Atolls

Question 1

Darwin’s canonical model

Answer 1

A volcanic island forms in deep tropical waters, giving coral polyps a foundation to grow on- because they photosynthesise they can’t grow if the water is deeper than about 120 feet.

In time, the volcano becomes dormant and the islands begin to subside.

Coral reefs, originally fringing the edges of the island, become a barrier reef outlining the contour of the original coastline.

After the original island slips entirely (erosion), all that is left is a coral atoll.

Question 2

Darwin’s canonical model of reef development proposes an evolutionary sequence of reef forms as a volcanic island ages and subsides:
Fringing reef ->

BUT many islands do not follow this sequence…

Answer 2

Fringing reef -> lagoon bounding barrier reef -> atoll

Means reefs must be shaped by more than island subsistence alone.

Question 3

Conceptual diagram of possible motu formation and evolution on reef flat:

Answer 3

The reef flat accretes vertically until reaching an equilibrium depth (threshold for sediment mobilisation);

Subsequent lateral growth as the reef flat depth is maintained.

During an extreme event , increased bottom shear stress -> mobilises coarser grained sediment from reef edge -> deposited at the shear minimum ~ halfway across reef flat

Afterward, even if the coral rubble is below sea level, may be shallow enough that deposition of fine sediment over the coarse -> shoaling of a “protumutu”.

Continued deposition of sediment -> formation of a sub-aerial landmass onshore fo the reef edge

Motu progrades laterally over the reef flat until the reef flat reaches a critical width (balance of onshore/ offshore directed shear stress.

Question 4

What is the primary control on the development of coral reefs over the time scales of glacial cycles?

Toomey et al. (2013)

Answer 4

The vertical rate of reef accretion, the integrated effect of coral growth, local production of detritus, and lithification.

Question 5

Coral growth rate, and thus reef accretion rate, is a decreasing function of…

Toomey et al. (2013)

Answer 5

Water depth (a proxy for reduced light intensity, which limits photosynthesis by coral’s symbiotic algae).