The Drainage Basin Continental Weathering In The Eastern Qinghai-xizang Plateau

The Changjiang river, Yellow river and Mekong river, the rare pristine rivers remaining in the middle latitude in the world, rise in the eastern Qinghai-xizang plateau and flow through the high mountains and deep gorges. Sampling in the rising stage, this paper studies the characteristics of the dissolved major ion geochemistry, especially stressing to study the Changjiang. The four monitoring stations are established in the upper reach and outlet in the Min jiang and Daduhe river, a major headwater tributary of Changjiang. The dissolved major element composition and the anthropogenic impact was determined through sampling in a monthly time series throughout 2001 year.Through sampling of the rivers and the rocks (the river samples including the dissolved loads, suspended loads and bed loads) and the comprehensive knowledge of the multi-disciplines, such as sediment logy, geochemistry, environmental chemistry and water chemistry, in combination with the characteristics of the geology, soil and climates of the drainage basins, this paper has made a detailed research on the data and comparison with the global data.The data discloses, that the Changjiang river, Yellow river and Mekong river are abundant with the HCO₃^-, the dissolved major ions of which are dominated by weathering of the carbonate in the drainage basins, with Tuotuohe river,Qumarhe river, the source water of the Changjiang river and a few of the tributaries of the Yellow river obviously impacted by the evaporate and with the Minjiang river and Daduhe river, the head tributaries of the Changjiang in calcite super saturation. The TZ⁺ of the Changjiang river might be correlated with the other orogenic belts in the world, e.g. Orinoco river in the south America Andes mountain, Mackenize river, Yukon river in the north American Rocky mountain and the eastern Siberian rivers draining Verkhoyansk and Cherskiy orogenic belts. The index of the Si,Si/TZ^+*,Si/Na^*+K etc of the dissolved loads and the clay mineral in the bed loads, indicate that the continental silicate weathering in Changjiang drainage is generally in the superficial weathering stage, i.e. in the illite-hydromica stage. Contribution to the major cations by the silicate weathering in the head tributaries of Changjiang is gradually increasing from of the Minjiang, Daduhe to Yalongjiang rivers. The monthly time series measurements throughout 2001 year at monitoring stations in Minjiang and Daduhe river indicate, that the ion concentrations of K, Na, Mg, Cl, SO₄, HCO₃ ,Sr decrease with increasing of discharge . This is best fit to equation, y=ax^-b(a, constant coefficient, o4 and Ca are significantly elevated as the river debouches onto the populated Sichuan Basin.This paper calculates the chemical denudation rates and the net atmospheric CO₂ consumption rates (ΦCO₂) in the eastern Plateau changjiang drainage basin. The calculation result of the chemical denudation rates is as follows: Yalongjiang river is 1.69×10³mol/yr/km², Jinshajiang river 1.74×10³mol/yr/km², Daduhe river 1.57×10³mol/ yr/km², Minjiang river, Qumarhe river 2.32×10³mol/yr/km², Tuotuohe river 1.37×10³mol/yr/km², which could correlate with the river draining the other orogenic belt in the world. The calculation result of the net atmospheric CO₂ consumption rates (ΦCO₂) consumed by silicate weathering is, that Daduhe river is the highest, 101.81mol/yr/km², Qumarhe river is the lowest, 7.55 mol/yr/km², which could correlate with Mackenzie river draining the north American Rocky mountain and the eastern Siberian rivers (such as Lena etc) draining Verkhoyansk and Cherskiy orogenic belts.Besides, this paper studies the relationship between the chemical denudation rates and tectonics, climate , and the primary controlling factor of the chemical denudation rates in the Changjiang drainage.