Calcite abiogenic

Although surfece waters are supersaturated with respect to calcium carbonate, abiogenic precipitation is imcommon, probably because of unfevorable kinetics. (The relatively rare formation of abiogenic calcite is discussed further in Chapter 18.) Marine organisms are able to overcome this kinetic barrier because they have enzymes that catalyze the precipitation reaction. Because fl declines with depth, organisms that deposit calcareous shells in deep waters, such as benthic foraminiferans, must expend more energy to create their hard parts as compared to surfece dwellers. 

Because warm surface seawater is usually supersaturated with respect to calcium carbonate, abiogenic precipitation of calcite and aragonite does occur, at least when supersaturations are very high. These conditions are limited to shallow water where temperatures can get sufficiently high, namely coastal tropical seas. 

Oolite A hydrogenous precipitate commonly found in carbonate sediments of continental shelves located in the tropics. They are composed primarily of calcium carbonate and are thought to be an abiogenic precipitate formed from warm seawater supersaturated with respect to calcite and aragonite. 

Sedimentary lamination is also caused by the deposition of calcium carbonate and is important in most freshwater lakes where the clastic input from rivers is low. Carbonate deposits may be chemical or biogenic (see Box 1.1). As with phytoplanktonic production, the abiogenic (chemical) precipitation of calcium carbonate is seasonal, occurring during the warmer months. During this period carbon dioxide is removed from the water by planktonic photosynthesis, which causes water acidity to decrease and calcium carbonate to precipitate. From the equilibrium in Eqn 3.9c (Box 3.12) it can be seen that if the amount of C02(aq) decreases, more C02(aq) and carbonate are produced from bicarbonate, which increases the concentration of carbonate and promotes its precipitation. Subsequently, the water becomes more acidic and no longer supersaturated with respect to calcite (the main form of crystalline calcium... 

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