A case study on how astronomical cycles affect the organic carbon accumulation

Yng Li , Ren-Cho Yng ,b,＊, Crlos Zvl , Ling Dong ,Vlentin Trobbini

a Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, Shandong Province, China

b Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology,Qingdao 266071, Shandong Province, China

c Geology Department, Universidad Nacional del Sur, Bahía Blanca 8000, Argentina

d Geología de Cuencas Sedimentarias (GCS) Argentina, Bahía Blanca 8000, Argentina

Abstract The paleoclimate change impacts the sedimentary environment and process, which in turn control the accumulation of organic carbon.Numerous studies have shown that the paleoclimate is controlled by astronomical cycles.Hence,understanding how these cycles impact the accumulation of organic carbon is a critical question that requires in-depth discussion.Previous studies have shown that Milankovitch cycle can be revealed from the sediments of the 7th Oil Member (Chang 7 Member for short) of the Triassic Yanchang Formation in the southern Ordos Basin,suggesting that the deposition was controlled by astronomically-forced climate changes.Building on previous research, this study collected natural gamma (GR) data of Chang 7 Member from the N36 Well to further analyze astronomical cycles, combined with X-ray fluorescence (XRF)analysis and total organic carbon (TOC) tests, to reconstruct the paleoenvironment and analyze the organic matter enrichment characteristics.The results of this contribution show that, paleoclimate, paleo-redox conditions, paleo-water level, paleo-productivity and sediment accumulation rate (SAR) collectively controlled the enrichment of organic matter.Notably, this study identified the presence of eccentricity,obliquity,precession,and the 1.2 Myr long obliquity cycle in the Chang 7 Member.These cycles controlled the paleoenvironmental changes at different timescales and influenced the enrichment of organic matter, which has implications for subsequent energy exploration.

Keywords Cyclostratigraphy, Milankovitch cycle, Organic matter enrichment, Paleoenvironmental, Ordos Basin, Chang 7 Member

1.Introduction

In the early 19thcentury,astronomers first realized that the Earth's orbital geometry changes periodically,and since then,many geologists began to pay attention to sedimentary cycles recorded in the strata (e.g.,Hinnov, 2013; Chen et al., 2019).Among them, the Serbian geophysicist Milankovitch made quantitative calculation of the Earth's orbital parameters and proposed the Milankovitch theory (Milankovitch, 1941),which greatly contributed to cyclostratigraphy.According to Milankovitch's theory, periodic changes of the Earth's orbital parameters can control the amount of sunshine received on the Earth's surface(Milankovitch, 1941).An increasing number of studies have shown that, under the influence of periodic changes in Earth's orbital parameters, lithofacies,sedimentary structures, geophysical and geochemical parameters of the sediments exhibit a clear rhythmicity (Hofmann et al., 2004; Val et al., 2017; Shi et al.,2018;Zhang et al., 2021; Zhong et al., 2022).

Periodic changes in the Earth's orbital geometry lead to periodic changes in the amount of solar radiation received by the Earth, which can substantially affect the paleoenvironment (Milankovitch, 1941;Cosentino et al., 2011; Westerhold et al., 2017; Li et al., 2019), and the paleoenvironment plays an important role in controlling the sedimentary evolution (Qin et al., 2021; Liu et al., 2022).On the one hand, paleoenvironment can control the rainfall,chemical weathering, sediment transportation and sedimentation processes (Burdige, 2007; Spychala et al., 2020).On the other hand, paleoenvironment not only affect the species and number of biological communities, but also impact the amount of detrital material, degradation degree of organic matter and paleo-productivity, which control the accumulation and distribution of organic matter (Burdige, 2007;Chen et al., 2020).Therefore, cyclostratigraphy has become an essential tool for analyzing the sedimentary framework.It not only helps to better understand sedimentary evolution but also provides a deeper comprehension of organic matter accumulation and distribution (Chen et al., 2019; Ravier et al., 2020;Zhang et al., 2021).

The Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin primarily consists of finegrained sediments such as shales, mudstones, and siltstones deposited in a deep lake setting(Li and Yang,2023).The organic-rich shales developed in this member are the primary hydrocarbon source rock in the Yanchang Formation (Guo et al., 2022), making it an ideal target for hydrocarbon exploration and production(Cui et al.,2019;Ju et al.,2020).The Chang 7 Member was deposited in a warm and humid temperate subtropical climate, with a paleotemperature higher than 15°C(Fu et al.,2018).The existence of warm and humid climatic conditions and reducing bottom waters probably favored the formation and preservation of organic-rich shales (Fu et al., 2018).Furthermore, the Chang 7 Member is well-preserved and can be an ideal target for Milankovitch cycle analysis.Currently, several scholars have previously identified Milankovitch cycle in the Chang 7 Member and analyzed its sediment accumulation rate (SAR)(Chen et al., 2019; Zhang et al., 2019; Chu et al.,2020).Using natural gamma (GR) data, Fe data and Al data from the N36 Well, our research team has successfully identified Milankovitch cycle in the Chang 7 Member (Fig.1), calculated its SAR (ranging from 0.5 cm/kyr to 2.0 cm/kyr), and furthermore,established an astronomical age scale spanning from 229.4 Ma to 239 Ma(corresponding to the mid-Ladinian through Carnian stages of the Triassic period) (Fig.2)(Li and Yang, 2023).

Fig.1 Multitaper method (MTM) power spectrum of GR data of the Chang 7 Member of Triassic Yanchang Formation from the N36 Well,southern Ordos Basin,China(modified from Li and Yang,2023).E－Long-eccentricity;e－Short-eccentricity;O－Obliquity;P－Precession.

Fig.2 Astronomical time scale of the Triassic Chang 7 Member, the Ordos Basin (modified from Li and Yang, 2023).

Building on prior research,GR data were collected from the N36 Well for further analysis of astronomical cycles in this paper, and the geochemical characteristics of the Chang 7 Member from the N36 Well were analyzed using a handheld XRF analyzer to reconstruct the paleoenvironment and analyze its cyclical variation pattern.Samples were collected for TOC test, in order to analyze the organic matter enrichment characteristics.Through a combination of these analyses and experimental results,this paper investigates the relationship between astronomical cycles and organic matter enrichment.These findings can reveal the cyclicity of paleoenvironmental evolution and its influence on the organic matter accumulation and preservation in the Ordos Basin during the Triassic,providing valuable insights for hydrocarbon accumulations exploration.

2.Geological setting

With an area of 25 × 104square kilometers, the Ordos Basin is situated in Central China(Fig.3A).This basin is one of the oldest cratonic basins in China, as well as the second largest sedimentary basin in the country,renowned for its abundant oil and gas reserves(Yin et al.,2020;Li et al.,2021).Due to plate-tectonic movement, the main structures such as the Yimeng High, the Weibei High, the Western Thrust Belt, the Tianhuan Depression, the Yishan Slope and the Jinxi Fold Belt have been formed(Liu et al.,2017;Wu et al.,2021).The Ordos Basin is bounded by the Helan Mountains at west, the Qingling Mountains at south,Lüliang Mountains at east and Yinshan Mountains at north (Li et al.,2022) (Fig.3B).

Fig.3 A) Location of the Ordos Basin(modified after the Standard Map Service of the National Administration of Surveying, Mapping and Geoinformation of China (http://bzdt.ch.mnr.gov.cn/), GS(2023)2763); B) Tectonic units of the Ordos Basin (M.= Mountains)(modified from Li et al., 2021).

The Triassic Yanchang Formation is a significant petroliferous reservoir in the Ordos Basin, which can be divided into 10 members,ranging from the Chang 10 Member at the base to the Chang 1 Member at the top(Fig.4A), and these members are in conformable contact with each other (Zhang et al., 2019).During the deposition of the Yanchang Formation, the lake level fluctuated several times, thus forming a set of fluvio-deltaic-lacustrine clastic rock system (Yang et al., 2017; Fu et al., 2020).This study focuses on the Chang 7 Member(which can be further divided into three submembers, Chang 73, Chang 72and Chang 71;Fig.2), which was deposited during the largest lake flooding event period, characterized by extensive organic-rich mud shale deposition(Zhangjiatan Shale)(Cui et al.,2019;Ju et al.,2020).In addition,this lake flooding event was accompanied by various types of event sedimentation, leading to the accumulation of numerous gravity flow sand bodies and tuff deposits(Fu et al., 2020).

