To understand how limestone is formed one has to look at the the global carbon cycle. You may already be familiar with the nitrogen cycle, sulfur cycle, and the water cycle
We all know that animals and humans exhale carbon dioxide. This is how we create energy. We take in oxygen and it combines with sugars and we exhale carbon and water. Glucose is a typical food and the metabolic reaction can be represented by:
C6H12O6+ 6 O2®6 CO2+ 6 H2O
Plants and green bacteria, produce oxygen and consumes carbon dioxide in its photosynthesis. Energy in the form of electromagnetic radiation or photons from the sun is supplied so that the plants convert low-energy-content carbon dioxide into high-energy-content glucose. The photosynthesis reaction is represented by:
6 CO2+ 12 H2O --h v®C6H12O6+ 6 O2+ 6 H2O
Plants and animals make food for each other. The plants convert sunlight into high-energy food for the animals. Water is a reaction medium and a product of photosynthesis. Radioactive labeled studies show that the oxygen in the water produced from photosynthesis is the same oxygen from the carbon dioxide.
You may be thinking that crops and plants with leaves, like trees, lawns, and flowers are the main source of oxygen production. In fact, primitive plants in the ocean play a more important role in in the photosynthesis process, because sheer numbers of them.
Speaking of the ocean, it is the largest sink of carbon dioxide in the world. Carbon dioxide is heavier than the surrounding atmosphere and sinks to the lowest spot in the landscape. Carbon dioxide sinks to reach sea level and then interacts with the water where it dissolves in water to form carbonic acid:
H2CO3®H++ HCO3- Ka1= 4.2x10-7
HCO3-®H++ CO32- Ka2= 4.8x10-11
The dissolved carbon dioxide or carbonic acid in the ocean further re
metal ions in the water forming calcium and magnesium carbonates. Extensive limestone (CaCO3) and dolomite (mixture of CaCO3 and MgCO3) have been formed this way. The Gilmore City quarry was nearthe equator and a shallow sea about 450 million years ago and our limestone was deposited in this way!