Walker-Trivett (2024)¹, based on observed anomalies in mercury concentrations, proposed that Kerguelen volcanism triggered Oceanic Anoxic Event 2 (OAE2), leading to a super-hot-house climate, which was accompanied by the deposition of organic-rich shales and mass extinction events. A crucial link in this logical chain is the assertion that the development of organic-rich shales was caused by the oceanic anoxic event. However, both modern and ancient evidence indicate that there is no direct correlation between the deposition of organic-rich shales and super-hot-house climate or oceanic anoxia. Their paper essentially argues for a potentially non-existent hypothesis, namely OAE2, and the claim that the paleoclimate during OAE2 was characterized by a super-hot-house climate is fundamentally flawed. It is now imperative to reevaluate the validity of the OAEs hypothesis and to investigate whether OAEs are genuinely linked to the formation of black shales. The reasons are as follows.

1. No causal link between the OMZ and the deposition of organic-rich black shales

Geology, while not characterized by the rigorous formulaic derivations of mathematics, must still adhere to the logical rules of limited extrapolation: proceeding from the near to the far, and from modern times to ancient times in a constrained manner. If there is not a single instance in the modern environment that demonstrates a correlation between the Oxygen Minimum Zone (OMZ) in warm, mid- to low-latitude regions and the development of black shales, we cannot affirm that oxygen deficiency led to the formation of black shales at any given period.

First, modern OMZ are found in mid- to low-latitude oceans at depths ranging from 600 to 1200 m. In this region, the sea surface temperature is high, while the seabed temperature is low (around 1°C), and the ocean stratification is stable. At this point, the dissolved oxygen (DO) content is directly proportional to water pressure and inversely proportional to water temperature. Due to high pressure and low temperature, the seabed is typically in a state of oxygen enrichment. According to theoretical formulas, the sea surface should have the lowest DO concentration, meaning that DO increases with water depth (as indicated by the dashed line in Fig. 1a). However, owing to the disturbance of waves, tides, and ocean currents, the surface ocean is rich in oxygen, and consequently, the least oxygenated zone appears within the unaffected water depth range (600-1200 m)²(Mao et al., 2023). In high-latitude regions, the annual mean sea surface temperature is low (approximately <-10°C), whereas the seabed temperature is relatively higher (also approximately 1°C), leading to unstable water column stratification and intense vertical convection. From shallow to deep waters, the environment is characterized by oxygen enrichment without any anoxic zones, and the DO concentration exceeds 8 mg/L (as shown in Fig. 1a and 1d).

Figure 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°N³. (c) Radiolarian abundance vs. depth at various stations in the South China Sea³. (d) Distribution of DO in a meridional vertical section of the western Atlantic Ocean⁴.

The distribution characteristics of dissolved oxygen (DO) from high latitudes to the equator in modern oceans (Fig. 1d) can be analogous to the relationship of climate transitioning from cold to warm. As the climate warms, the Oxygen Minimum Zone (OMZ) expands and thickens, yet the seabed remains in an oxygen-rich state. Conversely, when the climate cools, the OMZ contracts and may disappear altogether. Extensive test data indicate that DO levels in lakes and reservoirs are entirely governed by seasonal variations, with the entire water column, from surface to bottom, being oxygen-rich in winter³(Yu Xiao, et al., 2020). Walker-Trivett (2024)¹ suggests that "the carbon release from the Kerguelen Large Igneous Province likely induced ocean warming, which, during the Cretaceous, may have disrupted the thermocline, triggering the upwelling of nutrient-rich waters to sustain enhanced productivity." Clearly, this hypothesis contradicts contemporary environmental science. When the ocean warms, the thermocline is not disrupted; on the contrary, the surface stratification becomes more stable, leading to an expansion in the thickness and extent of the OMZ ⁵(Busecke et al., 2022). However, even with rising temperatures, the OMZ can only occur below the wave base and within the influence of ocean currents, typically at depths greater than 300 m ²(Mao et al., 2023).

