Darwin Point: A threshold for atoll formation

A threshold for atoll formation, herein termed the Darwin Point, exists at the northern end of the Hawaiian Archipelago at 29°N latitude. Hawaiian atolls and coral islands transported northwest by tectonic movement of the Pacific Plate appear to have "drowned" near the Darwin Point during the last 20 million years. Measures of gross carbonate production by corals across the archipelago show that growth rates decrease with increasing latitude. At the Darwin Point, corals may contribute only 20% of the calcium carbonate necessary to keep pace with recent changes in sea level and thus appear to be more important as builders of framework than producers of limestone. Reduction in this function rather than total carbonate production may be the determining factor in the formation of atolls and coral islands. Elsewhere in the world other Darwin Points may exist but probably not at the same latitude due to differences in ecological conditions, coral species composition, island area, rates of erosion and tectonic histories.