Late Miocene Intensified Tectonic Uplift and Climatic Aridification on the Northeastern Tibetan Plateau: Evidence From Clay Mineralogical and Geochemical Records in the Xining Basin

Abstract

The uplift of the Tibetan Plateau (TP) during the late Cenozoic is thought to be one of crucial factors controlling Asian climate. However, the complex interaction between tectonics and climate change is still unclear. Here we present the first record of clay mineralogy and elemental geochemistry covering 12.7-4.8 Ma in a fluvial-lacustrine sequence in the Xining Basin. Geochemical provenance proxies (Th/Sc, Zr/Sc, and Cr/Zr) in the <2-μm fraction show a significant provenance change at~8.8 Ma. Silicate-based weathering indexes (CIA, CIW, and PIA) displayed coeval changes with provenance but discrepant changes with regional climate. Since the clay mineralogy exhibits significant change at~7.8 Ma uncorrelated with modifications in provenance, it can be employed to reveal regional climate change. The rise in illite and associated decrease in the sum of smectite and illite/smectite mixed layers reflect gradual and slow aridification since~12.7 Ma with intensified drying since~7.8 Ma until approaching the modern climate status. Our results, together with other regional climatic and tectonic records, clearly illustrate that accelerated uplift of the northeastern TP since~8-9 Ma has mainly modulated the regional erosion, weathering, transportation, and sedimentation and amplified the global cooling and drying trend toward the regional climate of modern conditions. Our study suggests that in the tectonically active northeastern TP, a comprehensive mineralogical and geochemical investigation of the fine-grained fraction of the basin sediments could retrieve the interactions between tectonics and climate behind the complex change in exhumed lithology and sedimentary routing systems.

A Late Miocene Terrestrial Temperature History for the Northeastern Tibetan Plateau's Period of Tectonic Expansion

“…This differential uplift may explain the decoupling of the sustainable aridification and short-term uplift events throughout the Mid to Late Miocene. The uplift of the Xining Basin during thẽ 10.5-to 8-Ma period would have strongly affected the regional environment and led to the enhanced aridification suggested by rock magnetic records (Zan et al, 2018), clay mineral records (R. Yang et al, 2019), and our reconstructed paleo-pH and microbial lipid ratio values (see section 3.2). Furthermore, the divergence in the oxygen isotopic values observed between the Xunhua and Linxia basins, located on either side of the Jishi Mts.…”

“…However, climate change can also affect erosion by controlling the vegetation that protects the Earth's surface from erosion; conversely, climate change can also affect precipitation levels that can in turn strengthen erosion (P. Zhang et al, ; Nie et al, ). Also, the magnetic susceptibility, clay mineral, and element indicators all appear to have changed simultaneously at ~9 Ma (Ruan et al, ; Zan et al, ; R. Yang et al, ), asynchronous with the increases in sedimentation rates. Our data indicate that the drought in the Xining Basin reached a certain statistical point (i.e., MAP <600 mm) at ~9 Ma during the cooling process.…”

Monsoon‐Enhanced Silicate Weathering as a New Atmospheric CO2 Consumption Mechanism Contributing to Fast Late Miocene Global Cooling

“…The clay sample was prepared using pipetting before the X‐ray diffraction (XRD) analysis. The air‐dried (AD), ethylene glycol‐saturated (EG), and heated XRD patterns for each sample were used in clay mineral identification and semiquantitative calculations (see details in Yang et al., 2019). In brief, each sample was suspended and deposited onto glass slides and allowed to air‐dry at room temperature.…”

“…The East Asian summer monsoon (EASM) provides a majority of the moisture for the entire CLP and the eastern portion of the northeastern TP (e.g., F. Chen et al., 2008; Z. Li et al., 2015; Zan et al., 2015), while the East Asian winter monsoon and the westerlies transport dust from the arid Asian inland to the downwind northeastern TP and CLP regions since at least ∼9–7 Ma (An et al., 2001; M. Fan et al., 2006; Nie et al., 2014; Y. Yang et al., 2017). Furthermore, the 9–7 Ma mountain building in the northeastern TP (e.g., Lease et al., 2007; J. Li et al., 2014; Saylor et al., 2018; Yang et al., 2019; D. W. Zheng et al., 2006) also exposed a large amount of fresh upper crustal rocks that were transported to adjacent drainage basins by rivers and to the downwind CLP region by wind. Provenance studies also suggest that the northeastern TP acts as a dominant source area for Neogene fluvial‐lake sediments in adjacent fluvial basins (e.g., Fang et al., 2016; S. Liu et al., 2019; Yang et al., 2019) and eolian deposits on the CLP (e.g., Bird et al., 2020; Z. Chen & Li, 2013).…”

“…Furthermore, the 9–7 Ma mountain building in the northeastern TP (e.g., Lease et al., 2007; J. Li et al., 2014; Saylor et al., 2018; Yang et al., 2019; D. W. Zheng et al., 2006) also exposed a large amount of fresh upper crustal rocks that were transported to adjacent drainage basins by rivers and to the downwind CLP region by wind. Provenance studies also suggest that the northeastern TP acts as a dominant source area for Neogene fluvial‐lake sediments in adjacent fluvial basins (e.g., Fang et al., 2016; S. Liu et al., 2019; Yang et al., 2019) and eolian deposits on the CLP (e.g., Bird et al., 2020; Z. Chen & Li, 2013). Silicate minerals in these fluvial and eolian sequences can be weathered, which involves the removal of base cations (e.g., Ca, Na, K, and Mg) and the concomitant formation of clay minerals (Nesbitt & Young, 1982).…”