Cyclostratigraphy and astrochronology: Case studies from China.

A high-precision geologic time scale is the essential key for understanding the Earth's evolutionary history and geologic processes. Astronomical tuning of orbitally forced stratigraphic records to construct high-resolution Astronomical Time Scales (ATS) has led to a progressive refinement of the geologic time scale over the past two decades. In turn, these studies provide new insights regarding the durations and rates of major Earth events, evolutionary processes, and climate changes, all of which provide a scientific basis for contextualizing and predicting future global change trends. South China hosts some of the best-exposed and well-dated Neoproterozoic through Mesozoic stratigraphic sections in the world; many of which are suitable for cyclostratigraphy and calibrating the geologic time scale. In North China, several Cenozoic oil-bearing basins have deep boreholes with continuous sampling and/or well logging that enable derivation of astronomically tuned time scales for an improved understanding of basin evolution and hydrocarbon generation. This Special Issue focuses on case studies of astrochronology and applied cyclostratigraphy research using reference sections within China. In this introductory overview, we: (1) summarize all existing astrochronology studies of the Neoproterozoic through Cenozoic sections within China that have been used to enhance the international geologic time scale, (2) examine briefly the astronomically forced paleoclimate information recorded in various depositional systems and the modern techniques employed to analyze the periodicity of these signals encoded within the sedimentary record, and (3) summarize the 20 contributions to this Special Issue of Palaeogeography, Palaeoclimatology, Palaeoecology on 'Cyclostratigraphy and Astrochronology: Case studies from China'.

Introduction

Astronomically induced variations in Earth’s precession and obliquity modulate seasonal insolation and drive climate changes in the Earth system (Hinnov, 2018). The Earth's seasons are caused by the obliquity (tilt) of its rotation axis relative to the Sun. The Earth's orbit around the Sun is slightly elliptical (= eccentricity); therefore, the relative average summer temperature in a hemisphere depends upon whether its summer occurs during the Earth's furthest distance from the Sun (aphelion; which is the current situation for the Northern Hemisphere) or during its closest approach (perihelion). The precession of the Earth's rotation axis relative to its orbit causes a ca. 20-kyr cycle in the relative warmth of a hemisphere's summer (and coldness of its winter) as it oscillates between aphelion to perihelion conditions. The eccentricity of the Earth's orbit is influenced by the gravitational attraction of other planets, and changes from nearly circular (the current situation with a relatively low importance of precession-induced climatic change) to a more elliptical orbit that causes greater seasonal contrasts. Eccentricity varies with a ca. 100-kyr oscillation that is further modulated by a 405-kyr cycle. Superimposed on the precession-eccentricity signal are ca. 40-kyr periodic variations of the obliquity of the Earth's axis; and these are especially important in influencing seasonal contrasts at higher latitudes.

Together, these variations in the relative differences in summer heat versus winter cooling are called "Milankovitch cycles" after Milutin Milanković, the Serbian mathematician who did the initial detailed numerical analysis of the effects on solar insolation at different latitudes (Milankovitch, 1930, Milankovitch, 1941). These long-term orbital-induced climate cycles are also manifested as changes in many different geographic and sedimentary processes, including average seasonal rainfall and fluvial runoff, ice cap size, global sea level, types and amount of vegetation, rates and styles of continental weathering, the position and intensity of wind belts, monsoon patterns, coastal currents, and the supply and recycling of nutrients within both marine and lacustrine waters. In turn, sediment influxes into all depositional systems, from continental lacustrine sediments to deep-sea deposits, undergo shifts in the relative ratios of coarse-to-fine siliciclastics and of terrigenous to biogenic components, and in the relative importance of storm and flood events. Consequently, the periodic climatic changes attributable to orbital forcing are readily encoded in stratigraphic successions, and cyclostratigraphers can measure chemical and physical proxies (e.g., elemental concentrations, grain size, magnetic susceptibility, etc.) that track these cycles in order to identify astronomical signals. One challenge to this endeavor is that the recording of Milankovitch cycles in the sedimentary record is superimposed upon a wide spectrum of non-periodic variability, and sedimentary systems are prone to shifts in accumulation rates and sediment composition induced by non-Milankovitch factors such as regional tectonics, temporally irregular shifting of fluvial-delta depocenters, and other trends.

