Astrochronology for the Early Cretaceous Jehol Biota in

Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228
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Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo
Astrochronology for the Early Cretaceous 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
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
b
a r t i c l e
i n f o
Article history:
Received 14 September 2011
Received in revised form 10 May 2013
Accepted 15 May 2013
Available online 23 May 2013
Keywords:
Astrochronology
Early Cretaceous
Jehol Biota
Yixian Formation
Northeastern China
a b s t r a c t
The Early Cretaceous Jehol Biota in northeastern China provides an evolutionary window for ‘feathered’
dinosaurs, primitive birds, insects and early flowering plants. It also provides critical information for the biodiversity changes of the Early Cretaceous terrestrial ecosystem. Here we report a time series analysis for the
11.2-m-thick, fossil-bearing lacustrine deposits at the Sihetun section in western Liaoning, northeastern
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
was recorded in the MS and ARM series. With available radioisotope age constraints from the same section,
sedimentary cycles of 120–260 cm, 50–67 cm and 18–42 cm were interpreted as Milankovitch cycles of short
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
~0.67 Myr and the average depositional rate is ~1.70 cm/kyr. The duration of the 1.8-m-thick, main fossilbearing interval that contains 8 beds of ‘feathered’ dinosaur/primitive bird fossils can be estimated as short as
150 kyr. The results suggest that climate fluctuations manifested in paleobotanical, sedimentological and geochemical records of the Yixian Formation may have been controlled by orbital forcing during Early Cretaceous.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
The Early Cretaceous Jehol Biota, discovered from the Yixian Formation and its overlying Jiufotang Formation in northeastern China,
provides a unique opportunity to address questions about the evolution of ‘feathered’ dinosaurs and primitive birds, the diversification of flowering plants, and the radiation of placental mammals
(Zhou et al., 2003; He et al., 2004, 2006; Zhou and Wang, 2010)
(Fig. 1). Furthermore, exceptionally well-preserved fossils of the
Jehol Biota and the unique lithological contents (fine-grained
siliciclastic rocks and volcanic ash beds) open a rare window for understanding the evolution of Early Cretaceous terrestrial ecosystems and reconstruction of paleoecological history (Zhang et al.,
2010; Zhou and Wang, 2010). The mass mortality and remarkable
fossil preservation of the Jehol Biota have been ascribed to repetitive environmental crises induced by volcanic eruptions (Wang et
al., 1999; Guo et al., 2003).
Obtaining high-precision radiometric ages from the Yixian and
Jiufotang formations and evaluating the duration of biological events
are essential for delineating the evolutionary patterns of the Jehol
Biota. Enormous efforts using 40Ar/39Ar and U/Pb dating techniques
⁎ Corresponding author at: School of Ocean Science, China University of Geosciences,
Beijing 100083, China.
E-mail address: [email protected] (H. Wu).
0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.palaeo.2013.05.017
have been taken to constrain the absolute age of the Jehol Biota, and
the results from different sections indicate that the ages across
the Jehol Biota range from ~ 131 Ma to 120 Ma (Barremian to Early
Aptian) (Swisher et al., 1999, 2002; Wang et al., 2001; He et al.,
2004, 2006; Yang et al., 2007; Chang et al., 2009). However, further
refinement for the duration of fossil-bearing strata is hindered due
to uncertainties inherent to radioisotope age-dating methods (Erwin,
2006), systematic discrepancies between 40Ar/39Ar and U–Pb dating
methods (Min et al., 2000; Kuiper et al., 2008), and the limited number
of volcanic/tuff layers amenable for precise dating.
Astrochronology provides an alternative method for fine-tuning
the duration of stratigraphic units and geological events when continuous, fine-grained sedimentary records are available. Recent studies
indicated that orbital-scale climate cycles were well preserved in
terrestrial successions deposited from lacustrine and palustrine environments. Examples include the Late Triassic–Early Jurassic Newark Basin in
eastern North America (Olsen and Kent, 1996), the Late Cretaceous
Songliao Basin in northeastern China (Wu et al., 2007, 2009, 2013-this
issue), the Early Eocene Green River Formation in North America
(Machlus et al., 2008), and Miocene basins around the Mediterranena
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
down to 0.02–0.40 Ma (e.g., Olsen and Kent, 1996; Abdul Aziz et al.,
2004; Hinnov and Ogg, 2007; Wu et al., 2009, 2013-this issue).
