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166 CHAPTER 6. FORSCHUNGSSTELLE “RADIOMETRIE”
6.1.6 14 C in speleothems
Jens Fohlmeister (Participating scientists: Bernd Kromer, Denis Scholz, Renza Miorandi (Trento),
Silvia Frisia (Trento))
Abstract Using the absolute age of speleothem samples from Th/U-dating, we can determine their
14 C reservoir age. The 14 C reservoir age is controlled mainly by pCO2 in the unsaturated soil zone and
the dissolution system of the limestone. The main factors governing soil pCO2 are soil temperature and
precipitation. Hence, variations in the 14 C reservoir age may provide information on the variability
of these two climate variables.
d c f [% ]
1 7
1 6
1 5
1 4
1 3
1 2
1 1
1 0
9
8
5 0 0 0 4 0 0 0 3 0 0 0 2 0 0 0 1 0 0 0 0
c a l a g e [y B P ]
Figure 6.6: Dead carbon fraction (dcf) of stalagmite
ER-76. Dcf is increasing with time. Possible
causes are changes in the dissolution system or an
aging of soil organic matter.
Background Speleothems (carbonates formed
in caves) are excellent archives for studies of paleoclimate.
We use 14 C of stalagmites as a geochemical
tracer of climate-related processes in the unsaturated
soil zone. A useful tool for radiocarbon
interpretation is the dead carbon fraction (dcf).
It determines the percentual fraction of carbon in
a stalagmite, coming from the limestone or from
old organic matter of the soil above the cave.
In the soil the dcf is influenced by the soil respiration
rate (atmospheric 14 C content) and the
age spectrum of soil organic matter (depleted
by radioactive decay). By dissolution of the
limestone ( 14 C free) the dcf increases. The increase
depends on the open/closed dissolution ratio
[Hendy, 1971]. For the dcf we expect values
between 0 (for an open system) and 50% (for a
closed system).
Methods and results We choose two time approaches:
firstly we take a section of a stalagmite
in the Holocene and secondly we investigate the
present day situation by analysing drip water covering
an annual cycle.
We measured the 14 C content of the stalagmite
ER-76 of the Grotta di Ernesto (N-Italy), covering
a time span from 2.3 to 4.9 ky before present
and one sample at 180 y BP. For the second approach
we investigate the drip water of two sta-
a 1 4 C [p m C ]
1 0 2
1 0 1
1 0 0
9 9
9 8
9 7
9 6
9 5
9 4
E R -7 6 s lo w d rip ra te
E R -G 1 fa s t d rip ra te
N o v J a n M a r M a y J u l S e p
M o n th 0 5 /0 6
Figure 6.7: 14 C in drip water of two stalagtites
(ER-76, ER-G1). The 14 C activity imply different
processes, which are responsible for the annual
cycle.
lagtites (ER-76, ER-G1) of the same cave. Each
month we collect one sample of each source.
Sample preparation for Accelerator Mass
Spectrometry (AMS) 14 C dating was done in
the Heidelberg radiocarbon labaratory. The stalagmite
samples were drilled under a CO2 free
atmosphere in a glove box. By acidifying the
calcite powder and the drip water samples we obtain
CO2 which is converted to graphite (required
by the ion source) by the semi-automated AMStarget
preparation line. The AMS measurements
were done in Lund. The δ 18 O and the δ 13 C of the
drip water samples were measured in Heidelberg.
Figures 6.6 and 6.7 show first results of the
Holocene section and drip water, respectivly. The
results will be evaluated using additional information
from stable isotope information of both approaches
and from meteorological parameters for
the annual cycle.
Outlook/Future work We will follow another
annual cycle of drip water samples. It is planned
to study another Holocene section obtained from
a different cave.
Funding DFG Forschergruppe 668: ”Datierte
Speläotheme: Archive der Paläoumwelt”
Main publication Claussen et al. [2006]