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Cyclostratigraphy of the global stratotype section and point (GSSP) of the basal Guzhangian Stage of the Cambrian Period
Journal Pre-proof Cyclostratigraphy of the global stratotype section and point (GSSP) of the basal Guzhangian Stage of the Cambrian Period
Jichuang Fang, Huaichun Wu, Qiang Fang, Meinan Shi, Shihong Zhang, Tianshui Yang, Haiyan Li, Liwan Cao PII:
S0031-0182(19)30005-7
DOI:
https://doi.org/10.1016/j.palaeo.2019.109530
Reference:
PALAEO 109530
To appear in:
Palaeogeography, Palaeoclimatology, Palaeoecology
Received date:
6 January 2019
Revised date:
3 December 2019
Accepted date:
7 December 2019
Please cite this article as: J. Fang, H. Wu, Q. Fang, et al., Cyclostratigraphy of the global stratotype section and point (GSSP) of the basal Guzhangian Stage of the Cambrian Period, Palaeogeography, Palaeoclimatology, Palaeoecology (2019), https://doi.org/ 10.1016/j.palaeo.2019.109530
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© 2019 Published by Elsevier.
Journal Pre-proof
Cyclostratigraphy of the global stratotype section and point (GSSP) of the basal Guzhangian Stage of the Cambrian Period Jichuang Fanga,b, Huaichun Wua,b,*, Qiang Fanga,b, Meinan Shia,b, Shihong Zhanga, Tianshui Yanga, Haiyan Lia, Liwan Caoa a
State key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China School of Ocean Sciences, China University of Geosciences, Beijing 100083, China
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b
*
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Corresponding author: [email protected] (Huaichun Wu)
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ABSTRACT
The Cambrian is the first period of the Phanerozoic during which numerous
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major geological and biological events occurred. Understanding these events requires
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a high-resolution time scale. The Luoyixi section in Guzhang (Hunan Province, China) is the global stratotype section and point (GSSP) for the Guzhangian Stage. Here,
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magnetic susceptibility and δ13C data were used to conduct cyclostratigraphic analyses covering the upper Drumian through lower Guzhangian stages. The results of the power spectral analyses and average spectral misfit analyses indicate that the 25– 25.53 m, 6.25–6.91 m, 1.90–2.03 m, and 1.08–1.19 m sedimentary cycles may represent long-eccentricity, short-eccentricity, obliquity and precession cycles, respectively. A ~1400 kyr floating astronomical time scale was constructed using the stable 405-kyr eccentricity cycles to calibrate the agnostoid trilobite zones. The obtained fundamental obliquity period (30.7 ± 0.7 kyr) implies 370,180 ± 1,220 km (vs. 384,000 km for present day) for the Earth–Moon distance and 21.58 ± 0.18 h (vs. 23.93 h for present day) for the length of day during the middle Cambrian (500 Ma).
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Journal Pre-proof Key words: Floating astronomical time scale, Luoyixi section, Earth–Moon system, South China
1. Introduction The Cambrian, the first period of the Phanerozoic, includes four epochs corresponding broadly to significant steps in Earth’s biological and chemico-physical
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development (Babcock et al., 2015). The transition from Epoch 2 (520–509 Ma) to Miaolingian (509–497 Ma) is marked by major biotic turnover from archaeocyath
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sponges to olenelline and redlichiid trilobites. Marine ecosystems of the Miaolingian
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were stable compared with the subunit of the middle Paleozoic and evolutionary
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expansion of cosmopolitan marine arthropods (Brett and Baird, 1995; Brett et al.,
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1996). These biotic events had a deep influence on the ocean carbon inventory leading to multiple carbon isotope excursions in the Cambrian (Peng et al., 2004a; Babcock et
na
al., 2005; Zhu et al., 2006). However, the lack of a high-resolution temporal framework for the Cambrian hinders our understanding of these events. The available
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Cambrian chronostratigraphy has been built using radioisotopic ages with errors exceeding 2 Myr (Ogg et al., 2016); thus, a more powerful dating method is required for this important interval.
The studied period (Drumian–Guzhangian) exhibited a general greenhouse climate with relatively high sea level and quasi-stable ecosystem conditions comparable to the middle Paleozoic (Babcock et al., 2015). Thick cyclic sequences of shallow marine carbonate platforms were globally distributed (Derby et al., 2012), and their stacking patterns may be associated with orbitally-forced sea-level oscillations (Osleger and Read, 1991; Bazykin and Hinnov, 2002). The identified Milankovitch cycles in sedimentary strata could provide a continuous and high-
2
Journal Pre-proof resolution time scale for stratigraphic successions (Hinnov, 2013). The Neogene, Paleogene, and Mesozoic have been calibrated to the full astronomical solution, eccentricity solution, and stable 405-kyr eccentricity cycles, respectively (Gradstein et al., 2012). By 405-kyr calibration, a floating astronomical time scale (ATS) can be constructed for significantly improving the temporal resolution of the Paleozoic chronostratigraphy, including the Permian, Carboniferous, Devonian, Silurian and Ordovician (e.g., De Vleeschouwer et al., 2012; Wu et al., 2013, 2019; Svensen et al.,
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2015; Fang et al., 2015, 2016, 2017, 2018, 2019; Zhong et al., 2018).
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Milankovitch cycles recorded in rock can also be utilized to reconstruct the
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Earth–Moon evolution, which involves three major parameters, i.e., the precession
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constant k, Earth–Moon distance, and length of day (LOD). The accurate values of these parameters in deep time are unknown due to the lack of reliable astronomical
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solution and limited knowledge of the tidal dissipation history (Laskar et al., 2004;
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Waltham, 2015). A recent research conducted by Meyers and Malinverno (2018) indicated k = 85.79, an Earth−Moon distance of 340,900 km, and a LOD of 18.68
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hour for 1.4 Ga in the Proterozoic, which demonstrates the application of cyclostratigraphy in reconstructing the Earth–Moon evolution in geological history. In this study, we conducted cyclostratigraphic analyses on the Luoyixi section, the global stratotype section and point (GSSP) for the base of the Guzhangian Stage in South China (Peng et al., 2009). The main scientific objectives of this study were to: 1) reveal the depositional cycles in the Cambrian marine sedimentary records and their possible astronomical origins; 2) improve Cambrian chronostratigraphy with astrochronology; and 3) reconstruct the deep-time evolution of the Earth–Moon system.
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2. Geological setting and studied section During the Cambrian, the South China Block was an isolated island located on the equator near the west coast of Gondwana (Fig. 1A). Cambrian lithofacies in South China Block were affected by synchronous rift along the southeastern margin of the Yangtze Platform (Chen, 1991). The Yangtze Platform lies on the northwestern side of the rift while the northeast-trending Jiangnan Slope Belt and the Jiangnan Basin lie
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on the southeastern side of the rift. The studied Luoyixi section is located on the Jiangnan Slope Belt (Fig. 1B).
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The Luoyixi section (28°43.20´N, 109º57.88´E) is exposed along a roadcut
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situated on the south bank of the Youshui River in the Wuling Mountains. The
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Youshui River forms the boundary between Guzhang (to the south) and Yongshun (to
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the north) counties in this area (Fig. 1C). The roadcut along the opposite bank of the river, which contains the same succession of strata, is referred to as the Wangcun
na
section. Previously, the Luoyixi section was referred to informally as the Wangcun South section (Peng et al., 2004b, 2005) and the entire succession belongs to the
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Huaqiao Formation.
