Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Astrochronology for the Early Jehol Biota in northeastern China

Huaichun Wu a,b,⁎, Shihong Zhang a, Ganqing Jiang c, Tianshui Yang a, Junhua Guo d, Haiyan Li a a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China b School of Ocean Science, China University of Geosciences, Beijing 100083, China c Department of Geoscience, University of Nevada, Las Vegas, NV 89154, USA d Department of Geological Science, University of Missouri, Columbia, MO 65211, USA article info abstract

Article history: The Jehol Biota in northeastern China provides an evolutionary window for ‘feathered’ Received 14 September 2011 , primitive , insects and early flowering . It also provides critical information for the bio- Received in revised form 10 May 2013 diversity changes of the Early Cretaceous terrestrial ecosystem. Here we report a time series analysis for the Accepted 15 May 2013 11.2-m-thick, -bearing lacustrine deposits at the Sihetun section in western , northeastern Available online 23 May 2013 China on the basis of high-resolution magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) measurements. A hierarchy of sedimentary cycle bands of 120–260 cm, 50–67 cm and 18–42 cm Keywords: Astrochronology was recorded in the MS and ARM series. With available radioisotope age constraints from the same section, Early Cretaceous sedimentary cycles of 120–260 cm, 50–67 cm and 18–42 cm were interpreted as Milankovitch cycles of short Jehol Biota eccentricity (130 and 95 kyr), obliquity (36.6 and 46 kyr), and precession (22.1, 20.9 and 18 kyr), respectively. The 100 kyr-tuned ‘floating’ astronomical time scale indicates that the duration of the 11.2-m-thick section is Northeastern China ~0.67 Myr and the average depositional rate is ~1.70 cm/kyr. The duration of the 1.8-m-thick, main fossil- bearing interval that contains 8 beds of ‘feathered’ /primitive can be estimated as short as 150 kyr. The results suggest that climate fluctuations manifested in paleobotanical, sedimentological and geo- chemical records of the Yixian Formation may have been controlled by orbital forcing during Early Cretaceous. © 2013 Elsevier B.V. All rights reserved.

1. Introduction have been taken to constrain the absolute age of the Jehol Biota, and the results from different sections indicate that the ages across The Early Cretaceous Jehol Biota, discovered from the Yixian For- the Jehol Biota range from ~131 Ma to 120 Ma (Barremian to Early mation and its overlying Jiufotang Formation in northeastern China, Aptian) (Swisher et al., 1999, 2002; Wang et al., 2001; He et al., provides a unique opportunity to address questions about the evo- 2004, 2006; Yang et al., 2007; Chang et al., 2009). However, further lution of ‘feathered’ dinosaurs and primitive birds, the diversifica- refinement for the duration of fossil-bearing strata is hindered due tion of flowering plants, and the radiation of placental mammals to uncertainties inherent to radioisotope age-dating methods (Erwin, (Zhou et al., 2003; He et al., 2004, 2006; Zhou and Wang, 2010) 2006), systematic discrepancies between 40Ar/39Ar and U–Pb dating (Fig. 1). Furthermore, exceptionally well-preserved fossils of the methods (Min et al., 2000; Kuiper et al., 2008), and the limited number Jehol Biota and the unique lithological contents (fine-grained of volcanic/tuff layers amenable for precise dating. siliciclastic rocks and volcanic ash beds) open a rare window for un- Astrochronology provides an alternative method for fine-tuning derstanding the evolution of Early Cretaceous terrestrial ecosys- the duration of stratigraphic units and geological events when contin- tems and reconstruction of paleoecological history (Zhang et al., uous, fine-grained sedimentary records are available. Recent studies 2010; Zhou and Wang, 2010). The mass mortality and remarkable indicated that orbital-scale climate cycles were well preserved in fossil preservation of the Jehol Biota have been ascribed to repeti- terrestrial successions deposited from lacustrine and palustrine environ- tive environmental crises induced by volcanic eruptions (Wang et ments. Examples include the Late –Early Newark Basin in al., 1999; Guo et al., 2003). eastern North America (Olsen and Kent, 1996), the Late Cretaceous Obtaining high-precision radiometric ages from the Yixian and Songliao Basin in northeastern China (Wu et al., 2007, 2009, 2013-this Jiufotang formations and evaluating the duration of biological events issue), the Early Eocene Green River Formation in North America are essential for delineating the evolutionary patterns of the Jehol (Machlus et al., 2008), and Miocene basins around the Mediterranena Biota. Enormous efforts using 40Ar/39Ar and U/Pb dating techniques sea (Abdul Aziz et al., 2004; Abels et al., 2010). The preservation of orbital-scale climate cycles in terrestrial successions makes it possible to construct astronomical time scales with resolution potentially ⁎ Corresponding author at: School of Ocean Science, China University of Geosciences, – Beijing 100083, China. down to 0.02 0.40 Ma (e.g., Olsen and Kent, 1996; Abdul Aziz et al., E-mail address: [email protected] (H. Wu). 2004; Hinnov and Ogg, 2007; Wu et al., 2009, 2013-this issue).

