Obliquity-paced summer monsoon from the Shilou red clay section on the eastern Chinese Loess Plateau

Silu Xu; Jiasheng Chen

doi:10.1515/geo-2022-0616

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Obliquity-paced summer monsoon from the Shilou red clay section on the eastern Chinese Loess Plateau

Silu Xu und Jiasheng Chen

Veröffentlicht/Copyright: 2. April 2024

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Abstract

The red clay of the Chinese Loess Plateau (CLP) is an important geological archive for understanding the variability in the late Neogene East Asian monsoon. The periodicity of the summer monsoon of the red clay on the eastern CLP is dominated by eccentricity cycles within the constraints of the palaeomagnetic chronological framework, whereas global climate change characteristics represented by the deep-sea oxygen isotope record at that time show a dominating obliquity cycle. Here, we analyzed the East Asian summer monsoon proxies from the Shilou red clay section with the cyclostratigraphy method. The results show that the summer monsoon variation was dominated by obliquity, the optimum deposition rate was 4.451 cm/kyr, and the floating age of the Shilou red clay section was ca. 1.7 Ma. The late Neogene East Asian summer monsoon inferred from the eastern CLP was thus paced by the obliquity cycle, which is consistent with global change.

Keywords: cyclostratigraphy; East Asian summer monsoon; red clay; TimeOpt; obliquity-paced

1 Introduction

The Quaternary loess-paleosol and the late Neogene red clay across the Chinese Loess Plateau (CLP) are crucial geological archives for understanding monsoon variability in East Asia and aridification processes in the Asian inland [1,2,3,4]. Lu et al. [5] summarized studies on Chinese loess and late Neogene red clays, which have significantly enhanced our comprehension of alterations in Asian climate and their driving mechanisms. Precise dating of loess and red clay is essential for discussing these geological processes. Due to the inadequacy of radiometric dating material, magnetostratigraphy has become the principal technique for establishing age models of loess and red clay [2,3,6,7]. Magnetic stratigraphic dating studies rely on comparing the palaeomagnetic polarity zones of sediments with standard magnetic polarity scales. However, this method is often prone to bias as it is not always a one-to-one correspondence. Three distinct interpretations of the chronology of magnetic stratigraphy in the Shilou red clay section on the eastern CLP have been identified as follows: (1) relying purely on a comparison between the palaeomagnetic polarity zones of the profile and the standard magnetic polarity scale, Xu et al. [8] demonstrated that the bottom age of the profile can be dated up to 11 Ma, with an average deposition rate for red clay of ∼0.8 cm/kyr. (2) Anwar et al. [9] identified a series of abnormal polarity intervals with the data from Xu et al. [8]. Subsequently, Anwar et al. [9] investigated the age of the Shilou profile using paleomagnetic data with cyclostratigraphic analysis and provided an age of 5.2 Ma for the Shilou profile, with an average deposition rate for red clay of ∼2.5 cm/kyr. (3) The magnetostratigraphy of a newly discovered red clay section in this area (∼1 km from the section in Xu et al. [8]) suggests a bottom age of 8 Ma, with an average deposition rate for red clay of ∼2.5 cm/kyr [10,11]. The interpretation of the paleomagnetic signals of the Shilou red clay section displays a broad range of possibilities in contrast to the standard magnetic polarity scale, which led to a deviation in the sedimentation rates of the profile. These significant changes in chrono-stratigraphic interpretations can affect interpretations of climate change mechanisms.

Deep-sea oxygen isotope records indicate that obliquity cycles dominated the global climate record before the mid-Pleistocene climate transition [12]. Ao et al. [11] established the age of the Shilou red clay section through magnetostratigraphy and analyzed the summer monsoon proxies (Rb/Sr, Al/Na, and lightness). They revealed that the dominant cycle in the red clay was the eccentricity cycle. Anwar et al. [9] established the age of the Shilou red clay section through magnetostratigraphy and cyclostratigraphy. They analyzed the summer monsoon proxies (magnetic susceptibility) and the winter monsoon proxies (grain size), showing that the red clay exhibited a pronounced eccentricity cycle and a high amplitude 200 kyr period, which was considered to be a harmonic at 95 and 125 kyr. Laskar et al. [13] reported a modulation period of 173 kyr for the obliquity cycle; therefore, the period of 200 kyr could be the obliquity cycle. If this is the case, the climate change cycle of the late Neogene red clay record of the CLP may be consistent with that of the deep-sea oxygen isotopes record, which is worthy of further study.