The studied section is composed of argillaceous limestone, ribbon limestone and limestone. The studied intervals were deposited during a sea-level transgression as the result of a gradual warming trend (Zuo et al., 2008; Peng et al., 2009). Setting the base of the Huaqiao Formation as 0 m, the Luoyixi section ranges from 78 to 164.78 m with a total thickness of 86.78 m (Fig. 2). Four globally correlated trilobite biozones across the Drumian/Guzhangian boundary were recognized in the section, including Ptychagnostus punctuous (78–79.4 m), Goniagnostus nathorsti (79.4–111.9 m), Lejopyge armata (111.9–121.3 m) and Lejopyge laevigata (121.3–164.78 m) (Peng et al., 2009). The base of the Guzhangian Stage is defined by a GSSP that
4
Journal Pre-proof coincides with the first appearance datum (FAD) of the cosmopolitan agnostoid trilobite L. laevigata at 121.3 m above the base of the Huaqiao Formation in the Luoyixi section (Fig. 2) (Peng et al., 2009).
3. Data and Methods 3.1. Magnetic susceptibility and carbon isotope
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Magnetic susceptibility (MS) measures the degree to which particular materials can be magnetized when they are exposed to a magnetic field and has been widely
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applied as a paleoclimate and paleoenvironment proxy and has also been used for
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regional and global stratigraphic correlation. (e.g., Ellwood et al., 2006; Wu et al.,
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2012; De Vleeschouwer et al., 2012). MS values depend mainly on the concentration,
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composition, grain size and shape of the magnetic minerals in rocks (Da Silva et al., 2013). In this study, MS were measured directly along the section using an SM-30
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MS meter (ZH-Instruments, Czech Republic, sensitivity of 1×10−7 SI). A total of 4340 data points with identical spacing of 2 cm were measured, covering the entire
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thickness (86.78 m) of the Huaqiao Formation (Supplementary Table 1, Fig. 3). The carbon isotope trends in carbonate successions can be used for many purposes, such as an important proxy for stratigraphic division and correlation, the definition of key stratigraphic boundaries, reconstructions of geography and environments, mass extinction implications and ecosystem recovery (e.g., Hesselbo et al., 2002; Saltzman, 2002; Yang et al., 2005; Zhu et al., 2006). A δ13C record with a resolution of 0.5–1 m has been constructed for the establishment of the GSSP of the Guzhangian Stage (Zuo et al., 2008) (Fig. 3). Here we used δ13C data to reinforce our cyclostratigraphy study (Supplementary Table 1).
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Journal Pre-proof 3.2. Reconstruction of the Earth–Moon evolution Reconstruction of the Earth–Moon evolution needs to know the precession constant k, Earth–Moon distance, and length of day (LOD). Tidal dissipation in the Earth–Moon system results in lunar recession, deceleration in Earth’s rotation, and the increasing of precession and obliquity periodicities throughout the geological history (Berger et al., 1992; Laskar et al., 2004; Waltham, 2015). Thus, the k, Earth–Moon distance, and length of day are changing all the time. We can estimate them using the
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The detailed processed are described as follows:
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obtained precession and/or obliquity periodicities through cyclostratigraphic analysis.
Equation 4 in Hinnov (2018): k=
– ⁄ i
+
⁄
–e
] cos
(1)
lP
–e
⁄
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The precession constant k is defined in equation (1), which is equivalent to
The variable definitions are shown in Table 1. The variable k/cosis designated as the
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in Equation 8 of Laskar et al. (2004). The Earth’s shape is ellipsoidal and dynamic,
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commonly referred to as “dynamical ellipticity”, and depends on the Earth’s rotation rate, internal density and the consequent gravitational and centrifugal force balance. It is characterized largely by rotational flattening of the poles, and has been approximated as (based on Equation 9 in Berger and Loutre (1994)): ED =
–
= 6.094
105
2
(2)
Equation (3) is based on Equation 5 in Williams (2000), =
–
( )
⁄
–
( )
⁄
(3)
which calculates the Earth–Moon angular momentum exchange for a range of Earth– Moon separations from today to the Roche limit (1.5 of Earth radii).
6
Journal Pre-proof The above three equations determine the relationship among k, Earth–Moon di t ce,
d the
rth’ rot tio r te Using the obtained periodicity of the main
obliquity term (k + s3), we can calculate the value of k based on the equation (4), k=
– s3
(4)
and then input the k value back into Eqs. (1), (2) and (3) to figure out the Earth–Moon
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distance and length of day using Matlab (the codes were provided by Linda Hinnov).
0
e a e I m m⊙ ⨀
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7.2921150 × 10 rad/s 6.67408 × 10-11 m3/(kg s2 ) 0.003273787 1.4959802×1011 m
Earth rotation rate present-day Earth rotation rate gravitational constant Earth dynamical ellipticity semi- jor xi of rth’ orbit past lunar semi-major axis present-day lunar semi-major axis rth’ orbit l ecce tricity semi- jor xi of Moo ’ orbit ecce tricity of the Moo ’ orbit i cli tio of the Moo ’ orbit o the ecliptic mass of the Moon mass of the Sun obliquity angle of Earth symbol for the Sun symbol for the Moon
-p
-5
lP
G ED a
Name
3.833978 × 108 m 0.016708634 3.833978×108 m 0.05554553
na
0
Value
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Symbol
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Table 1 Definition of the variables in equation 1, 2 and 3 (Hinnov, 2018).
5.156690º
7.34767309 × 1022 kg 1.98855 × 1030 kg 23.43928º
3.3. Time-series analysis The MS series was detrended by removing a long-term trend using Matlab script smooth.m. To detect periods in the stratigraphic domain, spectral analysis was performed using the multitaper method (MTM) (Thomson, 1982) with classical red noise modeling reported at the 85%, 90%, 95%, and 99% confidence levels with 7
Journal Pre-proof Matlab
scripts
available
(http://mason.gmu.edu/~lhinnov/cyclostratigraphytools.html).
at Sliding-window
spectral analysis using evolutive fast Fourier transform (FFT) (Kodama and Hinnov, 2015) was conducted to identify changes in cycle frequencies due to possible variations in the sedimentation accumulation rate (SAR). The link between the observed sedimentary cycles and orbital forcing was investigated according to the cycle length ratio method (Weedon, 2003). The interpreted 405-kyr eccentricity
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cycles were extracted using a Gaussian bandpass filter and calibrated to a theoretical
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sinusoidal age model with a period of 405-kyr to construct a floating ATS.
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The average spectral misfit (ASM) provides a quantitative measurement for
re
fitting the observed sedimentary cycles to astronomical periods, given a range of plausible SARs (Meyers and Sageman, 2007). The result is expressed by an optimal
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SAR and corresponding null hypothesis significance level (Ho-SL), which represents
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the probability of no orbital forcing. Lower Ho-SL values signify a better match between the stratigraphic and orbital frequencies. ASM was used to test the presence
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of astronomical signals in the MS series with the “Astrochron” package developed by Meyers (2014). In this study, the 405-kyr calibrated astronomical cycles in the MS time series were compared to the orbital parameter periodicities of 125-kyr and 95-kyr for short-eccentricity, 32.6-kyr for main obliquity, and 20-kyr and 16.95-kyr for precession at 500 Ma (Waltham, 2015).
