Just before the vacation, I wrote about the science of coal. You may remember that I said that coal is made from plant material. Somehow, through processes not yet fully understood, such decomposed material is transformed by heat and pressure from deep burials over millions of years in a process called charring.
Paleontologists know that in the geological past our earth had dense forests in deep-lying wetlands. In these areas, the plant material is to be converted into peat with the help of layers of mud and acidic water. Because the organic matter was covered, it was protected from biodegradation and oxidation, and huge peat bogs formed, which were deeply buried by sediments. For millions of years, heat and pressure in deep spills caused the loss of water, methane and carbon dioxide and increased carbon levels.
Although coal can be found in most geological periods, almost all coal deposits were formed in the Carboniferous and Permian periods, which occurred 360 million and 250 million years ago, only 2 percent of the Earth’s geological history. Centered during this timeframe, something caused the greatest life changes the world has ever seen. Severe extremes of climate and environment began where the southern regions were cold and dry and the northern parts suffered from intense heat and large seasonal fluctuations between wet and dry conditions. This led to the fact that the lush swamp forests of the Carboniferous Period were gradually replaced by conifers, seed ferns and other drought-resistant plants.
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Textbooks suggest that during this period plants somehow developed the ability to produce lignin, creating a woodier structure, precursors to the first trees. When they died and fell, the wood did not completely deteriorate but was buried under sediment and eventually turned to coal.
As one might suspect, different types of coal require certain temperatures to form. Lignite, the lowest grade, can form at temperatures as high as 95 degrees Fahrenheit (a hot summer day), while the harder types like anthracite take around 400 degrees to produce, like a hot kitchen stove. The geological processes of heat and pressure over hundreds of millions of years are responsible for the formation of coal.
It’s easy to show that coal comes from plants. First, brown coal, the softest coal, often contains recognizable plant debris. Second, sedimentary rock layers above, below and next to coal seams contain plant fossils in the form of leaf prints, even with casts of larger parts such as roots, branches and trunks. Finally, when thin sections of anthracite are polished and examined microscopically, cell walls, cuticle spores, and other structures can be easily seen.
So far, most scientists agree that the charring process is mainly controlled by changes in physical conditions that occur with depth, such as heat and pressure over time. This is due to the idea that microbial activity stops within a few feet of the surface of the earth. However, the mechanism behind one of the early stages of coal production may not be what we thought, according to a team of researchers who found that microbes are actually primarily responsible for coal formation and methane production.
In a report published in Science last month, Professor Max Lloyd, a geologist at the University of Pennsylvania, and his team suggested that “communities in deep biospheres have been involved in the conversion of plant material to coal on geological timescales”. Your research was funded with a mission to identify the production and improvement of coal seam methane as it is an important economic resource as it is an important source of energy in the United States and other countries.
Dr. Lloyd examined the methoxyl groups in coal samples from around the world and, using stable isotopes, showed that the organic material eventually turns into coal – not through a thermal reaction, as everyone thought – but only through some form of microbial action.
You probably know from chemistry that carbon is one of the few elements that has a valence of four, meaning it can grab other atoms with so many bonds, making it the basis of an enormous number of different molecules. The methoxyl groups they studied consist of a carbon atom bonded to three hydrogen atoms and an oxygen atom, which by virtue of its own two bonds, can continue to bond to others.
When Lloyd investigated methoxyl groups in hard coal, he found that the profile of certain carbon isotopes did not match what would be found if methane had been formed from heat, acid, or catalytic reactions. However, the fact that they conformed to the patterns expected of microbial activity makes many view the formation of char in a different light.