Neogene tectonics and climate forcing of carnivora dispersals between Asia and North America

Introduction Conclusions References

. Sedimentary records on the basin flanks of the Himalaya and out into the Indian Ocean generally show a similar change around 20 Ma. About 69 % of the Himalayas south of the Indus-Yarlung suture zone, or about 6.7 × 10 6 km 3 , have been denudated since ∼ 20 Ma (Einsele et al., 1996). Records of isotopic ratio changes through time provide another window to observe the significant tectonic or environmental change in 5 Asia around 20 Ma. The steepest rise in the strontium isotopic ratio ( 87 Sr/ 86 Sr) of seawater during the Cenozoic was from 20 to 14.4 Ma (Hodell et al., 1991;Richter et al., 1992;Hodell and Woodruff, 1994). Similarly, lithium isotopes in seawater (δ 7 Li SW ) increased abruptly at ∼ 20 Ma, then generally decreased from 20 to 15 Ma (Misra and Froelich, 2012). Hence, the Himalaya and southern Tibet was significantly uplifted and 10 eroded at ∼ 20 Ma. This conclusion is consistent with a marked slowdown in the convergence rate between India and Eurasia by more than 40 % since 20 Ma (Molnar and Stock, 2009). Modeling of apatite fission track data from the Songpan-Ganzi Fold Belt suggests that exhumation accelerated ∼ 20 Ma in East Tibet, consistent with the mid-Tertiary tim- 15 ing inferred for reactivation of the Wenchuan-Maoxian Fault from zircon fission track data (Arne et al., 1997). Moreover, ages on the Anning transect suggest an early initiation of rapid cooling (ca. 20 Ma, Clark et al., 2005). Thus significant tectonic movements occurred along the eastern margin of the Tibetan Plateau at ∼ 20 Ma.
Along the northeastern margin of the Plateau, several basins also record significant 20 tectonic changes around 20 Ma, such as the transitions of sedimentary facies in the Lanzhou and Qaidam Basins (Yue et al., 2001;Qiu et al., 2001;Lu and Xiong, 2009), the onset of widespread contractional deformation in the Gonghe Basin (Craddock et al., 2011;Lu et al., 2012), the initiated deposition of Xunhua Basin (Hough et al., 2011), and the transition to alluvial facies in the Hualong Basin (Lease et al., 2012). Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | then supported by several studies from the Dahonggou section (changes in sedimentation facies and SUS, Lu and Xiong, 2009), the Wulan section (changes in sedimentation facies and mean declination, Lu et al., 2012), and the Huaitoutala section (changes in the δ 18 O of lacustrine carbonates, Zhuang et al., 2011) in the Qaidam Basin. That is to say, these regions in the northeast Tibetan Plateau experienced significant tectonic 5 movements at ∼ 12 Ma. Nevertheless, it is noteworthy that the East Antarctic Ice Sheet expanded significantly since 14 Ma and initiated the Mid-Miocene Climate Transition (MMCT), probably causing a marked cooling in East Asia during 14-11 Ma (Fig. 4a, Jiang and Ding, 2008). This aroused a wide curiosity about whether the Tibetan uplift or the global cooling has been 10 the first-order driver controlling stepwise drying in Asia (e.g. Jiang et al., 2008;Lu et al., 2010;Zhuang et al., 2011;Miao et al., 2012;Lu and Guo, 2014). In order to explore the evolution of climate through the MMCT, Jiang et al. (2007 analyzed multiple proxies from the 2900-m-thick fluviolacustrine sediment sequence at Sikouzi, Ningxia, China, such as pollen humidity index (Fig. 4a), redness (Fig. 4b), Lightness (Fig. 4c), 15 Susceptibility (Fig. 4e), TIC, and TOC. The results indicate that the palaeoclimate in East Asia has got cooler and drier since 12-11 Ma. This climate change also left imprints in many other regions of the world, probably linked with the marked expansion of the East Antarctic Ice Sheet and resultant positive feedbacks of vegetation change and greenhouse gas fluctuations (Jiang et al., 2008). This inference is supported by a good 20 correlation of the thick eolian silt sequences of Asian drying from the Early Miocene to Late Pleistocene with global cooling (Lu et al., 2010). Later, Zhuang et al. (2011) attributed the isotope-constrained intensified aridity in the Qaidam Basin at 12 Ma to retreat of Paratethys from central Asia, blocking moisture-bearing air masses by the elevated south-central Tibetan Plateau, and enhanced isolation and outward growth of Introduction  Miao et al., 2012), supporting the inference that global cooling provided a dominant driving factor for the drying of Eurasia (Jiang et al., 2008;Lu et al., 2010;Lu and Guo, 2014). Accordingly, global cooling is believed to have been responsible for the mammal exchanges between North America and Eurasia during 13-11 Ma.

