Earth and Planetary Science Letters 483 (2018) 52–63
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Earth and Planetary Science Letters
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Cyclostratigraphic constraints on the duration of the Datangpo Formation and the onset age of the Nantuo (Marinoan) glaciation in South China ∗ Xiujuan Bao a, Shihong Zhang a, , Ganqing Jiang b, Huaichun Wu a, Haiyan Li a, Xinqiang Wang a, Zhengze An c, Tianshui Yang a a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China b Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USA c 103 Geological Team, Guizhou Bureau of Geology and Mineral Resources, Tongren, Guizhou 554300, China a r t i c l e i n f o a b s t r a c t
Article history: Constructing an accurate timeline is critical for reconstructing the Earth systems through critical Received 20 July 2017 transitions in climate, geochemistry, and life. Existing dates constrain synchronous initiation (ca. 717 Ma) Received in revised form 26 November 2017 and termination (ca. 660 Ma) of the Sturtian glaciation from multiple continents. The termination of Accepted 3 December 2017 the younger Marinoan glaciation is also well dated at ca. 635 Ma, but the onset of this glaciation is Available online xxxx only roughly constrained as ≤ ca. 654 Ma (South China) and ≥ ca. 639 Ma (Namibia). To test if the Editor: M. Frank Marinoan glaciation started close to ca. 654 Ma or ca. 639 Ma, we have conducted a cyclostratigraphic Keywords: study on the Cryogenian non-glacial Datangpo Formation that conformably overlies and underlies Sturtian Cryogenian and Marinoan glacial diamictites, respectively, in a deep-water basin section in South China. A total Snowball Earth of 28,765 magnetic susceptibility (MS) measurements from a drillcore of the 292-m-thick, muddy Milankovitch cycles siltstone- and shale-dominated Datangpo Formation are used for cyclostratigraphic analysis. The results Datangpo Formation reveal significant decameter- to meter-scale sedimentary cycles of 16–12 m, 3.6–3.0 m, 1.0–0.8 m, and Nantuo Formation 0.6–0.4 m. The ratios of these cycle wavelengths match well with those of the Milankovitch cycles South China calibrated for the Cryogenian Period. The established astrochronologic time scale suggests that the duration of the Datangpo Formation is about 9.8 million years. Together with the radiometric age of ca. 660 Ma for the termination of the Sturtian glaciation, the cyclostratigraphic data suggest that the Nantuo (Marinoan) glaciation in South China initiated at ca. 650 Ma, which is slightly younger than but consistent with the ca. 654 Ma U–Pb age from the top of the Datangpo Formation in shelf sections. This age, however, is significantly older than the ages obtained from Marinoan-age glacial diamictites in South China (ca. 636 Ma) and Namibia (ca. 639 Ma). Given that most of the shelf sections may have suffered from glacial erosion, obtaining the onset age of the Marinoan glaciation should focus on relatively complete, deep-water successions. © 2017 Published by Elsevier B.V.
1. Introduction the synchronous initiation of the Sturtian glaciation at ca. 717 Ma (Bowring et al., 2007; Macdonald et al., 2010; Lan et al., 2014; The Cryogenian Period (717–635 Ma) witnessed episodes of ex- Rooney et al., 2015), and its termination at ca. 660 Ma (Zhou treme cold known as the Sturtian and Marinoan glaciations, during et al., 2004; Kendall et al., 2006; Rooney et al., 2014, 2015) on which ice sheets may have extended to equatorial latitudes, form- multiple continents (Fig. 1b). The synchronous termination of the ing the “Snowball Earth” (Hoffman et al., 1998). The synchroneity Marinoan glaciation is also well dated by U–Pb ID-TIMS ages of ca. of the Cryogenian glaciations is a key prediction of the Snow- 635 Ma from multiple continents (Fig. 1b; Hoffmann et al., 2004; ball Earth hypothesis (Rooney et al., 2015). Recent U–Pb (both Condon et al., 2005; Calver et al., 2013; Rooney et al., 2015), but ID-TIMS and SIMS) and Re–Os ages provide faithful constraints on the onset of the Marinoan glaciation is only roughly constrained as ≤654 Ma (U–Pb SIMS age) in South China (Zhang et al., 2008b; Liu et al., 2015) and ≥639 Ma (U–Pb ID-TIMS age) in Namibia * Corresponding author. (Prave et al., 2016). Because the ca. 654 Ma age constraining E-mail address: [email protected] (S. Zhang). the onset of the Marinoan glaciation in South China is obtained https://doi.org/10.1016/j.epsl.2017.12.001 0012-821X/© 2017 Published by Elsevier B.V. X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63 53
