model chosen takes account of the real phenomena sustained
by the sample during its geological history.
In conclusion, the analytical equipment available to us at
present provides high precision and potentially accurate
measures of U/Th ages of marine carbonates, with precisions
on the permil magnitude for ages ranging from current day
to several hundreds of thousands of years old. The limits on
dating are not technical but primarily imposed by the quality
of the samples. For recent samples, up to 15,000 years in
age, the U/Th dating system can be considered to be a closed
system, since the physicochemical alteration of corals and
the nuclear recoil effect have not yet had a significant impact
altering the U-series age.
For older corals, however, the impact of physicochemical
alteration and especially that of the nuclear recoil displacement result in a system partially open to exchange of
234
U
and
230 Th, requiring subsequent correction to obtain reasonable and accurate ages. The use of correction models
reduces precision in dating, as not only the analytical error
must be taken into account, but also the parameters of the
correction model used, such as the initial d
234
U and the
exchange factors f 234 and f 230 . Selection of the
best-preserved specimens possibly ensures the highest
quality of dating results that we can expect. If, however, the
sample shows signs of significant alteration, it is a prerequisite to measure several subsamples to better identify the
extent to which the system may have behaved open for
uranium.
Estimating the Change in Sea Level
from Tropical Corals
Changes in sea level come from the growth and melting of
polar ice caps when the climate of the Earth varies. Knowing
the exact variations in sea level during climate cycles can help
to reconstruct a precise chronology of climate changes and to
connect them with other parameters such as temperature
recorded in polar ice cores, the temperature and salinity of the
surface waters of the ocean, or even climate variations
recorded in continental archives. A chronological framework
established in this way helps to better constrain the phase
relationships between the various components of the climate
system and to determine the teleconnections between the
hemispheres. In addition, knowledge of sea level during climate cycles is essential to study the variations in the continental surfaces (expansion or reduction of the continental
margins) and their erosion. We have seen that U/Th dating of
corals is a powerful tool to determine the evolution of coral
reefs over time. But how can the ages of coral reef terraces
contribute to a reconstruction of changes in sea level?
Linking the age (its time of formation) and location of a
coral taken in a particular part of the world to sea level is
complex. In most cases, tropical corals are located in volcanic areas that can be very tectonically active (subsidence
or uplift). The sampled coral needs to have grown in situ and
not moved by erosion. Also, corals can live at different
depths depending on the species. On the continental margins, the processes of eustasy (change in volume of ocean
water) and isostasy (variation in altitude of the continent) are
also important. With changes in the volume of ice caps
during the climate cycles, the continent may well rise or fall
depending on the weight it supports. Given all these variables, the reconstruction of sea level from tropical corals
requires the selection of corals from a habitat whose characteristics are well known. Therefore, the tectonic processes,
which may have had an impact on their original position
compared to where they were collected, need to be identified. The parameters required can be defined as follows:
• chronological age t;
• the sampling height above present-day sea level h (+ or −);
• the rate of uplift or subsidence Dh/Dt;
• the average depth of the habitat of the species under
study d;
• the variability in the ecological habitat Dd.
The sea level over time m(t) can then be determined by
the following equation:
m t
ð Þ ¼ h þ d þ Dh=Dt  t:
The associated uncertainty is:
Dm t
ð Þ ¼ dh þ Dd þ t  d Dh=Dt
ð
ÞþDh=Dt  dt:
The reconstruction of sea level is highly dependent on
time and the Dh/Dt values. Moreover, it assumes that the rate
of uplift or subsidence has remained constant over the time
interval of the reconstruction.
The localization h and depth of habitat d can be well
constrained and are not, or only very little, influenced by
time. To minimize the impact of subsidence or uplift rates,
the ideal is to collect corals in the most stable places possible, in terms of tectonics, for the sea level (m) to be almost
completely a function of t. However, most studies are
located in places with strong uplift rates because these coral
terraces emerged and become easily accessible to sampling.
This sampling approach means that there is uncertainty
about the uplift rate and therefore about the reconstructed sea
level. Figure 6.7 shows reconstructions of sea levels
obtained from U/Th dating of tropical corals, as they have
been published for various regions ranging from the Pacific
(Tahiti, the Marquesas, Vanuatu, Australia, New Caledonia)
to the Atlantic (Barbados) (Bard et al. 1990; Cabioch et al.
2003; Frank et al. 2006; Gallup et al. 1994; Hanebuth et al.
2000; Thompson and Goldstein 2005). The graph also
6 Dating of Corals and Other Geological Samples …
97
by the sample during its geological history.