222
H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228
Depth (m)
75 OE
95 OE
115 O E
135 O E
a)
b)
3
Beipiao
0
40 ON
2
4
6 km
4
Section locality
Beijing
30 ON
5
China
6
20 ON
7
Nanling
8
Sihetun
Chaoyang
Shangyuan
9
10
0m
0
c)
Legends
Conglomerate or conglomeratic
Sandstone
2
124.2
2.5 Ma (A r-Ar)(Zhu et al., 2007)
124.6
125.0
124.6
124.1
124.7
125.2
0.1 Ma (Ar-Ar ) (Swisher et al., 1999)
0.1 Ma (Ar-Ar ) (Swisher et al., 2002)
0.25 Ma (Ar-Ar ) (Swisher et al., 2002)
0.3 Ma (Ar-Ar ) (Chang et al., 2009)
2.7 Ma (U-Pb) (Yang et al., 2007)
0.9 Ma (U-Pb) (Wang et al., 2001)
Fine sandstone
3
Jianshangou Unit
Silty shales
Shales
4
Mudston e
5
Lava
Lapilli
6
125.7
2.6 Ma (A r-Ar)(Zhu et al., 2007)
Tuff or tuffaceous
7
8
0
9
5
10
10 m
Main fossil
bearing layers
Lujiatun Unit Lower Lava Unit
L o w e r C r e t a c e o u s Yi x i a n F o r m a t i o n
Upper Lava Unit
1
Confuciusornis, Peipiaosteus pani
Confuciusornis
Confuciusornis, Sinosauropteryx,Sinornithosaurus millenii,
Beipiaosaurus inexpectus, Psittacosaurus, Caudipteryx zoui,
Caudipteryx dongi
Psittacosaurus, Protarchaeopyeryx robust
Confuciusornis, Sinosauropteryx
Confuciusornis
Confuciusornis, Liaoningornis longiditris
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 radioisotopic ages and location of main fossil-bearing layers are marked.
It provides a tool to estimate the duration of critical stratigraphic intervals and geological events at remarkable resolution and precision that
help to better understand the deep-time paleoclimatological and paleoecological changes (Hinnov and Ogg, 2007; Hinnov and Hilgen, 2012).
In this paper, we report a cyclostratigraphic study of fossilbearing, thinly laminated and fine-grained lacustrine deposits of the
Jianshangou unit, Yixian Formation, using high-resolution magnetic
susceptibility (MS) and anhysteretic remanent magnetization (ARM)
measurements obtained from the Sihetun section in northeastern
China. The main objectives of this study are (1) to construct a ‘floating’
astronomical time scale that provides an independent age constraint
for the duration of the Jehol Biota and its host strata, and (2) to discuss
the possible causes of paleoenvironmental and paleoclimatic fluctuations recorded in the fossil-bearing strata.
2. Geological background
During Late Jurassic–Early Cretaceous, a series of rifted sedimentary basins were formed in northeastern China in response to crustal
extension that was likely triggered by gravitational collapse of
H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228
3. Sampling, magnetic measurement and data processing
The rock magnetic parameters are extensively used in cyclostratigraphy research because their measurements are comparatively
fast, cost-effective and non-destructive, allowing analysis of large
sample populations (Maher and Thompson, 1999; Hinnov, 2004).
The MS refers to the capacity of a substance to become magnetized
when subjected to an external magnetic field. The ARM values reflect
the concentration of fine-grained (b 20 μm), low-coercivity ferromagnetic minerals that have a relatively simple origin, and it has
been successfully used in cyclostratigraphic research on continental
and marine successions (e.g., Latta et al., 2006; Kodama et al., 2010;
Wu et al., 2012). In this study, both MS and ARM series are used for
cyclostratigraphic analysis.
A total of 670 samples were collected from the 11.2-m-thick fossilbearing strata in the Sihetun section (41°35′20.2″ N, 120°47′35.7″ E)
(Fig. 1b and c). Samples were crashed and placed into plastic paleomagnetic cubes. The MS was measured on KLY-4S kappa-bridge.