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Noticeably, both the climate and tectonic records and the observed mammal fauna are relatively few in East Asia during the MMCT. With further investigations and more climatic and tectonic records published in the future, the timing interval of mammal exchange between North America and Eurasia during the MMCT would be narrower and clearer. The pollen record from Guyuan, Ningxia, China, indicates that the East-Asian summer monsoon declined significantly from 14. 25-11.35 Ma and kept weak since 11.35 Ma 15 (Fig. 4a, Jiang and Ding, 2008). This is well consistent with marked development of herbs and shrubs in the vast region north to the Yangtze River of South China during the late Middle to Late Miocene as synthesized by Jiang and Ding (2009), probably correlated with evident global cooling caused by significant expansion of the East Antarctic Ice Sheet during the MMCT (e.g. Woodruff and Savin, 1989;Flower and Kenett, 1994;20 Ohta et al., 2003;Shevenell et al., 2004;Zachos et al., 2001Zachos et al., , 2008. Following the MMCT, the climate evolution in East Asia during 11-8 Ma is pivotal to understanding the fauna exchange between North America and Eurasia at 8-7 Ma. In Ningxia Province, the redness (a*) record of the Sikouzi fluviolacustrine sediments showed a slight decrease from 11 to 8 Ma (Fig. 4b), possibly reflecting a declining oxidation caused by global cooling (Jiang et al., 2007(Jiang et al., , 2008. Such a declining oxidation increased magnetic minerals in the sediments, which is mirrored as a continuous in-  Fig. 4e, Jiang et al., 2008). The Sikouzi lightness (L*) record during 11-8 Ma maintained higher values than previously (Fig. 4c), implying high contents of carbonate in sediments and thus a more arid environment (Jiang et al., 2008). Its slight decreasing trend from 11 to 8 Ma is possibly related to the evident increase in sedimentation rate during this period, especially during the late period (Jiang and Ding, 2008). Such inference is confirmed by an evident increase of SUS during this period (Fig. 4e). Furthermore, the pollen record from the Linxia Basin on the northeastern margin of the Tibetan Plateau indicates that, during 11-8 Ma, the conifers showed a steep decline while the herbs and shrubs increased significantly (Ma et al., 1998), implying a rapid drying environment. Similarly, the coniferous pollen in the Qaidam Basin decreased while the xerophytes increased during 11-8 Ma (Miao et al., 2011), indicating that drying in the Qaidam intensified during this period. Therefore, it is clear that the climate evolution in East Asia during 11-8 Ma is characterized by slow cooling and gradual drying. This is well correlated with further enrichment of the integrated δ 18 O of marine benthic foraminifera (Fig. 5a, Zachos et al.,15 2008) and the significant sea-level fall during this period (Fig. 5b, Haq et al., 1987). Such a global declining climate during 11-8 Ma probably resulted in stepwise enhancement of the East Asian winter monsoon (transporting relatively coarse dust particles) and of the westerlies (transporting relatively fine dust particles), providing important transporting agents and arid geographic locations for widespread dust accumulation in 20 North China and even the western Pacific since ∼ 8 Ma.
Previous studies indicate that the Tibetan Plateau experienced significant tectonic movements at ∼ 8 Ma (e.g. Pan and Kidd, 1992;Harrison et al., 1995;Kirby et al., 2002;Fang et al., 2005;Zheng et al., 2006;Lease et al., 2011;Duvall et al., 2012). As shown in Table 2  Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ment at this time. Accordingly, it is speculated that tectonic activities in the eastern and northeastern Plateau generated large quantities of dust materials since 8.5-7.5 Ma and provided adequate material sources for widespread dust accumulation in North China and even the western Pacific. This is probably responsible for the significant increase of eolian deposit from 4 sites during 14-7.5 Ma to 14 sites during 7.5-3.6 Ma in North 5 China (Lu et al., 2010). Furthermore, at some sites, red clay overlies much older rock of a different type, such as Lingtai (7.05 Ma, Ding et al., 1998a, 1999, Xifeng (7.2 Ma, Sun et al., 1998), Jiaxian (8.35 Ma, Qiang et al., 2001, and Chaona (8.1 Ma, Song et al., 2007). Almost at the same time, both sedimentation rate and mean grain-size of sediments increased clearly in North China (e.g. Lu et al., 2004Lu et al., , 2007Guo et al., 2002;Qiao et al., 2006). Therefore, significant environmental events characterized by widespread dust accumulation occurred at 7-8 Ma in North China and the western Pacific (e.g. Ding et al., 1998b;Rea et al., 1998;Sun et al., 1998;Pettke et al., 2000;Guo et al., 2001;Qiang et al., 2001). Such events are responsible for the carnivoran dispersal from Eurasia 15 to North America at 8-7 Ma, probably driven by a combination of continuous global cooling and tectonic movements of the eastern and northeastern Tibetan Plateau.