The N36 Well is located on the Yishan Slope in the southern part of Ordos Basin(Fig.3B).The thickness of the Chang 7 Member in the N36 Well is 103.84 m(1592.38 m－1696.22 m),and the recovery rate is 100%which allows the interval serve as an ideal target for cyclostratigraphic analysis(Fig.4B).

3.Samples and analytical methods

3.1.TOC analysis

A total of 55 mudstone or shale samples were selected, with 19, 21, and 15 samples collected from the Chang 73submember, Chang 72submember, and Chang 71submember,respectively,from bottom to top.The samples were washed with distilled water and powdered to 200-mesh.Hydrochloric acid was added to the crushed samples and allowed to stand for one hour to eliminate inorganic carbon.After that,samples were washed again with distilled water to clean the residual hydrochloric acid.Then,after drying the samples,TOC test was carried out with Leco CS-230 carbon analyzer,having a test accuracy of about ±0.5%.The above analyses were completed at the Lanzhou Institute of Geology,Chinese Academy of Sciences.

3.2.GR data and XRF data collection

The GR data can sensitively reflect the clay content in sediments(Schnyder et al.,2006)and can serve as a valuable tool for cyclostratigraphy and paleoclimate investigations(Yuan et al.,2013;Chen et al.,2020).In this study, the GR data of Chang 7 Member from the N36 Well were sampled at 5 cm equal intervals using the FFG-1 intelligent radiometer.

X-ray fluorescence scanning (XRF) analysis is a commonly used method for obtaining geochemical parameters of rocks (Palin et al., 2016; Cheng et al.,2020).The handheld X-ray fluorescence spectrometer is well-suited for this research due to its small size,portability, and fast testing capabilities.In this study,we utilized a Niton XL 2 handheld XRF analyzer to collect and analyze major and trace element contents from the N36 Well at 5 cm intervals.

Fig.4 A)Representative stratigraphic columns of the Yanchang Formation(modified from Zhang et al.,2019);B)Lithological characteristics and GR data of Chang 7 Member in the N36 Well.

3.3.Multitaper method (MTM) spectrum analysis and amplitude modulation (AM)analysis

The multitaper method(MTM)spectrum analysis is a commonly used technique to analyze Milankovitch cycle.Previous studies have applied MTM spectrum analysis to GR data from the N36 Well (Li and Yang,2023), identifying peaks at 4.78 m, 1.62 m, 1.22 m,0.47 m,0.26 m,and 0.22 m(reciprocal of frequency),corresponding to 405 kyr long-eccentricity,124 kyr and 95 kyr short-eccentricity, 33 kyr obliquity, 21 kyr and 17 kyr precession, respectively, and the confidence level of these peaks are all above 90% (Fig.1).The identified peak ratio is about 20:5:2:1, which is consistent with the ratio of Milankovitch cycle during the Late Triassic(Laskar et al.,2004).In this research,MTM spectrum analysis was conducted on Fe/Mn, V/(V+Ni),Ti/Al and Baxsdata to identify the presence of Milankovitch cycle.

Aside from the commonly studied long eccentricity,short eccentricity,obliquity and precession,there are other long-term astronomical periodicities that can be identified through amplitude modulation(AM)analysis(Boulila et al., 2011; Fang et al., 2016; Shi et al.,2018).AM analysis can determine whether sediment cycles have astronomic origins and identify long-term periodicities (Shi et al., 2018).Typically, AM analysis involves bandpass filtering to extract the interpreted signals of both eccentricity and obliquity, using the Hilbert transform to extract AM envelops, and exploring the extracted AM envelops through MTM spectrum analysis (Shi et al., 2018).In this research,AM analysis was conducted on GR data to identify longterm astronomical periodicities.

