Marine organic rich sediments are widely preserved in strata around the world, and productivity and preservation conditions are important factors for organic matter enrichment^[1,2]. The factors affecting the enrichment of organic matter in sediments have always been a hot topic. Rahmstorf^[3] and Huang^[4] considered that the climate change driven by the astronomical orbital cycle is closely related to the marine productivity in middle and low latitudes based on cyclostratigraphic research of the Quaternary and Late Triassic. Li et al.^[5] and Wu et al.^[6] believed that the volcanic ash released by volcanic activities provided nutrients for the ocean, and the anoxic environment in the ocean related to volcanic activities provided favorable conditions for the preservation of organic carbon. The climate fluctuation and sea level change are closely related to variations of planetary orbital cycle^[7,8]. Orbital cycle variations have great impact on solar radiation received by the earth's surface, resulting in periodic fluctuations in climate and sea level, which are recorded in sediments^[9,10]. Organic rich black shales are widely deposited in the Upper Ordovician-Lower Silurian of the Yangtze plate in southern China^[11]. Crick et al.^[12], Nestor et al.^[13] and Svensen et al.^[14] have identified the astronomical orbital cycle in the Upper Ordovician-Lower Silurian. Was the oceanic productivity in the Middle to Upper Yangtze region during Late Ordovician-Early Silurian driven by orbital cycle? Besides, a large amount of volcanic ash has been found in the organic rich shale of the Upper Ordovician Wufeng- Lower Silurian Longmaxi Formation, which confirms the existence of volcanic activities. How did volcanic activity contribute to the enrichment of organic carbon? Did the volcanic activity contribute mainly to oceanic productivity or preservation efficiency of organic carbon? If it can be confirmed that the orbital cycle has a driving effect on marine productivity, then which is the main controller: orbital cycle or volcanic activity, or both?

To answer these questions, two outcrop sections, the Jienietuo section in Jinyang County, Sichuan Province in Upper Yangtze region and the Lücongpo section in Badong County, Hubei Province in Middle Yangtze region, were taken as research objects in this study. These two sections are thousands of miles apart and record different volcanic activity intensities, which is convenient for the comparative study of contributions from paleoclimate, paleomarine environment and volcanic activity to carbon enrichment. Based on the analysis of trace elements and organic carbon isotopic composition, the orbital parameters in stratigraphic records were identified, and the climate fluctuations, paleo marine productivity and paleo marine environment of the Middle and Upper Yangtze regions from late Katian to early Rhuddanian were reconstructed to analyze the driving mechanism of astronomical orbital cycle on marine productivity, the relationship between volcanic activity, marine productivity and paleo marine environment favorable to preserve organic carbon, and the main controlling factors of marine productivity and organic carbon preservation efficiency. The study is of great significance to the enrichment mechanism of organic carbon, and the selection of shale gas exploration targets.

The Yangtze platform is located in the northern part of the South China Plate^[15]. The Yangtze plate near the equator during Late Ordovician-Early Silurian collided with the Cathaysia plate, forming the central Sichuan uplift in the northwest and the Kangdian, Qianzhong old land and Xuefeng underwater highland in the south^{[11, 16]} (Fig. 1a, 1b). The plate collision caused the Yangtze plate to sink continuously during Hirnantian. However, with the arrival of the global ice age, the rate of sea level decline was still greater than that of the plate sinking. The sedimentary environment of the Yangtze region evolved from deep sea to shallow sea to deep sea, from the Upper Ordovician Wufeng Formation (late Katian) to Early Silurian Longmaxi Formation (early Rhuddanian)^[16]. The Wufeng-Longmaxi Formation black shale extensively deposited in the Yangtze block of South China, and the graptolites developed in the black shale were well preserved^[17,18,19]. Late Katian-erarly Hirnantian (Wufeng Formation) corresponds to Dicellograptus complexus-Normalogr extraordinaries graptolite zone (WF2-WF4 graptolite zones). The Guanyinqiao Member has abundant shells and rare graptolites, corresponding to Persculptogr pacificus graptolite zones. Late Hirnantian-Rudanian (Longmaxi Formation, LM1-LM5) corresponded to Persculptogr persculptus-Cystograptus vesiculosus graptolite zone (LM1-LM4 graptolite zone) and Coronograptus cyphus graptolite zone (LM5 and graptolite zones above)^{[20-21, 6]} (Fig. 1c).