Cyclostratigraphy is a methodology that modulates cyclic variations in the stratigraphic record with the periodicity of Earth’s orbit. Moreover, it is capable of establishing a continuous and high-precision astronomical time scale using the rhythms recorded in strata. The time scale optimization (TimeOpt) inversion method, which has been recently developed, can be utilized for the construction of a dependable test of astronomical hypotheses by merging multiple attributes of the astronomical signal [14,15,16,17,18]. TimeOpt is capable of assessing the dominant cycle and optimal deposition rate in the stratigraphic record through eccentricity and obliquity driving, respectively [16]. Combined with astronomical tuning, the floating age of geological events or processes can be further estimated.

Due to debate surrounding the interpretation of the magnetostratigraphic age of the Shilou red clay and variations between the dominant climatic cycle based on a palaeomagnetic framework and the deep-sea oxygen isotope records, the TimeOpt method can effectively address both of these issues. Therefore, we applied this method to analyze the climate records of the Shilou red clay section, aiming to determine the dominant climatic cycle and create a floating astronomical time scale. This study examines the floating age and climatic cycle of the red clay with cyclostratigraphy from an independent viewpoint.

2 Geological setting and materials

The Shilou red clay section is located ∼15 km northwest of Shilou County, Shanxi Province, and it is on the eastern boundary of the CLP, bounded by the Lüliang Mountains to the east and the Yellow River to the west [9] (Figure 1a). The reddish-red clay is accompanied by calcareous nodules, which are more weathered than the Quaternary loess. The red clay grain size shows a tri-peak or multi-peak distribution, with peaks at 1–2, 10, and 100 μm, respectively. The lithological distinction between the upper and lower sections of the Shilou profile is clearly expressed. The lower red clay is over 40 m thick and displays alternating light red and brown paleosols, which are rich in carbonate binding. They show 5R 3/4 and 10R 5/4 Munsell colors, respectively. The upper section is more than 20 m thick and contains developed ferromanganese colluvium and clay-grain colluvium, 5R 2/2 Munsell color, darker in color than the lower red clay, and has only a few thin (0.2–0.4 m) carbonate nodule layers (Figure 1b). For a more detailed description of the profiles, refer to Ao et al. [11].

Figure 1

Geographic location of the Shilou profile, lithological histograms, and climate proxy data for red clay from the (CLP). (a) Overview map of the study area. Dashed contour shows the extent of the Loess Plateau in China. Red pentagram shows the geographic location of the Shilou profile; (b) stratigraphic feature; (c) Rb/S, (d) Al/Na, and (e) lightness from the article by Ao et al. [11].

