Standfirst: The manifestation of oceanic anoxia is considered to be the result of the development of organic-rich shales. However, black shales are not directly associated with oceanic anoxic environments; instead, they are related to cool-temperate zones. Black shales do not develop under greenhouse conditions. Vleeschouwer et al.1 posited that ‘the Devonian warmhouse and the Late Cretaceous hothouse were characterised by recurrent episodes of marine anoxia, potentially paced by astronomical forcing’ and highlighted the fact that a key indicator of marine anoxia is the presence of ‘several globally recognised organic-rich shale layers deposited under varying pre-Cenozoic climate conditions’. However, we contend that their argument is erroneous.
1. No causal relationship between the oxygen minimum zone and black shale
The modern OMZ occurs within the 600–1,200-m water-depth range in mid- and low-latitude oceans. In this zone, the sea-surface temperature is high, while the seabed temperature is low, which results in stable ocean stratification. Under these conditions, the dissolved oxygen (DO) content is directly proportional to water pressure and inversely proportional to water temperature (FIG. 1a), with the seabed typically being in an oxygen-rich state. Because it is disturbed by waves, tides and ocean currents, the ocean surface is actually rich in oxygen, and the OMZ consequently appears at the bottom of the depth range that is unaffected2. In high-latitude regions, there exists strong vertical convection due to the instability of seawater stratification, resulting in an oxygen-rich state of the shallow to deep waters (FIG. 1a and FIG. 1d).
 
Fig. 1 | Modern Marine dissolved oxygen (DO) and biological distribution characteristics. (a) Vertical distribution of DO in the middle and low latitudes of different oceans. (b) Abundance of living radiolarians vs. water temperature in a section at 8°N. (c) Radiolarian abundance vs. depth at various stations in the South China Sea5. (d) Distribution of DO in a meridional vertical section of the western Atlantic Ocean.

The characteristics of DO distribution in modern oceans, from high latitudes to the equator (FIG. 1d), are analogous to those observed during a transition from cold to warm conditions following climate change. As the temperature increases, the OMZ expands and thickens, yet the seabed and upper ocean remain rich in oxygen. Because the epipelagic zone is influenced by waves and ocean currents, the OMZ can only appear at its bottom (water depth > 200 m).
Marine organisms primarily inhabit nearshore waters at depths ranging from 0 m to 100 m and within 300 km of the continental shelf, with higher concentrations observed nearer to the continent3. The depths corresponding to the OMZ are mainly characterised by continental shelves and slopes, where fine sand and silt are predominantly developed, and the total organic carbon (TOC) content is low (<1%), similar to open ocean environments. Black-shale formation primarily occurs in restricted lagoon environments at the continental margins2, which aligns with the conclusion by Hartnett et al.4 that over 90% of organic carbon in the ocean is buried in sediments at the continental margins.
Therefore, there is no causal relationship between black shales and the OMZ.
2. Organic-rich shale cannot develop under greenhouse conditions.
Vleeschouwer et al.1 proposed that the oceanic anoxic events during the Late Devonian and Late Cretaceous, accompanied by the development of organic-rich shales, indicate the existence of a greenhouse environment. However, this is actually contradictory to the carbon sequestration patterns observed historically and in modern times.
As an example, the Jurassic period is divided into five stages, each characterised by the development of coal in the high-latitude cold temperate zones, the formation of red beds in the low-latitude regions (FIG. 2a) and the presence of carbonaceous shales in the intermediate latitudes. Under most conditions, coal, black shales and red beds do not develop simultaneously. When temperature is low, coal and black shales tend to develop at low latitudes; conversely, when temperature is high, they develop at high latitudes, which is a universal feature. Li et al.6 studied the onset times of Cretaceous oceanic anoxic event 2 (OAE2) (the time when black-shale formation began) in various locations globally and found significant variations in them. Similar to climatic fluctuations during the Jurassic period, this process is essentially climate cooling (FIG. 2b), where black-shale formation progressed from high latitudes (70°N) to low latitudes (30°N) over a period of 0.55 Ma. The southernmost extent of this cooling event reached only 30°N, allowing for the development of the red beds of the Guanzhai Formation in the Hekou Basin, Fujian Province (26°N), which lies to the south of this latitude7.
 
Fig. 2 | Climate fluctuations affecting sedimentation. (a) Distribution of coal and red beds in different ages of the Jurassic period in the continental region of eastern China. (b) Relationship between OAE2 onset time and latitude.
Similar features have persisted from the Triassic period to the present (FIG. 3b–3h), aligning perfectly with the high TOC content observed in surface sediments of modern wetlands, which are primarily distributed in cold-temperate regions (FIG. 3j). Meanwhile, low-latitude tropical regions are characterised by the development of red beds or bauxite deposits (FIG. 3i). Red beds and black strata can coexist within the same geological period but not within the same climatic zone. This implies that coal and black shales cannot develop under tropical climatic conditions (FIG. 3h, i).
 
Fig. 3 | Comparison of ancient and modern sediments at different latitudes.
3. Cooling trend already existed prior to the development of black shales.
The development of organic-rich shales is associated with climate cooling. However, the process leading to this climate cooling was not determined by any single event during or prior to the deposition of the black shales. As an example, the Triassic period of the Sichuan Basin witnessed a transition from hot to cold and wet conditions10. The Feixianguan Formation at the base of the Triassic system is characterised by the development of thick red beds with low organic content(FIG. 4). In contrast, the Xujiahe Formation at the top of Triassic System is characterized by the development of organic-rich shale and coal seams. This cooling trend, which has been ongoing since the Early Triassic and has lasted for 51 million years, was not driven by volcanic activities recorded in the Xujiahe Formation or a single astronomical forcing cycle.
 
Fig. 4 | Relationship between climate evolution and carbon burying in the Triassic System of the Sichuan Basin
In summary, although Vleeschouwer et al. (2024) 1 did not directly address the depositional environment of organic-rich shales, their hypothesis regarding oceanic anoxic events (OAEs) primarily relies on the occurrence of black shales. However, both ancient and modern evidence suggests that the formation of black shales is more closely associated with cold and wet climates, than within anoxic conditions. This finding contradicts their assertion that the Devonian and Late Cretaceous periods were characterized by hothouse climates. Consequently, their premise that marine anoxia is paced by astronomical forcing appears to be flawed.
Acknowledgements
The authors acknowledge the support of the project ‘Hydrocarbon formation mechanism and resource evaluation of deep-ultra-deep ancient source rocks based on Shallow, Land, Sealed and Cold ideas’ funded by the State Energy Key Laboratory for Carbonate Oil and Gas.
Competing interests
The authors declare no competing interests.
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