Organic Matter

a necessary minor component of soils, surface and subterranean waters, most rocks (particularly sedimentary rocks), and also the atmosphere (see Figure 1). Plants are the primary source for organic matter; animals account for a significantly smaller proportion.

Figure 1. Content of organic matter in (1) soils, (2) marine sediment, (3) rock, (4) subterranean water, (5) soil solutions, (6) interstitial solutions, (7) sediment solutions, (8) surface water, (9) seawater, (10) atmospheric precipitation

Three types of organic matter are distinguished according to original composition. Humic organic matter is formed primarily from higher plants and soil organisms, mainly microorganisms; it makes up a varying proportion of soils and is a major component of brown and hard coals. The hydrogen/carbon ratio for humus is usually less than 0.9. Sapropelic organic matter is a product of transformation of lower plants, mainly planktonic algae. It is characteristic of sapropelites, combustible shale, and dispersed organic matter, and its hydrogen/carbon ratio is usually greater than 1.2. Liptobiolitic organic matter forms because of concentration of the most chemically stable components of plant matter.

Other types of organic matter are also known that were formed because of the existence of several groups of animal organisms in the past—for example, chitinous remains of graptolites and a number of crustaceans. Both organic matter dispersed in sedimentary rock and concentrated organic matter (coals) undergo catagenesis in a number of stages, from brown coal, in a zone with temperatures up to 60°-70°C, to anthracite, in a zone with temperatures higher than 250°C. Organic matter is finally transformed into graphite in the zone of metagenesis (metamorphism).

The average content of organic matter in sedimentary rocks is about 15–20 kg/m³; the total mass in the continental sector of the sedimentary crust (continents and shelves) is as high as 10¹⁶ tons. Organic matter is the source of petroleum and fuel gases.

In magmatic rocks, organic matter is most often allochthonous, carried to the place of deposition by water. Sometimes such organic matter is related to the intrusion of petroleum or other naphthides. In some cases, organic matter reaches magmatic rock by assimilation of sedimentary matter by the rocks.