4.Results

4.1.Lithology of the N36 well

The Chang 7 Member of the N36 Well is mainly composed of shales, mudstones, silty mudstones,argillaceous siltstones and siltstones, with frequent interbeds of tuff.According to the lithology,the Chang 7 Member can be divided into three submembers:Chang 73submember (1656.95 m－1696.22 m), Chang 72submember (1618.11 m－1656.95 m) and Chang 71submember (1592.38 m－1618.11 m) (Fig.4B).The Chang 73submember is dominated by black shales and mudstones and gray-black argillaceous siltstones, at the boundary between the Chang 7 Member and the Chang 8 Member, tuff beds are found with stable distribution (Fig.4B).The lower part of the Chang 73submember is dominated by organic-rich shales characterized by high GR values, known as the “Zhangjiatan Shale”(Fig.4B),and this shale layer is recognized as the most important Mesozoic oil source rock in the Ordos Basin(Zhang et al.,2019; Yuan et al.,2020).In the Chang 72submember, sand content significantly increased; it mainly consists of gray and gray-black argillaceous siltstones and siltstones, with a lesser amount of organic black shales(Fig.4B).The Chang 71submember is mainly composed of gray argillaceous siltstones and dark gray mudstones, with a small amount of gray siltstones and gray-black silty mudstones(Fig.4B).

4.2.Total organic carbon (TOC) content

Total organic carbon (TOC) content is a proxy for assessing the potential of a hydrocarbon source(Dembicki Jr, 2009), and can be used to evaluate the degree of organic matter enrichment.In this study,the TOC content of samples from the Chang 7 Member ranges from 0.13% to 7.9%, with an average of 1.73%.The TOC content of the Chang 73submember ranges from 0.51%to 7.9%,with an average content of 2.56%.The TOC content of three shale samples from the“Zhangjiatan Shale” is higher, ranging from 4.2% to 7.9%.The TOC content of Chang 72submember ranges from 0.62% to 5.01%, with an average content of 1.42%.The TOC content of Chang 71submember ranges from 0.13% to 3.59%, with an average content of 1.05%.Our analysis indicates that, the average TOC content of the Chang 7 Member is generally low, and the TOC content gradually decreases from Chang 73submember to Chang 71submember.However,the TOC content of the “Zhangjiatan Shale” developed in Chang 73submember is high,indicating that it is rich in organic matter and has good hydrocarbon generation potential, allowing it serve as the main hydrocarbonsource rock layer.

4.3.Geochemical characteristics

The content of different elements in sedimentary rocks can reflect the characteristics of paleoenvironmental conditions(Hinnov,2013;Chen et al.,2020).This paper analyzes the characteristics of elements in sedimentary rocks from Chang 7 Member, and studies the characteristics of their related paleoenvironment from four aspects: paleoclimate, paleo-redox conditions, paleo-water level and paleo-productivity.

The ratio of Ti/Al is widely used as a paleo-climatic indicator, with a higher ratio suggesting a wetter climate (Fu et al., 2017).This is due to the higher stability of Ti compared to Al in humid environments,where Al is more prone to migration, resulting in a higher ratio(Fu et al.,2017).The average values of Ti/Al ratios for Chang 73submember, Chang 72submember and Chang 71submember are 0.070, 0.074 and 0.071, respectively.These values are relatively similar, indicating that the overall paleoclimate change during this period was not significant.

Due to the difference of elemental chemistry, the sedimentary differentiation of elements Mn and Fe is obvious.Colloidal form of element Fe is mainly found in lake waters, and it is more enriched in humid environments.Conversely,element Mn is more enriched in lake waters during arid climates (Shi et al., 2019).Therefore, the ratio of Fe/Mn can be used as an indicator that reflects changes in paleo-water level,where the Fe/Mn ratio increases with the deepening of lake waters(Shi et al.,2019).The average values of Fe/Mn ratios for Chang 73submember, Chang 72submember and Chang 71submember are 56.02, 48.69 and 48.66,respectively, possibly indicating a gradual shallowing lake level.

The V/(V +Ni) ratio can be used to analyze paleoredox conditions (Hatch and Leventhal, 1992).Typically,a ratio of V/(V+Ni)greater than 0.77 suggests an anoxic and extremely oxygen-poor environment,while a ratio between 0.60 and 0.77 indicates an oxygenpoor and sub oxygen-rich environments.Conversely,a ratio less than 0.6 suggests an oxygen-rich environment(Zhang et al.,2021).The V/(V+Ni)ratio of Chang 7 Member ranges between 0.59 and 0.97,with an average value of 0.79, indicating that the paleo-redox conditions of Chang 7 Member were predominantly characterized by an anoxic and extremely oxygen-poor environment.

Commonly, paleoclimate, paleo-redox conditions,and paleo-water level are closely interrelated (Chen et al., 2020).As the climate becomes more humid,the fresh water flow into the lake increases (Nesbitt et al., 1996), resulting in a deeper and more anoxic environment(Nesbitt et al.,1996;Chen et al.,2020).