Secondly, the primary contributors to marine primary productivity are located within the depth range of 0 m to 50⁓100 m (Fig. 1b,1c), and in nearshore waters less than 300 km from the continent, productivity tends to be higher closer to the continent⁴(Ma et al., 2022). In modern oceans, within the depth range of the OMZ, the primary features are continental shelves and slopes, where shale development is absent²(Mao et al., 2023). These areas are influenced by ocean currents and tidal flows, with strong hydrodynamics. Due to the input of large amounts of terrigenous clastic material, sand waves develop, forming fine silt, and the total organic carbon (TOC) content of surface sediments is generally less than 1%. Pelagic surface sediments contain clay, but the TOC is generally below 0.6% (data from IODP). This phenomenon aligns with the conclusion by Hartnett et al.⁷ that more than 90% of organic carbon in the ocean is buried in continental margin sediments.

Therefore, there is no intersection or causal relationship between the production and preservation conditions of organic matter and the OMZ. There is no modern evidence to support the development of black shale in anoxic environments during OAE2.

2. Organic-rich shale cannot develop under greenhouse climate conditions.

Walker-Trivett (2024)¹ argues in their paper that during the OAE2 stage, the Earth was under super-hot-house conditions, and the Kerguelen LIP carbon release likely caused ocean warming, providing the material conditions for the development of black shales.  This notion is consistent with prevailing petroleum geology and sedimentology theories. According to mainstream thought, the enrichment of organic matter hinges on the proliferation of plants and algae. It is only when greenhouse plants flourish and algae bloom that organic matter can accumulate significantly, facilitating its burial. However, this is actually a common misconception in geology, as this view conflicts with environmental science. Environmental science contends that, amidst global warming, former carbon reservoirs can transform into carbon emissions, detrimental to carbon sequestration^9,10(Chen et al., 2024; Kai et al., 2016).

Based on the preservation and accumulation characteristics of organic carbon in modern sediments in recent years, this paper proposes the delineation of black shale primarily developing in high-latitude cold-temperate climates using three types of climate-sensitive sediments⁸(Mao et al., 2024).

Taking the Jurassic period as an example, this era is divided into five stages, each characterized 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 occurrence of carbonaceous shales in the intermediate latitudes. In low-latitude tropical areas, if conditions similar to those of the cold-temperate zone exist—such as high-altitude plateaus with cold-temperate climates or wetlands and continental shelves where deep waters result in lower temperatures at the seabed, approximating cold-temperate climatic conditions—carbonaceous shales can also form in these low-latitude regions. At this point, there is some overlap between the latitudinal zones conducive to the development of black shales and those favorable for red bed formation. However, the overall trend is that coal and black shales develop in high-latitude cold-temperate zones, while red beds develop in low-latitude tropical zones. Under most conditions, coal, black shales, and red beds do not develop simultaneously. As illustrated in Fig. 2a, when the climate cools, coal and black shales tend to develop at lower latitudes; whereas if the climate warms, coal and black shales form in high-latitude regions.

Figure 2. Impact of climate fluctuations on sedimentary rocks. (a) Distribution of coal and red beds across various ages of the Jurassic period in the continental region of eastern China. (b) Relationship between the onset time of OAE2 and latitude.

Apart from the Jurassic period, other geological periods exhibit similar characteristics. From the Triassic to the present, coal and black shale are primarily distributed in the high-latitude cold-temperate zone (Fig. 3b-3h)¹²(Parrish et al., 1982), indicated by black markings. This distribution aligns perfectly with the high TOC content observed in modern wetland surface deposits, which are also primarily found in cold-temperate zones (Fig. 3j). In contrast, the mid-latitude subtropical high-pressure belt is dominated by evaporites, denoted by white markings, while the low-latitude tropical zone is characterized by the development of red beds or bauxite (Fig. 3i)¹³(Zhang et al., 2021), indicated by red markings. Although red and black may coexist within the same geological period, they do not occur in the same climatic zone. This determines that coal and black shale cannot develop under tropical climatic conditions (Fig. 2a).