4. Results and discussions 4.1. Cyclostratigraphy of the Luoyixi section The measured MS values range from 0.008 × 10-3 to 0.233 × 10-3 SI (Fig. 3). Variations in MS show clear cyclic patterns with lithology with high values in
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Journal Pre-proof argillaceous and ribbon limestone and low values in limestone (Fig. 3). The δ13C values ranged from -1.46‰ to 1.95‰ (Fig. 3). The δ13C values co-varied with lithology and showed large-scale cyclic patterns in spite of its low resolution. Here, MS and δ13C data were used to conduct cyclostratigraphic analyses. The predicted periods of the orbital parameters for the Cambrian at 500 Ma are with a ratio of 23.89:7.37:5.60:1.92:1.18:1. The MTM power spectrum and evolutionary FFT spectrogram of the Luoyixi MS series reveal significant
(Fig.
4).
The
ratio
of
the
sedimentary
cycles
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level
of
wavelengths of 25.53 m, 6.25 m, 1.90 m, 1.19–1.12 m above the 95% confidence is
approximately
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22.79:5.58:1.70:1.06:1 that is comparable with that predicted from theoretical orbital
re
parameters. If the sedimentary 25.53 m cycle is interpreted as the 405-kyr eccentricity cycle with an assumed SAR of 6.3 cm/kyr, the sedimentary cycles of 6.25 m, 1.90 m
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and 1.19–1.12 m may represent short-eccentricity, obliquity and precession,
na
respectively. Furthermore, the δ13C spectrum showed significant peaks with a wavelength of 25 m, similar to that observed in MS data. ASM outputs provide an
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optimal SAR of 6.358 cm/kyr with a Ho–SL of 0.007% (Fig. 5), and hence supporting the assumption that 25.53 m cycles are related to long-eccentricity. The MTM power spectrum of the 405-kyr-calibrated MS series of the Luoyixi section showed significant peaks with periodicities of 405-kyr long-eccentricity (calibrated), short-eccentricity (100.9-kyr), main obliquity (30.7-kyr) and precession (19.2-kyr and 18.2-kyr) (Fig. 6). Additionally, the MTM power spectrum of the 405kyr-calibrated δ13C series also showed significant peak with predicted 405-kyr cycles (Fig. 6). These results are consistent with the predicted periodicities of the astronomical terms at 500 Ma (Waltham, 2015).
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Journal Pre-proof The 405-kyr eccentricity cycle is suggested to be stable throughout geological time (Laskar et al., 2004), and it is typically used as a metronome for establishing an ATS for the Mesozoic and Paleozoic eras. A floating ATS was built by calibrating the filtered 405-kyr sedimentary cycles to an artificial 405-kyr sinusoidal curve, and setting the age of the section base (78 m) as 0 kyr (Fig. 7). Bandpass filtering results showed that the Luoyixi section contains approximately 3.5 long-eccentricity cycles that spans ~1400 kyr (Fig. 7). The duration of two integrated agnostoid trilobite zones
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of
(G. nathorsti and L. armata) are estimated as ~524 kyr and ~152 kyr, respectively.
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4.2. Orbitally forced sea-level change during the Cambrian greenhouse climate
re
It has been widely demonstrated that the Earth’s orbital parameters (eccentricity, obliquity and precession) could control global climate changes induced by variations
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in incident insolation (Hays et al., 1976). In a greenhouse climate, sea-level
na
fluctuations can be controlled by orbitally forced thermal oceanic water expansion (Schulz and Schäfer-Neth, 1997), changes in lake and groundwater storage (e.g.,
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Hinnov and Park, 1999; Li et al., 2018a) and/or melting small-scale glaciers (Fang et al., 2016). The Drumian to Guzhangian time was a typical super-greenhouse climate with no ice sheets and thus, orbitally forced glacio-eustasy cannot explain meter-scale sea-level fluctuations. Schulz and Schäfer-Neth (1997) showed that a 2°C increase in deep-sea temperatures could induce 1.7 m of sea-level rise in tune with Milankovitch forcing. Thermo-eustasy and/or changes in lake and groundwater storage induced by insolation were likely the main mechanisms controlling sea-level fluctuations. The Huaqiao Formation was deposited in an outer slope-apron environment where argillaceous and ribbon limestone corresponding to high sea level and fossilrich limestone corresponding to low sea level (Zuo et al., 2008). The variations of the
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Journal Pre-proof MS series were consistent with the alternation of argillaceous/ribbon limestone with limestone beds, which may have been controlled by sea-level fluctuations associated with precession cycles, and modulated by short-eccentricity cycles (Fig. 8). During higher insolation periods, the climate was warmer, and the sea level was higher with strengthened runoff and precipitation that increased the transfer of magnetic minerals to the ocean and led to higher MS values. On the opposite effect, the climate was colder during lower insolation periods, and the sea level was lower with weaker
of
runoff and precipitation that reduced the input of magnetic minerals to the ocean and
ro
resulted in lower MS values. Therefore, the minimum of precession signal cycles in
-p
the MS series may be caused by the colder climate and lower continental runoff
re
during lower insolation periods, and vise versa.
Previous research showed that the depositional sequences were divided into six
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orders ranging in duration from tens of millions of years to a few tens of thousands of
na
years. The 405-kyr eccentricity-related sea level cycles were suggested to correlate with the fourth-order depositional sequences, the 100-kyr eccentricity with the fifth-
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order sequences and the fundamental precession with the sixth-order sequences (Boulila et al., 2011). Lithological alternations of limestone-argillaceous limestone and limestone-ribbon limestone couplets may represent the sixth-order sequences that match the interpreted precession cycles and six meter-scale bundles represent the fifth-order sequences that match the interpreted 100-kyr short-eccentricity cycles, reinforcing the solid relationship between orbital forcing and eustatic sequence hierarchy during the early Paleozoic Era. 4.3. Earth–Moon distance and length of day during the Drumian–Guzhangian Tidal dissipation in the Earth–Moon system results in lunar recession, deceleration in Earth’s rotation, and the increasing of precession and obliquity
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Journal Pre-proof periodicities throughout the geological history. Recovery of the cyclostratigraphic record of Earth’s astronomical parameters has grown rapidly over the past several decades. At first, the focus was on the Cenozoic Era, but advanced to the Mesozoic Era, and most recently, progressed to the Paleozoic Era and the Precambrian (Hinnov, 2018). The Milankovitch records over
rth’ hi tory appear to indicate a dominant
41-kyr obliquity cycle at present day (Hinnov et al., 2012), 35-kyr obliquity at 245 Ma (Anisian; Li et al., 2018b), 34-kyr obliquity at 252–260 Ma (Wuchiapingian–
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Changhsingian; Wu et al., 2013), and 31-kyr obliquity at 467 Ma (Dapingian–
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Darrivilian; Zhong et al., 2018), 28.2-kyr obliquity at 650 Ma (Cryogenian; Bao et al.,
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2018), showing a decreasing trend through the geological time. Thus, our result of
re
30.7-kyr obliquity cycles at 500 Ma (Drumian–Guzhangian) is consistent with this trend. With this information, we can infer the Earth–Moon distance and length of day
lP
(LOD) during the studied time period.
