decay of dissolved uranium is observed. Deep-water corals
therefore incorporate small amounts of
230 Th during the
formation of their skeleton, and a significant correction is
required to achieve precision and accuracy in equivalent
ages (Cheng et al. 2000a; Frank et al. 2004).
Shells of mollusks have another distinctive feature. The
organism precludes incorporation of uranium during the
formation of its aragonite shell through a still unknown
biological process. A modern shell contains very little
uranium (in the range of a few tens of ng/g), whereas an
aragonite coral skeleton contains a few micrograms/g. This
would not be problematic for U/Th dating if the shell
remained in a closed system during its preservation in
sediment. Unfortunately, it was observed that, after the
death of the organism, the shell takes up uranium from its
environment. The uranium concentration may increase by
up to 10 mg/g, which is 100 times higher than at the time of
its formation. As it is impossible to identify the source of
the excess uranium and how it has accumulated over time,
shells of mollusks are considered as open systems for
uranium. Thus, one of the two basic conditions of dating
mentioned above is violated (closed system), implying that
based on our current knowledge, U/Th dating is not a
suitable dating method for shells and mollusks (Kaufman
et al. 1996).
For continental secondary carbonates, such as stalagmites, travertine or tuff, dating by U/Th follows the same
‘analytical and theoretical’ principles presuming the same
two assumptions and using the same basic Eqs. (6.2) and
(6.3) to determine the age. In groundwater a disequilibrium
between
238 U,
234 U and
230 Th is created due to the fact that
uranium is easily dissolved by the weathering of rocks, soils
and sediments, while the less soluble thorium is essentially
absent. In fact, the precipitation of calcite in the form of
stalagmites, travertine or calcareous tuff leads to the
co-precipitation of uranium in the carbonate without its
radioactive daughter,
230 Th. Therefore, as in corals a
radioactive disequilibrium is created during the formation of
the carbonate. The concentration of uranium in these secondary carbonates is related to the uranium content of
groundwater and the type of mineral formed. It can vary
from a few ng/g to hundreds of µg/g. However, the application of the dating method is more complex than for marine
organisms. The isotopic composition of uranium in
groundwater can be highly variable depending on the
weathering processes that come into play in an aquifer,
leading to activity ratio values for (
234
U/
238 U) that vary
between 0.8 and more than 5, or in delta notation between
−200‰ and more than +5000‰ (Kaufman et al. 1996).
Another issue is that continental carbonates are often
contaminated with clay or even organic particles. These may
exhibit high uranium and thorium concentrations, without
being in radioactive disequilibrium. Moreover, in porous
carbonates, such as tuffs or travertines, several generations of
crystals can be found in a single layer of carbonate, Mallick
and Frank (2002). This means that U/Th dating of continental carbonates is far more difficult than for corals. The
isotopy of uranium cannot be used to test if the system is
open or closed, and the presence of an initial supply of
thorium by groundwater or contaminants needs to be
investigated. To estimate the importance of contaminants,
the isotope—
232 Th is used, Szabo et al. (1994). It is the most
abundant isotope of natural thorium. It is at the origin of a
decay chain called after
232 Th. As clay contains lots of
thorium (>5 lg/g), the appearance of
232
Th in a carbonate
sample is a sign of the presence of such contaminants
(Ludwig and Titterington 1994).
Therefore, as for corals, sampling is a crucial stage of
U/Th dating. It is essential to select a sample from a layer of
carbonate containing a minimum of
232 Th, indicator of
contaminants, and the sample must be representative of the
original carbonate, i.e. the first generation deposited. Within
these constraints, the U/Th dating of stalagmites, travertine
and tuff can be accurate, and nowadays allows for precise
determination of time frames for climate (Wang, et al. 2001),
and archaeological (McDermott et al. 1993), studies.
Conclusions
Dating by U/Th methodologies has become a very powerful
tool, widely used to obtain a fine chronology of the growth
of coral reefs and many other carbonates. Although technical
developments have been rapid in recent years, with precise
isotopic measurements of radionuclides from the decay of
uranium, the quality of the sample itself and the movement
of radionuclides induced by their own radioactivity cause
problems for dating. Today, we no longer refer to a dating
system that is strictly closed, since the decay itself is partly
the cause of exchanges of uranium and thorium with the
coral environment. Correction models, known as ‘open
system age models’ are emerging to incorporate this
important issue. However, prior to being studied, the sampling process and characterization of the sample are crucial
as dating samples significantly altered by diagenesis is
doomed to failure.
A precise geochronological framework established under
optimum analytical conditions and based on samples with as
good a level of preservation as possible, provides exceptional opportunities to determine geological parameters such
as the uplift or subsidence of a coral reef or variations in sea
level. The U/Th dating method has now become a key element to place significant changes in major components of
the climate system within a precise temporal context. This
permits a direct comparison with astronomical forcings for
the Quaternary.
