Climatically forced moisture supply, sediment flux and pedogenesis in Miocene mudflat deposits of south‐east Kazakhstan, Central Asia
Climatically forced moisture supply, sediment flux and pedogenesis in Miocene mudflat deposits of south‐east Kazakhstan, Central Asia
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DOI：
10.1002/dep2.34
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发表时间：
2017-12
期刊：
影响因子：
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通讯作者：
S. Voigt;Y. Weber;Konstantin Frisch;Alexander Bartenstein;Alexandra Hellwig;R. Petschick;A. Bahr
中科院分区：
文献类型：
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作者：
S. Voigt;Y. Weber;Konstantin Frisch;Alexander Bartenstein;Alexandra Hellwig;R. Petschick;A. Bahr
在中亚大陆背景下，由于山脉隆起和帕拉提提斯山脉的退缩，在新生代期间，沙漠化加剧。这些构造尺度过程与轨道强迫气候变化的相互作用及其对亚洲大气湿度分布的影响尚不清楚。阿克套山脉（哈萨克斯坦东南部伊犁盆地）中新世陆相泥滩沉积序列具有作为陆相古气候档案的巨大潜力。在1700米厚的序列中，约有90米由冲积泥滩沉积物组成，表现为粗片洪水、泥滩细粒和半干旱水形态土壤的循环交替。在这项研究中，通过确定沉积物排放强度、风化作用和成土作用的变化和强迫机制，利用大量沉积物矿物学和地球化学、磁化率、沉积物颜色和孢粉学来重建环境条件。结果表明，在高碱/高盐条件下，土壤有4个主要的干旱期和1个蒸发速率较高的沼泽期。风化指数与Mg/Al比值呈正相关，表明这些层位对应最大蒸散速率和干旱化速率。相反，泥滩细粒的形成被解释为在高降水时期由更强烈的冲积扇活动产生了更高的碎屑沉积物。对风化指数、颜色和磁化率数据进行时间序列分析，得出周期频率比，有可能代表米兰科维奇旋回，其中短偏心和长偏心是主要周期。以长偏心强迫为节奏的明显干旱期反映了水分可利用性的变化。在更长的构造时间尺度上，石膏的持续出现表明向更干旱的条件转变。这种气候趋势被认为是由于地中海通往印度洋的东部门户关闭，限制了环流并增加了东帕拉提斯的盐度。
The continental settings of Central Asia witnessed increased desertification during the Cenozoic as a result of mountain uplift and the Paratethys retreat. The interaction of these tectonic‐scale processes with orbitally forced climate change and their influence on Asia's atmospheric moisture distribution are poorly constrained. A Miocene succession of continental mudflat deposits, exposed in the Aktau Mountains (Ili Basin, south‐east Kazakhstan), has great potential as a terrestrial palaeoclimate archive. About 90 m of the 1700 m thick succession comprise alluvial mudflat deposits and appear as cyclic alternation of coarse sheet floods, mudflat fines and semi‐arid hydromorphic soils. In this study, bulk‐sediment mineralogy and geochemistry, magnetic susceptibility, sediment colour and palynology are used to reconstruct environmental conditions by determining changes and forcing mechanisms in the intensity of sediment discharge, weathering and pedogenesis. The results presented here indicate four major periods of arid soil formation and one palustrine interval characterized by higher evaporation rates under highly alkaline/saline conditions. A positive correlation between weathering indices and the Mg/Al ratio suggest that these horizons correspond to maximum rates of evapotranspiration and aridity. The formation of mudflat fines is, instead, interpreted as representing higher detrital sediment production by more intense alluvial fan activity during times of higher precipitation. Time series analysis of weathering indices, colour and magnetic susceptibility data yields cycle‐to‐frequency ratios with the potential to represent Milankovitch cyclicity with short and long eccentricity as dominant periodicities. Periods of pronounced aridity, paced by long eccentricity forcing, reflect changes in moisture availability. On longer tectonic timescales, the persistent appearance of gypsum indicates a shift towards more arid conditions. This trend in climate is considered to result from the closure of the eastern gateway of the Mediterranean to the Indian Ocean that restricted circulation and enhanced salinity within the Eastern Paratethys.