Ao et al. [11] collected 3,527 fresh samples at 2-cm intervals. Many studies have used Al/Na, Rb/Sr, and lightness as proxies for summer monsoon precipitation in Quaternary loess and underlying Neogene red clay on the CLP [19,20,21,22,23,24]. Increased precipitation enhances weathering on the Loess Plateau, resulting in a significant decrease in soluble Na and Sr but an increase in insoluble Al and Rb, which contribute to heightened Al/Na and Rb/Sr ratios [19,20,24]. Increased precipitation induces the enrichment of darker minerals (fine-grained magnetite, clay minerals, and organic matter) and the leaching of lighter-colored carbonates, which leads to more saturated red soil coloration and a lower lightness [21,23]. Buggle et al. [25] summarized the evaluation of geochemical weathering indicators in loess-paleosol research. However, the Neogene was warm and humid, resulting in more intense pedogenesis and darker soil coloring compared to the overlying Quaternary loess-paleosol sequence. Therefore, Al/Na, Rb/Sr, and lightness variations predominantly reflect post-depositional pedogenesis, with only minor pre-depositional pedogenic effects affected by dust sources. The Neogene red clays reflect dust transport similar to that of the Quaternary loess. This is mainly due to the northwesterly migration of the winter monsoon from the arid area to the west and north. Dust accumulation rate changes have unlikely had an impact on the substantial changes in CLP wind-formed deposition. The S1, S2, S3, and S4 CLP paleosoils exhibited varying rates of accumulation, but they all displayed a similar increase in pedogenic magnetic material. This indicates that they have had relatively consistent monsoon precipitation [22,26]. Thus, the Al/Na, Rb/Sr, and lightness of the Neogene red clay and the Quaternary loess-paleosol sequences are often employed as proxies for summer monsoon precipitation. The strong correlation between these proxies and precipitation has been extensively documented in various studies [21,22,23]. Ao et al. [11] provide high-resolution Al/Na, Rb/Sr, and lightness data for the Shilou red clay section (Figure 1c and d).

3 Methods

Previous research has indicated that the Shilou red clay section possesses distinctive and unparalleled stratigraphic cycles, developing distinct strong (red or dark red) and weak (light red) paleosol cycles, which can be easily observed in the field and are reportedly regulated by cyclical variations in Earth’s orbit [9,11]. The stratigraphic cycles of the Shilou red clay section are therefore highly appropriate for cyclostratigraphic analyses.

3.1 TimeOpt

TimeOpt initially computed deposition rates and dominant periods by analyzing the amplitude of the precession, eccentricity, and frequency modulation. The reliability of the orbital forcing and dominant period were tested against the null hypothesis. However, recent extensions have enabled TimeOpt analyses utilizing the amplitude and frequency of obliquity and its modulation periods [16]. TimeOpt analysis involves three primary stages. Foremost, the amplitude modulation (amplitude envelope) and spectral power are evaluated separately. Second, these outcomes are combined. Finally, the Monte Carlo simulation establishes the statistical significance of the best-fit results. Previous research has reported that the Shilou profile exhibits distinguishable sedimentary cycles [9,11]. In this study, TimeOpt was used to assess the dominating cycle and optimal deposition rate in the stratigraphic record through eccentricity and obliquity driving, respectively [16]. Laskar et al. [13] reported that the dominant period of the obliquity is 41 kyr, containing 54, 39, 29, and 28 kyr. The modulation periods of obliquity are mainly dictated by 172.4, 150.2, and 98 kyr. The precession period spans 23.6, 22.3, 19, and 18.9 kyr. The eccentricity period, also known as the modulation period of the precession, is 405.6, 130.7, 123.8, 98.8, and 94.8 kyr.

TimeOpt analysis was implemented using the “timeopt” and “timeOptSim” functions in the “astrochron” package of R. For TimeOpt analyses dominated by the obliquity period, the filter range was set to 1/70–1/25 cycles/kyr. For TimeOpt analyses dominated by the eccentricity and precession period, the filter range was set to 0.035–0.065 cycles/kyr. The deposition rate range was set as 0.5–5 cm/kyr, encompassing the range of sedimentation rates from previous studies on the Shilou red clay section. Here, 100 evenly spaced deposition rates were designated, and 2,000 Monte Carlo simulations were performed.

3.2 Spectral analysis in the depth domain

Spectral analysis of stratigraphic data sequences constitutes a pivotal stage in cyclostratigraphic research. Spectral analysis is based on the principle of transforming signals in a sequence from either the depth or time domain into frequency domain signals [27]. The relationship between the ratio of the sedimentary cyclic signals identified in the depth domain and the ratio of the period of Earth’s orbital parameter can be used to obtain a preliminary judgment on whether Milankovitch signals are recorded in the study profile [28,29]. The evolutive harmonic analysis (EHA) method evaluates the variation in the depositional rates for dominant cycles in the time or depth domain of stratigraphic data [30]. The technique performs spectral analysis on a moving window of data to obtain a dynamic spectral structure that varies with time or depth. This study utilized the “noLow” for detrending raw data, “trim” for removing extreme values, and the “mtm” and “eha” functions for conducting multi-taper method (MTM) spectral analysis and EHA analysis within the “astrochron” package of R. The depth-domain MTM analysis of the data series was configured with a confidence level of 90%.