In conclusion, the analytical equipment available to us at
present provides high precision and potentially accurate
measures of U/Th ages of marine carbonates, with precisions
on the permil magnitude for ages ranging from current day
to several hundreds of thousands of years old. The limits on
dating are not technical but primarily imposed by the quality
of the samples. For recent samples, up to 15,000 years in
age, the U/Th dating system can be considered to be a closed
system, since the physicochemical alteration of corals and
the nuclear recoil effect have not yet had a significant impact
altering the U-series age.
For older corals, however, the impact of physicochemical
alteration and especially that of the nuclear recoil displacement result in a system partially open to exchange of
234
U
and
230 Th, requiring subsequent correction to obtain reasonable and accurate ages. The use of correction models
reduces precision in dating, as not only the analytical error
must be taken into account, but also the parameters of the
correction model used, such as the initial d
234
U and the
exchange factors f 234 and f 230 . Selection of the
best-preserved specimens possibly ensures the highest
quality of dating results that we can expect. If, however, the
sample shows signs of significant alteration, it is a prerequisite to measure several subsamples to better identify the
extent to which the system may have behaved open for
uranium.
Estimating the Change in Sea Level
from Tropical Corals
Changes in sea level come from the growth and melting of
polar ice caps when the climate of the Earth varies. Knowing
the exact variations in sea level during climate cycles can help
to reconstruct a precise chronology of climate changes and to
connect them with other parameters such as temperature
recorded in polar ice cores, the temperature and salinity of the
surface waters of the ocean, or even climate variations
recorded in continental archives. A chronological framework
established in this way helps to better constrain the phase
relationships between the various components of the climate
system and to determine the teleconnections between the
hemispheres. In addition, knowledge of sea level during climate cycles is essential to study the variations in the continental surfaces (expansion or reduction of the continental
margins) and their erosion. We have seen that U/Th dating of
corals is a powerful tool to determine the evolution of coral
reefs over time. But how can the ages of coral reef terraces
contribute to a reconstruction of changes in sea level?
Linking the age (its time of formation) and location of a
coral taken in a particular part of the world to sea level is
complex. In most cases, tropical corals are located in volcanic areas that can be very tectonically active (subsidence
or uplift). The sampled coral needs to have grown in situ and
not moved by erosion. Also, corals can live at different
depths depending on the species. On the continental margins, the processes of eustasy (change in volume of ocean
water) and isostasy (variation in altitude of the continent) are
also important. With changes in the volume of ice caps
during the climate cycles, the continent may well rise or fall
depending on the weight it supports. Given all these variables, the reconstruction of sea level from tropical corals
requires the selection of corals from a habitat whose characteristics are well known. Therefore, the tectonic processes,
which may have had an impact on their original position
compared to where they were collected, need to be identified. The parameters required can be defined as follows:
• chronological age t;
• the sampling height above present-day sea level h (+ or −);
• the rate of uplift or subsidence Dh/Dt;
• the average depth of the habitat of the species under
study d;
• the variability in the ecological habitat Dd.
The sea level over time m(t) can then be determined by
the following equation:
m t
ð Þ ¼ h þ d þ Dh=Dt  t:
The associated uncertainty is:
Dm t
ð Þ ¼ dh þ Dd þ t  d Dh=Dt
ð
ÞþDh=Dt  dt:
The reconstruction of sea level is highly dependent on
time and the Dh/Dt values. Moreover, it assumes that the rate
of uplift or subsidence has remained constant over the time
interval of the reconstruction.
The localization h and depth of habitat d can be well
constrained and are not, or only very little, influenced by
time. To minimize the impact of subsidence or uplift rates,
the ideal is to collect corals in the most stable places possible, in terms of tectonics, for the sea level (m) to be almost
completely a function of t. However, most studies are
located in places with strong uplift rates because these coral
terraces emerged and become easily accessible to sampling.
This sampling approach means that there is uncertainty
about the uplift rate and therefore about the reconstructed sea
level. Figure 6.7 shows reconstructions of sea levels
obtained from U/Th dating of tropical corals, as they have
been published for various regions ranging from the Pacific
(Tahiti, the Marquesas, Vanuatu, Australia, New Caledonia)
to the Atlantic (Barbados) (Bard et al. 1990; Cabioch et al.
2003; Frank et al. 2006; Gallup et al. 1994; Hanebuth et al.
2000; Thompson and Goldstein 2005). The graph also
6 Dating of Corals and Other Geological Samples …
97