The ARM was acquired by applying a peak alternating field of 0.1 T
and a bias field of 50 μT on the D-2000 AF demagnetizer, and the remanence intensity measurements were made on a JR6 spinner magnetometer. In order to identify the composition of magnetic minerals,
rock magnetic experiments were carried out on representative samples. Temperature dependence of low-field magnetic susceptibilities
(χ − T) was measured on a KLY-4S Kappa-bridge with a temperature
apparatus (CS-3). Isothermal remanent magnetization (IRM) acquisition and direct field demagnetization were applied in an ASC
IM-10-30 impulse magnetizer. All measurements were performed
in the Paleomagnetism and Environmental Magnetism Laboratory at
China University of Geosciences (Beijing). Both MS and ARM values
were normalized by mass, given in 10−8 m3/kg and 10−6 Am2/kg,
respectively.
The average value of MS is 5.77 × 10−8 m3/kg, and the average
value of ARM is 1.37 × 10−6 Am2/kg, with 34 ‘abnormal’ peak values
(a1–a34 in Fig. 2). Samples with ‘abnormal’ high MS and ARM values
are mostly brownish to yellowish, showing strongly weathered features. These layers are commonly less than 2 cm, and their lithologies
are tuffs or tuffaceous siltstone/shale. High MS and ARM values from
these layers may have overridden the normal depositional information.
After removing these ‘abnormal’ points, the MS and ARM values show
distinct and similar cyclic variability throughout the section (Fig. 3).
The tuff/tuffaceous beds, which represent instantaneous volcanic
eruption events, were excluded when constructing the new MS and
ARM time series for cyclostratigraphy analysis. The MS and ARM data
series were detrended prior to time series analysis by removing a 35%
weighted mean from the data using the software KaleidaGraphTM
(Fig. 3). Power spectral was calculated for the MS and ARM time series
( 10-8 m3 /kg)
0
10
20
30
40
0
a34
1
a33
2
3
a32
a31
a30
a29
4
a28
a27
Depth (m)
previously thickened crust (Meng, 2003; Wu et al., 2008). The Cretaceous strata surrounding the Sihetun area in western Liaoning
Province were deposited from one of the extensional volcanicsedimentary basins (Wang et al., 1983, 1998) (Fig. 1). The Jehol
Biota in this region was excavated from the Yixian Formation and
the overlying Jiufotang Formation (Zhou et al., 2003).
One of the best fossil-preservation sites of the Jehol Biota is located
in a small region surrounding the village of Sihetun, about 25 km
south of the Beipiao City (Fig. 1a). The Yixian Formation in the Sihetun
area consists of four units including, in ascending order, the Lujiatun
Unit, Lower Lava Unit, Jianshangou Unit and Upper Lava Unit (Jiang
and Sha, 2007) (Fig. 1). Fossil-bearing strata in the Sihetun section
are from the Jianshangou Unit, which is 14.4 m thick and composed
of dark to light gray shale, siltstone, silty mudstone and fine-grained
sandstone that were deposited from lacustrine environments (Zhu
et al., 2007). Exceptionally well-preserved ‘feathered’ dinosaurs
Sinosauropteryx (Chen et al., 1998), Protarchaeopteryx and Caudipteryx
(Ji et al., 1998), primitive birds Confuciusornis (Hou et al., 1995), placental mammals Zhangheotherium (Hu et al., 1997), and the oldest
flowering plant Archaefructus (Sun et al., 1998), have been reported
from the Jianshangou Unit around this area.
A number of brownish to yellowish tuff layers have been found in
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,
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 ±
0.3 Ma (Chang et al., 2009) were obtained from the same interval.
The volcanic rocks underlying and overlying the fossil-bearing lacustrine deposits were dated (40Ar/39Ar) as 125.7 ± 2.6 Ma and
124.2 ± 2.5 Ma, respectively (Zhu et al., 2007; Fig. 1c). These ages
roughly constrain the duration of the Jianshangou unit as less than
1.6 Myr (Fig. 1), and provide a solid basis for constructing the astronomical time scale.
223
5
a26
a25
a24
a23
a22
6
a21
a19
a18
7
a20
a17
a16
a15
a14
a13
a12
8
a11
a10
9
a9
a8
a7
a6
10
a5
a3
a4
a2
a1
11
0
4
8
12
16
ARM( 10 -6 Am2 /kg)
Legends
mudstone shales
silty shales
fine sandstone Tuff or tuffaceous
Fig. 2. Variation of magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) in the fossil-bearing interval. High MS and ARM values of a1–a34 correspond
to brownish or yellowish tuff or tuffaceous beds.
