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Asian Aridification Linked to the First Step of the Eocene-Oligocene

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Asian Aridification Linked to the First Step of the Eocene-Oligocene Clim. Past, 6, 501–513, 2010 www.clim-past.net/6/501/2010/ Climate doi:10.5194/cp-6-501-2010 of the Past © Author(s) 2010. CC Attribution 3.0 License. Asian aridification linked to the first step of the Eocene-Oligocene climate Transition (EOT) in obliquity-dominated terrestrial records (Xining Basin, China) G. Q. Xiao1,2, H. A. Abels3, Z. Q. Yao1, G. Dupont-Nivet4, and F. J. Hilgen3 1State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, P.O. Box 17, Xian, 710075, China 2Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan, 430074, China 3Stratigraphy/Paleontology, Dept. of Earth Sciences, Utrecht University, Utrecht, The Netherlands 4Paleomagnetic Laboratory “Fort Hoofddijk”, Dept. of Earth Sciences, Utrecht University, Utrecht, The Netherlands Received: 21 March 2010 – Published in Clim. Past Discuss.: 20 April 2010 Revised: 22 June 2010 – Accepted: 9 July 2010 – Published: 29 July 2010 Abstract. Asian terrestrial records of the Eocene-Oligocene ume growth. Coincidence with lithofacies changes in our Transition (EOT) are rare and, when available, often poorly Chinese record would suggest that the atmospheric impact constrained in time, even though they are crucial in un- of the first step was of global significance, while the major derstanding the atmospheric impact of this major step in ice volume increase of the second step did not significantly Cenozoic climate deterioration. Here, we present a detailed affect Asian interior climate. cyclostratigraphic study of the continuous continental EOT succession deposited between ∼35 to 33 Ma in the Xining Basin at the northeastern edge of Tibetan Plateau. Lithol- 1 Introduction ogy supplemented with high-resolution magnetic suscepti- bility (MS), median grain size (MGS) and color reflectance ∗ The Eocene-Oligocene Transition (EOT) encompasses the (a ) records reveal a prominent ∼3.4 m thick basic cyclicity most pronounced cooling event during Cenozoic climate de- of alternating playa gypsum and dry mudflat red mudstones terioration (Miller et al., 1991; Zachos et al., 1996, 2001; of latest Eocene age. The magnetostratigraphic age model Lear et al., 2000, 2008). It was accompanied by the expan- indicates that this cyclicity was most likely forced by the 41- sion of ice sheets on the Antarctic continent (e.g. Zachos et kyr obliquity cycle driving oscillations of drier and wetter al., 2001; Coxall et al., 2005; Lear et al., 2008), a global sea- conditions in Asian interior climate from at least 1 million level drop (e.g. Miller et al., 1991; Katz et al., 2008), a de- year before the EOT. In addition, our results suggest a du- clining atmospheric CO2 concentration (Pearson and Palmer, ration of ∼0.9 Myr for magnetochron C13r that is in accor- 2000; Pearson et al., 2009), a deepening of the calcite com- dance with radiometric dates from continental successions pensation depth (Coxall et al., 2005; Tripati et al., 2005), in Wyoming, USA, albeit somewhat shorter than in current and a biotic reorganization (e.g. Meng and McKenna, 1998; time scales. Detailed comparison of the EOT interval in the Ivany et al., 2003; Hansen et al., 2004; Pearson et al., 2008). Tashan section with marine records suggest that the most pro- Geological records from different latitudes indicate signifi- nounced lithofacies change in the Xining Basin corresponds cant cooling during this transition (Zachos et al., 1996; Re- to the first of two widely recognized steps in oxygen isotopes tallack et al., 2004; Zanazzi et al., 2009; Katz et al., 2008; across the EOT. This first step precedes the major and second Lear et al., 2008; Schouten et al., 2008; Liu et al., 2009), im- step (i.e. the base of Oi-1) and has recently been reported to plying that this phenomenon had a global rather than regional be mainly related to atmospheric cooling rather than ice vol- impact on climate. Climate modeling results and ocean sed- iment records have demonstrated that the decline of pCO2 and a peculiar orbital configuration were the primary factors Correspondence to: H. A. Abels responsible for this transition (Coxall et al., 2005; DeConto ([email protected]) and Pollard, 2003; DeConto et al., 2008). Ocean sediment Published by Copernicus Publications on behalf of the European Geosciences Union. 502 G. Q. Xiao et al.: Asian aridification linked to the first step of the Eocene-Oligocene climate Transition (EOT) 101°30’ E 101°45’ E 102°00’ E 102°15’ E records further show that this transition occurred in two or 5000 (m) 37° possibly three steps within ∼400 kyr (Coxall et al., 2005; )a( 20’ N (b))b( Tibetan Plateau 4500 37° Katz et al., 2008; Lear et al., 2008; Pearson et al., 2008). 