The first published application of this cyclostratigraphic "clock in the rock" concept was by G.K. Gilbert (1895), who used it to estimate elapsed time in Upper Cretaceous deposits in North America. The discipline became more widely established after the seminal work by Hays et al. (1976), which demonstrated pervasive astronomial control on climate changes across the past ~0.5 million years. Since then, the astronomical theory has been widely studied and applied. During the past two decades in particular, astronomical calibration of stratigraphic successions has been successfully used to constrain the Cenozoic and Mesozoic geologic time scales, aided by the development of numerical solutions of Earth's orbit across the past ~50 million years by Jacques Laskar et al., 1993, Laskar et al., 2004, Laskar et al., 2011.

China hosts superb Neoproterozoic and Mesozoic stratigraphic sections in marine facies, as well as lacustrine deposits encompassing large portions of the Permian through Neogene, and thick accumulations of wind-blown loess spanning the Quaternary. Cyclostratigraphic analyses of proxy data from outcrops and boreholes within China have been important for calibrating the geologic time scale, understanding basin evolution and oil-forming mechanisms, and for unraveling the impacts of past glacial-interglacial climate oscillations. This special issue of Palaeogeography, Palaeoclimatology, Palaeoecology brings together 20 papers that highlight the importance of Chinese sections for cyclostratigraphy and time scales. The studies cover a wide range of stratigraphic records in marine and terrestrial deposits within China, and incorporate aspects of biostratigraphy, lithostratigraphy, chemostratigraphy, magnetostratigraphy, and radioisotopic dating.

In this introductory review paper, we present two summaries of cyclostratigraphy studies in China. We first summarize all the cyclostratigraphy studies of sections within China that present astronomical time scales that enable high-resolution enhancement of the geologic time scale (Ediacaran through Quaternary). Six of these are also part of this Special Issue (Fang et al., 2020; Lu et al., 2019; Ma et al., 2019; Ma et al., 2020a; Sui et al., 2019; Zhong et al., 2019b). The second group are those that apply cyclostratigraphy and astronomically tuned depositional rates to better understand basin development and Earth system evolution during portions of the Neoproterozoic through Cenozoic (Chu et al., 2020; Du et al., 2020; Gong et al., 2019; Li et al., 2020; Liu et al., 2020; Peng et al., 2020; Xu et al., 2019; Xu et al., 2020; Yao and Hinnov, 2019; Zhang et al., 2019a, Zhang et al., 2019b; Zhang et al., 2020; Zhang 2019; Zhao et al., 2019).

Geologic settings of the sedimentary records of China

The sedimentary history of regions within China were particularly conducive to recording climate cycles in relatively stable depositional settings. The South China Craton was a part of the Gondwana continental margin during the Cryogenian through Silurian; then, as a separate island mini-plate from the Devonian through middle Triassic, it accumulated a thick carbonate platform adjacent to eroding "oldlands". Consequently, South China hosts some of the best-exposed and well-dated Neoproterozoic, Paleozoic and Mesozoic stratigraphic sections in the world. The exceptional nature of these fossiliferous and well-preserved marine deposits is why South China hosts nearly a dozen GSSPs (Global Stratigraphic Section and Point) of stratigraphic boundaries for the Paleozoic and early Mesozoic geologic stages. Equally, these reference sections have provided cyclostratigraphic records for calibrating the durations of biozones and other events of the international geologic time scale.

Basins within other blocks that now comprise China contained large lakes at tropical to temperate latitudes. In addition to accumulating organic-rich deposits that became important oil and gas source rocks, these lacustrine deposits were particularly conducive to recording climate-induced fluctuations in the relative influx of clay and organic carbon. Cyclostratigraphic studies of these lacustrine deposits have often utilized records from deep boreholes and range from the Permian through Neogene. In particular, North China has several Cenozoic oil-bearing basins with deep boreholes that have been continuously sampled and/or well-logged for cyclostratigraphy studies.

