From desert to monsoon: irreversible climatic transition at ~ 36 Ma in southeastern Tibetan Plateau

Abstract

Although there is increasing evidence for wet, monsoonal conditions in Southeast Asia during the late Eocene, it has not been clear when this environment became established. Cenozoic sedimentary sequences constrained by radiometrically dated igneous rocks from the Jianchuan Basin in the southeast flank of Tibetan Plateau now provide a section whose facies and climatic proxies determine this evolution. Semi-arid conditions had dominated the region since Paleocene controlled by the northern sub-tropical high pressure system, culminating in mid Eocene when desert dunes developed. From 36 Ma, the basin began to accumulate swamp sediments with coals, together with synchronous braided river deposits and diversified pollen assemblages, indicating significant increase in precipitation. This remarkable transition from dry to wet conditions precedes the Eocene/Oligocene boundary at 34 Ma, thus excluding general global cooling as the prime driver. We propose that uplift of Tibetan Plateau might have reached a threshold elevation by that time, operating through thermal and dynamic forcing, causing the inception or significant intensification of monsoonal rains to penetrate into this downwind locality.

Eocene Tectonic Uplifts Caused the Early Oligocene Intrusion of the Indian Monsoon Into the Tuotuohe Basin and the Increased Evaporation, Central‐Northern Tibet: Insights From the Oxygen Isotope Record

“…The view that the monsoon originated during the Eocene (e.g., Licht et al, 2014) means that it would have been influenced by the ITCZ (Spicer et al, 2017), and it is thus not similar to the present Asian monsoon. After distinguishing this definition, the latest studies suggest that the Asian monsoon was disassociated from the ITCZ and extended into Yunnan 36 or 41 Ma (Fang et al, 2021;Zheng et al, 2022), affecting the Nangqian Basin after 35 Ma (Fang et al, 2022), and that the modern-like Asian monsoon pattern formed in the Late Oligocene-Early Miocene (e.g., Wu et al, 2022). the growth of the Tibetan Plateau (e.g., An et al, 2001An et al, , 2006Boos & Kuang, 2010;Farnsworth et al, 2019;Guo et al, 2002;Liu & Yin, 2002;Liu et al, 2017;Manabe & Terpstra, 1974;Molnar et al, 1993;Prell & Kutzbach, 1992;Spicer & Farnsworth, 2021;Wu et al, 2012;Zhang et al, 2007), orbital variation (Tardif et al, 2021), atmospheric CO 2 (Licht et al, 2014;Tardif et al, 2023), the ice-sheet state, and oceanic gate-ways (Thompson et al, 2021), all of which still impact the Asian monsoon.…”

“…The Indian summer monsoon is slightly different, with simulations showing contribution ratios of 39% for orography, 25% for precession, 21% for atmospheric CO 2 , 5% for the ice-sheet state, 5% for the oceanic gateways (Thompson et al, 2021). However, it is not clear how the Asian monsoon evolved after it left the ITCZ range to influence Yunnan Province 36 or 41 Ma (Fang et al, 2021;Zheng et al, 2022), before a modern-like monsoon formed ∼26 Ma (e.g., Wu et al, 2022). Oxygen isotope variations are related to factors such as temperature, precipitation, and elevation, and are therefore commonly used to reconstruct the paleoelevation of mountains and the paleoclimate variations within a region (e.g., Bershaw et al, 2012;Botsyun & Ehlers, 2021;Botsyun et al, 2019;Caves et al, 2014Caves et al, , 2015Chamberlain et al, 2012;Currie et al, 2005Currie et al, , 2016Dettman et al, 2003;Ding et al, 2014;Fan et al, 2017;Garzione et al, 2000;Graham et al, 2005;Hough et al, 2010;Ibarra et al, 2023;Kent-Corson et al, 2009;L.…”

“…However, it is not clear how the Asian monsoon evolved after it left the ITCZ range to influence Yunnan Province 36 or 41 Ma (Fang et al., 2021; Zheng et al., 2022), before a modern‐like monsoon formed ∼26 Ma (e.g., Wu et al., 2022). Oxygen isotope variations are related to factors such as temperature, precipitation, and elevation, and are therefore commonly used to reconstruct the paleoelevation of mountains and the paleoclimate variations within a region (e.g., Bershaw et al., 2012; Botsyun & Ehlers, 2021; Botsyun et al., 2019; Caves et al., 2014, 2015; Chamberlain et al., 2012; Currie et al., 2005, 2016; Dettman et al., 2003; Ding et al., 2014; Fan et al., 2017; Garzione et al., 2000; Graham et al., 2005; Hough et al., 2010; Ibarra et al., 2023; Kent‐Corson et al., 2009; L. Li et al., 2016; Liu et al., 2014; Mulch, 2016; Rowley & Currie, 2006; Quade et al., 2011, 2020; Sun et al., 2017; Zhuang et al., 2011).…”

“…The view that the monsoon originated during the Eocene (e.g., Licht et al., 2014) means that it would have been influenced by the ITCZ (Spicer et al., 2017), and it is thus not similar to the present Asian monsoon. After distinguishing this definition, the latest studies suggest that the Asian monsoon was disassociated from the ITCZ and extended into Yunnan 36 or 41 Ma (Fang et al., 2021; Zheng et al., 2022), affecting the Nangqian Basin after 35 Ma (Fang et al., 2022), and that the modern‐like Asian monsoon pattern formed in the Late Oligocene–Early Miocene (e.g., Wu et al., 2022).…”