Figure 3. Comparison of Ancient and Modern Sediments at Different Latitudes

The reason why high carbon sequestration intensity does not occur in the high-productivity equatorial or anoxic environments is that, in the low-productivity cold-temperate zone, although biomass is low, the activity of microorganisms that facilitate mineralization is weak, making it difficult for organic matter to decompose and thus allowing it to be preserved¹⁰(Kai et al., 2016). In contrast, the equatorial tropical zone has the highest biomass and primary productivity, but mineralization is extremely strong, preventing effective preservation of organic matter. Even under anoxic conditions, anaerobic microbial activity in tropical climates is intense¹⁴(Gudasz et al., 2010), leading to vigorous organic matter mineralization. This characteristic aligns with most cases where climate warming results in carbon storage being released as carbon emissions, consistent with environmental science. The only exception occurs during the period of global deglaciation when temperatures increase, analogous to the modern climatic shift from the polar cold zone to the cold-temperate zone, as depicted by points F to C in Fig. 3j.

Due to global climate change, when temperate zones appear at a given latitude, the development of black shales or coal occurs in that corresponding latitude belt.  Focusing on the OAE2 event, Li et al.¹⁵ investigated the onset times of OAE2 across multiple global locations and found significant variations in the initiation times (marked by the beginning of black shale development) among different sections. Based primarily on the test data from this paper, combined with multiple literature sources, a relationship between the onset time of OAE2 and latitude was plotted (Fig. 2b). The figure reveals that black shales initially developed at high latitudes (70°N, 94.55 Ma), subsequently progressing southward to mid-latitudes (37°N, 94.29 Ma), and finally reaching low latitudes (30°N, 94.0 Ma), corresponding to the coldest periods of the Jurassic, J₁¹ and J₂² (Fig. 2a). Considering the prior knowledge that black shales and coal develop in cool-temperate zones, we attribute this pattern not to calculation errors or atmospheric/ oceanic hypoxia, but to a rapid cooling process relative to the Jurassic, spanning 0.55 Ma. As global climate shifts, the cool-temperate zone leaves its imprint wherever it passes: black shales or coal. During the Late Cretaceous, frequent interbedding of black shales and red beds occurred, reflecting rapid climatic shifts between cold and warm conditions. Both red and black strata can be found in the same period but at different latitudes. For instance, during the OAE2 event (equivalent to the Turonian stage, 96-90 Ma), the Kukebai Formation black shales developed in the Tarim Basin, while the Qingshankou Formation black shales developed in the Songliao Basin¹⁶ (Song Chunhui et al., 2011). At the same latitude, black shales also developed at DSDP Site 398¹⁷ (Iván et al., 2024). During this OAE2 cooling event, it extended only as far south as 30°N. As a result, south of this latitude, in the Guangfeng Basin in northeastern Jiangxi Province (28°N), the Zhou Tian Formation red beds developed, and in the Hexi Basin in Fujian Province (26°N), the Guanzhai Formation red beds developed¹⁸(Wu Wenbin et al., 2020). This demonstrates that red and black strata can develop simultaneously at different latitudes during the same period.

Therefore, the enrichment of organic matter and the development of black shales and coal in the cool-temperate zone are not attributed to atmospheric or oceanic hypoxia. Similarly, the development of red beds with low organic content in tropical regions is not due to atmospheric or oceanic oxygen abundance.. In any geological epoch, as long as these four climatic zones exist, black shales and red beds will appear simultaneously, each developing in different latitude belts.

Tropical peat has been observed in tropical regions, which has led to the belief that tropical wetlands can accumulate organic matter, seemingly contradicting our proposition. To address this discrepancy, we have included three additional studies on tropical peat in this comment, concluding that tropical peat is not currently being formed in these settings; rather, it is undergoing decomposition or disappearance^19-21(Bai et al., 2004; Dommain et al., 2014; Hodgkins et al., 2018). Hodgkins et al.²¹ conducted research indicating that, even within the anaerobic environment at a depth of 5 meters in tropical peat, a significant degree of mineralization persists. As a result, the concentration of mineralization-resistant aromatic hydrocarbons at this depth is comparable to that found in the surface layer, and is relatively higher than that observed in peat deposits situated at higher latitudes.

