| The geochemistry of earth surface processes and element cycles has long been one of the key topics in the earth sciences, especially for chemical weathering. The loess, which covers 10% area of global continent, is characterized by high carbonates, loose structure, fine grains(i.e. large specific surface area) and easy denudation. The loess weathering would have a great impact on the chemical composition of surface water and sediment, even on the global mass cycle. However, the study of modern loess weathering process is deficiency at present.Since the loess covers 40% area of the Yellow River catchment and provides huge amount of sediment load to the river, the loess weathering would affect the river water chemistry greatly. Considering that the middle reaches of the Yellow River is the main channel flowing through the Chinese Loess Platuea(CLP), tracing river water chemistry in the middle reaches of the Yellow River will help us to better understanding loess weathering process and its controls. The contrast seasonal climate in the the middle reaches of the Yellow River provides a good opportunity to trace the loess weathering. However, little is known about the influences by loess weathering on the river chemistry and the controlling factors.In this thesis, the seasonal river water in the middle reaches of the Yellow River was chosen as the object of this study. The river water samples were collected weekly from the Longmen hydrological station over the whole year of 2013 and were measured major cation and anion compositions. These water chemical data, combined with synchronous hydrological data, were used to reveal the seasonal variations of river water chemistry. A forward model was used to calculate the contributions of rain, evaporite, silicate, carbonate, and anthropogenic inputs and discuss the seasonal chemical weathering of the catchment in CLP and its relation with hydro-climate and physical erorion rate(PER).The major ionic compositions of the dissolved load in the middle reaches of the Yellow River exhibited distinct seasonal variability over the whole year of 2013, reflecting various sources and weathering reactions within the catchment. All of the major ions, except for NO3-, declined gradually from the highest values in January and reached stable levels until the beginning of the monsoon season. During the monsoon season, with the increasing Q, Na+, Cl-, and SO42- dropped from their stable levels to a lowest value owing to a dilution, then increased in the peak of monsoon and returned to the levels of the monsoon beginning. Meanwhile, the concentrations of Ca2+, Mg2+ and HCO3- kept stable levels in the early and late monsoon seasons. The concentrations of Ca2+ and Mg2+ in the early monsoon were lower than those in the late monsoon, but HCO3- was contrary. After the monsoon season, all these major ions gradually rose to the pre-monsoon level until the end of this year, again resulting from the dominated input of the base flow when the Q was low.The calculation result of the forward model showed that the annual average contributions of rain, evaporite, silicate, carbonate, and anthropogenic inputs were 6.5 +4.0/-3.5%(4.0 +2.5/-2.2 – 8.1 +5.0/-4.4%), 48.2 +18.2/-19.7%(38.1 +16.6/-15.6 – 60.3 +23.8/-25.8%), 20.4 +5.3/-11.7%(0.7 +6.3/-14.5% – 40.4 +5.6/-11.3%), 23.8 +23.4/-17.0%(12.1 +22.6/-11.2 – 33.0 +27.9/-17.7%), and 5.1%(3.3 – 9.8%), respectively. The contributions of both rain and anthropogenic sources to the middle Yellow River were less than ~12% in average, with little seasonal variations in 2013. The silicate input during the pre-monsoon seasons was 29.1 +5.2/-11.0%, with strong correlations versus Q and PER. In the same period, the average contributions of carbonate and evaporite were 20.4 +22.0/-12.5% and 43.2 +13.1/-17.2%, respectively. We suggested that the river water in the pre-monsoon was potentially derived from plateau headstream, soil pore and ground water, which had the long water-rock interaction. In the late- and post-monsoon seasons, the contributions of carbonate were 25.0 +24.9/-19.5% and 27.1 +24.8/-20.2%, respectively, and the contributions of evaporite were 50.4 +20.9/-21.0% and 52.0 +21.7/-21.2%, respectively, both of them increasing with increasing Q. Meanwhile, the contributions of silicate decreased to 16.3 +5.5/-12.4% and 13.7 +5.5/-12.4%, respectively. This was because the rain accelerated the physical erosion of loess which contained high carbonates and evaporites. The seasonal variation in the relative contributions of carbonate and silicate sources clearly demonstrated that various water sources in different seasons and loess weathering were sensitive to seasonal hydrological processes. Carbonate weathering rate shows good correlation with water discharge(Q) and PER in the whole 2013 year, whereas silicate weathering rate shows different correlations with both two key parameters, also reflecting seasonal differences of silicate sources and loess weathering reaction.Our densely time-series data also highlighted a significant impact of two hydrological events on the middle Yellow River water chemistry in semi-arid area. During the ice melting interval in the springtime(16th March to 13 th April in 2013), more silicate-origin ions were invited to the river from melting water, leading that the daily average silicate contribution(32.6 +5.3/-11.2%) was nearly double of the normal monsoon days. Whereas during the stormy event interval in the peak monsoon(22nd–25th July in 2013), more carbonates and evaporites were introduced largely into the Yellow River from the loess plateau via accelerated physical erosion.Our research shed some light on the controls of seasonal variation of river chemistry of the Yellow River and modern loess weathering. In the middle reaches of the Yellow River with semi-arid condition, much abundant and easily-eroded loess provide large amounts of weatherable material, such that both of Q and PER become the key factors controlling the weathering rates in the basin, rather than temperature. |
PDF Full Text Request