3.3 Filtering and tuning

Stratigraphic deposition sequences frequently feature multiple superpositions of cyclic signals and contain environmental noise. However, studies usually need to extract the frequency signal of a single track; therefore, filtering methods are used to separate the signal from noise. Using the frequency range obtained from the previous spectral analysis, the cyclic signal was extracted from the original depth domain signal through filtering. Astronomical tuning was then used to establish the correlation between the separated cyclic signals and astronomical signals. First, the cyclic signal was extracted through Gaussian band-pass filtering. Second, depth domain periodic signals were assigned to theoretical orbital periodic signals. Finally, the depth domain was converted to the time domain to achieve astronomical tuning. Both filtering and tuning were implemented in MATLAB using the Acycle software [14].

4 Results

4.1 TimeOpt of Al/Na, Rb/Sr, and lightness of the Shilou red clay section

Distinguishable sedimentary cycles are observed in the Shilou section, which have been linked to Milankovitch cycles in previous studies [9,11], with specific cycles of 405, 100, and 41 kyr. Both 405 and 100 kyr are eccentricity cycles. However, if the profile cycle is mainly influenced by the obliquity cycle (i.e., the 405 kyr cycle analyzed in previous studies corresponds to a 173 kyr obliquity cycle), then the specific cycles would be 173, 41, and 17 kyr based on the prior ratios between cycles. Here, 173 and 41 kyr are obliquity cycles, and 17 kyr is close to the precession cycles. TimeOpt analyses and null hypothesis testing were carried out in this study to examine climate change across all indicators. Table 1 lists the results of the TimeOpt calculations and null hypothesis testing for different climate indicators and different climate-driven cycles.

Table 1

TimeOpt analysis for different dominant periods of Al/Na, Rb/Sr and lightness

Dominant periods	ρ value			Optimal deposition rates (cm/kyr)
Dominant periods	Al/Na	Rb/Sr	Lightness	Al/Na	Rb/Sr	Lightness
Obliquity	0.998	0.022	0.536	4.556	4.451	4.556
Precession and eccentricity	0.992	0.862	0.986	3.068	3.527	3.214

The ρ value means probability of running 2,000 Monte Carlo simulations with no astronomical assumptions; optimal deposition rate is calculated by TimeOpt analysis; dominant periods including obliquity, precession, and eccentricity.

In the obliquity period dominant mode, the TimeOpt analyses for Al/Na, Rb/Sr, and lightness were used in 2,000 Monte Carlo simulations with ρ values 0.998, 0.022, and 0.536, respectively. Although the optimal sedimentation rates were all in the range of 4.4–4.5 cm/kyr and the associated floating astronomical ages were between 1.68 and 1.72 Ma, only the ρ value for Rb/Sr was lower than 0.05. This indicates that the null hypothesis was rejected at a confidence level exceeding 95%. The summer monsoon variation reflected in Rb/Sr was driven by the obliquity cycle. In the model dominated by precession and eccentricity, the optimal deposition rates were all in the range of 3.0–3.5 cm/kyr, corresponding to floating astronomical ages of 2.17–2.5 Ma, but the ρ values were generally higher than 0.86, which failed the null hypothesis testing.

