Other data sources - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 198

''Coral growth proliferates when the rate of sea-level rise equals or exceeds
the rate of land uplift, but when sea-level rise cannot keep up only patchy and
thin reef veneers develop. Important examples are from Barbados, Sumba and
Papua New Guinea. Uplift at Huon Peninsula, Papua New Guinea, is as high
as 4mm/yr, resulting in a more detailed record than is available from other
localities. Huon terraces of last interglacial age are now at elevations up to
400m, whereas in tectonically stable areas such reefs occur near modern sea
level.''

Corals can be dated by radiocarbon for the past 40,000 years and by the
formation of 230 Th through the radioactive decay of uranium for the past 500,000
years. However, Henderson (2005) cautioned that: ''the longer a coral sits around
waiting for a passing geochemist to take it back to the lab, the more likely it is

to be altered, causing addition or loss of uranium or thorium and making
uranium-thorium ages inaccurate.'' Over the past million years or so, global sea
level changed almost exclusively in response to the volume of water stored in the
ice sheets. Therefore, by dating any remnant shorelines that formed in the past, it
should be possible to directly assess the amount of water tied up in ice caps in
historical times. Since the oceans are presently roughly 100m or more higher than
they were during the Last Glacial Maximum, most of the glacial age shorelines we
might wish to study lie beneath the present level of the oceans. Carrying out
underwater geologic studies on submerged shorelines to establish the relationship
between the elevation of the sample to be dated and the elevation of the sea at the
time it formed is a dicult task. In addition, the problem of assessing past ocean
levels is complicated by the fact that almost all the land on Earth has changed
elevation in the intervening years.
A breakthrough was made when the 230 Th/ 234 U ages of corals from raised
shorelines in many locations in the tropics revealed a prominent sea level high
stand with an age of about 124,000 years. These radiometric methods were
applied to corals that depend upon the presence of Acropora palmata which is
known to grow only within the top 2m of sea level. It is now generally accepted
that these reefs formed during a time when global ice volume was slightly smaller
than today's. These areas, as a result of rapid tectonic uplift, have preserved the
shorelines from the last interglacial maximum and are amenable to radiometric
dating. Quite a number of reports have been published based on radiometric
dating of coral terraces. In this connection, a major point of interest is the timing
of the peak ocean level at the termination of the last interglacial. One result is
shown in Figure 6.8 .
M&M discussed the results of sea level measurements from coral terraces in
relation to the astronomical theory of ice ages. Figure 9.12 shows that solar input
to high northern latitudes minimized around 138,000 ybp , peaked around
125,000 ybp , and minimized again around 114,000 ybp . Based on solar input
alone, one would expect a time lag for maximum sea level after peak solar
intensity; so, one might guess that the astronomical theory would predict a
maximum sea level somewhat more recently than 125,000 ybp . Figure 9.14 shows

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