Wet tropical climate in SE Tibet during the Late Eocene

Abstract

Cenozoic climate cooling at the advent of the Eocene-Oligocene transition (EOT), ~33.7 Ma ago, was stamped in the ocean by a series of climatic events albeit the impact of this global climatic transition on terrestrial environments is still fragmentary. Yet archival constraints on Late Eocene atmospheric circulation are scarce in (tropical) monsoonal Asia, and the paucity of terrestrial records hampers a meaningful comparison of the long-term climatic trends between oceanic and continental realms. Here we report new sedimentological data from the Jianchuan basin (SE Tibet) arguing for wetter climatic conditions in monsoonal Asia at ~35.5 Ma almost coevally to the aridification recognized northwards in the Xining basin. We show that the occurrence of flash-flood events in semi-arid to sub-humid palustrine-sublacustrine settings preceded the development of coal-bearing deposits in swampy-like environments, thus paving the way to a more humid climate in SE Tibet ahead from the EOT. We suggest that this moisture redistribution possibly reflects more northern and intensified ITCZ-induced tropical rainfall in monsoonal Asia around 35.5 Ma, in accordance with recent sea-surface temperature reconstructions from equatorial oceanic records. Our findings thus highlight an important period of climatic upheaval in terrestrial Asian environments ~2–4 millions years prior to the EOT.

Leaf and infructescence fossils of Alnus (Betulaceae) from the late Eocene of the southeastern Qinghai–Tibetan Plateau

“…All these fossil taxa indicate a much warmer and wetter climate during the late Eocene than now. Generally, our finding agrees with a recent paleoclimate reconstruction of the Jianchuan Basin along the southeastern margin of the QTP, which also suggested a warm climate during the late Eocene (Sorrel et al, ).…”

The origin of Asian monsoons: a modelling perspective

“…1c, d). We have chosen the 3W / 3D ratio among many available criteria to characterize the climate seasonality because it has also been used as an indicator of monsoonal climates (with a minimum threshold value close to 5) in previous investigations of palaeo-monsoons (Herman et al, 2017;Shukla et al, 2014;Sorrel et al, 2017). The modern monsoonal regions in our model are adequately characterized by a high 3W / 3D ratio, although this signature is stronger in southern Asia than in eastern China.…”

“…5c). This alternate moisture pathway toward western Africa rather than (Bougeois et al, 2018), 2: Xining Basin (Meijer et al, 2019;Licht et al, 2014a), 3: Maoming Basin (Herman et al, 2017;Spicer et al, 2016Spicer et al, , 2017, 4: Jiuchuan Basin (Sorrel et al, 2017) and 5: Pondaung Formation in Myanmar (Licht et al, 2014b).…”

“…(3) the Maoming and Changchang basins in southeastern China (Herman et al, 2017;Spicer et al, 2016), located in the transition zone between EAM and I-AM; (4) the Jiuziyan Formation (Sorrel et al, 2017) and finally (5) the Pondaung Formation in Myanmar (Licht et al, 2014b), presently located in the area of influence of the SAM. Although we lack Indian sites suggesting the presence of the SAM in the late Eocene, we acknowledge that such sites do exist for the early Eocene (i.e.…”

“…The inception of the SAM and EAM has been proposed to have occurred during the early Miocene (Guo et al, 2002) or the latest Oligocene (Sun and Wang, 2005), but recent field observations have suggested an earlier inception, as soon as the middle to late Eocene (∼ 40 Ma). These studies rely on different indices such as (a) records of high seasonality in precipitations from palaeovegetation and sedimentary deposits in China (Quan et al, 2012;Sorrel et al, 2017;Q. Wang et al, 2013) and Myanmar (Licht et al, 2015); (b) δ 18 O measurements showing high annual vari-ability in water availability in oyster shells from the Tarim Basin (Bougeois et al, 2018;Ma et al, 2019), in mammals tooth enamel and gastropod shells from Myanmar (Licht et al, 2014a).…”