Published on: 10-Feb-2020
Left photo: Coral atoll island.
Right photo: example of one of the larger fossil micoatolls underwater,
this one has a live coral (the purple mound) that has settled and begun
to grow upwards, but not yet reached the water surface.
The sea-level of the Indian Ocean is rising at a rate and magnitude
nearly twice the global average, but insufficient data records have
stood in the way of understanding this strong response to climate. In a
study recently published in Nature Geoscience, ASE/EOS
researchers Dr Kyle Morgan, Research Assistant Ke Lin, Assoc Prof
Xianfeng Wang and Keven Roy together with colleagues from Canada,
Australia and New Zealand tracked relative sea level change on coral
atoll islands in the Maldives over the past two millennia. The findings
are both surprising and alarming: the tropical Indian Ocean has a
history of much larger sea level variation in response to climate
compared to the Atlantic and Pacific Oceans. For the latter two, solid
records of sea level show no more than 25 cm deviation from current
relative sea levels in the past 2000 years. However, as this study
reveals, it’s quite a different story in the tropical Indian Ocean,
where the relative sea level dropped and rose almost 90 cm at times
during the same time interval. The study, titled Climate-forced sea-level lowstands in the Indian Ocean during the last two millennia,
found two historical phases of very low sea level that are associated
with the cold time periods known as the Late Antiquity Little Ice Age
and the Little Ice Age.
To track historical sea levels and create a continuous proxy over the
past 2000 years, the researchers measured fossil coral microatoll
growth in the Maldives. Microatolls are corals that have
been constrained by water levels, providing a reliable long-term marker
of sea-level. Corals with hard calcified skeletons live up to about 200
years, but if the conditions are right, they can be preserved as fossils
for thousands of years after death. The fossilised microatoll coral
samples used in this study were unique because they were very well
preserved under water and were more favourable for precise age control.
U-Th (Uranium-Thorium) dating was used to date the coral fossils. The
method is well-established at the ASE/EOS isotope geochemistry lab to
determine the age of calcium carbonates, for example in corals.
Studying past changes in sea level is our best tool for understanding
current and predicting future changes in sea level. The researchers
will continue to work on corals from Maldives to answer questions such
as: ‘Are current rates of sea level rise in the tropical Indian Ocean
unprecedented?’ and ‘Can the decline of the Indian summer monsoon in
past half-century be linked to human-caused climate change?’.
More about the project here
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