[H-CG22] Earth surface processes related to deposition, erosion and sediment transport

[HCG22-P05] Climate change in the southern Tibetan plateau since the middle Miocene period based on the classification of paleosol type

*Nozomi Hatano¹, Babu Ram Gyawali^2,3, Haruki Sugiyama⁴, Kohki Yoshida⁴ (1.Nagano Environmental Conservation Research Institute, Japan, 2.Tripbuvan University, Nepal, 3.Pokhala University, Nepal, 4.Shinshu University, Japan)

Keywords:Paleosol, Paleoclimate, Miocene–Pliocene, south Asian monsoon, Tibetan plateau, central Nepal

Introduction: The uplift of Himalaya and Tibetan plateau exerted significant influence on the onset and evolution of the monsoonal climate in the Asian region. Especially, the high elevations in the southern Tibetan plateau persisted since the middle Miocene period [1, 2], have an effect on the onset of the south Asian monsoon and the regional difference of rainfall. The Neogene and Quaternary deposits in Mustang, central Nepal, are composed of the alluvial, fluvial, and lacustrine deposits [3]. Paleosols in the middle Miocene-Pleistocene Tetang and Thakkhola formations can be expected to record the climate change caused by the onset and fluctuation of the monsoonal climate and the change in the arrangement of the surrounding mountains. In this study, the interpretation of sedimentary environment and the classification of paleosol type were carried out for these formations to identify the Neogene climate change in the southern Tibetan plateau.

Setting: Mustang district is located at the boundary between Himalaya and Tibetan plateau. This district is situated in sub-alpine and alpine regions of which elevation range from about 1500 to 8000 m under arid steppe cold climate or polar tundra climate zones in the present [3]. The Miocene-Pleistocene deposits are distributed in some grabens which are formed in an east-west extension of the stress field after the collision between India and Eurasia [4, 5]. The sedimentary age of the Tetang Formation is dated at 11-9.6 Ma and that of the Thakkhola Formation is dated at 8-2 Ma, based on the paleomagnetic age dating [6, 7].

Description of paleosols: The Tetang Formation shows 200 m in maximum thickness. This formation is divided into alluvial, meandering fluvial, and lacustrine deposits in an ascending order. Paleosols in the alluvial deposits correspond to Oxisol (tropical deeply weathered soil), because they are characterized by deeply weathered crust with rusty red color, root traces with bleached color, cutan composed of iron oxide, illuvial clay, and lack of organic matter and easily weathered minerals. Paleosols in the meandering fluvial deposits show soil horizonation (O, A, and B horizons), remnant of organic matter, mottling, siderite nodules, cutan composed of clay, and illuvial clay, corresponding to gleied Inceptisol (young soil) and Ultisol (base-poor forest soil). Paleosols in the lacustrine deposits are comparable with gleied Inceptisol, based on ferruginous and manganese nodules, rhizoconcretions and illuvial clay.
The Thakkhola Formation shows over 600 m in maximum thickness. This formation is divided into alluvial, braided fluvial, fan delta (1), meandering fluvial, lacustrine, and fan delta (2) deposits. The alluvial and braided fluvial deposits lack paleosols. The fan delta (1) deposits intercalate some paleosols showing bluish soil color, mottling, and drab-haloed root traces. These paleosols correspond to gleied Entisol (incipient soil). Paleosols in the meandering fluvial deposits show soil horizonation (A, Bk, and C horizons) and yield caliches. Hence, these paleosols are comparable with Aridisol (dryland soil). Gleied Entisols yielding mottling and rhizoconcretions are developed in these deposits, implying seasonal fluctuation of ground water level. Paleosols in the lacustrine and fan delta (2) cannot be compared to the soil type because the surface soil horizons were truncated by the erosion.

Discussion: Oxisols recognized in the Tetang Formation suggest humid tropical climates, where natural vegetation is rainforest and forest [8]. Ultisols are formed in humid and warm climates [8]. Accordingly, during the middle Miocene period, heavier rainfall and warmer climates are dominated than those in the present. Subsequently, arid climates with seasonality of rainfall occur during the late Miocene to Pleistocene, based on the occurrence of the Aridisols and gleyed paleosols in the Thakkhola Formation. The climate change can be ascribed to the formation of rain shadow caused by the uplift of Himalaya and Tibetan plateau and the change of the elevation in the region.


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