Oceanic and Terrestrial Carbon

This lesson examines the fast (biological) carbon cycle, focusing on the ocean as a carbon sink, terrestrial photosynthesis and respiration, and soil carbon dynamics. This material is central to Edexcel A-Level Geography (9GE0), Paper 1, Topic 6, addressing the Enquiry Question: "How does the carbon cycle operate to maintain planetary health?"

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The Fast Carbon Cycle

The fast carbon cycle involves the transfer of carbon between the atmosphere, biosphere, soils and surface oceans over timescales of days to centuries. Unlike the slow geological cycle (millions of years), the fast cycle is driven primarily by biological processes — photosynthesis, respiration and decomposition.

The fast carbon cycle transfers approximately 120 GtC per year between the atmosphere and terrestrial biosphere, and approximately 90 GtC per year between the atmosphere and oceans. Before human disruption, these fluxes were approximately balanced, maintaining atmospheric CO₂ at ~280 ppm.

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The Ocean as a Carbon Sink

The oceans absorb approximately 25–30% of anthropogenic CO₂ emissions — around 2.5 GtC per year. This makes them the largest active carbon sink after the terrestrial biosphere. The ocean absorbs CO₂ through two main mechanisms: the solubility pump and the biological pump.

The Solubility Pump

The solubility pump is a physical process driven by the fact that CO₂ dissolves more readily in cold water than in warm water.

1. At high latitudes (North Atlantic, Southern Ocean), surface waters are cold. Atmospheric CO₂ dissolves into these cold surface waters, forming dissolved inorganic carbon (DIC) — a mixture of dissolved CO₂, carbonic acid (H₂CO₃), bicarbonate ions (HCO₃⁻) and carbonate ions (CO₃²⁻).
2. These cold, CO₂-rich surface waters are dense and sink as part of the thermohaline circulation (global conveyor belt), carrying dissolved carbon to the deep ocean (>1,000 m depth).
3. Deep ocean water circulates slowly (residence time ~1,000 years), effectively sequestering the carbon from the atmosphere for centuries.
4. Eventually, deep water upwells in the tropics, warms, and releases some CO₂ back to the atmosphere.

Factor	Effect on CO₂ Solubility	Explanation
Temperature	Lower temperature → higher solubility	Cold water holds more dissolved gas (Henry's Law)
Salinity	Lower salinity → higher solubility	Freshwater dissolves more CO₂ than saltwater
Pressure	Higher pressure → higher solubility	Deep water holds more dissolved CO₂
Atmospheric CO₂	Higher CO₂ → more dissolves	Greater concentration gradient drives more absorption

Exam Tip: The solubility pump creates a crucial positive feedback in climate change: warming oceans → reduced CO₂ solubility → less oceanic CO₂ absorption → more CO₂ stays in atmosphere → further warming. This is a key feedback loop to include in extended answers on climate change.

The Biological Pump

The biological pump is driven by marine organisms, primarily phytoplankton (microscopic photosynthetic algae and cyanobacteria).

1. Photosynthesis — Phytoplankton in the sunlit surface layer (photic zone, 0–200 m) absorb dissolved CO₂ and convert it into organic carbon through photosynthesis. Phytoplankton are responsible for approximately 50% of all photosynthesis on Earth, despite representing <1% of global plant biomass.
2. Food web transfer — Zooplankton eat phytoplankton; fish eat zooplankton. Carbon passes through the marine food web.
3. Sinking — Dead organisms, faecal pellets and organic debris sink from the surface to the deep ocean as marine snow. Only about 1–2% of surface productivity reaches the deep ocean floor, but this is a significant flux (~2 GtC/year).
4. Burial — A tiny fraction of the sinking organic carbon is buried in ocean sediments, removing it from the active carbon cycle for millions of years.

Some marine organisms also contribute to the biological pump through the formation of calcium carbonate (CaCO₃) shells and skeletons:

Organism	Carbon Role
Coccolithophores	Build CaCO₃ plates; when they die, plates sink and accumulate as chalk/limestone
Foraminifera	Build CaCO₃ shells; important deep-sea sediment component
Pteropods	Build CaCO₃ shells; significant in polar waters
Corals	Build CaCO₃ reef structures; store ~2 GtC in living reefs globally

flowchart TD
    A[Atmospheric CO₂] -->|"Dissolves in<br>surface water"| B[Surface Ocean<br>dissolved CO₂]
    B -->|"Photosynthesis by<br>phytoplankton"| C[Organic carbon in<br>marine organisms]
    C -->|"Death, faecal pellets<br>marine snow"| D[Sinking particles]
    D -->|"~1-2% reaches<br>ocean floor"| E[Deep ocean<br>sediments]
    E -->|"Burial over<br>millions of years"| F[Sedimentary rock<br>lithosphere]
    B -->|"Thermohaline<br>circulation"| G[Deep ocean<br>dissolved carbon]
    G -->|"Upwelling in<br>tropics ~1000 yrs"| B
    C -->|"Respiration &<br>decomposition"| B

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Ocean Acidification

As the ocean absorbs more CO₂, a series of chemical reactions occur that lower the pH of seawater — a process called ocean acidification.

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ → 2H⁺ + CO₃²⁻

Since the Industrial Revolution, ocean surface pH has decreased from approximately 8.2 to 8.1 — a seemingly small change, but because the pH scale is logarithmic, this represents a 26% increase in hydrogen ion concentration.

Impact of Ocean Acidification	Detail
Reduced calcification	Lower CO₃²⁻ concentration makes it harder for organisms to build CaCO₃ shells and skeletons
Coral reef degradation	Corals struggle to build and maintain reef structures; increased dissolution
Pteropod shell thinning	Shells of these important polar organisms dissolve in acidified water
Food web disruption	Loss of shell-building organisms at the base of food chains affects entire ecosystems
Reduced ocean CO₂ uptake	More acidic water absorbs CO₂ less efficiently (positive feedback)

Projections suggest ocean pH could fall to 7.7–7.8 by 2100 under high-emission scenarios — a level not experienced for at least 14 million years.

Exam Tip: Ocean acidification is sometimes called the "other CO₂ problem" because it is a direct consequence of rising atmospheric CO₂ that is separate from (though related to) climate warming. It is an excellent example for discussing the interconnected impacts of carbon cycle disruption.

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Terrestrial Photosynthesis and Respiration

Photosynthesis

Photosynthesis is the primary mechanism by which carbon moves from the atmosphere to the terrestrial biosphere:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Terrestrial plants absorb approximately 120 GtC per year from the atmosphere through photosynthesis. This carbon is incorporated into plant biomass — leaves, stems, roots, fruits and seeds.

The total amount of carbon fixed by photosynthesis is called Gross Primary Productivity (GPP):

• Global terrestrial GPP ≈ 120 GtC/year

Respiration