The Carbon Cycle

The carbon cycle is a biogeochemical cycle that represents the exchange of carbon (C) between different reservoirs on Earth, such as the atmosphere, biosphere, oceans and rocks. It is a critical part of the Earth system that is required to sustain life and regulate climate. The carbon cycle can be subdivided into a short-term cycle and a long-term cycle, which operate on different spatial and temporal scales.

Short-term carbon cycle
The short-term cycle involves carbon transfer between the atmosphere, biosphere and oceans on timescales of days to tens of thousands of years. Carbon dioxide (CO2) from the atmosphere is taken up by plants on the continents or by phytoplankton in the oceans through photosynthesis, during which it is transformed into organic matter and oxygen (O2). Over time, plants and phytoplankton may be eaten by higher organisms, but eventually their organic matter will be respired. Respiration is the reverse process of photosynthesis and consumes O2 and produces CO2, which is often associated with microbes. The processes of photosynthesis and respiration may be expressed as follows:

CO2 + H2O <-> CH2O + O2

On land, carbon may also be transferred to soils by the falling of leaves, the death of plants and the development of soil biota. This soil carbon may is subsequently transported to the oceans in dissolved form by rivers. Gas exchange between the oceans and atmosphere completes the short-term carbon cycle.

Long-term carbon cycle
The long-term carbon cycle involves carbon transfer between the atmosphere, biosphere, oceans and rocks on timescales of millions of years. It consists of two subsycles: the silicate-carbonate subcycle and the organic subcycle.

In the silicate-carbonate subcycle, silicate rocks on the continents (CaSiO3) are subjected to chemical weathering over time, which consumes CO2 from the atmosphere. Rivers deliver the dissolved minerals to the oceans, where they are reprecipitated by biological activity as marine silicates and carbonates (SiO2 and CaCO3, respectively) and eventually buried in the geological record. These sedimentary rocks are ultimately returned to Earth’s surface through subduction, volcanism and metamorphism, which releases CO2 back to the atmosphere and results in the formation of new silicate rocks. These processes may be expressed as follows:

CaSiO3 + CO2 <-> CaCO3 + SiO2

In the organic subcycle, photosynthesis and respiration result in carbon transfer on geological timescales. Organic matter is eventually buried in sedimentary rocks, such as shales and coals, which consumes CO2 from the atmosphere. Old sedimentary rocks enriched in organic matter may become subjected to chemical weathering on the continents over time, which releases CO2 back to the atmosphere.

The long-term carbon cycle is able to exchange massive amounts of carbon between rocks and the other reservoirs on geological timescales. As a consequence, it governs atmospheric CO2 concentrations and regulates global surface temperatures and climate. Moreover, major perturbations of the carbon cycle are shown to have greatly affected life on Earth in the geological past, for example during mass extinctions.


Information source: Berner, R. A. (2004). The Phanerozoic Carbon Cycle: CO2 and O2. Oxford University Press.

Image: Paraná pines at sunrise in Serra da Bocaina National Park, Brazil. Credit: Heris Luiz Cordeiro Rocha, Wikimedia Commons.

El Niño Southern Oscillation

The El Niño Southern Oscillation (ENSO) is an irregular, recurring climate change phenomenon that is associated with variations in sea surface temperatures in the equatorial Pacific Ocean. It is caused by changes in the strength and direction of the Walker circulation, which governs zonal and vertical atmospheric circulation between the tropical eastern and western Pacific Ocean. The ENSO is characterized by a warming phase called El Niño (‘the boy’ in Spanish) and a cooling phase called La Niña (‘the girl’ in Spanish). By affecting the distribution of heat and precipitation across the Pacific Ocean, the ENSO is able to greatly influence weather and climate in many regions around the world.

The Walker circulation generally arises from a high air pressure system above the eastern Pacific Ocean near South America and a low air pressure system above the western Pacific Ocean near Indonesia and Australia. Under normal conditions, easterly equatorial winds result in the development of a warm water pool towards the western Pacific.

