Atmospheric pO2 is closely related to the evolution of lignocellulosic plants (Mills et al. 2016), and plays an indispensable role in biochemical cycles of the Earth (Berner 1999). The emergence of terrestrial plants, along with the development of lignin at the end Silurian, facilitated the formation of large-scale peat and coal deposits (Wang 2002). Subsequently, inertinite-containing strata began to appear and continued to the present day (Premovic 2006). Therefore, the long-term changes in atmospheric pO2 in geological history may potentially explain the distribution and abundance of inertinite in coal deposits (Diessel 2010).
Statistical tables of inertinite abundance and pO2 during different coal-forming periods in China were completed in Tables 1, 2, 3, 4, 5 and 6, and the spatial distributions of related data points were plotted in Fig. 1.
3.1 Devonian
During the Devonian, plants evolved systematically, and a large number of aquatic plants moved towards land. The diversity of plant systems, and their amazing abilities to reproduce and grow, laid the foundation for the formation of higher plants on land. It has been discovered that during the Late Silurian (Rimmer and Scott 2006) and Early Devonian (Glasspool et al. 2006), an inertinite in the form of charcoal appeared for the first time in the world.
The Devonian is considered to be one of the most important coal-forming periods in China, during which both southern and northern China were characterized by tropical climate conditions (Li and Jiang 2013). The Early Devonian plants were in the early stages of growth and were considered to be higher terrestrial plants. They grew in clusters around coastal wetland areas with limited coverage, and generally formed thin coal seams. During the Middle Devonian, sea level fluctuated frequently, and plants were further propagated in low-lying areas, such as coastal boughs and lagoons. Coal seams were also formed in those areas (Han et al. 1993). Tectonic movements and transgression in southern China had profound influence on the development of coal basins and the formation of coal deposits (Zhang 1995; Li et al. 2018). It has been determined that during the Devonian, coal-bearing strata in China were mainly distributed in the southwestern regions, which mainly developed during the Dongganglingian (Givetian) of Middle Devonian, such as the Haikou Formation in Panzhihua of Sichuan Province, and the Qujing Formation in Luquan of Yunnan Province.
Devonian coal seams are generally thin, ranging between 0.1 m and 0.3 m, characterized by the low abundance and limited distribution of inertinite. The majority inertinite abundance ranges between 0.1% and 3.0% (Table 1). Correspondingly, the overall pO2 was not high. It was approximately 15% during Early Devonian, and increased significantly during Middle and Late Devonian, and reached approximately 20% by Late Devonian.
3.2 Carboniferous–Permian
During the Carboniferous–Permian period, the southern and northern China were under tropical climate conditions (Tabor and Poulsen 2008) with significantly-increased diversity of terrestrial plants (mainly ferns and gymnosperms), leading to profound changes in inertinite type and abundance (Table 2).
3.2.1 Early carboniferous
During Early Carboniferous, coal-bearing strata were widely distributed in the southern China. Under humid and rainy tropical climates, plants as Calamites and Sublepidodendron formed dense forests (Yang 1996), which mainly distributed in coastal lowlands, and did not show obvious climatic differentiation and palaeogeographical isolation (Liu and Quan 1996). The preservation condition for lignin in the plants had been continuously improved, which was conducive to the formation and preservation of coal seams (Diessel 2010). In southern China, the coal seams were mainly developed in deltaic environments, followed by meandering river–lake environments, alluvial fan–braided river environments, and tidal flat–coastal environments, respectively (Liu 1990; Zhang 1995; Zheng 2008).
The coal-bearing strata of the Early Carboniferous were mainly distributed in the Ceshui Formation of Dewuan (Serpukhovian) Stage in central Hunan Province; Simen Formation of Visean Stage in Guangxi Province; and Wanshoushan Formation of Visean Stage in Yunnan Province. In addition, it has also been determined that in northwestern China, such locations as Chouniugou Formation of Visean Stage in Jingyuan, Gansu were also conducive to coal deposits.
The mean inertinite abundance in coals of the southern China during Early Carboniferous ranged between 5.6% and 20.2%, with the largest variation (5.0%–22.3%) observed in Ceshui Formation in different areas of Hunan Province. In Datang area of Guangxi, the value ranged between 13.0% and 15.0%, with an average of 14.0%. The pO2 during Early Carboniferous had increased compared to that of Devonian, and remained stable around 25%.
