Ocean Acidification and the Carbon Cycle:
Long-Term Carbon Balance (>100,000 years)
Over geological timescales exceeding 100,000 years, Earth maintains a natural carbon balance. Carbon dioxide (CO₂) released primarily by volcanic activity—the chief natural source—is gradually absorbed through two major processes:
- Photosynthesis by plants (organic matter production)
- Chemical weathering of rocks on land
However, the process of rock weathering occurs extremely slowly, over tens of thousands of years. Therefore, it is inadequate to counteract the rapid and large-scale anthropogenic (human-caused) CO₂ emissions currently affecting the atmosphere and oceans.
Medium-Term Feedback Mechanisms (>1,000 years)
On shorter timescales (more than a thousand years), the ocean regulates its carbon content through a feedback process called carbonate compensation. This links the oceanic carbon cycle to deep-sea carbonate-rich sediments.
- The surface ocean is typically supersaturated with calcium carbonate (CaCO₃), limiting its dissolution.
- In contrast, the deep ocean is undersaturated, leading to enhanced dissolution of carbonate materials.
The transition between these two layers is known as the lysocline—the depth at which dissolution of CaCO₃ increases significantly.
When marine organisms die, their calcium carbonate shells sink:
- In shallow waters, these shells are mostly buried in sediments and sequestered for long periods.
- In deep waters, however, nearly all CaCO₃ dissolves, preventing long-term carbon storage.
Carbonate Compensation Depth (CCD) and Ocean Imbalance
- The increasing uptake of atmospheric CO₂ by the ocean is disrupting the carbonate compensation depth (CCD)—the level below which all calcium carbonate dissolves.
As ocean pH continues to fall due to acidification, both the lysocline and CCD become shallower. This means more shells in ocean sediments are exposed to corrosive (undersaturated) conditions, causing them to dissolve. While this process can help buffer ocean acidification, it operates over a very long timescale (about 1,000 years), offering no immediate remedy.
Winners and Losers in Marine Life
Ocean acidification does not impact all marine organisms equally.
- Some marine phytoplankton and plant species may benefit from increased CO₂ through enhanced photosynthesis and growth.
- However, this is not universal. For many species, elevated CO₂ and rising acidity can have neutral or negative physiological effects.