The significance of mid-latitude rivers for weathering rates and chemical fluxes: Evidence from northern Xinjiang rivers

Abstract

Rivers draining the sedimentary platform of northern Xinjiang (the center of Asian continent) are characterized by low discharge under a temperate and arid climate. The influence of rock mineralogy, climate, relief and human activity on natural water composition and export as a result of weathering is a major scientific concern both at the local and the global scale. While comprehensive work on the controlling mechanism of chemical weathering has been less carried out in the sedimentary platform of northern Xinjiang. Thus, the effects of climate and rock weathering on the inorganic hydrogeochemical processes are not well quantified at this climatic extreme. To remedy this lack a comprehensive survey has been carried out of the geochemistry of the large, pristine rivers in northern Xinjiang, the Erlqis, Yili, Wulungu, Jingou and numerous lesser streams which has not experienced the pervasive effects of glaciation and subsequent anthropogenic impacts. The scale of the terrain sampled, in terms of area, is comparable to that of the Huanghe and includes a diverse range of geologic and climatic environments. In this paper the chemical fluxes from the stable sedimentary basin of the northern Xinjiang platform will be presented and compared to published results from analogous terrains in the monsoon basins of China and world. Overall, the fluvial geochemistry of northern Xinjiang in westerly climate is similar to that of the Chinese rivers (Huanghe and Yangtze) in the East-Asian monsoon Climate, both in property–property relationships and concentration magnitudes. The range in the chemical signatures of the various tributaries is large; this reflects that lithology exerts the dominant influence in determining the weathering yield from the sedimentary terrains rather than the weathering environment. The effect of different rock weathering ranges from rivers dominated by aluminosilicate weathering, mainly of granites, sandstones and shales, to those bearing the signatures of dissolution of carbonates and evaporites and of continental playa deposits. Carbonates are the general predominant lithology undergoing dissolution particularly within the lesser arid areas. The pCO2 in the study rivers is out of equilibrium with respect to atmospheric pCO2, about up to ∼20 times supersaturated relative to the atmosphere but not to such an extent as the Amazon in the floodplain. A roughly positive relationship is observed between solute concentrations and the drought index (DI) for natural waters in the region, indicating a coupled mountain-basin climate has a direct effect. The relative contributions of end-member solute sources to the total dissolved cations from each watershed have been quantitatively estimated using dissolved load balance models, showing the results as evaporite dissolution > carbonate weathering > silicate weathering > atmospheric input for the whole catchment. The areal total dissolved fluxes range from 0.05 to 2.53 × 106 mol/km2/yr, 0.02–2.09 × 106 mol/km2/yr and 0.01–1.04 × 106 mol/km2/yr in the Yili, Zhungarer and Erlqis, respectively, comparable to those of Chinese and Siberia rivers draining sedimentary platforms, even though they are in drastically different climatic regimes. In general, the fluxes from rivers in sedimentary basins are comparable to those from orogenic zones, but are much higher than in the shield regions. The CO2 consumption by aluminosilicate weathering (0.2–284 × 103 mol/km2/yr) is much smaller than in active orogenic belts (19–1750 × 103 mol/km2/yr in similar latitudes and 143–1000 × 103 mol/km2/yr in the tropical basins), but comparable to those of the Chinese (7–106 × 103 mol/km2/yr) and Siberia (16–112 × 103 mol/km2/yr) rivers.