December 4. Rise of grasses and some coal

Everyone knows grass. Common worldwide today, grasses include the cereal grains such as rice and wheat, bamboo, swamp sedges, and of course lawns, but as common as they are today, grasses were the last major group of plants to evolve. They were certainly present but uncommon during the Cretaceous – dinosaur coprolites, or fossil excrement, are known with grass components. But grasses really began to expand during the early part of the Cenozoic, the Paleocene and Eocene epochs.

Grasses continued to evolve through the Cenozoic, inventing novel ways to fix carbon during photosynthesis. Grasses including maize or corn, sugar cane, and sorghum use a more efficient method of carbon fixation than many plants, a method that is thought to be a relatively recent development – meaning probably the past 40 million years. 

Grasses diversified a lot in the middle to late Miocene, around 6 to 10 million years ago. Their dominance in prairies and savannahs may be a result of their drought tolerance and ability to use carbon dioxide more efficiently than some other plants, even in low CO2 conditions such as were developing over the course of the Cenozoic. Another factor might be a co-evolution with hoofed animals that grazed on grasses, and helped spread them. 

Coal mining, Powder River Basin, Wyoming (USGS photo)

In the western United States during the Paleocene, as grasses were beginning to expand, swamps and lakes were forming in what are now northeastern Wyoming and southeastern Montana, the Powder River Basin. This basin was one of the low-lying areas between two of the late Cretaceous Laramide uplifts, the Black Hills and Big Horn Mountains. Those mountains were actively uplifting into the early Paleocene, and shedding sediment into the adjacent swampy basins, where extensive forests grew in Paleocene and Eocene time. The package of rocks including fluvial or river-borne sediments, lake deposits, and organic material deposited in swamps is called the Ft. Union Formation. The climate was warm temperate to subtropical but with alternating warm and cooler intervals during the Paleocene and Eocene, the first epochs of the Cenozoic. The plant matter in the Ft. Union Formation has produced thick coal beds. One individual bed, near Gillette, Wyoming, is about 110 feet thick.

Similar coals of Cenozoic age can be found throughout the Rocky Mountains, but the Wyoming-Montana coals are the most important economically. Wyoming is the leading state in the US for coal, with 338 million tons produced in 2013, 39% of the U.S. total. West Virginia, at #2, produced 11% of the total, which amounts to about 1.1 billion tons of coal for the whole United States.

In other plant-related news, just this week a new discovery was announced of a carnivorous plant in Baltic amber, dating to about 40 million years ago, the Eocene epoch. LINK

Today, December 4, is St. Barbara’s Day. She’s the patron saint of artillerymen, mathematicians, and miners. Geologists sometimes adopt her, usually as an excuse for a party.

—Richard I. Gibson

LINKS and References:
Recent evolution of grasses
Carnivorous plant

Evolution of grasses 

Cretaceous and Tertiary coals of the Rocky Mountains and Great Plains regions, by R. Flores and T. Cross, 1991, in GSA DNAG volume P-2, Economic Geology, U.S.

Photo from USGS 

August 17. Glaciers and Coal

Today’s episode is a response to a listener’s question about the close juxtaposition of glacial deposits and coal beds in Australia.

Despite the abundance of coal in the Carboniferous, especially in the northern hemisphere, and despite the changing climate that meant coal formation there largely ended with the end of the Carboniferous, there’s plenty of Permian coal too. Most of it is in the former Gondwana – Australia, South Africa, India, South America, and Antarctica, but there is a lot of Permian coal in Russia as well. In Gondwana, the coal is pretty closely associated with glacial deposits.

We might expect that glacial deposits and coal swamps would reflect two very different environments, but so far as I can tell, they pretty much co-existed in Permian time at least in quite a few places.

In Australia and elsewhere, the coal-bearing rocks and glacially deposited layers actually interfinger. So at best, we might have had some relatively rapid changes in climate to switch from glacial times to warmer, coal-swamp times, and from what I read there were at least 8 specific glacial periods in the late Carboniferous and early Permian.

