Catastrophic construction: Storms can build reef islands in atoll
Boulder, Colo., USA: Tropical storms, with waves reaching up to
10-meters-high, can wallop coral reef islands. As global temperatures
increase, some scientists suggest that such storms will become more
frequent and intense over the next few decades. Additionally, potential sea
level rise is perceived as a threat to the continued existence of these
remote, low-lying communities.
Many coral reef islands, or atolls, are created by water moving sand and
gravel, piling it up into consecutive ridged layers, "a bit like onions,"
says Paul Kench, earth scientist and dean of science at Simon Fraser
University, Burnaby, Canada. However, new research by Kench and colleagues
recently published online ahead of print for Geology uncovered a
different type of island construction: storm-deposited boulders.
On Tutago island in Tuvalu's Funafuti atoll, giant coral blocks sit on top
of a flat reef. Researchers noted the jumble of upended corals didn't grow
there, but instead were tumbled onto the site. As storm waves hit the
reefs, it's not just fist or head-sized gravel chunks that are moving—Kench
says coral boulders, which can be meters in diameter, are getting plucked
and hurled into piles, forming the island.
To better understand how storms shaped Tutago, the team collected
radiocarbon dates on the piled coral blocks. Within these boulder deposits,
they uncovered distinct clusters of ages, revealing that Tutago island
formed about 750 years ago, and at least two major storm events occurred
about 600 and 350 years ago.
Kench says the 300-year hiatus between these storm events may have given
nearby reefs a chance to recover after being plucked by the storm. "For
coral to grow to two meters diameter, it may take it a hundred and fifty
years," he says. "Island genesis is not only dependent on the storm, it's
also dependent on the rate of coral production.”
Understanding how Tutago formed can be helpful to other researchers
interested in atoll behavior, coral reef regeneration, and ancient storm
events. "I think there's certainly purchase in this notion that the islands
are archives of storm sequences," says Kench, adding that storm records in
the remote ocean can be tough to find.
Storm events have built Tutago up vertically, but that isn't always the
case for islands. Kench says that understanding how an increase in storm
frequency and intensity might modify atolls—especially whether they add or
destroy land—is extremely important to island communities and governments.
"It's quite real for some of these communities," he says. "They're dealing
with environmental change on a daily basis."
Kench, P., McLean, R.F., Owen, S.D., Tuck, M., and Ford, M.R.,
Storm-deposited coral blocks: A mechanism of island genesis, Tutaga island,
Funafuti atoll, Tuvalu: Geology, https://doi.org/10.1130/G45045.1. Contact:
Paul Kench, firstname.lastname@example.org.
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