The uranium-series technique is based on the observation
that carbonates precipitating in nature, in particular
corals and oolites, show an initial disequilibrium between
Th²³⁰ and its parent U²³⁸.
The amount of Th²³⁰ initially
present in a coral is negligible in comparison to that subsequently
generated by the radioactive decay of uranium. If
the system remains closed then the ratio of Th²³⁰ to U²³⁸
is a simple function of time. Because there is a 15 percent
excess of U²³⁴ to U²³⁸ in sea water this has to be
taken into account when determining the age.
Corals from New Guinea, the Loyalty Islands, and the
east coast of Australia were provided for dating. The New
Guinea corals, from the Huon Peninsula, had been dated previously,
and so they provided a check on the reliability of
the techniques used in this study. With one exception the
ages from this study are within the error limits placed on
the original ages. The one sample that does not agree is
shown to have a high proportion-of void-filling low-Mg calcite
cement.
Corals from the Capricorn Group and Hayman Island
within the Great Barrier Reef province show relatively
young ages. One coral recovered by drilling at a depth of
17m on the Hayman Island reef indicates that the time of
recolonization of the reef towards the end of the Holocene
transgression is about 8,300 yr B.P. Coral samples below a
marked discontinuity at a depth of about 20m are extensively
recrystallized. Ages of corals from the Inner Barrier of New South
Wales show that this feature 'formed during the last inter-glacial
at about 120,000 yr B.P. The ages suggest that
there were two periods of high sea level at about this time.
Ages from reef terraces 2-6m above present sea
level from three islands of the Loyalty Archipelago show
the varying degrees of uplift of these islands. Corals
from +2m on Beautemps-Beaupre are older than 200,000 yr
B.P., while a coral at +6.5m from Ouvea gave an age of
117,000 ± 6,000 yr B.P. Ages from the +2m terrace on
Lifou support the interpretation of a relatively high sea
level at about 180,000 yr B.P.
Ages of corals from a slightly raised fringing reef
around Mud Island, Moreton Bay indicate a sea level about
one metre higher than present during the interval 4,000 -
6,000 yrs B.P. This slightly higher sea level could have
been the result of a changing.tidal regime within the bay