But the alternative explanation, and from what I can gather it seems to be the preferred one, is that these areas were on the margin of the ice, and plant life actually thrived there. The keys to making coal are 1) lots of plants and 2) rapid burial of the plant matter so it does not have time to decay. Our typical vision of warm swampy areas with low oxygen to prevent decay is just one way to do that. A cold climate, with plants buried by glacial debris, would work just as well, if not better.

Gondwana base map from Du Toit (1937, Our Wandering Continents). Blue line is highly generalized margin of glacial area; solid black are highly generalized coal deposits (based on Langford, 1992).

I think that while Australia was certainly part of the glaciation in the southern polar part of Pangaea, it was probably far enough away from the pole (which was more or less in South Africa, but there are coal deposits there, too) that the climate might not have been like modern Antarctica, but perhaps more like modern Patagonia but with glaciers. So abundant plant life could have been growing, even thriving, near the glacial margin. A modern analogy would be the peat bogs of temperate and even arctic climates. When glaciers receded, forests and peat on the tundra would advance. When glaciers advanced, the deposits the glaciers carried would have buried the forests. This would be a good way to get the interfingering of glacial and coal deposits that we do observe.

This would not strictly be cyclothems, which represent rises and falls of sea level, alternately allowing swamps to form and then burying them in river sediment, but they would be cyclic nonetheless, like cyclothems. I do not know if the coal-glacial sediment packages follow the 8 known glacial periods or are (likely) something more complex, but if they do it would be on a periodicity of a few million years. Standard cyclothems can show alternations that may represent changes on scales of a few tens of thousands of years, or even fewer, as well as the longer periods of millions of years. If the coal results from glacier-margin plant life, as I infer it does, then the alternations would not reflect sea-level changes as cyclothems do, but more directly would reflect changes in position of the glaciers (together with the dumping of sediment to bury the forests or other vegetation).

—Richard I. Gibson

Links:
Permian of Australia
Permian coal in South Africa 

Langford, 1992 - Gondwana’s Permian coal 

July 24. The end of coal

One possible explanation for the decline in carbon dioxide in the atmosphere toward the end of the Pennsylvanian, which we talked about yesterday, is that so much of the carbon was being tied up in coal and therefore not available to the atmosphere. Plants take CO2 in – and if they died and became coal rather than decomposing more completely, the carbon would be sequestered in the earth, in coal beds.

Some recent research by scientists at Clark University and the US Department of Energy, and elsewhere, published in 2012, suggests that one reason for all the plants and all the coal in the Pennsylvanian was a sort of arms race between plants and the enemies that could destroy them – specifically, fungi.

When plants evolved a biopolymer called lignin, it allowed them to better transfer fluids around their bodies. Lignin gives wood both strength and functionality for plants, and a vast amount of the world’s carbon is contained in lignin and related chemicals in plants. It may have been the evolution of lignin that allowed plants to grow to large sizes, beyond the tiny stems of Cooksonia back in the Silurian.

Having lignin may have given plants another advantage of sorts – protection against attack by some other organisms. The recent work suggests that the abundance of coal in the Pennsylvanian is because there was nothing around to attack dead woody tissue, because nothing had evolved the necessary chemicals to break it down. Dead wood simply accumulated and altered under physical weathering conditions, together with the acids formed by the plants themselves. Nothing was eating the wood, so it became coal.

Toward the end of the Carboniferous, a fungus evolved that had the chemicals needed to decompose lignin. Was this enough to mean that following periods yielded less coal because more wood was broken down, with carbon returning to the atmosphere more than to the earth? The authors of the recent work think so.

White-rot fungus photo by Sten Porse
via Wikipedia under Creative Commons license.

This has some significant consequences for earth history as well as for present-day energy studies. The Department of Energy was involved because they were studying ways of degrading wood to make better biofuels. But geologically, this might help account for the low point in atmospheric carbon dioxide at the end of the Pennsylvanian and early Permian Period. Later, the additional input due to fungal decomposition of wood could have brought CO2 levels up again. Never discount the power of plants over time – remember that the oxygenation of earth’s atmosphere was probably due to the action of algae and cyanobacteria back in the Precambrian.