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Here, we defined the 405-kyr calibration derived 30.7-kyr as the periodicity of the obliquity term k + s3 (Table 2 in Hinnov, 2018). Due to the bandwidth resolution
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limit in the spectral analysis, each spectrum peak has an uncertainty of ± Rayleigh frequency. Thus, the obliquity periodicity should be 30.7 ± 0.7 kyr in the entire time span of ca. 1400 kyr. The basic frequency s3 = –18.85 arcsec/yr comes from Laskar et al. (2004) and is shown in Table 1 in Hinnov (2018). Based on equation 4 (Section 3.3) with the obtained obliquity O, we can give a k value (k = 61.06 ± 0.94 arcsec/yr) and then input the k value back into Eqs. (1), (2) and (3) (Section 3.3) to calculate the Earth–Moon distance and LOD. The results showed that, during the studied interval, the Earth–Moon distance was 370,180 ± 1,220 km (vs. 384,000 km for present day) and the LOD was 21.58 ± 0.18 h (vs. 23.93 h for present day). This Earth–Moon distance is a little bit longer than that from Berger and Loutre (1994) (366,075 km)
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Journal Pre-proof and is comparable with the result from the Milankovitch Calculator (Waltham, 2015) at 500 Ma (371,600 ± 6,900 km). The LOD is similar to the result from Berger and Loutre (1994) (21.47 h) and is also comparable with the result from the Milankovitch Calculator (Waltham, 2015) (22.21 ± 0.91 h).
5. Conclusions
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In this study, we conducted cyclostratigraphic analyses on high-resolution MS data from Cambrian strata of the Luoyixi section. Both the MTM spectrum in the
ro
depth domain and the ASM results revealed the presence of Milankovitch signals, and
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the MTM spectrum exhibited significant periodicities consistent with those expected
re
for the Cambrian (500 Ma). A 405-kyr-calibrated floating ATS of the Cambrian for
lP
the Luoyixi section was constructed, which gave a duration of ~1400 kyr for the whole succession. The durations of agnostoid trilobite biozones of G. nathorsti and L.
na
armata are estimated as ~524 kyr and ~152 kyr, respectively. Under greenhouse climate conditions, sedimentary characteristics of the Luoyixi section strata were
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controlled by sea-level oscillations forced by precession and modulated by shorteccentricity. Using the obliquity period of 30.7 ± 0.7 kyr derived from the 405-kyr calibration, we calculated the Earth–Moon distance and LOD during the Cambrian (500 Ma); the results showed that the Earth–Moon distance was 370,180 ± 1,220 km and the LOD was 21.58 ± 0.18 h during the studied interval, which is comparable with previous research.
Acknowledgments We express our sincere appreciation to the editor in chief (Prof. Thomas Algeo), guest editors (Profs. James Ogg, Chunju Huang and David Kemp), Dr. Mingsong Li, 13
Journal Pre-proof and an anonymous reviewer for their careful comments and constructive suggestions that significantly improved the paper. The authors greatly appreciated the assistance from Junjie Xu, Chumeng Peng and Runjian Chu in the field. This study was supported by the National Natural Science Foundation of China (41925010, 41790451, and 41688103), and the fundamental research funds for the Central Universities (2652018072). This is a contribution to IGCP 652.
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the Paleo-Tethyan Realm. Geology 47, 83–86. Wu, H.C., Zhang, S.H., Hinnov, L.A., Jiang, G.Q., Feng, Q.L., Li, H.Y., Yang, T.S., 2013.Time-calibrated Milankovitch cycles for the late Permian. Nature Communications 4, e2452.
Yang, J.L., Xu, S.Q., 1997. Negative carbon isotope excursion in the base Cambrian of Guizhou province, China: implication for biological and stratigraphical significance. Acta geologica sinica 79, 157–164 (in Chinese with English Abstract).
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Journal Pre-proof Zhong, Y.Y., Wu, H.C., Zhang, Y.D., Zhang, S.H., Yang, T.S., Li, H.Y., Cao, L.W., 2018. Astronomical calibration of the Middle Ordovician of the Yangtze Block, South China. Palaeogeography, Palaeoclimatology, Palaeoecology 505, 86–99. Zhu, M.Y., Babcock, L.E., Peng, S.C., 2006. Advances in Cambrian stra-tigraphy and paleontology: Integrating correlation techniques, paleobi-ology, taphonomy and paleoenvironmental reconstruction. Palaeoworld 15, 217–222. Zuo, J.X, Peng, S.C., Zhu, X.J., Qi, Y.P., Lin, H.L., Yang, X.F., 2008. Evolution of
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carbon isotope composition in potential Global Stratotype Section and Point at
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Luoyixi, South China, for the base of the global seventh stage of Cambrian
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System. Journal of China University of Geosciences 19, 9–22.
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Figure captions
Fig. 1. A. Paleogeographic reconstruction for the Cambrian after Peng et al. (2012). B. Map showing the location of the Luoyixi section on the Jiangnan slope belt, modified from Zuo et al. (2008). C. Basic geological map of part of northwestern Hunan
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Province and the location of the Luoyixi section (marked as LYX) from Peng et al.
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(2009).
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Fig. 2. The photograph of the Luoyixi section and the GSSP of the Guzhangian Stage,
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coinciding with the FAD of Lejopyge laevigata, FADs and positions of the trilobite
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biozones in the Huaqiao Formation at the Luoyixi section. Strata underlying the
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Guzhangian GSSP belong to the Drumian Stage.
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Fig. 3. Integrated stratigraphy framework and data series of the Luoyixi section. The interpreted 405-kyr (red and green curves) long-eccentricity cycles (E)
were
extracted from MS and δ13C data using Gaussian bandpass filters with frequencies of 0.03917 ± 0.011 cycles/m and 0.04 ± 0.011 cycles/m to obtain the sedimentary cycles of ~25.53 m and ~25 m, respectively. The interpreted 100-kyr (blue curve) shorteccentricity cycles (e) was extracted from MS data using a passband of 0.16 ± 0.06 cycles/m to obtain the sedimentary cycles of ~6.25 m. The yellow and purple curves are 30-m “moving” trend of the MS and δ13C series.
Fig. 4. The 2π MTM power spectra of the MS (A) and δ13C (B) in the depth domain. Evolutionary FFT spectrogram of the MS series (C) and δ13C series (D) with 30 m 22
Journal Pre-proof sliding windows. The labels E, e, O and P represent long-eccentricity, shorteccentricity, obliquity and precession cycles, respectively. The solid red and dashed green, purple, blue and orange curves represent the median smoothed and fitted red noise spectra at 85%, 90%, 95% and 99% confidence levels, respectively.
Fig. 5. The average spectral misfit (ASM) results and null hypothesis test for the MS series of the Luoyixi section. (A) ASM result indicating an optimal fit at a
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sedimentation rate of 6.358 cm/kyr. (B) Null hypothesis significance levels for the
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ASM result, indicating an optimal sedimentation rate of 6.358 cm/kyr. The black
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dotted line in (B) indicates the critical null hypothesis significance level (H0-SL) of
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0.5%. (C) The number of Milankovitch terms that were available for calculation of ASM. (D) The fit of the frequencies above the 90% confidence level in Fig. 4A with
na
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the predicted orbital frequencies at 500 Ma (Waltham, 2015).
Fig. 6. The MS and δ13C series power spectra in the time domain with significant
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peaks labeled in kiloyears (kyr).
Fig. 7. A floating ATS for the Luoyixi section. Stages with relative boundary ages, lithology, trilobite biozones, extracted 405-kyr long-eccentricity cycles (red curve) using a passband of 0.002469 ± 0.001 cycles/kyr, 100-kyr short-eccentricity cycles (blue curve) using a passband of 0.0099 ± 0.003 cycles/kyr and 405-kyr-calibrated MS series (black curve) (Supplementary Table 1) are also shown.
