Paleoenvironmental reconstruction of a saline lake in the Tertiary: Evidence from aragonite laminae in the northern Tibet Plateau

The origin of aragonite has long been debated because it is precipitated and preserved under specific conditions. Aragonite laminae, first found from Eocene to Miocene strata in the western Qaidam Basin, northern Tibet Plateau, contain much information on paleolake signatures. Mineralogical and geochemical analyses were conducted on alternating yellowish and grayish aragonite layers. The yellowish layers are mainly composed of aragonite crystals, while the grayish layers contain less aragonite and fewer organic remnants that accumulate among debris with sporadic framboidal pyrite. The delta C-13 values of yellowish layers are remarkably positive by approximately 4.01%. (VPDB), and the delta O-18 values are slightly negative compared with base data of the Qaidam Basin. Considering the (CO2)-C-12 absorption of algal blooms, positive excursions of delta C-13 shown in aragonite indicate high C-13 values in depositional water. Therefore, a seasonal algal-influenced inorganic origin is proposed to explain the formation of aragonite laminae. During warm seasons, Mg/Ca ratios are elevated because of evaporation effects. The algal blooms decrease the CO2 content, leading to high pH values. These conditions promote the rapid crystal growth of aragonite instead of other carbonate minerals. Slightly negative delta O-18 values in yellowish layers are interpreted as the result of intense inflow during warm seasons, which leads to less precipitation of organic matter and debris. The grayish layers in cold seasons are the opposite. From the Eocene to Oligocene, the progressively decreasing delta O-18 values of aragonite reflect global cooling during this time. A conspicuously positive step in delta O-18 values indicates an arid environment coinciding with the uplift of the Himalaya, from the Oligocene to Lower Miocene. The results from this study show that understanding of aragonite in the Qaidam Basin is essential to reconstruct the high-resolution paleoenvironment and to reveal the Tertiary evolution of paleoclimates in the northern Tibet Plateau. (C) 2017 Elsevier B.V. All rights reserved.Ministry of Science and Technology of China [2011ZX05009-001]SCI(E)ARTICLE1-1235