224
H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228
a)
b)
( 10
Lithology
0
2
4
-8
c)
( 10
m3 /kg)
6
8
-2
-1
d)
-8
m3 /kg)
0
1
ARM (
0
2
0.4 0.8 1.2 1.6
f)
e)
10 -6 Am2 /kg)
ARM (
2
10 -6Am2 /kg)
-0.8
0
0.8
0
15
1
3
215
Depth (m)
4
5
315
6
415
7
465
8
~150 kyr
515
9
Short eccentricity (~100 kyr)-based astronomical time scale (ka)
115
2
615
10
665
-1 -0.5
0
0.5
-0.3
1
0
0.3
100 kyr filter output of ARM
100 kyr filter output of
Legends
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.,
2002), with robust red noise estimation reported at 90%, 95% and 99%
confidence levels for the interpretation of spectral peak significance
(Mann and Lees, 1996). The evolutionary fast Fourier transform (FFT)
spectrograms were constructed and used to track changes in sedimentation rate. The Gauss band-pass filtering was carried out with the
freeware Analyseries 2.0.4.2 (Paillard et al., 1996).
4. Results and discussion
4.1. Rock magnetism results
IRM measurement of representative samples shows rapid increases below a field of 100 mT and the remanence coercivity of the
same samples is less than 50 mT (Fig. 4a, b), which indicate that
low magnetic coercive minerals are the dominant magnetic minerals
in the samples. Thermal magnetic experiment shows a gradual
drop of magnetic susceptibility, indicating the existence of paramagnetic minerals. The susceptibilities on the heating curves decrease
rapidly between 560 °C and 580 °C, indicating the dominance of
titanomagnetite (Fig. 4c). The cooling curves are much higher than
the heating curves, which can be interpreted as a formation of strong
magnetic minerals during heating (Fig. 4c, d). The rock magnetic
experiments show that the dominant magnetic mineral is most likely
the low magnetic coercive titanomagnetite.
A normal paleomagnetic polarity was revealed from the Jianshangou
lacustrine interval at the Sihetun section, and the remanence carriers
were identified as titanomagnetite (Pan et al., 2001; Zhu et al.,
2007). Major and trace elemental analyses indicated that the source of
lacustrine deposits of the Sihetun section was mainly from weathering
H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228
b)
1
1
0.8
0.8
IRM(M/Mmax)
IRM(M/Mmax)
a)
225
S7-009
8.62 m
0.6
0.4
0.2
B0-054
10.32 m
0.6
0.4
0.2
0
0
200
400
600
800
0
1000
0
200
Applied field (mT)
400
600
800
1000
Applied field (mT)
c)
d)
3
4
S7-009
8.62 m
B0-054
10.32 m
Cooling
3
2
κ/κ0
κ/κ0
Cooling
2
1
Heating
0
0
100
200
300
Heating
1
400
500
600
700
o
Temperature (C )
0
0
100
200
300
400
500
600
700
o
Temperature (C )
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 interpret that variations in MS and ARM values of the Jianshangou Unit
at the Sihetun section record fluctuations in the flux of fine-grained
detrital titanomagnetic minerals in response to climate changes.
4.2. Cyclostratigraphic results
The MTM power spectral analysis and evolutionary FFT spectrum
analysis of the ARM series in the depth domain reveals a hierarchy of
cycles with 227 cm, 157 cm, 128 cm, 57 cm, 40 cm and 21 cm wavelengths above 99% confidence (Fig. 5a, b). The MTM power spectra and
evolutionary spectrum of the untuned MS series show significant
peaks at 255 cm, 120 cm, 21 cm and 18.3 cm above 99% confidence
level (Fig. 5c, d). The evolutionary FFT spectrum of the MS and ARM
shows that the dominant frequency of ~ 0.08 cycles/cm gradually
changed to lower frequency of ~ 0.05 cycles/cm, which may indicate
a higher sedimentary rate in the upper part of the section. Jiang et al.
(2012) also proposed that the upper part of the section has a higher
rate of sedimentation due to increased flooding events during the late
stage of the lake deposition.
The ratio of the major cycle bands of 120–260 cm:50–67 cm:
18–42 cm is similar to the ratio of the Late Cretaceous astronomical
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)
(Laskar et al., 2011) (Fig. 5a–e). Thus, we interpret these cycles
as Milankovitch sedimentary cycles of short eccentricity, obliquity
and precession according to the cycle length ratios (Hinnov, 2000;
Weedon, 2003).