Fig. 8. Sedimentary characteristics of the Luoyixi section. Curves in A represent the interpreted short-eccentricity (e) cycles (yellow) and precession (P) cycles (red).
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Journal Pre-proof Curves in B represent MS residuals (black), precession (green) using a passband of 0.86 ± 0.18 cycles/m and short-eccentricity (blue) using a passband of 0.16 ± 0.06
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cycles/m.
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Journal Pre-proof Highlights
• Reliable Milankovitch cycles were identified in the Middle Cambrian marine strata. • An ~1400 kyr ATS was constructed for the Drumian–Guzhangian transition. • The durations of the two complete agnostoid trilobite biozones were estimated.
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• Deep-time evolution of the Earth–Moon system was reconstructed.
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
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Jichuang Fang, Huaichun Wu, Qiang Fang, Meinan Shi, Shihong Zhang, Tianshui Yang, Haiyan Li, Liwan Cao PII:
S0031-0182(19)30005-7
DOI:
https://doi.org/10.1016/j.palaeo.2019.109530
Reference:
PALAEO 109530
To appear in:
Palaeogeography, Palaeoclimatology, Palaeoecology
Received date:
6 January 2019
Revised date:
3 December 2019
Accepted date:
7 December 2019
Please cite this article as: J. Fang, H. Wu, Q. Fang, et al., Cyclostratigraphy of the global stratotype section and point (GSSP) of the basal Guzhangian Stage of the Cambrian Period, Palaeogeography, Palaeoclimatology, Palaeoecology (2019), https://doi.org/ 10.1016/j.palaeo.2019.109530
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier.
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Cyclostratigraphy of the global stratotype section and point (GSSP) of the basal Guzhangian Stage of the Cambrian Period Jichuang Fanga,b, Huaichun Wua,b,*, Qiang Fanga,b, Meinan Shia,b, Shihong Zhanga, Tianshui Yanga, Haiyan Lia, Liwan Caoa a
State key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China School of Ocean Sciences, China University of Geosciences, Beijing 100083, China
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b
*
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Corresponding author: [email protected] (Huaichun Wu)
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ABSTRACT
The Cambrian is the first period of the Phanerozoic during which numerous
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major geological and biological events occurred. Understanding these events requires
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a high-resolution time scale. The Luoyixi section in Guzhang (Hunan Province, China) is the global stratotype section and point (GSSP) for the Guzhangian Stage. Here,
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magnetic susceptibility and δ13C data were used to conduct cyclostratigraphic analyses covering the upper Drumian through lower Guzhangian stages. The results of the power spectral analyses and average spectral misfit analyses indicate that the 25– 25.53 m, 6.25–6.91 m, 1.90–2.03 m, and 1.08–1.19 m sedimentary cycles may represent long-eccentricity, short-eccentricity, obliquity and precession cycles, respectively. A ~1400 kyr floating astronomical time scale was constructed using the stable 405-kyr eccentricity cycles to calibrate the agnostoid trilobite zones. The obtained fundamental obliquity period (30.7 ± 0.7 kyr) implies 370,180 ± 1,220 km (vs. 384,000 km for present day) for the Earth–Moon distance and 21.58 ± 0.18 h (vs. 23.93 h for present day) for the length of day during the middle Cambrian (500 Ma).
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Journal Pre-proof Key words: Floating astronomical time scale, Luoyixi section, Earth–Moon system, South China
1. Introduction The Cambrian, the first period of the Phanerozoic, includes four epochs corresponding broadly to significant steps in Earth’s biological and chemico-physical
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development (Babcock et al., 2015). The transition from Epoch 2 (520–509 Ma) to Miaolingian (509–497 Ma) is marked by major biotic turnover from archaeocyath
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sponges to olenelline and redlichiid trilobites. Marine ecosystems of the Miaolingian
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were stable compared with the subunit of the middle Paleozoic and evolutionary
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expansion of cosmopolitan marine arthropods (Brett and Baird, 1995; Brett et al.,
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1996). These biotic events had a deep influence on the ocean carbon inventory leading to multiple carbon isotope excursions in the Cambrian (Peng et al., 2004a; Babcock et
na
al., 2005; Zhu et al., 2006). However, the lack of a high-resolution temporal framework for the Cambrian hinders our understanding of these events. The available
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Cambrian chronostratigraphy has been built using radioisotopic ages with errors exceeding 2 Myr (Ogg et al., 2016); thus, a more powerful dating method is required for this important interval.
The studied period (Drumian–Guzhangian) exhibited a general greenhouse climate with relatively high sea level and quasi-stable ecosystem conditions comparable to the middle Paleozoic (Babcock et al., 2015). Thick cyclic sequences of shallow marine carbonate platforms were globally distributed (Derby et al., 2012), and their stacking patterns may be associated with orbitally-forced sea-level oscillations (Osleger and Read, 1991; Bazykin and Hinnov, 2002). The identified Milankovitch cycles in sedimentary strata could provide a continuous and high-
2
Journal Pre-proof resolution time scale for stratigraphic successions (Hinnov, 2013). The Neogene, Paleogene, and Mesozoic have been calibrated to the full astronomical solution, eccentricity solution, and stable 405-kyr eccentricity cycles, respectively (Gradstein et al., 2012). By 405-kyr calibration, a floating astronomical time scale (ATS) can be constructed for significantly improving the temporal resolution of the Paleozoic chronostratigraphy, including the Permian, Carboniferous, Devonian, Silurian and Ordovician (e.g., De Vleeschouwer et al., 2012; Wu et al., 2013, 2019; Svensen et al.,
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2015; Fang et al., 2015, 2016, 2017, 2018, 2019; Zhong et al., 2018).
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Milankovitch cycles recorded in rock can also be utilized to reconstruct the
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Earth–Moon evolution, which involves three major parameters, i.e., the precession
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constant k, Earth–Moon distance, and length of day (LOD). The accurate values of these parameters in deep time are unknown due to the lack of reliable astronomical
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solution and limited knowledge of the tidal dissipation history (Laskar et al., 2004;
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Waltham, 2015). A recent research conducted by Meyers and Malinverno (2018) indicated k = 85.79, an Earth−Moon distance of 340,900 km, and a LOD of 18.68
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hour for 1.4 Ga in the Proterozoic, which demonstrates the application of cyclostratigraphy in reconstructing the Earth–Moon evolution in geological history. In this study, we conducted cyclostratigraphic analyses on the Luoyixi section, the global stratotype section and point (GSSP) for the base of the Guzhangian Stage in South China (Peng et al., 2009). The main scientific objectives of this study were to: 1) reveal the depositional cycles in the Cambrian marine sedimentary records and their possible astronomical origins; 2) improve Cambrian chronostratigraphy with astrochronology; and 3) reconstruct the deep-time evolution of the Earth–Moon system.
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2. Geological setting and studied section During the Cambrian, the South China Block was an isolated island located on the equator near the west coast of Gondwana (Fig. 1A). Cambrian lithofacies in South China Block were affected by synchronous rift along the southeastern margin of the Yangtze Platform (Chen, 1991). The Yangtze Platform lies on the northwestern side of the rift while the northeast-trending Jiangnan Slope Belt and the Jiangnan Basin lie
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on the southeastern side of the rift. The studied Luoyixi section is located on the Jiangnan Slope Belt (Fig. 1B).
