Geochemical Study On Dissolved And Suspended Load From The Yangtze River Drainage:Weathering And Provenance Implications

As one of the biggest river that connects the Tibetan Plateau with the ocean,Yangtze River transport a large quantity of dissolved and suspended sediment into the sea.The erosion products not only record the weathering information of the plateau but also affect the environment of the maginal seas.Hence,one of the most highlights in global changes researches is unstanding the relation between weathering and the uplift of the Tibetan plateau as well as elucidating the provenance through the geochemistry and mineralogical analyses of erosion matters transported by the Yangtze River.Due to the diverse rocks,monsoon climate and hydrological sorting in the Yangtze River,matters transported by Yangtze can show great seasonal and spatial variations.Accurate explaintion about weathering and provencen can only be made when those variations were well understood.With a systematic sampling strategy of both water and suspended sediment in the Yangtze River,we try to obtain a better understanding regarding weathering conditions,carbon fluxes and provenance of the Yangtze River drainage area.A series of mineralogical and geochemical analyses have been conducted for three sample sets:?1?a temporal series of samples collected at Datong hydrological station in the lower mainstream of the Yangtze River from June 2010 to June 2011;?2?basin-wide sampling of water and suspended sediment along the mainstream and major tributaries during summer and winter;?3?depth sampling at hydrological stations in the mainstream over different reaches of the Yangtze River during different seasons.The main results are listed below:1?The results here show that the major element composition of the Yangtze River is mainly controlled by rock weathering.In the Jinsha Jiang,Na and Cl are the dominating major ions which indicate evaporate dissolution dominates the water chemistry while HCO₃^-and Ca²⁺ are the dominant ions downstream due to the abundance of carbonate rocks in the basin.Overall,water chemistry was controlled by carbonate weathering with small contribution from silicates and evaporites.As a result of alternating climate across the basin,chemical weathering rates in the Yangtze River show higher values in the middle-lower reaches than the upper reach.Similarly,weathering rates in summer are higher than that in winter.Carbonate and silicate weathering rates in the Yangtze River in summer?winter?are 19.01?9.10?and 7.07?2.92?t km-a^-1,respectively.Weathering intensity of silicates increased from the upper to the lower reach,with higher values in summer.Among all the investigated tributaries,the chemical weathering flux of the Jinsha Jiang is the highest in summer among tributaries while Han Jiang becomes the highest in winter.Through chemical weathering,atmospheric CO₂ is transformed to riverine HCO₃^-which is the major component of dissolved inorganic carbon?DIC?.Results show that the CO₂ consumption rates of carbonate weathering are 41.9×1010 mol/ain winter and 95.4×1010mol/a in summer while silicate weathering consume 34.4×1010mol/ain winter and 62.3×1010mol/a in summer.The flux of the CO₂,that is consumed by silicate weathering of the Yangtze River accounts for 7.8%of the total consumpution fluxes by continental silicate weathering.2?From the upper to the lower reach,a general trend was observed for the grain size and concentration of the suspended sediment.Grain size of SS from summer is slightly larger than that from winter.Both grain size and concentration are controlled by hydrological power of the river.The mechanical denudation rate?MDR?within the Yangtze River varies from 0.34 to 28.08×10-4 kg km^-2s^-1 in summer and from 0.07 to 9.11×10-4 kg km^-2s^-1 in winter.The mechanical denudation flux?MDF?shows spatial and seasonal variation in the Yangtze River.In summer,the MDF is high in the upper reach and decreases dramatically after passing the Three Gorges Dam?TGD?.The opposite is true for the winter months,when the MDF is small in the upper reach and increases father downward from the TGD.The opposite trend of MDF in winter is mainly caused by erosion of the river bed by clean water from the TGD and human activities,like dam building and sand digging in the middle-lower part of the catchment.Discharge-weighted annual sediment flux of the Yangtze River is 9.62×107t/a,lower than data published before.Comparing to the discharge weighted flux,annual flux calculated based on single samples lead to errors of-66.42%?152%,with smallest error for samples collected in flood season.3?Based on calculations of the fluxes of dissolved load and suspended sediment,we observed that the dissolved load in the Yangtze River mainly comes from the Chuan Jiang section,and does not show large seasonal variation.On the other hand,sources of suspended sediment show great seasonal variation with major contribution from Jinsha Jiang in summer and major contribution from the Dongting and Poyang Lake in winter.According to our investigation,it is clear that human activities were not affecting the sources of dissolved load in the Yangtze River but changed the proportion of suspended sediment from different sources.4?Concentration of riverine HCO₃^-in the Yangtze River shows a decreasing trend over the river course which is controlled by the distribution of rocks and the weathering rate over the drainage area.With regard to the depth variation,results indicate that HCO₃^-concentration is not uniform over the length of the water column,but lack a certain trend.The heterogeneity of HCO₃^-concentration may be the result of used DIC by photosynthesis.Temporal variations of HCO₃^-are observed from water samples taken biweekly,collected over a period of one year.HCO₃^-concentrations in the Yangtze River range from 1.4 to 2.0 mmol/L with low values during flood season and high values during dry season.The time series together with depth variation data from the Datong station allow us to generate a new estimate of discharge-weighted DIC flux of 18.6×10^-2 g/a.The content of dissolved organic carbon?DOC?in the water from the bottom is higher than that of water collected from the river surface as a result of the release of water-soluble organic carbon?WSOC?from the river bed sediment.The seasonal variation of DOC is quite large?0.95 to 2.03 mg/L?with higher value during flood season,which is driven by strong leaching of the soil as a result of high temperature and precipitation within the floodseason.The discharge-weighted