Late Pliocene Monsoonal Rainfall Gradients in Western China Recorded by the Eolian Deposits From the Linxia Basin, NE Tibetan Plateau

Abstract

Loess and red clay sediments from the Chinese Loess Plateau (CLP) contain a wealth of information about variations in the Asian monsoon and the aridification history of the Asian interior. However, relatively little is known about late Pliocene palaeoenvironmental evolution and the variations in the gradients of monsoonal rainfall in the western part of the CLP, due to the poor exposure of pre-Quaternary eolian deposits and uncertainties in the interpretation of paleoclimatic proxies in most parts of the region. To address these issues, we conducted a detailed rock magnetic investigation of newly drilled eolian sequences (spanning 3.7-2.4 Ma) from the Linxia Basin, NE Tibetan Plateau. Our results demonstrate that pedogenically formed fine magnetic grains are responsible for the magnetic enhancement of loess-red clay sediments from the basin. Spatially, a clear decrease in magnetic susceptibility and its frequency dependence is observed between the central and western CLP, indicating that the East Asian summer monsoon already exerted a major influence on the amount and distribution of late Pliocene rainfall in the Linxia Basin. After evaluating the validity of magneto-climofunctions, we attempt to reconstruct late Pliocene-early Pleistocene monsoonal precipitation in the NE margin of the Tibetan Plateau. The results demonstrate that the difference in late Pliocene monsoonal rainfall between the study area and Chaona in the central CLP was about 300-400 mm. This value is higher than that of the present day, indicating a steepened gradient in monsoonal rainfall between the central and western CLP during the late Pliocene.Recent rock magnetic investigations further confirmed that pedogenically formed fine magnetic grains were also responsible for the magnetic enhancement of Pliocene red clay deposits from the central CLP (Liu et al., 2003;Nie et al., 2007;. Moreover, the spatial variations in magnetic susceptibility values of the red clay deposits exhibit similar patterns to the overlying loess-paleosol sediments (Xiong et al., 2002), that is, an evident decrease from southeast to the northwest. Therefore, several routine rock magnetic parameters or interparametric ratios for the red clay deposits from the central CLP have been used successfully to reconstruct paleorainfall from modern soil-rainfall calibrations (Nie et al., 2008;. However, these previous studies did not consider the red clay sections from the northeastern part of the CLP. Recently, Peng et al. (2018) compared the spatial differences in heavy mineral assemblages between pre-Quaternary Red Clay sequences and Quaternary loess deposits from the central ZAN ET AL.

Chronological and rock magnetic constraints on the transition of the Quaternary paleoclimate in the western Qaidam Basin, NE Tibetan Plateau

“…The provenance alternations caused by the glacial-interglacial cycles during the Quaternary should transform the type of magnetic mineral or the size of magnetic grains, which is inconsistent with no more hematite in our SG-5 borehole and dominant SD grains in the SG-3 borehole (Tan et al, 2020). In addition, the pedogenesis model for magnetic properties is only present in the floodplain within the fluviolacustrine sediments (Ao et al, 2010) or in the loess-paleosol (Zan et al, 2018). Thus, this model is only one explanation for Asian interior aridification and global cooling for the χ variations in the western Qaidam Basin.…”

“…Irrespective of which mechanism drove the desertification of the Asian interior, Quaternary climates have experienced several significant transitions (Ruddiman and Kutzbach, 1989; Clark et al, 2006; Song et al, 2014). The drying process of central Asia has been recorded in the loess from the western Kunlun Mountains (Zan et al, 2010, 2013), the Tianshan Mountains (Fang et al, 2002; Song et al, 2018) and Tajikistan (Ding et al, 2002, Yang et al, 2020a), the Chinese Loess Plateau (Guo et al, 2002; Ding et al, 2005; Zan et al, 2018), and the fluviolacustrine sequences of the Gobi Desert (Sun and Liu, 2006; Chang et al, 2014) from the Late Pliocene. Except for these loess aeolian and fluviolacustrine deposits, uniform and fine lacustrine detritus convey more paleoclimatic information because the lithofacies are highly correlated with climatic change (Zhang et al, 2012b; Tan et al, 2020; Yang et al, 2020b); however, attaining complete and continuous Quaternary lacustrine sequences from the Tibetan Plateau has been a considerable challenge owing to poor exposure of the outcrop sections.…”

The Sources and Transport Dynamics of Eolian Sediments in the NE Tibetan Plateau Since 6.7 Ma

“…Much research attention has been directed to the Chinese Loess Plateau (CLP), and comprehensive investigations have been made of the origin of the red clay and the overlying loess‐paleosol sequence (Bird et al, ; Chen & Li, ; Li et al, , ; Nie et al, ; Shang et al, ; Sun, ; Sun, Tada, et al, ) and their paleoclimatic record (An et al, ; Ding et al, , ; Qiang et al, ; Vandenberghe et al, ; Zan, Fang, Li, et al, ; Zan, Fang, Zhang, et al, ; Zhang et al, ). Several primary conclusions have been drawn: (i) The ultimate source materials of these eolian sediments are the northern TP (NTP) and the Central Asian Orogen (CAO), and shifts in these sources were determined by processes of mountain erosion within the two regions, and by desertification processes in the Asian interior (Chen et al, ; Chen & Li, ; Fan et al, ; Li et al, , , ; Nie et al, ).…”