Re: Linking carbon cycle perturbations to the Late Ordovician glaciation and mass extinction: A modeling approach

The paper by Zhang et al. (2024) (hereinafter referred to as the ‘Z paper’) contains three serious errors:

1. The enrichment of organic matter in black shale is not caused by anoxic environments. Black shale is one of the climate-sensitive sediments and represents cold temperate zones.

The main conclusion of the Z paper is that the onset of the Hirnantian glaciation led to a decrease in global temperature, and its termination resulted in increased organic carbon export and deep-water anoxia. This viewpoint represents the mainstream opinion, but it is an incorrect hypothesis that contradicts modern sedimentary and environmental science as it lacks supporting evidence from modern sedimentary studies.”

Modern sedimentary types are related to climatic zoning. Roughly, from north to south, they can be divided into four zones: the cold zone, the cool-temperate zone, the subtropical high-pressure zone, and the tropical zone. (1) The cold zone represents polar climates, with no sedimentary records and minimal biomass (Figure 1f). (2) The cool-temperate zone is a cold and wet climate zone with the highest carbon sequestration in wetlands (Figure 1d). The coverage of peatlands and the content of soil organic matter are also the highest in this climate zone or in alpine cool-temperate zones (Figures 1c, e,Yin,1991). Furthermore, this zone is currently forming thick layers of peat and future black shale. (3) The subtropical high-pressure zone is a warm and dry climate zone, dominated by evaporites, poor in organic matter, and calcium-deficient. (4) The tropical zone represents low-latitude tropical rainforest climates with strong weathering effects. Besides the decomposition of organic matter and significant loss of major nutrients, silicates are also decomposed into silicic acid and leached away, leading to relative enrichment of aluminum and iron, and high kaolinite content. The oxidation of ferrous iron to ferric iron results in a red color, making red soil dominant in tropical regions.

The reason why organic matter is enriched only in the cold temperate zone is that, although biomass is low in the cold temperate zone (Figure 1f), the activity of microorganisms that cause mineralization is weak, leading to the preservation of organic matter that is difficult for microorganisms to decompose. Therefore, the coverage area of peatlands, the organic carbon content in wetlands, and the organic carbon content in soils are all the highest in the cold temperate zone (Figure 1c, d, e). Although the equatorial tropical zone has the highest biomass (Figure 1f), mineralization is extremely strong, making it impossible to effectively preserve organic matter. Even under anoxic conditions, microbial activity is high in tropical climates (Gudasz et al.,2010). Therefore, the enrichment of organic matter and the development of black shale and coal in the cold temperate zone are not due to oceanic or atmospheric anoxia; whereas the development of red beds and the scarcity of organic matter in tropical regions are not due to oceanic or atmospheric oxygenation.

Fig.1 Curve of sediment and biomass with latitude in ancient and modern times

Ancient strata and modern sediments exhibit similar patterns. Despite fluctuations in paleoclimate, from a statistical perspective, the distribution of climate-sensitive sediments from the Triassic period to the present is fundamentally consistent with that of modern deposits. In high-latitude cold temperate zones, coal and black shale are developed, represented by coldness and the color black; in mid-latitude subtropical high-pressure belts, arid conditions prevail, leading to the development of evaporite rocks, represented by warmth and the color white; in low-latitude tropical regions, bauxite and red beds are prevalent, denoted by heat and the color red (Figure 1a, b).

Deng et al. (2017) conducted a detailed study on the Jurassic strata, organizing the development of climate-sensitive sediments from various major basins in China and reconstructing the paleoclimate of the Jurassic. They divided it into five stages: the Badaowan Formation, Sangonghe Formation, Xishanyao Formation, Toutunhe Formation, and Tuchengzi Formation deposition periods, which were characterized as cold, hot, severe cold, hot, and extremely hot, respectively, as shown in Table 1. It can be concluded from the table that the development of coal and black shale is strictly controlled by climatic zones. When the climate becomes colder, the distribution range of coal and black shale shifts further south, and coal and black shale can also be found at lower latitudes. When the climate becomes warmer, the distribution range of coal and black shale shrinks northward, and evaporative salt rocks and red beds are widely distributed across China. However, the statistical trend of the latitude-based distribution of climate-sensitive sediments (as shown in Figure 1a,b) is consistent with modern sediment deposition. Coal and black shale are developed at high latitudes, evaporative salt rocks are found at mid-latitudes, and red beds or bauxite are present at low latitudes. Red and black are mutually exclusive in this context.

Table 1 Developmental Ages of Climate-Sensitive Sediments in Various Periods of the Jurassic in Mainland China

That is to say, the formation of black shale is not caused by oxygen deficiency, and it has no relationship with whether the ocean is anoxic or not. This is the first mistake in Z-paper.

1. Black shale formation and mass biological extinction are not the same geological event, and they should not be confused.

Z-paper argues that “increased productivity and sea-level changes leading to ocean anoxia further suppressed continental shelf habitats, which was a key factor in the second mass extinction.” This clearly treats the development of black shale and mass biological extinction as a single geological event with a causal relationship, while neglecting the fact that cold climate was the key factor.

