Carbonate Precipitation

Atmospheric carbon dioxide combines with water during rainfall and in breaking wave’s spray, producing carbonic acid. Carbonic acid spontaneously disassociates into hydrogen ions and bicarbonate, which in turn disassociates into hydrogen ions and carbonate. Dissolved carbonate combines with dissolved calcium to produce calcium carbonate, which crystallizes into calcite and aragonite, the core component of limestone. Adding carbon dioxide to water decreases carbonate production, while removing carbon dioxide increases carbonate production.

Factors that impact limestone formation are:

  • Temperature – Heat assists chemical reactions, so carbonate precipitation increases with temperature. More limestone is deposited in tropical regions than polar regions.
  • Pressure – Carbonate precipitation decreases with pressure, and pressure increases with depth. More limestone is deposited in shallower water than deeper water.
  • Agitation – Carbonate precipitation increases with agitation, where increased agitation increases atmosphere-water interactions for carbonic acid formation. More limestone is deposited on the break side of reefs, not the lee side.
  • Organic activity – Carbonate precipitation increases in response to direct and indirect organic activity. Direct organic activity is when creatures like clams and zooplankton directly extract and precipitate carbonate and limestone. Indirect organic activity is when creatures like phytoplankton and algae remove carbon dioxide from the water through photosynthesis, changing the local environment to conditions more favourable to carbonate precipitation.
  • Sediment masking & clogging – Other sediment rates can directly and indirectly impact carbonate sedimentation rates. Carbonate deposition rates can be masked by a faster rate of deposition by other types of sediments, or an influx of sediments can clog filter-feeding organisms that would otherwise increase carbonate precipitation.
  • Light (Photic Zone) – Light is necessary for photosynthesis, and light penetration depends on water depth and clarity. Limestone deposition is higher within the photic zone, where light can penetrate.
  • Carbonate Compensation Depth (CCD) – As pressure increases and temperature decreases, calcium carbonate dissolves. The Carbonate Compensation Depth is the balance point where the rate of precipitation is equal to the rate of deposition. This depth is typically around 500 meters depth, with variation depending on ocean circulation.

Carbonate precipitation is greatest in shallow, warm, clear waters, with minimal other sediment input. Living creatures and agitated water increase precipitation, exactly the places we find reefs.

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