Link Between Global Cooling and Mammalian Transformation Across the Eocene–Oligocene Boundary in the Continental Interior of Asia
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Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-012-0776-1 ORIGINAL PAPER Link between global cooling and mammalian transformation across the Eocene–Oligocene boundary in the continental interior of Asia Rui Zhang • Vadim A. Kravchinsky • Leping Yue Received: 11 August 2011 / Accepted: 1 April 2012 Ó Springer-Verlag 2012 Abstract Evidence in the world’s ocean current system reveal that perissodactyl faunas were abruptly replaced by indicates an abrupt cooling from 34.1 to 33.6 Ma across the rodent/lagomorph-dominant faunas during climate cooling, Eocene–Oligocene boundary at 33.9 Ma. The remarkable and that changes in mammalian communities were accel- cooling period in the ocean, called the Eocene–Oligocene erated by aridification in central Asia. The biotic reorgani- transition (EOT), is correlated with pronounced mamma- zation events described in the Burqin section are comparable lian faunal replacement as shown in terrestrial fossil to the Grande Coupure in Europe and the Mongolian records. For the first time within Asia, a section is mag- Remodeling of mammalian communities. That is, the faunal netostratigraphically dated that also produces mammalian transition was nearly simultaneous all over the world and fossils that span the Late Eocene—Early Oligocene tran- mirrored global climatic changes with regional factors sition. Three fossil assemblages revealed through the EOT playing only a secondary role. (34.8, 33.7, and 30.4 Ma) demonstrate that perissodactyl faunas were abruptly replaced by rodent/lagomorph- Keywords Eocene–Oligocene boundary Á dominant faunas during climate cooling, and that changes Eocene–Oligocene transition Á Junggar Basin Á in mammalian communities were accelerated by aridifica- Magnetostratigraphy Á Paleomagnetism tion in central Asia. Three fossil assemblages (34.8, 33.7, and 30.4 Ma) within the north Junggar Basin (Burqin section) tied to this magnetostratigraphically dated section, Introduction As recorded by a large-scale positive marine benthic d18O anomaly and gradual drops in sea level, a dramatic decrease in deep-sea temperature occurred across the Electronic supplementary material The online version of this article (doi:10.1007/s00531-012-0776-1) contains supplementary Eocene–Oligocene boundary (EOB) 33.9 Ma (Miller et al. material, which is available to authorized users. 1987; Zachos et al. 2001; Coxall et al. 2005; Tripati et al. 2005). At the same time, the major continental fauna & R. Zhang Á V. A. Kravchinsky ( ) alteration in North America, Europe, and Asia is attributed Department of Physics, University of Alberta, Edmonton, AB T6G 2E1, Canada to the climatic Eocene–Oligocene transition (EOT) (Meng e-mail: [email protected] and McKenna 1998; Grimes et al. 2005; Gale et al. 2006; Zanazzi et al. 2007). The global scale climate change R. Zhang Á L. Yue during the EOT caused one of the most significant mammal State Key Laboratory for Continental Dynamics, Department of Geology, Institute of Cenozoic Geology and Environment, biotic reorganization events since the extinction at the end Northwest University, Xi’an 710069, China of the Cretaceous period (Meng and McKenna 1998; Kraatz and Geisler 2010). Eocene–Oligocene land mam- L. Yue malian fossils clearly record this reorganization, termed the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Mongolian Remodeling in Asia and the Grande Coupure in Xi’an 710075, China Europe, when medium-sized perissodactyl communities 123 Int J Earth Sci (Geol Rundsch) were abruptly replaced by small-sized rodent/lagomorph- boundary, we studied the Burqin section (northern Junggar dominant faunas (Meng and McKenna 1998; Kraatz and Basin, central Asia). The section is situated 1,000 km to the Geisler 2010; Costa et al. 2011). The age of the Mongolian west of the Mongolian section described by Kraatz and Remodeling, however, is still not precisely reconstructed. Geisler (2010). Our section could be compared to sections A broad pattern of aridification across the late Eocene from similar time intervals in Europe and Mongolia in and early Oligocene epochs within central Asia was order to specify a linkage to faunal transformation during recently reported in Mongolia (Kraatz and Geisler 2010) the EOT (Wu et al. 2004; Ye et al. 2005; Ni et al. 2007; and the northeast Tibetan Plateau (Dai et al. 2006; Dupont- Costa et al. 2011). Nivet et al. 2007, 2008). Dupont-Nivet et al. (2007, 2008) The Junggar Basin, formed during late Paleozoic, and Abels et al. (2011) built an age model based on Mesozoic, and Cenozoic, is a large-scale piled sedimentary magnetostratigraphy and correlations between local/regio- basin bounded by the Tian-Shan Mountains to the south nal pollen records and a marine oxygen isotope reference and the Altay Mountains to the north. Bosboom et al. curve to perform their paleoenvironmental