6 Dating of Corals and Other Geological Samples …
99
therefore incorporate small amounts of
230 Th during the
formation of their skeleton, and a significant correction is
required to achieve precision and accuracy in equivalent
ages (Cheng et al. 2000a; Frank et al. 2004).
Shells of mollusks have another distinctive feature. The
organism precludes incorporation of uranium during the
formation of its aragonite shell through a still unknown
biological process. A modern shell contains very little
uranium (in the range of a few tens of ng/g), whereas an
aragonite coral skeleton contains a few micrograms/g. This
would not be problematic for U/Th dating if the shell
remained in a closed system during its preservation in
sediment. Unfortunately, it was observed that, after the
death of the organism, the shell takes up uranium from its
environment. The uranium concentration may increase by
up to 10 mg/g, which is 100 times higher than at the time of
its formation. As it is impossible to identify the source of
the excess uranium and how it has accumulated over time,
shells of mollusks are considered as open systems for
uranium. Thus, one of the two basic conditions of dating
mentioned above is violated (closed system), implying that
based on our current knowledge, U/Th dating is not a
suitable dating method for shells and mollusks (Kaufman
et al. 1996).
For continental secondary carbonates, such as stalagmites, travertine or tuff, dating by U/Th follows the same
‘analytical and theoretical’ principles presuming the same
two assumptions and using the same basic Eqs. (6.2) and
(6.3) to determine the age. In groundwater a disequilibrium
between
238 U,
234 U and
230 Th is created due to the fact that
uranium is easily dissolved by the weathering of rocks, soils
and sediments, while the less soluble thorium is essentially
absent. In fact, the precipitation of calcite in the form of
stalagmites, travertine or calcareous tuff leads to the
co-precipitation of uranium in the carbonate without its
radioactive daughter,
230 Th. Therefore, as in corals a
radioactive disequilibrium is created during the formation of
the carbonate. The concentration of uranium in these secondary carbonates is related to the uranium content of
groundwater and the type of mineral formed. It can vary
from a few ng/g to hundreds of µg/g. However, the application of the dating method is more complex than for marine
organisms. The isotopic composition of uranium in
groundwater can be highly variable depending on the
weathering processes that come into play in an aquifer,
leading to activity ratio values for (
234
U/
238 U) that vary
between 0.8 and more than 5, or in delta notation between
−200‰ and more than +5000‰ (Kaufman et al. 1996).
Another issue is that continental carbonates are often
contaminated with clay or even organic particles. These may
exhibit high uranium and thorium concentrations, without
being in radioactive disequilibrium. Moreover, in porous
carbonates, such as tuffs or travertines, several generations of
crystals can be found in a single layer of carbonate, Mallick
and Frank (2002). This means that U/Th dating of continental carbonates is far more difficult than for corals. The
isotopy of uranium cannot be used to test if the system is
open or closed, and the presence of an initial supply of
thorium by groundwater or contaminants needs to be
investigated. To estimate the importance of contaminants,
the isotope—
232 Th is used, Szabo et al. (1994). It is the most
abundant isotope of natural thorium. It is at the origin of a
decay chain called after
232 Th. As clay contains lots of
thorium (>5 lg/g), the appearance of
232
Th in a carbonate
sample is a sign of the presence of such contaminants
(Ludwig and Titterington 1994).
Therefore, as for corals, sampling is a crucial stage of
U/Th dating. It is essential to select a sample from a layer of
carbonate containing a minimum of
232 Th, indicator of
contaminants, and the sample must be representative of the
original carbonate, i.e. the first generation deposited. Within
these constraints, the U/Th dating of stalagmites, travertine
and tuff can be accurate, and nowadays allows for precise
determination of time frames for climate (Wang, et al. 2001),
and archaeological (McDermott et al. 1993), studies.
Conclusions
Dating by U/Th methodologies has become a very powerful
tool, widely used to obtain a fine chronology of the growth
of coral reefs and many other carbonates. Although technical
developments have been rapid in recent years, with precise
isotopic measurements of radionuclides from the decay of
uranium, the quality of the sample itself and the movement
of radionuclides induced by their own radioactivity cause
problems for dating. Today, we no longer refer to a dating
system that is strictly closed, since the decay itself is partly
the cause of exchanges of uranium and thorium with the
coral environment. Correction models, known as ‘open
system age models’ are emerging to incorporate this
important issue. However, prior to being studied, the sampling process and characterization of the sample are crucial
as dating samples significantly altered by diagenesis is
doomed to failure.
A precise geochronological framework established under
optimum analytical conditions and based on samples with as
good a level of preservation as possible, provides exceptional opportunities to determine geological parameters such
as the uplift or subsidence of a coral reef or variations in sea
level. The U/Th dating method has now become a key element to place significant changes in major components of
the climate system within a precise temporal context. This
permits a direct comparison with astronomical forcings for
the Quaternary.
6 Dating of Corals and Other Geological Samples …
99