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Why was the Great Barrier Reef’s continental shelf above sea level during the last Pleistocene Ice Age?

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Diagram showing ocean zonation by depth and region, with labeled layers: littoral, sublittoral, bathyal, abyssal, and hadal along the seafloor, and epipelagic, mesopelagic, bathypelagic, and abyssalpelagic in open water. The littoral zone is at the high-tide mark. Vertical scale marks photic and aphotic zones from surface to 10,000 meters. — Marine ecosystem Zonation of the ocean. Note that in the littoral zone the water is at the high-tide mark.

The Great Barrier Reef’s continental shelf was above sea level during the last Pleistocene Ice Age primarily due to the significant global drop in sea level associated with the expansion of continental ice sheets.12

During periods of maximum glaciation, such as 18,000 years ago, sea levels were more than 100 metres (328 feet) lower than they are today. This dramatic reduction in ocean volume caused extensive areas of the world’s continental shelves to become exposed. In the case of the Great Barrier Reef, this exposure resulted in the bank tops being left above water, where they were subjected to erosion by rainwater and wind, which in some areas formed dune ridges. As the climate warmed and the large ice sheets melted, the rising sea level eventually submerged these features, allowing the modern reef system to develop.12