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Text S1
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Geological setting
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The Tianshui Basin, situated at the southeastern corner of the broad Longzhong
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Basin, is surrounded by the Western Qinling Mt. to the south, the Liupan Shan Mt. to
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the east, and the Huajia Ling Mt. to the north. To the west, it is separated from the
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Linxia Basin by minor topographic highs composed of early Tertiary rocks (Fig. 1).
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Tertiary sediments are well preserved in the basin and are divided into the Paleogene
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Guyuan Group and the Neogene Gansu Group [Gansu Regional Geological Survey
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Team (GRGST), 1984]. The Gansu Group overlies unconformably on Paleogene and
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pre-Cenozoic rocks. The Neogene Gansu Group includes abundant mammal fossils,
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such as Platybelodon sp., Chilotherium sp. and Hipparion sp. [Chou and Chang, 1961;
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GRGST, 1984; Li et al., 2006; Qiu, 1979; Xie, 1991; Zhai, 1961; Zhang et al., 2011].
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The thickness of the Neogene sediments decreases from south to north. It is more than
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1400 m thick at the pediment of Western Qinling and decreases to only several
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hundred m thick at the northern part. At the same time, the lithology changes from
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mainly sandstones and conglomerates in the south to mainly siltstones and mudstones
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in the north [Alonso-Zarza et al., 2009].
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We studied biomarker characteristics of the Yanwan (105° 34′ E, 34° 58′ N), the
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QA-I (105° 27′ E, 35° 2′ N), and the Yaodian (105° 55′ E, 34° 38′ N) sections in the
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Tianshui Basin (Fig. 1). The Yanwan section is about 288 m thick with abundant
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mammal fossils in the upper part and calcified plant root fossils in the bottom.
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Constrained by the late Miocene fauna of Hipparion, Cervidae and Chilotherium et al.,
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high-resolution magnetostratigraphic research suggests that the Yanwan section spans
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from ~ 17.1 Ma to 6.1 Ma ago [Zhang, 2008]. Field investigation indicates that
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sediments of this section are mostly yellow-brown or reddish-brown mudstones with
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carbonate nodules. In addition, massive sandstones (~ 11 m thickness) and grey-green
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marls exist as well (Fig. 2 for lithology).
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The QA-I section, ~ 12 km northwestern of the Yanwan section, spans from 22 to
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6.2 Ma with a thickness of 253.1 m [Guo et al., 2002]. Guo et al. [2002] reported the
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QA-I section contains 231 reddish paleosols interbedded with yellow-brown or brown
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loess layers. The appearance, grain-size distribution, snail fossils, quartz morphology,
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magnetic properties, and geochemistry characteristics of this sequence are similar to
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those of the Quaternary loess, which have led to an interpretation of eolian origin for
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this set of sediments [Guo et al., 2002, 2010]. However, Alonso-Zarza et al. [2009]
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argued against an eolian source for this set of sediments and they postulated a
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fluvial-lacustrine origin instead on the basis of sedimentological evidence. For
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example, the appearance of carbonate interclasts, coarse clast, micritic carbonates,
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groundwater calcretes and palustrine limestones are difficult to be interpreted as an
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eolian origin [Alonso-Zarza et al., 2009, 2010]. In addition, Flynn et al. [2011] also
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observed pond marls and green clays near QA-I.
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The Yaodian section is located to the northeast of the Tianshui City (Fig. 1).
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Obvious fluvio-lacustrine texture markers, such as green sediment color, layered
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sediments, horizontal bedding, algae and ostracod fossils are observed. The detailed
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lithologic properties and magnetostratigraphic age (12.4 ~ <7.43 Ma) based on late
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Miocene fossils (including the new Eostylocerus sp. fossil) have been reported by Li
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et al. [2006] (Fig. 2).
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Materials and Methods
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A total of 26, 7, and 6 fresh samples were collected from the Yanwan, QA-I, and
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Yaodian sections, respectively. Our sampling strategy was based on differences in
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lithological characteristics and color. The Yanwan section was sampled at about 10 m
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intervals, whereas the QA-I and Yaodian sections were sampled at varying intervals.
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Air-dried samples were ground to <80 mesh, and then about 350-400 g were extracted
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for 72 h with chloromethane (CM) using the Soxhlet extraction method. The extracts
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were concentrated by evaporation in air and then transferred to a small vial. After
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weighing, the total extracts were separated into the aliphatic hydrocarbon, aromatic
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hydrocarbon, and non-hydrocarbon fractions via activated silica gel column
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chromatography (25 cm × 0.6 cm) by elution with redistilled n-hexane,
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dichloromethane (DCM), and methanol (MeOH), sequentially. The aliphatic
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hydrocarbon portions of the samples were directly analyzed using a Hewlett-Packard
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6890 gas chromatograph (GC) coupled to a Hewlett-Packard 5973 quadrupole mass
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spectrometer (MS). Individual components were identified by comparison of mass
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spectra with those obtained from the NIST02L library, according to retention time,
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and by comparison with published compound characteristics. All experiments were
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done in the Key Laboratory of Gas Geochemistry, Institute of Geology and
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Geophysics, Chinese Academy of Sciences. Biomarker parameters were calculated
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using peak heights of the respective compounds in the gas chromatograms.
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Table S1. Details of Neogene samples—Yanwan, QA-I and Yaodian sections from the
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Tianshui Basin and their biomarker proxies values.
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Figure S1. Relation plot of Pr/n-C17 and Ph/n-C18 in the Tianshui Basin. This figure is
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modified from Peters et al. [2005] and Duan et al. [2008]. The biodegradation degree
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increases as the points move towards the right. The cross represents the Yanwan
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samples, the filled cycle represents the QA-I samples, and the unfilled triangle
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represents the Yaodian samples. Pr is the abbreviation of Pristane, Ph is the
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abbreviation of Phytane; n-C17 means the n-alkane with 17 carbon number.
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