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The Luoyixi section (28°43.20´N, 109º57.88´E) is exposed along a roadcut
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situated on the south bank of the Youshui River in the Wuling Mountains. The
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Youshui River forms the boundary between Guzhang (to the south) and Yongshun (to
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the north) counties in this area (Fig. 1C). The roadcut along the opposite bank of the river, which contains the same succession of strata, is referred to as the Wangcun
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section. Previously, the Luoyixi section was referred to informally as the Wangcun South section (Peng et al., 2004b, 2005) and the entire succession belongs to the
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Huaqiao Formation.
The studied section is composed of argillaceous limestone, ribbon limestone and limestone. The studied intervals were deposited during a sea-level transgression as the result of a gradual warming trend (Zuo et al., 2008; Peng et al., 2009). Setting the base of the Huaqiao Formation as 0 m, the Luoyixi section ranges from 78 to 164.78 m with a total thickness of 86.78 m (Fig. 2). Four globally correlated trilobite biozones across the Drumian/Guzhangian boundary were recognized in the section, including Ptychagnostus punctuous (78–79.4 m), Goniagnostus nathorsti (79.4–111.9 m), Lejopyge armata (111.9–121.3 m) and Lejopyge laevigata (121.3–164.78 m) (Peng et al., 2009). The base of the Guzhangian Stage is defined by a GSSP that
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Journal Pre-proof coincides with the first appearance datum (FAD) of the cosmopolitan agnostoid trilobite L. laevigata at 121.3 m above the base of the Huaqiao Formation in the Luoyixi section (Fig. 2) (Peng et al., 2009).
3. Data and Methods 3.1. Magnetic susceptibility and carbon isotope
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Magnetic susceptibility (MS) measures the degree to which particular materials can be magnetized when they are exposed to a magnetic field and has been widely
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applied as a paleoclimate and paleoenvironment proxy and has also been used for
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regional and global stratigraphic correlation. (e.g., Ellwood et al., 2006; Wu et al.,
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2012; De Vleeschouwer et al., 2012). MS values depend mainly on the concentration,
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composition, grain size and shape of the magnetic minerals in rocks (Da Silva et al., 2013). In this study, MS were measured directly along the section using an SM-30
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MS meter (ZH-Instruments, Czech Republic, sensitivity of 1×10−7 SI). A total of 4340 data points with identical spacing of 2 cm were measured, covering the entire
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thickness (86.78 m) of the Huaqiao Formation (Supplementary Table 1, Fig. 3). The carbon isotope trends in carbonate successions can be used for many purposes, such as an important proxy for stratigraphic division and correlation, the definition of key stratigraphic boundaries, reconstructions of geography and environments, mass extinction implications and ecosystem recovery (e.g., Hesselbo et al., 2002; Saltzman, 2002; Yang et al., 2005; Zhu et al., 2006). A δ13C record with a resolution of 0.5–1 m has been constructed for the establishment of the GSSP of the Guzhangian Stage (Zuo et al., 2008) (Fig. 3). Here we used δ13C data to reinforce our cyclostratigraphy study (Supplementary Table 1).
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Journal Pre-proof 3.2. Reconstruction of the Earth–Moon evolution Reconstruction of the Earth–Moon evolution needs to know the precession constant k, Earth–Moon distance, and length of day (LOD). Tidal dissipation in the Earth–Moon system results in lunar recession, deceleration in Earth’s rotation, and the increasing of precession and obliquity periodicities throughout the geological history (Berger et al., 1992; Laskar et al., 2004; Waltham, 2015). Thus, the k, Earth–Moon distance, and length of day are changing all the time. We can estimate them using the
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The detailed processed are described as follows:
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obtained precession and/or obliquity periodicities through cyclostratigraphic analysis.
Equation 4 in Hinnov (2018): k=
– ⁄ i
+
⁄
–e
] cos
(1)
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–e
⁄
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The precession constant k is defined in equation (1), which is equivalent to
The variable definitions are shown in Table 1. The variable k/cosis designated as the
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in Equation 8 of Laskar et al. (2004). The Earth’s shape is ellipsoidal and dynamic,
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commonly referred to as “dynamical ellipticity”, and depends on the Earth’s rotation rate, internal density and the consequent gravitational and centrifugal force balance. It is characterized largely by rotational flattening of the poles, and has been approximated as (based on Equation 9 in Berger and Loutre (1994)): ED =
–
= 6.094
105
2
(2)
Equation (3) is based on Equation 5 in Williams (2000), =
–
( )
⁄
–
( )
⁄
(3)
which calculates the Earth–Moon angular momentum exchange for a range of Earth– Moon separations from today to the Roche limit (1.5 of Earth radii).
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Journal Pre-proof The above three equations determine the relationship among k, Earth–Moon di t ce,
d the
rth’ rot tio r te Using the obtained periodicity of the main
obliquity term (k + s3), we can calculate the value of k based on the equation (4), k=
– s3
(4)
and then input the k value back into Eqs. (1), (2) and (3) to figure out the Earth–Moon
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distance and length of day using Matlab (the codes were provided by Linda Hinnov).
0
e a e I m m⊙ ⨀
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7.2921150 × 10 rad/s 6.67408 × 10-11 m3/(kg s2 ) 0.003273787 1.4959802×1011 m
Earth rotation rate present-day Earth rotation rate gravitational constant Earth dynamical ellipticity semi- jor xi of rth’ orbit past lunar semi-major axis present-day lunar semi-major axis rth’ orbit l ecce tricity semi- jor xi of Moo ’ orbit ecce tricity of the Moo ’ orbit i cli tio of the Moo ’ orbit o the ecliptic mass of the Moon mass of the Sun obliquity angle of Earth symbol for the Sun symbol for the Moon
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-5
lP
G ED a
Name
3.833978 × 108 m 0.016708634 3.833978×108 m 0.05554553
na
0
Value
Jo ur
Symbol
ro
Table 1 Definition of the variables in equation 1, 2 and 3 (Hinnov, 2018).
5.156690º
7.34767309 × 1022 kg 1.98855 × 1030 kg 23.43928º
3.3. Time-series analysis The MS series was detrended by removing a long-term trend using Matlab script smooth.m. To detect periods in the stratigraphic domain, spectral analysis was performed using the multitaper method (MTM) (Thomson, 1982) with classical red noise modeling reported at the 85%, 90%, 95%, and 99% confidence levels with 7
Journal Pre-proof Matlab
scripts
available
(http://mason.gmu.edu/~lhinnov/cyclostratigraphytools.html).
at Sliding-window
spectral analysis using evolutive fast Fourier transform (FFT) (Kodama and Hinnov, 2015) was conducted to identify changes in cycle frequencies due to possible variations in the sedimentation accumulation rate (SAR). The link between the observed sedimentary cycles and orbital forcing was investigated according to the cycle length ratio method (Weedon, 2003). The interpreted 405-kyr eccentricity
of
cycles were extracted using a Gaussian bandpass filter and calibrated to a theoretical
ro
sinusoidal age model with a period of 405-kyr to construct a floating ATS.