Fig. 1. Summary of available age data from Cryogenian successions. (a) Paleocontinental reconstruction at ca. 635 Ma (Modified from Zhang et al., 2013, location changes for Tarim and Siberia are based on Wen et al., 2017). Circled numbers ➀–➅ mark the paleogeographic locations of dated Cryogenian successions. The location of the Zavkohan terrane of Mongolia is roughly restored to near Tarim (Bold et al., 2016). (b) Summary of ages and their relative stratigraphic position in Cryogenian successions marked in (a). Notice that the 643 ± 2.3 Ma age above the Sturtian glacial deposits in Australia has been questioned by Rooney et al. (2014). CA – Chang’an Formation; BX – Banxi Group; DST – Doushantuo Formation; Gp – Group; Fm – Formation; I.B. – Ice Brook Fm; LK – Lake; M.B. – Masirah Bay Fm; Mt. – Mount; NT – Nantuo Fm; Rav. – Ravensthroat Fm; XM – Xiangmeng Fm; References for ages: [1] Condon et al. (2005); [2] Zhang et al. (2008b); [3] Liu et al. (2015); [4] Zhou et al. (2004); [5] Lan et al. (2014); [6] Calver et al. (2013); [7] Kendall et al. (2009); [8] Fanning and Link (2008); [9] Hoffmann et al. (2004); [10] Prave et al. (2016); [11] Rooney et al. (2015); [12] Rooney et al. (2014); [13] Lund et al. (2003); [14] Macdonald et al. (2010); [15] Bowring et al. (2007). (c) Summary of the geological time scale for Cryogenian–middle Ediacaran based on available age data, covering three Neoproterozoic glaciations. The ca. 650 Ma age for the Marinoan glaciation is from this study. The age of the Gaskiers glaciation is from Pu et al. (2016). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.) 54 X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63
Fig. 2. (a) Generalized late Neoproterozoic paleogeographic reconstruction of South China. (b) Depositional model showing the paleogeographic position of the studied drillcore (modified from Jiang et al., 2011). References for ages: 635.2 ± 0.6 Ma (Condon et al., 2005); 654.2 ± 2.7 Ma (Liu et al., 2015); 654.5 ± 3.8 Ma and 636.3 ± 4.9 Ma (Zhang et al., 2008b); 662.9 ± 4.3 Ma (Zhou et al., 2004); 715.9 ± 2.8 Ma (Lan et al., 2014). (c) Stratigraphy of the Datangpo and Nantuo formations from drillcore ZK1909, DST – Doushantuo Fm; TSA – Tiesi’ao Fm. from shelf sections (Figs. 1 and 2) where glacial diamictites of Minguez and Kodama, 2017). Cyclostratigraphic analysis of the the Nantuo Formation (Fm) disconformably overlie the non-glacial Datangpo Fm reveals that the ratios of sedimentary cycles have Datangpo Fm, apotential concern is whether the upper Datangpo a similar hierarchy as that of orbital parameters predicted for the Fm was eroded and the Nantuo glaciation started much later than Cryogenian period, which is one of the few examples that demon- ca. 654 Ma (Zhang et al., 2008b)or even close to ca. 639 Ma. strate the response of Earth’s climate to orbital forcing in the Answering this question requires age constraints from deep-water distant past. In combination with available radiometric ages, the sections where a relatively conformable transition from non-glacial established astrochronological time scale provides constraints on to glacial deposits is preserved. the duration of the Cryogenian non-glacial Datangpo Fm and the In this paper, we present a cyclostratigraphic study of the Cryo- initiation age of the Nantuo (Marinoan) glaciation. genian non-glacial Datangpo Fm from a drillcore (ZK1909) of a basinal section in Songtao County, Guizhou Province, South China 2. Geological setting and the drillcore records (Fig. 2a), using centimeter-scale magnetic susceptibility (MS) mea- surements. The Datangpo Fm in this section preserves a complete 2.1. Regional stratigraphy non-glacial record and is dominated by thinly laminated muddy siltstone and shale amenable for cyclostratigraphic analysis, which The tectonic setting of the late Neoproterozoic Nanhua Basin has proven to be an efficient tool for calibrating the time series and has been interpreted as a back-arc basin (Zhou et al., 2002; duration of stratigraphic intervals and geological events from Ceno- Zhao et al., 2011)or a rift to passive marginal basin (Wang and zoic (e.g., Hinnov, 2013), Mesozoic (e.g., Wu et al., 2014), Paleozoic Li, 2003; Jiang et al., 2003, 2011). However, the rift to passive (e.g. Wu et al., 2013; Fang et al., 2016), to Precambrian time (e.g. marginal basin better explains the consistent southeast-deepening S.C. Zhang et al., 2015; Minguez et al., 2015; Fairchild et al., 2016; stratigraphic pattern in the Yangtze Block (Fig. 2a) from late X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63 55
