Black shales, selenium

The primary sources of selenium are volcanic emanations and metallic sulfides associated with igneous activity. Secondary sources are biological sinks in which it has accumulated. The selenium content of black shales, coal, and petroleum is 10-20 times the crustal abundance (0.05 ppm). Seleniferous black shales are the parent materials of the widespread seleniferous soils of the western plains of the United States. When burned, coal and petroleum containing selenium give rise to a redistribution of particulate Se and SeOg. The average selenium content of U. S. coal is about 3 ppm and of petroleum about 0.2 ppm. Selenium is an essential nutrient for animals and is required at a concentration of about 40 ppb in their diet at concentrations of 4000 ppb and above, however, it becomes toxic to animals. 

The selenium distributed by erosion of igneous rocks or by volcanic emanations is locally concentrated in carbonaceous deposits. Black shales, coal, and, to a lesser extent, petroleum often contain much selenium due in part, at least, to selenium introduced by biomaterials. 

Kim, K.-W., and Thornton, I. (1993). Influence of uraniferous black shales on cadmium, molybdenum and selenium in soils and crop plants in the Deog-Pyoung area of Korea. Environ. Geochem. Health 15 119-33. 

Wen, H. and Carignan, J. (2011) Selenium isotopes trace the source and redox processes in the black shale-hosted Se-rich deposits in China. Geochim. Cosmochim. Acta, 75, 1411-1427. 

Selenium (Se) Black shales, phosphatic sediments. Sometimes... 

Although the differences in physical and chemical properties of selenium and sulfur tend to separate these elements in weathering and erosion, their similar functions in biological reactions tend to unite them in organic-rich materials. Selenium is found often manyfold its crustal abundance in black organic-rich shales, in coal, and, to a lesser extent, in petroleum. 

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