11.6: Synthesis- Linking the carbon and oxygen cycles

Plants use photosynthesis to capture energy from the sun, building tissue. In the distant past the same was true for primordial cyanobacteria and phytoplankton in the oceans. Collectively, these organisms have been the most powerful agents of geochemical change: they extracted CO₂ from the atmosphere, shucking off its oxygen as waste, and retaining the carbon, first in the form of glucose. In the far past much of the carbon was buried, sequestered from the atmosphere, and transformed into fossil fuels. Today burning fossil fuels is reversing that. In the words of Roger Revelle

“. . . human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years. This experiment, if adequately documented, may yield a far-reaching insight into the processes determining weather and climate.”

Carbon burial

Figure \(\PageIndex{1}\): Carbon and oxygen fluxes between plants, the atmosphere, and the geosphere. Callan Bentley cartoon.

Consider this diagram, which highlights some of the fluxes (flows) between the atmosphere, plants, and layers of sedimentary rock. Plants pull CO₂ from the atmosphere and release O₂ as a waste product. The O₂ will eventually react with the plant carbon through decomposition unless the plant matter is buried. Once buried, it is isolated from free oxygen; it cannot decompose. Its carbon then becomes part of sedimentary rocks, and is locked in the geosphere until uplift (often, mountain-building) brings it back into contact with atmospheric oxygen. This allows it to be weathered (re-oxidized). This occurs on timescales of tens to hundreds of millions of years.

Impacts on the cryosphere and biosphere

Let us consider the impacts of carbon burial on three phenomena: fire, climate, and animal physiology.

Fire

Figure \(\PageIndex{2}\): A 2007 wildfire in Idaho. US Fish & Wildlife Service photo.

Fire is a natural part of the modern Earth system. Wildfires can be sparked by lightning strikes or volcanic eruptions. They are more likely to ignite and to spread in areas that are temporarily warm and dry, but have seen wet enough conditions in the past to build up a supply of plant fuel. Combustion occurs when the plant tissues react with atmospheric oxygen, and release the carbon in the plants back into the atmosphere as CO₂.

So oxygen is a pre-requisite for wildfire, and oxygen levels have changed throughout Earth history. How much oxygen is necessary? It turns out that fire requires at least 13% O₂ in the surrounding atmosphere. Over the long-term, our planet has bulked up the free oxygen content of its atmosphere, from an initial 0% to the current ~21%. It is estimated that the critical 13% threshold was crossed in the Silurian. Two lines of evidence support this interpretation:

First, the oldest terrestrial plant fossils are spores that date to the Ordovician, suggesting colonization of the land surface by organisms akin to club mosses. These plants provided fuel to burn, but also shelter for animals. The oldest terrestrial animal fossils date to shortly thereafter. As plants colonized the land, they led to more of the planet’s surface being devoted to (incidental) oxygen production. It is thought that oxygenation of the atmosphere would also have led to the development of an ozone (O₃) layer, which would have enabled animals to spread across the land due to lowering the influx of harmful ultraviolet radiation from the Sun.

Figure \(\PageIndex{3}\): Charcoal clasts in Silurian sandstone from Wales. Modified from Figure 3 of Glasspool & Gastaldo (2022).

Second, by 420 Ma (Silurian), charcoal starts showing up. Charcoal is a carbon-rich version of what used to be wood, but all the water and volatile components have been driven off through heating it in the presence of limited amounts of oxygen. It is our first evidence that fires had started burning on Earth.

With the exception of a “gap” in the mid-Devonian, charcoal has been a common constituent of terrestrial sediments for the rest of the Phanerozoic Eon. Thanks to the rise in oxygen levels, fire has become a standard phenomenon on the surface of Earth. So the burial of plant biomass preserves some carbon for the future, but leaves behind oxygen at the surface, and that oxygen that makes it more likely for unburied plants to burn.

Ice

In the mid- to late-Paleozoic, plants evolved and spread across the land area of our planet. These plants extracted CO₂ from the atmosphere, but if they burned or rotted over short timescales, that carbon went straight back into the atmospheric reservoir as CO₂. That’s an example of the short-term carbon cycle. On the other hand, if the plants were buried in swampy sediments, then the plant carbon could no longer can interact with the oxygen in the atmosphere. It had been sequestered.

There is evidence of the reduction of atmospheric CO₂ at several points in Earth history. During the late Devonian period, excessive formation of black shale (full of carbon captured by marine plankton) led to an ice age and one of the big five mass extinctions. During the ensuing Carboniferous period, repeated transgressive burials of coastal swamps led to cyclothems including the coal seams that powered the Industrial Revolution. The Carboniferous gets its name from the carbon-rich nature of the rocks laid down at that time in England. In the Neoproterozoic Era, the planet froze over when CO₂ levels moved close to zero due to excessive weathering of the continental crust. It wasn’t organic carbon burial in that case, but reaction of CO₂ with silicate minerals that caused the CO₂ drawdown and cooling.