Neoproterozoic until the late Ordovician (Zhang et al., 2008a; clude glaciogenic rocks of the uppermost Tiesi’ao Fm (equivalent Jiang et al., 2011). of the Chang’an Fm), the non-glacial Datangpo Fm, the Nantuo Neoproterozoic strata in the Nanhua Basin consist of three Fm, and Ediacaran–early Cambrian strata (Fig. 2c). The Cryoge- major units: the pre-Cryogenian siliciclastic units in the lower nian part of the drillcore is lithostratigraphically divided into 10 part, the Cryogenian glacial and non-glacial deposits in the mid- units (Fig. 2c). In ascending order, Unit 1 is the uppermost part dle, and the Ediacaran carbonate–siliciclastic rocks in the upper of the Tiesi’ao Fm, composed of siltstone and sandstone with part (Jiang et al., 2003). The Cryogenian strata are divided into subordinate pebble-sized clasts unevenly distributed in a silty- the lower glacial diamictite (the Chang’an/Jiangkou formations and sandy matrix. The studied Datangpo Fm in the drillcore con- their correlative equivalents), the middle non-glacial shale and silt- sists of Units 2–4. Unit 2 (1415–1404.92 m) consists of a 10-m- stone unit (the Datangpo Fm and its equivalents), and the upper thick carbonaceous manganese-bearing organic-rich black shale. glacial diamictite (the Nantuo Fm) (Jiang et al., 2003, 2011). The Unit 3 (1404.92–1373.02 m) is a 32-m-thick interval composed Chang’an/Jiangkou glacial deposits are commonly correlated with of organic-rich black shale (Fig. 2c), followed by a 260-m-thick the Sturtian glaciation, while the Nantuo Fm is correlated with the interval (Unit 4) composed mainly of laminated muddy siltstone Marinoan glaciation. with thin interbeds of shale and fine-grained sandstone (Fig. 2c), The Cryogenian strata are thin or partially missing in the shelf which is the major part of the Datangpo Fm (1373.02–1113.64 m). and are thick in the basin (Fig. 2b; Jiang et al., 2003, 2011; Zhang Compositionally, laminations of the Datangpo Fm are expressed by et al., 2011). The lower glacial deposits in the shelf, named as the interbedding of (quartz, albite, mica)-dominated laminae and clay- Dongshanfeng Fm in the west Hunan Province and Gucheng Fm dominated laminae. Small amounts (0–7%) of pyrite, calcite and in the Hubei province, vary in thickness from several centimeters dolomite are present. The overlying Nantuo Fm consists mainly to 10 m and have an erosional contact with the underlying silici- of diamictite, siltstone and mudstone with dropstones, laminated clastic strata (the Liantuo Fm or upper Banxi Group; Zhang et al., siltstone and mudstone, and fine-grained sandstone (Units 5–10, 2008a). The lower glacial unit in the basin, named as the Jiangkou Fig. 2c). Fm in the central and southern Hunan Province, Tiesi’ao Fm in A continuous transition from the Datangpo to the overlying the southeast Guizhou Province, and Chang’an Fm in the north- Nantuo Fm is recorded by gradual changes in lithology and MS val- ern Guangxi Province, are in general >2000 m thick (Fig. 2b; Jiang ues (Fig. 3). The lithology in the uppermost Datangpo Fm is charac- et al., 2011) and in some places have a conformable contact with terized by laminated siltstone and shale (Fig. 3i and Fig. 4a), which the underlying Banxi Group (Zhang et al., 2011). The Chang’an, changes to the diamictite of the Nantuo Fm (Fig. 3h). The 3-m- Jiangkou, and Tiesi’ao formations mainly consist of massive and thick transitional interval (Fig. 3d) consists of laminated siltstone stratified diamictites separated by siltstone and sandstone beds, with sparsely presented, pebble-sized lonestones (Fig. 3e–g), which with banded iron formation (BIF) strata locally present in basinal gradually changes to massive diamictite (Fig. 3h). There is no ob- sections (Zhang et al., 2011). vious unconformity or major erosional surface observed across the The non-glacial deposits vary in thickness from <20 m in the transition. We interpret that the transitional interval may record shelf (named as the Xiangmeng Fm in the Hubei and the north- the growth of ice sheets and sea-level fall, during which small ice west Hunan provinces) to >150 m in the basin (named as the rafts may have brought pebble-sized clasts into the deep-water en- Datangpo Fm in the Guizhou and the northern Guangxi provinces) vironments (Fig. 3a–c). (Fig. 2b; Jiang et al., 2011). In most sections, the Xiangmeng and Datangpo formations are marked by carbonaceous manganese- 3. Magnetic measurements and data processing bearing shale at the base, followed by fine-grained muddy sand- stone, muddy siltstone and mudstone (Zhou et al., 2004; Zhang Magnetic susceptibility (MS) of the drillcore was measured at et al., 2011). In some basinal sections, the base of the Xiang- an average resolution of 1.0 cm using an SM-30 magnetic suscep- meng/Datangpo formations is characterized by 0.5–5 m thick man- tibility meter. The SM-30 contains an oscillator with a 5 × 5cm ganese (Mn) carbonates that have negative δ13C values down to pickup coil, the frequency change of which is proportional to the − −10h (Yu et al., 2017). This 13C-depleted Mn-carbonate interval amount of MS of the rock. Sensitivity of SM-30 is 10 7 SI and − is considered to represent the cap carbonate of the Sturtian glacia- the measured MS values of the core range from 29 × 10 6 SI − − tion (Yu et al., 2017). to 286 × 10 6 SI, with an average value of 150 × 10 6 SI for The Nantuo Fm has a broader distribution into the shallow- the Datangpo Fm. Some separated samples were reshaped for water regions than its underlying units (Jiang et al., 2003). The anisotropy of magnetic susceptibility (AMS) measurements in order thickness of the Nantuo Fm varies from 