During the Quaternary and continuing to the present, windblown dust from the arid interior regions of central Asia accumulated as thick loess deposits in the middle watershed of the Yellow River and elsewhere. During more humid intervals, such as major interglacial episodes, reddish soil horizons developed on these loess plains; and the resulting alternation of arid-climate loess and humid-climate soil has enabled astronomical calibration of Eurasia's glacial episodes.

Astronomical Time Scale studies using reference sections in China

As of July 2020, there have been over 30 cyclostratigraphy studies of marine, lacustrine or loess deposits that have direct application to enhancing the age model for the geologic time scale (Table 1 , Fig. 1 ). Studies of these are part of this Special Issue of Palaeogeography, Palaeoclimatology, Palaeoecology, including contributions on the Ediacaran (Sui et al., 2019), Cambrian (Fang et al., 2020) and Ordovician-Silurian (Lu et al., 2019; Zhong et al., 2019b; Ma et al., 2020b; Ma et al., 2019). Only the Cenozoic and the Jurassic of China, for which there is a lack of extensive uplifted exposures of fossiliferous marine strata, have not yielded cyclostratigraphic studies that are directly applicable to improving the geologic time scale. However, the entire Cenozoic time scale and much of the Jurassic has been cycle-tuned based on ocean drilling boreholes, uplifted oceanic sediments in the Mediterranean region, and borehole and outcrop studies in Britain and other regions.

Table 1

Summary of astrochronology studies on Chinese reference sections that are used to enhance the geologic time scale. Abbreviations: (1) Proxies – MS = magnetic susceptibility, GR = natural gamma-ray radiation, ARM = anhysteretic remanent magnetization, XRF = X-ray fluorescence; (2) Method – MTM = multi-taper method, Evol. spectra = evolutive spectra; (3) Cycles – E = long-eccentricity, e = short-eccentricity, O = obliquity, P = precession; (4) Other – U-Pb = uranium-lead dating (Ding et al., 1994, Lu et al., 1999, Heslop et al., 2000; Deng et al., 2019a; Deng et al., 2019b; Sun et al., 2006; Wu et al., 2014; Wang et al., 2016; Deng et al., 2013; Wang et al., 2016; Wu et al., 2009; Xi et al., 2019; Wu et al., 2013a; He et al., 2012; Wu et al., 2013b; Zhu et al., 2007a, Zhu et al., 2007b; Liu et al., 2017a, Liu et al., 2017b; Huang, 2019; Li et al., 2017; Tong et al., 2019; Zhang et al., 2015; Lehrmann et al., 2015; Li et al., 2016a, Li et al., 2016b; Li et al., 2007; Wu et al., 2012; Zhao et al., 2005; Wu et al., 2013c; Shen et al., 2011; Mundil et al., 2004; Yuan et al., 2019; Xue et al., 2015; Shen et al., 2019; Xue et al., 2015; Fang et al., 2017; Mei et al., 1994; Fang et al., 2015; Fang et al., 2012; Ueno et al., 2013; Wang et al., 2019a, Wang et al., 2019b; Zhang et al., 2019a, Zhang et al., 2019b, Zhang et al., 2019c, Zhang et al., 2019d, Zhang et al., 2019e, Zhang et al., 2019f; Ueno et al. (2013); Wu et al., 2018; Qi et al., 2014; Ross and Ross, 1988; Li et al., 1997; Rygel et al., 2008; Ueno et al., 2013; Bai, 1995b; Qie et al., 2019; Gong et al., 2001; Gong et al., 2005; Huang et al., 2016; Rong et al., 2019; Chen et al., 2018; Chen et al., 2011; Ma et al., 2019a; Wang et al., 2018; Ma et al., 2016; Ma et al., 2019b; Ma et al., 2019c; Peng et al., 2009; Chen et al., 2006; Zhu et al., 2019a, Zhu et al., 2019b; McFadden et al., 2008; Condon et al., 2005; Zhou et al., 2019;Bai et al., 1982; Bai et al., 1994; Li et al., 2009; Zhong et al., 2018; Bai, 1995a; Chen et al., 2015; Fang et al., 2019, Gong et al., 2017, Hu and Qi, 2017; Sui et al., 2018, Wang et al., 2009, Zhong et al., 2018).