0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.05.017 222 H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228

Depth (m) 75O E 95O E 115 O E 135O E a) b) Beipiao 3 0246km 40O N Section locality 4 Beijing 30O N China 5

20O N 6

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Shangyuan Sihetun 9 Chaoyang 10

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o

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o

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r

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o h 124.7 2.7 Ma (U-Pb) (Yang et al., 2007)

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10 b Confuciusornis Confuciusornis, Liaoningornis longiditris 10 m Confuciusornis 11

Fig. 1. (a) Location of the Sihetun section in northeastern China (after Zhu et al., 2007). (b) Photograph of the sampling outcrop. (c) Simplified stratigraphic column of the Yixian Formation (left) and the study interval of Jianshangou Unit (right) at Sihetun section (modified from Zhang et al., 2004; Jiang and Sha, 2007; Zhu et al., 2007). Published radioiso- topic ages and location of main fossil-bearing layers are marked.

It provides a tool to estimate the duration of critical stratigraphic inter- astronomical time scale that provides an independent age constraint vals and geological events at remarkable resolution and precision that for the duration of the Jehol Biota and its host strata, and (2) to discuss help to better understand the deep-time paleoclimatological and paleo- the possible causes of paleoenvironmental and paleoclimatic fluctua- ecological changes (Hinnov and Ogg, 2007; Hinnov and Hilgen, 2012). tions recorded in the fossil-bearing strata. In this paper, we report a cyclostratigraphic study of fossil- bearing, thinly laminated and fine-grained lacustrine deposits of the 2. Geological background Jianshangou unit, Yixian Formation, using high-resolution magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) During Late Jurassic–Early Cretaceous, a series of rifted sedimen- measurements obtained from the Sihetun section in northeastern tary basins were formed in northeastern China in response to crustal China. The main objectives of this study are (1) to construct a ‘floating’ extension that was likely triggered by gravitational collapse of H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228 223 previously thickened crust (Meng, 2003; Wu et al., 2008). The Creta- were normalized by mass, given in 10−8 m3/kg and 10−6 Am2/kg, ceous strata surrounding the Sihetun area in western Liaoning respectively. Province were deposited from one of the extensional volcanic- TheaveragevalueofMSis5.77×10−8 m3/kg, and the average sedimentary basins (Wang et al., 1983, 1998)(Fig. 1). The Jehol value of ARM is 1.37 × 10−6 Am2/kg, with 34 ‘abnormal’ peak values Biota in this region was excavated from the Yixian Formation and (a1–a34 in Fig. 2). Samples with ‘abnormal’ high MS and ARM values the overlying Jiufotang Formation (Zhou et al., 2003). are mostly brownish to yellowish, showing strongly weathered fea- One of the best fossil-preservation sites of the Jehol Biota is located tures. These layers are commonly less than 2 cm, and their lithologies in a small region surrounding the village of Sihetun, about 25 km are tuffs or tuffaceous siltstone/shale. High MS and ARM values from south of the Beipiao City (Fig. 1a). The Yixian Formation in the Sihetun these layers may have overridden the normal depositional information. area consists of four units including, in ascending order, the Lujiatun After removing these ‘abnormal’ points, the MS and ARM values show Unit, Lower Lava Unit, Jianshangou Unit and Upper Lava Unit (Jiang distinct and similar cyclic variability throughout the section (Fig. 3). and Sha, 2007)(Fig. 1). Fossil-bearing strata in the Sihetun section The tuff/tuffaceous beds, which represent instantaneous volcanic are from the Jianshangou Unit, which is 14.4 m thick and composed eruption events, were excluded when constructing the new MS and of dark to light gray shale, siltstone, silty mudstone and fine-grained ARM time series for cyclostratigraphy analysis. The MS and ARM data sandstone that were deposited from lacustrine environments (Zhu series were detrended prior to time series analysis by removing a 35% et al., 2007). Exceptionally well-preserved ‘feathered’ dinosaurs weighted mean from the data using the software KaleidaGraphTM Sinosauropteryx (Chen et al., 1998), and Caudipteryx (Fig. 3). Power spectral was calculated for the MS and ARM time series (Ji et al., 1998), primitive birds Confuciusornis (Hou et al., 1995), pla- cental mammals Zhangheotherium (Hu et al., 1997), and the oldest ( 10-8 m3 /kg) flowering (Sun et al., 1998), have been reported 010203040 from the Jianshangou Unit around this area. A number of brownish to yellowish tuff layers have been found in 0 the Jianshangou Unit (Wang et al., 1998)(Fig. 1). Radiometric age dating of a tuff layer overlying the ‘feathered’ dinosaur-bearing bed of the Jianshangou unit yielded 40Ar/39Ar ages of 124.6 ± 0.1 Ma, 1 a34 124.6 ± 0.25 Ma and 125.0 ± 0.1 Ma (Swisher et al., 1999, 2002). Subsequent U–Pb ages of 125.2 ± 0.9 Ma (Wang et al., 2001), 124.7 ± 2.7 Ma (Yang et al., 2007) and 40Ar/39Ar age of 124.1 ± 2 a33 0.3 Ma (Chang et al., 2009) were obtained from the same interval. The volcanic rocks underlying and overlying the fossil-bearing la- 40 39 custrine deposits were dated ( Ar/ Ar) as 125.7 ± 2.6 Ma and 3 a31 a32 124.2 ± 2.5 Ma, respectively (Zhu et al., 2007; Fig. 1c). These ages a30 roughly constrain the duration of the Jianshangou unit as less than 1.6 Myr (Fig. 1), and provide a solid basis for constructing the astro- 4 a29 nomical time scale. a28