Volcanic ash has been widely recorded in the Upper Ordovician-Lower Silurian in areas such as Yangtze region of China^{[15, 22]}, Sweden^[23], Kentucky and Virginia, United States^{[23,24,25,26]}, etc., confirming that this period was a global large-scale volcanic eruption window. The bentonite interbeds are common in organic-rich shale of Wufeng-Longmaxi Formation in the Yangtze region of China, mainly distributed in Upper Katian of M. Extraordinarius grapolite zone, upper Hirnantian and Lower Rhuddanian of Persculptogr Pacificus graptolite zone^[6]. For example, the Wangjiawan section in Yichang City, Hubei Province^[27], the Nanbazi section in Tongzi City, Guizhou Province^[28], and the key parts of Jienietuo section in Jinyang County, Sichuan Province (Fig. 1c-1e), Lücongpo section in Badong County, Hubei Province, and so on.

Samples used in this study were collected from the Jienietuo section in Sichuan Province and Lücongpo section in Hubei Province. The Jienietuo section is located next to a new highway in Jienietuo village, Zhaizi Town, Jinyang County, Xichang Basin (Fig. 1b), while the Lücongpo section is located in Lücongpo Town, Badong County, western Hubei Province of eastern Sichuan Basin. The two sections were located in low latitudes during the Late Ordovician-Early Silurian^[29] (Fig. 1b). The samples were collected along the road-cut outcrop at an interval of 10-15 cm excluding the weathering layer on the surface. 292 fresh samples were taken from the 36 m thick Katian to Rhuddanian of Jienietuo section, and 168 fresh samples were taken from the 18 m thick Lücongpo section. Organic Carbon isotopic composition (δ¹³C), major and trace elements and total organic content (TOC) tests on these samples were carried out in the Test Center of the Sichuan Coalfield Geological Bureau. The δ¹³C value was tested with an isotope mass spectrometer (MAT 252). Major and trace elements were analyzed by an automatic X-ray fluorescence spectrometer (XRF-1500). The TOC was tested by a carbon-sulfur analyzer (LECO CS230) with the analytical precision of ±0.5%, and the tested results meet the Chinese National Standard (GB/T 19145-2003)^[30]. In this research, major elements were used to calculate chemical index of alteration (CIA) index, and the formula is as follows.

CIA, as a substitute index of climate, reflects the relationship between the degree of chemical weathering of sediment and temperature and humidity, and is widely used in paleoclimate reconstruction^{[31,32,33,34]}. The m_CaO* is CaO in silicate minerals^[35,36], excluding calcium in carbonate and phosphate minerals^[37,38]. When the sediment experienced strong chemical weathering, A-CN-K triangle diagram can be used to correct CIA (Fig. 2) to eliminate the influence of potassium metasomatism. The CIA values in this paper are corrected values if there is no special explanation.

The spectra of δ¹³C, CIA and TOC values series were analyzed with the multi-taper method (MTM)^[39,40], and the peaks in the spectra may represent different astronomical orbital cycles. The spectral peaks with a 95% confidence level were analyzed, while the spectral peaks with a 90%-95% confidence level were used selectively. The data series were processed by sliding-window spectrum analysis and Gaussian filtering, and the 405 kyr long eccentricity period obtained by Gaussian filtering was used to modulate the short eccentricity, obliquity and precession cycles^[7], and to build a floating astronomical time scale (ATS) during late Kaitian to early Rhuddanian. δ¹³C and CIA value series were converted from depth domain to time domain by “Age Scale” module. The above data processing was repeated to convert the cyclostratigraphy from depth domain to time domain.

The global climate turned cold from the Late Ordovician to Early Silurian, and the whole Gondwana entered the glacial age, the long-term greenhouse climate disappeared^[41]. This research reconstructs the paleoclimate of this period by CIA index, which indicates chemical weathering intensity is related to climate condition^{[31, 35]}. The CIA values of sediments depositing in hot and humid climate, warm and humid climate, and cold and dry climate are in general 80-100, 70-80, and 50-70, respectively^[35,36]. The CIA values of shale samples from Upper Katian, Upper Hirnantian and Lower Rhuddanian of Jienietuo and Lücongpo sections are 75-85 (Fig. 2), higher than the average value of shale (70-75)^[36]. This shows that the clastic materials experienced strong weathering before deposition, indicating hot and humid tropical greenhouse climate. This result is consistent with the conclusion from δ¹³C and δ¹⁸O data of Qing and Veizer^[42], Brenchley et al.^[43]. The CIA values of samples from Upper Hirnantian are 50-70 (Fig. 2), especially those of samples from Guanyinqiao layer are 50-70, closing to the CIA values of Pleistocene glacial clay and moraine^{[34-35, 44]}. This shows that the clastic materials experienced moderate to weak weathering before deposition, indicating cold and dry ice climate.