Fig. 1

Locations of cyclostratigraphy studies in China that are relevant to enhanced calibration of the geologic time scale. The color of each location dot is the same as the color of the geologic epoch. Coloring of the bars for the span of each study (next to the geologic time scale) indicates the general depositional setting (blue = marine, green = lacustrine, tan = other terrestrial). For details on each study, see the corresponding number in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

As expected from its geologic history, nearly all of the Ediacaran, Paleozoic and Triassic cyclostratigraphy studies have been undertaken on the South China Craton (Fig. 1). The exceptions are Ordovician studies in the Tarim Basin, which included the auxiliary GSSP for the Middle/Late Ordovician boundary (Sandbian GSSP), and in Hebei Province of North China. North China has cyclostratigraphy studies of Cretaceous terrestrial basins that helped to calibrate the evolution of feathered dinosaurs (Wu et al., 2013b) and of magnetic polarity chrons. The thick loess deposits in Shaanxi Province allowed calibration of the major Quaternary climate oscillations and the development of desert regions of eastern Asia (e.g., Ding et al., 2002).

Nearly all studies on pre-Cenozoic Chinese reference sections recognized the dominant 405-kyr long-eccentricity modulation of the precession cycle within marine or lacustrine deposits. In contrast to the theoretical lengthening of the periods for Earth's precession and obliquity through geologic time due to the influence of its Moon, the 405-kyr long-eccentricity cycle has had a stable period for at least the past half-billion years (e.g., Hinnov, 2018). This 405-kyr signal in the sedimentary record enables an "astronomical tuning" of a meter-scale record yielding a "floating" time scale. The resulting durations of geologic stages and of corresponding biozones, geochemical excursions, and magnetic polarity zones derived by these studies generally have a precision of ~0.1 Myr (i.e., one quarter of a long-eccentricity cycle). In the case of Songliao Basin borehole in northeastern China, radio-isotopic dating has enabled the 405-kyr cycles within the Upper Cretaceous record to be anchored in absolute time by tying these to the corresponding cycles in the numerical solution of astronomical forcing (Laskar et al., 2011). This assigns an age of 82.9 Ma to the base of magnetic polarity Chron C33r, which is the candidate marker for the base of the Campanian Stage (Wu et al., 2020). Several of these cycle-scaled durations of stages and of other geologic episodes have been incorporated into the period-level syntheses of the integrated stratigraphy and timescale of China (right-hand column in Table 1) and into Neoproterozoic and Phanerozoic time scales (e.g., Ogg et al., 2016; and chapters within Gradstein et al., 2020).

Applications of cyclostratigraphy within China

The 20 cyclostratigraphy studies in this Special Issue (Table 2 ) span the Ediacaran through Pleistocene (Table 2). Of these, 6 are directly applicable to enhancing the geologic time scale, and therefore were also included in Table 1. The locations range from the Tibetan Plateau near the Himalayas to offshore Guangdong in the South China Sea (Fig. 2 ).

Table 2

Summary of cyclostratigraphy and astrochronology studies in this Special Issue of Palaeogeography, Palaeoclimatology, Palaeoecology. For abbreviations, see caption to Table 1. (Zhang et al., 2019b; Xu et al., 2020; Du et al., 2020; Zhao et al., 2019; Liu et al., 2017a, Liu et al., 2017b; Xu et al., 2019; Peng et al., 2020; Qu et al., 2014; Chu et al., 2020; Zhu et al., 2019a, Zhu et al., 2019b; Zhang et al., 2019a; Liu et al., 2018; Zhang et al., 2020; Wu et al., 2017; Kametaka et al., 2009; Lu et al., 2019;Zhong et al., 2019b; Tang et al., 2017; Ma et al., 2019; Fang et al., 2020; Peng et al., 2009; Condon et al., 2005; Sui et al., 2019; McFadden et al., 2008; Condon et al., 2005); Deng et al., 2018