So, there is apparently a coincidence in timing between the evolution of genes in these fungi that could degrade lignin and the end of abundant coal, toward the end of the Pennsylvanian. But there are probably other factors in the decline of rainforests, such as we discussed yesterday, and while a coincidence in timing is interesting, it isn’t proof. And the work was based on genome studies – there are not many fungi in the fossil record. Probably more telling is the fact that there is hardly anything other than the enzymes from funguses that can really attack lignin and break it down. So the evolution of those chemicals must have indeed been an important event in the history of plant life, and in the evolution of the carbon cycle on earth. It’s definitely an intriguing idea.

—Richard I. Gibson

White-rot fungus photo by Sten Porse via Wikipedia under Creative Commons license.   

Wood-decay fungus 

Links (all to reports on the 2012 study):

http://science.energy.gov/ber/highlights/2012/ber-2012-06-a/

http://www.sciencemag.org/content/336/6089/1715.short

http://www.sciencedaily.com/releases/2012/06/120628181721.htm

July 18. Coal

Coal is known from most geologic periods, even the Precambrian, when it probably derived from algal accumulations. But the Carboniferous, especially the later part of the period called Pennsylvanian in the United States, was probably the time when more coal formed than any other period. That was pretty much directly proportional to the widespread environments that encouraged the growth of plants in settings that led to their burial and alteration to coal.  

Depending on how much the original plant matter is compressed and heated up, you get various grades of coal. The least compacted type with the most impurities is called lignite, which is essentially compressed peat, a loose accumulation of plant matter. Lignite has only about 30% carbon content and high moisture. The next grade is called bituminous, around 70% carbon and fewer impurities and lower moisture than lignite. The word bituminous means “containing bitumen,” and bitumen was the name of the tarry substance used as pitch or glue in Roman times and earlier. Lignite comes from a Latin word meaning wood.

The highest grade of coal, anthracite, can have carbon content as high as 95% or more. It is essentially a low-grade metamorphic rock, low in moisture and volatiles and other impurities. The grades of coal are proportional to their heat content when burned, and consequently the price of coal follows its grade, with anthracite most expensive. Anthracite’s name comes from the Greek word for coal.

The biggest deposits of anthracite in the world are in the Pennsylvanian rocks of Pennsylvania, but today more anthracite is mined in China than any other country. China is also the world’s largest consumer of coal. Anthracite accounts for only about 1% of all the coal reserves known, but its value means about 9% of total coal production is anthracite.

Bituminous coal is the bulk of coal mined and burned for fuel and energy generation. About a third of the world’s energy comes from coal, with 40% of electricity generated by burning coal. And about 70% of all the steel made relies on coal to fuel the blast furnaces and refineries where steel is fabricated.

Today the leading producers of coal, in order, are China, with almost half of the total world production, followed by the US, India, Indonesia, and Australia. Close to eight billon tons of coal is mined worldwide every year.

While burning coal is the primary end use, there are lots of products that come from coal and the coal tars derived from coal. Heat-resistant black plastics, aspirin pills, dyes, mothballs, and more were ultimately made from chemicals distilled from coal. There’s a fair amount about these topics in my other book, What Things Are Made Of.

—Richard I. Gibson

July 10. Joggins Formation, Nova Scotia

With the collision of Gondwana and the combined North America-Europe, a pretty significant mountain range was formed. The deformation was mostly the Alleghenian Orogeny in eastern North America and the Variscan in southern Europe, and while it made some new mountains it probably rejuvenated some old mountains as well, including parts of the Caledonian mountain belt between Maritime Canada – Nova Scotia and Newfoundland – and Greenland on one side and the British Isles and Scandinavia on the other.  

Within the mountain belts, quite a variety of environments formed, including extensive swampy basins in which abundant plant life lived, and died to make the coal that gives the Carboniferous its name. One such swampy basin was in what is now western Nova Scotia, where a low-lying area received sediments that became the Joggins Formation. 

Photo of upright Pennsylvanian tree fossil in
Joggins Formation by Michael C. Rygel.