3.2.1. Characteristics of astronomical orbital cycles

The δ¹³C and CIA value series of Upper Katian-Lower Rhuddanian of Jienietuo and Lücongpo sections in the Yangtze region were analyzed by MTM. There are prominent peaks in the 6.54, 1.71 and 0.50 m cycles of the δ¹³C value series of Jienietuo section, which represent 405 kyr long eccentricity cycle, 106 kyr short eccentricity cycle and 31 kyr obliquity cycle (Figs. 3 and 4a). The 6.39, 1.79 and 0.51 m cycles with prominent peaks of CIA series of Jienietuo section, representing 405 kyr long eccentricity cycle, 113 kyr short eccentricity cycle and 33 kyr obliquity cycle (Figs. 3 and 4a). For the Lücongbo section, the 2.91, 0.79 and 0.25 m cycles of δ¹³C series representing 405 kyr long eccentricity cycle, 110 kyr short eccentricity cycle and 33 kyr obliquity cycle have prominent peaks (Figs. 3 and 4b); the 2.91 m, 0.77 m and 0.25 m cycles of CIA series representing 405 kyr long eccentricity cycle, 107 kyr short eccentricity cycle and 33 kyr obliquity cycle have prominent peaks (Figs. 3 and 4b). The 405 kyr long eccentricity cycle is not only stably recorded in strata of the Mesozoic and Cenozoic depositing under the effect of astronomical cycles (After 250 Ma)^{[7, 45]}, but also identified in the Paleozoic Upper Ordovician-Lower Silurian strata^[12-14,18]. The 405 kyr long eccentricity cycle was used to establish a floating astronomical time scale in this study to convert the Upper Katian-Lower Rhuddanian of Jienietuo and Lücongpo sections from the depth domain to the time domain. The age of bentonite (443.2±1.6 Ma) at the top of Guanyinqiao layer^[2] was taken as the anchor to establish the chronological framework of Upper Katian-Lower Rhuddanian in the Yangtze area (Fig. 5). The results calculated using astronomical cycles tally well with the results of Lu et al., Ogg et al., and Zhong et al.^{[2, 46-47]}.

3.2.2. Astronomical response of climate change

Precession and eccentricity have a great effect on climate change in the middle and low latitudes, while obliquity is likely to affect climatic oscillation in polar regions^{[7, 48]}. However, paleo-climate changes controlled by obliquity have also been identified in sedimentary strata of low latitudes, including Cenozoic and Mesozoic^[49,50], Triassic^[8], Permian^[51] and Ordovician-Silurian^[47]. The Jienietuo and Lücongpo sections in the Yangtze region are located in the low latitude region (Fig. 1b). From them, 2-3 long obliquity cycles (1.2 myr) and 6 long eccentricity cycles (405 kyr) have been identified (Fig. 5c-5l). The 1.2 myr of long obliquity cycle modulated by obliquity is the result of inclination variations^[52] between the Earth and Mars orbit, while the 405 kyr long eccentricity cycle modulated by short eccentricity cycle is originated from the result of interaction of the Venus and Jupiter at orbital perihelion^[10].