Fig. 2

Location of cyclostratigraphy studies in China in this Special Issue of Palaeogeography, Palaeoclimatology, Palaeoecology. The color of each location dot is the same as the color of the geologic epoch. Coloring of the bars for the span of each study (next to the geologic time scale) indicates the general depositional setting (blue = marine, green = lacustrine, tan = other terrestrial). For details on each study, see the corresponding number in Table 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The majority of the 12 Mesozoic and Cenozoic studies in this Special Issue applied cyclostratigraphy to deposits in terrestrial basins, especially lacustrine and fluvial-delta sequences. Scientific questions include: establishing the relative timing of seismic horizons and the duration of oil- and gas-generating organic-rich source rocks (Du et al., 2020; Zhao et al., 2019; Xu et al., 2019; Peng et al., 2020; Liu et al., 2020; Chu et al., 2020; Zhang et al., 2019b), determining how glacial-interglacial oscillations influenced the influx of coarser-grained siliciclastics (Zhang et al., 2019a; Zhang et al., 2019a), and constraining the depositional history of deltas (Xu et al., 2020; Zhang et al., 2020). The biostratigraphic constraints in many of these deposits are limited to poorly age-calibrated terrestrial faunal and floral remains. As such, some of the floating astronomically tuned time scales are anchored using the "traditional" ages based on assigning certain seismic or other horizons to geologic stage boundaries. However, with the aid of the constraints on durations and relative timing of events from cyclostratigraphy, the widespread non-marine Mesozoic-Cenozoic facies of China have the potential to be tied to contemporaneous marine rocks elsewhere (and vice versa), emphasizing the utility of cyclostratigraphy for building timescales to correlate and understand climate evolution in terrestrial environments. In the future, it may be possible to improve those age models through recognition of longer-period astronomical cycles (so-called "grand cycles").

The eight Ediacaran and Paleozoic cyclostratigraphy studies in this Special Issue are on marine successions composed of carbonate, shale or chert. Scientific questions range from durations of graptolite, conodont and radiolarian zones (Yao and Hinnov, 2019; Lu et al., 2019; Zhong et al., 2019a; Ma et al., 2019; Fang et al., 2020) to the durations of carbon-isotope excursions (Ordovician GICE by Ma et al., 2020b; Ediacaran Shuram CIE by Gong et al., 2019, and Sui et al., 2019). The two studies on the Shuram CIE yielded quite different durations, estimated at ca. 9 Myr by Gong et al. (2019) versus ca. 20 Myr by Sui et al. (2019) (note: the shorter estimate was based on extrapolating a sedimentation rate from only a portion of the entire CIE).

General methods

Paleoclimatic proxies

Cyclostratigraphy is dependent on recognizing orbital cycles in climatically sensitive physical, chemical or biological proxies. In order to understand how paleoclimatic proxies can record astronomical signals, cyclostratigraphers need to know how the paleoclimate changed in response to astronomically forced insolation variations, how these paleoclimatic variations influenced climate-sensitive components of the sedimentary record, and how the recording of these climate-sensitive components in the resulting deposits are linked to different proxies. Due to the Earth’s rotation and its orbit around the Sun, the solar irradiance has strong daily, seasonal and annual cycles. Non-linear mechanisms rectify the seasonal modulation, thereby enhancing precessional-period and other Milankovitch cycle variability (Huybers and Wunsch, 2003). This seasonal-scale climatic transformation of the modulation of insolation forcing can amplify the astronomical frequencies and subsequently be preserved in stratigraphic records (Hinnov, 2018).

Paleoclimate proxies of widespread use in cyclostratigraphy include color, lithology, stable isotopes, rock magnetism, paleontology, gamma ray logs, elemental geochemistry, and organic carbon content (e.g., Hinnov, 2013; Li et al., 2019a). In this Special Issue, many types of paleoclimatic proxies were measured as a basis for analysis by spectral methods. Physical proxies include lithologic characteristics (i.e., chert-clay couplet coding, bed bundling, color), borehole well logs (i.e., resistivity, and natural gamma ray (GR) of Th, K and U contents), and magnetic parameters (i.e., magnetic susceptibility (MS), anhysteretic remanent magnetization (ARM), and hard Isothermal remanent magnetization/magnetic susceptibility (HIRM/χ)). Geochemical proxies include total organic carbon (TOC), carbon isotopes (δ13C), and XRF scanning data of elemental concentrations and ratios (Fe, Ca, Si, Ti, Rb/Sr, Sr/Al, Zr/Al, V/Cr, Ni/Ti, Cu/Ti, Fe/Ca).