60–100 m in the shelf to analyse the magnetic fabric of the rocks. The AMS was measured to >2000 m toward the basin (Jiang et al., 2011). The Nantuo at the Paleomagnetism and Environmental Magnetism Laboratory Fm consists of massive to stratified diamictites with sandstone, at China University of Geosciences, Beijing (CUGB) using a KLY-4S siltstone and shale interbeds (Zhang et al., 2011). The contact be- kappa-bridge. tween the Nantuo Fm and its underlying Datangpo/Xiangmeng Fm MS variations in sedimentary rocks have proven to be an effec- on the shelf is lithologically sharp and erosional (Zhang et al., tive paleoclimate proxy record for astronomical cycle studies (e.g., 2008b, 2011), but in basinal sections, a gradual transition from Heller et al., 1991; Wu et al., 2013; Kodama and Hinnov, 2015). MS the Datangpo to the Nantuo Fm is observed, marked by the pres- is the acquired magnetization of a material in a per unit magnetic ence and gradual increase of pebble-sized clasts in silty shales of field, representing a substance’s capacity to become magnetized. In the Datangpo Fm to the first massive diamictite of the Nantuo Fm siliciclastic sedimentary rocks, MS values are related to weathering (Zhang et al., 2011; F. Zhang et al., 2015). intensity, sea-level changes and lithologies of source areas (Zhang et al., 2000; Wu et al., 2013; Kodama and Hinnov, 2015). Although 2.2. Drillcore records post-depositional processes may affect the absolute MS values, many studies have demonstrated that the temporal MS trends are The analyzed drillcore (ZK-1909) has a diameter of 4.9 cm and well-preserved in laminated, fine-grained sedimentary rocks (e.g., is collected from the Daotuo manganese ore district in Songtao Heller et al., 1991; Wu et al., 2013; Kodama and Hinnov, 2015; County, northeastern Guizhou Province, which is paleogeograph- Minguez and Kodama, 2017). ically located in the deep-water (lower slope to basin) setting Several lines of evidence indicate that the MS values of the of the Nanhua Basin (Fig. 2a, b). Recovered drillcore strata in- Datangpo Fm in ZK1909 are not significantly modified by diage- 56 X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63
Fig. 3. Transitional changes in lithology and MS values from the topmost Datangpo Formation to the lower Nantuo Formation. (a–c) A depositional model for the gradual transition from the laminated Datangpo muddy siltstones (a) to muddy siltstones with sparsely distributed, pebble-sized muddy siltstones (b) and to massive diamictite of the Nantuo Formation (c). The 3-m-thick transitional interval is interpreted as recording cooling and sea-level fall, during which small ice rafts brought clasts to the deep-water environments (b). (d) The MS series across the Datangpo-Nantuo transition (1125–1105 m). The gradual decrease of MS values in the 3-m-thick transitional interval is consistent with cooling and decrease of weathering input of clay minerals. (e–g) Petrographic micrographs of the transitional interval showing pebble-sized clasts in fine-grained matrix. (h) Petrographic micrographs of massive diamictite at the basal Nantuo Formation. (i–k) Polished drillcore segments (scale: 1cm) showing laminated silty shales of the topmost Datangpo Formation (i) and the transitional interval (j and k). The positions of e, f, g, h, i, j, and k are marked in (d). netic or metamorphic alteration. First, the mudstone and siltstone sedimentary fabric of the rocks (Fig. S1). Third, XRD analysis from of the Datangpo Fm show thin laminations that do not have ob- 30 specimens with variable mass MS values demonstrate that the vious post-depositional disruption. Fine-grained mineral particles MS values are positively correlated with the clay content but neg- such as quartz, albite and mica show their original angular shapes atively correlated with the contents of quartz, albite and mica without recrystallization. No metamorphic minerals indicative of (Fig. 4; Fig. S2; Table S1). This indicates that the MS values are high temperature and pressure (e.g., garnet and amphibole) are likely controlled by variations in detrital sediment supply to the identified through SEM/EDS and XRD analyses. Second, measure- basin, which is in turn, may have been controlled by climate cycles ments and the AMS analyses from 113 specimens show a typical during which warm and wet periods had more intense weather- X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63 57