Cyclostratigraphic analysis of Jienietuo and Lücongpo sections during Upper Katian-Lower Rhuddanian in the Upper Yangtze region shows that the orbital cycle has a significant effect on the climate change, and the 1.2 myr long obliquity cycle and 405 kyr long eccentricity cycle control climate changes jointly. The 1.2 myr long obliquity cycle modulates the transition of heat and moisture by modulating climate change^{[8, 49, 53-54]}. This study shows that the 1.2 myr long obliquity modulated climate change controlling the climate transformation between greenhouse and icehouse at a longer time scale. When 1.2 myr long obliquity was the maximum (early-middle Hirnantian), the energy of solar radiation migrated to high latitudes to the greatest extent, and solar radiation in the Yangtze region with low latitudes weakened, leading to a colder climate. Conversely, without considering the impact of volcanic activity, when the 1.2 myr long obliquity is the minimum, the solar radiation energy was the weakest in the high latitude region, and the strongest in the low latitude Yangtze region, leading to climate warming (Fig. 5d-5m)^[55]. Eccentricity modulates the earth’s climate by controlling the earth’s orbit around the sun, which is particularly prominent in low to middle latitude regions^{[3, 12]}. When the eccentricity is large, the solar radiation energy to the low latitude areas is weak, which is conducive to icehouse climate; when the eccentricity is small, the solar radiation energy to the low latitude areas is strong, which is conducive to greenhouse climate. The 405 kyr long eccentricity is positively correlated with the amplitude of precession cycle^{[10, 56]}. The Yangtze region had small amplitude of precession cycle and low long eccentricity in late Katian (E1) and late Hirnantian (E4), indicating this region was greenhouse climate in this period. On the contrary, this region had large amplitude of precession cycle and high long eccentricity in early-middle Hirnantian, indicating it was icehouse climate in this period (Fig. 5c-5k).

Sea level fluctuations are closely related to climate changes^{[3, 8]}. The third-order and fourth-order sea level fluctuations are greatly related to different time scales^[57]. The origins of third-order sea level fluctuation have been long controversial, but more and more evidences from Mesozoic and Paleozoic support the view that the astronomical orbital cycles controlled sea level fluctuations by modulating climate changes^{[47, 58-59]}. Moreover, Lu et al.^[2] and Zhong et al.^[47] proved that the third-order sea level fluctuation in the Late Ordovician-Early Silurian was controlled by climate change modulated by 1.2 myr long obliquity cycle. In the Jienietuo and Lücongpo sections of Yangtze area, the 1.2 myr long obliquity cycle curves (Fig. 5f, 5l) have consistent fluctuations with the third-order third-order sea level fluctuation (Fig. 5o) proposed by Haq and Schutter^[60] and Ogg et al.^[46]; and the 405 kyr long eccentricity cycle curves (Fig. 5c, 5e, 5i, 5k) show consistent oscillations with the fourth-order third-order sea level fluctuation (Fig. 5o) proposed by Loi et al.^[61]. It follows that climate changes driven by astronomical orbital cycle change control the sea level fluctuations in different orders during late Katian to early Rhuddanian in Yangtze area.

The study shows that obliquity modulating thermohaline circulation dominated the distribution of marine productivity in the Middle and Upper Yangtze regions, while volcanic activities had limited effect on marine productivity.

4.1.1. Distribution of marine productivity dominated by obliquity modulating thermohaline circulation

Paleo-marine productivity and preservation are two key factors affecting the abundance of organic matter enrichment^{[1, 6]}. Continental weathering debris is a major source of nutrients for the formation of organic-rich sediments^{[51, 62]}. Alexandre et al.^[63] reconstructed the ocean currents circulation during Late Ordovician-Early Silurian transition (Fig. 6) based on FOAM ocean-atmosphere model with data of atmospheric CO₂ content, which showed there were NE direction ocean currents in South China. Zhang et al.^[11] also found evidence of NE direction ocean currents during Late Ordovician-Early Silurian transition (Wufeng-Longmaxi Formation) in the Yangtze region, further confirming the existence of thermohaline circulation in this period. The bottom current driven by thermohaline circulation played an important role in the accumulation of organic matter enrichment in the Yangtze Sea with low latitudes. Climate change modulated by obliquity cycle controlled ice freezing rate and physical properties (temperature, salinity, density) of the water masses under ice sheet in the poles^{[51, 62]}. Bottom current/contourite driven by thermohaline circulation transported the low temperature water mass, with rich nutrients in the hypoxic deep sea, from high latitude areas to middle and low latitude areas, which was conducive to the accumulation of organic carbon^{[2, 65-66]}. In the Middle and Upper Yangtze regions, as obliquity decreased and obliquity amplitude increased in Hirnantian glaciation (E3, Guanyinqiao layer) (Fig. 5b, 5d, 5h, 5j), colder climate led to ice sheet expansion in the polar regions, and contourite driven by thermohaline water mass in early glacial period was more active. Thermohaline water mass under ice sheet in high latitudes transported seafloor nutrients to Yangtze Sea at low latitude, enhancing the paleo marine productivity. As a result, Ba/Al and Ni/Al, which represent paleo-productivity, showing positive shift (Fig. 7e-7l). The earth rapidly turned into greenhouse climate after Hirnantian glaciation. As obliquity increased and obliquity amplitude decreased in early E4 (Fig. 5b, 5d, 5h, 5j), global warming led to the melting of ice sheets in the polar regions, and contourite driven by thermohaline water mass became inactive. The capacity of thermohaline water mass in high latitudes transporting nutrients to Yangtze Sea at low latitude got week, so the paleo marine productivity declined. Consequently, Ba/Al and Ni/Al, which represent paleo-productivity, showing negative shift (Fig. 7e-7l).