In terrestrial or lacustrine settings, variations in some of these paleoclimate proxies are responses to astronomical-forced insolation changes in weathering, precipitation and temperature. In marine depositional settings, some of these paleoclimate proxies are records of variations in the relative productivity of carbonate organisms and variable fluxes of sediments derived from continental weathering and runoff (Li et al., 2019a).

Spectral analysis

Cyclostratigraphy requires the detection and separation of astronomical signals in paleoclimate proxy data (Hinnov and Hilgen, 2012). The most common procedures involve Fourier analysis of the proxy data to establish the frequency distribution of variance. High variance within a relatively narrow band of frequencies (i.e. spectral peaks) are used to identify likely cyclic signals within the dataset that represent the astronomical forcing. These spectral analysis methods are designed primarily to isolate signals in evenly-spaced time series. In contrast, proxy data measured through a stratigraphic record are in height/depth. Therefore, depending upon the type of deposit, well-defined, rapidly deposited event beds such as debris flows or turbidites are removed from the records. Methods such as evolutive harmonic analysis (EHA) and correlation coefficient (COCO) analysis (Meyers et al., 2001; Li et al., 2018b, Li et al., 2019b) can be performed to identify temporal changes in the potential dominant cyclic frequencies preserved in stratigraphic sections, to reconstruct variations in sedimentation rates, and to select a suite of subintervals to analyze separately using spectral analysis.

Associated methods of time-series analysis of paleoclimate proxies employed in cyclostratigraphy include interpolation, integral-sampling, smoothing, detrending, filtering, and demodulation, correlation and tuning (Hinnov and Ogg, 2007). Common software for such signal processing includes KaleidaGraph 4.0, Analyseries 2.0.8, and Acycle 2.0. In this Special Issue, most studies utilize the multi-taper method (MTM) of spectral analysis, which offers high-resolution and statistical estimates that are independent of spectral power (Ghil et al., 2002). The MTM method can provide a well-resolved spectrum of variance within a dataset, allowing clear distinction of peaks even with small amplitude oscillations. The results are often more optimized than those based on other, more-classical spectral methods (Thomson, 1982). The statistical significance of spectral peaks, and thus whether they are likely to have been caused by astronomical cycles or perhaps relate to some random process instead, can be determined by establishing by how much the peak exceeds the expected background red-noise spectrum for an absence of cycles (e.g., Mann and Lees, 1996).

Most studies in this Special Issue also apply evolutionary spectral methods in order to visualize the time-frequency landscapes of the stratigraphic signals (e.g., Sui et al., 2019; Xu et al., 2019; Zhang et al., 2019b). Such analyses can provide important information that guide recognition of Milankovitch cycles through a section, as well as track variable sedimentation rates and the presence of hiatuses.

Astronomical tuning and construction of floating astronomical time scales

The main components of astronomical parameters are Earth’s orbital cycles of precession, obliquity and eccentricity and their long-term modulation cycles. As noted above, Laskar et al., 1993, Laskar et al., 2004, Laskar et al., 2011 has progressively developed enhanced astronomical solutions (called La1993, La 2004, and La 2011, respectively) that now span from 250 Myr in the past to 250 Myr in the future. These numerical solutions provide an important astronomical time scale target and comparator to cyclostratigraphic results. Tuning of cyclostratigraphically identified cycles in proxy data to the astronomical solution allows for the construction of calibrated timescales anchored in absolute time. However, even the most recent solution of La2011 becomes uncertain for the high-frequency components of precession and obliquity beyond 50 Myr into the past, making any time-calibration of paleoclimatic proxy data to the Astronomical Time Scale difficult. Many stratigraphic records of paleoclimatic proxy series (e.g., Mediterranean sapropel sedimentary records, marine oxygen isotope and carbon isotope data, natural gamma ray (GR) logs, magnetic susceptibility, core-scanning XRF data, etc.) in the Cenozoic can be directly tuned to astronomical targets such as 65°N summer insolation, obliquity and eccentricity solutions (Ding et al., 2002; Lourens et al., 2004; Hilgen et al., 2012; Lisiecki and Raymo, 2005; Zachos et al., 2001).

