Origin of black coal

Origin of Black Coal

     Many people think that the process of coal formation is fully understood. The paradigm is that coal is essentially a product formed through the burial of plants and, according to this conventional view, the common presence of plant fossils associated with coal deposits therefore would explain intrinsically throught biogenic origin. However, there are much evidences related to the coal origin that are not yet understood.

     The American scientist Thomas Gold proposed in his book "The Deep Hot Biosphere" an insight into the processes of coal formation very different from the conventional view. He states that the existence of fossils with an excellent state of preservation, including textures at the cellular tissues level, proves that coal can not be formed through biogenic origin.


He said in an interview:

“The coal we dig is hard, brittle stuff [but] it was once a liquid, because we find embedded in the middle of a six-foot seam of coal such things as a delicate wing of some animal or a leaf of a plant. They are undestroyed, absolutely preserved, with every cell in that fossil filled with exactly the same coal as all the coal on the outside... The fact that coal contains fossils does not prove that it is a fossil fuel; it proves exactly the opposite. Those fossils you find in coal prove that coal is not made from those fossils. How could you take a forest and much it all up so that it is a completely featureless big black substance and then find one leaf in it that is perfectly preserved? That is absolute nonsense.”

 Highly preserved plant fossil in black coal

    The abiogenic theory then, combined with the deep, hot biosphere theory is as Gold succinctly phrases it; “not biology that has been reworked by geology but geology that has been reworked by biology”.  One might expect coal to be the exception; surely coal is the result of degraded plant life and ancient swamps. No, says Gold, but he does make a partial exception for peat and lignite, which are indeed reworked plant life with some help from primordial hydrocarbons. But black coals come from the same upwelling of hydrocarbons as petroleum and methane, originating far below the sedimentary layers. The process is essentially a sequential loss of hydrogen atoms as hydrocarbons upwell through porous rock, and this is the primary reason why so many petroleum fields are configured in a “layer-cake” manner. Methane is at the lowest depth, layered on this is light crude, next come the heavier oils, and then often on top of all is blackcoal. This correlation of coal with petroleum fields can be seen in many parts of the world (see pictures of US oil and coal maps below). The blacker the coal the greater the hydrogen loss and the greater the carbon to hydrogen ratio. How do the hydrocarbons lose their hydrogen atoms? Though many factors are involved, and we can go no further into the technical details here, there is a gradual process of oxidation as the hydrocarbons upwell, and carbon deposits left behind tend to be a catalyst for more carbon deposits, not unlike what happens in an internal combustion engine.

Coal formation

Biogenic (Orthodox): Coal is a material derived from organic detritus (plant material) that was buried and compressed. 

 

 Coal mining in Indonesia

Abiogenic: Coal (black only) is a material that may contain the presence of organic compounds, but that was filled by inorganic hydrocarbons that migrated by continuous upwelling come from great depth and reached these deposits in the surface and preserving fine debris and cellular tissues of plants. Such a situation may occur in the surface migration of methane and oil on areas of marshes or peat.

Several metals such as Nickel, Vanadium, Chromium, Cadmium, Mercury, Arsenic, Lead, Selenium, among others, are also present in coal. Many coals are sometimes bituminous and also have high sulfur content. As with oil, these metals come from deep inside the Earth (mantle) and black coal only represent stages in high loss of hydrogen of primordial hydrocarbons and intense biodegradation at shallower levels as postulated by Thomas Gold.
It's interesting that the same biomarkers found in oil are present in coal and represent, of course, parts of prokaryotic archaea that re-worked primordial hydrocarbons.

It's not rare association of uranium with black coal deposits. Association of biocide and poisonous element mercury with coal is also common evidence. In many coal deposits in the world are commonly found thin white layers called tonsteins that consisting of kaolin material, sometimes interpreted as volcanic ash.
There are some occurrences of coal in Precambrian, Neoproterozoic. According to fossil record of planet Earth there's no superior plant  at that time, then the Proterozoic coal is surely abiotic and represents probably  ancient oil accumulation with high hydrogen loss and biodegradation of primordial hydrocarbons.

Coal sometimes occurs in thick layers, as shown in the pictures below. It would be hard to imagine a swamp or anarea with thick ancient forests accumulated and its volume decreased after the water loss and compaction of the layers to form a thick coal layer.

Only the brown coal (lignite) should be considered dominantly biogenic.

Coal layer over 100 ft - Powder River, Wyoming, USA

Thick coal layer. See car and person as scale

   It is also common association of coal over oil and gas production areas. See below a comparison between maps of oil and coal occurrences in the United States.

Oil and natural gas production areas in the United States

   Main coal basins in the United States

"Petroleum and coal were made from materials in which heavy hydrocarbons were common components. We know that because the meteorites are the sort of debris left over from the formations of the planets and those contain carbon in unoxidized form as hydrocarbons as oil and coal-like particles. We find that in one large class of meteorites and we find that equally on many of the other planetary bodies in the solar system. So it’s pretty clear that when the Earth formed it contained a lot of carbon material built into it." (Thomas Gold)