a27 3. Sampling, magnetic measurement and data processing 5 a26 a25 The rock magnetic parameters are extensively used in cyclostra- a24 a23 tigraphy research because their measurements are comparatively a22 Depth (m) 6 fast, cost-effective and non-destructive, allowing analysis of large a21 sample populations (Maher and Thompson, 1999; Hinnov, 2004). a19 a20 a18 The MS refers to the capacity of a substance to become magnetized a17 7 fi fl a16 when subjected to an external magnetic eld. The ARM values re ect a15 the concentration of fine-grained (b20 μm), low-coercivity ferro- a14 magnetic minerals that have a relatively simple origin, and it has a13 8 been successfully used in cyclostratigraphic research on continental a12 and marine successions (e.g., Latta et al., 2006; Kodama et al., 2010; a11 Wu et al., 2012). In this study, both MS and ARM series are used for a10 cyclostratigraphic analysis. 9 a9 A total of 670 samples were collected from the 11.2-m-thick fossil- a7 a8 bearing strata in the Sihetun section (41°35′20.2″ N, 120°47′35.7″ E) a6 (Fig. 1b and c). Samples were crashed and placed into plastic paleo- 10 a5 a4 magnetic cubes. The MS was measured on KLY-4S kappa-bridge. a3 The ARM was acquired by applying a peak alternating field of 0.1 T a2 and a bias field of 50 μT on the D-2000 AF demagnetizer, and the rem- 11 a1 anence intensity measurements were made on a JR6 spinner magne- tometer. In order to identify the composition of magnetic minerals, 0481216 -6 2 rock magnetic experiments were carried out on representative sam- ARM( 10 Am /kg) ples. Temperature dependence of low-field magnetic susceptibilities Legends (χ − T) was measured on a KLY-4S Kappa-bridge with a temperature apparatus (CS-3). Isothermal remanent magnetization (IRM) acquisi- tion and direct field demagnetization were applied in an ASC mudstoneshales silty shales fine sandstone Tuff or tuffaceous IM-10-30 impulse magnetizer. All measurements were performed Fig. 2. Variation of magnetic susceptibility (MS) and anhysteretic remanent magnetiza- in the Paleomagnetism and Environmental Magnetism Laboratory at tion (ARM) in the fossil-bearing interval. High MS and ARM values of a1–a34 correspond China University of Geosciences (Beijing). Both MS and ARM values to brownish or yellowish tuff or tuffaceous beds. 224 H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228 a) b) c) d) e) f) -8 3 2 ( 10-8m3 /kg) ( 10 m /kg) ARM ( 10 -6 Am /kg) ARM ( 10 -6Am2 /kg) -2 -1 0 1 2 0 0.4 0.8 1.2 1.6 2 -0.8 0 0.8 Lithology 2468 0 0 15

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mudstone shales silty shales fine sandstone

Fig. 3. (a) Lithological column of fossil-bearing lacustrine deposits of the Jianshangou Unit, Yixian Formation. The thickness of the tuff/tuffaceous layers with high MS and ARM values have been removed. (b, d) The raw MS and ARM series with a 35% weighted average fit curve (red dashed line). (c, e) the MS and ARM series after subtracting the 35% weighted mean with filtered short eccentricity cycles (red dashed line) in the depth domain. Filter passband for 100 kyr eccentricity cycles are 0.006 ± 0.003 cycles/cm for ARM series, and 0.0055 ± 0.0025 cycles/cm for MS series using the free solftware Analyseries 2.0.4.2 (Paillard et al., 1996). (f) ARM short eccentricity (100 kyr)-based astronomical time scale. Shading area indicates the eight main fossil-bearing horizons that host primitive bird or ‘feathered’ dinosaur fossils.

in the depth and time domains using the SSA-MTM toolkit (Ghil et al., low magnetic coercive minerals are the dominant magnetic minerals 2002), with robust red noise estimation reported at 90%, 95% and 99% in the samples. Thermal magnetic experiment shows a gradual confidence levels for the interpretation of spectral peak significance drop of magnetic susceptibility, indicating the existence of paramag- (Mann and Lees, 1996). The evolutionary fast Fourier transform (FFT) netic minerals. The susceptibilities on the heating curves decrease spectrograms were constructed and used to track changes in sedimen- rapidly between 560 °C and 580 °C, indicating the dominance of tation rate. The Gauss band-pass filtering was carried out with the titanomagnetite (Fig. 4c). The cooling curves are much higher than freeware Analyseries 2.0.4.2 (Paillard et al., 1996). the heating curves, which can be interpreted as a formation of strong magnetic minerals during heating (Fig. 4c, d). The rock magnetic 4. Results and discussion experiments show that the dominant magnetic mineral is most likely the low magnetic coercive titanomagnetite. 4.1. Rock magnetism results A normal paleomagnetic polarity was revealed from the Jianshangou lacustrine interval at the Sihetun section, and the remanence carriers IRM measurement of representative samples shows rapid in- were identified as titanomagnetite (Pan et al., 2001; Zhu et al., creases below a field of 100 mT and the remanence coercivity of the 2007). Major and trace elemental analyses indicated that the source of same samples is less than 50 mT (Fig. 4a, b), which indicate that lacustrine deposits of the Sihetun section was mainly from weathering H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228 225 a) b)