Looking "backwards" in geologic time, Earth’s orbital precession and obliquity periods become shorter relative to the Present due a reduction of the Earth-Moon distance and tidal dissipation, which caused changes in Earth’s rotation rate and shape (Laskar et al., 2004). Therefore, for cyclostratigraphic analysis of deposits older than 50 Ma, the stable 405-kyr long-eccentricity cycles have emerged as the most prominent and stable orbital oscillation (Hinnov, 2018). This 405-kyr long-eccentricity cycle is caused by the combined interactions of the large mass of Jupiter and the perihelia of Venus and Jupiter on the Earth's orbit. Therefore, most cyclostratigraphy studies of Mesozoic and Paleozoic sediments tune their paleoclimatic proxy series to the 405-kyr long-eccentricity cycle (Bao et al., 2018; Boulila et al., 2014; De Vleeschouwer et al., 2015; Fang et al., 2018; Grippo et al., 2004; Huang et al., 2010a, Huang et al., 2010b; Husson et al., 2011; Li et al., 2016a, Li et al., 2016b, Li et al., 2018a; Pas et al., 2018; Ruhl et al., 2016; Wu et al., 2013c).

The predominant frequencies in rhythmic sedimentary stratigraphic records are observed from the power spectra and evolutionary spectral analysis results of paleoclimate proxies. Based on the estimates of variable sedimentation rates from methods such as COCO and EHA analyses, it is possible to determine whether the 405-kyr long-eccentricity, ~100-kyr short-eccentricity, or obliquity cycles are the dominant orbital parameter in high-resolution paleoclimatic proxy series. these cycles become the tuning term to convert the depth-domain signals into the time-domain, thereby enabling the construction of an astronomically tuned time scale. If the stratigraphic record of interest has a calibration based on a radio-isotopically dated horizon, or if the tuned-record of magnetic polarity zones can be matched to the calibrated geomagnetic polarity time scales (or similar types of age-control), then it is possible to apply cyclostratigraphy to generate a precise age model at every level within a stratigraphic succession.

In this Special Issue, nearly every study resolves a dominant 405-kyr long-eccentricity cycle and utilizes this term to construct floating astronomically tuned time scales. However, in some cases where multiple Milankovitch signals were identified, the tuning made use of the ~100-kyr short eccentricity cycle or obliquity cycle. This was the case for Quaternary fluvial-lake deposits (Zhang et al., 2019b), Cretaceous outer-shelf marine carbonates (Li et al., 2020), Middle Triassic lacustrine deposits (Chu et al., 2020), middle Permian deep-marine chert-shale couplets (Yao and Hinnov, 2019), and an Ediacaran marine carbonate succession (Gong et al., 2019).

Some caveats and challenges

Even though astronomically induced climate signals can be recognized in nearly every depositional setting in terrestrial and marine environments, the preservation and fidelity of that signal in the geologic record are influenced by many other natural processes. Strata do not accumulate steadily, and interruptions or disruptions in sediment accumulation can result in "missing cycles", time-averaging (e.g. bioturbation), and unstable sediment accumulation rates. Tectonics can cause fault-displaced records within an outcrop or borehole, and distortion of some types of paleoclimate proxies can occur through redox processes or diagenesis. In dynamic settings such as terrestrial and shallow-marine environments, non-cyclic fluvial-delta lobe switching and facies migration can occur, obfuscating astronomical signals. Most spectral analysis methods and bandpass separation of frequencies implicitly assume a semi-stable and continuous signal in the proxy being analyzed.

As a consequence of these issues, it is important to verify the interpretations from spectral-analysis, bandpass and tuning methods. If the methods are applied to more than one type of proxy, does one obtain the same results? Are each of the assigned cycles visibly present in the proxy records, and ideally also seen as features in the outcrop? Can the same cycles be resolved in an independent coeval section from another part of the basin? These types of verification and testing procedures have enabled development of a reliable astronomically tuned time scale for the Cenozoic and portions of the Mesozoic-Paleozoic. However, it is common for a study to concentrate on only a single reference section (e.g., over a third of the studies summarized in Table 1, Table 2), and these studies therefore await future verification. Confirmation of features in multiple sections is an issue that applies not only to cyclostratigraphic analyses, but also to many studies of geochemical excursions, magnetic polarity patterns, and impact horizons, more than a few of which were initially reported only from a single reference section.