3. Some black shales are not deep-sea sediments.

Since Schlanger and Jenkyns²² proposed the view that during the Aptian-Albian and Cenomanian-Turonian periods, organic carbon-rich sediments developed in pelagic sedimentary sequences on a global scale, and inferred that these sediments formed under various paleo-water depth conditions as a result of "Oceanic Anoxic Events" (OAEs), this viewpoint has been widely accepted for a long time. Modern pelagic sea beds are not anoxic, and conditions conducive to the formation of organic-rich black shales do not exist in pelagic environments (Fig. 1a, 1d). Previous research in this paper concludes that almost all black shales are developed in relatively enclosed lagoon and bay environments²(Mao Xiaoping et al., 2023), which is generally consistent with the findings of Hartnett et al.⁷ They found that over 90% of organic carbon in the ocean is stored in sediments on continental margins, aligning with the aforementioned phenomenon that primary productivity is concentrated within 300 kilometers of continental margins. This underscores the close link between carbon burial and "terrestrial" factors, which cannot be ignored. Through the analysis of extensive drilling data from the Ocean Drilling Program (ODP), it was found that black shales with high total organic carbon (TOC) content were indeed discovered in Cretaceous strata in the deep Pacific Ocean, with TOC values exceeding 10% and some even reaching 50%. However, based on previous analyses of their depositional environments, many high-TOC black shale formations were deposited in terrestrial, meaning these formations were not deposited in pelagic environments during the Late Cretaceous. For example, at Site 865, currently at a water depth of 4000 m, black shales from the Late Albian period were encountered with a TOC as high as 50.6%, indicating a terrestrial depositional environment. Similarly, at Site 462, currently at a water depth of 5,200 m, the shale TOC is 0.25% and is also terrestrial. The Aptian shale formations at both Site 317 and Site 463 are determined to contain terrestrial environments²³(Dumitrescu M & Brassell S C, 2006).

4. Having correlation does not equate to having causal linkage.

In enclosed aquatic environments (such as lagoons), shale formation is prone to occur. Under such conditions, it is inevitable that certain metal and non-metal elements will experience abnormal enrichment or depletion²(Mao et al., 2023). Simultaneously, sediments or depositional phenomena such as volcanic ash and gravity flows can also be well-preserved in these relatively enclosed aquatic environments. There are up to 25 types of mineral resources, including the mercury element mentioned by Walker-Trivett et al.¹, which has a close relationship with black shale and often forms large to super-large sedimentary ore deposits²¹(Li Zhixing et al., 2022).

Under cold and wet climatic conditions coupled with relatively enclosed aquatic environments, black shales rich in organic matter are prone to develop. Conversely, under greenhouse climates condition, enclosed water environments also exist, where gray and red mudstones devoid of organic matter tend to form, often accompanied by volcanic ash, gravity flows, and abnormal enrichment of various metal elements. However, in non-shale formations like limestone and sandstone, these element enrichments or depositional phenomena collectively disappear²⁵(Wang et al., 2015). The reason for this is not due to the absence of volcanic activity or gravity flows during those periods, but rather that these geological records are difficult to preserve in open aquatic environments. That is, our observation that black shale exhibits a higher frequency of volcanic ash depositions and develops gravity flows compared to surrounding rocks does not imply more frequent volcanic activities at that time, nor does it directly equate to a significant geological event. Black shale only shows correlation with these phenomena, without implying a causal relationship. Walker-Trivett et al.¹ inference violates the exclusivity principle in causal analysis.

Due to the relatively thin stratigraphic layers and rapid climatic changes during the OAE2 period (Fig. 2b), this is not an ideal example. To illustrate that a cooling trend existed prior to the development of black shale, the climatic changes of the Triassic period are used as an example here.

As previously mentioned, the lithology developed within stratigraphic layers is correlated with climate. The transition from red beds to black shale indicates a cooling process, whereas the reverse transition suggests a warming process. The Sichuan Basin during the Triassic period experienced a cooling process from hot to cold-wet conditions (Fig. 4). Starting from the lower Triassic Feixianguan Formation, the red beds are widely distributed, organic matter is scarce, and gypsum layers are present. However, by the upper strata, the red beds and gypsum layers disappear, organic matter content increases, and the organic-rich black shales and coal of the Xujiahe Formation are developed²⁶(Bo Jinfang et al., 2019). This cooling process lasted for 51 Ma.  From the Early to Middle Triassic, the tropical rainforest climate that developed red beds gradually cooled to a warm, arid climate, marking the beginning of the cooling process. The Middle to Late Triassic merely continued this process. Thus, the development of black shales is a result of global climate cooling. This cooling process was not determined by short-term events such as volcanic activity records or abnormal enrichment of Hg elements found in the Xujiahe Formation at the top of the Upper Triassic, as this would violate the principle of temporal sequence in causal analysis.