-p
The average spectral misfit (ASM) provides a quantitative measurement for
re
fitting the observed sedimentary cycles to astronomical periods, given a range of plausible SARs (Meyers and Sageman, 2007). The result is expressed by an optimal
lP
SAR and corresponding null hypothesis significance level (Ho-SL), which represents
na
the probability of no orbital forcing. Lower Ho-SL values signify a better match between the stratigraphic and orbital frequencies. ASM was used to test the presence
Jo ur
of astronomical signals in the MS series with the “Astrochron” package developed by Meyers (2014). In this study, the 405-kyr calibrated astronomical cycles in the MS time series were compared to the orbital parameter periodicities of 125-kyr and 95-kyr for short-eccentricity, 32.6-kyr for main obliquity, and 20-kyr and 16.95-kyr for precession at 500 Ma (Waltham, 2015).
4. Results and discussions 4.1. Cyclostratigraphy of the Luoyixi section The measured MS values range from 0.008 × 10-3 to 0.233 × 10-3 SI (Fig. 3). Variations in MS show clear cyclic patterns with lithology with high values in
8
Journal Pre-proof argillaceous and ribbon limestone and low values in limestone (Fig. 3). The δ13C values ranged from -1.46‰ to 1.95‰ (Fig. 3). The δ13C values co-varied with lithology and showed large-scale cyclic patterns in spite of its low resolution. Here, MS and δ13C data were used to conduct cyclostratigraphic analyses. The predicted periods of the orbital parameters for the Cambrian at 500 Ma are with a ratio of 23.89:7.37:5.60:1.92:1.18:1. The MTM power spectrum and evolutionary FFT spectrogram of the Luoyixi MS series reveal significant
(Fig.
4).
The
ratio
of
the
sedimentary
cycles
ro
level
of
wavelengths of 25.53 m, 6.25 m, 1.90 m, 1.19–1.12 m above the 95% confidence is
approximately
-p
22.79:5.58:1.70:1.06:1 that is comparable with that predicted from theoretical orbital
re
parameters. If the sedimentary 25.53 m cycle is interpreted as the 405-kyr eccentricity cycle with an assumed SAR of 6.3 cm/kyr, the sedimentary cycles of 6.25 m, 1.90 m
lP
and 1.19–1.12 m may represent short-eccentricity, obliquity and precession,
na
respectively. Furthermore, the δ13C spectrum showed significant peaks with a wavelength of 25 m, similar to that observed in MS data. ASM outputs provide an
Jo ur
optimal SAR of 6.358 cm/kyr with a Ho–SL of 0.007% (Fig. 5), and hence supporting the assumption that 25.53 m cycles are related to long-eccentricity. The MTM power spectrum of the 405-kyr-calibrated MS series of the Luoyixi section showed significant peaks with periodicities of 405-kyr long-eccentricity (calibrated), short-eccentricity (100.9-kyr), main obliquity (30.7-kyr) and precession (19.2-kyr and 18.2-kyr) (Fig. 6). Additionally, the MTM power spectrum of the 405kyr-calibrated δ13C series also showed significant peak with predicted 405-kyr cycles (Fig. 6). These results are consistent with the predicted periodicities of the astronomical terms at 500 Ma (Waltham, 2015).
9
Journal Pre-proof The 405-kyr eccentricity cycle is suggested to be stable throughout geological time (Laskar et al., 2004), and it is typically used as a metronome for establishing an ATS for the Mesozoic and Paleozoic eras. A floating ATS was built by calibrating the filtered 405-kyr sedimentary cycles to an artificial 405-kyr sinusoidal curve, and setting the age of the section base (78 m) as 0 kyr (Fig. 7). Bandpass filtering results showed that the Luoyixi section contains approximately 3.5 long-eccentricity cycles that spans ~1400 kyr (Fig. 7). The duration of two integrated agnostoid trilobite zones
ro
of
(G. nathorsti and L. armata) are estimated as ~524 kyr and ~152 kyr, respectively.
-p
4.2. Orbitally forced sea-level change during the Cambrian greenhouse climate
re
It has been widely demonstrated that the Earth’s orbital parameters (eccentricity, obliquity and precession) could control global climate changes induced by variations
lP
in incident insolation (Hays et al., 1976). In a greenhouse climate, sea-level
na
fluctuations can be controlled by orbitally forced thermal oceanic water expansion (Schulz and Schäfer-Neth, 1997), changes in lake and groundwater storage (e.g.,
Jo ur
Hinnov and Park, 1999; Li et al., 2018a) and/or melting small-scale glaciers (Fang et al., 2016). The Drumian to Guzhangian time was a typical super-greenhouse climate with no ice sheets and thus, orbitally forced glacio-eustasy cannot explain meter-scale sea-level fluctuations. Schulz and Schäfer-Neth (1997) showed that a 2°C increase in deep-sea temperatures could induce 1.7 m of sea-level rise in tune with Milankovitch forcing. Thermo-eustasy and/or changes in lake and groundwater storage induced by insolation were likely the main mechanisms controlling sea-level fluctuations. The Huaqiao Formation was deposited in an outer slope-apron environment where argillaceous and ribbon limestone corresponding to high sea level and fossilrich limestone corresponding to low sea level (Zuo et al., 2008). The variations of the
10
Journal Pre-proof MS series were consistent with the alternation of argillaceous/ribbon limestone with limestone beds, which may have been controlled by sea-level fluctuations associated with precession cycles, and modulated by short-eccentricity cycles (Fig. 8). During higher insolation periods, the climate was warmer, and the sea level was higher with strengthened runoff and precipitation that increased the transfer of magnetic minerals to the ocean and led to higher MS values. On the opposite effect, the climate was colder during lower insolation periods, and the sea level was lower with weaker
of
runoff and precipitation that reduced the input of magnetic minerals to the ocean and
ro
resulted in lower MS values. Therefore, the minimum of precession signal cycles in
-p
the MS series may be caused by the colder climate and lower continental runoff
re
during lower insolation periods, and vise versa.
Previous research showed that the depositional sequences were divided into six
lP
orders ranging in duration from tens of millions of years to a few tens of thousands of
na
years. The 405-kyr eccentricity-related sea level cycles were suggested to correlate with the fourth-order depositional sequences, the 100-kyr eccentricity with the fifth-
Jo ur
order sequences and the fundamental precession with the sixth-order sequences (Boulila et al., 2011). Lithological alternations of limestone-argillaceous limestone and limestone-ribbon limestone couplets may represent the sixth-order sequences that match the interpreted precession cycles and six meter-scale bundles represent the fifth-order sequences that match the interpreted 100-kyr short-eccentricity cycles, reinforcing the solid relationship between orbital forcing and eustatic sequence hierarchy during the early Paleozoic Era. 4.3. Earth–Moon distance and length of day during the Drumian–Guzhangian Tidal dissipation in the Earth–Moon system results in lunar recession, deceleration in Earth’s rotation, and the increasing of precession and obliquity
11
Journal Pre-proof periodicities throughout the geological history. Recovery of the cyclostratigraphic record of Earth’s astronomical parameters has grown rapidly over the past several decades. At first, the focus was on the Cenozoic Era, but advanced to the Mesozoic Era, and most recently, progressed to the Paleozoic Era and the Precambrian (Hinnov, 2018). The Milankovitch records over
rth’ hi tory appear to indicate a dominant
41-kyr obliquity cycle at present day (Hinnov et al., 2012), 35-kyr obliquity at 245 Ma (Anisian; Li et al., 2018b), 34-kyr obliquity at 252–260 Ma (Wuchiapingian–
of
Changhsingian; Wu et al., 2013), and 31-kyr obliquity at 467 Ma (Dapingian–
ro
Darrivilian; Zhong et al., 2018), 28.2-kyr obliquity at 650 Ma (Cryogenian; Bao et al.,
-p
2018), showing a decreasing trend through the geological time. Thus, our result of
re
30.7-kyr obliquity cycles at 500 Ma (Drumian–Guzhangian) is consistent with this trend. With this information, we can infer the Earth–Moon distance and length of day
lP
(LOD) during the studied time period.