An El Niño may occur when the Walker circulation weakens or reverses, resulting in a lower air pressure above the eastern Pacific and a higher air pressure above the western Pacific. During El Niño, the Pacific warm water pool moves east towards South America and the surface waters of the eastern Pacific warm up as the upwelling of colder deep waters is reduced. Therefore, an El Niño is characterized by warmer and wetter climates in South America and colder and drier climates in Indonesia and Australia.

A La Niña may occur when the Walker circulation grows especially strong. During La Niña, the Pacific warm water pool moves further west towards Indonesia and Australia and the surface waters of the eastern Pacific cool down as the upwelling of colder deep waters is enhanced. As a consequence, a La Niña is characterized by colder and drier climates in South America and warmer and wetter climates in Indonesia and Australia.

El Niño and La Niña may vary in duration and intensity, but each phase generally lasts one to a few years. Extreme shifts of the ENSO may result in severe floods or droughts and may therefore have a large impact on society. Because the occurrence of El Niño and La Niña is irregular, the ENSO is difficult to predict longer than a year in advance.


Information source: National Oceanic and Atmospheric Administration (NOAA).

Image: El Niño sea surface temperature (SST) anomaly in the eastern Pacific Ocean on December 24th, 2015. Credit: National Oceanic and Atmospheric Administration (NOAA).

Mount Erebus, Ross Island, Antarctica

Mount Erebus is the southernmost active volcano on Earth and is located on Ross Island in Antarctica. It has an elevation of 3794 meters and is surrounded by three inactive volcanoes, Mount Terror, Mount Bird and Mount Terra Nova. Volcanism on Ross Island is related to the presence of the Erebus hotspot in the deep subsurface and as a result, continuous eruptions occur from the persistent lava lake in the inner crater of Mount Erebus. The volcano is also known for its ice fumaroles, towers of ice that form around gases escaping from vents in its surface.


Information source: Mount Erebus Volcano Observatory

Image: Mount Erebus and the surrounding landscape on Ross Island, Antarctica. Credit: Hannes Grobe, Wikimedia Commons.

Hydrated salts are evidence for flowing water on Mars

Scientists at NASA have discovered evidence for flowing water on the surface of Mars. Spectral data recorded by the imaging spectrometer of the Mars Reconnaissance Orbiter indicates the presence of hydrated salts in recurring slope lineae at four different locations on the planet. These narrow streaks of low reflectance on the surface of Mars grow in the downslope direction during warm seasons when temperatures reach 250 K – 300 K and fade away during cold seasons. The hydrated salts most likely represent a mixture of magnesium perchlorate (Mg(ClO4)2.H2O), magnesium chlorate (Mg(ClO3)2.H2O) and sodium perchlorate (NaClO4.H2O), and appear to be most abundant when the recurring slope lineae are most extensive. This suggests that these structures are formed as a result of water flowing at the surface or in the shallow subsurface of Mars. The discovery of liquid water is a major step in the search for extant life on Mars.


Journal reference: Ojha, L., Wilhelm, M. B., Murchie, S. L., McEwen, A. S., Wray, J. J., Hanley, J., … & Chojnacki, M. (2015). Spectral evidence for hydrated salts in recurring slope lineae on Mars. Nature Geoscience.

Image: Recurring slope lineae in the Garni Crater on Mars as seen from the Mars Reconnaissance Orbiter. Source: NASA/Jet Propulsion Laboratory-Caltech/University of Arizona.

Fossil remains of new hominin species Homo naledi discovered in South Africa

Paleontologists have discovered the fossil remains of a new hominin species in a dark cave system in South Africa and published their preliminary results in eLife. The new species has been named Homo naledi, which means “star man” in Sotho, after the Dinaledi Chamber of the Rising Star cave system where it was found. Homo naledi is characterized by a small body and brain size, but it also features several adaptations of the hands and feet that are relatively similar to modern humans. The age of Homo naledi and its position in the phylogenetic tree of hominins is still unresolved, but it is believed to be one of the more primitive ancestors of mankind. So far, an unprecedented 1550 remains of at least 15 individuals of Homo naledi have been unearthed, representing the largest fossil assemblage of a single hominin species ever found in Africa. Given the richness and exceptional preservation state of these fossils, it is speculated that Homo naledi may have been capable of performing primitive burial rites.