1 1

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Fig. 4. Normalized acquisition curve and opposite field demagnetization of isothermal remanent magnetization (IRM) (a, c) and temperature-dependence of low-field magnetic susceptibility (χ − T) (b, d) of representative samples. products of the underlying volcanic rocks (Ke et al., 2008). Thus, we in- It has been recommended that long eccentricity (405 kyr) cycles terpret that variations in MS and ARM values of the Jianshangou Unit should be used for the calibration of Mesozoic astronomical time scales at the Sihetun section record fluctuations in the flux of fine-grained (Laskar et al., 2004; Hinnov and Ogg, 2007; Hinnov and Hilgen, 2012), detrital titanomagnetic minerals in response to climate changes. but examples have shown that astronomical time scales calibrated by ~100 kyr eccentricity cycles are identical to those tuned by long eccentricity cycles (e.g., Wu et al., 2009; Huang et al., 2010). Because 4.2. Cyclostratigraphic results no long eccentricity cycles are identified in this study, we use the 100 kyr eccentricity cycles to tune the ARM and MS series to construct The MTM power spectral analysis and evolutionary FFT spectrum a “floating” astronomical time scale (Fig. 3). The Gaussian band-pass analysis of the ARM series in the depth domain reveals a hierarchy of filters were designed to extract the signal of short eccentricity cycles cycles with 227 cm, 157 cm, 128 cm, 57 cm, 40 cm and 21 cm wave- of 120–260 cm wavelength. As shown in Fig. 3, the studied succession lengths above 99% confidence (Fig. 5a, b). The MTM power spectra and recorded 6.7 short eccentricity cycles. The 120–260 cm cycles were evolutionary spectrum of the untuned MS series show significant then tuned by assigning 100 kyr to neighboring peaks in the filter- peaks at 255 cm, 120 cm, 21 cm and 18.3 cm above 99% confidence outputs in the depth domain (Fig. 3). The power spectral analysis level (Fig. 5c, d). The evolutionary FFT spectrum of the MS and ARM reveals 102 kyr, 85 kyr, 39.4 kyr, 35.3 kyr, 20.9 kyr and 18 kyr peaks shows that the dominant frequency of ~0.08 cycles/cm gradually above 99% confidence in 100 kyr-tuned ARM series, and significant changed to lower frequency of ~0.05 cycles/cm, which may indicate 100 kyr and 18 kyr periods in the 100 kyr-tuned MS series (Fig. 5f, g). a higher sedimentary rate in the upper part of the section. Jiang et al. These spectra match well with Early Cretaceous astronomical parame- (2012) also proposed that the upper part of the section has a higher ters of the La2010a solution (Fig. 5e; Laskar et al., 2011) and support rate of sedimentation due to increased flooding events during the late our cyclostratigraphic interpretation on the MS and ARM data. stage of the lake deposition. The ratio of the major cycle bands of 120–260 cm:50–67 cm: 4.3. Time constraints for the Jehol Biota and geomagnetic polarity chron 18–42 cm is similar to the ratio of the Late Cretaceous astronomical M3n parameters of short eccentricity (95 kyr and 130 kyr): obliquity (36.6 kyr and 46 kyr): precession (18 kyr, 20.9 kyr and 22.1 kyr) Based on the 100 kyr-tuned ‘floating’ astronomical time scale (Laskar et al., 2011)(Fig. 5a–e). Thus, we interpret these cycles (Fig. 3), the duration of the 11.2-m-thick fossil-bearing strata is as Milankovitch sedimentary cycles of short eccentricity, obliquity 0.67 Myr and the average depositional rate for the studied interval and precession according to the cycle length ratios (Hinnov, 2000; is 1.70 cm/kyr. The duration of the 14.4-m-thick Jianshangou Unit Weedon, 2003). outcropped at the Sihetun section can be estimated as 0.86 Myr. 226 H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228

a) 1.5 c) 8 e) 60 e: 120-250 cm e: 110-260 cm 99% Ee OP 255 cm 95% 50 O: 50-67 cm 120 cm 36.6 kyr 1.2 90% 405 kyr 227 cm 6 40 157 cm P: 18-42 cm M 130 kyr 22.1 kyr 0.9 128 cm 30 20.9 kyr 57 cm P: 18-22 cm 4 Power 95 kyr 18 kyr 40 cm

Power 0.6 Power 20 21 cm 18.3 cm 46 kyr 2 10 0.3 21 cm 0 102 kyr 0 0 f) b) d) 85 kyr 2 200 200 39.4 kyr 35.3 kyr 300 300 20.9 kyr Power 1 18 kyr 400 400

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4 900 900 0 0 0.01 0.02 0.03 0.04 0.05 0.06 0 0.01 0.02 0.03 0.04 0.05 0.06 0 0.01 0.02 0.03 0.04 0.05 0.06 Frequency (cycles/cm) Frequency (cycles/cm) Frequency (cycles/kyr)

Fig. 5. 2π MTM power spectra of the ARM (a) and MS (c) time series in depth domain and 100 kyr-tuned ARM (f) and MS (g) time series. (b, d) Evolutionary spectrum for the ARM (b) and MS (d) series in depth domain. (e) Power spectra of Earth's orbital parameters from 120 to 125 Ma presented in ETP format according to the La2010a model (Laskar et al., 2011). The purple, red, green and blue (dashed) curves represent the median smoothed, linear fitted red noise spectrum at 90%, 95% and 99% confidence levels. The red and blue colors in evolutionary spectrum represent high and low power, normalized to 1.

