The Valley of Lakes is approximately a 500-km elongate depression in Central Mongolia, where Eocene to Miocene continental sediments are long known for their outstanding fossil richness. The palaeontological record of this region is an exceptional witness for the evolution of mammalian communities during the Cenozoic global cooling and regional aridification. In order to precisely elucidate the climatic evolution of the region, we studied the mostly siliciclastic sediments with several levels of paleosols for their sedimentology, mineralogy, major and trace element composition and δ¹³C and δ¹⁸O composition. The obtained results show that temperate hydrothermal fluids induced a strong illitization of the fluvial and lacustrine sediments. This finding contradicts the current conceptual view that the fine fraction of the sediments is of aeolian origin. Moreover, the diagenetic growth of illite resulted in a strong overprinting of the sediments and, subsequently, largely disturbed the pristine mineralogical and geochemical composition of the sediments that could have carried any palaeo-climatic information. An exception is the δ¹³C (and δ¹⁸O) isotope values of authigenic carbonate found in calcrete horizons that still record the ambient climatic conditions prevailing during paleosol formation. Our novel δ¹³C and δ¹⁸O record suggests an early Oligocene aridification in Central Asia at ∼31 Ma, whereas the Oligocene glacial maximum shows no increase in aridification. A second, regional-scale aridification occurs at ~25 Ma and corresponds to a late Oligocene marked mammalian turnover in the Valley of Lakes sediments.

中文翻译：

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蒙古湖谷渐新世-中新世沉积物的地球化学和矿物学。

湖谷是蒙古中部一个长约 500 公里的狭长洼地，这里的始新世至中新世大陆沉积物长期以来因其丰富的化石而闻名。该地区的古生物学记录是新生代全球变冷和区域干旱期间哺乳动物群落演化的特殊见证。为了准确阐明该地区的气候演化，我们对多级古土壤中硅质碎屑沉积物的沉积学、矿物学、主量和微量元素组成以及δ ¹³ C和δ ¹⁸ O组成进行了研究。结果表明，温带热液引起了河流和湖泊沉积物的强烈伊利石化。这一发现与目前认为沉积物的细小部分是风成物的概念观点相矛盾。此外，伊利石的成岩生长导致了沉积物的强烈叠印，随后在很大程度上扰乱了沉积物的原始矿物学和地球化学成分，而这些成分本来可以携带任何古气候信息。一个例外是在钙质结层层中发现的自生碳酸盐的δ ¹³ C（和 δ ^{18 O）同位素值，它仍然记录了古土壤形成期间盛行的环境气候条件。}我们新的 δ ¹³ C 和 δ ¹⁸ O 记录表明中亚在 ∼31 Ma 发生了早期渐新世干旱化，而渐新世冰川盛期则表明干旱化没有增加。第二次区域性干旱发生在约 25 Ma 时，对应于渐新世晚期，湖谷沉积物中明显的哺乳动物更替。