10’ Age models for paleoclimatic records across the EOT have G0 1 4000 37° G7 00’ increased in both resolution and accuracy, which is crucial G9 G10 G11 36° G12 for unraveling cause and effect during such a complicated 3500 50’ G13 G14 transitional interval. Sediments recovered during ODP Leg Shuiwan 36° G15 40’ 3000 Xining City 199 (Palike¨ et al., 2006) and the Tanzanian Drilling Project G16 G17 Tashan 36° 30’ (TDP) (Pearson et al., 2008; Lear et al., 2008) are excellent 2500 Xiejia examples of this approach. The coming years are expected G18 36° 20’ G19 2000 to shed more light as multiple continuous carbonate-rich G20 G21 36° records have been recovered during IODP Leg 320. Equally 10’N G22 1500 0 25 50 km promising are continental EOT records that would allow di- 101°30’ E 101°45’ E 102°00’ E 102°15 ’ E rect evaluation of modeled atmospheric climate, which is Fig. 1. (a) Location of the Tashan (TS), the Xiejia, and the Shui- currently mostly based on data from the oceanic domains. Xiao et al., Figure 1 Reliable global marine to continental comparison requires wan sections, (b) View of the TS section. The end of regular, thick gypsum-mudstone cyclicity occurs above bed G indicated as event increased age control at orbital scale resolution of key conti- 7 1 (see Fig. 7). nental successions such as in North America (e.g. Zanazzi et al., 2009), in the UK (Gale et al., 2006; Hooker et al., 2009), and in China (Dupont-Nivet et al., 2007). The Eocene to Early Miocene lacustrine records from 2 Geological setting the Xining Basin at the northeastern margin of the Tibetan Plateau (Fig. 1a) provide an excellent opportunity to in- The Xining Basin is part of a larger Paleocene-Miocene vestigate climate change during the EOT (Dai et al., 2006; basin system characterized by upward fining sediments asso- Dupont-Nivet et al., 2007). The Eocene successions consist ciated with decreasing rates of accumulation (1–10 cm/kyr) of red mudstones with intercalations of gypsum and gypsif- attributed to regional post-rift thermal subsidence subse- erous layers (Qinghai Bureau of Geology and Mineral Re- quent to Mesozoic extension (Dupont-Nivet et al., 2004; sources, 1985). Detailed lithofacies analyses of the Xiejia Horton et al., 2004). The stratigraphic base is found dis- and Shuiwan sections showed that gypsum/gypsiferous lay- conformable on the Upper Cretaceous Minhe group. This ers were formed in shallow playa lake settings, while mud- basal relationship extends to the east in the adjacent Lanzhou stone layers developed in distal alluvial fan and dry mudflat Basin where thicker conglomeratic series are found uncon- environments (Dupont-Nivet et al., 2007). Dupont-Nivet et formable on folded Cretaceous rocks suggesting post Cre- al. (2007) interpreted gypsum intercalations within red mud- taceous tectonism east of the Xining Basin followed by re- stones to indicate periods of significantly higher water sup- gional subsidence and basin initiation or re-activation (Zhai ply. They further showed that the regional disappearance of and Cai, 1984). Magnetostratigraphic dating of the partic- intercalated gypsum beds from red mudstone stratigraphy co- ularly well-developed >1000 m thick Cenozoic stratigraphy incides with the EO-boundary based on a stratigraphic analy- of the Xining Basin indicates sub-continuous deposition be- sis of the Xiejia and Shuiwan sections. This coincidence was tween ∼52 Ma to 17 Ma (Dai et al., 2006). The stratig- interpreted as to reflect regional aridification related to cli- raphy consists of basal sandy successions (Qijiachuan for- mate change. Their magnetostratigraphic age model further mation) overlain by red mudstones with distinctive gypsif- suggested that the regular lithofacies alternations between erous intercalations (Honggou and Mahalagou formations) mudstone and gypsum in the Late Eocene were forced by the overlain by light brown to yellow mudstones with occa- 100-kyr eccentricity cycle. These results provide the frame- sional sandy lenses (Xiejia, Chetougou and Xianshuihe for- work for a more detailed analysis of the E/O-boundary inter- mations). Apart for the disappearance of gypsum interca- val. Here we present a cyclostratigraphic study of the upper lation precisely correlated to the 34 Ma Eocene-Oligocene Eocene succession in the Xining Basin enabling a compar- climate transition (Dupont-Nivet et al., 2007), deposition ison with recently refined marine records. We investigate a is virtually undisturbed with slow accumulation (average well-exposed section near the village of Tashan (TS section; 2.2 cm/kyr) until 17 Ma suggesting that important deforma- Fig. 1) using high-resolution magnetic susceptibility (MS), tion did not affect the Xining Basin until that time. A post- median grain size (MGS) and color reflectance (a∗) records, 17 Ma age is hence indicated for the observed deformation of regional correlation of lithostratigraphy, and detailed magne- the strata by a set of regional E-W and local NW-SE struc- tostratigraphic time control. tures. The TS section (101◦500 E, 36◦330 N) is located ∼3 km west of the Xiejia and ∼13 km south of the Shuiwan section (Dai et al., 2006; Dupont-Nivet et al., 2007) in the Xining Clim.
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