The ‘feathered’ dinosaur/primitive bird fossils in this section were layers in the succession were formed under very shallow-water and discovered from eight layers in the 1.8-m-thick interval from 8.4 m to arid conditions. However, Li and Batten (2007) suggested a warm and 10.2 m (Fig. 1c) (Zhang et al., 2004). The floating astronomical time seasonal climate (semi-arid) system with fairly short wet phases scale (Fig. 3) suggests that the duration of these ‘feathered’ dinosaur/ and long dry (arid) periods. Ding et al. (2003) indicated that most of primitive bird fossil beds is ~150 kyr, with each fossil-bearing bed sporopellen, plant, and wood fossils of the lacustrine deposits in this re- appearing at an average period of ~18.7 kyr (Figs. 1cand3). gion recorded warm and wet habitats, and that only a few fossils such Recently, three new M-sequence geomagnetic polarity time as xerophilous Gnetales, Bennettiales with membranous leaves, and scales were proposed, including GPTS12 (Ogg, 2012), MHTC12 and Conifersles with scaled leaves were representative of arid or semi-arid MHTC12-125 (Malinverno et al., 2012). The main difference of these climatic conditions. Fürsich et al. (2007) also proposed a semi-arid cli- new polarity time scales was the assignment of the onset age of mate in which dry periods with little air movements alternated with M0r, i.e., 125 Ma (GPTS12 and MHTC12-125) or 121 Ma (MHTC12). stormier wet seasons. Amiot et al. (2011) obtained the oxygen isotope Zhu et al. (2007) reported normal to reverse magnetic polarity composition of apatite from various reptile remains of the Jehol Biota changes from the fossil-bearing strata to the overlying volcanic from China, Thailand, and Japan, which shows that the climate in this rocks at the Sihetun section. In combination with40Ar/39Ar ages period of the Early Cretaceous was cold and similar to that of today at from overlying and underlying volcanic rocks (125.7 ± 2.6 Ma and equivalent latitude. The inferred low temperatures are in agreement 124.2 ± 2.5 Ma), the normal and reversed polarities could correspond with the marine records of late Barremian–Early Albian (e.g., Price, to geomagnetic polarity chron M3n (123.92–124.58 Ma) of MHTC12 1999; Pucéat et al., 2003; Dumitrescu et al., 2006). (Malinverno et al., 2012), but could not correlate well with the polarity The causes of climate changes recorded in the Yixian Formation chrons of the GPTS2012 (Ogg, 2012) and MHTC12-125 (Malinverno still remain uncertain. It has been proposed that frequent volcanic et al., 2012). The duration of ~0.86 Myr for the normal polarity of activities around western Liaoning area played an important role in cli- the Jianshangou section is consistent, in first order, with the duration mate change by inducing ‘volcanic winters’ (e.g., Ding et al., 2003; Guo of 0.66 ± 0.0929 Myr for M3n (Malinverno et al., 2012) and supports et al., 2003; Zhou et al., 2003). The Milankovitch cycles identified from the polarity time scale of MHTC12, which was also preferred by the Jianshangou Unit in this study offers an alternative interpretation: Malinverno et al. (2012). climate variations were likely driven by Early Cretaceous orbital forcing, similar to the astronomical forcing recorded in Cretaceous marine sys- 4.4. Orbital forcing of climate recorded in the Yixian Formation tems (e.g., Fiet and Gorin, 2000; Gale et al., 2002; Miller et al., 2005). The Sihetun paleolake represents a relatively small and closed volcanic Sedimentological, paleobotanical and geochemical studies revealed valley formed during volcanic eruptions of the Lower Lava Unit of the dynamic climate fluctuations during the deposition of the Yixian Yixian Formation. The depositional processes and lake level were very Formation. Wu (1999) proposed warm and arid climate alternations sensitive to climate and environmental changes (Jiang et al., 2012; based on the size, root system and membranous leaves of plants. Jiang Zhang and Sha, 2012). The rock magnetic cyclostratigraphy of the and Sha (2007) proposed that thin gypsum and calcareous mudstone Jianshangou Unit suggests that climate variations are linked to orbital H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228 227 forcing climate cycles which are encoded by the concentration of He, H.Y., Wang, X.L., Zhou, Z.H., Wang, F., Boven, A., Shi, G.H., Zhu, R.X., 2004. Timing fi – of the Jiufotang Formation (Jehol Group) in Liaoning, northeastern China, and its ne-grained detrital titanomagnetite (Figs. 3 5). During periods of implications. Geophysical Research Letters 31, L12605. http://dx.doi.org/10.1029/ relatively cold and dry climates (precession maxima or low obliquity), 2004GL019790. decrease in weathering rate, precipitation and riverine discharge He, H.Y., Wang, X.L., Jin, F., Zhou, Z.H., Wang, F., Yang, L.K., Ding, X., Boven, A., Zhu, R.X., 2006. The 40Ar/39Ar dating of the early Jehol Biota from Fengning, Hebei Province, would