and evaporates）.The chemical weathering rate of the UIRB is estimated to be 70.1 t km^-2 a^-1for carbonate weathering and 6.0t km^-2 a^-1for silicate weathering,which consumes 729×10³ mol km^-2 a^-1and 148×10³ mol km^-2 a^-1of atmospheric CO₂,respectively.Sulphuric acid,generated via oxidation of pyrites,acts as an important agent for chemical weathering in the catchment.When pyrite oxidation is considered,the CO₂ consumption rates for the UIRB are estimated to be 503×10³ mol km^-2a^-1and 91×10³ mol km^-2 a^-1for carbonate weathering and silicate weathering.The highest carbonate weathering rate(128.2 t km^-2 year^-1)in the UIRB is observed in the Suru River which has the steepest catchment average slope.Theδ¹³C-DIC in the water samples ranged from-8.3‰to-1.5‰with an average of-4.3‰.The findings show that CO₂ consumption by silicate and carbonate weathering has a large impact on the basin.After pyrite oxidation weathering effect deduction,the actual CO₂consumption rates by carbonate weathering and silicate weathering in the basin are only around 80%of the amount estimated under the assumption that carbonic acid donates all protons involved in the weathering cycle.The second study focused on moisture sources,hydrological processes,and paleoelevation reconstructions in the UIRB.This study also focused on the water isotopic compositions and its variations in the UIRB,which significantly varied among individual ecosystems.The Indus River is previously unexplored with respect of the variation in water isotopic composition.Results showed thatδ¹⁸O signatures of the mainstream,tributaries and groundwater ranged from-15.1‰to-12.3‰,-16.5‰to-12‰,-13.8‰to-12‰,with a mean value of-13.6‰,-14.7‰,-12.8‰,respectively.The UIRB and its major tributaries（i.e.,the Zanskar,Nubra,and Shyok rivers）were characterized by relatively lowerδ¹⁸O values,whereas Tang Tse and other small streams were relatively enriched inδ¹⁸O.The River water line for the UIRB was found to beδD=8.1δ¹⁸O+16.4,which suggests the meteoric origin of water and low evaporation in the basin.There was a northward rise inδ¹⁸O by 1.30‰per degree of latitude in the tributary waters.The d‐excess andδ¹⁸O values suggested that the key source of moisture in the studied river basin was mainly from westerlies and southwest monsoons.Thus,regional moisture recycling and subcloud evaporation contributed little in supplying moisture to the precipitation.The isotopic signature(δ¹⁸O)of the tributary stream and its surrounding regions conformed to a best-fit second‐order polynomial relationship betweenδ¹⁸O and elevation over 3500 masl.Moreover,anδ¹⁸O elevation slope of-2‰/km was also established in this study area.This isotope–elevation relationship will be highly useful and can be applied to the Indus River and other Himalayan rivers for paleoelevation reconstruction.The third study focus on dissolved carbon dynamics.The glaciers globally are contributing to the storage,transformation,and transport of carbon,contributing global carbon cycle.To date,many studies are conducted on dissolved organic matter（DOM）sources and carbon dynamics along the adjacent downward margins,and their optical-molecular level characterization.But the integrated mechanistic insights into the origin of OM on glaciers usingδ¹³C-DIC and DOM transformation dynamics o along with,dissolved organic nitrogen（DON）,N-nutrients,dissolved organic phosphorus（DOP）and PO₄-P are unexplored to date.The results showed DOC fluctuated from glaciers to streams-tributaries-mainstream Indus River by approximately 2.5-20.6%,which are greatly masked by the transformation of its molecular structural components of photochemically-microbially active DON and DOP.DON highly fluctuated along with NO₃-N,NH₃-N,and NO₂-N fluctuations,showing to increase in DON by112%and high production of NH₃-N by 44%in the mainstream Indus River whilst DOP gradually increased,but PO₄-P highly fluctuated.This indicates the occurrence of DOM production-degradation transformation along with N-and P-nutrients during water transportation from glaciers to streams-tributaries-mainstream Indus River.These transformations are further supported by the detection of fluorescent DOM（FDOM）by the measurement of three-dimensional fluorescence（excitation-emission matrix,EEM）spectroscopy coupled with parallel factor（PARAFAC）,EEM-PARAFAC modelling.EEM-PARAFAC model identified a maximum of six fluorescent components and minimal two fluorescent components in individual samples and they included C-type autochthonous humic acid-like substances AHLS,M-type autochthonous humic acid-like substances（AHLS）,tryptophan-like substances（TLS）,newly-released protein-like substances（PLS）,phenylalanine-like or tyrosine-like substances and degradative derivatives.These components are simultaneously transformed from glaciers to streams-tributaries-mainstream Indus River,particularly,degradative derivative is highly observed in mainstream Indus River,indicating the occurrence of high transformation in mainstream Indus River.Such all transformations are evidenced by subsequent production of ending byprodycts CO₂,DIC(CO₂+H₂CO₃+HCO₃^-+CO₃^2-),autochthonous DOM or FDOM,nutrients,other products e.g.pigments.Such transformation is further evidenced by theδ¹³C-DIC of the UIRB.This indicates that the carbon transformation is occurred,particularly by emission of lighter CO₂ into the atmosphere,contributing to global carbon cycling.Based on the results from glaciers to streams-tributaries-mainstream Indus River excluding individual ecosystems,a conceptual model is provided depicting the production processes of photosynthetically-derived PP/OM（e.g.phytoplankton,algae）via photosynthesis in glaciers by uptake of atmospheric CO₂and then its subsequent release of FDOM,DIC and nutrient either photoinduced and/or biological respiration which belongs to very nearly similar ranges of glacier’sδ¹³C-DIC（-9.5‰to-3.1‰）.Furthermore,the fluorescent components were similar to hot springs,saline lake,pond water,and groundwater,but the occurrence of DOM along with N-nutrients and P-nutrients are greatly varied as they are an individual ecosystem that is regulated by diverse biogeochemical processes and factors.Finally,the chemical weathering processes constrained by hydro-geochemistry,stable isotopes,and calculated CO₂ consumption rates will provide thoughtful information on the intensity of chemical weathering rates and their controlling factors in the Himalayas River,which had implications for the global carbon cycle.The water isotopic composition will provide useful information on the moisture sources in adjoining areas and this isotope–elevation relationship will be highly useful for paleoelevation reconstruction in the Western Himalayan zones.DOM transformation along with N-nutrients and PO₄-P are constantly occurring,showing in-situ production from photosynthetically-derived PP/OM and its subsequent degradation into DIC,thereby contributing to the global carbon cycle either directly or by emission of lighter CO₂ into the atmosphere or input into downward margins that greatly influenced the downstream carbon metabolism,primary productivity,and carbon dynamics.These overall information and understanding will greatly assist in the proper management and diverse uses of glaciers-fed meltwaters in the western Himalayan regions.