In summary, high rates of carbon burial lead to lower CO₂ levels, which causes a reduction in the greenhouse effect, and thus deep ice ages. Sometimes this global cooling is severe enough to cause a mass extinction.

Big bugs

But burial of organic carbon doesn’t just lower global CO₂ levels. It also means that the waste product of photosynthesis, oxygen, builds up and has a lesser amount of carbon available to react with. With less organic carbon available for oxidation, atmospheric oxygen levels would be expected to go up.

In the planet’s earliest years, there was no free oxygen in its atmosphere. Gradually it built up through time, largely thanks to the photosynthetic work of cyanobacteria. By the Paleoproterozoic/ Mesoproterozoic boundary, it had accumulated to something like 1% of the atmosphere by volume. This has been dubbed the Great Oxygenation “Event.” Currently, the atmosphere has an atmosphere that consists mainly of nitrogen (N₂) but also includes ~21% free oxygen (O₂).

However, it hasn’t been a straight path from the GOE to our present oxygenation. During the Carboniferous period, rates of carbon burial were high, so that means atmospheric CO₂ was low and therefore atmospheric O₂ was high. Estimates are that it got as high as 35% of the atmosphere. In such an atmosphere, you wouldn’t have to inflate your lungs as often in order to get the oxygen you need.

Insects don’t have lungs. Instead, they have a network of branching tubes that runs throughout their body, and through which air circulates. The maximum size of insects today is controlled by the efficiency of this tube network in delivering sufficient oxygen to the tissues that need it. The Goliath beetle is the current record-holder as “biggest bug.” It can get up to 4.5 inches (11 cm) long. While that’s certainly impressive, it doesn’t hold a candle to this:

Figure \(\PageIndex{4}\): Large fossilized Meganeura dragonfly. Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.

That’s a dragonfly the size of a hawk. Its wingspan is 28 inches (70 cm). It could never survive today, but during the Carboniferous, with an atmosphere of 35% oxygen, such a huge insect was plausible. Other examples of giant arthropods include Arthopleura and Megarachne.

Fossil fuels

Buried carbon does not necessarily stay buried. By oxidizing that carbon vis combustion, energy originally bound up in chemical bonds as much as ~350 million years ago is released. We refer here to and coal, petroleum, and natural gas: the “fossil fuels.” They get this name because all the carbon in each fuel came originally from living things extracting CO₂ from the atmosphere.

Coal

Coal is a solid fossil fuel that forms from compressed plant matter called peat. Frequently coal layers (“seams”) still retain the impressions of leaves, stems, seeds, spores, and other botanical details. Coal comes in several “grades” that reflect varying degrees of compression. The lowest grade is lignite (brown coal), which is just a step up from peat, the original mix of plant bits. If it is more compressed still, it will become bituminous coal. And the highest grade of coal, anthracite, forms only when a coal seam is subject to metamorphism under the influence of mountain-building.

The plants that are the progenitor of coal live on the land, in wet, aquatic, or swampy settings and thus are poorly oxygenated. Carbon-rich plant fragments falling into that water are therefore more likely to avoid oxygenation long enough to be buried. The rate of new carbon being added as plants shed debris outstrips the rate at which oxygen diffuses across the surface of the water.

But peat can also form in cool, wet climates such as Ireland or Scotland. There, a layer of vegetation clings to the topography of the land. The damp weather means that sphagnum and heather, the main peat-building vegetation, grow on slopes and hilltops as well as in the valley bottoms, with underlying peat layers averaging a meter or two thick. The entire landscape is essentially blanketed in bog. It grows upward at about 1 cm/year.

Oil

Oil has a different origin story. Here, the photosynthesis was conducted by marine or lake algae, phytoplankton floating as single-celled organisms in the uppermost, sunlit portion of the water column. When they died, their bodies rained down to the bottom. This transferred the carbon they contained to the dark depths. Often, the bottoms of lakes or the ocean have far less dissolved oxygen than waters closer to the surface. As a result, carbon entering these anoxic waters tends to be buried before it can oxidize. As with swamp coal, it’s a case of the carbon burial rate exceeding the flow of dissolved oxygen to the same place.

Once buried, the algae have to be warmed up to a critical temperature – about 100°C. If it’s too cold, their dead cells won’t transform into petroleum. If it’s too hot, the mixture will turn into natural gas (described below) and graphite.

Figure \(\PageIndex{5}\):The Conodont Alteration Index of Anita Harris (then Anita Epstein), et al., 1977. USGS image.

