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188 CHAPTER 6. FORSCHUNGSSTELLE “RADIOMETRIE”
14 C in the present-day carbon cycle The present-day 14 C level is largely determined by the
re-equilibration of the atmosphere following strong 14 C input during the bomb testing up to 1962,
and the dilution of the natural 14 C level by anthropogenic, 14 C free, CO2 emissions (see contribution
by I. Levin in this report). In our laboratory continuous 14 C times series from several sites around
the globe have been measured up to today, now covering more than 40 years [Levin & Kromer, 2004].
Recently, 14 C has become an important marker to determine the ratio of fossil to present-day sources
of carbon, e.g. in the emissions trading. Here we are involved in pilot studies to establish legislative
procedures.
Th/U-Lab The Th/U-Lab works on continental archives, such as speleothems and travertines, as
well as on marine samples, such as deep sea sediments, Mn-nodules and corals.
One principal focus is the determination of the natural variations of Holocene and Late Pleistocene
climate using the stable isotopic composition of speleothems. Furthermore, we determine the magnitude,
timing, and duration of past sea level fluctuations from the position and age of fossil coral reefs
as well as the intensity of the Earths magnetic field during the past 350,000 years from 10 Berecords.
These studies deliver a precise time scale for the variations of past climate. This is a basic requirement
for the understanding of the causal relationships and the complex interplay between the forcing and
feedback mechanisms in the climate system during the past 350,000 years and during the Holocene. We
work in close cooperation with climate modelers from Hamburg and Berlin in the DEKLIM program
(through 5/2006). With modelers from Potsdam we apply their CLIMBER2 model to study the
abrupt climate changes that occurred during the Last Glacial (Dansgaard/Oeschger events).
Methods We use dating methods, relying on the disequilibrium of the natural decay chains (TIMS-
230 Th/U and 231 Pa/U). In addition, we use the decay of 10 Be, a radioactive product of cosmic rays.
Speleothems, an archive of paleoclimate Speleothems are an excellent climate archive because
they can be dated very precisely with the Th/U method. Stable isotope signals recorded in these
stalagmites can be measured at a resolution of 100 µm and hence provide a climate signal of one-yearresolution.
Most isotope signals in stalagmites display significant kinetic effects. These kinetic signals have been
related to the intensity of precipitation within the last ten years. For example, periods of enhanced
kinetic were ascribed to periods of less intense precipitation in speleothems from Oman and from
Central Germany [Burns et al.,2002; Fleitmann et al., 2003; Neff et al., 2001; Niggemann et al,2003].
The combination of this high resolution proxy for precipitation and the precise Th/U dating lead to
a number of internationally regarded publications. For example, we found a very good correlation
between the intensity of precipitation in Oman and the intensity of solar irradiation [Neff, 2001].
This relationship has been confirmed by a number of following studies [e.g. Holzkämper et al., 2004;
Mangini et al., 2005].
In November 2005 we were granted by the DFG for a Forschergruppe (www.fg-Daphne.de) to study the
basic processes affecting speleothem formation as well as the isotopic and the chemical signals. These
studies will establish and improve the applicability of speleothems as archives for past precipitation
and temperature. The DAPHNE project is funded for the next three years, with an option for funding
for three additional years. We will work in close collaboration with groups from Bochum, Trento (Italy)
and Innsbruck (Austria).
Reconstruction of Sea level from fossil corals Reconstructions of past sea level allow to determine
the magnitude, timing and duration of interglacial periods. which are essential for the understanding
of the causal relationships and the complex interplay between the forcing and feedback
mechanisms in the climate system.
Some reef coral genera grow in upper 5m below sea surface only and record past sea level fluctuations.
Thus, the determination of accurate U-series ages provides a direct method for sea level reconstruction.
Unfortunately, many fossil reef corals show clear evidence for post-depositional open-system behaviour.
There-fore, conventional Th/U-ages obtained from such corals cannot be considered as strictly reliable.
We dealt with this problem (i) by identifying corals that have not been altered [Scholz and Mangini,
in press] and (ii) by developing an appropriate correction technique [Scholz et al., 2004]. The results
enabled to reconstruct sea level during Marine Isotope Substage 6.5 (∼175,000 years before present).
The intensity of the Earth’s magnetic field in the past Accumulating evidence suggests that
solar activity is responsible for at least some climatic variability. These include correlations between