3.2.2 Late Carboniferous–Early Permian
From Late Carboniferous to Early Permian, coal-bearing strata mainly developed in the basins of North China, with humid climate conditions (Chang and Gao 1993; Tan 2017), and extended stable tectonic setting. Coal-forming processes were active in the following formations: Taiyuan Formation of Xiaoyiaoan–Longlinian (Kasimovian–Artinskian) Stages; Shanxi Formation of Zisongian–Longlinian (Asselian–Artinskian) Stages; and Liangshan Formation of Luodianian–Xiangboan (Kungurian–early Roadian) Stages (which spanned the Lower and Middle Permian). (1) In Taiyuan Formation of Xiaoyiaoan Stage (Kasimovian–Gzhelian), the mean inertinite abundance in coals ranges between 19.1% and 58.8%, with relatively high values occurring in the Ordos Basin. In the area of Jiangsu–Shandong–Henan, this value ranges between 8.3% and 28.6%. (2) In Taiyuan Formation of Zisongian–Longlinian (Asselian–Artinskian) Stages, the mean inertinite abundance remains high with significant variations, with the highest value of 51.6% occurring in Junggar Basin, and the lowest of 8.5% occurring in northern Jiangsu Province. (3) In Shanxi Formation of Zisongian–Longlinian (Asselian–Artinskian) Stages, the inertinite abundance slightly decreased when compared to the values of previous periods, ranging between 15.9% and 45.0%, (4) The inertinite abundance decreased from west to east from Taiyuan Formation to Shanxi Formation (Han 1996). (5) In the Liangshan Formation of Luodianian Stage (early Kungurian), the inertinite abundance was relatively low, ranging between 6.4% and 10.0%.
During Late Carboniferous–Early Permian, the pO2 increased significantly compared with the Early Carboniferous data. In Late Carboniferous, it reached the maximum value of almost 30%. It was above 27% throughout the Early Permian with no significant decrease and began to decrease since the late Early Permian, dropping down to approximately 24%.
3.2.3 Middle Permian
During the Middle Permian, coal-bearing strata of China were mainly developed in large-scale delta environments (Li et al. 2018), including the southern regions of northern China, west of Henan Province, southern and northern Anhui Province, Xuzhou area of Jiangsu Province, with semi-humid climate (Chang and Gao 1993; Tan 2017). Coal was mainly distributed in the following formations of Middle Permian: (1) Lower Shihezi Formation of Xiangboan Stage (late Kungurian–early Roadian), with mean inertinite abundance ranging between 7.9% and 45.3%; (2) Upper Shihezi Formation of Lengwuan Stage (Capitanian), with mean inertinite abundance ranging between 23.0% and 42.1%, and the inertinite mainly comprised of macrinite; (3) Tongziyan Formation of Lengwuan Stage (Capitanian), which was mainly distributed in Guangdong and Fujian provinces. The mean inertinite abundance varied greatly in different regions, ranging from 6.5% to 28.5%; (4) Liangshan Formation of Luodianian–Xiangboan Stages (Kungurian), of which the coal-bearing strata mainly developed during Early–Middle Permian. Coal seams were thickest in the Middle Permian, with the mean inertinite abundance ranging from 13.8% to 58.3%.
During the early Middle Permian, the pO2 increased slightly, and remained at approximately 28% until the late Middle Permian.
3.2.4 Late Permian
During the Late Permian, coal-bearing strata in China were mainly distributed in the southern regions (Liu 1990; Zhang 1995), dominated by a semiarid climate at the early stage, and an arid climate at the late stage (Chang and Gao 1993; Tan 2017). It was also formed in an epicontinental marine environment with humid climate conditions in the early Late Permian (Golonka 2011). The distribution of coal-bearing strata was directly controlled by palaeogeographical environment, with the transgression and regression causing continuous migration of coal-rich belts (Li et al. 2018).
Coal-bearing strata were mainly developed in Longtan Formation of Wuchiapingian Stage, and in Changxing and Wangjiazhai formations of the Changhsingian Stage. (1) In Longtan Formation, the mean inertinite abundance varied among different regions: 20.3%–20.7% in eastern Jiangxi; 13.0%–15.6% in southern Anhui, southern Jiangsu and Zhejiang; and 11.8%–21.1% in Yunnan, Guizhou, and Sichuan. (2) In Changxing Formation, the value was 2.4%–33.3% in Sichuan and Chongqing with large variations. (3) In Wangjiazhai Formation, the value was 23.7%–42.4% in Guizhou, with an average of 29.3%.
