The conclusion "coal records were associated with a median temperature of 25°C," reached by Bao et al. [1], is erroneous, due to its violation of the coal-forming environments from the Permian period to the present and modern peat accumulation.

Numerous scholars have documented that since the Permian, coal seams have primarily developed at high latitudes [1,2,3]. The situation in the Paleozoic era was distinct. The paleogeographic locations of most coal-producing basins in the Northern Hemisphere during this period are believed to have been situated near the equator. Consequently, it has been concluded that coal can form under tropical conditions. In conjunction with the modern development of tropical peat, there is a near-universal consensus that peat and coal can form under a variety of climatic conditions, primarily in tropical regions, and serve as an indicator of past rainforests [2]. Furthermore, organic-rich black shales are also considered to develop predominantly in warm regions characterized by high primary productivity.

However, peat can only develop in plateau cold temperate zones or cold temperate zones (abbreviated as: Cold Rule) [3]. Yu et al. [4] documented 116 of the world's most notable tropical peat cases. Among them, there are only four peat samples with relatively younger ¹⁴C ages, ranging from 250 to 1000 years Before Present (BP), while the vast majority of the peat samples are older than 1000 years BP. At least ten cold events have occurred in the past 12,000 years [5], which may have provided conditions for the development of low-latitude peat. Furthermore, substantial evidence indicates that tropical peat is currently in a stage of decomposition [6,7]. The largest tropical peatlands are located in Kalimantan and Sumatra, Indonesia, where the peat reservoir has transitioned from being a net carbon sink to a significant source of atmospheric carbon, currently facing the risk of elimination [6]. While the existence of tropical peat cannot be completely denied, one thing is certain: the amount of tropical peat is relatively small, accounting for only one-tenth of the total global peat reserves [4]. Mao Xiaoping et al. [3] conducted a statistical analysis of the total organic carbon in surface sediments of wetlands and lakes globally, revealing that peat TOC content is highest in the cold temperate zone (Fig. 1d). Therefore, peat or coal can serve as reliable lithologic indicators of cold and wet climates. Evaporative salt rocks are indicative of the mid-latitude subtropical high-pressure belt; whereas red beds, bauxite, and coral reefs represent low-latitude tropical rainforest climates (Fig.1 b-d) [3]. This limits the development environment of peat or coal to boreal zones with an annual average temperature of <5°C or a latitude of >45°.

The Cold Rule also applies to the Paleozoic era. The dominance of ferns in the Paleozoic did not result in significantly stronger mineralization resistance in tropical regions compared to modern times. Numerous environmental studies have demonstrated that lower temperatures are more conducive to carbon sequestration [3,8,9]. The statistics compiled by Ziegler et al. [2] indicate that the latitudes of coal development from the Permian to the Cenozoic are primarily within the boreal zone, ranging from 45° to 70°. Generally, coal seams develop at high latitudes, while red beds develop at low latitudes (Figure 1). When global climate cools, the boreal zone shifts towards the equator, resulting in coal development at lower latitudes. Conversely, during global warming, the boreal zone migrates towards the poles, leading to coal formation at higher latitudes, while red beds develop at mid-to-low latitudes. The lithologic indicators of climate composed of coal and red beds can provide more refined corrections to paleoclimate inferences. It can be observed that the Jurassic period was generally a process of global warming (Figure 1), contrary to the mainstream view represented by Scotese [10].

North China and South China do not possess the conditions at their current latitudes for the extensive accumulation of peat or the development of black shale, unless the climate here shifts to a cold temperate zone. The development of coal seams or black shales demands extremely stringent conditions and forms during an extremely short time window of global climate cooling. For example, the deposition of black shales in the Late Ordovician Wufeng-Longmaxi Formation took less than 2 million years (Ma), and the coal-forming periods of the various members of the Upper Triassic Xujiahe Formation in the Sichuan Basin lasted for less than 0.5 Ma. Climate fluctuations can transform subtropical South China into a cold temperate zone, as evidenced by the development of carbonaceous shales and coal seams in Guangxi during the Late Triassic [3]. Regardless of the paleolatitudes of various landmasses, the Cold Rule has been applicable from the Paleozoic era to the present. If South China was located near the equator during the Paleozoic, the possibility of such a drastic climate fluctuation down to a cold temperate zone and develop coal would be extremely low, as the global mean surface temperatures (GMSTs) used by Bao et al. [1] indicate a minimum of 20°C even in equatorial regions. Sacrificing the Cold Rule, which aligns with the distribution of modern peat and Mesozoic-Cenozoic coals, is not advisable. Therefore, we must re-examine the accuracy of paleocontinental reconstructions primarily supported by paleomagnetic data.

Fig. 1 (a)Distribution of Jurassic coal and red beds in Eastern China; (b) Distribution of coal and evaporites from the Permian to the present; (c) Distribution of bauxite; (d) Distribution of TOC in wetlands.

In summary, the conclusion that coal-forming environments can only be in cold temperate zones must be treated as a fundamental constraint in paleoclimate reconstructions. Errors in calculating the paleolatitudes of coal-producing plates and inaccuracies in paleotemperature estimates derived from paleoclimate models in Bao’s study [1], which fail to account for the extremely short time window for coal formation, result in numerical simulations based on these multiple unreliable outcomes inevitably leading to the erroneous conclusion.

References

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