reconstructions (2011) demonstrate the remnants of a large epicontinental in the Xining Basin of northeastern Tibet. The mammalian sea at the end of Eocene in the middle of the Asian con- fossils found in the upper part of the Tibetan section cor- tinent. In this paper, we base the timing of terrestrial respond to the late Oligocene. Dai et al. (2006) demon- cooling and aridification on our magnetostratigraphic dat- strated that late Eocene to early Oligocene fossils were ing of mammalian fauna findings in the Burqin section. missing in Xining. Therefore, other studies are required in order to find a section with both late Eocene and early Oligocene fossils and to clarify the timing of the EOT in Mammalian fossils of the Burqin section Asia. Kraatz and Geisler (2010) reported on several sec- tions across the EOT from Mongolia. Their age model was The terrestrial Tertiary strata on the northern bank of the based on a magnetostratigraphic compilation and direct tie Irtysh River near Burqin City (Fig. 1) consist of lower points to the geomagnetic polarity time scale (GPTS) via rusty-yellow and purplish red beds of the Irtysh River radioisotopically dated basalts for a few short sedimentary Formation, and upper yellowish-green sandstones with sequences that sometimes contained only one or two nor- variegated green mudstones beds of the Keziletuogayi mal/reversed polarity zones. Although the authors descri- Formation (Ye et al. 2005; Ni et al. 2007). All stratigraphic bed faunal transitions during the EOT, late Eocene faunas layers in the northern part of the Junggar Basin are hori- were not found. Finding both late Oligocene and early zontal, and there are no signs of sedimentary discontinu- Eocene faunas in one section would confirm the timing of ities or tectonic deformations in the region that makes the the EOT and determine the upper/lower limits of the Burqin section one of the key sections to study the EOT in transition in continental Asia. Asia. The sedimentary strata are folded and steep only in To better understand the character and timing of the the southern periphery of the basin (Carroll et al. 1990). faunal transformation in Asia compared to Europe, as well Three mammalian fossil assemblages were found across as differences and similarities between continental and the EOB during 1998 to 2006 within the Keziletuogayi oceanic climate changes around the Eocene–Oligocene Formation in the Burqin section (Wu et al. 2004; Ye et al. Fig. 1 a Study area location in Asia. b Location of the Junggar Basin and the Burqin section. The key Eocene–Oligocene localities in Asia: 1 Mongolia (Kraatz and Geisler 2010); 2 Xining Basin (Dai et al. 2006; Dupont-Nivet et al. 2007, 2008). Our study area is marked as a star 123 Int J Earth Sci (Geol Rundsch) 2005; Ni et al. 2007). The first assemblage was discovered reflects a faunal change from the lower two assemblages. in a 70-mm thin layer of black manganese siltstone in The dominant small-sized mammals of the third assem- grayish green mudstone at a depth of 58 m. These findings blage included 16 species similar in size to Palaeoscaptor are described in detail by Ye et al. (2005) and Ni et al. cf. P. acridens and Tupaiodon cf. T. morrisi, and analogous (2007) (see Table 1 in Ni et al. (2007) and Figure SM-1 in to the Shandgolian from Mongolia (Meng and McKenna the online Supplementary Materials). The first assemblage 1998; Ye et al. 2005; Kraatz and Geisler 2010). is similar to the Khoer-Dzan and Ergilin faunas of Mon- The faunas transition in the Burqin basin correspond to golia (Wu et al. 2004; Ye et al. 2005; Ni et al. 2007). Six the Mongolian Remodeling and the Grande Coupure from species representing four families include Cadurcodon cf. Europe (Meng and McKenna 1998; Wu et al. 2004;Ye C. ardynensis, Amynodontidae gen. et sp. indet., Gigant- et al. 2005; Ni et al. 2007) linking those areas to the same amynodon giganteus, Indricotheriinae gen. et sp. indet., global cooling event across the Eocene–Oligocene bound- Rhinocerotidae gen. et sp. indet., and Brontotheriidae gen. ary. In all of these sections, Late Eocene faunas were et sp. indet.; all are perissodactyls. There were no fossils of dominated by large perissodactyls, whereas the Early Oli- small mammals found in the paleontological screen- gocene faunas are rich in rodents and lagomorphs. washing. The apparent lack of forest-related species like euprimates, chiropterans, and plesiadapiformes suggests an absence of dense forest in Burqin during the studied The magnetostratigraphic result interval. Rhynchosia and Elaeagnus pollen findings and evidence of perissodactyl-dominant mammals indicate a In the Burqin section, we were able to sample a 128 m warm subtropical climate (Ye et al. 2005). interval that corresponds to the Irtysh River (lower part of The second mammalian assemblage includes fossils the section) and Keziletuogayi (upper part of the section) found at a depth of 45 m in a 1.7 m layer of brown mud- formations at the north bank of the Irtysh River.