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decreased towards the present day. By dating sediments within the relict chan-
nels it is possible to constrain the age of this previously enhanced flooding
regime.
Coral luminescence
Massive corals contain annual growth layers similar to rings in trees.
Sometimes these growth layers display luminescent lines that can be seen when
the sawnsurface of massive corals is illuminated with long-wavelength ultra-
violet (UV) light (Isdale, 1984; Lough et al ., 2002). Luminescent lines, previously
known as fluorescent bands, were originally attributed to terrestrial humic sub-
stances incorporated into coral skeletons (Boto and Isdale, 1985). In this model,
humic acids are incorporated into river flood plumes as sediments are eroded
from river banks and floodplains during floods. Larger floods create plumes of
water which extend well offshore and the humic acids carried by these plumes
affect the outside layer of calcium carbonate or that year's growth layer on a
massive coral. Hence, large floods will not only extend further offshore but also
contain a higher quantity of humic acids and the resulting luminescence inten-
sity reflects the size or discharge of the terrestrial flood. Using this technique,
Isdale (1984)andIsdale et al .(1998)reconstructed river flood records for the
Burdekin River, North Queensland for the last 350 years.
More recent research has cast doubt on the notion that the humic acids
are responsible for the luminescent lines. Barnes and Taylor (2001)suggest that
variations in the luminescence of the annual growth layer are more likely to be
caused by variations in the growth of the coral skeletal architecture, which may
in turn be influenced by the volume of freshwater entering the sea. The skeletal
architecture is essentially a function of the size of the spaces between skele-
tal elements in the coral growth layer. Variations in this architecture probably
arise from reduced calcification during periods of decreased salinity resulting
from the dilution of seawater by freshwater run-off. This effect probably bet-
terexplains situations where the occurrence of luminescence in corals that
grow significantly offshore could not be influenced by terrestrial humic acids.
Even if humic acids are not responsible, the skeletal architecture hypothesis still
accommodates the idea that the luminescent lines record flood plumes into the
ocean. Hence, the timing and intensity of the lines can be closely related to the
timing and intensity of river run-off (Isdale 1984).
Lough et al .(2002)examined slices from 232 similar-sized colonies of massive
Porites corals from 30 reefs on the Great Barrier Reef (GBR), Australia. The coral
slices were viewed under UV light in a darkened room. For each colony and year,
theappearance of the luminescent lines was graded into one of four categories:
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