Coccolithophores calcite

Figure 2. Examples of typical calcium carbonate tests which are deposited in pelagic sediments. (1) and (2) are foraminifera (calcite) (3) is a coccolithophore (calcite) (4) and (5) are pteropods (aragonite). Note the difference in size as...

Among the organs of living things consisting of calcite, the spicules of sea urchins and the exo-skeleton of coccolithophores attract particular interest. A sea... 

Coccolith, an exo-skeleton of coccolithophores, consists of calcite crystals of uniform size showing a most unusual morphology, which resembles a trug. The structure of the exo-skeleton consists of about thirty calcite crystals of equal size, which are regularly aligned and conjugated. This unusual form of calcite crystals (shown in Fig. 14.5) has stimulated particular interest, and many studies have been conducted on this structure [8]. 

Figure 14.5. Crystal orientation and mechanism of formation of calcite constituting the segment of coccolithophores. Emiliania huxleyi is presented as an example [8].

Biogenous oozes are either calcareous or siliceous. Calcareous oozes are predominantly the calcitic tests of coccolithophores and/or foraminifera, or the aragonitic tests of pteropods. The solubility of CaCOs increases with decreasing temperature and increasing pressure, and thus with increasing depth in the oceans. Aragonite is 1.45 times more soluble than calcite (Morse and Mackenzie, 1990), so aragonitic oozes are confined to shallower depths than the calcitic oozes. The compensation depth for each mineral is defined as the depth at which the rates of... 

Almost all deep-sea carbonate-rich sediments are composed of calcite low in magnesium (> 99% CaCOa). This material is primarily derived from pelagic skeletal organisms. Coccolithophores... 

Particulate Calcium in WCR 82-B. Calcium is a major component of particulate matter and occurs largely as calcite (5). Most particulate Ca is small and is predominantly present in 10-20-jLtm sized coccolithophores in surface waters and in 1-5- im sized coccoliths below the euphotic zone (5). Individual coccoliths sink only fractions of a meter per day (2, 5). Hence, the <53-fim Ca concentration in waters below 100 m would be expected to show the least temporal variability of all particulate elements sampled. For this reason, Ca data from below 100 m was used for comparison of samples collected with GF and MQ filters and by the LVFS and MULVFS. 

Coccolithophores are, with rare possible exceptions, exclusively marine algae, chiefly planktonic although some benthonic forms are known. They secrete minute calcitic plate scales a few microns across. The mineralized plate scales are called coccoliths and may be extremely abundant in some soil parent materials, particularly chalk. Chalk commonly consists almost exclusively of the tests of planktonic foraminifers, discussed below, and coccoliths the two components may be present in approximately equal-weight abundances, or either component may strongly predominate. Although both kinds of fossil are calcitic, they have remarkably different resistance to solution, and local conditions may favor concentration of one or the other type in soils derived from a chalky parent material. A characteristic Cretaceous coccolith is illustrated in Figure 5. 

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