During the early Late Permian, pO2 did not vary much compared to the Middle Permian data, and remained stable at approximately 26%. During the mid Late Permian, it decreased to approximately 20%, and then increased once again to approximately 28% in late Late Permian.
3.3 Triassic
During the Triassic, gymnosperms quickly emerged and became dominant. At a global scale, the Triassic was characterized by dry and arid climate, although Middle Triassic was under increased rainfall and occasional humid conditions (Preto et al. 2010). The mass extinction event at the end Permian resulted in a 10 Ma coal-forming gap during the Triassic, when most regions were covered by widespread desert conditions (Kutzbach and Gallimore 1989). The coal was scarce during the Middle Triassic (Retallack et al. 1996).
Coal-bearing strata mainly developed during the Late Triassic in the humid, hot, and rainy southern regions in China. In particular, it occurred in the Xujiahe Formation of Peikucuo Stage (Norian–Rhaetian) in Sichuan, Yunnan, Hubei, Guangdong, and Guizhou (Zhang 1995; Shao et al. 2014). Among those, the strata in Sichuan had the best coal-bearing properties, under coal-forming environments including coastal plains, coastal lake–delta plains, and coastal delta plains (Lu et al. 2008). However, during the same period, northern China was mostly under arid or semi-arid climate. It was only at the end of Late Triassic that coal-bearing strata of river and lake facies were formed in Wayaobao Formation of the Ordos Basin (Tian et al. 2011).
The inertinite abundance varied between the northern and southern China. For example, in the Wayaobao Formation in the north-central Ordos Basin, it was about 24.8%, and was mainly semi-fusinite. However, in southern China, the coal-bearing strata almost covered the entire southwestern regions, with the mean inertinite abundance in coals ranging between 5.8% and 26.3%. In regard to the Maantang Formation of Yazhiliangian Stage (Carnian), the value was 7.9%–19.5% in Zixing (Hunan Province), and 23.2%–28.4% in Sichuan Province, respectively. It varied most in Anyuan Formation of Jiangxi Province, ranging from 1.1% to 21.2% (Table 3).
During the Triassic, the pO2 recorded a declining trend, from 25% in the early Late Triassic, to 18% at the end Late Triassic, which was the lowest value of the entire Mesozoic Era.
3.4 Jurassic
During the Jurassic, gymnosperms like cycads, conifers, and ginkgo were extremely abundant. The coal-bearing strata mainly developed in the northern and northwestern regions in China during the Early and Middle Jurassic, under a subtropical–warm temperate humid climate (Huang and Hou 1988). The coal-bearing basins were generally dominated by large- and medium-sized inland lake basins; while the coal-forming environments were dominated by alluvial-lake delta systems, followed by lacustrine systems (Li et al. 2018).
The coal-bearing strata during the Early Jurassic were mainly distributed in Badaowan Formation of Yongfeng Stage (Hettangian–Sinemurian) in the Junggar Basin of Xinjiang; Xishanyao Formation of Liuhuanggou Stage (Pliensbachian–Toarcian) in the Tuha and Yili basins, Xinjiang; as well as Xiahuayuan Formation of Shihezi Stage (Pliensbachian–Toarcian) in Hebei and Shandong regions (Shao et al. 2009; Shi et al. 2011). During that period, inertinite abundance in coal deposits varied significantly (Table 4), with mean values of 3.3%–14.8% in Badaowan Formation of Junggar Basin, and 4.7%–49.0% in Xishanyao Formation of Yili Basin.
During the Middle Jurassic, coal was well accumulated in entire northern China (Wu et al. 2008; Qin et al. 2009). Most of the coal-bearing strata were found in Yan’an Formation of Shihezi Stage (Pliensbachian–Toarcian) in the Ordos Basin. In Inner Mongolia, the mean inertinite abundance in coals ranged between 31.9% and 84.0%, with maximum values of up to 86.9% in Longdong Coalfield of Gansu Province. In Xishanyao Formation of Shihezi Stage (Pliensbachian–Toarcian) (Xinjiang), the mean inertinite abundance varied greatly, ranging between 12.2% and 68.8%. In Yaojie Formation of Shihezi Stage (Pliensbachian–Toarcian) in Tianzhu county (Gansu), the value was between 1.1% and 3.4%, which was the lowest values during that period. In Toutunhe Formation of Manasi Stage (Bathonian–Callovian) in Minhe Basin (Qinghai), the value was between 24.0% and 66.5%.
pO2 changed drastically during the Jurassic compared to earlier periods. Since the Early Jurassic, it began to rise and reached approximately 28% by the end Early Jurassic. During the Middle Jurassic, it rapidly increased to 30% or more, which was the highest value ever recorded and was maintained throughout the Late Jurassic.