The orbital cycle parameters from the δ¹³C and CIA series of Jienietuo and Lücongpo sections after filtering have not only similar number of orbital cycles but also similar variations of obliquity amplitude, indicating that the two areas had similar nutrients input capacity associated with thermohaline circulation driven by obliquity (Fig. 5b, 5d, 5h, 5j). Although the Upper Yangtze region where the Jienietuo section is located is nearly 1000 km away from the Middle Yangtze region where the Lücongpo section is located, they differ only about 3° in latitude, the two regions were both at low latitude during Late Ordovician-Early Silurian transition. At that time, thermohaline circulation driven by orbital cyclic variation dominated global material redistribution, transporting terrigenous weathering detritus of Gondwana in high latitude to middle and low latitude areas, resulting in undiscriminating nutrient input in middle and low latitude area (Middle and Upper Yangtze regions) (Fig. 6).

4.1.2. Limited contribution of volcanic activities to marine productivity

Frequent volcanic activities occurred in the Upper and Middle Yangtze regions from late Katian to early Rhuddanian, this is supported by abundant bentonite layers in organic-rich shale intervals of Jienietuo section (Fig. 5a, 5g). But the volcanic activities at the same period in different areas still differed in intensity. This viewpoint has been confirmed in the southeastern margin of the Sichuan Basin^[67]. During late Hirnantian (E4), southwestern area in Upper Yangtze region where Jienietuo section is located had obvious volcanic activities, developed abundant bentonite layers; while Lücongpo section in western Hubei province had no bentonite layers in the black shale depositing in this period. The two sections both had volcanic ash layers depositing in late Katian (E4), specifically, dense bentonite intervals in black shale of southwestern Sichuan Basin, and sparse bentonite intervals in western Hubei province (Fig. 5a, 5g).

Some researchers proposed that volcanic activities released a large amount of CO₂, which dissolved into ocean system, promoting conversion of carbon from atmospheric carbon reservoir to oceanic carbon reservoir, and volcanic ash is important nutrient that improves the original productivity of the ocean surface^[68]. The soluble salt in the volcanic ash releases a large amount of micronutrient in ocean, promoting the proliferation of phytoplankton which provides materials for organic carbon accumulation^[69]. But based on research of geochemical characteristics of Upper Katian-Lower Rhuddanian organic-rich shale in the Middle and Upper Yangtze Region, we believe that volcanic activities made limited contribution to marine productivity, and thermohaline circulation driven by obliquity was the key factor affecting the marine productivity in this region. Ba/Al and Ni/Al are widely used to characterize paleo marine productivity^[70]. Higher values of Ba/Al and Ni/Al indicate that southwestern Sichuan Basin and western Hubei regions had strong paleo marine productivity during E4 period (Fig. 7e, 7f, 7k, 7l). The Lücongpo section in western Hubei has no bentonite layers, indicating this area had no strong volcanic activities during E4 period. But this area still had strong paleo marine productivity, which means that there is no direct relationship between volcanic activity and paleo marine productivity. The paleo- marine productivity of western Hubei in E4 period mainly came from thermohaline circulation. Similarly, although volcanic activities have been identified in both southwestern Sichuan and western Hubei areas during E1 period, the high paleo marine productivity was driven by thermohaline circulation from obliquity modulation, but not volcanic activity.

Volcanic activity controlled the conditions for the organic carbon preservation by affecting the redox environment of the paleo-ocean. The difference in volcanic activity intensity in different regions was the main factor leading to the differential enrichment of organic carbon in these regions.