Fig. 4. (a) Micrographs showing laminated muddy siltstone of the Datangpo Formation near 1123.4 m, the stratigraphic position marked in Fig. 3d. (b) The relationship between mass magnetic susceptibility (MMS) and clay mineral contents (red dots) and the total content of quartz, albite, mica (green dots). The measured data are listed in Table S1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) ing and more abundant clay input, while cold and dry periods had of the major cycle bands of 3.6–3.0 m: 1.0–0.8 m: 0.6–0.4 m is less clay and more silicate mineral input. For these reasons, we 6–5:1.7–1.3:1–0.7, matching well with the ratios of the Cryogenian consider that the MS data from the Datangpo Fm are suitable for astronomical cycles of 6.4–4.7:1.5–1.4:1–0.7 (Table 2). This indi- cyclostratigraphic analysis. cates that the 3.6–3.0 m, 1.0–0.8 m and 0.6–0.4 m wavelength The transition between the Datangpo and the Nantuo Fm is bands are most likely short eccentricity, obliquity and precession characterized by gradual decrease in MS values. The average MS cycles, respectively. A peak with 12 m wavelength cycles above − value prior to the transitional interval is about 175 ×10 6 SI, while the 85% confidence level together with 16 m wavelength cycles − the average MS value after the transition period is 135 × 10 6 SI may represent the 405 kyr long eccentricity band cycle if the (Fig. 3d). The gradual (instead of sharp) change in MS values 3.6–3.0 m wavelength is taken as the ∼100 kyr short eccentric- (Fig. 3d) across the transitional interval is consistent with cooling ity signal (Fig. 6a). and decrease of weathering input of clay minerals. Therefore, we For the shale-dominated portion of the Datangpo Fm (Unit 3), are confident that the transition between the Datangpo and Nan- MTM power spectral analysis reveals cycles with wavelengths of tuo Fm does not contain a significant time gap. 3.0 m, 1.6 m, 1.2 m, 0.9 m and 0.8 m above the 90% confidence A total of 28,765 MS data points were obtained at an average level, and a significant cycle period of 0.4 m above the 85% con- resolution of 1.0 cm from the 292-m-thick Datangpo Fm (Units fidence level (Fig. 6b). The ratios of the major wavelength bands 3–4; Fig. 5). The MS series are linearly interpolated every 5 cm of 3.0 m, 0.9–0.8 m and 0.5–0.4 m have ratios of 5:1.5–1.3:0.8–0.7, using Matlab. The interpolated data were detrended by subtracting which is consistent with the ratios of short eccentricity, obliquity smooth curves (Fig. 5) using Robust Locally Weighted Regression and precession cycles of 6.4–4.7:1.5–1.4:1–0.7 (Table 2). in the software KaleidaGraphTM (Cleveland, 1979). The amount of Wavelet analyses (Grinsted et al., 2004) further identify the smoothing of the Datangpo Fm is 20% ( f = 0.2). interpreted orbital signals in the dataset. Wavelet scalograms of Unit 4 and Unit 3 reveal relatively continuous cycles with periods 4. Cyclostratigraphy of comparable spectral bands of the interpreted Milankovitch cy- cles (Fig. 6c–d). 4.1. Cyclostratigraphic analysis in the stratigraphic domain 4.2. Astronomical calibration and spectral analysis in the time domain Power spectral analysis is implemented with the multitaper method (MTM) in Matlab with red noise modeling reported at 85%, The direct cycle counting method was used to establish the 90%, 95% and 99% confidence levels of spectral peak significance cyclostratigraphic framework of the Datangpo Fm in ZK1909. Gaus- (Mann and Lees, 1996; Fig. 6a–b). Frequency ratios are evaluated sian bandpass filters were designed to isolate the long eccentricity to compare the observed spectral frequencies with orbital frequen- and short eccentricity cycles of Unit 4 and the short eccentric- cies estimated for the Cryogenian based on available radiometric ity cycles of Unit 3 are based on our interpretation of procession age constraints. This is the commonly used method to test whether cycle and cycle hierarchy (Fig. 7). The cycles are numbered accord- the observed cycles in ancient strata were formed by astronomical ing to their positions relative to the top of the Datangpo Fm in forcing (Hinnov, 2000; Weedon, 2003). ZK1909. The long eccentricity cycles are labeled as E0, E1, E2, etc., MTM power spectral analysis of the detrended MS series from while the short eccentricity cycles are labeled as e0, e1, e2, etc. the laminated muddy siltstone-dominated portion of the Datangpo (Fig. 7). Fm (Unit 4) reveals a hierarchy of cycles with 3.6 m, 1.8 m, The 405-kyr long eccentricity cycle is stable and believed to 0.9 m, 0.5 m and 0.4 m wavelengths above the 99% confidence have remained nearly constant through geological time (Laskar et level, and 16 m, 3m, ∼1.2 m, 1.0 m, 0.8 m, 0.59 m and 0.42 m al., 2011). The MS series of Unit 4 are astronomically calibrated via wavelengths above the 95% confidence level (Fig. 6a). The ratios the interpreted 405 kyr orbital eccentricity cycles while the MS 58 X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63