The element Ba is commonly used to study paleoproductivity (Dymond et al., 1992; Chen et al.,2019).During the decay of organic matter, SO42-is produced, which can react with Ba2+in the water column, forming BaSO4that is often preserved in the sedimentary record.The element Ba in sediments is derived from both terrestrial detritus and biological activity.However,only the element Ba associated with biological activity(i.e.Baxs)can be used as a proxy to reflect paleo-productivity (Chen et al., 2019).The formula for calculating Baxsis as follows (Murray and Leinen, 1996).

Where Baypand Alyprepresent the values of element Ba and element Al of a measured sample,(Ba/Al)pass= 0.0077 (Murray and Leinen, 1996).After calculation, the Baxscontent of Chang 7 Member ranges between 112.68 ug/g and 732.51 ug/g,with an average of 413.89 ug/g, indicating high paleoproductivity.

4.4.Results of MTM spectrum analysis and AM analysis

Zircon isotope analysis of the tuffs located at the base of the Chang 7 Member indicates an age of approximately 241－239 Ma (Wang et al., 2014; Deng et al., 2018; Zhu et al., 2019; Jin et al., 2021; Li and Yang, 2023).Therefore, according to the astronomical solution (Laskar et al., 2004), the calculated theoretical long-eccentricity (E), short-eccentricity(e), obliquity (O), and precession (P) are 405 kyr, 124 kyr and 95 kyr, 33 kyr, 21 kyr and 17 kyr.

The MTM spectral analysis of Fe/Mn,V/(V+Ni),Ti/Al and Baxsdata concluded that, the ratios of the identified wave peaks (confidence level >90%) in the above data can all conform to 20:5:2:1(Fig.5),which are extremely similar to the theoretical astronomical cycles (Laskar et al., 2004).

Fig.5 The MTM power spectrum of each index(modified from Li et al.,2023).E－Long-eccentricity;e－Short-eccentricity;O－Obliquity;P－ Precession.

Fig.6 A－B) The MTM spectrum analysis of the extracted AM envelops of the 405-kyr-long eccentricity and the MTM harmonic F-test confidence level(modified from Li et al.,2023);C－D)The MTM spectrum analysis of the extracted AM envelops of the 124-kyr-short eccentricity and the MTM harmonic F-test confidence level(modified from Li et al.,2023);E－F)The MTM spectrum analysis of the extracted AM envelops of the 33-kyr-obliquity and the MTM harmonic F-test confidence level (modified from Li et al., 2023).

According to the tuned GR data, the interpreted signals of long-eccentricity, short-eccentricity and obliquity,corresponding to 405 kyr,124 kyr and 33 kyr respectively, were extracted and their AM envelops were obtained using the Hilbert transform.MTM spectrum analysis was then conducted on the extracted AM envelops, revealing the presence of peaks at 1171.61 kyr,1143.0 kyr and 1089.9 kyr with confidence levels exceeding 95% (Fig.6), which are very close to the 1.2 Myr long obliquity cycle caused by the interaction of the orbits of the Earth and Mars (Shi et al.,2018; Zhang et al., 2021).Additionally, the MTM spectrum analysis of GR,Fe/Mn,V/(V +Ni), Ti/Al and Baxsdata yielded a peak at 0－0.1 cycles/m(Figs.1 and 5),suggesting the existence of long obliquity cycles in the Chang 7 Member.

5.Discussion

5.1.Factors contributing to organic matter enrichment

In lacustrine sediments, the abundance of organic matter is influenced by various factors.Regardless of the controlling factors,they are mainly analyzed from the perspective of organic matter production, preservation and dilution(Zhang et al.,2016).

