Atolls Flashcards
Darwin’s canonical model
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.
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…
Fringing reef -> lagoon bounding barrier reef -> atoll
Means reefs must be shaped by more than island subsistence alone.
Diversity of modern reef morphology arises from the combined effects of…
Island subsidence
Coral growth
Glacial sea level cycles
Formation feedback
if you relate water depth to reef accretion rate…
If your water depth is 0 your coral growth rate is going to increase to match whatever the sea level rise is.
Coral growth rate (-> reef accretion rate) is a decreasing function of water depth (a proxy for reduced light intensity, which limits photosynthesis by coral symbiotic algae).
If you relate water depth to reef accretion rate, what two things can happen:
Reefs can “catch up” with slow to moderate rates of relative SLR, growing toward the stable fixed point close to the surface.
Or the reef is submerged sufficiently deeply that is “gives up”, at least until sea level falls again.
If your water depth is 0, your reef accretion rate is going to increase to match
the pace of sea level rise
The deeper the water, the slower the accretion rate because the polyps need light.
Reefs beginning at depths below drowning depth can grow faster than SLR -> rise toward stable equilibrium near the surface (reaching balance between relative SLR and accretion).
These dynamics are driven by these specific, relatively simple interactions.
Coral growth
Wave erosion
Uplift (or subsidence)
Reef profiles and formation of barrier reefs are controlled by
subsidence and vertical coral growth and accumulation.
Why do the society islands follow Darwin’s progression but the Hawaiian Islands do not?
Subsidence and accretion rates in the society islands happen to fall within a narrow range that favors the formation of barrier reefs when islands experience.
Within the broader parameter spaces of uplift and growth rates, this “Goldilocks zone” for barrier reef formation is quite limited.
They follow this particular band of coral growth rate relative to subsidence rate that is just in that sweet spot.
Conceptual diagram of possible motu formation and evolution on reef flat:
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.
Atoll persistence and growth
Results challenge existing narratives of island loss
Island expansion has been the most common physical alteration throughout Tuvalu over the past four decades
Absence of a uniform or widespread erosion response indicates that sea lvel change alone cannot account for the observed island changes
Suggests a set of higher frequency processes affecting island change -> may mask the possible effects of incremental sea level change
Island are dynamic features that will persist as sites for habitation over the next century
Alternative opportunities that “embrace the heterogeneity of island types and their dynamics”?