Interestingly, one way geologists can make a quick but accurate assessment of the temperature to which potentially-oil-bearing rocks have been warmed is by examining fossils. Specifically, the spiky “elements” of the conodont animal. These are made of hydroxylapatite, a phosphate mineral which changes color when heate. Geologist Anita Harris developed a tool, the Conodont Alteration Index, a 6-point color scale that uses the various colors of conodonts to see if their host rocks warmed up to the appropriate temperature for oil generation.

Conodonts can be collected from numerous sites in a region, their color assessed using the Index, and then the presence of each color can be contoured. Here’s an example for the sedimentary strata of the Appalachian Plateaus + the Valley & Ridge provinces:

Figure \(\PageIndex{6}\): Conodont Alteration Index color distribution across the Appalachian Basin. Modified from a USGS figure.

Once produced, oil must migrate to a spot where it can collect in amounts where humans will find it economically worthwhile to drill it out and extract it. There are places where oil naturally seeps out at Earth’s surface, such as the legendary tar pits of Rancho La Brea in Los Angeles, California.

Figure \(\PageIndex{7}\):An anticlinal petroleum trap. C.Bentley modification of an original diagram by GreenMagenta via Wikimedia.

Most of the oil pumped today found a stable residence underground in some variety of subterranean oil trap. These have various shapes due to various geological circumstances, but one of the easiest to grasp is that of an anticline. If the arch-like shape of an anticline consists of a stack of layers with carbonaceous source rocks at the bottom (such as black shale), impermeable cap rock strata (such as shale) at the top, and a spongey reservoir layer (such as sandstone) in between, that makes for an ideal petroleum trap. Petroleum geologists are charged with finding these traps and directing the company’s engineers to tap into them.

Natural gas

Figure \(\PageIndex{8}\): Outcrop of the Devonian-aged Marcellus Shale in Pennsylvania. Yellow field notebook for scale. Modified from a Terry Engelder photo.

The third and final fossil fuel is neither a solid like coal, nor a liquid like oil. Instead, it’s a mixture of gases, principally methane (CH₄). Like oil, its progenitor is phytoplankton. While these algae live, they photosynthesize. When they die, their bodies rain down into anoxic deep waters, imparting a high carbon content to the sediments deposited there. Typically, the resulting rock is a black shale.

Black shales are more common in the geologic record during times of sluggish ocean circulation or widespread stagnation, such as the end-Permian or the mid-Cretaceous. Black shales can also be deposited in lakes; they are not necessarily marine in origin.

The Marcellus Shale of the Appalachians and the Bakken Shale of the Dakotas are both Devonian-aged black shales with lots of natural gas trapped inside them. In the past decade, the advent of horizontal drilling has allowed fossil fuel companies to drill into these shales and intentionally shatter them, allowing the gas to flow out, up the drill hole, where it can be collected, transported, and sold. The shattering process is achieved by (a) injecting fluids under very high pressures, along with (b) sand, which props open the fractured shale once the cracks open up. This process is called hydraulic fracturing, or “fracking.”

Natural gas is also produced in coal seams. This variety is referred to as “coal-bed methane.” Coal miners have been very conscious of coal-bed methane since coal mining began – because it is invisible gaseous reduced carbon, one spark from mining equipment could trigger an explosion. Far too many miners lost their lives to this hazard. They called it (and any other inflammable gas) “firedamp.”

Anthropogenic climate change

Because of the rich energy content of fossil fuels, humans are burning them, and depending intensely on the energy released in the combustion. The modern global economy and the lifestyles of modern humans are powered in large part by the combustion of fossil fuel — solids, liquids, and gases that derive their energy content from sunlight that shone on Earth tens to hundreds of millions of years ago. As plants do with oxygen waste, the CO₂ waste goes into the atmosphere. Because it’s invisible, we’ve been able to ignore it for a long time. Out of sight, out of mind.

CO₂ rise

Figure \(\PageIndex{9}\): The record of the past half-century of changing CO₂ levels in Earth’s atmosphere. Scripps / NOAA graph.

Since humanity began burning fossil fuels in earnest (the start of the Industrial Revolution), CO₂ levels have increased in the atmosphere of our world. The seasonal signal of photosynthesis is still there, but every year, CO₂ ends up at a higher level than the previous year. From a pre-Industrial level of ~280 parts per million (ppm) to the current ~420 ppm, it has increased by about 50%.

It would have been an even larger increase, had not the Earth system been so intimately inter-connected. Much of the CO₂ that humanity has emitted through the burning of fossil fuels has been re-extracted from the atmosphere: some has been taken out by plants doing photosynthesis more efficiently, and some has been soaked up by the oceans.

