Traveling around the world with carbon

Carbon, with the chemical symbol C, is a non-metallic element located at the 6th position in the periodic table.

Image source: Tuchong Creative

It is almost ubiquitous and vital in our lives - the bodies of animals and plants everywhere are made of it;

Daxinganling autumn forest image source: Tuchong Creative

Breathtaking karst landscapes are composed of it;

Nandan Karst Peaks in Hechi, Guangxi Photo source: Tuchong Creative

It is also present in the fossil fuels (coal, oil, etc.) that supported the Industrial Revolution;

Coal loaded on ocean-going cargo ships Image source: Tuchong Creative

The element carbon, in its unique way, has left its footprints in the four major spheres of the Earth: biosphere, lithosphere, atmosphere and hydrosphere, profoundly affecting all aspects of the Earth's environment.

1. From the atmosphere to the biosphere

Before plants appeared, carbon and two oxygen atoms (O) were in close proximity, forming a group called carbon dioxide (CO₂);

Molecular model of carbon dioxide Image source: Visual China

The fate of carbon dioxide changed after the emergence of plants.

Carbon dioxide enters the plant through its stomata, and under the action of sunlight, the well-known photosynthesis occurs, which includes two stages.

Stomata of plant leaves under a microscope Image source: Visual China

The first is the light reaction stage, in which the photosynthetic pigments present in the chloroplasts of plant cells absorb light energy, excite high-energy electrons in the body, and transfer them within the body to provide energy for the photolysis of internal water, ultimately converting light energy into chemical energy, which is stored in the form of ATP (adenosine triphosphate) and NADPH (reduced coenzyme II), and produces oxygen (O₂) which is released into the air.

Illustration of the two stages of photosynthesis in plant chloroplasts: He Lin

The second is the dark reaction stage, during which no light is required, but ATP and NADPH, the products of the light reaction, are needed to reduce carbon dioxide to sugars.

Through photosynthesis of plants, the carbon dioxide that originally suffocated animals is converted into oxygen and energy sources (sugars) that are essential for their survival.

Scenery of Kalajun grassland in Yili, Xinjiang (vertical picture, horizontal screen view) Image source: Tuchong Creative

After carbon dioxide is converted into sugars in plants, it has a specific body, but it can only move inside the plant. When plants are eaten by herbivores, the organic matter (polysaccharides) in the animals is broken down into monosaccharides, which, under the action of oxygen, generate carbon dioxide and water, as well as energy that can be used by animals.

Rabbits and mice on the Qinghai-Tibet Plateau feed on plants. Image source: Tuchong Creative

The mantis stalks the cicada, unaware of the oriole behind it. There is a complete food chain in nature. While herbivores are enjoying the buffet carefree, carnivores are hiding in the distance, watching them hungrily. The herbivores cannot escape the fate of being hunted, and the organic matter and energy in their bodies are transferred to the latter in the predator-prey relationship.

Tibetan fox is hunting rabbits and mice. Image source: Tuchong Creative

The starting point of the energy of this food chain is carbon dioxide in the air, and the end point is also carbon dioxide. The carbon element completes a long cycle in the biosphere.

2. From Biosphere to Lithosphere

Plants have a limited lifespan; they emerge from the soil and eventually become buried underground; the same is true for animals.

Carbon in the remains of plants and animals mostly exists in the form of organic matter. If it is completely decomposed by microorganisms under aerobic conditions, it will produce inorganic substances such as water and carbon dioxide.

Fungal communities growing from the "remains" of trees. Image source: Tuchong Creative

Organic matter that decays quickly is almost completely decomposed into inorganic matter and becomes rock; the movement of the earth's crust crushes it, and the extremely high temperature and pressure deep underground causes the rock to melt into magma.

La Palma volcano eruption Image source: Tuchong Creative

When the earth's crust moves actively, magma erupts from the ground, and a new cycle of carbon begins. Large-scale volcanic eruptions can penetrate different layers of the earth system (lithosphere, hydrosphere, biosphere, and atmosphere), promote material exchange and circulation among the earth's layers, and transport large amounts of volcanic gases rich in carbon dioxide to the atmosphere.

Basalt columnar joints in Changle ancient crater, Weifang City, Shandong Province, China Image source: Tuchong Creative

The erupting magma brought disaster to the earth's creatures, and no grass grew wherever it went. But the magma also brought a large amount of minerals from the earth's crust. After cooling, the magma will form a large amount of volcanic rocks (such as basalt). After weathering, the volcanic rocks will form mineral-rich soil, which is very conducive to plant growth.

