Coral Reefs, Markers of Tropical Climate
195
o·
Fig. 7.6. Distribution of coral reefs (hachured areas). 20°C isotherm for coldest month of the year
shown as heUl' Y line. Note the general correspondence. Areas of coastal upwelling (triang els) inhibit
coral growth. Hence the eastern margins are unfavorable for reef de velopment. Present maximum
equatorward extent of iceberg drift also is shown. [E. Seibold in R. Brinkmann led]. 1964, Lehrbuch
der allgemeinen Geologie . F. Enke, Stuttgart; coral reef distribution after J. W. Wells, 1957, Geol.
Soc. Am. Mem. 67: 609]
influence of the Canary Current and of upwelling, reef organisms begin to grow only
south of Dakar (15 ° N). However, on the whole, the equator does denote the mid-line
of the distribution, an observation which is useful in paleogeographic reconstructions.
7.4.2 Reef Building. "Coral reefs" are built up by many different kinds of organisms
and corals do not necessarily contribute the bulk of the material. Up to 10 000 g of
calcium carbonate per square meter can be produced annually, with a rate of upbuilding of 10 mrn per year. Calcareous ooze, for com~arison, builds up a thousand times
more slowly, with accumulation rates near 10 g/m /yr. Part of this high production of
calcium carbonate is based on photosynthesis by unicellular symbiotic dinoflagellates
living within the corals. During daytime, they use C02 and promote thereby precipitation of calcium carbonate. Skeletons are deposited up to five times faster during
daytime than during the night. Seasonal temperature variations are indicated in annual layers (see Fig. 6.8).
About 50 % of the reef is solid matter and 50 % is empty space (that is, pore space
filled with water). Where the sea floor sinks, reefs can build enormous shelf bodies
(on continental margins), or mountain peaks (on sunken volcanoes).
How is the reef built? To begin with, we have to distinguish between the framework (the supporting structure) and the fill , much as in building construction. The
corals proper, and, just as importantly, the calcareous algae, are involved in producing
the framework. Massive or (in protected places) branching stony corals can grow up
by more than 2.5 cm/yr, as individual colonies. In the shallowest water, encrusting
algae occur which grow over dead portions of the reef and cement it to such a degree
that it can resist even the strongest surf. These encrusting algae, too, can reach a high
195
o·
Fig. 7.6. Distribution of coral reefs (hachured areas). 20°C isotherm for coldest month of the year
shown as heUl' Y line. Note the general correspondence. Areas of coastal upwelling (triang els) inhibit
coral growth. Hence the eastern margins are unfavorable for reef de velopment. Present maximum
equatorward extent of iceberg drift also is shown. [E. Seibold in R. Brinkmann led]. 1964, Lehrbuch
der allgemeinen Geologie . F. Enke, Stuttgart; coral reef distribution after J. W. Wells, 1957, Geol.
Soc. Am. Mem. 67: 609]
influence of the Canary Current and of upwelling, reef organisms begin to grow only
south of Dakar (15 ° N). However, on the whole, the equator does denote the mid-line
of the distribution, an observation which is useful in paleogeographic reconstructions.
7.4.2 Reef Building. "Coral reefs" are built up by many different kinds of organisms
and corals do not necessarily contribute the bulk of the material. Up to 10 000 g of
calcium carbonate per square meter can be produced annually, with a rate of upbuilding of 10 mrn per year. Calcareous ooze, for com~arison, builds up a thousand times
more slowly, with accumulation rates near 10 g/m /yr. Part of this high production of
calcium carbonate is based on photosynthesis by unicellular symbiotic dinoflagellates
living within the corals. During daytime, they use C02 and promote thereby precipitation of calcium carbonate. Skeletons are deposited up to five times faster during
daytime than during the night. Seasonal temperature variations are indicated in annual layers (see Fig. 6.8).
About 50 % of the reef is solid matter and 50 % is empty space (that is, pore space
filled with water). Where the sea floor sinks, reefs can build enormous shelf bodies
(on continental margins), or mountain peaks (on sunken volcanoes).
How is the reef built? To begin with, we have to distinguish between the framework (the supporting structure) and the fill , much as in building construction. The
corals proper, and, just as importantly, the calcareous algae, are involved in producing
the framework. Massive or (in protected places) branching stony corals can grow up
by more than 2.5 cm/yr, as individual colonies. In the shallowest water, encrusting
algae occur which grow over dead portions of the reef and cement it to such a degree
that it can resist even the strongest surf. These encrusting algae, too, can reach a high