Figure 4. Relationship between Climate Evolution and Carbon Burying in the Triassic System of the Sichuan Basin

Therefore, we believe that the paper by Walker-Trivett et al.¹ continues to perpetuate misconceptions aligned with the mainstream view, and timely correction is necessary.

References

1. Walker-Trivett, C. A., Kender, S., Bogus, K. A., Littler, K., Edvardsen, T., et al. Oceanic Anoxic Event 2 triggered by Kerguelen volcanism. Commun. 15(5124), 1-12. https://doi.org/10.1038/ s41467-024-49032-3 (2024).
2. Mao, X. P., Chen, X. R., Li, Z., Li, S. X., Li, S. S., et al. Shale sedimentary patterns and organic matter enrichment patterns of the Wufeng Longmaxi Formation in the Sichuan Basin. Sedim., 1-46. https://doi.org/10.14027/j.issn.1000-0550.2023. 060 (2023).
3. H, W. F., Zhang, L. L., Chen, M. H., Zeng, L. L., Zhou, W. H., et al. Distribution of living radiolarians in spring in the South China Sea and its responses to environmental factors. Science China: Earth Sciences, 58, 270–285. https://doi.org/10.1007/s11430-014-4950-0 (2015).
4. Sferdrup, H. U., Johnson, M. W. & Fleming, R. H. Translated by Mao Hanli. 1958. Oceans and Seas, Volume 2 . Beijing: Science Press, 471~475(1946).
5. Busecke, J. M., Resplandy, L., Ditkovsky, S. J. & John, J. G. Diverging fates of the Pacific Ocean oxygen minimum zone and its core in a warming world. Agu Adv. 3, 1-20. https://doi.org/10.1029/2021AV000470 (2022).
6. Ma, W. T., Xiu, P., Yu, Y., Zhen, Y. L. and Cai, F. Production of dissolved organic carbon in the South China Sea: A modeling study. Science China Earth Sciences, 65(2): 351-364. https://doi.org/10.1007/s11430-021-9817-2 (2022).
7. Hartnett, H. E., Keil, R. G., Hedges, J. I. & Devol, A. H. Influence of oxygen exposure time on organic carbon preservation in continental margin sediments.Nature, 391(6667): 572-575. https://doi.org/10.1038/35351 (1998).
8. Yu, X.,Zhu, G. Y. S., Liu, X. B., Du, Q. & Tan, H. Evolution mechanism of dissolved oxygen stratification in a large deep reservoir. Lake Sci. 32(5), 1496-1507. https://doi.org/ 10.18307/2020.0521 (2020).
9. Chen, Y., Qin, W. K., Zhang, Q. F., Wang, X. D., Feng, J. G., et al. Whole-soil warming leads to substantial soil carbon emission in an alpine grassland. Nat. Commun. 15(1). https://doi.org/10.1038/s41467-024-48736-w (2024).
10. Kai, X., Yuan, M. M., Shi, Z. J., Qin, Y. J., Den, Y., et al. Tundra soil carbon is vulnerable to rapid microbial decomposition under climate warming. Nat. Clim. Change. https://doi.org/ 1038/nclimate2940 (2016).
11. Mao, X. P., Chen, X. R., Wang, Z. J., Yang, Y., X., Li, S. X., et al. Relationship between organic matter enrichment degree of black shale and paleoclimate: Taking the shale of the Wufeng-Longmaxi Formation in the middle and upper Yangtze region as an example. J. Geol. 59(5), 1-25. https://link.cnki.net/urlid/11.1937.p. 20240627.1807.002 (2024).
12. Parrish, J. T., Ziegler, A. M. & Scotese, C. R. Rainfall patterns and the distribution of coals and evaporites in the Mesozoic and Cenozoic. Palaeoclimatol. Palaeoecol. 40, 67-101. https://doi.org/10.1016/0031-0182(82)90085-2 (1982).
13. Zhang, H. K., Hu, P., Jiang, J. S., Cheng, X., Wang, J. X., et al. Distribution, genetic types and current situation of exploration and development of bauxite resources. Geol. 48(1), 68-81. https://doi.org/doi: 10.12029/gc20210105 (2021).