3. From the lithosphere to the atmosphere

The decomposition of plant and animal remains is not completely aerobic. Some microenvironments buried underground are almost entirely oxygen-deficient. Many decomposition processes are completed under anaerobic conditions. This process is relatively slow and produces a lot of organic acids, alcohols, aldehydes and other substances.

These decomposed carbon-containing organic matter mix with materials such as silt or carbonate sediments to form sedimentary layers. As the sediments continue to accumulate and thicken, the temperature and pressure rise. This process continues, and the sedimentary layers become sedimentary rocks, thereby forming sedimentary basins, providing a basic geological environment for the generation of fossil energy (oil, coal, and natural gas).

Plant fossils in coal seams Image source: Tuchong Creative

The carbon elements that become oil, coal, and natural gas are not as "violent" as the carbon elements in magma. They remain dormant in the strata until people mine and utilize them.

One of the thousands of oil wells in the Central Valley of California, where oil extraction is underway. Image source: Visual China

The extensive use of fossil energy has led to a sharp increase in carbon dioxide production, which is accumulating more and more at the top of the earth and inside the atmosphere.

Originally, carbon dioxide can absorb infrared radiation in the atmosphere, allowing the earth to provide a habitable place for humans. This is a "natural greenhouse effect."

On June 22, 2012, an iceberg broke off from the Petermann Glacier in northwest Greenland. Image source: Visual China

After entering the industrial age, humans' extensive use of fossil energy has released excessive carbon dioxide, disrupting the carbon cycle: more infrared radiation is reflected back to the ground, exacerbating the "greenhouse effect". Like a domino effect, it has produced a series of chain reactions: glaciers retreat, sea levels rise, waves hit, crops are flooded...

4. From the atmosphere to the hydrosphere

The atmosphere and hydrosphere are inextricably linked, and carbon has left its traces here.

During the period of 4.6-3.8 billion years ago, there was a primitive volcanic atmosphere, whose main components were water vapor, carbon dioxide and highly acidic gases, and the hydrosphere had not yet appeared;

Schematic diagram of the original earth. Image source: Tuchong Creative

3.8-2.6 billion years ago, the hydrosphere appeared, which was a highly acidic-reducing seawater, and the atmosphere evolved into a carbon dioxide-reducing atmosphere. Because the seawater was too acidic, the carbon dioxide in the atmosphere could not provide a carbon source for the hydrosphere.

2.6-1.8 billion years ago, the atmospheric carbon dioxide content began to decline, and the seawater was characterized by an acidic-reducing environment with low pH and low oxygen fugacity. Carbon dioxide has a low solubility in a strongly acidic liquid environment and is always trying to escape. The atmosphere and the hydrosphere still cannot "get along happily";

Until 1.8-6 billion years ago, the earth's environment underwent major changes. The carbon dioxide content in the atmosphere dropped sharply, oxygen began to accumulate, the pH and oxygen fugacity of seawater increased, and it turned into a weakly acidic and weakly oxidizing environment. The atmosphere and hydrosphere began to establish a stable interactive relationship.

From 600 million years ago to 10,000 years ago, the carbon dioxide content in the atmosphere continued to drop sharply, the oxygen content fluctuated and rose, and the nitrogen content continued to accumulate until the modern level. The seawater evolved into a weakly alkaline, strongly oxidizing environment, and the carbon elements in the atmosphere and hydrosphere could interact freely.

Coral reefs in Egypt Image source: Jonnysek/Adobe Stock/TuChong Creative

When a heavy rain falls, carbon dioxide from the atmosphere is carried into the hydrosphere. The rain falls to the ground, and the carbon elements in the rocks flow into the ocean, providing an indispensable carbon source for the formation of coral reefs, and thus making it possible for the formation of coral ecosystems based on reef-building corals.

Let's move on to more recent times. Since the Industrial Revolution, human activities have caused an excess of carbon dioxide in the atmosphere. The seawater has absorbed a large influx of carbon dioxide, causing its pH value to drop, which has had a series of impacts on the ecological balance of the ocean. For example, corals, which are vital to the marine ecosystem, have become bleached due to the departure or death of the symbiotic algae in their bodies, and eventually died from loss of nutrient supply.

Schematic diagram of the coral bleaching process: Xiao Chu

Coral bleaching in Australia's Great Barrier Reef in October 2016. Image source: Visual China

Carbon dioxide is the cause of success and failure. The journey of carbon is still going on silently...

If the earth is to regain its vitality, humans and the environment will have to go through a long process of reconciliation.