annual DOC flux of the Yangtze River is 1.88×1012 g/a,which is larger than data published before.Particulate organic carbon?POC?content decreases along the water depth and show the lowest in the sand from the river bed.POC content have negative correlation with concentration and grain size of suspended sediment.Differece of annual POC fluxes calculated using samples from different depth amounts to 17%.Hence,we report an annual POC flux of the Yangtze River about 1.38×1012 g/a based on the average POC content of SS collected from different depths.When comparing our data with others,we conclude that the flux of organic carbon transported by the Yangtze River,shifted from a dominant POC to DOC.5?1)dissolved 87Sr/86Sr in the Yangtze River is homogeneous along the river depth profile.Unlike dissolved matter,the 87Sr/86Sr ratios of suspended sediment decrease from surface to bottom,which means that the 87Sr/86Sr ratios increase when grain size decreases;while the 143Nd/144Nd ratios increase with water depth.Depth variations can be as large as spatial variations and seasonal variations of the suspended surface sediments.The stratification of Sr-Nd isotopic composition may indicate different proportion of different sources to sediments of different depth.2)Spatially,the dissolved Sr concentrations of the Yangtze River range from 0.75 to 8.03 ?mol/L with 87Sr/86Sr ratios from 0.709983 to 0.711647.The dissolved 87Sr/86Sr ratios of the Yangtze River mainly reflect the control of source rocks,with low 87Sr/86Sr values in the upper reach corresponding to its widely distributed carbonate.Higher 87Sr/86Sr values can be distinguished in the middle and lower reaches which are mostly covered by silicates.3)Temporal analyses indicate a variation of Sr concentrations from 1.74 to 2.92?mol/L,with high values in the dry season.The variation in concentration relates to monsoonal dilution.The 87Sr/86Sr ratios range from 0.7101 to 0.7109,with abrupt changes correlating to extreme climatic events such as flooding and droughts.This temporal variation can be explained by the changes of relative contributions from the upper reach and middle-lower reaches that results from spatially and seasonally variable rainfall over the basin.A mixing model based on discharge data during sampling time and Sr values deduced from basin-wide samples predicts similar temporal variation.6?Calculation of Sr fluxes based on the basin-wide samples indicates that the Sr flux from the upper reach is about three times higher than that of the middle-lower reaches.A notable increase in Sr flux has been found among the Three Gorges Reservoir,indicating that this area might be a major Sr source for the Yangtze River.Since the Three Gorges Dam was finished in 2009,this potential Sr source has not been recognized before and should be taken into consideration in future provenance studies.Discharge-weighted 87Sr/86Sr ratio based on the time series data is 0.7106 which is lower than the global rivers.Annual Sr flux of the Yangtze River is 1.9×109 mol·a^-1 which is one magnitude higher than that of the Brahmaputra and Ganges while its Sr ratio is much lower.The Sr flux of the Yangtze River accounts for 54.75%of the total Sr flux of the eleven rivers draining the Tibetan Plateau,being 5.59%of that transported by global rivers.The 87Srex flux of the Yangtze River is 3.1×106 mol?a^-1,which accounts for 11.48%of the total 87Srex of the eleven rivers draining the Tibetan Plateau,being 3.73%of that transported by global rivers.The results show that the dissolved annual Sr fluxes based on a single sample at Datong station vary from 1.47×109 to 2.46×109 mol·a^-1,leading to an uncertainty of 29.45?^-22.9%in the annual Sr flux with respect to the discharge-weighted annual Sr flux.Fluxes calculated using the sample collected during peak flow period,are close to the discharge-weighted annual flux.Large uncertainties are evident in samples collected during a major flood(^-22.9%)and during a period with alternating drought and flood?29.45%?.According to our analyses,when time series sampling is not feasible,sampling should be carried out during the monsoon season and extreme weather climatic events should be avoided.7?The magnetic mineralogy is dominated by low coercivity minerals?magnetite?in all samples,with a small proportion of high coercivity minerals?hematite and goethite?.Affected by the vanadium-titanium magnetite around Panzhihua,the concentration of ferrimagnetic minerals in the main stream increased rapidly near Panzhihua with x and SIRM reaching up to 246×10-8 m3/kg and 48833 Am2/kg respectively.The high concentration is followed by a moderate decline downstream and a sharp drop at the TGD.Grain size of magnetic mineral particles show a fining trend from source to sink,and is influenced by catchment lithology,geomorphology and hydrology.Magnetic parameters of the suspended sediment also show clear depth and temporal variations.? of suspended sediment collected from the bottom is larger than that of samples from the river surface,with the largest variation amount to 21×10-8 m3/kg.Concentration and grain size of magnetic minerals increased during monsoon period compared to the nonmonsoon period.The largest temporal variation of x is 37×10-8 m3/kg which is large enough and should receive extra attention when discussing environmental and provenance issues with samples collected from two dfifferent seasons.Though the magnetic properties of suspended sediment in the Yangtze River can indicate the source information to some extent,they are also controlled by weathering conditions and,in modern settings,anthropogenic impacts?such as TDG?which made this method restricted when applied to provenance studies.Compared to spatial variation of magnetic properties,variations in depth and temporal scale are more affected by hydrological conditions of the river itself.In a word,based on the detailed investigation of the spatial and seasonal variations of weathering in the Yangtze River,we calculate the distribution of dissolved load and suspended sediment flux of the Yangtze River in different season.This allowed us to have a better understanding about seasonal variation of relative contribution from different sources.Besiedes,a systematic investigation on the provenace indicators such as Sr-Nd isotope and magnetic properties show great variation on different scales?temporal,spatial and depth?.Without a complete knowledgement of those variations will lead to a misunderstanding when apply these methods to explain their provenance implication.