The tectonic deformation that was associated with the continental collisions wasn’t all just a big crunch, a big squeeze. In places, because of the angle of collision, actual extension could occur, leading to down-dropped areas that became basins. In some places including parts of what is now Nova Scotia, older salt deposits might have been dissolved and flowed so that there was very rapid subsidence and infill of sediments. This is one explanation, suggested by geologists John Waldron and Michael Rygel, for the many upright tree fossils that are found in the Joggins Formation. The trees could have been buried during life by incursions of sediment. The sediment was brought from the Caledonian highlands by complex river systems, resulting in the alternating coal swamps and the sands and silts that buried them, pretty much the standard example of Pennsylvanian coal deposits.

The package of rocks is one of the best examples of Pennsylvanian coal strata in the world. At least Charles Lyell thought so. Lyell was the author of the books entitled the Principles of Geology, which arguably made geology into a modern science, and Lyell wrote that these were the finest examples of coal-age rocks on earth.

The upright fossil trees at Joggins contain animal fossils as well, including the oldest known reptile. We’ll talk more about them in a couple days.

The fossiliferous cliffs at Joggins are a World Heritage Site, designated in 2008.

—Richard I. Gibson

References:
Joggins Formation 
Alluvial sedimentology and basin analysis of Carboniferous strata near Joggins, Nova Scotia, Atlantic Canada, by Michael C. Rygel, 2005
Five more articles
Historical perspective on the Joggins cliffs geology 
Jogins Fossil Cliffs

Photo of upright Pennsylvanian tree fossil in Joggins Formation by Michael C. Rygel.

July 3. Coal Swamps

The carbon that gives its name to the Carboniferous is found in abundant coal beds from around the world. Coal is a sedimentary rock composed almost entirely of plant matter. Some older coal beds exist, but it wasn’t until the Pennsylvanian Period that plants were abundant enough to form extensive deposits. And it wasn’t just a matter of piles of wood and leaves – a stagnant, low-oxygen environment was needed to prevent complete decomposition by oxidation. Such a setting was found in the swamps that were common – and huge – during Pennsylvanian time around the world. 

Partially decayed plant matter initially produces peat. As the pile is buried by later sediment, higher temperatures drive off water and coal forms.

Mark Twain’s 1903 description of coal measures is really pretty accurate. “In the first place,” Twain wrote, “a coal bed is a slow and troublesome and tiresome thing to construct. You have to grow prodigious forests of tree-ferns and reeds and calamites and such things in a marshy region; then you have to sink them under, out of sight and let them rot; then you have to turn the streams on them, so as to bury them under several feet of sediment, and the sediment must have time to harden and turn to rock; next you must grow another forest on top, then sink it an put on another layer of sediment and harden it; then more forest and more rock, layer upon layer, three miles thick – ah, indeed, it is a sickening slow job to build a coal-measure and do it right!”

Twain was describing sequences of rock known today as cyclothems, alternating layers of coal and sediment that reflect alternations in the environment, from coal swamps to river lowlands and mud flats and even occasional incursions by the sea. These alternating environments produced rhythmically layered coal, shale, sandstone and occasional limestone called cyclothems. The individual layers, representing distinct environments, can be as thin as a few inches, but the total package might be 5 to 10 feet or more thick, and the pile of packages reaches many hundreds to thousands of feet in places. It’s possible to recognize specific erosion surfaces – unconformities – that may not represent the vast amounts of time we’ve talked about previously with unconformities, but that may be regionally extensive even if they represent time spans of less than a million years.

The alternating environments most likely indicate rhythmic variations in sea level, and those variations in turn were probably caused by episodes of glaciation in the southern hemisphere that locked up water to lower sea level, then ended to allow sea level to rise again.

Predictable variation in the kinds of rock in a sequence is a great tool for understanding not only where you should expect to find coal, but also for understanding the variations in rock type that can point to oil source rocks and reservoir rocks. This study is an important aspect of oil exploration, where it’s given the general name sequence stratigraphy.

* * *

Today’s geological birthday is Charles Schuchert, born July 3, 1858, in Cincinnati, Ohio. Schuchert was a brachiopod specialist and spent much of his career at Yale University, but he’s probably best known as one of the first geoscientists to make comprehensive paleogrographic maps – maps that show the distribution of lands and seas and other environments at various points in the geologic past.

—Richard I. Gibson

Coal swamp image from 4th edition of Meyers Konversationslexikon (1885–90, public domain)