Global and Planetary Change 158 (2017) 36–46 Contents lists available at ScienceDirect Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha Magneto- and litho-stratigraphic records of the Oligocene-Early Miocene T climatic changes from deep drilling in the Linxia Basin, Northeast Tibetan Plateau ⁎ Fuli Wua, Xiaomin Fanga,b, , Qingquan Mengb, Yan Zhaoa, Fenjun Tanga,c, Tao Zhanga, Weilin Zhanga, Jinbo Zana a CAS Center for Excellence in Tibetan Plateau Earth Sciences, Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China b School of Earth Sciences, Key Laboratory of Western China's Mineral Resources of Gansu Province, Lanzhou University, Lanzhou 730000, China c University of Chinese Academy of Sciences, Beijing 100049, China ARTICLE INFO ABSTRACT Keywords: The East Asian monsoon is generally regarded to have initiated at the transition from the Late Oligocene to the Oligocene/Miocene boundary Early Miocene. However, little is known about this process because of a lack of continuous strata across the Paleomagnetism boundary between the Late Oligocene and the Early Miocene in Asia. Based on previous drilling (core HZ-1) in Hematite and redness records the Miocene sediments in the southern Linxia Basin in NW China, we drilled a new 620 m core (HZ-2) into the Early monsoon history Late Oligocene strata and obtained 206 m of continuous new core. The detailed paleomagnetism of the new core Linxia Basin reveals eleven pairs of normal and reversed polarity zones that can be readily correlated with chrons 6Bn-9n of the geomagnetic polarity time scale (GPTS), define an age interval of 21.6–26.5 Ma and indicate continuity from the Late Oligocene to Early Miocene. The core is characterized by the remarkable occurrence of brownish-red paleosols of luvic cambisols (brown to luvic drab soils) above reddish-brown floodplain siltstones and mud- stones, which suggest that the East Asian monsoon likely began by 26.5 Ma. In contrast to the siltstone and mudstone of the Late Oligocene strata, the Miocene strata begin with a thick fine sandstone bed, which marks sudden increases in erosion and loading that most likely reflect a response to tectonic uplift. The hematite content and redness index records of the core further demonstrate that the monsoonal climate in the Late Oligocene to Early Miocene in this area was mainly controlled by global temperature trends and events. 1. Introduction the Early Miocene was a critical interval for the climate transition and change in the Cenozoic era. The Oligocene was a period of climatic deterioration that witnessed In Asia, evidence from both temporal and spatial investigations the initiation of the continent-wide ice sheet on Antarctica and the shows that a climate transformation from a zonal pattern to a monsoon- formation of the Antarctic circumpolar current. Deep-sea δ18O records dominated pattern occurred roughly from the Late Oligocene to the (e.g., Miller et al., 1987; Zachos et al., 2001) and terrestrial palyno- Early Miocene (Liu et al., 1998; Guo et al., 2002, 2008; Wang et al., morphs recovered from Antarctic margin drill holes (e.g., Mildenhall, 2003a, 2003b, 2005; Sun and Wang, 2005; Wang, 2009; Qiao et al., 1989; Raine, 1998; Raine and Askin, 2001) indicate climatic ameli- 2006; Qiang et al., 2011), though several studies suggest that the East oration in the Late Oligocene, which may be attributable to deep-sea Asian monsoon had already appeared in the Eocene (Licht et al., 2014; warming coupled with a collapse of the Antarctic ice sheet (e.g., Miller Quan et al., 2012). Therefore, the transition between the Oligocene and et al., 1987; Zachos et al., 2001). Subsequently, until the middle Mio- Miocene is a critical interval for global and East Asian climate change. cene, the global ice volume remained low, and the bottom water tem- However, a definite determination of the timing of the onset of the East peratures increased slightly but were punctuated by several short in- Asian monsoon is required to understand the mechanisms and re- tervals of glaciations (called Mi-events) (Flower and Kennett, 1994; lationships (phases) with global climatic change and the uplift of the Zachos et al., 2001). Therefore, the period from the Late Oligocene to Tibetan Plateau. This has long been impeded by a widespread shortage ⁎ Corresponding author at: CAS Center for Excellence in Tibetan Plateau Earth Sciences, Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. E-mail address: [email protected] (X. Fang). http://dx.doi.org/10.1016/j.gloplacha.2017.09.008 Received 29 March 2017; Received in revised form 16 August 2017; Accepted 10 September 2017 Available online 18 September 2017 0921-8181/ © 2017 Elsevier B.V. All rights reserved. F. Wu et al. Global and Planetary Change 158 (2017) 36–46 Fig. 1. (A) Location of the Linxia Basin in the topographic and climatic transitional zone between the Tibetan Plateau and the humid monsoon front (modified from Fang et al., 2015). (B) Geologic map of the Linxia Basin showing the distribution of the Cenozoic stratigraphy. The red circle indicates the location of the Heilinding section (modified from Fang et al., 2016). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) of well-dated and continuous terrestrial sediment sequences that record zone between the East Asian monsoon and non-monsoon Asian inland the climate changes across the boundary between the Oligocene and dry climate (Fig. 1). Detailed stratigraphic, biostratigraphic and mag- Miocene. netostratigraphic studies suggest that the exceptionally long continuous The Linxia intracontinental foreland basin (Fang et al., 2016) pro- sedimentary succession in this basin was deposited from the Oligocene vides an ideal opportunity for this purpose. It is located in a transitional to the Pliocene (Li et al., 1995; Fang et al., 2003, 2016; Deng et al., 37 F. Wu et al. Global and Planetary Change 158 (2017) 36–46 (caption on next page) 38 F. Wu et al. Global and Planetary Change 158 (2017) 36–46 Fig. 2. (I), Stratigraphic succession of the HLD outcrop section (226.5 m) and its relation to drilling cores HZT (129.2 m), HZ-1 (477.7 m) and HZ-2 (620 m) at its top and bottom, blue and red arrows show the location of the collected samples; (II), google map of the study area; (III), the photos show the main lithologies of the cores HZ-1 and HZ-2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 2004, 2013; Qiu et al., 2004; Garzione et al., 2005). In particular, the a cambic Bw horizon, an underlying distinct calcic Bk horizon, and Heilinding (HLD) section (Figs. 1, 2) in the foredeep of the southern occasionally an additional calcic Ck horizon (Fang et al., 2016). The Bt Linxia basin includes continuous thick floodplain sediments with three horizon is approximately 0.5–1 m thick, more reddish in color, has levels of fossil mammals but few lithofacies variations, so it is suitable black Fe/Mn mottling stains and contains a well-developed granular for revealing climate change (Fang et al., 2016). Unfortunately, the ped structure (Fig. 2III-b and e). The Bw horizon is similar to the Bt stratigraphy at Heilinding outcrops over a thickness of only 226.5 m. horizon but is lighter red (mostly dull reddish-brown) and has less Therefore, in 2010 and 2011, we drilled a 129.2 m core (HZT) at the top pedality (ped to block structures). The Bk horizon is located im- of the section and a 477.7 m core (HZ-1) (Fig. 2) at its bottom. Detailed mediately below the Bt or Bw horizon, is light brownish gray or yel- paleomagnetic measurements demonstrated an age of 23.3 Ma for the lowish-gray, and contains many white carbonate coatings, small no- bottom of core HZ-1 (Fang et al., 2016). Encouraged by those results, dules and impregnations that cement the sediments (Fig. 2III-b). Down- we performed additional deeper drilling (core HZ-2, drilling depth leaching carbonate occasionally reaches the soil's parent materials 620 m) at the same site as core HZ-1 in December 2015 using the same (mudstone or siltstone) below the Bk horizon, but its content is less than drilling technology (three-in-one tube drilling, in which the core is di- that in the Bk horizon, and it forms a Ck horizon (Fig. 2III-f). The lower rectly recovered into an inner plastic tube for better protection and ZZ Fm. is composed of alternating thick reddish-brownish parallel- recovery rate) and obtained 206 m of new core with an recovery rate of bedding fine sandstones (Fig. 2III-d) and massive siltstones, which are over 95% (Fig. 2). In this paper, we carried out a detailed paleomag- classified as a river channel facies over a floodplain (Fig. 2)(Fang et al., netic analysis of this core and analyzed its lithofacies, hematite contents 2016). and redness proxies. Based on these data, we attempted to reconstruct the climatic changes across the boundary between the Late Oligocene 3. Sampling and methods and the Early Miocene. Core HZ-2 can be linked to core HZ-1 by the sandstone marker bed 2. Geological setting and stratigraphy (Fig. 2I). We collected samples starting at a depth of 414.5 m in core HZ-2, which overlaps with core HZ-1 by 63.7 m. Paleomagnetic samples – The Linxia Basin is located on the northeastern margin of the were collected at intervals of 0.5 1 m within the overlapping section – Tibetan Plateau and has an average elevation of 2000–2600 m above and at 0.25 0.5 m intervals elsewhere, and a total of 554 block samples sea level (asl).