It is also important for all studies to publish/archive their raw data and exact location information, so that future techniques can be reapplied to these datasets. In compiling our summaries, it was disturbing how many important studies, both legacy and current, did not provide the supporting raw data in an accessible form or indicate the exact geographic location of the sections with precise Google-Earth-verified latitude-longitude coordinates.

Outlook

A recent international collaborative effort within the cyclostratigraphic community (The Cyclostratigraphy Intercomparison Project, CIP) has sought to lay a foundation for standardization, both in terms of approach and reporting (Sinnesael et al., 2019). The CIP workshops explored how different approaches adopted for cyclostratigraphic analysis by the community can lead to different results, and thus this community-led approach has the specific aim of developing best practice protocols. A key outcome of the CIP work was the finding that cyclostratigraphy is a trainable skill. The rapid emergence of cyclostratigraphy and timescale development in China speaks to the importance of unifying the discipline in a way that ensures optimal, reproducible results that will add data and value to the geologic time scale.

Of particular note is the recent development of new software tools; in particular the Astrochron package for R (Meyers, 2014) and the Acycle code written for Matlab (Li et al., 2019b). Both packages provide a complete set of ready-to-use tools for cyclostratigraphic analysis of proxy data. The use of these tools helps to ensure reproducibility, eases communication of results, and also guides new researchers in cyclostratigraphy. We note that all but 3 of the papers in this Special Issue utilized either the Acycle or the Astrochron package. This emphasizes the clear demand and acceptance for these tools. Moreover, 12 studies utilize the COCO correlation coefficient method for constraining sedimentation rates recently introduced by Li et al. (2018b), which is integrated into the Acycle code. Although all workers must be wary of adopting ‘black box’ approaches in the statistical treatment and analysis of data (i.e., where the tools are used but not fully understood), the provision of these tools to the community means that integration and understanding of results should ultimately improve.

Future applications

In the near future, it should be possible to compile a global composite of Paleozoic-Mesozoic reference sections for a complete record of the 405-kyr "metronome" relative to the Present that encompasses all major biozones and is constrained by radioisotopic dates. This will extend the current Cenozoic astronomically tuned scale back to the Ediacaran. Such a preliminary "proof-of-concept" synthesis with standardized statistical processing of numerous overlapping sections was accomplished for the entire Mesozoic by Huang (2018), and the major 405-kyr "cyclothems" in the late Carboniferous through early Permian await a direct calibration to an enhanced astronomical solution.

There is also the exciting possibility of augmenting these compilations with the so-called "grand cycles" that modulate the amplitudes of the 405-kyr long-eccentricity cycles (ca. 1 to 2 Myr periodicities) and of obliquity (ca. 1.2 Myr periodicity). Such very-long-term cycles can act as additional time-scale and correlation constraints. It has already been shown that these modulations appear to influence global Phanerozoic sea level (e.g., Boulila et al., 2018) and govern the relative importance of obliquity versus eccentricity-precession cycles within basins (e.g., Li et al., 2016b). Some of these very-long-period modulations have been observed in longer sedimentary records within China (see the column with "other recorded cycles" in Table 1, Table 2). With constraints from radio-isotopic dates, it may be possible to bridge current gaps in the Mesozoic astronomically tuned time scale to begin a direct calibration of portions of Paleozoic time.

In turn, a verified master 405-kyr metronome may help to resolve the existing systematic "external uncertainty" of about 0.3 Myr or greater on most pre-Cenozoic radioisotopic dates. The analytical precision on such dates is often better than 0.1 Myr, but the added external uncertainty is a result of uncertainties in decay constants and of currently used "standards".

Looking further back in time, there is the challenge of analyzing pre-Cryogenian cyclostratigraphic successions. China hosts thick Mesoproterozoic deposits that appear to have cyclic characteristics; but studies are inhibited by the current lack of horizons that can be dated by radioisotopic methods to constrain sediment accumulation rates and the true nature of the cycles. This will probably be the next major frontier in China-hosted cyclostratigraphic studies, and it opens the possibility of understanding climate change in the pre-Cryogenian world.

Declaration of Competing Interest

None.