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Abstract

Quaternary reef complexes are primarily Darwinian oceanic or shelf reefs, on slowly subsiding foundations. “Glacial control” of reef morphology has been important, but not in the sense that R. A. Daly visualized. Corals dated by the Uranium-series method prove that many, perhaps most, reefs were not truncated at the low sea levels of the last glacial age. Dates from such scattered and classic reef localities as Western Australia, the Tuamotu Archipelago, Eniwetok Atoll, the Florida Keys, the Bahamas, and Barbados demonstrate that reef limestones and eolianites were emerged for 100,000 years or more, several hundred feet above the glacially lowered sea, without being dissolved away. The permeability of reef frameworks is a major reason for their durability under subaerial tropical weathering. Although Pleistocene and older reefs were not eroded to sea level during glacial ages, the extensive drowned karst topography on them, as identified from Florida, Bermuda, the Bahamas, and the Marshall Islands, probably has been a primary control in the postglacial evolution of modern reef complexes.

Reef growth probably was not suppressed during glacial low sea levels, at least not in the tropical Pacific. Changes in O18 isotopic composition of sea water attributable to the growth of continental ice sheets minimize the effects formerly attributed to cooling of the sea surface. The central Pacific, and quite possibly other reef provinces as well, was neither too cold nor too turbid for coral-reef development, even during full glacial ages. Pelagic carbonate productivity in the equatorial Pacific actually increased during glacial ages, and it is likely that reefs flourished on the exposed flanks of emerged atolls. The submarine slopes of reef-rimmed Pacific islands and atolls may provide evidence by which we can measure the true Pleistocene fluctuations of sea level, and thereby infer the amount of Quaternary warping of continental shelves.

The concept that a postglacial sea level higher than the present is necessary to cause modern reef morphology needs to be rigorously reexamined. Rather than being relict from a “10-foot stand of the sea” a few thousand years ago, modern reefs show every indication of being in dynamic equilibrium with the forces that now act on them. Modern reef-building organisms have reoccupied the weathered surfaces of reef limestones that had been exposed for 100,000 years, and have thinly veneered the tops of ancient reefs or eolianite dune fields. As the postglacial rise of sea level has approached its present level, upward growth has slowed and reefs have expanded laterally to rebuild their precipitous upper slopes over eroded ancient platforms. Carbonate productivity by reef builders is generally adequate to keep pace with rising sea level, but at several localities, atolls were “drowned” and have not reached present sea level. Drowned atolls and banks remind us that even today reef complexes may be in the process of being preserved in the stratigraphic section.

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