686 ABRHENIUS [CHAP. 25 



D. Carbonates 



The abundant planktonic and benthic organisms which build skeletal 

 structures of calcite or aragonite provide a mechanism for deposition of these 

 minerals over the ocean floor. Quantitatively most important in this respect are 

 Foraminifera, which are Protozoa with calcareous, arenaceous, or chitinous 

 tests (Fig. 16), and coccolithophorids, which are unicellular algae with minute 

 calcite platelets — coccohths — that have been secreted from the cell 



Fig. 16. Calcareous ooze with tests of Foraminifera. Thin section, polarized light. 



wall (Fig. 17). Most Foraminifera contributing to the pelagic sediments are 

 planktonic, and all of the jolanktonic species form calcite tests. Benthonic 

 Foraminifera constitute less than 10% of the fossil assemblages in pelagic deep- 

 sea sediments but are more abundant in shallow-water pelagic sediments such 

 as coral reefs. The variation in species distribution as a function of ambient 

 water temperature has successfully been used, initially by Schott and later by 

 a number of workers (ref. in Phleger, 1960), to trace variations in the surface- 

 water temperature of the ocean in the past. Recently, similar information has 

 been obtained by mass spectrometric determination of the temperature- 

 dependent ratio Oi^/Qi^ in the carbonate of Foraminifera in deep-sea pelagic 

 sediments (Emiliani et al., see ref. in Emiliani, 1955 and 1956; Ericson and 

 Wollin, 1956) and in the skeletal structures of a number of reef organisms 

 (Lowenstam and Epstein, 1957). The usefulness of pelagic coccolithophorids for 

 this purpose has recently been indicated by Craig (unpubhshed). A compre- 

 hensive survey of marine paleo -temperature work is presented by Emiliani 

 and Flint elsewhere in this volume (Chapter 34). 