result in less input of magnetic minerals, leading to lower MS northern China. Geochemistry, Geophysics, Geosystems 7, Q04001. http://dx.doi.org/ and ARM values. In contrast, high MS and ARM values suggest increased 10.1029/2005GC001083. input of magnetic minerals, which can be explained by increased pre- Hinnov, L.A., 2000. New perspectives on orbitally forced stratigraphy. Annual Review of – cipitation, weathering and riverine input during warm and wet climates Earth and Planetary Sciences 28, 419 475. Hinnov, L.A., 2004. Earth's orbital parameters and cycle stratigraphy. In: Gradstein, corresponding to periods of precession maxima or high obliquity. F.M., Ogg, J.G., Smith, A.G. (Eds.), A Geologic Time Scale 2004. Cambridge University Press, pp. 55–62. Hinnov, L.A., Hilgen, F.J., 2012. Cyclostratigraphy and astrochronology. In: Gradstein, 5. Conclusion F.M., Ogg, J.G., Schmitz, M., Ogg, G. (Eds.), The Geologic Time Scale 2012. Elsevier, pp. 63–83. Spectral analyses of high-resolution MS and ARM time series in Hinnov, L.A., Ogg, J.G., 2007. Cyclostratigraphy and the astronomical time scale. Stratigra- – the depth and time domain reveal significant orbital forcing climate phy 4, 239 251. Hou, L.H., Zhou, Z., Martin, L., Feduccia, A., 1995. A beaked bird from the Jurassic of China. cycles in the lacustrine deposits that host the Early Cretaceous Jehol Nature 377, 616–618. Biota. A 100 kyr-tuned floating astronomical time scale of the ARM Hu, Y.M., Wang, Y.Q., Luo, Z.X., Li, C.K., 1997. A new symmetrodont mammal from – series indicates that the duration of the 11.2-m-thick fossil-bearing China and its implications for mammal evolution. Nature 390, 137 142. Huang, C.J., Hesselbo, S.P., Hinnov, L.A., 2010. Astrochronology of the late Jurassic strata is 0.67 Myr and the average depositional rate is 1.70 cm/kyr. Kimmeridge Clay (Dorset, England) and implications for Earth system processes. The duration of the 1.8-m-thick interval that contains abundant and Earth and Planetary Science Letters 289, 242–255. well-preserved feathered dinosaur/primitive bird fossils is as short Ji, Q., Currie, P.J., Norell, M., Ji, S.A., 1998. Two feathered dinosaurs from northeast – fl China. Nature 393, 753 761. as 150 kyr. The climate uctuations recorded in the Yixian Formation Jiang, B.Y., Sha, J.G., 2007. Preliminary analysis of the depositional environments of may have been controlled by orbital forcing during Early Cretaceous. the Lower Cretaceous Yixian Formation in Sihetun area, western Liaoning, China. Cretaceous Research 28, 183–193. Jiang, B.Y., Fürsich, F.T., Hethke, M., 2012. Depositional evolution of the Early Cretaceous Acknowledgments Sihetun Lake and implications for regional climatic and volcanic history in western Liaoning, NE China. Sedimentary Geology 257–260, 31–44. We express our sincere appreciation to three anonymous re- Ke, L., Gao, S., Pan, Y.X., Hu, Z.C., 2008. Geochemistry and provenance of the Early Cretaceous clastic sedimentary rocks of Yixian Formation from the Sihetun section, viewers for their careful review and constructive suggestions that West Liaoning. Earth Science-Journal of China University of Geosciences 33, 504–514 significantly improved the paper. This work was supported by the (In Chinese with English Abstract). National Key Basic Research Development Program of China (Grant Kodama, K.P., Anastasio, D.J., Newton, M.L., Pares, J.M., Hinnov, L.A., 2010. High-resolution rock magnetic cyclostratigraphy in an Eocene flysch, Spanish Pyrenees. Geochemis- 2012CB822002), the National Science Foundation of China (Grants try, Geophysics, Geosystems 11, Q0AA07. http://dx.doi.org/10.1029/2010GC003069. 91128102, 40802012) and the Fundamental Research Funds for the Kuiper, K.F., Deino, A., Hilgen, F.J., Krijgsman, W., Renne, P.R., Wijbrans, J.R., 2008. Central Universities. Synchronizing rock clocks of earth history. Science 320, 500–504. Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A.C.M., Levrard, B., 2004. A long term numerical solution for the insolation quantities of the Earth. Astronomy and References Astrophysics 428, 261–285. Laskar, J., Fienga, A., Gastineau, M., Manche, H., 2011. La2010: a new orbital solution for Abdul Aziz, H., Van Dam, J.A., Hilgen, F.J., Krijgsman, W., 2004. Astronomical forcing in the long-term motion of the Earth. Astronomy and Astrophysics 532, A89. upper Miocene continental sequences: implications for the Geomagnetic Polarity Latta, D.K., Anastasio, D.J., Hinnov, L.A., Elrick, M., Kodama, K.P., 2006. Magnetic record Time Scale. Earth and Planetary Science Letters 222, 243–258. of Milankovitch rhythms in lithologically non-cyclic marine carbonates. Geology Abels,H.A.,AbdulAziz,H.,Krijgsman,W.,Smeets,S.J.B.,Hilgen,F.J.,2010.Long- 34 (1), 29–32. period eccentricity control on sedimentary sequences in the continental Li, J.G., Batten, D.J., 2007. Palynological evidence of an Early Cretaceous age for the Yixian Madrid Basin (middle Miocene, Spain). Earth and Planetary Science Letters Formation at Sihetun, western Liaoning, China. Cretaceous Research 28, 333–338. 