In this series, we’ve explored the mysterious depths of the ocean, discovering unique creatures and the complex ecosystems they inhabit. We’ve also touched on the history of ocean exploration. However, one of the most important processes happening in our oceans often goes unnoticed: the marine carbon cycle.
The marine carbon cycle is crucial for capturing carbon and helping to reduce climate change. Through biological processes like photosynthesis, predation, and decomposition, along with the movement of ocean currents, the oceans absorb more carbon than they release. This makes them a major carbon sink, which is vital for protecting our planet from the harmful effects of climate change.
The cycle starts at the ocean’s surface, where large amounts of carbon dioxide diffuse into the water. This carbon can either return to the atmosphere in a process called the fast carbon cycle or stay in the ocean for thousands of years. Phytoplankton, tiny plant-like organisms, play a key role in storing carbon through photosynthesis. Amazingly, phytoplankton are responsible for producing about 80% of the oxygen in our atmosphere.
Through photosynthesis, carbon is turned into sugars, which are then used by larger organisms that eat phytoplankton. This starts the movement of carbon through the ocean’s food web.
Zooplankton and filter feeders mainly eat phytoplankton, and these organisms are then eaten by larger animals, like baleen whales. A whale can eat krill equal to 4% of its body weight each day, keeping much of this carbon from going back into the atmosphere. However, this is only temporary; for long-term carbon storage, the carbon must be moved to deeper ocean layers.
One important process that helps move carbon is called diel vertical migration. Every night, many organisms from the twilight zone (200 to 1,000 meters deep) move to the surface to feed on plankton. This is the largest migration on Earth and results in a net transfer of organic carbon from the surface waters to the deeper ocean.
As these creatures eat and grow, they store carbon in their bodies. When they die or release waste, carbon-rich materials are released into the ocean, helping move carbon downward instead of back into the atmosphere.
Marine snow, a constant shower of organic material and debris falling from shallow waters, plays a big role in the slow carbon cycle. Most of this material is eaten by detritivores, like vampire squid and polychaete worms, which trap and feed on marine snow. Each year, about 50 gigatons of carbon are drawn down into the biological pump, though only a small part reaches the sea floor.
Over time, this sediment builds up, and larger events, like whale falls, support diverse scavenger communities that take in carbon. The carbon stored in a typical 40-ton whale carcass can represent around 2,000 years’ worth of marine snow material, making it a significant part of the biological carbon pump.
Once carbon reaches the sea floor, it gets buried under layers of sediment. Over millions of years, these deposits turn into hard limestone rock and carbonate structures, effectively locking carbon away for long periods. This process helps form geological features, like the White Cliffs of Dover, which are made of ancient organic material.
Besides sedimentation, corals and shell-building organisms sequester carbon by using dissolved carbon to create shells and skeletons. When these organisms die, they leave behind large carbonate structures that store carbon long-term, further enhancing the ocean’s role as a carbon sink.
Besides biological processes, physical mechanisms also help the ocean act as a carbon sink. Ocean circulation, especially through upwelling and downwelling, moves carbon. Upwelling brings carbon from deep waters to the surface, while downwelling moves carbon from the atmosphere into the deep ocean.
The thermo-haline circulation, driven by temperature and salinity differences, plays a crucial role in this process. As warm water moves towards the poles, it cools and becomes denser, allowing it to sink and carry dissolved carbon to the deep sea, where it can stay for hundreds of years.
While the marine carbon cycle is essential for reducing human-driven carbon emissions, it faces challenges. Increased carbon absorption leads to ocean acidification, which can cause coral bleaching and significant biodiversity loss. The marine carbon cycle can only absorb so much carbon, and with many of its inhabitants and interactions threatened by human activity, its ability to function effectively may be compromised.
The marine carbon cycle is a complex mix of biological and physical processes that allows the oceans to capture carbon and help reduce climate change. However, as human activities continue to impact marine ecosystems, it’s crucial that we address carbon emissions to preserve the delicate balance of the ocean’s carbon cycle and protect our planet’s future.
Conduct a simple experiment to observe photosynthesis in action. Use a clear container filled with water and add a small amount of algae or phytoplankton culture. Place the container in sunlight and observe the oxygen bubbles forming. Discuss how this process contributes to the marine carbon cycle and the production of oxygen.
Create a simulation of the ocean food web using role-play. Assign each student a role, such as phytoplankton, zooplankton, or a larger marine animal. Use string to represent the transfer of carbon as students “consume” each other. Discuss how carbon moves through the food web and the importance of each organism in the cycle.
Build a model to demonstrate the concept of marine snow. Use materials like cotton balls and glitter to represent organic material and debris. Drop these materials through a column of water to simulate how marine snow falls through the ocean. Discuss the role of marine snow in carbon transfer and long-term storage.
Organize a debate on the impact of human activities on the marine carbon cycle. Divide the class into two groups: one arguing for the importance of reducing carbon emissions and the other discussing the natural resilience of the marine carbon cycle. Use evidence from the article to support your arguments.
Explore the concept of ocean circulation and its role in carbon transfer. Use a large container of water, ice cubes, and food coloring to simulate thermo-haline circulation. Observe how the cold, colored water sinks and moves through the container. Discuss how this process helps transfer carbon to the deep ocean.
Marine – Related to the sea or ocean, especially in terms of ecosystems and organisms that inhabit saltwater environments. – Marine ecosystems are crucial for global biodiversity, providing habitat for countless species.
Carbon – A chemical element that is the fundamental building block of life, forming the basis of organic molecules and playing a key role in the Earth’s climate system. – The carbon cycle involves the movement of carbon among the atmosphere, oceans, soil, and living organisms.
Cycle – A series of processes that repeat in a predictable pattern, often involving the movement of substances through different parts of an ecosystem. – The nitrogen cycle is essential for converting atmospheric nitrogen into forms usable by plants and animals.
Phytoplankton – Microscopic photosynthetic organisms that float near the surface of aquatic environments and form the base of the marine food web. – Phytoplankton are responsible for producing about half of the Earth’s oxygen through photosynthesis.
Photosynthesis – The process by which green plants, algae, and some bacteria convert light energy, carbon dioxide, and water into glucose and oxygen. – Photosynthesis in plants can be represented by the equation: $$6CO_2 + 6H_2O + text{light energy} rightarrow C_6H_{12}O_6 + 6O_2$$.
Zooplankton – Small, often microscopic animals that drift in aquatic environments and feed on phytoplankton and other small organisms. – Zooplankton play a critical role in aquatic food chains, serving as a primary food source for larger marine animals.
Sediment – Particles of organic and inorganic material that settle at the bottom of a body of water, often forming layers over time. – Sediment cores from the ocean floor can provide valuable information about past climate conditions.
Ocean – A vast body of saltwater that covers about 71% of the Earth’s surface and is home to diverse ecosystems and species. – The ocean plays a crucial role in regulating the Earth’s climate by absorbing heat and carbon dioxide.
Biodiversity – The variety of life in a particular habitat or ecosystem, encompassing the diversity of species, genetic variation, and ecosystem complexity. – Coral reefs are known for their high biodiversity, supporting thousands of marine species.
Acidification – The process by which water becomes more acidic, often due to increased levels of carbon dioxide, affecting marine life and ecosystems. – Ocean acidification poses a significant threat to shell-forming marine organisms, such as corals and mollusks.