4.2 Dominating obliquity cycle of the Rb/Sr data in the Shilou red clay section

Figure 2 illustrates the specific results of the TimeOpt calculations for the Rb/Sr variation in the Shilou red clay section in the obliquity-driven mode. Figure 2a shows the Rb/Sr variation in the depth domain. Figure 2b depicts the obliquity signals and obliquity amplitude envelopes extracted from the Rb/Sr data for the Shilou red clay section through obliquity filtering and the Hilbert transform. The amplitude envelope fit ( r envolope 2 , red line in Figure 2c) was obtained from envelope regression models. This line displays a maximum r envolope 2 value of 0.160 at the average deposition rate of 4.885 cm/kyr. The spectral power fit ( r sepctral 2 , gray line in Figure 2c) was obtained from spectral power regression models. This line displays a maximum r sepctral 2 average value of 0.139 at an average deposition rate of 4.451 cm/kyr. The maximum r opt 2 value of 0.021, which corresponds to a deposition rate of 4.451 cm/kyr (Figure 2d), was determined by calculating the product of r envolope 2 and r sepctral 2 . During the process, 2,000 Monte Carlo simulations were performed with AR1 surrogates, resulting in a ρ value of 0.022. This indicates a 97.8% probability of rejecting the null hypothesis of no astronomical driving (Figure 2e). Figure 2g–h shows the fit between the obliquity amplitude envelope corrected for a deposition rate of 4.451 cm/kyr and the obliquity model.

$Figure 2 TimeOpt analysis of the Rb/Sr data of the Shilou red clay section. (a) Rb/Sr data; (b) the precession signal (black line) and its amplitude envelope (red line) vary with time; (c) correlation between r envolope 2 {r}_{\text{envolope}}^{2} (red dots) and r sepctral 2 {r}_{\text{sepctral}}^{2} (black dots) with deposition rate; (d) r opt 2 {r}_{\text{opt}}^{2} changes with deposition rate; (e) results of 2,000 Monte Carlo simulations were used to evaluate the confidence level of r opt 2 {r}_{\text{opt}}^{2} 0.021; (f) periodic spectrogram of Rb/Sr series corrected by deposition rate of 4.451 cm/kyr, the black line represents the linear spectrum and the gray line represents the logarithmic spectrum, the yellow shaded area represents the band width of the calculated obliquity amplitude envelope, and the red dashed line indicates the target frequency; (g) the Rb/Sr data amplitude envelope (red line) versus the TimeOpt-reconstructed obliquity amplitude modulation model (black line); and (h) cross plot of the Rb/Sr data amplitude envelope with the TimeOpt-reconstructed obliquity amplitude modulation mode.$

Figure 2

TimeOpt analysis of the Rb/Sr data of the Shilou red clay section. (a) Rb/Sr data; (b) the precession signal (black line) and its amplitude envelope (red line) vary with time; (c) correlation between r envolope 2 (red dots) and r sepctral 2 (black dots) with deposition rate; (d) r opt 2 changes with deposition rate; (e) results of 2,000 Monte Carlo simulations were used to evaluate the confidence level of r opt 2 0.021; (f) periodic spectrogram of Rb/Sr series corrected by deposition rate of 4.451 cm/kyr, the black line represents the linear spectrum and the gray line represents the logarithmic spectrum, the yellow shaded area represents the band width of the calculated obliquity amplitude envelope, and the red dashed line indicates the target frequency; (g) the Rb/Sr data amplitude envelope (red line) versus the TimeOpt-reconstructed obliquity amplitude modulation model (black line); and (h) cross plot of the Rb/Sr data amplitude envelope with the TimeOpt-reconstructed obliquity amplitude modulation mode.

In conclusion, TimeOpt analyses of the Rb/Sr data from the Shilou red clay section indicate that the optimum sedimentation rate for the section is 4.451 cm/kyr and the stratigraphic cycle is dominated by the obliquity cycle.

