Some aspects of biomineralization exemplified by deepwater scleractinian corals

Abstract

Scleractinian corals belong to organisms with the most intense biomineralization (1). Carbonate mineral- ization is one of the earliest geochemical processes, which started in the Early Cambrian and continues at present. In the oceanic and marine ecosystems, living organisms with the carbonate mineralization function (coccolithophorids, foraminifers, corals, pteropods, and others) provide heterogeneous accumulation to form carbonate skeletons that serve as a basis for bio- phases in all the examined samples. The total content of quartz, amphibolite, albite, and clay minerals does not exceed 14%. The analysis of different zones in the coral stem from sample 35 revealed that its central part is composed of X-ray amorphous matter represented likely by organic macromolecules alternating with car- bonate packets. The beige and white zones demonstrate practically equal proportions of aragonite (98 and 99%, respectively). The calcite content in scleractinian corals varies from almost zero to 30% (average 9.2%). The arago- nite/calcite ratio range is small. The Stephanophyllia com- plicata sample (water depth 680 m, Station Mend-1265) taken from the central part of the Fiji Basin is charac- terized by the maximal calcite content (30%) and min- imal aragonite content (55%). It should be noted that the same sample contains maximal concentrations of some chemical components, such as Si, Al, Fe, Mn, and Mg (figure). The figure shows that maximums of these elements coincide with minimums of the Sr/Ca value. It is conceivable that this area of the Fiji Basin is characterized by local upwelling, which can be respon- sible for the defined anomaly. As is known, deep waters rising in upwelling zones are enriched in biogenic ele- ments and many microelements that are consumed by corals in the course of biomineralization. The minimal Sr/Ca value is probably explained by the lower (30- 45 times) aragonite/calcite ratio as compared with other samples. Such a value is substantially lower than the level that permits the replacement of Ca by Sr in the crystal lattice of aragonite. It is known that Sr can replace Ca in the aragonite structure due to closeness of their ionic radii, while Ca can be replaced by Mg, Fe, and Mn in the calcite structure (7).