The Structure and Distribution of Coral Reefs - Darwin's Findings and Modern Views

Darwin's Findings and Modern Views

Darwin's interest on the biology of reef organisms was focussed on aspects related to his geological idea of subsidence; in particular, he was looking for confirmation that the reef building organisms could only live at shallow depths. FitzRoy's soundings at the Keeling Islands gave a depth limit for live coral of about 20 fathoms (37 m), and taking into account numerous observations by others, Darwin worked with a probable limit of 30 fathoms (55 m). Modern findings suggest a limit of around 100 m, still a small fraction of the depth of the ocean floor at 3000–5000 m. Darwin recognised the importance of red algae, and he reviewed other organisms that could have helped to build the reefs. He thought they lived at similarly shallow depths, but banks formed at greater depths were found in the 1880s. Darwin reviewed the distribution of different species of coral across a reef. He thought that the seaward reefs most exposed to wind and waves were formed by massive corals and red algae; this would be the most active area of reef growth and so would cause a tendency for reefs to grow outwards once they reach sea level. He believed that higher temperatures and the calmer water of the lagoons favoured the greatest coral diversity. These ecological ideas are still current, and research on the details continues.

In assessing the geology of the reef, Darwin showed his remarkable ability to collect facts and find patterns to reconstruct geological history on the basis of the very limited evidence available. He gave attention to the smallest detail. Having heard that parrotfish browsed on the living coral, he dissected specimens to find finely ground coral in their intestines. He concluded that such fish, and coral eating invertebrates such as Holothuroidea, could account for the banks of fine grained mud he found at the Keeling Islands; it showed also "that there are living checks to the growth of coral-reefs, and that the almost universal law of 'consume and be consumed,' holds good even with the polypifers forming those massive bulwarks, which are able to withstand the force of the open ocean."

His observations on the part played by organisms in the formation of the various features of reefs anticipated modern studies. To establish the thickness of coral barrier reefs, he relied on the old nautical rule of thumb to project the slope of the land to that below sea level, and then applied his idea that the coral reef would slope much more steeply than the underlying land. He was fortunate to guess that the maximum depth of coral would be around 5,000 ft (1,500 m), as the first test bores conducted by the United States Atomic Energy Commission on Enewetak Atoll in 1952 drilled down through 4610 ft (1,405 m) of coral before reaching the volcanic foundations. In Darwin's time no comparable thickness of fossil coral had been found on the continents, and when this was raised as a criticism of his theory neither he nor Lyell could find a satisfactory explanation. It is now thought that fossil reefs are usually broken up by tectonic movements, but at least two continental fossil reef complexes have been discovered to be about 3,000 ft (1,000 m) thick. While these findings have confirmed his argument that the islands were subsiding, his other attempts to show evidence of subsidence have been superseded by the discovery that glacial effects can cause changes in sea level.

In Darwin's global hypothesis, vast areas where the seabed was being elevated were marked by fringing reefs, sometimes around active volcanoes, and similarly huge areas where the ocean floor was subsiding were indicated by barrier reefs or atolls based on inactive volcanoes. These views received general support from deep sea drilling results in the 1980s. His idea that rising land would be balanced by subsidence in ocean areas has been superseded by modern plate tectonics, which he did not anticipate.