4.3 Astronomical tuning of Rb/Sr in the Shilou red clay section

The spectral analysis in the depth domain on the Rb/Sr data from the Shilou red clay section revealed that the stratigraphy had five significant cyclic signals, confidently exceeding 90% (Figure 3a). These signals corresponded to 7.67, 4.26, 1.81, 1.66, and 1.21 m, respectively. TimeOpt analysis indicated an optimal sedimentation rate of 4.451 cm/kyr for the profile. This finding revealed that the cyclic signal of 7.67 m corresponds to 173 kyr. The ratio of the above sedimentary cyclic signals is 173:96:41:37:27, where the cyclic signals represented by 173 (7.67 m), 96 (4.26 m), 41 (1.81 m), 37 (1.66 m), and 27 (1.21 m) were closer to the cyclic signals of the 172.4, 98, 41, 39, and 28 kyr obliquity. For the signal represented by 96 (4.26 m) and 27 (1.21 m), an alternative interpretation is that they are in closer proximity to a short eccentricity cycle and precession cycle, respectively. According to the depth domain spectral analysis, a perfect match could be found between the ratio of stratigraphic cycles and that of obliquity and its modulation cycles. Therefore, the variation in the summer monsoon represented by the Rb/Sr of the Shilou red clay section was driven by the obliquity cycle.

Figure 3

Cyclostratigraphy of Rb/Sr data of the Shilou red clay section. (a) MTM power spectrum from 0 cycles/m to 1 cycles/m; (b) EHA amplitude results using five 3π tapers and a 15 m moving window; (c) the detrended Rb/Sr data; (d) 173 kyr filtering results for Rb/Sr depth domain sequences; (e) sedimentation rate; and (f) duration of the profile after tuning.

EHA analysis indicated that the stratigraphic cycles of the 7.67 m depth domain Rb/Sr data for the Shilou red clay section, which corresponds to the 173 kyr obliquity, remained stable across the section (Figure 3b). Figure 3c shows the detrended Rb/Sr data. The 7.67-m cycle signals were extracted by a Gaussian band-pass filter at 0.13 ± 0.0125 cycles/m. The filtering results showed that a total of 10 complete 173 kyr obliquity cycles were recorded based on the Rb/Sr data from the Shilou red clay section (Figure 3d). The ca. 173 kyr obliquity cycle is present in numerous records from the Mesozoic and Cenozoic eras; thus, the ca. 173 kyr cycle could serve as a “Geological Timekeeper” [31,32,33]. By assigning a time scale of 173 kyr to each sedimentary cycle, the sedimentation rate of the Shilou red clay section ranged from 4.41 to 4.54 cm/kyr, with an average sedimentation rate of 4.447 cm/kyr (Figure 3e). The floating age of the Shilou red clay section was ∼1.7 Ma (Figure 3f), which is consistent with the results calculated by the TimeOpt analysis.

5 Discussion

Huang et al. [34] compiled a high-resolution dataset of total organic carbon and stable carbon isotopes in mid-to-high latitude regions, finding a robust periodicity of 173 kyr. This phenomenon could potentially be caused by the amplification of the forcing signal associated with the ca. 173 kyr obliquity modulation. This is further influenced by the internal climate feedback of the carbon cycle under varying geographical and climatic conditions, which controls a multitude of sensitive climate processes. In the northwestern Loess Plateau, with a relatively arid climate, three distinct paleosol horizons are well preserved within the fossil pedo-complex S1 of the loess. The development of these paleosol horizons is associated with the precession cycle.

However, in the humid central and southern regions of the Loess Plateau, the interlayer between the three paleosol horizons became indistinct due to the subsequent leaching and sedimentation of the well-developed soil, resulting in the three paleosol horizons fusing into a single horizon. In a humid climate with strong pedogenesis, S1 is the result of multiple soil horizon formation events, with paleosol intervening in the underlying older loess. This condition is likely to have been more severe during the formation of the well-developed red clay. Consequently, the 41 kyr modulation cycle would have had greater significance, showing a robust 173 kyr cycle.

Anwar et al. [9] observed a high-amplitude (170–200 kyr) cycle between the 405 kyr long eccentricity and the 100 kyr short eccentricity in the magnetic susceptibility record of the Shilou red clay section. They suggest that this cycle may be a harmonic of the 95 and 125 kyr cycles. Our results suggest that this could be the 173-kyr obliquity cycle.

