唐古拉山发源的河水主要元素与锶同位素来源及环境意义

Origin and Environmental Significance of Major Elements and Sr Isotope Ratios in Rivers Originating from Tanggula Mountains

发源于唐古拉山山脉的河流在青藏高原地区具有流域面积大、径流距离长以及流量大等特点,与喜马拉雅山地区河流共同组成全球河源区最高的水流系统。它们的物理与化学侵蚀在很大程度上代表了在全球气候变化的条件下,大陆的侵蚀作用所发生的变化,即大陆与海洋的物质运移和平衡。发源于唐古拉山北坡的长江源头主要支流楚玛尔河与沱沱河的物理化学特点是溶解的化学成分含量较高,一般为10.66～410.81mmol/L,主要阳离子为Na+、Ca2+和Mg2+,占阳离子总量的97%以上。Ca2+/Na+、Mg2+/Na+、K+/Na+的比值较低,87Sr/86Sr值为(0.708954±0.000020)～(0.711860±0.000011),表明唐古拉山北坡的河水成分以蒸发岩类溶解为主。发源于唐古拉山南坡的怒江源头以及雅鲁藏布江及其支流等的主要阳离子为Ca2+、Mg2+、Na+,占阳离子总量的97%以上。Ca2+/Na+、Mg2+/Na+、K+/Na+的比值较高,87Sr/86Sr值为(0.705534±0.000016)～(0.722856±0.000014),表现为以碳酸盐岩和硅酸盐岩的溶解为主。长江河源区河水中主要化学成分来自蒸发岩,其中Na+和Cl-在河流水化学成分中占比例最大,经计算,唐古拉山北坡长江源头地区的物理侵蚀率为77kg/(km.2a),蒸发岩的化学侵蚀率为49～72.7t/(km.2a),碳酸盐岩的化学侵蚀率为15～20t/(km.2a),硅酸盐岩的化学侵蚀率为2～5t/(km.2a);南坡物理侵蚀率为34kg/(km.2a),碳酸盐岩的化学侵蚀率为25～30t/(km.2a),硅酸盐岩的化学侵蚀率为7～10t/(km.2a)。这些特征反映出北坡高寒干旱环境下河流蒸发岩的化学侵蚀作用较强,南坡碳酸盐岩和硅酸盐岩的化学侵蚀作用大于北坡。

The elementary compositions of river water in the Yangtze River headwater area are derived from evaporite, carbonate and silicate dissolved in rain （ snow, ice）. Major cations in main tributaries of Tongtian River named Qumar River and Tuotuo River, Na^＋ , Ca^2＋ and Mg^2＋ , account for 97% or more in total cations. Ratios of Ca^2＋/Na^＋ , Mg^2＋/Na^＋ and K^＋/Na^＋ are relatively low, and the values of ^87Sr/^86Sr range from（0. 708,954 ±0. 000,020） and to（0. 711,860 ±0. 000,011 ） , indicating that elements of rivers flowing on the northern slope of the Tanggula Mountains mainly come from evaporite rocks. Major cations of Nujiang River headwater area originating from southern slope of Tanggula Mountains, Brahmaputra River and its tributaries, Ca^2＋ , Mg^2＋ and Na^＋ , account for 97% or more in total cations. But the ratios of Ca^2＋/Na^＋ , Mg^2＋/Na^＋ and K^＋/Na^＋ are high and ^87Sr/^86Sr ranges from（0. 705,534 ±0. 000,016） to （0. 722,856 ±0. 000,014）. Elementary compositions of these rivers are mostly derived from carbonate and silicate. Na^＋ and Cl^ - rooted in evaporite rocks are the most abundant chemical compositions in the Yangtze River headwater area. Calculations show that the physical erosion rate is 77 kg/（ km^2·y） , and the chemical erosion rates of evaporite rocks, carbonate rocks and silicate rocks are 49 -72.7, 15 -20 and 2 -5 t/（ km^2·y）, respectively. However, on the southern slope of the Tanggula Mountains, the physical erosion rate is 34 kg/（ km^2 ·y） , and the chemical erosion rates of car- bonate rocks and silicate rocks are 25 -30 and 7 -10 t/（ km^2 ·y）, respectively. These features are explained by the facts that in the high elevation and dry environment, the evaporite has a higher chemical erosion on the northern slope; on the other hand, carbonate and silicate have higher chemical erosion on the southern of the mountains because the precipitation on the southern slope is higher than that on the northern slope.