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Calcium is the fifth most abundant element in nature, found primarily in limestone (calcium carbonate), gypsum (calcium sulfate) and fluorite (calcium-chloro-(or fluoro) phosphate). It also occurs in a number of other natural minerals including calcite, aragonite, dolomite and sandstone. It is used by many forms of life to make their shells and bones, but there is little demand for the pure metal. Instead, it is largely produced commercially by reacting lime with mercuric oxide in hot low pressure conditions. It is a key ingredient in cement and a major raw material in the manufacture of steel, aluminium and other alloys. It is also widely used as a dietary supplement.
Stable Ca isotope variations and their implication for the elemental cycling of the lithosphere, hydrosphere and biosphere have received increasing attention over the past few decades. Recent studies documented significant d44/40Ca anomalies in both silicate and marine sediments as well as in terrestrial igneous rock samples. The magnitude of the d44/40Ca variation diminishes with an increase in temperature. Thus, igneous silicate Ca reservoirs show much narrower ranges of d44/40Ca values (about 2 %0) compared to the heaviest Ca accumulated in marine carbonates (Fantle and Tipper 2014).
The isotopic composition of biologically derived Ca varies significantly. It is fractionated between the bones, urine and blood of animals as well as between soils, roots and leaves of plants by up to 3000 ppm for the 44Ca/40Ca ratio. These isotopic signatures provide useful information on Ca transport and fluxes through the lithosphere, hydrosphere and the biosphere in both modern and geological settings.