14. Gudasz, C., Bastviken, D., Steger, K., Premke, K., Sobek, S., et al. Temperature-controlled organic carbon mineralization in lake sediments. Nature, 466(7310), 1134-1134. https://doi.org/10.1038/nature09186 (2010).
15. Li, X. S., Zhang, T. & Ma, C. Astrochronological comparative study of Cretaceous Oceanic Anoxic Event 2 (OAE 2). Sci. 43(6), 1597-1613. https://doi.org/10.11928/j.issn. 1001-7410.2023. 06.08 (2023).
16. Song, C. H., Zhang, M., Wei, Y. J., Fang, X. M., Meng, Q. Q., et al. Late Cretaceous Oceanic Anoxic Event (OAE₂) in northwestern of Tarim Basin. Journal of Earth Environment, 2(4): 541-548. https://doi.org/CNKI:SUN:DQHJ.0.2011-04-009 (2011).
17. Barreiro, I. R., Santos, A. A., Amadoz, U. V., Louwye, S., Robinson, S. A., et al. Lead-up and manifestation of the Oceanic Anoxic Event 2 at the DSDP Site 398 (Vigo Seamount, NW Iberian offshore): Palynological and geochemical insights. Palaeogeography, Palaeoclimatology, Palaeoecology,2024,647:1-17. https://doi.org/10.1016/j.palaeo. 2024.112223 (2024).
18. Wu,W. B., Chen, L. Q., Ding, T., Li, W., H., & Wang, Y. J. Sedimentary Characteristics and Paleoclimatic Significance of the Late Cretaceous Zhoutian Formation Red Beds in the Guangfeng Basin. Acta Sedimentologica Sinica, 38(3), 485-496. DOI:10.14027/j.issn. 1000-0550.2019.074 (2020).
19. Bai, G. R., Wang, S. Z., Gao, J. & Yu, J. L. Liquid-heat conditions and microbic decomposition on the forming of turf deposits. Shangahi Norm. Univ. (Natural Sci.) 33(3), 91-97. https://doi.org/10.3969/j.issn.1000-5137.2004.03.018 (2004).
20. Dommain, R., Couwenberg, J., Glaser, P. H., Joosten, H. & Suryadiputra, I. N. N. Carbon storage and release in Indonesian peatlands since the last deglaciation. Sci. Rev. 97, 1-32. https://doi.org/10.1016/j.quascirev. 2014.05.002 (2014).
21. Hodgkins, S. B., Richardson, C. J., Dommain, R., Wang, H. J., Glaser, P. H., et al. Tropical peatland carbon storage linked to global latitudinal trends in peat recalcitrance. Commun. 9, 3640. https://doi.org/10.1038/s41467-018-06050-2 (2018).
22. Schlanger, O. & Jenkyns, H. C. Cretaceous oceanic anoxic events: causes and consequences. Geologie en Mijnbouw, 55(3/4): 179-184. (1976).
23. Dumitrescu, M. & Brassell, S. C. Compositional and isotopic characteristics of organic matter for the early Aptian Oceanic Anoxic Event at Shatsky Rise, ODP Leg 198. Palaeoclimatol. Palaeoecol. 235, 168-191. https://doi.org/10.1016/j.palaeo. 2005.09.028 (2006).
24. Li, Z. X., Qin, M., K., Liu, X. Y., Wang, W. Q., Wang, J., et al. Characteristics, genesis, and research significance of multi-element enrichment layers in black rock formations. World Nucl. Geosci. 39(01), 14-26. https://doi.org/10.3969/j.issn.1672-0636.2022.01.002 (2022).
25. Wang, Z. T., Zhou, H. R., Wang, X. L., Zhang, Y. & Xing, E. Ordovician geological event group coupling at the west and southern edges of the Ordos Basin. Acta Geol. Sin. 89(11), 1987-2001. https://doi.org/19762/j.cnki.dizhixuebao.2015.11.010 (2015).
26. Bo, J. F.,Yao, J. X.,Lin, B. Y., Liu, W. Q. & Li, M. Distribution and age of the Triassic marine red beds in eastern Sichuan and Chongqing. Acta Geologica Sinica, 93(2), 285-301. https://doi.org/CNKI:SUN:DZXE.0.2019-02-001 (2019).