Carnivoran exchanges between Eurasia and North America at ∼ 4 Ma possibly driven by global cooling
Previous studies indicate that climate was relatively warm and wet during the Early 20 Pliocene and declined during the Late Pliocene, especially in East China (e.g. Yu and Huang, 1993;Ding et al., 2001;Guo et al., 2004;Wu et al., 2006;Jiang and Ding, 2009;Xiong et al., 2010). This arouses a wide interest in the beginning of climate recession during the Late Pliocene. The grain-size record of the Sikouzi section at Guyuan, Ningxia, China suggests that Md (median grain size) ranged from 1.6 to ate content, increase in terrigenous sediments and a lowering of sea level controlled by global cooling (Hay et al., 1988;Tian et al., 2008). This inference is supported by the benthic δ 18 O record of the same core (Tian et al., 2008) and the grain-size record at ODP Site 1146 (Wan et al., 2007). Similarly, oscillating amplitude of the grain-size record of core BDP98 (600 m) from Academician ridge ( Lisiecki and Raymo, 2005;Zachos et al., 2008), and is also correlated with strengthened periodicity of sea-level fluctuations since ∼ 4 Ma (Fig. 5b, Haq et al., 1987). In general, the above data suggest that Late Cenozoic global climate probably entered a new state at ∼ 4 Ma. The factor responsible for this significant climate change deserves further investigation. The change in depositional facies and increase in sed-5 imentation rate of the Yecheng section in the western Kunlun Mountains reflects the main uplift of the northwestern Tibetan Plateau ca. 4.5-3.5 Ma (Zheng et al., 2000(Zheng et al., , 2006. Nevertheless, more studies indicate that the Tibetan uplift subsequent to ca. 3.6 Ma was intense, such as the upper reaches of the Yellow River (Li et al., 1996(Li et al., , 1997, the Linxia Basin (Fang et al., 2005), the Guide Basin (Pares et al., 2003), the 10 Guyuan Basin (Jiang et al., 2007;Jiang and Ding, 2010), and the Sanmenxia Basin (Wang et al., 2002). Regional unconformities at ∼ 4 Ma are observed in the Great Plains and western United States (Hanneman et al., 2003;Hanneman and Wideman, 2006). However, all of these apparently could not explain the increases in sedimentation rates as well as in grain sizes of sediments at 4-2 Ma in a variety of settings around the 15 globe (Zhang et al., 2001). Increase in erosion rates caused by global cooling is a major feature of environmental changes in various regions around the globe at ∼ 4 Ma (Zhang et al., 2001;Jiang et al., 2010). Recent climate modeling results suggest that the progressive closure of the Central American Seaway (CAS) initiated strengthening of Atlantic meridional overturning circulation (AMOC) between 4.8 and 4.0 Ma, lead-20 ing to both warming of the Northern Hemisphere (NH) and cooling of the Southern Hemisphere (SH) (Steph et al., 2010). Cooling of the SH would induce a marked development of the Antarctic Ice Sheets at ∼ 4 Ma, pushing the Intertropical Convergence Zone northward. This was superimposed on warming of the NH and brought more precipitation to the middle latitudes of the NH, resulting in increases in coarse-grained    Lett., 25, 85-88, 1998 Tang, Z. H., Ding, Z. L., White, P. D., Dong, X. X., Ji, J. L., Jiang, H. C., Luo, P., and Wang, X.: Late Cenozoic central Asian drying inferred from a palynological record from the northern Tian Shan, Earth Planet. Sc. Lett., 302, 439-447, 2011. Tang, Z. H., Huang, B. C., Dong, X. X., Ji, J. L., and Ding, Z Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | River running through the Sanmenxia Gorge eastward into the sea, Sci. China, 45, 595-608, 2002. Wang, W.-M. and Deng, T.: Palynoflora from the stratotype section of the Neogene Xiejian stage and its significance, Acta Palaeontologica Sinica, 48, 1-8, 2009 (in Chinese with English abstract).   Age (Ma) Figure 5. Correlation of (a) the composite oxygen isotope curve from Zachos et al. (2008) and (b) the Neogene sea-level record from Haq et al. (1987).