4.2.1. Regulation of paleo-ocean oxygen content by volcanic activity

Volcanic activity is one of the important factors affecting the ocean oxygen content. Strong volcanic activities would release a mass of SO₂ and H₂S, which would form aerosols in the stratosphere and circulate into the ocean in the form of acid rain, causing ocean anoxia^[71,72]. The redox environment of the ocean is usually determined by contents or ratios of environmental sensitive elements, for example, Th/U, V/Cr and V/(V+Ni) are important indicators to identify redox conditions of sedimentary water. Th/U from 0 to 2 indicates anoxic environment, from 2 to 8 indicates weak oxygen environment, and greater than 8 indicates oxygen-rich environment. V/Cr greater than 4.25 and V/(V+Ni) greater than 0.60 indicate anoxic environment, 2.00-4.25 and 0.45-0.60 indicate anoxic environment, and less than 2.00 and 0.45 indicate oxygen-rich environment^[73,74]. During the Late Ordovician to Early Silurian, volcanic activities in the Middle-Upper Yangtze region mainly happened in E1 and E4 period. In E4 period, Jieinietuo section in southwestern Sichuan developed sparse bentonite intervals, with negative shift of Th/U and an average Th/U value of 0.65 (Fig. 7b), positive shift of V/Cr and an average V/Cr value of 4.32 (Fig. 7c), and positive shift of V/(V+Ni) value and an average V/(V+Ni) value of 0.68 (Fig. 7d), all indicating anoxic environment (Fig. 8). The Lücongpo section in western Hubei had no bentonite interval developing, with no significant shift of Th/U, V/Cr and V/(V+Ni), with average values of 2.11, 2.84 and 0.48, respectively (Fig. 7h-7j). All these indexes above indicate oxygen-poor environment (Fig. 8). In E1 period, the Jieinietuo section in southwestern Sichuan developed dense bentonite intervals, with an average Th/U value of 0.52, showing negative shift of Th/U (Fig. 7b); an average V/Cr value of 4.75, showing positive shift of V/Cr (Fig. 7c); and an average V/(V+Ni) value of 0.79, showing positive shift of V/(V+Ni) (Fig. 7d). All these indexes above indicate anoxic environment (Fig. 8). The Lücongpo section in western Hubei developed sparse bentonite intervals, with an average Th/U value of 1.42, showing negative shift of Th/U (Fig. 7h), an average V/Cr value of 4.41, showing positive shift of V/Cr (Fig. 7i), and an average V/(V+Ni) value of 0.59, showing positive shift of V/(V+Ni) (Fig. 7j). All the indexes above indicate anoxic environment (Fig. 8).

In E4 period, the southwestern Sichuan area had volcanic activities, but the western Hubei area had no volcanic activities. The paleo-ocean environments of two areas were obvious different, indicating that volcanic activities directly affected the redox environment of paleo- ocean. In E1 period, volcanic activities were strong in southwestern Sichuan, but weaker in the western Hubei. Although paleo-oceans of the two areas were both anoxic environment, characteristics of trace elements indicate that the ocean in southwestern Sichuan was more anoxic than the ocean in western Hubei. In conclusion, the volcanic activity affects the redox environment of the paleo- ocean, and the intensity of volcanic activity has a negative correlation with the oxygen content of the paleo-ocean.

4.2.2. Key factors controlling organic carbon enrichment

Obliquity cycle parameters have been identified from δ¹³C, CIA and TOC data series of both Jieinietao and Lvongpo sections during Upper Katian-Lower Rhuddanian (Fig. 5b, 5d, 5f, 5h, 5j, 5l). But the organic-rich shale in the two sections developed in the transition period from interglaciation to greenhouse (δ¹³C negative shift and CIA positive shift), rather than glaciation period (δ¹³C positive shift and CIA negative shift) which was favorable for marine nutrients input (Fig. 5f, 5l, 5n). In addition, the organic-rich shale interval has higher TOC of transition zone of δ¹³C negative shift (transition stage from glaciation to greenhouse) than that of the interval with the maximum negative shift (greenhouse stage) (Fig. 5f, 5l, 5n). Obliquity modulating thermohaline circulation was the major driving mechanism for nutrient input in the Yangtze Sea. Although the high-intensity bottom currents driven by thermohaline circulation carried abundant nutrients into the low-latitude Yangtze Sea during glaciation, Yangtze Sea was in a relatively oxygen-rich sedimentary environment, which was not conducive to the preservation of organic-rich sediments (Figs. 5a and 7).