Fluctuations in ancient climate can cause climatic zones to shift southward or northward, resulting in the deposition of sediments from different climatic zones at the same location over different periods of time. For example, a climate change from hot to warm to cold constitutes a temperature decline, forming an inverse cycle from red to black sediments. In stratigraphic sequences, this would correspond to the sequential development of red beds or bauxite, evaporite rocks, and black shale. Conversely, this would represent a positive cycle of temperature increase. During the Late Ordovician period, there was a black-white-red climate inverse cycle, indicating a cooling process. From the late Middle Ordovician to the early Late Ordovician, the development of red beds in the Baota Formation (Xu et al, 2024) transitioned to the development of stone coal and black shale in the Wufeng Formation, which is rich in graptolite fossils, indicating a general trend from hot to cold. If the climate continued along this trend, the Upper Yangtze region would have transitioned from a cool temperate zone to a cold zone, similar to modern polar climates. Research by Shen et al. (2023) suggests that the Guanyinqiao Formation represents the strongest period of the Hirnantian glaciation, with minimal biomass. Additionally, the thin stratigraphic thickness of the Guanyinqiao Formation is consistent with the characteristic of little or no sedimentary records in cold zones, indicating that most of the Earth was covered by cold zones at this time. This period corresponds to the first mass extinction event (Harper et al.,2014).

Therefore, the mass biological extinction and the development of black shale (which Z-paper mistakenly attributes to ocean anoxia) are not the same geological event. The Wufeng Formation and Longmaxi Formation black shale are rich in graptolites, while the Guanyinqiao Formation represents the coldest period of the glaciation. These two belong to different stages and have no causal relationship. The mass biological extinction simply follows the overall trend of temperature decline preceding the development of shale, which is the process of transitioning from a cool temperate zone to a cold zone.

3.The author of the Z-paper lacks understanding of the aquatic environment in which shale develops and mistakenly assumes a causal relationship between the positive correlation between bentonite stripe density and the degree of anoxia. According to their Figure 5, there is a positive correlation between volcanic activity intensity and the carbon isotope of black shale in the Hirnantian stage, and they propose that “persistent volcanic activity terminated glaciation.” Many scholars have also observed similar phenomena, where the bentonite stripes developed within the black shale of the Late Ordovician Wufeng-Longmaxi Formation show a positive correlation with TOC of shale, leading to the proposal that volcanic ash promotes productivity (Qiu et al., 2019). This violates the exclusivity principle in logic.

In reality, this is merely the preservation of these materials and geological phenomena in a relatively enclosed environment. Whenever black shale develops, volcanic ash, polymetallic mineralization, and gravity flow phenomena can be observed in most cases. And the more enclosed the environment is, the denser these phenomena become.

Shale develops in water-filled depressions such as wetlands, small lakes, lagoons, and other enclosed aquatic environments(Mao et al., 2023). These aquatic environments are relatively enclosed, with quiet water bodies undisturbed by ocean currents or waves. Fine-grained sediments, along with organic matter, vertically settle to the bottom of the water through suspension and flocculation. Because of this special environment, it can record a variety of geological information and sedimentary phenomena, and can preserve them well. Some elements (such as U, Cr, Cu, Pb, Zn, etc.) tend to abnormally accumulate in this environment. Up to 25 types of mineral deposits (including non-ferrous metals, ferrous metals, rare, scattered, and dispersed metals, precious metals, non-metallic minerals, and energy minerals) have a close relationship with black rock series, often forming large to super-large mineral deposits (Li et al., 2022). This environment can also record detailed volcanic activity and gravity flow phenomena in the surrounding area. They coexist but do not have a causal relationship; they are associated rather than symbiotic.

During the Sandbian Stage of the Late Ordovician (a period of great warmth), the Baota Formation limestone(Katian stage) was developing in the Upper Yangtze region, indicating an open water environment with no record of volcanic activity. At the same time, the Pingliang Formation was developing in the Ordos Basin, which consisted of interbedded sandstone and mudstone. The shale section, being in a relatively closed water environment, also recorded abundant tuff and gravity flow phenomena (Wang et al., 2015), while these phenomena “collectively disappeared” in the non-shale sections. The main reason is that the non-shale sections developed in an open aquatic environment with strong hydrodynamic forces, which prevented the recording of these information (Mao et al., 2023). However, the absence of records does not mean that the events did not occur. In this environment, organic matter is decomposed, ore-forming elements and volcanic ash are diluted, and gravity flow slump deposits are destroyed. Only when the deposition is close to the volcanic crater and has a large thickness, geological records of volcanic activity can be preserved, while volcanic ash deposits far from the volcanic crater are difficult to retain. If we only focus on Southern China, we might erroneously conclude that volcanic activity during the Hirnantian Stage was stronger than that during the Katian Stage. Therefore, to compare the intensity of volcanic eruptions, it is necessary to exclude other interferences and ensure exclusivity.

In summary, the use of black shale as an indicator of anoxia and volcanic activity intensity as evidence for anoxia in this paper is a misconception. The carbon isotope evidence, δ¹³C_carb must be subject to stratigraphic evidence.

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