Fig. 5. Magnetic susceptibility (MS) profile of the Datangpo Formation from drillcore ZK1909. (a) MS series of Unit 4 of the Datangpo Formation with the smoothing curve (blue, upper), and the MS series after detrended (lower). (b) MS series of Unit 3 of the Datangpo Formation with the smoothing curve (blue, upper), and the MS series after detrended (lower). The raw MS data are available in supplementary file Table S2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) series of Unit 3 are astronomically calibrated via the interpreted The 405-kyr eccentricity period, resulted from interactions be- precession cycles. Because there is no obvious expression of the tween the orbital perihelia of Venus and Jupiter, is relatively stable long eccentricity cycle in Unit 3, the precession cycle is the best and believed to have remained nearly constant through time, rep- choice for astronomical calibration. resenting the metronome of astronomical forcing in the distant The power spectrum of the tuned MS series of Unit 4 shows past (Laskar et al., 2011). Previous studies have successfully iden- significant peaks with periods of eccentricity (405 kyr, 118 kyr, tified the long eccentricity period (405 kyr) from Cenozoic to late 109 kyr), obliquity (36.4 kyr, 34.6 kyr, 30.3 kyr, 28.2 kyr) and pre- Ediacaran strata (Hinnov, 2013; Minguez and Kodama, 2017). The cession (21.8 kyr, 20.5 kyr, 18.4 kyr, 16.0 kyr) above the 95–99% signal of long eccentricity was also identified during the Meso- confidence level (Fig. 8). The power spectrum of the precession proterozoic (e.g., S.C. Zhang et al., 2015). Therefore, the signal of (17 kyr)-tuned MS series of Unit 3 reveals significant peaks of the 405-kyr cycles should be significant enough to be identified in obliquity (37.3 kyr, 35.1 kyr, 29.6 kyr) and precession (20.3 kyr, Cryogenian strata. 17–16 kyr) periodicities above the 95–99% confidence levels The short eccentricity period includes multiple components of (Fig. 8), which also show significant short eccentricity (116 kyr) cy- 95 kyr and 99 kyr (e-I), 124 kyr and 130 kyr (e-II) (Laskar et al., cles. Wavelet analyses of the tuned MS series of Unit 4 and Unit 3 2011). These components commonly overlap with each other and show comparable spectral bands (Fig. 8). The results of spectral express as a ∼100-kyr period that commonly forms a strong signal and wavelet analyses are consistent with those of the predicted in sedimentary successions of Cenozoic to Paleozoic age (Hinnov, Cryogenian astronomical parameters and further support our cy- 2013). Recent studies indicate that the ∼100-kyr signal is also clostratigraphic interpretation. well-preserved in Ediacaran (Minguez et al., 2015; Minguez and Kodama, 2017) and Mesoproterozoic strata (S.C. Zhang et al., 2015). 5. Orbital periods of the Cryogenian non-glacial interlude Consequently, the signal should also be strong in the Cryogenian strata. Orbital parameters change through time in Earth history. In The main obliquity period is 41 kyr, with a weak component the Cenozoic, the major wavelengths of Milankovitch cycle periods of ∼54 kyr, and the precession period has multiple components are 405 kyr for long eccentricity, ∼100 kyr for short eccentricity, of 23.7 kyr, 22.4 kyr, 19.1 kyr and 18.9 kyr at the present day 41 kyr for obliquity, and ∼20 kyr for precession. (Laskar et al., 2011; Table 1). Both obliquity and precession periods X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63 59
Fig. 6. (a–b) 2π MTM power spectra of the MS series of (a) Unit 4 and (b) Unit 3 of the Datangpo Formation. The red curve and the orange, green, yellow and pink dashed curves represent the first-order autoregressive (AR(1)) red noise spectrum, and 85%, 90%, 95%, and 99% confidence levels. (c–d) Morlet wavelet scalograms of the MS series of (c) Unit 4 and (d) Unit 3. The shaded contours in wavelet scalograms are normalized linear variance, with blue representing low variance and red representing high variance. Regions below the curves indicate the cone of influence where edge effects become significant. The dashed white bands labeled with E, e, O and P indicate long eccentricity, short eccentricity, obliquity and precession cycles, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 60 X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63
Fig. 7. Smoothed MS series of the Datangpo Formation of the ZK1909, showing the interpreted ∼405 kyr (E0–E20) and ∼100 kyr (e0–e92) cycles. The long eccentricity and short eccentricity cycles of Unit 4 are extracted with Gauss bandpass filter outputs with passbands of 0.079 ± 0.02 cycles/m and 0.31 ± 0.08 cycles/m, respectively. The interpreted short eccentricity cycle of Unit 3 is extracted with passbands of 0.34 ± 0.08 cycles/m. Labels: E – long eccentricity cycle; e – short eccentricity cycle; P – precession cycle; green arrow – obliquity cycle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