Organic productivity(paleo-productivity) refers to the ability of living organisms to produce organic matter by acquiring materials and energy, which serves as the foundation for the accumulation of organic matter (Ding et al., 2015).High paleoproductivity plays a decisive role in the formation of high-quality source rocks (Zhang et al., 2016).The supply of nutrients directly affects paleo-productivity,when terrestrial detritus is injected into lakes, nutrients are also injected into the lakes(Schoepfer et al.,2015).Among these nutrients,phosphates and nitrates contribute most to organic productivity(Zhang et al.,2016).Additionally, phosphorus can limit and determine the productivity of lakes,the higher the input of phosphates,the higher the paleo-productivity of lakes(Zhang et al.,2016).The input of phosphates is mainly affected by the rate of weathering of surrounding parent rocks,and climate can affect the rate of parent rock weathering (Riebe et al., 2001).For example, a more humid climate can accelerate parent rock weathering and increase the input of terrestrial detritus and nutrients (Nesbitt et al., 1996; Riebe et al., 2001), thus increasing paleo-productivity.

High paleo-productivity in a basin does not necessarily mean that the abundance of organic matter in the sediment is high, and only a small portion of the generated organic matter can be preserved in the sediment, while the vast majority will be destroyed and cannot be preserved (Keast and Eadie, 1984).Therefore, appropriate conditions for preserving organic matter are another important condition for the enrichment of organic matter(Zhang et al., 2016).Paleo-redox conditions play a decisive role in the preservation of organic matter(Zhang et al., 2021).There is generally a redox boundary in lake water, and the position of this boundary will change with climate change (Simon et al., 1994; Zhang et al., 2021).Above the redox boundary is an oxidizing environment, which can cause oxidative decomposition of organic matter and is not conducive to its preservation (Zhang et al.,2016; Tang et al., 2020).In contrast, an anaerobic environment below the boundary can slow down the rate of organic matter decomposition, promoting its preservation (Zhang et al., 2016; Tang et al., 2020).Paleo-water level also affects the preservation of organic matter.A greater water depth can lead to the formation of anoxic conditions at the bottom of the lake.Furthermore,hydrodynamic conditions decrease with increasing water depth, and strong hydrodynamic conditions can cause organic matter to be resuspended and accelerate oxidation (Zhang et al.,2016).

For lacustrine sediments, the input of terrestrial debris can provide nutrient elements for the lake basin,but it can also dilute the enrichment of sedimentary organic matter (Tyson, 2001).Previous studies have shown that,the input of terrestrial debris of the Chang 7 Member in the southern Ordos Basin was proportional to the SAR(Chen et al.,2019).To eliminate the impact of paleo-productivity on organic matter enrichment,the TOC/Baxs(TOC per unit paleo-productivity) was applied to studying the effect of the SAR on the organic matter enrichment(Chen et al.,2019).It is found that when the SAR(SAR data sourced from Li and Yang,2023)in the study area is less than 1.36 cm/kyr, TOC/Baxsincreases with the increase of SAR (Fig.7).This is because the higher the SAR, the more efficient the burial of organic matter deposited on the sediment surface, and the less it suffered from microbial decomposition damage.When the SAR is greater than 1.36 cm/kyr, TOC/Baxsdecreases with the increase of SAR, indicating that excessive SAR will dilute organic matter, which is detrimental to the enrichment of organic matter(Fig.7).

In summary, paleoclimate, paleo-redox condition,paleo-water level, paleo-productivity and SAR jointly affect the production, preservation, and dilution of organic matter.

5.2.About the control of astronomical cycles on climatic cycles and organic matter enrichment

5.2.1.Astronomical cycles controlling climatic cycles

Milankovitch cycle has been successfully identified in Fe/Mn, V/(V + Ni), Ti/Al and Baxsdata (Fig.5),suggesting that paleoclimate,paleo-redox conditions,paleo-water level, and paleo-productivity were all controlled by the periodic variations of the Earth's orbit.As different proxies have varying sensitivities to the integral orbital forces (Cheng et al., 2020), the astronomical cycles identified by different data are varying, but they are highly similar.

Moreover, the climate change caused by the 1.2 Myr long obliquity cycle plays an important role in the sedimentary filling of large terrestrial basins(Laskar et al., 2004; Shi et al., 2018; Zhang et al.,2021).

The interpreted signal of long obliquity cycle of 14.16 m was extracted from the GR data (1.2 Myr is 2.96 times 405 kyr, and 2.96 times the long eccentricity cycle of 4.78 m is 14.16 m)and found that it has 8 cycles (Fig.8).Similarly, the interpreted signals extracted from Fe/Mn, V/(V + Ni), Ti/Al and Baxsof long eccentricity cycle also has 8 cycles(Fig.8).These observations suggest that the climate during the deposition of the Chang 7 Member in the southern Ordos Basin was also controlled by the 1.2 Myr long obliquity cycle.