na
Here, we defined the 405-kyr calibration derived 30.7-kyr as the periodicity of the obliquity term k + s3 (Table 2 in Hinnov, 2018). Due to the bandwidth resolution
Jo ur
limit in the spectral analysis, each spectrum peak has an uncertainty of ± Rayleigh frequency. Thus, the obliquity periodicity should be 30.7 ± 0.7 kyr in the entire time span of ca. 1400 kyr. The basic frequency s3 = –18.85 arcsec/yr comes from Laskar et al. (2004) and is shown in Table 1 in Hinnov (2018). Based on equation 4 (Section 3.3) with the obtained obliquity O, we can give a k value (k = 61.06 ± 0.94 arcsec/yr) and then input the k value back into Eqs. (1), (2) and (3) (Section 3.3) to calculate the Earth–Moon distance and LOD. The results showed that, during the studied interval, the Earth–Moon distance was 370,180 ± 1,220 km (vs. 384,000 km for present day) and the LOD was 21.58 ± 0.18 h (vs. 23.93 h for present day). This Earth–Moon distance is a little bit longer than that from Berger and Loutre (1994) (366,075 km)
12
Journal Pre-proof and is comparable with the result from the Milankovitch Calculator (Waltham, 2015) at 500 Ma (371,600 ± 6,900 km). The LOD is similar to the result from Berger and Loutre (1994) (21.47 h) and is also comparable with the result from the Milankovitch Calculator (Waltham, 2015) (22.21 ± 0.91 h).
5. Conclusions
of
In this study, we conducted cyclostratigraphic analyses on high-resolution MS data from Cambrian strata of the Luoyixi section. Both the MTM spectrum in the
ro
depth domain and the ASM results revealed the presence of Milankovitch signals, and
-p
the MTM spectrum exhibited significant periodicities consistent with those expected
re
for the Cambrian (500 Ma). A 405-kyr-calibrated floating ATS of the Cambrian for
lP
the Luoyixi section was constructed, which gave a duration of ~1400 kyr for the whole succession. The durations of agnostoid trilobite biozones of G. nathorsti and L.
na
armata are estimated as ~524 kyr and ~152 kyr, respectively. Under greenhouse climate conditions, sedimentary characteristics of the Luoyixi section strata were
Jo ur
controlled by sea-level oscillations forced by precession and modulated by shorteccentricity. Using the obliquity period of 30.7 ± 0.7 kyr derived from the 405-kyr calibration, we calculated the Earth–Moon distance and LOD during the Cambrian (500 Ma); the results showed that the Earth–Moon distance was 370,180 ± 1,220 km and the LOD was 21.58 ± 0.18 h during the studied interval, which is comparable with previous research.
Acknowledgments We express our sincere appreciation to the editor in chief (Prof. Thomas Algeo), guest editors (Profs. James Ogg, Chunju Huang and David Kemp), Dr. Mingsong Li, 13
Journal Pre-proof and an anonymous reviewer for their careful comments and constructive suggestions that significantly improved the paper. The authors greatly appreciated the assistance from Junjie Xu, Chumeng Peng and Runjian Chu in the field. This study was supported by the National Natural Science Foundation of China (41925010, 41790451, and 41688103), and the fundamental research funds for the Central Universities (2652018072). This is a contribution to IGCP 652.
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Figure captions
Fig. 1. A. Paleogeographic reconstruction for the Cambrian after Peng et al. (2012). B. Map showing the location of the Luoyixi section on the Jiangnan slope belt, modified from Zuo et al. (2008). C. Basic geological map of part of northwestern Hunan
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Province and the location of the Luoyixi section (marked as LYX) from Peng et al.
ro
(2009).
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Fig. 2. The photograph of the Luoyixi section and the GSSP of the Guzhangian Stage,
re
coinciding with the FAD of Lejopyge laevigata, FADs and positions of the trilobite
lP
biozones in the Huaqiao Formation at the Luoyixi section. Strata underlying the
na
Guzhangian GSSP belong to the Drumian Stage.
Jo ur
Fig. 3. Integrated stratigraphy framework and data series of the Luoyixi section. The interpreted 405-kyr (red and green curves) long-eccentricity cycles (E)
were
extracted from MS and δ13C data using Gaussian bandpass filters with frequencies of 0.03917 ± 0.011 cycles/m and 0.04 ± 0.011 cycles/m to obtain the sedimentary cycles of ~25.53 m and ~25 m, respectively. The interpreted 100-kyr (blue curve) shorteccentricity cycles (e) was extracted from MS data using a passband of 0.16 ± 0.06 cycles/m to obtain the sedimentary cycles of ~6.25 m. The yellow and purple curves are 30-m “moving” trend of the MS and δ13C series.
Fig. 4. The 2π MTM power spectra of the MS (A) and δ13C (B) in the depth domain. Evolutionary FFT spectrogram of the MS series (C) and δ13C series (D) with 30 m 22
Journal Pre-proof sliding windows. The labels E, e, O and P represent long-eccentricity, shorteccentricity, obliquity and precession cycles, respectively. The solid red and dashed green, purple, blue and orange curves represent the median smoothed and fitted red noise spectra at 85%, 90%, 95% and 99% confidence levels, respectively.
Fig. 5. The average spectral misfit (ASM) results and null hypothesis test for the MS series of the Luoyixi section. (A) ASM result indicating an optimal fit at a
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sedimentation rate of 6.358 cm/kyr. (B) Null hypothesis significance levels for the
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ASM result, indicating an optimal sedimentation rate of 6.358 cm/kyr. The black
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dotted line in (B) indicates the critical null hypothesis significance level (H0-SL) of
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0.5%. (C) The number of Milankovitch terms that were available for calculation of ASM. (D) The fit of the frequencies above the 90% confidence level in Fig. 4A with
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the predicted orbital frequencies at 500 Ma (Waltham, 2015).
Fig. 6. The MS and δ13C series power spectra in the time domain with significant
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peaks labeled in kiloyears (kyr).
Fig. 7. A floating ATS for the Luoyixi section. Stages with relative boundary ages, lithology, trilobite biozones, extracted 405-kyr long-eccentricity cycles (red curve) using a passband of 0.002469 ± 0.001 cycles/kyr, 100-kyr short-eccentricity cycles (blue curve) using a passband of 0.0099 ± 0.003 cycles/kyr and 405-kyr-calibrated MS series (black curve) (Supplementary Table 1) are also shown.
Fig. 8. Sedimentary characteristics of the Luoyixi section. Curves in A represent the interpreted short-eccentricity (e) cycles (yellow) and precession (P) cycles (red).
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Journal Pre-proof Curves in B represent MS residuals (black), precession (green) using a passband of 0.86 ± 0.18 cycles/m and short-eccentricity (blue) using a passband of 0.16 ± 0.06
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cycles/m.
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Journal Pre-proof Highlights
• Reliable Milankovitch cycles were identified in the Middle Cambrian marine strata. • An ~1400 kyr ATS was constructed for the Drumian–Guzhangian transition. • The durations of the two complete agnostoid trilobite biozones were estimated.
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• Deep-time evolution of the Earth–Moon system was reconstructed.
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