change with the evolution of the Earth–Moon system (Berger and 6. Duration of the Datangpo Formation Loutre, 1994; Waltham, 2015). Theoretical values of obliquity and precession periods have been estimated throughout the geologi- In the MS series of Unit 4, most long eccentricity cycles contain cal history (Berger and Loutre, 1994), but additional observational 4 short eccentricity cycles, which may represent a predominant ex- constraints are needed for pre-Cenozoic records. The Marinoan- pression of 95/99 kyr (e-II) cycles. E1, E2 and E3 each contains 3 aged Elatina tidal rhythmites in South Australia provide one of short eccentricity cycles, which are probably 124/130 kyr (e-I) cy- the best opportunities to estimate the Earth–Moon distance at a cles. E13–14 and E19–20 consist of 7 short eccentricity cycles and day-length of 21.9 h (Williams, 2000; Waltham, 2015). The preces- they contain both e-I and e-II cycles. The precession period during sion constant (k) of the Marinoan-aged period is thus estimated the Cryogenian is 15–20 kyr (Williams, 2000; Table 1). There are two types of short eccentricity cycles identified from the Datangpo as 62.03–64.60 /yr (Williams, 2000; Table 1) and the major peri- Fm, based on the number of precession cycles they contain (Fig. 7). ods of obliquity and precession were estimated as 30–28 kyr and One type of short eccentricity cycles (type e-I) contains 7–9 pre- 20–15 kyr, respectively. These estimations are consistent with the cession cycles, while the other type (type e-II) contains 5–6 pre- theoretical prediction by Berger and Loutre (1994) for this time cession cycles. period (Table 1). Unit 4 of the Datangpo Fm consists of 21 long eccentricity cy- Taking into account the theoretic predictions and available tests cles, 81 short eccentricity cycles, and 506 precession cycles. If each on pre-Mesozoic orbital parameters, the Milankovitch cycles iden- long eccentricity cycle represents 405 kyr, the duration of Unit 4 tified from the Cryogenian Datangpo Fm are interpreted as long can be calculated as 8505 kyr. An average period of 16.8 kyr of eccentricity (∼405 kyr), short eccentricity (130–95 kyr), obliquity the precession cycle can be calculated from the long eccentricity, (30–28 kyr) and precession (20–15 kyr) cycles, despite a poten- which is consistent with the theoretical prediction of precession tial impact of chaotic drift. The ratios of these main periods are for the Cryogenian Period (15–20 kyr; Table 1; Williams, 2000). ∼20:6.4–4.7:1.5–1.4:1–0.7 (Table 2). Unit 3 contains 10.8 short eccentricity cycles and 77 precession X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63 61
Fig. 8. 2π MTM power spectra and Morlet wavelet scalograms of the astronomically tuned MS series of Unit 4 (upper, 405 kyr tuned) and Unit 3 (lower, precession tuned) of the Datangpo Formation. Frequency peaks are labeled in kyr. See legends and notes in Fig. 6.
Table 1 of 2.8 cm/kyr from the Cryogenian non-glacial strata in NE Sval- Computed value of K, obliquity and precession periods for present day and bard (Fairchild et al., 2016). 635–650 Ma. Present day ∼650 Ma 635–650 Ma 7. Onset age of the Nantuo (Marinoan) glaciation in South China (Berger and Loutre, 1994) (Williams, 2000) K( /yr) 50.47 67.15 62.03–64.60 The Datangpo Fm in the measured drillcore provides a continu- O-I (kyr) 53.73 31.76 33.88–36.32 ous sedimentary record between the Chang’an (Sturtian) and Nan- O-II (kyr) 40.99 26.83 28.33–30.01 tuo (Marinoan) glaciations. Cyclostratigraphic analysis (Figs. 5–8) P-I (kyr) 23.68 18.15 18.82–19.55 ∼ P-II (kyr) 22.37 17.37 17.99–18.65 indicates that the duration of the Datangpo Fm is 9.8 myr, P-III (kyr) 19.10 15.33 15.81–16.32 which represents the non-glacial time between the two Cryoge- P-IV (kyr) 18.95 15.24 15.71–16.21 nian glaciations. Increasing evidence supports a globally synchronous termina- tion of the Sturtian glaciation at ∼660 Ma (Fig. 1b–c; Zhou et al., 2004; Fanning and Link, 2008; Kendall et al., 2006; Rooney et al., cycles. Assigning an age of 16.8 kyr for each precession cycle, 2014, 2015). In South China, a U–Pb TIMS age of 662.9 ± 4.3 Ma consistent with Unit 4, the duration of Unit 3 is calculated as was obtained from a tuffaceous bed within the cap carbonate of 1,293.6 kyr. In total, the duration of the Datangpo Fm is estimated the Tiesi’ao Fm from the Zhailanggou section (Zhou et al., 2004), as 9,798.6 kyr or ∼9.8 million years. Using the number of type e1 ∼5km away from the site of the drillcore. In Laurentia, sam- (95–99 kyr) and e2 (124–130 kyr) short eccentricity cycles (Laskar ples from the organic-rich cap carbonate of the Rapitan Group et al., 2011), the calculated duration is nearly identical. in Canada yielded a Re–Os age of 662.4 ± 3.9 Ma (Rooney et al., The calculated duration of the Datangpo Fm is consistent with 2014). In Australia, a U–Pb SIMS age of 659.7 ± 5.3 Ma was ob- the available ages from the base and top of this unit and the de- tained from Sturtian non-glacial volcanoclastic rocks (Fanning and positional rates of pelagic sediments. The basal Datangpo Fm is Link, 2008), but analytical details of this age has never been ± dated at 662.9 4.3 Ma (U–Pb TIMS; Zhou et al., 2004) and the fully published. Re–Os ages of 657.2 ± 5.4 Ma and 643 ± 2.4 Ma ± top of the Datangpo Fm is dated at 654.5 3.8 Ma (U–Pb SIMS; were also obtained in central and southern Australia from the ± Zhang et al., 