289, 220–231. Machlus, M.L., Olsen, P.E., Christie-Blick, N., Hemming, S.R., 2008. Spectral analysis of the Amiot, R., Wang, X., Zhou, Z.H., Wang, X.L., Buffetaut, E., Lécuyer, C., Ding, Z.L., Fluteau, F., lower Eocene Wilkins Peak Member, Green River Formation, Wyoming: support for Hibino, T., Kusuhashi, N., Mo, J.Y., Suteethorn, V., Wang, Y.Q., Xu, X., Zhang, F.S., 2011. Milankovitch cyclicity. Earth and Planetary Science Letters 268, 64–75. Oxygen isotopes of East Asian dinosaurs reveal exceptionally cold Early Cretaceous Maher, B., Thompson, E. (Eds.), 1999. Quaternary Climates, Environments and Magnetism. climates. Proceedings of the National Academy of Sciences of the United States of Cambridge University Press, Cambridge, pp. 1–402. America 108 (13), 5179–5183. Malinverno, A., Hildebrandt, J., Tominaga, M., Channell, J.E.T., 2012. M-sequence geo- Chang, S.C., Zhang, H.C., Renne, P.R., Fang, Y., 2009. High-precision 40Ar/39Ar age for magnetic polarity time scale (MHTC12) that steadies global spreading rates and Jehol Biota. Palaeogeography, Palaeoclimatology, Palaeoecology 280, 94–104. incorporates astrochronology constraints. Journal of Geophysical Research 117, Chen, P.J., Dong, Z.M., Zhen, S.N., 1998. An exceptionally well-preserved theropod dino- B06104. http://dx.doi.org/10.1029/2012JB009260. saur from the Yixian Formation of China. Nature 391, 147–152. Mann, M.E., Lees, J.M., 1996. Robust estimation of background noise and signal detec- Ding, Q.H., Zhang, L.D., Guo, S.Z., Zhang, C.J., Peng, Y.D., Jia, B., Chen, S.W., Xing, D.H., tion in climatic time series. Climate Change 33, 409–445. 2003. Paleoclimatic and palaeoenvironmental proxies of the Yixian formation Meng, Q.R., 2003. What drove late Mesozoic extension of the northern China–Mongolia in the Beipiao area, western Liaoning. Geological Bulletin of China 22, 186–191 tract? Tectonophysics 369, 155–174. (In Chinese with English Abstract). Miller, K.G., Wright, J.D., Browning, J.V., 2005. Visions of ice sheets in a greenhouse Dumitrescu, M., Brassell, S.C., Schouten, S., Hopmans, E.C., Sinninghe Damste, J.S., 2006. world. Marine Geology 217, 215–231. Instability in tropical Pacific sea-surface temperatures during the early Aptian. Min, K., Mundil, R., Renne, P., Ludwig, K.R., 2000. A test for systematic errors in Geology 34 (10), 833–836. 40Ar/39Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga Erwin, D.H., 2006. Dates and rates: temporal resolution in the deep time stratigraphic rhyolite. Geochimica et Cosmochimica Acta 64, 73–98. record. Annual Review of Earth and Planetary Sciences 34, 569–590. Ogg, J.G., 2012. Geomagnetic polarity time scale. In: Gradstein, F.M., Ogg, J.G., Schmitz, Fiet, N., Gorin, G., 2000. Lithological expression of Milankovitch cyclicity in carbonate- M., Ogg, G. (Eds.), The Geologic Time Scale 2012. Elsevier, pp. 85–113. dominated, pelagic, Barremian deposits in central Italy. Cretaceous Research 21 Olsen, P.E., Kent, D.V., 1996. Milankovitch climate forcing in the tropics of Pangea (4), 457–467. during the Late Triassic. Palaeogeography, Palaeoclimatology, Palaeoecology 122, Fürsich, F.T., Sha, J.G., Jiang, B.Y., Pan, Y.H., 2007. High resolution palaeoecological 1–26. and taphonomic analysis of Early Cretaceous lake biota from western Liaoning Paillard, D., Labeyrie, L., Yiou, P., 1996. Macintosh program performs time-series analysis. (NEChina). Palaeogeography, Palaeoclimatology, Palaeoecology 253, 434–457. Eos 77, 379. Gale, A.S., Hardenbol, J., Hathway, B., Kennedy, W.J., Young, J.R., Phansalkar, V., 2002. Pan, Y.X., Zhu, R.X., Shaw, J., Zhou, Y.X., 2001. Magnetic polarity ages of the fossil-bearing Global correlation of Cenomanian (Upper Cretaceous) sequences: evidence for strata at the Sihetun section, west Liaoning: a preliminary result. Chinese Science Milankovitch control on sea level. Geology 30 (4), 291–294. Bulletin 46, 1473–1476. Ghil, M., Allen, M.R., Dettinger, M.D., Ide, K., Kondra-shov, D., Mann, M.E., Robertson, Price, G.D., 1999. The evidence and implications of polar ice during the Mesozoic. A.W., Saunders, A., Tian, Y., Varadi, F., Yiou, P., 2002. Advanced spectral methods Earth-Science Reviews 48, 183–210. for climatic time series. Reviews of Geophysics 40 (1) (3-1–3-41). Pucéat, E., Lécuyer, C., Sheppard, S.M.F., Dromart, G., Reboulet, S., Grandjean, P., 2003. Guo, Z.F., Liu, J.Q., Wang, X.L., 2003. Effect of Mesozoic volcanic eruptions in the Thermal evolution of Cretaceous Tethyan marine waters inferred from oxygen isotope western Liaoning Province, NE China on paleoclimate and paleoenvironment. composition of fish tooth enamels. Paleoceanography 18, 1029. http://dx.doi.org/ Science in China-Earth Science 33, 59–71. 10.1029/2002PA000823. 228 H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228