It has been recommended that long eccentricity (405 kyr) cycles
should be used for the calibration of Mesozoic astronomical time scales
(Laskar et al., 2004; Hinnov and Ogg, 2007; Hinnov and Hilgen, 2012),
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
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
a “floating” astronomical time scale (Fig. 3). The Gaussian band-pass
filters were designed to extract the signal of short eccentricity cycles
of 120–260 cm wavelength. As shown in Fig. 3, the studied succession
recorded 6.7 short eccentricity cycles. The 120–260 cm cycles were
then tuned by assigning 100 kyr to neighboring peaks in the filteroutputs in the depth domain (Fig. 3). The power spectral analysis
reveals 102 kyr, 85 kyr, 39.4 kyr, 35.3 kyr, 20.9 kyr and 18 kyr peaks
above 99% confidence in 100 kyr-tuned ARM series, and significant
100 kyr and 18 kyr periods in the 100 kyr-tuned MS series (Fig. 5f, g).
These spectra match well with Early Cretaceous astronomical parameters of the La2010a solution (Fig. 5e; Laskar et al., 2011) and support
our cyclostratigraphic interpretation on the MS and ARM data.
4.3. Time constraints for the Jehol Biota and geomagnetic polarity chron
M3n
Based on the 100 kyr-tuned ‘floating’ astronomical time scale
(Fig. 3), the duration of the 11.2-m-thick fossil-bearing strata is
0.67 Myr and the average depositional rate for the studied interval
is 1.70 cm/kyr. The duration of the 14.4-m-thick Jianshangou Unit
outcropped at the Sihetun section can be estimated as 0.86 Myr.
H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228
c)
1.5
e: 120-250 cm
0.9
0.6
99%
95%
90%
M
6
227 cm
157 cm
P: 18-42 cm
128 cm
57 cm
40 cm
21 cm
P: 18-22 cm
4
e) 60
Ee
OP
50
36.6 kyr
405 kyr
130 kyr
40
22.1 kyr
20.9 kyr
18 kyr
30
95 kyr
20
46 kyr
18.3 cm
10
2
0.3
Depth (cm)
e: 110-260 cm
255 cm
120 cm
21 cm
0
f)
0
200
200
300
300
400
400
500
500
600
600
700
700
800
800
900
900
102 kyr
85 kyr
2
Power
d)
0
39.4 kyr
35.3 kyr
20.9 kyr
1
18 kyr
0
g) 20
16
Power
Power
1.2
b)
8
O: 50-67 cm
Power
a)
Power
226
100 kyr
12
18 ky r
8
4
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0
Frequency (cycles/cm)
0.01
0.02
0.03
0.04
0.05
Frequency (cycles/cm)
0.06
0
0.01
0.02
0.03
0.04
0.05
0.06
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
discovered from eight layers in the 1.8-m-thick interval from 8.4 m to
10.2 m (Fig. 1c) (Zhang et al., 2004). The floating astronomical time
scale (Fig. 3) suggests that the duration of these ‘feathered’ dinosaur/
primitive bird fossil beds is ~150 kyr, with each fossil-bearing bed
appearing at an average period of ~18.7 kyr (Figs. 1c and 3).
Recently, three new M-sequence geomagnetic polarity time
scales were proposed, including GPTS12 (Ogg, 2012), MHTC12 and
MHTC12-125 (Malinverno et al., 2012). The main difference of these
new polarity time scales was the assignment of the onset age of
M0r, i.e., 125 Ma (GPTS12 and MHTC12-125) or 121 Ma (MHTC12).
Zhu et al. (2007) reported normal to reverse magnetic polarity
changes from the fossil-bearing strata to the overlying volcanic
rocks at the Sihetun section. In combination with40Ar/39Ar ages
from overlying and underlying volcanic rocks (125.7 ± 2.6 Ma and
124.2 ± 2.5 Ma), the normal and reversed polarities could correspond
to geomagnetic polarity chron M3n (123.92–124.58 Ma) of MHTC12
(Malinverno et al., 2012), but could not correlate well with the polarity
chrons of the GPTS2012 (Ogg, 2012) and MHTC12-125 (Malinverno
et al., 2012). The duration of ~0.86 Myr for the normal polarity of
the Jianshangou section is consistent, in first order, with the duration
of 0.66 ± 0.0929 Myr for M3n (Malinverno et al., 2012) and supports
the polarity time scale of MHTC12, which was also preferred by
Malinverno et al. (2012).