During the relatively arid Quaternary, evidence from pollen assemblages and loess-paleosols suggests that the northern boundary of summer monsoon precipitation in the Loess Plateau may have retreated to the southernmost part of the CLP during periods of dry and cold climates. During the warmer climate, the northern boundary of summer monsoon precipitation extended well beyond the CLP [4]. Oscillatory changes in the intensity of the summer monsoon described by the loess-paleosol alternation essentially reflect changes in the movement of monsoon precipitation fronts to higher and lower latitudes. Davis and Brewer [35] showed that the summer solar radiation difference at 60–30°N exhibits a significant 41 kyr cycle. Thus, the migration of the monsoon precipitation zone due to changes in the solar radiation difference between high and low latitudes may be the main reason for the obliquity cycle of Rb/Sr. Stronger soil development may have allowed multiple soil horizons to leach and deposit into a single horizon, which made the 41-kyr modulation cycle more significant, showing a robust 173-kyr cycle.

The results of Ao et al. [11] indicate that the climate of the Shilou red clay section exhibits a distinct eccentricity cycle with a time span of 4.7 Myr. The ratio of the 405 kyr eccentricity cycle to the 173 kyr obliquity cycle is 2.34. If converted according to this ratio, the time span of 4.7 Myr corresponds to (4.7/2.34≈) 2 Myr, which is relatively close to 1.7 Ma in this study. Combining the sedimentary age of 3.4 Ma at the top of the Shilou profile from Ao et al. [11], the lower boundary age of the Shilou red clay section can be calculated as 5.1 Ma, which is relatively consistent with the profile age from Anwar et al. [9]. and Zhang et al. [36] provide a detailed explanation of the paleomagnetism of the Shilou red clay section; therefore, it will not be significantly discussed here.

The TimeOpt analysis of Rb/Sr indicates that the climatic cycles in the Shilou red clay section are driven by obliquity and have passed the null hypothesis testing. Other indicators, dominated by obliquity and eccentricity cycles, did not pass the null hypothesis testing in the TimeOpt calculation. The failure to pass the null hypothesis testing may be due to issues with the indicators themselves, or it could be that the cyclic modulation relationship has not been preserved or has been disrupted, which requires further research. The obliquity-driven Rb/Sr provides at least a new possibility and perspective for the study of climatic cycles and chronology in the Shilou red clay section.

6 Conclusions

According to the TimeOpt method, the Al/Na, Rb/Sr, and lightness of the Shilou red clay section were examined using the eccentricity cycle mode and obliquity cycle mode. Only the cyclostratigraphic study of Rb/Sr obliquity passed the null hypothesis testing. The obliquity cycle was identified as the dominant cycle following the Rb/Sr cycle analysis of the Shilou red clay section.

Based on obliquity as the dominant cycle, the floating astronomical time scale of the Shilou red clay section, representing the changes in the East Asian summer monsoon, was established through cyclostratigraphy methods as ca. 1.7 Ma, with an average sedimentation rate of 4.451 cm/kyr.

Acknowledgements

This work was supported by the National Science Foundation of China (grant 42130507, 41602190), the Special Project of Fujian Public Welfare Research Institute (2019R1002-5), and Project of Innovation Team of Fujian Normal University (IRTL1705). We would like to thank Editage (www.editage.cn) for English language editing.

Author contributions: Silu Xu: performed the data analysis and wrote the manuscript. Jiasheng Chen: contributed to the conception of the study and revision of the manuscript.
Conflict of interest: Authors state no conflict of interest.
Data availability statement: Data openly available in a public repository, we use public data from the article by Ao et al. [11].

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Received: 2023-10-15

Revised: 2024-02-07

Accepted: 2024-02-08

Published Online: 2024-04-02

This work is licensed under the Creative Commons Attribution 4.0 International License.

Artikel in diesem Heft

https://doi.org/10.1515/geo-2022-0616

Schlagwörter für diesen Artikel

cyclostratigraphy; East Asian summer monsoon; red clay; TimeOpt; obliquity-paced

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