Sun, G., Dilcher, D.L., Zheng, S.L., Zhou, Z.K., 1998. In search of the first flower: a Jurassic Wu, H.C., Zhang, S.H., Jiang, G.Q., Huang, Q.H., 2009. The floating astronomical time angiosperm, Archaefructus, from northeast China. Science 282, 1692–1695. scale for the terrestrial Late Cretaceous Qingshankou Formation from the Songliao Swisher, C.C., Wang, Y.Q., Wang, X.L., Xu, X., Wang, Y., 1999. Cretaceous age for the Basin of northeast China and its stratigraphic and paleoclimate implications. Earth feathered dinosaurs of Liaoning, China. Nature 400, 59–61. and Planetary Science Letters 278, 308–323. Swisher, C.C., Wang, X.L., Zhou, Z.H., Wang, Y.Q., Jing, F., Zhang, J.Y., Xu, Q., Zhang, F.Y., Wu, H.C., Zhang, S.H., Feng, Q.L., Jiang, G.Q., Li, H.Y., Yang, T.S., 2012. Milankovitch Wang, Y., 2002. Further support for a Cretaceous age for featured dinosaur beds of and sub-Milankovitch cycles of the Early Triassic Daye Formation, South China Liaoning Province, China: new 40Ar/39Ar dating of the Yixian and Tuchengzi forma- and their geochronological and paleoclimatic implications. Gondwana Research tions. Chinese Science Bulletin 47, 135–138. 22, 748–759. Wang, H.Z., Yang, S.N., Li, S.T., 1983. Mesozoic and Cenozoic basin formation in east Wu, H., Zhang, S., Jiang, G., Hinnov, L., Yang, T., Li, H., Wan, X., Wang, C., 2013. China and adjacent regions and development of the continental margin. Acta Astrochronology of the Early Turonian–Early Campanian terrestrial succession in Geologica Sinica 57, 213–223. the Songliao Basin, northeastern China and its implication for long-period behavior Wang, X.L., Wang, Y.Q., Wang, Y., 1998. Stratigraphic sequence and vertebrate-bearing of the Solar System. Palaeogeography, Palaeoclimatology, Palaeoecology 385, beds of the lower part of the Yixian Formation in Sihetun and neighbouring 55–70 (this issue). area, western Liaoning, China. Vertebrata PalAsiatica 36, 81–101 (in Chinese with Yang, W., Li, S.G., Jiang, B.Y., 2007. New evidence for Cretaceous age of the feathered English Summary). dinosaurs of Liaoning: zircon U–Pb SHRIMP dating of the Yixian Formation in Wang, X.L., Wang, Y.Q., Xu, X., Wang, Y., Zhang, J.Y., Zhang, F.C., Jin, F., Gu, G., 1999. Sihetun, northeast China. Cretaceous Research 28, 177–182. Record of the Sihetun vertebrate mass mortality events, western Liaoning, China: Zhang, X.L., Sha, J.G., 2012. Sedimentary laminations in the lacustrine Jianshangou bed caused by volcanic eruptions. Geological Review 45, 458–466 (Suppl., In Chinese of the Yixian Formation at Sihetun, western Liaoning, China. Cretaceous Research with English Abstract). 36, 96–105. Wang, S.S., Wang, Y.Q., He, H.G., Li, H.M., 2001. The existing time of Sihetun vertebrate in Zhang, L.J., Li, Y.X., Qiu, J., Ye, J., 2004. Accurate division and correlation of bird-bearing western Liaoning — evidence from U–Pb dating of zircon. Chinese Science Bulletin horizons at Sihetun, Beipiao, Liaoning. Geological Bulletin of China 23, 184–188 46, 779–782. (In Chinese with English Abstract). Weedon, G.P., 2003. Time-series Analysis and Cyclostratigraphy. Cambridge University Zhang, H.C., Wang, B., Fang, Y., 2010. Evolution of insect diversity in the Jehol Biota. Press, Cambridge (256 pp.). Science China-Earth Sciences 53, 1908–1917. Wu, S.Q., 1999. A preliminary study of the Jehol flora from western Liaoning. Zhou, Z.H., Wang, Y., 2010. Vertebrate diversity of the Jehol Biota as compared with Palaeoworld 11, 7–57 (in Chinese with English abstract). other lagerstätten. Science China-Earth Sciences 53, 1894–1907. Wu, H.C., Zhang, S.H., Sui, S.W., Huang, Q.H., 2007. Recognition of Milankovitch cycles Zhou, Z.H., Barrett, P.M., Hilton, J., 2003. An exceptionally preserved Lower Cretaceous in the natural gamma-ray logging of Upper Cretaceous terrestrial strata in the ecosystem. Nature 421, 807–814. Songliao Basin. Acta Geologica Sinica 81, 996–1001. Zhu, R.X., Pan, Y.X., Shi, R.P., Liu, Q.S., Li, D.M., 2007. Palaeomagnetic and 40Ar/39Ar dating Wu, F.Y., Xu, Y.G., Gao, S., Zheng, J.P., 2008. Lithospheric thinning and destruction of constraints on the age of the Jehol Biota and the duration of deposition of the Sihetun the North China Craton. Acta Petrologica Sinica 24, 1145–1174 (in Chinese with fossil-bearing lake sediments, northeast China. Cretaceous Research 28, 171–176. English abstract).