A Pedogenic Model for the Magnetic Enhancement of Late Miocene Fluvial‐Lacustrine Sediments From the Xining Basin, NE Tibetan Plateau

Abstract

Continuous sequences of Cenozoic fluvial‐lacustrine sediments are well preserved in the northeastern edge of the Tibetan Plateau (TP), and they provide a good opportunity to improve our understanding of the tectonic uplift and paleoenvironmental evolution of the NE TP. To date, however, the factors controlling the variations in the magnetic properties of these fluvial‐lacustrine sediments remain complex and ambiguous. Here we systematically assess the contribution of pedogenesis to the magnetic enhancement of late Miocene fluvial‐lacustrine sediments in the Xining Basin, using a combination of rock magnetic, diffuse reflectance spectroscopy, and geochemical methods. Our results demonstrate that the magnetic concentration parameters show a long‐term increasing trend since the late Miocene, with two dramatic increases at ~11 and 9 Ma. Based on the examination of several factors that may affect the magnetic properties of fluvial‐lacustrine sediments in the NE TP, we argue that an accelerated drying process in waterlogged environments, induced by late Miocene global cooling or/and tectonic uplift, may have caused the changes from anaerobic to aerobic conditions and resulted in the observed stepwise increases in pedogenic intensity. The two dramatic increases in magnetic parameters are closely related to intensified aridification and tectonic activity in the NE TP at ~11 and 9 Ma, respectively. These observations suggest that the pedogenic magnetic enhancement model, caused by the disappearance of a waterlogged and reducing pedogenic environment since the late Miocene, should be reconsidered as a possible factor explaining the magnetic properties of fluvial‐lacustrine sediments in the NE TP.

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

“…B. Yang, Galy, et al, ), the North Pacific (Rea et al, ), the South China Sea (Wan et al, ), and the Japan Sea (Shen et al, ), indicating enhanced aridification in central Asia and/or more dust available on the source areas (e.g., Nie et al, ). Magnetic enhancements could also be seen in the Qaidam Basin (Nie et al, ), the Jianzha Basin (Fu et al, 2017), the Xining Basin (Zan et al, ), the Linxia Basin (Yan et al, ), and the CLP (Nie et al, ) at ~8 Ma, yet the mechanisms of this enhancement are still debated. Eolian dust input (Fu et al, 2017; Nie et al, ), provenance change (Yan et al, ), and pedogenic enhancement (Zan et al, ) are all thought to be the main potential factors.…”

“…Magnetic enhancements could also be seen in the Qaidam Basin (Nie et al, ), the Jianzha Basin (Fu et al, 2017), the Xining Basin (Zan et al, ), the Linxia Basin (Yan et al, ), and the CLP (Nie et al, ) at ~8 Ma, yet the mechanisms of this enhancement are still debated. Eolian dust input (Fu et al, 2017; Nie et al, ), provenance change (Yan et al, ), and pedogenic enhancement (Zan et al, ) are all thought to be the main potential factors. Despite multiple mechanisms, such modifications may reflect complex responses of fluvial and atmospheric systems to climate‐tectonic coupling at ~8 Ma on the northeastern TP and the CLP (e.g., Nie et al, ).…”

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.…”

Paleoclimatic and provenance implications of magnetic parameters from the Miocene sediments in the Subei Basin

“…Recently, it was reported that the MMCO can be identified using pollen records (Sun JM and Zhang ZQ, 2008; Larsson et al, 2011; Miao YF et al, 2011; Hui ZC et al, 2018), geochemical records (Wan SM et al, 2009; Song YG et al, 2018), and magnetic records (Zan JB et al, 2015; Guan C et al, 2019). Compared with investigations of other climatic proxies, studies of environmental magnetism are convenient, nondestructive, and effective in treating loess‐paleosol sequences (Heller and Tungsheng, 1984; Verosub et al, 1993; Sun JM and Liu TS, 2000; An ZS et al, 2001; Guo ZT et al, 2002; Deng CL et al, 2005, 2006; Liu QS et al, 2007; Hao QZ et al, 2008) and fluviolacustrine sediments (Jiang HC et al, 2008; Ao H et al, 2010; Song CH et al, 2014; Fang XM et al, 2015; Zhang T et al, 2016; Fu CF et al, 2018; Zan JB et al, 2018), enabling us to reconstruct past climatic changes. Therefore, we performed rock magnetic measurements in the Tiejianggou Section.…”