In modern oceans, 56% of the organic-rich sediments dissolve in the euphotic layer before reaching seafloor, the organic matters depositing on the seafloor continue to dissolve, and only 3% can be preserved^[62]. The enrichment of organic carbon is closely related to the preservation efficiency controlled by the content of ocean oxygen. The Middle and Upper Yangtze regions witnessed volcanic activities of different intensities in the transition period of Late Ordovician-Early Silurian, and the difference in volcanic activity intensity was an important factor causing the differential preservation of organic carbon. For example, in E4 period, although obliquity and obliquity cycle amplitude were small (Fig. 5b, 5d, 5h, 5j), bottom currents had a strong capacity of organic carbon input to the low latitude areas, and the whole Middle and Upper Yangtze region had a strong marine productivity (Fig. 7e, 7f, 7k, 7l). The western Hubei region and southwestern Sichuan region had large difference in degree of organic carbon enrichment, the main reason is that, compared with western Hubei region, southwestern Sichuan region had intense volcanic activities, which caused ocean anoxia and improving the efficiency of organic carbon preservation (Fig. 7a-7d). In contrast, the western Hubei region had no volcanic activities, so the ocean was poor in oxygen (Fig. 7h-7j), and the efficiency of organic carbon preservation was much lower than southwestern Sichuan region (Fig.7a, 7g and Fig. 9a, 9c, 9e). Similarly, in E1 period, the western Hubei region and southwestern Sichuan region had similar input ability of organic carbon and marine productivity (Fig. 5b, 5d, 5h, 5j and Fig. 7e, 7f, 7k, 7l), and they both had volcanic activities and ocean anoxia environment (Figs. 7b-7d, 7h-7j), and high efficiency of organic carbon preservation (Fig. 7a, 7g and Fig. 9b, 9d, 9f).

The enrichment of organic carbon is controlled by the coupling of organic carbon input ability and preservative efficiency. The climate change modulated by orbital cyclic change controlled the input of organic carbon in different periods, and volcanic activities control preservative efficiency of organic carbon by regulating ocean oxygen content. In the vertical sequence, the shale interval with high organic carbon content can be discriminated by obliquity variation, the formation with high obliquity and small obliquity amplitude may have high organic carbon content. On the plane, anoxic environment conducive to organic carbon preservation can be sorted out by clarifying the distribution of bentonite layers, and orbital parameters and distribution of volcanic ash can be combined to figure out spatiotemporal distribution law of organic-rich shale. Based on this understanding, the enrichment model of organic carbon during Late Ordovician-Early Silurian transition in Middle and Upper Yangtze region was established in this study (Fig. 10).

Late Katian (E1) and late Hirnantian (E4) are the most favorable periods for organic carbon enrichment, so the zones with volcanic ash in these two periods should be taken as the target intervals for shale gas exploration.

The accumulation of organic carbon was controlled by the coupling of organic carbon input ability and preservative efficiency. Based on δ¹³C and CIA data series, the astronomical orbit parameters in the stratigraphic record of the Yangtze region during Late Ordovician- Early Silurian transition have been identified. The astronomical orbit parameters have an important impact on the late Katian-early Rhuddanian climate change in the Yangtze region. The 1.2 myr long obliquity cycle and 405 kyr long eccentricity modulated climate changes at different time scales, and respectively drove the fluctuations of third-order and four-order sea levels. Climate changes caused by orbital cycle drove oceanic thermohaline circulation, and thermohaline circulation dominated global material redistribution, transferring nutrients in areas of high latitudes to areas of low and middle latitudes, and thus increasing marine productivity in low and middle latitudes.

Volcanic activities made little contribution to marine productivity. But the intensity of volcanic activity has an important impact on preservative efficiency of organic carbon by regulating ocean oxygen content. The difference in volcanism intensity in different areas at the same period is an important factor behind the differential preservation of organic carbon.

The enrichment of organic carbon is controlled by the coupling of organic carbon input ability and preservative efficiency. In the vertical sequence, the shale interval with high organic carbon content can be discriminated by obliquity variation. On the plane, anoxic environment conducive to organic carbon preservation can be sorted out by clarifying the distribution of bentonite layers, and orbital parameters and distribution of volcanic ash can be combined to figure out spatiotemporal distribution law of organic-rich shale.

CIA—chemical chemical alteration index, dimensionless;

m_Al2O3, m_CaO*, m_Na2O, m_K2O—the mole fractions of Al₂O₃, CaO*, Na₂O, and K₂O in the samples, %.