2008b) and 654.2 2.7 Ma (U–Pb SIMS; Liu et al., black shale overlying the Sturtian glacial diamictite (Kendall et al., 2015). Given that the top ages of the Datangpo Fm are from strata 2006). These Re–Os ages, however, were challenged due to ana- below the Nantuo/Datangpo contact that is an erosional unconfor- lytical uncertainties and problems related to stratigraphic correla- mity, these ages give the duration of 8–17 myr for the Datangpo tion (Rooney et al., 2014). Considering the stratigraphic position Fm. The 9.8-myr duration from the cyclostratigraphic analysis is of available ages, a recalculated age of 659.7 ± 5.3 Ma is consid- consistent with the radiometric ages and may represent a more ered as the closest to the termination age of the Sturtian glaciation precise estimation because the basinal section presumably pre- in Australia (Hoffman and Li, 2009). In addition, samples from serves a more complete sedimentary record of the Datangpo Fm. the Sturtian cap carbonate in Mongolia yielded a Re–Os age of With the thickness of 292 m, the average sedimentary rate of the 659.0 ± 4.5 Ma (Rooney et al., 2015). In summary, the end of the Datangpo Fm is 3.0 cm/kyr (Fig. S3), which is within the range of Sturtian glaciation in South China, Canada, Mongolia and Australia pelagic sedimentation rate of 0.5–25 cm/kyr (Fairchild et al., 2016) seems to be synchronous at ∼660 Ma (within errors of dating and is consistent with the deep-shelf mudrock accumulation rate methods). 62 X. Bao et al. / Earth and Planetary Science Letters 483 (2018) 52–63
Table 2 Periods and ratios of the main Milankovitch cycles of present day and ∼650 Ma and the interpreted main Milankovitch cycles in the Datangpo Formation. Term Period Period (∼650 Ma) Period (Datangpo Formation) (m) (kyr) (kyr) Unit 4 Unit 3 (Williams, 2000) E ∼405 ∼405 12 a e 130–95 130–95 3.6–3.0 3.0 O-II 41 30–28 1.0–0.8 0.9–0.8 P 24–18 20–15 0.6–0.4 0.5–0.4 Ratios 20:6.4–4.7:2:1.2–0.9 20:6.4–4.7:1.5–1.4:1–0.7 20:6–5:1.7–1.3:1–0.7 a:5:1.5–1.3:0.8–0.7 a Unit 3 is too short to identify long eccentricity cycles.
Accepting the termination age of the Sturtian glaciation at ca. Appendix A. Supplementary material 660 Ma and our estimated non-glacial duration of 9.8 myr, the on- set age of the Nantuo (Marinoan) glaciation in South China would Supplementary material related to this article can be found on- be 650.2 Ma, or roughly, ca. 650 Ma. This astrochronological age line at https://doi.org/10.1016/j.epsl.2017.12.001. is consistent with the existing U–Pb ages in South China, includ- ing the U–Pb SIMS age of 654.5 ± 3.8 Ma from a tuff bed at the References top of the Xiangmeng Fm in Guzhang, Hunan Province (Zhang et al., 2008b) and the U–Pb SIMS age of 654.2 ± 2.7 Ma from a tuff Berger, A., Loutre, M.F., 1994. Astronomical forcing through geological time. In: De Boer, P.L., Smith, D.G. (Eds.), Orbital Forcing and Cyclic Sequences, vol. 19. Black- bed in the middle Xiangmeng Fm in Gucheng, Hubei Province (Liu well Scientific Publications, Oxford, pp. 15–24. et al., 2015). The two dated tuff beds are possibly synchronous; Bold, U., Crowley, J.L., Smith, E.F., Sambuu, O., Macdonald, F.A., 2016. Neoproterozoic their different relative stratigraphic positions (top and middle) in to early Paleozoic tectonic evolution of the Zavkhan terrane of Mongolia: impli- the Xiangmeng Fm likely record differential erosion in shelf sec- cations for continental growth in the Central Asian orogenic belt. Lithosphere 8 (6), 729–750. tions during the Nantuo glaciation (Liu et al., 2015). The onset age Bowring, S.A., Grotzinger, J.P., Condon, D.J., Ramezani, J., Newall, M.J., Allen, P.A., of ca. 650 Ma for the Marinoan glaciation is consistent with but 2007. Geochronologic constraints on the chronostratigraphic framework of the significantly older than the U–Pb SIMS age of 636.3 ± 4.9 Ma from Neoproterozoic Huqf Supergroup, Sultanate of Oman. Am. J. Sci. 307, 1097–1145. the basal Nantuo Formation (Zhang et al., 2008b) and the U–Pb Calver, C.R., Crowley, J.L., Wingate, M.T.D., Evans, D.A.D., Raub, T.D., Schmitz, ID-TIMS age of 639.3 ± 0.26 Ma from the Marinoan-age glacial di- M.D., 2013. Globally synchronous Marinoan deglaciation indicated by U– Pb geochronology of the Cottons Breccia, Tasmania, Australia. Geology 41, amictite of the Ghaub Fm in Namibia (Fig. 1b; Prave et al., 2016). 1127–1130. Thus, the astrochronological data suggest that in most shelf sec- Cleveland, W.S., 1979. Robust locally weighted regression and smoothing scatter- tions Cryogenian non-glacial strata may have suffered from glacial plots. J. Am. Stat. Assoc. 74, 829–836. erosion and obtaining the onset age of the Marinoan glaciation Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., Jin, Y., 2005. U–Pb ages from the Neoproterozoic Doushantuo Formation, China. Science 308, 95–98. should focus on relatively complete, deep-water successions. 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