3.5 Cretaceous
The Cretaceous was an important period of plant evolution. Angiosperms appeared and flourished, which provided important materials for coal production. During the Early Cretaceous, a series of continental fault basins were formed due to rifting and faulting activities in northeastern China, which provided sites for coal formation (Li et al. 1987). The coal-bearing strata were mainly formed in alluvial fans, fan deltas, lakeside deltas, and lacustrine environments. The coal-rich belts were mainly located on the sides of main faults near basin margins, with their distribution directions consistent with basin trends (Cai et al. 2011; Shao et al. 2013).
During the Early Cretaceous, coal-bearing strata were mainly distributed in northeastern China, e.g., on the western side of Greater Khingan Mountains and the southern margin of Songliao Basin. They occurred in the Muling Formation, Yixian Formation, and Jiufotang Formation of Rehei Stage (lower Aptian) in Heilongjiang; Fuxin Formation and Shahai Formation of Liaoxi Stage (upper Aptian) in Hebei; and the Huolinhe Formation of Jibei Stage (Berriasian–Hauterivian) and the Yimin Formation of Rehei Stage (Barremian) in Inner Mongolia. The mean inertinite abundance in coal is relatively higher in the west than in the east of northeastern China (Table 5). It varied greatly in some coalfields in western Inner Mongolia, e.g., ranging between 26.1% and 55.8% in Yimin Formation of Jalainur Coalfield, with 42.4% on average. However, this value was relatively low in coalfields in eastern Inner Mongolia. For example, it ranges between 2.2%–16.5% in Fuxin and Tiefa Coalfields of Liaoning Province, and 9.4%–25.0% in Jixi and Hegang Coalfields of Heilongjiang Province. During the Early Cretaceous, pO2 was slightly lower than that during the Jurassic, which, however, still kept at a relatively high level of approximately 25%.
3.6 Paleogene to Neogene
Since the Cenozoic, evergreen and deciduous broad-leafed plants of modern angiosperms have gradually flourished, which indicated the obvious seasonal climate changes, as well as the rapid development of modern plants which are more adapted to severe climatic conditions (Zhao et al. 1995).
From Paleogene to Neogene, coal-bearing strata mainly developed in small continental basins in the northeastern and southwestern China. The coal-forming processes mainly occurred in the lake and delta marsh environments (Zhang 1995; Li et al. 2018).
During the Paleogene, the coal-bearing basins were mainly distributed to the east of Greater Khingan Mountains and north of Qinling Mountains and southwest of Guangxi. The coal-forming periods include Eocene and Oligocene. (1) The Eocene coal-bearing strata mainly included Hunchun and Shulan formations of Ashantou Stage (Ypresian–Lutetian) in northeastern China; Nadu Formation of Yuanquan Stage in Baise and Nanning basins in Guangxi; and Lijiaya Formation of Yuanquan (Bartonian) in Liangjia Coal Mine in Shandong. (2) The Oligocene coal-bearing strata included the Pinghu Formation of late Caijiachongian Stage (Priabonian) within Xihu Depression of East China Sea, and the Yacheng Formation of Tabenbuluckian Stage (Chattian) in Qiongdongnan Basin. The inertinite abundance in coal was generally low (Table 6), with the mean values ranging between 1.1% and 11.4%. (3) During the Oligocene, a few coal seams developed in Huerjing Formation of Wulanbulagean Stage (Rupelian) in Erlianhot, Inner Mongolia, within which the inertinite abundance was very high, ranging from 23.8% to 38.7%, with a mean value of 30.4%, which was significantly higher than that in other regions during the same period.
During the Neogene, coal-bearing strata were mainly distributed in the Miocene and Pliocene coal basins in the eastern coastal and the southwestern areas of southern China (Zhang 1995), mainly in continental lacustrine basins. The Miocene coal-bearing strata included the Xiaolongtan Formation of Tonggurian Stage (Langhian–Serravallian) in Yunnan Province and the Nanzhuang Formation of Shanwangian Stage (Burdigalian) in the northwestern basins of Taiwan. The Pliocene coal-bearing strata mainly included the Shagou Formation of Mazegouan Stage (Piacenzian) in Shaotong–Qujing Basin of Yunnan. During the Neogene, the total inertinite abundance was relatively low, with the mean value ranging between 0.2% and 10.2%.
