Questions: Marine Biological Pump and Carbon Sequestration

Question 1

Ocean warming is projected to increase bacterial decomposition rates, remineralizing sinking organic particles more rapidly and at shallower depths. What would be the likely climate consequence?

• A. Decreased atmospheric CO₂, because faster decomposition releases more nutrients, stimulating greater phytoplankton productivity and CO₂ uptake
• B. Increased atmospheric CO₂, because more carbon would be remineralized within the shallow, well-mixed surface layer and returned to the atmosphere more quickly
• C. No net climate effect, because the total amount of carbon fixed by photosynthesis remains unchanged
• D. Increased ocean acidification only, with no direct effect on atmospheric CO₂

Answer: (B)

The depth of remineralization is the critical variable for climate impact. Carbon remineralized above the thermocline returns to the surface mixed layer and equilibrates with the atmosphere on timescales of years to decades. Carbon remineralized below the permanent thermocline is sequestered from the atmosphere for the duration of the ocean's deep overturning circulation — centuries to a millennium. Shallower remineralization means more carbon cycles back into the atmosphere quickly, weakening the biological pump as a climate regulator. This is a key positive feedback in warming scenarios: higher temperatures reduce pump efficiency, releasing more CO₂, causing more warming.

Question 2

Coccolithophores are calcifying phytoplankton that build CaCO₃ shells. What is the direct effect of CaCO₃ formation at the sea surface on the surrounding seawater's CO₂ concentration?

• A. It decreases seawater CO₂ by incorporating dissolved inorganic carbon into the shells, which are then removed when they sink
• B. It has no immediate effect on seawater CO₂; only the sinking and dissolution of shells affects carbon chemistry
• C. It increases seawater CO₂ by shifting the carbonate equilibrium — forming CaCO₃ from bicarbonate releases CO₂ to the surrounding water
• D. The effect depends on depth: it releases CO₂ in shallow water but absorbs CO₂ in deep water

Answer: (C)

The reaction Ca²⁺ + 2HCO₃⁻ → CaCO₃ + H₂O + CO₂ releases CO₂ at the surface, counterintuitively making CaCO₃-forming organisms a local source of CO₂ even while they are sequestering carbon in their shells. This is why the carbonate pump is sometimes called the 'carbonate counter-pump' — it partially offsets the soft-tissue pump's CO₂ drawdown at the surface. The sinking and dissolution of CaCO₃ at depth does increase deep-ocean alkalinity and the ocean's long-term CO₂ absorption capacity, but the surface signal is a CO₂ release.

Question 3

The climate significance of the biological pump depends more on the depth at which organic carbon is remineralized than on the total amount of carbon that leaves the surface euphotic zone.

• T. True
• F. False

Answer: True

This is the key insight for understanding the pump's climate relevance. Carbon remineralized at 100 m depth is back in the surface ocean within months to years, effectively bypassing sequestration. Carbon remineralized at 1,000 m or deeper is removed from atmospheric contact for centuries. The Martin curve shows that ~90% of exported carbon is remineralized above 1,000 m — only the remaining fraction that reaches depth contributes to long-term sequestration. A pump that exports a lot of carbon but remineralizes it all at 200 m provides far less climate regulation than a pump that exports less carbon but delivers a higher fraction to deep waters.

Question 4

The biological pump transfers most of the organic carbon fixed by phytoplankton in the euphotic zone to the deep seafloor, where it is stored for centuries.

• T. True
• F. False

Answer: False

This dramatically overstates the pump's efficiency. Of the organic carbon fixed by phytoplankton, roughly 10–20% is exported below the euphotic zone (~200 m) as sinking particles, and of that, approximately 90% is remineralized before reaching 1,000 m. Only about 1–3% of total surface production ultimately reaches the seafloor. The pump is a highly 'leaky' system — the vast majority of photosynthetically fixed carbon is recycled within the surface ocean. What matters climatically is not that the pump is efficient, but that even the small fraction that escapes to depth can significantly reduce atmospheric CO₂ compared to a world without any biological pump.

Question 5

Explain why the carbonate pump is sometimes called the 'carbonate counter-pump' — what does it counteract, and through what mechanism?

Model answer: The soft-tissue pump removes CO₂ from the surface ocean by fixing it into organic matter that sinks. The carbonate pump partially opposes this at the surface: when calcifying organisms form CaCO₃ shells from dissolved bicarbonate ions, the reaction releases CO₂ into the surrounding seawater (Ca²⁺ + 2HCO₃⁻ → CaCO₃ + H₂O + CO₂). This increases surface ocean pCO₂, promoting outgassing to the atmosphere — the opposite of what the soft-tissue pump does. On longer timescales, the dissolution of CaCO₃ at depth raises deep-ocean alkalinity and enhances the ocean's overall capacity to absorb atmospheric CO₂, but the surface signal remains a net CO₂ source. The ratio of organic carbon to CaCO₃ in sinking material (the 'rain ratio') therefore significantly affects whether the biological pump is a net atmospheric CO₂ sink.

The counter-pump concept is one of the non-obvious features of marine carbon chemistry and a frequent source of confusion. The intuitive expectation — that organisms building carbon-based shells should remove CO₂ — is wrong for the inorganic carbonate pathway because of carbonate chemistry. This distinction has practical importance for climate projections: ocean acidification (lower pH) slows calcification in many organisms, which might seem like it would reduce the counter-pump effect and increase net CO₂ uptake, but the reality is more complex because acidification also reduces carbonate ion concentrations needed for shell formation.