Aridification signatures from middle – late Eocene pollen indicate 1 widespread drying across the Tibetan Plateau after 40 Ma 2

Central Asia experienced a number of significant elevational and climatic changes during the Cenozoic, but much 17 remains to be understood regarding the timing and driving mechanisms of these changes, as well as their influence on 18 ancient ecosystems. Here we describe the palaeoecology and palaeoclimate of a new section from the Nangqian Basin in 19 Tibet, northwestern China, here dated as late Lutetian–Bartonian (late middle–late Eocene) based on our palynological 20 analyses. Located on the east-central part of the Tibetan Plateau, this section is excellently placed for better understanding 21 the palaeoecological history of Tibet following the India-Asia collision. Our new pollen record reveals that a strongly 22 https://doi.org/10.5194/cp-2019-138 Preprint. Discussion started: 20 January 2020 c © Author(s) 2020. CC BY 4.0 License.

Paleogene or Neogene phenomenon, and its relationship with TP uplift (e.g., Dupont-Nivet et al. 2007;Xiao et al., 2010; indicating a productivity ratio of 30%. Nevertheless, well-preserved palynological assemblages were recovered throughout 166 the section, enabling a representative portrayal of vegetation changes through time to be reconstructed. In total 26 spore and 167 81 pollen taxa (5 gymnosperm and 76 angiosperm morphospecies) were able to be identified, which are illustrated (Plate I, 168 II, III) and grouped into seven different Plant Functional Types (PFTs) that represent various ecological groups (Fig. 2).

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Overall trends for the RZ section include rare conifers and a general dominance of steppe-desert pollen in all zones. Ferns 170 are abundant and diverse, particularly in the lower part of the section (Zone I), while temperate and warm broad-leaved 171 forest are relatively diverse and present throughout, but not particularly abundant in any zone. Steppe-desert pollen decreases 172 concurrently with a spike in tropical forest in Zone II, and then resurges to dominance in Zone III.

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with Nitrariadites/Nitraripollis increasing steadily through the section. Temperate broad-leaved forest is now much more 244 common than warm or tropical forest pollen, while ferns are least common in this zone but still plentiful.  (Xu et al., 2016). In the latter study, zircon U-Pb 252 age data were derived from felsic intrusions sampled at two localities in the Nangqian Basin (Boza and Nangqian). The 253 syenite porphyries from the Boza area (further south of the RZ section) show an emplacement age of 35.58± 0.33 Ma, while

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Our results indicate that the MECO prompted a considerable change in the vegetation in east-central Tibet, encouraging 345 the temporary spread of (dry) forests while steppe-desert vegetation contracted (Zone II). Warming is reflected by an atypical 346 spike in tropical forest that coincides with a warm forest spike in northeastern Tibet (tropical forest is exceedingly rare in the 347 latter area during the middle-late Eocene), which demonstrates the regional influence of the MECO across (at least) the 348 northern and central parts of the TP. In order to estimate relative humidity in arid environments such as these, the 349 Nitraria/Ephedra (N/E) ratio can be used to distinguish between desert/semi-desert (< 1) and steppe-desert (> 1; Li et al., 350 2005;Hoorn et al., 2012). Although both genera occupy arid environments today, Ephedra is currently distributed primarily 351 throughout deserts, semi-deserts and grasslands globally (Stanley et al., 2001), while Nitraria is a relatively more humid 352 steppe-desert taxon (Cour et al., 1999;Sun and Wang, 2005;Jiang and Ding, 2008;Li et al., 2009;Zhao and Herzschuh, 353 2009). In the RZ section, the proportion of temperate forest in relation to warm and tropical forest became much greater from 354 ca. 39 Ma (Fig. 2), indicating a cooler climate than prior to the MECO, which matches the cooling trend recorded by 355 clumped isotopes to the north (Page et al., 2019). Importantly, the N/E ratio in the RZ section is lowest immediately 356 following the MECO (Fig. 2) and persists for an extended period, indicating rapid, prolonged aridification, and an overall 357 expansion of steppe-desert vegetation is observed in Zone III, corresponding with patterns observed on the northeastern TP the proto-Paratethys Sea was reduced as far as central Tibet. This provides further support for the argument that this sea was

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Long-term aridification after the MECO exerted further influence on vegetational composition in east-central Tibet with 367 regards to the proportions of the ancestral vs. derived types of Ephedripites. In modern and Quaternary settings, this has been 368 developed as a ratio to distinguish between desert and steppe-desert environments, termed the Ephedra fragilis-type 369 s.l./Ephedra distachya-type (Ef/Ed) ratio (whereby E. fragilis represents the ancestral type and E. distachya, the derived 370 type). Tarasov et al. (1998) found the E. fragilis-type s.l. to be common in arid climates with mean temperatures of the 371 warmest month above 22°C. Herzschuh et al. (2004) applied the Ef/Ed ratio to Holocene pollen spectra from the Alashan 372 Plateau and tested its reliability with a regional modern pollen dataset, finding Ef/Ed ratios > 10 in most samples from desert 373 sites, and values < 5 in most samples from the sites with more favourable climates (e.g., forest-steppe, steppe, and alpine 374 meadow).

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In the middle-late Eocene of Central Asia, the ancestral type of Ephedripites never comprises more than 25% of the 376 ephedroid pollen sum in northeastern Tibet while the derived type makes up at least 60% (Xining Basin;Han et al., 2016 and 377 Qaidam Basin;Zhu et al., 1985;Miao et al., 2013a;Jiuquan Basin;Miao et al., 2008), and this also appears true for 378 northwestern Tibet (Tarim Basin;Wang, et al., 1990b;Hoh Xil Basin;Miao et al., 2016) and east-central Tibet (Yuan et al.,

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2017; this study). Therefore, Ef/Ed ratios > 10 (supposedly indicative of desert ecosystems) are never observed, despite the 380 N/E ratio indicating regular existence of deserts or semi-deserts in northern Tibet (Zhu et al., 1985;Hoorn et al., 2012;Miao