Fig.7 The linear diagram showing relationship between TOC/Baxs ratios and the sediment accumulation rate (SAR).

Fig.8 Changes in lithology, paleoclimate, paleo-redox conditions, paleo-water level, paleo-productivity, TOC and SAR of the Chang 7 Member.The blue dotted line represents the long obliquity cycle periodic curve extracted from GR data, the other four black dotted line curves represent the long obliquity cycle periodic curve extracted from the other four data(Fe/Mn,V/(V+Ni),Ti/Al and Baxs),and the pink color band indicates the corresponding stage when the long obliquity is the maximum (SAR data sourced from Li and Yang, 2023).

5.2.2.The control of astronomical cycles on organic matter enrichment

During the Middle－Late Triassic, the Ordos Basin was located around 30°N and belonged to the mid-low paleo-latitudes (Zhang et al., 2019), where obliquity usually has a significant control on climate fluctuations in high-latitude regions (Zhang et al., 2021).Despite this, obliquity-controlled paleoclimatic fluctuations have also been identified in the sedimentary records from low-latitude areas (Boulila et al., 2011; Fang et al., 2016).Generally, when the obliquity is small,the climatic differences between different latitudes are significant, but the seasons at a fixed latitude are not distinct (Sun et al., 2017).Conversely, when the obliquity is large, the climatic differences between different latitudes are small,but the seasons at a fixed latitude are distinct, and seasonal variations are significant (Sun et al., 2017).

Table 1 Paleoenvironment characteristics of the eight long obliquity cycles in Chang 7 Member of the Triassic Yanchang Formation,the Ordos Basin reflected by different factors(SAR data sourced from Li and Yang, 2023).

The long obliquity cycles were extracted from different data, and it was found that they all exhibit eight cycles and highly consistent trends (Fig.8).Specifically, when the obliquity is large, the corresponding climate is generally more humid (indicated by higher Ti/Al ratios), and the paleo-water depth increases due to increased rainfall (indicated by higher Fe/Mn ratios) resulting in a more reducing environment (indicated by higher V/(V + Ni) ratios).Furthermore, the distinct seasons resulting from a large obliquity are favorable for the proliferation of seasonal algae(Zhang et al.,2016),contributing to higher paleo-productivity (indicated by higher Baxsvalues).Due to the influence of the above factors, the TOC content will in turn change.

In this study,55 samples were collected for the TOC content analysis.Compared to XRF data collected at 5 cm intervals, the number of samples used for TOC content analysis is relatively less,making it difficult to achieve high-resolution reflection of changes in organic matter abundance.The positive correlation between the element Ba and organic matter enrichment has been confirmed:Baxsnot only represents the paleo-productivity, but also reflects the enrichment characteristics of organic matter to a certain extent(Dymond et al., 1992).As shown in Fig.8, the change trend of Baxsvalue is highly consistent with the change trend of the long obliquity cycle, indicating that long obliquity cycle can control the enrichment of organic matter.

There are 8 long obliquity cycles(cycle 1－cycle 8)identified in the Chang 7 Member (Fig.8), and the organic matter enrichment pattern was discussed based on the characteristics of paleoclimate, paleoredox conditions, paleo-water level, paleoproductivity, SAR and TOC content in each cycle(Table 1).

The Chang 73submember contains three long obliquity cycles(cycles 1－cycle 3)(Fig.8).During stage of cycle 1, the “Zhangjiatan Shale” and mudstones were mainly developed, and the climate first became relatively humid and then relatively arid.Additionally,the paleo-water level also increased and then decreased, and the SAR also increased gradually, but did not exceed the critical value (1.36 cm/kyr).The variation range of Baxsvalue in the first cycle stage was large, the V/(V + Ni) ratio reveals an anoxic and extremely oxygen-poor environment,thus favoring the enrichment of organic matter with the highest TOC content (the average value of TOC content is 4.24%).Stage of cycle 2 mainly developed mudstones and argillaceous siltstones.The paleo-productivity of this stage increased compared with stage of cycle 1,but the reduction of lake environment decreased,and the SAR of some strata was greater than the critical value thus diluting the organic matter, resulting in a significant decrease of TOC content (the average value of TOC content is 1.64%).The stage of cycle 3 was dominated by mudstones,silty mudstones,argillaceous siltstones and a small amount of shales.Compared with the second stage,the paleo-productivity of this stage has little change, but the reducibility of the lake water increased,and the SAR was relatively high,but did not exceed the critical value, resulting in the increase of TOC content(the average TOC content is 2.83%).

