Biological Processes & the Carbon Cycle
The Biological Pump: Phytoplankton at the ocean surface fix into organic tissue. When these organisms die, they sink as 'marine snow' to the deep ocean, transporting approximately annually into intermediate and deep-water stores.
The Carbonate Pump: Marine organisms use dissolved carbonates to build shells and skeletons (calcium carbonate). Upon death, these hard parts sink and accumulate on the ocean floor, eventually forming sedimentary rocks like limestone through lithification.
The Physical (Solubility) Pump: This relies on the thermohaline circulation. Cold, dense water at high latitudes absorbs more and sinks (downwelling), while wind-driven upwelling brings carbon-rich deep water back to the surface to be released.
Thermohaline Circulation: A global system of surface and deep-water currents driven by temperature and salinity gradients that distributes dissolved carbon and nutrients across the planet's oceans.
Photosynthesis vs. Respiration: Land plants absorb during the day to create glucose, but they also release a portion back into the atmosphere through respiration. The net difference determines the amount of carbon stored in biomass.
Litter Fall and Decomposition: When plants shed leaves or die, carbon is transferred to the soil as litter. Micro-organisms and detritivores break this down, releasing some carbon as while the rest becomes part of the soil organic matter.
Temporal Variations: Carbon fluxes in terrestrial biomes vary diurnally (peaking during daylight) and seasonally (peaking during spring and summer growing seasons in temperate zones).
Biome Productivity: Tropical rainforests are the most productive terrestrial biomes due to year-round high temperatures and rainfall, which maximize both growth and rapid nutrient cycling.
Soil Organic Matter (SOM): Soils store of global carbon. This store includes dead organic matter that can remain for years, decades, or even centuries depending on environmental conditions.
Climate Influence: Decomposition is fastest in warm, humid climates with high oxygen levels. In contrast, cold or waterlogged environments (like peatlands) inhibit microbial activity, leading to massive long-term carbon accumulation.
Soil Type and Texture: Clay-rich soils have a higher capacity to store carbon than sandy soils because clay particles chemically bind with organic molecules, protecting them from rapid decomposition.
Human Impact: Cultivation and soil disturbance have historically led to the loss of of carbon globally since the Industrial Revolution by accelerating oxidation and erosion.
| Feature | Biological Pump | Physical Pump | Carbonate Pump |
|---|---|---|---|
| Primary Driver | Living organisms (Phytoplankton) | Temperature and Salinity | Shell-building organisms |
| Mechanism | Photosynthesis and sinking | Diffusion and Downwelling | Inorganic sedimentation |
| Timescale | Days to years (fast) | Centuries (slow) | Millions of years (geological) |
| Key Store | Intermediate/Deep Ocean | Deep Ocean Currents | Ocean Floor Sediments |
Distinguish the Pumps: When discussing ocean sequestration, clearly separate the biological (organic), carbonate (inorganic/shells), and physical (solubility/currents) mechanisms to avoid losing marks.
Scale Awareness: Always specify the timescale. Terrestrial sequestration can happen in seconds (photosynthesis), while soil storage can last centuries, and oceanic sediment storage lasts millions of years.
Check the Variables: If a question asks about soil health, mention both inputs (litter fall, residues) and outputs (decomposition, erosion) to show a balanced understanding of the system.
Link to Climate: Be prepared to explain how global warming affects these processes, such as how rising temperatures might slow the physical pump by reducing the solubility of in warmer water.