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6.1. FORSCHUNGSSTELLE “RADIOMETRIE” 171
6.1.11 231 Pa/ 235 U-Dating of fossil corals with AMS
Kerstin Bauer (Participating scientists: Denis Scholz, Jörg Lippold)
Abstract The ages of fossil reef corals from Aqaba, Jordan, and Barbados, West Indies, are determined
by uranium-series dating. The samples are prepared with ion exchange chemistry and analysed
by thermal ionisation mass spectrometer (TIMS) for 231 Th, 234 U, and 238 U and by accelerator mass
spectometry (AMS) for 231 Pa and 235 U. The ages obtained by both methods are compared for consistency.
If they differ, the coral is assumed to show open-system-behaviour.
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Figure 6.12: Concordia diagrams for the U-Th-Pa system. The straight lines show the timely development
of the ( 230 Th/ 238 U), ( 234 U/ 238 U), and ( 231 Pa/ 235 U) activity ratios with an initial ( 234 U/ 238 U)
activity ratio of 1.1496 (modern seawater). The red circle indicates a coral which has behaved as a
closed system for 100 kyr. The dashed lines indicate the 100-kyr-closed-system isochrons.
Background The main goal of U-series dating
of fossil reef corals is to reconstruct past sea level
changes during the last 300,000 years. Some coral
species only inhabit the upper five metres below
the sea surface. If their age and their location relative
to the current sea level are known and if a
correction for the uplift or subsidence of the land
mass can be applied, the change of the sea level
can be calculated. It is an established procedure
to determine the age of carbonates by measuring
the ratio between 238 U and its decay series daughters
230 Th and 234 U. This has been done with the
TIMS of the group for several years. Still, the
age of a coral determined by this method can be
inaccurate due to open system behaviour and geochemical
processes of isotope mobilization which
alter the activity ratios which are used for dating.
Numerous criteria to identify diagenetically
altered corals can be applied. In addition, several
so-called open-system models have been developed
to correct the diagenetic changes and calculate
the true coral age [Scholz, 2005]. The subject
of this diploma thesis is another access to validate
the accuracy of fossil coral ages: the decay
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of 235 U to 231 Pa, which provides a second ”clock”.
Methods and results The 231 Pa/ 235 U dating
method can be applied for the last 250,000 years
and in this timeframe it can be compared with
the results of 238 U/ 230 Th method. If the coral behaved
as a closed system, both ages should be concordant
and different ages of a coral system should
plot on a concordia line (see Fig. 6.12). This technique
is planned to be tested on corals collected at
Barbados (West Indies) and Aqaba (Jordan, Red
Sea). Thereby the dating with 231 Pa is associated
with some difficulties as there must be a different
chemical sample preparation. In addition, there
is no long-living isotope which can be used as a
spike. The only possibility is to use 233 Pa (t 1/2
= 26,97 d) which must be separated shortly before
the measurement from a Np mother. Furthermore,
the abundance of Pa is lower than that of
Th and is not measured at the local TIMS, but at
an AMS at the ETH in Zuerich, Switzerland.
Funding Diploma thesis, therefore not applicable.