During the Quaternary, coal seams were generally developed in Yuanma Formation of Nihewanian Stage (Gelasian–Calabrian) in Tengchong, Yunnan. The inertinite was mainly composed of filamentous bodies and fungi, and was of low abundance (mean value of 4.0%) (Jin and Qin 1989).
Throughout the Cenozoic, pO2 fluctuated around 21%, which was roughly the same as that in the current atmosphere.
3.7 Inertinite abundance and pO2 evolution
During the entire Phanerozoic, the abundance and distribution of inertinite in coal varied greatly. In terms of time, there were four major cycles in which the inertinite abundance first increased and then decreased, i.e., during Early Devonian–Late Permian; Late Triassic–Early Jurassic; Middle Jurassic–Late Cretaceous; and Paleogene–Neogene, respectively (Figs. 2, 3a). During these periods, the evolution process of pO2 also changed a lot (Figs. 2, 3b).
3.7.1 Early Devonian–Late Permian
During the Paleozoic, the inertinite abundance in coal was at a very low level in the Early Devonian. During Middle Devonian, this value was as low as 0.1% in Qujing Formation of Dongganglingian Stage in Yunnan. From Early Carboniferous to Late Permian, accompanied by the unprecedented diversity occurring in the plant kingdom, this value increased rapidly and reached its peak of 58.8% in Taiyuan Formation of Xiaoyiaoan Stage in Junggar Coalfield, Inner Mongolia. It remained high throughout the Permian, e.g., in Changxing Formation of Changxingian Stage in Guizhou Province. Until Late Permian, it slowly decreased to 30.7%. From Devonian to Permian, the pO2 kept increasing and reached a maximum value of approximately 29% during the Middle Permian. Throughout Permian–Triassic, the pO2 values fluctuated several times. However, prior to the Early Triassic, the pO2 values had rapidly decreased to 23%.
3.7.2 Late Triassic–Early Jurassic
During the Mesozoic, the inertinite abundance in coal-bearing strata of China was quite different from that of the Paleozoic. During the early Late Triassic, it rapidly decreased to 1.1%, the lowest value of the entire Phanerozoic, in Anyuan Formation of Peikucuo Stage in Leping (Pingxiang, Jiangxi). Immediately after the global coal gap of the Early and Middle Triassic, the inertinite abundance of the Late Triassic increased rapidly until reaching the same level as before the Early Triassic, with the peak value of 28.4% in Sichuan. From Late Triassic to Early Jurassic, the value decreased rapidly, only 4.7% in Xishanyao Formation of Liuhuanggou Stage in Tuha Basin (Xinjiang). Correspondingly, during the early Late Triassic, pO2 was at the lowest level of only 18%, which, however, increased rapidly since the Early Jurassic, reaching up to 26% by the end of Early Jurassic.
3.7.3 Middle Jurassic–Late Cretaceous
During the entire Jurassic, the inertinite abundance displayed an increasing trend. It was not high at the Early Jurassic, but increased rapidly since the early Middle Jurassic, and reached 84%, i.e., the maximum value of the entire geological history at Yan’an Formation of Shihezi Stage in Gansu Province. However, the value significantly decreased since the Early Cretaceous. By the Late Cretaceous, it reduced to 28.3% in Fuxin Formation of Liaoxi Stage in Wanquan Coalfield, Hebei Province. Correspondingly, from Middle Jurassic to Late Cretaceous, pO2 also changed greatly, which first increased to over 30% during the Middle Jurassic, and then slowly decreased but still remained as over 25%.
3.7.4 Paleogene–Neogene
The inertinite abundance in coal-bearing strata decreased during the Cenozoic when compared with prior periods, but was relatively stable. However, this value increased since the beginning of the Paleocene. During Oligocene, it reached 30.4%, the maximum value of the entire period, in the Huerjing Formation of Wulanbulagean Stage in Erenhot, Inner Mongolia, and then decreased again. During Neogene, it decreased to 1.8% in Shagou Formation of Mazegouan Stage. During the entire Cenozoic, pO2 did not change significantly, remaining between 20% and 22%, which was equivalent to the current oxygen levels in the atmosphere.