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August 09, 2016 | By:  Jessica Carilli
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What are coral atolls made of?

Hello out there! We haven't forgotten about you, wonderful blog readers - but sometimes (or a lot of the time) other deadlines keep us from writing as much as we wish. I'm currently focused on publication- and proposal-writing (this, plus fieldwork, is what a lot of academics do all summer!), but I keep thinking of things I want to write about for the blog - so here goes!

Much of my recent research has been focused on islands in the central Pacific Ocean - most of which are classic coral atolls. The original understanding of atoll formation is attributed to Charles Darwin, who reasoned that atolls are remnants of fringing reefs that once grew around islands; over time, these islands eroded away and/or sank (Darwin didn't know this, but islands sink as the tectonic plates on which they ride cool and contract and sit lower in the mantle). As the islands sink, corals continue to grow upwards to stay in the sunlight shallow parts of the ocean, leaving a ring of reefs surrounding a lagoon.

But, why is there any emergent land associated with this ring of reefs, if the original subaerial land has sunk far below the surface? Waves from storms near and far impinging on the reefs transport coral and sediment material from the living reef lagoonward, where it can build up into islets. For instance, 18 km of new land was created in Funafuti, Tuvalu after cyclone Bebe.

An important question, then, is whether coral atolls can keep up with sea level rise. Theoretically, just as the islands have kept themselves above sea level while the crust below them sinks, rising seas could provide more vertical space for growth. If atoll-forming material - sand and corals - grow fast enough, they could theoretically keep pace with, or at least slow down the island-swallowing effects of sea level rise. If corals and reef sand isn't produced fast enough to keep up with sinking islands or rising seas, the atolls will eventually drown - just as has happened in the most northwest of the Hawaiian Islands.

Coral skeletal growth is influenced by a number of factors, but water temperature is a strong determinant. As water temperatures warm, coral skeletal growth generally increases - hence the chilly reefs in the northwestern-most Hawaiian Islands aren't keeping pace with subsidence and sea level rise. However, if water temperatures are too warm, corals bleach and skeletal growth is inhibited. Outright loss of corals and large foraminifera due to bleaching, disease, algal overgrowth, pollution, and other problems will also strongly reduce the amount of carbonate material available for atoll growth.

Thus, protection of living coral reefs around atolls is vital for their continued survival as far into the future as possible. Some researchers have even gone a step further from advocating for coral reef protection, and are making strides towards culturing large benthic foraminifera - a major component of reef sand in places like Tuvalu - on land. Some days when I've feeling particularly exasperated by the state of reefs around the world, I dream up crazy plans to culture foraminifera, calcareous algae like Halimeda, and scrubby, fast-growing corals in raceways that would also treat sewage at the same time...made out of household waste! Too bad I'm not an engineer, or I might just do it...

What crazy ideas do you have to save the world's imperiled coral reefs and atolls?


Grigg, R. W. (1982). Darwin Point: a threshold for atoll formation. Coral reefs, 1(1), 29-34.

Lough, J. M. (2008). Coral calcification from skeletal records revisited. Marine Ecology Progress Series, 373, 257-264.

McLean, R., & Kench, P. (2015). Destruction or persistence of coral atoll islands in the face of 20th and 21st century sea-level rise? Wiley Interdisciplinary Reviews: Climate Change, 6(5), 445-463.

Maragos, J. E., Baines, G. B., & Beveridge, P. J. (1973). Tropical cyclone Bebe creates a new land formation on Funafuti Atoll. Science, 181(4105), 1161-1164.

Terry, J. P., & Goff, J. (2013). One hundred and thirty years since Darwin: ‘Reshaping' the theory of atoll formation. The Holocene, 23(4), 615-619.

Woodroffe, C. D., McLean, R. F., Smithers, S. G., & Lawson, E. M. (1999). Atoll reef-island formation and response to sea-level change: West Island, Cocos (Keeling) Islands. Marine Geology, 160(1), 85-104.

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