The Chang 72submember also contains three long obliquity cycles (cycle 4－cycle 6) (Fig.8).Stage of cycle 4 mainly developed mudstones,silty mudstones,argillaceous siltstones and siltstones, which have the highest SAR levels.Most of the strata have SAR values that exceed the critical value, and water reducibility decreased,leading to a lower TOC content(the average TOC content is 1.62%).Stage of cycle 5 mainly consists of mudstones, silty mudstones, argillaceous siltstones and siltstones.Although the paleo-productivity during this stage increased,the water reducibility decreased and the SAR is relatively low, resulting in a lower TOC content (the average TOC content is 1.28%).Stage of cycle 6 is mainly composed of argillaceous siltstones and silty mudstones.Compared with the previous stage, little change occurred regarding the SAR and paleo-productivity of this stage, but the reducibility continued to decrease, resulting in further lower TOC content(the average TOC content is 0.94%).

The Chang 71submember contains two long obliquity cycles(cycle 7－cycle 8)(Fig.8).Stage of cycle 7 is mainly composed of mudstones, silty mudstones and argillaceous siltstones.Compared with stage of cycle 6,the paleo-productivity is generally higher, the water reducibility and SAR increased, and the SAR of most strata has not exceeded the critical value,due to which the TOC content increased (the average value of TOC content is 1.35%),but the overall change is not significant.Stage of cycle 8 is mainly composed of argillaceous siltstones,siltstones and mudstones.During this stage,the reducibility and paleo-productivity reduced,and the SAR was low,resulting in the decrease of TOC content(the average TOC content is 0.87%).

The above analysis indicates that the long obliquity cycle variability can influence the production, preservation,and dilution of organic matter by controlling paleoclimate, paleo-water level, paleo-redox conditions, paleo-productivity, and terrigenous input (sediment accumulation rate).Apart from the long obliquity cycle, other cycles such as eccentricity,obliquity, and precession cycles can also be identified in GR, Fe/Mn, V/(V + Ni), Ti/Al, and Baxsdata, suggesting their control on climate changes at different timescales and then affecting the enrichment of organic matter.

6.Conclusions

Based on amplitude modulation(AM)analysis of the GR data of the Chang 7 Member in the southern Ordos Basin, it is inferred that 1.2 Myr long obliquity cycle controlled the climate during its deposition.The paleoenvironment was reconstructed using X-ray fluorescence(XRF)data and total organic carbon(TOC)content tests, and the characteristics of the climate cycles controlled by astronomical cycles and their impact on organic matter accumulation were examined, leading to the following primary conclusions.

The enrichment of organic matter is influenced by several factors.Paleo-productivity mainly affects the production of organic matter,while paleo-water level,paleo-redox conditions and sediment accumulation rate(SAR) mainly affect the preservation and dilution of organic matter.

The astronomical orbital parameters have an important impact on the mid-Ladinian through to Carnian climate change in the Ordos Basin.The long eccentricity, short eccentricity, obliquity, precession and 1.2 Myr long obliquity cycles play a crucial role in modulating the climate change across various timescales.The changes in climate result in alterations of paleo-water depth, paleo-redox conditions, paleoproductivity and sediment accumulation rate, which affect the production, preservation and dilution of organic matter, and ultimately influence the accumulation of organic matter.

Funding

This work was supported by the National Natural Science Foundation of China(Grant No.41972146)and the China Scholarship Council (Grant No.202008370261).

Availability of data and materials

Data supporting the findings of this work are available upon request from the corresponding authors.

Authors'contributions

Yang Li: Methodology, software, data curation and writing.Ren-Chao Yang: Supervision, conceptualization, visualization and investigation.Carlos Zavala:Writing-reviewing and editing.Liang Dong: Field investigation and sampling.Valentin Trobbiani: Software.All authors read and approved the final manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful for the help provided by Changqing Oilfield, China National Petroleum Corporation.