Journal references:

Berger, L. R., Hawks, J., de Ruiter, D. J., Churchill, S. E., Schmid, P., Delezene, L. K., … & Zipfel, B. (2015). Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. eLife4, e09560.

Dirks, P. H., Berger, L. R., Roberts, E. M., Kramers, J. D., Hawks, J., Randolph-Quinney, P. S., … & Tucker, S. (2015). Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa. eLife4, e09561.

Image: Fossil remains of Homo naledi, the new hominin species discovered in South Africa. Source: Lee Berger, Wikimedia Commons.

Angel Falls, Guiana Highlands, Venezuela

Angel Falls, also known as Salto Ángel, is the highest waterfall on Earth and is found in Canaima National Park in the Guiana Highlands of Venezuela. At Angel Falls, the waters of the Churun river fall down the cliffs of the Auyán-tepui mountain from a staggering total height of 979 meters. The waterfall consists of three parts: the main plunge of 807 meters, a section of sloped rapids and a final plunge of 30 meters.


Information source: Canaima National Park Service

Image: Angel Falls flowing down the Auyán-tepui mountain in the Guiana Highlands, Venezuela. Credit: Diego Delso, Wikimedia Commons.

Mount Nyiragongo, Virunga Mountains, Democratic Republic of the Congo

Mount Nyiragongo is an active stratovolcano in the Virunga Mountains in the Democratic Republic of the Congo. The volcano has an elevation of 3470 meters and its main crater usually contains a large lava lake. Volcanism at Mount Nyiragongo is related to the presence of the Albertine Rift, the western branch of the East African Rift system, where the African tectonic plate is splitting into two.


Information source: Virunga National Park Service

Image: Lava lake in the crater of Mount Nyiragongo in the Virunga Mountains, Democratic Republic of the Congo. Credit: Caj Tjeenk Willink, Wikimedia Commons.

Uluru / Ayers Rock, Northern Territory, Australia

Uluru / Ayers Rock is a massive rock structure located in Northern Territory, Australia. Consisting of red sandstone that is relatively resistant to weathering, Uluru / Ayers Rock prominently rises 348 meters above the surrounding lowlands. The structure has likely been formed by sediment deposition in alluvial fans associated with mountain uplift during the late Neoproterozoic to early Cambrian (~ 550 million years ago), followed by tilting in the Paleozoic (~ 400 million years ago). Subsequent erosion of the surrounding rocks and the formation of iron oxides further shaped Uluru / Ayers Rock into its present state as a majestic red mountain.


Information source: Uluru-Kata Tjuta National Park Service

Image: Uluru / Ayers Rock at sunset in Northern Territory, Australia. Credit: Weyf, Wikimedia Commons.

Mount Everest, Himalayas, Nepal and China

With a summit at 8848 meters, Mount Everest is the highest mountain peak on Earth. It is located in the Himalayas on the border between Nepal and China and is surrounded by several other high peaks, including the Lhotse, Nuptse and Changtse. The lower part of Mount Everest consists of metamorphic rocks such as schists and gneisses, as well as numerous granite intrusions. The middle part of the mountain consists of schists, phyllites and marbles with a lesser metamorphic grade, while the upper part of the mountain is mostly characterized by a succession of limestones and siltstones. Mount Everest and the Himalayas have been formed by the continental collision of the Indian and Eurasian tectonic plates in the Cenozoic.


Information source: Sagarmatha National Park Service

Image: Mount Everest in the Himalayas as seen from Tibet, China. Credit: Luca Galuzzi, Wikimedia Commons.

Monte Fitz Roy, Andes, Argentina and Chile

Monte Fitz Roy, also known as Cerro Chaltén, is a mountain in the Andes, located near the village of El Chaltén in Patagonia along the Argentina and Chile border. The summit of Monte Fitz Roy reaches a height of 3405 meters, but is generally covered in clouds. While it is not as high as other mountain peaks in the Andes, it is especially notorious among mountaineers for its jagged peaks and sheer granite faces.


Information source: Los Glaciares National Park Service

Image: Monte Fitz Roy in the Andes along the Argentina and Chile border. Credit: Todor Bozhinov, Wikimedia Commons.

1 2 3 4 5 7