4.4. Orbital forcing of climate recorded in the Yixian Formation
Sedimentological, paleobotanical and geochemical studies revealed
dynamic climate fluctuations during the deposition of the Yixian
Formation. Wu (1999) proposed warm and arid climate alternations
based on the size, root system and membranous leaves of plants. Jiang
and Sha (2007) proposed that thin gypsum and calcareous mudstone
layers in the succession were formed under very shallow-water and
arid conditions. However, Li and Batten (2007) suggested a warm and
seasonal climate (semi-arid) system with fairly short wet phases
and long dry (arid) periods. Ding et al. (2003) indicated that most of
sporopellen, plant, and wood fossils of the lacustrine deposits in this region recorded warm and wet habitats, and that only a few fossils such
as xerophilous Gnetales, Bennettiales with membranous leaves, and
Conifersles with scaled leaves were representative of arid or semi-arid
climatic conditions. Fürsich et al. (2007) also proposed a semi-arid climate in which dry periods with little air movements alternated with
stormier wet seasons. Amiot et al. (2011) obtained the oxygen isotope
composition of apatite from various reptile remains of the Jehol Biota
from China, Thailand, and Japan, which shows that the climate in this
period of the Early Cretaceous was cold and similar to that of today at
equivalent latitude. The inferred low temperatures are in agreement
with the marine records of late Barremian–Early Albian (e.g., Price,
1999; Pucéat et al., 2003; Dumitrescu et al., 2006).
The causes of climate changes recorded in the Yixian Formation
still remain uncertain. It has been proposed that frequent volcanic
activities around western Liaoning area played an important role in climate change by inducing ‘volcanic winters’ (e.g., Ding et al., 2003; Guo
et al., 2003; Zhou et al., 2003). The Milankovitch cycles identified from
the Jianshangou Unit in this study offers an alternative interpretation:
climate variations were likely driven by Early Cretaceous orbital forcing,
similar to the astronomical forcing recorded in Cretaceous marine systems (e.g., Fiet and Gorin, 2000; Gale et al., 2002; Miller et al., 2005).
The Sihetun paleolake represents a relatively small and closed volcanic
valley formed during volcanic eruptions of the Lower Lava Unit of the
Yixian Formation. The depositional processes and lake level were very
sensitive to climate and environmental changes (Jiang et al., 2012;
Zhang and Sha, 2012). The rock magnetic cyclostratigraphy of the
Jianshangou Unit suggests that climate variations are linked to orbital
H. Wu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 385 (2013) 221–228
forcing climate cycles which are encoded by the concentration of
fine-grained detrital titanomagnetite (Figs. 3–5). During periods of
relatively cold and dry climates (precession maxima or low obliquity),
decrease in weathering rate, precipitation and riverine discharge
would result in less input of magnetic minerals, leading to lower MS
and ARM values. In contrast, high MS and ARM values suggest increased
input of magnetic minerals, which can be explained by increased precipitation, weathering and riverine input during warm and wet climates
corresponding to periods of precession maxima or high obliquity.
5. Conclusion
Spectral analyses of high-resolution MS and ARM time series in
the depth and time domain reveal significant orbital forcing climate
cycles in the lacustrine deposits that host the Early Cretaceous Jehol
Biota. A 100 kyr-tuned floating astronomical time scale of the ARM
series indicates that the duration of the 11.2-m-thick fossil-bearing
strata is 0.67 Myr and the average depositional rate is 1.70 cm/kyr.
The duration of the 1.8-m-thick interval that contains abundant and
well-preserved feathered dinosaur/primitive bird fossils is as short
as 150 kyr. The climate fluctuations recorded in the Yixian Formation
may have been controlled by orbital forcing during Early Cretaceous.
Acknowledgments
We express our sincere appreciation to three anonymous reviewers for their careful review and constructive suggestions that
significantly improved the paper. This work was supported by the
National Key Basic Research Development Program of China (Grant
2012CB822002), the National Science Foundation of China (Grants
91128102, 40802012) and the Fundamental Research Funds for the
Central Universities.
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