Reduced chemical weathering intensity in the Qaidam Basin (NE Tibetan Plateau) during the Late Cenozoic
Reduced chemical weathering intensity in the Qaidam Basin (NE Tibetan Plateau) during the Late Cenozoic
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晚新生代柴达木盆地（青藏高原东北部）化学风化强度减弱
DOI：
10.1016/j.jseaes.2018.10.018
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发表时间：
2019-02-01
影响因子：
3
通讯作者：
He, Pengju
中科院分区：
文献类型：
--
作者：
Bao, Jing;Song, Chunhui;He, Pengju
青藏高原新生代盆地沉积是揭示构造、气候和风化作用长期相互作用的重要资料。然而，在这个构造活动区的硅酸盐风化历史的重建一直受到阻碍的存在下，经常嵌入的砾岩和砂岩在长期的沉积序列。在这里，我们提出了详细的地球化学组成的粘土组分的沉积物中怀头塔拉剖面（约。15.3- 1.8Ma），重建了柴东北地区中中新世以来的化学风化史。主量元素比值代理揭示了四个阶段的变化证据：从约。15.3至12.6 Ma，其中粘土成分的特征是相对较高的化学蚀变指数（CIA）值（75-88，平均= 81），但较低的移动的/非移动的氧化物比率（例如，K2 O/Al 2 O3、Na 2 O/Al 2 O3和CaO/Al 2 O3），从而指示了强烈化学风化条件的存在; 12.6至6.6 Ma，其中CIA值下降，同时移动的/非移动的氧化物比率增加，表明源区硅酸盐风化强度发生了长期下降; 6.6 ~ 2.6Ma，沉积物中粘土组分的CIA值持续较低（66-81，平均= 76.8）和高氧化物比率，这表明轻度至中度化学风化; 2.6Ma以来，CIA值（64-80，平均75.5）随氧化物比值的增加而持续下降，反映了较温和的化学风化条件。地球化学代用记录表明，柴达木盆地化学风化作用强度持续减弱，自1980年以来经历了持续干旱化过程。12.6 MA.我们认为，中中新世全球变冷相关的干燥化对化学风化强度的下降趋势产生了重大影响。12.6 MA.另外，晚新生代青藏高原东北部的快速隆升和折返，限制了硅酸盐岩在沉积前的风化剖面中的出露时间，导致其化学风化强度减弱。
Cenozoic basin sediments around the Tibetan Plateau are valuable archives that can be used to reveal long-term interactions among tectonics, climate and weathering. However, the reconstruction of silicate weathering histories in this tectonically active region is consistently hindered by the presence of frequently embedded conglomerate and sandstone in the long sedimentary sequences. Here, we present detailed geochemical compositions of the clay components of the sediments in the Huaitoutala section (ca. 15.3-1.8 Ma) of the northeastern Qaidam Basin to reconstruct the chemical weathering history of the catchment since the Middle Miocene. Major element ratio proxies reveal evidence of four stages of change: from ca. 15.3 to 12.6 Ma, in which the clay components are characterized by relatively high Chemical Index of Alteration (CIA) values (75-88, average = 81) but low mobile/immobile oxide ratios (e.g., K2O/Al2O3, Na2O/Al2O3 and CaO/Al2O3), thus indicating the occurrence of intense chemical weathering conditions; from ca. 12.6 to 6.6 Ma, in which decreasing CIA values, together with increasing mobile/immobile oxide ratios, suggesting that a long-term decrease in silicate weathering intensity occurred in the source area; from 6.6 to 2.6 Ma, in which the clay components in the sediments exhibit persistently low CIA values (66-81, average = 76.8) and high oxide ratios, which are indicative of mild to moderate chemical weathering; and since 2.6 Ma, in which the CIA values (64-80, average = 75.5) have continued to decrease with increasing oxide ratios, thus reflecting mild chemical weathering conditions. The geochemical proxy records demonstrate that the intensity of chemical weathering has continually decreased and that the Qaidam Basin has experienced continuous aridification since ca. 12.6 Ma. We suggest that Middle-Miocene global cooling-related drying has exerted a significant influence on the trend of decreasing chemical weathering intensity since ca. 12.6 Ma. In addition, the Late Cenozoic rapid uplift and exhumation of the NE Tibetan Plateau could have limited the time that the silicate rocks were exposed in the weathering profile prior to the deposition, which may have led to their weakened intensity of chemical weathering.