Figure \(\PageIndex{10}\): Thanks to CO₂ absorption by the oceans and photosynthesis by plants, the human-induced carbon loading of the planet’s atmosphere isn’t as severe as it might otherwise have been. NASA graphic.

Estimates of the flow of carbon between reservoirs in the Earth system suggest that human activities release about nine billion tons (gigatons) of carbon into the atmosphere per year, of which three is then pulled out again by plant photosynthesis, and two is dissolved in the oceans. That leaves a net atmospheric gain of four gigatons per year. Examine the graphic at left to explore the complexities of carbon fluxes between reservoirs, but follow the red numbers, which show the changes that result from the human economy’s dependence on burning ancient trees and algae. This imbalance is why the graph above shows a relentless climb: the rates of carbon absorption can’t match the rates of carbon emission.

Oceanographers are well aware of one of the consequences of this natural carbon sequestration: those extra two gigatons of carbon soaked into the oceans each year produce carbonic acid, which is making the ocean overall more acidic. Already surface ocean pH has dropped by 0.1 unit, with a forecast total of -0.7 pH units change over the next 750 years. Remember that pH is -log[H⁺] so 0.1pH units is roughly a 20% increase in [H⁺]. This potentially has enormous effects for the viability of life in the oceans. Many of our greatest episodes of mass extinction are correlated with times of intense ocean acidification.

But the more significant issue is probably global warming. Because of the selective transparency of the CO₂ molecule, the higher level of this greenhouse gas has resulted in Earth retaining more energy than previously. This has caused the average global temperature to increase.

Figure \(\PageIndex{11}\):Ranking the last 142 years of global average temperatures in chronological order. NASA animation of NASA data.

There are many consequences of this change in the planet’s climate, including: melting glacial ice and permafrost, rising sea levels, intensification of tropical storms, increasing desertification, and a spur to the migration of species toward more pole-ward latitudes and higher elevations. It is an interesting time to be alive and scientifically aware on planet Earth.

O₂ fall

There is another aspect of the burning of fossil fuels which is also noteworthy. The basic combustion reaction of C + O₂ → CO₂ generates CO₂ of course, but it also consumes O₂. For every atom of carbon we oxidize to power our energy grid or heat our homes or propel our vehicles, we lose one molecule of free oxygen from the atmosphere. Thus, as the past six decades of measurements show a rise in CO₂, we should also expect a commensurate decline in O₂.

Indeed, that is what we have observed:

Figure \(\PageIndex{12}\):Measurements of atmospheric oxygen levels show a long-term decline. Graph by Scripps O₂ program, annotated by C. Bentley.

At first glance, this graph might seem alarming, but remember that this is a parts-per-million level decline in a very large reservoir of oxygen: ~21%. It represents a drop of a few ten-thousandths of the total amount of oxygen. So you don’t have to worry about running out of breathable air. We will exhaust our supply of fossil fuels long before atmospheric oxygen levels change by even 1%.

Instead, save your worrying for the impacts of global warming.

Further reading

Wally Broecker, 1996. “Et tu, O₂?” 21^stC, Columbia University.

Ken Caldeira and Michael Wickett, 2003. “Anthropogenic carbon and ocean pH,” Nature 425, p.365. https://doi.org/10.1038/425365a

Anita Epstein, Jack Epstein, and Leonard Harris, 1977. “Conodont color alteration — an index to organic metamorphism.” USGS Professional Paper 995. 27 pages.

Ian Glasspool and Robert Gastaldo, 2022. “A baptism by fire: fossil charcoal from eastern Euramerica reveals the earliest (Homerian) terrestrial biota evolved in a flammable world,” Journal of the Geological Society pre-print. Accessed online 14 January 2023.

Iman Gosh, 2021. “All the Biomass of Earth, in One Graphic,” Visual Capitalist. (20 August 2021) Retrieved 10 January 2023.

Elizabeth Kolbert, 2006. “The Darkening Sea,” The New Yorker. November 20, 2006, p.66.

Museum of the Earth, 2023. “Anita Harris,” Daring to Dig Paleontologist Profiles. Retrieved 12 January 2023.

Holly Riebeek, 2011. “The Carbon Cycle,” NASA’s Earth Observatory. Retrieved 12 January 2023.

A.A. Venn, J.E. Loram, A.E. Douglas, “Photosynthetic symbioses in animals,” Journal of Experimental Botany, Volume 59, Issue 5, March 2008, Pages 1069–1080, https://doi.org/10.1093/jxb/erm328

Original Source

Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts.Historical Geology A free online textbook for Historical Geology courses, https://opengeology.org/historicalge...gen-interplay/ Accessed December 2023 CC-BY-SA-NC

with editing and additions by LibreTexts